ASSESSMENT OF LITHOLOGIC CONTROLS ON TRILOBITE PRESERVATION IN THE KIMMSWICK LIMESTONE, JEFFERSON COUNTY, MISSOURI

The Kimmswick Limestone (Middle Ordovician) in the eastern portions of Missouri has been considered a relatively homogenous unit with a coarse crystalline texture dominating within a heavily bedded system. While highly fossiliferous throughout, variation is present within and across sections in the region, with lower portions of the unit generally more abundant in fossil material. Typically the most prevalent fossil species include trilobites, brachiopods, echinoderms and bryozoans. The goals of this project are 1) to analyze the degree and types of lithologic variation present at both the outcrop and regional scales testing the degree homogeneity assumed within the unit. 2) To assess the environmental and taphonomic controls on trilobite species stratigraphically throughout the unit.

For this analysis three outcrops are studied along county roads MM and M in Jefferson County, Missouri, creating a cross-section running approximately east-west respectively. Sections were measured and logged to approximate centimeter scale for individual beds. Units recorded within sections vary across individual outcrops to include the Plattin, Kimmswick and potentially the Fernvale Formations in respective stratigraphic order, the focus of this analysis is the Kimmswick Limestone. Individual horizons were collected from the Kimmswick for each outcrop, with a focus on trilobite material, sedimentary structures, and lithologic textures. Lithologic samples are cut and polished to highlight textural variability.

Lithologic variability within sections occurs at higher degrees than previously recorded. Individual beds are often separated by erosional hardground surfaces and occur in with a wide range of variation in clast origin (bioclastic/intraclastic) size, abrasion, and crystalline texture. Analyses of the type and preservation of trilobite material within these individual horizons provides evidence on the environmental and taphonomic controls across the unit, likely heavily influenced by the ranges of energy input into the system for a given horizon.