PLEISTOCENE LAKE LOMAX: FACT OR FICTION

The southern High Plains of Texas are well-known for the thousands of generally < 10-ha playas and saline lake basins. The Lake Lomax basin is one of the largest depressional areas encompassing approximately 140,000 ha. Many origins for this basin have been proposed. Five deep cores and 2 surface transects were sampled to examine this basin.

Surface soils on the west side of the basin exhibit properties relating to the adjacent escarpment and are colluvial soils formed from backwearing of the escarpment, subsequent erosion and redeposition, and soil formation. The gravels in the soils include both Ogallala and Cretaceous lithologies. Soils in the eastern half of the basin have fewer petrocalcic horizons in the near surface and less well-developed calcic profiles. Source material is most likely down valley redeposition of finer materials rather than the coarse colluvial gravels and sediments from the escarpment fans. Several sand sheets and dunes blanket the center and eastern margins of the basin. An age of 8620±50 (Beta 133060) from a buried soil at 1.4 m in the dunes indicates Holocene landscape stability occurred around 8500 YBP followed by a period of aridity and eolian activity.

An escarpment site represents a nearly complete Cenozoic rock-stratigraphic section for the High Plains escarpment. Rock fragments from these units are found in the gravels of the colluvial soils represented by several transect sites. The other 4 cored sites are within the basin and represent unconsolidated sections resting unconformably on Triassic shale. A series of paleosols, some welded, with alternating argillic and calcic horizons are present throughout the section. In many of the paleosols, carbonate coats argillic clay. There is some evidence for ponded water in 1 core, however its relative elevation and extent is suspect for a large Pleistocene-age lake. Sand and gravel sequences suggest alluvial fan debouchment into streams rather than large lake bodies. The presence of multiple stacked paleosols in sequences of eolian and reworked alluvial/colluvial deposits in the basin indicate that the basin has been a low-relief area for an extended time. Climatic change and periods of relative landscape stability occurred throughout the Quaternary and Tertiary as represented by the multiple paleosol sequences in this basin.