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Field Trip No. 2
STRUCTURAL GEOLOGY AND TECTONICS OF THE
SALTON TROUGH, SOUTHERN CALIFORNIA
by
S. Biehler and R.W. Rex
Department of Geological Sciences
University of California
Riverside, California 92502
March 1971
30 CONTENTS
Introduction...... 32 .32 Regional Geologic Setting Physiography ...... 32 .35 Regional Tectonics . . Geology ...... 37 Road Log ...'...... 37 Stop 1 ...... 38 Stop 2 ...... 38 Stop 3 ...... 39 Stop 4 ...... 39 Stop 5 ...... 39 Stop 6 ...... 40 Stop 7 ...... ,. . 40 Stop 8 ...... 40 Stop 9 ...... 41 Bibliography ...... 41
List of I Ilustrations Figure 1. Index map showing route of travel. F igure 2. Generalized Geologic Map of the Salton Trough. Figure 3. Columnar Section for the Northeastern Coachella Valley.
31 Introduction
One of the most striking topographic and tectonic features of southern California is the Salton trough. This structural depression is the northward continuation of the Gulf of California and extends from the Mexican tidelands at the head of the Gulf to San Gorgonio Pass, 80 miles east of Los Angeles. The Salton trough is an area of diverse geological complexity. Its major faulting, thick sedimentary deposits, recent volcanism and potential geothermal reservoirs, all have interesting geophysical expressions of local and regional importance.
REGIONAL GEOLOGIC SETTING
Physiography
The Gulf of California is a relatively narrow structural depression less than 100 miles wide and over 700 miles long. Although the Gulf terminates 60 miles south of the international border the characteristic structural depression continues for 240 miles northwest from the Gulf to San Gorgonio Pass. This northern segment of the Gulf of California structural province, called the Salton trough, is marked by a broad flat alluviated valley with an area of 10,000 square miles, of which 2,000 square miles lie below sea level. The Colorado river delta south of the international border rises to a height of 40 to 50 feet above sea level, forming a natural dam which prevents this sink from being inundated by sea water from the Gulf of California. Geophysically and physiographically the Salton trough can be divided into four units: the Coachella Valley, Salton Sea, Imperial Valley and the Colorado River delta proper. The Coachella Valley extends southeast from San Gorgonio Pass to the north end of the Salton Sea. It is the narrowest segment of the Salton trough with an average width of less than 15 miles. The borders of the Coachella Valley are well defined on the southwest by the San Jacinto and Santa Rosa mountains of the Peninsular Range province and on the northeast by the Little San Bernardino and Orocopia mountains of the Transverse Range province. The mountains which border the Coachella Valley have an average elevation which is approximately 1500 feet higher than their counterparts to the southeast which form the borders of the Imperial Valley. As the Coachella Valley narrows in the northwest the average elevation of the bordering ranges increases. At San Gorgonio Pass where the two highest peaks of southern California, San Gorgonio and San Jacinto, form the borders of the pass, there is 10,000 feet of relief in a few miles. The Salton Sea is located in the transition zone between the narrow Coachella Valley in the north and the broad Imperial Valley in the south. This topographic center of the Salton trough is covered by a large man-made lake approximately 35 miles long and 10 to 15 miles , wide, which forms a sink for all of the drainage in the area. The surface of the sea is presently 234 feet below sea level with a maximum measured depth of 44 feet. Prior to the flooding of the sink, this area was a broad flat sandy playa which at one time formed the bottom of an ancient fresh-water lake. Before the disastrous inundation of the area in 1905-1907 by the accidental diversion of the Colorado River, the lowest exposed land area
32 ->'• RIVERSIDE RI»\ A Stop I 996070 0 110 20 30 PALM SPRINGS 1 I 1 1 INDIO MILES N Stop9 195 Stop 8 Stop7 Stop 2 01.-P' 04 e #e Q. -7 0 Stop 0•e 6 0 56' 99 % WESTMORLAND -'FA f< Stop•5 GLAMIS BRAWLEY Stop•3 IMPERIAL HOLTVILLE 8 .Stop4 SAN DIEGO EL CENTRO 80 98 8 CALEXICO -- -,-1------.-i ....,------U....----S.A.-- - -- I------I. -MEXICO
Figure 1. Index Map Showing Route of Travel 1 111-v. ' 1 7 9 I< Fig.2. GENERALIZED GEOLOGIC MAP 1150423601V.----5 • t·' _, 680 115° t\ OF THE SALTON TROUGH -- »*r--1•»==--ff•T44 -- fts-,1\ . i., (,•.'.•. ,.J...... ». 34, 33 'i fbj-L•; 0 5 10 15 20 25km .#/-"6•- -,i . . ..,; 1... 1-.'...'.rli•"•=.·...f.'.> 1. :.nty-...... " . .-. ·. , J ./.\... .1. ', 1.ir,"il '4, tt •>,»&73 0 5 10 15mi '.3.*/\ '3 11 ... EQ i.3 ..45. 4.-\ .1. , . 1,.· r .r 9 0. , t.. -., \0 -·-0, 0.-'.443 -...i/\. ..1 1 Rgle&* 20km Universal Tronsverse Mercctor Grid i ,- 9. 4,2 ,\, // 1 • 011.,1 - 17«'. . 5/64 S. BIEHLER 02111. ': .te MOUNTAINS .. c"b. 2- 12 4 340- £/4- :\.A . %41, \ PINTO C_21 S.X-S *\S \ 35 •37692 4+ -' r\.. + ".0 ..\'.• .140• +"\D·580..\ ..,- . tee + L-5 < + /'.1. . •i 3760- 4 /71...... :. .r1 \. 2 400 D036036tHot Brl»l__1 .../-7 . '·r *ll"94 -t/ C»«,254. Springs - 74..) . -....-. 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S..P, X4.:-'.ru'-' · - 0Alpine : 0 '06 *- , S \ p x700C *78068 ElCentro Ho0lvI e Ogilby ***1 .••\ / 1100 Yplostere city tcp 44313P -'.A . 1.r'\ *8015M 172:.... +$11 4/ l .. 7323M 't"L,-:., \0 # i *1363M ------427-7- .--" I S t- 1-7,2«OL\1 . + \ + >16,000 jlus:'1-- + 1 Colexico .------======1 - MEXICO 1 31:.:d». ------930 560 . 1- ''1'. 1•1"-t..... 'c 175058 64-!12039' 680 1150 1 of the Salton trough was only 4 feet higher than Death Valley. The Salton trough is still the second lowest land area in the United States. South of the Salton Sea lies the broad Imperial Valley, bounded by the Chocolate and Cargo Muchacho Mountains on the northeast and by the Peninsular Ranges on the southwest. The borders of the I mperial Valley, however, are not as clearly defined as those of the Coachella Valley. Along the southwest margin are several fault-controlled hills of crystalline rock within the valley. In the southeast the bordering mountains gradually decrease in relief until at Yuma, Arizona, the eastward extent of the valley is poorly defined. Within the Imperial Valley and southwest of the Chocolate and Cargo Muchacho Mountains is a narrow northwest-trending band of low ridges called the Sand Hills or Algodones Dunes. They are an almost unbroken mass of sand 45 miles long and 4 to 8 miles wide, which has been studied geologically by Norrit and Norris (.1961 ). Four volcanic cones protrude over 100 feet above the valley floor at the south end of the Salton Sea. The northeast dome, Mullet Island, is now partially submerged in the Salton Sea. Associated with these domes are extensive hot springs activity and potential geothermal reservoirs. The Colorado River delta proper designates that area in Mexico between the Imperial Valley on the north and the head of the Gulf in the south. This area is daily receiving large quantities of sediments from the Colorado River. The Colorado River has digressed many times across this cone, at times emptying into the Salton sink and then returning to the Gulf of California. This area is also referred to as the Mexicali Valley.
Regional Tectonics
The general trend of the major faults in the Peninsular Ranges and Salton trough indicate that the surface expression of the Salton trough is not of a simple rift valley marked by parallel breaks along the borders of a dropped block. The general fault trend cuts obliquely across the axis of the Gulf and this northern extension. On the basis of topography it appears that this fault pattern is continued on the floor of the Gulf. The predominant displacement along these major faults is right-handed strike slip and may be as great as 150 to 300 miles ( Crowell, 1962). The narrowing and termination of the Salton trough at its northern end are caused by truncation and conflict between northwest-trending faults of the San Andreas system and east-trending faults of the Transverse Range province. At about the mid point of the Gulf of California south of Tiburon Island, seismic evidence indicates a transition from continental to oceanic structures ( Phillips, 1963). It is in this area also that the gravity anomalies become positive ( Harrison and Spiess, 1959, 1963). This suggests that sea floor spreading is active in the Gulf and that the Gulf floor is undergoing a gradual transition from continental to oceanic type crust by rifting and northwest movement of Baja relative to the Mexican mainland. The emplacement of more basic material into the upper portion of the crust is occurring along zones of weakness. A similar mechanism has been suggested by Girdler (1964) for the formation of the Red Sea
35 COLUMNAR SECTION NORTHEASTERN COACHELLA VALLEY Indio Hills and Mecco Hills REG ALLUVIUM, TERRACES & LAKE COAHUILA BEDS 0-300 feet tn'.- UtlewifuzElly W --c.'-le...... e...... OCOTILLO CONGLOMERATE (Qo) 0-2,400 feet ./.-...... *..'. 2 gray conglomerate of granitic and Orocopia ...... 0.000.0...... , 0 schist debris; grades eastward into fanglomerate; I -042O.··-*.0'. .I. .OIlt,042.... 2,400 feet thick west of San Andreas fault, and ...... QO .....0.. 0-900 feet thick east of San Andreas fault 0'.0...... 0 t ....--*.../...I...... 0...... '...... unconformity east of San Andreas fault ...... -
PALM SPRING FORMATION (Tp) 0-4,800 feet ------
Geology The geology of the Salton trough has been studied by Dibblee ( 1954), Tarbet ( 1944, 1951 ), Woodard ( 1961), Durham and Allison (1961) and Downs and Woodard ( 1961). The Peninsular Range province west of the Salton trough has been studied by Jahns ( 1954), Larsen ( 1948), Merriam ( 1958), Beal ( 1948). The geology of the mountains to the north and east of the trough is covered by Bailey and Jahns ( 1954), Rogers (1961) and Crowell ( 1962). Only a brief summary of the rock types is given here. A stratigraphic section for the Cenozoic sedimentary rocks of the Coachella Valley as given by Dibblee (1954) is shown in Figure 3. Similar sections for the Imperial Valley have also been published ( Dibblee, 1954). The detailed stratigraphy of the Salton trough is incompletely known. The geologic samples from the few deep wells within the trough are not easily interpreted stratigraphically. Most of the columnar sections are based on exposures in the western side of the I mperial Valley and the eastern side of the Coachella Valley. One of the most economically important stratigraphic units is the Imperial Formation which in places is over 3000 feet thick. This formation was deposited under marine conditions during early Pliocene time and probably represents a major marine incursion of the Gulf. It is thought to contain potential petroleum reserves. Following this marine incursion there has been intermittent and interfingering deposition from the Colorado River, alluvial fans, and occasional marine inundations by the Gulf waters. The possibility of large continuous horizontal displacements along the major strike-slip faults certainly adds much complexity to stratigraphic correlation within the Cenozoic section. The crystalline rock types of the Peninsular Ranges are primarily granitic intrusives of the Southern California batholith probably of Mid-Cretaceous age. I n places large remnants of older sedimentary rocks now metamorphosed are observed. The crystalline rocks of the ranges east of the trough comprise more diverse igneous, metamorphic and volcanic types.
ROAD LOG
Start ( Riverside ) to Stop 1 ( Palm Springs Tramway )
Traveling eastward from Riverside on U.S. 60, the route of travelcrosses the Box Springs Mountains just east of the city limits. Exposed here on both sides of the freeway are some of the northernmost exposures of the Mesozoic granitic rocks of the Southern California batholith. The highway leads down across the flat floor of the Perris plain and crosses the San Jacinto Fault just prior to rising through the San Timoteo badlands ( Pliocene non-marine). After passing through the badlands the highway joins I nterstate 10 at Beaumont. This is the western end of the Banning pass. Traveling through Banning the two highest peaks of Southern California flank the pass-Mount San Gorgonio ( 11,502') on the north and San Jacinto Peak ( 10,831') on the south. The Banning pass marks the northern
37 limit of the Salton trough. Leaving U.S. 60 traveling southeast on highway 111 the route passes through several excellent exposures of pre-Cretaceous metamorphic rocks. These remnants flank the Mesozoic granitic rocks of the Southern California batholith. Just north of the town of Palm Springs the road turns southwest up Chino Canyon to the base of the Palm Springs Aerial Tramway. As the road winds up the canyon it passes through outcrops of pre-Cretaceous metamorphic rocks and injected lenses of batholithic rock.
Stop 1. Palm Springs Aerial Tramway.
The Valley Station and lower portion of the Tramway are located in Paleozoic or older metamorphic rocks. Mica and hornblende schist and some limestone and quartzite are exposed near the valley station. Along the first half of the tramway ride, as the cars ride up the face of San Jacinto they cross over granitic rocks of the Southern California batholif:h. Rising above the Coachella Valley a good view of the entire Salton Trough and Salton Sea area can be seen to the southeast. At the mountain station an excellent panoramic view of the surface structure of the Coachella Valley can be seen. The alignment of oases along the San Andreas fault are clearly evident and demonstrate the predominant northwest-southeast alignment of faulting in the valley. Across the valley lie the Little San Bernardino Mountains of the Transverse Range province.
Stop 1 to Stop 2. Palm Springs to Travertine Rock.
Returning to highway 111 the route goes south towards Indio. The highway parallels the outcrops of pre-Cenozoic metamorphic and portions of Mesozoic granitic rocks. Just west of Indio the high shoreline of ancient Lake Cahuilla is crossed. Indio lies about in the center of the Coachella Valley on Quarternary alluvium and lake deposits. Traveling south down the axis of the Coachella Valley the route drops below sea level. The Santa Rosa Mountains on the west and the Mecca Hills on the east mark the boundaries of the valley.
Stop 2. Travertine Rock.
From this vantage point there is an excellent view of the narrow Coachella Valley and its broadening into the wider I mperial Valley through the Salton Sea transition zone. When this region lay under the waters of Lake Cahuilla (900 A.D. - 1500 A.D.) only a small section of these rocks extended above the water line. Below the top of this outcrop the ancient shoreline is visible, as an encrustation of travertine.
Stop 2 to Stop 3. Travertine Rock to Superstition Hills Fault.
Traveling south on Highway 99 the ancient shoreline of Lake Cahuilla is visible near the foot of the Santa Rosa Mountdins on the west side of the highway. Here spiral shells, relics of the ancient lake litter the desert floor. The travertine deposits have discolored the crystalline rocks of the Santa Rosa marking the high water line of the lake. Near Salton City the sharp boundary of the Coachella Valley broadens westward toward the
38 Anza Borrego area. To the west of the highway numerous oil test wells have been drilled unsuccessfully in the folded and faulted Tertiary deposits. To the east of the highway in the vicinity of Salton Sea Naval Test Base is an area of excellently developed barchan dunes. As the highway turns eastward at the south end of the Salton Sea it enters the broad flat-alluviated Imperial Valley.
Stop 3. Superstition Hills Fault-New River.
The New River gorge was formed in a period of less than two years (1906-1907) when the Colorado River flooded into the preliminary construction of a canal system into the Imperial Valley. In June of 1906 the entire Colorado River was emptying into the Salton trough. The channel to the Gulf had become completely dry. The presence of the Superstition Hills fault has been exposed in this location because of the erosion of the New R iver. Here up-turned beds of Quaternary and Pleistocene deposits form an angular unconformity with recent lake and alluvium deposits. This area indicates some of the complexities of structure which underlie the apparent flat valley floor, and make the interpretation of geophysical and geological data very difficult.
Stop 3 to Stop 4. El Centro, California.
Traveling from the New River to El Centro the route passes through some of the most fertile and productive farmlands in the United States. Along this route an appreciation of the intricate canal system of irrigation can be obtained.
Stop 4. El Centro, Overnight Stop at the Holiday Inn.
A series of short talks on the geological and geophysical framework of the Salton trough and its geothermal potential will be given.
Stop 4 to Stop 5. El Centro to Sand Hills.
Traveling east from El Centro and north on highway 111 about three miles north of the intersection of highway 8 and 111, the route crosses the 1940 break of the Imperial Fault. Turning westward on the Glamis road the route leaves the irrigated portion of valley as it crosses the East H ighland canal onto the East Mesa and then across the Coachella Canal into the Algodone dunes. The very straight and sharp western boundary of the Sand Hills is apparent. The route travels up into the major portion of the Sand Hills to a lookout on top of one of the larger dunes.
Stop 5. Sand Hills.
The sand hills which have been called the Algodones dunes form a belt which is 40 miles long in a northwesterly direction and 3-6 miles wide. Some of the individual dunes are 200 - 300 feet high. The northwest trending front of the dunes and its sharpness has lead to the inference that the dune formation is fault controlled. From the top of the dunes the entire Imperial Valley can be observed.
39 Stop 5 to Stop 6.
Returning to the agricultural portion of the Imperial Valley the route heads northwards to the town of Calipatria and crosses the Alamo River. The New and Alamo R ivers form the major drainage for the irrigation system into the Salton Sea. To the west the outline of the Salton volcanic domes can be seen.
Stop 6. Salton Volcanic Domes.
The Salton Volcanic domes are a series of four separate rhyolitic and obsidian outcrops which trend northeast across the south end of the Salton Sea. I n this area some of the first geothermal exploration in the valley took place. Because of the high saline brines encountered, this area appears to be non-commercial in terms of power production. The rocks of the Buttes included xenoliths of apparent basement rocks and some basaltic rocks. The area is associated with both a large gravity positive anomaly and a broad magnetic positive. The geophysics indicates that the anomalous mass extends at least 6 km in depth-which coincides in this area with the thickness of alluvium.
Stop 6 to Stop 7..
Traveling northward along highway 111 the route parallels the Salton Sea and the projected trace of the San Andreas Fault. To the east can be seen the Chocolate mountains which form the eastern limit of the Imperial Valley.
Stop 7. Hot Mineral Spa.
Numerous Hot Springs and mud volcanoes are aligned along the traceof the San Andreas fault on the eastern side of the Valley. These extend from Desert Hot Springs in the north into Mexico. They are considered by many to have therapeutic value. They are considered an indication of the potential geothermal reservoirs in the valley.
Stop 7 to Stop 8. San Andreas Fault.
Returning to the highway the route swings westward and then north crossing the San Andreas Fault. Bat Cave Buttes on the east are faulted structures associated with the most southerly surface evidence for the San Andreas fault.
Stop 8. Salton Creek.
The fault structure can be observed just off the highway where it has been exposed by erosion in Salton Creek. This makes available a three dimensional exposure of the Recent faulting along this break of the San Andreas system. At least 1800 feet of Recent horizontal separation across the San Andreas is indicated by the offset of Salton Creek and at least 3700 feet of vertical separation is indicated by stratigraphic offset ( Babcock, 1969).
40 Stop 8 to Stop 9.
Traveling northward along highway 111 the Orocopia mountains on the east form the boundary of the valley. Studies of the rocks of this area by Crowell (1962) indicate offsets of over a hundred miles on the San Andreas fault system. The rocks in this area have been correlated with similar rocks on the west side of the San Andreas in the Soledad and Tejon regions.
Stop 9. Mecca Hills.
The complex structure along the San Andreas is well-exposed in the Mecca Hills numerous small fault slices and drag folds along the fault are exposed here. Segments of pre-Cenozoic metamorphic rocks crop out in fault slices.
Stop 9 to Riverside.
The route rejoins highway 111 and travels northwest up the axis of the Coachella Valley passing through Indio and paralleling the Banning Fault on the east. The upper Tramway station can be seen on the southwest. The route returns via the Banning pass back to R iverside.
BIBLIOGRAPHY
Babcock, E.A., 1969, Structural Geology and Geophysics of the Durmid Area, Imperial Valley, Calif.: University of California, Riverside, Ph.D. Thesis. Bailey, T. L. and Jahns, R.H., 1954, Geology of the Transverse Range province, southern California, in Geology of southern California: California Div. Mines Bull. 170, Chapt. 2, p. 83-106. Beal, Carlton, 1948, Reconnaissance of the geology and oil possibilities of Baja California, Mexico: Geol. Soc. Am. Memoir 31. Crowell, J.C., 1962, Displacements along the San Andreas fault, California: Geol. Soc. America Spec. Paper 71. Dibblee, T.W., Jr., 1954, Geology of the Imperial Valley region, California, in Geology of southern California: California Div. Mines Bull. 170, chapt. 2, p. 21-28. Down, T. and Woodard, G.D., 1961, Middle Pleistocene extension of the Gulf of California into the Imperial Valley: paper presented annual meeting Cordilleran section Geol. Soc. Am., San Diego. Durham, J.W. and Allison, E.C., 1961, Stratigraphic position of the Fish Creek gypsum at Split Mountain gorge, Imperial County, California: paper presented annual meeting Cordilleran section Geol. Soc. Am., San Diego. Girdler, R.W., 1964, Geophysical studies of rift valleys, in Physics and Chemistry of the Earth, v. 5: Macmillan Company, New York, p. 121-156. Harrison, J.C., and Spiess, F.N., 1959, Gravity measurements in the Gulf of California: I nterim report, Institute of Geophysics and Planetary Physics, University of California, Los Angeles.
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