Gallery of Geology - Late Triassic pisolites in Socorro County, New Mexico

One of the most visually distinctive outcrops of sedimentary rock in New Mexico is located in Socorro County, a few kilometers northwest of Carthage, at Cañon Agua Buena. Strata of the Chinle Group, of Late Triassic age, crop out in the ravines here and are assigned primarily to the San Pedro Arroyo Formation, a unit of mudstone, limestone, and sandstone that overlies sandstone and conglomerate of the Shinarump Formation, with the limestones concentrated in the Ojo Huelos Member. Lucas (1991) first named this unit and established the type section in southern Valencia County at Ojo Huelos (Lucas 1991; Lucas et al. 2004). The Ojo Huelos Member is a persistent, distinctive marker unit throughout the Upper Triassic section in both Valencia and Socorro Counties, from Hubbell Springs near Belen and Carrizo Arroyo in the Lucero uplift to its southernmost outcrops near Carthage (Lucas 1991; Lucas et al. 2004; Cather and Osburn 2007; Spielmann and Lucas 2009; Tanner and Lucas 2012). The member consists mainly of micritic lime mudstone, ostracode-bearing wackestone to grainstone, peloidal grainstone, and distinctive pisolitic rudstone, interbedded with fine-grained siliciclastic mudstone. The age of