PALEOENVIRONMENTAL IMPLICATIONS OF A UNIQUE OSTRACODE- AND TRILOBITE- RICH GRAINSTONE BED IN THE LOWER LIBERTY FORMATION (UPPER ORDOVICIAN), PREBLE COUNTY, OHIO

A unique ostracode- and trilobite-rich grainstone bed in an outcrop of the Waynesville and Liberty formations (Upper Ordovician) at Acton Lake Spillway, in Oxford, Ohio, documents a brief period of hypersaline conditions in a restricted environment.  The unique grainstone lies in the lowermost Liberty Formation.  It has several characteristics which differentiate it from the grainstone beds that are typical of the Liberty Formation.  Grainstones found in this unit are dominated by echinoderm ossicles, small fragments of brachiopods, and bryozoans.   Thin section analysis of this unique bed has revealed a fossil composition of 90% ostracode valves with no preferred orientation.  The laminations are made distinct by thin, dark laminae of pyrite. The pyrite may be a remnant of bacterial mats or horizons which underwent reducing conditions. Although brachiopods and bryozoans are the dominant components of the Liberty Formation, there is a near absence of these fragments in this bed.  The top of the bed appears to be an erosional surface that has been bored, which suggests that it may have been a hardground.  The base of the bed is composed of disarticulated Flexicalymene exoskeletons that lie normal to bedding in a convex-up position. The abundance of trilobites on the bottom of this bed is currently being investigated. The dominance of a single type of organism, the ostracodes, represents a change in the paleoenvironmental conditions to one that was inhospitable to other organisms.   A change to hypersaline conditions would have favored the proliferation of ostracodes. The lack of imbrication or orientation of the ostracode valves and the fine laminations suggests that there was little to no bottom current.  The observations made during the initial investigation of this unique bed suggest a shallow, restricted marine environment, rather than the subtidal, normal marine environment typical of the Liberty Formation.