Ravenna Quadrangle: Supplement 1 from "Geology of the Ravenna quadrangle, Los Angeles County, California" (Thesis)

Description

In the Ravenna quadrangle, north central Los Angeles county, approximately 8,400 feet of middle Miocene (?) fanglomerates and 3,600 feet of curiously interbedded basalts are exposed. They lie unconformably on a pre-Tertiary basement and unconformably under the Mint Canyon formation (upper Miocene). For this group of rocks the name "Vasquez Series" is suggested to replace the term "Escondido Series" of O.H. Hershey, which is preoccupied. The fanglomerates of the Vasquez Series are composed of angular fragments of anorthosite, quartz diorite, granite, and gneiss, commonly a foot or two in diameter though larger fragments are abundant. The fragments are embedded in a sparse matrix of sand and gravel cemented by calcite and iron oxides. The rude and irregular strata of the series are typically red, brown, or white in color. These sediments accumulated rapidly under semi-arid conditions in a large canoe-shaped basin and formed a series of coalescing alluvial fans which sloped in general toward the west. The upper part of the series contains local occurrences of fine silty beds of lacustrine origin. Some of the interbedded basalts exhibit phenomena suggestive of intrusive emplacement, others are clearly extrusive. These lavas range from massive hypohysline porphyricitc rocks to spongy amygdaloidal masses. Subsequent to deposition, the Vasquez Series was uplifted and deformed. After an epoch of erosion the basaltic conglomerate of the basal Mint Canyon (middle Miocene (?) was laid down. Faulting is greatly predominant over folding in the Ravenna quadrangle. Since upper Miocene time two periods of faulting have occurred. The first characterized by the Soledad fault, a normal fault, and the second characterized by a series of northeast trending strike slip faults. This last group of faults is clearly due to shearing stresses. Physiographically the area has a mature topography which was formed by the end of the Pleistocene. This mature topography is now being dissected by headward working streams rejuvenated in recent time.

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