Journal of rhe Geological Society, London, Vol. 144, 1987, pp. 531-542, 10 figs, 2 tables. Printed in Northern Ireland Tectonic environment of the Devonian Gramscatho basin, south Cornwall: framework mode and geochemical evidence from turbiditic sandstones P. A. FLOYD & B. E. LEVERIDGE' Department of Geology, University of Keele, Staffordshire ST5 5BG, UK 1 British Geological Survey, St Just, 30 Pennsylvania Road, Exeter EX4 6BX, UK Abstra& ThePortscatho Formation, within the allochthonous unit of the Middleand Upper Devonian Gramscatho Group, is a thick sequence of deep-water sandstones and interbedded slates deposited by southerly-derivedturbidity currents into the Gramscatho basin of southCornwall. Throughout an approximately 3.5 km thick sequence, the Portscatho Formation is petrographically and chemically coherent, except that the upper section shows a higher proportion of metamorphic clasts, high, but variable Cr, and low, uniform Zr abundances. Complementary framework mode and bulk geochemistry indicate that the sandstones were derived from a dissected continental magmatic arc of predominantly acidic composition, similar to averageupper continental crust, but with an admixture of minor intermediate/basic material. Flysch deposition took place in a fore-arc setting. Thepresence of an arc to the south of Cornwall during the Devonianimplies that there was subduction at the margin of the Gramscatho basin, whose ultimate closure was accommodated by the northward stacking offlysch-ophiolite nappes. The DevonianGramscatho Beds in south Cornwall were nappes of flysch and olistostrome, ophiolite and continental defined and described in some detail by Hendricks (1937), basement are related to theclosure of anoceanic basin who referred to the thicksuccession of alternating situated to the southof Cornwall. greywacke sandstones and slates as a flysch facies sequence. Contemporaryattention was, however, focused onthe superincumbentrocks as indicators of regional tectonics, Sandstone discrimination andthey were interpreted wholly, or in part, as tectonic The relationship of sandstone composition to tectonic mBlange beneath the Lizard thrust nappe (Flett 1933) or the environment, namely thatquartz-rich rocks characterize Lizard-Dodman-Startnappe (Hendriks 1939). Onlylater continentalmargins, quartz-poor rocks are derived from were the character and associations of flysch sequences fully island arcs and intermediate quartz contents are associated described(e.g. Kuenan & Migliorini1950; Crowell 1957) with active continental margins and orogenic belts, had been and their close relationship with tectonically active regimes recognizedby the mid-seventies (e.g. Schwab1975). The detailed (see Aubouin 1965). Following recognition that the concept was developedand aclose correlationbetween Cornish mClangewas sedimentary(Lambert 1965) the sandstoneframework mode proportions and plate setting Gramscatho Beds and the mClange breccias were interpret- was demonstrated, particularlyby Dickinson & Suczek ed as a pre-tectonic flysch/wild-flysch sequence(Dearman (1979) and Ingersoll & Suczek(1979), using standard and et al. 1969; Dearman 1971). An earlyplate model by discriminatory ternaryplots of framework grains. The Mitchell (1974) proposed that the Gramscatho breccias and general validity of these diagrams has been confirmedby Lizard Complex constituted a subduction association related subsequent work, but so-called error populations have been to closure of a VariscanRheic ocean and subsequent recognizedby Mack (1984). Thesepopulations represent continental collision. sandstonesdeposited in the transitionbetween tectonic It has since been established that a major pari of the regimes, thosefound in settings unrepresented on the Lizard Complex is an ophiolite segment (Strong et al. 1975; diagramsand sediments enriched in quartz as a result of Kirby 1979; Hoyd 1984) of probable Silurian/Devonian age weathering on depositionalreworking. Work on modern (Halliday & Mitchell 1977; Davies 1984) and the presence of deep-seasands has shownsimilara close correlation a RheicOcean south of Cornwallhas been indicated by betweencomposition and environment (Maynard et al. distribution of faunas(Cocks & Fortey 1982) and 1982), although the figured compositional fields may be less palaeomagnetic evidence (Tarling 1979; Scotese et al. 1979). distinct than those proposed for ancient sandstones. Nevertheless,plate models have tended to emphasize the It has been suggested that the bulk chemical composition intracratonicnature of theGramscatho depositional of turbiditic sandstones, as well as finer-grained deep-water environment, be it in a back-arc basin related to northerly argillites, can be used to designate environmental features subduction in S Brittany (Leeder 1982; Floyd 1982, 1984) or (Bhatia 1983,1985). Abundances of rareearth elements an extensional basin generatedin a strike-slip/transfortn (REE)and other stable elements (Ti, Zr, Hf, Y, Sc, Nb, regime (Badham 1982; Sanderson 1984; Barnes & Andrews Ga, Th, U), or their ratios, are particularly promising in this 1986). However, Holder & Leveridge (1986) have proposed contextand have been used to distinguish the tectonic a model for south Cornwall involving southward subduction setting of sedimentary basins in Australia (Bhatia & Taylor during the Devonian.Sedimentation of theGramscatho 1981; Bhatia 1985). The above elements have intermediate flysch and concomitant northward migration and stacking of ionic potential and low ocean residence times (Henderson 531 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/144/4/531/4888992/gsjgs.144.4.0531.pdf by guest on 02 October 2021 532 P. A. FLOYD & B. E.LEVERIDGE 1982; Mason & Moore 1982) and as they are readily thickness up to 2 m, being largely classic turbidites (Walker incorporated into sediments they also provide an indication 1979). Bedding,where traceable, is planeparallel, with of the source composition (McLennan et al. 1980; Taylor & scour grove and flute moulds apparent on inverted bases. McLennan 1981, 1985). Internal structures are referrable to the Bouma(1962) model Thepetrographic character of37 GramscathoGroup butthe simple pattern may be disturbed by delayed or sandstonesamples from the allochthonous Portscatho multiple grading, the presence of a coarse traction carpet or Formation has been investigated during this study whereas wavy laminatedbedding whichin wispy siltstone is 44 sampleshave been analysed geochemically. Bulk interspersed with sandstone.Towards the top of the geochemical analysis was determined by XRF spectrometry formation in Gerrans Bay grain-flow sandstones(Stauffer (on an ARL 8420 Quantometer at University of Keele) for 1967) are also common. major and trace elements (method modified after Norrish & Theformation contains fining-up, coarsening-up and Hutton 1969) anda selected subset of20 sampleswere mudstonesubfacies, although there is anoverall upward anslysedfor total rare earthelements (REE) (at Kings coarsening and thickening apparent. It typifies deposition in College, London; Walsh et al. 1981) and Hf, Ta, Th, U and adeep-water fan environment (see Rupke 1977; Walker CS (atUniversities Research Reactor, Risley; Duffield & 1978)with ageneral progradation from outer to mid-fan Gilmore 1979). Modal compositions of samples from the ‘A’ regimes through the formation. The deep-water character is divisions of graded turbidite flows have been determined by furtherindicated by thedark grey mudstonebackground a standard 500 point count of thin-sections. The sandstones sediment,containing sapropelic organic residues through- are largely greywackes, with anaverage of 26% clastic out,the considerable thickness of the succession and the matrix. Apparent matrix proportion is not a constraint on lack of reworking or shallow water depositional structures. thedetermination of frameworkmodal compositions, The PendowerFormation is ahemipelagic sequence because fine-grained lithic grains and even altered feldspars (Holder & Leveridge 1986) of grey, green, brown and black are still recognizable in sandstones with up to 35% matrix. slates with interbeddedautochthonous radiolarian cherts, However, those sandstones with a high proportion of matrix turbiditicpelagic limestones, and coarse lithic greywacke phyllosilicate,white micas andchlorite, do have a strong sandstoneforming sparse, thin, graded beds, lenses and dependent tectonic fabric with recrystallization overprinting small channel fills. The succeeding CarneFormation is a andobscuring grain boundaries, and have been omitted sequence of turbiditesandstone, channel-fill sandstone, from the petrographic study. Also omitted are a few samples slumpfolded and phacoidal sandstone, interbedded with with a secondary carbonate replacement overprint. interlaminatedmudstone, siltstone and sandstone.It is Thepresent investigationattempts to combineboth interpreted (Holder & Leveridge 1986) as an association of petrographic and geochemical data for sandstones from an middle to upper fan distributary channel and interchannel allochthonous unit of the Gramscatho Group as an aid to deposits(Mutti & Ricci-Lucchi 1972; Rupke 1977), slope theidentification of thetectonic environment in south deposits and normal proximal turbidites (Walker 1978). Cornwallduring the Devonian. A representative selection of samples spanning the full stratigraphic sequence are listed in Tables l (frameworkmodes) and 2(corresponding Sandstone petrography and framework modes chemical analyses). ThePortscatho sandstones