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The Sabine-Neches fluvial-estuary system is composed of deposits that represent fluvial, deltaic, central-basin, tidal inlet/delta, and chenier depositional systems. The Holocene deposits and associated environmental changes preserved in the drowned Sabine-Neches alluvial valley provide a valuable analog for present and future environmental changes. These deposits are bounded by flooding surfaces that record episodes of dramatic environmental reorganization during the Holocene. The most dramatic environmental changes are manifested as stratigraphic back stepping in which central-basin sediments overlie deltaic sediments. The magnitude of flooding varies from a few tens of kilometers to less pronounced back stepping followed by rapid progradation.

Initial flooding of the onshore Sabine-Neches incised valley occurred around 9800 cal yr B.P., and by ca. 8900 cal yr B.P., a vast bayhead delta occupied the southern half of the valley. This delta backstepped up the valley during the relatively rapid sealevel rise of the early to middle Holocene, and by ca. 7100 cal yr B.P., it occupied the entire valley. After ca. 7100 cal yr B.P., the bayhead delta shifted up the valley again, and a central-basin setting existed in the lower half of the valley. The middle basin expanded episodically between ca. 5500 cal yr B.P. and 1700 cal yr B.P., and a brief period of delta growth occurred ~300 yr ago.

Controlling mechanisms for flooding surface formation include sea-level rise, changes in the antecedent topography of the incised valley, and sediment supply variations. Antecedent topography was influential in controlling estuarine evolution between ca. 7800 and 7500 cal yr B.P., when an extensive fluvial terrace was inundated. The fact that some flooding surfaces appear to be synchronous, within a few centuries, in several estuaries across the northern Gulf of Mexico suggests a eustatic rather than local control. Flooding events at ca. 8900 cal yr B.P. and ca. 8400–8000 cal yr B.P. were likely caused by rapid, sub–meter-scale sea-level rise events. Sediment supply variations controlled by climatic forcing appear to have been the main cause of other flooding events. Unfortunately, the Holocene climate record for the east Texas–west Louisiana coastal region is poorly documented, and a direct relationship to central and western Texas climate records may be complex. So the exact nature of climate control on sediment flux to the estuary system remains elusive.

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