Page 203 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 199
taceous age, probably Mancos shale, have been brought into posi- tion alongside of the Permian beds. These beds outcrop extensively along the east slope of the Sierra Caballos and at the southern end of the Fra Cristobal Range.
Igneous Rock.—Nearly 3 miles west of Cutter, monzonite porphyry projects through beds of Cretaceous age as a low, rounded and weathered hill slightly less than half a mile in diameter. This may be the top of a small cupola or stock. About 12 miles southeast of Cutter and 4 miles northeast of Upham, three outcrops of porphyry, the largest being a mile in length, project through the sands of the Jornada del Muerto as low irregular hills. Approximately 6 miles south of Upham is a group of low hills consisting of rhyolite flows and tuffs, these rocks belonging to the upper part of the Tertiary lava flows that at one time probably completely covered the Sierra Caballos in a series of thin sheets. These flows approximately mark the east- ern limits of the vast flows which still cover the western half of the county and extend into Grant County. Faulting and folding have depressed these flows in places, and they have subsequently been buried beneath Quaternary alluvium, and on the tilted fault blocks of the range erosion has completely removed them. Remnants of basaltic flows cap several small buttes and mesas east of the Elephant Butte dam.
STRUCTURAL RELATIONS
In general the Sierra Caballos gives the appearance of being
a very large faulted block with the steep escarpment face along the western side and gentle dip slope to the eastward. In detail, however, the dips of the beds diverge slightly, and the range was probably at one time a much-elongated dome, which later was cut by a series of faults parallel to the major axis. The general fault pattern in the Fra Cristobal Range and Sierra Caballos and the relations of the faulted blocks may be seen on the generalized County map, Plate I. The cross sections, figure 16, A-A' and B-B', and figure 17, D-D' and E-E', show additional details of the relations of the blocks to each other. In the Fra Cristobal Range the block of pre-Cambrian granite in front of the sediments is the upthrown side, while along the corresponding fault in the Sierra Caballos the block on the northwest side is the down- thrown block. At the north end of the Fra Cristobal range the vertical displacement is 1,000 feet or more. South of Hot Springs the downthrown block has a displacement of approximately 700 feet. The hinge nature of the movement is particularly evident when viewing the Fra Cristobal range from the west side of the Elephant Butte Lake. The block of Magdalena and later sedi- ments on the east side of the fault dips radically to the east and southeast, and the block has a domed appearance as viewed from the west. The Sierra Caballos also shows the domed structure to the east of the main fault scarp. The northern