Abstract

Mid-Holocene facies underlying Matagorda Peninsula and Galveston Island record a complex history of transgressive and regressive fluvial, deltaic, and estuarine sedimentation along the central Texas coast. Data from 80 borings and jet-down holes reveal a dissimilar sequence of depositional events for northeastern and southwestern Matagorda Peninsula. Along northeastern Matagorda Peninsula and the area now occupied by the Colorado-Brazos delta plain, all major river valleys were flooded during the Holocene transgression; gulfward, a barrier system developed and was driven landward by rising sea level. As the rate of sea-level rise decreased, the Colorado and Brazos Rivers completely filled their common estuary and prograded across ancestral Matagorda and West Bays. Deltaic progradation terminated after the landward-migrating barrier system overrode the delta front; at that time, sediment was delivered directly to the Gulf of Mexico. Shortly thereafter, beach and shoreface progradation began along barrier island systems lateral to the now rapidly eroding deltaic headland. This was accompanied by renewed deposition of estuarine mud in Matagorda and West Bays.

Facies underlying southwestern Matagorda Peninsula indicate Pass Cavallo was initially established over the axis of the Lavaca River valley. Southwestward migration of the pass accompanied by landward migration of the barrier peninsula has resulted in a well-preserved facies tract of bay, distal and proximal flood tidal delta, and spit-related sediments now underlying this coastal segment.

Modern physiographic features, as well as rates of shoreline erosion and recession, are directly related to these middle and late Holocene depositional events.

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