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The Soledad basin is situated north of the San Gabriel Mountains, in Los Angeles County, California, and its center is about 35 miles north of the Los Angeles civic center. Roughly an elongate parallelogram in shape, this basin has dimensions of about 10 by 30 miles, with the longer dimension oriented east-west. The Sierra Pelona and the San Gabriel Mountains form the northern and southern boundaries, respectively. The San Andreas fault and the San Gabriel fault, both of which trend northwest in this region, bound the basin on its northeast and southwest borders, respectively. Only the northern part of the basin is discussed in this paper.
The pre-Cretaceous Pelona schist, the oldest unit in the area, is a thick sequence of muscovite schist, muscovite-chlorite schist, and chlorite-actinolite schist with rare layers of quartzite and marble. This unit makes up the Sierra Pelona, an elongate mountain mass that trends east. Granitic intrusive rocks, probably late Mesozoic in age, underlie a complex section of Tertiary sedimentary and volcanic rocks in most of the eastern Soledad basin. Gneisses, some of which may represent highly injected Pelona schist, also are present in a belt that trends essentially parallel with and south of the Sierra Pelona.
In general, the sedimentary beds at the surface are younger from east to west in the Soledad basin. The marine Martinez formation of Paleocene age is the oldest sedimentary unit in the region. It is overlain by the Vasquez formation of doubtful Oligocene age, which crops out over a wide area and has a maximum known thickness of about 16,700 feet. This younger unit is composed of interlayered fanglomerate and volcanic rocks. Where the Martinez formation was removed by pre-Vasquez erosion the Vasquez beds rest directly on the pre-Tertiary crystalline rocks. The Vasquez beds were deposited in three basins separated by ridges. Burial of the intervening ridges by thick alluvial fans building northward and westward from the ancestral San Gabriel Mountains coalesced these basins into a singl broad area of sedimentation late in Vasquez time.
The upper lower Miocene Tick Canyon formation and the upper Miocene Mint Canyon formation which lie unconformably above the Vasquez formation, also represent non-marine deposition of dominantly coarse-grained sedimentary material. These units are widespread in the western part of the Soledad basin. Marine sandstone and siltstone of the Castaic formation rest unconformably on the Mint Canyon beds, and reflect an eastward encroachment of a late Miocene sea over the western part of the basin. West of the area mapped, the marine Pliocene Towsley formation and the non-marine Plio-Pleistocene Saugus formation, which grades westward into a marine facies, overlie the older rocks. Terrace deposits of late Pleistocene age are common over much of the area. Recent alluvium is present in all of th major valley bottoms and locally in some of the minor valleys.
The date of the folding and metamorphism of the Pelona schist is not known, but it assuredly is pre-Tertiary. Tertiary rocks have been either tilted or deformed into broad open folds, although nearly isoclinal folds are found locally near the major faults. Almost without exception, these folds plunge gently west or southwest.
The most prominent structural features of the Soledad basin are numerous faults. High-angle dip-slip faults that trend generally east were formed in post-Martinez, pre-Vasquez time. Displacements occurred throughout Vasquez time and ceased prior to Tick Canyon time. Net slip on these faults may be as much as 10,000-15,000 feet. The normal faults indicate that the minimum compressive stress in this region was oriented nearly north-south prior to Mint Canyon time. On the other hand, the post-Mint Canyon, pre-Saugus faults indicate a maximum compressive stress oriented north-south, which resulted in a number of northeast-trending left-lateral strike-slip faults with displacements up to 10,000 feet. None of the faults of the Soledad basin has been reactivated in spite of Pleistocene and R cent offsets along the nearby San Andreas and San Gabriel faults.
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