SEQUENCE STRATIGRAPHY AND PALEOENVIRONMENTAL CONTROLS OF NEOGENE CORAL THICKET ASSEMBLAGES IN THE CHESAPEAKE GROUP OF VIRGINIA AND NORTH CAROLINA

Rare, monospecific ahermatypic coral thickets and biostromes have been reported in Pliocene and Pleistocene Coastal Plain strata from Virginia to Florida. These Neogene assemblages occur in shallow to mid-shelf siliciclastic facies and in all cases appear to represent brief (strata 0.3 to 1m thick) and somewhat unusual expansions of normally limited coral growth. Similar coral dominated communities do not occur in shallow to mid-shelf siliciclastic environments off the modern Atlantic coast, although tropical hermatypic corals are displaced northward on hardgrounds off North and South Carolina and locally extensive thickets of Acropora cervicornis occur in very shallow water off Ft Lauderdale, FL. Evaluation of Septastrea coral thickets in the Yorktown and Chowan River Formations of Virginia and North Carolina respectively, suggests that the conditions for rapid coral growth and thicket elaboration are best met in early development of a highstand systems tract. Shell gravel or shelly firmgrounds of the transgressive systems tract are still available for colonization near the maximum flooding surface. Conditions at this position in the sequence cycle result in a brief period of limited sediment supply and clastic facies progradation. Larval settling is enhanced because of abundant and appropriate substratum and reduced sediment input promotes coral growth and survival for up to several hundred years (based on age increment studies of coralla). Later in the history of a sequence or parasequence, sedimentation rate increases as highstand facies prograde and coral thickets or coral dominated assemblages are quickly killed and buried. Other Neogene coral assemblages in somewhat similar facies in NC and FL occur early in sequence cycles, are associated with or are near lower sequence bounding unconformities, and may have been similarly constrained.