TAPHONOMY AND BIOEROSION OF A PLIOCENE CORAL THICKET FROM VIRGINIA, USING COMPUTER BASED AREA ANALYSIS

A Septastrea coral thicket (0.2 to 0.5 m thick and 250 m long) is exposed in sediments of the Sunken Meadow Member of the late Pliocene Yorktown Formation along the Piankatank River in Virginia. Prior studies of the sequence stratigraphy and paleoecology (Bailey and Blow, 1996; Bailey, 2006) suggest thicket development within shallow to mid-shelf siliciclastic facies as part of a transgressive systems tract during an interval of low sedimentation rate at or on a maximum flooding surface. Many of the coralla are invaded by a suite of macroboring species, indicating a period of prolonged exposure before sedimentation increase and final burial. Collectively these borers reflect a hard substrate trypanites ichnofacies and include the ichnogenera Entobia (sponges), Gastrochaenolites (bivalves), Caulostrepsis, and Trypanites (worms?).

Slabs of coral branch fragments cut at roughly 1 cm intervals were photographed and analyzed using the computer program MATLAB. Percentage of area bioeroded (defined as % of total slab removed) was determined for each specimen. In one sample Entobia bioerosion led to loss of nearly 25% of the sample. In addition to the identification of species that generally appear late in successions of boring organisms (i.e. clionid sponges and lithophaginid and gastrochaenid bivalves) the bioerosion results confirm a hypothesis of a long period of exposure on the seabed during a sea level highstand and associated interval of sediment bypassing.