Quaternary shelves and the return to grade

Abstract

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