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

摘要: ABSTRACT Modern sediment-starved continental shelves represent developing condensed sections analogous to those considered key stratigraphic markers in many models. Condensed and their associated hardbottoms on the modern, high-energy North Carolina margin provide important benthic habitats that are modified time scales of days centuries by interrelated sedimentological, biological, physical processes. Outcropping Upper Cenozoic strata varying lithologies form distinct hardbottom morphologies that, through differential bioerosion, contribute significant volumes new sediment surficial sand regime shelf. (1) Vertical sloped mudstone muddy sandstone Miocene Pungo River Formation dominated endolithic fauna Jouanettia quillingi (bivalve) Upogebia sp. (shrimp). (2) consisting harder Pleistocene limestone bivalves Lithophaga bisulcata, Gastrochaena stimpsoni, G. ovata. (3) The highly lithified, flat Plio-Pleistocene epifloral macroalgal species Dictyopteris hoytii, Zonaria tournefortii, Sargassum filipendula. These three groups bioeroders physically and/or chemically degrade respective lithologies, develop relief surfaces, produce large-scale morphological features shelf, recycle ancient into system. rate production resulting from bioerosion varies 5.5 kg/m2/yr vertical hardbottoms, 0.4 limestone, 0.03 flat, lithified hardbottoms. Depending lithology bioerosional processes, excavate exposed surfaces ranging millimeters meters, whereas rates between different lithologic units results meters tens meters. Recession measured mudstones at Chapel site range 2 4 cm per year. For outcrop exposure, which is 132 m long, this would a ten-meter overhang overlying 250-500 years removing 13,400 metric tons eroded (25% fine sand) be contributed sediments. ultimately break off during storm next row rubble blocks ramp front receding scarp. rapid enough bury producing sediment. However, surface generally not accumulating shelf; it present only as thin (0-1 m), variable, ephemeral bodies. Major storms modify abundance distribution habitats, routinely export large these sediments shelf system, depositing them fine-sand clinoforms prograding edge. Exposed free diverse communities epilithic flora, with 2-6 scattered small growths flora irregularly distributed topographic highs, > 6 softbottom communities. Storms bottom sediments, either exposes or buries additional controls expansion contraction Thus, intensity, frequency, character individual seasonal pattern determine amount location accumulation, community structure. In turn, determines type, rate, volume production.