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University Microfilms International A Bell & Howell Information C om pany 300 North Zeeb Road, Ann Arbor, Ml 48106-1346 USA 313/761-4700 800/521-0600 Order Number 8916511 Systematic v a r ia ti o n in clay-mineral composition o f Wisconsinan age t i l l s h e et s in the Lake Michigan Basin. Monaghan, George William, Ph.D. Michigan S t a t e U n iv e r si t y , 1989 UMI 300 N. Zeeb Rd. Ann Arbor, MI 48106 SYSTEMATIC VARIATION IN CLAY-MINERAL COMPOSITION OF WISCONSINAN AGE TILL SHEETS IN THE LAKE MICHIGAN BASIN. By G. William M o n a g h a n A DISSERTATION S u b m i t t e d to M ic h i g an S t a t e U n i v e r s i t y in p a r t i a l f u l f i l l m e n t of t h e r e q u i r e m e n t s f o r t h e d e g r e e of DOCTOR OF PHILOSOPHY D e p a r t m e n t of Ge ological S c i e n c e s 1989 ABSTRACT SYSTEMATIC VARIATION IN CLAY-MINERAL COMPOSITION OF WISCONSINAN AGE TILL SHEETS IN THE LAKE MICHIGAN BASIN. By G. William M o n a g h a n X-ray diffraction exposures along the analyses of till s a m p l e s collected f ro m multi-till L ak e M ic h i g an s h o r e l i n e in Wi sco ns in a n d M ich ig an s h o w a s y s t e m a t i c v a r i a t i o n in t h e c l a y - m i n e r a l c o m p o s i t i o n of s u c c e s s i v e till s h e e t s . A g e n e r a l i n c r e a s e in t h e r e l a t i v e a m o u n t of 10A c l a y o c c u r s b e g i n n i n g w it h e a r l y o r mi ddl e W i s c o n s i n a n (Altonian) G len n S h o r e s till a n d t h r o u g h la te W i s c o n s i n a n ( m id - W o o d f o r d ia n ) G a n g e s - N e w B e r l in till a n d S a u g a t u c k - O a k C r e e k till. A s i g n i f i c a n t d e c r e a s e in t h e r e l a t i v e a m o u n t of 10A post-M ackinaw clay occurs within Interstade (lat e Wo od fo rd ia n ) O z a u k e e - H a v e n a n d Two R i v e r s tills. L a c u s t r i n e s e d i m e n t s sa m p l e d from b e t w e e n till s h e e t s m a i n t a i n e d a generally constant from the m i n e r a l o g y w it h e q u a l a m o u n t s of 7A a n d mid-Wisconsinan until the Two Creeks Interstade. 10A c l a y s Post-Two C r e e k a n l a c u s t r i n e s e d i m e n t s , h o w e v e r , s h o w a s i g n i f i c a n t i n c r e a s e in 10A clay co ntent. T h i s s u g g e s t s t h a t i n c o r p o r a t i o n of l a c u s t r i n e c l a y w i t h i n t h e till in n o t r e s p o n s i b l e f o r t h e o b s e r v e d s y s t e m a t i c v a r i a t i o n in till c l a y mineralogy. A s tro n g , positive relationship, however, exists between the a m o u n t of s h a l e / s i l t s t o n e c l a s t s in t h e till a n d t h e a m o u n t of 10A c l a y in t h e tills. T h i s i n d i c a t e s t h a t t h e v a r i a t i o n in till c l a y m i n e r a l o g y r e s u l t s f r o m e r o s i o n o f d i f f e r i n g p r o p o r t i o n s of l a c u s t r i n e c l a y a n d s h a l e d u r i n g W i s c o n s i n a n a d v a n c e s o f t h e L a k e M ic h i g a n Lobe. A progressive readvances increasing of the am ounts unroofing Lake of of shale M ic h i g a n shale clasts Lobe and outcrops is during believed 10A clay successive responsible minerals for within s u c c e s s i v e l y y o u n g e r till s h e e t s in t h e s o u t h e r n h a l f of t h e L a k e M ich ig an basin. A la c k of s h a l e o u t c r o p p i n g s a s well a s c o m p o s i t i o n a l s i m i l a r i t y b e t w e e n P o r t H u r o n a n d G r e a t l a k e a n a g e tills a n d L a k e C h ic a g o s e d i m e n t s , h o w e v e r , s u g g e s t t h a t c l a y m i n e r a l s in tills of t h e n o r t h e r n h a l f of t h e b a s i n w e r e d e r i v e d p r i m a r i l y fr o m r e w o r k i n g u n d e r l y i n g l a c u s t r i n e c l a y a n d s ilt d e p o s i t s . ACKNOWLEDGEMENTS I w is h , f i r s t of all, t o t h a n k t h e m e m b e r s o f my co m mi tt ee, Dr. Max M o r t l a n d , Dr. Michael Velbel, a n d help free ly g iven d u r in g Dr. J o h n Wilband f o r t h e a d v i c e a n d t h e c o u r s e of t h i s r e s e a r c h . Special th a n k s , h o w e v e r , m u s t b e g i v e n to t h e co m mi tt ee c h a i r , Dr. G r a h a m e J. L a r s o n f o r h is a d v i c e , b o t h p r o f e s s i o n a l a n d p e r s o n a l , a n d f r i e n d s h i p w i t h o u t w h i c h n o n e of t h i s presented Larson. two research here were could have hammered been completed. Most of t h e o u t of me ( s o m e t im e s p a i n f u l l y ) ideas b y Dr. T h a n k s al so go to my wife, Heidi Ro ss , who c a m p e d w it h me f o r summers assistant. in northern M ic hi ga n and was my most faithful field T h e g e r m s of m a n y of t h e i d e a s p r e s e n t h e r e i n w e r e so m et im e s o b t a i n e d d u r i n g c o n v e r s a t i o n s o v e r b e v e r a g e s w ith f r i e n d s a n d c o l l e a g u e s am o n g whom Dr. Steven Mattson, Greg Gephart, John Gellispi, Dave V e s t j o h n , a n d M an ri co De lco re s t a n d o u t in my mind. Acknowledgment is also ma d e to Chevron Oil Company and the D e p a r t m e n t of Geological S c i e n c e s , M ic hi ga n S t a t e U n i v e r s i t y , f o r f u n d i n g s u p p o r t to c a r r y o u t m u c h of t h e field w o r k a s s o c i a t e d w it h t h i s r e s e a r c h . S t i p e n d s u p p o r t w as a l s o s u p p l i e d b y U n i t e d S t a t e s Geological S u r v e y , Coal Division. iv TABLE OF CONTENTS LIST OF F I G U R E S ......................................................................................................................vii LIST OF T A B L E S ......................................................................................................................... viii INTRODU CTIO N .......................................................................................................................... 1 LITHOLOGY AND STRATIGRAPHY OF TILL SHEETS IN THE LAKE MICHIGAN B A S I N ........................................................................... 4 4 M e t h o d s .......................................................................................................................... S o u t h w e s t e r n M i c h i g a n ......................................................................................... 9 S o u t h e a s t e r n W i s c o n s i n ..................................... 19 E a s t e r n a n d N o r t h e a s t e r n W i s c o n s i n ................................................................... 23 N o r t h w e s t e r n M i c h i g a n ............................................................................................... 25 S u m m a r y of C l a y - M i n e r a l V a r i a t i o n in t h e L ak e M ich ig an Bas in . 28 THE ORIGIN OF CLAY MINERALS FROM TILLS IN THE LAKE MICHIGAN B A S I N ........................................... 31 P e d o g e n i c M o d e l ............................................................................................................. 31 L a c u s t r i n e - M i x i n g M o d e l ...............................................................................................34 B e d r o c k - M i x i n g M o d e l ....................................................................................................39 D i s c u s s i o n ........................................................................................................................... 49 Ic e Flow V a r i a t i o n Model .......................................................................................... 47 U n r o o f i n g Model ............................................................................................................. 49 G len n S h o r e s Till? .........................................................................................................54 SUMMARY AND C O N C L U S I O N S ............................................................................................... 57 B I B L I O G R A P H Y ................................................................................................................................ 62 APPENDIX A: TABLES OF CLAY MINERAL DAT A ..........................................................63 S a u g a t u c k Till (Glenn S h o r e s S e c t i o n ) .............................................................. 69 S h a l e C l a s t s ( S a u g a t u c k till; G len n S h o r e s S e c t io n ) ............................. 69 G a n g e s Till (Glenn S h o r e s S e c t io n ) ................................................................... 69 S h a l e C l a s t s ( G a n g e s Till; Glenn S h o r e s S e c t i o n ) .......................................70 Middle W i s c o n s i n a n L a c u s t r i n e (Glenn S h o r e s S e c t i o n ) ............................70 G len n S h o r e s Till (Glenn S h o r e s S ec t io n ) ..................................................... 70 S h a l e C l a s t s (Glenn S h o r e s Till; G lenn S h o r e s S e c t i o n ) ............................70 S a u g a t u c k till ( C e d a r Bluf f S e c t i o n ) ...................................................................... 71 S a u g a t u c k till (Mimoi P a r k S e c t i o n ) .......................................................................71 S a u g a t u c k till ( P a r k Cliffs S e c t io n ; A r e a G ) ................................................ 71 S a u g a t u c k till ( P a r k Cliffs S e c t io n ; A r e a N ) ................................................ 71 L a k e M ilw au k ee (?) S e d i m e n t s ( P a r k Cliffs S e c t i o n ) .................................. 72 M iddle(?) W i s c o n s i n a n L a c u s t r i n e ( P a r k Cliffs S e c t i o n ) ............................72 v Middle W i s c o n s i n a n L a c u s t r i n e ( S o u t h H a v e n S e c t i o n ) .......................... 72 La k e B o r d e r M o ra in e ( S o u t h w e s t e r n M i c h i g a n ) .......................................... 73 V a l p a r a i s o M o r a in e ( S o u t h w e s t e r n M i c h i g a n ) ...............................................73 V a l p a r a i s o M o r ai n e ( I n d i a n a a n d I l l i n o i s ) .........................................................74 S t u r g i s - K a l a m a z o o S y s t e m ( S o u t h w e s t M i c h i g a n ) ......................................74 T e k o n s h a M o r ai n e ( S o u t h w e s t M i c h i g a n ) .........................................................74 Oak C r e e k till (St. F r a n c i s S e c t i o n ) .................................................................. 75 La k e Mil wa uk ee S e d i m e n t s (St. F r a n c i s S e c t i o n ) ......................................75 New B e r l in till (St. F r a n c i s S e c t i o n ) .................................................................. 75 O z a u k e e till ( S h o r e w o o d S e c t io n ) .......................................................................75 Glenwood S e d i m e n t s ( S h o r e w o o d S e c t i o n ) .........................................................75 "Oak C r e e k ( ? ) " till ( S h o r e w o o d S e c t i o n ) .........................................................76 Two R i v e r s till (Two C r e e k s S e c t i o n ) ............................................................. 76 Cal um et S e d i m e n t s (Two C r e e k s S e c t i o n ) .........................................................76 Glenwood S e d i m e n t s (Two C r e e k s S e c t i o n ) ....................................................76 H av en till (Two C r e e k s S e c t i o n ) ............................................................................77 Two R i v e r s (?) till ( P e t e r s o n P a r k S e c t i o n ) .................................................. 78 Two R i v e r s ! ? ) Till ( I n d i a n C e m e t e r y S e c t i o n ) ...............................................78 ................................78 Two R i v e r s ( ? ) till ( S o u t h M anitou I s l a n d S e c t i o n ) Two R i v e r s f ? ) till (Pt. B et si e S e c t i o n ) ............................................................78 Calu met( ?) S e d i m e n t s (Pt. B e t s i e S e c t i o n ) .................................................... 78F il e r till ( O r c h a r d B ea c h S e c t i o n ) .......................................................................79O r c h a r d Be ac h till ( O r c h a r d Beac h S e c t i o n ) ............................................... 79 vi LIST OF FIGURES F i g u r e 1 Map of t h e G r e a t L a k e s R eg io n s h o w i n g t h e l o c a t i o n s of s e c t i o n s s t u d i e d a n d m o r a i n e s of t h e E r i e , S a g i n a w , a n d L a k e M ic h i g a n l o b e s ................................................................................................................. 6 F i g u r e 2 Map of a x e s of m a j o r m o r a i n e s in s o u t h w e s t e r n M ich ig an s h o w i n g t h e d i s t r i b u t i o n of S a u g a t u c k till ( o p e n c i r c l e s ) a n d G a n g e s till (c lo s ed c i r c l e s ) ..................................................................................... 3 F i g u r e 3 7A/10A f r e q u e n c y h i s t o - g r a m s of till a n d l a c u s t r i n e u n i t s e x p o s e d a t s e c t i o n s w i t h i n s o u t h w e s t e r n M i c h i g a n .....................................12 F i g u r e 4 7A/10A f r e q u e n c y h i s t o - g r a m s of till a n d l a c u s t r i n e s a m p l e s f r o m s o u t h w e s t e r n M ic h i g an ( u p p e r p a r t of d i a g r a m ) a n d S t . F r a n c i s s e c t i o n (low er p a r t of d i a g r a m ) ...................................................... 18 F i g u r e 5 7A/10A f r e q u e n c y h i s t o - g r a m s of till a n d l a c u s t r i n e s a m p l e s fr o m t h e S h o r e w o o d s e c t i o n ( u p p e r p a r t of d i a g r a m ) , Two C r e e k s s e c t i o n (middle p a r t of d i a g r a m ) , a n d NW. M ic h i g an s e c t i o n s (low er p a r t of d i a g r a m ) ................................................................................................. 22 F i g u r e 6 C om po sit e 7A/10A f r e q u e n c y h i s t o g r a m s f o r till s h e e t s a n d i n t e r v e n i n g l a c u s t r i n e u n i t s in t h e L a k e M ic hi ga n b a s i n ......................... 30 F igure 7 Map showing distribution of A n t r i m - E l l s w o r t h and C o l d w a t e r F o r m a t i o n s w i t h i n t h e L ak e M ic h i g an b a s i n ............................. 33 F i g u r e 8 7A/10A f r e q u e n c y h i s t o g r a m s of A n t r i m - E l l s w o r t h a n d C o l d w a t e r s h a l e s ( u p p e r p a r t of t h e d i a g r a m ) a n d s h a l e p e b b l e s c o l l e c t e d f r o m t h e t h r e e tills a t t h e G len n S h o r e s s e c t i o n (lower p a r t of d i a g r a m ) .................................................................................................................40 F i g u r e 9 Plot s h o w i n g r e l a t i o n s h i p b e t w e e n %shale c l a s t s (>3.0 mm) f o u n d in till a n d a v e r a g e 7A/10A r a t i o v a l u e s f o r t h e till u n i t a t th e section from which d a ta c o l l e c t e d ................................................... 44 F i g u r e 10 C u m u l a t i v e p e r c e n t d i a g r a m of r e l a t i v e a b u n d a n c e of illite (I), k a o l i n i t e (K), a n d C h l o r i t e (C) in till a n d l a c u s t r i n e u n i t s d e t e r m i n e d b y HCL m e t h o d ......................................................................................... 56 v ii LIST OF TABLES T able 1. C o r r e l a t i o n c h a r t o f s e l e c t e d till s h e e t s i n t h e G r e a t L a k e s R e g i o n ................................................................................................................................... 3 T able 2. C l a y - m i n e r a l , m a t r i x g r a i n siz e, a n d m a j o r c l a s t l i t h o l o g y d a t a f r o m s e l e c t e d till u n i t s in t h e L a k e M ic h i g a n b a s i n ..................... ]_j T a b l e 3. P e b b l e c o u n t d a t a f r o m c o m p o s i t e till s a m p l e s c o l l e c t e d a t s e c t i o n s w i t h i n t h e L a k e M ic h ig an b a s i n ........................................................... 13 v iii INTRODUCTION R e s e a r c h c o n d u c t e d i n t h e G r e a t L a k e s r e g i o n o v e r t h e p a s t 25 y e a r s h a s b o t h r e v i s e d a n d r e f i n e d t h e W i s c o n s i n a n s t r a t i g r a p h i c s y s t e m (Willman a n d F r y e 1970; D r e im a n is a n d G o l d t h w a i t 1973; E v e n s o n 1973; Acomb e t a l . 1982; M ic k el s on e t a l . 1984; M o n a g h a n e t a l . 1986; M o n a g h a n a n d L a r s o n 1986; S c h n e i d e r a n d Need 1985). east of and west margins P a r t of t h i s s y s t e m d e f i n e d a l o n g t h e Lake M ic hi g an , together with probable c o r r e l a t i o n s of till u n i t s fr o m l a k e b a s i n s e a s t of La ke M ic hi ga n, is s h o w n in T a b l e 1. F o r i n s t a n c e , S c h n e i d e r (1983) a n d S c h n e i d e r a n d Need (1985) h a v e d e s c r i b e d two m i d - W o o d f o r d i a n a g e till u n i t s a l o n g t h e w e s t s h o r e of Lak e M ic hi ga n in s o u t h e a s t e r n W is co n s in (tills of t h e New B e r l i n a n d Oak C r e e k F o r m a t i o n s ; Ta b l e 1) a n d c o n c l u d e d t h a t a s i g n i f i c a n t r e t r e a t of t h e L a k e M ic hi ga n Lobe o c c u r r e d b e t w e e n t h e d e p o s i t i o n of t h e s e tills. n o r t h of t h i s a r e a , in e a s t e r n a n d (1982) have defined (O z a u k e e , H a ve n , a n d three n o r t h e a s t e r n W isc ons in , Acomb e t a l . till u n i t s r e l a t e d Valders Just to t h e P o r t H u r o n a d v a n c e till m e m b e r s of t h e Kewaunee F o r m a t io n ; Ta b l e 1) a n d o n e till u n i t r e l a t e d to t h e G r e a t l a k e a n a d v a n c e (Two R i v e r s Till M ember, K e w a u n e e F o r m a t i o n ) . T h e y h a v e al so n o t e d t h e o c c u r r e n c e of till u n i t s o l d e r t h a n t h e M ac k in a w I n t e r s t a d e . Along the east s h o r e of L a k e M i c h i g a n in s o u t h w e s t e r n M ic hi ga n, M o n a g h a n e t a l . (1986) h a v e d e f i n e d t h r e e W i s c o n s i n a n till s h e e t s (Glenn Shores, Ganges, and Saugatuck; T a b l e 1) 1 and have suggested that a s i g n i f i c a n t r e t r e a t o f t h e L a k e M ic h i g a n Lo be o c c u r r e d b e t w e e n d e p o s i t i o n o f e a c h of t h e s e tills. In addition, th e y have noted th a t a system atic v a r i a t i o n in c l a y - m i n e r a l c o m p o s i ti o n , a p p a r e n t l y r e l a t e d to t h e s e r e t r e a t s , o c c u r s f r o m o n e till s h e e t to t h e ne xt . A l t h o u g h t h e a b o v e s t u d i e s h a v e c o n t r i b u t e d to a n u n d e r s t a n d i n g of the W i sc o n s i n a n , they are based prim arily on s tratig rap h ic fram eworks d e v e l o p e d from e m p i r i c a l a n a l y s i s of li th ol o gi c c h a r a c t e r i s t i c s of till u n i t s . I m pl ic it in t h i s e m p i r i c a l a p p r o a c h to s t r a t i g r a p h y is t h e a s s u m p t i o n n o t o n l y t h a t t h e li t h o l o g y of i n d i v i d u a l till s h e e t s r e m a i n s c o n s t a n t o v e r lo ng d i s t a n c e s , b u t al so t h a t c h a n g e s in li th o lo g i c c o m p o s i ti o n r e s u l t from some b a s i c c h a n g e in ice d y n a m i c s . R e c e n t l y , E y l e s e t a l . (1983) h a v e c r i t i c i z e d s u c h " p r o c e e d u a l a p p r o a c h e s ” to d e s c r i p t i o n of g l a c i g e n i c s e d i m e n t s b e c a u s e they often are not related in any simple fashion to depositional e n v i r o n m e n t s a n d do n o t g e n e r a l l y c o n s i d e r f a c i e s v a r i a b i l i t y . Th e p u r p o s e of t h i s p a p e r is two fold. clay-mineral sheets composition deposited during system atically the T h e f i r s t is to sh ow t h a t varies Wisconsinan between Stage successive throughout the till L ak e M ic h i g a n b a s i n a n d t h a t t h i s v a r i a t i o n is r e l a t e d to s i g n i f i c a n t f l u c t u a t i o n s of t h e L ak e M ic h i g an accounts framework. for this Lobe. variation The second in a is to p r o p o s e a model w h ic h basin-w ide erosional/ depositional 3 LAKE MICHIGAN LOBE NE H I . ' SE H I . 2 N IL .3 SW M I.4 Two R ive rs NH H I . 3 SAGINAH LOBE ERIE LOBE S M l.6 ONT.7 "Red V a ld e rs " 11.0 THG CREEKS INTERSTADE V alde rs Haven Ozaut.ee M a n itow o cShorewood "P o rt Huron" 13.3 MACKINAW INTERSTADE "F 're-F 'ort Huron" Oak Creek S augatuck U adsw orth HID-HOQDFORDIAN INTERSTADE Hew B e r lin Haeger York v i l i e Ganges MID-HI SCONS I NAN INTERSTADE ________ L TIME kB.P. _ L "P re -P o rt Huron" B edford P o rt S ta n le y ERIE INTERSTADE F u lto n 15.5 C a tfis h Creek PT. TALBOT INTERSTADE >40 Glenn Shores . Ta b l e 1. C o r r e l a t i o n c h a r t o f s e l e c t e d till s h e e t s in t h e G r e a t L a k e s Region. 'Acomb e t a l . (1982); 2S c h n e i d e r (1983), Mick el so n e t a l . (1984), a n d S c h n e i d e r a n d Need (1985); 2Willman a n d F r y e (1970), a n d J o h n s o n et al. (1985); •'Monaghan e t al. (1986); 5M elh o rn (1954); 6M o n a g h a n a n d L a r s o n (1986); 7D re im a ni s a n d G o l d t h w a i t (1973). 4 LITHOLOGY AND STRATIGRAPHY OF TILL SHEETS IN THE LAKE MICHIGAN BASIN M ethods Clay M ineralogy: compositions Most involve diffraction reflections 10 A verm iculite), c h l o r i t e 002 r e f l e c t i o n ) techniques peak-height from to quantify measurements of (smectite), 14A 18A (illite), clays employed and ( C a r r o ll 7A 1972; basal (001) X -ray (chlorite (kaolinite Brindley clay-m ineral and 001 and/or and/or Brown 1980). T h e s e m e a s u r e m e n t s a r e u s u a l l y r e p o r t e d e i t h e r a s r e l a t i v e p e r c e n t a g e s of t h e s e r e f l e c t i o n s (Glass 1981; Acomb e t a l . 1982; Willman e t a l . 1963) o r a s simpl e r a t i o s between two of t h e p e a k s (Rieck e t a l . 1979; C h u n g 1973; M o n a g h a n a n d L a r s o n 1986; M o n a g h a n e t a l . 1986). T h e l a t t e r m e t h o d , in t h i s c a s e 7A/10A p e a k - h e i g h t r a t i o s of t h e c l a y - s i z e f r a c t i o n (<2 m i cr o n s ) of till, has been used in t h i s a m o u n t s of 7A a n d 10A c l a y s . paper to s h o w the variation in r e l a t i v e S a m p l e s of f r e s h , g e n e r a l l y u n l e a c h e d till a n d l a c u s t r i n e s e d i m e n t w e r e p r e - t r e a t e d w it h s o d iu m a c e t a t e , w a s h e d with distilled w ater, dep o sited on glass slides as a s l u r r y and air d ried . They w e r e t h e n p l a c e d in a n e t h y l e n e - g l y c o l a t m o s p h e r e f o r 72 h o u r s a n d Xrayed. De tails of t h e s e m e t h o d s a r e d i s c u s s e d in M o n a g h a n (1984) a n d M o n a g h a n e t a l . (1986). Matrix a n d C la st L i t h o l o g y : Both c l a s t a n d m a t r i x l i t h o l o g y a s well a s t h e d i s t r i b u t i o n of g r a i n - s i z e s in till m a t r i x (<2 mm) a r e com monly e m p l o y e d b y r e s e a r c h e r s w i t h i n t h e L a k e M ic h i g an b a s i n to e i t h e r d i f f e r e n t i a t e till units (M urray 1953; Willman a n d Frye 1970; B u r g i s and Eschman 1981; Acomb e t a l . 1982; S c h n e i d e r 1983; S c h n e i d e r a n d Need 1985; M ic k el s o n e t 5 al. 1984) or to distinguish A n d e r s o n 1957). provenance ( S la w s o n 1933; Krumbein 1933; G enerally, lithologies a r e determ in e d by s e p a r a t in g the p a r t i c l e s i z e of i n t e r e s t ( u s u a l l y <2mm f o r m a tr ix a n d >3-4 mm f o r c l a s t s ) f r o m a b u l k s am pl e a n d s i m p l y c o u n t i n g t h e n u m b e r of c l a s t s p r e s e n t from e a c h litho lolo gic c l a s s . M at ri x g r a i n - s i z e d i s t r i b u t i o n d a t a p r e s e n t e d in t h i s s t u d y a r e all r e p o r t e d e l s e w h e r e a n d t h e t e c h n i q u e s u s e d a r e d i s c u s s e d in t h e a p p r o p r i a t e r e f e r e n c e s . In this study, the lithology of clasts in the till was exa m in ed p r i m a r i l y to q u a n t i t a t i v e l y a s s e s s t h e r e l a t i v e a b u n d a n c e of s h a l e c l a s t s p r e s e n t in till. T h e li th o lo g i c c l a s s e s c o u n t e d w e r e c h o s e n b o t h b e c a u s e t h e y r e p r e s e n t t h e d o m i n a n t li th o l o g i e s p r e s e n t in till w i t h i n t h e r e g i o n ( A n d e r s o n 1957) a n d to e n a b l e d e t e r m i n a t i o n of t h e a m o u n t of s h a l e a n d siltstone compared to D e t e r m i n a t i o n of t h e discussed below), other relative shale bedrock types occurring in the tills. a m o u n t of s h a l e is i m p o r t a n t b e c a u s e may represent the d o m i n a te source for (as cl ay m i n e r a l s f o u n d in till. C la st li th o l o g i e s w ere analyzed for clay-m ineral because it better determ ined using c o m p o s i ti o n . represents the a c o m p o s i te A c o m p o s i te bulk of sam pl e c o m p o s i ti o n of samples w as used the till. A p p r o x i m a t e l y e q u a l a m o u n t s of i n d i v i d u a l s a m p l e s w e r e p l a c e d t o g e t h e r in a 16 l i t e r v e s s e l a n d w a s h e d in t a p w a t e r f o r s e v e r a l h o u r s to d i s a g g r e g a t e t h e c o m p o s i te . fraction was T h e c o m p o s i t e w a s t h e n w e t - s i e v e d a n d t h e >3 mm s iz e saved. c o m p o s i t e samp le. At least 75 clasts were then counted fr om the N.W. Mich. PPK -TR.-Mn.^ Shorewood ' St . Franis ^2 co X o <5 G l en n l S h o r e s ^ /T ^>J Ml 25 50 50 KM F i g u r e 1 Map of t h e G r e a t L a k e s Region s h o w i n g t h e lo c a t i o n s of s e c t i o n s s t u d i e d a n d m o r a i n e s of t h e Erie, S a g in a w , a n d L a k e Michigan lob es . Section shown by closed circles. T r e n d of m o r a i n e s s h o w n by h a t c h e r e d line ( d a s h e d w h e r e a p p r o x i m a t e ) ; i n t e r l o b a t e b o u n d a r i e s s h o w n b y d a s h - d o t t e d line ( a f t e r L e v e r e t t a n d T a y l o r 1915). Ka Kalamazoo Moraine ( S a g i n a w Lobe) Ka-S S t u r g i s - K a l a m a z o o Mor ain al S y s t e m MSW M is s i s s i n e w a M o ra in e PH P o r t H u ro n M oraine PPK P e t e r s o n P a r k S e c t i o n PWL TK Powell M or ain e T e k o n s h a M or ai ne (S ag in aw a n d La ke M ic h i g an lob es ) TR-MnTwo R i v e r s (WI) a n d M an is te e (MI) M o r a in e s VLP V a lp a r a is o M or ain e (IN, IL) 7 F ig u r e 2 Map of a x e s of m a j o r m o r a i n e s in s o u t h w e s t e r n M ic h i g a n s h o w i n g t h e d i s t r i b u t i o n of S a u g a t u c k till ( o p e n c i r c l e s ) a n d G a n g e s till ( c lo s e d c i r c l e s ) . 7A/10A h i s t o g r a m s s h o w c o m p a r i s o n of t h e c l a y - m i n e r a l c o m p o s i t i o n of t h e e a c h of t h e m o r a i n e s . H o r iz o nt a l ax i s s h o w s 7A/10A r a t i o v a l u e s ; v e r t i c a l axis s h o w s n u m b e r of s a m p l e s w i t h a p a r t i c u l a r r a t i o v a l u e . Mora ine na m e s s h o w n a l o n g r i g h t s i d e of h i s t o g r a m s . Generalized c ro s s s e c t i o n s h o w s t h e s t r a t i g r a p h y a n d r e l a t i o n s h i p to s e c t i o n s d i s c u s s e d in t h e t e x t f o r t h e till u n i t s a l o n g t h e L a k e M ic hi ga n shoreline. M or ain e p o s i t i o n s a f t e r L e v e r e t t a n d T a y l o r (1915); c r o s s s e c t i o n o f s o u t h w e s t e r n M ic hi ga n a f t e r L a r s o n a n d M o n a g h a n (1988). LU l U t I f i i r ItrilM F iq u r e t i i l v I M Ac ri l M Iw rittl l i l j v i i a W i i e i <4 l i i i M IIII l.U 7 / I OA S o u th w e s te r n M ichigan T h e s e q u e n c e o f till s h e e t s i n s o u t h w e s t e r n M ic h i g a n (Tab le 1) is b e s t exposed at the G le n n Shores section, shoreline about 16 km n o r t h of S o u t h H ere units three till (Glenn Shores, located along the L a k e Michi gan H av en , M ic hi ga n ( F i g u r e s Ganges, and 1 a n d 2). S a u g a t u c k tills) are e x p o s e d a n d a r e s e p a r a t e d f ro m e a c h o t h e r b y l a c u s t r i n e s a n d , silt, a n d clay (Glenn Saugatuck) Shores and deposits. Ganges) Saugatuck or by till, lacustrine w h ic h sand represents (G a n g e s the su rface and till t h r o u g h o u t m o st of s o u t h w e s t e r n M ic hi ga n, h a s b e e n t r a c e e a s t w a r d a l o n g the ground surface from the Lake M ic hi ga n shoreline to the S t u r g i s - K a l a m a z o o m o ra i n a l s y s t e m ( F i g u r e s 1 a n d 2) a n d c o r r e l a t e d with t h e W a d s w o r t h Till Memb er of t h e W ed r on F o r m a t i o n in Ill inois (M o n ag h an et ah 1986). Ganges till has been traced in t h e subsurface beneath S a u g a t u c k till from t h e s h o r e l i n e e a s t w a r d to e x p o s u r e s in t h e T e k o n s h a M or ain e ( F i g u r e 2) a n d c o r r e l a t e d w i t h t h e u p p e r p a r t of Y o rk v ill e Till Member (Wedron F o r m a t io n ) of t h e J o l i e t S u b l o b e in Illinois ( M o n a g h a n et al. 1986). Gephart et al. (1982) have proposed W i s c o n s i n a n a g e f o r Glenn S h o r e s till ( a n d o v e r l y i n g a mi dd le o r early s a n d , silt , a n d clay) b a s e d on r a d i o c a r b o n d a t e s r a n g i n g fr o m 37,150 to >48,000 y e a r s B.P. on detrital organic H ow ev er, y e a r s B.P. a m a te r ia l more (Larson within s tra tifie d recently obtained 14C deposits age overlying date of a n d M o n a g h a n 1988) f r o m t h i s d e p o s i t t h e till. 64,500 + 1000 suggests that Gl enn S h o r e s till may be o l d e r t h a n m id d le W is co n s i n an . B a s e d o n t h e r e g i o n a l s t r a t i g r a p h i c r e l a t i o n s h i p s of t h e tills a n d in p a r t on t h e f a c t t h a t s t r a t i f i e d d e p o s i t s s e p a r a t e t h e till u n i t s , M o n a g h a n e t a l . (1986) p r o p o s e d t h a t a s i g n i f i c a n t r e t r e a t of t h e L a k e M ic hi ga n Lobe 10 o c c u r r e d b e t w e e n d e p o s i t i o n of e a c h till s h e e t . the Tekonsha Saugatuck till M o ra in e of the represents the Lake S i n c e G a n g e s till m a k e s u p M ic hi ga n surface Lo be till fr o m the (Figure Sturgis 2) while Kalamazoo Mor ain al S y s t e m ( F i g u r e 2) w e s t w a r d to L ak e M ic h i g a n a n d o v e r l i e s G a n g e s till, M o n a g h a n e t a l . (1986) w e r e a l s o a b l e to p l a c e a minimum d i s t a n c e f o r t h e r e t r e a t r e c o r d e d b e t w e e n G a n g e s a n d S a u g a t u c k tills. They proposed that ( F i g u r e 2) fo llowi ng the formation of the Tekonsha M or ain e and d e p o s i t i o n of G a n g e s till, t h e L a k e M ic hi ga n Lobe r e t r e a t e d a t l e a s t 60 km w e s t w a r d i n t o t h e L ak e M ic h i g a n b a s i n a n d t h a t a p r o - g i a c i a l l a k e f o r m e d in front of the retreating readvanced east across System ( F i g u r e 2). ice margin. The ice margin subsequently s o u t h w e s t e r n M ic h i g an to t h e S t u r g i s - K a l a m a z o o Monaghan and Larson (1986) suggested that th i s r e t r e a t o c c u r r e d p r i o r to a p p r o x i m a t e l y 15,000 y e a r s a g o a n d is c o r r e l a t i v e w ith t h e Erie I n t e r s t a d e of t h e L a k e E ri e b a s i n . Average reported matrix for th e grain-size three distribution and clay-m ineral analyses till u n i t s a t G le nn S h o r e s a r e l i s t e d in T ab l e 2. Also f r e q u e n c y h i s t o g r a m s c o m p a r i n g t h e c l a y m i n e r a l c o m p o s i t i o n of t h e tills f r o m Glenn S h o r e s a r e s h o w n in F i g u r e 3. A p p li c a ti o n of t h e T - t e s t to t h e c l a y - m i n e r a l d a t a s h o w t h a t t h e t h r e e tills e x p o s e d a t G lenn S h o r e s h a v e d i f f e r e n t c o m p o s i t i o n s to t h e 99% c o n f i d e n c e lev el, a n d i n d i c a t e s t h a t a s i g n i f i c a n t i n c r e a s e in 10A c l a y c o n t e n t , r e l a t i v e to 7A cl ay , o c c u r s from the lo w e r m o s t till u n i t to the upperm ost till ( F i g u r e 3). Clay-mineral a n a l y s e s al so s h o w t h a t t h e l a c u s t r i n e s e d i m e n t s o c c u r r i n g b e t w e e n Glenn Shores and Ganges tills have b e t w e e n t h e two tills ( F i g u r e 3). a clay-m ineral c o m p o s i ti o n interm ediate 11 TILL SHEET CLAY-HINERAL DATA MATRIX 6RAIN SIZE DATA (71/lOU Z SAND T**o R iv e rs 2 * ‘ 'V a ld e rs * (NH H i . ) 3 * 1 0.89 1 0 .3 1 * 31 1.01 t 0 .2 8 7 Haven2 ’ 1 Ozaukee2 ’ 1 Z SILT Z CLAST LITHOLOGY Z CLAY I/H CB SH OT 50 19 14* 79 1 5 — — — 127 86 0 0 0.99 i 0 .3 4 * 0.87 t 0 .3 0 ° 16 13 56 47 28 40 15* 15s 80 74 0 10 6 1 Oak C reek4 - 1 Saugatuck3 ’ * 0.51 t 0 .2 0 ’ 0.58 + 0 .1 2 > ° 12 36 44 42 44 22 15’ 14>° 49 36 30 45 6 4 Nen B e r lin 4 ’ 1 Ganges3 ’ 3 0.91 t 0 .3 0 ’ 0.85 i 0 .1 8 ‘ ° 58 59 29 22 13 19 12’ 17l ° 82 52 3 27 3 4 Glenn S hores3 ,3 1.22 + 0 .3 2 * ° 46 34 20 15>° 70 13 2 T a b l e 2. C l a y - m i n e r a l , ma tri x g r a i n s iz e, a n d m a j o r c l a s t li t h o l o g y d a t a from s e l e c t e d till u n i t s in t h e L ak e M ic h ig an b a s i n . G r ai n size b o u n d a r i e s : S a n d 2.0 mm; Silt 0.062 mm; c l a y <0.004 mm. C la st l i t h o l o g y b a s e d >3 mm siz e f r a c t i o n ; I/M I g n e o u s / M e t a m o r p h i c , CB Lime sto ne- D olo m ite , SH S h a le , OT O t h e r ; d a s h r e p r e s e n t s <1%. S u p e r s c r i p t s in t h e "Till S h e e t " Column r e f e r to d a t a s o u r c e s ; f i r s t n u m b e r , m a tr ix g r a i n - s i z e d a t a ; s e c o n d n u m b e r , clay-m ineral data. S u p e r s c r i p t s in t h e "Clay Min era l D at a” a n d " P e b b l e C o u n t Data" c o l u m n s r e f e r s e c t i o n from w h ic h d a t a c o l le c t e d ; lo c at io n of s e c t i o n s f o u n d in F i g u r e s 1 a n d 2. JT h i s S t u d y ; zAcomb e t a l . 1982; 3M e lh o r n 1954; 4S c h n e i d e r a n d Need 1985; 5M o n a g h a n et al. 1986; 6Two C r e e k s ; 7P e t e r s o n P a r k ; ^S ho re w o od ; 9St. F r a n c i s ; 10G lenn S h o r e s . 12 Saugatuck t i l l ",i 11 (23) (0.38) (0.04) jm , n Ganges t i l l (39) (0.83) (0.11) _ J iu . aid-kisconslnan lake clay JLAx (9) (1.04) (0.08) Glenn Shores t i l l ( 21 ) ( 1. 21 ) ( 0 . 10 ) F i g u r e 3 7A/10A frequency h isto g ram s of ti ll and lacustrine units exposed at se c tio n s within s o u t h w e s t e r n M ic h i g a n . H o r iz o n ta l axi s shows 7A/10A ratio values; v e r t i c a l ax i s s h o w s n u m b e r of s a m p l e s w ith a p a r t i c u l a r r a t i o v a l u e . U n it n a m e s s h o w n a l o n g r i g h t of histogram s; section from which data collected shown along le ft side of d i a g r a m w ith line s e p a r a t i n g i n d i v i d u a l s e c t i o n s . U n i t s from i n d i v i d u a l s e c t i o n a r r a n g e d in s t r a t i g r a p h i c o r d e r . Data from G le n n S h o r e s fr o m M o n a g h a n e t al. (1986) i^ i I -n -frq Saugatuck t i l l 3 (10) (Cedar Bluff) 10.63) (0.04) Saugatuck t i l l (flioei Park) (IS) (0.561 (0.08) f r q 3 Saugatuck t i l l — Area M (Park C l if t s ) | J jj (9) (0.61) (0.06) Saugatuck t i l l — Area 6 (Park C liffs ) (15) (0.78) (0.08) ik . Lake Nil«aukee (Park C lif f s ) (12) 10.98) (0.17) JUUL-jijLtuU-j aid-Hisconsinan lake? (Park C lif fs ) (7) (1.01) (0.10) j 1^1 ■ , frq 7-i aid-Hisconsinan lake (South Haven) g ; n ........ - '■ i ^ 0.30 (10) (0.97) (0.11) r 1.00 ^>1 o 1 A ■ i 1.30 13 NAME OF TILL S HEET P E RCE N T CLAS T LITHOLOGIES I/M CB SH SS 79 80 86 81 1 1 0 0 4 4 O il Filer 14 15 12 14 Haven Ozaukee Orchard 15 15 15 80 74 80 - 10 4 1 o .j> 14 13 14 17 17 36 45 40 28 49 56 45 37 39 46 30 24 52 47 51 82 27 26 New B e r l i n 17 14 15 12 Glenn 15 70 Two R i v e r s "Valders" ( NW. MI.) Beach Saugatuck Oak [ ------------------- ----------------- ----------------- ----------------- ---------------- --------------- Creek 15 Ganges Shores CH 0T 1 - - 1 1 1 _ - - - 4 1 - o ? I - 3 6 *7 - c U 4 4 O ,3 - - n 9 £ ■j n cr .i. U - _ - 1 -I NAME OF S AMP LED SECTION Two C r e e k s P e t e r s o n Park P e t e r s o n Park Orchard Beach Two C r e e k s Shorewood Orchard Beach Glenn Sh ores Park C l i f t s ( A) Park C l i f f s ( N) B1oomi n g d a l e St Francis Shorewood Glenn S h ores Cedar B l u f f B1oomi n g d a l e St Francis Glenn Shores T a b l e 3. P e b b le c o u n t d a t a from c o m p o s i t e till s a m p l e s c o l le c t e d a t s e c t i o n s wi thi n t h e La ke M ic h i g an basin. Clast l i t h o l o g y b a s e d on p e r c e n t a g e of e a c h l i t h o l o g y in t h e >3 mm size f r a c t i o n ; I/M I g n e o u s M etam or phi c; CB Li m es to ne -D ol o m ite ; SH Shale ; SS S a n d s t o n e ; CH C h e r t ; OT O t h e r . Dash r e p r e s e n t s <1%. In o r d e r to a s s e s s t h e r e l a t i v e a m o u n t of s h a l e c o m p a r e d to o t h e r l i th o lo g i e s f o u n d in e a c h o f t h e till u n i t s a t Glenn S h o r e s , t h e li th o lo g i e s of pebble clasts outlined above. and show t h a t younger (>3 mm) were determ ined and counted using methods T h e r e s u l t s of t h e s e d e t e r m i n a t i o n s a r e l i s t e d in Ta bl e 3 the till u n i t s . r e l a t i v e a m o u n t of s h a l e Im portantly, the i n c r e a s e s in p r o g r e s s i v e l y i n c r e a s e in the rela tiv e a m o u n t of s h a l e g e n e r a l l y c o r r e s p o n d s w it h t h e i n c r e a s e in the rela tiv e a m o u n t of IDA clay in e a c h till u n i t ( T ab l e 3 a n d F i g u r e 3). C o r r e la ti o n of G a n g e s a n d S a u g a t u c k till with s p e c i f i c e n d m o r a i n e s of t h e Lake M ich ig an L o b e ( F i g u r e 2) is b a s e d on s i m i l a r i t i e s in c l a y - mineral comp osi tio n b e t w e e n t h e s e till u n i t s e x p o s e d a l o n g t h e l a k e s h o r e 14 a n d till c o m p r i s i n g s p e c i f i c m o r a i n e s . T h e c l a y - m i n e r a l d a t a p r e s e n t e d in M o n a g h a n e t a l . (1986), t o g e t h e r w it h a d d i t i o n a l d a t a c o l l e c t e d d u r i n g t h e course of t h i s Saugatuck study, and southw estern shows that Ganges M ic h i g an tills t h e c l a y - m i n e r a l c o m p o s i t i o n of b o t h remains (Figure generally 2). In constant addition, the throughout clay-m ineral c o m p o s i t i o n of till c o m p r i s i n g t h e o u t e r r i d g e of t h e V a l p a r a i s o M or ain e a r o u n d t h e s o u t h e r n e n d of L a k e M ic h i g a n ( W a d s w o r t h till in n o r t h e a s t e r n I n d i a n a a n d n o r t h w e s t e r n Ill inois) w a s al so d e t e r m i n e d . 2) show that Wadsworth till in I n d i a n a a n d These data (Figure Ill ino is is sim il ar i n c l a y - m i n e r a l c o m p o s i t i o n to S a u g a t u c k till. In order to exposed along the assess variability lakeshore in in clay-m ineral southw estern composition Mich ig an, e x p o s e d a t G le n n S h o r e s w e r e t r a c e d s o u t h w a r d . the of till till units A few km s o u t h of t h e Glenn S h o r e s s e c t i o n G a n g e s a n d Glenn S h o r e s tills d i p u n d e r t h e p r e s e n t beach. S a u g a t u c k till, h o w e v e r , c a n be followed s o u t h w a r d f r o m t h e Glenn Shores section ( F i g u r e 2). through several At a few p l a c e s km of relatively ( C e d a r Bluff, Miomi P a r k , a n d F i g u r e 2), S a u g a t u c k till w a s s a m p l e d a n d a n a l y z e d . analyses continuous exposure P a r k Cliffs; T h e r e s u l t s of t h e s e ( F i g u r e 3) s h o w t h a t in g e n e r a l t h e c l a y - m i n e r a l c o m p o s i t i o n of Saugatuck till remains relatively constant. One notable exception, h o w e v e r , is f o u n d a t t h e P a r k Cliffs e x p o s u r e , t h e s o u t h e r n most e x t e n t of c o n t i n u o u s e x p o s u r e of S a u g a t u c k till, a n d s h o w s t h a t t h e c l a y m i n e r a l o g y of S a u g a t u c k till c a n s i g n i f i c a n t l y v a r y o v e r a s h o r t d i s t a n c e . T h e P a r k Cliffs e x p o s u r e o c c u r s in b l u f f s a t t h e V a r i e t y P a r k t r a i l e r p a r k a l o n g t h e L a k e M ic h i g a n s h o r e l i n e a b o u t 2 km n o r t h of S o u t h H av en in A ll e g an County ( F i g u r e 2). The generalized stratigraphy of the e x p o s u r e b e g i n s a t l a k e l e v e l (177 m) w it h a b o u t 1-1.5 m of d i s c o n t i n u o u s deformed, interbedded sand and silt. The in t e r b e d s f o l d e d a n d t h e t o p of t h e f o l d s commonly t r u n c a t e d . of clay also o c c u r w ithin th e deposit. within yielded 1988). this an deposits, age one of w h i c h are often Some r o u n d e d c l a s t s Small a b r a d e d wood c h i p s o c c u r (300 mg of c a r b o n ) of >24,700 y e a r s B.P. tightly (WAT-1918; w as dated Larson and and Monaghan B a s e d o n t h e a g e wood w i t h i n it, t h e d e f o r m e d s i l t may r e p r e s e n t e v i d e n c e f o r a m i d - W i s c o n s i n a n (or o l d e r ) l a k e in t h e L a k e M ic h i g a n b a s i n . At t h e north and south e n d s of th e e x p o s u re th e silt d ip s u n d e r the p r e s e n t b e a c h a n d is n o t e x p o s e d . T h e d e f o r m e d s il t is u n c o n f o r m a b l y o v e r l a i n b y 2-4 m of l a m i n a t e d , t a b u l a r b e d d e d cl ay. often sep arated T h e t a b u l a r b e d s of c l a y a r e 2- 8 cm t h i c k a n d a r e by thin b e d s of f i n e s a n d a n d silt. T h e b e d s of c l a y becom e t h i n n e r t o w a r d s t h e to p of t h e d e p o s i t while t h e i n t e r b e d d e d s a n d and s il t l a y e r s beco me t h i c k e r . T hi n l a y e r s of d i s c o n t i n u o u s d ia m ic t o n o c c u r w it h t h e c l a y l a y e r s a n d o f t e n e x h i b i t flow s t r u c t u r e s . grades upw ards The clay i n t o a 2 -5 m t h i c k d e p o s i t of c r o s s b e d d e d a n d r i p p l e d medium to c o a r s e s a n d . T h e c r o s s b e d s a r e most common in t h e lo w er p a r t t h e d e p o s i t while r i p p l e d s a n d is more common in t h e u p p e r p a r t . clay often d r a p e th e rip p les. Silt a n d T h i s s e q u e n c e of l a c u s t r i n e s e d i m e n t c a n be t r a c e d n o r t h w a r d a n d is p r o b a b l y e q u i v a l e n t to t h e l a c u s t r i n e s a n d w h ic h o c c u r b e t w e e n G a n g e s a n d S a u g a t u c k till a t G len n S h o r e s . If so, t h e s e s e d i m e n t s w e r e d e p o s i t e d i n a n e x t e n s i v e p r o g l a c i a l l a k e f o r m e d p r i o r to 15,000 y e a r s a g o w h e n t h e L ak e M ic hi ga n Lobe r e t r e a t e d d u r i n g t h e E ri e Interstade. 16 The rip p le d Saugatuck till. and crossbedded The contact sand between is o v e r l a i n the b y a b o u t 5-6 m of till a n d underlying sand is g e n e r a l l y s h a r p ; h o w e v e r , d i s c r e t e l a y e r s o f c l a y a n d d ia m ic t o n o c c u r in t h e t o p s e v e r a l cm of t h e s a n d . sand pinches out and the At t h e s o u t h e r n e n d of t h e e x p o s u r e , t h e till directly overlies the la m i n a t e d cl ay. S a u g a t u c k till is o v e r l a i n b y a few m e t e r s of s a n d a n d p e b b l e to c o b b l e g r a v e l i n t e r p r e t e d to be Glenwood sh al lo w w a t e r a n d / o r b e a c h s e d i m e n t . Two a r e a s w e r e s a m p l e d , o n e a l o n g t h e s h o r e l i n e a t t h e n o r t h e n d of t h e e x p o s u r e (A re a N) a n d t h e o t h e r a b o u t 200 m to t h e s o u t h in a small gully (Are a G). Saugatuck Clay-m ineral till as well as Wisconsinan laminated clay. difference deposit. in analyses were performed mid-Wisconsinan deformed on samples s il t and of late T h e s e a n a l y s e s i n d i c a t e t h a t no s i g n i f i c a n t clay-m ineral c o m p o s i ti o n exists between either lacustrine I n f a c t , t h e c o m p a r i s o n of t h e c o m p o s i t i o n of t h e s e l a c u s t r i n e d e p o s i t s w it h t h e m i d - W i s c o n s i n a n l a c u s t r i n e s e d i m e n t s from Glenn S h o r e s al so shows no difference composition of tills (Figure sampled fr om 3). H o w e v er , w h e n Areas N and the G at Park c o m p a r e d d i s t i n c t i v e d i f f e r e n c e s a r e o b s e r v e d ( F i g u r e 3). till from A r e a N is si m i la r to Saugatuck till fr om clay-mineral the Cliffs are While S a u g a t u c k G lenn Shores S e c t i o n , S a u g a t u c k Till f r o m A r e a G h a s a c l a y - m i n e r a l c o m p o s i t i o n more li k e Ganges simi lar till. am ounts Till f r o m of shale both (39% a n d Areas N and G, however, 37% r e s p e c t i v e l y ; Ta b l e each 3) a n d have are sim i la r to t h e a m o u n t of s h a l e o b s e r v e d f o r S a u g a t u c k till a t Glenn S h o r e s a n d Miomi P a r k . of till can vary T h e s e d a t a s h ow t h a t w hi le t h e c l a y - m i n e r a l c o m p o s i ti o n significantly over a relatively short distance, the d i s t r i b u t i o n of p e b b l e l i t h o l o g i e s a p p a r e n t l y r e m a i n s r e l a t i v e l y c o n s t a n t . 17 A f in a l s i t e of i n t e r e s t , t h e S o u t h H a v e n P e a t Bed ( Z u m b u r g e a n d P otzger south 1956), i s l o c a t e d a l o n g t h e L a k e M ic h i g an s h o r e l i n e a b o u t 3 km of S o u t h Haven in Van B u r e n C o u n t y , M ic h ig an . Here, the top m e t e r of a c o n t o r t e d s i l t b e d c o n t a i n i n g small, a b r a d e d wood c h i p s d a t e d to 30,000 + 800 y e a r s B.P. ( E s c h m a n 1980) is e x p o s e d . The r a d io - c a r b o n d a t e o n t h i s d e p o s i t i n d i c a t e s t h a t i t may r e p r e s e n t s e d i m e n t d e p o s i t e d in a m i d - W i s c o n s i n a n l a k e a s h a s b e e n s u g g e s t e d b y E s c h m a n (1980). T he silt is u n c o n f o r m a b l y o v e r l a i n b y a d i s c o n t i n u o u s l a y e r of p e a t a n d lo g s d a t e d a t 11,200 + 600 to 10,860 + 350 y e a r s B.P. w h i c h is in t u r n o v e r l a i n b y 2 to 9 m of medium to c o a r s e s a n d w i t h s t r i n g e r s and l e n s e s of f in e to medium g r a v e l i n t e r p r e t e d to r e p r e s e n t sh a ll o w l a c u s t r i n e a n d / o r l i t t o r a l d e p o s i t f o r m e d d u r i n g t h e Cal um et t r a n s g r e s s i o n . were performed c l ay . Clay-mineral a n aly se s on s a m p l e s fr om t h e m i d - W i s c o n s i n a n d e f o r m e d sil t a n d T h e r e s u l t s of t h e s e a n a l y s e s s h o w t h a t t h i s d e p o s i t h a s a simi lar c l a y - m i n e r a l c o m p o s i t i o n to t h e l a c u s t r i n e s e d i m e n t s ( b o t h m i d - a n d late W i s c o n s i n a n ) fr o m G le nn S h o r e s a n d P a r k Cliffs ( F i g u r e 3). A s u m m a r y of the clay-m ineral variation observed in till e x p o s e d a l o n g t h e la k e s h o r e in s o u t h w e s t e r n M ic h ig an , t o g e t h e r w i t h i n t e r v e n i n g lacustrine silts and clays, is p r e s e n t e d in F i g u r e 4. It shows that a s i g n i f i c a n t i n c r e a s e in t h e r e l a t i v e a m o u n t of 10A c l a y o c c u r s b e g i n n i n g with m i d - W i s c o n s i n a n G le n n S h o r e s till a n d c o n t i n u e s t h r o u g h d e p o s i t i o n of Saugatuck till. The clay-m ineral c o m p o s i t i o n of i n t e r v e n i n g d e p o s i t s , h o w e v e r , s h o w s no s u c h t r e n d . area of southw estern M ic h ig an , the lacustrine I n f a c t , a t l e a s t in t h i s small composition of l a c u s t r i n e sedim ent d e p o s i t e d in t h e L a k e M ic h i g a n b a s i n a p p a r e n t l y r e m a i n s c o n s t a n t from t h e mid-W isconsinan t h r o u g h th e late Wisconsinan. 13 ■f rq Saugatuck t i l l (70) (0.63) (0.10) l a k e Hil»aukee* uediaeflt | F igure 4 7A/10A frequency histo-gram s of till and lacu strin e sam ples from s o u t h w e s t e r n M ich ig an ( u p p e r p a r t of d i a g r a m ) a n d S t . F r a n c i s s e c t i o n (lo w er p a r t of d i a g r a m ) . Named u n i t s s h o w n a l o n g r i g h t side of each histogram; section or area from w h ic h data collected show n along left side of diagram. U n it s from i n d i v i d u a l s e c t i o n a r r a n g e d in s t r a t i g r a p h i c o r d e r . H o r iz o nt a l ax i s s h o w s 7A/10A r a t i o v a l u e s ; v e r t i c a l ax i s r e p r e s e n t s t h e number of s a m p l e s with a particular ratio value. Southw estern Michi gan data r e p r e s e n t s a c o m p o s i te of d a t a s h o w n in F i g u r e 3. j n i g li (12) i 10.98) (0.17) mi ki-r-X Ganges t i l l (39) (0.83) (0.11) jiij. •aid-ttisconsinan* lake sediaent (19) (1.00) (0.10) JUL Slenn Shares t i l l (21) 11.21) (0.16) fr q J Li Oak Creek t i l l (13) (0.31) (0.13) Lake Hilaaukee sediaent ( 6 ) ( 1. 0 3 ) (0 . 10 ) New Berlin t i l l U R 0.30 1. 0 0 (12) (0.91) (0.11) “7 1.30 19 S o u th e a s te r n W iscon sin The sequence of mid-W oodfordian age till sheets in southeastern W i s c o n s i n (New B e r l i n a n d Oak C r e e k ; T ab l e 1) is well e x p o s e d a t t h e St. Francis section, ( F i g u r e 1). located about 8 km south of Milwaukee , Wi sconsin H ere , two till u n i t s r e p r e s e n t a t i v e of t h e Oak C r e e k a n d New B e r l i n f o r m a t i o n s (M ic k el so n e t a l . 1984) a r e s e p a r a t e d b y a t h i c k s e q u e n c e of glacial-lacustrine deposits S c h n e i d e r a n d Need 1985). a n d Need (C hristensen and Schneider 1984 and T h e two tills w e r e d i f f e r e n t i a t e d b y S c h n e i d e r (1985) b a s e d o n m a t r i x g r a i n - s i z e (Table 2). T h e p r e s e n c e of s t r a t i f i e d d e p o s i t s b e t w e e n t h e two ti ll s a t St. F r a n c i s a s well a s s e v e r a l other places suggest that in a the M il w au k ee a r e a retreat d e p o s i t i o n o f New of the led La ke Schneider M ic h ig an B e r l i n a n d Oak C r e e k tills. gl a c ia l l a k e w h i c h formed d u r in g this r e t r e a t and Lobe Need (1985) occurred to between T hey have named a p r o ­ L a k e Milwaukee. In p a r t b e c a u s e of t h e s i m i l a r i t i e s in m a t r i x g r a i n - s i z e , S c h n e i d e r (1983), M ickelson e t a l . (1984), a n d B er li n S c h n e i d e r a n d Need till c o r r e l a t e s w ith Haeger (1985) have suggested till of t h e H a r v a r d t h a t New S u b l o b e in Illinois a n d t h a t Oak C r e e k till c o r r e l a t e s w ith W a d s w o r t h till in Ill ino is (Table 1). M onaghan (1985, in press) has further suggested that Oak Creek till c o r r e l a t e s w it h S a u g a t u c k till in s o u t h w e s t e r n M ic h i g a n while New B er li n till p r o b a b l y c o r r e l a t e s w i t h G a n g e s till (Table 1). Th e c o r r e l a t i o n of S a u g a t u c k a n d Oak C r e e k till is r e a s o n a b l e s i n c e b o t h h a v e b e e n c o r r e l a t e d w i t h W a d s w o r t h till in Ill inois a n d all t h r e e of these units represent the youngest s o u t h e r n e n d of L a k e M ic h i g a n . till is more problem atic, however, surface till around m uc h of the C o r r e l a t i o n of G a n g e s w i t h New Be rl in since each has been correlated with 2Q different till units in I ll in oi s B e rl in till w it h H a e g e r till). (Ganges till w it h Y o r kv ill e till a n d New T h e c o r r e l a t i o n of G a n g e s till w it h Y o r kv il l e till b y M o n a g h a n e t a l . (1986) is b a s e d p r i m a r i l y o n a s u m m a r y b y J o h n s o n (1976) w ho s u g g e s t e d t h a t H a e g e r till c o r r e l a t e s in time w i t h W a d s w o r t h till a n d Y o r k v il l e till d i r e c t l y u n d e r l i e s b o t h H a e g e r a n d W a d s w o r t h tills. More r e c e n t l y , t h e r e l a t i o n s h i p s b e t w e e n Y or k v il l e, H a e g e r , a n d W a d s w o r t h tills h a s b e e n r e v i s e d to s h o w t h a t H a e g e r till (or i t ’s e q u i v a l e n t s o u t h of Ch ic ag o, Le m on t northeastern Vvadsworth Drift) Illinois actually and underlies Y o r k v il l e till is tills ( J o h n s o n e t a l . 1985). Wadsworth older than till throughout both Haegar and If c o r r e c t , G a n g e s till p r o b a b l y c o r r e l a t e s w ith H a e g e r till ( a n d / o r L em ont d r i f t ) . I n fa c t , J o h n s o n e t a l . (1985) p r o p o s e d t h a t a r e t r e a t , e q u i v a l e n t in time to t h a t r e c o r d e d b e t w e e n d e p o s i t i o n of New B e r l i n a n d Oak C r e e k tills, o c c u r r e d in Ill ino is b e t w e e n d e p o s i t i o n of H a e g e r a n d W a d s w o r t h ti lls a n d h a v e s u g g e s t e d t h a t La ke M ilw auk ee was formed during this interval. A recently published r a d i o c a r b o n d a t e of 15,240 + 120 y e a r s B.P. (ISGS-465; S p r i n g e r a n d Flemal 1981) p r o v i d e s a minimum a g e f o r W a d s w o r t h till ( J o h n s o n e t a l . 1985) a n d indicates that the Saugatuck-Oak retreat Creek 15,000 y e a r s B.P. recorded tills must between have Ganges-New occurred B erl in before and about If so, it p r o v i d e s f u r t h e r e v i d e n c e t h a t t h e r e t r e a t is a p p r o x i m a t e l y time e q u i v a l e n t to t h e E r i e I n t e r s t a d e of t h e L ak e Erie b a s i n (Table 1) a s s u g g e s t e d b y M o n a g h a n a n d L a r s o n (1986) a n d more r e c e n t l y b y M o n a g h a n (1985, in p r e s s ) . In order to test w hether a system atic, stratigraphic variation in c l a y - m i n e r a l c o m p o s i t i o n s im i la r to t h a t o b s e r v e d in s o u t h w e s t e r n Michi gan o c c u r s in e a s t e r n W isc o ns in , X - r a y d i f f r a c t i o n a n a l y s e s w e r e p e r f o r m e d on 21 s a m p l e s of New B e r l i n a n d Oak C r e e k tills c o l l e c t e d a t St. F r a n c i s . The results Oak of t h e s e a n a l y s e s ( F i g u r e 4) i n d i c a t e t h a t New B e r l i n a n d C r e e k tills n o t o n l y h a v e s i g n i f i c a n t l y d i f f e r e n t c l a y - m i n e r a l c o m p o s i t i o n s , b u t also th a t the two ti lls e x h i b i t a sim il ar s y s t e m a t i c i n c r e a s e in t h e r e l a t i v e a m o u n t of 10A c l a y i n c o r p o r a t e d w i t h i n t h e y o u n g e r till a s was o b s e r v e d in s o u t h w e s t e r n M ic h i g an ( F i g u r e 3 a n d F i g u r e 4). A p p l i c a t i o n of t h e T - t e s t to t h e two tills a t St. F r a n c i s w i t h t h o s e fr om G le n n S h o r e s s h o w s t h a t to t h e 95% c o n f i d e n c e le v el Oak C r e e k a n d S a u g a t u c k till h a v e the same general c l a y - m i n e r a l c o m p o si ti o n , as do a l s o New B e r l in and G a n g e s tills. P e b b l e c o u n t s of New B e r l in a n d Oak C r e e k ti ll s (Ta b le 3) indicate the that d o e s New Berl in. Oak C r e e k till c o n t a i n s significantly more shale than A l t h o u g h d i f f e r i n g in a b s o l u t e a m o u n t , b o t h t h e Glenn S h o r e s a n d St. F r a n c i s s e c t i o n s e x h i b i t a t r e n d t o w a r d i n c r e a s i n g s h a l e a n d 10A c l a y c o n t e n t in y o u n g e r till u n i t s . C l a y - m i n e r a l a n a l y s e s w e r e al so p e r f o r m e d on s a m p l e s of L ak e Milwaukee c l a y w h i c h o c c u r s b e t w e e n New B e r l in a n d Oak C r e e k tills. that these sedim ents T h e r e s u l t s of t h e s e a n a l y s i s ( F i g u r e 4) sh ow h a v e a simi lar c l a y - m i n e r a l c o m p o s i t i o n to all th e l a k e s e d i m e n t s a n a l y z e d from s e c t i o n s in s o u t h w e s t e r n M ich ig an . I n p a r t b a s e d on t h e i r litho log ic s i m i l a r i t i e s a s well a s t h e r e g i o n a l c o r r e l a t i o n s p r o p o s e d a b o v e , Oak C r e e k till is c o r r e l a t e d wdth S a u g a t u c k till, while New Be r l in till is c o r r e l a t e d a t l e a s t in p a r t w ith G a n g e s till. If these correlations are correct then the retreat recognized in s o u t h w e s t e r n M ic h ig an b e t w e e n d e p o s i t i o n of G a n g e s a n d S a u g a t u c k tills is p r o b a b l y e q u i v a l e n t to t h a t r e c o r d e d in W is co n s in b e t w e e n d e p o s i t i o n of New B e r l i n a n d Oak C r e e k tills (Tab le 1). 22 fr q 3 Ozaukee t i l l (10) (0.88) (0.03) Sleouood Phase sediaent (11) I ■A 1 (0.95) (0.13) I i •Oak Creek(?)* t i l l (12) (0.67) (0.07) IX fr q Tao River* t i l l F igure 5 7A/10A frequency histo-gram s of till and l a c u s t r i n e s a m p l e s fr om t h e Shorewood section (u p p e r p a r t of diagram), Two Creeks section (middle part of diagram), a n d NW. M ich ig an sections (lower part of diagram). Named u n i t s s h o w n along r ig h t side of each h i s t o g r a m ; s e c t i o n o r a r e a from w h ic h d a ta collected show a l o n g l e f t s i d e of d i a g r a m . U n i t s fr om i n d i v i d u a l s e c t i o n a r r a n g e d in s t r a t i g r a p h i c o r d e r . H o ri z o n ta l ax i s s h o w s 7A/10A ratio values; vertical axi s r e p r e s e n t s n u m b e r of s a m p l e s w ith a p a r t i c u l a r r a t i o v a l u e . Data f r o m NW M ichigan r e p r e s e n t s a c o m p o s i t e from s e c t i o n s d i s c u s s e d in te x t. (8) (0.89) (0.09) mu} Caluaet Phase sediaent 13) (0.70) (0.03) Slenaood Phase sediaent 19) (0.99) (0.11) JU t Jk Haven t i l l (10) (1.01) (0.16) I UUI i fr q 'Tao Rivers’ t i l l (29) (0.97) (0.14) 3 J lu L i 1 C aluaet(?) Phase sediaent (3) (0.76) (0.04) lip fr q Filer t i l l ■*- H a ■ , (14) (0.92) (0.17) , I Orchard Beach t i l l (27) (1.03) (0.17) 3 "' 1_I— 1— I— 1— a 0.30 I flji 1.00 —1 1.30 A 23 E a ste r n a n d N o r th e a s te r n W iscon sin A s e q u e n c e of t h r e e P o r t H u r o n a n d o n e G r e a t l a k e a n a g e till u n i t s f r o m e a s t e r n a n d n o r t h e a s t e r n W is co n s in (Table 1) h a s b e e n d e s c r i b e d b y Acomb e t a l . (1982). Two of t h e P o r t H u r o n ti lls, O z a u k e e a n d Hav en, o c c u r r e l a t i v e l y c o n t i n u o u s l y n o r t h w a r d a l o n g t h e L a k e M ic h i g an s h o r e l i n e from the P o r t H u r o n M o r ai n e to well n o r t h of t h e Two R i v e r s Moraine ( F i g u r e 1). G reatlakean age Two Rivers till, however, n o r t h w a r d from t h e Two R i v e r s M or ain e ( F i g u r e 1). only occurs T h e s e till u n i t s c a n be d i f f e r e n t i a t e d b a s e d on g r a i n - s i z e d i s t r i b u t i o n (Table 2) a n d t h e r e l a t i v e a m o u n t of e x p a n d a b l e c l a y m i n e r a l s (Acomb e t a l . 1982). To t e s t w h e t h e r a s y s t e m a t i c v a r i a t i o n in c l a y - m i n e r a l co m p o si ti o n , s u c h a s t h a t o b s e r v e d a t S t. F r a n c i s a n d Glenn S h o r e s , a l s o o c c u r s w i t h i n y o u n g e r s e q u e n c e s of till s h e e t s , two s e c t i o n s r e p r e s e n t a t i v e of P o r t H u ro n and Greatlakean s tr a t ig r a p h y were studied. The f i r s t , i n f o r m a l l y n am ed t h e S h o r e w o o d s e c t i o n , i n c l u d e s t h e o l d e r s e q u e n c e of till u n i t s , while t h e s e c o n d , t h e Two C r e e k s S e c t i o n , i n c l u d e s t h e y o u n g e r p a r t of t h e s e q u e n c e ( F i g u r e 1). The S h o r e w o o d s e c t i o n , l o c a t e d a b o u t 16 km n o r t h of t h e St. F r a n c i s s e c t i o n a n d n o r t h of t h e P o r t H u r o n Mora ine ( F i g u r e 1), i n c l u d e s a t l e a s t two till u n i t s s e p a r a t e d b y a t h i c k s e q u e n c e of g l a c i a l - l a c u s t r i n e d e p o s i t s (Acomb e t a l . 1982). T h e u p p e r till, O z a u k e e Till M em ber of t h e K e w au n ee F o r m a t i o n , is c o n s i d e r e d Huron M o r ai n e e q u i v a l e n t i n time to till c o m p r i s i n g (M ick el son et al. 1983) while the the Port lo w er till ( u n d i f f e r e n t i a t e d ) is c o n s i d e r e d to b e m i d - W o o d f o r d i a n ( p r e - P o r t H u ro n) in age (Acomb e t a l . 1982). Thus these two till u n i t s a r e separated by 24 Glenwood l a c u s t r i n e s e d i m e n t s d e p o s i t e d d u r i n g t h e M ac ki n a w I n t e r s t a d e (Ta bl e 1). C l a y - m i n e r a l a n a l y s e s o f Glenwood s e d i m e n t s w h i c h s e p a r a t e t h e p r e P o r t H u r o n fr o m O z a u k e e till ( F i g u r e 5) s h o w t h a t t h e y h a v e a simi lar c o m p o s i t i o n to L ak e M ilwaukee a n d m i d - W i s c o n s i n a n l a c u s t r i n e s e d i m e n t s fr o m southeastern Wi sco n si n and southw estern M ic h i g a n (Figure 4). C l a y - m i n e r a l a n a l y s e s p e r f o r m e d o n t h e two till u n i t s , h o w e v e r , s h o w t h a t each has a significantly different clay-m ineral composition ( F i g u r e 5). U n li k e t h e g e n e r a l i n c r e a s e in t h e r e l a t i v e a m o u n t of 10A c l a y o b s e r v e d a t G len n Shores Shorewood and section incorporated St. Francis, the sequence sh o w a d e c r e a s e in t h e within the younger till. of tills exposed at the r e l a t i v e a m o u n t of 10A c l a y Although s o m e w h a t lo w er in 10A c l a y c o n t e n t , t h e c o m p o s i ti o n of t h e lo w e r till is si m i la r to Oak C r e e k till fr o m t h e St. F r a n c i s S e c t i o n ( F i g u r e 4) a n d is p r o b a b l y e q u i v a l e n t to Oak Creek till. P e b b l e c o u n t d a t a co m p il ed f o r O z a u k e e a n d Oak C r e e k till f ro m S h o r e w o o d Creek (Tab le 3) al so s u p p o r t t h i s r e l a t i o n s h i p s i n c e b o t h Oak tills h a v e a similar s h a l e d i s t r i b u t i o n . Ozaukee and T h e s h a l e d i s t r i b u t i o n of Oak C r e e k ti ll s fr o m S h o r e w o o d a l s o s h o w s a d e c r e a s e in s h a l e c o n t e n t w i t h i n t h e y o u n g e r till. T h e Two C r e e k s S e c t i o n o c c u r s a p p r o x i m a t e l y Shorewood H ere , two section, just till u n i t s , north of the separated by lake F o r e s t Bed, a r e e x p o s e d . Two Rivers 125 km n o r t h of t h e M o r ai n e sedim ents an d the (Evenson below t h e 1973; forest Mick el so n Two C r e e k s T h e u p p e r till h a s b e e n c o r r e l a t e d R i v e r s till a n d is t h e r e f o r e G r e a t l a k e a n in a g e ( E v e n s o n 1973). till o c c u r s ( F i g u r e 1). et bed al. and 1983) is c o n s i d e r e d and w it h Two T he lo w er P o r t H u r o n in a g e correlated w it h H av en till 25 (Acomb et al. 1982). Based on the ages of the till units, the lake s e d i m e n t s w h i c h s e p a r a t e t h e tw o tills m u s t b e r e l a t e d to L a k e C h ic ag o a n d a r e s e p a r a t e d b y t h e Two C r e e k s F o r e s t Bed i n t o two d i f f e r e n t p h a s e s . L ak e C h i c a g o s e d i m e n t s related are be lo w t h e f o r e s t b e d a r e p r e - T w o C r e e k a n a n d to t h e Glenwood p h a s e w hil e s e d i m e n t s o v e r l y i n g t h e f o r e s t b e d post-Tw o Creekan an d related to t h e Ca lu m et P h a s e (H ans el e t a l . 1986). P e b b l e c o u n t d a t a ( T ab l e 3) of s a m p l e s t a k e n f ro m t h e two till u n i t s e x p o s e d a t Two C r e e k s s h o w t h a t b o t h h a v e o n l y t r a c e a m o u n t s of s h a l e a n d c o n tain mostly c a r b o n a te clasts. C l a y - m i n e r a l a n a l y s e s ( F i g u r e 5) of t h e two tills s h o w t h a t t h e Two R i v e r s a n d H av en tills b o t h e x h i b i t t h e sa m e g e n e r a l c l a y - m i n e r a l c o m p o s i t i o n a n d t h a t b o t h a r e simi lar in c l a y mineral composition to Shorewood section sedim ents, however, different composition the Port ( F i g u r e 5). show age Clay-mineral th a t each (Figure Huron Ozaukee analyses of till fr o m Lake the C hi c a g o p h a s e of t h e l a k e h a s a d i s t i n c t l y 5). Glenwood Phase sedim ent is c o m p o s i t i o n a l l y si m i la r to Gl enwood s e d i m e n t s a m p l e d a t S h o r e w o o d a n d to m i d - a n d l a t e W i s c o n s i n a n l a c u s t r i n e d e p o s i t s from t h e s o u t h e r n e n d of t h e basin. than other C al u m et P h a s e s e d i m e n t , h o w e v e r , h a s s i g n i f i c a n t l y more 10A c l a y Glenwood lacustrine Phase sedim ent (Figure 5). While it is d i s s i m i l a r to all d e p o s i t s , it is c o m p o s i t i o n a l l y s im il ar to S a u g a t u c k - O a k C r e e k till f ro m t h e s o u t h e r n e n d of t h e b a s i n ( F i g u r e 4 a n d F i g u r e 5). N o r th w e ste r n M ichigan I n 1954 M e l h o r n c o r r e l a t e d t h e s u r f a c e till in L e e l a n a u a n d Ben zie c o u n t i e s in n o r t h w e s t e r n M i c h i g a n w it h t h e " V a l d e r s " till (as d e f i n e d b y 26 T h w a i t e s 1943) in Wi sc o ns in . been correlated with Two More r e c e n t l y , M e l h o r n ’s " V a l d e r s ” till h a s Rivers till in Wi sco n si n and was d e p o s i t e d fo ll o w i n g t h e Two C r e e k s i n t e r v a l ( E v e n s o n 1973). therefore A n a l y s e s of t h e u p p e r m o s t till u n i t s f r o m s e v e r a l s e c t i o n s in n o r t h w e s t e r n M ich ig an ( F i g u r e 1) s h o w t h a t t h e s e ti lls h a v e a c l a y - m i n e r a l c o m p o s i ti o n s im il ar to Two R i v e r s , O z a u k e e , a n d H a v e n tills f r o m Wisc on si n ( F i g u r e 5). section (just south m u l ti p l e till units, of Peterson separated Park by in Leelanau stratified County, deposits, At o ne F i g u r e 1), are exposed. C l a y - m i n e r a l a n a l y s e s of t h e lo w e r a n d m id d le till of t h i s s e c t i o n s h ow a s im il ar s y s t e m a t i c i n c r e a s e in t h e r e l a t i v e a m o u n t of 10A c l a y i n c o r p o r a t e d in t h e y o u n g e r till u n i t a s w as o b s e r v e d a t Glenn S h o r e s a n d St. F r a n c i s , while t h e u p p e r till s h o w a d e c r e a s e in 10A c l a y c o n t e n t similar to t h a t o b s e r v e d a t t h e S h o r e w o o d s e c t i o n in Wi sco ns in ( F i g u r e 5). data (Tab le 3), h o w e v e r , shows that unlike the Pebble count h i g h a m o u n t s of s h a l e o b s e r v e d in 10A r i c h tills in s o u t h e r n L a k e Mic hi gan , all tills a t P e t e r s o n P a r k h a v e o n l y t r a c e a m o u n t s of s h a le . At o n e of t h e s e c t i o n s a n a l y z e d in n o r t h w e s t e r n M ichigan, Pt. B et sie (Figure 1), a t h i c k u n i t o f l a c u s t r i n e c l a y , silt, a n d "Two R i v e r s ” till. sand occurs Clay-mineral a n aly se s perform ed below on s a m p l e s of th i s l a c u s t r i n e d e p o s i t s i n d i c a t e s t h a t t h e c l a y - m i n e r a l c o m p o s i ti o n is similar to the Ca lu m et sedim ents analyzed at Two Creeks. S in c e no distinctive s t r a t i g r a p h i c b r e a k was o b s e r v e d w i t h i n t h i s s e c t i o n , t h e s e s e d i m e n t s a r e p r o b a b l y r e l a t e d to t h e Ca lu m et P h a s e . Recently, Taylor (1979, 1981) has proposed a more f o r m a l iz e d s t r a t i g r a p h i c f r a m e v ' o r k f o r P o r t H u r o n a n d G r e a t l a k e a n tills o c c u r r i n g in northw estern M ich ig an . He d e s c r i b e d two tills, M o n t a g u e a n d Orchard 27 Beach , r e l a t e d to t h e P o r t H u r o n a d v a n c e a n d o n e till, F i l e r , r e l a t e d to the Greatlakean advance. O rchard Be ac h till, r e l a t e d b y T a y l o r to t h e i n n e r P o r t H u r o n M or ain e a n d p r o b a b l y c o r r e l a t i v e w i t h e i t h e r O z a u k e e o r H av en till f r o m Wisc on sin , is e x p o s e d a t s e v e r a l p l a c e s a l o n g t h e s h o r e l i n e fro m L u d i n g t o n n o r t h w a r d to F r a n k f o r t . F il e r till, c o r r e l a t e d b y T a y l o r w it h Two R i v e r s till, is a l s o e x p o s e d a l o n g t h e s h o r e l i n e b u t g e n e r a l l y r e s t r i c t e d to e x p o s u r e s n o r t h of M an sit e e. T h e s o u t h e r n m o s t e x p o s u r e d e s c r i b e d b y T a y l o r (1981) w h e r e Port Huron and Greatlakean O r c h a r d B ea c h s e c t i o n Mic hi g an . age till units are exposed occurs both at the ( F i g u r e 1), l o c a t e d a b o u t 3 km n o r t h of M a n s it e e Here, O r c h a r d B e a c h a n d F i l e r tills a r e e x p o s e d in a 20-25 m h i g h b l u f f a l o n g t h e L a k e M ic hi ga n s h o r e l i n e a t t h e O r c h a r d B eac h S t a t e Park. T he s t r a t i g r a p h y b e g i n s l a k e le v el (177 m) w it h a b o u t 6 m of d e n s e , b r o w n s a n d y till. A d i s c o n t i n u o u s l a y e r of g r a v e l u p to s e v e r a l cm t h i c k in p l a c e s s e p a r a t e s t h e till i n t o two u n i t s . of c r o s s - b e d d e d s a n d a n d p e b b l e g r a v e l . T h e till is o v e r l a i n b y 3-4 m O cc as io na l d i s c o n t i n u o u s l a y e r s of sil t a n d c l a y a l s o o c c u r w i t h i n t h i s u n i t . T h e s a n d a n d p e b b l e g r a v e l is o v e r l a i n b y 2- 3 m of r e d s a n d y till. bedded sand and s ilt w it h o c c a s i o n a l l e n s e s of c r o s s b e d d e d diamicton c o n c e n tr a te d w i t h i n t h e mi dd le of t h e u n i t . capped b y 2-3 m of r e d Taylor (1979, Wi sconsin. 1981) T h e till is o v e r l a i n b y 10-12 m of s a n d y till. to F i l e r till a n d gravel and T h e e x p o s u r e is T h e u p p e r m o s t till w a s r e l a t e d by c o r r e l a t i v e with Two R i v e r s till in T a y l o r a l s o p r o p o s e d t h a t t h e 2 (or 3) till u n i t s u n d e r l y i n g F i l e r till e a c h represent some p h a s e O r c h a r d Beac h till d e p o s i t i o n a n d s u g g e s t e d t h a t t h e y w e r e d e p o s i t e d a s t h e ice f r o n t of t h e L a k e M ic h ig an Lob e o s c i l l a t e d d u r i n g t h e P o r t H u r o n S t a d e . 23 T h e till u n i t s e x p o s e d a t O r c h a r d B ea c h c a n b e t r a c e d s o u t h w a r d f o r a b o u t 2 km a n d w e r e s a m p l e d a n d a n a l y z e d f r o m a f e w p l a c e s a l o n g t h i s exposure. The results significant difference of these exists in O r c h a r d B ea c h o r F i l e r ti lls. analyses the (Figure clay-m ineral 5) indicate composition that of no either C o m p a r i s o n o f t h e s e r e s u l t s w it h t h o s e from Two R i v e r s e q u i v a l e n t till s a m p l e d f u r t h e r n o r t h a n d w it h O z a u k e e , H av en , a n d Two R i v e r s tills from Wisc on si n i n d i c a t e t h a t all h a v e t h e same b a s i c c l a y - m i n e r a l c o m p o s i t i o n ( F i g u r e 5). of till from O rchard B ea c h al so P e b b l e c o u n t s p e r f o r m e d on s a m p l e s show th a t, like o t h e r P ort Huron and G r e a t l a k e a n ti ll s f r o m t h e L a k e M ic hi ga n b a s i n , O r c h a r d B ea c h a n d F i l e r tills a r e low in s h a l e - c l a s t c o n t e n t (Table 3). S u m m a r y o f C l a y - M i n e r a l V a r i a t i o n in t h e L a k e M ic h i g a n B a s in A composite summary of the system atic variation of clay-mineral c o m p o s i t i o n s in s u c c e s s i v e till s h e e t s of t h e L ak e M ic hi g an b a s i n , t o g e t h e r w i t h t h e c l a y - m i n e r a l c o m p o s i t i o n of i n t e r v e n i n g l a c u s t r i n e u n i t s , is s h o w n in F i g u r e 6. I t w as c o n s t r u c t e d b y g r o u p i n g t o g e t h e r tills a n d l a c u s t r i n e d e p o s i t s of sim il ar a g e a n d operated during the s h o w s t h a t two d i f f e r e n t s y s t e m m u s t h a v e Wisconsinan, one controlling the clay-m ineral co m p o s i ti o n of till a n d a n o t h e r c o n t r o l l i n g t h e c o m p o s i t i o n of l a c u s t r i n e sediment. I n d e t a i l , F i g u r e 6 s h o w s a g e n e r a l i n c r e a s e in t h e r e l a t i v e a m o u n t of 10A c l a y i n c o r p o r a t e d in till b e g i n n i n g w it h e a r l y o r mi ddle W i s c o n s i n a n (Altonian) G le n n S h o r e s till a n d c o n t i n u i n g t h r o u g h l a te W i s c o n s i n a n (mi dW o od f or di a n) Ganges-New B er l in and Saugatuck-Oak Creek tills. A s i g n i f i c a n t d e c r e a s e in t h e r e l a t i v e a m o u n t of 10A c l a y , h o w e v e r , o c c u r s 29 f o r p o s t - M a c k i n a w I n t e r s t a d e O z a u k e e - H a v e n - O r c h a r d B ea c h ( P o r t H u r o n ) a n d Two R i v e r s - F i l e r ( G r e a t l a k e a n ) tills. B e c a u s e a r e t r e a t o f t h e La ke M ic h i g an Lobe h a s b e e n p r o p o s e d b e t w e e n e a c h of t h e s e g r o u p i n g s of till, m arked changes in c l a y - m i n e r a l c o m p o s i t i o n of till s h e e t s apparently o f t e n c o i n c i d e w i t h s i g n i f i c a n t f l u c t u a t i o n s o f t h e L a k e M ic h i g an Lobe (i.e. t h e m i d - W i s c o n s i n a n P o r t T a l b o t I n t e r s t a d e a n d l a t e W i s c o n s i n a n " E r ie " a n d M ac ki n a w I n t e r s t a d e s ; T a b l e 1). U n li k e mineral the variation composition intervals occurring constant. of observed lacustrine in s u c c e s s i v e sedim ent till s h e e t s , associated with the clay- interstadial b e t w e e n t h e till s h e e t s a p p e a r s to r e m a i n r e l a t i v e l y B eginning with unnamed m i d - W i s c o n s i n a n la k e s e d i m e n t s a n d c o n t i n u i n g t h r o u g h l a te W i s c o n s i n a n a g e L ak e M ilwaukee (Erie I n t e r s t a d e ) s e d i m e n t s a n d t h e Glenw ood P h a s e of L a k e C hi c a g o (Mackinaw I n t e r s t a d e ) sedim ents no significant change in clay mineralogy w as found. The c o m p o s i t i o n of e a c h of t h e s e u n i t s is s im il ar to l a te W i s c o n s i n a n G a n g e s New B e r l i n a n d to P o r t H u r o n a n d G r e a t l a k e a n a g e tills. A significant i n c r e a s e in t h e r e l a t i v e a m o u n t of 10A c l ay , h o w e v e r , o c c u r s f o r Calumet Phase of Lake C h ic a g o c o m p o s i t i o n of t h e s e till. (Two Creeks sedim ents a p p e a rs Interstade) s im il ar sedim ents. to S a u g a t u c k - O a k The Creek 30 frq Tao R i v e r s - f il e r t i l l (31) (0.94) (0.14) Caluaet Phase sediaent (8) (0.72) (0.04) I 61enaood Phase sediaent (9) (0.99) (0.11) L j JU t 1 Oaaukee-Haven-Qrchard Beach t i l l (47) (0.99) (0.14) i k Jill Figure 6 C o m p os it e 7A/10A f r e q u e n c y h i s t o g r a m s f o r till sh eets and in terv en in g l a c u s t r i n e u n i t s in t h e L a k e M ic h i g a n basin. H o r iz o n ta l ax i s s h o w s 7A/10A r a t i o v a l u e s ; v e rtic a l axis r e p r e s e n t s n u m b e r o f s a m p l e s w ith a p a r t i c u l a r r a t i o v a l u e . U n i t s a r r a n g e d in strati-g rap h ic order. Slenaood Phase sediaent (ID (0.95) (0.13) u u j l f .-JL.1 Saugatuck-Oak Creek t i l l (80) (0.39) (0.09) Lake hilaaukee sediaent (18) (1.00) (0.13) A-lfl I Sanges-Nea Berlin t i l l (51) (0.86) (0.11) • a id -d lic o n sin a n l? )' lake sediaent (19) (1.00) (0.10) ill UL Slenn Shores t i l l (21) (1.21) (0.16) 4 0 . 34) 1.00 o J U i4 1.30 A 31 THE ORIGIN OF CLAY MINERALS FROM TILLS IN THE LAKE MICHIGAN BASIN Three variation basic in m od el s can clay-m ineral be formulated composition of to explain ti lls the deposited in the L ak e M ic h i g a n b a s i n . T h e s e model a r e : mineral v a ria tio n in till is d u e s o le l y to f o r m a t i o n o r a l t e r a t i o n of c l a y s after deposition, variation to 2) a 1) a p e d o g e n i c system atic lacustrine-m ixing incorporation by the ice model of model in w h i c h c l a y - which previously attributes the fractionated lake s e d i m e n t s w it h d i f f e r e n t c l a y - m i n e r a l c o m p o s i ti o n , a n d 3) a b e d r o c k - m i x i n g model w h i c h amount of ascribes shale the during variation to e r o s i o n each read v a n ce. by Eac h the of ice of d i f f e r i n g these mod els are d i s c u s s e d below. P e d o g e n ic Model The composition pedogenic related of processes sedim ents associated could the variation in clay-m ineral s e l e c t i v e a l t e r a t i o n of c l a y minerals T h e model, t h e r e f o r e , a s c r i b e s t h e u l t i m a t e c l a y - m i n e r a l to w e a t h e r i n g mechanism attributes of till to f o r m a t i o n o r a fte r deposition. c o m p o s i ti o n model to post-depositional rather w it h account than ice basic dynamics. for the c o m p o s i t i o n of till; o n e r e l a t e d to a erosional or Two observed to time a n d transform ation basic variation processes depositional transform ation in clay-m ineral the o th e r related to d e p t h . Both p r o c e s s e s r e l y o n t h e f a c t t h a t , a t l e a s t i n t h e G r e a t L a k e s r e g i o n , c h l o r i t e is g e n e r a l l y o n e o f t h e l e a s t s t a b l e c l a y m i n e r a l s ( D r o s t e 1956a, 1956b) while k a o l i n i t e o n e of t h e m o s t s t a b l e . i n t e r m e d i a t e betw'een c h l o r i t e a n d k a o l in it e . T h e s t a b i l i t y of illite is 32 C h l o r i t e c a n w e a t h e r to e x p a n d a b l e c l a y m i n e r a l s s u c h a s v e r m i c u l i t e and smectites through transform ation verm iculite often (or conditions. to are can of by also alter and the brucite developing structures vermiculite in t e r la y e r sites. cations proceeds smectite) Ill ite ultimately hydrolysis and to is a smectite a interlayer. mixed mo st rapid mixed-layered by layer s im il ar This chlorite- under ac i di c structure hydration and of th e U n d e r i n t e n s i v e l e a c h i n g c o n d i t i o n s , m o no - a n d d i - v a l e n t leached from the parent material which promotes c o n d i t i o n s a n d d e s i l i f i c a t i o n a n d allows t h e f o r m a t i o n o f k a o l in it e . ac id If, on t h e o t h e r h a n d , l e a c h i n g r a t e s a r e low, n e o f o r m e d v e r m i c u l i t e a n d s m e c t i t e may be s t a b l e e n d p r o d u c t s . If r e l a t i v e l y l a r g e a m o u n t s of K -io n s a r e p r e s e n t , h o w e v e r , t h e y may b e s c a v e n g e d a n d allo\v illite to form. process of K - f ix a ti o n is particularly promoted if the parent Thi s m a te r ia l u n d e r g o e s a lte rn a tin g w e t- d r y cycles. T h e t r a n s f o r m a t i o n p r o c e s s e s d i s c u s s e d a b o v e a r e well d o c u m e n t e d an d have been u sed by n u m e ro u s r e s e a r c h e r s th r o u g h o u t the Great Lakes r e g i o n to e x p l a i n c l a y - m i n e r a l d i f f e r e n c e s in tills (eg. D r o s t e 1956a, 1956b; Willman e t a l . 1963; Q u i g e l y 1968). If t h e s e p r o c e s s e s a r e s i g n i f i c a n t , t h e v a r i a t i o n in c l a y - m i n e r a l c o m p o s i t i o n of tills, a t l e a s t from t h e s o u t h e r n p a r t of t h e L ak e M ic h i g a n b a s i n , c o u l d r e l a t e to g r e a t e r a l t e r a t i o n of o l d e r till u n i t s r e l a t i v e to y o u n g e r u n i t s . originally illite-rich, alternatively, an then alteration the F o r exam ple , if t h e all t h e tills w e r e alteration sequence of of c h l o r i t e illite to kaolinite to e x p a n d a b l e clay (or, to k a o l in it e ) c o u l d p r o d u c e a n a p p a r e n t p r o g r e s s i v e i n c r e a s e in th e r e l a t i v e a m o u n t of 7A c l a y w i t h i n o l d e r tills. Such a p a tte rn o b s e r v e d a t t h e G le n n S h o r e s a n d St. F r a n c i s s e c t i o n s . is simi lar to t h a t A l t e r n a t i v e l y , if 33 these tills were all originally low in illite (wi th a c o m p o s i ti o n , for ex am p le, s im il ar to t h e G le n n S h o r e s till) t h e n a n a p p a r e n t i n c r e a s e in illite w i t h i n y o u n g e r ( a n d more s h a l l o w l y b u r i e d ) ti lls may h a v e r e s u l t e d from an a c c e le ra te d t r a n s f o r m a t i o n of c h l o r i t e i n t o e x p a n d a b l e m i n e r a l s a n d / o r n e o f o r m a t i o n o f illite b y K - f i x a t i o n p r o c e s s e s w i t h i n n e a r s u r f a c e deposits. A l t h o u g h some w e a t h e r i n g of t h e o r i g i n a l p a r e n t m a t e r i a l m i g h t h a v e o c c u r r e d , t h e p e d o g e n i c model is p r o b a b l y n o t p l a u s i b l e s i n c e it c a n no t a d e q u a t e l y a c c o u n t f o r t h e v a r i a t i o n in c l a y - m i n e r a l c o m p o s i t i o n of tills w i t h i n t h e L a k e M ic h ig an b a s i n . F o r exam ple , if t h e i n c r e a s e in 7A c l a y c o n t e n t w i t h i n o l d e r ti lls r e s u l t e d f r o m n e o f o r m a t i o n of k a o l i n i t e , t h e n t h e youngest till u n i t s should have t h e lo w e s t a m o u n t of 7A c l ay . Th is is c l e a r l y n o t t h e c a s e s i n c e t h e y o u n g e s t till u n i t s ( O z a u k e e - H a v e n a n d Two R i v e r s ) h a v e n e a r l y t h e same p r o p o r t i o n of 7A a n d 10A c l a y s a s t h e o l d e s t (Glenn S h o r e s till; F i g u r e 6). Th e f a c t t h a t t h e y o u n g e s t a n d o l d e s t tills h a v e s im il ar p r o p o r t i o n s of 7A a n d 10A c l a y s a l s o s h o w s t h a t t h e s e c o n d m e c h a n is m f o r p e d o g e n i c f o r m a t i o n of c l a y m i n e r a l s , n e o f o r m a t i o n of illite r e l a t e d to n e a r s u r f a c e w eathering, is q u e s t i o n a b l e . basin high-illite most Although ti lls o c c u r near within the the ground s o u t h e r n e n d of the s u rfa c e , within the n o r t h e r n e n d of t h e b a s i n n e a r s u r f a c e tills a r e r e l a t i v e l y r i c h in 7A cl ay while more illitic s e d i m e n t s a r e f o u n d a t g r e a t e r d e p t h Two C r e e k s , a n d Peterson Park section). (e.g. S h o r e w o o d , A pedogenic origin f o r cl ay m i n e r a l s w i t h i n ti lls in s o u t h w e s t e r n M ic hi ga n h a s al so b e e n q u e s t i o n e d b y Monaghan (1984) and Monaghan et al. (1986). They showed that no 34 r e g u l a r t r e n d in c l a y - m i n e r a l c o m p o s i t i o n o c c u r r e d a l o n g c l o s e l y s p a c e d v e r t i c a l s a m p l i n g t r a n s e c t s w i t h i n i n d i v i d u a l till u n i t s . L a cu strin e-M ix in g Model T h e l a c u s t r i n e - m i x i n g model r e q u i r e s t h a t e x t e n s i v e l a k e s y s t e m s fo rm in t h e L a k e M ic h i g a n b a s i n d u r i n g m a j o r ice r e t r e a t s a n d t h a t t h e c l a y mineral composition of sedim ents s y s t e m a t i c a l l y c h a n g e t h r o u g h time. deposited It, t h e r e f o r e , within these lakes a s c r i b e s t h e u l t im a te r e a s o n f o r c h a n g e in t h e c o m p o s i t i o n of till to s e d i m e n t o l o g i c a l p r o c e s s e s a c t i n g in t h e l a k e b a s i n d u r i n g i n t e r s t a d i a l s . D i f f e r e n t i a l s e t t l i n g of c l a y m i n e r a l s in a n a q u e o u s s y s t e m (Gibb s 1977; P r a h a m 1966) may h a v e a c t e d as th e fra ctio n atio n p ro c e s s for th e se sedim ents. D uring sedim ents larger readvances of were in c o rp o ra ted scale incorporation processes such the Lake M ic h ig an Lobe, in t h e ice b y r e g e l a t i o n as accretation (We ert man (Moran 1971, M oran e t a l . 1980). these lacustrine processes and/or 1961) and bloc k If f r o z e n , t h e y may al so h a v e b e e n p l u c k e d a s " c l a s t s " fr om t h e la ke be d . The e r o d e d s e d i m e n t s w e r e t h e n t r a n s p o r t e d b y t h e ice, h o m o g e n i z e d , a n d d e p o s i t e d a s till with a c l a y - m i n e r a l c o m p o s i ti o n sim il ar to t h a t of t h e s o u r c e l a c u s t r i n e b e d s . T h i s m e c h a n i s m seem p l a u s i b l e p a r t i c u l a r l y s i n c e c h a n g e s in c l a y c o n t e n t of some tills in t h e G r e a t L a k e s r e g i o n , a s well a s c h a n g e s in t h e c l a y m i n e r a l o g y of till, h a v e b e e n a t t r i b u t e d to e r o s i o n a n d i n c o r p o r a t i o n of u n d e r l y i n g d r i f t d e p o s i t s ( S c h n e i d e r 1983; Acomb e t a l . 1982; Willman e t a l . 1963). I n o r d e r to f u n c t i o n , h o w e v e r , t h e model r e q u i r e s t h a t , a t l e a s t in t h e s o u t h e r n e n d of t h e b a s i n , t h e a m o u n t of 10A c l a y m u s t i n c r e a s e in 35 successively y ounger lacustrine units. I t a l s o im pl ie s t h a t t h i s 10A c l a y is p r o b a b l y d e r i v e d f r o m o u t s i d e of t h e L a k e M i c h i g a n b a s i n . D u r i n g m a j o r i n t e r s t a d i a l p e r i o d s in t h e G r e a t L a k e s r e g i o n , e x t e n s i v e lacustrine system s s y s t e m s w e r e f o r m e d w i t h i n t h e L a k e M ic h i g a n b a s i n . al so developed periodically drained within w estw ard the Lakes into Lake Huron and M ic h ig an . Erie The Such basins and thickness of sedim ent d ep o site d within th e s e lakes was p r o b a b l y controlled by s e v e ra l factors areas, including: the t h e a m o u n t of v e g e t a t i o n extent and height of bluffs d u r a t i o n a n d m a g n i t u d e of t h e r e t r e a t . m agnitude of the retreat) thickness of la ke probably sedim ents since covering along the surrounding shoreline, and la n d the These later factors (duration and had the longer g r e a te s t control over time intervals allow the more s e d i m e n t to a c c u m u l a t e . T h e m a g n i t u d e a n d e x t e n t of r e t r e a t w o ul d n o t o n l y h a v e a f f e c t e d t h e t h i c k n e s s of s e d i m e n t w h ic h a c c u m u l a t e d in t h e L ak e M ic h i g an b a s i n b u t c o u l d a l s o h a v e a f f e c t e d t h e m i n e r a l o g y of s e d i m e n t d e p o s i t e d . When e a s t e r n o u t l e t s f o r t h e G r e a t L a k e s ( s u c h a s t h e St. L a w r e n c e Valley) w e r e blocked as was o f t e n (D rei ma nis 1958; the case d u rin g D re im a n is and t h e mi dd le a n d Goldthwait 1973; la te W is c o n s i n a n D re im a n is and K arr o w 1972; W i n t e r s e t a l . 1986, 1988; G e p h a r t e t a l . 1982), d r a i n a g e w as f o r c e d w e s t w a r d t h r o u g h b a s i n s of t h e G r e a t L a k e s . M ic h i g an basin w as often the last basin in D u r i n g s u c h times t h e Lake the drainage system. If d i f f e r e n t i a l s e t t l i n g of c l a y m i n e r a l s in a n a q u e o u s s y s t e m is a s i g n i f i c a n t f r a c t i o n a t i o n p r o c e s s t h e n d u r i n g ti m es of w e s t w a r d d r a i n a g e t h e e a s t e r n basins may have acted as a sink for either kaolinite or chlorite (the p r i n c i p a l 7A c l a y - m i n e r a l s ) a n d s u p p l i e d i l l i t e - r i c h (10A cl ay ) to t h e La ke 36 M ic hi ga n b a s i n . T h i s f r a c t i o n a t i o n p r o c e s s is b a s e d in p a r t o n t h e f a c t t h a t illite h a s a s m a l l e r g r a i n s iz e r e l a t i v e to e i t h e r k a o l i n i t e o r c h l o r i t e (Grim 1968; G ib b s 1977) a n d w o ul d t h e r e f o r e r e m a i n in s u s p e n s i o n l o n g e r . A l t e r n a t i v e l y , t h e e a s t e r n b a s i n s may h a v e s i m p l y c o n t a i n e d m o s tl y il li t e rich sedim ent which was then washed into the L ak e M ic hi ga n basin. W e a t h e r i n g of c h l o r i t e to v e r m i c u l i t e p r i o r to o r d u r i n g e r o s i o n o f d e p o s i t s s u r r o u n d i n g t h e b a s i n s , p a r t i c u l a r l y if a c c o m p a n i e d b y K - f ix a ti o n d u r i n g e r o s i o n a n d / o r t r a n s p o r t , may a l s o h a v e a l t e r e d t h e r e l a t i v e a m o u n t of 7A a n d 10A c l a y s d e p o s i t e d in t h e b a s i n s . A l t h o u g h a n y , a n d p e r h a p s all, of t h e a b o v e f r a c t i o n a t i o n p r o c e s s e s may have operated significant. during the Wisconsinan, apparently none was As i l l u s t r a t e d on F i g u r e 6, d u r i n g most of t h e W i s c o n s i n a n no w i d e s p r e a d , s i g n i f i c a n t c h a n g e in t h e c l a y - m i n e r a l c o m p o s i t i o n of l a c u s t r i n e s e d i m e n t o c c u r r e d in t h e L a k e M ich ig an b a s i n ; t h e o n l y e x c e p t i o n b e i n g Cal um et P h a s e (L ake C hi c ag o ) s e d i m e n t s . C o m p a r e d to o l d e r l a k e d e p o s i t s , t h e s e s e d i m e n t s s h o w a r e l a t i v e i n c r e a s e in 10A c l a y c o n t e n t . F a th e r than f r a c t i o n d u e to s e d i m e n t a l o g i c p r o c e s s e s , h o w e v e r , t h e i n c r e a s e in 10A c l a y may sim p l y r e f l e c t i n c r e a s e d e r o s i o n of tills w it h h i g h 10A c l a y c o n t e n t such as the Saugatuck-Oak Creek till. If these data ( F i g u r e 6) are i n d i c a t i v e of t h e b a s i n a s a w hol e t h e n t h e l a k e s y s t e m may h a v e o p e r a t e d a s a n h o m o g e n i z i n g b a s i n f o r e r o d e d c l a y m a t e r i a l w h o s e c o m p o s i t i o n was i n f l u e n c e d b y t h e c l a y m i n e r a l o g y of e r o d e d , p r e - e x i s t i n g s e d i m e n t . C le a r ly , based on the data presented in F i g u r e 6, t h e system atic v a r i a t i o n in c l a y - m i n e r a l c o m p o s i t i o n of tills is n o t s o le l y r e l a t e d to t h e c o m p o s i ti o n of u n d e r l y i n g factor for stratigraphic lacustrine variation, sediments. the c o m p o s i ti o n A l t h o u g h n o t t h e sole of t h e s e underlying 37 sedim ents a p p a re n tly did s i g n i f i c a n t l y i n f l u e n c e t h e c l a y m i n e r a l o g y of s u b s e q u e n t l y d e p o s i t e d till. local scale by the T h i s i n f l u e n c e , f o r i n s t a n c e , is i l l u s t r a t e d o n a dramatic change in composition sampled from two a r e a s o n l y a few h u n d r e d Cliffs in section southw estern M ic hi ga n of Saugatuck till m eters a p a rt at the Park (Figures 2 and 3). Here, the co m p o s i t i o n of till s a m p l e d fr o m A r e a G s h o w s a r e l a t i v e l y g r e a t e r a m o u n t of 7A c l a y s t h a n A r e a N a n d may h a v e r e s u l t e d from a l a r g e r a m o u n t of u n d e r l y i n g l a k e s e d i m e n t s b e i n g e r o d e d a t A r e a G t h a n at. A r e a N ( F i g u r e 3). T he f a c t t h a t t h i s o c c u r r e d o n l y loca lly is e v i d e n c e d both by the p e r s i s t e n c e of S a u g a t u c k till " m i n e r a l o g y " e l s e w h e r e a l o n g t h e s h o r e l i n e ( s u c h a s a t G len n S h o r e s , C e d a r B lu f f s , a n d Miomi P a r k ; F i g u r e s 2 a n d 3) and by the throughout account uniform ity of southw estern for the the c o m p o s i ti o n M ic h i g a n relatively high (Figure amount 2). of sampled from A simi lar 10A c l a y moraines process observed at may the P e t e r s o n P a r k s e c t i o n ( F i g u r e 5). B a s e d on s i m i l a r i t i e s of G a n g e s - N e w B e r l i n till a n d P o r t H u r o n a n d Greatlakean ti lls with underlying lake sedim ents ( F i g u r e 6), th e i n c o r p o r a t i o n of l a k e s e d i m e n t s b y t h e ice may a l s o s i g n i f i c a n t l y (if not completely) regional c o n t r o l t h e c l a y - m i n e r a l c o m p o s i ti o n of some till u n i t s on a s ca le . H ow e ve r, the significant increase in 10A c l a y content o b s e r v e d in S a u g a t u c k - O a k C r e e k till c l e a r l y c a n n o t be a c c o u n t e d f o r b y this process since s e d i m e n t s ( F i g u r e 6). no s im il ar mineralogy exists for L ak e Milwaukee 38 C oldw ater Shale A n l f Im^E I a w o r t h y / S h a l e / ””^ F i g u r e 7 Map s h o w i n g d i s t r i b u t i o n of A n t r i m - E l l s w o r t h a n d C o l d w a t e r F o r m a t i o n s w i t h i n t h e La ke Michi gan b a s i n . ( A fte r Wold e t a l . 1981) 39 B ed rock -M ix in g Model The bedrock-m ixing model a t t r i b u t e s t h e v a r i a t i o n in c l a y - m i n e r a l c o m p o s i ti o n of till to e r o s i o n o f d i f f e r i n g a m o u n t of s h a l e a n d s i l t s t o n e bedrock during r e a d v a n c e s of t h e L a k e M ic h i g a n Lobe. T h i s model is p a r t i c u l a r l y a p p e a l i n g s i n c e h i g h a m o u n t s of s h a l e h a v e b e e n o b s e r v e d a s s o c i a t e d w it h L a k e M ic h i g a n Lobe ti lls ( A n d e r s o n 1957; K r u m b e i n 1933; t h i s s t u d y ) a n d h a v e b e e n u s e d to a c c o u n t f o r t h e h i g h c l a y c o n t e n t of t h e till. S h a l e is al so a m o n g t h e mo st e r o d i b l e rocks s i n c e it is well jointed, extrem ely fissile, rela tiv e ly soft, and often has tensile s t r e n g t h lo w er t h a n s t r e s s e s e x e r t e d b y a n o v e r - r i d i n g ice s h e e t s ( D r e w r y 1986). When s a t u r a t e d , t h e t e n s i l e s t r e n g t h of s h a l e is e v e n lo w er (as m u c h a s 30% of t h e u n s a t u r a t e d s t r e n g t h ; Mesri a n d Gibala 1972) al lo w in g it to fail e a s i l y u n d e r t h e s t r e s s e s a p p l i e d b y an' o v e r r i d i n g ice s h e e t . During a r e a d v a n c e , s h a l e c o u l d be d i r e c t l y i n c o r p o r a t e d in t o t h e ice b y p l u c k i n g ( q u a r r y i n g ) a n d / o r l a r g e r s c a l e p r o c e s s e s s u c h a s a c c r e t a t i o n (We ert man 1961) o r bl oc k i n c o r p o r a t i o n (Moran 1971, M or an e t a l . 1980). As t h e ice flowed a c r o s s o u t c r o p a r e a s , a b r a s i o n a t t h e sole of t h e g l a c i e r s may also h a v e g r o u n d t h e r o c k i n t o a f i n e f l o u r o r d i s p l a c e d small c h i p s w h i c h w e r e then incorporated into the ice by regelation processes. In order to f u n c t i o n , t h e b e d r o c k - m i x i n g model r e q u i r e s t h a t t h e s h a l e f o r m a t i o n s m u s t h a v e a r e l a t i v e l y h i g h a m o u n t of 10A c l a y a n d m u s t , a t l e a s t in p l a c e s , o u t c r o p u n d e r L ak e M ich ig an . An implicit a s s u m p t i o n of t h e model is t h a t most of t h e o u t c r o p s of t h e s e s h a l e s m u s t be c o n c e n t r a t e d in t h e s o u t h e r n e n d of t h e b a s i n . The M ich ig an principal basin are bedrock shales clay of sources the which occur within Devonian-M ississippian the A n tr im Lake and Ellsworth Formations and Mississippian Coldwater F o r m a t io n . The d i s t r i b u t i o n of t h e s e d e p o s i t s is Shown in F i g u r e 7 a n d i n d i c a t e s t h a t most of t h e s o u t h e r n underlain ma inly h a l f of L a k e M ic hi ga n, p a r t i c u l a r l y t h e e a s t e r n p a r t , is by shale. Except along the eastern shore of La ke M ic hi ga n, t h e n o r t h e r n h a l f of t h e b a s i n is u n d e r l a i n b y middle De v on ian d o lo m ite s a n d l i m e s t o n e s a s well a s u p p e r a n d mi ddl e S i l u r i a n do lo m ite s a n d e v a p o r i t e s (Wold e t a l . 1981). IM * o o c o LU 03 Figure 8 7A/10A frequency h i s t o g r a m s of A n t r i m - E l l s w o r t h and Coldwater sh ales (u p p e r p a r t of the diagram) and shale pebbles c o l le c t e d fr om th e t h r e e tills a t t h e G le nn S h o r e s s e c t i o n (lower p a r t of d i a g r a m ) . H or iz o n ta l axi s s h o w s 7A/10A ratio values; vertical axis represents the number of s a m p l e s with a p a r t i c u l a r r a t i o v a l u e . Note c h a n g e in s c a le of h o r i z o n t a l axi s f o r p r e v i o u s histogram s. C o ld > « tir Shale llta n • 0.40 3 t0 * .« 0 . IS N • 12 JLjJj A n trlo - E lia o rth Shale /lean • 0.21 S tfo .* 0.20 8 » 13 ]k lu . 8 II S ha lt Pebbles * Saugatuek t i l ! 8 • ¥ R u n ■ 0.47 StD?.« 0.14 tri S halt Pebbles * Ganges t i l l 8 *7 R u n • 0.43 StD»." 0.04 i j l , a, < Shale Pebbles - 61>nn Shores t i l l R • 7 neao • 0.41 S tO r .' 0.04 X C/3 jL 0.30 0.00 10 The Coldwater F o rm a t io n c o n s i s t s ma inly of g r e y ‘ s h a l e with minor i n t e r b e d s of dolo m ite , lim es ton e, a n d s a n d s t o n e a n d is u p to 150 m th i c k . It subcrops under from 30 to 100 m of drift throughout most of 41 M ich ig an . S h a l e s of t h e C o l d w a t e r F o r m a t i o n a l s o o c c u r u n d e r a fe w to u p to 30 m of s e d i m e n t in t h e s o u t h e r n h a l f o f t h e L a k e M ic h i g a n b a s i n . Clay-m ineral a n a ly se s for th e Coldwater have been rep o rte d by Chung (1973) a n d a r e b a s e d o n c u t t i n g s s a m p l e d f r o m t h e u p p e r , m id dle a n d lo w er 1 /3 of oil we lls t h r o u g h o u t t h e l o w e r p e n i n s u l a of M ich ig an . T h e 7/10A r a t i o r e p o r t e d f o r wells u n d e r l y i n g L a k e M ic h i g a n Lobe d e p o s i t s a r e s h o w n in F i g u r e 8. T h e s e d a t a i n d i c a t e t h a t a h i g h a m o u n t of 10A ( r e l a t i v e to 7A) c l a y c h a r a c t e r i z e indicates that Mich ig an. the If Coldwater relative correct, shale. amount the of Im portantly, 10A c l a y a m o u n t of 10A Chung increases clay in (1973) al so westw ard Coldwater in shale s u b c r o p p i n g u n d e r la k e M ic hi g an ( f u r t h e r w e s t t h a n C h u n g s a m p l e d ) m i g h t h a v e e v e n more 10A c l a y t h e n s h o w n in F i g u r e 8. The A ntrim -Ellsw orth F o r m a t io n , which underlies the Coldwater F o r m a t i o n t h r o u g h o u t mo st of s o u t h w e s t e r n M ic h i g an ( F i g u r e 7), c o n s i s t s of u p to 100 m of m o s tl y g r e y shale. to b l a c k (Antrim) a n d g r e e n ( E l ls w o r t h ) It o u tc ro p s extensively along a band exten d in g a c ro s s the n o r th e r n p a r t of t h e lo w e r p e n i n s u l a fr o m A l p e n a to C h a r l e v o i x a n d s u b c r o p s a s a n i r r e g u l a r a r c u a t e b a n d u n d e r p a r t o f t h e s o u t h e r n e n d of L ak e M ich ig an (Figure 7). between An e r o s i o n a l o u t l i e r of A n tr im s h a l e a l s o o c c u r s o f f - s h o r e Milw auk ee and Sheboygan, Wisc ons in. The presence of this o u t l i e r , a s well as the fact th a t fish fossils and plant fragm ents found within shale do A n tr im proximity to a offshore shoreline, fr om Mil wa uk ee suggest that that Antrim -Ellsworth not indicate shales once e x t e n d e d m uc h f u r t h e r w e s t t h a n s h o w n in F i g u r e 7 (Wold e t a l . 1981). Along mo st o f t h e e a s t s h o r e of L a k e M ic h ig an , A n t r i m - E l l s w o r t h a n d C o l d w a t e r s h a l e s a r e b u r i e d b y a t h i c k m a n tl e ( u p to 200 m) of d r i f t (Wold 42 e t a l . 1981). I n t h e s o u t h e r n m o s t e n d of t h e b a s i n , h o w e v e r , t h e j - a r e o n l y b u r i e d b y a t h i n l a y e r o f s e d i m e n t ( L i n e b a c k e t a l . 1972; L i n e b a c k e t al. 1974) a n d may a c t u a l l y o u t c r o p in p l a c e s a l o n g a n a r r o w northeast- s o u t h w e s t e s c a r p m e n t e x t e n d i n g fr o m L u d i n g t o n , M ic hi ga n, to Milwaukee , Wi sco ns in in t h e c e n t r a l p a r t of L a k e M ic h ig an (Wold e t a l . 1981; L i n e b a c k e t a l . 1974). T h i s e s c a r p m e n t m a r k s t h e n o r t h w e s t e r n limit of A n t r i m - E l l s w o r t h s h a l e u n d e r L a k e M ic hi g an a s well a s t h e n o r t h e r n e d g e of t h e " m i d - l a k e h i g h " , a 25-50 km wid e t o p o g r a p h i c r i s e in t h e l a k e b o tt o m ( an d b e d r o c k s u r f a c e ) e x t e n d i n g a c r o s s L ak e M ic h i g a n ( F i g u r e 7). The p r e s e n c e of t h i s e s c a r p m e n t i n d i c a t e s t h a t e x t e n s i v e e r o s i o n of t h e s h a l e , p o s s i b l y a s s o c i a t e d w ith g l a c i a t i o n , m u s t h a v e o c c u r r e d . Clay-mineral Routsaulla (1980) analyses of indicates that, Antrim -Ellsworth like r e l a t i v e l y h i g h a m o u n t of 10A cl ay. the the lo w er peninsula Coldwater reported shale, it by has a The d a t a w as c o l l e c t e d b y s a m p l i n g well c o r e s a t a p p r o x i m a t e l y 3 m (10 ft) throughout shale of intervals M ich ig an . from wells d i s t r i b u t e d Originally, Routsaulla r e p o r t e d t h e c l a y - m i n e r a l d a t a a s r e l a t i v e p e r c e n t a g e s of p e a k - h e i g h t s of e a c h m i n e ra l s t u d i e d , h o w e v e r , to f a c i l i t a t e c o m p a r i s o n w it h t h e p r e s e n t study, these data were converted a v e r a g e of s a m p l e s r e p o r t e d to 7/10A peak-height b y Routsaulla for the u p p e r ratios. 15 m of wells u n d e r l y i n g L ak e M ic h i g an Lobe d e p o s i t s a r e s h o w n in F i g u r e 8. to provide outcrops additional of A n tr im checks shale on were the data sampled reported and by analyzed; The In o r d e r Routsaulla, one fr om two the n o r t h e a s t e r n M ic hi ga n n e a r A l p e n a a n d o n e from a l o n g t h e L a k e M ich ig an s h o r e l i n e in n o r t h w e s t e r n M ic h i g a n n e a r t h e v i l l a g e of Norwood in A ntrim County. T he results of these analyses compare well with the range 43 d e r i v e d fr o m R o u t s a u l l a ’s d a t a ( F i g u r e 8) a n d i n d i c a t e t h a t A n tr im s h a l e c o n t a i n s r e l a t i v e l y h i g h a m o u n t s of 10A clay. Both the spatial distribution of A ntrim -Ellsworth and Coldwater F o r m a t i o n s a n d t h e f a c t t h a t b o t h o f t h e s e f o r m a t i o n s c o n t a i n s h a l e s with r e l a t i v e l y h i g h a m o u n t s o f 10A c l a y f u lf il l t h e b a s i c r e q u i r e m e n t s of t h e b e d r o c k - m i x i n g model. If t h e b e d r o c k - m i x i n g p r o c e s s a c t u a l l y o p e r a t e d d u r i n g t h e W i s c o n s i n a n t h e n tills w it h r e l a t i v e l y h i g h a m o u n t s of 10A cl ay, such as Saugatuck-O ak C r e e k , s h o u l d al so h a v e a h i g h a m o u n t of s h a l e while tills w it h lo w e r a m o u n t s of 10A c l ay , s u c h a s G a n g e s - N e w Be r l in a n d G lenn Shores tills, Tills d e p o s i t e d Rivers should well n o r t h till, s h o u l d have concom itantly of the " s h a l e limit" have only lo w er a m o u n t s of s ha le . ( F i g u r e 7), s u c h a s Two t r a c e a m o u n t s of s h a l e . In o r d e r to t e s t w h e t h e r s u c h a r e l a t i o n s h i p in f a c t e x i s t s , c o m p o s i t e till s a m p l e s d e r i v e d from representative sections were wet sieved and the distribution l i t h o l o g i e s in t h e p e b b l e - s i z e (>4 mm) f r a c t i o n w a s d e t e r m i n e d . of The a c t u a l te c h n iq u e s employed w ere d e s c r ib e d above. T h e r e s u l t s of t h e s e d e t e r m i n a t i o n s a r e l i s t e d on T ab l e 3 a n d i n d i c a t e that in section general have G reatlakean tills w it h the correspondingly highest high am ounts amount of of 10A c l a y shale. Port fr o m each Huron and tills s a m p l e d f r o m g e n e r a l l y n o r t h of t h e " s h a l e limit" s ho w v e r y low p e r c e n t a g e s of s h a l e w it h t h e h i g h e s t a m o u n t of s h a l e a s s o c i a t e d with Ozaukee section). till Da ta from from the sections southernm ost south sampling local (Shorewood of t h e s h a l e limit (St. F r a n c i s a n d G lenn S h o r e s ) , o n t h e o t h e r h a n d , s h o w a g e n e r a l i n c r e a s e in t h e a m o u n t of b o t h s h a l e a n d 10A c l a y f o u n d in p r o g r e s s i v e l y y o u n g e r till u n i t s . The 44 r e l a t i o n s h i p b e t w e e n t h e a m o u n t of s h a l e p e b b l e s a n d t h e 7/10A r a t i o is i l l u s t r a t e d in F i g u r e 9. • ll/IN. Ml . « Q AC A T L A K C A N R O A T MU R OM SHORCS •5 0 •7 0 • 90 7/l0 A F i g u r e 9 Plot s h o w i n g r e l a t i o n s h i p b e t w e e n %shale c l a s t s (>3.0 mm) f o u n d in till a n d a v e r a g e 7A/10A r a t i o v a l u e s f o r t h e till u n i t a t t h e s e c t i o n from w h i c h d a t a c o l l e c t e d . T r e n d a t s e l e c t e d s e c t i o n s s h o w n b y solid line; nam ed s e c t i o n s s h o w n a l o n g line. Data from s o u t h w e s t e r n Michigan s e p a r a t e d from r e s t of t h e b a s i n b y a h e a v y solid line. The a s s o c i a t i o n of h i g h a m o u n t s of s h a le c l a s t s w ith r e l a t i v e l y h i g h am ounts of presented 10A c l a y by in Krumbein till is (1933). al so He supported sampled by till pebble at count data several-kilom eter i n t e r v a l s a l o n g t h e f r o n t of t h e V a l p a r a i s o Moraine in Illinois, I n d i a n a , a n d s o u t h w e s t e r n M ic h ig an ( F i g u r e 1) t h e n s e p a r a t e d a n d c o u n t e d p e b b l e - s i z e clasts. His r e s u l t s i n d i c a t e t h a t s h a l e c l a s t s in till from t h e V a lp a r a is o M or ain e in Illinois a n d I n d i a n a ( W a d s w o r th till) a v e r a g e 72% of all c l a s t s w hil e till s a m p l e s c o l l e c t e d a v e r a g e 62% s h a l e . from s o u t h w e s t e r n Michigan (Saugatuck till) In o r d e r to c o m p a r e t h e r e l a t i v e a m o u n t of s h a l e to 45 the clay-m ineral composition collected foll owi ng a p a t t e r n analysis of t h e s e averages samples 72% s h a l e has a of till from this area, till samples simi lar to t h a t o f K r u m b e i n . show that till in I ll in o is a n d mea n 7A/10A r a t i o were Clay-mineral Indiana v a l u e of 0.39 which ( F i g u r e 2). A n a l y s i s of till c o l l e c t e d f r o m t h e same a r e a a s s a m p l e d b y K r u m b e i n in southw estern M ich ig an which averages somewhat less shale s h o w s a c o r r e s p o n d i n g l y h i g h e r mean 7A/10A r a t i o (0.48). well as the rest included in F i g u r e 9, indicate (62%) al so These data, as that an apparent r e l a t i o n s h i p e x i s t s b e t w e e n h i g h a m o u n t s of s h a l e a n d 10A c l a y c o n t e n t of till. A l t h o u g h t h e e x a c t q u a n t i t a t i v e r e l a t i o n s h i p b e t w e e n a m o u n t s of s h a l e a n d t h e c l a y - m i n e r a l c o m p o s i ti o n of till v a r i e s g e o g r a p h i c a l l y ( p a r t i c u l a r l y b e t w e e n t h e M ic hi g an a n d Wi sco n si n s i d e s of L ak e M ichigan; F i g u r e 9), a t r e n d t o w a r d s i n c r e a s e d 10A c l a y c o n t e n t w ith i n c r e a s e d a m o u n t s of s h a l e is p r e v a l e n t t h r o u g h o u t t h e b a s i n . similarity in clay mineralogy of S u c h a r e l a t i o n s h i p , t o g e t h e r w ith t h e shales from the Antrim -Ellsworth and C o l d w a t e r F o r m a t i o n s ( F i g u r e 7) a n d S a u g a t u c k - O a k C r e e k till ( F i g u r e 6), s u p p o r t s t h e b e d r o c k mixing model. T h e s e d a t a a l o n e , h o w e v e r , do n o t d e m o n s t r a t e t h a t t h e model a c t u a l l y o p e r a t e d d u r i n g t h e W i s c o n s i n a n . In o r d e r to h a v e o p e r a t e d , t h e b e d r o c k s o u r c e m u s t n o t o n l y h a v e r e l a t i v e l y h i g h a m o u n t of 10A cl ay, be c l a y - r i c h , a n d be " e r o d i b l e " b u t m u s t al so be t e m p o r a l l y a n d s p a t i a l l y u n i f o r m in c l a y - m i n e r a l c o m p o s i ti o n . I n o r d e r to t e s t w h e t h e r t h e s h a l e c l a s t s f o u n d in till w e r e in f a c t derived from the A ntrim -Ellsw orth a n d / o r Coldwater sh a le s (or a t l e a s t fro m b e d r o c k w it h a simi lar c l a y - m i n e r a l c o m p o s i t i o n ) , h a v e r e l a t i v e l y h i g h am ounts of 10A c l ay , and have a generally uniform clay mineralogy 46 t h r o u g h time, s e v e r a l s h a l e c l a s t s f ro m e a c h of t h e ti lls a t G len n S h o r e s were analyzed. These clasts were ground into a s l u r r y w it h d is t i l l e d w ater, d e p o site d on g la ss slides, glycolated, and X -rayed. these analyses are shown on Figure 8 and indicate s i m i l a r i t y in c o m p o s i t i o n , t h e y w e r e p r o b a b l y derived T h e r e s u l t s of that 1) based on fr o m t h e A n t r i m - E l l s w o r t h o r C o l d w a t e r s h a l e s a n d 2) t h e i r c l a y - m i n e r a l c o m p o s i t i o n d o es n o t v a r y b e t w e e n ti lls u n i t s . This la t t e r o b s e rv a tio n shows th a t th e shale s o u r c e w h i c h was e r o d e d b y ice a p p a r e n t l y did n o t v a r y in c l a y - m i n e r a l composition during the Wisconsinan. This is a n im portant observation b e c a u s e it i n d i c a t e s t h a t b o t h m a jo r c l a y - m i n e r a l s o u r c e s ( s h a l e a n d lake c l a y s ) w h i c h c o u l d h a v e b e e n e r o d e d b y ice m a i n t a i n e d a c o n s t a n t c l a y m i n er al c o m p o s i t i o n t h r o u g h o u t t h e W i sc o n s i n a n . T herefore, since n eith er c h a n g e d a n d b o t h h a v e s u c h r a d i c a l l y d i f f e r e n t c o m p o s i t i o n s , n e i t h e r a l on e can account for the system atic variation observed in clay-m ineral c o m p o s i t i o n s h o w f o r till u n i t s ( F i g u r e 6). Discussion B as ed on the above discussion, c l a y m i n e r a l s in tills of t h e Lake M ic h ig an b a s i n w e r e d e r i v e d l a r g e l y fr o m e r o s i o n b y ice of s h a l e b e d r o c k and la cu strin e clays. A l t h o u g h s i m i l a r i t y in c o m p o s i t i o n c a n be o b s e r v e d b e t w e e n i n d i v i d u a l till s h e e t s a n d o n e o r t h e o t h e r of t h e s e s o u r c e s , th e s y s t e m a t i c v a r i a t i o n in c l a y - m i n e r a l c o m p o s i ti o n s h o w n f o r s u c c e s s i v e till s h e e t s ( F i g u r e 6) c a n n o t b e f u l l y a c c o u n t e d f o r b y e r o s i o n of e i t h e r on e of these sources composition of a lo n e. Lake C le a r ly , M ic h i g a n therefore, basin tills are variations probably in clay-m ineral related tc the e r o s i o n of d i f f e r i n g p r o p o r t i o n s of s h a l e a n d l a c u s t r i n e c l a y t h r o u g h time. 47 T h e r e a s o n f o r e r o s i o n of d i f f e r i n g p r o p o r t i o n s of t h e s e s o u r c e b e d s may r e l a t e to o n e of tw o p r o c e s s e s : 1) c h a n g e s i n i c e - f l o w v e c t o r s d u r i n g t h e Wisconsinan within the basin or 2) p r o g r e s s i v e "unroofing" of shale o u t c r o p s b y e r o s i o n of d r i f t m a n t l i n g b e d r o c k d u r i n g s u c c e s s i v e r e a d v a n c e s o f t h e L a k e M ic h i g a n Lobe. T h e s e mo de ls a r e b r i e f l y o u t l i n e d below. Ice Flow V ariation Model Ba s ed o n t h e o u t c r o p p a t t e r n o f s h a l e in t h e La ke M ic hi g an b a s i n ( F i g u r e 7), ice f lo w in g N-S a l o n g t h e ax i s o f L a k e M ich ig an wo uld h a v e b e e n c h a n n e l e d o b l i q u e to b e d r o c k s t r i k e w h i c h would h a v e r e s u l t e d in a re la tiv e ly s h o r t t r a v e r s e a c r o s s th e shale. A m or e NE-SW flow p a t t e r n , h o w e v e r , w o ul d d i r e c t ice a l o n g s t r i k e of t h e e r o d e r e l a t i v e l y more s h a l e . shale (Glenn S h o r e s , b e d r o c k a n d would h a v e T h u s , till w ith low a m o u n t s of 10A c l a y a n d Ganges-New B e r l in , a n d Ozaukee-Haven) may h a v e b e e n d e p o s i t e d b y ice fr o m a n o r t h o r n o r t h w e s t e r n s o u r c e a r e a w h i c h was c h a n n e l e d s o u t h d o w n t h e axi s ( c e n t e r ) of L a k e M ich ig an . Till with h ig h a m o u n t s of 10A c l a y a n d s h a l e ( S a u g a t u c k - O a k C r e e k ) , on t h e o t h e r h a n d , may h a v e been deposited b y ice f r o m a n o r t h e a s t e r n s o u r c e a r e a w hic h flowed s o u t h w e s t a c r o s s n o r t h w e s t e r n M ic hi ga n a n d i n t o t h e L ak e Michigan b a s in . S h i f t s in i c e - f l o w p a t t e r n s s im il ar to t h o s e s u g g e s t e d a b o v e h a v e b e e n proposed by several au th o rs. F o r ex am pl e, D e n t o n a n d H u g h e s (1980) h a v e p r o p o s e d a s h i f t in i c e - f l o w d i r e c t i o n o f t h e L a k e M ic h ig an Lobe f r o m a northern y e a r s ago source region (deposition to a n o r t h e a s t e r n of Ganges-New source region between B e rl in ( d e p o s i t i o n of S a u g a t u c k - O a k C r e e k till). till) and 14,000 17,000 y e a r s ago T h e y h a v e a l so s u g g e s t e d t h a t 48 b y a p p r o x i m a t e l y 10,000 y e a r s a g o t h e s o u r c e a r e a of t h e L a k e M ich ig an Lobe h a d s h i f t e d b a c k to t h e n o r t h w e s t . Li k ew is e, D w o r k i n e t a l . (1986) h a v e s u g g e s t e d t h a t t h e s o u r c e r e g i o n f o r S a u g a t u c k till in s o u t h w e s t e r n M ic h i g a n was north of s o u t h w e s t flow p a t h region had shifted for Lake Huron t h e ice. w estw ard and have proposed They also p ro p o s e d during a northeast- that the source d e p o s i t i o n of P o r t H u r o n tills. A s im il ar w e s t w a r d s h i f t in s o u r c e r e g i o n f o r t h e L a u r e n t i d e ice s h e e t was s u g g e s t e d b y Gwyn a n d D r e im a n is (1979). for Two Rivers E s c h m a n (1981). till (Greatlakean) has A n o rth w e s te rn source region been suggested by Burgis and A s o u t h e a s t flow p a t h d u r i n g t h e l a s t r e t r e a t of ice from n o r t h e r n M ic h ig an h a s a l s o b e e n s u g g e s t e d b y K a r r o w (1987). A l t h o u g h , a s d i s c u s s e d a b o v e , t h e c h a n g e s in s o u r c e a r e a r e q u i r e d b y the i c e - f lo w variation model have been suggested several authors, the model d o e s n o t o f f e r a p a r t i c u l a r l y co m p e ll i n g e x p l a n a t i o n f o r t h e c l a y m i n e r a l v a r i a t i o n in till. I n o r d e r f o r t h e model to h a v e a c t u a l l y o p e r a t e d , it r e q u i r e s t h a t ice m u s t flow o b l i q u e to t h e M ic hi ga n. Because the Lake M ic h i g a n basin t o p o g r a p h i c axi s of La ke forms a lo n g , regional, t o p o g r a p h i c t r o u g h , a n y ice flow p a t t e r n o t h e r t h a n d i r e c t l y s o u t h dow n t h e axi s ( c e n t e r ) of L ak e M ic h ig an s e e m s p r o b l e m a t i c . Th is wou ld h a v e b e e n t r u e r e g a r d l e s s of t h e d i r e c t i o n fr o m w h i c h ice e n t e r s t h e b a s i n . Fo r ex am p le, a c c o r d i n g to t h e model, ice w h ic h d e p o s i t e d S a u g a t u c k - O a k C r e e k till m u s t f i r s t h a v e flo w ed s o u t h w e s t a c r o s s t h e t o p o g r a p h i c u p l a n d s of northw estern M ic hi ga n a n d then entered the trough o f L a k e Mich ig an. M o r e o v e r , o n c e t h e L a k e M ic h i g a n Lobe e n t e r e d t h e b a s i n , it m u s t h a v e m a i n t a i n e d a g e n e r a l n o r t h e a s t - s o u t h w e s t flow p a t h a n d m u s t h a v e b e e n completely u n a ffe c te d by t h e t o p o g r a p h y of t h e lake; a s i t u a t i o n w h i c h 49 s ee m s u n l i k e l y . T he model a l so implies t h a t s h a l e is g e n e r a l l y e x p o s e d a n d available for erosion o v e r la rg e g e o g ra p h ic a r e a s t h r o u g h o u t the basin. S h a le b e d r o c k , h o w e v e r , o u t c r o p s e x t e n s i v e l y in o n l y a few a r e a s w h ic h , most n o t a b l y , a r e a l o n g t h e m i d - l a k e h i g h in c e n t r a l L ak e M ich ig an ( F i g u r e 7) a n d in e x t r e m e n o r t h e r n n o r t h w e s t e r n Michigan. Th e ic e- f lo w F i g u r e 9. model is a l so q u e s t i o n e d based on d a t a p r e s e n t e d in T h e s e d a t a i n d i c a t e t h a t no simple r e l a t i o n s h i p e x i s t s b e t w e e n th e a m o u n t of s h a l e c l a s t a n d c l a y - m i n e r a l c o m p o s i ti o n of till. Although a few m e c h a n i s m s m i g h t e x p l a i n t h e a b s o l u t e d i f f e r e n c e s in a m o u n t s of s h a l e c l a s t s in sim il ar a g e tills with sim il ar c l a y - m i n e r a l c o m p o s i t i o n s b e t w e e n Wisconsin and M ic h ig an differences in am ounts c o m p o s i ti o n co u l d be ( F i g u r e 9), of shale explained by n o n e is s a t i s f a c t o r y . clasts a and associated "threshold" volum e of s h a l e a n d c l a y - m i n e r a l co m p o si ti o n . F or example, clay-mineral relationship between Such a relationship derives from t h e f a c t t h a t t h e c l a y - m i n e r a l c o m p o s i t i o n of s h a l e c l a s t s f o u n d in till is cl o se to t h a t of b o t h S a u g a t u c k a n d Oak C r e e k till ( F i g u r e s 6 a n d 8 ) a n d , t h e r e f o r e , r e g a r d l e s s of t h e volum e of s h a l e e r o d e d , no s i g n i f i c a n t c h a n g e in c o m p o s i ti o n c o u l d r e s u l t . Michi gan ) and Oak C r e e k H ow ev er, if c o r r e c t t h e n G a n g e s (in (in Wisconsin) till s h o u l d e x h i b i t simi lar c l a y m i n e ra l c o m p o s i t i o n s s i n c e t h e y h a v e sim il ar a m o u n t s of s h a l e c l a s t s . is c l e a r l y n o t t h e c a s e a n d Th is i n d i c a t e s t h a t a more complex p r o c e s s m u s t have operated. U n r o o fin g Model Much r e c e n t r e s e a r c h c o n c e r n i n g t h e W i s c o n s i n a n in t h e G r e a t L a k e s r e g i o n h a s c o n c l u d e d t h a t t h e m i d - W i s c o n s i n a n (55,000 to 25,000 y e a r s ago; 50 D r e im a n is a n d G o l d t h w a i t 1973) g e n e r a l l y r e p r e s e n t s a n i c e - f r e e i n t e r v a l ( B e r g e t a l . 1985; E s c h m a n 1980; G e p h a r t e t a l . 1982; Rieck a n d W i n te r s 1982; W i n t e r s e t a l . 1986; W i n t e r s e t a l . 1988). If c o r r e c t , t h a n a s mu ch a s 20-30,000 y e a r s o f l a c u s t r i n e s e d i m e n t w as d e p o s i t e d w i t h i n t h e L ak e M ic h i g a n b a s i n d u r i n g t h i s i c e - f r e e i n t e r v a l . m a n tl e of l a c u s t r i n e within the basin. T h i s p r o b a b l y al lowed a t h i c k s e d i m e n t to m a s k a l r e a d y s p a r s e As the in i ti a l late Wisconsinan bedrock outcrops (early W ood fo rd ia n) a d v a n c e filled t h e b a s i n a t a b o u t 25,000 y e a r s a g o , it would h a v e b e g u n to erode this sedim ent m a n tl e . Thus, tills deposited early in the l a te W i s c o n s i n a n ( s u c h a s G a n g e s - N e w Be rl in ) s h o u l d r e f l e c t t h e m i n e r a l o g y of these la cu strin e sedim ents. S i n c e m u c h of t h e s e d i m e n t o v e r l y i n g s h a l e o u t c r o p s wou ld h a v e b e e n e r o d e d t h r o u g h time, h o w e v e r , l a t e r a d v a n c e s of the Lake M ic h i g an lobe wo ul d have eroded relatively m o re shale and d e p o s i t e d till ( S a u g a t u c k - O a k C r e e k ) w ith a g r e a t e r i n f l u e n c e of b e d r o c k mineralogy. S u c h a p a t t e r n is i l l u s t r a t e d in F i g u r e s 4 a n d 7. of l a c u s t r i n e - t y p e m i n e r a l o g y in t h e y o u n g e s t A dominance ( a n d most n o r t h e r n ) tills p r o b a b l y r e f l e c t s t h e f a c t t h a t t h e s h a l e e i t h e r did n o t o c c u r w it h i n t h e n o r t h e r n p a r t of L a k e M i c h i g a n o r s u b c r o p p e d u n d e r t h i c k , p r e - e x i s t i n g drift. T h e q u a n t i t a t i v e d i f f e r e n c e in t h e r e l a t i v e a b u n d a n c e of s h a l e c l a s t s o c c u r r i n g w i t h i n e q u i v a l e n t a g e till in M ic h ig an a n d Wisc ons in ( S a u g a t u c k and Oak C r e e k , respectively; Figure 9) may relate to t h e p r o x i m i t y of s h a l e o u t c r o p s to s i t e s of d e p o s i t i o n of t h e s e tills a n d to t h e local b a s a l c o n d i t i o n s of t h e L a k e M ic h i g a n Lobe. F o r exam ple , a l t h o u g h a simple, p o s i t i v e r e l a t i o n s h i p may e x i s t b e t w e e n t h e a m o u n t of a g i v e n r o c k t y p e e r o d e d b y ice a n d t h e a r e a l e x t e n t of o u t c r o p of t h a t r o c k t y p e " u p - i c e " ( H a r r i s o n 1960; P e lt o n ie m i 1985), t h e a c t u a l d i s t a n c e c l a s t s a r e t r a n s p o r t e d a n d t h e r a t e o f co m m in u t io n a p p e a r i n d e p e n d e n t of o u t c r o p s ize . T h i s was s u g g e s t e d b y C l a r k (1987) w h o f u r t h e r p r o p o s e d t h a t t r a n s p o r t d i s t a n c e of c l a s t s a r e n o t c o n t r o l l e d b y i n i ti a l c o n c e n t r a t i o n b u t r a t h e r b y t o p o g r a p h y o f t h e s u b s t r a t e a n d b a s a l ice v e l o c i t y . Topographic o b s tr u c tio n s along t h e b e d c a u s e t h e ice to d e c r e a s e b a s a l v e l o c i t y w h i c h r e s u l t s in i n c r e a s e d frictional d r a g and tra c tiv e forces along the bed, relatively g r e a t e r d ebris e n t r a i n m e n t , a n d i n c r e a s e d "mixi ng" of b a s a l d e b r i s (Clark 1987). Large t r a c t i v e f o r c e s al so r e s u l t in i n c r e a s e d c r u s h i n g a n d g r i n d i n g of s e d i m e n t s ( D r e w r y 1986). conditions C r u s h i n g a n d g r i n d i n g is p r o b a b l y e n h a n c e d u n d e r t h e s e because particularly of low larger basal velocity clasts ( Bo ult on increases 1975), r e w o r k i n g of d e b r i s a l o n g t h e b a s e of t h e ice. rates which of lodgement, allow extensive C o n v e r s e l y , a s ice e n t e r s t o p o g r a p h i c d e p r e s s i o n s , b a s a l v e l o c i t y is e n h a n c e d . Rapi d b a s a l v e l o c i t y is a c o n s e q u e n c e of d e c r e a s e d f r i c t i o n a l d r a g b e t w e e n c l a s t s in t h e ice a n d the substrate and r e s u l t s in l o n g e r t r a n s p o r t d i s t a n c e s a n d mixing of d e b r i s w i t h i n t h e ice ( C la rk 1987). decreased Low er co m m in u t io n r a t e s p r o b a b ly also o c c u r u n d e r s u ch conditions. Although the relationship between basal velocity and topographic o b s t r u c t i o n s is b e s t u n d e r s t o o d f o r s m a l l - s c a l e f e a t u r e s (e.g. l e s s t h a n a few m e t e r s in s iz e) , it h a s al so b e e n a p p l i e d on a r e g i o n a l s c a l e to th e Great Lakes M ic h i g an area basin ( Bo ult on et al. 1985) (Mickelson e t a l . 1981). and specifically to the L a k e M ic h i g a n c o n s i s t s Lake of two d e e p b a s i n s (on e r e s t r i c t e d to t h e n o r t h e r n h a l f of t h e l a k e a n d t h e o t h e r to t h e s o u t h e r n half) w h i c h a r e s e p a r a t e d b y t h e m i d - l a k e h i g h ( F i g u r e 7). Water d e p t h in t h e two b a s i n s e x c e e d s 150 m b u t is o f t e n <60 m a l o n g th e 52 mid-lake high (Wickham e t a l . 1978). Rapid v e l o c i t y is p r e d i c t e d a s ice flo w s t h r o u g h t h e two b a s i n s a n d in p l a c e s may h a v e e v e n u n c o u p l e d fr o m the bed (Mickelson e t a l . 1981). H o w e v er , a s ice c r o s s e d the regional to p o g rap h ic o b s tr u c tio n r e p r e s e n t e d by the mid-lake high, basal velocity w o u ld decrease increased. as frictional T h i s, a s o u t l i n e d drag above, and tractive wou ld r e s u l t forces in along relatively the bed greater e n t r a i n m e n t a n d mixing of b a s a l d e b r i s a s well a s i n c r e a s e d c r u s h i n g a n d g r i n d i n g of t h e e n t r a i n e d s e d i m e n t s . As t h e L a k e M ic h i g an Lobe f low ed a c r o s s t h e m i d - l a k e h i g h d u r i n g t h e l a t e r p a r t of t h e W o o d f o r d ia n ( a b o u t 14-15,000 y e a r s a g o ) , it p r o b a b l y e r o d e d a c o n s i d e r a b l e a m o u n t of n e w l y e x p o s e d s h a l e (as p r e d i c t e d b y t h e u n r o o f i n g model). F u t h e r m o r e , if e n h a n c e d r a t e s of c r u s h i n g a n d g r i n d i n g a c t u a l l y o c c u r r e d , t h e s h a l e would h a v e b e e n r a p i d l y c o m m i n u t e d to f i n e ­ grained debris. C o n t i n u e d s o u t h w e s t w a r d flow of t h e L a k e M ic hi ga n Lobe t o w a r d St. F r a n c i s would h a v e c a u s e d it to p a s s from a t e r r a i n d o m i n a t e d b y s h a l e to o n e d o m i n a t e d b y mid dle D ev o ni an c a r b o n a t e ( F i g u r e 7; Wold e t a l . 1981). T h i s w ou ld h a v e r e s u l t e d in d i l u t i o n of t h e s h a l e - r i c h p e b b l e - s iz e d e b r i s w it h c a r b o n a t e c l a s t s . Because th e se c a rb o n a te s a re relatively low in c l a y m i n e r a l s (Wold e t a l . 1981), h o w e v e r , no s i g n i f i c a n t c h a n g e in t h e c l a y - m i n e r a l c o m p o s i t i o n of t h e v e r y f i n e f r a c t i o n wro u l d h a v e r e s u l t e d . Flow from t h e m i d - l a k e h i g h s o u t h e a s t w a r d t o v ' a r d G len n S h o r e s , on t h e o t h e r h a n d , wo uld h a v e c a u s e d t h e L ak e M ic h i g a n Lobe to p a s s o v e r t h e s o u t h e r n b a s i n w h i c h is n o t o n l y t o p o g r a p h i c a l l y low b u t a l s o u n d e r l a i n m a in ly by shale bedrock ( F i g u r e 7). The broad, low, topographic c o n f i g u r a t i o n of t h i s a r e a p r o b a b l y r e s u l t e d in e n h a n c e d b a s a l flow v e l o c i t y a n d c o n s e q u e n t l y n o t o n l y all ow ed ice to c a r r y c l a s t s f u r t h e r , b u t al so 53 decreased mixing a n d c l a s t c o m m in u t io n . B e c a u s e t h e s o u t h e r n b a s i n is s u c h a b r o a d f e a t u r e , h o w e v e r , m i n o r b e d o b s t r u c t i o n s p r o b a b l y lo ca lly a f f e c t e d t h e b a s a l flow p a t t e r n o f t h e L a k e M ic h i g an Lobe a n d i n p l a c e s increased the obstructions rate w ou ld existing drift. of have erosion and c o m m in ut io n . Im portantly, these b e e n c o m p o s e d m a in ly of e i t h e r s h a l e o r p r e ­ T h e r e f o r e , a l t h o u g h ice c r o s s e d a s i g n i f i c a n t d i s t a n c e (75— 100 km; F i g u r e 7) f r o m t h e m i d - l a k e h i g h to G len n S h o r e s a n d may h a v e c r o s s e d a n d e r o d e d c o n s i d e r a b l e s h a l e o u t c r o p , it al so c r o s s e d a n d e r o d e d e x t e n s i v e la k e c l a y s . T h u s , a d y n a m i c e q u i l i b r i u m b e t w e e n e r o s i o n of s h a l e a n d la ke c l a y s may h a v e b e e n a c h i e v e d a n d 10A c l a y d e r i v e d fr o m e r o s i o n of s h a l e may h a v e b e e n d i l u t e d b y 7A c l a y d e r i v e d fr om e r o s i o n of la k e clays. As t h e L ak e M ic h i g a n Lobe c o n t i n u e d to flow s o u t h t o w a r d I n d i a n a a n d Illinois it p a s s e d o u t o f t h e s o u t h e r n b a s i n a n d b e g a n to climb o u t of the lake basin. T h i s a r e a is n o t o n l y t o p o g r a p h i c a l l y a n a l o g o u s to t h e m i d - l a k e h i g h b u t is a l s o u n d e r l a i n b y v e r y sh al lo w d r i f t ( s e v e r a l m e t e r s ; L i n e b a c k e t a l . 1972, 1974) a n d a t l e a s t in p a r t b y s h a l e ( F i g u r e 7). Th is wo uld r e s u l t in b a s a l ice c o n d i t i o n s s im il ar to t h o s e s u g g e s t e d a l o n g t h e m i d - l a k e h i g h a n d allow s i m i l a r l y h i g h r a t e s of e r o s i o n a n d c o m m in u t io n of bedrock. R e g a r d l e s s of a n y d i f f e r e n c e in a b s o l u t e a m o u n t s of s h a l e , h o w e v e r , t h e d a t a s h o w n in f i g u r e 9 a n d t h e s p a t i a l d i s t r i b u t i o n of s h a l e w i t h i n t h e b a s i n d e m o n s t r a t e t h a t s h a l e w a s a p p a r e n t l y r a p i d l y c o m m i n u t e d to c l a y siz e m a t e r i a l ( w i t h i n a fe w to s e v e r a l 10’s of km). This s u g g e s ts that c l a y - s i z e m a t e r i a l in till is n o t n e c e s s a r i l y r e p r e s e n t " f a r - t r a v e l e d ” m a t e r i a l a n d a s i g n i f i c a n t c o m p o n e n t c o u l d b e d e r i v e d locally. T h i s is al so implied, 54 if a s s u g g e s t e d , t h e d i f f e r e n c e s in c l a y m i n e r a l o g y b e t w e e n t h e two a r e a s a t P a r k Cliffs (A re a N a n d G; F i g u r e 3) r e s u l t e d lo ca lly f r o m e r o s i o n of lake clays. Glenn S h o r e s Till? Although above the mod els Wisconsinan unroofing co u l d tills. account Neither, m i n e r a l o g y o f G lenn S h o r e s clay than any model s e e m s m o s t p l a u s i b l e , of the for the variation however, of c l a y - m i n e r a l s adequately accounts till s i n c e it c o n t a i n s sources discussed e i t h e r of t h e for in la te the cl ay h i g h e r a m o u n t s of 7A above. This c o m p o s i ti o n a l d i s s i m i l a r i t y w ith o t h e r s e d i m e n t s o f t h e L a k e M ic h i g a n b a s i n may r e l a t e to t h e r e l a t i v e l y g r e a t a g e of t h e till. F o r exam ple , if G len n S h o r e s till was d e p o s i t e d d u r i n g t h e E a r l y W i s c o n s i n a n (as p r o p o s e d b y G e p h a r t e t a l . 1982), it may r e p r e s e n t o n e of t h e e a r l i e s t p o s t - S a n g a m o n ice a d v a n c e s in the L ak e M ic h i g an basin. This ice advance probably traversed an e x t e n s i v e l y w e a t h e r e d s u r f a c e w h ic h , if w e a t h e r i n g h a d b e e n of s u f f i c i e n t i n t e n s i t y , m ay h a v e b e e n r e l a t i v e l y k a o l i n i t e - r i c h . This w e a th e r in g s u r f a c e may, t h e r e f o r e , r e p r e s e n t a n a d d i t i o n a l , b u t now a b s e n t , clay-m ineral source bed and could account for the kaolinite-rich, anomalously high a m o u n t of 7A c l a y f o u n d i n G len n S h o r e s till. A lthough appealing, the above model is n o n e the less speculative s i n c e no S a n g a m o n a g e w e a t h e r i n g s u r f a c e h a s b e e n r e p o r t e d w i t h i n t h e L a k e M ic h i g a n b a s i n . soils h a v e b e e n s t u d i e d . S o u t h of t h e b a s i n in Ill inois, h o w e v e r , S a n g a m o n F o r exam ple , B r o p h y (1959), u s i n g h e a v y m i n e r a l s a s a n i n d e x o f w e a t h e r i n g , h a s s u g g e s t e d r e l a t i v e l y i n t e n s e w e a t h e r i n g of S a n g a m o n soils, b u t i n d i c a t e s t h a t t h e p r i n c i p a l c l a y - m i n e r a l w e a t h e r i n g 55 product is m o n t m o r il lo n it e (smectite). Elsewhere in the region, however, kaolinite-rich saprolite has been rep o rte d . and LaSa ll e developed et on a l. (1985) shield a u t h i g e n i c k a o l in it e . rocks have of described southern scattered Quebec that Great Lakes LaSalle (1984) saprolite contain deposits abundant D u r i n g i n t e r g l a c i a l p e r i o d s , t h e s e s a p r o l i t e s may h a v e b e e n mo re w i d e - s p r e a d a n d may h a v e r e p r e s e n t e d a s i g n i f i c a n t k a o l in it e s o u r c e w h i c h is now a l m o s t c o m p l e t e l y g o n e . In o r d e r to t e s t w h e t h e r G len n S h o r e s till in f a c t d o e s c o n t a i n a s i g n i f i c a n t q u a n t i t y of k a o l i n i t e , t h e r e l a t i v e c o n t r i b u t i o n of c h l o r i t e a n d k a o l i n i t e to 7A p e a k i n t e n s i t y m u s t be d e t e r m i n e d . Th is is b e c a u s e b o t h k a o l i n i t e a n d c h l o r i t e a r e p r e s e n t in all s a m p l e s a n d c h l o r i t e a n d k a o l in it e b o t h s h a r e b a s a l X - r a y d i f f r a c t i o n r e f l e c t i o n a t 7.14A (002 c h l o r i t e a n d 001 k a o l in it e ) and 3.58A (004 chlorite and 002 k a o l in it e ) . V a r io u s heat t r e a t m e n t s a r e com monly e m p l o y e d to d e t e r m i n e if k a o l i n i t e is p r e s e n t in the sample ( C a r r o ll 1972; B r i n d l e y and B ro w n 1980). Such treatm ents, h o w e v e r , u s u a l l y o n l y allow d e t e r m i n a t i o n of t h e p r e s e n c e of k a o l i n i t e a n d a r e not quantitative. chlorite is poorly Heat t r e a t m e n t s a r e a l s o n o t p a r t i c u l a r l y r e l i a b l e if crystalline, ( B r i n d l e y a n d B ro w n 1980). as in often the case in "soil” material A s e p a r a t i o n of k a o l i n i t e a n d c h l o r i t e c a n al so b e o b t a i n e d u s i n g w ar m HC1 ( B r i n d l e y a n d B ro w n 1980). This tre a tm e n t r e l i e s o n t h e f a c t t h a t HCl s e l e c t i v e l y d i s s o l v e s c h l o r i t e while g e n e r a l l y l e a v i n g k a o l i n i t e u n t o u c h e d a n d w as e m p l o y e d in t h i s s t u d y to d e t e r m i n e t h e r e l a t i v e a m o u n t s o f k a o l i n i t e a n d c h l o r i t e in s am p l e s . P r e v i o u s l y s e p a r a t e d c l a y - s i z e (<2 um) f r a c t i o n of s a m p l e s w e r e s p l i t in t o 2 p o r t i o n s . One p o r t i o n w a s p l a c e d on a g l a s s s li d e a n d X - r a y e d . Th e o t h e r w as p l a c e d in a s o l u t i o n of warm, IN HCl f o r 4 h o u r s , w a s h e d 56 with d is t i l l e d w a t e r , p l a c e d on a g l a s s slide a n d X - r a y e d . ray pattern was then compared with the T h e a c i d i f i e d X- non-acidified. The relative r e d u c t i o n of t h e 7A p e a k of aci d ifi ed s a m p l e s r e p r e s e n t s t h e a m o u n t of c h l o r i t e r e m o v e d by HCl wh ile t h e p e a k r e m a i n i n g r e p r e s e n t s t h e a m o u n t of k a o l in it e in t h e s am p le . T h e % in t e n s it y of t h e 7A p e a k d u e to ka o l in it e (%K7A) a n d c h l o r i t e {%Cta) w e r e d e t e r m i n e by t h e following r e l a t i o n s h i p s : %K7A = [ (7 A / 1 0 A ) hci] / [ (7A/10A)noHcil %C7a = 1-%K7a t Two Ri ve r s LAKE