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 in the Lower River

Abstract

The Brazos River empties into the Gulf of Mexico (Gulf of Mexico) forming a wave-influenced delta dominated by a muddy subaqueous portion. Recent work in the lower river and subaqueous delta (SAD) however, have discovered evidence for sedimentary processes more often associated with higher sediment discharge rivers and/or high-gradient rivers. These studies utilized 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. From the SAD results showed the eastern portion was dominated by high side scan sonar backscatter features, an erosional scarp along the upper shoreface, and a thinning of the Holocene sediment package immediately downslope of the scarp. The thickness of the Holocene sediment package increased in deeper water. These features suggest sediment mass wasting events on the delta front. After rapidly prograding during the early and mid 20th century, reductions in sediment load, and a shift in the primary depocenter lead to erosion on these abandoned portions of the delta. The result is the erosional scarp upslope of the high backscatter features representing exposed relict consolidated sediments. In the lower river we observed a separate extreme event. During an elevated fluvial discharge event, data from the lower river showed an anomalously high suspended sediment layer. This fluid mud layer was >1 m thick in areas, and located along a 6 Km span of the river ~ 2 Km upstream from the mouth. The location corresponded to a reach of the river impacted by salt-water intrusions from the GOM. This salt-water intrusion was shown to inhibit sediment export from the river to the GOM, and facilitate deposition of fine-grained sediment in the lower river. Based on our observations we believe that this is a mechanism for the development of hyperpycnal flow in the river. The mud layer in the lower river builds during moderate and low discharge periods and remobilized during increased discharge. 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. These results highlight the potential for sedimentary processes not typically associated with a river in this environmental setting.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015