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ABSTRACT: Fluvial Response to Tectonics and Sea Level Change in Foreland Basins

Charles L. Angevine, H. W. Posmentier

Fluvial responses to a variety of tectonic and sea level variations have been modeled for foreland sedimentary basins. Sea level cycles encompass three orders of magnitude: fifth-order cycles (^sim 104-105 yr), fourth-order cycles (^sim 105-106 yr), and third-order cycles (^sim 106-107 yr). For cycles in the fourth- to fifth-order range, the rates of eustatic change overwhelm the rates of subsidence and, consequently, tectonic considerations are insignificant. However, during third-order cycles of sea level change, the rates of sea level change and tectonic subsidence may be comparable, and the evolution of the fluvial section can be complicated. Two end-member responses to eustatic fall are considered: (1) the situation where the point to which the streams are adjusted (i.e., the shoreline) is located seaward of the zone of maximum subsidence rate due to flexural loading by the fold and thrust belt, and (2) the situation where the shoreline lies within the zone of maximum subsidence rate. In the first case, modeling suggests that fluvial aggradation continues, unaffected by eustatic change. Consequently, sequence boundaries associated with sea level change are not recognized here. In the second case, the response is more complex, and a variety of responses are possible depending on rates of subsidence and sediment flux, and the slope of the profile exposed by relative sea level fall. Under most circumstances, fluvial aggradation will continue, albeit at lower rate than had sea level remain d constant.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990