The Upper Cretaceous Gallup Sandstone in northwestern New Mexico is interpreted as coastal-barrier or delta-front deposits. A study of 160 closely spaced sections along nearly continuous exposures in the Gallup Sag, New Mexico, suggests that these deposits are related to wave-dominated delta and low-sinuosity braided fluvial systems.
The Gallup Sandstone in this area is divided into three prograded depositional packages. The lowermost package consists of shoreface sheetlike sandstones, siltstones, and shales, which coarsen upward into coalesced distributary-mouth-bar and beach sandstones that are locally dissected by fluvially and tidally influenced channel sandstones. This package is overlain by constructional delta-plain deposits and coastal back-barrier deposits, the latter being associated with the destructional phase of the delta system. The constructional delta-plain deposits consist of meandering distributary channels interspersed with interdistributary crevasse-splay sandstones and backswamp coals, carbonaceous shales, siltstones, and shales. The destructional coastal back-barrier deposits consist of heavily bioturbated lagoonal or bay sequences of sandstones, siltstones, shales, and carbonaceous shales. Coal beds associated with this destructional facies are as much as 4 ft (1.2 m) thick and extend laterally as much as 4.2 mi (6.7 km). These coal beds are blanketlike in contrast to the constructional delta-plain coal beds, which are thin, discontinuous, and lenticular. This overall package is overlain by an alluvial depositional package consisting of numerous stacked, overlapped, lenticular, varicolored, pebbly sandstones. These sandstones, which are blanketlike in extent and contain a few poorly developed coals, probably represent low-sinuosity braided stream deposits.