ECOLOGIC AND TAPHONOMIC SETTING FOR HIGH-FIDELITY CALCITE REPLACEMENT OF MOLLUSCAN ARAGONITE: A CLOSER LOOK AT THE UNUSUALLY-PRESERVED MOLLUSKS OF THE MARBLE HILL BED IN THE UPPER ORDOVICIAN OF NORTHERN KENTUCKY

The Marble Hill Bed is a Richmondian-age shell-bed limestone unit at least 2.5 meter thick that crops out in vicinity of Bedford, Kentucky. This unit has several unusual characteristics as compared to other Cincinnatian units. First, it is one of the thickest nearly pure limestone units in the succession. Second, it is distinctive in that it contains almost exclusively mollusks and not the typical brachiopods and bryozoans of the Cincinnatian. Third, it is the only unit in the succession where originally aragonitic shells are recrystallized finely enough to preserve the lamellar microstructure. The purpose of this investigation is to document the occurrence of this fine preservation and hypothesize about the cause. This study is based on a continuous rock sample collected from an interval about 2.5 m thick in an outcrop on US Highway 42 near Bedford.

The bed is dominated by two species of gastropod, and includes a minor component of other taxa which were all originally aragonitic. This is unusual as the typical brachiopods and bryozoans that comprise Cincinnatian shell beds were calcitic. All originally aragonitic shell material in the Cincinnatian is replaced by calcite or completely dissolved. The molluscan assemblage found in the Marble Hill bed is relatively rare but is only unique in abundance, to other shell beds of the Cincinnatian.

One hypothesis is that diagenesis of the molluscan material was controlled by pore water chemistry. An environment so rich in shells allowed for a buffering of pore waters that slowed crystal replacement compared to the rapid dissolution commonly represented in other Cincinnati units. The abundance of shell material present during diagenesis supports this hypothesis. Further support may come from preliminary petrographic studies showing that fine preservation predominates near the top of the bed while lower shells tend to be dissolved. The unit consists of broadly cross-bedded grainstones and packstones, and represents deposition in a nearshore environment. This is not the only Cincinnatian unit showing similar depositional features, so it is difficult to explain ecologically why mollusks were so concentrated in this particular unit. This leaves an open question: is the abundance an artifact of preservation, or is preservation an artifact of abundance?