THE EFFECT OF THE MID-PLEISTOCENE TRANSITION ON THE CHARACTER OF SEDIMENTATION ON THE SOUTHERN HIGH PLAINS

The Southern High Plains (SHP) is a large plateau (130,000 km²), encompassing southeast New Mexico to northern Texas. The SHP was largely a site of sediment aggradation since the Pecos River cut off the Rocky Mountain highlands to the west ~1.4 Ma, and forming the Blackwater Draw Formation, a regional mantle composed of eolian deposits separated by paleosols intervals. A change in grain size, geochemistry, and weathering character occurs at ~ 4 meters above the base of the type section in Blackwater Draw, ~ 17 miles north of Lubbock, TX. This change in character is coincident with the Matuyama to Brunhes R to N field reversal at ca. 778 ka and is coincident with the interval of the Mid-Pleistocene Transition (MPT) (from 1.25 to 0.70 Ma). Here, we place the changes in grain size, geochemistry, and weathering character in the stratigraphic section into context of the MPT.

With 10-cm sampling spacing for resolution, the grain-size distribution demonstrates a change in mean grain size from 3ϕ (fine sand) below the MPT to 5ϕ (coarse silt) above the MPT, indicating that post-MPT sediment is best characterized as wind-blown silt or loess. The chemical index of alteration (CIA) increases upward from near 60 to around 70 through each loess-paleosol couplet above the MPT. For comparison, unweathered granite is 50 and average shale is 70. These results indicate that relatively unweathered silt provided the substrate for each weathering event. Comparing immobile element geochemistry of the unweathered silt in this study to Nebraska and Kansas loess deposits suggest a common provenance for loess deposits in both regions. Given this, we present a testable hypothesis that sediment of the post-1.0 Ma strata of the Blackwater Draw Formation was derived during the increased duration and volume of northern hemisphere ice that characterized glacial cyclicity during and since the MPT. Strata older than 1.0 Ma appears to represent sand sheets derived from the Pecos River valley to the southwest and west. Thus, on the SHP, the MPT represents an interval characterized by a change in eolian transport direction and style.