Abstract

Previously undocumented cobbles and boulders of partly welded to welded rhyolitic tuff have been discovered within dominantly fine-grained deposits of the Middle Jurassic Carmel Formation. These large clasts occur in mudflow deposits and fluvial conglomerates of the upper member of the Carmel Formation in south-central Utah. The composition and texture of the monolithic tuff clasts indicate that they were derived from ignimbrite deposits. An increase in abundance and size of the volcanic clasts, as well as paleocurrent indicators in enclosing sedimentary rocks, suggest a southwestern source. The clasts were most likely derived from a volcanic source associated with the Jurassic magmatic arc that lay in a belt across contiguous parts of Nevada, California, and southern Arizona. The paleogeographic implications are that the magmatic arc lay just off the edge of the Colorado Plateau and the associated volcanic apron likely overlapped the plateau.

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