Extreme Events on a Low-Gradient River and Delta: Evidence for Sediment Mass Movements on the Subaqueous Delta and a Mechanism for Creating Hyperpycnal Flow onto the Shelf
Timothy Michael Dellapenna, Texas A & M University-Galveston Campus, Marine Science and Oceanography Depts, Galveston, TX, United States, Joseph A Carlin, California State University Fullerton, Geological Sciences, Fullerton, CA, United States and Joshua R Williams, Virginia Institue of Marine Science, Gloucester Point, VA, United States
Abstract:
The Brazos River empties into the Gulf of Mexico (GOM) forming a wave-influenced, muddy, subaqueous delta (SAD). Recent research in the estuarine reach of the river and on the SAD, however, found evidence for significant mass wasting of the delta-front and potential evidence of hyperpycnal flow, a processes typically associated with higher gradient and higher sediment yield rivers. The study used high-resolution geophysics on the SAD and water-column profiling in the lower river to investigate the transfer to and fate of fluvial sediment on the shelf. The SAD side scan mosaic combined with core data reveal that the eastern portion was dominated by exposed relict, consolidated sediment; an erosional scarp along the upper shoreface; and a thinning of the Holocene strata immediately downslope of the scarp. Holocene strata thickness increases into deeper water. These features suggest sediment mass wasting on the delta front. After rapidly prograding during the early and mid 20
th century, reductions in sediment load due anthropogenic influences, and a shift in the primary depocenter lead to erosion on these abandoned portions of the delta.
During an elevated fluvial discharge event, a >1 m thick fluid mud layer was found along a 6 km span of the river ~ 2 km upstream from the mouth. The river’s salt wedge was shown to inhibit sediment export from the river to the GOM, and facilitate deposition of mud in the lower river. We believe that the mud layer in the lower river builds during moderate and low discharge periods and remobilized during increased discharge, potentially resulting in hyperpyncnal flow to the shelf. We observed suspended sediment concentrations up to 100 g/l in the fluid mud layer during this event. While our observations did not capture the transition from fluid mud to hyperpycnal flow, we believe that with persistent increased discharge the fluid mud layer could transition to hyperpycnal flow.
