Stratigraphy and Paleontology of the Lower Chinle Group (adamanian: Latest Carnian) in the Vicinity of St. Johns, Arizona
نویسنده
چکیده
—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 Petrified Forest formations. Petrified 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 Petrified 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 identified 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 Petrified 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 Petrified 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. �������������� ������������������� ������� �
منابع مشابه
A Revision of the Upper Triassic Ornithischian Dinosaur Revueltosaurus, with a Description of a New Species
Ornithischian dinosaur body fossils are extremely rare in Triassic rocks worldwide, and to date the majority of such fossils consist of isolated teeth. Revueltosaurus is the most common Upper Triassic ornithischian dinosaur and is known from Chinle Group strata in New Mexico and Arizona. Historically, all large (>1 cm tall) and many small ornithischian dinosaur teeth from the Chinle have been r...
متن کاملBiostratigraphic Utility of the Upper Triassic Aetosaur Tecovasuchus (archosauria:stagonolepididae), an Index Taxon of St. Johnsian (adamanian:late Carnian) Time
The recently recognized aetosaur Tecovasuchus chatterjeei Martz and Small occurs at several localities in lower Chinle Group strata across Texas, New Mexico, and Arizona. Based on lithostratigraphic correlation and other vertebrate occurrences, all Tecovasuchus occurrences are of Adamanian age. Indeed, all occurrences are consistent with assignment to the St. Johnsian sub-land vertebrate faunac...
متن کاملA New Aetosaur from the Upper Triassic (adamanian: Carnian) of Arizona
Adamanasuchus eisenhardtae is a new genus and species of aetosaur from the Upper Triassic (Adamanian: late Carnian) Blue Mesa Member of the Petrified Forest Formation (Chinle Group) in the Petrified Forest National Park, east-central Arizona. Known from part of its dorsal armor, caudal vertebrae, and a femur, A. eisenhardtae is distinguished by relatively narrow and arched paramedian osteoderms...
متن کاملUpper Triassic Tetrapods from the Lucero Uplift, Central New Mexico
Six localities in the lower Chinle Group of central New Mexico yield a tetrapod fauna consisting of the diminutive metoposaurid cf. Apachesaurus sp., and indeterminate large metoposaurids (probably Buettneria sp.), the phytosaur Rutiodon sp., indeterminate phytosaurs, the aetosaurs Desmatosuchus sp. and cf Stagonolepis sp., a probable theropod, and other, indeterminate archosaurs. Five of these...
متن کاملRedescription of Calyptosuchus (Stagonolepis) wellesi (Archosauria: Pseudosuchia: Aetosauria) from the Late Triassic of the Southwestern United States with a discussion of genera in vertebrate paleontology
Calyptosuchus wellesi is a medium-sized desmatosuchian aetosaur common in Adamanian (early to middle Norian) age rocks from the Chinle Formation and Dockum Group of the Western United States. Known chiefly from osteoderms, this taxon has never been fully described and non-osteoderm material assigned to Calyptosuchus has been done so based on questionable criteria. Mapping of aetosaurian element...
متن کامل