SEDIMENTS AND PALEOSOLS OF PLEISTOCENE LAKE LOMAX

The Lake Lomax study site is one of the largest low-relief basins in the southern High Plains of Texas encompassing approximately 140,000 ha. The name for the study area was derived from Frye and Leonard (1957; 1964) who believed that the depressional basin had been formed by a Pleistocene lake during the Quaternary Period. Later work (Cronin, 1961; Finley and Gustavson, 1981) suggested that the depressional area was older and predated the deposition of the Ogallala Formation, a Pliocene-age unit. Within the basin, colluvial, eolian, and alluvial units comprise the Holocene surficial materials. Petrocalcic horizons are exposed at or near the surface discontinuously in road cuts throughout the basin. The Lake Lomax site represents an unconsolidated section resting unconformably on a truncated Triassic shaley paleosol, a part of the Dockum Group. A series of paleosols with alternating and superimposed argillic and calcic horizons are present throughout the section. Some paleosols are welded. Field evidence for A horizons is lacking. These stacked sequences represent aggradational sequences of eolian and alluvial/colluvial deposits that have been deposited in this basin throughout the Quaternary and include both Blackwater Draw and Ogallala Formation materials. At least 4 similar groups or sequences of paleosols and the Triassic paleosols and parent materials are present. Individually, these soil sequences represent a record of significant changes in source, weathering, and/or possibly climate from overlying and underlying sequences during the Cenozoic Era. Among these are at least two periods in which gleyed soils developed, representing significantly wetter conditions. These periods were followed by more arid conditions resulting in sedimentary carbonate accumulations. Samples from 3 cores were collected and dated using OSL. Results indicate a mid-Pleistocene age for much of the sequence, although near saturation and bleaching appeared to be a significant problem.