STRUCTURAL GEOLOGY OF THE NORTHWESTERN ’ PORTION OF THE MICHIGAN BASIN ‘ Thesis for the Degree of M.‘ s. MICHIGAN STATE UNIVERSITY NOBLE F, LEWALLEN II 1983 114:8“; , Y- its“ 4% This is to certify that the thesis entitled STRUCTURAL GEOLOGY OF THE NORTHWESTERN PORTION OF THE MICHIGAN BASIN presented by Noble F. Lewallen II has been accepted towards fulfillment of the requirements for Masters deg”? in Geology c? Major professor Date ' / 0-7639 MSU is an Affirmative Action/Equal Opportunity Institution IVIESI.J RETURNING MATERIALS: Place in book drop to LIBRARJES remove this checkout from w your record. FINES WIII be charged if book is returned after the date stamped below. ABSTRACT STRUCTURAL GEOLOGY OF THE NORTHWESTERN PORTION OF THE MICHIGAN BASIN BY Noble F. Lewallen II Previous studies have proposed that the Michigan Basin subsided on basement faults, and that these faults controlled structure in the overlying sediments. The findings of this study support such a theory, based on structural mapping of various horizons in northwestern lower Michigan. These maps reveal that the major struc- tural trends of this area have a northwest-southeast orientation and that the structural features become more pronounced with depth. Such observations are interpreted to result from northwest-southeast trending basement faults which parallel the Mid-Michigan Gravity Anomaly - a proposed rift zone. Cross-sections of two fields in Missuakee County reveal little or no structural effect of salt solution or flowage associated with these structures. However, salt movement may be more significant nearer the deposi- tional limits of the various salt units. Future hydrocarbon exploration in northwestern Michigan will be directed toward fault related anticlines and associated porosity zones, or Niagaran pinnacle reefs. STRUCTURAL GEOLOGY OF THE NORTHWESTERN PORTION OF THE MICHIGAN BASIN By Noble F. Lewallen II A THESIS Submitted to Michigan State University in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE Department of Geology 1983 AC KNOWLE DGMENTS I am indebted to Dr. James H. Fisher, Committee Chairman, for accepting me as his last graduate student before retiring, and for sharing part of his immeasurable knowledge of geology with me. Thanks also to the other committee members: Dr. James Trow and Dr. William Cambray, for their valuable comments and time: and to Dr. John Wilband who served as substitute during Dr. Cambray's sabatical. Scott Cranswick and Paul McMahon, Tenneco Oil Company, read the manuscript and provided valuable sugges- tions. Rick Clark, Robert Gomez and Lynette Fudge, helped draft the final figures and plates. The Petroleum Geology staff of the State Geological Survey, Michigan Department of Natural Resources, was particularly helpful during my research at the survey. Special thanks to my parents Noble and Jane Lewallen, who have always stressed the importance of education and who provided the encouragement and financial support necessary to complete my educational endeavors; and to my wife, Suzanne, who provided moral support throughout the thesis project, helped collect well data, typed the rough draft and aided in editing the final draft. ii .‘3 i . L I “('9' If " “I ‘1‘ 5‘1 45 I}! I R; a J I TABLE OF CONTENTS LIST OF FIGURESOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO v LIST OF PIA'I‘ESOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO Vi KEY To SYMBOLSOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO Vii INTRODUCTION...................................... 1 Purpose of Study.............................. 2 Area of Study................................. 4 Method of Study............................... 4 Error Analysis................................ 10 Previous Work................................. 12 STRATIGRAPHYO O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O 22 STRUCTURE. O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O 32 DATA ANALYS I S O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O 4 4 Regional Structure Maps-General............... 44 Sunbury Structure Map (Plate l)............... 45 Traverse Limestone Structure Map (Plate 2).... 48 Dundee Structure Map (Plate 3)................ 49 A-2 Carbonate Structure Map (Plate 4)......... 49 Trenton Structure Map (Plate 5)............... 50 Detailed Structure Maps-General............... 51 Missaukee County-Dundee Structure Map (Plate 6)OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO 51 Falmouth Field-Trenton Structure Map (Figure 11)OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO 52 Oceana County-Dundee Structure Map (Plate 7).. 54 Oceana County-A-Z Carbonate Structure Map (Plate 8)OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO 55 Cross-Section Analysis-General................ 57 East-West Falmouth Field Cross-Section: Dundee to Niagaran (Plate 9)................ 57 North-South Falmouth Field Cross-Section: Dundee to Niagaran (Plate 10)............... 58 East-West Falmouth Field Cross-Section: Dundee to Base of Detroit River Salts (Plate 11)OOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO 58 iii iv North-South Falmouth Field Cross-Section: Dundee to Base of Detroit River Salts (Plate 12)................................... East-West Enterprise Field Cross-Section: Dundee to Base of Detroit River Salts (Plate 13)................................... North-South Enterprise Field Cross-Section: Dundee to Base of Detroit River Salts (Plate 14)................................... CONCLUSIONS AND PETROLEUM POTENTIAL. . . . . . . . . . . . . . . . BIBLIOGRAPHYOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO 60 61 62 62 74 LIST OF FI GURES Figure Page 1O StUGY AreaO O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O O 5 2. Stratigraphic Succession in Michigan........... 7 3. Salt Cored Anticline, Overisel Field, Allegan CountYOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO 19 4. Rectilinear Pattern of Faulting in the Canadian ShieldOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO 20 S. Michigan Basin and Surrounding Structural FeatureSOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO 33 6. Structural Trends of the Dundee Formation...... 35 7. Bouguer Gravity Anomaly Map.................... 36 8. Schematic Diagram of Mid-Michigan Gravity High. 37 9. Total Magnetic Intensity Anomaly Map........... 38 10. Michigan Oil and Gas Fields.................... 46 ll. Trenton Formation Structure Map of Falmouth Field Area, Missaukee County................... 53 12. Extent of Lower Salina and Detroit River Salt UnitSOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOOO 67 13. Asymmetrical Anticline......................... 7o Plate Plate Plate Plate Plate Plate Plate Plate Plate Plate Plate Plate Plate Plate on \I 0‘ U1 vb w N H O LIST OF PLATES Sunbury Structure Map Traverse Limestone Structure Map Dundee Structure Map A-2 Carbonate Structure Map Trenton Structure Map Missaukee County - Dundee Structure Map Oceana County - Dundee Structure Map Oceana County - A-2 Carbonate Structure Map East-West Falmouth Field Cross-Section: Dundee to Niagaran North-South Falmouth Field Cross-Section: Dundee to Niagaran East-West Falmouth Field Cross-Section: Dundee to Base of Detroit River Salts North-South Falmouth Field Cross-Section: Dundee to Base of Detroit River Salts East-West Enterprise Field Cross-Section: Dundee to Base of Detroit River Salts North-South Enterprise Field Cross-Section: Dundee to Base of Detroit River Salts vi KEY TO SYMBOLS Jj LIMESTONE r ] DOLOMITE SHALE ANHYDRITE SALT SANDSTONE VARIATIONS : I—-l Rgtfi'fmgous ME -u- cuearv I” STONE ANHYDRITIC oomurnc I SHALE vii INTRODUCTION The Michigan Basin is an intracratonic basin whose mode of origin is poorly understood. Similarly, the origin of the Basin's folded and faulted structures is unclear. An important key to solving such problems is a detailed study of the structures within the Basin. A number of workers have proposed that the Michigan Basin subsided on faults in the Precambrian basement and that these faults have controlled structure in the overly- ing Phanerozoic sediments throughout the evolution of the Basin. It follows then that by studying the location and orientation of the structures within the Basin sedi- ments, this model can be tested. Additionally, because much of Michigan's petroleum is produced from structural traps, a study of this nature can shed valuable insight on the potential of future exploration efforts. The Albion-Scipio oilfield, Michigan's largest, produces from a fault-controlled structural trap, and some workers have suggested that similar traps may be the most likely prospect for future large fields in Michigan. Mescher (1980) and Fisher, J.A. (1981) studied the structural geology of southeastern Michigan which contains the best deep well control in the state, and related the structures to complex faulting in the Precambrian basement. They, and many others, also related the Mid-Michigan Gravity Anomaly to a rifted graben-like structure that probably filled with Keweenawan basalts. These studies however, do not address the possible factors that could modify structure, such as salt solution and flowage. The area for this study was chosen in order to further test the work of Mescher and Fisher. It is well suited for such a study because of its recent deep drilling activity, the presence of the Mid-Michigan Gravity Anomaly, and the presence of thick Devonian salts which may possibly alter structures if they have flowed. Purpose of Study Since the drilling of the Dart-Edwards 7-36 discovery well in Reeder Township of Missaukee County in 1980 (tested 12 MMCFPD of gas) numerous deep tests have been drilled in northwestern lower Michigan. This deep well control makes this area the best suited for deep structural studies outside the area studied by Fisher in southeastern Michigan. As in southeastern Michigan, the Mid-Michigan Gravity Anomaly extends through northwestern Michigan. Thus, structural mapping of northwestern Michigan will determine the location and orientation of anticlines, synclines, and faults, as well as the relationship of these structures to the Mid-Michigan Gravity Anomaly. Salt solution and flowage are important modifiers of structure in Allegan County of southwestern Michigan, and this has led to speculation that similar modification of structure may occur in other areas of the basin. Further, salt solution may indicate the presence of faults or fractures in the area and therefore be an important exploration tool in locating dolomitized reservoirs such as the Northville oilfield which produces from the dolomitized Trenton Limestone and exhibits extensive salt solution and thinning. Northwestern Michigan might be particularly prone to salt modified structure due to the presence of thick Devonian salts which are not present in the southern half of the basin. Good control on both Devonian and Silurian salts is possible in northwestern Michigan because of the hundreds of wells drilled to the Middle Silurian Niagara Formation along the northern Niagara reef trend. This good well control, the presence of the Mid- Michigan Gravity Anomaly, and Devonian salts, make the northwestern portion of the Michigan Basin a good location to study structures within the basin and their possible modification. Knowledge of the cause, location and trends of structural features and how these features have changed through time is of great economic value, for such knowledge can guide future oil and gas exploration efforts. Area of Study The area selected for this study is the northwestern portion of the lower peninsula of Michigan in an area bounded by the eastern boundary of Emmet, Charlevoix, Antrim, Kalkaska, Missaukee, Osceola, and Mecosta Counties, and by the southern boundary of Mecosta, Newaygo (less the eight southernmost townships), and Oceana Counties (Figure 1). Method of Study Because a thick mantle of glacial drift covers virtually the entire basin, data collection for this study was limited to information obtained from oil and gas exploration wells. Approximately 7000 wells lie within the study area. Data for wells were examined at the State Geological Survey in Lansing, Michigan. Selection of wells for use in this study was made by choosing only dry holes wherever possible. This was done in order to eliminate the localized structural effects that pinnacle reefs have on the overlying forma- tions. Also, wells with mechanical logs were preferred to wells without such logs because of the better an OT. CL". an I'll Figure l . Study Area reliability of correlations between these wells. Where no mechanical logs were available, driller's logs were used. Because of the large study area, and the scale used for regional mapping (one inch equals six miles), only wells in every other section were used in order to avoid cluttering. When more than one well was present in the desired section, the deepest, most recently drilled well was chosen. From these data, formation tops were identi- fied and used to prepare a series of structure contour maps. Five regional structure maps were made in order to delineate the structural configuration at different levels within the stratigraphic column. (Figure 2) The selected formations were the Sunbury Formation (Early Mississippian), Traverse Limestone (upper Middle Devonian), Dundee Formation (Middle Devonian), A-2 Carbonate (Middle Silurian), and the Trenton Formation (Middle Ordovician). Unfortunately, data for formations deeper than the Trenton were too sparse to be of use in the study area. Correla- tions of these tops were based on the stratigraphic cross-sections prepared by Lilienthal (1978). All five of these formations produce easily recognizable patterns on geophysical well logs. Additionally, the mwm STRATIGRAPHIC SUCCESSION IN MICHIGAN mm m WIS 51““ "1 WWW § .1: i g 5‘1 ‘lQ—---—I -lal—h—O— mm l d -~d~—du~ ‘ dfin—d_u—Ih N" u- “ u—n—nd a... u...— uha—a = —. —|- ~- - .—_—.O- ‘h -1- I-—‘ “I— -—-- u- hul- ——-—-—_-u- —.u In. _-' |—— —o--——— n.——___-u. — l_-—.._-u. -hM————_-Ib l—u “‘- nun-uni. —O|-hfl _—fl-——-Ih —- “-0- ‘-———-u- . —¢- un— —- __.|n- ”h __——— tidb- .— ~n~ __.-~—__- n—dn (~- .— “~_—~ I-In— “- Mb——fl— _—-lb tu-J ~— _— .—|—-—- I-—‘“" la. Figure 2. Stratigraptfic Necession in Michigan Traverse, Dundee, A-2 Carbonate, and Trenton Formations produce oil and gas, making them economically interesting. Four detailed (one inch equals one mile) structure maps were constructed to show the structure in Missaukee and Oceana Counties, which have the best well control in the study area. Additionally, Missaukee County contains the Falmouth and Enterprise fields on which stratigraphic cross-sections were constructed. In Missaukee County, structure was mapped on the Dundee, and on the Trenton in the Falmouth field area. In Oceana County, Dundee and A- 2 Carbonate Formation structure maps were prepared. Six stratigraphic cross-sections were constructed in order to determine whether or not salt thicknesses change significantly near structure. The Falmouth and Enterprise fields of Missaukee County were selected for use in this study. Two cross-sections approximately perpendicular to one another, were constructed for both fields. Additionally, two detailed cross-sections of the Detroit River Salts unit were prepared for the Falmouth field. The Falmouth field was chosen because recent deep drilling in and near the field penetrated not only the Devonian Detroit River Salts, but also the entire Salina salt sequence. The Enterprise field was chosen because it was the only other non-reef field in the study area with sufficient well control to allow construction of cross-sections of the Detroit River Salts. Niagaran reef fields were not selected for cross-sections because of their relatively small size and the fact that salt thicknesses near such reefs vary depositionally rather than post-depositionally. In summary, it is proposed that most structures within the Michigan Basin were developed, and controlled by an ancient pattern of faults in the Precambrian base- ment rock. Therefore, the location and orientation of these structures in the overlying sediments should exhibit characteristics that reflect such basement influence. The major characteristic of such influence would be an increasingly subdued diSplacement of sediments upward in the stratigraphic column. Thus, the Trenton Formation should show the greatest relief, followed by the A-2, Dundee, Traverse, and Sunbury Formations, respectively. Basement structure might also be evident in the overlying sediments along the Mid-Michigan Gravity Anomaly, thus revealing important clues as to the nature of this feature. It is further proposed that movement of salt can significantly modify structure. Determination of the extent to which salt movement has affected known oil producing structures can provide important clues to locating other salt modified structures. Therefore, 10 stratigraphic cross-sections were constructed for the known structures associated with the Falmouth and Enter- prize fields of Missaukee County. Anomalous thickening or thinning of salt horizons near these fields would indicate a need for further study of the effects of salt solution and flowage. Error Analysis Data collection procedures for this study were subject to three potential sources of error. First, and most importantly, inaccurate drillers' logs can introduce significant errors in formation top elevations. This is particularly true in the older wells which were drilled early in the exploration history of Michigan before the stratigraphy of the basin was well understood. Where data from drillers' logs were obviously in error, the data were omitted. Another problem with drillers' logs was the lumping of several formations under one heading. Commonly, the Sunbury Shale was not differentiated from the overlying Coldwater Shale, nor the Traverse Limestone separated from the overlying shales of the Traverse Formation. Therefore, some wells in this study were not useful for the shallow formations such as the Sunbury and Traverse Limestone, yet yielded good data for deeper formations such as the Dundee. 11 Secondly, correlation of formation tops in poorly recorded or poorly printed geophysical logs can introduce errors. Formation tops can also be inaccurate where the borehole has been deviated from vertical (which exaggertes the thickness of formations). To minimize this error, only non-directional boreholes were utilized in this study. However, most wells are not perfectly vertical and thus introduce minor errors. Finally, contouring of well data is an individual interpretation, and is therefore subject to error. In areas of sparse well control, interpretations by different workers can vary significantly. In areas of good control, interpretations vary less and accuracy is improved. In this study, the best control, and therefore the greatest accuracy, is obtained in those portions of Kalkaska, Grand Traverse, and Manistee Counties where the Niagaran reef trend has been extensively drilled. Good control is also present in the counties south of the reef trend. Control is sparse north of the reef trend in Leelanau, Antrim, Charlevoix, and Emmet Counties, and is absent for the Sunbury and Traverse Limestone Formations Where they have been eroded. Control on the Trenton FOr'mation is sparse throughout the study area. 12 Previous Work The Michigan Basin area has been studied geologically for more than one hundred and forty years. Consequently, much has been written about the geology of the region. Part of this information has been published and is well known throughout the geological community. However, much of the geological knowledge of the Michigan Basin is contained in the files of the many companies that have explored here for a variety of resources. Such informa- tion is unavailible to the public and is obviously not reported here. Likewise, much of the published work on the region is out of date. Therefore, only the most important published works, relevant to this study, are included here. In 1923, Robinson suggested that the structural elements in the Michigan Basin were folds that resulted from vertically acting forces rather than compressional forces. He described five fold types that could be formed by such vertical forces as subsidence and downwarping. Pirtle (1932) stated that the major structural elements did not result simply from subsidence, but rather were controlled by trends of folding or lines of structural weakness in the basement rocks. He proposed that a system of Precambrian mountains extended southeastward 13 from central Wisconsin to northwestern Indiana, the remains of which are the Wisconsin and Kankakee arches. Paralleling these mountains, Pirtle suggested a geosyn- cline which later became the Michigan Basin. Such a geosyncline would have an axis and structures parallel to the mountain ranges. Thus, Pirtle explained the dominant northwest-southeast trending structural elements of the basin by relating them to basement features that moved throughout time as horizontal forces were applied to affect the overlying sediments. Despite the time of Newcombe's (1933) work, "Oil and Gas Fields of Michigan,“ his study provides an excellent account of the geology of the Michigan Basin. Many of the predictions and hypotheses put forth by Newcombe have been proven correct by the data provided by about thirty- five thousand wells drilled since 1933. One such hypoth- esis is that the Lake Superior Basin is a rift valley that connects with the Michigan Basin. This idea was later supported by geophysical surveys conducted by Hinze in 1963. Like Pirtle, Newcombe believed that the basin originated during the Precambrian and that the structure was related to zones of weakness in the basement rocks. In 1945, Landes, Ehlers, and Stanley wrote the first Couuprehensive report exclusively about the geology of the 'Nbrthern part of the southern peninsula of Michigan. 14 Of particular interest to this study is their work concerning the relationship between salt solution and the Mackinac Breccia. They preposed that the Mackinac Breccia resulted from the leaching of the Salina salts near the edge of the basin and the subsequent collapse of the overlying sediments. They further proposed that the Detroit River Evaporites are the reprecipitated Salina salts which were transported in solution to their present position south (i.e. down-dip) from the Mackinac area. Lockett (1947) related the formation of the basin and its structures to the positive areas around it. Like Pirtle, Lockett suggested that the Wisconsin and Kankakee arches lie along an ancient mountain range. As these mountains eroded, the sediment load collected in the surrounding low areas, causing the basin to sink. Lockett argued that the northwest-southeast structural trends of the basin resulted when fractures occurred parallel to the positive areas to the west. Both Pirtle and Lockett overlooked the lack of geosynclinal-type sediments in the Michigan Basin. If the erosion of the Wisconsin range was associated with the initiation of subsidence, a coarse conglomerate should be present in the basin. No such conglomerate is known if! Michigan. A further problem with the sediment load- subsidence theory is that sediment loading by itself 15 appears to be insufficient to cause the entire subsidence of the basin based on geophysical studies (Sleep, 1976 and Sleep, Nunn, and Chou, 1980). Cohee (1944-1948) prepared a series of maps and cross-sections of the Michigan Basin for the 0.5. Geolo- gical Survey. Cohee and Landes (1958) summarized this work and acknowledged the northwest-southeast structural trends of the basin. They also compared different struc- tural horizons over the Howell anticline in southeastern Michigan and showed that the axis of the structure had migrated one and one-half miles west from Niagaran time to Dundee time. Migration of structural axis through time has obvious importance to the exploration for oil and gas. If a migration pattern could be found in the Michigan Basin, it would greatly aid the extrapolation of shallow fields to deeper horizons. Such a migration pattern is closely examined in this study. Cohee and Landes proposed a late Silurian origin of the basin with the major period of folding occurring during Late Mississippian time. Based on gravity and magnetic maps of the southern peninsula of Michigan, Hinze (1963) examined the regional structure of the Michigan Basin Precambrian basement. The most pronounced feature of the study was the anomalous 16 gravity and magnetic high transecting the Basin from northwest to southeast. Hinze proposed a mass of mafic rock in the basement as the probable cause of such an anomaly. This suggested a link between the anomaly and the exposed mafic rocks of the Keweenawan rift zone in the Lake Superior Basin. Additionally, the anomaly is strikingly similar to the Mid-Continent gravity high in form and magnitude and thus Hinze suggested a relation- ship to this feature. Sanford (1965) briefly reported the geology of the Salina salt beds of southwestern Ontario. He described the structures associated with leaching of salts and the subsequent collapse of the overlying sediments or the gradual filling during the deposition of the overlying Bass Islands Formation. Apparently, leaching first occurred near the margin of the basin soon after salt deposition and moved basinward through post-Upper Devonian time. Leaching was most intense above structural irregularities such as patch reefs, or along joints and faults. Sanford reported that most of Ontario's Devonian oil fields produce from dome structures associated with salt leaching and collapse. Ells (1967) summarized much of what was known about Michigan's Silurian oil and gas fields. Although written prior to the discovery of the northern Silurian reef 17 trend, the paper provides valuable information on the geology of the rocks of the Silurian group based on data from the southern reef trend. Of particular interest to this study is the analysis of reef associated structures and salt structures. 3113 stated that structural closure is formed in nearly all Salina units overlying Niagaran pinnacle reefs, with some reef fields showing closure in formations as young as Mississippian age. Reef height, and the thickness of overlying sediments apparently control the degree of closure in formations above reef structures. Thus, the most structural closure is encountered in basin margin reef structures where the overlying sediments are thinner: closure decreases basinward as the overlying sediment thickness increases. Ells attributed anomalous salt thicknesses and the occurrence of small salt-cored structures to either the result of solution and collapse, or by flowage of salt. The Mackinac Breccia apparently resulted from salt solu- tion and collapse of the overlying sediments, while the salt-cored anticlines of Allegan County appear to be due to salt flowage and subsidence rather than solution and collapse. Both the Mackinac Breccia and the Allegan County salt-cored anticlines occur along the depositional edge of the salt. This is true of the other known 18 salt-related structures in the Michigan Basin. Figure 3 is a cross-section of the salt-cored 'pseudoanticline" of the Overisel Field in Allegan County showing the anomalous thickness of A-l Evaporite salt causing a doming of the overlying sediments. In 1969, Ells summarized much of the previous structural work done in the Michigan Basin. He discussed the Basin's origin and framework based on his analysis of the Howell anticline of southeastern Michigan. He suggested that structure in this area is controlled by three major fault blocks which have moved relative to one another through time, and that other similar fault blocks may control structure throughout the basin. Fisher, J.H. (1969) used isopach maps to show that the Michigan Basin began during Cambrian time and evolved into its present size and shape during the Ordovician. The most rapid subsidence occurred during the Salina period. He suggested that the structure of the basin is controlled by a pattern of faulting in the basement rock that changes direction regionally. Such a pattern is evident in the exposed Canadian Shield north of the basin (Figure 4). Examination of Figure 4 reveals that the fault pattern of the Shield is predominantly northwest-southeast with a lesser northeast-southwest trend. NO SCALE 19 II. II Ii, [I'll/I III I “I I” I" I IIIgIIIIIII /.II,/,5,III’,’I, / I I", III I I .' W I I ‘ I". 'fil‘ I w IIIIIIIIEIIOIIHIII I IN "I I II I I ,I . -.‘A '2.- .4 ~ . .~- ~ ‘ ~ ‘ ~ A \“ ‘ \ ~ \ “\I I‘ III .I . \ I IL. I Ells, 1967.) Figure 3. Salt Cored Anticline, Overisel Field, Allegan County. (From 20 LAKE Qv \ ¢Q~ (v 'f \v enon.‘ manual! 84" HIOAO 88° 4P “”5”” (Adopted frcm Illectonic Map of Canada, 1969.) Figure 4. Rectilinear Pattern of Faulting in the Canadian Shield. 21 Hinze and Merritt (1969) and Hinze, Kellogg, and O'Hara (1975) related the Mid-Michigan Gravity Anomaly to a rift zone. Such a rift zone would contain dense mafic rocks (basalt) that would give a positive gravity anomaly. Haxby, Turcotte, and Bird (1976) suggested the subsidence of the basin was related to a heating event caused by diapiric penetration of the lithosphere by hot asthenospheric mantle rock. This heating transformed the rocks of the lower crust, metastable gabbroic rocks, to a more dense rock, eclogite. The basin then subsided under the load of the eclogite. Lilienthal (1978) prepared the first extensive series of cross-sections of the Michigan Basin based on geOphysi- cal well logs. Picks for formation tops in this study were the same as those used by Lilienthal. Sleep, Nunn, and Chou (1980) reviewed many of the theories of intracratonic basin formation, particularly ' as they related to the Michigan Basin. They examined such mechanisms as sediment loading, thermal contraction, phase changes, crustal stretching and loading, and sublithospheric processes. They favored a thermal contraction model to explain intracratonic basin formation, not because of strong positive evidence, but by analogy to subsidence along Atlantic margins and mid-oceanic ridges. 22 Mesher (1980) studied the structural evolution of southeastern Michigan. Using a series of structure contour and isopach maps, he concluded that an irregular, highly faulted basement played a major role in forming the structures visible in the overlying sediments. Fisher, J.A. (1981) also studied southeastern Michigan with structure contour and isopach maps. She showed that the faults and folds of this area parallel the Mid-Michigan Gravity Anomaly and are probably controlled by a combina- tion of vertical movements of basement fault blocks and a horizontal shearing force derived from outside the basin. She concluded that structure in the Michigan Basin as a whole is controlled by a rectilinear pattern of faults and fractures in the Precambrian basement. Such a theory is evaluated in this study. STRATIGRAPHY The Michigan Basin contains sediments with an estimated maximum thickness of 15,000 feet. All periods from Cambrian to Pennsylvanian are represented. Jurassic red beds are present in the central portion of the basin, however Permian and Triassic rocks are not known to exist within the basin (Figure 2). The rocks are predominantly carbonates, with lesser amounts of shale, evaporites, and sandstones. The entire Southern Peninsula is covered 23 with glacial drift which averages 200-300 feet thick, however, it reaches over 1000 feet in some areas. What follows is a brief description of those sedimentary units which were mapped or contained within the cross-sections of this study. The Trenton Formation of Middle Ordovician age is the oldest formation examined in this study. It consists of light brown to brown and grey, fossiliferous, fine to medium crystalline limestone (Cohee, 1945) (Fisher, J.H. et a1, 1969). It contains thin beds of carbonaceous shale which increase in number near the base of the formation in the northern part of the basin. Dolomites occur locally and seem to be confined to the axes of folds and faults. The largest oil field in Michigan, the Albion-Scipio field, produces from a dolomitized zone in the Trenton. The formation ranges from 200-475 feet in thickness. The Middle Silurian age Niagaran group is the oldest formation shown on the cross-section of the Falmouth field. The Niagaran consists of carbonate beds in the upper portion and carbonates, argillaceous carbonates, shales and chart in the lower section (Lilienthal, 1978). Niagaran rocks consist of thick barrier reef and pinnacle reef buildups along the margin of the basin. Basinward, the Niagaran group thins considerably. The pinnacle reefs 24 have been a major exploration target for oil and gas in Michigan. This intense exploration effort is reflected in the dense well control along the reef trend seen in the structure contour maps. The A-l Evaporite overlies the Niagaran group and is the lower-most formation of the Salina group of Middle Silurian age. In the center of the basin the A-l reaches a thickness of 475 feet and is predominantly salt, which grades into anhydrite and pinches out near the reef complex along the margin. It is generally absent over the crests of pinnacle reefs. Its absence over the reef appears to be due to lack of deposition rather than a post-depositional alteration and is therefore not examined here. Overlying the A-l Evaporite is the A-l Carbonate. It is typically a dark colored dolomite in the margin areas and a limestone in the basin center. The unit sometimes contains thin beds of anhydrite near the shelf margins and adjacent to pinnacle reefs. These so-called "rabbit ears” are sometimes indicative of a nearby reef. The A-l Carbonate is thickest near the basin margins where it reaches 160 feet, and is thinnest in the basin center where it is about 60 feet thick. Oil and gas are produced from a few A-l Carbonate fields in Michigan. Some are associated with Niagaran pinnacle reefs, others are 25 associated with folds and faults where the unit is dolomite or where A-l Salt flowage has resulted in doming of the unit. The overlying A-2 Evaporite is the cap rock for such fields. The A-2 Evaporite is very similar to the A-l Evapor- ite. It reaches a thickness of 475 feet in the basin center and consists almost entirely of pure salt. Salt changes to anhydrite near the basin margins and pinches out. The A-2 Evaporite often thins drastically and changes to anhydrite over pinnacle reefs. The A-2 Carbonate consists of grey to brown lime- stones and dolomites. Where the unit overlies the reef complex, it is usually dolomite. Near the center of the basin, the A-2 Carbonate reaches a maximum thickness of 150 feet, and may contain some poorly developed shale and anhydrite beds. The unit generally thins to 50-75 feet near the reef complex, except in localized areas, where it may thicken to as much as 275 feet. Oil and gas have been produced from the A-2 Carbonate, especially in southwestern Michigan where the unit is dolomite. The B-Unit overlies the A-2 Carbonate and is of late Middle Silurian age. It consists predominantly of salt, however in the upper portion of the unit, interbedded shales, anhydrite, and dolomite also occur. The unit is 475 feet thick in the center of the basin, but thins to 26 50 feet along the basin margins as the lower salt pinches out. In southeastern Michigan, the distribution of the B-Unit is irregular, probably as a result of solution (Fisher, J.A. 1981). The B-Unit is overlain by the early Late Silurian age C-Unit. The C-Unit is a greenish-grey dolomitic shale noted for its variation in thickness (50-200 feet) and extremely widespread nature. Irregularities in its thickness have been related to the solution of the underlying B-Unit by Fisher,J.A. (1981). In contrast to the underlying C-Unit, the D-Evaporite has the smallest areal extent of all the evaporite units of the basin. It is found only in the basin interior and averages about 40 feet in thickness (Lilienthal, 1978). It consists of salt and is usually split by a thin dolomite bed. The D-Evaporite is overlain by the E-Unit which is composed of a series of grey, greenish-grey and red shales interbedded with thin dolomites (Lilienthal, 1978). In the western portion of the basin the unit contains a porous dolomite near its base which has produced some oil. The E-Unit has a fairly constant thickness of 90- 120 feet. The F-Evaporite overlies the E-Unit and shows a variation in thickness of 0-970 feet within the basin. 27 It consists of a succession of salt, thin anhydritic shale, dolomitic shale and dolomite beds. The shales are generally grey, reddish or greenish-grey, while the dolomites are grey or brown. Thinning is due mostly to depositional thinning of the salt beds, however it has been completely removed by erosion in parts of the basin (Ells, 1967). The G-Unit is the uppermost formation of the Salina Group. It consists of a grey dolomitic shale in the basin center which grades into a thinner dolomite at the edges of the basin. Overlying the Salina Group is the Bass Islands Group of late Late Silurian age. The Bass Islands Group is composed of the Raisin River and Put-In-Bay Dolomites, but these formations are rarely separated in the subsurface. The rocks consist of typically dense buff dolomites which are sometimes oolitic in the upper portion. The lower portion is more argil- laceous, and in the basin interior contains thin anhydrites and salt beds (Lilienthal, 1978). In the northern part of the basin, the Bass Islands rocks are 350 feet thick and thicken to 500 feet in the center of the basin. The Silurian and Devonian rocks of the Michigan Basin are separated by an unconformity in most areas of Michigan. The Bois Blanc Formation is the basal formation 28 of the Devonian. It is a cherty carbonate which is inter- bedded and gradational with the Sylvania sandstones or the Amherstburg, where the Sylvania is absent (Lilienthal, 1978). The central basin contains 800 feet of Bois Blanc which thins toward the margins of the basin. To the northwest, it thins to 300 feet in Leelenau County, and to the southeast it disappears completely in Monroe County. The Bois Blanc forms much of the well-known Mackinac Breccia which has been discussed previously (Landes et al, 1945). Rocks of the Detroit River Group overlie the Bois Blanc Formation. The group is composed of the Lucas, Amherstburg, and Sylvania Formations. The Sylvania Sandstone is the basal formation of the Detroit River Group and is composed of well-rounded and sorted, fine to medium grained sandstone, with lesser amounts of silt, chert, and carbonate (Landes, 1951; Lilienthal, 1978). Not found everywhere throughout the basin, the Sylvania is located in northwestern, central and southeastern Michigan (Landes, 1951). The Sylvania reaches a maximum thickness of 300 feet in the central basin, and pinches out in all directions. The Amherstburg Formation lies stratigraphically above the Sylvania Formation and is present everywhere in the Southern Peninsula except in the southeast and 29 southwest corners of the state, where it has been eroded. It consists of mostly limestone in the north and east, but is nearly all dolomite in the western and southern portions of the basin. The Amherstburg has been termed ”The Black Lime" because of its very dark brown to black color. The thin Filer sandstone is contained within the Amherstburg in western Michigan. The Amherstburg Formation produces hydrocarbons in Michigan and is the productive formation of the Enterprise field which is examined in this study. The Lucas Formation comprises the uppermost rocks of the Detroit River Group. The formation consists of beds of dolomite, anhydrite, salt, limestone, and sandstone. It varies in thickness from 20 feet in southwestern Michigan to over 1000 feet in the central basin area. In the central basin area, much of the section is composed of salt and anhydrite, making the Lucas a candidate for possible salt solution or flowage. Such a possibility is examined in this study. Salt in the Lucas is usually confined to the upper portion of the evaporite sequence in the central basin area, where it reaches 400 feet in thickness. Anhydrite, like salt, is more prominent in the central basin area, but it extends further marginward than does the salt. The greatest abundance of anhydrite is found in the lower 30 half of the evaporite sequence, particularly in the zone termed “the massive anhydrite“. Lilienthal (1978) made no attempt to correlate the individual salt beds of the Lucas. In this study, the individual salt beds were correlated in the cross-sections of Falmouth and Enter- prise fields in order to analyze whether thickening or thinning of salt beds occurs across these fields. A group of argillaceous dolomite beds separated by anhydrites is present below the Detroit River Salts in northern and central Michigan and is termed DR-2. The unit is correlated together with the ”massive anhydrite“ as the unnamed unit directly overlying the Richfield in the cross-sections of this study. The Richfield member of the Lucas Formation is a dolomite which contains several porosity zones which produce hydrocarbons. These porosity zones seem to be best developed in northern Michigan (Lilienthal, 1978). In central and western Michigan, the Richfield contains beds of sandstone which are sometimes called the Freer Sandstone. Besides the Richfield member of the Lucas Formation, several other zones produce oil and/or gas, including the “sour zone” and the ”Reed City” zone (if the Reed City zone is placed in the Detroit River Group). Additionally, the Lucas has produced significant quantities of salt 31 and brine, making this formation a valuable economic resource. The Dundee Limestone is the most prolific producer of oil and gas in the Michigan Basin. This Middle Devonian formation consists predominately of brownish-grey, fine to coarsely crystalline limestone. Dolomite is present with limestone in the central basin area, and the Dundee is entirely dolomite in the west and south (Lilienthal, 1978). The formation varies considerably in thickness, from less than 40 to more than 475 feet. It is present everywhere except in the extreme southwestern portion of the state. For this study, the Reed City zone is included in the Dundee rather than the Detroit River group. The Dundee is overlain by rocks of the Traverse group which consists of the Traverse Formation, Traverse Limestone, and Bell Shale. The Traverse Limestone is predominantly shale in eastern Michigan, but grades progressively to nearly a pure limestone or dolomite in western Michigan. Small reefs have been found in Alpena County quarries and others probably exist in the subsurface in other parts of the state (Lilienthal, 1978). In fact, some Traverse Limestone oil fields seem to produce from small bioherms. Most Traverse fields are located in the southwest quandrant of the state where they have produced a significant portion of Michigan's oil and gas. 32 The youngest formation examined in this study is the Early Mississippian Sunbury Shale. The Sunbury is a brown to dark grey or black pyritic shale. It ranges in thick- ness from 0 feet in western Michigan, where it pinches out or grades into the Ellsworth Shale, to 120 to 140 feet in the eastern portions of the state (Lilienthal, 1978). The Sunbury is locally thick in the southern portion of Newaygo County (Newcombe, 1933). STRUCTURE The Michigan Basin has been variously classified in the past, but is probably best described as a shallow, intracratonic basin. It is similar in shape and magnitude to the Illinois and Williston Basins; it is nearly circular, encompasses approximately 122,000 square miles, and contains a maximum thickness of about 15,000 feet of gently dipping strata. Figure 5 shows the shape of the basin and its relationship to the surrounding positive structures. The basin includes the entire Southern Peninsula, the eastern part of the Northern Peninsula, eastern Wisconsin, northeastern Illinois, northern Indiana, northwestern Ohio, and western Ontario. It is surrounded by the Canadian Shield to the north, the Algonquin Arch to the east, the Findlay Arch to the southeast, the Kankakee 33 LAKE suesmon CANADA UPPER MICHIGAN IOCONGIN HIGHLAND MICHIGAN BASIN HOWELL ANTICLINE ALBION- scmo / ,Luc - menace uouoctme ’ a uw sxrsquu ILL. BOWLING GREEN FAULT Figure 5 . Michigan Basin and Surrounding Structural Features . 34 Arch to the southwest, and the Wisconsin Arch and Dome to the west. The nearly circular shape of the basin shows a slight northwest to southeast elongation. Dominant structural trends of the basin parallel the elongation trend as shown by the Dundee trends of Figure 6. The Mid-Michigan Gravity Anomaly also transects the basin from northwest to southeast (Figures 7 and 8). Associated with this gravity high is a magnetic anomaly as shown in Figure 9. Sedimentary rocks of the area dip toward the center of the basin which lies just west of Saginaw Bay. Dips average about 25 to 60 feet per mile (Ells, 1969), but folding, faulting, solution and removal of salt layers, erosion, and other factors greatly modify the uniformity of the rock sequence. It is these first three factors which are examined in the data analysis portion of this study. Before analyzing the data, however, it is worthwhile to review some of the preposed models of basin subsidence. Some of the factors to be considered in basin subsidence are: the location of the basin within the continental plate, the time and amount of subsidence, the size and shape of the basin, the structures around and within it, and its relation to other basins on the continental plate. 35 “I" If. O‘lll “(I I'll Figure 6. Structural Trends of the Dundee Formation. 36 . ‘— ‘ " 00qu um" lull" In ' In I Pod-sun - T ' ‘ nun... LII-1'27 UH ,1 "‘III‘IH ( unu- W 1 In... M' T J V . 3.5;":15'21". 3'33; 33:}; . I . I I I 7‘ I ‘ . V O a In I. II..- . V _ \ = *‘ II; "it I ~ fia— Figure 7- Bouguer Gravity Anmaly mp. 37 H To HI a]g,, LANE .' ‘ .‘. ..- ‘ . . '. ’4 I u. l. .. v '. -. -. . ‘. ~‘ -.'0 u u ‘- 3' .' .. . . 1.. .- I .. .. I ,- l I'O . . . ‘ t o ' ' ‘—I¥ ‘ . ‘- I ' i .0 O I I I \ . - . l . I..O..O. ' a g n n u . I c. - . ' .1 ‘I .u ‘ . _ . '. . O I n a I p c . I ' - a ' . .' o . c a. .I - . n o . a a ’n ' o EAMADA LAKE ERIE Figure 8 . Schematic Diagram of Mid-Michigan Gravity High. 38 dd . I! o u. a ... a...“ Efi “ we“ _ nt_ . ., i/Avgm . A I... 1% A ,.,r. JI M\\ .i,.III l . II.%i/?‘§ v sJ a a w {a - . W/Mrl/A l I. A t . I. a . A. .A . A/ A/ / ///// / / V / o u A A / , . AA A/ A/ //H //AH/‘ A . ...// AA AA/AA AAA/AA I. .. . / / / / //// /// / / / . . / / / / / / / . . . // // / //// / / . . /// 500 / f‘p ;- . 36+250 EEWEE h .Em >n< Egan—mud“) PLATE 3 DUNDEE STRUCTURE MAP mu 3% M. a 3%. % 3% ii": =’ ../ / w / / Z/ / AH / N H/ I// / // / ”m w. Lm A TV ~ ( \ #m (3 J \ fit —. *2».— ”REV: \(Eidm MAI . (AL SMWEMBWNW I: PLATE 4 A-2 CARBONATE STRUCTURE TTTTTTTTTTTTTTTTTTTTTT OOOOOOOOOOOOOOOOOOOOO IIIIIIIIIIIIIIII CONTOUR INTERVAL 11111 A A Ame/mm? 3&2: >9fl§flfififllWT PLATE 5 TRENTON STRUCTURE MAP N l SU STRU‘ STRUCTURAL NORTHWEST THE MICHI CONTOUR l SCALE: 1" NF LEWA HI CHIGRN STATE UNIV. LIBRARIES I III III II IIIIII III IIII III IIIIIIII IIII III III