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Introduction
The purpose of this study was to determine the environments of deposition of the lowermost bed of the Americus Limestone Member (Wolfcampian) in Kansas and to discern the effects of buried structural features on the lateral distribution of paleoenvironments. Fisher (1980) studied the Americus limestone in the vicinity of the Nemaha anticline, and his interpretation of paleoenvironments was used to determine the paleoenvironmental significance of stromatolite morphologies (Denver, 1985) and species of ostracodes and foraminifers (Peterson, 1978; Peterson and Kaesler, 1980; Kaesler and Denver, 1988). This project extends the paleoenvironmental framework southward into south-central Kansas.
The lower Americus Limestone Member was deposited in supratidal to subtidal environments during transgression of an epeiric sea. The lateral distribution of depositional environments of the Americus was influenced by more nearly normal marine conditions toward the south and by paleotopography. Areas of relatively high paleotopography, although quite subtle, evidently were the locations of higher depositional energy. Areas of positive seafloor paleotopography coincided with anticlinal structures, indicating that these structures were the ultimate control on the lateral differentiation of paleoenvironments. The locations of relatively high seafloor paleotopography and buried anticlinal structures are characterized by thin deposits of shale, low mud content in limestone, distinctive stromatolite morphologies, and distinctive fossils. These characteristics can be used in similar strata to locate minor anticlines and arches that are not otherwise clearly expressed at the surface.
Stratigraphy
Although the primary concern of this study was the lowermost limestone bed of the Americus Limestone Member of the Foraker Limestone (Council Grove Group), overlying shales and limestones of the Americus and the underlying Hamlin Shale Member of the Janesville Shale (Admire Group) also were examined where possible. Figure 1 shows the position of the Americus within the Foraker Limestone and associated stratigraphic units.
Figure 1--Generalized stratigraphic section of the Americus Limestone Member of the Foraker Limestone and associated units [after Mudge and Yochelson (1962)].
The limestones of the upper part of the Americus Limestone Member characteristically weather into distinctive large slabs that form the first prominent bench of the Flint Hills. These features, along with a typical stromatolite layer at the base of the lower limestone bed of the Americus and an orange lime-sand mudstone to grainstone at the top of the Hamlin Shale Member, facilitate recognition and correlation of units in the field (Mudge and Yochelson, 1962). A summary of the lithology of the Hamlin shale and the Americus limestone is provided in appendix F.
Methods
I measured and sampled 35 outcrops of the Americus Limestone Member along an outcrop belt 210 km (130 mi) long from Wabaunsee County to Cowley County, Kansas (fig. 2). I also examined several localities within the area studied by Fisher (1980) and Denver (1985). The field procedure included photographing, sketching, and measuring outcrops; describing vertical and lateral relationships, especially the in situ position of stromatolites; and sampling representative rocks, fossils, and stromatolites.
Figure 2--Index map of study area showing sample localities (filled circles). See appendix E for detailed locations and appendix G for measured sections.
More than 80 polished slabs and more than 150 5 x 7.5 cm (2 x 3 in.) thin sections were prepared and examined. Stromatolite morphologies were characterized in detail and compared with those studied by Denver (1985). Ostracodes were picked from washed residue for future study. Lithology, paleontology, stromatolite morphology, and corresponding lateral and vertical facies relationships were used to interpret depositional environments.
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Kansas Geological Survey, Geology
ISBN: 1-58806-107-8
Placed on web Sept. 8, 2011; originally published 1992.
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