Lithofacies heterogeneity, fluvial style variations, and floodplain vegetation distributions : deposition and diagenesis of the Chinle Formation at Petrified Forest National Park, Arizona.
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Trendell, Aislyn M.
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The Upper Triassic Chinle Formation in the Petrified Forest National Park (PFNP) was evaluated using sedimentologic, stratigraphic, paleopedologic and petrographic criteria. The study interval, which includes the Blue Mesa Member and overlying Sonsela Member, consists of paleosol-bearing alluvial strata whose characteristics vary markedly. The Sonsela Member is a fining upward succession that contains a higher proportion of coarse and conglomeratic sandstones than the Blue Mesa. Evidence is presented that suggests that the Sonsela was deposited within a mixed-load fluvial system that was influenced by tectonism. Sonsela sandstones have undergone an almost complete diagenetic loss of porosity due to the precipitation of authigenic clays that likely occurred at or immediately after burial and contemporaneous with the silicification of fossil logs in channel deposits. The study succession from the Blue Mesa Member to the Sonsela Member 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 paleosol/ overbank drainage. Mean annual precipitation remained stable throughout deposition of the succession despite changing paleosol drainage. These features and an upsection decreasing sandstone mineralogic maturity are consistent with deposition within a progradational large fluvial fan system. Sediment accumulation rates within the study interval suggest decreased subsidence within the upper Blue Mesa Member that may have promoted progradation of the fluvial fan system. The Sonsela Member contains few paleobotanical fossils other than abundant silicified conifer logs within channel sandstones and, as such, its ecosystem is poorly understood. Sonsela paleosols contain fossilized root traces that provide information regarding plant size and densities. Paleosol maturity suggests that the Sonsela fluvial system experienced high rates of lateral migration and cannibalization of overbank sediments. Fossilized root characteristics suggest that small-stature plants were living on channel-proximal paleosols while distal floodplain paleosols may have hosted herbaceous understory, small-stature shrubby and fewer arborescent plants. There is little evidence to suggest that Sonsela paleosols hosted a dense coniferous forest and conifer logs may have been sourced from uplands. Rhizohalos within the Sonsela Member appear diagenetic.