Bedrock Geology by Nancy R. Hasenmueller

Figure 1.
Map showing regional tectonic features.

The principal bedrock units found in Allen County are composed of Paleozoic limestone, dolomite, and shale ranging in age from the Silurian to Upper Devonian. (Limestone is composed of calcium carbonate, dolomite is composed of calcium-magnesium carbonate, and shale is composed of clay and some silt.) The predominantly limestone and dolomite bedrock was deposited in an ancient sea basin centered in Michigan (Bleuer and Moore, 1978). These marine rocks dip, or are slightly tilted, to the north.

The dip of the bedrock ranges from 10 ft per mile in the southern part of the Allen County to 30 ft in the northeastern part of the county (Fleming, 1994). The dip of the rock is influenced by the position of Allen County in relation to two structural features: the Michigan Basin, a structural low, to the north, and the Cincinnati Arch, a structural high, to the south (fig. 1) (Rupp, 1997).

Rupp and others (1994) mapped several faults in Allen County. The large fault in northeastern Allen County can be demonstrated to have at least 75 ft of offset of bedrock strata, but the history of faulting cannot be conclusively established from current data. The fault is clearly post-Late Devonian, but whether it has had more recent episodes of motion similar to other better-studied faults in the region is currently unknown.

Figure 2.
Map showing the bedrock geology of Allen County. From Fleming (1994).

Nowhere in the county is bedrock exposed at the land surface. In the northwestern part of the county, the bedrock is covered by as much as 300 ft of glacial material; to the south the glacial materials thin and bedrock lies 60 to 80 ft below the surface in much of southern Allen County. Bedrock is typically less than 50 ft below the surface under much of eastern Allen County, corresponding generally to the Maumee Lacustrine Plain, and is about 30 ft below the surface where the Maumee River and Flatrock Creek cross the state line into Ohio.

Four rock units have been mapped by Fleming (1994) in Allen County (fig. 2). Information about the characteristics of the buried bedrock units is obtained by geologists through the examination of drill cuttings and cores, geophysical logs taken from well bores, and rock exposures in limestone quarries in the county (fig. 3). The youngest unit exposed at the bedrock surface is the Upper Devonian Antrim Shale (fig. 4).

The Antrim is predominantly brownish-black shale; however, in some places medium-gray calcareous shale or limestone is in the lower part of the unit (Hasenmueller, 1986). The Antrim is present in northern and central Allen County where it forms the bedrock surface; the formation ranges from zero to 90 ft in thickness (Fleming, 1994). The Antrim Shale, both conformably and with minor unconformity, overlies the Traverse Formation (Devonian) according to Orr (1971).

Figure 3.
An exposure of limestone and dolomite of the Muscatatuck Group (Devonian) and the Salina Group (Silurian) in a quarry in Allen County. The contact between the units is shown by a black line.

The Traverse Formation (Devonian) consists of a variety of limestones, some dolomite, and thin shale beds (Droste and Shaver, 1986a) and unconformably overlies the Detroit River Formation. The unit reaches a maximum thickness of about 75 ft in northern Allen County (Fleming, 1994). The Detroit River Formation (Devonian) is composed of dolomite, limestone, and evaporite rocks (gypsum and anhydrite) (Droste and Shaver, 1986b). The thickness of the Detroit River ranges from 35 ft in southern Allen County to 70 ft in the northern part of the county (Fleming, 1994).

The oldest unit at the bedrock surface in Allen County is the Salina Group of Silurian age. The Salina includes a variety of dominantly carbonate rocks ranging from fine-grained shaly rocks to coarse-grained fossiliferous rocks Droste and Shaver, 1986c). The unit forms the bedrock surface in southernmost Allen County and ranges from 400 to 600 ft in thickness (Fleming, 1994).

The dolomitic limestone bedrock and the sand and gravel in the overlying glacial sediment are sources of aggregate in Allen County (Moore and Ault, 1978). Crushed stone is used as fundamental materials for most structures and roads in Indiana.





System Group Formation view image
Devonian   Antrim Shale Core Example Photograph of a core of brownish-black to black, organic-rich calcareous shale of the Devonian Antrim Shale from a well near Helmer in Steuben County, Indiana.
Muscatatuck Group Traverse Formation Core Example
Photograph of a core of light-gray to tan medium-grained crystalline dolomite from the Devonian Traverse Formation in Allen County. Note the vugs and residual oil associated with the vugs. Photograph of a core of light-tan fine-grained limestone from the Devonian Traverse Formation in Allen County. Note the colonial coral.
Detroit River Formation Core Example Photograph of a core of brown, fine-grained dolomite from the Devonian Detroit River Formation in Allen County.
Silurian Salina Group Wabash Formation Salina Group
Core Photograph of a core of light gray, fine-grained, slightly vugular dolomite from the Silurian Salina Group in Allen County.
Pleasant Mills Formation
Figure 4. Stratigraphic column showing the relationships of Paleozoic rock-units
that occur in Allen County. Additional stratigraphic information about these
units can be found at the Indiana Geological Survey
Indiana Geologic Names Information System Web page.




References:

Bleuer, N. K., and Moore, M. C., 1978, Environmental geology of Allen County, Indiana: Indiana Geological Survey Special Report 13, 72 p.

Droste, J. B., and Shaver, R. H., 1986a, Traverse Formation, in Shaver, R. H., and others, 1986, Compendium of Paleozoic rock-unit stratigraphy in Indiana-a revision: Indiana Geological Survey Bulletin 59, p. 156-157; Indiana Geological Survey Web page, date accessed March 8, 2007.

Droste, J. B., and Shaver, R. H., 1986b, Detroit River Formation, in Shaver, R. H., and others, 1986, Compendium of Paleozoic rock-unit stratigraphy in Indiana-a revision: Indiana Geological Survey Bulletin 59, p. 35-37; Indiana Geological Survey Web page, date accessed March 8, 2007.

Droste, J. B., and Shaver, R. H., 1986c, Salina Group, in Shaver, R. H., and others, 1986, Compendium of Paleozoic rock-unit stratigraphy in Indiana-a revision: Indiana Geological Survey Bulletin 59, p. 133-135; Indiana Geological Survey Web page,  date accessed March 8, 2007.

Fleming, A. H., 1994, The hydrogeology of Allen County, Indiana–a geologic and ground-water atlas: Indiana Geological Survey Special Report 57, 111 p.

Hasenmueller, N. R., 1986, Antrim Shale, in Shaver, R. H., and others, 1986, Compendium of Paleozoic rock-unit stratigraphy in Indiana-a revision: Indiana Geological Survey Bulletin 59, p. 5; Indiana Geological Survey Web page, date accessed March 8, 2007.

Moore, M. C., and Ault, C. H., 1978, Mineral Aggregates, in Bleuer, N. K., and Moore, M. C., 1978, Environmental geology of Allen County, Indiana: Indiana Geological Survey Special Report 13, p. 21-31.

Orr, R. W., 1971, Conodonts from Middle Devonian strata of the Michigan Basin: Indiana Geological Survey Bulletin 45, 110 p.

Rupp, J. A., 1997 [Copyright date], Tectonic features of Indiana: Indiana Geological Survey Web page, <https://igs.indiana.edu/structuralFeatures/tectonic.cfm>, date accessed March 21, 2007.

Rupp, R. F., Steen, W. J., and Fleming, A. H., 1994, Map of Allen County, Indiana showing the topography and geology of the bedrock surface, in Fleming, A. H., The hydrogeology of Allen County, Indiana-a geologic and ground-water atlas: Indiana Geological Survey Special Report 57, plate 2.

Indiana Geological Survey Geologic Names Committee, 2009, Geologic Names Information System: Indiana Geological Survey Web page, <https://igs.indiana.edu/geology/geologicNames/>, date accessed October 21, 2013.