AGE OF THE GLEN CANYON AND LOWER SAN RAFAEL GROUPS, REGIONAL CORRELATION, AND TECTONIC IMPLICATIONS

The presumption of an Early Jurassic age for the Glen Canyon Group (GCG) because it is (1) a conformable body of strata, (2) containing middle Early Jurassic palynomorphs, and (3) is bounded above and below by regional unconformities is neither supported by field observations nor basic stratigraphic principles. The J-1 and J-2 unconformities of southwestern Utah are transgressive surfaces. The Sinawava Member of the Temple Cap Sandstone and much of the Page Sandstone are highstand eolian tongues of the Navajo Sandstone within the lower SRG. The J-2 unconformity, which places Middle Jurassic strata on Triassic strata at many localities elsewhere in the Western Interior, is correlative with an unconformity within the San Rafael Group (SRG). The upper GCG interfingers with the lower SRG. Ash-fall tuffs within the lower SRG yield isotopic ages ranging from 170 to 166 Ma. A cryptic unconformity within the Moenave Formation separates the upper GCG from the lower part of the GCG (Moenave Formation). Palynomorphs in the lower Moenave Formation, once thought to be middle Early Jurassic are now interpreted to be early Jurassic (Hettangian) in age. The cryptic unconformity cuts down section southwestward, truncating lower Moenave strata, ultimately placing lower Jurassic rocks of the GCG on Paleozoic rocks in the Mojave Desert. The J-0 unconformity at the base of the GCG may be Late Triassic in age. Foregoing stratigraphic relations imply correlation of the upper GCG, including the Navajo Sandstone, with the Aztec Sandstone of southern Nevada, and interbedded eolian sandstone and intermediate volcanic rocks of the Mojave Desert and southern Arizona, intermediate volcanic rocks of the southern Panamint Range and Inyo Mountains of east-central California, shales, sandstones, and conglomerates of the Sunrise, Dunlap, and Boyer Ranch formations of western Nevada, and the upper Antimonio formation of northwestern Sonora, Mexico. The lower SRG is correlative with overlying 165 to 170 Ma ignimbrite dominated volcanic rocks of the Mojave Desert, southern Arizona, and east-central California. Stratigraphic relations reveal the forebulge geometry of the Temple Cap and Page Sandstones east of the lower Carmel (SRG) trough. Transgression within the trough terminated eolian deposition within the arc and back arc.