THE REMARKABLE LATE HOLOCENE SHIFT FROM TRANSGRESSIVE TO HIGHSTAND SEDIMENTATION IN THE NORTHWESTERN GULF OF MEXICO

During the early-mid Holocene, the Brazos, Colorado and Rio Grande rivers formed fluvial-dominated deltas on the continental shelf while smaller rivers formed large bayhead deltas that partially filled their incised valleys. This was followed by a landward shift in deposition and fluvial aggradation within river valleys that was in pace with sea-level rise. Modern barriers of the central Texas coast evolved while the shoreline off east Texas and western Louisiana was located tens of kilometers offshore of its current location (e.g. Sabine and Heald banks).

The current highstand began ~ 4 ka when the rate of sea-level rise decreased from an average rate of 1.4 mm/yr to 0.5 mm/yr, coincident with a shift toward higher frequency climate oscillations. It was during this time that modern strandplains, chenier plains, peninsulas and barrier islands of the east Texas-western Louisiana coast formed. The lower Brazos and Colorado valleys were filled with sediment, increasing sediment bypass from these rivers to the Gulf. Bays grew wider and shallower during the current highstand and experienced a more restricted connection with the Gulf, making them more efficient sediment traps. Baffin Bay shifted from fluvial dominated to its current unique suite of arid depositional environments. The Texas Mud blanket became the dominant depocenter on the shelf, accumulating approximately 172 km³ of fine-grained sediment that was derived mainly from the Mississippi River in response to an increase in westerly winds and near-shore currents.