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Home , Franciscan Complex, Great Valley Sequence

Provenance of Franciscan graywackes in coastal

WILLIAM R. DICKINSON Department of Geosciences, University of Arizona, Tucson, Arizona 85721 RAYMOND V. INGERSOLL Department of , University of New Mexico, Albuquerque, New Mexico 87131 DARREL S. COWAN Department of Geological Sciences, University of Washington, Seattle, Washington 98195 KENNETH P. HELMOLD Exploration and Production Research Laboratory, Cities Service Company, P. O. Box 3908, Tulsa, Oklahoma 74150 CHRISTOPHER A. SUCZEK Department of Geology, Western Washington University, Bellingham, Washington 98225

ABSTRACT known Great Valley counterparts and were tinite, minor pelagic , and rare probably derived from segments of the arc polymict conglomerate—are distinctly sub- A systematic comparison of available where exposures of plutons were ordinate. detrital modes for graywacke of more extensive than within typical Great One line of thought, based in part on the the Franciscan complex and for Valley sources. Higher proportions of non- work of Taliaferro (1943), holds that the coeval sandstones of the Great Valley volcanic to volcanic lithic fragments in sandy detritus in the graywackes came from sequence in the some Franciscan sandstones probably re- some unknown western landmass lying off indicates that both were apparently derived flect complex recycling processes on the the coast. Initially, of course, this supposed from the same general sources. The inferred trench slope. Diagenetic effects in many landmass was viewed as a borderland on the provenance terrane was the ancestral Sier- Franciscan suites include apparent whole- edge of North America. In recent , ran-Klamath magmatic arc, from which sale replacement of K-feldspar by albite. speculation has centered instead upon in- mixed volcanic and plutonic detritus readily Present age control is inadequate to test ferred offshore island arcs (Blake and entered the adjacent Great Valley forearc fully for time-dependent trends in the com- Jones, 1974) or other crustal blocks that basin. At intervals along the trend of the positions of Franciscan sandstones analo- possibly lay within or beyond the oceanic arc-trench system, arc-derived detritus also gous to the known stratigraphie variations region west of North America. Plate- bypassed the forearc region through sub- in the composition of Great Valley sand- tectonic theory implies that marine canyons that fed the Franciscan trench. stones. This question ought to be investi- spread out upon the ocean floor could have Longitudinal flow along both the Great Val- gated in future studies. been rafted into the Franciscan subduction ley trough and the Franciscan trench zone from sites of deposition that were achieved wide dispersal of the INTRODUCTION located at almost any arbitrary distance sediment. from the coast. Indeed, there is now clear Suites of both Franciscan and Great Prior to the advent of plate-tectonic the- evidence that some pelagic with- Valley samples include an array of subquart- ories, the origin of the Franciscan assem- in the Franciscan assemblage were depos- zose compositions ranging from feldspath- blage (or ; Berkland ited at paleolatitudes, implying tectonic olithic to lithofeldspathic. Mean framework and others, 1972) of the California Coast transport of several thousand kilometres modes of 17 Franciscan suites comprising Ranges was almost wholly enigmatic. At before their incorporation into the Francis- 203 individual samples, and of 23 Great present, the Franciscan terrane is widely can subduction complex along the conti- Valley suites comprising 410 individual regarded as a subduction complex de- nental margin (Alvarez and others, 1980). samples, range from 14% to 44% quartz formed within and beneath the inner slope On the other hand, Jacobson (1978), and grains, 15% to 54% feldspar grains, and 7% of a late and early others before him, have argued that the to 71% total lithic fragments. The ratio of trench that lay along the western margin of sources for most sandy detritus in Francis- quartz to feldspar remains relatively con- North America. Even so, the provenance of can graywackes lay within the orogenic ter- stant as the proportion of lithic fragments the terrigenous detritus in Franciscan gray- rane along the western margin of North changes. The compositional variations re- wackes has remained uncertain. This unre- America—regardless of the distance that flect differences mainly in the admixture of solved question is important because all some or all associated oceanic pelagites and lithic fragments derived principally from workers report that turbidite successions of pillow lavas may have moved across the sea volcanic cover with quartz, and feldspar graywacke and associated argillite or floor. This view is based upon general anal- derived principally from of underly- form the overwhelming bulk of the Francis- ogies between the compositions of the ing plutons. Despite major overlap in the can assemblage (for example, see Bailey and Franciscan sandstones and compositions of compositions of the two sets of samples, others, 1964). Other characteristic coeval sandstones within the Great Valley some Franciscan sandstones are somewhat types—which include metabasaltic green- sequence (or Great Valley Group; Ingersoll more feldspathic and less lithic than any stone, radiolarian , ophiolitic serpen- and Dickinson, 1981). The Great Valley

Geological Society of America Bulletin, v. 93, p. 95-107, 6 figs., 4 tables, February 1982.

95 96 DICKINSON AND OTHERS

rocks are exposed mainly in a belt thai: is of several concepts that have evolved con- pound composed of now geographically adjacent to the elongate currently during the past few decades. successive parallel belts of deformed ocean- Franciscan terrane, although the juxtaposi- Taliaferro (1943) championed the er- ic materials. These belts become younger tion of the two was achieved along a tec- roneous view that the Franciscan assem- to the west and were added sequentially to tonic contact (see below). Detailed petro- blage was exclusively Late the as they became de- logic studies indicate that the provenance (Tithonian) in age, and thus entirely older tached from a descending oceanic plate at for the Great Valley sandstones was the than the . Irwin the trench. Dickinson (1970b) then indi- ancestral Sierran-Klamath terrane (Oja- (1957) laid the essential foundation for cated how the deformational history of the kangas, 1968; Dickinson and Rich, 1972; understanding the geology of the California Franciscan assemblage, the depositional Mansfield, 1979), across the flank of which Coast Ranges by showing that the Francis- history of the Great Valley sequence, and the Great Valley sequence onlaps landward. can assemblage and Great Valley sequence the igneous history of the Ingersoll (1978a, 1979a, 1979b, in press) has instead are facies equivalents. Each includes batholith can be interpreted together as the provided full summaries and analyses of strata ranging in age from coherent record of an integrated arc-trench data documenting these relationships. through , and grading into system along the continental margin. Ernst In this paper, we make a systematic com- Paleogene analogs. (1970) similarly concluded that structural parison of available detrital modes for In their masterful review of Franciscan relations between the Franciscan assem- Franciscan and Great Valley sandstones geology, Bailey and others (1964) discussed blage and Great Valley sequence reflect (using our own point counts and those of the concepts that Franciscan strata were deformation associated with a convergent others). Our analysis reinforces the conclu- deposited on a basaltic substratum, and that plate boundary during the late Mesozoic. sion of Jacobson (1978) that the two suites they were underthrust beneath the coeval Travers (1972) presented evidence that the had roughly the same provenance. In our Great Valley sequence along a tectonic con- plate convergence extended into Paleogene view, continued speculation that any signi- tact commonly marked by a sheet of serpen- time. ficant fraction of the sandy detritus in the tinite. This was later shown to Subsequent work has verified the general Franciscan came from sources outside be part of the implications of these concepts for the evolu- North America (compare Blake and Jones, (Bailey and others, 1970), upon which the tion of the Franciscan assemblage. The 1978) is thus unwarranted. The reader thickest parts of the Great Valley prism Franciscan terrane is now known to consist should bear in mind, however, that the accumulated. The Coast Range thrust sepa- of several subparallel belts composed of dif- methodology we develop here is a relatively rating the Franciscan assemblage from the ferent lithologic associations and containing insensitive means to detect tectonic trans- Great Valley sequence lies structurally be- of different ages (Blake and Jones, port of segments of either terrane by strike low the ophiolite slab wherever the latter is 1974). These lithostratigraphic belts or sub- slip along faults that are subparallel to the present. represent the eroded edges of fault- regional tectonic grain. Ernst (1965) was the first to suggest, by bounded panels that generally become analogy with ideas then well developed in younger in sequence westward (Maxwell, FRANCISCAN SUBDUCTION Japan, that the Franciscan rocks probably 1974). The subterranes evidently were in- accumulated in a trench, where the expected corporated into the continental margin as Figure 1 is our version of the currently low geothermal gradient could allow blue- successively underthrust imbricate slices. accepted model for the genesis of the Fran- to proceed at depth. Subsequent episodes of deformation that ciscan assemblage and Great Valley Hamilton (1969) later argued cogently that affected the whole accretionary mass, or at sequence. The scheme represents the union the Franciscan terrane represents a com- least significant parts of it jointly, were

FRANCISCAN SIERRAN-KLAMATH •SUBDUCTION MAGMATIC ARC COMPLEX

Figure I. Schematic profile of a typical transect across the California Coast Ranges, Great Valley, and Sierra Nevada, showing general structural relationships of the Franciscan assemblage and Great Valley sequence in their inferred tectonic setting during the late Mesozoic (after Dickinson and Seeley, 1979, Fig. 111). PROVENANCE OF FRANCISCAN GRAYWACKES, CALIFORNIA 97 intricate and are still not fully understood can is a major defect of the present study. (Cowan and Silling, 1978). Individual struc- Additional work based on improved age tural units include bands of dislocated controls can be expected to bring many of melange as well as coherent slabs of foliate the relations that we discuss here into tectonite or bedded strata (Cowan, 1974). sharper focus. Recent work indicates that the Franciscan Subtle areal variations in com- assemblage probably includes sandstone position have also been detected within the and argillite deposited on the trench slope Great Valley sequence (Ingersoll, 1978a). and within slope basins as well as along the These are thought to reflect derivation from trench axis or on the ocean floor (Howell different segments of an elongate orogenic and others, 1977; Underwood, 1977; provenance. To allow for these areal differ- Bachman, 1978; Smith and others, 1979). ences, and to test for similar differences among Franciscan sandstones, we here treat CORRELATION PROBLEMS the data from both the Great Valley sequence and the Franciscan assemblage in Stratigraphically controlled variations in three areal sets (Fig. 2): (I) Northern Coast detrital compositions of sandstones are well Ranges and , (2) Diablo documented within the Great Valley se- Range and , and (3) quence (Ojakangas, 1968; Dickinson and Nacimiento block along the central Califor- Rich, 1972; Ingersoll, 1978a; Mansfield, nia coast. The Diablo Range and Naci- 1979). These variations reflect time- miento block of the Southern Coast Ranges dependent changes in the detritus as the are separated by the Salinian block of gra- provenance evolved through time. To be nitic and metamorphic similar to fully meaningful, therefore, comparisons of the bedrock of the Sierran-Klamath terrane. Franciscan and Great Valley sandstone Restoration of Cenozoic strike slip on the compositions ought to be made for rocks of along the eastern edge of equivalent age. However, the difficulties the Salinian block would thus align the involved in establishing the ages of various three discrete areas of Franciscan and Great Valley rocks as successive segments of a parts of the Franciscan terrane remain Figure 2. Geologic sketch map of Cali- continuous elongate tectonic system. How- severe. Whereas Great Valley horizons can fornia showing three chief areas of exposure ever, this alignment does not preclude the be assigned to specific faunal stages, only of Franciscan assemblage and Great Valley possibility of additional strike-slip offsets broader age assignments are feasible for sequence (to which compilations of Tables 3 within the Franciscan assemblage. much of the Franciscan assemblage. More- and 4 and plots of Figs. 4 and 5 are keyed). over, Franciscan sandstones can seldom be Salinian block is underlain by granitic and dated directly using fossils imbedded in metamorphic basement similar to that of DETRITAL MODES sandstone. More commonly, the fossils the Sierra Nevada and Klamath Mountains, occur in calcareous nodules within asso- and is bounded on the east by the San General procedures for interpreting detri- ciated argillites, and in radiolarian chert or Andreas fault (see text). tal modes of arkoses and graywackes were pelagic limestone successions. Such occurrences are clearly unsatisfactory for dating sandstone samples collected mainly from blocks within melanges or from tec- TABLE 1. CLASSIFICATION OF DETRITAL GRAIN TYPES tonic slabs that typically are separated from the fossiliferous horizons by fault contacts. Sums Symbols Grain types With present knowledge of age relations, Total quartzose grains Franciscan sandstones can be subdivided Q Q = Qm + Qp Qm Monocrystalline quartz grains with full confidence into only two broad age Polycrystalline quartzose lithic fragments (chert, etc.) groupings. The younger group includes QP F Total feldspar grains uppermost Cretaceous (Campanian- F = P + K P Plagioclase grains Maastrichtian) slope-basin deposits (see K K-feldspar grains above) in central California, and the Coast- L Total unstable aphanitic lithic fragments al Belt of the Franciscan terrane in northern Volcanic, hypabyssal, and metavolcanic aphanitic California where the rocks are partly of sim- L = Lv + Ls Lv lithic fragments ilar age but are mainly Paleogene (Evitt and Total aphanitic lithic fragments (both unstable and Pierce, 1975). The older group includes a Lt = L + Qp Lt quartzose) spectum of pre-Campanian Cretaceous and uppermost Jurassic (Tithonian) rocks Note: Classification and notation after Graham and others (1976); the notations Lv and Ls used forming the bulk of the Franciscan assem- here after Dickinson and Suczek (1979) represent the same categories as the notations Lvm and Lsm of Ingersoll and Suczek (1979), who use Lm for metamorphic aphanitic lithic fragments and reserve blage. Our inability to further resolve age the notations Lv and Ls for unmetamorphosed volcanic and sedimentary lithic fragments only. relations of sandstones within the Francis- 98 DICKINSON AND OTHERS

TABLE 2. GRAIN PARAMETERS AND POPULATIONS from the Great Valley sequence and Fran- PLOTTED ON TRIANGULAR DIAGRAMS ciscan assemblage combined. We concen- trated attention, however, on data for about Diagram Population 200 Franciscan samples and about 400 Great Valley samples, or about 600 in all. QFL Total framework with all quartzose grains grouped together The degree of selectivity we exercised was QmFLt Total framework with all aphanitic lithic fragments grouped together dictated solely by the need to restrict our analysis to detrital modes generated in QpLvLs Polycrystalline aphanitic lithic fragments only comparable detail by comparable method- QmPK Monocrystalline mineral grains only ology. Although this criterion led us to exclude the pioneering data of Bailey and Note: "Total framework" here includes only sum of grain types defined by Table 1. Irwin (1959), it was they who first tried to use detrital mineralogy as a means to com- pare inferred provenances for Franciscan outlined by Dickinson (1970a). Table 1 from our compilations. Table 2 defines the and Great Valley sandstones. presents the scheme used here for classifying grain parameters and grain populations Three special problems arise in compar- sand grain types reported from point counts used to display the data that we have com- ing detrital modes of Franciscan and Great of Franciscan and Great Valley samples. piled on triangular diagrams. Valley rocks (compare Jacobson, 1978): Interstitial components and rare constitu- The total available data base includes 1. Percentages of interstitial matrix differ ents not mentioned in Table 1 were omitted detrital modes for perhaps 1,000 samples systematically but not uniformly. Matrix

TABLE 3. MEAN FRAMEWORK MODES OF SELECTED SUITES OF SANDSTONES FROM GREAT VALLEY SEQUENCE

Suite Area* Nf QFL QmFLt QpLvLs QmPK Reference

A. Uppermost Cretaceous (Campanian-Maastrichtian) and Paleogene suites} Al NC 9 38-41-21 35-41-24 11-66-23 46-39-15 Baker, 1975 A2 NC 15 30-38-32 26-38-36 10-38-52 41-42-17 K. P. Helmold data A3 NC 10 35-25-40 33-25-42 5-46-49 57-27-16 Ingersoll, 1978a A4 NC 53 40-41-19 39-41-20 5-65-30 48-28-24 Ingersoll, 1978a A5 DR 13 37-43-20 736-43-21? 5-65-30 40-35-25 Mansfield, 1979 A6 NB 2 45-37-18 44-37-19 5-84-11 54-27-19 Gilbert and Dickinson, 1970 A7 NB 5 40-42-18 39-42-19 5-69-26 48-35-17 Mackinnon, 1978 B. Pre-Campanian Upper Cretaceous suites poor in lithic fra,gments§

Bl NC 52 35-38-27 33-38-29 6-66-28 45-34-21 Ingersoll, 1978a B2 NC 29 35-31-34 32-31-37 8-65-27 51-36-13 Ingersoll, 1978a B3 DR 25 33-39-28 31-39-30 7-40-53 44-39-17 Ingersoll, 1978a B4 NB 4 31-36-33 29-36-35 4-89-7 45-29-26 Gilbert and Dickinson, 1970 C. Pre-Campanian Upper Cretaceous suites rich in lithic fragments**

CI NC 18 22-31-47 20-31-49 4-71-25 39-35-26 Ingersoll, 1978a C2 NC 16 22-31-47 20-31-49 4-80-16 39-43-18 Ingersoll, 1978a C3 DR 37 27-30-43 25-30-45 4-36-60 45-45-10 Ingersoll, 1978a C4 DR 19 30-28-42 27-28-45 7-41-52 49-41-10 Ingersoll, 1978a C5 NB 3 28-34-38 27-34-39 3-82-15 52-35-13 Mackinnon, 1978 D. Upper Jurassic (Tithonian) and Lower Cretaceous suitestf D1 NC 17 40-21-39 27-21-52 25-22-53 56-36-8 R. V. Ingersoll data D2 NC 32 37-21-42 29-21-50 15-36-49 59-35-6 R. V. Ingersoll data D3 NC 33 25-22-53 16-22-62 15-51-34 41-54-5 R. V. Ingersoll, data D4 DR 7 22-27-51 717-27-56? 10-60-30 40-50-10 Mansfield, 1979 D5 DR 5 21-35-44 716-35-49? 10-60-30 30-65-5 Mansfield, 1979 D6 NB 3 17-15-68 14-15-71 4-64-32 50-44-6 Gilbert and Dickinson, 1970 D7 NB 3 23-26-51 22-26-52 2-90-8 46-50-4 Mackinnon, 1978

*NC = Northern Coast Ranges and Sacramento Valley, DR = Diablo Range and San Joaquin Valley, NB = Nacimiento block (Southwest of Salinian block). |N = Number of point-counted samples averaged. J A suites: AI, Capay Formation, Capay Valley; A2, Subsurface Forbes, Kione, Winters, and Starkey sands, Sacramento Valley; A3, Rumsey Hi L petrofacies in Sacramento Valley; A4, Rumsey Lo L petrofacies of Central Valley; A5, Joaquin Ridge petrofacies near Coalinga; A6, Upper Great Valley unit, southern ; A7, Campanian section on Mono Creek. §B suites: Bl, Cortina Lo L petrofacies; B2, Boxer Lo L petrofacies; B3, Los Gatos petrofacies; B4, Middle Great Valley unit, southern Santa Lucia Range. **C suites: CI, Cortina Hi L petrofacies; C2, Boxer Hi L petrofacies; C3, Studhorse petrofacies; C4, Grabast petrofacies; C5, Upper Estrada Forma- tion near Santa Barbara. tfD suites: Dl, Platina petrofacies, Klamath Mountains; D2, Lodoga petrofacies (Aptian-Albian); D3, Stony Creek petrofacies (Tithonian- Neocomian); D4, Center Peak petrofacies (Albian) near Coalinga; D5, Gravelly Flat petrofacies (Tithonian) near Coalinga; D6, Lower Great Valley unit, southern Santa Lucia Range; D7, Lower Espada Formation near Santa Barbara. PROVENANCE OF FRANCISCAN GRAYWACKES, CALIFORNIA 99

typically forms 15% to 20% of Franciscan is difficult to prove (Cowan, 1974). The stones comprising 203 individual samples. sandstones, although its content ranges problem involves rocks that lack Paleogene samples are included in the Great downward toward 5% to 10% in some of the or other metamorphic textures, and thus Valley compilation because Paleogene younger suites of samples. By contrast, appear generally suitable for determination rocks occur among the Franciscan samples. matrix commonly forms only 5% to 10% of of detrital modes by point counting. Detri- Most of the Great Valley suites are regional Great Valley sandstones, although its con- tal modes affected by loss of K-feldspar by or subregional petrofacies defined to in- tent ranges upward toward 15% to 20% in replacement or removal presumably would clude rocks having consistent petrologic some of the older suites of samples. Where show either anomalously high contents of similarity. Most of the Franciscan suites are the presence of interstitial matrix reflects albitic plagioclase or anomalously low con- merely geographically associated collections diagenetic modification or destruction of tents of total feldspar, respectively. showing relative petrologic homogeneity. unstable lithic fragments, its greater average 3. Some Franciscan metagraywackes are Figure 3 compares the overall range of abundance in the Franciscan samples may too foliated or otherwise too modified tex- detrital modes from the Great Valley bias point counts toward lower apparent turally by metamorphism to allow reliable sequence and Franciscan assemblage. As proportions of lithic grains in the sand determinations of detrital modes to be plotted, the Great Valley field represents the frameworks. made. If such metagraywackes include some tight spread among a control group of spe- 2. Most Franciscan graywackes contain varieties not represented at all among the cial samples counted by one operator only diagenetic albite as the common plagioclase less metamorphosed rocks suitable for (Ingersoll, in press). Even so, the overlap of but are entirely lacking in K-feldspar (Bai- study, then our compilations inadvertently Great Valley and Franciscan fields is nota- ley and Irwin, 1959). It has never been clear ignore them. ble at the level of one standard deviation for whether the complete absence of K- QFL values in the various suites. In particu- feldspar in some or all of these rocks is a DATA COMPARISON lar, the means of all Great Valley petrofa- primary compositional property or a secon- cies, except for two closely related upper dary metamorphic one. Widespread re- Tables 3 and 4 are compilations of mean Lower Cretaceous ones, fall within the placement of K-feldspar by albite, or detrital modes for 23 suites of Great Valley Franciscan field as well. As upper Lower wholesale removal of K-feldspar by intra- sandstones comprising 410 individual sam- Cretaceous rocks are apparently rare within stratal solution, has long been suspected but ples, and for 17 suites of Franciscan sand- the Franciscan assemblage (Blake and

TABLE 4. MEAN FRAMEWORK MODES OF SELECTED SUITES OF GRAYWACKES FROM FRANCISCAN ASSEMBLAGE

Suite Area* Nt QFL QmFLt QpLvLs QmPK Reference

E. Uppermost Cretaceous (Campanian-Maastrichtian) and Paleogene SuitesJ El NC 8 30-42-28 27-42-31 11-58-31 39-48-13 W. R. Dickinson data E2 NC 5 30-47-23 28-47-25 8-62-30 39-49-12 D. S. Cowan data E3 NB 7 41-47-12 39-47-14 12-66-22 45-47-8 Gilbert, 1973 E4 NB 35 41-45-14 738-45-17? -20-20-60? 746-42-12? after Smith, 1978 E5 NB 17 39-50-11 736-50-14? -20-10-70 742-41-17? Lee-Wong and Howell, 1977 F. Upper Jurassic to Cretaceous suites poor in lithic fragments§ Fl DR 17 42-53-5 40-53-7 29-14-57 43-57-0 Jacobson, 1978 F2 DR 6 35-42-23 28-42-30 23-44-33 40-60-0 Jacobson, 1978 F3 DR 38 38-49-13 29-49-22 39-35-26 37-63-0 Jacobson, 1978 F4 DR 4 35-54-11 35-54-11 3-22-75 40-60-0 Cowan, 1974

F5 DR 6 30-51-19 29-51-20 4-39-57 37-63-0 Cowan, 1974 F6 DR 30 33-54-13, 29-54-17 24-41-35 35-65-0 Jacobson, 1978 F7 NB 5 33-40-27 31-40-29 6-80-14 44-56-0 Gilbert, 1973 F8 NB 2 42-48-10 37-48-15 31-26-43 44-54-2 D. S. Cowan data G. Upper Jurassic to Cretaceous suites rich in lithic fragments** Gl NC 8 730-31-39? 23-31-46 -15-55-30 43-57-0 Suppe, 1973 G2 NC 4 29-40-31 24-40-36 12-59-29 38-61-1 D. S. Cowan data G3 NC 5 27-19-54 16-19-65 17-55-28 44-56-0 Swe and Dickinson, 1970 and D. S. Cowan data G4 DR 6 33-33-34 28-33-39 11-52-37 46-54-0 Cowan, 1974

*NC = Northern Coast Ranges, DR = Diablo Range, NB = Nacimiento block (southwest of Salinian block). f N = Number of point-counted samples averaged. JE suites: El, Coastal Belt Franciscan (Paleogene), Cape Mendocino region; E2, Coastal Belt Franciscan (Paleogene), Noyo and Navarro Rivers area; E3, Pfeiffer Point area (Cretaceous), central California coast; E4, Cambria slab (Cretaceous), central California coast; E5, Point San Luis area (Creta- ceous) and Cambria slab, central California coast. §F suites: Fl, Burnt Hills sequence, Hamilton Range; F2, Isabel-Hamilton melange blocks, Hamilton Range; F3, Mount Eylar sequence, Hamilton Range; F4, Orestimba metagraywacke near Pacheco Pass; F5, Mustang Peak metagraywacke slab and Garzas melange blocks near Pacheco Pass; F6, Las Aguilas Creek area near Panoche Pass; F7, Lucia to Lopez Point area, central California coast; F8, Morro Bay to Cayucos area, central California coast. **G suites: Gl, Hellhole graywacke facies northeast of Round Valley; G2, area northeast of Willits toward Hearst; G3, area southeast and northeast of Clear Lake; G4, Zimba metagraywacke slab and Garzas melange blocks near Pacheco Pass. 100 DICKINSON AND OTHERS

Jones, 1974), this discrepancy may be sig- eroded from the exposed flank of the arc ciscan suites may alternatively reflect nificant but not surprising (Jacobson, massif. Less lithic rocks in parts of the diagenetic modification of framework 1978). Franciscan assemblage seemingly indicate modes (see below). Both the Great Valley and Franciscan deeper erosion of arc roots to expose Figure 4 displays the data of Tables 3 and fields of Figure 3 overlap the field for detri- plutons or older basement from which 4 in a format that allows the mean detrital tus shed from active magmatic arcs. Mixed quartzofeldspathic or arkosic sand could be modes of Franciscan and Great Valley volcanic and plutonic sources are thus derived. However, mean detrital modes of suites from the three discrete areas delin- implied for both. More quartzose rocks in Cretaceous and Eocene sandstones derived eated on Figure 2 to be compared separately upper Lower Cretaceous petrofacies of the from the plutonic basement of the Salinian or to be considered jointly. Standard devia- Great Valley sequence apparently reflect block fall just outside the Franciscan field, tions of suite means for QFL values average admixture of arc detritus with recycled sed- which is slightly more feldspathic. Conceiv- about 5 percentage points with respect to imentary and metasedimentary materials ably, the less lithic character of some Fran- the various constituents plotted, but range up to 10 percentage points in exceptional cases.

QFL Diagram

As expected, the QFL diagram (Fig. 4a) repeats the pattern of Figure 3 in different format. Mean framework modes for about half the Franciscan suites either fall within the field defined by mean framework modes of Great Valley suites, or are statistically indistinguishable when approximate stan- dard deviations are taken into account. The mean framework modes for the other half of the Franciscan suites are less lithic and more feldspathic than the Great Valley field. In general, Franciscan suites are just as quartzose as most Great Valley suites, although a few Great Valley suites are less quartzose than any Franciscan counter- parts. The more feldspathic and less lithic char- acter of some Franciscan sandstones in comparison to Great Valley sandstones (compare Jacobson, 1978) could be the result of diagenetic alterations that involved destruction of unstable lithic fragments and growth of metasomatic albite. However, close scrutiny of Figure 4a reveals that Franciscan suites from the Northern Coast Figure 3. QFL diagram showing overall range in framework modes of 232 Great Valley Ranges have mean QFL compositions quite sandstones and 203 Franciscan sandstones. Great Valley field (horizontal rules) is joint similar to those of Great Valley suites from envelope of QFL standard deviations (Ingersoll, in press) for eight Great Valley petrofacies the same area. Discrepancies between Great whose mean framework modes are shown individually. Franciscan field (vertical rules) is Valley and Franciscan framework modes joint envelope of QFL standard deviations computed for the selected sandstone suites occur only for suites from the Diablo Range whose mean framework modes are given in Table 4. Heavy dashed line shows field for and Nacimiento block, though not for every sandstones derived from magmatic arcs as inferred by Dickinson and Suczek (1979), As- one of those suites. Although it is perhaps terisks show mean framework modes of Cretaceous (K) sandstones (Lee-Wong and Howell, conceivable that effective diagenetic modifi- 1977) and Eocene (E) sandstones (Link and Nilsen, 1980) on the Salinian block (see Fig. 2). cations occurred only in those two areas, it Light dashed line with arrows shows time-dependent trend of straitigraphically controlled seems at least as likely that sources for variations in framework modes within the Great Valley sequence (Ingersoll, in press): (1) Franciscan detritus were different enough Upper Jurassic to lowermost Cretaceous (Neocomian) Stony Creek petrofacies (SC); (2) to account for the contrasts shown (see Lower Cretaceous Lodoga (L) and Platina (P) petrofacies; (3) basal Upper Cretaceous below). (Cenomanian) Grabast (G) and Boxer (B) petrofacies; (4) Upper Cretaceous Cortina (C) In any case, the separation of discrepant and Los Gatos (G) petrofacies; (5) uppermost Cretaceous (Campanian-Maastrichitian) Franciscan suites from the nearest Great Rumsey petrofacies (R). Valley suites is not as great as the spread Figure 4. Triangular plots showing mean framework modes of selected Great Valley (Table 3) and Franciscan (Table 4) sandstone suites: (a) QFL, (b) QmFLt, (c) QpLvLs, (d) QmPK. See Tables 1 and 2 for definitions of detrital grain types and modal grain populations. Open symbols denote Great Valley suites and solid symbols denote Franciscan suites (see legend). On QFL plot, "K"" and "is" are mean compositions (see Fig. 3) of Cretaceous (K) and Eocene (E) sandstones on the Salinian block (see Fig. 2). QFL plot also includes an Upper Jurassic Great Valley suite (Q23, F17, L60) and and Upper Jurassic (?) Franciscan suite (Q23, F23, L49) from the Nacimiento block after Seiders (1981). 102 DICKINSON AND OTHERS

within the Great Valley field on the QFL detritus derived from magmatic arcs is suites richest in Qp or Ls should also be diagram. Moreover, if the generally higher composed of mixtures of lithic sand derived richest in Lt, for the background Lv content matrix contents of Franciscan sandstones from volcanic cover and quartzofeldspathic would still remain. are interpreted as mainly due to alteration sand derived from plutons in proportions On the other hand, quartzose lithic frag- of lithic fragments, then the plotted differ- that vary through time as volcanisrn and ments (Qp) and phyllosilicate-rich lithic ences would be further reduced, though not erosion vie for dominance along the trend fragments (Ls) are more resistant to diage- removed entirely. of the arc (Dickinson and Rich , 1972). netic reactions than volcanic lithic frag- ments (Lv), which contain unstable compo- QmFLt Diagram QpLvLs Diagram nents, including igneous minerals and even glass. Thus, enrichment in Qp or Ls in some Transfer of quartzose lithic fragments On the QpLvLs diagram (Fig. 4c) the suites of Franciscan sandstones could repre- (Qp) from the quartzose Q pole to the Lt Great Valley suites define a field near the Lv sent residual concentration of relatively sta- pole for total lithic fragments produces the pole for volcanic lithic fragments. Such a ble lithic fragments, as the unstable Lv QmFLt diagram (Fig. 4b). The sets of Fran- distribution is diagnostic of derivation from component was destroyed during diagene- ciscan and Great Valley suites both show a an arc orogen rather than a collision oro- sis. In this case, suites richest in Qp or Ls large variation in content of lithic frag- gen, which would yield detritus tending to should be poorest in Lt, for the Lv content ments, but a relatively constant ratio of lie along or near the QpLs leg of the triangle would have been drastically reduced. quartz to feldspar grains. Again, about half (Graham and others, 1976; Dickinson and Figure 5 tests the two alternative explana- the Franciscan means plot within or very Suczek, 1979; Ingersoll and Suczek, 1979). tions for high contents of Qp or Ls in Fran- near the field defined by Great Valley About half the points for Franciscan suites ciscan sandstone suites by plotting Lv/Lt means, whereas the other half are slightly scatter away from the Great Valley field against Lt for all the suites of Tables 3 and less lithic or somewhat more feldspathic or toward the Qp or Ls pole. This tendency for 4. Among the Franciscan suites for which both. Once again, the most discrepant scatter might reflect a tendency for sedi men- the plotted parameters diverge from the Franciscan means represent samples from tary or metasedimentary materials like Great Valley field, it is clear that Lv/Lt is a the Diablo Range. argillite and chert to be recycled in cannibal- direct function of Lt, and not the inverse. As illustrated, the mean framework istic fashion by uplift and erosion on the Thus, high values of Qp and Ls cannot modes for plotted Franciscan suites are dis- trench slope. In this way, volcanic lithic reflect simple dilution of arc-derived sand tributed across the QmFLt diagram remark- fragments derived from the magmatic arc with recycled Qp and Ls, but the high Qp ably close to a straight line connecting the would be diluted with other types of lithic and Ls values seemingly could have de- Lt pole with the point denoting 41.5% Qm fragments in the recycled debris before veloped through diagenetic destruction of and 58.5% F on the QmF leg of the triangle. deposition as parts of Franciscan sand- Lv. This point accurately represents the mean stones. This kind of mixing would not occur However, scrutiny of the raw point proportion of quartz and albite grains in to the same degree in most Great Valley counts reveals that there is no systematic Franciscan sandstones that wholly lack K- sandstones. In this case of simple dilution, correlation, direct or inverse, between Lt feldspar (see Fig. 4d). This striking coinci- dence suggests two key inferences: (1) the large variations in content of lithic frag- ments within the Franciscan suites take place against a constant background of unchanging quartz/feldspar ratio — regardless of whether the variation is attri- buted to differences in source rocks or contrasts in diagenesis, or both in combina- tion; and (2) the development of albite-rich, K-spar-free feldspar mineralogy in Francis- can sandstones does not appreciably change the overall quartz/feldspar ratio—again regardless of whether the modification stems from changes in provenance or dia- genesis, or both. It is clear from past work and from asso- ciated changes in feldspar mineralogy that the variation in content of lithic fragments among the Great Valley suites is a func- tion of provenance and not diagenesis % Lt in Qm-F-L.t Population (Qm + F+Lt = 100) (Dickinson and Rich, 1972; Ingersoll, 1978a; Mansfield, 1979). By analogy, most Figure 5. Plot of the ratio Lv/Lt versus framework percentage of total lithic fragments of the variation in the content of lithic (Lt) for the Great Valley and Franciscan suites of Figure 4 and Tables 3 and 4 to show fragments among the Franciscan suites can decrease in volcanic lithic fragments (Lv) by modification of Franciscan sandstone frame- be attributed to provenance also. The sandy works along variation trend depicted by heavy arrows; symbols same as Figure 4. PROVENANCE OF FRANCISCAN GRAYWACKES, CALIFORNIA 103 values, or the Lv/Lt ratio, and percent any K-feldspar at all is distinctly anoma- tion of paleocurrent indicators impossible matrix. Therefore, if the component Lv was lous. We infer that K-feldspar has been sys- in most areas and render the restoration of removed diagenetically to effect a residual tematically replaced by chessboard-twinned paleocurrent vectors at least questionable concentration of Qp and Ls, the process albite (Moore and Liou, 1979), altered to nearly everywhere. In the most systematic must have involved wholesale intrastratal sericitic aggregates (Cowan, 1974), or re- paleocurrent study published to date, solution of volcanic lithic materials, rather moved by intrastratal solution. Telleen (1977) inferred a net flow of Fran- than simple physical deformation or even Ratios of quartz to total feldspar are sim- ciscan turbidity currents within the Diablo local chemical redistribution. This hypothe- ilar in Great Valley and Franciscan suites, Range toward S33W, a direction quite sis cannot be tested. If it is not accepted, a although Qm averages perhaps 5% higher suitable for sources on the edge of the conti- diagenetic explanation for high Qp and Ls in the Great Valley suites. However, the nent. However, the sequence of strata he contents in some Franciscan suites is not overlap in Qm percentages is complete for studied is apparently in age viable. pre-Campanian suites. An essentially con- and possibly represents slope-basin deposits Perhaps the correct answer is a sediment stant ratio of quartz to feldspar is compati- (Jacobson, 1978). Thus, they may not be mixing process more complex than simple ble with the hypothesis that K-feldspar was typical of the bulk of the Franciscan ter- dilution with Qp and Ls. For example, replaced by albite, and is further supported rane, either in age or in depositional setting. recycled materials rich in Qp and Ls relative by the remarkable linear scatter of Francis- Sparse paleocurrent indicators in Late Cre- to Lv also may have had reduced contents can suites as plotted on the QmFLt diagram taceous slope-basin deposits of the Naci- of total lithic fragments (Lt) as a result of (Fig. 4b). There is no hint of a residual miento block similarly reflect mean flow of processes that operated during transport increase in the ratio of quartz to feldspar as turbidity currents toward the southwest and deposition. In effect, proportions of would occur if K-feldspar were altered to away from the adjacent continental block mineral grains (Qm and F) may have been phyllosilicates or dissolved away. We provi- (Smith, 1978). Paleocurrent indicators in increased at the expense of Lv during recy- sionally conclude that detrital K-feldspar in the Coastal Belt Franciscan of mainly Ter- cling events that also added chert (Qp) and the Franciscan suites has been replaced by tiary age in the Northern Coast Ranges gen- argillite (Ls) debris from uplifted oceanic albite showing chessboard twins. This mode erally reflect southerly flow, and thus materials exposed on the trench slope. of albite occurrence has been linked specifi- suggest derivation from the region of the Although plausible, this hypothesis is also cally to replacement of K-feldspar at least Klamath Mountains to the north (Bach- untestable using the modal data alone. On locally within the Franciscan terrane man, 1978). other grounds, however, several previous (Moore and Liou, 1979). Taliaferro (1943) rested his original case workers have suggested that sedimentary for a western source on the fact that rare recycling did occur at least locally within DISCUSSION conglomerates within the Franciscan as- the Franciscan terrane (Cowan and Page, semblage are coarser and more abundant 1975; Smith and others, 1979). Figure 4 reveals that consistent overlap toward the west in the Southern Coast occurs between detrital compositions of Ranges, and on the fact that the Franciscan QmPK Diagram Great Valley and Franciscan suites with sediments coarsen generally toward the west respect to all significant grain parameters. in the Northern Coast Ranges. In doing so, On the QmPK diagram (Fig. 4d), all the The differences that exist are not great he assumed that the strata in question were Franciscan suites of Campanian or younger enough to suggest major differences in all of roughly the same age. As discussed age plot within the overall Great Valley provenance. Of course, the modal data can earlier, however, there is a systematic varia- field. Although some of these younger suites only show that each suite had an eroded tion in the age of the Franciscan across the are reported to be slightly richer in plagio- magmatic arc as its provenance, but cannot Coast Ranges. Taliaferro also believed that clase than Great Valley suites of strictly prove that the two were derived from the the sediments were deposited in shallow comparable age, the effect is of marginally same magmatic arc. The data do indicate marine waters, whereas all modern workers statistical significance if standard deviations that hypotheses involving separate prove- interpret them as deep-marine turbidites. It are taken into account. However, all the nances, except for odd and unusual compo- is well known that coarseness of grain in older, pre-Campanian Franciscan suites are nents of the overall terranes, are turbidites reflects the locations of deep-sea K-spar-free, or nearly so. The contrast with unnecessary in the absence of some compel- channels and fans with respect to starved Great Valley suites is dramatic and consist- ling independent evidence. Otherwise, their slopes and interfan basinal areas, rather ent, as Bailey and Irwin (1959) noted. If the close geographic association and the pre- than strictly their proximity to source lack of K-feldspar is an initial depositional vailing geotectonic model of Figure 1 make (Ingersoll, 1978b). On two counts, then, property, then the Franciscan sandstones the assumption of a common provenance Taliaferro's observations on grain size are clearly did not have the same provenance as both logical and satisfying, as Jacobson irrelevant to the question of source direc- the Great Valley sandstones. (1978) argued. As observed previously, the tion. However, the consistently albitic nature notion of a common arc provenance should Blake and Jones (1974, 1978) have raised of the plagioclase and its association with be held to allow for the possibility of unde- more recent objections to the idea of a secondary laumontite, pumpellyite, or var- tected strike-slip offsets along faults parallel common source for the Franciscan sand- ious minerals indicate that signifi- to the trend of the arc. stones and the Great Valley sequence. Their cant metamorphism has occurred. For Unfortunately, paleocurrent studies are principal argument rests on their conclusion graywackes from elsewhere with the other unable to test this assumption of a common that the Franciscan clastics are generally compositional attributes of the Franciscan provenance. Deformation and dislocation coarser than the Great Valley sequence, samples, the total and uniform absence of of the Franciscan strata make the observa- parts of which include a large proportion of 104 DICKINSON AND OTHERS

mudstone. They question whether it is pos- Slope and shelf facies onlapping the Kla- but also for minor differences in detail. For sible to envision ways to transport coarse math block document the northern end of example, Franciscan sandstones of the detritus across the forearc region and into the Great Valley forearc trough (Ingersoll, Northern Coast Ranges are inferred to have the trench while depositing distinctly finer 1978c), but the southern end shown near the been derived from some part of the Kla- sediment within the forearc basin. Mojave block is conjectural. math block; they have QFL and QmFLt We think that it is indeed quite possible. The scheme implies that Franciscan and compositions (Figs. 4a, 4b) similar to adja- Of course, if one views the geotectonic sys- Great Valley sands were derived from the cent Great Valley sandstones of the Sacra- tem in profile (Fig. 1), the difficulty of same magmatic arc of the elongate Sierran- mento Valley known to have been derived bypassing coarse sediment into the trench Klamath terrane, but that Franciscan sourc- from both the Klamath Mountains and the seems obvious. Such sediment could cross es were principally from segments of the geologically similar northern Sierra Nevada the forearc basin only at times when there arc lying farther north and farther south (Ingersoll, 1978a, 1978b, 1978c, 1979a, was no bathymetric closure at the trench than the location of the sources for the main 1979b). On the other hand, Franciscan slope break, which acts as a structural sill Great Valley belt. This inference allows for sandstones of the Southern Coast Ranges on the outer flank of the forearc basin the general similarity of the detrital modes, are inferred to have been derived from some (Dickinson and Seely, 1979). Turbidites could cross the forearc region only if there were an unbroken forearc slope from arc to trench. Such a condition probably existed at the inception of Franciscan subduction in the Late Jurassic, but it did not persist long into the Cretaceous (Ingersoll, 1978c). When the forearc basin later filled to the level of a shallow sill, as it eventually did during the Paleogene (Dickinson and oth- ers, 1979), coarse sediment might have locally crossed the resulting forearc shelf. Neither of these conditions prevailed, how- ever, as long as a prominent forearc trough existed during the Cretaceous. However, the profile view is an incom- plete one from the standpoint of sediment dispersal. At most modern subduction zones, turbudite sediment enters the trench mainly at selected points where submarine canyons or other transverse depressions allow turbidity currents to break through the forearc region (Underwood and Karig, 1980). The turbidite sediment then moves longitudinally along the trench axis for dis- tances up to hundreds of kilometres. In this way, relatively coarse sediment could by- pass parts of forearc basins that could serve as sediment traps for finer sediment. The coarseness of sediment in adjacent segments of trench and forearc basin need bear no direct relationship to each other. Figure 6 is a conceptual model to explain the delivery of coarse sediment to the Fran- ciscan trench while comparable or finer sed- iment accumulated in the Great Valley forearc basin. Parts of the scheme illus- trated are well documented, but others are speculative. The paleocurrent trends shown within the Great Valley sequence are based on abundant outcrop observations (Inger- soll, 1979b) recording flow toward the south-southeast and southwest for the Figure 6. Diagrammatic view of continental margin in Cretaceous California showing Sacramento Valley (Ojakangas, 1968), and inferred dispersal paths for Franciscan and Great Valley turbidites (Salinian block restored toward the southwest and northwest for the prior to late Cenozoic strike slip on San Andreas transform; true position Nacimiento San Joaquin Valley (Mansfield, 1979). block is still unknown, but it may have been even farther south). PROVENANCE OF FRANCISCAN GRAYWACKES, CALIFORNIA 105 part of the Mojave block; they are more to suggest derivation of most of the detritus a fertile field for future research (com- feldspathic and less lithic than Great Valley in both terranes from related sources in a pare Jacobson, 1978). sandstones of the San Joaquin Valley and common provenance. Nacimiento block presumably derived from 2. The provenance terrane was a dissected ACKNOWLEDGMENTS the southern Sierra Nevada. magmatic arc where associated volcanic and In sandstones derived from magmatic plutonic rocks, together with subordinate We thank Peter C. Van de Kamp for sub- arcs, quartz grains and much of the feldspar metamorphic and sedimentary rocks, were surface sandstone samples from the Great are derived from dissected plutons, whereas exposed in proportions that varied from Valley sequence in the Sacramento Valley, lithic fragments are derived mainly from place to place and from time to time as and Mark Dalrymple for pétrographie work volcanic cover, augmented by some debris magmatism and erosion competed for dom- on Franciscan sandstones from the Coastal from flanking sedimentary strata or from inance. Belt near Cape Mendocino. Collaboration metamorphic envelopes adjacent to the 3. Regional geologic relations imply that through the years with V. B. Cherven, plutons. In general, the proportion of lithic the provenance was the ancestral Sierran- W. G. Gilbert, C. F. Mansfield, R. W. fragments, as opposed to quartz and feld- Klamath terrane and its extension to the Ojakangas, E. I. Rich, R. J. Stewart, Win spar grains, is a measure of the ratio of vol- south. Swe, and E. V. Tamesis on studies of the canic and other supracrustal rocks, as 4. Arc-derived sediment reached the Great Valley sequence formed an essential opposed to plutons, exposed in the source. Franciscan trench by transport along trans- background for the present study. This The linear array of the Franciscan modes on verse dispersal routes, probably submarine research was supported by the Earth Scien- the QmFLt plot (Fig. 4b) can thus be inter- canyons, which bypassed the sediment trap ces Section, National Science Foundation, preted as a reflection of the degree of dissec- of the Great Valley forearc basin at intervals NSF Grants GA-1567, DES72-01728, tion of the provenance at different times along the trend of the arc-trench system. EAR76-22636, and EAR79-22848. The Re- and in different places. Generally, the 5. Franciscan turbidites of the California search Allocations Committee of the Uni- inferred Mojave provenance for the South- Coast Ranges were probably deposited in versity of New Mexico provided additional ern Coast Ranges was thus apparently more large measure by longitudinal flow parallel funds. Reviews by G. deV. Klein and R. L. plutonic and less volcanic than the inferred to the regional tectonic grain from sources Brenner improved the text. Klamath provenance for the Northern in or near the Klamath and Mojave blocks Coast Ranges. lying north and south of the Sierra Nevada REFERENCES CITED Without better knowledge of the actual and the Great Valley. ages of different Franciscan suites, little can 6. The more feldspathic and less lithic Alvarez, Walter, Kent, D. V., Silva, I. P., be inferred with confidence about the evolu- character of some Franciscan sandstones Schweickert, R. A., and Larson, R. A., 1980, Franciscan Complex limestone depos- tion of the provenance terranes through suggests that the segments of the arc terrane ited at 17° South paleolatitude: Geological time. 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