Quaternary shelves and the return to grade

摘要: Abstract The first attempt to propose a general theory of shelf sedimentation was Johnson 's (1919) concept the equilibrium profile, and its corollary, size-graded shelf. graded challenged when Shepard (1932) pointed out that chart notation indicated non-graded condition, discredited field studies many workers showed central outer shelves be mantled with sediments relict from Pleistocene low stands sea. Recently, however, Curray (1965) has nearshore belts fine modern sediments, suggested these will prograde seaward form texturally finally an profile. Sufficient data concerning dynamics now accumulated so some tentative models may proposed for processes might drive towards grade. In zone, wave-driven null line mechanism sediment sorting dispersal been proposed. Attempts establish validity in have not yielded consistent results. However, generally recognized response model described here as sand prism suggests such process must applicable marine sedimentation. There is believed little movement zone. Moore (1964) shown this area “natural cyclic period” considered, on order century's duration. “Cataclysmic” storm then becomes steady-state process. It where occurs only during storms, wave-drift currents dominant control dispersal. With conditions linear source at shoreline, sink break, slowly rising water interface, random, intermittent movements by storm-generated would result net movement. Shelf grading occur progressive sorting. Models based tides or other agents are probably also valid, but considered briefly here. Shelves morphology transport disrupted sea-level rise which rapid relative rate supply. Consideration ancient deposits following sequence partially stages ensue, terminated climax A stage appears situ grading, origin become terms texture. stage, characterized portions present Atlantic shelf, lens least part derived winnowing sediments. second begins estuaries bypass suspended fines amounts adequate generate prograding blankets mud. Climax well mud bypassed, spreads all parts This hypothetical cycle ideal rather than normative degree realization depends energy imput, rates subsidence sedimentation, factors