HARDBOTTOM MORPHOLOGY AND RELATIONSHIP TO THE GEOLOGIC FRAMEWORK - MID-ATLANTIC CONTINENTAL-SHELF

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 Terti ary 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 pattern s of stratification, and subsequent weathering and erosion. Hardbottom s vary in surface relief from smooth, flat surfaces to scarped surface s with up to 10 m of relief, The morphology ranges from sloping and st epped 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 morpho logies. Hardbottoms developed on Pleistocene units unconformably overl ie the Tertiary sequences as Bat-lying marine carbonates, or cut into them as channel systems backfilled with fluvial and estuarine sediment s, Initial dissection of hardbottoms produced highly convoluted surfac es that resulted from subaerial weathering, stream erosion, and karst formation during sea-level lowstands, During subsequent sea-level high stands, these primary morphologies were greatly modified through the i nteraction of bioerosion and storms. Understanding continental shelf h ardbottoms is critical for interpreting the sedimentology and stratigr aphy of the Atlantic Coastal Plain and for reconstructing paleoceanogr aphic conditions, for the following reasons, (1) They are an extensive part of the stratigraphic record on shelves that are not actively sub siding and have small volumes of terrigenous input with low sediment a ccumulation rates, (2) They are important stages in the formation of m ajor stratigraphic unconformities, condensed sections, and sequence bo undaries, (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.