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ABSTRACT: Orbital Forcing of Glacioeustasy: Evaluation of the Alluvial Clastic Source/Sink Term

R. K. Matthews, Cliff Frohlich

Orbital forcing of climate change at key latitudes has long been recognized as a probable cause of variation in continental ice budget and thereby sea level. The precession cycle, variation in tilt of the earth's axis relative to the sun, and variation in the eccentricity of the earth's orbit about the sun all affect seasonal solar insolation. The seasonal solar insolation signal at any particular latitude is complex, with periods ranging upward from 20,000 yr to hundreds of thousands of years. Further, nonlinear responses of the earth system (e.g., slow buildup and rapid destruction of continental ice sheets) can convert modulation of short-period signal into long-period sea level variation.

The alluvial valley/floodplain can be an important factor in sand supply to the marine environment. The alluvial system is constantly linked to the marine depositional system by sea level. With rising sea level, the alluvial system may store sand, thus depriving the marine system. With falling sea level, incisement may release sand previously sequestered in the alluvial system. To investigate these relationships in the context of numerous, rapid, and complex glacioeustatic sea level fluctuations, we have incorporated into our two-dimensional forward model an interactive, model-driven alluvial source/sink sediment supply term which includes numerous user-specified parameters. Valley/floodplain geomorphic evolution includes incisement, terrace erosion, and floodplain sedimentation. Sens tivity tests suggest the system seldom reaches equilibrium for geologically reasonable parameterizations. Source/sink relations, which might be ignored if one assumed sea level to be moving slowly in one direction for long periods, can drastically modify availability of sand to the marine environment.

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