New Mexico Geological Society Annual Spring Meeting — Abstracts

Triassic-Jurassic strata exposed just south of Palo Duro Canyon on the Sevilleta Grant (El/2, sec. 22 [unsurveyed], TIN, R2E) pertain to the Moenkopi, Chinle and Morrison Formations. Myers et al. (1986, USGS Map I-1567) incorrectly mapped these rocks as Upper Triassic Dockum Formation (=Moenkopi) and Cretaceous Tres Hermanos Formation (=Chinle and Morrison). The Moenkopi Formation rests disconformably on reddish brown, bioturbated and laminar siltstone and fine sandstone of the Permian "Bernal Formation" (Artesia Formation) and is 61 m dominated by grayish red, trough crossbedded litharenite. As much as 5 m of "mottled strata" (pedogenically modified siltstone and nodular calcrete) of the Chinle Formation rest on the Moenkopi and are overlain by 25.6 m of variegated purple and yellow, bentonitic mudstone of the lower Petrified Forest Member of the Chinle. The 3.2 m of ostracodal/oolitic limestone that overlie these strata contain Late Triassic vertebrate fossils: skull and girdle fragments of large metoposaurid amphibians and teeth, scutes and vertebrae of phytosaurian reptiles (NMMNH locality 1330: SE1/4 NW1/4 SEI/4, sec. 22, T1N, R2E). This limestone bed is the same bed that occurs in the lower Chinle near Carthage (sec. 5, T4S, R2E), at Mesa del Yeso (sec. 28, T1S, R2E), at Ojo Huelos (34°43'46" N,106°42'42" W) and at Carrizo Arroyo (sec. 6, T6N, R2W). It is a
regional marker bed in the lower Petrified Forest Member of the Chinle Formation. At Palo Duro Canyon it is overlain by 3.6 m of purple, bentonitic mudstone terminated by a strike-slip fault. East of the fault are 4.5 m of green, ledge-forming claystone, siltstone and micrite that are remarkably similar to the Brushy Basin Member of the Jurassic-cretaceous Morrison Formation. These Morrison strata are disconformably overlain by trough crossbedded quartzarenite and conglomerate that mark the base of the Cretaceous Dakota Group.