Evolution of Holocene Bay Complexes in the Trinity Incised Valley, Texas

RODRIGUEZ, ANTONIO B. and JOHN B. ANDERSON, Rice University, Houston, TX

Examination of how bay complexes (bay-head delta, middle bay, barrier shoreline, and tidal delta) retreat landward during transgression will help predict the occurrence and location of sand bodies associated with incised valleys. During the last transgression (18,000 yr. BP to present) the Trinity/Sabine incised valley backfilled with continuous fluvial and bay-head delta facies and discontinuous middle-bay and coastal (tidal delta complex and barrier facies) deposits. In places on the inner shelf, banks (Sabine, Heald, and Shepard banks) lie adjacent to and over the incised valley. These banks represent submerged paleoshorelines. The modern Galveston Bay complex formed above the Trinity incised valley during the most recent stage of marine incursion, and because of its protected waters, is an ideal area to examine incised valley fill in detail. The top of the bay-head delta surface has been mapped regionally within Galveston Bay and is characterized by a series of flat steps and inclined risers. The flat steps are flooding surfaces that represent landward translation of the bay-head delta environment. These flooding surfaces correlate with ravinement surfaces located within the offshore banks and indicate rapid landward translation of a bay complex as the result of sea-level rise. The risers represent periods of bay-head delta aggradation. This correlates with deposition of middle bay and barrier island sediments indicating formation of a new bay complex. Landward translation of bay-head delta facies and barrier island submergence events occur simultaneously; therefore, recognition of bay-head delta steps within incised valley fill may aid in predicting the location of barrier sands stranded offshore as banks.

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