Of Rocks: Conclusion

The under­ly­ing rock of the Puente Hills from the Bed­ford and South­ern Cal­i­for­nia Batholith base­ment to the Miocene and Pliocene sand­stone, silt­stones, and shale tells us a story about the nat­ural his­tory of the Puente Hills, about how they were made and how they came to be Hills. This was not a short process. Time passed slowly but surely for mil­lions of years, tec­tonic plates floated on molten rock, bump­ing and grind­ing against each other. Where two pieces rubbed edges, the earth twisted, slipped, folded, and buck­led over and up. The land was, for the most part, cov­ered by an ocean. Water crea­tures lived in forests of sea­weed, and when the crea­tures and the forests died, their bod­ies set­tled to the bot­tom. When mixed with sand and mud, and heated and pres­sur­ized, they became the thick lay­ers of rock and oil under­ly­ing the Hills. This hap­pened for 245 mil­lion years, and some hun­dreds of thou­sands of years ago, the Puente Hills emerged from the ocean, as it fol­lowed one of its many ebbs and flows, slowly becom­ing dry land 1.

As the Hills emerged from their oceanic exis­tence, wind and water imme­di­ately began to shape them. The soft sed­i­men­tary rock under­ly­ing the Hills would ulti­mately com­bine with the cli­matic effects of wind and rain and the sur­face streams and creeks, result­ing in the deep ero­sion evi­dent in the Hills today. Ulti­mately, the rock and water would com­bine, cre­at­ing the land­scape nat­ural his­tory of the Puente Hills. Wind and rain and streams carry their own sto­ries of the nat­ural his­tory of the Puente Hills.

Excerpted from Richard H. Ross. “From Rock, Wind, and Water: A Nat­ural His­tory of the Puente Hills.” Clare­mont Grad­u­ate Uni­ver­sity, 2006.

Notes:

  1. Kim Stan­ley Robin­son, The Gold Coast, (New York: Tom Doherty Asso­ciates Book, Inc., 1988), 43 — 45 ↩

Third Rocks and Uplift

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. 1 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. 2

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” 3 the most sig­nif­i­cant being the San Andreas Fault and the recently dis­cov­ered Puente Hills Blind Thrust Fault 4—formed in the plate col­li­sions over the past 240 My between the var­i­ous chunks of rock that under­lay the Puente Hills. 5

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. 6 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. 7

Excerpted from Richard H. Ross. “From Rock, Wind, and Water: A Nat­ural His­tory of the Puente Hills.” Clare­mont Grad­u­ate Uni­ver­sity, 2006.

Notes:

  1. 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. ↩
  2. 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/​d​o​w​n​l​o​a​d​s​.​php. ↩
  3. Wright, “Struc­tural Geol­ogy and Tec­tonic Evo­lu­tion of the Los Ange­les Basin,” 45. ↩
  4. 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. ↩
  5. See Inger­soll and Rumel­hart, “Three-​stage Evo­lu­tion of the Los Ange­les Basin, South­ern Cal­i­for­nia,” see also, Baldridge, Geol­ogy of the Amer­i­can South­west. ↩
  6. 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. ↩
  7. 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.” ↩

Sec­ond Rocks and Rotation

The sec­ond event recorded in the rocks of the Puente Hills began some 23 mil­lion years ago. The trench sys­tem, which had for so long dom­i­nated the for­ma­tion of South­ern Cal­i­for­nia, ended around 28 Ma, when the Pacific Plate trans­form con­ver­gence replaced the Far­al­lon Plate trench sys­tem. As the “trail­ing edge of the Far­al­lon Plate entered the sub­duc­tion zone,” 1 the Pacific Plate was brought into con­tact with the North Amer­i­can Plate. The Pacific Plate’s col­li­sion, in con­trast to the Far­al­lon Plate, which had col­lided with the North Amer­i­can plate almost per­pen­dic­u­larly, was “nearly par­al­lel” 2 to the North Amer­i­can Plate. This new trans­form fault — the “noto­ri­ous San Andreas Fault of Cal­i­for­nia” 3—would live up to its name; it would trans­form the greater Los Ange­les region and give rise to the greater Puente Hills. This story is recorded in the thick marine rocks that under­lie the Hills and in the vol­canic rocks that form the bound­ary between the old plate sub­duc­tion sys­tem and the new plate trans­form sys­tem. When the Pacific Plate slid in behind the rem­nants of the Far­al­lon Plate, it made con­tact with the North Amer­i­can Plate, broke off sliv­ers and blocks of con­ti­nen­tal crust, and began drag­ging them away toward the north­west. Thus, as the Pacific Plate moved almost par­al­lel to the North Amer­i­can plate bound­ary, it picked up and began to drag South­ern Cal­i­for­nia and the greater Los Ange­les region to the north­west. 4

The first result of this north­west­erly trans­fer and sub­se­quent drag­ging of chunks of crust caused the West­ern Trans­verse Range 5 to break away from the Penin­su­lar Range. At one time the West­ern Trans­verse Range had lain par­al­lel to the Penin­su­lar Range with its south­ern rim, the Chan­nel Islands, lying near San Diego. When the blocks and chunks of crust form­ing the bedrock of South­ern Cal­i­for­nia were trans­ferred from the North Amer­i­can Plate, the Pacific Plate picked up the Trans­verse Range blocks. In effect, the Pacific Plate, scrap­ing against the North Amer­i­can Plate, vio­lently wrenched the West­ern Trans­verse Range from its posi­tion next to the Penin­su­lar Range along the con­ti­nent, rotat­ing it clock­wise 110 degrees to its present east to west con­fig­u­ra­tion. 6 This block rota­tion stretched, thereby thin­ning, the remain­ing crust, draw­ing lava and the lower base­ment rock up to fill the gap. This process of rotat­ing is known through the vol­canic rocks — the Glen­dora and El Modeno vol­canic rocks — which spewed out of the cracks that had opened in the bedrock of the newly-​opened marine basin. These for­ma­tions were directly laid over the bedrock for­ma­tions; their story is one of rift­ing, raft­ing, and rota­tion, which would ulti­mately result in the open­ing of the Los Ange­les Basin and the for­ma­tion of the thick marine sed­i­ments that con­sti­tute the roof rock of the Puente Hills. 7

Excerpted from Richard H. Ross. “From Rock, Wind, and Water: A Nat­ural His­tory of the Puente Hills.” Clare­mont Grad­u­ate Uni­ver­sity, 2006.

Notes:

  1. Baldridge, Geol­ogy of the Amer­i­can South­west, 215. ↩
  2. Baldridge, Geol­ogy of the Amer­i­can South­west, 215. ↩
  3. Baldridge, Geol­ogy of the Amer­i­can South­west, 216. ↩
  4. McPhee, Basin and Range, 180. See Baldridge, Geol­ogy of the Amer­i­can South­west, and T. L. Wright, “Struc­tural Geol­ogy and Tec­tonic Evo­lu­tion of the Los Ange­les Basin,” K.T. Bid­dle, ed. Active Mar­gin Basins. Amer­i­can Asso­ci­a­tion of Petro­leum Geol­o­gist Mem­oir 52. (Tulsa: The Amer­i­can Asso­ci­a­tion of Petro­leum Geol­o­gists, 1991), 35 – 134. ↩
  5. “Trend­ing essen­tially east-​west across the regional grain of South­ern Cal­i­for­nia is the Trans­verse Range province, which com­prises elon­gate moun­tain ranges and val­leys, chains of hills, an broad basins that are geo­log­i­cally very com­plex. The province as a whole resem­bles the adjoin­ing Coast Range and Penin­su­lar Range regions in sev­eral aspects, but is dis­tin­guished from them by pre­vail­ing east-​west struc­tural trends.” Jahns, “Inves­ti­ga­tions and prob­lems of south­ern Cal­i­for­nia Geol­ogy,” 17). The Trans­verse Ranges are sep­a­rated into east­ern and west­ern seg­ments. The east­ern Trans­verse Ranges are, from east to west: the Eagle, Pinto, Lit­tle San Bernardino, and San Bernardino. The west­ern Trans­verse Ranges are, from east to west: the San Gabriel, Santa Mon­ica, Santa Susana, Topatopa, Pine, and Santa Ynez Moun­tains. Only the west­ern Trans­verse Range blocks were rotated dur­ing the switch from a sub­duc­tion to trans­form mar­gin. The Penin­su­lar Ranges are, from south to north: Laguna, Val­lecito, Santa Rosa, Agua Tibia, San Jac­into, and Santa Ana Moun­tains. In addi­tion, the Penin­su­lar Range extends, through the South­ern Cal­i­for­nia Batholith for­ma­tion, into the Baja Penin­sula of Mex­ico and is geo­log­i­cally part of the over­all Penin­su­lar Range block sys­tem. (Richard H. Jahns, “Inves­ti­ga­tions and prob­lems of south­ern Cal­i­for­nia Geol­ogy,” Sec­tion 1 in Chap­ter I, Geol­ogy of South­ern Cal­i­for­nia, Richard H. Jahns, ed., Cal­i­for­nia Divi­sion of Mines Bul­letin 170. (San Fran­cisco: Depart­ment of Nat­ural Resources, 1954), 11. ↩
  6. The rota­tional axis was the San Gabriel to Chino Hills to Cris­tian­i­tos faults east of the Puente basin. This fault sys­tem — though each indi­vid­ual mem­ber would remain active between 18 to 0 Ma — was replaced by a series of other faults. Fore­most among those begin­ning 6 Ma was the San Andreas Fault sys­tem. When the San Andreas became active, the Pacific Plate con­tin­ued to drag the Penin­su­lar Range block north­ward open­ing the Gulf of Cal­i­for­nia as Baja Cal­i­for­nia was detached from its ini­tial posi­tion along main­land Mex­ico and added to the Pacific Plate. For more infor­ma­tion on the rota­tion of the Trans­verse Range blocks. See Yeats, “Tec­ton­ics of the San Gabriel basin and Sur­round­ings, South­ern Cal­i­for­nia,” 1158 – 1182, Baldridge, Geol­ogy of the Amer­i­can South­west, Wright, “Struc­tural Geol­ogy and Tec­tonic Evo­lu­tion of the Los Ange­les Basin,” and Ray­mond Inger­soll and Peter E. Rumel­hart, “Three-​stage Evo­lu­tion of the Los Ange­les Basin, South­ern Cal­i­for­nia,” Geol­ogy. 27(7) July 1999. ↩
  7. Peter W. Weigand, et. al., “The Conejo Vol­canics and other Miocene Vol­canic Suites in South­west­ern Cal­i­for­nia,” in Con­tri­bu­tions to Crustal Evo­lu­tion of the South­west­ern United States, Andrew Barth ed., Spe­cial Paper 365, (Boul­der Col­orado: The Geo­log­i­cal Soci­ety of Amer­ica, 2002), 197. See Inger­soll and Rumel­hart, “Three-​stage Evo­lu­tion of the Los Ange­les Basin, South­ern Cal­i­for­nia,” 595. See also, Baldridge, Geol­ogy of the Amer­i­can South­west, and Andrew J. Meigs and Michael E. Oskin, “Con­ver­gence, Block Rota­tion, and Struc­tural Inter­fer­ence Across the Peninsular-​Transverse Range Bound­ary, East­ern Santa Mon­ica Moun­tains, Cal­i­for­nia,” in Con­tri­bu­tions to Crustal Evo­lu­tion of the South­west­ern United States, Andrew Barth ed., Spe­cial Paper 365, (Boul­der Col­orado: The Geo­log­i­cal Soci­ety of Amer­ica, 2002) 279 – 295. ↩