CLINOFORM RESPONSE TO SEA LEVEL: PHASE RELATIONS BETWEEN SHORELINE AND ROLLOVER, DEVELOPMENT OF COMPOUND CLINOFORMS, AND THE TIMING OF MARGIN PROGRADATION

We present a morphodynamic theory of clinoform development in which characteristic floods and coastal storms of specified magnitude and frequency drive evolution of the coupled fluvial and marine sediment surfaces. Our theory treats the shoreline as a dynamic moving boundary. We analyze clinoform response to eustatic cycling and the corresponding development of stratigraphic sequences in a passive-margin setting. The interplay of sea level with coupled fluvial and marine morphodynamics generates a wide range of clinoform behavior and sequence geometries. For small-amplitude eustatic cycles, a single rollover characterizes the subaqueous clinoform geometry. Significant phase lags develop between rollover and shoreline when the period of eustatic forcing is comparable to the intrinsic response time of the clinoform. As the amplitude of eustatic forcing increases, the style of clinoform response changes profoundly, with the clinoform rollover becoming abandoned during rapid sea level rise and associated shoreline transgression. As the rate of sea level rise wanes, renewed progradation generates a compound clinoform with an actively prograding rollover separating a regressive shoreline from the previously abandoned clinoform rollover. The morphologic evolution of this compound clinoform and the resultant timing of overall margin progradation via rejuvenation of the abandoned rollover are sensitive to the relative efficacy of fluvial and shallow-marine sediment dynamics. Small, infrequent coastal storms and/or large, frequent floods generate a fluvio-deltaic clinoform that overruns the abandoned clinoform rollover near sea-level lowstand. In contrast, vigorous marine sediment dynamics favor formation of a subaqueous delta with an actively prograding rollover that outpaces the regressive shoreline and merges with the abandoned rollover significantly prior to lowstand.