Abstract

The Upper Triassic Chinle Formation of the Colorado Plateau contains voluminous volcanic detritus evidently derived from a source to the south. Volcanic rocks exposed in southern Arizona and northern Sonora have been assumed to represent this source terrane, but U-Pb isotopic geochronology and regional stratigraphic correlations indicate that these volcanic rocks are distinctly younger than the Chinle, and thus not a source for the volcanic detritus in the Chinle. Igneous rocks of known or possible Late Triassic age in Nevada, California, or northeastern Mexico are possible sources, but a clearly defined source terrane for the volcanic detritus in the Chinle has not been identified. Tectonic removal of the source terrane by rifting or strike-slip offset, though not proven, is a possibility.

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