Evaporite facies of the Michigan Basin
Published:May 10, 2018
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William B. Harrison III, Peter J. Voice, 2018. "Evaporite facies of the Michigan Basin", Paleozoic Stratigraphy and Resources of the Michigan Basin, G. Michael Grammer, William B. Harrison, III, David A. Barnes
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The Michigan Basin is one of the world’s important sedimentary basins that contains significant quantities of evaporites. Here, evaporites are found in deposits of Ordovician through Mississippian age rocks; however, most of the thick evaporite accumulations occur in Silurian and Devonian intervals. Halite is most significant in the Silurian Salina Group, with a maximum aggregate thickness of halite exceeding 650 m (2150 ft). During the earliest evaporite deposition in the Salina Group (A-1 Evaporite), sylvite was widely deposited in the north-central portion of the basin within the upper 91.4 m (300 ft) of the formation. Devonian salt is also present in the north-central portion of the basin in the Horner Member of the Lucas Formation, where maximum aggregate net thickness of halite reaches 125 m (410 ft). Recrystallization of much of the halite obscures the primary depositional crystal geometry; however, some well-preserved beds do show crystal growth that is interpreted as bottom-growth chevrons, which likely suggest shallow-water deposition. Throughout the rest of the Michigan Basin, in both space and time, the evaporite phase deposited is CaSO4. In the shallowest portions of the Mississippian Michigan Formation, the sulfate mineral phase is gypsum; everywhere else in the basin, all the evaporitic sulfate deposits are anhydrite. Although the dehydration of the gypsum to anhydrite has slightly altered the original depositional morphology, some primary growth geometry is still evident. Subtidal and sabkha morphologies can be documented in all the anhydrite/gypsum deposits of the Michigan Basin. Based on historic production, evaporite minerals have added an estimated $15.5 billion (in 2013 dollars) to the industrial mineral economy of Michigan since the first commercial development in the 1860s.
Sedimentary successions that are made up partly or entirely of evaporitic deposits may form within widely diverse settings—within sediments at or below the water table, in lacustrine or marine environments, in supratidal, shallow-water, or even deep-water settings—the only controlling feature is that the water from which they formed was supersaturated. On Earth, the most common minerals are halite and gypsum (or anhydrite), but numerous other compounds may form, depending on the specific sources of the water.
Michigan Basin evaporite deposits occur in strata from the Ordovician to Mississippian geological intervals, although most of the evaporites, by volume and economic significance, occur in Silurian, Devonian, and Mississippian formations. Michigan evaporites in the Ordovician through Devonian intervals are primarily associated with carbonate depositional systems, whereas the Mississippian units are mixed siliciclastics with minimal carbonate deposits.
The Michigan Basin provides examples of evaporites deposited in the supratidal and intertidal zones, the “sabkha” (Salina Group, Detroit River Group; Reed City Anhydrite, Michigan Formation), shallow-marine (Salina Group, Detroit River Group), and deeper-water settings (lower Salina Group), as well as examples of secondary evaporites in units throughout the stratigraphic section. This review provides an overview of major primary evaporite facies found within the stratigraphic section of the Michigan Basin, including historical discussion of evaporite studies in Michigan as well as discussion of the primary textures, sedimentological context, and paleoenvironmental interpretation of the evaporite units in the basin.
The paleographic position and structural history of the Michigan Basin have had major influence on sources of sediments and depositional environments throughout the Paleozoic Era. Michigan was consistently in tropical or equatorial regions throughout most of the middle Paleozoic Era. Because the Michigan Basin is bounded to the north by the Canadian Shield and surrounded to the west, south, and east by persistent structural arches (Catacosinos et al., 1991), the basin was periodically and substantially restricted during times of global sea-level drawdown. Also, due to a late Precambrian failed rift running through the center of the basin, Michigan exhibits periodic basin-centered subsidence (Howell and van der Pluijm, 1999), creating accommodation space even during times of falling sea level. The isolation of this basin from the world ocean during lowstands and the subsidence-induced accommodation space have allowed for substantial thicknesses of basin-centered evaporite deposits, especially during Silurian and Devonian time. In addition, the major Silurian and Devonian evaporite deposits of the Michigan Basin formed during greenhouse climates characterized by aridity (on the basis of fluid inclusion data from the Salina Group halites [Losey and Benison, 2000] and brine chemistry [Lowenstein and Timofeeff, 2008]). Additionally, at any time in Earth history, the rate of evaporite deposition, as compared to other sediments, must also be considered. Evaporites form at a much greater rate than all other sediments (two to hundreds of times faster), so that the great thicknesses of evaporitic sediments present in the Michigan Basin do not serve as an indicator of relative depositional time (Schreiber and Hsü, 1980).
ECONOMIC SIGNIFICANCE OF MICHIGAN BASIN EVAPORITES
Evaporites in the Michigan Basin have been historically studied for stratigraphic and paleoenvironmental reasons to understand regional subsurface correlation and for reconstructing paleoclimatic conditions and depositional environments. In addition to providing considerable knowledge for interpreting the geological history of the Michigan Basin, evaporitic facies have significant economic impact on the development of geological natural resources. Several of the halite and anhydrite strata in the Late Silurian Salina Group and the Middle Devonian Detroit River Group are seals for oil- and gas-producing reservoirs. The thick evaporite deposits themselves are also exploited by mining the sulfate (gypsum/anhydrite) and salt in several stratigraphic horizons.
Michigan has been a significant producer of calcium sulfate (both gypsum and salt) for much of the last century. Cohee (1965) estimated that the Salina Group (Fig. 1A) includes 60 trillion metric tons (66 trillion tons) and the Detroit River Group (Fig. 1B) contains 4.5 trillion metric tons (5 trillion tons) of salt. A tiny fraction of that salt has been harvested through solution mining and excavations. Salt and sylvite have been produced through solution mining from the Salina A-1 Evaporite, and halite has been produced from underground mining in the F Salt of the Salina Group. Solution mining of halite (salt) and sylvite (potash) occurred at a plant near Hersey, Osceola County, from 1989 to 2013, with maximum potash production of ~160,000 metric tons per year and salt production roughly four times the potash level. Statewide salt production between 1860 and 2013 yielded 308 million metric tons from rock salt mining and brine evaporation. Peak production of salt (Fig. 2B) occurred in 1956 at 5 million metric tons (5.55 million tons). The earliest salt production in Michigan was from natural salt seeps and produced brine from saline aquifer wells. Later, solution mining of the salt beds became popular. Underground mining of rock salt began with the construction of the Detroit mine shaft in 1906, which was completed to 323 m (1060 ft) in 1910.
Gypsum production (Fig. 2A) has yielded ~110 million metric tons of gypsum since 1865, with peak production occurring in 1978, with almost 2.5 million metric tons (2.8 million tons) mined (see Fig. 2A). Cumulative value of Michigan’s salt and gypsum mineral production, in 2013 dollars, is just over $15.5 billion, using average annual prices from Bolen (2014). With the exception of a few minor gypsum deposits (Cabot Head Shale, Pointe aux Chênes Formation), the majority of the gypsum mined in Michigan has come from the Mississippian Michigan Formation (Fig. 1B).
Evaporite deposits play a huge role in oil and gas reservoirs of the Michigan Basin. Carbonate rocks produce 95% of the oil and 45% of the natural gas that has come from Michigan reservoirs. Oil and gas fields producing from the Silurian Niagaran Pinnacle reefs and lower Salina restricted peritidal carbonates are capped by salt and/or anhydrite of the Salina A-1 and A-2 Evaporite formations. Devonian reservoirs in the Dundee Formation (Reed City Member) and Lucas Formation are platform and restricted carbonates also sealed by salt and/or anhydrite in structural or stratigraphic traps. Together, these Silurian and Devonian formations have produced nearly 700 million barrels of oil and 3.5 trillion cubic feet (~99 billion cubic meters) of natural gas. Those volumes represent nearly half of the total historic production of oil and 40% of the natural gas in Michigan. In addition to the role as seals in hydrocarbon reservoirs, some of the salt formations now have solution caverns that have been created for underground storage of natural gas or natural gas liquids. Two salt cavern natural gas storage fields currently exist in Michigan, Lacey Station in Barry County and Morton in Macomb County. Both fields are in salt caverns of the Salina “B-Salt.” There are five salt storage caverns that currently store liquified petroleum gas (LPG). One field, Alto LPG Storage, is in Kent County in the Salina A-2 Evaporite. There are two fields in Wayne County, Woodhaven LPG Storage and Inkster Junction LPG Terminal, both with caverns in the Salina “B-Salt.” There are also two storage fields in St. Clair County, the St. Clair LPG Terminal in the Salina A-2 Evaporite and the Marysville LPG Terminal, in the Salina “B-Salt.”
GEOLOGICAL AND STRATIGRAPHIC OCCURRENCE
Geologists have long recognized the existence of evaporite deposits in the Michigan Basin, and economic exploitation of these resources was one the earliest commercial developments of Michigan’s mineral resources. Published accounts of evaporite deposits from the Michigan Basin by Bigsby (1824, p. 193) may have been the earliest, when he reported, “At the isles of St. Martin [northeast of St. Ignace], however, we find a large deposit of gypsum. It is an extensive bed of the granular kind, white, gray and brown, interspersed with frequent masses of red, white, and brown selenite, occurring in shapeless lumps, in veins or in small and very thin tables, having three or more sides and sharp angles.”
Winchell (1861) reported the first descriptions of the gypsum in the Grand Rapids area (Michigan Formation). Winchell (1862a, 1862b) published lithologic descriptions from borings near East Saginaw. He described the “Michigan Salt Group” as “argillaceous, pyritous, and gypseous shales, within beds of arenaceous and magnesian limestone and thick beds of gypsum” (Winchell, 1862b, p. 308). This interval was recorded as 52 m (170 ft) thick. Winchell (1862b) also described the stratigraphic distribution of salt and brine in the Michigan Basin, describing three intervals: the Michigan Salt Group, the Onondaga Salt Group, and brines in the Parma Sandstone. The Onondaga Salt Group was described as “well stocked with gypsum and … known to be saliferous” (Winchell, 1862b, p. 308) and was found 230 m (750 ft) beneath the Michigan Salt Group. Rominger (1873) created a preliminary stratigraphic chart and detailed descriptions for Paleozoic rocks in Michigan. He described the Onondaga Salt Group from outcrops in the Pointe aux Chênes region as consisting of “dolomites, green and red variegated marls, alternating with more massive ledges of a calcareous rock…” and “interstratified with these are nodular concretionary masses, and narrow bands of gypsum” (Rominger, 1873, p. 29). The Onondaga Salt Group was stratigraphically above the Niagara Group dolomites and below the Mackinac Breccia (which Rominger correlated to the Helderberg Group). Lane (1900, p. 14) renamed Winchell’s Michigan Salt Group as the Michigan Series and provided the following description: “Toward the bottom of the series … the shales are darker and more bituminous. In the shales also occur lenticular beds of gypsum, which is white when pure, but when impure bluish and hardly to be distinguished from shale.” From well records, Lane (1900) mapped the distribution of the Michigan Series from Huron County (Saginaw Valley area) to Bayport and Grand Rapids to the west.
Grimsley (1903) provided a thorough discussion of the gypsum industry in Michigan and the geology and distribution of gypsum deposits in the state. Grimsley focused on two sedimentary packages, the Mississippian Michigan Group of the central Lower Peninsula of Michigan and the Silurian Monroe Group (Winchell’s Onondaga Salt Group) in the southern Upper Peninsula of Michigan. He described the Michigan Group as a “group of shales, thin bedded limestones, and gypsum layers” (Grimsley, 1903, p. 56). Similar lithologic relationships were observed in the Alabaster area (Arenac County) from wells and outcrops. At the Alabaster quarry, Grimsley described the gypsum as “dark colored, impure gypsum with a large percentage of clay…” (p. 68). Wells at the site show that beneath the main gypsum layer (7 m [23 ft] thick), there is an interbedded package of sandstone and cherty limestone with beds of gypsum. In the Pointe aux Chênes area near St. Ignace, Grimsley described a gypsum mine opened in the 1850s that actively mined gypsum from the Monroe Group (Pointe Aux Chênes sediments). He also examined well records from several wells drilled in the St. Ignace area and observed that the distribution of gypsum was patchy geographically and stratigraphically, with some wells recording the presence of multiple gypsum-bearing horizons and others exhibiting no evidence of gypsum.
Lane and Seaman (1909) provided a detailed review of the stratigraphic section in Michigan, including new well data and outcrop samples. Lane’s concept of the Salina Formation (or the lower Monroe Formation) consisted of Silurian rocks above the Niagara Group and below the Bass Islands Group. The upper Monroe Formation was placed by Lane into the Devonian and was described from a series of outcrops along the Detroit River and in the subsurface from wells. In the upper part of the upper Monroe Formation, dolomite with sulfur and gypsum occurred as part of the Lucas Beds. Cook (1914) reported on brine-bearing units in the Carboniferous and Devonian Systems of Michigan (Parma Sandstone, Marshall Sandstone, Berea Sandstone, Dundee Formation, and the upper Monroe Formation [Detroit River Group]), as well as the rock salt deposits of the Salina Formation. His descriptions came from well borings in the upper Salina Formation in southeastern Michigan. He described the Salina Formation as consisting mainly of limestone, “interbedded with dolomitic limestones and bands of salt and anhydrite. The beds of salt vary from a fraction of a foot to several hundred feet in thickness” (p. 86).
Cohee (1965) briefly described gypsum deposits in the Pennsylvanian and Jurassic intervals of the Michigan Basin; however, they are volumetrically insignificant and have never been commercially utilized.
STRATIGRAPHIC OCCURRENCE OF EVAPORITIC FACIES
Ordovician Evaporites of the Michigan Basin
Ordovician evaporites were first observed in Michigan Basin sedimentary deposits when core was collected in the drilling of the Brazos et al.-State Foster #1 well (permit 25099) in Ogemaw County in 1964. The core recovered from this well contained over 450 m (1350 ft) of mixed carbonate, siliciclastic, and evaporitic sedimentary rock interpreted as having been deposited in a peritidal system (Catacosinos, 1973; Lynch, 1992; Harrison and Grammer, 2012).
Catacosinos (1973) defined the “Anhydrite Member” of the Munising Formation from the core in this well. He described the anhydrite as nodules and thin lenses interbedded with sandstones and pelletal and stromatolitic dolomites. He interpreted the deposits to be of shallow-marine origin, in lagoonal to supratidal environments. Catacosinos (1973) dismissed reports that Ordovician ostracodes were present in the interval and assigned the “Anhydrite Member” to the Cambrian Munising Formation.
Fisher and Barratt (1985) provided an updated description of the cored interval of the Brazos St. Foster #1 and included stratigraphic descriptions from the more recently cored JEM-Bruggers #3-7 (permit 34078). They assigned the lower interval of the Brazos St. Foster #1 to a new stratigraphic formation—the Foster Formation. The Foster Formation was defined as dominantly dark-gray dolomitic siltstone, black shale, and dark gray dolostone. Nodular and bedded anhydrite (Lynch, 1992) was observed through the entire cored interval from 3.55 to 3.96 km (11,637–12,996 ft; for example, see Fig. 3C). Examination of the density wireline log from the State Foster #1 well in the cored interval showed that 21.3 net meters (70 net feet) of strata within the cored interval had a bulk density greater than 2.9 g/cm3. That high value for density is generally assumed, in a sedimentary rock succession, to contain a significant amount of anhydrite (Fig. 3A). These high-density layers can be correlated to anhydrite-bearing zones (Fig. 3C) in the core that are at the top of meter-scale shallowing-upward depositional cycles (Lynch, 1992; Harrison and Grammer, 2012). The Foster Formation was composed of repeated, shallowing-upward cycles consisting of quartz sandstone, laminated mudstone, pelletal carbonates, oolitic grainstones, and intraclastic grainstones, with capping algal laminates/laminated or nodular anhydrite (Fig. 3B), with an average cycle thickness of 2.6 m (9 ft). These cycles are readily distinguishable in gamma-ray wireline logs (Fig. 3A), with the sandstones and clean carbonates having low gamma-ray values and the organic-rich mudstones and tidal flat caps showing increased gamma-ray counts on wireline logs. Harrison and Grammer (2012) recognized 144 depositional cycles in the Foster Formation in the Brazos St. Foster #1 core and interpreted them as likely fifth-order cyclicity. Ordovician conodonts from the State Foster #1 core reported by Repetski and Harris (1981) showed the upper Foster Formation to be Whiterockian in age and the lower Foster to be Ibexian in age. Fisher and Barratt (1985) assigned the Foster Formation to the Prairie du Chien Group of Wisconsin, with the upper Foster Formation correlating to the Shakopee Dolomite.
Silurian Evaporites of the Michigan Basin
During the Silurian, the Michigan Basin experienced fluctuating degrees of restriction, which led to evaporation and the development of substantial evaporite deposits. Small amounts of primary evaporites (gypsum) as well as secondary, diagenetic evaporite minerals (gypsum or anhydrite and halite) have been observed in pre–Salina Group strata; however, the extensive salt and anhydrite formations of the Salina Group have been the most thoroughly studied of all the evaporite deposits in the Michigan Basin and will be the main focus of this review. Minor amounts of nodular anhydrite have been observed from cores in Early Silurian Cataract Group sediments, especially the Cabot Head Shale. In addition, diagenetic gypsum and anhydrite have been observed in both the Cataract and Burnt Bluff Groups, in both the subsurface and outcrop. These diagenetic sulfates include vug-filling gypsum cements, satin spar veins, and anhydrite laths (Ehlers and Kesling, 1957; Ehlers et al., 1967; Voice and Harrison, 2014). At the end of the Silurian, the Bass Island Group, in the subsurface, has nodular anhydrite identified from core and an interval on the bulk density wireline log (Harrison et al., 2009). Secondary halite has been observed throughout the Silurian package in Michigan as vug- and fracture-filling cements.
Salina Group Evaporites
Sonnenfeld and Al-Aasm (1991) provided a detailed historical overview of the Salina Group rocks. The Salina Group in Michigan was formally subdivided by Landes (1945) into the A through G Units. Landes (1945) treated the Bass Islands Group as the overlying H Unit. The Salina Group consists of interbedded carbonate, anhydrite, rock salt, and shales and becomes more dominantly shale to the north, where it transitions into the Pointe aux Chênes Formation that outcrops in the southernmost Upper Peninsula of Michigan. The Pointe Aux Chênes Formation may also be traced a short distance into the northernmost Lower Peninsula of Michigan using wireline logs. Evans (1950) further subdivided the A Unit into the A-1 and A-2 units, and subsequent workers have further subdivided the A-Unit into the A-0 Carbonate, A-1 Evaporite, A-1 Carbonate, A-2 Evaporite, and A-2 Carbonate (Gill, 1977; Budros, 1974). During deposition, the Michigan Basin reached halite saturation during the deposition of the A-1 Evaporite, A-2 Evaporite, B Evaporite, D Salt, and F Salt and reached sylvite saturation only during A-1 Evaporite deposition in the northern central basin. The C Shale, E Unit, and G Unit are dominated by shales and shaley dolomites with interbedded anhydrite and minor halite.
Lower Salina: A-1 Evaporite, A-2 Evaporite, and B Evaporite. The Salina A-1 Evaporite is the basal evaporitic unit in the Salina Group. In basinal and shelf-margin settings, it has a gradational lower contact with the underlying A-0 Carbonate (Cain Formation), which is a restricted, laminated micritic carbonate that grades upward over several feet into millimeter-scale laminae of carbonate and anhydrite that then grade further upward into millimeter-scale laminae of anhydrite and carbonate interbedded with decimeter-scale beds of halite (Fig. 4A). The halite often shows chevron crystal growth forms (Fig. 4B), suggesting shallow subaqueous deposition (Schreiber and El Tabakh, 2000). Sylvite is interbedded with halite (Fig. 5) throughout the upper half of the A-1 Evaporite in the north-central part of the basin. Chemical assays of cores from the producing area show these deposits to be one of the highest-grade potash deposits in the world, at an average of nearly 45% KCl by volume (30% K2O).
Elowski (1980) mapped “fingers” of thin sylvite beds along the basinward edge of the northern “pinnacle reef” trend, suggesting that the sylvite-producing brines extended a short distance into the reef trend. Matthews (1970) suggested that potash occurred in parts of 22 counties covering over 13,000 mi2 (over 33,700 km2). Recent mapping for this study showed commercial volumes of potash occur in only about eight counties in north-central Michigan (~11,700 km2). There are two main target horizons that have been evaluated for commercial development of potash in Michigan. The western area of the sylvite occurrence is limited to a single unit, informally named the “Borgen Bed,” near the top of the Salina A-1 Evaporite interval. It is generally one continuous bed 3–10 m (10–30 ft) thick. Average estimated sylvite content in the “Borgen” interval from core observation is 55% KCl or 35% K2O. The other potassic zone is an interval informally termed the “Basin-Centered Beds” that occur several hundred feet stratigraphically below the top of the Salina A-1 Evaporite. This interval has numerous thin beds (centimeters to meters thick) in a gross interval 21.3–45.7 m (70–150 ft) thick with net log measured sylvite bed thickness of 12–21 m (40–70 ft). The average estimated sylvite content calculated from core observation is 43.8% KCl or 27.7% K2O.
The A-1 Evaporite attains a thickness of over 136 m (450 ft; Fig. 6A) in the basin center, where it is predominately halite or halite interbedded with sylvite in the central and northwestern portions. The evaporite interval gradually thins toward the basin margin, where it reaches a zero edge at the Niagaran platform shelf edge. The Niagaran “pinnacle reefs” that lie on the Niagaran slope, basinward of the shelf edge, have A-1 Evaporite between them, but the A-1 Evaporite does not overtop these reefs. In the southern portion of the reef trend, the A-1 Evaporite is exclusively anhydrite, but in the northern trend, the A-1 Evaporite is anhydrite near the reefs but grades into halite a short distance away from the reefs.
Nurmi and Friedman (1977) also mapped the extent of the sylvite facies in the center of the basin, as well as the limits of the Salina A-1 Evaporite, Salina A-2 Evaporite, and B Evaporite deposits. Throughout most of the basin, the B Evaporite has the greatest areal extent, and the A-1 Evaporite covers the smallest area. This pattern is reversed in the southwestern portion of the basin due to “leach-back” dissolution of the salt edges (Mesolella and Weaver, 1975). This dissolution has created collapse structures in the overlying A-1 Carbonate, A-2 Carbonate, and the Devonian Dundee and Traverse Limestones. The collapse toward the updip margin of the basin created closure for hydrocarbon traps in numerous fields in southwestern Michigan (Mesolella and Weaver, 1975).
The Salina A-2 Evaporite has similar thickness and areal distribution (Fig. 6B) to the A-1 Evaporite, although it occupies slightly more of the basin, especially in the southeastern region (Figs. 6A, 6B, and 6C). Also, the A-2 Evaporite is more uniform, at ~120 m (over 400 ft) thick in the eastern part of the basin. Additionally, the A-2 Evaporite has no known sylvite beds. Anhydrite is the predominate lithology of the A-2 Evaporite overtopping the Niagaran “pinnacle reefs” and near the shelf platform margins. Anhydrite is also widespread across the southern margin of the basin, associated with the off-reef–interreef areas of the Southern Pinnacle reef trend. The lower gradient to the southern Niagaran shelf to basin ramp/slope allowed for widespread shallow water during the sea-level drawdown during deposition of Salina A-1 Evaporite, producing extensive restricted and evaporitic tidal flats. The steeper seafloor gradient in the Northern Pinnacle reef trend allowed halite-saturated waters to invade the areas adjacent and between many of the reefs. A-1 Anhydrite deposition is restricted to areas very near the reefs and along their flanks.
The B Evaporite is the most extensive of the Lower Salina evaporite units and has a connection to the Northern Ohio salt basin through the southeastern corner of Michigan (Fig. 6C). The B Evaporite is divided into a lower “B Salt” and an upper “B Unit” that is composed of interbedded salt, carbonate, and anhydrite. The “B Salt” is predominately halite, which is commonly very coarsely recrystallized (Fig. 7A), distorting and obscuring the original depositional textures. However, in some intervals, possible primary bottom growth gypsum crystals pseudomorphed by halite may be preserved, suggesting original deposition of these evaporites in shallow water (Figs. 7B and 7C). The “B Unit” is easily distinguished on gamma-ray logs, where the carbonate/anhydrite beds have a high gamma-ray signature that stands out from the low gamma-ray salt beds. The B Evaporite thickness is nearly the same as the A-1 and A-2 Evaporite thicknesses at over 135 m (450 ft; Fig. 6C).
Upper Salina: C Shale, D Salt, E Unit, and F Salt. The C Shale does not contain much primary evaporite; however, near the top of the interval, there is some nodular anhydrite that has been observed in cores. The C Shale is mostly composed of calcareous mudstone in intervals of anhydrite and dolomite. The C Shale in the Salina Group of Michigan has distinctive orange to reddish-brown salt that mainly fills secondary fractures (Fig. 8A). The C Shale is easily distinguished on gamma-ray wireline logs by its relatively high gamma-ray values (>70 API [American Petroleum Institute] units) compared to the overlying D Salt and the underlying B Evaporite at very low values (<10 API units).
The D Salt is the thinnest of the major Salina evaporite units (Fig. 6D), with a maximum thickness of ~23 m (76 ft) in the area just west of Saginaw Bay. There is a dense dolomitic carbonate that divides the unit into the upper and lower D Salt (Sonnenfeld and Al-Aasm, 1991). This carbonate unit is easily observed on gamma-ray wireline logs. In a core from foundation test boring TB-7 for the Detroit International Bridge Crossing, Wayne County (southeastern Michigan), the carbonate interval is ~3 m (10 ft) thick and composed of structureless carbonate mudstone in the lower portion and continuous, small, laterally linked hemispheroidal stromatolites in the upper part (1.5 m [5 ft]). The D Salt in both the upper and lower units is very coarsely recrystallized, and primary sedimentary structures have been obscured. Core observation shows that the D Salt (Fig. 8B) in southeastern Michigan has very similar fabric characteristics to the “B Salt” in that same area (Fig. 7A).
The E Unit is also not primarily an evaporitic unit, being mostly fine-grained dolomitic carbonate and minor amounts of shaley carbonate mudstone, similar to the dominant lithology in the C Shale. There are, however, significant intervals in the E Unit of primary bluish-gray nodular and bedded anhydrite, and some nodules and secondary fractures filled with reddish-brown salt similar to the fracture-fill salt in the C Shale (Fig. 8C).
The F Salt is the uppermost evaporite-prone stratigraphic unit in the Michigan Salina Group and is, by far, the thickest, reaching 330 m (1100 ft) near the southwestern edge of Saginaw Bay in Bay County (Fig. 6E). The F Salt interval is composed of from three to six halite beds separated by dolomitic and anhydritic intervals. The F Salt is one of the more intensively studied of the Salina evaporite units (Brennan and Lowenstein, 2002; Satterfield et al., 2005), mostly because it is readily accessible in the Detroit underground salt mine, where the “F-2 Salt” bed, which is the second salt package from the bottom of the F Salt interval, is currently being mined. The entire F Salt interval is preserved in the Detroit River International Crossing Bridge foundation core number TB-7. In that core, the “F-1 Salt” bed is coarsely recrystallized, even showing recrystallized crystal displacement and disturbance of some of the interbedded carbonate laminae (Fig. 9A). In the “F-2 Salt” interval of the TB-7 core, the salt is less recrystallized and shows some small, vertically oriented crystals emanating from a thin seam of anhydrite. This growth form suggests bottom growth halite in shallow water after the deposition of the thin anhydrite layer (Fig. 9B). Salt deposition must have been frequently punctuated with freshening of the seawater because of the thin interbedded laminae of anhydrite that occur throughout the salt strata (Fig. 9C). Other evidence of shallow water has also been reported from the “F-2 Salt” bed in the Detroit underground salt mine by Kaufmann and Slawson (1950). They observed extensive ripple marks on surfaces at the contact between thin salt beds and anhydrite laminae. Dellwig (1955) also reported these ripple marks in layers from the Detroit salt mine; additionally, he reported hopper crystals of salt being found throughout the mine (Dellwig, 1953, 1955). Dellwig indicated that the hopper crystals had to have formed at the surface of a salt-saturated water body and then settled to the bottom, suggesting some depth to the water column. Examination of core material from wells and the rock walls in the Detroit mine show frequent fluctuation in evaporite mineral deposition from halite to anhydrite, even to dolomite, indicating numerous and substantial variations in seawater salinity during F Salt deposition.
The Salina Group in Michigan represents the most lithologically diverse, intensively researched, and economically important evaporitic depositional package in the basin. The abundance of salt in the group has been a significant economic driver for Michigan’s industrial mineral portfolio for over 150 yr. The Salina Group deposits reach a maximum of over 900 m (3000 ft) in the depocenter of the basin (Fig. 6F). Careful mapping of lithofacies distribution and sequence stratigraphic patterns can help to define depositional environment distribution and extreme changes in ocean water chemistry within the basin through this time interval of the Upper Silurian. Dramatic sea-level fluctuations occurred during the deposition of the Salina Group, which reflect global eustatic perturbations as well as important climatic enhancement of evaporative drawdown within the basin. The thickness and diversity of depositional environments showcase the Salina Group of the Michigan Basin as a “classic” package of rocks from Earth’s past that can help us to understand global processes and their climatic impact on Earth’s surface environments.
Bass Islands Group Evaporites
The Upper Silurian Bass Islands Group in the subsurface of the Michigan Basin is not differentiated into formations. In the outcrop belts (southernmost Upper Peninsula and southeasternmost Lower Peninsula of Michigan), outcrops of the Bass Islands Group have been subdivided. In the Lower Peninsula of Michigan, the stratigraphic nomenclature follows the terms used in northern Ohio: the Greenfield, Tymochtee, Put-in-Bay, and Raisin River Formations (after Sparling, 1970). In the Upper Peninsula of Michigan, subsurface samples and clasts in the Upper Silurian–Lower Devonian Mackinac Breccia have been used to define the St. Ignace Formation (Landes et al., 1945) by micritic dolomites that are nonfossiliferous and include decussate molds of gypsum crystals. In the subsurface, the Bass Islands Formation in the Michigan Basin does contain beds of anhydrite, most likely in the stratigraphic equivalent of the Tymochtee Formation, which is known to contain substantial beds of anhydrite or gypsum (Sparling, 1970). No gypsum or anhydrite is known from outcrops in southeasternmost Michigan, but an extensive layer of anhydrite is widely mappable throughout the central and northern basin using wireline logs with a bulk density track. A core obtained from the Bass Islands interval in Otsego County (St. Charlton #4-30, permit 57916) recovered a nodular anhydrite interval near the top of the Bass Islands strata conformably underlying the upper Bass Islands dolomite (Fig. 10). This is consistent with the environmental interpretation by Sparling (1970) of a shallow, restricted peritidal system during Bass Islands deposition.
Devonian Evaporites of the Michigan Basin
Two intervals are characterized by thick sequences of evaporite deposits in the Devonian section of the Michigan Basin: the Lucas Formation of the Detroit River Group and the “Reed City Anhydrite” of the Dundee Formation. Significant work has been done on the Lucas Formation evaporites, while fewer studies have been done on the “Reed City Anhydrite.”
Lucas Formation, Detroit River Group
Prosser (1903) defined the Lucas Limestone in rocks outcropping in Lucas County, Ohio, and included all the rocks between the Sylvania Sandstone and the Dundee Formation (or Columbus Limestone). Lane et al. (1908) and Grabau and Sherzer (1910) further subdivided the Monroe Formation and proposed the upper part of the Monroe be called the Detroit River Series, with the Lucas Dolomite as the uppermost member in the Detroit River Series.
Landes (1951) defined the Richfield Member of the Lucas Formation as the dolomitic rocks at the base of the unit, separated from the dominantly evaporitic units in the upper Lucas Formation. Anhydrite was the more common evaporite mineral in the upper portion of the Lucas Formation, both vertically and laterally within the formation. Salt was observed in the upper half of the upper Lucas Formation. The total aggregated thickness of evaporite lithologies (salt and anhydrite) in the Lucas Formation is up to 122 m (400 ft) thick in the central basin (Missaukee and Roscommon Counties), though this value may include the “Reed City Anhydrite” (which Landes separated from the Dundee Formation).
Haney and Briggs (1964) provided the first detailed examination of the evaporites in the Lucas Formation. They recognized that the Lucas Formation consisted of repeated packages of dolomite, anhydrite, and halite, with the thicknesses of the packages ranging from 1 to 46 m (3–150 ft) thick. Gardner (1974) provided a well-illustrated, updated overview of Early to Middle Devonian units in the Michigan Basin from review of existing subsurface data (cores, cuttings, and wireline log data). Gardner proposed new stratigraphic terminology in order to further subdivide the Lucas Formation. His Lucas Formation included a lower Richfield Member, a middle Iutzi Member, and an upper Horner Member. He also placed the “Reed City Anhydrite” into the Dundee Formation in the western Lower Peninsula of Michigan. Gardner (1974) described the Richfield Member as consisting of a series of interbedded anhydrites and dolomudstones. He defined the Iutzi Member as the interbedded anhydrite and dolomite from the top of the Richfield Member to the base of the Horner Member. The Iutzi Member consisted of reticulate and nodular anhydrite in the lower half, with a transition toward micritic carbonates up section (especially in the basin center). The Horner Member, as defined by Gardner (1974), is a package of predominately evaporite strata (salt and anhydrite; Fig. 11E), underlain by the Iutzi Member and overlain by the Dundee Formation. He interpreted the Lucas Formation as a package of shallow subtidal to supratidal environments, including oolitic sand shoals, restricted lagoons, algal-dominated tidal flats, and sabkha environments. Gardner suggested that the massive and mosaic anhydrites were likely precipitated under subtidal conditions.
Matthews (1977) recognized evaporite cyclicity with beds grading from dolomitic carbonate to anhydrite to halite and then back to anhydrite and then carbonate as increasing and decreasing salinity cycles. He divided the Lucas Formation into the Richfield zone and “Massive Anhydrite,” “Lower Salts” or “Sour Zone,” “Massive Salt,” “Middle Salts,” and “Upper Salts.” The salt-containing cycles in the Horner Member of the Lucas Formation showed the best examples of this cyclicity (Fig. 11E). Matthews (1977) mapped at least eight salinity cycles that contained halite in the central Michigan Basin. Many other cycles in the Lucas only reach gypsum saturation (anhydrite).
Sullivan (1986) described four Richfield fields in northeastern Isabella County in the central basin. He recognized that the anhydrite in the Richfield Member exhibited relict textures from the primary gypsum crystals precipitated in the lagoonal–tidal flat–sabkha complex. He described palmate to vertically aligned, upward-growing crystals that he believed formed after subaqueous gypsum crystals (Figs. 11B and 11C). More equant nodules associated with microbial-laminated carbonates were interpreted as displacively grown gypsum crystals subsequently recrystallized to anhydrite (Figs. 11A and 11D).
Melvin (1989) recognized four major facies associations in the Richfield Member: subtidal, lower intertidal, upper intertidal, and supratidal. The subtidal facies included vertical and randomly aligned mosaic anhydrite, lime mudstones, oolitic grainstones, and pelletal carbonates, which were interpreted as forming in lagoonal environments. The lower intertidal facies consisted of pelletal carbonates with anhydrite laths, bioturbated dolomudstones with some authigenic anhydrite, and laminated algal mats with isolated laths. The upper intertidal facies consisted of mottled and algal-laminated dolomudstones with authigenic anhydrite and some eolian quartz. The supratidal facies consisted of nodular “chicken-wire” anhydrite and massive anhydrite with well-rounded, probably eolian quartz grains. Melvin (1989) interpreted the intertidal-supratidal facies as a tidal flat–sabkha complex.
Melvin (1989) interpreted the crystal morphology and fabric of the anhydrite in the Richfield Member to assist in assigning depositional environments to the facies. Melvin recognized relict palmate structures in the mosaic anhydrite of the subtidal facies; these crystals were aligned with the long axes perpendicular to the paleoseafloor, suggesting shallow subaqueous precipitation. Scattered anhydrite nodules in algal-laminated carbonates defined the intertidal facies. Coalesced anhydrite nodules with faint laminar bedding overlying the algal-laminated carbonates were interpreted as being deposited on a sabkha flat. The coalesced anhydrite nodules were truncated by erosional surfaces. In a few instances, Melvin (1989) recognized porosity that was plugged with halite crystals, usually associated with oomoldic porosity in the subtidal facies.
“Reed City Anhydrite” of the Dundee Formation
Landes (1951) provided a brief description of the “Reed City Anhydrite,” now an informal member of the Dundee Formation (Catacosinos et al., 2001). Landes considered the “Reed City Anhydrite” as part of the upper 31 m (100 ft) of the evaporite package of the Lucas Formation. Dolomitic zones beneath the “Reed City Anhydrite” are important hydrocarbon reservoirs in western Michigan. Hixon (1964) and Ehman (1964), as well as a report by Knapp (1947), were the earliest works we could find that included the Reed City Member as the lower part of the Dundee Formation in the western Lower Peninsula of Michigan. In the western basin, the Reed City Member overlies the Horner Member of the Lucas Formation and underlies the undifferentiated Dundee Formation, which is most likely the Rogers City Formation (Kirschner and Barnes, 2009). The Reed City Member of the Dundee Formation consists mainly of dolomite with an anhydrite layer near its top. The “Reed City Anhydrite” averages 6 m (20 ft) thick, but it may grade eastward into a paraconformity or hardground that is pyritized and bored (Curran and Hurley, 1992). The lithologic character of the “Reed City Anhydrite” suggests that it was primarily formed in sabkha settings in the west-central half of the Lower Peninsula of Michigan.
Carboniferous Evaporites of the Michigan Basin
As the early literature suggests, evaporites have been recognized in the Michigan Formation since the 1860s, due to their accessibility as surficial or shallow subsurface deposits. Unfortunately, the Mississippian Michigan Formation has been poorly studied since. Cohee (1979) described the Michigan Formation as an interval composed of interbedded gray to dark gray and greenish-gray shale, limestone, and dolomite, with thin lenses of sandstone, anhydrite, and gypsum. He assigned a Meramecian age to this unit and recognized that the Michigan Formation was underlain unconformably by the Marshall Sandstone and overlain conformably by the Bayport-Parma succession. The gypsum-anhydrite interval has an aggregate thickness of 12 m (40 ft) in the Grand Rapids (Kent County) and Alabaster (Iosco County) areas, but it thickens to 30 m (100 ft) in the central basin. Recent palynomorph data suggest that the Michigan Formation is actually Chesterian in age (Towne et al., 2013; see Towne et al., this volume).
Moser (1963) presented a detailed lithologic and petrophysical characterization of the Michigan Formation from subsurface data. He recognized the complex lithologic relationships within the Michigan Formation. Moser recognized three major intervals of anhydrite in the Michigan Formation that could be traced over large regions: the “National City Gypsum” (quarried at Alabaster), the “Triple Gyp” (consisting of three anhydrite beds interbedded with shale; quarried at Grand Rapids), and the “Pencil Gyp” (a red anhydrite bed, roughly 4 in. [0.1 m] thick, observed directly above the “Triple Gyp” in the mines). The rest of the Michigan Formation consisted of interbedded anhydrite, dolomite, and shale.
In a review paper, Briggs (1970) described the gypsum deposits of the Lucas Formation (Detroit River Group, Devonian) and the Michigan Formation (Mississippian). He described the Michigan Formation sequence as interbedded immature sandstones, shales, and evaporites (anhydrite or gypsum). He followed the classification of Moser (1963) in delineating three major gypsum intervals in the Michigan Formation. The “Triple Gyp” was the interval with the greatest areal extent in the Michigan Basin.
Van Regenmorter et al. (2008) briefly reported stratigraphic properties of the Michigan Formation in the gypsum mines in the Grand Rapids area. They observed repetitive successions of bedded and nodular gypsum, overlain by siltstones and dolomites. The siltstones and dolomites recorded a variety of marine (shark and other fish teeth, fish scales, coprolites, and occasional burrows) and terrestrial (plant) fossil fragments. Van Regenmorter et al. (2008) preferred a subaqueous origin for the gypsum deposits. Examination of core samples from the Michigan Formation in the central Michigan Basin show the interbedded nature of the evaporite deposits with siliciclastic layers. The morphology of the nodular anhydrite suggests pseudomorphing of the originally precipitated gypsum crystals. Vertically oriented geometries suggest subaqueous, bottom growth “swallowtail” crystals that were subsequently recrystallized as anhydrite (Fig. 12).
SUMMARY AND CONCLUSIONS
Evaporites in the Michigan Basin range in age from Lower Ordovician to Mississippian. CaSO4 is found through all of the evaporite-bearing sedimentary units and is anhydrite in the deeper subsurface units and is only gypsum in shallow Mississippian strata that have had contact with meteoric water to rehydrate it from its anhydrite phase formed during deeper burial. Halite is abundant in Silurian and Devonian strata and occurs in depositional cycles that indicate fluctuating water salinities during deposition. The evaporitic deposition only reached sylvite concentrations during the latter part of deposition of the Lower Salina A-1 Evaporite.
Preservation of primary depositional fabrics in the gypsum and anhydrite shows a shallow subtidal to sabkha growth pattern of both “bottom growth” “swallowtail” crystals and displacive growth within laminated, stromatolitic, and massive micritic carbonate sediment deposits. The halite beds are more extensively recrystallized, obscuring the primary depositional fabric. However, occasionally, original halite crystal fabrics are preserved and show chevrons and other crystal growth features from carbonate or anhydrite bedding planes that indicate bottom crystal growth in shallow water. The cyclicity in evaporite mineralogy from carbonate to anhydrite to halite in the Devonian Lucas Formation and various Silurian Salina Group units suggests frequently fluctuating salinities, which would be more common in shallow-water rather than deep-water depositional systems. It must also be remembered that evaporites are deposited far more rapidly than both carbonates and siliciclastics, so that the great evaporite thicknesses do not represent equally long spans of time (Schreiber and Hsü, 1980).
Michigan Basin evaporites have had significant impact on the economic value of industrial mineral production in Michigan since the late nineteenth century. Mining of salt and gypsum is estimated to have had a value of over $15.5 billion in 2013 dollars. Nearly half of the oil and gas reservoirs in the Michigan Basin owe their seals to evaporites, and the oil and gas industry has produced over $35 billion in historic wellhead value.
We wish to thank Charlotte Schreiber and Ted Pagano for reviewing this manuscript; their comments greatly improved the paper. In addition, Charlotte Schreiber generously provided encouragement in proposing the paper. Discussions and viewing selected salt cores with her greatly improved our understanding of evaporite sedimentology. Jennifer Trout and Linda Harrison assisted greatly in selecting cores to highlight in this review. Linda Harrison provided photographs and advice for photographing salt cores, which were greatly appreciated. Andrew Caruthers provided additional insight and valuable discussions on various Salina units. The Michigan Geological Repository for Research and Education, part of the Michigan Geological Survey at Western Michigan University, provided access to all the core, samples, and data used in this study.
Figures & Tables
Paleozoic Stratigraphy and Resources of the Michigan Basin
- chemically precipitated rocks
- Lucas Formation
- Michigan Basin
- mineral composition
- mineral economics
- North America
- sedimentary rocks
- United States