Triassic stratigraphy and biostratigraphy in Socorro County, New Mexico
— Justin A. Spielmann and Lucas
The Triassic outcrops in Socorro County, New Mexico have received relatively little study compared to correlative strata in the north-central and east-central parts of the state. Triassic sections to the north of Carthage, Socorro County (T5S, R2E), encompass the Middle Triassic Moenkopi Formation and Upper Triassic Chinle Group (Shinarump and San Pedro Arroyo formations). The Moenkopi Formation (Anton Chico Member) disconformably overlies the siltstone-dominated Middle Permian Artesia Formation and is mostly cross-bedded sandstones and less common beds of mudstone, siltstone and intraformational conglomerate. Fragmentary capitosauroid amphibian material near the top of the unit near Carthage are the only identifiable fossils collected from the Moenkopi Formation in Socorro County and are consistent with assigning it a Perovkan (Anisian) age. The Shinarump and San Pedro Arroyo formations of the Chinle Group disconformably overlie the Moenkopi Formation in Socorro County; this is the Tr-3 unconformity, a hiatus of approximately 10 million years. The San Pedro Arroyo Formation is interbedded mudstone and sandstone except for the Ojo Huelos Member, which is a prominent limestone interval that can be used regionally as a marker bed. The San Pedro Arroyo Formation can be subdivided into three member-rank units (in ascending order): the Araya Well, Ojo Huelos and Cañon Agua Buena members; two of which are named here (the Araya Well and Cañon Agua Buena). The Araya Well Member is mudstone dominated with a few siltstones, thinly laminar sandstones and pebble conglomerates. Lithology within the Ojo Huelos Member ranges from lime mudstone to brecciated and pisolitic limestone; the member is a calcrete complex. The Cañon Agua Buena Member consists of thick (>10 m) mudstone slopes with numerous micaceous sandstone ledges. Fossils from the San Pedro Arroyo Formation are characteristic of Late Triassic tetrapod faunas in being metoposaur- and phytosaur-dominated. Abundant fossils of large metoposaurid amphibians have been recovered from the Araya Well and the Ojo Huelos members, and this corresponds to the previously established “metoposaurid acme zone” within the Chinle, which, in addition to a record of Desmatosuchus in the Cañon Agua Buena Member, suggests an Adamanian age for much of the San Pedro Arroyo Formation, including the Ojo Huelos Member. The Cretaceous Dakota Formation and/or a thin section of Upper Jurassic Morrison Formation unconformably overlies the San Pedro Arroyo Formation near Carthage.
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- Spielmann, Justin A.; Lucas, 2009, Triassic stratigraphy and biostratigraphy in Socorro County, New Mexico, in: Geology of the Chupadera Mesa, Lueth, Virgil W.; Lucas, Spencer G.; Chamberlin, Richard M., New Mexico Geological Society, Guidebook, 60th Field Conference, pp. 213-226.