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Franklin County Geology

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The following discussion of subsurface rocks in Franklin County is abstracted from Lee (1943) and Jewett (1954).

Sedimentary rocks of Pennsylvanian and older Paleozoic age and of Cenozoic age underlie Franklin County. Cambrian rocks comprise the Lamotte Sandstone, Bonneterre Dolomite, and Eminence Dolomite (lower part of the Arbuckle Group), and range in thickness from approximately 150 feet in the southwestern part of Franklin County to 260 feet in the northeast. Ordovician strata include the upper part of the Arbuckle Group, St. Peter Sandstone, Platteville Formation, and Viola Limestone; and they range in thickness from approximately 435 to 705 feet. A maximum of slightly less than 100 feet of limestone and dolomite is included in the "Hunton" Group, which in Franklin County is thought to be Devonian in age but which may contain some undifferentiated Silurian strata. The Chattanooga Shale, Devonian or Mississippian in age, is believed to underlie all of Franklin County and is about 50 feet thick. Mississippian rocks in Franklin County are almost exclusively limestone and range from slightly less than 250 to 400 feet in thickness. The subsurface Pennsylvanian section has an aggregate thickness of approximately 900 feet and consists of shale, sandstone, limestone, and coal.

Extensive erosion surfaces within the subsurface section include the sub-Paleozoic surface, sub-Chattanooga surface, and sub-Pennsylvanian surface. Numerous intrasystemic unconformities are firmly documented or suspected.

Upper Pennsylvanian rocks cropping out in Franklin County are approximately 700 feet thick. Neogene surficial deposits include floodplain and terrace alluvium along the streams, river-laid chert gravels locally on the upland areas, and soils, which mantle broad upland bedrock areas to a depth of 3 feet or less.

Exposed bedrock in Franklin County is exclusively sedimentary. About 70 percent of the outcropping pre-Neogene rocks consists mainly of shale and sandstone but includes lesser amounts of claystone, siltstone, and coal; the other 30 percent is marine limestone that probably formed in shallow water (Moore, 1929). Fossils, physical aspects of lithology, and what is known of the dimensions and shapes of the stratigraphic units indicate that most of the noncarbonate rocks were deposited in marine and mixed environments. Mixed environments (Dunbar and Rodgers, 1957, p. 67) are defined as those environments transitional between the marine and nonmarine. Some of the terrigenous detritus represents nonmarine deposits.

Salient features of the strata exposed in the county, excepting Douglas Group rocks, are: (1) continuity of the individual stratigraphic units, (2) lack of marked lateral variation in lithology, and (3) vertical sequences of strata in which rock types are repeated in the same relative order, which Moore (1936, p. 29) has termed megacyclothems. Of the 17 stratigraphic units mapped (Pl. 1), 11 have continuous lines of outcrop and 9 are known to be essentially continuous in the subsurface. Even many of the individual members are continuous. Shale and sandstone units show the greatest thickness ranges; limestone and black fissile shale show the least.

Relation of bedrock to topography is well shown by generally east-facing escarpments of relatively resistant limestone which are separated by gently sloping plains developed on less resistant shale. The regional landform which includes Franklin County is called the Osage Plains (Schoewe, 1949, p. 280). The present stream pattern of Franklin County is thought to have evolved during Pleistocene time from a generally southwestward-flowing to a generally eastward-flowing drainage (Frye and Leonard, 1952, p. 194-195).

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Kansas Geological Survey, Geology of Franklin County
Web version July 2002. Original publication date Jun. 1963.
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