Pliocene Conglomerates of the Los Angeles Basin and their Paleographic Significance. Foraminifera of the Repetto Hills

Citation

Edwards, Everett Carlyle (1933) Pliocene Conglomerates of the Los Angeles Basin and their Paleographic Significance. Foraminifera of the Repetto Hills. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/DGBV-5P24. https://resolver.caltech.edu/CaltechETD:etd-07192007-093245

Abstract

Pliocene conglomerates of the Los Angeles basin and their paleographic significance:

The Los Angeles Basin of southern California is 50 miles long and 25 miles wide, and is bounded by the Pacific Ocean, the Santa Monica, San Gabriel and Santa Ana Mountains and the Ferris Block. It contains an enormous thickness of Miocene, Pliocene and Pleistocene strata.

Physical conditions of origin and the nature of sediments are as closely related as cause and effect. Since the physical conditions of past epochs no longer persist, the sediments may bear them witness. Conglomerates are especially suited for the purpose of locating the source areas of strata, because they preserve not only the original minerals, but the texture, structure and general appearance of the original rocks.

Conglomerates contain only a partial representation of the list of formations present in a source area. The relative abundance of the various rock types occurring in a conglomerate is not proportional to their relative quantities in the original district. This is due mainly to the selective action resulting from difference in resistance to weathering and from differential destruction during transportation.

The Santa Monica Mountains furnished quantities of pholas-bored limestone breccia from the limestone lenses of the Modelo formation, to the Pliocene beds of the west side of the Los Angeles Basin. Conglomerates derived from this range are also characterized by pebbles and cobbles of igneous rocks, released by the break-down, of Miocene conglomerates, reworked and recomposed in the Pliocene sea.

The San Gabriel Mountains shed conglomerate material characterized by an abundance of pink quartz-orthoclase granite, pink quartz-orthoclase pegmatite and aplite, and white spotted albiclase diorite. These mountains were emergent, but of too low relief during the Pliocene and lower Pleistocene to be of importance as a source for elastic sediments. They were, however, a dominant source during Miocene and upper Pleistocene.

Conglomerates of Ferris Block origin are characterized by large quantities of white quartzalbite pegmatite. During the Pliocene and lower Pleistocene the northwest part of the Perris Block was mountainous. From these mountains enormous quantities of coarse and fine elastic material were removed, to be deposited in the northeast quarter of the Los Angeles Basin, in the area now occupied by the Puente Coyote, Monte Bello, and Repetto Bills, The Ferris Mountains were the most important single source for the Pliocene and lower Pleistocene sediments.

The Santa Ana Mountains have been contributing terrestrial material to the Los Angeles Basin since before the advent of the Pliocene Epoch. Conglomerates from these mountains are characterized by quantities of feldspathic porphyries ranging from andesite to latite, quartzite, Triassic sandstone and siliceous slate.

In the middle Pleistocene, a diastrophic revolution occurred in southern California, and the modern physiographic expression developed. The Santa Monica, San Gabriel, San Bernardino, San Jacinto and Santa Ana Mountains were thrust upward along fault planes. The rejuvenated mountains released floods of coarse and fine terrestrial material to the Los Angeles Basin, and fanglomerates began to accumulate at the mountain fronts. The Ferris Block, formerly a positive and mountainous element became depressed as a result of the structural revolution. The Santa Ana River maintained its course, became an antecedent stream, and grew headward into the San Bernardino Mountains.

Foraminifera of the Repetto Hills:

The use of Foraminifera for correlation purposes has rapidly increased during the past eight years. The Foraminiferal assemblages for the Miocene, Pliocene, Pleistocene and Recent epochs have been studied rather fully by the micropaleontologists of several of the oil companies located in Los Angeles, California. Less is known of the Foraminifera of the Oligocene, Eocene and Cretaceous of the Pacific coast.

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