PALEOHYDROLOGY AND GEOARCHAEOLOGY AT HORN SHELTER NO. 2 ALONG THE BRAZOS RIVER IN CENTRAL TEXAS

Horn Shelter Number 2 is a rockshelter situated above the floodplain of the Brazos River, about 105 km (65 mi) SSW of Dallas, Texas in Bosque County. Geologic analysis included the lithostratigraphic reconstruction of the shelter sediment fill from walls exposed after archaeological excavations. Radiocarbon and artifact aging provided the depositional chronology, ranging from about 11,000 BP to the present. A dominant depositional process for each lithostratigraphic unit was identified utilizing lithological characteristics evident in the field and in petrographic thin section. Numerous slackwater paleoflood deposits originating from the Brazos River were identified, and used in conjunction with HEC-2 hydraulic modeling to provide data for basin-wide flood frequency reconstruction. Flood frequency curves were calculated using statistical algorithms imbedded in the MAX and EMA computer programs for temporally ramified segments of the late Quaternary as represented in the paleoflood record of the shelter. Flood frequency on the Brazos River and depositional processes once active in Horn Shelter are used to interpret regional late Quaternary climatic oscillations. The terminal Pleistocene interval from approximately 11,000 BP to 10,000 BP is interpreted as regionally cooler and drier than present based on the paucity of flood deposits. The early Holocene interval from 10,000 BP to 8,000 BP is interpreted as cooler with more available moisture than the previous interval based on high flood frequencies. The middle and late Holocene is interpreted as regionally more arid than present, with peak aridity sometime before 2,300 years BP based on low flood frequencies. The latest Holocene (2,300 BP to 64 BP) is interpreted as returning to a slightly less arid climate regime because flooding becomes more frequent. However, historical agriculture practices may have resulted in abnormally frequent flood deposition in the shelter from 200 BP to 64 BP, the result of the most severe pattern of Brazos River flooding in the past 11,000 years.