Lacustrine sediments from six pluvial lake basins of the southern High Plains, Texas, contain illite, montmorillonite, interstratified illite-montmorillonite, kaolinite, glauconitic mica, and in two basins, sepiolite. Sepiolite, illite-montmorillonite, and glauconitic mica are authigenic; some of the montmorillonite is derived from volcanic ash, but most of the lacustrine clay is detrital and representative of ancient soils of the Llano Estacado. Increase in kaolinite and montmorillonite with depth and corresponding decrease in illite is indicative of a Pleistocene pluvial period antedating Tahoka Clay time (14,000 to 22,000 years B.P.). Lacustrine strata of post-Tahoka age, formed in a closed lake environment, consist of illite, illite-montmorillonite, montmorillonite, sepiolite with gypsum, sodium sulfate, glauconitic mica, and dolomite.

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