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

The presumption of an Early Jurassic age for the Glen Canyon Group (GCG) because it (1) is a conformable body of strata, (2) contains Early Jurassic palynomorphs, and (3) is bounded above and below by regional unconformities is neither supported by field observations nor by basic stratigraphic principles. The J-1 and J-2 unconformities are here interpreted as transgressive surfaces. The White Throne Member of the Temple Cap Sandstone and the Thousand Pockets tongue of the Page Sandstone are here interpreted as highstand tongues of the Navajo Sandstone at the GCG – SRG contact. The J-2 unconformity, which places Middle Jurassic on Triassic strata at several localities elsewhere in the Western Interior, is correlative with an unconformity within the 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 lower GCG (Moenave Formation) separates the lower GCG from the upper GCG. This unconformity and the unconformity with the SRG define the lower and upper boundaries of the Glen Canyon tectonosequence (GCt), respectively. The Moenave Formation below the basal GCt boundary is Hettangian in age. The J-0 unconformity at the base of the Moenave Formation may be of Triassic age. The basal GCt sequence boundary cuts down section southwestward, ultimately placing Lower Jurassic rocks of the GCt on Paleozoic and Precambrian rocks in the Mojave Desert. The foregoing stratigraphic relations imply that the Navajo Sandstone of the GCt is correlative with (1) the Aztec Sandstone in southern Nevada. (2) eolian sandstones interfingering with volcanic rocks in the Mojave Desert, and (3) intermediate volcanic rocks in the southwestern Great Basin. In addition, the Sunrise, Dunlap, and Boyer Ranch formations of western Nevada and the upper Antimonio Formation of northwestern Sonora are assigned to the GCt. The lower SRG (= upper GCt) is correlative with 170 to 166 Ma ignimbrites of the Mojave Desert, southern Arizona, and east-central California. The intertonguing relationship of the GCG and SRG reveal the forebulge geometry of the Temple Cap and Page sandstone east of the Carmel foredeep implying crustal loading to the west.