SEDIMENT PRODUCTION ON SEDIMENT-STARVED CONTINENTAL MARGINS - THE INTERRELATIONSHIP BETWEEN HARDBOTTOMS, SEDIMENTOLOGICAL AND BENTHIC COMMUNITY PROCESSES, AND STORM DYNAMICS

Modern sediment-starved continental shelves represent developing conde nsed sections analogous to those considered key stratigraphic markers in many stratigraphic models. Condensed sections and their associated hardbottoms on the modern, high energy North Carolina continental marg in provide important benthic habitats that are modified on time scales of days to centuries by interrelated sedimentological, biological, an d physical processes. Outcropping Upper Cenozoic strata of varying lit hologies form distinct hardbottom morphologies that, through different ial bioerosion, contribute significant volumes of new sediment to the surficial sand regime of the continental shelf. (1) Vertical and slope d mudstone to muddy sandstone hardbottoms of the Miocene Pungo River F ormation are dominated by the endolithic fauna Jouanettia quillingi (b ivalve) and Upogebia sp. (shrimp). (2) Vertical and sloped hardbottoms consisting of harder Miocene and Pleistocene limestone are dominated by the endolithic bivalves Lithophaga bisulcata, Gastrochaena stimpson i, and G. ovata. (3) The highly lithified, flat hardbottoms of Plio-Pl eistocene limestone are dominated by the epifloral macroalgal species Dictyopteris hoytii, Zonaria tournefortii, and Sargassum filipendula. These three groups of bioeroders physically and/or chemically degrade their respective lithologies, develop relief on hardbottom surfaces, p roduce large-scale morphological features on the shelf, and recycle an cient sediment into the modern, surficial sediment system. The rate of sediment production resulting from bioerosion varies from 5.5 kg/m(2) /yr on the vertical and sloped Miocene mudstone hardbottoms, to 0.4 kg /m(2)/yr on vertical and sloped Pleistocene limestone, to 0.03 kg/m(2) /yr on the flat, highly lithified Plio-Pleistocene limestone hardbotto ms. Depending on lithology and associated bioerosional processes, bioe roders excavate exposed hardbottom surfaces and develop relief ranging from millimeters to meters, whereas differential rates of bioerosion between different lithologic units results in relief ranging from mete rs to tens of meters. Recession rates measured on Miocene mudstones at the Chapel site range from 2 to 4 cm per year. For the outcrop exposu re, which is 132 m long, this would produce a ten-meter overhang of th e overlying Pleistocene Limestone in 250-500 years by removing 13,400 metric tons of eroded sediment (25% fine sand) that would be contribut ed to the surficial sediments. The overhang would ultimately break off during a storm to produce the next row of the limestone rubble blocks that form a ramp in front of the receding mudstone scarp. These rates of sediment production are rapid enough to bury the hardbottoms produ cing the sediment. However, surface sediment is generally not accumula ting on the shelf; it is present only as thin (0-1 m), highly variable , and ephemeral sand bodies. Major storms modify the abundance and dis tribution of surface sediment on hardbottom habitats, and routinely ex port large volumes of these sediments from the shelf system, depositin g them as fine-sand clinoforms off the prograding shelf edge. Exposed hardbottom habitats free of sand are dominated by highly diverse commu nities of endolithic fauna and epilithic fauna and flora, those habita ts with 2-6 cm of sand are generally dominated by scattered epilithic fauna with small growths of epilithic flora irregularly distributed on topographic highs, and those habitats with > 6 cm of sand are general ly dominated by softbottom benthic communities. Storms modify the dist ribution of bottom sediments, which either exposes or buries additiona l hardbottom surfaces and controls the expansion or contraction of har dbottom benthic communities. Thus, the intensity, frequency, and chara cter of individual storms and the seasonal storm pattern determine the amount and location of sand accumulation, which controls the benthic community structure. In turn, the benthic community determines the typ e, rate, and volume of hardbottom bioerosion and resulting sediment pr oduction.