The bench deposits at Berger Bluff: Early Holocene -Late Pleistocene depositional and climatic history.

摘要

This study reports original field and laboratory studies on paleoenvironmental aspects of the Berger Bluff archeological site (Goliad County, Texas). It uses data from geoarcheology (stratigraphy, grain-size analysis, magnetic susceptibility and sediment thin sections) and several different biotic indicators (chiefly diatoms, snails, freshwater mussels, and vertebrates) of environmental change to reconstruct past environments of the site and upstream drainage, and relate these events to regional and global paleoenvironment.;Berger Bluff is a sandy bluff about 9 m high on the west, or Goliad County side of Coleto Creek, west of Victoria. The bluff (41 GD 30A) records continuous deposition of sediments by Coleto Creek from the Late Pleistocene until the Late Holocene. The lowest 2.45 m of deposits form a prominent erosional bench from floodplain sediments cemented by phreatic carbonate, probably representing the fossilized damp margin of a spring or seep area, although no actual spring conduit was visible. Based on a series of radiocarbon assays, they are estimated to date from about 8500-11,000 radiocarbon years before present (or 9500-13,000 calendar years), but there are no assays from the upper or lower part of the bench, and there are a number of inversions among the assays. This corresponds to the Younger Dryas, Preboreal, and part of the Boreal period in the climatic chronology, and the Folsom and Late Paleoindian periods in the cultural chronology, although none of the artifacts from the bench are time-diagnostic.;The bench consists of fine-grained overbank deposits (cyclically bedded sandy and muddy units) formed by vertical accretion near the south valley wall. Sedimentary evidence suggests the creek was probably narrower, deeper, more sinuous, and carried a much greater suspended load during the Younger Dryas/Early Holocene compared to today. Rainfall was probably less seasonally concentrated, discharge less flashy, and flood duration longer. The floodplain was probably wider, flatter, less drought-prone, and (based on evidence from snails and vertebrates) covered with a heavier, more continuous deciduous tree canopy than today. These conditions probably lasted some 1500 years into the Holocene, until regional drying and flash flooding stripped the wet riparian habitat out of Coleto Creek and other small tributaries on the Gulf Coastal Plain.;Keywords. Berger Bluff, Goliad County, Younger Dryas, paleoenvironment, geoarcheology.