The subsurface geology of Marshall County is known largely through the interpretation of data on oil and gas test holes. Records are available on 21 such test holes drilled in Marshall County.
Marshall County is underlain by Precambrian rocks at known depths ranging from 1,225 to 2,698 feet. These rocks comprise chiefly granite and metamorphosed sediments such as schist. An oil test hole in sec. 4, T. 4 S., R. 7 E. was drilled 1,770 feet into Pre-Cambrian rocks, all of which probably comprise metamorphosed sediments (Jewett, 1949).
The St. Peter sandstone of Ordovician age is probably the oldest sedimentary formation underlying Marshall County. Any Cambrian rocks deposited in this area were removed by erosion prior to Ordovician deposition. Rocks of Silurian and Devonian age are found in the subsurface of Marshall County. Rocks of Mississippian age were deposited and have been removed by erosion in Marshall County, but are present in the area to the west. Structural movements at the end of Mississippian deposition resulted in the formation of the Nemaha anticline, the crest of which trends slightly southwest from southeastern Nebraska to central Oklahoma. Immediately after its formation the Nemaha anticline consisted of a Pre-Cambrian core with the sedimentary rocks arched over, standing several hundred feet higher than rocks of the same age to the east and west that had not been uplifted. The sedimentary rocks over the anticline were eroded, and by the beginning of Pennsylvanian deposition the Pre-Cambrian rocks were exposed along the crest. This beveling of upturned sedimentary beds resulted in the Pennsylvanian sediments being deposited on successively younger rocks east or west from the anticline. Marshall County is on the west flank of the anticline, the crest of which passes through western Nemaha County. About 1,000 feet of Pennsylvanian rocks underlie all of Marshall County, and the upper 10 to 50 feet of the rocks is exposed at a few places along the eastern edge of the county (Table 10).
Pennsylvanian System (Virgilian Series)
The Brownville limestone, which is the youngest formation of the Pennsylvanian System, is the only recognizable Pennsylvanian unit cropping out in Marshall County. The Brownville limestone crops out along deep ravines in sec. 36, T. 5 S., R. 10 E. and in sec. 13, T. 3 S., R. 10 E. (Pl. 1). This formation consists of about 2 feet of impure limestone with an abundance of brachiopod, crinoid, and echinoid remains. At the locality in T. 3 S., R. 10 E. the Brownville limestone is underlain by about 12 feet of varicolored, calcareous shale, which is the upper part of the Pony Creek shale.
No wells in Marshall County are known to obtain water from Pennsylvanian rocks.
Kansas Geological Survey, Geologic History of Kansas
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Web version March 2004. Original publication date March 1954.