Paper No. 340-6
Presentation Time: 2:45 PM
INCISED VALLEY AND BACKBARRIER LAGOON HISTORY OF THE WEST GALVESTON BAY COMPLEX, NORTHERN GULF OF MEXICO
The West Galveston Bay (WGB) back barrier lagoon system is part of a broader Holocene sedimentary sequence that also contains incised valley and deltaic sequences. WGB is the modern-day micro tidal back-barrier lagoon of Galveston Island, which is located along the northern coast of Gulf of Mexico. During the Last Glacial Maximum (LGM) the incised valleys of Chocolate, Halls, Mustang, and Wharton Bayous either formed or were reoccupied. These tributaries converged into a trunk channel beneath the modern day Chocolate Bay, situated directly landward of WGB. A rapid period of sea level rise drowned this confluence around 6.5 kya, resulting in a dramatic increase in accommodation space, and dividing a 1,086 km2 drainage basin into four individual drainage basins of 420 km2, 334 km2, 284 km2, and 48 km2, respectively. At that time, deposition within the incised valley transitioned from a fluvial into a predominately marine regime. The paleo-Brazos River then began flowing through the area around 4 kya, increasing the associated drainage basin area to 110, 852 km2, and resulting in a reintroduction of predominately fluvial sediments within the incised valley. Stream piracy then caused the paleo-Brazos River to change its course, resulting in a shift back to four relatively small associated drainage basins. At this point, Galveston Island was fully established as a barrier island, and an ephemeral tidal inlet formed where the island intersected the incised valley. This inlet then migrated west until stabilizing within the paleo-Brazos River incised valley at the current position of San Luis Pass. This migrating tidal inlet was the controlling mechanism of the sedimentation of WGB from the departure of the paleo-Brazos River until present. The result of the constantly changing balance between accommodation space and drainage basin area is a complex depositional history over a relatively brief timespan.