Sediment production on sediment-starved continental margins; the interrelationship between hardbottoms, sedimentological and benthic community processes, and storm dynamics

ABSTRACT Modern sediment-starved continental shelves represent developing condensed 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 margin provide important benthic habitats that are modified on time scales of days to centuries by interrelated sedimentological, biological, and physical processes. Outcropping Upper Cenozoic strata of varying lithologies form distinct hardbottom morphologies that, through differential bioerosion, contribute significant volumes of new sediment to the surficial sand regime of the continental shelf. (1) Vertical and sloped mudstone to muddy sandstone hardbottoms of the Miocene Pungo River Formation are dominated by the endolithic fauna Jouanettia quillingi (bivalve) 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 stimpsoni, and G. ovata. (3) The highly lithified, flat hardbottoms of Plio-Pleistocene 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, produce large-scale morphological features on the shelf, and recycle ancient sediment into the modern, surficial sediment system. The rate of sediment production resulting from bioerosion varies from 5.5 kg/m2/yr on the vertical and sloped Miocene mudstone hardbottoms, to 0.4 kg/m2/yr on vertical and sloped Pleistocene limestone, to 0.03 kg/m2/yr on the flat, highly lithified Plio-Pleistocene limestone hardbottoms. Depending on lithology and associated bioerosional processes, bioeroders 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 meters 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 exposure, which is 132 m long, this would produce a ten-meter overhang of the overlying Pleistocene limestone in 250-500 years by removing 13,400 metric tons of eroded sediment (25% fine sand) that would be contributed 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 producing the sediment. However, surface sediment is generally not accumulating on the shelf; it is present only as thin (0-1 m), highly variable, and ephemeral sand bodies. Major storms modify the abundance and distribution of surface sediment on hardbottom habitats, and routinely export large volumes of these sediments from the shelf system, depositing them as fine-sand clinoforms off the prograding shelf edge. Exposed hardbottom habitats free of sand are dominated by highly diverse communities of endolithic fauna and epilithic fauna and flora, those habitats 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 generally dominated by softbottom benthic communities. Storms modify the distribution of bottom sediments, which either exposes or buries additional hardbottom surfaces and controls the expansion or contraction of hardbottom benthic communities. Thus, the intensity, frequency, and character 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 type, rate, and volume of hardbottom bioerosion and resulting sediment production.