Changing Depositional Environments in an Upper Ordovician Stratigraphic Sequence, Ashlock Formation, Madison County, Kentucky
We investigate the sedimentology, stratigraphy, and depositional environments of a 7-meter, Upper Ordovician limestone sequence cropping out in Richmond, Kentucky. The stratigraphic section lies within the Ashlock Formation with good lateral exposure stretching along 200 meters of a highway roadcut. We took approximately 20 samples from the measured section, focusing on representative samples and lithologic transitions. We use standard laboratory procedures in slabbing rock samples and making thin sections.
The Ashlock Formation at this locality consists of alternating layers of limey mudstone and limestone. Megafossils - brachiopods, bryozoans, trilobites, gastropods, ostracodes, coralline algae, and bivalves - are abundant in various limestone units. The observed transitions from limestones and limy muds to lithologies with more terrigenous mud suggests any combination of: (1) migration of depositional environment with a slight increase in water depth; (2) climatic change resulting in more runoff; or (3) tectonic activity delivering more mud to the basin. These shallow water environments change to glauconitic mudstone and laminated shales, which we interpret as deeper shelf deposits. The measured section is capped by shaley limestones and mudstones that signal a return to shallow subtidal environments.