Abstract

The Upper Triassic Sonsela member of the Chinle Formation in the Petrified Forest National Park was evaluated using sedimentologic, stratigraphic, paleopedologic, and petrographic criteria along a continuous 0.5-km (0.3-mi) outcrop. The study interval consists of interbedded sandstones and mudstones and is composed of a two-tiered hierarchy of cyclic alluvial deposits with bounding paleosols. The succession is composed of 15 fluvial aggradational cycles (FACs) that comprise two FAC sets (FAC-Sets). The FAC-Sets are composed of architectural elements suggestive of a mixed-load fluvial system that is alternately dominated by bed-load deposits and suspended-load deposits. A thinning and fining stacking pattern within FAC-Sets is accompanied by an upward increase in pedogenic modification, suggesting that cycles systematically stack in response to a longer period decrease in the rate of accommodation gain. Sandstones are classified as litharenites, feldspathic litharenites, and lithic subarkoses, and occur within recycled orogen, dissected arc, and transitional arc provenance fields. Sandstone compositional maturity increases upward through the FAC-Sets. Point counts of intergranular volume (as a proxy for primary porosity) within channel facies and subsequent transform to syndepositional permeability provide a two-dimensional depiction of the lateral variability in reservoir quality. Paleosols are weakly to moderately developed and have little stratigraphic variation. These characteristics suggest that climatic fluctuations are not responsible for evolving fluvial depositional style or associated reservoir quality. Trends in sandstone compositional maturity suggest that fluvial stacking patterns and depositional style are related to pulses of tectonism. Sandstones are volcanogenic rich and have undergone an almost complete diagenetic loss of porosity caused by the precipitation of authigenic clays. Paragenetic reconstruction suggests that porosity loss occurred contemporaneous with the silicification of fossil logs in channel deposits. Log compaction at the time of silicification averaged 9.1%, suggesting that log silicification and porosity loss occurred soon after deposition.

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