Stratigraphy and paleoenvironments of late Quaternary valley fills on the Southern High Plains

The dry valleys or "draws" of the Southern High Plains (in northwestern Texas and eastern New Mexico), headwater tributaries of the Red, Brazos, and Colorado Rivers, contain late Quaternary sediments that accumulated over the past 12,000+ years. A few previous, scattered stratigraphic investigations of the draws strongly suggested synchroneity in late Quaternary depositional and soil-forming events and regionwide environmental changes. This volume reports on a systematic study conducted from 1988 to 1992 aimed at better documenting the late Quaternary geomorphic evolution and stratigraphic record of the draws, investigating their paleoenvironmental significance, and determining whether there were synchronous, regional, geomorphic, and soil-forming events in these dry valleys. The work focused on the past 12,000 years because most of the valley fill dates to this time, but older deposits occur locally and were investigated as well. Most of the research was in Running Water, Blackwater, and Yellowhouse Draws (tributaries of the Brazos River), and Sulphur and Mustang Draws (tributaries of the Colorado River), with additional coring on McKenzie, Seminole, Monument, Monahans, and Midland Draws (all tributaries of the Colorado). Approximately 410 cores and exposures at 110 localities were studied. Samples collected from these sections underwent a variety of sedimentological and pedological analyses. Age control is provided by 53 new radiocarbon ages and scores of ages already available from several archaeological sites. Efforts also were made to recover pollen, phytoliths, molluscs, insects, ostracodes, and vertebrate faunal remains, and stable-carbon isotope trends were determined for four sites. Several geomorphic processes and features exerted some influence on late Quaternary drainage development. Quaternary jointing and subsidence controlled drainage patterns around the margins of the Southern High Plains, particularly on the northern edge (Red River system). Major segments of most draws roughly parallel paleodrainages on the buried Tertiary erosion surface. Factors influencing the older drainage likely influenced development of the present drainage. Segments of Running Water, Blackwater, and Sulphur Draws also probably follow ancient drainageways that once connected the plains with the mountains to the west. Most of the draws intersect paleolake basins or extant lake bains, which may have exerted control on drainage development by directing water to paleotopographic lows or by overtopping and interconnection of basins. The last phase of incision by the draws began after 20,000 yr B.P. but before 12,000 yr B.P. and aggradation began ca. 12,000 yr B.P. Valley fill predating this final incision is common locally and includes alluvial sand and gravel (stratum A) and lacustrine carbonate (stratum B). Eolian sheet sand with strong pedogenic modification (stratum C) accumulated on the uplands adjacent to some reaches during valley aggradation. After ca. 12,000 yr B.P. the draws filled with a variety of sediments but produced a similar stratigraphic sequence among all of the drainages. Five principal lithostratigraphic units were identified: strata 1-5, oldest to youngest. Sandy and gravelly alluvium (stratum 1) is the oldest fill postdating final incision. There were several cycles of alluviation contemporaneous with or following the downcutting. The beginning of stratum 1 deposition is undated, but the top of stratum 1 ranges in age, with a few exceptions, from ca. 11,000 to ca. 9,500 yr B.P. Stratum 2 contains beds of diatomaceous mud and noncalcareous or low-carbonate paludal mud conformably overlying stratum 1. Valley-margin facies of eolian and slopewash sands are common locally. A weakly developed soil formed at the top of stratum 2. These deposits are well known for containing extinct vertebrates and Paleoindian archaeological materials. Stratum 2 is quite rare, however. Of the >100 study localities only 12 yielded stratum 2. Beginning of stratum 2 deposition varied from ca. 11,000-ca. 10,000 yr B.P., and the end of deposition ranged from ca. 10,000-ca. 8,500 yr B.P. Stratum 3 is a marl deposited by precipitation in marshes or shallow ponds along the valley axes. Locally the marl has a sandy, relatively low carbonate eolian facies along valley margins. A weakly developed soil formed at the top of stratum 3 (Yellowhouse soil). Most stratum 3 deposition occurred between ca. 10,000 and ca. 7,500 yr B.P., but both the beginning and end of deposition was time transgressive. Stratum 4 is a thick (1-3 m), loamy to sandy, eolian layer. A moderately to strongly developed soil (Lubbock Lake soil: ochric or mollic A horizon over argillic and calcic Bt or Btk horizons) formed in stratum 4 and usually is the surface soil along the draws. Stratum 4 generally dates to ca. 7,500-4,500 yr B.P. The Lubbock Lake soil developed throughout the rest of the Holocene, except where buried by stratum 5. Stratum 5 includes localized accumulations of late Holocene paludal mud (beginning 3,900 yr B.P.) and slope wash and eolian sediment (beginning 3,000 yr B.P.). The late Quaternary fill in the draws provides evidence of significant environmental change. From the latest Pleistocene to the early Holocene there was a hydrologic shift from flowing water (deposition of stratum 1) to standing water (deposition of strata 2 and/or 3), then almost complete disappearance of surface water and the accumulation of eolian sediment (stratum 4). Very broadly, the shifts in depositional environment were time transgressive (younger down draw). These environmental changes resulted from a decrease in effective regional precipitation from the late Pleistocene to the middle Holocene. In the late Pleistocene and early Holocene, local variability in the types and ages of the deposits was controlled by the presence or absence of springs and by time-transgressive decline in spring discharge. The early to middle Holocene eolian fill resulted from desiccation and wind deflation of the High Plains surface. By about 4,500 yr B.P., effective precipitation increased and vegetation became more dense, denoting establishment of the modern environment. There were brief climatic departures toward aridity in the late Holocene.