The Holocene Texas Mud Blanket (TMB) is a large (∼ 300 km3 containing ∼ 5 × 1011 t of sediment) depocenter on the central Texas shelf and represents a stratigraphic anomaly in that it is the only significant transgressive-highstand depocenter in the northern Gulf of Mexico other than the Mississippi River Delta. Given its size and remoteness from large rivers, the origin of the TMB has remained problematic. Our results demonstrate that the TMB accumulated mainly in the late Holocene as a result of increased sediment supply from rivers and an increase in delivery of Mississippi River sediments to the region by marine currents.
Approximately 3000 km of high-resolution 2D seismic data was used to construct detailed isopach maps of the TMB. A robust radiocarbon age model derived from long sediment cores is used to measure its accumulation through time. Mineralogical (XRD) results provide information about temporal variability in the source of sediments to the TMB and indicate that sediments came mainly from the Colorado, Brazos, and Mississippi rivers, with the Rio Grande and smaller rivers of Texas having been minor contributors. The second largest source is from transgressive erosion of the shelf.
The TMB accumulated in a shelf depression that exists between the falling-stage Colorado and Brazos deltas to the north and the Rio Grande Delta to the south. Initial inundation of the outer shelf occurred ∼ 17 ka when terrestrial and lagoonal sediments filled the deepest parts of the depocenter. Between ∼ 17 and ∼ 9 ka was a time of rapid eustatic rise and low sedimentation (VAR, volumetric accumulation rate, = 0.4 km3/ka; MAR, mass accumulation rate, = 0.7 × 106 t/y). At ∼ 9 ka, sediment accumulation dramatically increased to 41 km3/ka (69 × 106 t/y). This sediment was mainly generated by ravinement of shelf strata, including the ancestral Brazos and Colorado deltas. Between ∼ 7.5 and ∼ 4 ka, Texas was experiencing maximum temperature and minimum precipitation for the Holocene (the Climatic Optimum). This was a time of minimum sedimentation in the TMB. This was followed by a period of remarkable growth during the late Holocene (∼ 3.5 ka to Present) when the TMB accumulated 172 km3, total volume, or 2.86 × 1011 t of sediment, accounting for 57% of its volume. Mineralogical results, plus the sheer volume of sediment deposited during this time, calls for a corresponding increase in sediment delivery to shelf from the Brazos and Colorado rivers and an increase in the delivery of sediment from the Mississippi River. By the late Holocene the Brazos and Colorado rivers had filled their lower valleys with sediment, thus eliminating onshore accommodation and increasing sediment delivery to the Gulf. The dramatic increase in sediment delivery from the Mississippi River to the TMB during the late Holocene is best explained by an increase in southeasterly winds, which drive westward-flowing marine currents in the northwestern Gulf.