Abstract

Buried Vertisols, which are commonly preserved in smectite-rich clay and sandy clay facies of the upper Tertiary Fort Hancock Formation, are products of the late Tertiary depositional and paleoclimatic environment in the Hueco Bolson, West Texas, United States, and Chihuahua, Mexico. Fort Hancock clayey facies are in part laminated and contain abundant desiccation cracks and locally gypsum beds, suggesting deposition in an ephemeral or playa lake. Buried Vertisols apparently resulted from repeated shrink/swell cycles due to drying and wetting of expansive clays in the ephemeral lake basin. Lacustrine facies are interbedded with gravel, sand, and sandy mud facies that were likely deposited as alluvial fans and fan deltas at the lake margin. Calcic soil horizons, which form mostly in arid to subhumid modern climates and consist of CaCO3 nodules and filaments, are present in most buried Vertisols of the Fort Hancock Formation. Collectively, buried Vertisols with calcic soil horizons and ephemeral-lake and alluvial-fan deposits suggest that the Fort Hancock Formation accumulated in an arid to semiarid climate.

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