Page 38 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

GENERAL FEATURES 37
In front of the Fra Cristobal Range and diagonally across the north end of the Sierra Caballos, a hinge fault appears to have dropped the younger sediments in the Hot Springs region, while elevating a long narrow segment of pre-Cambrian rocks in front of the Fra Cristobal. The movement of this faulted block in the neighborhood of Hot Springs has effected a longitudinal splitting of the sedimentary beds at the north end of the bierra Caballos, and these have been dragged down and under, so that over half of the normal thickness is now concealed from view. At the same time the length of the Caballos dome was shortened by compression with the development of transverse overthrust faulting and folding. This is plainly seen in Palomas Gap, where the northern part of the range has been overthrust onto the central mass, and again in the southern part about opposite Derry, where the southern part has been pushed several hun- dreds of feet to the north.
The development of the main north-south faulting in this region was accompanied in many places by transverse breaks which served to relieve the various strains set up within the rock masses. It is along these transverse planes of movement that the drainage pattern of Sierra County has been very largely developed.
In addition to the regional dislocation, many of the igneous intrusions by their thrusting action have produced local faulting, usually of rather simple radial pattern. In the Hillsboro district this radial arrangement of the fractures is definitely associated with the intrusive monzonite of the district. In the Macho dis- trict the intrusions are radially arranged latite dikes quite similar to the apophysal dikes at Hillsboro. In the Black Range at the Silver Monument mine and near the town of Roundyville, clusters of radially striking veins occur superimposed on the gen- eral northward-striking vein system of the region, and these may be the result of unexposed intrusions.
GEOLOGICAL SEQUENCE
The oldest rocks in Sierra County are the pre-Cambrian granites and schists that outcrop near Kingston, in the Mud Springs Mountains, and along the western base of the Fra Cris- tobal Range and the Sierra Caballos. These rocks underlie the younger formations throughout the county. Resting on their eroded surfaces is the thick sequence of Paleozoic quartzites, shales, and limestones, with some conglomerates and arkoses. It is probable that these sedimentary rocks were deposited over the entire county, although rocks of Mississippian age and earlier have not been identified north of Hermosa in the Black Range or north of the Mud Springs Mountains near Hot Springs. These sediments were not laid down in a continuous succession, for the terrane was subjected to several periodic uplifts, when non-