Abstract

Fluvial flood-plain deposits of the upper Aguja, Javelina, and El Picacho Formations in Brewster and Presidio Counties of West Texas record multiple episodes of soil formation during Late Cretaceous (Maastrichtian) time. These well-differentiated alluvial paleosols are characterized by pale gray leached (albic) A horizons and purple or red, clay-and iron oxide-enriched (cambic, argillic) B horizons. These characteristics indicate that podzolization and lessivage were important pedogenic processes and suggest that these paleosols are comparable to some modern AI-fisols. Texturally diverse accumulations of calcite nodules, coalesced masses of nodules, rhizocretions, and thin discontinuous hard-pans indicate that calichification also occurred. The extreme development of the petrocalcic horizons, partial silicification, association with sulfate minerals, and high position within the soil profiles indicate that calichification was in many cases not con-temporaneous with iron and clay illuviation. The compound paleosols thus formed indicate that the Maastrichtian climate in this region may have fluctuated between regimes of humid and semiarid character, each at least several thousand years in duration. This inferred long-term climatic cyclicity may correspond to climatically induced sedimentation cycles observed in the marine realm that are ascribed to Milankovitch cycles. Fossil wood suggests that the flood-plain paleosols supported coniferous forests, whereas adjacent stream courses were lined by angiosperm woodlands.

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