Page 175 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

172 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
derived from the disintegration and erosion of the higher peaks and slopes has formed flat valley floors that partly obscure the underlying formations. The bed of Berenda Creek takes a north- easterly course through the main valley that lies at the foot of the northwest-facing escarpment of the tilted rocks. (See Plate X.)
The elevation of Lake Valley is given as 5,412 feet above sea level by the United States Weather Bureau. The climate is mild and the rainfall slight, averaging only 13.25 inches per year. Roads leading into the district are open at all times, and outside work can be carried on effectively all the year round.
GEOLOGY THE ROCKS
The rocks of the Lake Valley area include sedimentary and igneous varieties. The sedimentary rocks range in age from Ordovician to Mississippian, 52 all the intervening periods being represented. The stratigraphy, together with intervals of ero- sion or non-deposition, is illustrated in Plate II.
El Paso Limestone.—The oldest sedimentary formation showing, in the district, is the El Paso limestone of Lower Ordo- vician age, visible along the foot of the northwest escarpment of the tilted block of sediments that rises from the Berenda Valley 11/2 miles northwest of the town of Lake Valley. In this area the formation is about 150 feet thick and is composed of slabby lime- stone. Many of the beds are covered with a thin meshlike growth of brown silica, and where exposed they have weathered to a pale-gray color. These two features are distinctive of the El Paso limestone in this region.
Montoya Limestone.—Above the El Paso limestone and lying unconformably upon it is the Montoya limestone of latest Ordovician (Richmond) age, which consists of 20 feet of hard gray sandstone at the base, overlain by 25 feet of cherty lime- stone. This cherty limestone consists of alternating thin beds of limestone and chert in the upper part and very dark massive limestone below. The lower part of the Montoya limestone con- tains few fossils, but in the upper cherty layers fossils of the Richmond fauna are relatively abundant.
Fusselman Limestone.—Strata of Middle Silurian (late Niagara) age, known as the Fusselman limestone, rest upon the Montoya beds, and are separated from them by an erosional un- conformity. In places the basal part of these beds consists of a conglomerate made up of rounded pebbles of the underlying Montoya limestone in a limestone cement. At Lake Valley the exposures of Silurian limestone are highly silicified and in places much brecciated, especially along the fault scarp on the north- west side of the ridge. Quartz has recemented the fault breccia,
522Darton, N. H., op. cit. (U. S. G. S. Bull. 794), p. 326. Clark, Ellis, op. cit., pp. 139-142.