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Detrital framework modes have been determined for the sand fraction of Quaternary terrigenous clastic sediments from the Cascadia accretionary wedge. The sands and sandstones analyzed are moderately quartz-dominated, with smaller fractions of both feldspars and lithic fragments. Triangular discrimination plots indicate provenance from a recycled-orogenic crust, perhaps with a contribution from a deeply dissected magmatic arc. The composition of the modern active continental margin of British Columbia, Washington, and Oregon is reflected well by sandstone composition in the Cascadia trench-forearc system. INTRODUCTION The mineralogy of a provenance area and its plate-tectonic setting strongly influences sandstone compositions (e.g., Dickinson and Suczek, 1979; Dickinson et al., 1983). In continental-margin settings, sand detrital modes can be sensitive indicators of plate-tectonic processes affecting the margin itself (e.g., Marsaglia, 1991; Marsaglia et al., 1995), and reflect protracted phases of arc magmatism, lulls in volcanic activity, or major continental uplift and subsequent erosion. In their study of Phanerozoic North American sandstones, Dickinson et al. (1983) showed that Neogene onshore occurrences in the Pacific northwest of the United States and Canada are clearly magmatic-arcrelated, with recycled orogenic sands and transitional continental suites north of the Queen Charlotte fault and south of the Mendocino transform faults, respectively (see Fig. 1A). We report modal composition data of Pleistocene framework-supported turbiditic sands and sandstones from the Cascadia accretionary prism, and we attempt to make inferences about their provenance and the tectonic context of the source areas. Scientific offshore drilling in the Cascadia accretionary prism during Leg 146 of the Ocean Drilling Program (ODP) (Westbrook, Carson, Musgrave, et al., 1994) was located in the North American forearc above the downgoing Juan de Fuca Plate (Fig. IB). Sites 888-890 were drilled off the coast of Vancouver Island and to the north of the Nitinat Fan. Sites 891 and 892 were drilled off the coast of Oregon to the south of the Astoria Fan. The penetrated sediments principally consist of abyssal-plain clays and silts, and of middle-to-outer-fan hemipelagic turbidites and mass-flow deposits. For a detailed description of lithostratigraphy and facies, see Westbrook, Carson, Musgrave, et al. (1994). Samples analyzed in this study came from Sites 889, 891, and 892. METHODS Framework modal compositions were determined from 16 Leg 146 sand samples (out of a batch of 80) that are well sorted and sufficiently free of matrix to allow provenance analysis. Sample identi1 Carson, B., Westbrook, G.K., Musgrave, R.J., and Suess, E. (Eds.), 1995. Proc. ODP, Sci. Results, 146 (Pt. 1): College Station, TX (Ocean Drilling Program). Institut für Geowissenschaften und Lithosphàrenforschung, Universitát Giessen, Senckenbergstrasse 3, D-35390 Giessen, Federal Republic of Germany. fications and results of the modal analysis are given in Tables 1 and 2. Counting was restricted to monocrystalline quartz (Qm), poly crystalline quartz (Qp), Plagioclase (P) and orthoclase (K) feldspars, and lithic fragments derived from sediments (Ls) or volcanics (Lv). The other parameters shown in Table 1 are total quartz (Q = Qm + Qp), total feldspar (F = K + P), lithic fragments (L = Ls + Lv), and total lithics (Lt = L + Qp). Lithic fragments of metamorphic rocks appear under the Ls and Lv categories. Where detected, however, their proportion is small. Sand-sized grains of bioclastic calcite, and other terrigenous and volcanic minerals were not counted. Thin sections were not stained, but ambiguities in feldspar identification were avoided by using conoscopic techniques where necessary. The samples were freeze-dried and then impregnated with epoxy resin. Petrographic thin sections were cut in orientations parallel to the core axis (i.e., perpendicular to bedding) and described for their mineralogical composition (see below). Point counting was carried out on a rectangular grid with an equal spacing of 2 mm. The field of view at each station was 1.2 mm in diameter, and within this field of view all sand-sized (0.06 to 2 mm) grains were counted and classified. Depending on mean grain size, between 81 and 1911 grains were counted per sample. This counting procedure introduces a bias towards fine grain sizes (cf. Ingersoll et al., 1984). However, we feel that this does not constitute a major problem in the well-sorted and generally fineto medium-grained sand and sandstone samples chosen for the analysis. SAND AND SANDSTONE PETROGRAPHY The sands and sandstones analyzed varied between arkosic to quartzose, with a minor component of lithic fragments. The quartz grains are subrounded, often polycrystalline, and have sizes ranging from 0.07 to 2 mm with a maximum at sizes between 0.1 and 0.4 mm. Plagioclase and potassic feldspar occur in variable amounts, generally with angular grain shapes. Plagioclase commonly shows polysynthetic twinning. Most of the potassic feldspar is microcline. Carbonates occur as bioclasts, as authigenic concretions or as cement of extensional veins and fractures (Kopf et al., this volume). Although the total amount of carbonate is in the l%-10% range in some of the samples analyzed it was ignored in the counting procedure, as the concentration of carbonate may be dominantly an effect of diagenesis. Accessory minerals are clinopyroxene and amphiboles, detrital micas (muscovite and biotite in the case of Sample 891B-14X1, 66-70 cm), pyrite, and Fe-oxides. Siliceous microfossil fragments, mainly diatoms, also occur. Lithic fragments are well-rounded or

Data Report: Provenance Analysis of Quaternary Sands and Sandstones from Cascadia Margin