THE LATE QUATERNARY PLANT COMMUNITIES OF THE CAPROCK CANYONLANDS ECOTONE: MULTIPLE PROXY PALEOENVIRONMENTAL DATA FROM TWO DRAWS ON THE EASTERN ESCARPMENT OF THE SOUTHERN HIGH PLAINS OF TEXAS, USA

The Caprock Canyonlands are an ecotone and physiographic boundary in northwest Texas, USA, between the Southern High Plains to the west and the Central Lowlands to the east. The canyonlands are defined by the steep Ogallala caprock escarpment, remnant mesas, and alluvial outwashes, and have undergone significant periods of erosion during the late Quaternary. In stark contrast to the flat, featureless Southern High Plains surface, the canyonlands contain abundant springs, lithic resources, shelter, and plant and animal food sources that attracted hunter-gatherer groups. New pedologic, lithologic, and multiple proxy paleoenvironmental data are presented from the upper reaches of two draws on the edge of the caprock escarpment: Spring Creek and Middle Creek. Grain-size, stable carbon isotope, and microbotanical data, as well as radiocarbon ages determined on soil organic matter, are presented. Both the upper reaches of Middle and Spring Creeks contain a late Pleistocene through middle Holocene soil and sediment record that has otherwise been removed downstream. At Middle Creek, buried soils in lacustrine and colluvial deposits ranging from ~11,000-2800 ¹⁴C yr B.P. (uncalibrated) contain well-preserved phytolith and diatom assemblages. Microbotanical analyses and stable carbon isotope values of soil organic matter suggest a cool/moist environment at the Pleistocene-Holocene transition, followed by a slight increase in aridity with seasonal wetting and drying of a marsh or shallow lake. The most abundant diatom species, Diadesmis gallica, likely thrived under a thick moss covering near the edge of a shallow lake under a cooler climate than today. At Spring Creek, a series of laterally-inset fills span ~22,000-3000 ¹⁴C yr B.P. (uncalibrated). Stable carbon isotope results from upper Spring Creek indicate that the C₄ photosynthetic pathway emerged during the Last Glacial Maximum, potentially under low atmospheric pCO₂, but a C₃ and mixed plant community persisted in Holocene microenvironments from seasonal moisture availability. The results of the high-resolution paleoenvironmental data set from soils and sediments are providing additional context for the geomorphic and archaeological record that will be compared to adjacent regions.