Page 92 - The Geology and Ore Deposits of Sierra County, New Mexico - Bulletin 10

MINING DISTRICTS 91
the old settlement of Palomas Camp, about a mile east of Her- mosa on the north bank of Palomas Creek.
GEOGRAPHY
The altitude at Hermosa is 7,200 feet. Palomas Creek and its tributaries have cut deep canyons in the sedimentary rocks of the region, and just east of the town the main stream has formed cliffs which rise almost vertically to a height of 1,000 feet above the stream bed. The tributaries in general flow north or south, following along the lines of the regional faults that have sliced the east flank of the range into long narrow blocks. Erosion by these streams has formed a cuesta-like topography in the easterly dipping overlying formations along the east bank, while the west bank is more apt to be an easterly dip-slope of the underlying formation.
GEOLOGY
Gordon39 describes the general geology of the Hermosa
region as follows :
Hermosa lies on the western boundary of a belt of sedimentary rocks which have been exposed by the removal of extensive deposits of lavas that once covered the region. On the west are volcanic rocks, chiefly andesite, with scattered patches and embayments of limestone. Igneous rocks, both intrusive and eruptive, occur also within the sedimentary belt. At Hermosa the area of sedimentary rocks is 3 or 4 miles wide. About 2 miles below Palomas camp these beds are terminated abruptly by the Palomas gravel, which abuts directly against the cliff formed by the erosion of the limestone. Along the sides of the valley here the Palomas gravel has been deeply trenched, exposing a thickness of approximately 800 to 900 feet. North of
Palomas Creek are several buttes capped with basalt. 40
At Palomas camp the walls of the canyon are about a thousand feet in
height and are composed in large part of massively bedded limestone. In the lower half some shale and quartzite beds alternate with the limestones. At the base of the escarpment is a thick-bedded fine-grained grayish-blue limestone overlain by dark carbonaceous shales. The ores at this camp occur in pockets and shoots in the limestone below the shale. Fossils ob- tained from the limestone were identified by George H. Girty as forms occurring near the base of the Pennsylvanian series. From this it would appear that the base of the section at Hermosa is to be correlated with the Sandia formation at Magdalena and farther north, indicating a change in the character of the sedimentation toward the south whereby the shales and quartzites give place to limestones.
The strata dip from 20° to 30° N. 62° E. At Palomas camp the gen- eral slope of the beds is interrupted by a low arch which pitches to the north. At this point two systems of faults occur, the major faults running N. 38° W., and a series of lesser breaks trending approximately with the dip. One of these, the "Kendall break," contains one of the principal ore deposits of the district.
About 2 miles above Palomas camp, toward the west, the limestone is cut off by andesite tuff. Along the boundary occurs a remnant of an earlier flow of andesite about 600 feet wide, which runs in a nearly north-south direction. On the west side of the mass the surface rock is andesite tuff, showing well-marked stratification planes which bend up abruptly on meet- ing the old ridge. The south fork of the river on which the town is situated
39Gordon, C. H., op. cit. (U. S. G. S. Prof. Paper 68), pp. 266-267.
40 Just east of the limestone-gravel contact in Palomas Creek, dikes of basaltic rock may be seen, which are perhaps the feeders for the flow remnants noted by Gordon.