Page 41 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

40 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
time the material derived from the tearing down of the higher extrusive masses contributed to the building of the gravels, sands and silts of the Santa Fe formation. A moderate amount of movement and adjustment continued along the old fault planes of the region, and a few new breaks were formed, but in general, faulting was much less important in this period than during the previous ones. Some deposition of ore minerals followed the extrusion of the rhyolites, and the resulting deposits are gen- erally found along the walls of rhyolite dikes or in close prox- imity to them. Tin deposits are found in the rhyolite flows of the Black Range and Sierra Cuchillo, and along dikes of rhy- olite are found the silver-copper-gold deposits of the Gray Eagle mine near Kingston, the silver telluride and gold deposits of the Lookout mine in the Tierra Blanca district, and small but high- grade gold veins cutting the earlier deposits in the Chloride, Tierra Blanca and Carpenter districts.
Quaternary time was essentially a period of erosion, during which the projecting fault blocks of the ranges were planed off. Except on the west slope of the Black Range and in the San Mateo Mountains, only remnants of the rhyolite flows, tuffs and breccias are left, capping the tilted fault blocks of the region. In many places erosion has cut deeply into the underlying ande- sites or has removed them altogether, exposing sediments of Paleozoic age. The debris from this process accumulated over the beds of Santa Fe age in the Jornada del Muerto, the present Rio Grande valley, and in the small bolsons east of the Black Range, and these deposits are known as Palomas gravel. Locally, flows of basalt poured out over the Palomas gravel, remnants of which now cap the buttes and mesas of the county. One of these flows appears to have cut off the Rio Grande in its former course through the Jornado del Muerto and to have diverted it through the old bolson plain west of the Fra Cristobal Range and the Sierra Caballos, where it now flows. Faulting continued, as is shown by the extension of the main fault of the Hillsboro district up through the Tertiary and Quaternary sediments. Faulting of this age may be more general than heretofore recognized, and may well have provided the channels through which the basalt was enabled to pour out quietly over the surface. It is interest- ing in this connection to note how the remnants of the basalt flows form clusters along the known faults of the region or in areas through which they would pass if extended. General eleva- tion of the entire region during this period is indicated, and the Rio Grande drainage system cut to new depths in the gravels along its course, leaving terraces to mark its cyclic development.
Throughout the region periodic floods continue to build up and tear down the alluvial fans at the base of the mountain ranges, while in the valleys and bolsons, intermittent streams and tributaries continue to carve their way headword. playa deposits are from time to time built up in the interior bolsons, and dur-