Page 30 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

GENERAL FEATURES 29
thickness of the formation ranges from 80 to 100 feet, and in general it is a hard, massive gray to buff bed of fine-grained sandstone, with locally a bed of conglomerate at the base.
Mancos Shale and Mesaverde Formation.—Overlying the Dakota sandstone wherever it is exposed is the Mancos shale of Colorado age. Its thickness ranges from 900 to 1,300 feet, and in general it consists of dark-gray sandy marine shale, with con- siderable slabby to shaly sandstone near the top. Lee16 believes that the coal-bearing beds at the north end of the Sierra Caballos, known as the Engle field, are of Benton (Colorado) age. They are largely sandstones of gray to buff color separated by beds of shaly sandstone and shale. Near the Elephant Butte dam is a small area of what appears to be still later beds consisting of white, brown and red sandstones and conglomerates, which grade upward into brown and chocolate-colored shales and sandy shales. These beds are believed to be of Mesaverde (Montana) age.
TERTIARY SYSTEM
On the northeast flank of the Mud Springs Mountains, the small group of hills west of Hot Springs, is an area in which numerous bones and much petrified wood are reported to have been found. On the southeast flank of the Sierra Cuchillo, ap- parently similar beds contain petrified wood. These exposures may be of Tertiary age, although no direct evidence to support this opinion is at hand.
Underlying many of the terraces along the Rio Grande, especially in the neighborhood of Hot Springs and from there southward to Rincon, are horizontal or nearly horizontal beds of sand, soft sandstone and conglomerate. The colors of these beds are buff, pink, salmon and red, some of the red beds with thin layers of greenish clay, or spotted with small to large greenish to grayish circular patches. This greenish color may be due to the reduction of the normal red ferric oxide by local accumulations of organic matter or other reducing agents. The writer believes that these beds in part at least are the equivalent of the Santa Fe formation of late Tertiary (Miocene and Plio- cene) time, as designated by Hayden in 1869 in the northern part of the State. In Sierra County they heretofore have been generally held to be of Quaternary age.
Near the mountain ranges and abutting against them are alluvial fans and beds of conglomerate and sandstone, which were formed during quiescent periods between the various erup- tions of Oligocene time. Lenses of sediments were laid down on the erosion surfaces that developed during breaks in the se-
is 16W. T., The Tijeras coal field, Bernalillo County, N. Mex.: U. S. Geol. Survey Bull. 471, p. 571. 1912.
Lee, W. T., and Knowlton, F. H., Geology and paleontology of the Raton Mesa and other regions in Colorado and New Mexico: U. S. Geol. Survey Prof. Paper 101, p. 199, 1917. Lee, W. T., Stratigraphy of the coal fields of north-central New Mexico: Geol. Soc. Amer-
ica Bull., vol. 23, p. 031, 1912.