The Effects of Varying Base Level and Sediment Supply on Incised-Valley Evolution in an Experimental Basin

Incised valleys are important stratigraphic markers of relative sea-level fall, are genetically linked with lowstand deltas downdip, and commonly contain stratigraphically-trapped hydrocarbons in their fill. Several experimental studies of incised valleys have been done over the last two decades. Though different in setup, all examined the effect of base-level variation on valley evolution but none considered variation in the sediment-supply rate. We build on this work by presenting results from an experimental study of incised valleys formed during cyclic changes in both base level and sediment supply. We performed four experiments in which base level and sediment supply varied sinusoidally with the same period but a different phase, so that between experiments the maximum sediment-supply rate occurred at different times during a base-level cycle. In a fifth (control) experiment sediment supply was held constant. All experiments were performed on a semi-cohesive erodible substrate molded into a shelf-slope configuration with a break (3 to 30 deg) that was exposed during base-level fall. High-resolution laser scans of the drained topography were collected at fixed intervals during each experiment. We use the scans to track surface evolution, construct a synthetic stratigraphy, and compute bulk sediment fluxes in the basin.

Changes in valley morphology, the valley-bounding unconformity, and the ratio of bulk erosion to bulk sediment supply in our control experiment were similar to those observed in previous experiments. Results varied, however, when sediment supply was varied. Maximum valley widening occurred around the time of maximum sediment-supply rate. When the sediment-supply rate peaked during base-level fall widening accompanied incision; when the sediment-supply rate peaked during base-level rise, valley widening accompanied aggradation. So while high sediment-supply rates augmented erosion caused by base-level fall, they caused erosion of valley walls during base-level rise even as the valley floor aggraded. These sediment-supply driven differences in valley evolution were translated into differences in the age of the valley-bounding unconformity and hence the stratigraphic organization of the valley fill and lowstand deltas. Our findings expand on those from previous experiments and demonstrate how the relationship of sediment supply to base level is recorded in the morphology and stratigraphy of incised valleys.
 

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California