Fluvial Deltaic Evolution Through Event Sedimentation: Understanding the Interplay of Sediment Supply and Remobilization in Shaping the Growth of a Muddy Subaqueous Delta

Abstract

The Brazos River, has the highest water and sediment discharge of all rivers in Texas, and ranks second behind the Mississippi River in terms of sediment load delivered to the Gulf of Mexico (GOM) forming a wave-dominated delta. Through high-resolution geophysical data, sediment cores, and historic shoreline data this study investigated the evolution of the Brazos River delta. The delta has experienced rapid changes over the last 100-plus years resulting from changes directly to the river, the coastal zone proximal to the river mouth, and within the watershed. These changes have led to rapid, but relatively short periods of delta progradation and transgression. The most significant change occurred in 1929 when the river mouth was moved 10 km west of its prior location. Following the mouth diversion, the Old Brazos Delta began to retreat, while the New Brazos Delta began to prograded. Sediment accumulated rapidly within the new subaqueous delta during this period. With a characteristic red color, Brazos River sediment was easily distinguished in the sediment record from grey, GOM mud. The delta sedimentary sequences contain alternating layers of Brazos River and GOM sediment, indicating deltaic accumulation resulting from deposition of episodic fluvial events separated by quiescent periods of GOM sedimentation. Widespread progradation of the new delta ceased, initiating a transgressive phase in the 1950s from a significant reduction in sediment supply attributed to drought and subsequent changes in agricultural practices. Over the past ∼50 years delta progradation has been limited to the western portions of the delta, with much of the delta experiencing abandonment. Fluvial sediment delivered to these portions of the delta may be completely remobilized within a year, indicating remobilization from marine processes outpacing fluvial supply. Data from within the river show a salt-water intrusion reducing the export of sediment to the delta under a majority of discharge regimes. Outside of major fluvial flood events fluvial sediment is deposited proximal to the mouth, and quickly remobilized. These conditions have resulted in a significant deficit in the sediment budget where an estimated 90% of the sediment load from the river is not accumulating on the delta. This study highlights the importance of understanding of short-term variability in the link between fluvial and marine processes in governing the accumulation of sediment within a delta.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014