Geology of the Le Brun and Mint Canyon Quadrangles, Los Angeles County, California

Abstract

The area discussed is in the Le Brun and Mint Canyon Quadrangle which in turn form the southwest corner of the Elizabeth Lake Quadrangle. This area lies about 45 miles north west of Los Angeles and comprises a part of Sierra Pelona Ridge, Sawmill and Jupiter Mountains. These mountains form a portion of the transverse ranges. The Pelona schists make up about two-thirds of the area, and are probably of Archeozoic age. A series of migmatite forming inclusions in the granitic country rock are probably of Pre-Cambrian age also. These old metamorphics were intruded during Jura-Cretaceous time by batholith whose average composition is that of a monzonite although different facies of it vary from dioritic to granitic. The Martinez formation was deposited in lowermost Eocene time and is made up of 9000+ feet of sandstones, shales and a few intercalated conglomerate beds. These are marine sediments. Between Martinez and Mint Canyon times a thick series of fanglomerates, sands, silts, and muds were laid down in local basins. These continental beds are red in color, and make up the Le Brun formation and Vasquez series. Although this series contains no lava in this area, it contains large thicknesses of basic lava south and east of this locality. The Mint Canyon formation is also continental in origin and lies on the truncated edges of the Vasquez series. It is composed of a basal conglomerate overlain by well bedded sandstones and shales. Terrace materials of two different ages can be recognized. In addition there is reason to believe that this region was pen plained toward the end of Pliocene time. The strata of the above formations strike east-west or slightly southwest-northeast, and with the exception of the Mint Canyon formation they dip very steeply, in places standing vertical. The foliation of the metamorphic rock also has the same general strike end steep dip of the sediments. The Mint Canyon beds dip off relatively gently to the south. Faulting has been very active in this region and has taken place from middle Miocene time, or perhaps earlier, to the recent. The San Andreas Rift, which passes along just to the north of this area, provides the key to the structural history of the region. Adjacent to the San Andreas Rift large wedges of basement complex shoved up and over the Martinez formation and the Pelona schist, while the faults south of Sierra Pelona Ridge are of the normal type they are not tensional, but compressional faults due to the large horizontal displacement which has taken place along them. Compressional effects such as these are typical along the San Andreas Rift. Folding has played but a minor role in this area. South of the Sierra Pelona Ridge, long slim wedges of igneous and metamorphic rock have been faulted into the Vasquez series; these horses are found sometimes half a mile from the nearest outcrop of igneous or metamorphic rock.