Event-driven deltaic sedimentation on a low-gradient, low-energy shelf: The Brazos River subaqueous delta, northwestern Gulf of Mexico

• Sediment accumulated from flood events and reworked proximal shelf sediments
• The depocenter migrated over time due to changes in the fluvial sediment supply
• Current event beds can be completely remobilized within a year of deposition
• Study shows delta accumulation through fluvial and marine-derived sediment inputs

The Brazos River Delta is located in the northwestern Gulf of Mexico. Sedimentation on the Brazos River subaqueous delta has been largely understudied. Swath bathymetry data collected across the subaqueous delta reveals a noticeable asymmetry with much deeper water depths and greater cross-shelf slopes along the eastern, up drift side of the delta than along the western, down-drift side. End-member sampling reveals that Brazos River derived sediment is red, red-brown, or brown in color, exhibits increased bulk density, and is enriched in calcium a function of carbonate-rich areas of the watershed, whereas the Gulf of Mexico inner shelf muds are gray to black and have a relatively lower calcium content. Analyses of a series of 33 submersible vibra cores collected across the subaqueous delta reveal alternating beds of Brazos River derived sediment and Gulf of Mexico shelf dominated sediment. Excess 210Pb geochronologies of select cores reveal that sediment accumulation has been a combination of fluvial flood event deposition, and deposition of remobilized sediment from the proximal shelf and older portions of the delta rather than steady state accumulation. Because the Brazos River is susceptible to local climatic cycles, fluvial flood events that create the event layers are intermittent between prolonged periods where marine processes drive accumulation. Results also show that the primary depocenter of the subaqueous delta has migrated southwestward over time, and the erosion of earlier depocenters and proximal shelf sediment has contributed to accumulation on the delta. Over the past ~ 50 years the subaqueous delta has reached an equilibrium where flood event beds can be completely remobilized within a year, and net accumulation approximates relative sea level rise. This study highlights relatively rapid clinoform migration due to natural and anthropogenic changes to the system, and demonstrates subaqueous delta growth through a combination of direct fluvial and marine-derived sediment input.