Testing depositional and provenance models of the Quaternary Blackwater Draw Formation of the Southern High Plains

The Blackwater Draw Formation (BDF) composes 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, southwest of the SHP, eroded through the Caprock Caliche and isolated the SHP from the Rocky Mountains. At the Type Section of the BDF, located near Lubbock, TX, a distinct transition in grain-size, mineralogy, and geochemistry occurs within the section, which suggests the BDF comprises at least two distinct lithostratigraphic units. To test this idea, a 13.9-m push core was recovered from Bushland Playa west of Amarillo, TX. The newly cored interval was physically and petrographically described as having four buried soils and a modern soil. Following grain-size analysis of the core, it was determined that the coarsest 10% of sediment grain size fines upward from nearly medium sand at the base of the Bushland Core to very fine sand at the top of the core. In comparison to the Type Section, the Bushland Core exhibits generally finer grain size and lacks the coarse sand component observed towards the base of the Type Section. Geochemical data from XRF and LA-ICP-MS analysis indicates that the fourth buried soil is distinct from the other soils in the Bushland Core. The presence of an upper and lower member of the BDF at the Bushland Core is consistent with data from the Type Section. Stark differences between relatively immobile elements of the upper and lower members of the BDF indicate they must have had distinct parental input. In this study, multiple northern and southern sources were geochemically compared to both the Bushland Core and the Type Section, which showed that the upper member of the BDF likely had influence from southern and northern provenance areas, potentially as far away as Nebraska loess, while the lower member of the BDF is mostly sourced from the Pecos River valley and other local parental material.