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Abstract

Jurassic strata in the southern part of the Rocky Mountain region include such colorful and well-known sedimentary rocks as the Glen Canyon and San Rafael Groups and the Morrison Formation—rock units that contribute greatly to the scenic beauty of the many popular recreation areas in the region. Most of these rocks were deposited in continental environments bordering an epeiric seaway that, at times, migrated into the region from the north or northwest.

For purposes of synthesis and discussion, the rocks are here separated into six divisions, labeled A through F from oldest to youngest, that are bounded by the J-0 to J-5 unconformities of Pipiringos and O’Sullivan (1978) and by sub-Cretaceous unconformities at the top (Fig. 1). Where not cut out by younger erosion surfaces, most of the Jurassic unconformities extend throughout much of the region, although the J-3 unconformity fades out eastward in southeasternmost Utah (O’Sullivan, 1980a). Additionally, most of these unconformities correspond fairly well to the short-term eustatic falls in sea level postulated by Vail and others (1984). An exception is the J-4 unconformity that apparently was not related to any of the major sea-level fluctuations and, instead, may have been produced by tectonic processes entirely within the Western Interior.

This chapter includes important stratigraphic revisions and nomenclatural changes made especially in the Colorado Plateau region in recent years (Green, 1974; O’Sullivan, 1980a, b; Pipiringos and O’Sullivan, 1978; Peterson and Pipiringos, 1979; O’Sullivan and Pierce, 1983; Condon and Peterson, 1986; Condon and Huffman, 1988; Peterson, 1988a). Scarcity of

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