CALIBRATING DEPTHS OF ORDOVICIAN BENTHIC ASSEMBLAGES IN THE UPPER ORDOVICIAN OF THE CINCINNATI ARCH REGION, USA

Ancient marine benthic assemblages arrayed gradationally along environmental gradients, can be quantified to provide proxies of ecological parameters, especially those related to water depth. However, while it is commonly possible to determine the relative position of a given facies or fossil community, it is far more difficult to assign absolute depths in terms of meters below sea level. Absolute depth estimates can be made using a variety of exposure-related sedimentary structures, including indicators of shoreline, normal wave base, and storm wave base, as well as biologic evidence of light-related zones.

The Upper Ordovician (Upper Katian) Rowland Member (Drakes Formation) in northern Kentucky provides an excellent opportunity to quantify biotic gradients in relation to absolute depth because recent high-resolution correlations permit recognition of transects that range from peritidal to deeper subtidal (30 to 40 m depth range based on microendoliths) within single small-scale cycles (Brett et al., 2014). In the Rowland Member, the abundance of dasyclad algae within the deeper organic-rich clay facies indicates relatively shallow water (< 15 m depth), while the Skolithos facies, which contains desiccation cracks, can be placed near the shoreline (~ 0 to 2 m above sea level). The configuration of depth-related facies within this interval thus provides a unique opportunity to assign fossil assemblages to quantitatively defined depth zones along a gently dipping carbonate ramp.

The Rowland Member displays four well-defined biofacies, each representative of a distinct lithofacies, including: (a) scavenging ostracods and infaunal filter feeders (e.g., Skolithos and lingulid brachiopods) in the shallowest shaly facies; (b) abundant deposit feeding organisms, epibyssate bivalves, free-resting bryozoans, and non-calcified algae in the shallow subtidal "lagoonal" facies (c) prolific herbivorous? gastropods in high energy carbonate shoal facies; and (d) a higher diversity (10-15 species) of filter feeding epifaunal brachiopods and ramose bryozoans, in offshore areas. The distinct biofacies present within this depositional system provide a rich temporal and environmental framework in which to study ecological-evolutionary patterns.