The Sonsela Member of the Upper Triassic Chinle Formation in Petrified Forest National Park, Arizona, USA, forms a distinctive part of an extensive ancient river system that once flowed, at least in part, from a major volcanic arc bordering the western margin of North America (Cordilleran magmatic arc).
The Sonsela Member is characterized as a relatively coarse-grained unit compared to other mudstone-dominated members of the Chinle Formation, and is therefore thought to reflect a unique period in the evolution of the basin.
Modern exposures of the Cordilleran arc are poorly preserved, the result of subsequent tectonic deformation, erosion, and sedimentation in western North America since the Late Triassic. Sedimentary sequences of the Chinle Formation, however (particularly those of the Sonsela Member), preserve the dynamic evolution of the basin as the volcanic arc was established in southwestern North America.
The Sonsela Member therefore serves as the most reliable indicator of fluctuations in arc dynamics of a poorly preserved and disjointed portion of the Cordilleran magmatic arc.
Sedimentological constraints on the evolution of the Cordilleran arc: New insights from the Sonsela Member, Upper Triassic Chinle Formation, Petrified Forest National Park (Arizona, USA),Evan R. Howell, Noble Energy and Ronald C. Blakey, DOI:10.1130/B30714.1.