Review of Upper Triassic stratigraphy and biostratigraphy in the Chama Basin, northern New Mexico

Abstract

Triassic strata in the Chama Basin of Rio Arriba County, New Mexico, pertain to the Upper Triassic Chinle Group (in ascending order, the Zuni Mountains, Shinarump, Salitral, Poleo, Petrified Forest and Rock Point formations). The base of the Chinle Group locally is the Zuni Mountains Formation (formerly mottled strata), a pedogenic weathering profile as much as 7 m thick, developed in the top of the Pennsylvanian-Permian Cutler Group. Where the Zuni Mountains Formation is absent, the base of the Chinle Group is the Shinarump Formation. In the Chama Basin, the Shinarump Formation (= Agua Zarca Formation of previous usage) is as much as 13 m thick and consists mostly of trough-crossbedded, quartzose sandstone and siliceous conglomerate. The Salitral Formation is as much as 31 m of mostly greenish and reddish brown, bentonitic mudstone. The Salitral Formation is divided into two members: a lower, Piedra Lumbre Member of greenish mudstone with a peristent sandstone bed (the El Cerrito Bed) at its top, and an upper, Youngsville Member, which mostly consists of reddish-brown mud- stone. The Poleo Formation is up to 41 m thick and is mostly grayish yellow, trough-crossbedded litharenitic and subarkosic sandstone with minor amounts of both intrabasinal and siliceous conglomerate. Above the Poleo Formation, as much as 200 m of strata, dominated by reddish brown, bentonitic mudstone, constitute the Petrified Forest Formation. In the Chama Basin, the Petrified Forest Formation consists of two members, the lower Mesa Montosa Member, up to 24 m of thin-bedded sandstone, siltstone and mudstone, which is overlain by up to 176 m of the bentonitic mudstone-dominated Painted Desert Member. The Rock Point Formation in the Chama Basin disconformably overlies the Petrified Forest Formation and is as much as 70 m thick and mostly laterally persistent, repetitive beds of reddish brown and grayish red siltstone and ripple-laminar sandstone that essentially lack volcanic detritus. In the Chama Basin, unionid bivalves from the Petrified Forest Formation are consistent with a Revueltian age, and paly- nomorphs from the Painted Desert Member of the Petrified Forest Formation and from the Rock Point Formation are of Norian age. Three formations of the Chinle Group in the Chama Basin contain biochronologically important vertebrate fossils, notably the aetosaur Desmatosuchus haplocerus (Adamanian) in the Salitral Formation, the aetosaurs Typothorax coccinarum and Desmatosuchus chamaensis and the phytosaur Pseudopalatus buceros (Revueltian) in the Petrified Forest Formation, and the phytosaur Redondasaurus (Apachean) in the Rock Point Formation. These fossils and lithostratigraphy allow precise correla- tion of the Chinle Group strata exposed in north-central New Mexico with other Upper Triassic strata in New Mexico.