INCISED-VALLEY FILL PARASEQUENCE FORMATION AND IMPLICATIONS FOR ESTUARINE EVOLUTION: GALVESTON ESTUARY, TEXAS

Late Quaternary incised-valley fill architecture is commonly attributed to the interplay between sea-level rise, sediment supply, and hydrodynamic processes. The control variable antecedent topography has on incised-valley fill architecture is examined in the Trinity incised valley, Texas. The Trinity valley is characterized by a series of downward-stepping terraces and the Galveston Estuary formed above this irregular antecedent topography. Flooding surfaces, recognized in core by a decrease in sedimentation rates and a change from delta plain to central basin facies, formed at ~ –14 m, 8,200 cal. yr. BP and ~ –10 m, 7,700 cal. yr. BP, matching depths of the relatively flat fluvial terraces. Flooding surfaces formed rapidly and represent entire reorganization of the estuarine complex. Across the –10 m flooding surface, the river mouth and bayhead delta shifted landward at a rate of ~ 6.5 km per century and the associated barrier shoreline was stranded on the inner continental shelf forming Heald Bank. Flooding surfaces formed as the rate of sea-level rise was decreasing, and are not associated with a decrease in sediment delivery to the estuary. As sea level inundates relatively flat fluvial terraces, rates of shoreline transgression increase dramatically resulting in a sudden increase in accommodation space, and an associated landward shift in coastal facies. Terraced incised valley-fill architecture is inherent to backstepping parasequences, in spite of external forcing mechanisms.