sign in sign up
<<
Home , Diablo Range, Franciscan Complex

Tectonic outlier of in Franciscan terrain, Diablo Range,

JANET M. BAUD ER* , ^ , • c r J ,-r • m,,,- J G LIOU I Department of , Stanford University, Stanford, California 9-1 >(b

ABSTRACT between the Franciscan Complex and the Great Valley sequence have long been a subject of controversy. Taliaferro (1943) thought A dismembered ophiolite, tectonically overlain by Great Valley that the Franciscan was overlain conformably by Upper strata, rests upon Franciscan rocks southeast of Cedar Mountain in beds of the lower Great Valley sequence. Irwin (1957) disproved the northern Diablo Range. The klippe of sedimentary rocks is the this hypothesis by demonstrating that parts of the Franciscan are first reported outlier of the Great Valley sequence in the interior of in age. Irwin (1964) and Bailey and others (1964) noted the range. The dismembered ophiolite is composed of serpentinized that faults generally separate the Franciscan from the Great Valley ultramafic rocks, hornblende gabbro, diorite, and plagiogranite, sequence, and they suggested that the two units are juxtaposed and it is partially altered to facies assemblages contain- along a regional , later named the Coast Range thrust ing chlorite + epidote + albite + actinolite. (Bailey and others, 1970). The Franciscan Complex northeast of the ophiolite is charac- Within the last decade, the concept of plate tectonics has revo- terized by lithologic heterogeneity, and it includes melanges com- lutionized the interpretation of Coast Range geology. According to prised of mixed fragments of varied metamorphic grade in- current theories, the Franciscan Complex was assembled in a sub- corporated in a pervasively sheared matrix. The coherent duction zone, while the Great Valley strata accumulated farther Franciscan terrain southwest of the ophiolite may belong to a struc- east in a fore-arc basin (see, for example, Hamilton, 1969; Blake tural unit of Late Jurassic age which overlies younger Franciscan and Jones, 1974). The ultramafic and mafic rocks that commonly rocks in other parts of the Diablo Range. All Franciscan samples separate the two sequences represent the and the examined petrographically contain facies minerals such upper mantle that lie below the base of the westernmost Great Val- as , pumpellyite, sodic amphibole, and jadeitic pyroxene. ley sequence. Probably beginning in the Late Jurassic and continu- The Great Valley strata consist of interbedded and ing into the early Tertiary, the Franciscan Complex was subducted shale. Arkoses of the Great Valley klippe are generally calcareous beneath the oceanic crust and the Great Valley sequence as a result and contain well-preserved detrital potassium feldspar and biotite. of ocean-floor spreading. At depth within the zone, the Locally, the sandstone contains megafossils of probable Valan- Franciscan was successively subjected to the conditions of zeolite, ginian and Cenomanian age, includingLinearia multicostata (Gabb), prehnite-pumpellyite, and blueschist facies , and Pterotrigonia oregana (Packard), Turritella hearni Anderson, then somehow returned to the surface (Ernst, 1973). This model Trigonia sp., Panope sp., and small ammonite scraps similar to accounts for the different deformational styles and metamorphic Thurmanniceras (?) sp. Incipient low-grade metamorphism is indi- mineral assemblages in the Franciscan Complex and Great Valley cated by alteration of calcic plagioclase to albite + calcite + white sequence. mica and by partial chloritization of biotite. The lithology and the The present study covers 29 km2 in the Diablo Range (Fig. 1), age, structural, and metamorphic relationships indicate that the which is a northwest-trending antiform in the central Coast Great Valley outlier and its underlying dismembered ophiolite are Ranges. Rocks of the area consist of typically heterogeneous, dis- erosional remnants of the Coast Range thrust which once extended rupted, and metamorphosed Franciscan lithologies, which are over- across the Diablo Range. lain by ophiolitic rocks and the Great Valley sequence. Although much of the northern Diablo Range has been mapped (for example, INTRODUCTION Huey, 1948; Maddock, 1964; Cotton, 1972; Raymond, 1973; Cowan, 1974; Crawford, 1976), no outlier of fossiliferous Great Western California is underlain in large part by the Franciscan Valley sandstone and shale within the interior of the Diablo Range Complex and the Great Valley sequence. Both units consist mainly was recognized prior to the present study. The purposes of this of marine clastic rocks, but they differ in style of deformation and paper are to describe the structural and age relations of lithologic degree of metamorphism. The origin of and tectonic relationship units in the Cedar Mountain area, to discuss the metamorphic min- eral assemblages that distinguish the Franciscan Complex from the ophiolite and the Great Valley sequence, and to relate the discovery ,f Present address: Continental Oil Company, 555 17th Street, Denver, of the Great Valley klippe to the tectonic and metamorphic history Colorado 80202. of the Diablo Range.

Geological Society of America Bulletin, Part I, v. 90, p. 561-568, 4 figs., 1 table, June 1979, Doc. no. 90608.

561

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/6/561/3434088/i0016-7606-90-6-561.pdf by guest on 29 September 2021 562 BAUDER AND LIOU

DESCRIPTION OF LITHOLOGIC UNITS blueschist, and metaconglomerate. This area contains melanges with fragments up to 50 m in length enveloped in a pervasively Franciscan Complex sheared shale matrix. However, Franciscan strata on Taylor Ridge are fairly undeformed and may be the westward extension of a As shown in the map and cross sections of Figure 2, almost coherent metagraywacke unit in the adjacent Lone Tree Creek three-fourths of the area is underlain by Franciscan metaclastic quadrangle (M. E. Maddock, 1977, oral commun.). rocks, metachert, greenstone, and some exotic blocks such as Detrital texture in most metasandstones has been disrupted by blueschist, , and hornblendite. Exposures are generally shearing and recrystallization. In the Cedar Mountain area, the poor, consisting of isolated blocks scattered across grassy hill metagraywackes range from textural zone 1 to low zone 3, with slopes, within stream beds, and along roadcuts. The Franciscan most belonging to zone 2 (Blake and others, 1967). The areal dis- Complex northeast of the dismembered ophiolite consists mainly of tribution of textural zones apparently is unsystematic (Fig. 3), as metagraywacke and argillite with some greenstone, metachert, samples from all three zones may occur within a 600-m radius

Figure 1. Index map of Cedar Mountain area and its environs, northern Diablo Range, California. Distribution of jadeitic pyroxene is modified from Ernst (1971, Fig. 7) with contributions from Raymond (1973, Fig. 7) and Cowan (1974, Fig. 2).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/6/561/3434088/i0016-7606-90-6-561.pdf by guest on 29 September 2021 EXPLANATION

GREAT VALLEY SEQUENCE Sondstone and shale DISMEMBERED OPHIOLITE Xjp^ Gabbro.diorite, and plagiogranite 3000'- UM^ Serpentinized ultramafic rocks 2000- FRANCISCAN COMPLEX ssMetograywacke and shale 1000'-

CG Metoconglomerote A

Greenstone Hornblendite

Blueschist EC Eclogite ••• Lithologic contact La-1' High angle fault (hachures on downthrown side) *—>•— Thrust foult (borbs on upper plate) —r^ Strike and dip of bedding 40-1 Sample locality £ Landslide deposits Figure 2. Geologic map and cross sections of Cedar Mountain area (mapped by J. Bauder, 1975).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/6/561/3434088/i0016-7606-90-6-561.pdf by guest on 29 September 2021 564 BAUDER AND LIOU

(examples may be seen near Mines Road). This randomness is metachert are part of a large, relatively coherent tabular body un- further evidence that the region includes melanges. derlain by melange. The Franciscan Complex southwest of the ophiolite is fairly The greenstone bordering the ophiolite contains lawsonite ± coherent, containing neither metaconglomerate nor blueschist. A sodic amphibole, blueschist facies minerals characteristic of large greenstone body with minor interbeds of metagraywacke, ar- Franciscan metamorphic rocks. The greenstone-ophiolite contact is gillite, and metachert lies along the southwest border of the ophio- inferred to be a fault because of the disparity in metamorphic phase lite body. This greenstone is apparently overlain by an extensive assemblages across the contact. metachert unit which includes discontinuous outcrops of meta- (that is, rocks with sodic amphibole visible in hand graywacke and minor greenstone. The metachert and metagray- specimen) were observed only northeast of the ophiolite body. wacke in this unit commonly have similar bedding attitudes, They are sporadically distributed, typically as 1- to 10-m-wide piles suggesting that they are interbedded rather than tectonically jux- of boulders. An excellent example of a blueschist knocker occurs taposed. Mapping by Crawford (1976) in the adjacent Eylar about 150 m northeast of Mines Road at locality 40-1 (Fig. 2). The Mountain quadrangle suggests that the greenstone and overlying boulder at this location is about 5 m in diameter, has a steely-blue

METAMORPHIC MINERAL SYMBOLS 2 Km LAWSONITE + ALBITE ± SODIC AMPHIBOLE

JADEITIC PYROXENE + LAWSONITE + ALBITE ± SODIC AMPHIBOLE

• JADEITIC PYROXENE + ALBITE ± SODIC AMPHIBOLE o ARAGONITE IDENTIFIED BY BIAXIAL FIGURE o PUMPELLYTE POSITIVELY IDENTIFIED

TEXTURAL ZONE CLASSIFICATION (AFTER BLAKE AND OTHERS, 1967)

2- LOW ZONE 2 2 MID ZONE 2 2+ HIGH ZONE 2 [GEOLOGIC SYMBOLS SAME AS FIGURE 2]

Figure 3. Map showing textural zones (af- ter Blake and others, 1967) and distribution of blueschist facies minerals (lawsonite, pumpellyite, glaucophane, jadeitic pyroxene, and aragonite) in the Franciscan meta- graywackes, Cedar Mountain area.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/6/561/3434088/i0016-7606-90-6-561.pdf by guest on 29 September 2021 TECTONIC OUTLIER OF THE GREAT VALLEY SEQUENCE, CALIFORNIA 565

interior, well-defined planar , and small porphyroblasts of some concretions were processed for but and pyrite. The planar fabric is transected by 1- to 24-cm- were barren. thick veins of quartz and pumpellyite. The exterior of the knocker Numerous megafossils were found in fine-grained, calcareous is covered by a 10- to 24-cm-thick shell of coarse-grained actinolite sandstone at locality 74-2 (for a detailed locality description, see + chlorite , which is foliated parallel to the margins of the Bauder, 1975). D. L. Jones of the U.S. Geological Survey considers boulder. The veins and internal foliation do not extend into this the age of the assemblage to be (probably outer selvage. The existence of the selvage of retrograde minerals Cenomanian), on the basis of his identification of the following suggests that such blueschist bodies were tectonically emplaced fauna: Linearia multicostata (Gabb), Pterotrigonia oregana (Pack- rather than being metamorphosed in situ (Bailey and others, 1964; ard), Turritella hearni Anderson, Trigonia sp., and Panope sp. Coleman and Lanphere, 1971). These and other unidentified clams and snails constitute a Tectonic blocks of hornblendite and eclogite occur near the con- shallow-water assemblage that was probably redeposited in deep tact between the Franciscan Complex and the of the water by slumps or turbidity currents. At locality 117-1, Jones ophiolite body (Fig. 2). The hornblendite consists mainly of brown found a small ammonite scrap similar to Thurmanniceras (?) sp. hornblende with minor sodic amphibole, white mica, chlorite, indeterminable, suggestive of a late Valanginian (Early Cretaceous) sphene, and opaques. The eclogite contains omphacitic pyroxene + age (D. L. Jones, 1975, oral comraun.), garnet retrograding to chlorite + epidote + sphene + opaques. Dismembered Ophiolite Great Valley Sequence A northwest-trending body of ultramafic, mafic, and felsic rocks Prior to the present study, Great Valley rocks were well known dominates the southwest half of the area. The assemblage, which on the flanks of the Diablo Range but not in its interior. Huey is interpreted to be a dismembered ophiolite, consists largely (1948) and Cotton (1972), who mapped the Cedar Mountain area (~ 70%) of serpentinized harzburgite, but it includes dunite, previously, reported only Franciscan rocks. However, during our pyroxenite, gabbro, diorite, and plagiogranite (for definition of investigation, an outlier of fossiliferous Great Valley sandstone and plagiogranite, see Coleman and Peterman, 1975). Locally, shale was discovered associated with a dismembered ophiolite. This harzburgite is massive and only partly altered; however, it becomes isolated exposure of Great Valley rocks occupies approximately increasingly sheared and serpentinized near the tectonic contacts 0.35 krtf on the Dudney Ranch in the NW1/4 sec. 6, T. 5 S., R. 4 E. with adjacent units. Mafic to felsic intrusive rocks, which form dis- (Fig. 2). crete bodies along the western side of the dismembered ophiolite, Except for outcrops in roadcuts and ditches, the Great Valley were mapped as a single unit because they have complex mutual clastic rocks are poorly exposed in the study area. However, along relations. Roadcut exposures show felsic dikelets cutting mafic one roadcut, Great Valley shale with regular interbeds of sandstone rocks, indicating the intrusion of late-stage differentiates. The gab- is continuously exposed for about 400 m. Sandstone beds are typi- bros, diorites, and plagiogranites are faulted against the underlying cally 50 cm thick, whereas shale beds average 15 cm. This flysch- ultramafic rocks. The nature of the contact is indicated by inter- like sequence dips ~ 30° to the northeast and is right side up, as tonguing fault slivers of gabbro and serpentinized peridotite. Locally, indicated by graded bedding. Assuming that the bedding attitude is hornblende gabbro is separated from underlying harzburgite by fairly consistent throughout the area, the maximum thickness of sheared dunite and pyroxenite. Similar features characterize the the Great Valley section here is ~ 300 m. fault zone between ultramafic rocks and overlying cumulate gabbro On the southwestern margin, the sedimentary outlier is clearly in the Red Mountain ophiolite 8 km southeast of the Cedar Moun- faulted against the underlying sheared serpentinite. Sedimentary tain area (Evarts, 1976; see Fig. 1 for location). rocks near the base of the Great Valley outlier, which are well ex- Near the contacts between the ophiolite and the Franciscan posed in the walls of a reservoir near locality 17-1, exhibit bedding Complex, the ultramafic rocks are highly sheared and serpen- cleavage that dips ~ 23° northeast. The eastern boundary between tinized, indicating that they are faulted against the Franciscan. The clastic rocks and serpentinized ultramafic rocks is not exposed; western boundary forms a fairly linear pattern on the map, suggest- however, it appears to be steep from the configuration of the con- ing that the contact is a high-angle fault. However, the sinuous tact and may represent a high-angle fault. To the north, Great Val- eastern border provides neither consistent nor conclusive evidence ley sandstone is separated from Franciscan metachert by ~ 30 m of as to the orientation of the contact. Along the northern border, the serpentinite; to the northwest, the sandstone seems to be in direct contact appears to dip ~ 50° toward the south, with Franciscan contact with Franciscan greenstone. These boundary relationships rocks beneath the ophiolite body. Fensters of Franciscan rocks suggest that some underlying Franciscan rocks have been brought within the northern part of the ophiolite indicate the thinness of the up along high-angle faults to their present structural positions (see ultramafic sheet. The interpretation of the complex contact rela- cross sections AA' and BB' of Fig. 2). tionships is presented in cross sections AA' and BB' of Figure 2. Great Valley sandstone is fine- to coarse-grained, angular to sub- rounded, moderately sorted to well-sorted lithic arkose. Unlike MINERAL PARAGENESES AND METAMORPHISM Franciscan metagraywacke in the area, the Great Valley sandstone commonly contains plant fragments and detrital biotite grains, plus Fifty-four thin sections were examined to determine parental 1 to 15% potassium feldspar. rock types, mineral parageneses, and textural relationships. De- The dark gray, brown-weathering Great Valley shale is silty and tailed descriptions of these rocks are presented in Bauder (1975); a is less indurated than Franciscan argillite. Four shale samples and summary of mineral parageneses and metamorphism follows.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/6/561/3434088/i0016-7606-90-6-561.pdf by guest on 29 September 2021 566 BAUDER AND LIOU

Franciscan Complex Great Valley

The Franciscan rocks studied include metagraywacke, green- Great Valley sandstone differs markedly from Franciscan meta- stone, blueschist, eclogite, and sodic amphibole-bearing horn- graywacke. The samples studied have epiclastic textures with no blendite. The various rock types have different primary textures evidence of deformation or extensive recrystallizaton. Unlike and mineral assemblages reflecting differences in origin and bulk Franciscan clastic rocks, Great Valley sandstone commonly has chemical composition. Nonetheless, they all contain characteristic calcareous , derrital biotite, and potassium feldspar but blueschist facies minerals which distinguish them from Great Val- lacks blueschist facies minerals. The presence of calcareous cement ley sequence rocks. suggests a high chemical potential of CQ>, which would inhibit the All metagraywackes contain albite + quartz + phyllosilicates + formation of zeolites, prehnite, or pumpellyite (Zen, 1961). A few sphene + opaques. Key metamorphic minerals include pumpellyite, calcic plagioclase crystals have altered to albite + calcite + sericite, lawsonite, sodic amphibole, and jadeitic pyroxene (Fig. 3). Pumpel- and some biotite has altered to chlorite. These reactions suggest lyite appears as very fine-grained, clear granules in albite and phyl- that the Great Valley rocks underwent low-grade metamorphism. losilicates. Clear idioblastic lawsonite prisms up to 0.9 mm long occur extensively in the groundmass intergrown with white mica. Dismembered Ophiolite Smaller lawsonites are present within lithic fragments and within albite grains, having formed from the original Ca-bearing plagio- The gabbro and plagiogranite in the dismembered ophiolite con- clase. In some samples, lawsonite porphyroblasts are bent and frac- tain chlorite and epidote, with minor incipient growths of albite in tured, indicating deformation after crystallization. Both sodic am- calcic plagioclase (An42 to An64) and actinolite in hornblende. phibole and jadeitic pyroxene occur as inclusions in quartz and Apparently these rocks were subjected only to the initial stages of albite and as intergrowths with phyllosilicates. Dense fans of blue greenschist facies metamorphism. Later retrograde alteration is amphibole project inward from the borders of some quartz clasts. demonstrated by the presence of prehnite and zeolite veins and of Many clusters of jadeitic pyroxene have a radial pattern, whereas pumpellyite in the groundmass. The occurrence of calcium-bearing others are straight and intersect one another. Jadeitic pyroxene and silicates rather than carbonate in the gabbro and plagiogranite is lawsonite are intergrown in some albitic clasts, indicating their indicative of low partial pressures of C'Oh during ocean-floor stable coexistence. metamorphism (see, for example, Coleman, 1977). Most of the greenstones have a well-preserved volcanic texture consisting of phenocrysts and/or amygdules set in a fine-grained, REGIONAL CORRELATIONS AND murky groundmass. The plagioclase phenocrysts are altered to TECTONIC SYNTHESIS albite + phyllosilicates, with some also containing lawsonite and/or jadeitic pyroxene. Amygdules consist of chlorite ± quartz ± law- Recent investigations in the Diablo Range have differentiated the sonite + pumpellyite ± sphene ± carbonate; larger amygdules Franciscan Complex into mélange and coherent units (Cotton, commonly have layers of different minerals. The aphanitic 1972; Raymond, 1973; Cowan, 1974; Crawford, 1975, 1976; M. groundmass consists of chlorite, sphene, and minor opaques. Sev- E. Maddock, 1977, oral commun.). Correlating results was difficult eral greenstones also have omphacitic pyroxene partly retrograded because researchers differ on the definition of mélange (for exam- to chlorite and sphene. ple, Garzas tectonic mélange of Cowan, 1974, versus broken for- Blueschists consist mainly of sodic amphibole + sphene + mation of Raymond, 1973). In addition, some structural units opaques + chlorite ± white mica ± lawsonite ± albite ± pumpel- probably constitute local fault slivers which do not extend to other lyite ± jadeitic pyroxene. Based upon variations in texture and parts of the Diablo Range. Nevertheless, the greenstone, metachert, mineralogy, the original lithologies were identified as , gab- and metagraywacke southwest of the Cedar Mountain ophiolite bro, diorite, and ironstone. Many mafic metaigneous rocks contain appear to be the northern extension of Crawford's coherent, pumpellyite in the matrix or in the veins. Remarkably coarse pum- jadeite-bearing Eylar Mountain sequence (Crawford, 1976). This pellyite occurs in the blueschist knocker at locality 40-1. The rock coherent unit is thought to be correlative with similar metagray- contains abundant chlorite and pumpellyite interlayered with sodic wackes and metacherts iri the northern Diablo Range that contain amphibole, whereas veins are composed mainly of pumpellyite. rare megafossils and numerous of Tithonian (Late Several 2-mm-long pumpellyites exhibit characteristic oak leaf Jurassic) age (D. L. Jones, 1977, oral commun.). The Eylar Moun- twins and fair cleavage perpendicular to the c-axis. Both host rock tain sequence is separated by a narrow mélange from underlying and veins contain pumpellyite locally enveloped in aragonite. The lawsonite-bearing Franciscan flysch of middle Cretaceous (Albian? vein pumpellyite retains its idioblastic habit and seems to coexist to Coniacian?) age (Crawford's Burnt Hills sequence; Table 1). stably with carbonate. However, pumpellyite in the rock is ragged Except for the scarceness of keratophyre and the absence of tuffs and fractured, suggesting replacement by CaCQ,. and chert, the ophiolite of the Cedar Mountain area is very similar All of the Franciscan samples contain characteristic blueschist to the 160-m.y.-old Red Mountain ophiolite, which is 8 km south- facies minerals, including lawsonite, sodic amphibole, aragonite, east of Cedar Mountain (Lanphere, 1971; Hopson and others, and jadeitic pyroxene. Jadeitic pyroxene is widespread in the Cedar 1975). By extrapolation, the remnant of oceanic crust and upper Mountain area, although Ernst (1971, Fig. 7) did not designate this mantle near Cedar Mountain probably formed ~ 160 m.y. ago. terrain as jadeite bearing in his map of the areal distribution of The remnants of Valanginian and Cenomanian sedimentary jadeite in the Diablo Range. Some of the Franciscan in this area rocks in the Cedar Mountain outlier are correlative with parts of constitutes a melange; therefore, it is not surprising that jadeitic the Great Valley sequence on the flanks of the Diablo Range. A nar- and nonjadeitic rocks are interspersed. row belt of Upper Jurassic and Cretaceous Great Valley strata

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/6/561/3434088/i0016-7606-90-6-561.pdf by guest on 29 September 2021 TECTONIC OUTLIER OF THE GREAT VALLEY SEQUENCE, CALIFORNIA 567

spans the western side of the range. Along the eastern border, the SW ,-GREAT VALLEY OUTLIER NE Great Valley sequence consists of 8,000 to 10,000 m of Albian to \(upper Jurassic - Mid Cret.) Maastrichtian (Lower to Upper Cretaceous) rocks, which are lo- cally unconformably underlain by Buchia-beating shelf deposits of J/'' Tithonian (Late Jurassic) to Valanginian (Early Cretaceous) age. Hauterivian through Aptian strata were either eroded or never de- posited, because rocks of this age have not been reported bordering „ „. C 0 s \ s the Diablo Range. At , 50 km southeast of Cedar ^M^ss ^% ^ Wv ^\n o\c Mountain, rocks similar to those in the Cedar Mountain klippe in- ^ i * ° - - - - - " elude Çenomanian through Campanian age strata unconformably ^ ^ PLAN ATION overlying Tithonian and Valanginian (Schilling, 1962; see Fig. 1 for locations). Although the contact between the Lower and [S3 GREAT VALLEY SEQUENCE FRANCISCAN COMPLEX Upper Cretaceous strata in the Cedar Mountain outlier is not ex- „ iïliïl Coherent Unit , , \M " (Upper Jurossic) posed, it may be a major unconformity. .—. uu,u""'' 0 5 10 15 LJ Melange Tectonic Significance ' 'KM' ' ^ ^T^tacLs) , .. „ ,, ,. , , . ,. . , „ . Figure 4. Diagrammatic cross section of northern Diablo Ranee T11 he klipp e ofc Great Valley sequence and ophiohte in the Cedar . ° ° . . ° Mountain area is comparable to other outliers in the northern and Pnor ° eogene ìg ang e au ing. southern Coast Ranges (for example, see Bailey and others, 1964). age (Burnt Hills sequence), which is separated by mélange from the These outliers commonly have basal serpentinized ultramafic rocks overlying Eylar Mountain sequence of presumed Tithonian age. which are in fault contact with underlying Franciscan rocks. In the Although a high-angle fault now separates the Eylar Mountain se- Geysers-Clear Lake area in , isolated Tithonian quence from the Cedar Mountain ophiolite, the Franciscan Com- to Valanginian Great Valley strata depositionally overlie an Upper plex originally lay beneath a thrust contact with the ophiolite and Jurassic ophiolite (McLaughlin, 1976). However, in the Cedar overlying Great Valley sequence. Hence, the ophiolite of probable Mountain outlier and at other localities in the Coast Ranges, the Kimmeridgian (pre-Tithonian) age overlies the younger Franciscan basal sedimentary beds are separated by a fault from the ophiolite Complex. Furthermore, Great Valley deposits of probable Valan- complex. Although the oldest clastic sediments above the ophiolites ginian age are apparently underlain at depth by middle Cretaceous are commonly Tithonian (for example, Swe and Dickson, 1970; Franciscan rocks (Fig. 4). Page, 1972), in some cases, they are Neocomian (Early Cretaceous) Any theory for the origin of Great Valley outliers must account (Bailey and others, 1964). for the established age, metamorphic, and structural relations be- The ages of Great Valley outliers are well documented, but the tween the juxtaposed units. Maxwell (1974, p. 1203) postulated age of the underlying Franciscan Complex is often only approxi- that isolated Great Valley deposits accumulated on a tectonically mately known due to poor control and the inherent active shelf made up of older Franciscan mélange above sediments difficulties of dating mélanges. Nonetheless, several klippen of the that were being actively underthrust from the seaward side. This Great Valley sequence and associated ophiolite in the northern hypothesis is not supported by the results of the present investi- Coast Ranges have been shown to be underlain by younger Fran- gation nor by previous studies (Bailey and others, 1964; ciscan rocks (for example, near Dry Creek, Bailey and others, McLaughlin, 1976). If the ophiolite in the Cedar Mountain outlier 1964; and in the Geysers-Clear Lake area, McLaughlin, 1976). formed ~ 160 m.y. ago, the ophiolite must have been emplaced Paleontologie data, K-Ar ages, and structural relationships in the over the Tithonian-aged Franciscan rocks after Late Jurassic time. Diablo Range suggest that younger Franciscan rocks occur beneath The Great Valley sequence in the outlier has undergone low tem- the Cedar Mountain outlier. The lowest structural unit in the Di- perature and low pressure burial metamorphism, whereas the un- ablo Range appears to be Franciscan flysch of middle Cretaceous derlying Franciscan Complex was metamorphosed at great depths

TABLE 1. SUGGESTED CORRELATION OF FRANCISCAN UNITS MAPPED IN THE NORTHERN DIABLO RANGE, CALIFORNIA

Investigation Cotton Cowan Crawford Present study (1972) (1974) (1976)

Locality Reconnaissance of Northwest of Eylar Mountain Cedar Mountain t central Diablo Range quadrangle quadrangle -c Structural KJfm — mainly massive semischistose Eylar Mountain Franciscan OJ0 J; units to thick bedded metagraywacke sequence southwest of ophiolite (Tithonian) (Albian or older) "ra 2 i3j KJfs — mélange Garzas mélange Garzas mélange Franciscan mélange(s) northeast of ophiolite KJfs flysch-like sequence Orestimba metagraywacke Burnt Hills sequence (Albian? to Coniacian?) (Cenomanian or older) (Albian? to Coniacian?)

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/6/561/3434088/i0016-7606-90-6-561.pdf by guest on 29 September 2021 568 BAUDER AND LIOU

under low temperature and high pressure conditions. The Francis- Coleman, R. G., and Peterman, Z. E., 1975, Oceanic plagiogranite: Journal can Complex and Great Valley sequence in the Cedar Mountain of Geophysical Research, v. 80, no. 8, p. 1099-1108. area could not have been in their present structural positions during Cotton, W. R., 1972, Preliminary geologic map of the Franciscan rocks in the central part of the Diablo Range, Santa Clara and Alameda Coun- Franciscan metamorphism because of the disparity in metamorphic ties, California: U.S. Geological Survey Miscellaneous Field Studies phase assemblages. Franciscan metagraywackes near Pacheco Pass Map MF-343. (Fig. 1) have minimum metamorphic ages ranging from ~ 76 to Cowan, D. S., 1974, Deformation and metamorphism of the Franciscan 122 m.y. (Suppe and Armstrong, 1972; Cowan, 1974). By extrapo- subduction zone complex northwest of Pacheco Pass, California: Geological Society of America Bulletin, v. 85, p. 1623-1634. lation, most Franciscan rocks in the study area were probably Crawford, K. E., 1975, The geology of the Franciscan tectonic assemblage metamorphosed during or after deposition of the Valanginian and near Mount Hamilton, California [Ph.D. thesis]: Los Angeles, Cenomanian Great Valley sandstones and shales. Therefore, the California, , Los Angeles, 137 p. Franciscan Complex and overlying Great Valley deposits must have 1976, Reconnaissance geologic map of the Eyiar Mountain quad- been tectonically juxtaposed after the Franciscan rocks were rangle, Santa Clara and Alameda Counties, California: U.S. Geologi- cal Survey Miscellaneous Field Studies Map MF-764. metamorphosed. Dickinson, W. R., 1965, Folded thrust contact between Franciscan rocks Based upon such circumstantial reasoning, outliers of the Great and Panoche Group in the Diablo Range of [abs.J: Valley sequence are generally thought to represent erosional rem- Geological Society of America Special Paper 82, p. 248—249. nants of a gigantic thrust plate or system of thrust plates that once Ernst, W. G., 1971, Petrologic reconnaissance of Franciscan meta- graywackes from the Diablo Range, central : covered the entire Coast Ranges (Bailey and others, 1970; Swe and Journal of Petrology, v. 12, no. 2, p. 413-437. Dickinson, 1970). Although exposures of the "Coast Range thrust" 1973, Blueschist metamorphism and P-T regimes in active subduction have been described on the eastern border of the Diablo Range zones: Tectonophysics, v. 17, p. 255-272. (Dickinson, 1965; Raymond, 1973), the ophiolite and Great Valley Evarts, R., 1976, Great Valley ophiolite sequence at Del Puerto Canyon, sedimentary rocks in the study area offer substantial evidence that California: EOS (American Geophysical Union Transactions), v. 57, p. 1026. this thrust surface once extended across the Diablo Range. Hamilton, W., 1969, California and the underflow of Pacific mantle: Geological Society of America Bulletin, v. 80, p. 2409-2429. ACKNOWLEDGMENTS Hopson, C. A., Mattinson, J. M,, and Pessagno, E., 1975, Record of Late Jurassic sea-floor spreading, California Coast Ranges: Geological So- ciety of America Abstracts with Programs, v. 7, p. 326. We wish to acknowledge E. E. Brabb of the U.S. Geological Sur- Huey, A. S., 1948, Geology of the Tesla quadrangle, California: California vey, who proposed the field area for investigation. We are espe- Division of Mines and Geology Bulletin 140, 75 p. cially grateful to D. L. Jones of the U.S. Geological Survey, who Irwin, W. P., 1957, Franciscan group in Coast Ranges and its equivalents in determined fossil ages and made many valuable suggestions. We Sacramento Valley, California: American Association of Petroleum Geologists Bulletin, v. 41, no. 10, p. 2284-2297. thank R. Evarts of Stanford University, K. Crawford of San Fran- 1964, Late Mesozoic orogenies in the ultramafic belts of northwestern cisco State University, and M. E. Maddock of San Jose State Uni- California and southwestern Oregon: U.S. Geological Survey Profes- versity for stimulating discussions and field excursions. Critical sional Paper 501-C, p. C1-C9. comments from D. L. Jones, Loren A. Raymond of Appalachian Lanphere, M. A., 1971, Age of the Mesozoic oceanic crust in the California State University, W. G. Ernest of UCLA, Ben M. Page of Stanford, Coast Ranges: Geological Society of America Bulletin, v. 82, p. 3209-3211. Jason Saleeby of University of California at Berkeley, and K. Craw- Maddock, M. E., 1964, Geology of the Mt. Boardman quadrangle, Santa ford were most helpful. The field study was supported by the U.S. Clara and Stanislaus Counties, California: California Division of Geological Survey, by the Geology Department of Stanford Uni- Mines and Geology Map Sheet 3. versity, and by the Shell Foundation. Preparation of the manuscript Maxwell, J. C., 1974, Anatomy of an orogen: Geological Society of was supported by NSF Grant EAR76-22650/Liou. The authors are America Bulletin, v. 85, p. 1195-1204. McLaughlin, R. J., 1976, Significance of age relationships of rocks above most grateful to the above named institutions and individuals for and below Upper Jurassic ophiolite in the Geysers-Clear Lake area, their assistance. California: Geological Society of America Abstracts with Programs (Cordilleran Section), no. 3, p. 394-395. REFERENCES CITED Page, B. M., 1972, Oceanic crust and mantle fragment in subduction com- plex near San Luis Obispo, California: Geological Society of America Bailey, E. H., Irwin, W. P., and Jones, D. L., 1964, Franciscan and related Bulletin, v. 83, p. 957-972. rocks and their significance in the geology of western California: Raymond, L. A., 1973, Tesla-Ortigalita fault, Coast Range thrust fault, and California Division of Mines and Geology Bulletin 183, 177 p. Franciscan metamorphism, northeastern Diablo Range, California: Bailey, E. H., Blake, M. C., Jr., and Jones, D. L., 1970, On-land Mesozoic Geological Society of America Bulletin, v. 84, p. 3547-3562. oceanic crust in California Coast Ranges: U.S. Geological Survey Pro- Schilling, F. A., Jr., 1962, The Upper Cretaceous stratigraphy of the fessional Paper 700-C, p. C70-C81. Pacheco Pass quadrangle, California [Ph.D. thesis]: Stanford, Bauder, J. M., 1975, Geology of the Cedar Mountain region, northern Di- California, Stanford University, 253 p. ablo Range, California [M.S. thesis]: Stanford, California, Stanford Suppe, J., and Armstrong, R. L., 1972, Potassium-argon dating of Francis- University, 93 p. can metamorphic rocks: American Journal of Science, v. 272, Blake, M. C., Jr., and Jones, D. L., 1974, Origin of Franciscan melanges in p. 217-233. northern California, in Dott, R. H., Jr., and Shaver, R. H., eds., Swe, W., and Dickinson, W. R., 1970, Sedimentation and thrusting of late Modern and ancient geosynclinal sedimentation: Society of Economic Mesozoic rocks in the Coast Ranges near Clear Lake, California: Paleontologists and Mineralogists Special Publication 19, p. 345-357. Geological Society of America Bulletin, v. 81, p. 165-188. Blake, M. C., Jr., Irwin, W. P., and Coleman, R. G., 1967, Upside-down Taliaferro, N. L., 1943, Franciscan-Knoxville problem: American Associa- metamorphic zonation, blueschist facies, along a regional thrust in tion of Petroleum Geologists Bulletin, v. 27, no. 2, p. 109-219. California and Oregon: U.S. Geological Survey Professional Paper Zen, E-an, 1961, The zeolite facies: An interpretation: American Journal of 575-C, p. 1-9. Science, v. 259, p. 401-409. Coleman, R. G., 1977, Ophiolites, ancient oceanic lithosphere?: New York, Springer-Verlag, Inc., 229 p. Coleman, R. G., and Lanphere, M. A., 1971, Distribution and age of high- grade blueschists, associated , and from Oregon MANUSCRIPT RECEIVED BY THE SOCIETY AUGUST 29, 1977 and California: Geological Society of America Bulletin, v. 82, REVISED MANUSCRIPT RECEIVED FEBRUARY 14, 1978 p. 2397-2412. MANUSCRIPT ACCEPTED MARCH 30, 1978

Printed in U.S.A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/90/6/561/3434088/i0016-7606-90-6-561.pdf by guest on 29 September 2021