The maps below are part of Muehlberger's paper.
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 ten by
thirty 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 map
area, is a thick sequence of muscovite schist, chlorite-muscovite
schist and actinolite-chlorite schist with rare layers of quartzite and
limestone. This unit underlies the Sierra Pelona, an elongate mountain
mass which trends east-west. Granitic intrusive rocks of probable
late Jurassic age underlie a complex section of Tertiary rocks in mo st
of the eastern Soledad basin. Gneisses, some of which may represent
highly injected Pelona schist, also are present in a belt that in general
trends parallel to the Sierra Pelona.
In general, the sedimentary beds exposed 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 series which crops out over a wide area,
and comprises interlayered fanglomerates and volcanic rocks. Where
the Martinez formation has been removed by erosion prior to the
deposition of the Vasquez beds, the Vasquez series rests on the preTertiary
crystalline rocks. This unit is of doubtful Oligocene age,
and has a maximum known thickness of nearly 16,000 feet: The Vasquez
beds were deposited in three basins separated by ridges. Late in
Vasquez time, these ridges were buried by thick alluvial fans built
northward from the San Gabriel Mountains, and the basins thus
coalesced into a single broad alluvial apron.
The upper Lower Miocene Tick Canyon formation and the
Upper Miocene Mint Canyon formation, which lie above the Vasquez
series, also represent nonmarine deposition of dominantly coarsegrained
sedimentary material. These units are widespread in the
western part of the Soledad basin. Sandstone and siltstone of the
"Modelo" formation rest unconformably on the Mint Canyon beds, and
reflect an eastward encroachment of marine waters over a part of the
basin in late Miocene time. West of the map area, the marine Pliocene
Pico formation and the nonmarine 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 the 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 locally
near the major faults nearly isoclinal folds are found. Almost without
exception, these folds plunge to the west or southwest.
The numerous faults are the most prominent structural
features of the Soledad basin. Normal faults which trend generally
east were formed in post-Martinez, pre-Vasquez time. Displacements
occurred throughout Vasquez time and ceased prior to Tick Canyon
time. The Pelona fault may have been reactivated just prior to Mint
Canyon time. Offsets on these normal faults are as much as 10,000 to
15,000 feet. The normal faults indicate that the minimum compressive
stress was oriented nearly north-south. 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 northeasttrending
left-hand faults with displacements up to 10,000 feet. None of
the faults of the Soledad basin have been reactivated in spite of Pleistocene
and Recent offsets along the San Andreas fault.