The geology of a part of Seminole quadrangle, Los Angeles County, California. Geology of Black Butte quadrangle, Los Angeles County, California
Creators: name: Oder, Frederic Carl Emil
Publisher: name: California Institute of Technology
Year: 1940
Subjects: subject: Modelo shales
subject: Seminole Quadrangle
subject: Rancho Las Virgenes
subject: Modelo sandstones
subject: Medea Creek
subject: Geology
subject: FOS: Earth and related environmental sciences

description: The Geology of a part of Seminole Quadrangle, Los Angeles County, California:

The work was done in partial fulfillment of the requirements for a Senior Thesis problem at the California Institute of Technology. The area mapped lies roughly as a strip in the central portion of Seminole Quadrangle, Los Angeles County, California. It is bounded on the north by the Los Angeles - Ventura County line, on the east by a line drawn southward from the intersection of the east boundary of Rancho Las Virgenes and the county line, on the south by a line approximately two thousand feet south of the San Bernardino Base Line, and on the west by Medea Creek. The area described is approximately six and one-half square miles in extent. The field work was done in the spring of 1940 and took approximately eighty hours to complete.

Mapping was done by Brunton Compass on U.S.G.S. base maps
photographically enlarged to a scale of one inch equals one thousand feet (1/12000).

The formations within the area are middle Miocene basalt flows with interbedded pyroclastics overlain unconformably by Modelo shales and sandstones. The relief is slightly less than six hundred feet. Exposures are good, with the exception of the alluviated valley adjacent to Ventura Boulevard.

Geology of Black Butte, Black Butte Quadrangle, Los Angeles County, California:

The work entailed in mapping the geology of Black Butte
was done in partial fulfillment of the requirements for a Senior Thesis problem at the California Institute of Technology. This desert butte has its center located at the intersection of the first parallel north of the San Bernardino Base Line end a north-south line at the longitude 117°43’30" west of the Greenwich Meridian,
and covers an area of approximately three-quarters of a square mile. The field work was done in April of 1940 and required approximately twenty-five hours to complete.

Mapping was done by Brunton Compass on U.S.G.S. base maps
photographically enlarged to a scale of one inch equals five hundred and sixty-three feet (1/6760).

The rocks found within the area are an igneous series together with a very small amount of metamorphosed sedimentary rock which is apparently present as inclusions in one member of the igneous series. This igneous series, together with the metamorphics, have been intruded
by later dikes of aplite, pegmatite, quartz, and aplite-pegmatite. Since the intrusion of the dikes there has been minor faulting followed by mineralization chiefly in the form of epidotization.

The butte is roughly oval in plan view with the long axis of the oval extending in a northwest direction. Towards the southeast end there is a narrow projection extending east-south-east. The butte is a mountain remnant projecting out from the alleviated floor of the Mojave Desert here formed by the ends of fan deposits extending from the base of the San Gabriel to the south. Rising steeply from the alluvial desert floor Black Butte has a relief of five hundred feet. Seen from a distance the butte appears dark colored due to the extensive coating by “desert varnish” of the boulders of the gabbrodiorite intrusive which cover a large part of the surface.

In general, exposures were very poor and pertinent information is obscured. The mapping of the dikes was best accomplished by first determining their actual presence and character at the few existing exposures, and then following the traces of their courses by observation from a not too distant vantage point.