Numerous studies have documented the lateral and vertical facies sequences of the Guadalupian-age Capitan-proximal carbonates of the Yates and Tansill formations on the Northwest Shelf of the Delaware Basin, New Mexico (Dunham, 1972; Hurley, 1978; Smith, 1974; Neese and Schwartz, 1977; Pray and Esteban, 1977). However, in contrast, relatively few detailed sedimentologic studies of the less abundant siliciclastic units that are interbedded with the shelf carbonates have been reported (Smith, 1974; Candelaria, 1982, 1983, 1988; Thorkelson, 1983). The lack of sedimentologic studies of the shelf sandstones has been due, no doubt, to the notable, and disconcerting apparent widespread, absence of visible primary stratification and bedforms indicative of physical processes and depositional environment. As a result, most interpretations of the shelf sandstone depositional environment have been made a priori and have remained largely unchallenged and unverified.

Candelaria (1982, 1983, 1988) described the sedimentological characteristics and lateral facies distribution of the upper two siliciclastic intervals of the Yates Formation as exposed along the outermost 5 km of the Northwest Shelf in the southeastern Guadalupe Mountains, New Mexico (Fig. 1). The two upper Yates sandstones are continuously traceable across the study area and well beyond (east) into the subsurface. The lateral continuity and uniformity of interval thickness on depositional strike is remarkable as is the absence of channeling within the sands or incisement into the subjacent carbonate units.