Page 26 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

GENERAL FEATURES 25
the base are alternating thin beds of quartzite and limestone about 15 feet in total thickness. Locally this part of the forma- tion consists of calcareous shales alternating with shaly lime- stone. Above this transition zone the formation is gray slabby and massive magnesian limestone. The surface of many of the beds is covered with thin reticulating brown deposits of silica, and most of the rock weathers to a pale gray tint, two features that are quite distinctive. The El Paso limestone is between 300 and 400 feet thick in the west face of the Sierra Caballos and 150 to 200 feet thick in the Lake Valley district. In Carbonate Creek in the Kingston district, there is a series of calcareous shales with interbedded limestones 100 feet thick. Overlying this is 300 feet of gray crystalline limestone, the lower part of which may be El Paso limestone. At Hillsboro and at other points along the Black Range, the El Paso limestone was not recognized, although it may be present in places under the exposed beds.
Montoya Limestone.—The Montoya limestone, of latest Ordovician (Richmond) age, lies on the slightly irregular sur- face of the El Paso limestone and is 200 to 300 feet thick at most of the outcrops. The lower part of the formation consists of dark massive limestone in which are many thin layers of chert. The strata are all hard, and at most places the outcrop is a dark cliff in the mountain side. In the Sierra Caballos the formation outcrops prominently with its usual characteristics. At Lake Valley there are 200 feet of gray hard sandstone at the base over- lain by 25 feet or more of cherty limestone of strong Montoya aspect. Near Kingston the base is a gray crystalline limestone, which is apparently not separable from the underlying El Paso limestone, overlain by 30 feet of thin-bedded cherty limestone. A small exposure of Montoya limestone has been found near the southeastern corner of the Hillsboro district, but nothing is known as to its thickness.
SILURIAN SYSTEM
Fusselman Limestone.— Only a small part of Silurian time
is represented by the Fusselman limestone, which contains abun- dant fossils of Lower Silurian (Niagara) age. The formation lies on the Montoya limestone, from which it is separated by a surface of erosional unconformity marked in places by a con- glomerate consisting of pebbles of the underlying formation. In most areas two members are present, a lower one 85 feet thick of compact fine-grained gray limestone that weathers nearly white, and an upper one about 50 feet thick of hard dark mas- sive limestone with fossils. In the southern part of the Sierra Caballos the Fusselman limestone shows in a prominent cliff and consists of characteristic dark-colored massive limestone 50 feet thick. The thickness of the Fusselman limestone varies greatly but probably averages between 250 and 300 feet. At Hillsboro and southward pronounced silicification of the upper