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Effects of structural features
Tectonic structures affected the strata of the midcontinent during the Pennsylvanian and Early Permian (Lee, 1943; Jewett and Abernathy, 1945; Jewett, 1949; Jewett and Merriam, 1959; Mudge, 1967; Harris and Larsh, 1979), and several locations of high paleotopography of the Americus Limestone Member (see fig. 21) correspond to known structural features. North of locality 4, in the region of the carbonate-shoal, lagoonal, and tidal flat facies (Fisher, 1980), is the Alma-Davis Ranch anticline and the southeastern flank of the Nemaha anticline. The paleotopographically high area at localities 9-17 corresponds to the general location of the Bourbon arch, a broad feature that separates the Forest City basin to the north from the Cherokee basin to the south. Localities 24-28 are the site of a shallow-water to subaerial paleoenvironment that existed at the location of the Beaumont anticline. The minor paleotopographic high indicated between localities 19 and 22 might also be the location of a previously unrecognized structural uplift.
The structural features caused lateral differentiation of paleoenvironments by forming topographically high areas directly over uplifts, probably by differential compaction or by subtle tectonic movements. The paleotopographically high areas were the locations of shallow-water high-energy to subaerial environments of deposition; deeper-water low-energy environments of deposition developed between anticlines. Figure 43 shows the lateral distribution of paleoenvironments in the lowermost bed of the Americus Limestone Member and associated structural features.
Figure 43--distribution of depositional environments of the lowermost limestone bed of the Americus Limestone Member and associated structural features from north-central Kansas to extreme south-central Kansas [see Jewett and Merriam (1959) and Fisher (1980)].
Conclusions
The upper Hamlin Shale and Americus Limestone members record a vertical succession of paleoenvironmental conditions ranging from freshwater to normal marine. The lateral distributions of fossils and other constituents in the lower limestone bed of the Americus indicate that more nearly normal marine conditions existed toward the south. Differences in paleotopography of the seafloor caused lateral differentiation of depositional environments by forming localized areas of shallow-water high-energy to supratidal environments. Paleotopographically high areas are characterized by thin intervals of overlying shale or terrigenous mudstone, packstones and grainstones, distinctive boundstone types, and the presence of Globivalvulina foraminifers and pectinoid and myalinid bivalves. The coincidence of such structural features as the Nemaha anticline, the Alma-Davis Ranch anticline, the Bourbon arch, and the Beaumont anticline with the locations of shallow-water, high-energy, and supratidal depositional environments indicates that these structural features were ultimately responsible for the lateral differentiation of the paleoenvironments. It should be possible to use such relationships in similar strata to locate similar minor anticlines and arches.
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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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