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Garnet-Bearing Trondhjemite and Other Garnet-bearing trondhjemite and other conglomerate clasts from the Gualala basin, California: Sedimentary record of the missing western portion of the Salinian magmatic arc? Ronald C. Schott* Clark M. Johnson Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA ABSTRACT magmatic arc is our preferred source for German Rancho Formation conglomerates are the Eocene clast assemblage. also compositionally distinct from contempo- Eocene conglomerates from the German raneous conglomerates elsewhere in Califor- Rancho Formation, currently exposed in Keywords: conglomerate, Eocene, isotope nia (Bachman and Abbott, 1988). Because it the Gualala basin of coastal northern Cal- geochemistry, Salinian block, sedimentary is the most outboard sedimentary basin in Cal- ifornia, are compositionally distinct from provenance. ifornia, and is west of the San Andreas fault, both underlying Upper Cretaceous con- the Gualala basin has a variety of potential glomerates and coeval conglomerates else- INTRODUCTION provenance areas. where in the California Coast Ranges. Zir- A number of provenance studies place the con U/Pb ages as well as isotopic (O, Sr, Nd, Investigations of the petrology of ®rst-gen- source for Gualala clasts in the nearby Sali- Pb) and chemical characteristics of German eration conglomerate clasts offer many in- nian block (e.g., Wentworth, 1966; Ross, Rancho Formation conglomerates indicate sights into the nature of rocks exposed in their 1970; Ross et al., 1973; Schott and Johnson, an origin in the central and western por- source terrain at the time of deposition. Pa- 1998a, 1998b; Burnham, 1998; Schott, 2000). tions of the Cretaceous Cordilleran batho- rameters such as age, texture, composition, The Salinian block is composed of rocks that liths. The majority of clasts in the Eocene and primary pressure-temperature conditions are characteristic of the eastern and central section are ca. 100 Ma hornblende- and bi- of formation for a group of conglomerate portions of the Cretaceous Cordilleran batho- otite-bearing tonalites and granodiorites, a clasts can be useful in identifying the location liths (Ross, 1972, 1984; Mattinson, 1990; common rock type in the Cordilleran bath- of deposition of suspect terranes (e.g., Seiders James, 1992). Palinspastic reconstructions that and Blome, 1988; Seiders and Cox, 1992; oliths of the southwestern United States. A remove Neogene offset on the San Andreas Schott and Johnson, 1998a, 1998b), and re®n- subordinate, but distinctive, clast type is fault system restore the Gualala basin, along ing paleogeographic and paleotectonic recon- made up of garnet-bearing trondhjemites with the Salinian block, to a pre-Neogene po- structions. In favorable circumstances, ther- that have Early Cretaceous U/Pb zircon sition outboard of, and adjacent to, the south- ages and isotopic compositions that are mobarometric and age data can be used in ernmost Sierra Nevada and western Mojave characteristic of the western (oceanic) mar- conjunction with the depositional age to de®ne Desert (Ross, 1984; Powell, 1993). These gin of the Cordilleran batholiths. The oc- minimum exhumation rates (e.g., Lovera et studies imply as much as 600 km of displace- currence of igneous almandine-rich garnet al., 1999). Because conglomerates preserve ment on faults of the San Andreas and pro- and epidote in the clasts may suggest crys- intact fragments of their source terrain, they to2San Andreas systems to restore the Gual- tallization at high pressure. Conclusive may provide the most detailed record of the ala basin to a Late Cretaceous depositional provenance ties for the Eocene conglomer- nature of terrains that have been lost to ero- location at the latitude of the southernmost Si- ates at Gualala are elusive because the most sion, burial, or tectonic dismemberment. distinctive clasts (i.e., garnet-bearing trond- Conglomerates in the German Rancho For- erra Nevada batholith. Total northward migra- hjemites) lack currently exposed source mation of the Gualala basin that have an early tion of the basin based on this reconstruction, bodies. Temporal changes in conglomerate Eocene depositional age (ca. 49 Ma) afford a supplemented by Miocene rotations in the clast provenance imply early Paleogene tec- unique opportunity to address temporal and southernmost Sierra Nevada and extension in tonism in the vicinity of the basin. The pos- spatial issues in the evolution of the western the Basin and Range province (Dickinson, sibility of large-scale Paleogene translation margin of California. Eocene German Rancho 1996; Cowan et al., 1997; Dickinson and But- of the basin or an Eocene sediment source Formation conglomerates are compositionally ler, 1998), probably does not exceed 800 km. in a passing exotic terrane appears unlike- distinct from the earlier Upper Cretaceous Alternatively, an exotic origin for the Gualala ly, but cannot be ruled out. A western con- conglomerates at Gualala, which have re- basin, involving thousands of kilometers of tinuation of the currently exposed Salinian ceived the most attention (Ross, 1970; Kistler northward transport, is based primarily on the et al., 1973; Ross et al., 1973; James et al., presence of detrital clasts and fragments of *E-mail: [email protected]. 1993; Schott and Johnson, 1998a). Eocene Tethyan gastropods and rudistid bivalves in GSA Bulletin; July 2001; v. 113; no. 7; p. 870±880; 7 ®gures; 1 table; Data Repository item 2001076. For permission to copy, contact Copyright Clearance Center at www.copyright.com 870 q 2001 Geological Society of America SEDIMENTARY RECORD OF THE MISSING WESTERN PORTION OF THE SALINIAN MAGMATIC ARC? isotopic methods to determine provenance characteristics of Eocene German Rancho For- mation conglomerate clasts in an attempt to resolve these disparate tectonic models. GEOLOGIC SETTING Upper Cretaceous through middle Eocene rocks of the Gualala basin (Fig. 1) consist of conglomerates, sandstones, and mudstones that were deposited by turbidity currents at bathyal depths (Wentworth, 1966; Wentworth et al., 1998). Paleogene turbidite sedimenta- tion into restricted basins in western Califor- nia has been interpreted to record the uplift of the Cretaceous magmatic arc and localized blocks west of the Great Valley forearc basin (Nilsen, 1987). Elsewhere in California, Up- per Cretaceous through Eocene conglomerate clast populations in the Coast Ranges gener- ally re¯ect a shift from local provenance to increasingly far-traveled clast assemblages that were derived from inboard (continental) sediment source areas (Abbot and Peterson, 1978; Bachman and Abbott, 1988). However, the Eocene clast assemblages at Gualala are distinct from others of comparable age, and therefore correlation with other conglomerates appears unlikely. Wentworth (1966) assigned the Upper Cre- taceous rocks to the Gualala Formation (sub- divided into the Stewarts Point and Anchor Bay Members), and Paleocene through middle Eocene rocks to the German Rancho Forma- tion (Fig. 2). The conglomerate exposures that are the focus of this study contain lower Eo- cene (ca. 49 Ma) fossils (Elder et al., 1998; McDougall, 1998) and are interpreted to rep- Figure 1. Present-day location of the Gualala basin (GB) in California showing principal resent a single(?) inner and middle fan channel faults and the current distribution of Mesozoic batholithic rocks. SAFÐSan Andreas fault, complex that has been repeated along the SGHFÐSan Gregorio2Hosgri fault, RRFÐReliz-Rinconada fault, SNFÐSur-Nacimiento coast by faulting (Wentworth, 1966; Went- fault, GFÐGarlock fault. SFÐSan Francisco, FRÐFresno, BKÐBakers®eld, LAÐLos worth et al., 1998; Anderson, 1998). Paleo- Angeles, SDÐSan Diego. currents generally indicate northwestward ¯ow parallel to the modern axis of the basin (Wentworth, 1966), although paleomagnetic the Gualala Formation (Elder et al., 1998) and in Eocene time, or compaction shallowing of data suggest that variable clockwise block ro- shallow paleomagnetic inclinations from tur- magnetic inclinations (e.g., Kodama and Davi, tation has affected the basin since Eocene time bidite sandstones of the German Rancho For- 1995) affected the Gualala turbidites. It has (Kanter, 1983). mation (Kanter and Debiche, 1985). Paleo- been suggested that the Baja2British Colum- Conglomerates are present in both the Up- magnetic data suggest 1810 6 200 km of bia superterrane may have shed detritus into per Cretaceous and Eocene sections at Gual- northward displacement of the Gualala basin the Gualala basin as it migrated from Mexico ala, but their deposition was not continuous since Eocene time (Kanter and Debiche, to Canada (Maxson and Tikoff, 1996), al- and their composition is not uniform. Upper 1985), implying a depositional location at the though the time of Baja2British Columbia Cretaceous conglomerate clasts are generally latitude of the northern or central Peninsular docking in Canada (Cowan et al., 1997) was well rounded and rarely exceed 50 cm in di- Ranges batholith. Contrasting results for the roughly contemporaneous with deposition of ameter, whereas lower Eocene clasts are gen- middle to upper Eocene Butano Sandstone the Eocene German Rancho Formation clasts erally less well rounded and range to3min from the northern Salinian block indicate only that are the focus of this study, so this terrane diameter. Compositionally, Upper Cretaceous 590 6 300 km of northward transport (Kanter, is effectively precluded as a source (Schott conglomerates of the Gualala Formation
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