QUATERNARY STRATIGRAPHY, GEOCHRONOLOGY, AND CARBON ISOTOPE GEOLOGY OF ALLUVIAL DEPOSITS IN THE TEXAS PANHANDLE (RADIOCARBON).
AuthorSTAFFORD, THOMAS WIER, JR.
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PublisherThe University of Arizona.
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AbstractSedimentology, stratigraphy, and stable-carbon isotopy were used to reconstruct geologic and climatic events on the Texas southern High Plains from ca. 13,000 yr B.P. to the present. The alluvial sediments in Yellowhouse and Blackwater Draws were used to construct the geologic history. The oldest valley alluvium comprises the > 13,000-yr-B.P. fluvial sediments that were incised and buried by fluvial and lacustrine sediments dating ca. 13,000 to 4900 yr B.P. Lacustrine waters changed from oligotrophic to eutrophic and finally calcalitrophic. Regional valley erosion at 4900 yr B.P. developed a widespread disconformity within the Yellowhouse Draw formation, which separates lower fluvial and lacustrine sediments (ca. 13,000-4900 yr B.P.) from the overlying sediments dating 4900 yr B.P. to present. After 4900 yr B.P., intermittent streams and eolian processes deposited several meters of sand the length of each valley. Cienegas returned to downstream reaches of both draws after 1500-2000 yr B.P. Methods were developed to extract purified collagen residues and hydroxyproline from heavily contaminated fossil bones. Reliable δ¹³C measurements on collgen require isolation of single amino acids, whereas less specific purifications may yield accurate bone collagen ¹⁴C dates. Collagenous residues were extracted from 13,000-200-yr-B.P. fossil bison bones from the Lubbock Lack Site at Lubbock, Texas, and δ¹³C values were determined. Collagen δ¹³C values changed from -8 per mil at 200 yr B.P. to -10 per mil at 4900 yr B.P. and to -17 per mil at 12,500 yr B.P. The δ¹³C changes imply that the Lubbock area grasslands contained 30 to 40 percent C₄ grass biomass at 12,500 yr B.P. in contrast to the 95 percent C₄ grass biomass in today's grasslands. The stratigraphic and isotopic results gave similar paleoecological histories for the Texas southern High Plains. At 12,500 yr B.P. permanent streams existed and grasslands may have resembled those in the northern central Great Plains today. The climate warmed gradually, and the water table dropped until 5000 yr B.P. when a major hydrologic shift occurred. After 4900 yr B.P., modern climatic depositional and vegetation communities were developed. Geomorphic thresholds apparently controlled the regional disconformities, depositional events, and pedogenetic episodes. Climatic change was the ultimate cause of stratigraphic changes, but individual geologic events were not coeval with any similar climatic shift.