Geology of the Aliso and Browns Canyons area, Santa Susana Mountains, California
Creators: name: Cabeen, William Ross
Publisher: name: California Institute of Technology
Year: 1939
Subjects: subject: Aliso Canyon
subject: Pico Formation
subject: Santa Susana Mountains
subject: Browns Canyon
subject: Llajas Quadrangle
subject: Geology
subject: FOS: Earth and related environmental sciences
subject: Topanga sandstone
subject: Newhall Quadrangle
subject: Chico sandstone
subject: Modelo shale

description: NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document.

The most important factor in the area to influence both structure and stratigraphy is the Santa Susana overthrust. Overthrusting along the Santa Susana fault has in many cases taken place along more than one plane producing an imbricate structure. Structurally the period of orogeny which culminated in this overthrusting resulted in close and overturned folding and faulting. The most prominent anticlinal fold is that which extends in a nearly east-west direction across the entire width of the area and passes just below the Holt Ranch, in Browns Canyon.  Faults which are older than the Santa Susana orogeny are found, and include both the normal and reverse types. Some of these earlier faults have cut out varying thicknesses of formation, introducing local stratigraphic gaps. Several periods of folding, uplift arid erosion, indicated by unconformities, are evident, and provide lines of weakness which have been influential in guiding and controlling the later folding. Thin development of formations and the numerous unconformities point to a very unstable area. Recent diadtrophic movement is indicated by folding and faulting of the (Pleistocene) Santa Susana overthrust plane.

Through the medium of this overthrusting, an entirely different stratigraphy from that native to the area has been introduced. Above the thrust, a thick section composed of Modelo shale, (lower Monterey) (Valvulineria californica zone) shale, and non-marine Topanga sandstone, conglomerate, and basalt is found, This section, which has been overthrust from the north, presents an entirely different environmental lithology. The lithology below the thrust, and native to the area, is much less well-developed, and differs considerably from the introduced section found above the thrust. The poor development of these formations, and the fact that they thicken northward and that they are nearly all separated by unconformities, suggests a marginal oscillating environment in the old Miocene-Pliocene depositional basin. The overthrust section and sections to the north, as at Pico and Elsmeree canyons, are much thicker and lack many of the unconformities present in this area. These areas undoubtedly occupied a position nearer the center of the Miocene-Pliocene depositional basin.

The oldest rock native to this area is the great thickness of Chico (Upper Cretaceous) sandstone, which makes up the boldly outcropping Simi Hills. Unconformably overlying this is the basal Martinez conglomerate (lower Eocene), which, in the particular area mapped, is in fault contact with younger Eocene to Pliocene formations. The only other Eocene formation mapped was the thick section (1300 feet [...]) of Llajas (middle Eocene)shales and silts, which grade downward to sandstone and conglomerate near the base of the formation. The Llajas is abundantly fossiliferous. A big stratigraphic break separates the Liajas from an unconformably overlying upper Topanga conglomerate and silt, 50 to 100 feet thick. This thin section of marine fossiliferous Topanga, limited above and below by unconformities, is a marked contrast to the thick section of non marine sandstone, conglomerate, and basalt which lies above the thrust and was introduced from an area to the north by overthrusting. Disconformably above the Topanga, in the section native to the area, is found a thin section of argillaceous Modelo shale, varying in thickness from about 25 feet in the southern part of the area to more than 100 feet in the northern portion, where sandstone lenses and layers are found interbedded. This relatively thin section is in marked contrast to the thick overthrust section of Modelo, which is in excess of a thousand feet thick. The Valvulineria californica shale section found between the Topanga non-marine and Modelo shale lying above the thrust is not found in the section below the thrust.

Disconformably above the "native" Modelo shale is found a late lower and early middle Pliocene concretionary silt which is equal in age to about the middle of the Pico formation at the type section in Pico Canyon. This partial equivalent of the type Pico has been designated as the Pico formation in the area mapped. An unconformity separates these Pico silts from an overlying sandstone, conglomerate and fossil reef formation which has been designated the San Diego formation because of the similarity of its abundant fossil content with that of the type San Diego. This 75 to 100 foot thickness of upper middle Pliocene-San Diego strata is, however, an age equivalent of the upper Pico formation at its type locality. Both mega- and micro- fossil evidence points to this. Thus it may be said that the time represented in the accumulation of the strata at the type locality of the Pico formation has its equivalent in the Aliso and Browns Canyons area. This time equivalent is represented by the San Diego formation, the so-called Pico silts of the area, the unconformity separating the two, and by part of the time represented in the unconformity below the Pico silts and above the Miocene Modelo formation. The San Diego formation grades upward into brackish and non-marine Saugus deposits composed of 500 feet of sands, conglomerates and mudstones.

All of these previously mentioned formations were involved in the latest severe folding which culminated in the Santa Susana overthrust. Lying on the erosion surface formed by the partial beveling of these folded deposits are found younger Pleistocene terrace and river channel deposits consisting of large, hard, sandstone boulders, mud, and Modelo shale fragments.