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Provenance of Franciscan Graywackes in Coastal California

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Provenance of Franciscan Graywackes in Coastal California Provenance of Franciscan graywackes in coastal California WILLIAM R. DICKINSON Department of Geosciences, University of Arizona, Tucson, Arizona 85721 RAYMOND V. INGERSOLL Department of Geology, 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 limestone, and rare probably derived from segments of the arc polymict conglomerate—are distinctly sub- A systematic comparison of available terrane where exposures of plutons were ordinate. detrital modes for graywacke sandstones of more extensive than within typical Great One line of thought, based in part on the the Franciscan subduction 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 California Coast Ranges 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 years, 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 turbidites 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 turbidite 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 limestones 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 Franciscan Complex; 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 Mesozoic and early Cenozoic 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 shale floor. This view is based upon general anal- derived principally from erosion 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 rock 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 chert, 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 accretionary wedge 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 Jurassic the continental margin 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 Great Valley sequence. 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 Sierra Nevada 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 Late Jurassic coherent record of an integrated arc-trench data documenting these relationships. through Cretaceous, 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 serpentinite was later shown to Subsequent work has verified the general Franciscan came from sources outside be part of the Coast Range ophiolite 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- fossils 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. terranes represent the eroded edges of fault- regional tectonic grain. Ernst (1965) was the first to suggest, by bounded
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