Stratigraphy and paleontology of the lowero Chinle Group (Adamanian: Latest Carnian) in the vicinity of St. Johns, Arizona
Andrew B. Heckert and Spencer G. Lucas
Topographically low exposures of Upper Triassic Chinle Group strata near St. Johns, Arizona, overlie the Middle Triassic Moenkopi Formation and can be assigned to four formation-rank units. These are, in ascending order, the Zuni Mountains (= “mottled strata”), Shinarump, Bluewater Creek (= “Mesa Redondo”), and Petrifi ed Forest formations. Petrifi ed Forest Formation strata in the vicinity of St. Johns pertain to three members, the basal Blue Mesa, medial Sonsela, and upper Painted Desert members. There are two principal unconformities in this section, the Tr-3 unconformity at the base of the Chinle and the Tr-4 unconformity at the top of the Blue Mesa Member. The Sonsela Member of the Petrifi ed Forest Member disconformably overlies the Blue Mesa Member throughout this region. However, although the Sonsela consists of basal extrabasinal conglomerates overlain by coarse-grained sandstones, it does not always form a persistent bench or ledge, and often must be identifi ed by lithology alone, not merely by topographic expression. In the northern and easternmost Blue Hills northeast of St. Johns, the Painted Desert Member of the Petrifi ed Forest Formation crops out as badlands of reddish-brown, bentonitic mudstone.
Numerous important vertebrate fossils are known from the St. Johns area. The Placerias quarry south and west of St. Johns, arguably the richest Late Triassic tetrapod quarry in the world, is very low in the Bluewater Creek Formation, only a few meters above the top of the Zuni Mountains Formation. The presence of the aetosaur Stagonolepis and the phytosaur Rutiodon at the Placerias quarry establish an Adamanian age for this fauna. Vertebrate fossils found in the Blue Hills northeast of St. Johns are stratigraphically high in the Bluewater Creek Formation and very low in the Blue Mesa Member of the Petrifi ed Forest Formation. Fossil vertebrate collections from the Blue Hills include the type specimen of the phytosaur Rutiodon (=Machaeroprosopus) zunii and an incomplete skeleton of the aetosaur Stagonolepis wellesi. These index fossils of the Adamanian land-vertebrate faunachron indicate an Adamanian age for the fossil assemblages of the Blue Hills. Furthermore, the National Museum of Natural History (Smithsonian) fossil vertebrate collection includes two Rutiodon skulls from the vicinity of St. Johns, further supporting the Adamanian age of lower Chinle Group strata in this area.
- Heckert, Andrew B.; Lucas, Spencer G., 2003, Stratigraphy and paleontology of the lowero Chinle Group (Adamanian: Latest Carnian) in the vicinity of St. Johns, Arizona, in: Geology of the Zuni Plateau, Lucas, Spencer G.; Semken, Steven C.; Berglof, William R.; Ulmer-Scholle, Dana S., New Mexico Geological Society, Guidebook, 54th Field Conference, pp. 281-288.