ABSTRACT

High resolution (uniboom) seismic reflection profiles and vibracores acquired from Sabine Lake, Texas, during 1987 aid in the reconstruction of the bay's history. Seismic records show that the bay is located over an incised valley, initially carved during an early lowstand, tentatively assigned to ¹⁸O Stage 5d. Thus, the valley fill sequence represents more than one stage of incision and fill.

Sediment cores collected along the flanks of the valley penetrated bay muds resting directly on Pleistocene clays. Radio-carbon dates suggest that the broad, shallow bay floor was flooded approximately 4,000 years ago. Prior to this time the bay was confined to the narrow, deep incised valley. None of the cores that penetrated beyond the Holocene fill contain transitional (deepening upwards) deposits, implying either that flooding was rapid or that erosion had removed these transitional deposits. Cores from the upper portion of the bay were characterized by coarsening-upward sequences which appear to reflect increasing fluvial input of sand with time. Cores from the marsh area at the southern end of the lake penetrated marsh deposits resting on flood-tidal delta and bay deposits.

A strong dominance of the bivalve Rangia cuneata (a species generally found in fresh to brackish waters) occurs throughout the Holocene section of sediment cores. Carbon isotopic values of these molluscan shells are generally low and consistent with low salinities. The only exception to this is seen in the very base of these cores, where the microfossil assemblage and isotopic results indicate a brief period during which salinities were higher than at present. Otherwise, the salinity structure of the bay and its fauna have changed little during the past 4,000 years.