Circular 131—Pennsylvanian system of Chloride Flat, Grant County, New Mexico

By D. V. LeMone, W. E. King, and J. E. Cunningham, 1974, 18 pp., 9 tables, 15 figs., 1 appendix, sheets.

Analysis of the Oswaldo carbonate sequence indicates a shallow water, normal marine, low to moderate energy, open shelf environment. Occurrences of phylloid algae suggest a regional examination should be made to determine if biohermal bank structures are present and forming possible stratigraphic reservoirs for petroleum. The Oswaldo Formation is exposed in faulted, eroded remnants immediately north of Silver City, Grant County, New Mexico. The Oswaldo unconformably overlies the Lake Valley Formation. The Beartooth Quartzite unconformably overlies the Oswaldo Formation. The fauna of the Oswaldo consists primarily of foraminifera, rugosan and tabulate corals, crinoids, brachiopods, bryozoa, gastropods, ostracods, some sponge spicules, and trilobite fragments. The fusulinids are represented in three reasonably well-defined zones: Millerella-Eostaffella zone, Profusulinella zone, and Fusulinella zone. Three biohermal to biostromal zones of Chaetetes milleporaceous were observed. The algal flora is represented by dasycladacean and phylloid algae. Petrographic analysis of the Oswaldo carbonate sequence indicates a shallow water, normal marine, low to moderate energy, open shelf environment. Silicification, apparently confined to the surface, is recorded. The upper surface of the Oswaldo Formation has a zone of limonite weathering. Patchy recrystallization is noted throughout the sequence. The presence of this Morrow-Atoka sequence of carbonates prompts two interesting paleogeographic speculations. The Oswaldo Formation at Silver City could represent a sag in the positive Florida Islands-Zuni arch axis. The Silver City sag could be an interconnecting link between the marine sediments of the Florida shelf of the Pedrogosa Basin and the Robledo shelf of the Orogrande Basin. The Silver City sag could also represent the westernmost known Robledo shelf.

Stratigraphic studies completed in 1970 revealed the presence of structurally uncomplicated sequence of the Oswaldo Formation in the Chloride Flat fault block of the southern part of the Silver City Range. Chloride Flat is a valley formed on the easily-eroded Percha Shale at the south end of the town of Silver City. The Silver City Range is an eroded east-dipping monocline which exposes a sequence of rocks from Precambrian to Mesozoic age. Eighty-nine ft of Pennsylvanian beds have been observed beneath the Beartooth Quartzite in the Bear Mountain area 6 mi northwest of Silver City. The earlier map does not separate the Pennsylvanian. Four known areas of occurrence of Pennsylvanian rocks are now recorded in the Silver City Range.

The base of the Pennsylvanian is not exposed in the 80 Mountain Area. The Cretaceous-Pennsylvanian unconformity and a partial section are reasonably well exposed in this area. Pennsylvanian is also recorded in the Cleveland mine area. The Cleveland mine area is too structurally complex to construct a section. Pennsylvanian rocks underlying the Cretaceous are present. Structurally, the Bear Mountain fault block is somewhat complex, having been involved with local intrusive activity. The stratigraphic sequence of the Pennsylvanian is not well exposed. Exposures are noted in the Little Walnut Creek Area and the South Fork of the Little Walnut Creek. These two exposures exceed the previously reported 89 ft. Possibly, the Syrena Formation and the Abo Formation both may be represented in the Little Walnut Creek Area.