Page 39 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

38 GEOLOGY AND ORE DEPOSITS OF SIERRA CO., N. M.
deposition or erosion interrupted the normal succession. The alternating periods of deposition and non-deposition are shown on Plate II. Deposition in Triassic and Jurassic time is not known to have occurred anywhere in Sierra County, and only upper Cretaceous beds remain to represent the Mesozoic period. The alternate elevation and depression of the land sur- face during Paleozoic and Mesozoic times must have been in the nature of gentle pulsating movements of continental extent, un- accompanied by folding, tilting or fracturing of the strata, be- cause throughout this long interval there is little or no dis- cordance in the attitudes of the beds above and below the various erosion surfaces.
During late Cretaceous or early Tertiary time, possibly as a phase of the Cretaceous igneous activity that is manifest around Silver City in southwest New Mexico, the sediments were warped into long anticlines or domes in the Black, Fra Cristobal and Caballos ranges, with gentle synclines in the Jornada del Muerto and the present Rio Grande valley. Minor folds and irregulari- ties probably developed within this area, as on the east flank of the Black Range, and on the west flanks of the Fra Cristobal and Caballos domes. As the igneous activity of the Silver City area continued and became more extended, these minor folds probably continued to develop, and finally the north-south regional faulting so characteristic of the structural conditions of the present day in Sierra County occurred.
Extrusions of andesite inaugurated the period of igneous activity in Sierra County, probably in early Tertiary (Oligocene) time, when the lavas commenced to well up along the regional fractures at many places, and it is quite probable that at the time of their maximum development they covered the entire county. These andesites were poured out in a succession of flows, and in places there was an interval of time between flows sufficient for erosion and sedimentation to accumulate lenses of detrital mate- rial in depressions in the surface of the flow rock and as fans along the edges. Faulting continued along the fractures already developed, and new fractures were opened up. Extravasation of lava from below and the piling up of this material on the sur- face broke the long segments developed by the regional faulting into typical fault blocks, which settled unequally and were tilted in places. Cooling and shrinking of the lava developed minor fracturing within the blocks.
Further igneous activity consisted of the rise along zones of regional fracture of dikes, sills, laccoliths and stocks of inter- mediate rocks such as monzonite and granodiorite, but locally these rocks varied in composition between granite porphyry and diorite. These magmas rose through the Paleozoic sedi- ments, or as in the case of the Cuchillo sill-laccolith, they were intruded into and between the beds, and it is believed that most of the intrusions came to rest with their uppermost portions well