Abstract: Storm-Generated Shell Ridge on Open Lagoonal Coast of Bulls Bay, South Carolina
John H. Barwis, Joseph H. Makurath
Bulls Bay is a shallow (<2 m), relatively unrestricted lagoon fronting the extensive salt marsh adjacent to Cape Romain. Although baymouth sand stringers provide some wave protection, the area is essentially a tidal flat-marsh complex on an open coast. Interior parts of the bay, which are relatively calm under prevailing wave conditions, are rimmed by lunate, regularly spaced oyster banks that act as fine sediment baffles. Along the southern edges of the bay, these banks are exposed to northeast storm waves and have been reworked into shell ridges that have transgressed as much as 50 m over the present marsh surface.
These storm ridges are as much as 2 m thick and more than 1 km long, and are composed almost entirely of disarticulated but unbroken oyster shells. Profiles across the ridges indicate longshore transport of shell material in the longshore direction. Wave baffling of backridge areas has caused the marsh surface to aggrade above spring high water, usually as a zone of sand flats devoid of Spartina and heavily reworked by burrowing crustaceans.
Fronting the ridges are wide tidal flats which can be divided into two zones. The high intertidal area, between mean water level and mean high water, is the old marsh surface and is characterized by intensely burrowed pelleted silty muds. Dominant sedimentary structures are symmetrical ripples, tracks and trails, and algal laminae. Oysters are present as scattered clumps about 30 cm in diameter. The low intertidal area is a mudflat free of oysters and dominated by mollusks and echinoderms. The relatively small size of the ridge notwithstanding, it has potential as an oil reservoir both because of its high porosity and permeability, and because it directly overlies organic-rich marsh muds.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC