New Mexico Geological Society Annual Spring Meeting — Abstracts

Nonmarine deposition following a regression of the Cretaceous Interior Seaway during Late Turonian time left a sedimentary sequence consisting of fluvial channel sandstones, thin overbank sandstones, and paludal shales containing minor coal. This sequence is locally well exposed in the Zuni Basin of westcentral New Mexico where it rests on the marine Gallup Sandstone and is overlain by the distinctive, feldspathic, Torrivio Member of the Crevasse Canyon Formation (formerly of the Gallup Sandstone). New Ramah, New Mexico the sequence overlies the F Member of the Gallup but progressively northward it overlies slightly younger members. These members are discrete sandstone units representing minor readvances of the seaway during a major regional-scale regression. North and east of Puerco Gap, near Gallup, New Mexico, this nonmarine mudstone dominated sequence thins appreciably and where unmappable it may be included with the Torrivio Member. Southward from Gallup, within the Zuni Basin, the interval locally approaches 150 ft in thickness, contains minable coal beds, and is herein designated the Ramah Member of the Crevasse Canyon Formation for exposures near the community of Ramah, New Mexico. The interval was previously referred to as the coal-bearing member of the Gallup or the Ramah unit.

In the northern part of the Zuni Basin a problem may exist locally in determining the top of the Ramah Member. This is due to the presence of a fluvial sandstone with coarse grained facies that looks much the same as the, Torrivio Member, but underlies it. Two criteria may be employed to distinguish the lower sand from the Torrivio and properly place it in the stratigraphic sequence. These are (1) the lower sand is not as feldspathic as the Torrivio nor do the coarse grained facies contain granule size material; and (2) the lower sand is not nearly as widespread as the overlying Torrivio which has a blanket geometry.

Reference sections for the Ramah Member have been established in the SE1/4 sec. 28, T11N, R17W, and in the NW1/4 sec. 16, T10N, R16W, McKinley county, New Mexico. At these localities the member is 78 ft thick and contains two coaly-carbonaceous zones. The upper zone was mined in nearby Coal Mine Canyon during the 1920's to supply coal for the Zuni Pueblo Schools.