Triassic stratigraphy in southeastern New Mexico and southwestern Texas

Abstract

— Upper Triassic strata exposed in southeastern New Mexico and southwestern Texas are assigned to the Santa Rosa, San Pedro Arroyo and Dockum Formations of the Chink Group. In southeastern New Mexico (Chaves, Eddy and Lea Counties) the Santa Rosa Formation is as much as 25 m thick and is mostly trough-crossbedded extraformational conglomerate and sandstone with minor beds of mudstone or siltstone. It disconformably overlies Upper Permian (Artesia Group or Quartermaster Formation) strata. The San Pedro Arroyo Formation conformably (?) overlies the Santa Rosa Formation and is at least 50 m of variegated smectitic mudstone and minor sandstone/conglomerate. Regional geologic maps have greatly overstated the extent of Upper Triassic exposures in southeastern New Mexico. In southwestern Texas (area from Pecos to Mitchell Counties) the Dockum Formation consists of the basal Camp Springs Member and overlying strata here assigned to a new stratigraphic unit, the Iatan Member. The Camp Springs Member is at least 15 m thick and is dominantly extraformational, siliceous conglomerate. It disconformably overlies the Upper Permian Quartermaster ( = Dewey Lake) Formation and is conformably(?) overlain by the Iatan Member, which is 80- 100 m thick and characterized by intercalated, persistent intervals of red smectitic mudstone and trough-crossbedded micaceous sandstone. Fossil vertebrates indicate the Camp Springs and Iatan Members are of late Carnian (Tuvalian) age. Physical stratigraphy and lithology suggest correlation of the Santa Rosa Formation with the Camp Springs Member and the San Pedro Arroyo Formation with the Iatan Member.