Introduction:

The principal bedrock units found in Marion County are composed of Paleozoic limestone, dolostone, siltstone, shale, and sandstone ranging from the Silurian to Mississippian age (Hasenmueller, 2003a, b). These rocks dip to the southwest at 10 to 20 ft per mile (3.0 to 6.1 m per 1.6 km), with the dip increasing from the northeast to southwest. The dip of the rock is influenced by the position of Marion County in relation to two structural features: the Illinois Basin, a structural low, to the southwest, and the Cincinnati Arch, a structural high, to the northeast (fig. 1) (click to view IGWS web page on the Tectonic Features of Indiana). Because of the southwesterly dip, the oldest units at the bedrock surface occur in the northeastern part of the county, whereas the youngest units occur in the southwestern part of the county (Fleming, Brown, and Ferguson, 2000).

The Fortville Fault, an inactive normal fault downthrown to the southeast, extends into southeastern Marion County. Normal faults form when the crust is being extended instead of being compressed. The fault trace, or the intersection of the Fortville Fault with the bedrock surface, was slightly modified by Hasenmueller (2003a, b) from previously published maps of the area (Hartke and others, 1980; Gray, Ault, and Keller, 1987; and Hasenmueller and James, 2002). Displacement on the fault is as much as 60 ft (18.3 m).

Stratigraphic units:

Overview

The following rock units were mapped in Marion County by Hasenmueller (2003a, b) and are listed from youngest to oldest: (1) upper part of the Borden Group (Mississippian) (Edwardsville and Spickert Knob Formations undifferentiated), (2) New Providence Shale (Mississippian), (3) Rockford Limestone (Mississippian) and New Albany Shale (Devonian and Mississippian) undifferentiated, (4) Muscatatuck Group (Devonian), (5) Wabash Formation (Silurian), and (6) Pleasant Mills Formation (Silurian) (fig. 2). Information about the characteristics of these buried bedrock units was obtained from petroleum well drillers' logs, water well drillers' logs, geophysical logs, exposures in quarries, and cores and seismic refraction records collected and maintained by the Indiana Geological Survey. Because the bedrock surface is usually overlain by 50 ft (15.2 m) to more than 300 ft (91.4 m) of unconsolidated glacial drift, the bedrock geology map of Marion County is a summary and interpretation of the geologic information available in the public domain (Hasenmueller, 2003a, b).

Edwardsville and Spickert Knob Formations undifferentiated (Borden Group) and New Providence Shale (Borden Group)

The youngest unit at the bedrock surface in Marion County is the Borden Group of Mississippian age (fig. 3). The stratigraphic unit is chiefly greenish-gray shale and siltstone and fine-grained sandstone (Rexroad, 1986a; Indiana Geologic Names Information System, 2013). The Borden Group forms the bedrock surface in the southwestern part of Marion County (Fleming, Brown, and Ferguson, 2000; Hasenmueller, 2003a); however, a complete section of the Borden is not present within the county. As much as 250 to 300 ft (76.2 to 91.4 m) of the unit is present below some elevated parts of the bedrock surface (Fleming, Brown, and Ferguson, 2000).

The upper part of the Borden Group is composed of the Edwardsville and Spickert Knob Formations, which consist of siltstone, shale, and fine-grained sandstone with minor amounts of limestones (Hasenmueller, 2003a, b). In Marion County, the two formations are difficult to differentiate and are mapped as a single interval. The thickness of the upper part of the Borden ranges from zero feet at its erosional edge to a maximum eroded thickness of 98 ft (29.9 m). The sandstone bodies in the upper portions of the unit form locally productive aquifers (Fleming, Brown, and Ferguson, 2000). The lower part of the Borden is composed of the New Providence Shale, a greenish-gray to dark-gray shale that borders on claystone (Rexroad, 1986b) (fig. 4). The New Providence ranges in thickness from zero at its erosional edge to a maximum of 227 ft (69.2 m) in the southwestern part of Marion County. The greater part of the Borden Group consists chiefly of low-permeability shale and siltstone and acts as a confining unit (Fleming, Brown, and Ferguson, 2000).

Rockford Limestone and New Albany Shale undifferentiated

The Rockford Limestone of Mississippian age underlies the Borden Group. In a bedrock core from Marion County, the Rockford Limestone is a light gray to light brownish-gray limestone and is about 7 ft (2.1 m) in thickness. It is fossiliferous and is characterized by green mottling (fig. 5). Additional information about the stratigraphy and distribution of this limestone throughout Indiana can be found at the Indiana Geologic Names Information System Web page (https://igs.indiana.edu/IGNIS/).

Because the Rockford Limestone is rarely more than 10 ft (3.0 m) thick, it was too thin to map as a separate unit and was mapped with the New Albany Shale in Marion County (Hasenmueller, 2003a, b). The New Albany Shale of Devonian and Mississippian age underlies the Rockford Limestone and overlies the Devonian carbonate rocks. It consists of brownish-black carbon-rich shale, greenish-gray shale, and minor amounts of dolostone and dolomitic quartz sandstone (Lineback, 1968, 1970; Hasenmueller, 1986; Indiana Geologic Names Information System (https://igs.indiana.edu/IGNIS/) (fig. 6). The unit is typically between 110 and 130 ft (33.5 and 39.6 m) thick in the Marion County area (Hasenmueller and Bassett, 1979) and is poorly permeable. It forms the lower part of a thick confining unit that also includes siltstone and shale of the overlying Borden Group (Fleming, Brown, and Ferguson, 2000).

There are no natural outcroppings of the New Albany Shale in Marion County; however, the lower 40 ft (12.2 m) of the formation are exposed in the Harding Street Quarry near the junction of Interstate 465 and State Road 37 on the south side of Indianapolis. Here, the shale is removed as overburden in the quarrying of the underlying Devonian carbonates, which are a source of crushed stone in the greater Indianapolis area (fig. 7).

Muscatatuck Group

The Devonian Muscatatuck Group is composed of two formations, the North Vernon Limestone and the Jeffersonville Limestone. The North Vernon Limestone is medium light gray to yellowish-gray, fine- to medium-grained, slightly shaly limestone (Becker, 1974). In central Indiana, the Jeffersonville is composed of light to medium gray to brownish-gray, fine-grained dolostone and limestone in the upper part and pale brown, finely to medium crystalline dolostone in the lower part (Droste and Shaver, 1975). Scattered, frosted, and well-rounded quartz sand grains are a diagnostic constituent in the laminated beds (Becker, 1974). Because these two formations are thin and not well documented in the study area, they were mapped as a single unit (Hasenmueller, 2003a, b). Throughout Marion County the Muscatatuck overlies the Wabash Formation of Silurian age.

Wabash Formation

The Wabash Formation is composed of two members, these are, in descending order, the Liston Creek Limestone and Mississinewa Shale Members. At its type section in Wabash County, Indiana, the Liston Creek is a limestone and dolomitic limestone that is light gray and tan, fine- to medium-grained, and contains fossil fragments (Droste and Shaver, 1986a) (fig. 8). The type lithology is present in the counties immediately surrounding the type locality, but Mississinewa-like rocks are intimately interspersed in places. Like the Mississinewa, the Liston Creek has a reef facies (Huntington Lithofacies) that continues stratigraphically upward without break from the Mississinewa into the Liston Creek. These three lithologies (Mississinewa, Liston Creek, and reef facies) are so intimately mixed in many areas that the Wabash members are not separable (Droste and Shaver, 1986a). The Wabash boundaries were extrapolated where the Silurian reef facies replace the Liston Creek Limestone and Mississinewa Shale Members (Hasenmueller, 2003a, b).

The contact of the Wabash Formation with the overlying Muscatatuck Group is a regional unconformity between the Silurian and Devonian rocks (Hasenmueller, 2003a, b) (fig. 3).

Pleasant Mills Formation

The Pleasant Mills Formation is composed of dolostone, limestone, and argillaceous dolostone (fig. 9). The formation was named by Droste and Shaver (1982) for exposures of dolomitic rocks in the Meshberger Bros. Stone Corp. quarry 3 miles (4.8 km) south of Pleasant Mills, Adams County, Indiana (Droste and Shaver, 1986b). As defined by Droste and Shaver (1986b) the Pleasant Mills consists of the following members in ascending order: the Limberlost Dolomite Member, the Waldron Member, and the Louisville Member. It was rarely possible to differentiate the Limberlost Dolomite and the underlying Salamonie Dolomite carbonates in the Marion County mapping area (Hasenmueller, 2003a, b). Hasenmueller (2003a, b) noted that the lithologic change that marks the base of the Waldron Shale Member of the Pleasant Mills Formation is distinct and commonly noted in drillers' logs and other geologic records from the Marion County area. Therefore, he restricted the Pleasant Mills Formation to the rocks from the top of the Louisville Limestone Member to the base of the Waldron Shale Member and mapped the Limberlost Dolomite as part of the Salamonie Dolomite. Thus, both the upper and lower contacts of the Pleasant Mills Formation as defined by Hasenmueller (2003a, b) are conformable.

References

Becker, L. E., 1974, Silurian and Devonian rocks in Indiana southwest of the Cincinnati Arch: Indiana Geological Survey Bulletin 50, 83 p.

Droste, J. B., and Shaver, R. H., 1975, The Jeffersonville Limestone (Middle Devonian) of Indiana-stratigraphy, sedimentation, and relation to Silurian reef-bearing rocks: American Association of Petroleum Geologists Bulletin, v. 59, p. 393-412.

Droste, J. B., and Shaver, R. H., 1982, The Salina Group (Middle and Upper Silurian) of Indiana: Indiana Geological Survey Special Report 24, 41 p.

Droste, J. B., and Shaver, R. H., 1986a, Liston Creek Limestone Member, in Shaver, R. H., Ault, C. H., Burger, A. M., Carr, D. D., Droste, J. B., Eggert, D. L., Gray, H. H., Harper, Denver, Hasenmueller, N. R., Hasenmueller, W. A., Horowitz, A. S., Hutchison, H. C., Keith, B. D., Keller, S. J., Patton, J. B., Rexroad, C. B., and Wier, C. E., Compendium of Paleozoic rock-unit stratigraphy in Indiana-a revision: Indiana Geological Survey Bulletin 59, p. 81-82.

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