ABSTRACT

High-energy, sediment-starved continental shelves of the mid-Atlantic region have abundant hardbottoms that extend from the shoreface to the shelf edge. Because of the thin and irregularly distributed Holocene sand sheet, shelf morphology is determined mainly by outcropping Tertiary and Pleistocene stratigraphic units. Each unit and combination of units produces different hardbottom morphologies that depend upon the geometry and spatial relationships of the units, lithology and patterns of stratification, and subsequent weathering and erosion.

Hardbottoms vary in surface relief from smooth, flat surfaces to scarped surfaces with up to 10 m of relief. The morphology ranges from sloping and stepped erosional ramps to vertical and undercut scarps with associated broad rubble ramps. Hardbottoms associated with each of the different gently dipping Tertiary depositional sequences have distinctive morphologies. Hardbottoms developed on Pleistocene units unconformably overlie the Tertiary sequences as flat-lying marine carbonates, or cut into them as channel systems backfilled with fluvial and estuarine sediments. Initial dissection of hardbottoms produced highly convoluted surfaces that resulted from subaerial weathering, stream erosion, and karst formation during sea-level lowstands. During subsequent sea-level highstands, these rimary morphologies were greatly modified through the interaction of bioerosion and storms.

Understanding continental shelf hardbottoms is critical for interpreting the sedimentology and stratigraphy of the Atlantic Coastal Plain and for reconstructing paleoceanographic conditions, for the following reasons. (1) They are an extensive part of the stratigraphic record on shelves that are not actively subsiding and have small volumes of terrigenous input with low sediment accumulation rates. (2) They are important stages in the formation of major stratigraphic unconformities, condensed sections, and sequence boundaries. (3) They support diverse biological communities that produce primary carbonate sediments and are rapidly degraded and modified by bioerosion and physical processes supplying abundant "new sediment" to the continental shelf.