Abstract

The lower Cutler beds (i.e., Rico and Halgaito formations) and uppermost Honaker Trail Formation represent the final phase of Paradox Basin sediment fill during the latest Pennsylvanian to Permian and Late Paleozoic Ice Age. Outcrop exposures in the San Juan River canyon near Mexican Hat, Utah, record mixed marine and coastal-plain deposition across the southwestern shelf of the Paradox Basin. The succession was deposited downwind of a major eolian erg and near the probable terminus of an alluvial fan complex derived from the Uncompahgre Uplift, which bounded the Paradox Basin to the northeast. The coastal plain siliciclastics are commonly described as glacially derived “loess.” However, fine-grained sediments were more likely derived from eolian processes similar to those of modern temperate to subtropical dryland settings due to the paleogeographic position relative to an eolian erg. Paleosols consist of paleoEntisols, paleoInceptisols, and a paleoVertisol and provide clear evidence of a predominantly arid to semiarid climate including pedogenic carbonate, eolian-derived detrital carbonate, and minerals consistent with weak chemical weathering. A new cyclostratigraphic approach centered around discontinuity-bounded alluvial units is applied to investigate continental depositional cyclicity. The stratal succession provides evidence for possible third-order wetter–drier paleoclimatic shifts based on assessment of facies proportions, alluvial grain-size trends, and inferred depositional processes. Paleosol attributes suggest generally dry conditions during this time, but alluvial grain-size trends, resulting from sediment dispersal via fluvial and eolian processes, may suggest shorter-lived periods of increased precipitation, thus highlighting the complicated nature of precipitation patterns for paleoequatorial Pangea before establishment of the Pangean monsoon.