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Full article available in PDF version.

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Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

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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.