CLIMATIC RESPONSE TO THE PLEISTOCENE-HOLOCENE TRANSITION EVIDENT BY SEDIMENTOLOGICAL AND SLACKWATER PALEOFLOOD MODELING AT HORN SHELTER, CENTRAL TEXAS

Horn Shelter offers a unique opportunity to investigate the climatic response to the last glacial-interglacial transition in Texas. The shelter is an archaeological site (41BQ46) adjacent to the Brazos River, about 105 km SSW of Dallas, Texas. Paleoclimatic study of the shelter utilized both depositional trend and slackwater paleoflood modeling in a mutually supporting approach. Depositional processes active in the shelter on either side of the Pleistocene-Holocene boundary were characterized by the microstratigraphic correlation of the sediment fill, and selected petrographic thin section analysis. Depositional trends were objectively derived from three-dimensional stratigraphic modeling in RockWorks99©. Paleoflood modeling consisted of estimating large statistical flood recurrence intervals with MAX 3.3 based on slackwater sediments on either side of the Pleistocene-Holocene boundary in the shelter. MAX 3.3 requires both the absolute chronology of the slackwater events, and an estimation of the minimum discharge thresholds (HEC-2) for the Brazos to deposit flood sediment in the shelter. Sediments deposited from about 11,200 radiocarbon years B.P. to 10,000 radiocarbon years B.P. consists of ebolis breccia floating in a matrix of sand fraction cave-attrition sediments. Significant cave-deterioration deposition ceased after the onset of the Holocene (10,000 B.P.), and the shelter sediments become dominated by slackwater flood deposits. Paleohydrologic modeling suggests a lower flood regime existed during the terminal Pleistocene (11,200 B.P. to 10,000 B.P.), with the Q100 for that interval estimated at 38,000 cfs. Flood severity increases during the early Holocene (10,000 B.P. to 8,200 B.P.), with the Q100 for that interval estimated at 70,000 cfs. The sediment and flood modeling suggest relatively cool and dry conditions persisted during the terminal Pleistocene (11,200 B.P. to 10,000 B.P.). The early Holocene (10,000 B.P. to 8,200 B.P.) is interpreted as warmer with probably more precipitation in the basin because of decreased cave-deterioration sedimentation, accompanied by increased slackwater deposition.