The geology of the upper Las Llajas Canyon area, Santa Susana Mountains, California

Abstract

The Upper Las Liajas Canyon Area, California is some six and one-half miles long and three and one-half miles wide. It is situated five to twelve miles north-west of the town of Chatsworth. It gives a representative picture of the geology of the California Coast Ranges. The main Santa Susana Range here trends almost due west. It is cut by steep-sided, youthful canyons which generally produce a relief of some five hundred feet. The highest point in the area is 3741 feet in elevation; the greatest relief, however, is not over 2500 feet. The dominant rocks are, east-west striking, faulted and folded sediments of Tertiary age; some late Mesozoic and Quaternary sediments also appear. The oldest rocks in the area, a series of sandstones and intercalated shales, comprise a part of the Chico formation. The portion mapped attains a thickness of more than 750 feet. Apparently unconformably above the Cretaceous lies a section of 350 feet of very coarse massive ferruginous conglomerates of lower Eocene Martinez age. The Martinez seems to grade into the middle Eocene Llajas formation. The Llajas consists of a series of fossiliferous shales, silts, sandstones, and black limestones which crop out over a large portion of the area south of the Santa Susana thrust zone. In the western portion of the area the Sespe formation appears and is composed of variegated sands and silts, possibly varying in age from upper Eocene to lower Miocene. The Sespe lies disconformably on the Llajas formation and wedges out to the east. Sediments, ranging in age from Pliocene to middle Miocene, overlie the middle Eocene with pronounced angular unconformity. Of these, the lowermost formation, the Topanga, consists of a basal fossiliferous conglomerate some 50 feet thick overlain by 100 feet of fossiliferous sandy silts. The uppermost member of the Topanga is a zone of silt from 25 to 100 feet thick characterized by the foraminifera species, Valvulineria calitornica Cushman. The upper Modelo diatmites overlie this zone in places with possible slight angular discordance. The average thickness of the Modelo is 200 feet. North of the Santa Susana fault zone, a thick series of upper Miocene shales, sandstones, and grits, which have apparently been pushed into the area by overthrusting, crop out. In most localities above the Modelo diatomite, and generally in unconformable relationship with it, are a series of fossiliferous sandy silts some 250 feet thick and of lower middle Pliocene Pico age. Unconformably above the Pico lie upper middle Pliocene sediments which become nonmarine upward in the section and grade into the Pleistocene (?) Saugus sands and gravels. The whole series exposed here is over 1150 feet thick. At least two ages of alluvial deposits were mapped. The main structural feature in the area is the northward-dipping Santa Susana thrust fault which became active in the Pleistocene epoch. The thrust zone itself is, as the name implies, not one clean-cut fault but a zone of roughly parallel faults forming a somewhat braided pattern. Most of the faults are rather steep-angled, northward dips from 30 to 70 degrees being recorded and the average being around 50 degrees. The total displacement of this zone is not known but is at least several thousand feet. The strata along the front of the overthrust block are in many places overturned. Numerous strike-slip faults, trending normal to the strike of the Santa Susana thrust zone appear to have offset the structures in the block itself and also those in front of the overthrust mass including the Quaternary alluvial deposits in places as much as 3000 feet. The area to the south of the Santa Susana fault zone is faulted and folded but not to the degree of the region to the north. The topographic features suggest that the region owes most of its present relief to uplift by faulting and folding in the Pleistocene epoch. The presence of terraces and inner gorges in several of the main canyons as well as the courses of the main streams indicates an erosional history that is far from simple. The relatively older topography, found south of the overthrust zone, and the younger topography appearing in the overthrust block itself, corroborate the geologic evidence indicating late movement along the Santa Susana thrust zone.