Calcic paleosols of the Plio-Pleistocene Camp Rice and Palomas Formations, southern Rio Grande rift, USA

Abstract

Seventy-four separate paleosols are recognized in the Plio-Pleistocene Camp Rice and Palomas Formations of the southern Rio Grande rift. Seven types of profiles are present: A-Bw-Bk-C, A-Bt-Bk-C, Bw-Bk-C, Bt-Bk-C, Bt-K-C, Bk-C, and Ak. Calcic and petrocalcic horizons (Bk and K) commonly overlain by argillic horizons (Bt) dominate most paleosol profiles. Greater pore size and enhanced permeability, typical of sandy and silty parent material, favored the development of argillic horizons. However, micrite is the dominant form of calcium carbonate in Bk and K horizons regardless of parent material grain size. Non-calcified A horizons are typically lighter in color than adjacent subsurface pedogenic intervals due to their oxidized nature or lower clay content. They are only rarely preserved, probably due to erosion prior to final burial. Calcified root mats (Ak) are not directly associated with other pedogenic horizons. Abundant horizontal roots suggest root mats formed under high water table conditions. Restriction of Ak horizons to the distal piedmont lithofacies supports their development as spring or pond deposits near the toes of alluvial fans. Individual pedogenic horizons can often be traced hundreds of meters to kilometers normal and parallel to a basin's axis. Paleosol maturity and preservation potential are greatest for symmetrical basins, especially where pedogenesis was far removed from active channels within the axial fluvial system or within distal piedmont settings of asymmetrical basins.