Lithology and Depositional Environments of a Portion of the Clays Ferry Formation (Middle and Upper Ordovician) Exposed at Silver Creek, Madison County, Kentucky
Undergraduate thesis, Eastern Kentucky University
Abstract
We measure, describe, and interpret a carbonate stratigraphic section within the Clays Ferry Formation (Middle and Upper Ordovician) cropping out in Madison County, Kentucky (USGS Kirksville 7.5” quadrangle). The total thickness of the measured section is 4.76 m. We sampled the stratigraphic section at approximately half-meter intervals, also taking samples at lithology changes. We collected a total 20 samples, all of which were slabbed, and then selected 12 samples for thin section analysis.
Observed lithologies represent discrete depositional environments. The rocks are dominantly limestones with some carbonate shales deposited in shallow-water depositional environments that are generally open-marine, subtidal with perhaps some intertidal units. Burrowed mudstones and wackestones are more common lower in the stratigraphic section and perhaps represent the shallowest depositional environments. Upsection, laminated pelloidal packstones/grainstones occur and contain varying amounts of fragmented fossils. The next prominent unit is a 1.5-meter-thick interval, where shaly carbonate is interbedded with ~10-cm thick limestone beds containing a diverse fossil assemblage, indicating subtidal, open-marine conditions. Several 15-to-25-cm thick grainstone beds mostly comprised of nested, strophomenid brachiopods are prominent ledge-formers being deposited under turbulent conditions. Fossiliferous packstones and grainstones with brachiopods, bryozoans, and crinoids then dominate indicating open-marine, subtidal environments; one such horizon displays 10- to 15 cm-high dune bedforms. Upsection for the next ~1.5 meters these lithologies reoccur and are interbedded with one another, representing migration of depositional environments over a shallow-marine platform.
