Geology of the Repetto and Montebello Hills
Creators: name: Quarles, Miller Winthrop
Publisher: name: California Institute of Technology
Year: 1941
Subjects: subject: Repetto Formation
subject: Repetto Hills
subject: Montebello Hills
subject: Pico Formation
subject: Miocene Formation
subject: Saugus Formation
subject: Geology
subject: FOS: Earth and related environmental sciences
subject: Puente Formation

description: The Repetto and  Montebello Hills are an east-west band of low hills four miles east of the Los Angeles city hall. As the hills lie well within the oil producing Los Angeles basin, their geology is of interest to petroleum geologists as well as to residents of the area.

Only sedimentary rocks are exposed, ranging in age from upper Niocene to Recent. The only Miocene formation in the area is the Puente, which is divided into three members; a white diatomaceous lower shale, a middle sandstone with shale lentils, and a silty upper shale.

Conformable above the Miocene is the Pliocene series, represented by two formations: 1) the lower Pliocene Repetto; and 2) the upper Pliocene Pico. The type locality of the Repetto formation lies within the area where it is composed almost entirely of foraminifera-bearing massive siltstone. The Pico formation is represented by siltstones and is divided into an upper and lower member by means of foraminifera and megafossils. The top of the Pliocene section is marked by an angular unconformity.

The two Pleistocene formations are the Saugus and the terrace gravels. The Saugus conglomerates and siltstones, of lower Pleistocene age, are separated from the upper Pleistocene to Recent non-marine terrace gravels by an angular unconformity.

The two	Pleistocene conglomerate formations are distinguished from one another by the petrology of their pebbles. The Saugus contains pebbles of dacite porphyry; the terrace gravels are characterized by a granite with pink orthoclase and a distinctive "dappled" diorite.

Foraminifera are used to fix the gradational Niocene-Pliocene contact and to divide the Pliocene into Repetto and Pico formations. Molluses as well as foraminifera are abundant in the Pico and make possible a division into upper and lower Pico members in spite of uniform lithology. The Molluscan faunas of the lower Pico are warm water types; the upper Pico contains a cold water assemblage. Megafossils from four of the sixteen new localities were studied, and several species not previously found in the Los Angeles basin were recognized.

Two east-west anticlines with a poorly defined syncline between them are the principle structural units in the area. Only he eastern closures of the large East Los Angeles Anticline is found in the northwestern part of the area. A short strip of lower Puente shale is exposed along the axis of this anticline. The 8000-foot series of Puente and Pliocene formations dip away from the axis to the south. Only the lower 1000 feet of the Puente section is exposed on the north limb of the anticline.

The oil-producing Montebello anticline in the southwestern part of the area has a complex structure and history. The surface exposures of Pico siltstone and Saugus conglomerate show an asymmetric fold with a steep south limb. Sub-surface contours an Repetto horizons, however, show that, at depth, the steeper flank of the fold is on the north. This anomaly is causes by by post-Saugus movement on a north dipping normal fault parallel to, and 600 feet south of, the anticline's crest. The Saugus beds which were previously on the flat top of the anticline were dropped 700 feet and are now adjacent to steeper dipping Pico sediments.

Three major disturbances dominated the structural history: 1) after the Pico was deposited, 2) after Saugus deposition, and 3) after the terrace gravels were deposited. Uplift to dry land and slight folding caused the erosion of the Pico siltstone to a surface of low relief. The ensuing subsidence and overlap of cross-bedded Saugus conglomerate produced an angular unconformity above the Pliocene series.

The second deformational period, which occurred after the Saugus formation was deposited, was the most intense. Both the major anticlines in the area were formed, and the beds in the Repetto Hills were steeply tilted to the south. A very long or very active period of erosion followed, for apparently 8000 feet of sediments were stripped off the East Los Angeles anticline to expose the lower puente shale beds. The final post-Saugus erosion surfaces before the terrace gravels were deposited wore I) in the Repetto Hills a topography similar to the present, and 2) a near-peneplain where the Montebello Hills now Stand.

The nature and amount of movement during the third major uplift, in post-terrace or early Recent time, can be accurately determined by the positions of numerous remnants of a terrace surface. The entire Repetto Hills area was raised about 150 feet along the southern border and tilted as a block to the northeast. At the same time the present Montebello Hills were formed when the flat terrace surface was uplifted by folding at least 300 feet, forming an anticline whose crest roughly coincided with the axis of the earlier post-Saugus anticline. The present topography is that anticlinal ridge modified by recent erosion.