Facies Assemblages for Two Transgressive Barrier Islands

Davin J. Wallace and John B. Anderson
Earth Science, Rice University, Houston, TX

Texas has an extensive coastline representing transgressive, progradational, and aggradational barrier islands. Each represents unique facies undergoing a complex evolution both on and offshore. Facies assemblages for Follet’s and South Padre Island, two transgressive barrier islands, are presented here. Previously collected data suggest minimal sand offshore both systems. Longshore transport is also thought to be minor. Subsidence for both study areas is negligible: 1.08 mm/yr and .38 mm/yr on Follet’s and South Padre Islands, respectively. Both barriers transgressed onto Holocene lowstand fluvial surfaces: Follets on a 2960±90 year old (calibrated) paleo Brazos River surface, and South Padre on a 5645±55 year old (calibrated) paleo Rio Grande surface. The sedimentary architecture beneath Follet’s Island consists of two facies: a 265 cm washover deposit (thickest) overlaying bay clay. We determine maximum average washover rates to be on the order of .90 mm/yr. South Padre Island facies are similar: 30 cm of humic clay overlaying 170 cm of washover deposit (thickest) resting on bay clay. Here, we calculate maximum average washover rates to be on the order of .30 mm/yr.

Therefore, over longer timescales (i.e. centuries to millennia), hurricane washover has had only a minor roll in the evolution of both barriers, which is a surprising outcome. It is hard to reconcile current rates of shoreline retreat using washover accumulation rates, so the current rate of retreat must be unprecedented.

AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas