Stratigraphy and Structure of the Central San Jose Hills, Los Angeles, California

Suzanne M. Baltzer

and

David R. Jessey

Introduction

The San Jose Hills are located at the eastern edge of the San Gabriel Valley between Interstate 10 and California 60. The Cal Poly campus lies along the northeast edge of the hills. The San Gabriel Mountains are situated approximately 10 kilometers to the north and the Puente Hills 2-3 kilometers to the south. The study area lies in the central portion of the San Jose Hills and includes mush of the hilly topography at the crest of hills. Elevations range from 200 to 400 meters. The area is presently being developed for custom homes and utilized as a land fill. Access is provided by a series of paved and ungraded dirt roads.


General Statigraphy and Structure

The oldest rocks in the San Jose Hills are the Early Miocene Topanga Formation. They outcrop in the core of the San Jose anticline at the west end of the San Jose Hills. Shelton (see 1955 map) was the first to recognize Topanga Formation, previous workers erroneously identifying the rocks as Puente Formation. The Middle Miocene Puente Formation overlies the Topanga and comprises much of the Puente Hills. In the type area. the Puente Hills, the Puente Formation has four members, the La Vida, Soquel, Yorba and Sycamore Canyon, Shelton made no attempt to map individual members within the San Jose Hills. Locally, in the western San Jose Hills Miocene Glendora Volcanics intrude the sedimentary units. No volcanics outcrop in the central San Jose Hills. Quaternary gravels and alluvial deposits lie to the north and south of the San Jose Hills

Notice that although Shelton did not differentiate the Puente, he did recognize the presence of a massive ledge-forming sandstone he mapped simply as tongues of sandstone.

Olmsted (1950) prepared the first detailed geologic map of the San Jose Hills showing the west plunging anticline that forms the core of the hills. Shelton mapped the San Jose fault along the southeast margin of the hills splitting it into two branches near Mt. San Antonio College. He did not project the fault to the west beyond Grand Avenue. Subsequent trenching to the west along the main (southern) branch of the fault by the City of Walnut and BKK Landfill Corporation never revealed a continuation of the fault zone. The Walnut Creek fault lie to the north of the San Jose hills and some have suggested its trace may enter the San Jose Hills west of Citrus Avenue (note: the actual trace of the Walnut Creek fault has never been located with any accuracy).


Geology of the Central San Jose Hills

Our interest in the San Jose Hills began with examination of aerial photographs taken of the San Jose Hills in the 1950's and 1960's. These photographs clearly reveal the trace of the northern branch of the San Jose fault as mapped by Shelton. The southern branch is more obscure. Upon closer examination three other distinct lineations show up to the west of the known trace of the San Jose fault. The two northernmost lineations were mapped by Pacific Soils Engineering (1972 company report) as faults exposed in outcrop and trenches. The southern lineation (the most striking on the aerial photographs) was not mentioned. Note that it appears to lie along strike of the northern branch of the San Jose fault.

The California Division of Mines and Geology maintains a file of oil and gas drill logs throughout California. The drill logs for the central San Jose Hills were plotted on a base map showing the location of the possible fault (Oil and Gas/Fault Map). Interpretation of the drill logs was not without problems since drillers are not geologists and their terminology is often obscure. Further, there were few usable logs for the central San Jose Hills, as no production was ever recorded from the area. In general, gravels probably represent conglomerate; sands are sandstones; and muds are siltstone and/or shale. Our map shows marked lithologic differences for drill holes north and south of the fault. North of the fault, thick gravels were encountered at fairly shallow depths; possibly the conglomerate beds of the Topanga Formation. South of the fault only minor gravels are present. For comparison, all logs were cut off at 2000 feet, but two of the holes south of the proposed fault reached depths in excess of 6000 feet with no appreciable gravel reported.

Field mapping was facilitated by the presence of numerous trenches resulting in a better exposure of strata than was available to earlier workers. Our geologic map shows that the west plunging anticline of Olmsted is present in the central San Jose Hills. It has, however, been offset by a N 70-80°E striking fault. Left-slip along the fault has moved the southern block eastward juxtaposing LaVida Member and Yorba Members of the Puente Formation. Evidence for the presence of the fault can be seen in trenches at the "tank site" where intense shearing has resulted in a 20 meter wide zone of faulted and folded rocks. Detailed mapping of this area has revealed discontinuities on either side of the fault zone. Immediately to the south of the fault beds strike northeast and dip northwest while north of the fault beds strike roughly east-west and dip to the south. Furthermore although the LaVida and Yorba Members are lithologically quite similar, north of the fault diatomaceous beds are common, while to the south of the fault they are not.

Additional evidence for a major fault can be seen on a larger scale. One hundred attitudes taken on either side of the fault clearly show the two populations of data when plotted as density contours on a pole-to-plane diagram. (Note the large population of northeast strikes and the smaller population of east-west strikes.) Furthermore, even where more poorly exposed, the fault trace is often characterized by vegetation lineations, topographic saddles and landslides that lie transverse to dip of the beds, Exposures at the tank site suggest the fault plane is nearly vertical (see Cross Section). In the absence of clear cut evidence from mullions or slickensides, we acknowledge the possibility that some dip-slip or oblique slip motion may have occurred along the fault.

Two additional faults are shown on the geologic map and cross section. Only the southernmost of the two was exposed during trenching. The northern fault has been added from aerial photographs and geologic maps by Pacific Soils. Attempts to locate the fault in the field were unsuccessful since the area has been developed. It was apparent, however, that the location of the fault coincides rather well with the southern contact of the "tongues of sandstone" as mapped by Shelton. Although poorly exposed in trenches, the southernmost fault appears to dip steeply to the south. The northern fault is shown as having a similar dip based on guilt by association. Movement along these faults is even more problematic. Initially, we felt these faults might be left slip as was the major fault to the south, but the style of deformation is markedly different. We now believe dip-slip reverse motion best fits the field relationships we have observed.

Our interpretation of the stratigraphy was facilitated by a paper by Critelli et. al., (1995). Critelli found that rock fragments contained in sandstones could be used to differentiate members of the Puente Formation. Although his work was done in the Puente Hills, we believed it might have application to the San Jose Hills. Thin sections were made of the "tongues of sandstone". These sections revealed that the dominant rock fragments (>75% by volume) were metamorphic in origin. According to Critelli, this places the sandstone within the Soquel Member. Hence the siltstone and diatomaceous shale mapped to the south of the sandstone is most likely LaVida. Further to the south, south of the proposed left slip fault, the stratigraphy has been studied extensively by consultants for the BKK Landfill. They map the units as Yorba and Sycamore Canyon. Although the authors were not permitted access to the landfill we have no reason to question this interpretation as the Sycamore Canyon is one of the most easily recognized units in the field. Thus, our map juxtaposes LaVida and Yorba.


Summary and Conclusions

Our field studies have shown that the central San Jose Hills are a part of west plunging anticline first proposed by Olmsted. Further, the Puente Formation can be differentiated into LaVida, Soquel, Yorba and Sycamore Canyon Members. A major left-slip fault just to the north of the crest of the San Jose anticline has juxtaposed LaVida and Yorba Members. We believe this fault to be an extension of the San Jose fault mapped to the east by Shelton. Smaller reverse faults to the north of the left-slip fault have offset both LaVida and Soquel strata. We propose the following sequence of events.


REFERENCES

Critelli, Salvatore, Rumelhart, Peter E., and Ingersoll, Raymond, 1995, Petrofacies and provenance of the Puente Formation (Middle to Upper Miocene), Los Angeles Basin, southern California: implications for rapid uplift and accumulation rates: Journal of Sedimentary Research, vol. A65, no. 4, p. 656-667.

Olmsted, F.H., 1950, Geology and oil prospects of western San Jose Hills, Los Angeles County, California: California Journal of Mines and Geology, vol. 46, no. 2, p. 191-212.

Shelton, John S., 1955, Glendora volcanic rocks, Los Angeles basin, California: Geological Society of America Bull., vol. 66, p. 45-90.

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