1984 Midyear Meeting San Jose, California

By Ralph E. Hunter, H. Edward Clifton, N. Timothy Hall and John L. Chin


Pleistocene Shoreline and Shelf Deposits at Fort Funston and Their Relations to Sea-Level Changes–Latest Cretaceous Eartly Tertiary Systems of the Northern Diablo Range, California Depositional Facies of Sedimentary Serpentinite: Selected Examples from the Coast Ranges, California

    1. Page 1

      This field-trip guidebook discusses the Merced Formation (of Pliocene and Pleistocene age) in its type section (Lawson, 1893) and associated Pleistocene beds in the same sea-cliff outcrops. The outcrops extend from Mussel Rock on the south to Fort Funston on the north (Fig. 1A). This section is notable for its thickness and excellence of exposure and for the wide variety of shallow marine and coastal depositional environments represented. Although the section contains a potentially important record of changes in relative sea level, the record has not yet been fully interpreted because of a dearth of precisely dated horizons.

      On this field trip we will proceed stratigraphically upward through the section, starting at Woods Gulch, proceeding northward past Thornton Beach State Park (presently closed because of landsliding and gullying), and ending at the northwest corner of the old Fort Funston military reservation, now part of Golden Gate National Seashore (Fig. 1A). We will not see the lower part of the section between Mussel Rock and Woods Gulch. Although the exposures are often excellent where newly eroded by ocean waves, they tend to disappear rapidly because of landsliding and sand deposition on the beach. Because of the rapid changes, all the features described in this guidebook cannot be expected to be seen at any one time. The exposures are usually best at the base of the sea cliffs in winter, when much of the beach sand has been removed by erosion.

      The Merced Formation and associated Pleistocene beds crop out in a belt that trends northwest-southeast for a distance of 25 km across the northern San Francisco Peninsula (Fig. IB).

    1. Page 31

      The continuous southern Sacramento-northern San Joaquin Basin shoaled from minus 1000 m to sea level during deposition of 2 km of deltaic, slope, and submarine fan deposits during the Maastrichtian Epoch of theLate Cretaceous Period. Basin filling was accomplished by a combination of 1) progradation of sandy deltas and muddy slopes, and 2) aggradation of mostly sandy submarine fans. The basin filled from north to south as fluvial systems flowing southwest out of the Sierra Nevada built a deltaic complex that prograded more rapidly in the Sacramento Basin, filling it before more stable deltas to the south could fill the San Joaquin Basin. As the Sacramento Basin became filled, the fluvial-deltaic systems turned more southerly and prograded into the San Joaquin Basin.

      Deltaic systems consist of 1) prodelta shale, 2) upward-coarsening delta-fron sandstone, 3) delta-plain carbonaceous shale and lignite and upward-fining distributary-channel sandstone, and 4) very fine-grained (transgressive) shelf shale. Slope systems consist of 1) hemipelagic shale, 2) massive to upward-fining slope-channel sandstone, and 3) slump deposits. Submarinefan systams consist of 1) thick-bedded, upward-fining upper-fan-channel sandstone, 2) medium-to thick-bedded, massive- to upward-fining middle-fan sandstone and shale in discrete packets separated bby thin shale beds, and 3) thin- to medium-bedded, lower-fan fine-grained sandstone and shale in discrete packets separated by shale.

    2. Page 41

      Local middle Eocene tectonic activity within the southern Sacramento Basin divided it into eight deposltlonal provinces during deposition of the Domenglne Formation. These provinces include the 1) Southeastern Channel Area, which grades laterally (southwest) to the 2) Southern Marsh Area and offshore (northwest) to the 3) Northeastern Bar Area. This latter area grades still farther northwest to the relatively stable 4) Northwestern Shelf Area, but was disrupted by active faults Into the subsident 5) Rio Vista Basin and 6) Sherman Island Trough to the west and southwest. This subsldent region was bordered on the south by the 7) Mt. Diablo Uplift and on the west by the 8) Kirby Hills Uplift. The Domenglne can be divided into eight distinct members, not all of which are present in each depositional provinee.

      Sedimentary facies within the eight members of the Domengine include 1) upward-fining coarse-to fine-grained sandstone beds, 2) upward-coarsening bar or delta-front/shoreline sandstone beds, 3) foramin perferal shelf shale, 4) carbonaceous marsh deposits, 5) fIaser-bedded subtidal deposits, 6) shelf sandstone, and 7) fluvial conglomerate and sandstone.

      The Domengine Formation is probably a tide-wave-dominated deltaic system. Although marine processes were Important in controlling the types and distribution of grain sizes and sedimentary structures, the overall sediment distribution system and resulting formational geometry were controlled by tectonics and subsidence related to the growth of the Stockton Arch, sediment compaction in the Meganos Gorge (submarine canyon), active faulting, and distribution of basin-margin up lifts.

    3. Page 53

      A late Paleocene -early Eocene submarine canyon and fan complex, the Meganos Formation (or “Channel”) is exposed in the homoclinal sequence of Mesozoic to late Eocene age sediments that form the northern flank of Mount Diablo. Along this outcrop, the Meganos Formation comprises a narrow belt, 16 km long and one km wide. Exposures of the canyon-fill mudstones are largely masked by the alluvium of Deer Valley, but the resistant sandstones and conglomerates of the fan are well exposed along the southwest ridge flanking the valley.

      Canyon fed sediments spilled onto the late Paleocene -early Eocene basin floor(?) and subsequently filled the lower canyon. These sediments may be divided into two facies: (1) a lower submarine fan facies of coarse-grained clastic material; and (2) an upper mudstone canyon-fill sequence. The submarine fan facies is particularly well exposed from its basal contact, which unconformably overlies the lower Paleocene Martinez Formation, to the base of the overlying mudstone or canyon-fill facies. Within this basal sequence, fan channels are filled with disorganized conglomerates and pebbly sandstones. Boulders over 2 m in diameter are the largest clasts found in the basal channel-fills near Oil Creek. The canyon fan sequence fines vertically upwards in general aspect from basal inner fan channels to the thin sandstones of the outer fan facies and finally the mudstone of the basin plain(?) and fill.

      Based upon palinspastic reconstructions a minimum wall height of the submarine canyon in the vicinity of the inner fan is estimated to have been 800 m. The submarine fan was 450 m thick and the overlying mudstone of the canyon-fill facies was at least 675 m thick.

      The series of north-trending high angle normal faults that cut the outcrop belt are dated as Maastrichtian-earliest Paleocene to middle Eocenein age. Palinspastic restoration of Paleocene and lower Eocene units demonstrates uplift to the east, west and possibly south during this time. Uplift related to the normal faulting controlled the position of the submarine fan system and possibly the canyon. The location of the early Paleogene Meganos and underlying Martinez canyon-fan complexes and younger (Eocene) canyons in the deepest portion of southwest tilted graben that forms the Sacramento basin suggests regional and long term tectonic control. Perhaps this southern basin edge was bounded by an ancestral Stockton Arch-Mt. Diablo uplift. This major tectonic feature may be the expression of a change of subduction rate or direction of the Early Tertiary trench system.

    1. Page 73

      This field trip examines spectacular exposures of sedimentary serpentinites that occur in contrasting tectonic regimes of the Neogene transform-dominated San Joaquin Basin and the late Mesozoic forearc of the Great Valley Sequence, Sacramento Valley (Fig. 1). Emphasis will be on depositional mechanisms which range from intrusive/extrusive protrusions to detrital accumulations, in subaerial to deep marine environments. The interplay between tectonic events and the deposition of sedimentary serpentinite will be stressed. For it is this interrelationship--the tectonic mobilization of serpentinized ultramafic masses from deep structural levels, their forceful protrusion to the surface, and the generation of active extrusive serpentinite flows into the sedimentary environment--that underscores the importance of these deposits in the stratigraphic record. Voluminous, monomineralic accumulations of serpentinous strata, such as the Big Blue Formation and the foliate breccias of the Wilbur Springs area, should be viewed not merely as compositional curiousities but rather as unique sedimentologic responses to tectonic events.

      The first day we will examine homoclinal exposures of the Big Blue Formation along the west side of the central San Joaquin Valley (Fig. 1). Six stops are planned in the Big Blue serpentinous strata. In the southern portion of the area between Anticline Ridge and Domengine Ranch, recent work by Bate describes interfingering of distal alluvial fan and tidal flat environment. Dickinson and Casey's (1976) descriptions and discussion of the Big Blue Formation in the area near Cantua Creek are recapitulated. North of Martinez Creek, they recognize a main body of subaerial protrusive serpentinite than can be traced along strike into fringing alluvial aprons that grade, with increasing distance from the source protrusion, into shallow marine facies.

    1. Page 77

      The generally homoclinal eastern flank of the Diablo Range along the west side of the San Joaquin Basin of California (Fig. 6) is modified by a series of en echelon folds. One of these folds, the Coalinga anticline, is the southeast plunging extension of the Idria (or Joaquin Ridge) uplift, cored by a large serpentine diapir (Eckel and Myers, 1946). In Miocene time, basin margin shallow-marine and non-marine environments interfingered in the area of Coalinga anticline depositing a sequence of siliciclastic sediments. This lithological pattern was interrupted, when a major episode of diapiric movement extruded an enormous volume of serpentine foliate breccia, eroded the previously deposited sediments, and provided a source for sedimentary serpentinite in the same near-shore environments (Dickinson and Casey, 1976). These serpentinite deposits together make up the Big Blue Formation.

      The initial development of the Diablo uplift of serpentinite and Franciscan rocks occurred in post middle Eocene time (Nilsen and others, 1974), changing the continental margin from an open shelf to a partially enclosed basin. Subsidence followed (Hackel, 1966). The resulting transgressive middle Eocene Domengine Formation is the first Franciscan-derived sediment in this area (Dickinson and others, 1979). It is conformably overlain by the siliceous and calcareous shales of the Kreyenhagen Formation, which are up to a thousand meters thick (Wilson, 1943). Through the Oligocene and early Miocene, uplifts and growth of Coalinga anticline resulted in numerous angular unconformities in the Vaqueros Formation. During the Saucesian the whole northern end of the basin was subjected to 300 to 600 meters of shoaling (Bandy and Arnal, 1969), resulting in a widespread unconformity above the Vaqueros, and exposing the Kreyenhagen Formation along the flanks of the Diablo uplift.

    2. Page 81

      The Big Blue Formation conformably overlies the Temblor Formation along the northeast flank of the Coalinga anticline, but is progressively truncated beneath the late Miocene Santa Margarita Formation. The near continuous exposures of yellow, orange, brown, red, and blue sediments are almost exclusively composed of detrital serpentinite. Several authors (Eckel and Myers, 1946; Cowan and Mansfield, 1970) noted the proximity of the thickest Big Blue exposures to the large (19 × 6 km) serpentine body atop San Joaquin Ridge, near the mining town of New Idria, and suggested a relationship between the two. This inferred, but poorly understood relationship between the technically emplaced serpenti nite mass at New Idria, exposed up-plunge from the San Joaquin-Coalinga anticline and the sedimentary serpentinite, was significantly clarified by Dickinson and Casey,(1976). They recognized the non-detrital character of much of the Big Blue Formation and distinguished a chaotic facies that represents protrusive serpentinite flows that resulted from massive protrusive emplacement of serpentine, associated with a major episode of diapiric movement. The major uplift occured in late middle Miocene (Luisian) time, depositing between 5 and 10 cubic miles of serpentinite debris (Eckel and Myers, 1946). Dickinson (1966) likewise called upon extrusive sheets of serpentine to explain similar deposits to the south on Table Mountain, west of Reef Ridge.

      The upper Temblor Formation is grey-green silty claystone overlain by 22 m of an increasingly coarse sand and pebble interval. The lowest 5 m are very fine-grained sandstone, planar bedded with thin silt-stone layers. The next 3 m are generally medium-grained sandstone, with wel 1-cemented si Itstone inter-beds.

    3. Page 86

      Coalinga Oil Field (Fig. 11) has been oil and gas productive from the Temblor Formation on Coalinga anticline and the contiguous homocline to the southeast since the early part of this century. Oil was first discovered in 1890 in the Oil City area in Upper Cretaceous strata, up-plunge from the present Coalinga Field (Anderson, 1952). Development of the Eastside and Westside Fields began in 1900 and 1901 respectively, close to the Oil City area, but soon spread north, south, and southeast to the limits of the field (Anderson, 1952). The producing area is divided into the Eastside Field on the crest and flanks of the plunging anticline, and the Westside Field along the homocline into the apex of the Coalinga syncline (Kaplow, 1945). Approximately >98% of oil production in the Coalinga Oil Field is from the Temblor Formation (Cal. Oil and Gas Fields, Maps and Data Sheets, 1960), although recent efforts have been directed toward the shallower Etchegoin Formation (Taschman, 1982).

      Trapping is accomplished by a combination of structural and strati graphic mechanisms. The structural configuration of Coalinga anticline and its position up-dip from the Buttonwillow depocenter through much of the Tertiary (Zieglar and Spotts, 1978) resulted in migration of oil to the Coalinga Field. Zieglar and Spotts (1978) suggested that hydrocarbon generation occurred in Tertiary beds of the depocenter within the last few million years (perhaps 5 m.y.), but other structures “competing” for the oil, such as the Kettleman Hills, only developed during the Pleistocene episode of folding (Harding, 1976). The Temblor is overlapped by the Monterey and Etchegoin Formations along the Westside Field and is not exposed.

    4. Page 92

      Between Anticline Ridge on the south and the Ciervo Hills on the north, serpentinous strata of the Big Blue Formation lie strati graphically between Middle Miocene Temblor Formation and Upper Miocene Santa Margarita Formation in homoclinal exposures of Tertiary strata that flank the western margin of the Great Valley. Mapping of three distinctive lithologic units within the Big Blue Formation near Cantua Creek defines key aspects of its unusual origin (see Fig. 30). Foliate but unstratified serpentinite breccia, thickest on the outcrop between Salt and Martinez Creeks, forms the body of a protrusive serpentinite extrusion that flowed as a sheared mass across an unconsolidated substratum of Temblor sand, which was partly scraped away beneath the mass-flow and was locally plowed into chaotically rumpled folds that piled up at the advancing margin of the protrusive mass. Bedded serpentinite-clast conglomerates and breccias, with intercalated serpentine-grain sandstones and associated serpentinous debris-flow deposits, were formed by fluvial reworking of the protrusive serpentinite. Detrital serpentinite locally underlies but more commonly overlies and most typically flanks the latter as a facies equivalent. Interbedded serpentine-grain sandstones and serpentinous claystones of probable marine origin exposed laterally along strike apparently formed as an extensive facies fringe of serpentinite detritus dispersed widely from a central core of protrusive and coarser detrital serpentinite. Santa Margarita beds rest conformably on the finer detrital serpentinite but uncon-formably on protrusive serpentinite and associated deposits. Easterly paleocurrents from clast imbrications in detrital serpentinite suggest that the serpentinite source lay west of the present outcrop belt. Dispersal of serpentinite debris perhaps was fed by protrusive diapiric movement of Mesozoic serpentinite mobilized in the core of an ancestral Joaquin Ridge anticline, whose initial growth thus may have coincided with the Miocene onset of rapid motion along the late Cenzoic San Andreas fault system.

    5. Page 98

      Bedded serpentinite-clast conglomerate and sedimentary breccia with subordinate debris flow deposits form most of the Big Blue Formation between Salt and Cantua Creeks (see Fig. 32). Rusty brown outcrops in resistant ledges are characteristic (Fig. 39). A total thickness of 50 to 100 m is estimated. The bedded serpentinite-clast rudites, in general, lap depositionally upon the flank of the thick lens of protrusive serpentinite. About 15 m of foliate serpentinite breccia persists locally beneath bedded serpentinite-clast conglomerate and breccia as far north as the Lillis Ranch (Fig. 40).

      The bedded rudites consist of subangular to sub-rounded serpentinite-clast gravel associated with interstitial and interstratified serpentine-grain sand and cemented by carbonate (Fig. 41). The largest clasts are about 1 m in diameter, but most are much smaller. The texture of the sandy matrix is plainly fragmental (Fig. 42), and there is no hint of a shear fabric. Lenticular bedding and channel structures characteristic of braided-stream deposits are prominent (Figs. 43-45). Nearly all clasts are serpentinite, although rare fragments, of other rock types occur in some beds, and the chrome garnet, uvarovite, is common in the sand fraction.

      The proportion of serpentinite-clast conglomerate and sedimentary breccia declines and the proportion of serpentine-grain sandstone increases with distance laterally away from the sequence of foliate serpentinite breccia. In the canyon of Cantua Creek (see Fig. 33), more than half the unit is composed of serpentine-grain sandstone that is intercalated between more prominent layers of coarser serpentinite detritus.

      Crudely stratified serpentinite-clast rudites that represent debris-flow deposits locally occur strati graphically between foliate breccia and bedded conglomerate and breccia.

    1. Page 104

      The Northern Coast Ranges (Fig. 61) are a composite geomorphic province consisting of structurally controlled, northwest trending, enechelon ridges and valleys that extend from the Klamath Mountains in the north to San Francisco Bay in the south. The core of the Coast Ranges consists of complexly deformed Upper Jurassic to Early Tertiary rocks of the Franciscan Complex. This core is structurally overlain to the east by less deformed, late Mesozoic marine, clastic rocks of the Great Valley Sequence. These two essentially coeval terranes are juxtaposed along a major low-angle fault, the Coast Range Thrust (Bailey et al., 1964), whose present geometry is extensively modified by Tertiary deformation of the Coast Ranges and extensive strike slip displacement along reactivated Mesozoic structural discontinuities (McLaughlin, 1974, 1981; Suppe and Foland, 1978; Suppe, 1979).

      The Franciscan Complex is a highly deformed, lithologically heterogeneous assemblage. It consists predominately of graywacke and metagraywacke with subordinate shale, altered mafic volcanic rocks, radiolarian chert and minor limestones, which range from unmetamorphosed through zeolite, prehnite-pumpellyite, greenschist, and blueschist facies. It also includes minor blueschist and eclogite facies exotic blocks. The distribution and relationships of these rocks is complicated by numerous thrusts and chaotic melange zones. Vestiges of original stratigraphic relations are only locally preserved in large tectonic slabs and thrust sheets. Thrust sequences “top” eastward and “young” to the west and the regional tectonic fabric dips eastward at high angles. Overall, the high P, low T metamorphic grade increases west to east but the metamorphic facies progression is more complicated in detail.

    2. Page 108

      The local appearance of voluminous, monomineralic, foliate serpentinite breccias in the Wilbur Springs section of the Great Valley Sequence record a direct sedimentological response to a late Neocomian accretion event in the Coast Ranges. Field and petrologic studies establish the following events: 1) Early Cretaceous emplacement of lower plate Franciscan rocks (Indian Valley terrane) beneath the proto-Coast Range thrust, 2) deformation of both upper and lower plate rocks into the southeast plunging Wilbur Springs anticline, and 3) contemporaneous protrusion of serpentinite breccias.

      Recently discovered modern examples of diapiric serpentinite protrusions of similar magnitude from the Mariana arc-trench system suggest many parallel analogies between the tectonics of active forearcs and the late Mesozoic of the California margin.

      Two modes of occurrence of serpentinite have been recognized in the Wilbur Springs area. The first type, serpentinite derived from the alteration of ultramafic tectonite, is widely distributed throughout the Coast Ranges (Bailey et al., 1964). Once considered to be altered ultramafic intrusions and included as part of the Franciscan Complex, these serpentinite masses occur as regionally extensive, allochthonous fault slices or fault-bounded mèlange belts, and tend to be preferentially concentrated along or near contacts between the Franciscan complex and the Great Valley Sequence (Fig. 61). Based on the characteristic spatial association of the serpentinite with layered mafic plutonic rocks, submarine basaltic lavas, and pelagic sedimentary sequences, these serpentinized ultramafic rocks are now generally accepted to represent the basal, mantle tectonite of an ophiolite sequence, the Coast Range Ophiolite, upon which the sediments of the Great Valley Sequence were deposited (Bezore, 1969; Bailey et al., 1970; McLaughlin, 1974; Evarts, 1977; Hopson and Frano, 1977; Hopson et al., 1981).

    3. Page 113

      Voluminous serpentinite strata interfinger with Lower Cretaceous terrigenous clastics of the Great Valley sequence along the western edge of the Sacramento Valley. These deposits are exposed in the core of a regional, southeast-plunging structure, the Wilbur Springs anticline. The sedimentary serpenti-nites form a distinctive lithologic unit of foliate serpentinite breccias in road cuts along Highway 20 just west of its intersection with Highway 16.

      Extensive serpentinite masses of sedimentary origin were first reported in the area of Wilbur Springs by Taliaferro (1943). He recognized their association with fault-bounded serpentinite of the ophiolite belt but was unable to establish sufficient criteria to distinguish between the two types in many cases. Thus the origin of much of the exposed serpentinite remained unresolved. Lawton (1956), citing numerous fossil discoveries within serpentinite breccia and gradational contacts with surrounding Great Valley strata, suggested that all of the serpentinite bodies south of Wilbur Springs were the products of sedimentary processes. Although he noted textural variation between chaotic and stratified units, he did not attach genetic significance to these facies changes. He considered all the deposits detrital accumulations despite the poor stratification, chaotic nature, schistose fabric, and the admitted lack of internal clastic textures in many of the bodies. The foliation was attributed to later tectonism, which he thought had obliterated original depositional features.

      Moiseyev (1966), in his study of the mercury mineralization of the Wilbur Springs District, concurred with Lawton's findings and provided further documentation of the detrital nature of some of the ultramafic rocks. Stressing the inherent difficulty in discriminating sedimentary serpentinites, he concluded that lateral gradation of foliate breccia into coarse elastics of ultramafic composition and the presence of fossils in the serpentinite are the only unambiguous criteria of sedimentary origin (Moiseyev, 1970).

    4. Page 117

      The sedimentary serpentinites we will visit are exposed in the southeast plunging Wilbur Springs anticline, where they interfinger with lower Cretaceous turbidites of the Great Valley Sequence. The Wilbur Springs area (Fig. 65) lies astride this anticlinal structure on the western edge of the Sacramento Valley approximately 175 km north of San Francisco (Fig. 61). The area is named for a hot springs resort on Sulfur Creek, which is located upstream from the Lodoga Road connecting Bear Valley with Highway 20. Barren, block-strewn, grass-free slopes or dense brush characterize areas underlain by serpentinite. Outcrops are sparse, and mainly confined to road and stream cuts. However, even in areas of poor exposure, contacts are easily traced in soils and subcrop. Because of the serpentinite, the area is highly suceptible to landslides and soil creep which locally bury or displace contacts.

      The main outcrops of sedimentary serpentinite occur near Sulphur and Bear Creeks, south of Wilbur Springs. Here, thick bedded to massive, crudely stratified, foliate serpentinite breccia is the dominant lithologic facies. A quarry on Highway 20 and adjacent roadcuts provide excellent exposures of foliate serpentinite breccia and its stratigraphic relation with Great Valley strata.

      Turn out at the large vista point and walk back across the road to examine the sandstone-shale sequence and the foliate serpentinite breccia exposures in the quarry and in roadcuts at either side. Relocation of the highway and backfill of the original quarry cut has removed some of the section and considerably degraded what remains. Nevertheless, the relationships seen here, though now with some difficulty, are significant enough to justify a look.

Purchase Chapters

Recommended Reading