From Contributions to Economic Geology, USGS, 1922
The following notes concerning the metal prospects of the Los Burros district, in the southwestern part of Monterey County, Calif., are based on observations made during a visit of a few days to this section of the Santa Lucia Range in February, 1921. The author is under obligation to the few operators in the region for their many courtesies, particularly to the owners of the Gorda properties and the caretaker at the Buclimo mine.
The district lies on the west side of the rugged Coast Range, near the south line of Monterey County, in Tps. 23 and 24 S., R. 25 E. Mount Diablo meridian. Most of the properties are near the Government mail trail between Jolon, east of San Antonio River, and Gorda, a small ranching settlement on the coast about midway between Monterey and San Luis Obispo. It is reached from King City, a station on the Coast Line of the Southern Pacific Railroad at the south end of Salinas Valley, by 45 miles of road and. 14 miles of rough, steep trail. Travel except on the trails is next to impossible, because of the dense tangle of scrub oak, madrona, and manzanita. There is a fair stand of spruce and pine on the upper slopes of the mountains, and redwood is found in the canyon bottoms on the coast side of the range at elevations as great as 2,000 feet. Most of the prospects are confined to the drainage basin of Willow Creek and the headwaters of Alder Creek, a smaller stream south of Willow Creek.TOPOGRAPHY
The Coast Range in this region is 12 miles wide and has a rather even sky line approximately 3,300 feet above the sea and 2,000 feet above the low country to the east. The summit of the range is 5 to 6 miles east of the coast line. This narrow mountain wall is intricately cut by deep, narrow gorges, and in consequence the surface is extremely rugged, as is well shown on the Cape San Martin topographic map issued by the United States Geological Survey.
The only flat areas in the mountain region are remnants of old beaches, now found elevated above the sea. The highest remnants noted are at an elevation of 900 feet, but the largest, upon one of which Gorda is located, are about 100 to 200 feet above sea level. South of Cape San Martin, however, there are well-developed beaches which stand 500 feet above sea level.GEOLOGY
The whole district is underlain by rocks of the Franciscan formation. No intrusive rocks other than the serpentinized basic dikes so characteristically associated with the Franciscan were noted, though intrusive granite occurs in the mountains at the headwaters of Nacimiento River. No attempt was made to map the irregularly distributed masses of serpentine, but there appear to be two more or less continuous belts or dikes, lying about 2,000 feet apart and trending in a west-northwest direction, which run through the south-central part of the district. The most conspicuous exposures of the southern belt were noted along Spruce Creek, in the vicinity of the Gorda mine, and at the head of Alder Creek, near the Buclimo camp. The northern belt is well exposed along the south wall of the canyon of the South Fork of Willow Creek. The serpentinized rocks are almost completely altered, but a few specimens taken from the center of a particularly large outcrop indicate that the original rock was a peridotite.
The Franciscan formation as exposed here is made up of dark greenish-gray arkosic, micaceous sandstones, in general fine grained but passing into fine conglomerates that are usually noted as lenses rather than as continuous beds. Although sandstone predominates, a large amount of the rock weathers like shale, because of the fineness of the constituent grains and the parallel distribution of the mica. The most abundant constituent of the sandstone is quartz, but orthoclase and plagioclase feldspar is only slightly less abundant. The dark color of the rocks is due to an unusual amount of hornblende and biotite. Near the serpentine areas the sandstone is altered to contorted schist, much crushed and sheared and cut by veinlets of quartz and calcite. In places in the vicinity of the basic dikes considerable bodies of massive sandstone have been thoroughly impregnated with minute crystals of garnet and pyrite.
All the rocks weather rusty brown, but there is a noticeable difference between the colors developed by weathering of the rocks in the deeper canyons and those near the summits of the ridges. The sandstone is massive, with little evidence of bedding, but weathers rapidly in small angular pieces that disintegrate readily into a brown sandy clay. The conglomerate lenses are usually composed of small rounded or egg-shaped pebbles of pinkish feldspar, white quartz, and black slate. In a few places, however, lenses of coarse arkosic sandstone, with small, flat, angular pieces of black slate, were noted. Only one small lens of radiolarian chert Was seen; this lies at the head of Alder Creek a short distance from the New York mine.
There seem to be two rather distinct facies of the Franciscan formation in this district. The rock exposed in the lower parts of the canyon weathers in larger and more angular blocks of darker color than the overlying rocks that are exposed along the mail trail near the Buclimo mine and on the upper part of the ridge north of Willow Creek canyon. So far as known, serpentine does not occur in the overlying rocks. These upper rocks weather in small fragments, more like shales, though fresh exposures show that they are fine-grained sandstones, composed of essentially the same materials as the slightly coarser sandstones of the lower series.
The heavy growth of underbrush and the deep accumulations of broken rock and soil, even on steep slopes, make study of the structure very difficult. Practically all the readings taken on the west side of the summit gave strikes of N. 50°-70° W. and dips of 20°-60° NE. East of the summit the strike is more nearly N. 45° W. and the dip 65°-70° NE. It is judged that this particular part of the Santa Lucia Range is a monoclinal tilted block, lying between a fault whose escarpment forms the northeastward-facing wall along Nacimiento River and another fault represented by the scarp along the coast. These faults trend approximately N. 40° W. The trace of a major fault trending N. 70° W. is indicated by a series of benches and depressions on the south wall of the South Fork of Willow Canyon, about midway between the two larger serpentine dikes. The structure is probably complex in detail, for much evidence of minor movement, both parallel to and across the general structure lines, is seen in every exposure.
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