Abstract

The Upper Triassic Blue Mesa Member and the overlying Sonsela Member of the Chinle Formation consist of paleosol-bearing alluvial strata whose characteristics vary markedly. Strata exposed within a 4 km² study area at Petrified Forest National Park were evaluated using sedimentologic, stratigraphic, paleopedologic, and petrographic criteria to formulate potential depositional models that may account for varying alluvial deposition. The study succession records a progressive up-section increase in grain size, increase in channel depth and width, increase in lateral and vertical connectivity of channel deposits, decrease in overbank preservation and crevasse-splay and/or sheetflood deposition, and increase in overbank drainage. Mean annual precipitation from the CALMAG paleosol weathering proxy suggests precipitation averages approximately 1300 mm/yr (1.3 m/yr) throughout the succession despite changing paleosol drainage. These features are consistent with deposition in a progradational fluvial system such as a large fan or splay complex. Sediment accumulation rates in the study interval suggest decreased subsidence in the upper Blue Mesa Member. Reduced rates of accommodation gain may have promoted progradation of the fluvial system. An up-section decrease in sandstone mineralogical maturity may be related to increasing sediment bypass in proximal fan positions or increased rates of erosion and sedimentation. Increasingly better drained paleosols are attributed to deposition in more upland positions during fan or avulsion-complex progradation, rather than changing climate.