Geology of the Northeast Margin of the Salton Trough, Salton Sea, California

ELKANAH A. BABCOCK Department of Geology, University of Alberta, Edmonton 7, Alberta, Canada

ABSTRACT mately 6,100 m, about 35 km east-southeast of the Salton Sea. Sharp (1967) documented of Mexicali, Baja California (Biehler, 1964; about 24-km right-lateral separation on this The San Andreas fault zone, in the Biehler and others, 1964, p. 132). From the fault. Other important faults of the San Durmid area northeast of the Salton Sea, deepest part of the trough, the irregular Andreas system in the Salton trough are forms the northeast boundary of the Salton basement surface rises to crop out at San the Imperial, Cucapa, Superstition Moun- trough (the landward continuation of the Gorgonio Pass (Fig. 1; Biehler and others, tain, Superstition Hills, and Elsinore faults Gulf of California rift). The zone is 1964, Fig. 6). Recent work suggests that the (Fig- 1). characterized by subparallel faults having Gulf of California and the San Andreas fault The Mecca Hills lie immediately north of both right-lateral and vertical separations. are the result of sea-floor spreading and the Durmid area. Within the Mecca Hills, Stratigraphic offsets indicate at least 1,100 associated transform faulting (Menard, Cenozoic clastic rocks, tightly folded and m of vertical separation on the San Andreas 1960; Hamilton, 1961; Harrison and cut by numerous closely spaced faults of the fault, southwest side downthrown, and Mathur, 1964; Rusnak and Fisher, 1964; San Andreas system, form an anticlinorium. 850-m right-lateral strike-slip separation is Wilson, 1965; Larson and others, 1968). Crystalline basement rocks are exposed in indicated by offset drainage. The eastern margin of the Salton trough is Painted Canyon at the core of the An anticlinal structure southwest of the characterized by a complex of subparallel anticlinorium (Hays, 1957, PI. 2). This study San Andreas fault is probably caused by branches of the San Andreas fault. The zone indicates that the style of deformation in the drape folding over an upfaulted basement of faulting is about 8 to 16 km wide and Mecca Hills continues with little change block or a recently intruded pluton. Small extends from the north end of the Coachella southward into the Durmid area. folds on the southwest flank of the anticline Valley to the Durmid area northeast of the are caused by drag along the San Andreas Salton Sea (Fig. 1). Its trace southeast of the Previous Work fault and gravity sliding. An angular Durmid area is commonly hidden by Little has been published about the discordance of 18° between the Shavers Well Pleistocene lake silts and dune sand. geology of the Durmid area. Dibblee (1954) and Borrego Formations indicates that Evidence of active faulting can be seen, briefly described the sedimentary rocks and tilting adjacent to the San Andreas fault however, southeast of Niland, as offset structure. He assigned the older coarse began prior to Borrego deposition in cultural features and as lineaments on clastic rocks to the Palm Spring Formation Pleistocene time. Folding followed deposi- oblique infrared aerial photographs (Bab- and the younger lacustrine clay and silt to tion of the lacustrine Borrego Formation. cock, 1971). the Borrego lacustrine facies of the Palm Uplift halted, and the area was planed off by The two major faults lying along the Spring Formation and the unconformably erosion. Renewed uplift followed, probably northeast border of the Coachella Valley are overlying Brawley Formation. Dibblee's within the last few thousand years. Key the Banning and Mission Creek faults. data for the Durmid area are summarized on words: structural geology, stratigraphy, San Southward in the Indio Hills, they join to the Salton Sea sheet (see Rogers, 1967). Andreas fault, folds. become the principal fault of the San Andreas fault zone, which is parallel to the STRATIGRAPHY INTRODUCTION AND eastern shore of the Salton Sea (Fig. 1). Sedimentary rocks within the Salton GEOLOGIC Right-lateral displacements have been trough range in age from probable late SETTING measured on faults of the San Andreas zone Pliocene to Holocene. The sediments near The Durmid area (Fig. 1) is an excellent in the Indio Hills (Stotts, 1965) and Mecca the axis of the trough are deltaic sand, silt, location to study structural relations along Hills (Hays, 1957). The total right-lateral and clay deposited by the Colorado River the eastern margin of the Salton trough, a separation on the San Andreas fault in this (Merriam and Bandy, 1965; Muffler and landward continuation of the Gulf of area may be as much as 257 km (Crowell, Doe, 1968). At the margins of the trough California rift. The Salton trough (Fig. 1), an 1962). these fine-grained sediments interfinger with isostatically compensated depression caused Relatively high seismicity characterizes locally derived coarse-grained detritus. by thinning of the crust, contains a the San Jacinto fault zone which enters the Formations recognized within the Dur- maximum sediment thickness of approxi- Salton trough through Borrego Valley west mid area by the author include the lacustrine

Geological Society of America Bulletin, v. 85, p. 321—332, 9 figs., March 1974

321

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Fault „»^ Fault located approximately " Fault existance uncertain Approximate axial trace of Durmid Anticline Generalized strike and dip of bedding linilllllll Generalized outline of Durmid Hill

Scale in kilometers J 2 3

Figure 2. Sketch map of tectonic elements in the Durmid area.

Borrego Formation named by Tarbet and is predominantly coarse-grained arkose and Borrego Formation Holman (1944, p. 1781) and the coarse- arkosic conglomerate, containing cobbles of grained Shavers Well Formation, defined by coarse-grained plutonic rock derived from The Borrego Formation crops out in the Hays (1957). the eastern side of the Chocolate Mountains Durmid area between the San Andreas fault or from ranges east of the Orocopia and the Salton Sea and on the plain Shavers Well Formation Mountains (Fig. 1), where similar rocks are northeast of Durmid Hill (Figs. 2 and 5). exposed today. Coarse-grained sandstone Exposed in the Durmid area is a The Shavers Well Formation, of late with interbeds of pebble conglomerate and 1,475-m-thick section which has been Pliocene or early Pleistocene age, crops out siltstone make up the upper 330 m of the Sea divided into six informal units (Fig. 4). Units northeast of the San Andreas fault at the Bat View Member. 1 and 2, the lowermost, crop out near Salt Caves Buttes (Figs. 2 and 5). The section Finer grained than the Sea View Member, Creek, northeast of the San Andreas fault; exposed at the Bat Caves Buttes is the Skeleton Canyon Member consists of and units 3, 4, 5, and 6 crop out at the south approximately 765 m thick (Fig. 3); but the medium- to coarse-grained sandstone in the end of Durmid Hill (Figs. 2 and 5). The base of the formation is not exposed, and the lower half and mostly fine-grained sand- complete Borrego section is not exposed in upper surface is eroded. On the basis of stone and siltstone in the upper half of the the Durmid area because the contact lithologic similarities, the lower 550 m of the member. Conglomerate beds in the top 15 m between units 2 and 3 is covered. formation was assigned to the Sea View of the Shavers Well Formation contain Based on faunal evidence, the Borrego Member and the upper 215 m was assigned pebbles of Orocopia schist which indicates Formation is a brackish water lacustrine to the Skeleton Canyon Member of Hays uplift of the Orocopia Mountains, and deposit of Pleistocene age (Tarbet and (1957). possibly the Chocolate Mountains, when Holman, 1944, p. 1782). Mineralogical The lower 220 m of the Sea View Member these sediments were being deposited. analyses of samples of the Borrego Forma-

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sand and coarse, bouldery alluvium on the alluvial fans near the mountains. -5 E Description Depositional History unconformify meters- Depositional events in the Durmid area are summa rized below. During late Pliocene or early Pleistocene time, sediments carried Tan, medium-to fine-grained quartz into the area were mostly coarse clastics sandstones, which contain lenses of derived from ranges east of the Chocolate 220+3 granitic conglomerate, interbedded with grey end olive-cobred laminated and Orocopia Mountains, indicating that siltstones. these areas were exposed and shedding coarse detritus at: that time. Near the end of Shavers Well deposition, uplift of :he - -600 Orocopia Mountains is indicated by :he presence of clasts of Orocopia schist in the Shavers Well Formation. Prior to deposition of the Borrego Formation, beds of the LU Shavers Well Formation were tilted and eroded, creating an erosional surface upon Tan, medium-to coarse-grained sandstones. which the Borrego lacustrine sediments were u Granitic pebble conglomerate in lenses. deposited. During deposition of the Borrego OC o Beds '5-25-meters thick. Extensively trough Formation, the Salton trough probably had < I— 330+3 cross bedded. Coarse - grained sandstones a configuration similar to that of today. The oo II® - -400 are arkosic M edium-grained sandstones trough underwent repeated periods of I— are quartzose. dissication as shown by thin evaporite beds CtC Q_ of considerable lateral extent. Dessication was interspersed with influxes of mostly I clayey and silty sediments, largely derived o from the Colorado River (Merriam and Bandy, 1965).

STRUCTURAL GEOLOGY 200 Major structural features of the Durmid Maroon-cclored arkosic conglomerate area are illustrated in Figure 2; geological and coarse-grained arkose. Beds T5—3 features, in Figure 5. The San Andreas fault meters thick. Conglomerate is in bands is the major structure in the area. Right- 225+3 up tol5meters thick. Minor interbedded lateral separation of drainage and vertical O greyish-tan fine-grained sandstone separation, (northeast side relatively up- and siltstone. thrown) of approximately 1,000 m are observable along the fault. Vertical separations on the Hidden Spring and Powerline faults created a horst between these and the covered San Andreas fault. Other faults of the San Andreas system in the Durrnid area have Figure 3. Columnar section of Shavers Well Formation, Bat Caves Buttes, California. geomorphic expression, but their traces are hidden by Lake Cahuilla sediments and Quaternary alluvium. In some places, these tion in the Durmid area by Muffler and Doe Lake Cahuilla Sediments sediments also cover the San Andreas fault. (1968) suggest thai: much of the detritus is of and Alluvium. The Durmid anticline lies southwest of the local origin. At most other localities in the San Andreas fault near the south end of Salton trough, the source of Borrego Durmid Hill. Geometrically complex, the sediments is the drainage basin of the During late Pleistocene to early Holocenc; fold is poorly defined, and its axial trace can Colorado River (Merriam and Bandy, 1965; time, a large, brackish water lake, Lake only approximately be shown. Muffler and Doe, 1968). The widespread Cahuilla, occupied the Salton trough, fed primarily by inflow from the Colorado distribution of the lacustrine sediments San Andreas Fault indicates deposition in a topographic River. Lake Cahuilla sediments crop out in depression similar to the modern Salton the Durmid a-ea as sheetlike deposits, such The San Andreas fault zone is 30 m wide trough. The presence of thin evaporite layers as the silt near Salt Creek; as shoreline; at Salt Creek, increasing to 150 m at the interbedded with clay and silt, with uniform deposits, such as spits and bars; and as south end of Durmid Hill (Figs. 2 and 5). thickness for greater than 2,000 m along isolated patc'ies of sand and silt (Fig. 5) The zone is nearly vertical at Salt Creek, strike, indicates repeated dessication of the Composition of these sediments ranges from where it consists of dark reddish-brown lacustrine basin during Borrego deposition. clayey silt to anlithified pebble conglomer- clayey gouge. Rocks within the zone at the It also suggests that during Borrego ate and generally reflects the composition ol: south end of Durmid Hill are pervasively deposition, subsidence of the basin occurred bedrock in the immediate vicinity. sheared and resemble sheared claystone of at approximately the same rate as deposi- Quaternary alluvium within the Durmic. the Borrego Formation. Within this sheared tion. area consists of re-worked Lake Cahuilla clayey matrix are isolated sparse sandstone

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blocks with sheared margins, ranging ir Thickness Unit Description maximum dimension from a few centime- in meters Upper 18 meters of unit is very fine yellowish - brown ters to 20 m (Fig. 6). meters quartz sandstone. The lower 69meters is tan, 90 ±1-5 6 Right-lateral separation of Salt Creek by clayey siltstones. the San Andreas fault is 850 m. The creek has cut a meandering channel in the sheared

Borrego Formation at the fault and does not This unit consists entirely of brown daystones having make a sharp bend to join the fault trace. a few silty interbeds except for two marker horizons. -1400 Separation of Salt Creek has occurred since The upper marker horizon is 150centimeters of white deposition of the Borrego Formation in volcanic ash. The lower is a 27meters interval of massively bedded orangeish - tan, medium-grained Pleistocene time. Brune (Wyss and Brune, quartz sandstones interbedded with claystones. 1968, p. 4692) calculated the rate of recent right-lateral slip for the entire Imperial Valley to be about 2.2 cm per yr. Had this slip occurred exclusively along the San Andreas fault, the separation of Salt Creek would have taken approximately 30,000 yr -1200 to occur. In historic time, however, the Imperial and San Jacinto faults have been VaCANlC ASH HORIZON the most active members of the San Andreas system in the Imperial Valley. Therefore, it is

probable that during late Pleistocene time, 640+6 much of the lateral slip in the Imperial Valley 5 SANDSTONE MARKER HORIZON occurred along faults other than the San

Andreas, and hence the offset of Salt Creek -1000 probably took considerably longer than 30,000 yr. Ash beds within the Borrego G ypsum Formation might be datable and could thus establish a maximum possible age of - separation of Salt Creek. Vertical stratigraphic separation across the San Andreas fault is approximately 500 m at Salt Creek and 1,120 m at the south end >- -800 Interbedded brown claystone (75 %) and laminated of Bat Caves Buttes, the northeast side of the cr reddish-tan weathering laminated siltstones. fault having moved relatively upward. At < 4 A few gypsum layers less than 30centimeters thick are scattered throughout the unit. Some thin Salt Creek, and at the south end of Bat Caves beds of gypsum-cemented quartz sandstone are Buttes, the Shavers Well and Borrego cr 230±6 present near the top of the unit. Formations, juxtaposed across the San Andreas, strike nearly parallel to the fault, so that stratigraphic separation of the top of < the Shavers Well Formation approximates ID vertical fault separation. The amount of O -600 vertical separation cannot be estimated

north of Salt Creek and south of Bat Caves The upper half of the unit is interbedded fine- Buttes because the rocks are covered by Lake grained quartz sandstone {50 %)and yellow and Cahuilla sediments. grey clays(50%). 3 The lower half of the unit is massive tan sandstone, Vertical stratigraphic separations mea- which weathers to a blocky surface. The sandstone is medium to fine grained and consists of quartz sured across the San Andreas fault could 310 ±30 also be explained by lateral slip; in this case, with a little biotite + feldspar.

rocks folded at an earlier time would have -400 been juxtaposed by large strike-slip movement to show apparent vertical slip. However, geomorphic evidence discussed later in this paper, and the presence of positive gravity and aeromagnetic anomalies between the San Andreas and Hidden Spring and Powerline faults (Bab- cock, 1969) suggest that vertical move- -200 Interbedded chocolate brown claystone and tan ments have been large. laminated siltstones consisting of quartz and biotite. 2 Occasional gypsum beds 15-30centimeters thick. Other Faults 165±V5 A single bed of granite pebble conglomerate is present near the top of the unit. The trace of the Hidden Spring fault extends approximately 19 km northwest from Salt Creek into the Mecca Hills (Figs. 2 Interbedded chocolate brown claystones 85% and and 5). The fault was mapped in the Mecca 60 ±1-5 1 ten sandstones 15% , consisting of well-sorted medium-sized quartz grains- unconformity Figure 4. Columnar section of Borrego Formation east of Salton Sea, California.

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Figure S. Gcologic map of the Durmid area, Imperial Valley, California.

Legend CONTACTS BOSS SAN ANDREAS FAUT ZONt FAULT — — FAULT LOCATED APPROXIMATELY ? — FAULT EXIStANCE UNCERTAIN » ANTlClM SHOWNG AXIAL TRACE AND *"T PLUNGE Of AXIS I SYNCLINE SHOWING AXIAL TRACE AND ~T PLUNGE OP AXIS OVBTURNED SYNC ANO KCMGI OP AXIS

-1- STRIKE AKODT OF REDOING STRIKE D» OF OV5RTU»lEO BEDDING

SEDIMENTARY ROCKS

RECENT Sffifijj QUATERNARY ALLUVIUM PLEISTOCENE EARLY RECENT ^ LAKE CAHUIliA SANDS jOjijJlAKE CAHUILIA SILTS PLEISTOCENE ^^^tORICGO FORMATION PLIO-PLEISTOCENE f@fl SHAVERS WaL FORMATION

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line between Frink Spring, southeast of the appears to die out to the northwest. Because area mapped, and the Old Imperial Spa, and of the many tight folds within the southwest by drill data. The trend of the Hot Springs limb of the anticline and the lack of a fault suggests that it is part of the San well-defined hinge, the fold is not apparent Andreas system. Drilling indicates the on geologic maps (Figs. 5 and 7), and the northeast side of the fault is upthrown (J. F. axial trace is only approximately located on Mann, Jr., unpub. data). Figure 2. However, the presence of the A buried fault, roughly parallel to the San southeast-plunging anticlinal nose is indi- Andreas fault, extends southeast from cated by an ash marker bed (Fig. 7). The Hunter's Spring to a prominent area of anticlinal nature of the fold is also indicated diffuse ground-water seeps about 8 km by younger rocks exposed progressively away (Fig. 5). On two gravity profiles farther from the core of the fold. The trending perpendicular to the fault trace, the average dip of the southwest limb of the fold fault is marked by very sharp, narrow, is 25° (Fig. 8); the fold plunges southeast the positive gravity anomalies (Babcock, 1969, same amount. p. 61). The sense of displacement on this Gravity and aeromagnetic data suggest fault is not known. the Durmid anticline is probably a drape The Powerline fault extends southeast 6.5 fold caused by passive folding of the km from near the Eagle Mountain Railroad sedimentary cover over an upfaulted base- tracks (Figs. 2 and 5). Near the railroad, it ment block or possibly a near-surface lies on the northeast side of a line of springs intrusive body similar to Obsidian Butte at in permeable sandy beds of lower units of the south end of Salton Sea (Babcock, 1969). the Borrego Formation. These beds are Near Bat Caves Buttes, the primary uplifted on the southwest side of the fault, structures are two monoclines dipping away whereas lake silt crops out on the from the San Andreas fault on opposite sides downthrown northeast side of the fault. The of the fault. Near the fault, bedding dips Figure 6. San Andreas fault zone 1.6 km northwest of Powerline fault coincides with the steepest the Bertram mine. An exotic block of sandstone is seen about 55° NE. and flattens to approximately suspended in a matrix of sheared claystone of the gradients on the eastern side of a positive 9° at 1.5 km from the fault. Southwest of Borrego Formation. gravity ridge which underlies Durmid Hill, the fault, the Borrego Formation is highly which also indicates that the southwest side contorted and poorly exposed. Younger Hills by Hays (1957, p. 298), where it is a of the fault is upthrown (Babcock, 1969, beds of the Borrego Formation are exposed zone of gouge or breccia up to 10 cm thick. p. 63). progressively farther from the fault, indicat- In the Durmid area, the fault trace is marked ing predominant dip toward the southwest. by a fault-line scarp near the Coachella Unconformity between the Shavers Well Canal, and by springs occurring along a line Small Folds from near the south end of the fault-line and Borrego Formations scarp to the Eagle Mountain Railroad An angular discordance of about 18° Small folds are developed in the Borrego tracks. Western margins of the springs are between the Shavers Well and Borrego Formation on the southwest limb of the aligned, suggesting that the fault is located a Formations indicates that uplift along the Durmid anticline (Figs. 7 and 8). The folds few meters west of the springs and that the San Andreas fault and tilting of the die out near the shore of the Salton Sea, fault acts as a barrier to ground water sedimentary rocks northeast of the fault where, in most places, the rocks dip about flowing southwest away from the moun- occurred prior to deposition of the Borrego 20° SW. These folds are of the passive flow tains. Nowhere is the fault surface visible. In Formation. The surface of unconformity variety, as defined by Donath and Parker Hidden Spring Canyon, in the southern appears to be unfolded on aerial photos. The (1964). Low in the Borrego section, near the Mecca Hills, Hays (1957, p. 300) found unconformity is 500 m higher in the section San Andreas fault, the style of folding is facies offsets suggesting 1,200 m of at Salt Creek than at Bat Caves Buttes, an nearly quasiflexural, but the folds retain right-lateral separation, but found no indication that the northeast side of the San enough characteristics of passive flow indications of horizontal slip in the Durmid Andreas fault has been uplifted more at Salt folding to be termed such. The folds range in area. The apparent right-lateral offset of the Creek than at the south end of the buttes. length from less than 90 m to 1,500 m and Lake Cahuilla sand-silt contact (Fig. 5) is This agrees with stratigraphic separations are generally more open near the San due to removal of the lake sand by erosion observed across the fault. Andreas fault than farther southwest near west of the fault and the exposure of the California Highway 111. underlying silt. Hays (1957, p. 301) found Large Folds Associated The sections shown in Figure 8 were dip-slip separation to be 75 to 105 m in with the San Andreas Fault interpreted from map geometry of folds Hidden Spring Canyon, the east wall having The Durmid anticline, a poorly defined because the folds cannot be seen in vertical moved down. The fault-line scarp west ol anticlinal structure lacking a well-defined section because of the flatness of Durmid Hidden Spring fault in the Durmid area hinge line, is developed southwest of the San Hill. The average dip of Durmid anticline, indicates the same sense of vertical separa- Andreas fault near the Bertram Mine which controls the general configuration of tion. The horst between the Hidden Spring (Babcock, 1969). Only the southwest limb the sections, was taken to be the minimum and San Andreas faults is a southerly and southeast-plunging nose of the fold are dip the ash marker bed could have and still continuation of the fault-bounded basement visible because the San Andreas fault cuts off maintain the measured stratigraphic thick- anticlinal structure mapped by Hays (1957) the east limb. It is unlikely that a northeast ness between the ash and siltstone marker in the Mecca Hills. limb is present opposite the southwest limb beds; below this depth, folding presumably The trace of the Hot Springs fault is because of large lateral displacement on the dies out. The dip of the major structure is hidden by bouldery alluvium, but its San Andreas fault. The anticline makes up only an approximation because unobserved position is indicated by hot springs along a the southern 4 km of Durmid Hill and tectonic thickness changes within the

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stratigraphic section between the ash and along the San Andreas fault, or (3) gravity movement of the San Andreas fault initiated siltstone horizons may be possible. sliding of the sedimentary cover off an drag folding. The drag folding, combined High in the stratigraphic section, in a zone upfaulted basement block or a rising with dewatering and osmotic effects, could about 460 m wide, parallel to California 111 intrusive body. have created abnormally high pore pressures and west of the ash bed (Fig. 7), the flow It is highly improbable that the folding within the rocks. As uplift of the Durmid folds are similar in style. The folds are tightly was caused by regional compression within anticline occurred in response to fault compressed, having high amplitudes relative the Salton trough because no localities movements or intrusive activity, downhill to wave lengths. Most folds have wave within the trough, except those associated creep of the water-saturated Borrego lengths of about 60 m and axial length to with faults, exhibit folding similar to that at Formation occurred. wave length ratios of 15 to 20, with fairly the Durmid anticline. Similar folding was uniform geometry along strike. The direc- described in association with faulting in the GEOMORPHIC EVIDENCE FOR tion of tectonic transport is seen to be Indio Hills (Stotts, 1965) and Mecca Hills RECENT TECTONIC ACTIVITY toward the southwest from observations of (Hays, 1957). Extensive areas of folded The relations between landforms and overturning and asymmetry of folds in that Borrego Formation, which might be the structural features provide a basis for direction. Average dip of 66 axial surfaces of result of regional compression, are present construction of a partial tectonic history of folds for the entire area of small folds is in the Anza-Borrego desert centered about the Durmid area. 55° NE. 15 km west of the Salton Sea. Within this Durmid Hill, the largest landform in the The folds decrease in amplitude and area, however, the style of folding differs area, is low and elongate and is parallel to increase in wave length lower in the section, from that in the Durmid area. The dying out the northeast side of the Salton Sea (Fig. 2). closer to the San Andreas fault. In some of of the Durmid folded belt about 3,200 m The hill rises to a height of about 60 m above these folds, there appears to be little thinning from the San Andreas fault casts doubt on the Salton Sea and about 15 m above the of competent beds. However, no evidence of the hypothesis of regional compression and plain to the northeast. The San Andreas fault bedding-plane slip or other planes of suggests that the small-scale folding is due to roughly follows the crest of the hill. At its decollement was observed, indicating that proximity to the San Andreas fault and the south end, Durmid Hill is coincident with the dominant mechanism of folding was Durmid anticline. the Durmid anticline. flowage. Most folds near the fault have wave Frictional drag acting parallel to a wrench Salt Creek is the major stream in the lengths of 90 to 360 m. Fold axes are more fault can cause greater movement farther Durmid area. It heads east of the Chocolate nearly parallel, and the degree of asymmetry from the fault than close to it, which pro- and Orocopia Mountains and drains much and overturning is less than in folds higher in duces drag folds adjacent to the fault of these ranges as well as most of the area the Borrego Formation. (Freund, 1965). The angle between these between Durmid Hill and the mountains. Because the Shavers Well Formation is fold axes and the fault is small (0° to 15°) and Where Salt Creek crosses Durmid Hill, it has more competent than the Borrego Forma- increases away from the fault (Freund, cut a gorge about 40 m deep (Fig. 9). tion, the small-scale folds present at Durmid 1965, p. 197). The folds are also tighter near Three possibilities might account for the anticline may not extend into the underlying the fault. This mechanism could produce deep canyon cut by Salt Creek. Salt Creek Shavers Well Formation, suggesting a those folds on the southwest limb of the may have captured another drainage net- surface or surfaces of slip between the Durmid anticline. However, two aspects of work by headward erosion through the hill formations. At Bat Caves Buttes, the steeply the folding do not fit the geometry predicted from the west, or it may be a superimposed dipping Shavers Well Formation, near the by Freund. First, the rocks farthest from the stream or an antecedent stream. San Andreas fault, contains no small folds. San Andreas are most tightly folded. Second, It is highly improbable that Salt Creek Small-scale folding at Durmid anticline the folds farthest from the fault most nearly captured a major drainage system to the appears to belong to a single phase. The parallel the fault. east. No geomorphic evidence exists which plunge of folds undulates slightly, but no The third mechanism by which small- would indicate that prior to the establish- cross folds are evident. The decrease in scale folding might have occurred is gravity ment of the present Salt Creek drainage tightness of folds low in the Borrego sliding. The folded rocks are mostly system, the plain east of Durmid Hill Formation and close to the San Andreas claystone and lesser amounts of unconsoli- drained in a different direction. In addition, fault cannot be fully explained because it is dated or semiconsolidated siltstone and geomorphic evidence (discussed later) impossible to separate the effects of sandstone. These rocks have low strength shows that the uplift of Durmid Hill was a differences in lithology, depth of burial, and when dry. When wet, the claystone behaves very recent geologic event. There would not proximity to the fault. Folds in the more plastically and should flow when subjected have been sufficient time for a small stream competent sandstone and siltstone low in to a small differential stress. The upfaulting flowing down the southwest side of the hill the Borrego Formation would be expected of a basement block or emplacement of an to erode headward through the hill and to be more open and less overturned. They intrusive body beneath Durmid anticline capture drainage to the east. Furthermore, would also show less evidence of flowage could have provided the slope necessary for none of the other streams draining Durmid than folds in the very ductile overlying gravity sliding to occur. Gravity sliding Hill has a valley more than 6 m deep or has claystone. However, differences in folds low could have been facilitated by possible eroded headward beyond the crest of the in the section could be related to proximity greater than hydrostatic pore pressures hill. to the San Andreas fault. developed during dewatering of the clayey If Salt Creek occupies its present course sediments (Hubbert and Rubey, 1959). because of superposition, it would be Possible Causes of Osmotic pressures caused by the clay acting necessary for the area east of Durmid Hill to Small-Scale Folding as a semipermeable membrane (Zen and have been lowered about 40 m by erosion. Three possible mechanisms may have Hanshaw, 1965) could also have contri- No evidence for such an erosional lowering acted in concert to produce small-scale buted to the development of greater than exists. Hence, Salt Creek must be an folding on the southwest flank of the hydrostatic pore pressures. antecedent stream which has maintained its Durmid anticline. The folding could have A combination of drag folding and gravity course during the uplift of Durmid Hill. been caused by (1) post-Borrego regional sliding probably folded the rocks on the The well-developed radial drainage pat- compression in the Salton trough, (2) drag southwest limb of Durmid anticline. Lateral tern eroded into claystone and siltstone of

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Direction of Tectonic Transfert-

—fit

200 SCALE IN METERS

1 1 h 0 200 400 600

ASH HORIZON SILTSTONE MARKER HORIZON

Figure 8. Generalized cross sections of folds at Durmid Hill.

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the Borrego Formation on Durmid Hill, from Bat Caves Buttes to its southern terminus, is an excellent example of consequent drainage. Almost no structural control of drainage is present (excepi: for washes 0.5 m deep or less) in spite of the fact that the bedrock is highly folded and consists of lithologies that have considerable differences in resistance to erosion. The larger washes are 30 to 150 m apart and are generally 1.5 to 3.0 m deep at their deepest point, halfway down the hillside. This youthful drainage system, in which most of the original upland surface is undissected, is interpreted as the initial developmental stage of a drainage system on a smoothly sloping, recently uplifted hill. Geomorphology provides insight into the geologic and tectonic history of the Durmid area. Folding of the Borrego Formation in Figure 9. Canyon formed where Salt Creek has cut through the Borrego Formation at Durmid Hill. Deepest portion response to fault movements probably of the canyon is to the west, behind camera position. occurred initially during latest Pleistocene time, forming the Durmid anticline and the clasts of Orocopia schist in the uppermost Planetary Physics at the University of small-scale folds associated with it. This was part of the Shavers Well Formation. Prior to California at Riverside. The Imperial Valley followed by a period of erosion that left the deposition of the Borrego Formation, beds Project, at the time of this study, was funded area as a west-sloping plain on which of the Shavers Well Formation were tilted by the U.S. Bureau of Reclamation, the the Salt Creek drainage was established. and eroded, creating an unconformity upon National Science Foundation, the Standard Renewed upfolding and vertical movement which the Borrego lacustrine sediments were Oil Company of California, and the on the San Andreas fault followed, creating deposited. During deposition of the Borrego Chevron Oil Field Research Company. Durmid Hill. During this time, Salt Creek Formation, the Salton trough probably had REFERENCES CITED maintained its original course and became a configuration similar to that of today. It deeply incised. The presence of Lake underwent repeated periods of dessication, Babcock, E. A., 1969, Structural geology and Cahuilla silt deposited in small tributary as evidenced by thin evaporite beds of geophysics of the Durmid area, Imperial Valley, California [Ph.D. dissert.]: River- valleys at the Salt Creek canyon indicates considerable lateral extent. Dessication side, Univ. of California, 149 p. that uplift and entrenching of the stream was interspersed with influxes of mostly 1971, Detection of active faulting using occurred prior to the last high-stand stage of clayey and silty sediments, largely derived oblique infrared aerial photography in the Lake Cahuilla 450 ± 200 yr ago (Hubbs from the Colorado River (Merriam and Imperial Valley, California: Geol. Soc. and others, 1965). Bandy, 1965). America Bull., v. 82, p. 3189-3196. During late Pleistocene time, the Borrego Biehler, S., 1964, Geophysical study of the Salton SUMMARY AND CONCLUSIONS Formation was folded, probably in response trough of Southern California [Ph.D. thesis]: The northeast margin of the Salton trough to vertical fault movement or intrusive Pasadena, California Inst, of Technology, 139 p. in the Durmid area consists of a series of activity along the San Andreas fault zone. Biehler, S., Kovach, R. L., and Allen, C. R., 1964, subparallel faults belonging to the San This folding formed the Durmid anticline. A Geophysical frame work of the northern end Andreas system. Both horizontal and tectonically quiet period followed during of the Gulf of California structural province, which the Salt Creek drainage was estab- vertical separations were observed across in van Andel, Tj. H., and Shor, G. G., Jr., these faults. Structural relief at the Shavers lished and the area was eroded to a smooth eds., Marine geology of the Gulf of Well—Borrego Formation contact is proba- surface. The present configuration of California — A symposium: Am. Assoc. bly at a maximum at the south end of Bat Durmid Hill is the result of renewed uplift Petroleum Geologists Mem. 3, p. 126-143. Caves Buttes. There is a vertical separation adjacent to the San Andreas fault in latest Crowell, J. C., 1962, Displacements along the San of approximately 1,120 m on the San Pleistocene time. Salt Creek was offset 850 Andreas fault, California: Geol. Soc. Andreas fault, southwest side downthrown. m right-laterally by movement of the San America Spec. Paper 71, 61 p. Dibblee, T. W., Jr., 1954, Geology of the Imperial Folds are present at two scales within the Andreas fault. The area has been seismically Valley region, California, in Jahns, R. H., Durmid area: the large-scale Durmid active in recent time, and the author ed., Geology of Southern California: Calif. observed small en echelon cracks along the anticline and the small folds on its southwest Div. Mines Bull. 170, p. 21-28. flank. The timing of events is known only San Andreas fault near Salt Creek following Donath, F. A., and Parker, R. B., 1964, Folds and within broad limits because of a lack of the April 9, 1968, Borrego Valley earth- folding: Geol. Soc. America Bull., v. 75, p. accurate paleontological or radiometric quake. The Durmid anticline may still be 45-62. dates from rocks in the area. rising, and repeated accurate surveying Freund, R., 1965, A model of the structural During late Pliocene or early Pleistocene could verify this. development of Israel and adjacent areas since Upper Cretaceous times: Geol. Mag., time, sediments carried into the area were v. 102, no. 3, p. 189-205. mostly coarse clastics derived from ranges ACKNOWLEDGMENTS Hamilton, W., 1961, Origin of the Gulf of The author gratefully acknowledges the east of the Chocolate and Orocopia California: Geol. Soc. America Bull., v. 72, Mountains, indicating that these areas were advice and suggestions of Sebastian Bell, p. 1307-1318. exposed and shedding coarse detritus. Uplift Tsvi Meidav, and Robert W. Rex. The study Harrison, J. C., and Mathur, S. P., 1964, Gravity of the Orocopia Mountains near the end of was supported by the Imperial Valley anomalies of the Gulf of California, in van Shavers Well deposition is indicated by Project of the Institute of Geophysics and Andel, Tj. H., and Shor, G. G., Jr., eds.,

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Marine geology of the Gulf of California — Muffler, L.J.P., and Doe, B. R., 1968, Composi- Stratigraphy and micropaleontology of the A symposium: Am. Assoc. Petroleum tion and mean age of detritus of the west side of the Imperial Valley, California: Geologists Mem. 3, p. 76-89. Colorado River Delta in the Salton trough, Am. Assoc. Petroleum Geologists Bull., v. Hays, W. H., 1957, Geology of the central Mecca south-eastern Ca'ifornia: Jour. Sed. Petrol- 28, p. 1781-1782. Hills, Riverside County, California [Ph.D. ogy, v. 35, p. 38^-399. Wilson, J. T., 1965, A new class of faults and their dissert.]: New Haven, Yale Univ., 324 p. Rogers, T. H., 1967, Geologic map of California, bearing on continental drift: Nature, v. 207, Hubbert, M. K„ and Rubey, W. W., 1959, Role of in Olaf P. Jenk ns ed., 1:250,000 Series, p. 343-347. fluid pressure in mechanics of overthrust Salton Sea sheet: California Div. Mines and Wyss, M., and Brune, J. N., 1968, Seismic faulting: Geol. Soc. America Bull., v. 70, p. Geology. movement, stress, and source dimensions for 115-166. Rusnak, G. A., and Fisher, R. L., 1964, Structural earthquakes in the California-Nevada re- Hubbs, C. L., Bien, G. S., and Suess, H. E., 1965, history and evolution of the Gulf of gion: Jour. Geophys. Research, v. 73, p. La Jolla natural radiocarbon measurements California, in van Andel, Tj. H., and Shor, 4691-4694. IV: Radiocarbon, v. 7, p. 66-1 17. G. G., Jr., eds., Marine geology of the Gulf Zen, E., and Hanshaw, B. B., 1965, Osmotic Larson, R. L., Menard, H. W., and Smith, S. M., of California — A symposium: Am. Assoc. equilibrium and mechanics of overthrust 1968, Gulf of California: A result of Petroleum Geo'ogists Mem. 3, p. 144-156. faulting: Geol. Soc. America, Abs. for 1964, ocean-floor spreading and transform fault- Sharp, R. V., 1967, San Jacinto fault zone in the Spec. Paper 82, p. 232-233. ing: Science, v. 161, p. 781. Peninsular Ranges of Southern California: Menard, H. W., 1960, The East Pacific Rise: Geol. Soc. America Bull., v. 78, p. 705-730. Science, v. 132, p. 1737. Stotts, J. L., 1965, Stratigraphy and structure of Merriam, R., and Bandy. O., 1965, Source of the northwest I idio Hills, Riverside County, MANUSCRIPT RECEIVED BY THE SOCIETY upper Cenozoic sediments in the Colorado California [M.S. thesis]: Riverside, Univ. of JANUARY 4,1971 Delta region: Jour. Sed. Petrology, v. 35, no. California, 208 p. REVISED MANUSCRIPT RECEIVED SEPTEMBER 12, 4, p. 911-916. Tarbet, L. A., and Holman, W. H., 1944, 1973

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