Page 201 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 197
SIERRA CABALLOS
PALOMAS GAP DISTRICT LOCATION AND TOPOGRAPHY
The Sierra Caballos is carved out of a prominent easterly dipping faulted block situated east of the Rio Grande valley, and extending from the Elephant Butte dam to the southern end of the county. The west or escarpment face of the range is steep, while on the east or dip-slope side, the slopes are more gentle, varying between 15° and 20°. The maximum elevation is about 10,000 feet.
GEOLOGY
THE ROCKS
Pre-Cambrian Rocks.—The western front of the Sierra Caballos consists in its lower portion of pre-Cambrian rocks, which outcrop for a distance of 15 miles and in places rise pre- cipitously nearly 1,000 feet above the river valley. These rocks consist of granite and smaller amounts of gneiss and schist, all of which are cut by dioritic dikes, pegmatites and quartz veins.
Bliss Sandstone.—The sandstone at the base of the sedimen- tary section was formerly called the Shandon quartzite, but in this report it is called the Bliss sandstone. (See page 24.) Its thickness varies between 55 and 100 feet, and it can be generally divided into a basal member of dark quartzite and conglomerate 10 feet thick, an intermediate member of white quartzite 4 to 5 feet thick, and an upper member of dark brown and green sandy shale and thin-bedded quartzite 40 to 90 feet thick. Many of the dark strata contain glauconite. In places the beds are highly ferruginous and resemble the Clinton oolitic iron ores of Ala- bama.
El Paso Limestone.—Limestone of Lower Ordovician age lies unconformably upon the Bliss sandstone, and has a thickness of 300 to 400 feet. It consists of a lower member of massive limestone which outcrops in a high cliff, but on weathering breaks into slabby layers of light gray color, the surfaces showing characteristic brown reticulated chert. Near Palomas Gap the upper portion of this formation is more massive, and in this area it also forms a cliff.
Montoya Limestone.—The Montoya limestone of Upper Ordovician age rests disconformably upon the El Paso lime- stone. The thickness of these beds is 200 to 300 feet. The lower member, 200 feet thick, consists of dark massive limestone with a thin sandy limestone or sandstone member at its base. In places an upper member, consisting of cherty beds of limestone, is as much as 100 feet thick.
Fusselman Limestone.—In the southern end of the range the Fusselman limestone makes a prominent cliff of dark-colored, massive limestone. It is 50 feet thick as exposed in this region.