, , . . . -. . V ...._....., -ME “WWW” V.‘ , .. . ., . . a“) .. THE NIAGARAN REEFS AND OVERLYING CARBONATE EVAPORITE ~- SEQUENCE IN SOUTHEASTERN MICHIGAN Thesis for the Degree of M. S. MICHIGAN STATE UNIVERSITY EDWARD R. BATES 1970 9 . I a ‘V Michigan Stat LIBRAR Y Umvcmt)’ THE NIAGARAN REEFS‘ AND OVERLYING CARBONATE EVAPORITE SEQUENCE IN SOUTHEASTERN MICHIGAN aG/flfl/ gyaq’X’774 For their help and critical advice in the research and preparation of this paper the writer wishes to express his indebtedness and gratitude to Dr. Chilton E. Prouty, Dr. Robert Erlich, and Dr. William J. Hinze of the Depart- ment of Geology, Michigan State University, and a special thanks to Garland Elle of the Michigan Geological Survey for his aid and interest in the research for this paper. II III IV VI VII VIII TABLE OF CONTENTS Introduction Stratigraphy of Formations Cross Sections Graphs of Apparent Relationships Summary and Conclusions Bibliography Appendix Back Pocket Reef Plot Map (Figure 1) 16 35 54 56 59 10. ll. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. LIST OF ILLUSTRATIONS Reef Plot Map Marine City Reef Belle River Mills Reef Boyd Reef Peters and Puttygut Reefs Lenox Reef Big Hand Reef Columbus Reef Ray Reef Depth Top G Shale Thickness C Unit Depth Top B Salt Depth Top A - 2 Carbonate Thickness A - 2 Carbonate Depth Top A - 2 Evaporite Thickness A - 2 Evaporite Depth Top A - 1 Carbonate Thickness A - 1 Carbonate Depth Top A - 1 Evaporite Thickness A - l Evaporite Depth Top Niagaran A - 2 Evaporite Thickness vs. Reef Height B Salt Thickness vs. Minimum Reef Width B Salt Thickness vs. Log Minimum Reef Width Standard Gamma-Ray Log Reef Height vs. Minimum Width Back Pocket 18 21 25 26 28 3O 32 34 37 38 39 4O 41 42 43 44 45 46 47 48 51 52 53 God 61 INTRODUCTION It is the intent of this study to examine in some detail a small part of the Michigan Basin where reefs are known to have developed in abundance during the Niagaran phase of the Silurian Period. The intent of the study is to determine the extent to which the Niagaran reefs are re- flected by thinning and draping of overlying formations in the Silurian Period. Sharma (1966) in his study of the Peter's Reef reports no reflection in the overlying Salina units above the D Salt; while Jodry ( 1969) indicates in his study of several reefs in eastern Michigan no reflection above the C unit. Though there may be closure of structur- al contours in the Devonian and Mississippian near the lo- cation of Niagaran reefs, this is not a direct effect of the Niagaran reefs themselves and is not included in this inves- tigation. This research was focused on a small area in St. Clair and Macomb Counties as this is the only area for which there is sufficient information to make such a study. The following is a general description of the Michigan Basin presented by Garland D. Elle (1967) of the Michigan Geological Survey: "The Michigan Basin is a relatively shal- low intracratonic basin encompassing all of the Southern Peninsula, the Northern Peninsula, and parts of Wisconsin, Illinois, Indiana, Ohio, and Ontario. It is bounded on the north and northeast by the Canadian Shield; on the east and southeast by the Algonquin Arch in Ontario and the Findlay 2 Arch in northern Ohio, on the southwest by the Kankakee Arch in northern Indiana and northwestern Illinois; and on the west and northwest by the Wisconsin Arch and Wisconsin Dome. The Basin, though circular, has a slight northwest- southeast elongation." "The deepest part is believed to underlie a part of Clare and Gladwin Counties in central Michigan where an es- timated 14,000 - 15,000 feet of sedimentary rocks mantle the Precambrian. The Paleozoic series are represented by Cam- brian, Ordivician, Silurian, Devonian, Mississippian, and Pennsylvanian sediments. A small amount of Mesozoic (Jur- assic) rock overlies the Pennsylvanian in the central area of the Lower Peninsula.“ "Rocks of Early, Middle, and Late Silurian Age occur throughout most of the Basin. They account for over 30 per cent of the estimated 108,000 cubic miles of sediment in the Basin (Cohee and Landes, 1958). Probably one third to one half of this volume consists of nearly pure rock salt in the Upper (Late) Silurian sequence.” The area of detailed study in this paper lies in St. Clair and Macomb Counties in southeastern Michigan and lies on the margin of the Michigan Basin. The exact area included and the location of the reefs studied is shown on Figure One in the back pocket of this report. This paper consists of several parts. Following the introduction is a section presenting a detailed description of all the formations included in this study. This is followed by cross sections and descriptions of the nine reefs included in this study. The reef locations, the location of the cross sections, and the permit numbers of the wells used in construction of the cross sections are all included on the map at the back of the paper. (Figure 1). Following the sections is a series of graphs which depict the trends or relationships between the reefs and the overlying formations observed by the writer. At the back there is also an Appen- dix containing all the data obtained and used for this study and also a sample standard gamma-ray log showing where the writer selected his formation tops. All formation thickness and depth to formation tops presented in this study are based on gamma-ray logs corrected to current sea level. STRATIGRAPHY OF FORMATIONS The following is a description both general and detailed of the formations studied. The information is drawn from the writer's examination of cuttings from a few wells and from the printed log descriptions of cuttings and cores of wells both over and off reef structures throughout the area of study which are published and distributed by the Michigan Geological Survey. The information on thickness variation is mainly from the writer's cross sections. The formations are listed in order oldest to youngest though not in all locations are all formations present. The descriptions apply only to the small area of study and not necessarily to all of the Michigan Basin. 5 MIDDLE SILURIAN Clinton Formation The Clinton Formation is the oldest formation considered in this study. Few wells in the area of study penetrate into the Clinton as most and in the overlying Niagaran. The Clin- ton is a shale usually gray in color though sometimes green- ish or brownish. It is granular and dolomitic in part. In places there are thin buff to brownish gray beds of finely crystalline dolomite in a glauconitic shale containing a lit- tle pyrite. Though the Clinton has a distinctive gamma-ray log kick in the area of study this is not true elsewhere in the Basin where it is hard to distinguish from the Cataract group below or from the Niagaran above. In this study, the Clinton, which is commonly considered as a part of the Niagaran, is treated separately. Niagaran Group In this study the Niagaran is undivided and used in the formational sense. In outcrop studies the Niagaran is often broken into formations but because of the complexity of the group in the area of study where it is the principal reef- forming rock it was not feasible to subdivide it on gamma- ray log characteristics. Though the Niagaran has occasional patches of limestone it consists mostly of dolomite in the area studied. There appears to be no distinct relationship as to where it is lime- stone or where it is dolomite. The dolomite is usually 6 medium gray to brown finely to very finely crystalline. It is often argillaceous and thin shale beds are not infrequent. The dolomite varies from dense, almost impermeable, to very vuggy with vugs ranging up to fist size. The vugs are usually filled with salt water or anhydrite, the latter being more common. Occasionally cores from over reefs bleed oil or gas or show carbonaceous partings. Thin lenses of gray to blue anhydrite one to four feet thick and often argillaceous oc- cur occasionally. There is vertical fracturing in some of the reefs and the fractures are usually filled with anhydrite. Fossil remains consist of both secondary infillings in frac- tures (usually crinoids) and as remains of the algal-stroma- toporoid life forms that probably created the reefs. The thickness of the Niagaran Group is highly variable since it is the rock of which the reefs in this study are com- posed. The only cross section in this study that shows the full section of the Niagaran is the marine City Field. How- ever, this field, being located on the extreme margin of the Michigan Basin, cannot give a true picture of the Niagaran in other reefs. In the Marine City Field the Niagaran varies from about 40 feet presumably at the reef edge to about 200 feet over the crest. This rapid increase in thickness occurs in about 0.7 miles. The increase in Niagaran section occurs through the area of study whenever a reef is encountered. The Big Hand cross section, for example, shows an increase in thickness of 240 feet in 0.25 miles. Lenox, Columbus, 7 and Puttygut all show increases of about 260 feet while Peters, Belle River Mills, Boyd, and Ray all show an increase of over 300 feet in the Niagaran section. 8 LATE SILURIAN A - 1 Evaporite Formation The A - l Evaporite lies immediately above the Niagaran except on the reefs where it is absent. It consists of a blue gray, white, or buff anhydrite often with dolomite stringers and pyrite specks. The cross sections between Puttygut and Peters and between Peters and Boyd, and the cross sections at the edges of the other reefs show that the usual thickness is between 15 and 25 feet with a slight dip towards the center of the Michigan Basin. The A - 1 Evaporite gener- ally pinches out about half way up the reef. The t0p of the A - l Evaporite occurs at increasing depth towards the Basin center being less than 1850 feet below sea level at Marine City Field but over 2600 feet at Ray Field. 9 A - l Carbonate Formation The A - l Carbonate lies immediately above the A - l Evaporite except over the Niagaran reefs where it lies im- mediately on the reefs, the A - 1 Evaporite not being present. The A - l Carbonate consists of gray to brown finely crystal- line dolomite occasionally with slight porosity and some an- hydrite. Off reefs it is sometimes a black or brown lime- stone though dolomite is equally common. Brown and black partings sometimes occur in off reef structure. It often has a gas or oil odor and sometimes contains a gas pool over a large reef structure. Anhydrite nodules often occur in the A - l Carbonate near the contact with the A - l Evaporite. The depth to the top of the A - 1 Carbonate increases towards the Basin center, being about 1600 feet (below sea level) at Marine City Field, but 2200 - 2500 feet at Ray Field. The A - l Carbonate covers the crests of all the Niagaran reefs but thins greatly in all cases. The most extreme example is the Darine City Field where the A - 1 Carbonate thins from over 200 feet at the reef edge to less than 40 feet over the crest. More commonly it thins from its usual 100 - 200 feet off-reef thickness to 20 - 60 feet over the reefs. 10 A — 2 Evaporite The A - 2 Evaporite lies immediately above the A - l Carbonate. The A - 2 Evaporite consists of both anhydrite and salt with salt predominating. The anhydrite is white, gray, or brown and often is dolomitic or intermingled with dolomite. The salt is white to clear and usually occurs be- low an anhydrite bed. It is about three times as thick as the anhydrite both over reefs and between reefs. The A - 2 Evaporite is usually between 120 and 180 feet off reef but thins to about 40 feet over reef crests. The Marine City section at the edge of the Michigan Basin is an exception since the Niagaran reef was not nearly so high and is not reflected by the A - 2 Evaporite. The thickness of the A - 2 Evaporite shows a tendency to increase farther to- wards the Basin center from about 100 feet near Belle River Mills Field near the Basin edge to over 180 feet at Ray Field in Macomb County. The depth to the top of the A - 2 Evapo- rite increases from 1500 feet at Marine City Field at the Basin edge to about 2200 feet at Ray Field farther out in the Michigan Basin. 11 A - 2 Carbonate The A - 2 Carbonate lies immediately above the A - 2 Evaporite throughout the area of study. Over reefs it con- sists of a gray to brown dense to slightly porous dolomite sometimes with gray shale partings and often with substantial amounts (up to 30%) of salt or anhydrite. Between reefs the A - 2 Carbonate consists either of a tan or buff dolomite often anhydritic, or a gray to brown limestone usually fine grained and with some shale near the base. Though it is not as common to find limestone over the reefs this writer did not see evidence that Niagaran reef location can be predicted from the amount of limestone versus dolomite in the A - 2 Carbonate or any other formation. This study, however, does not preclude the possibility of such a relationship existing elsewhere. The thickness of the A - 2 Carbonate is usually 170 to 140 feet between reefs and 100 to 120 feet over the Niagaran reefs. The depth to the top of the A - 2 Carbonate increases steadily from 1400 feet below current sea level at Marine City Field near the Michigan Basin edge to about 2100 feet at Ray Field. 12 §__§Naporite Formation The B Evaporite Formation, or B Salt as it is commonly called, lies above the A - 2 Carbonate Formation throughout the area of study. The B Evaporite Formation over reefs consists mostly of white salt with a few thin beds of anhy- drite and tan to brown dolomite. Off reef the B Evaporite also consists mostly of white salt with stringers of buff to white anhydrite, tan fine grained dolomite with leached porosity, and a gray shale. The thickness of the B Evaporite varies from 260 to 380 feet, but usually is about 300 feet, no consistent change being apparent over reefs. The depth to the top of the B Evaporite increases steadily towards the Michigan Basin cen- ter from about 1050 feet below sea level at Marine City Field to about 1800 at Ray Field. C Shale Formation The C Shale Formation lies above the B Evaporite For- mation through the area of study. Off reef the C Shale Formation consists of a green to gray shale with anhydrite and salt inclusions. Over reefs it is a gray to red shale with some thin beds of brown finely crystalline dolomite. These differences between over and off reef sections are minor and not always consistent so should not be used to distinguish whether a well is over or off reef. The thickness of the C Shale Formation varies from about 30 to 80 feet but usually is 50 to 60 feet. The 13 thickness is usually less over a reef crest as shown on a graph in a later section. The depth to the top of the C Shale increases steadily towards the Basin center from less than 1000 feet below sea level at Marine City Field to over 1700 feet at Ray Field. The depth to the top of the C Shale is also generally less over a reef than off reef by about 30 to 50 feet. The C Shale is the last formation to reflect the buried Niagaran reefs in some fields, though they may be detected even higher in some other fields. 2__§alt Formation The D Salt Formation lies immediately above the C Shale Formation throughout the area of study. The D Salt both off reef and over reefs consists mostly of a white translucent salt with a thin (about 4 foot) bed of tan to brown dolomite near the center of the formation. There is an occasional trace of white anhydrite in the salt and the dolomite bed is sometimes argillaceous. The D Salt Formation is absent over the crest of the Marine City Field but is present everywhere else in the area of study. The thickness of the D Salt varies from 14 to 50 feet but it is usually about 25 to 30 feet thick. The depth to the top of the D Salt increases from less than 1000 feet below sea level at Marine City to about 1700 feet at Ray Field. The D Salt does reflect the Niagaran reefs in some cases but not in all so is not diagnostic. 14 E Unit The E Formation or E Unit lies immediately above the D Salt Formation throughout the area of study. The E Unit consists of gray to brown finely crystalline non-porous dolomite sometimes with gray to green shale stringers or a little white anhydrite. The E Unit varies from 64 to 140 feet in thickness but is usually between 80 and 95 feet thick. The depth to the top of the E Unit increases steadily basinward from about 900 feet below sea level at Marine City Field to about 1600 feet at Ray Field. F Unit The F Formation or F Unit lies immediately above the E Unit throughout the area of study. The upper part of the F Uhit consists of interbedded light to medium brown dolomite, medium gray shale, salt, and white dolomitic anhydrite. The lower part of the F Unit consists almost entirely of a white salt. The upper interbedded part still has a high salt and anhydrite content and extends from i to over 5 way down the formation. Occasional beds of dolomite, shale, or anhydrite occur farther down the formation. Sometimes the F Unit is broken down into several parts usually called the F - 1 Salt, F - 2 Salt, etc., but for this study they are grouped and treated simply as the F Unit. The thickness of the F Unit varies from a little under 200 feet to over 400 feet generally becoming thicker away 15 from the Basin margin. The depth to the top of the F Unit increases from about 700 feet below sea level at Marine City Field to over 1200 feet at Ray Field. G Unit The G Formation or G Unit lies immediately over the F Unit throughout the area of study. The G Formation consists of interbedded gray to brown finely crystalline dolomite and gray shale often anhydritic. The thickness of the G Unit varies from 10 to 30 feet usually being about 20 to 25 feet thick. The depth to the top of the G Unit increases from about 700 feet below sea level at Marine City Field at the Basin margin to about 1200 feet at Ray Field. . 15 REEF CROSS SECTIONS The following is an explanation and description of the cross sections for each of the nine reefs studied. The depth to a particular formation or its thickness can be read quite accurately from the cross sections, but if more exact information is desired one can refer to the data sec- tion of this paper where all wells used in this study are listed by reef and permit number and the depth to all for- mations and thickness of formation are recorded. The location of all reef cross sections and the wells used are shown on Figure 1 (Back Pocket). Reference should be made to this map as the cross sections are discussed. It should be noted that the horizontal scale on all sections is the same and the vertical scale is likewise the same on all sections. It should also be noted that the vertical exaggeration is 6.6 times on all cross sections. 17 Marine City The Marine City Field is located at the extreme edge of the Michigan Basin. It is unique among the sections studied in that there were enough deep wells into the Clinton Formation to establish the Clinton top under the Niagaran reef. The dip towards the Michigan Basin center is approximately 50 on the Clinton top. The thickness of the Niagaran reef section is nearly 200 feet but thins to less than 40 feet at the west edge of the reef which is about 1 mile by 2 miles in extent. The A - l Evaporite thins on the reef flank and pinches out before it crosses the reef crest. At its thickest the A - l Evaporite is about 30 feet. The A - 1 Carbonate generally thins over the reef crest but shows an additional drastic thinning over the crest possibly due to solution of the A - 1 Carbonate, perhaps also with collapse causing about a 50 foot depression in the top of the A - 2 Carbonate. The A - 2 Evaporite fills in this depress- ion causing it therefore to actually thicken over the Niagaran reef crest. The A - 2 Carbonate thins slightly and the B Salt thickens slightly over the reef crest, and the G Unit is the highest unit to reflect the underlying reef and does so by thinning slightly and draping. Q I T. j .1 2 l I F I I I 800 I I/e 4/IN I,,...-D I E I I | I I I I I I I I I 0.5 MILE. I I I I" I 23043 allfl‘i‘l 23/?! 22.986 3347:). 2299/ MARINE CITY F I G. 7. 19 Belle River Mills Like Marine City the Belle River Mills Reef is also located at the Michigan Basin edge. However, in this case, it is a far larger reef and does not seem stunted in its growth as was the Marine City Reef. The Niagaran reef is about 300 feet high and about 1 mile wide by about 2.3 miles long. It is one of the largest reefs in the area of study, and this cross section is one of the best because of 9 wells used and the good control off both sides of the reef. As expected the A - l Evaporite pinches out on the begin- ning of the reef flanks. A very curious feature was obser- ved by the writer on this reef cross section and several others which is difficult to explain. If one observes closely he will see that the A - 1 Evaporite thickens sub- stantially at the edge of the reef before it pinches out on the flank. Though both the top of the A - l Evaporite and the top of the underlying Niagaran dip downward as one approaches the reef on the east side, the Niagaran dips more steeply. Thus it would seem that the weight of the reef caused the surrounding rock to be depressed, and that this occurred Just previous to the deposition of the A - l Evaporite which filled this depression. However, as over- lying sediment added to the weight of the reef, a second period of depression seems to have occurred previous to the deposition of the A - 2 Carbonate. All formations from the A - 1 Carbonate through the E Unit show varying amounts 20 of thinning over the reef crest with stratigraphically closer formations showing the most thinning. The draping effects on overlying sediments are clearly shown through the C Unit. Of special interest is the apparent solution and perhaps collapse feature of over 100 feet in the top of the A - 2 Carbonate which is now filled with B Salt. A similar though smaller feature in the tOp of the A - 1 Car- bonate was mentioned in the Marine City Field. I139! ‘7 BELLE RIVER MILLS FIG. 3 0.50 MILE 22 Boyd - Peters - Puttygut The combined sections running approximately southwest to northeast from Boyd through Peters to Puttygut have the best control in the area of study as three reefs plus the areas between the reefs are shown on one continuous cross section. Thus, altogether, data from 24 different wells was used for this section which is nearly eight miles long. The exact location of the reefs, the continuous section, and the wells are shown on the large map in the back pocket. The continuous section clearly shows that the area between reefs consists of very regular formations laid on top of each other. There is only slight variance in the depth to a formation top or in the thickness of formations. Here, as in other sections, off reef edges or between reefs, there is no evi- dence for collapse or solution as is sometimes found on reef crests. Bgyg The Boyd Reef is about 260 feet high and 1.5 miles at its longest by 0.75 miles at its narrowest. Its original height may have been a little higher as explained later. The A - l Evaporite pinches out on the reef flank as ex- pected. Here again, as observed on the Belle River Mills Reef, the A - l Evaporite top and the Niagaran top are depressed as they approach the reef edge. In this case the depression is about 40 feet and is filled by the A - 1 Car- bonate. No later depressions are observed and this one is 23 probably due to the weight of the Niagaran reef and the A - 1 Carbonate over it causing depression during the deposition of the A - l Carbonate. The units from A - l Carbonate through the E Unit reflect the underlying Niagaran reef by draping over it. The A - l Carbonate and the A - 2 Evapo- rite thin as they approach the crest. The A - 2 Carbonate, however, thickens greatly from 135 feet to 370 feet over the crest due to a huge collapse of the Niagaran reef center during deposition of the A - 2 Carbonate. The A - 1 Carbon- ate and the A - 2 Evaporite do not show any thickening but rather thin, hence must have been draped over the crest before collapse. The collapse appears to extend deeply in- to the Niagaran reef itself. A much later and apparently unconnected solution depression in the E Unit causes it to thin while the F Unit above fills this depression thick- ening over the same location as the older collapse. Peters The Peters Reef is about 300 feet high and 1.5 miles long by 0.5 to 0.75 miles wide. There is apparently no de- pression of the surrounding formations by the reef's weight as observed in Belle River Mills and Boyd. The A - l Evapo- rite pinches out part way up the reef flank. All other units from the A - l Carbonate through the F unit reflect the underlying Niagaran reef by thinning and by draping. No solution or collapse appears on this section. 24 Puttygut The Puttygut Reef is about 260 feet high and 1.7 miles long by 0.5 miles wide. The A - 1 Evaporite pinches out part way up the reef flank as expected. The A - 1 Carbon- ate thins slightly above the crest but thins greatly and disappears on the south edge of the reef. The A - 2 Evapo- rite thickens greatly here indicating that the A - 1 Car- bonate was completely eroded or dissolved away before de- position of the A - 2 Evaporite. How much A - l Evaporite, if any, is still left is not known as it is not easily dis- tinguished from the A - 2 Evaporite now filling this de- pression. The A - 2 Evaporite thins over the Niagaran reef crest as expected. The remaining formations through the D Salt also thin over the reef crest. Formations through the F Uhit reflect the underlying Niagaran reef by draping. I I I I I I I I I I I I I fl 4 I we avvws A— I we I A—I EVAP. NIAGI PUTTYGUT III I II III I III .I III. I I I I2 2.1823 n 7 I1 III II 1 I. 3 q. [III-Ill 1 III 1 The L! :iles in e: up the res Reef and t1 the aurroc: thickens s the south ncrt‘n edge nations. ncrth flan dips down "Fae A - 2 a“ ‘he A fills in I All format reflect tr 27 229225. The Lenox Reef is about 300 feet high and 1.4 by 1.0 miles in extent. The A - l Evaporite pinches out part way up the reef flank. Here again, as in the Belle River Mills Reef and the Boyd Reef, the weight of the reef has depressed the surrounding rock. In this case the A - l Evaporite thickens substantially from 20 feet to about 35 feet on the south edge of the reef and to a lesser extent on the north edge. There are no signs of depression in higher for- mations. There appears to be solution and collapse on the north flank of the reef as the top of the A - 2 Evaporite dips down sharply along with the top of the A - 2 Carbonate. The A - 2 Evaporite thins indicating much of it was removed and the A - 2 Carbonate collapsed into the void. The B Salt fills in the depression on the top of the A - 2 Carbonate. All formations from the A 1 Carbonate through the E Unit reflect the underlying Niagaran reef by draping and thinning. I I I I I I I m; I I I I I I I I522” W I I IF I I I I I I I I I I I I I I I’m M I I I I I WI I PL”... W I I I I I I I I I I I I I I I IB I I I I . I I I I I I ' I 4w I X I I I I 'I I I NIAG I I I I ....... I. ....I. Jen... I... .,,.I mIImJ LENOX I 050 MILE J FIG. 6 ' 29 Big, Hand The Big Hand Reef is about 270 feet high and 1.5 miles long by 0.5 to 0.75 miles wide. The A - l Evaporite pinches out very low on the reef flank. The formations from the A - l Carbonate through the E unit reflect the underlying Niagaran reef by draping, and the formations from the A - l Carbonate through the C Uhit thin over the reef crest. There is no evidence of solution or collapse on the section but information is limited by the few wells in the cross section. 11 m N .1 m m > II (p TI— "I: I I I FIG.7 IQFT—I II I I 0.50 MILE INIAGI I I- 1'1??? 1???? 246/5 9. BIG HAND 31 Columbus The Columbus Reef is about 360 feet high (the highest in the area of study) and 2.25 miles long by 1.0 miles wide. The A - l Evaporite pinches out low on the reef flank. For- mations from the A - l Carbonate through the G Unit reflect the underlying Niagaran reef by draping and by thinning with the A - 1 Carbonate and the A - 2 Evaporite thinning the most. There are no signs of collapse or solution though information is restricted by the small number of wells in the cross section. I A-Q CARE I I I A‘Q EVA? I A“ “RB...----°' N 1‘730 L IAG 0.50 MILES J FIGB I I QJ‘HS 9.5319 '1 COLUMBUS JVO‘ 33 331 The Ray Reef is about 290 feet high and 2.5 miles by 0.5 to 1.0 miles in extent. The A - l Evaporite pinches out low on the reef flank. All formations from the A - l Carbonate through the 3 Unit reflect the underlying Nia- garan reef by draping and thinning. There is an indi- cation of the weight of the reef and overlying sediments depressing the underlying rock on the east side of the reef. The tops of the Niagaran, A - l Evaporite, A -l Carbonate, A - 2 Evaporite, and A - 2 Carbonate dip downward towards the reef. The B Unit fills in this depression indicating that depression occurred after the A - 2 Carbonate was de- posited and as the 8 unit was being deposited. No other signs of solution or collapse appear on the cross section of the Ray Reef. I I M I F I I , I I I 221°" I I I I I I Liz. I I I I , I I I I I I I I I. I I m I I’—° I I | I I I I I I I I I l I I I I 9.4!! 77 9.5L?! QSIO I 9 Q3l‘03 9.339.? 9.94239 2.2710 F I 6. CI R A Y I J 0.50 MILE It: 2 ‘. (.7 “i A «I ‘ \. 35 APPARENT TRENDS IN SEDIMENTS OVERLYING THE NIAGARAN REEFS The following collection of graphs is designed to demonstrate some apparent trends or associations between the Niagaran reefs and overlying formations up through the C Unit which is the highest unit showing a consistent rela- tionship throughout the area of study. The graphs are one method of presenting the relationship of formation thickness and depth to location over reef crests, or flanks, and to off-reef areas in a visual, easily understood manner. Though the graphs may possibly be used to obtain an in- dication of whether a well being drilled between known reefs is over an undiscovered Niagaran reef this is not their in- tent. Referring to Figure 10 it can be seen that the top of the C Shale is encountered at a greater depth where no reef is present than it is over a reef. Figure 11 shows that the C Unit is thinner over reefs than it is where no reefs are present, except where the lines are dotted and the trend is temporarily reversed. The fields are spaced equally along the horizontal axis of each graph with the fields closer to the Basin margin on the right and those farther from the margin on the left. The lines of depth and thickness are projected between the fields. Referring to Figure 4 and Figure 5 it can be seen that areas between reefs have only a regional dip and slight variations in thickness and depth 36 of units so that projecting between reefs appears justified. It should be noted that the closer the unit is to the Niagaran the greater is the difference in depth and thick- ness between over reef and off reef locations. 0_ .w _ L 35m. 0a 2.02 (mmiumm< usmxm u .10thde Lulu “4.2! misc! 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F02 (MM‘Uhmlx _.m mv_ L .w<_ Z QOF IHQmQ “five: WM»?! 3 gum u a: mu Q>0h L531?» .633 4 92$ 65 “39.3.36 ‘5‘ m 4 m>m 4 < m m 33m m w \ hum“. [quad \ \ a: \ 4a.. N u mum umo.\ .53 muu\ Goad muum Kmo x\\\\ \\\N 00‘“ \ \ \ mumm mm>0 IUdOd \1 \ 00F. 49 Reef Height and Width Three special graphs of reef height and width versus formation thickness over reef crests were plotted. These are the only associations between reef characteristics and the overlying formations that the writer was able to deter- mine. Though the data does not fit any curve perfectly, it does, in each case, fit the curve well enough to merit spec- ial attention. The plot of reef height versus thickness of the A - 2 Evaporite formation (Figure 22) makes it possible to estimate the height of an underlying reef from the thick- ness of the A - 2 Evaporite formation though some variation must be allowed for since the data does not fit the curve perfectly. The plot of the reef width versus the thickness of the 8 Salt formation (Figures 23 and 24) makes it possible to estimate the minimum reef width measured through the reef crest. This plot is also shown as a full logarithmic plot with a straight line Joining the points. It must be noted that the Marine City and Belle River Mills fields do not fit the curve of reef width. This is probably the result of their position on the extreme edge of the Michigan Basin. The explanation of these associations of formation thickness to reef height and width is not certain. The A - 2 Evaporite formation becoming thinner over high reefs is not too surprising as most formations thin over the reefs. So, it is not surprising that the A - 2 Evaporite thins more over higher reefs than it does over lower reefs. 50 The increase in thickness of the 8 Salt formation over reefs that are wider is not so easily explained. Perhaps the wi- der reefs tended to trap sea water in pools over their crests as the water level in the Michigan Basin became lower. These pools often dried up and then were replenished so that salt was deposited. The smaller reefs would not have been able to do this effectively so did not build up as thick a de- posit of the B Salt formation. Though these are Just pos- sible explanations, the associations of thickness of the B Salt formation to reef width and of the A - 2 Evaporite for- mation to reef height deserve special attention. A plot of reef height versus width was also prepared (Figure 26 in Ap- pendix) and it may be significant in that no apparent rela- tionship was observed. .dfl 0_m can 95 otd Oh.“ FT.w_MmI end On.“ 0..“ o+~ .mmmwm 0*. 0h~ of Luna afln _ (1" _ _ A _ a .06 0? Om 111 00 l 00‘ I O“. hunk umuzxuatk m<>m di< md d.m IFD_?/ mmmm PLDZ_Z_Z unit 0.“ mix 02 M6 _ _ .t.¢.u— _ II, can I! con l1 - - [own [firm lawn 3...: .t mfimmiiid .ui. 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