Shoreface Ravinement of Backbarrier Bay Deposits of Follets Island, a Transgressive Island Barrier Island Along the Northern Gulf of Mexico, Cannot Source Sand Needed for Island Growth
Transgressive (retrogradational) barrier islands migrate landward as sediment is transported during washover events (e.g. hurricanes and storms), from the ocean-side to the bay-side of the island. Ravinement of these deposits, if they are sand bearing, can potentially help to maintain the island. Follets Island is a classic transgressive/retrogradational barrier island located along the Texas coast of the northern Gulf of Mexico (GOM). Christmas Bay is the back barrier bay of the eastern 13 km of Follets Island. At generally >200 m wide, Follets Island is in full retreat mode, migrating across a formerly much wider Christmas Bay, at a rate of ~1-2 m/y. This study addresses the question, has back barrier bay deposits been preserved on the Gulf-side of Follets Island and can these deposits provide significant sand sources to support island growth, when eroded. We use the results of the analyses of a series of vibra cores and high-resolution CHIRP seismic lines collected both on the bay and the ocean side to address these questions. Results find that Christmas Bay began as two channels incised into the Pleistocene aged Beaumont Formation paleosol, which is found at the base of the Holocene sequence. An early Holocene middle estuarine facies sits atop this Pleistocene unconformity. This is capped with the a red clay delta sequence of the Brazos delta. During the middle Holocene the nearby Brazos River emptied into both Christmas and the connected West Galveston Bay, and as much as 1.6 m of Brazos clay is present in the cores. Within the Christmas Bay cores, this is topped with 10-50 cm of modern estuarine mud. Offshore of Follets Island, there is ~5 km of the inner-shelf that sits shoreward of the ravinement front. These deposits contain +4 m of Holocene bay fill deposits, containing the same stratigraphic sequence found in Christmas Bay, with the Brazos clay generally found at the base of the barrier island sequence. As the ravinement front continues to migrate landward and these deposits are eroded, they will not be contribute to the island's maintenance. This may help to explain why Follets Island is disappearing at such a fast rate.
AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017