Almost all Paleozoic strata in the Michigan Basin display elevated levels of organic maturity that cannot be explained by present-day burial depths, geothermal gradients, and heat flow. Likewise, higher heat flow from the basement in the past is unsatisfactory as an explanation of the elevated maturities, particularly for the younger sediments. Previous studies have concluded that a significant amount of Permo-Carboniferous overburden has been removed from this region by early Mesozoic regional uplift and erosion. If the missing overburden were sufficiently thick and thermally resistive, a “thermal blanket” effect would have caused elevated temperatures throughout the underlying stratigraphic section during late Paleozoic and early Mesozoic time. Models of organic maturation that take into account this “thermal blanket” effect as well as other variations in thermal conductivity attributable to lithologic differences can explain the anomalous maturity of most strata in the Michigan Basin without postulating any increase in ancient heat flow. An overburden thickness of 2,000 m with a gradient the same as that observed in the present-day Carboniferous section would provide an adequate explanation for the elevated maturities. Lesser thicknesses of overburden would require correspondingly higher geothermal gradients, a reasonable possibility if the missing overburden was a fluvio-deltaic sequence containing low-conductivity carbonaceous strata.

A substantial section of Precambrian rock is exposed over an area of approximately 19,400 km 2 (7,500 mi 2 ) in the western part of the Northern Peninsula of Michigan. This province is a portion of the exposed southern terminus of the Canadian Precambrian Shield and contains a large variety of igneous, sedimentary, and metamorphic rocks. Significant amounts of iron and copper from Precambrian rocks of Michigan have provided important contributions to the growth of the state and national economy for nearly 150 years. Archean rocks consist of volcanics, sediments, and younger felsic and mafic intrusives, some of batholitic dimensions. Volcanic and associated sedimentary rocks occur as greenstone belts included in the Ramsay Formation, Gogebic County; Dickinson Group, Dickinson and Iron Counties; and Marquette Greenstone Belt, Marquette County. Volcanic rocks consist of mafic to felsic lava flows and pyroclastics and sediments derived from volcanic rocks. Volcanic flows include amygdaloidal and ellipsoidal varieties. Pyroclastics consist of agglomerate, conglomerate, breccia, and tuff. Sediments are described as graywacke, argillite, siltstone, conglomerate, quartzite, iron formation, and chert. Granite and granitic gneiss, principally tonalite and granodiorite, intrude the periphery and interiors of the greenstone belts. Mafic intrusives, including peridotite, are subordinate. Shearing is prominent in some areas, and metamorphic grade ranges from lower-greenschist to upper-amphibolite facies. Minor amounts of gold and silver have been produced from the Marquette Greenstone Belt. Early Proterozoic strata are subdivided into four groups, in ascending order: the Chocolay, Menominee, Baraga, and Paint River Groups. Copper mineralization occurs in the Kona Dolomite of the Chocolay Group. The Menominee Group contains three major iron formations of equivalent age, the Negaunee Iron Formation of the Marquette Iron Range; the Vulcan Iron Formation of the Menominee Iron and Felch Mountain Districts and the Ironwood Iron Formation of the Gogebic Iron Range. In the Baraga Group, the Goodrich Quartzite contains concentrations of monazite, and the Michigamme Formation has vast amounts of graphitic carbon. The Paint River Group includes the highly productive Riverton Iron Formation. Iron was discovered in 1844 on the Marquette Iron Range, and an early pig-iron industry flourished. The east-west–trending Marquette syncline, containing the Negaunee Iron Formation, is more than 65 km (40 mi) long. The Negaunee has a maximum thickness of 1,060 m (3,500 ft), and iron-formation resources have been estimated at 205 billion long tons. There are four iron formations on this range, three of which have been productive. However, 97 percent of the 588 million tons mined came from the Negaunee. The east-west–trending Menominee Iron-bearing District, in southern Dickinson County, consists of a north and south range segmented by longitudinal faulting. The Vulcan Iron Formation is exposed over a strike length of 28 km (16 mi) and has a maximum thickness of 180 m (600 ft). Production amounted to nearly 82 million long tons. In the Felch Mountain District of central Dickinson County, only eroded remnants of the Vulcan Iron Formation remain. Production of 36 million tons was principally from the Groveland low-grade iron mine. The Gogebic Iron Range, in Gogebic County, is an essentially east-west–trending, northward-dipping sequence of sediments containing the Ironwood Iron Formation. In Michigan, the Ironwood has a strike length of about 40 km (25 mi) and a maximum thickness of about 490 m (1,600 ft). Iron ore production totals 255 million long tons. The Negaunee, Vulcan, and Ironwood iron formations are considered to be stratigraphically equivalent. The Iron River–Crystal Falls District in Iron County is primarily composed of the Paint River Group containing the Riverton Iron Formation. The Paint River Group is outlined in a triangular-shaped basin approximately 260 km 2 (100 mi 2 ) in area. The Riverton has a maximum thickness of 240 m (800 ft) and has been intensely and complexly folded. A high phosphorous and manganese content characterizes the Riverton and its naturally derived iron ores. Production amounted to 207 million long tons. Middle Proterozoic rocks in Michigan consist of a very thick sequence of volcanics and sediments. For the most part, strata dip uninterrupted toward Lake Superior at varying degrees. Native copper was the exclusive mineral produced from the Portage Lake Volcanics in Michigan’s Keweenaw Peninsula. Stratabound native copper mineralization forms ore bodies in amygdaloidal and brecciated tops of lava flows, and in interflow conglomerates. Minor amounts were produced from transverse fissures. Production of refined copper through 1976 amounted to 4,769,465 metric tons (5,257,438 short tons). Sulfide copper (chalcocite) with some native metal is mined from the Nonesuch Formation several thousand feet about the Portage Lake Volcanics in the Porcupine Mountain area. Copper mineralization is confined to siltstone and shale of the basal portion of the Nonesuch. Small amounts of disseminated native copper are produced from the uppermost sandstone of the underlying Copper Harbor Conglomerate. Through 1987, 1,364,800 metric tons (1,504,433 short tons) of refined copper has been produced.

Continental rifting in the area now known as the Michigan Basin occurred some 1.1 b.y. ago (Van Schmus and Hinze, 1985), along with similar tectonism in other portions of the mid-continental United States. Although little is known of the subsequent 500 m.y., it appears that a change from a continental to a marine depositional regime took place during the Late Cambrian (Dresbachian) when northerly transgressing epeiric seas advanced into a slowly developing ancestral Michigan Basin. The record of those seas is documented by Late Cambrian to Middle Ordovician formations. These are, in ascending order, the Mt. Simon, Eau Claire, Galesville, Franconia, Trempealeau–Prairie du Chien (T-PDC), St. Peter, and Glenwood. On the margins of the basin, the basal Mt. Simon Sandstone rests disconformably on older Precambrian basement. There, also, T-PDC rocks (Late Cambrian–Early Ordovician age) were eroded, producing a major interregional unconformity (the post-Sauk unconformity) on which the St. Peter Sandstone lies and which marks the top of the Sauk sequence. In the central Michigan Basin, however, deposition of the Sauk sequence was continuous and the post-Sauk unconformity was not developed. Again, on the margins of the basin, a younger (Middle Ordovician) post–St. Peter unconformity was developed between the St. Peter and Glenwood Formations, but again is not present in the central basin where essentially continuous deposition of the entire section took place. The configuration of the present-day Michigan Basin was established during the Early Ordovician. Since that initial configuration, however, significant structural elements have been added during subsequent Paleozoic time.

The Trenton and Black River Formations of the Michigan Basin have been diagenetically altered by a complex sequence of events related to both the stratigraphic and structural history of the basin. The physical distribution and chemical composition of dolomite in the Trenton and Black River Formations are variable and suggest multiple episodes of dolomitization. The most extensive diagenetic alteration of both Trenton and Black River limestones has occurred in fracture-controlled hydrocarbon reservoirs. Within reservoirs several stages of dolomitization were followed by carbonate and sulfate cementation, and sulfide mineralization. Although the general patterns of reservoir alteration have been recognized for some time, possible causes of such alteration have not been adequately addressed. Several lines of evidence indicate that mineralization and hydrocarbon migration are related and occurred in the late Paleozoic, perhaps in response to compressional deformation caused by Appalachian tectonism. During such episodes, fluids were mobilized and channeled vertically through preexisting fracture zones. This fluid migration also served to drive maturing hydrocarbons out of Trenton–Black River source beds and into previously dolomitized, high-porosity intervals. This general mechanism could be applied to other fracture-related reservoirs in the Michigan Basin area based on the regional distribution of Mississippi Valley–type (MVT) reservoir alteration and compressional stress fabrics.

The pinnacle reefs of the Michigan Basin form small, isolated hydrocarbon reservoirs encased in impermeable evaporites and mudstones, and account for most of Michigan’s hydrocarbon reserves. The temporal relations between the reef sequence and the evaporites are still in dispute, but in the currently favored model, deposition of reefs and evaporites follow each other closely in a cyclic manner but are not synchronous. Pinnacle development occurred in four stages and included periods of subaerial exposure, which enhanced reef porosity and permeability through leaching and dolomitization. Subsequent evaporite precipitation filled much of this porosity; many reefs are completely salt plugged and impermeable. Sea-level history and depositional environments of Salina evaporites are disputed, but a model of shallow-water evaporite deposition in the basin is favored. Regional trends have been recognized across the pinnacle-reef belt, and these predict increased salt plugging of the reefs basinward, increased dolomitization and preserved secondary porosity toward the basin margin, and in the northern trend, zones of production that pass from gas to oil and finally to water toward the basin margin. The producing reefs have porosities that range from 3 to 37 percent (average 6 percent) and average permeabilities of 11 to 12 mD (ranges to more than 1 D).

The First Silurian carbonate buildup in North America to be correctly identified as a reef was in eastern Wisconsin, in 1862. In the ensuing 125 years, many hundreds of studies were presented on other Silurian reefs in the Michigan Basin and environs. Successive trends in both scientific and economic interests have characterized the quest to learn what riches these reefs may yield. Three periods are recognizable: (1) An early period of discovery, extending until 1926, saw more incorrect ideas to explain the once-mysterious reefs than correct ideas, e.g., ideas of volcanism and upheavals. James Hall, who advanced both incorrect and correct ideas, best typified this period, but T. C. Chamberlin should be credited most for his lasting insight. (2) A middle period of enlightenment, 1927 to 1960, saw a convincing reef proof and a systematic set of biologic reef parameters set forth by (especially) E. R. Cumings, R. R. Shrock, and H. A. Lowenstam. (3) A modern period of integration, 1961 to present, could have been designated as one of proliferation, so numerous were the new reef models and ideas concerning reef geometry, distribution, diagenesis, evaporite relations, deep- versus shallow-water environments, basin-to-shelf differences, cyclicity in deposition, sea-level changes, tectonism, and hydrocarbon accumulation. Many of these ideas conflict; thus, I choose the regionally broad stratigraphic integration that developed as the most significant key to the modern period and the several debates that yet require reckoning against the modern stratigraphic framework. The stratigraphic relations favored in this chapter depart from tradition, but they suggest several kinds of studies that need to be undertaken.

Evaporitic conditions in the Michigan Basin commenced in Late Ordovician time and continued into the Early Devonian. Five major evaporite (anhydrite/halite) cycles in the Silurian Salina Group (termed “A-1,” “A-2,” “B,” “D,” and “F”) can be recognized. They are further subdivided into two or more cycles of evaporite deposition by intercalations of less soluble members. Only the lowermost Salina evaporite produced a sylvinite deposit. The water surface was enlarged in subsequent evaporite cycles, which did not go beyond halite saturation, suggesting an enlargement of the water supply. Additional seawater came from the Kokomo Sea in Indiana; continental waters likely also entered at times from the Moose River Basin. The Chatham sag eventually fed Michigan Basin brines into Ohio and Pennsylvania. A progressive overstepping of one evaporite unit over the one below indicates either subsidence affecting a wider area of rising sea level, the latter suggested by the inundation of northern Ohio beginning with the “B” unit of salt deposition. In no case does brine depth appear to have exceeded some tens of meters. The present morphology of reefs is dictated as much by compaction of surrounding micrites as by renewed growth on older reef mounds where rates of subsidence remained moderate. The amount of clay influx in the upper part of the Salina suggests that humid periods became more frequent at the expense of the duration of dryer periods. Post-Salina rocks initially contain numerous short-lived evaporite cycles, and then grade to open-marine sediments, deposited before the Michigan Basin became emergent land in the late Paleozoic

The Late Devonian Michigan Basin was floored by the Middle and Upper Devonian Squaw Bay Limestone, which was deposited during the downwarping that produced the basin within a former Middle Devonian carbonate platform. The Squaw Bay comprises three beds, each having a different conodont fauna. The two upper beds, deposited during the transitans Zone, have different conodont biofacies that reflect this deepening. The basin was largely filled by the deep-water, anaerobic to dysaerobic, organic-rich, black Antrim Shale, which has a facies relationship with the prodeltaic, greenish gray Ellsworth Shale that prograded into the basin from the west. The Upper Devonian (Frasnian to Famennian) Antrim Shale is divided into four members, from base to top: the Norwood, Paxton, Lachine, and upper members. These members are more or less precisely dated by conodonts. The Norwood was deposited during the transitans Zone to Ancyrognathus triangularis Zone, and the Paxton was deposited from that zone probably through the linguiformis Zone at the end of the Frasnian. The overlying Lachine was deposited during the early Famennian and has yielded faunas of the Upper crepida and Lower rhomboidea Zones. Only the lower part of the upper member is exposed, and near Norwood, Michigan, it yielded conodonts of the Lower marginifera Zone. The widespread Famennian floating plant Protosalvinia (Foerstia) has not yet been found in outcrops of the Antrim, and should not be expected to occur except in the upper member or highest part of the Lachine Member. Its range in terms of conodont zones is from the Upper trachytera Zone through the Lower expansa Zone and possibly into the Middle expansa Zone. One known subsurface occurrence might be datable as rhomboidea or Lower marginifera Zone, depending on gamma ray correlations to outcrops. Black shale deposition ended when the Late Devonian mud delta of the Bedford Shale prograded across the Michigan Basin from the east and then retreated as the regressive Berea Sandstone was being deposited during the major eustatic sea-level fall that ended the Devonian. The Bedford was deposited during the Upper expansa to Lower praesulcata Zones, and the Berea was deposited during the Middle to Upper praesulcata Zones. Both formations contain the spore Retispora lepidophyta, which is a global indicator of latest Devonian age.

The Upper Devonian sequence in the Michigan Basin is a westward extension of coeval cyclical facies of the Catskill deltaic complex in the Appalachian basin. Both basins and the intervening Findlay arch express the tectonic and sedimentational effects of foreland compression and isostatic compensation produced by the Acadian orogeny. The Late Devonian Michigan Basin formed as one of several local deeps within the long Eastern Interior seaway that separated the North American craton, backboned by the Transcontinental arch, on the west from the Old Red continent, Avalon terrane (microplate), and possibly northwest Africa on the east. Basin development began in the late Middle Devonian (late Givetian varcus Zone) with subsidence of a shallow-water carbonate platform formed by rocks of the Traverse Group. Subsidence was contemporaneous with Taghanic onlap of the North American craton. During subsidence, a thin transitional sequence of increasingly deeper water limestones separated by hardgrounds was deposited in the incipient Michigan Basin during the latest Givetian to earliest Frasnian disparilis to falsiovalis Zones. Deposition of this sequence culminated during the early Frasnian transitans Zone with a calcareous mudstone bed at the top of the Squaw Bay Limestone. Subsidence was followed by a 12-m.y.-long Late Devonian episode of slow, hemipelagic, basinal sedimentation of organic black muds that formed the Antrim Shale, interrupted basinwide only by deposition of its prodeltaic Paxton Member. Westward, the basinal Antrim black muds intertongued with greenish gray, deltaic and prodeltaic muds of an eastward-prograding delta platform formed by the Ellsworth Shale. Basinal black shale deposition ceased in latest Devonian (late Famennian Lower praesulcata Zone) time, when the Bedford deltaic complex prograded westward, completely filling the Antrim Basin and even covering part of the older Ellsworth deltaic complex on the west. As sea level was lowered eustatically near the end of the Devonian, the regressive Berea Sandstone terminated deltaic deposition. After an Early Mississippian erosional episode, widespread deposition of the unconformably overlying Lower Mississippian Sunbury Shale began during the next transgression, associated with a major eustatic rise in the Lower crenulata Zone.

The Mississippian System has the largest subcrop area of any Phanerozoic system in the Michigan Basin, and attains a maximum thickness of 719 m (2,360 ft) northeast of the basin center. The Mississippian formations include, in ascending stratigraphic order: Antrim Shale, the laterally equivalent Bedford and Ellsworth Shales (all Upper Devonian to Kinderhookian); Berea Sandstone (Kinderhookian); Sunbury Shale (Kinderhookian); Coldwater Shale (Kinderhookian to Osagian); Marshall Sandstone (Osagian); Michigan Formation (Osagian to Meramecian); and Bayport Limestone (Meramecian). There are no Chesterian sediments in the Michigan Basin. The Mississippian sediments accumulated conformably on Devonian strata but are overlain with disconformity by Pennsylvanian and, very locally, Jurassic strata. The Kinderhookian, Osagian, and Meramecian series record a decreasing rate of Michigan Basin subsidence through time. Subsidence ceased temporarily during the Chesterian Epoch, and some Mississippian units were eroded from local anticlines in the central basin area during this interval of nondeposition. As a result, basal Pennsylvanian strata rest directly on Meramecian rocks and locally on older Mississippian formations. The Mississippian sediments are primarily shallow-marine deposits consisting largely of shale with subordinate amounts of sandstone, siltstone, carbonates, and evaporites. Fluvial-deltaic deposits make up a significant portion of the section only in the eastern half of the basin. Terrigenous clastics were derived mainly from a source to the northeast of the basin in the Canadian Shield and, to a lesser extent, from the northwest in the Wisconsin Highlands. Significant quantities of oil and gas have been produced from sandstones in the Berea, Marshall, and Michigan Formations, and from carbonates in the Ellsworth Shale. Sandstones in the Coldwater and Marshall Formations were, at one time, extensively quarried for grindstones and construction flagstone, respectively. The Michigan Formation is the chief source of gypsum in Michigan, and the Bayport supplies some of the state’s limestone.

A northeast-trending graben was hypothesized to extend southwest of Saginaw Bay to the Mid-Michigan Gravity High, based on interpretation of Landsat 1 imagery, stream drainage maps, and sparse well-log data. The edges of the graben were thought t o extend along and southwest of the Pinconning oil field on the northwest side, and the Quanicassee River on the southeast side. Subsequent analysis of digital terrain, magnetic, gravity, seismic, and well-log data showed that no unequivocal evidence for a discrete, simple graben within the originally defined limits could be found. However, the new data indicated that the proposed edges of the “graben” correspond to structural lineaments (monoclines and anticlines) expressed within the Paleozoic section and on the bedrock surface. These structural features correlate with basement contacts and/or fault zones inferred from interpretation of magnetic and gravity images. These possibly basement-controlled structural lineaments influenced depositional patterns intermittently during the Paleozoic, as evidenced by the presence of northeast-trending highs within the limits of the “graben” on isopach maps of Pennsylvanian, Mississippian, and Devonian stratigraphic units. Rapid thinning and facies changes in Middle and Lower Ordovician units across the southeastern edge of the “graben,” coupled with its correlation with northeast-trending positive gravity and magnetic anomalies, suggest that this is a significant structural feature, possibly controlled or influenced by the Grenville Front.

Abstract The Michigan basin is in the upper midwestern part of the United States. The thickest part of this almost circular basin contains a lens of about 16,000 ft (4800 m) that consists mostly of marine sedimentary rock, ranging in age from Cambrian to Pennsylvanian. A small patch of terrestrial Jurassic rocks is also present in the central basin. Most of the sedimentary section is overlain by glacial deposits. Precambrian sedimentary rocks are preserved in the subsurface in an ancient rift zone that can be defined by geophysical techniques. Although the exact origin of the basin is still in question, an embryonic form was present by Late Cambrian time, and the basin attained its modern configuration during Ordovician time. The main structural grain of the basin is oriented northwest-southeast, and a complementary northeast-southwest trend also exists. In 1886 oil and gas were discovered in the Port Huron field in the Michigan basin; the modern era of oil and gas exploration in the basin began in 1925 with the discovery of the Saginaw field. Oil and gas have been found throughout the Paleozoic section except in Pennsylvanian- and Cambrian-age rocks, which have not yet yielded economically significant quantities of hydrocarbons. Exploitation of the basins reserves has been a gradual process, developing from shallower to deeper horizons. At present, however, one of the shallowest formations (Antrim Shale) and one of the deepest (St. Peter Sandstone) are both the focus of intense exploration and exploitation.

The Keweenawan-age Oronto Group of northern Michigan and Wisconsin includes the Copper Harbor Conglomerate, Nonesuch (Shale) Formation, and Freda Sandstone. These formations are part of a volcanic-clastic sequence created in response to the formation of the Midcontinent Rift System. Although intercalated volcanics are found in the lower one-third of the Oronto Group, a sedimentary depositional regime was dominant. Along the Keweenaw Peninsula, paleocurrent indicators for all three formations show that the predominant depositional directions were northerly. On Isle Royale, the opposite side of the rift, sedimentary structures in the Copper Harbor Conglomerate indicate that flow was to the south and east. Lithologically, the Copper Harbor Conglomerate is a red-brown, basinward-thickening wedge of volcanogenic clastics and subordinate volcanics that fines distally and upsection. Maximum thickness for this formation is about 1830 m. The dominant sandstone type is lithic graywacke. Conglomerate facies are primarily clast-supported and comprised of volcanic clasts with a ratio of mafic to silicic + intermediate clasts of about 2:1. The heavy-mineral suite for the Copper Harbor Conglomerate (as well as the other Oronto Group formations) mainly consists of ilmenite and similar opaque minerals and epidote. Depositionally, the Copper Harbor Conglomerate represents a prograding alluvial fan complex. Interfingering with the Copper Harbor Conglomerate is the Nonesuch (Shale) Formation, an unoxidized sequence of gray-black siltstone, shale, and sandstone with a maximum thickness of 215 m. Besides having been deposited in a reducing environment, the Nonesuch differs from the enclosing redbed sequences by its increased textural maturity and its sulfide and hydrocarbon content. The heavy-mineral suite of the Nonesuch also differs from that of the redbeds only by relative enrichment of chlorite. The Nonesuch is perceived as a rift-flanking lacustrine environment, probably initiated through disruption of existing drainages by alluvial, volcanic, or tectonic processes. As with the underlying Copper Harbor Conglomerate, the contact with the overlying Freda Sandstone is gradational in character. The Freda Sandstone is a ferruginous, lithic sequence of cyclic sandstone and mudstone exceeding 3660 m in maximum thickness. Although similar in appearance to some sandstones of the Copper Harbor Conglomerate, the Freda, overall, is of greater compositional maturity, and conglomerate facies are uncommon. The Freda is dominantly fluvial in origin and appears to have “overridden” the Nonesuch environments. Although complex in detail, the overall depositional model for the Oronto Group is one of simple transgressive-regressive relationships between alluvial fan/lacustrine/fluvial environments. Important aspects of such a model are that (1) all the Oronto Group formations are genetically related with no major unconformities between them; and (2) the intervening Nonesuch Formation is, at least in part, equivalent in age to the upper Copper Harbor and the lower Freda.