Abstract

The Rancheria Formation is a distinctive carbonate unit representating deposition in a relatively deep basin (depths probably less than 1000 feet) in south-central New Mexico and West Texas. It typically consists of 200-500 feet of dark gray, sparsely fossiliferous, cherty, fine grained limestone. These limestones crop out in four major areas, the Sacramento and San Andres Mountains in New Mexico, and the Franklin and Hueco Mountains in West Texas. There are also possible outcrops in the Florida Mountains in southwestern New Mexico (Kottlowski, 1963, Fig. 8) and in the Placer de Guadalupe Range, Mexico (Wilson, 1971). Data is limited but the Rancheria may extend several hundred miles to the east of these exposures in the subsurface as the Mississippi Limestone.. Meramec deposits north and west of the Rancheria are represented by the Hachita and Terrero Formations, both composed of shallow water carbonates (Armstrong, 1972; Armstrong and Mamet, 1974).

The Rancheria was dated Meramecian and considered to be unconformable with and to onlap the shelf deposits of the Lake Valley Formation (Osagean) northward into New Mexico by Laudon and Bowsher (1949) and Pray (1961). Armstrong (1962, 1965), however, reinterpreted the Rancheria Formation as a basin equivalent of the Lake Valley Formation and to be Osagean to Meramecian or older based on field relations in the Sacramento Mountains. The author’s work, also based on field relations in the Sacramento Mountains, indicates the Rancheria is unconformable with the Lake Valley Formation as first interpreted (Yurewicz, 1973, 1975). This interpretation is further supported by the biostratigraphic work of Lane (1974, 1975).

Four major lithofacies can be recognized in the Rancheria Formation. One consists of deep-water lime mudstones and spiculitic wackestones deposited from suspension and possibly low density, low velocity turbidity currents (45 percent of the Rancheria). A second lithofacies consists of lime-silt grainstones that represent deep-water turbidity- and traction- current deposition (22 percent of the Rancheria). These two lithofacies reflect deposition below wave base under partially euxinic conditions. A third, but minor lithofacies, occuring predominately along the edge of the basin, consists of coarse skeletal grainstones that represent allodapic sands deposited by turbidity currents and possibly traction currents or sand flows (5 percent of the Rancheria). A fourth lithofacies, composed of lime-silt grainstones, represents shallower water traction current deposition during the final phases of basin filling (22 percent of the Rancheria). The sedimentary structures and trace fossils in this lithofacies suggest that it was deposited above wave base in more agitated and better oxygenated water. Fluctuations in benthonic fossil densities and bioturbation indicate that conditions ranged from anerobic to aerobic during Rancheria deposition.

Outcrops of the Rancheria in the Sacamento Mountains not only provide a look at typical deep water carbonate lithologies, but they also afford a glimpse at some unique basin margin features. The Rancheria here thins from 300 feet in the south to a feather edge in the north. This appears to be largely depositional onlap thinning across the edge of a basin. The Rancheria in the Sacramento Mountains contains many features characteristic of basin margin sedimentation and erosion. These include submarine channels that dissect the basin margin, slump features, and sheets (fans?) of allochthonous skeletal shelf debris.

A deep-water origin for most of the Rancheria is suggested by its dark color, the predominance of mudand silt-size sediment, its very low faunal density, the paucity, of wave- and surf-formed structures, the absence of typical shallow-water features, the amount of relief along the basin margin, and its similarity to other known deep-water carbonates. Depths are postulated to have been approximately 550 feet in the Sacramento Mountains along the basin margin and reaching a maximum near 800 feet in the Franklin Mountains. The presense of shallow water lithofacies, possibly representing depths between 50 and 100 feet, at the top of the Rancheria in the southern outcrop areas indicate shoaling of the Rancheria basin by basin filling during the final phase of Rancheria deposition in this region.