AAPG ACE 2018

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Effects of Highstand Mud Accumulation on the Evolution of the Central Texas Coast

Abstract

Fifteen transects of sediment cores from back-barrier to beyond the toe of the shoreface and a seismic line, between Matagorda Peninsula and North Padre Island, located on the central Texas coast were investigated. The sequence boundary (SB) separates Holocene sediments of the Transgressive System Tract (TST) from Late Pleistocene sediments of the preceding Highstand System Tract, HST). Stratigraphic architecture reveals two prominent transgressive surfaces with significant sand sandwiched between two marine muds that represent two episodes of transgression. Facies interpretations are based on lithology, grain size, and fossil content. These data indicate significant variability in shoreface migration along the coast during the Holocene.

The Holocene Mud blanket (referred to as Texas Mud Blanket, TMB) consists of up to 50 meters of mud that occupies an embayment in the continental shelf between the Rio Grande and Colorado/Brazos paleo-deltas. It is estimated to have accumulated nearly 200 km3 in total volume during the middle to late Holocene; most of this sediment came from the Mississippi River.

Core stratigraphy and radiocarbon ages indicate significant progradation (~5km) of the shoreface off Mustang Island between ~6-5ka followed by a 4-3 ka event when the Texas mud blanket (TMB) experienced significant expansion, coinciding with landward retreat of the shoreface (the upper transgressive surface). The TMB expansion coincided with a significant decrease in the rate of sea-level rise and a period of growth of North Texas coastal barriers.

Growth and expansion of the TMB resulted in onlap of the central Texas lower shoreface, potentially impacting sand supply to coastal barriers. This study explores how the growth of the TMB influenced the central Texas coast. The results have important sequence stratigraphic implications and amplify the importance of autocyclic controls on nearshore stratigraphic architecture. In addition, they amplify the seldom recognized importance of highstand mud dispersal and accumulation on coastal evolution.