GRAIN-SIZE ANALYSIS OF THE QUATERNARY BLACKWATER DRAW FORMATION, SOUTHERN HIGH PLAINS, TX: EVIDENCE FROM A NEWLY CORED INTERVAL

The Blackwater Draw Formation (BDF) comprises the surface deposit of the Southern High Plains (SHP). It covers roughly 120,000 km² and is described as an entirely aggradational eolian sequence deposited after the Pecos River Valley to the southwest of the plains eroded through the Caprock Caliche and isolated the SHP from the Rocky Mountains. At the type section of the BDF, a distinct switch in grain-size, mineralogy, and geochemistry occurs within the section. These results suggest that two distinct lithostratigraphic units may comprise the BDF, a southern eolian unit as well as a northern loess. To test these ideas, a new 13.9 m push core was recovered near Bushland, Texas. There is limited previous data from the northern part of the SHP, and the new push core represents the thickest known portion of the BDF that does not contain playa fill. Here, we present a detailed laser particle grain-size analysis from that core.

Overall, the mean and coarsest 10% fine upward. The coarsest 10% fines upward from medium sand (2ϕ), at the base, to very fine sand, (4ϕ) at the top. There are a few spikes in sand grain size associated at the base of suspected paleosols. The median grain size fines upward from coarse silt (5.5 ϕ), at the base, to medium silt (6ϕ), at the top of the section. The finest 10% stays relatively constant at clay grain-size, oscillating around 9ϕ. Frequency histograms are nearly always positively skewed with the mode displaced from the mean towards the coarser fraction. The coarsest ten percent is comprised of 28% fine sand and 72% medium sand. The 50th percentile is relatively constant, and is 100% silt. The finest 10% is 100% clay and occasionally shifts to a finer silt in relationship to specific soil horizons. The core is punctuated by multiple soils that are defined by three carbonate horizons. In comparison to the type section, located farther south near Lubbock, TX, the Bushland Playa site exhibits a general finer grain size throughout the core and lacks the coarse sand component observed towards the base of the section to the south. The results of the coarsest 10% indicate incorporation of sand into a silt-rich unit throughout the section, which could be a result of sand recycling on the SHP mixing with the eolian silt from suspension.