Geology of the Las Flores and Dry Canyon Quadrangles, Los Angeles County, California
Creators: name: Bolles, Lawrence William
Publisher: name: California Institute of Technology
Year: 1932
Subjects: subject: Eocene Martinez Formation
subject: Topanga fault
subject: Las Flores fault
subject: Las Flores Canyon
subject: Geology
subject: FOS: Earth and related environmental sciences
subject: Topanga Formation
subject: Old Topanga Canyon
subject: Santa Monica Mountains
subject: Dry Canyon
subject: Cretaceous Chico Formation
subject: Sespe beds

description: The Santa Monica Mountains trend in an east-west direction and in this region lie north of and parallel to the sea. The area included in this report consists of a strip of land six miles wide and ten miles long reaching from the sea northward to the west end of San Fernando Valley. It is, then, a section six miles wide directly across the axis, and in the center of the Santa Monica Mountains in southern California.

The rocks are highly faulted, the faulting increasing in intensity toward the sea. The older sediments are on the south side of the range, the younger on the north. A fairly complete section is present from upper Cretaceous to upper Miocene in age.

The Cretaceous Chico and Eocene Martinez formations have not been separately mapped. They are represented by a maximum possible thickness of about 6000 feet of conglomerates, sandstones and shales, the thickness probably being far less than that figure. A few limestone reefs are present, particularly in the Martinez. In both the Chico and Martinez beds conglomerates of well rounded cobbles are very characteristic and serve, generally, to identify immediately the age of the sediments containing them.

About 4000 feet of Sespe beds, mostly continental in origin, are present. The strata consist of a coarse, arkosic sand containing scattered pebbles, the entire body being usually a pink or pinkish purple color. The age may be anywhere from upper Eocene to lower Miocene but it is thought that at least the upper part of the section present is Vaqueros (lower Miocene) in age. No fossils wore found in the formation.

The middle Miocene is represented by a thickness of Topanga beds varying in thickness from 3,630 feet to 9000 feet. In Old Topanga Canyon the strata consist of alternating beds of yellow sandstone, shale and some conglomerate. Fossils are fairly abundant and are characterized by a Turritella ocoyana fauna. To the west the beds thicken remarkably and a lower member 4000 feet thick is present. This member consists almost entirely of a coarse, massive, yellow sandstone containing a few specimens of Turritella  ocoyana near its base. It is overlain by 5000 feet of sandstones, shales and conglomerates, much the same lithologically as the beds in Old Topanga Canyon. Intrusions of a black basalt occurred in middle Topanga time. The basalt appears to have been nearly all intrusive and was accompanied by considerable minor faulting.

The Topanga formation is overlain with marked unconformity by siliceous and punky shales of Modelo (upper Miocene) age. The shales grade very rapidly laterally into sandstone lenses, some of them of considerable size. Both pre-Modelo and post-Modelo folding has occurred. No beds younger than Modelo in age are found in the area. As the Modelo was the last formation to be deposited it is now being rapidly eroded and the thickness of Modelo beds exposed is exceedingly variable. At no place in the area does it exceed 3000 feet.

Faults are numerous and, in general, form two systems, one east-west, the other north-south in direction. The two largest faults in the area are the Topanga (east-west) and the Las Flores (north-south) faults. The latter has the largest displacement of any fault in the area, namely 2000 feet.

Several large folds have been developed with their axes trending in a north-northwest direction. Some of the folding occurred in pre-Modelo time but most of it occurred after Modelo deposition, the original folds being further steepened, the axes of folding remaining the same.