The third event recorded in the rock of the Puente Hills is the laying down of thick rock layers, which were subsequently uplifted. For over 10 million years in the marine basin opened by the rotation of the Transverse Ranges, the Los Angeles Basin and the nascent Puente Hills accumulated thick layers of sedimentary rock. The most important of these layers is the Puente Formation, named after the Puente Hills where these sedimentary deposits are thickest.[ref]The Puente Formation is divided into four members: the La Vida, Soquel, Yorba, and Sycamore Canyon members. Below the Puente Formation lies the Topanga Formation, which is interbedded with the El Modeno and Glendora volcanic intrusions. The Puente Formation and the Topanga formations are from the Miocene age (23.5 – 5.3 Ma). Lying above the Puente Formation are the Pliocene age (5.3 – 1.8 Ma) Fernando, San Pedro, and La Habra Formations. Bjorklund, “Four Dimensional Analysis of the Inversion of a half-graben to form the Whittier Fold-Fault System of the Los Angeles Basin,” 1370.[/ref] These rock layers are thick units of sandstone and siltstone and are easily seen in outcrops throughout the greater Puente Hills. This marine basin had varying water depths throughout the years with the deepest being about 6,000 feet around four million years ago. The rock that forms these layers was washed and drained off the highlands and mountains surrounding the marine basin. In this marine basin lived creatures, small and large, from whales, squid, and sharks to mollusks, oysters, microscopic floating plants called diatoms, and tiny single-celled organisms called foraminifera. When they died, their carcasses sank to the sea floor, and the rock, being washed in, overlaid these organic remains. This alternating process continued until roughly 27,000 feet of rock was accumulated. The weight of these layers combined with the Earth’s heat rising from below stewed this rock and organic material together for some 8 to 10 million of years, creating the sedimentary rock layers and trapping within those layers: oil—“black gold.” The nature of the subsequent uplift of the Hills is revealed in the buckled, cracked, and folded nature of the rock.[ref]Bjorklund and Burke, “Four Dimensional Analysis of the Inversion of a half-graben to form the Whittier Fold-Fault System of the Los Angeles Basin,” 1370–1. Wright, “Structural Geology and Tectonic Evolution of the Los Angeles Basin,” 100 – 101. See Harold W. Hoots and Ted L. Bear, “History of Oil Exploration and Discovery in California,” section 1 in chapter IX, Geology of Southern California, Richard H. Jahns, ed., California Division of Mines Bulletin 170, (San Francisco: Department of Natural Resources, 1954), 5 – 11. See also, Tanya Atwater, “Santa Barbara Channel Oil: Structural Evolution.” University of California, Santa Barbara. Dr. Atwater has created a series of Apple Quicktime™ animations that dramatize the events of the last 85 My and the building of Southern California, see http://emvc.geol.ucsb.edu/downloads.php.[/ref]
The uplift of the Puente Hills is the result of the Pacific Plate and its parallel movement to the northwest along the North American Plate, which began to squeeze the Los Angeles region and its thick marine sediments between the Transverse and Peninsular Ranges. The Pacific Plate caused this big squeeze as it dragged its newly acquired chunks of crust along the transform boundary formed between it and the North American Plate. This forced the Peninsular and Transverse Range blocks up against the deep granite roots of the Sierra Nevada Mountains. The intervening basinal regions between the Transverse and Peninsular Ranges were squeezed together as though in a vise. This compression took place along the joints—“zones of crustal weakness”[ref]Wright, “Structural Geology and Tectonic Evolution of the Los Angeles Basin,” 45.[/ref] the most significant being the San Andreas Fault and the recently discovered Puente Hills Blind Thrust Fault[ref]The Puente Hills Blind (a fault that does not breach the surface) Thrust Fault (PHT) was discovered in 1999 by John Shaw et. al., and was determined to be the cause of the 1987 Whittier Narrows Mw 5.9+ event; and was, thus, named after the nearest major structural element in the region, the Puente Hills. The PHT “extends for more than 40km along strike in the northern Los Angeles basin from downtown Los Angeles east to Brea in northern Orange County. The fault consists of at least three distinct geometric segments, termed Los Angeles, Santa Fe Springs, and Coyote Hills, from west east.” John H. Shaw, et.al, “Puente Hills Blind-Thrust System, Los Angeles, California,” Bulletin of the Seismological Society of America, 92(8), December 2002: 2946. Research on this fault has determined that it is, next to the San Andreas Fault, the most significant fault structure in the greater Los Angeles and Southern California region and poses a significant earthquake threat (it is believed to be capable of generating a significant earthquake (Mw 6.0 to 7.0+ ). Geologists are still investigating its structure and are undecided as to its full extent and role in the tectonic evolution to the greater Los Angeles region. The geologist Robert S. Yeats, who has studied numerous geologic aspects of the eastern Los Angeles Basin including the San Gabriel Basin (see Yeats, “Tectonics of the San Gabriel Basin and Surroundings, Southern California”) and its relationship to the Puente Hills, as well as participating in studies of the Puente Hills themselves with Tom Bjorklund (see T. Bjorklund, et. al., “Miocene Rifting in the Los Angeles Basin: Evidence from the Puente Hills half-graben, Volcanic Rocks, and P-wave Tomography,” Geology, 27(7), July 1999: 593-596), has found no evidence (yet) indicating that the PHT played a role, major or minor, in the geologic development of the Puente Hills. Dr. Yeats states, “[t]he blind thrust is generally assumed to pass beneath the San Gabriel Basin as a décollement. If so, there does not appear to be a close correlation between the blind thrust and uplifted terrain east and west of the San Gabriel Basin. Uplift accompanying the blind thrust affects the Coyote Hills and Santa Fe Springs anticline, but not the Puente Hills, which are more likely to owe their uplift to the restraining bend in the Whittier fault. Yeats, “Tectonics of the San Gabriel Basin and Surroundings, Southern California,” 1177. More research remains to be done on the extremely complex nature of the PHT and its intersection with other fault structures in the greater Los Angeles Region and what role, if any, it plays or played in the uplift of the Puente Hills. In addition, the most recent geologic study completed specifically on the greater Puente Hills by Tom Bjorklund, et. al. (see Bjorklund and Burke, “Four Dimensional Analysis of the Inversion of a half-graben to form the Whittier Fold-Fault System of the Los Angeles Basin,” and T. Bjorklund, et. al., “Miocene Rifting in the Los Angeles Basin: Evidence from the Puente Hills half-graben, Volcanic Rocks, and P-wave Tomography”) does not mention nor credit the PHT with any significant role in the uplift of the Puente Hills but credits the uplift to the actions of the Whittier Fault system.[/ref]—formed in the plate collisions over the past 240 My between the various chunks of rock that underlay the Puente Hills.[ref]See Ingersoll and Rumelhart, “Three-stage Evolution of the Los Angeles Basin, Southern California,” see also, Baldridge, Geology of the American Southwest.[/ref]
The most important of these joints in the Puente Hills was the Whittier fold and fault thrust system which runs their entire length on the southern side of the Hills.[ref]The main structural elements of the greater Puente Hills include the Puente Hills anticline, the La Habra syncline, and the Whittier Fault system. The Whittier Fault is located along the southern edge of the greater Puente Hills and runs their entire length (40km) from their emergence north of the Santa Ana Mountains. It is a steeply dipping fault (~50-55º) that extends to a depth of two kilometers. The Whittier fault system can be divided in three structurally distinct segments, a southeastern segment, a central segment, and a northwestern segment. The central segment of the Whittier fault runs for 18km from Telegraph Canyon to La Mirada Creek and forms the southern boundary of the Puente Hills anticline. The southeastern segment runs for 9km, from Telegraph Canyon to the Santa Ana River where in the vicinity of the Santa Ana Canyon, the Santiago Peak Volcanics are exposed. The northwestern segment runs for 15km from La Mirada Creek to the Whittier narrows. The northwestern most exposure is found in the Turnbull Canyon area, where the La Vida Member is juxtaposed against the Sycamore Canyon Member of the Puente Formation. The Whittier fault has been traced north to just short of the San Gabriel River, where it breaks north, becoming the East Montebello fault. There are several smaller faults: the Workman Hill fault, Whittier Heights fault, and the Handorf fault, all three of which are located in the northwestern section of the Puente Hills. Bjorklund, “Four Dimensional Analysis of the Inversion of a half-graben to form the Whittier Fold-Fault System of the Los Angeles Basin,” 1371- 1385.[/ref] Thrust faults result from the movement of one block of rocks being pushed up and over another. Generally, fold and thrust faults are the result of compression. In the case of the Puente Hills this movement and compression forced the rock material on the south side of the greater Puente Hills under the material on the north side, folding the bedrock and sediment of the northern side upward—not unlike an errant foot kicking up a rug. Not only were the Hills uplifted but this uplift also allowed oil to migrate through pores and cracks in the buckled rock, to collect in traps within the folded rock of the Hills. Where the rock layers were broken by faulting or breached by erosion, the oil escaped to the surface forming oil and tar seeps. These events, these deeply folded and faulted rocks and Hills, are the results of two massive plates colliding.[ref]Wright, “Structural Geology and Tectonic Evolution of the Los Angeles Basin,” 45. Bjorklund and Burke, “Four Dimensional Analysis of the Inversion of a half-graben to form the Whittier Fold-Fault System of the Los Angeles Basin,” 1383 – 1384. Mellor, American Rock: Region, Rock, and Culture in American Climbing, 51. See Frank S. Parker, “Origin, Migration, and Trapping of Oil in Southern California,” section 2 in Chapter IX, Geology of Southern California, Richard H. Jahns ed., California Division of Mines Bulletin 170, (San Francisco: Department of Natural Resources, 1954), 11 – 21, and Ingersoll and Rumelhart, “Three-stage Evolution of the Los Angeles Basin, Southern California.”[/ref]
Excerpted from Richard H. Ross. “From Rock, Wind, and Water: A Natural History of the Puente Hills.” Claremont Graduate University, 2006.