Abstract

The Sonsela Member of the Upper Triassic Chinle Formation in Petrified Forest National Park, Arizona, forms a distinctive part of an extensive fluviolacustrine complex that blanketed the Colorado Plateau region and surrounding areas. Upper Triassic sediments were deposited within a continental backarc setting, synchronous with the development of the Cordilleran magmatic arc. Although the Chinle Formation was derived from a broad variety of source terranes, petrographic and geochemical data clearly fingerprint the Cordilleran arc as a region important to provenance. The Sonsela Member exhibits a stratigraphically abrupt change in depositional style and sedimentary architecture that likely reflects the dynamics of the evolving backarc fluvial system and syndepositional arc magmatism. The stratigraphic boundary between the two disparate depositional styles is marked by a laterally persistent red silcrete horizon that is also associated with a biostratigraphically significant faunal turnover. The lower Sonsela Member—a coarse-grained assemblage dominated by persistent sheet sandstone and conglomerate bodies—reflects a coarse-grained progradation of sediment that suggests low rates of dynamic subsidence and high rates of sediment input. The upper Sonsela Member—characterized by an overall finer-grained assemblage and ribbon sand bodies with an increased proportion of encasing mudstone—reflects an increasing rate of basin subsidence that likely persisted throughout the deposition of the overlying Petrified Forest Member. These packages reflect a sharp change from lower- to higher-sinuosity river systems that closely coincides with the increased rate of subsidence across the regional stratigraphic marker. Previous studies have suggested the importance of the silcrete horizon as a key biostratigraphic marker associated with a significant faunal turnover. This study supports the hypothesis that this horizon may represent a regional unconformity as opposed to local channeling, perhaps the product of an abrupt change in basin geometry followed by an increase in basin subsidence. Because the Sonsela Member is the stratigraphically lowest unit of the Chinle Formation dominated by volcanic clastic detritus on the Colorado Plateau, architectural analysis of the Sonsela Member serves as the most reliable indicator of short-term fluctuations in arc dynamics of the poorly preserved and disjointed Mesozoic portion of the Cordilleran magmatic arc.

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