New Mexico Geological Society Annual Spring Meeting — Abstracts

In the San Jon Hill to the Mesa Redonda region of Quay County, the Upper Triassic Redonda Formation has been divided into four formal members (in ascending order): Red Peak, San Jon Creek, Duke Ranch and Wallace Ranch members. The type section of the members is at Red Peak, where the Red Peak Member is 64.4 m thick and disconformably(?) overlies the Upper Triassic Bull Canyon Formation. It is predominantly siltstone, with substantial mudstone and less common sandstone, limestone and conglomerate that are very pale orange, moderate red and reddish brown. The San Jon Creek Member is a 2.6-m-thick bench of pale greenish-yellow, sandy lime mudstone. The Duke Ranch Member is a siltstone-dominated, slope-forming unit that is 20.2 m thick and is mostly siltstone, with colors ranging from pale reddish brown to pale greenish yellow and very pale orange. The Wallace Ranch Member is 7.7 m thick and consists mostly of very fine-grained silty sandstone that forms a prominent bench; this sandstone is massive or crossbedded and is pale reddish brown to yellowish brown. Flaggy siltstone at the top of the Wallace Ranch Member is disconformably overlain by the Middle Jurassic Entrada Sandstone. Vertebrate fossils come from high in the Red Peak Member, low in the Duke Ranch Member, in the middle Duke Ranch Member and in the middle Wallace Ranch Member. The vertebrate fauna from the Redonda Formation is the “type” assemblage of the Apachean landvertebrate faunachron of Late Triassic (late Norian-Rhaetian?) age.