Page 27 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

26 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
portion of the limestone bed has formed a highly siliceous drusy rock, often called a "quartzite" or "limestone quartzite." Under- lying this siliceous phase of the rock, the limestone in many places is broken, and fragments of the limestone 1 to 6 feet in diameter have been cemented together by calcareous material derived from the rock itself. In some places the breccia includes blocks of white grit or sandstone. The upper silicified member of the Fusselman limestone constitutes the footwall limestone for the ores of the Kingston district and elsewhere, and at the Lookout mine in the Tierra Blanca district the ore occurs in fractures within and near the top of this bed. At Lake Valley there is a bed of quartzite 5 to 10 feet thick near the top of the Silurian formation, which in turn is overlain by 12 feet of pink or red limestone, carrying fossils which indicate its age as Silurian.
DEVON(AN SYSTEM
Percha Shale.—Resting upon the eroded surface of the Fus- selman limestone is a black shale formation known as the Percha shale from the type locality in Percha Creek near Kingston. This shale represents late Devonian time, and although accord- ant in attitude with the overlying and underlying formations, it is separated from them by breaks in sedimentation. It attains a thickness of 250 feet in the Sierra Caballos, 160 feet at Lake Val- ley, and 200 feet at Hillsboro and Kingston. In most places the lower beds are fissile black shales, and the upper beds, though chiefly gray shale, contain layers of slabby and nodular lime- stone. These upper beds weather to buff or brown with little or no fissility. The lower black shales contain no fossils, but the upper blue or gray shales near Hillsboro and Kingston 'have an abundant and well-preserved Upper Devonian fauna. In many places the basal portion of the Percha shale is brecciated and altered, presumably by heated siliceous waters, to a fine- grained red to black siliceous rock resembling jasper or flint. The brecciated fragments may be red, gray or black, in a matrix of like substance but of different color. Numerous quartz vein- lets and drusy cavities occur in this part of the formation.
MISSISSIPPIAN SYSTEM
Lake Valley Limestone.—Exposures of Lake Valley lime- stone are widespread in Sierra County, with the typical exposure in the Lake Valley district. Other known occurrences are in the Sierra Caballos, and in the Hillsboro, Kingston and Hermosa districts. At Lake Valley, where the top of the formation is eroded and in part overlain by Tertiary igneous rocks, the thick- ness is about 210 feet. At this place the lower member, 50 feet thick, is a compact massive gray limestone with nodular chert, which may possibly be Devonian. Above this is 5 feet of coarse crystalline yellowish-white limestone, 20 feet of grayish-blue limestone which is more or less siliceous, 25 feet of blue shale