USING PALEOSOLS AS TOOLS FOR UNDERSTANDING CLIMATE, STRATIGRAPHY, AND BASIN EVOLUTION IN SYNTECTONIC FLUVIAL SEDIMENTS IN THE SAN JUAN BASIN, NEW MEXICO, USA

Periods of widespread soil development in active sedimentary basins represent times when neither aggradation nor degradation occur, yet not every inferred period of depositional/erosional inactivity is marked by a paleosol in the rock record. To address some of the factors affecting soil development and preservation in a syntectonic succession of fluvial sediments, we present a basin-scale investigation of paleosols, stratigraphy, and basin evolution in early Paleocene (Danian-Selandian) fluvial sediments of the San Juan Basin, New Mexico, USA. Paleosols with vertic and alfic properties in the Paleocene San Juan Basin become more abundant and better drained through time, a change that could be caused by 1) changing climate conditions, 2) changing non-climatic environmental conditions, or 3) some combination of these, among others. Although the paleosols and their host rocks show no obvious evidence of significant diagenetic alteration, geochemical climate analyses of the clay-rich B horizons of these paleosols are affected by as-yet-unidentified chemical alterations and produce paleoclimate estimates in disaccord with other local indicators and proxies. The lack of ultic paleosols in this sequence adds to the perceived dichotomy that exists between its paleosols, which suggest temperate conditions, and other paleoclimate indicators and proxies that suggest humid subtropical conditions. While our analyses of these paleosols are currently unable to provide quantitative paleoclimate estimates, the physical properties and stratigraphic position of the paleosols suggest that the evolution of a fluvial system and its basin played an important role in both the development and preservation of pedogenic features in the early Paleocene.