ABSTRACT

Studies in the Zuni Basin of West-central New Mexico have resulted in interpretations of the depositional environments for coal accumulation in the Tres Hermanos Formation (and in the landward, partly equivalent, Moreno Hill Formation) and Gallup Sandstone. The Tres Hermanos Formation (middle and late Turonian) is a regressive-transgressive wedge composed of three members, 1) the Atarque Sandstone Member at the base, a regressive coastal barrier deposit 25 to 75 feet thick, 2) the overlying nonmarine Carthage Member which consists of 90 to 200 feet of paludal shales, channel and overbank sandstones, and thin lenticular coals, and 3) the Fite Ranch Sandstone Member, a transgressive barrier complex 2 to 48 feet thick. The Tres Hermanos Formation is separated from the slightly younger Gallup Sandstone by a thin wedge of marine shale—the Pescado Tongue of the Mancos Shale.

The Pescado Tongue thins from 90 feet in the northeast to 28 feet in the southwestern part of the basin and thus during this transgressive cycle the sea inundated a large portion of the present basin. One of the principal effects of a reversal of the direction of the shoreline migration (a turnaround) is an increase in extent and thickness in the laterally equivalent non- marine rocks and a corresponding increase in coal accumulation. In accordance with this pattern the thickest coals in the Salt Lake coal field at the southern end of the basin occur in the Moreno Hill Formation just landward of the Fite Ranch-Gallup turnaround. The F sandstone (late Turonian) forms the basal member of the Gallup Sandstone and is a 10 to 45 foot regressive coastal barrier sequence. The overlying, regressive, nonmarine rocks, in a interval as thick as 140 feet are informally called the Ramah unit of the Gallup Sandstone. Coal near the base of the member accumulated in a back-barrier environment; coal near the top on the member accumulated in an alluvial plain environment. The upper coal zone was rained for local use around the turn of the century and again between 1928 and 1951, and has a maximum thickness of 7 feet, variable ash content, low sulfur, and an apparent rank of high volatile C bituminous. The Ramah unit is overlain throughout much of the Zuni Basin by the distinctive, medium to very coarse grained, reddish- brown, feldspathic Torrivio Member of the Gallup Sandstone. In the northern and northeastern portions of the basin the Dilco Coal Member of the Crevasse Canyon Formation is present above the Torrivio and at the far northern end the upper members of the Crevasse Canyon are also present.