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
Figures & Tables
Sedimentary Cover—North American Craton
The “sedimentary cover” refers to the stratified rocks of youngest Proterozoic and Phanerozoic age that rest upon the largely crystalline basement rocks of the continental interior. The early chapters of the volume present data and interpretations of the geophysics of the craton and summarize, with sequential maps, the tectonic evolution of the craton. The main body of the text and accompanying plates and figures present the stratigraphy, structural history, and economic geology of specific sedimentary basins (e.g., Appalachian basin) and regions (e.g., Rocky Mountains). The volume concludes with a summary chapter in which the currently popular theories of cratonal tectonics are discussed and the unresolved questions are identified.