CARBONATE FACIES OF THE LATE TRIASSIC (CARNIAN) OJO HUELOS MEMBER (CHINLE GROUP), CENTRAL NEW MEXICO: REGIONAL AND PALEOCLIMATIC CONTEXT

The Upper Triassic (late Carnian=Adamanian) Ojo Huelos Member, San Pedro Arroyo Formation (Chinle Group) is a distinctively carbonate-rich unit that occurs in the lower Chinle of central New Mexico (Valencia and Socorro counties). The member consists mainly of micritic lime mudstones, ostracodal wackestones to grainstones, peloidal grainstones and distinctive pisolitic rudstones, interbedded with fine-grained siliciclastic mudstones. Most limestones exhibit some evidence of pedogenic brecciation and root penetration, and porous fabrics similar to those of modern limestone tufas occur locally. The interbedded mudstones are typically lenticular and commonly display a blocky ped fabric in which subequant peds are separated by sparry calcite veins. Fossils from the Ojo Huelos Member are freshwater (darwinuloid) ostracodes, various fishes and aquatic tetrapods---metoposaurs and phytosaurs. We interpret the carbonate facies as the deposits of carbonate lakes, ponds and wetlands that were at least partly spring-fed, whereas the interbedded and surrounding mudstones were alluvial in origin. The groundwater and overland hydrology of the region was likely controlled by the relative proximity to an upland recharge area in the Mogollon Highlands to the south, but sedimentary fabrics record strong overprinting by desiccation and pedogenic reworking. Consequently, we interpret the Ojo Huelos Member, which was deposited at a subtropical paleolatitude, as recording a semi-arid, probably strongly seasonal climatic regime, rather than an increase in humidity.

Following initial infill of paleovalleys by the Late Triassic Shinarump river system, the lower part of the San Pedro Arroyo Formation, the Araya Well Member, consisting predominantly of fine-grained siliciclastic lithofacies, was deposited by sheetflood and high-suspended load streams in a rapidly aggrading basin. Thus, deposition of the Ojo Huelos lacustrine limestones, which are unique in Chinle stratigraphy, represents a decrease in the rate of deposition, possibly due to a change in regional base level. The return to siliciclastic-dominated sedimentation during deposition of the overlying Cañon Agua Blanca Member records the resumption of active basin aggradation.