Fluvial Systems in the Rock-Record: Can Progradation of a Mega-Fan Explain Millions of Years of Evolving Alluvial Deposition in the Upper Triassic Chinle Formation in the Petrified Forest National Park, Arizona
The Upper Triassic Blue Mesa Member and overlying Sonsela Member of the Chinle Formation consist of paleosol-bearing alluvial strata whose characteristics vary markedly and suggest changing alluvial deposition. Strata exposed within a 4km2 study area at Petrified Forest National Park are evaluated using sedimentologic, stratigraphic, paleopedologic and petrographic criteria to determine what, if any, autocyclic or allocyclic forcing mechanisms may account for varying alluvial deposition. The middle Blue Mesa Member contains a large channelized scour infilled with heterolithic lateral accretion deposits with a prominent, red paleosol weathered into terminal channel fill and overbank deposits. The overlying upper Blue Mesa Member consists predominantly of drab, bluish-colored overbank paleosols with thin, discontinuous sandstones. In contrast, the Sonsela member contains sand-prone facies and immature, moderately drained purple paleosols. This succession is interpreted to record deposition during progradation of a distributive fluvial system (Hartley et al, 2010; Weissmann et al, 2010) or mega-fan as a result of allocyclic tectonic forcing. A subsidence curve suggests low rates of subsidence with decreasing rates of accommodation gain upsection. This low accomodation gain may have aided progradation of a clastic wedge into the Chinle basin. Upsection decreasing sandstone mineralogic maturity suggests increasing proximity to the fan apex and source area. Paleosol macromorphology suggests more poorly drained conditions from the middle Blue Mesa to the upper Blue Mesa followed by improved paleosol drainage within the Sonsela. Estimations of paleo-precipitation from geochemical proxies show only slight variation throughout the succession, and it is interpreted that trends of paleosol drainage may reflect changing depth to groundwater with fan progradation rather than changing climate.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California