Journal ofrhe Geological Society, London, Vol. 145, 1988, pp. 591-602, 10 figs, 2 tables. Printed in Northern Ireland

Relationships between the Cumberland Bay and Sandebugten Formations, , and some tectonic implications

1. M. TURNBULL & D. CRAW' N.Z. Geological Survey, DSIR, Private Bag, Dunedin, New Zealand 'Geology Department, University of Otago, Dunedin, New Zealand

Abstrad: Thelate Jurassic to early Cretaceous Cumberland Bay and Sandebugten formations of South Georgia have different detrital petrography, although some of their detrital components are common to both units. Cumberland Bay strata become progressively more deformed with depth, and themore deformed portions strongly resemble Sandebugten rocks. Estimates of pressureand temperature of metamorphism suggest that Sandebugten rocks underwent pumpellyite-actinolite or greenschist facies metamorphism, at least 1 kbar and about 50°C higher than prehnite-pumpellyite facies metamorphism of the Cumberland Bay sediments. Both units are now separated by a low angle fault.Palaeocurrent data from Cumberland Bay strata show definite patterns, but data from Sandebugten Formation are equivocal. Variations in petrography are no more than occur in similar island arc-derived sediments from other areas around the Pacific margin. It is postulated that both units,and possibly other formations on South Georgia, may have been derived from one evolved volcanicarc with a sialic basement, rather than from two sides of amarginal basin, as has been previously suggested.

Since the earliest systematic geological work on South geochemistry (Clayton 1982; Storey & Tanner 1982). The Georgia by Ferguson (1915), two distinct units have been unit is considered to have been derived from a volcanic arc recognized within the Mesozoic metasedimentary rocks that terraneto the south-eastand south of the present-day make up most of the island. These units are known as the position of the island (Winn 1978; Stone 1980; Macdonald & Cumberland Bay Formation(CBF) and Sandebugten Tanner 1983). Depositionoccurred in a back-arc or Formation (SBF), names formalized by Stone (1980) after marginal basin fromturbidity currents flowing tothe usage by Trendall (1959), and others. Stone (1980) also north-eastand north-west, across and along the basin described a third sedimentary unit, the Barff Point Member (Macdonald & Tanner 1983; Macdonald 1986). Tuff units of theCBF, which is intermediate in sedimentologic and are common (Stone 1980; Craw & Turnbull 1986) and much petrographiccharacter between theCBF and the SBF. of the volcaniclastic material implies contemporaneous Recently, another formationhas been distinguished on volcanic activity in the source area (Tanner 1982). The CBF Ducloz Head(Tanner 1982; Storey 1983a, after Trendall stratigraphic sequence is predominantly upright, dipping and 1959). The distribution of theseunits, and of other younging tothe south (Tanner & Macdonald 1982; BAS sequences on South Georgia, is shown in detail by British 1984) and is about 8 km thick (Tanner 1982). It is deformed Antarctic Survey (BAS) (1984, 1987) and is summarized in into simple upright folds which become progressively tighter Fig. 1. Regional interpretations show CBF and SBF as facies to the north-east where they are overprinted by a second variations within a back-arc or marginal basin (e.g. Dalziel phase of folding (Dalziel et al. 1975; Tanner 1982). The unit et al. 1975; Stone 1980; Tanner 1982; Storey & Macdonald hasbeen metamorphosed to prehnite-pumpellyite facies 1984). (Suarez & Pettigrew 1976; Stone 1980; Tanner 1982; Craw Because of theapparent differences between the SBF & Turnbull 1986). Its age is generally acceptedas late andCBF units,they are usually assumed to have been Jurassic to early Cretaceous, probably Albian (Thomson et derived from separate sources on either side of this marginal al. 1982). basin. Based on our own observations from South Georgia, An isolated area of CBF, named the Barff Point from a study of the available literature, and by comparison Member., occurs on Barff Point (Stone 1980; Fig. 1). These with other Palaeozoic-Mesozoic sequences on theGond- sediments are thinner-bedded and generally more siliceous wana margin, we offer an alternativeinterpretation for than typical CBF, and are interpretedby Stone (1980) to be consideration. This is that the CBF and SBF may represent transitional in composition between typical CBF and typical different stratigraphic, metamorphic and structural levels of SBF. a single sedimentary sequence, derived from a single though In contrast theSBF has mixed volcaniclastic and complex source on the Pacific side of the marginal basin, and granitic-metamorphic-sedimentary provenance withsig- since juxtaposed by thrusting or tectonicerosion during nificant amounts of detrital quartzand feldspar(Trendall closure of that basin. 1959; Dalziel et al. 1975; Winn 1978; Stone 1980). It is also turbiditic in nature, and more thinly bedded than the CBF (Stone 1980), with tuff beds occurring in some areas (Craw General description of units & Turnbull 1986). Palaeocurrent data are distorted due to The CBFis predominantly volcaniclastic, with minor detrital complex folding or are non-polar,and suggest sediment quartz and plutonic-metamorphic material (Skidmore 1972; transport indirections varying from N-S to NE-SW and Stone 1980; Tanner 1982), and has a calc-alkaline NW-SE (Dalziel et al. 1975; Winn 1978; Stone 1980; Tanner 591

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Head Fmn.

54 30'

80UTH OEOROIA -S ..

500 km a7 W a0 W V . I I Fig. 1. Generalized geological map of South Georgia (after BAS 1987) showing main units and areas mentioned in the text.

1982), covering a range similar to the CBF (Tanner 1982). Metamorphic grade is zeolite facies. The The SBF is more complexly deformed than the CBF and is sediments were once thought to be associated with the CBF of slightly higher metamorphicgrade (pumpellyite- (e.g. Tanner et al. 1981). However, Storey & Tanner (1982) actinolite or greenschist facies; Stone 1980; Craw & show that the chemistry of Annenkov Island sediments and Turnbull 1986). The age of the SBF is unknown although it CBF are dissimilar in some respects. In southernmost South is generallyassumed to be late Jurassic to Cretaceous Georgia, the Formation (Fig. 1) is possible (Tanner & Rex 1979; Thomson et al. 1982). ocean floor or ophiolitic basement tothe SouthGeorgia The Ducloz Head Formation (Fig. 1) consists of pillow sequence (Bell et al. 1977; Storey et al. 1977). lavas, volcaniclastic breccia,andestitic and felsitic tuffs, black mudstone,and massive and thin-bedded sandstone, and is highly sheared along theCooper Bay Dislocation Petrography of sedimentary units (Tanner et al. 1981; Storey in Tanner 1982; Storey1983a; The detrital petrography of the SBF, CBF and Barff Point BAS 1984; Storey & Macdonald 1984). The sediments have units has been described in detail and modal analyses given a mixed volcanic and plutonic-metamorphic source, and the by numerous authors, including Skidmore (1972), Stone & unit therefore has a provenance similar to that of the Barff Willey. (1973), Dalziel et al. (1975), Winn (1978), Winn & Point Member. Its age is unknown; metamorphic grade is Dott (1978), Tanner et al. (1981) and particularly by Stone prehnite-pumpellyite facies (Tanner et al. 1981; Storey (1980). Modal analyses were tabulatedand discussed by 1983a). Stone (1980) and are repeated here (Fig. 2). Most analyses Two other sedimentaryunits occur on South Georgia; werepresented on a matrix-free basis, using parameters theCooper Bay Formation(Trendall 1959; Stone 1982; modelled on those of Dickinson (1970). However, because Storey 19836), and the Annekov Island Formation (Suarez of alterationand deformationsome point counts are not & Pettigrew 1976; Pettigrew 1981; Tanner et al. 1981) (Fig. replicable(Winn 1978). Also, Dalziel et al.(1975), Winn 1). The Cooper Bay Formation is highly sheared, gneissic (1978) and Winn & Dott (1978) did not recognize a Barff and quartzose; it has been likened to both the SBF (Bell et Point Member of theCBF. Consequently the only 'true' al. 1977; Stone 1982; Storey1983b), and to the CBF SBF data are those of Stone (1980) (Fig. 2). Nevertheless, (Clayton 1982; BAS 1984). It is also involved along the the non-CBF data of Winn (1978) and others must either be Cooper Bay Dislocation, and is fault-bounded (Tanner CBF (Barff Point Member)or SBF, andextend the 1982; BAS 1984, 1987). The Annenkov Island Formation is petrographic fields of either (or both) of thoseunits relatively undeformed, and is a shallow marine volcaniclastic considerably.Various aspects of the petrography are sequence with an early Cretaceous fauna (Pettigrew 1981). summarized below, and in Figs 2-4.

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Skidmore 1972

Dalriel et 01.1975.Wnn 1978

A CBF

Stone l980 X SBF 0 brff Pt Mbr CBF A.

/ ;o0* + \

A $0 A.+ \

Fig. 2. Quartz-Feldspar-Lithin (Q-F-L) fields of South Georgia metasediments. Data points on left,fields summarized on right. Note that 'SBF' probably includes SBF and Barff Point Member.

CZmt type

Sandebugten Formation. Clasts in the SBF include mono- and polycrystalline quartz, plagioclase, rare orthoclase and microcline,felsitic volcanics with quartz, albiteand oligoclase phenocrysts, andrare metamorphic fragments. Accessory minerals reported include pale green pyroxene, hornblende,sphene, zircon,rutile and tourmaline.The source areas include silicic tointermediate (trachytic- dacitic) and andesitic volcanics, high-grade and lower grade metamorphics, granitoids, and quartzose sediments (Dalziel et al. 1975; Winn 1978; Stone 1980).

Ti norporogenic tholeiites

0.02 0 / 0

0 O. 0.0 1 0 0 Ca 0.8:

I calcalkalme basalts

0 island arc tholeiitic basalts

0 A I(tota0 0.1 o. (C) L. 0 m Fig. 4. Photomicrographs of some clasts from Cumberland Bay sandstones and granule conglomerates,Ross Glacier. Fig. 3. Microprobe analysesof detrital pyroxenes (see Table1) (A) graphic granite, 2mm wide; (B) probable metamorphic from Cumberland Bay metasediments near theRoss Glacier. Data quartz grain, 5 mm long; (C) semi-schist fragment,2 mm long. are plotted in fields defined by Letemeret al. (1982). All photographs with crossed polars.

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CumberlandFormation.Bay CBF clasts are Table 1. Representativeelectron microprobe analyses (wt%) of predominantly volcanic rockfragments and plagioclase detrital clinopyroxenes in CBF metasedimentary rocks, RoyalBay feldspar; rare polycrystalline quartz, pale green pyroxene, area,South Georgia. Energydispersion micro-analysis, at 15kV, brown hornblendeand detrital mica arethe main with sample current= 2 nA on periclase and beam diameter = 10 pm. accessories. The volcanic component includes dacitic, All iron is calculated as ferrous. trachytic,trachyandesitic and andesiticfragments and devitrified glass. Rare ophitic doleriteand graniticdebris SiO, 51.1 51.7 50.6 53.7 53.2 has also been reported. A predominantly volcanic source is A1203 3.8 3.0 4.4 0.8 0.5 implied (Skidmore 1972; Winn 1978; Stone 1980; Tanner et TiO, 0.6 0.4 0.7 0.2 0.0 al. 1981). Howeverit is significant that somecomponents Fe0 8.3 8.0 8.4 11.0 11.7 (pyroxene, dacite and trachyte fragments, slate and the rare MgO 13.4 13.7 13.1 13.6 13.1 CaO 22.9 22.7 -23.0 21.0 21.2 granitic component) are common to both the CBF and the Total 100.1 ~ 100.2 100.3 99.7 SBF, as shown by Winn (1978, fig. 7). The field of detrital -99.5 QFL modes of the CBF, and the Barff Point Member, is Cations on the basis otsix oxygens: shown in Fig. 2. Si 1.90 1.93 1.88 2.00 2.00 AI 0.17 0.13 0.19 0.04 0.02 Ba~f PointMember. Clastsin this unit include more Ti 0.02 0.01 0.02 0.01 0.00 basic to intermediate volcanics than occur in the SBF, and Fe 0.26 0.25 0.26 0.34 0.38 morequartz than in theCBF. A clastic petrography Mg 0.75 0.76 0.73 0.75 0.74 intermediatebetween the SBF and CBF is suggested by Ca 0.900.91 0.92 0.84 0.86 Stone (1980). Some ‘quartz-rich Cumberland Bay’ samples Total 4.00-3.99 4.00 3.98 4.00 described by Winn (1978) may also come fromthe Barff Point Member (see above).

Ducloz HeadFormation. Sedimentaryunits in the of the system used to obtain them. Alternative parameters Coastal Member of this formation contain quartz, and minor (e.g. discriminant function analysis; see below) may be more amounts of garnet, biotite andmetamorphic and plutonic reliableas provenance indicators. Nevertheless, traditional debris, in addition to felsitic volcanic fragments (Tanner et modal data are still of some value. al. 1981; Storey 1983~;Storey & Macdonald 1984). The numerousmodal analyses of SouthGeorgia Comparisonshave beendrawn with theSBF detrital metasediments(see above) have beencompared and petrography by Tanner et al. (1981). contrasted by Stone (1980) (see Fig. 2).They show that most CBF sediments are quartz-deficient,dominated by New observations from the CBF. Work by the authors in rock fragments, and most also have a relatively low feldspar the Royal Bay area (Fig. 1) provides additional information content.The SBF rocks in contrast are relatively .onCBF petrography. Althoughmost rocks show quartz-rich,although still with fairly low feldsparcontent considerablerecrystallization and were tooaltered for (<30%). Barff Point Member compositions given by Stone modal analysis, some coarse-grained sandstone and pebble (1980) cover thegap between theSBF and CBF modal conglomerate fromsouth of the RossGlacier contain fields, as do some intermediate CBFmodes and many ‘SBF‘ unaltered clasts. Most of theseare basic tointermediate modes given by Dalziel et al. (1975) and Winn (1978). These volcanics, but the following lithologies are also present. (i) latter probablyinclude numerous Barff PointMember Subrounded graphic granite and granite clasts up to 5 mm. modes (Stone 1980, p. 27). The data clearly show that the (ii) Polycrystalline quartz of probable metamorphic origin SBF-CBF (plus Barff Point) sediments cover a wide range which formsup to 50% of recognizable clasts insome of compositions. samples. (iii) Clasts of chlorite up to 3 mm (in one sample Subtle petrographic changes take place across the CBF, only). Although some other mafic clasts have altered in part in that older rocks on the north side of the island are more to chlorite (e.g. pyroxene, hornblende), these chlorite clasts feldspathic than younger rocks around Royal Bay (Skidmore havea detrital origin andare productnot a of 1972; Stone 1980). Rocks Capeat Darnley onthe post-depositional alteration. (iv) Very fresh pyroxenes (most south-west coast, probably the youngest of the CBF (Tanner CBF pyroxenes are altered). Microprobe analyses show the et al. 1981), aredominated by andesitic clasts, with pyroxene to be of arc tholeiitic origin, representative of the subordinate silicic volcanic clasts (Tanner et al. 1981, p. most primitive portions of a volcanic arc (Fig. 3, Table 1). 106). These differences are reflected in the geochemistry of (v) Rare fragments of slateor semi-schist, composed of the CBF (Clayton 1982), although they were discounted by fine-grained muscovite with incipient recrystallization Storey & Tanner (1982). Similar changes have notbeen segregation. Mica alignment is at a high angle to incipient recordedin theSBF, probably because themore intense cleavage in the hostsediments and is not of post- deformation and recrystallization largely obscuredetrital depositional origin. Some of these clast types are illustrated petrography.Some SBF specimens from near StAndrews in Fig. 4. Bay (Fig. 1)are highly feldspathic (Fig. 5) and are distinguished from nearby strongly-deformed CBF only by the presence of prehnite in the latter, and the presence of Modal analyses actinolite in interlayered SBF lithologies. We note that many South Georgia sedimentary rocks are The conclusions we draw from the detrital petrography highly alteredand sheared, and metamorphosed to (subject to the reservations expressed above)concur with pumpellyite-actinolite facies; underthese circumstances those of Stone (1980) andare that: (i) thereare distinct modal analyses are bound to be rathersubjective, regardless differences‘ betweenaverage end-memberCBF and SBF

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(A) (B) Fig. 5. Photomicrographs of CBF (A) and SBF (B) feldspathic semischists,on either side of fault, nearSt Andrews Bay. Crossed polars, horizontal field of viewis 4 mm in both photographs. All coarsegrains are detrital feldspars.

models;(ii) there is alsosome petrographicoverlap, and intermediatesource, in agreement with petrographic certain detrital components are definitely common to both observations. However, there is very little separationof SBF units. andCBF geochemistry using this technique.The highly feldspathic SBFsample illustrated in Fig. 5 defines the right-hand end of theSBF field,almost at theboundary Geochemistry betweenintermediate and basic provenanceand nearly In a study of the geochemistry of South Georgia sediments, overlapping with some CBF samples from Royal Bay. Both Clayton (1982) presented evidence that the CBF and SBF SBF and CBF fields overlap the data range for the Caples were quite different, and that the Barff Point Member was terrane, a sequence of volcanogenic greywackes from New geochemically similar theCBFto rather than of Zealand (Roser 1986). intermediate composition. Further work by Storey &L Tanner (1982) supported the conclusions drawn by Clayton (1982) and others (e.g. Stone 1980) from petrography, i.e. thatthe CBF was of calc-alkalic parentage.In addition, Clayton (1982) showed that there is a general decrease in silica contenttowards the south-east in theCBF (i.e. Nap0 towardsthe top of thesequence). Thus the greatest A geochemical differencesbetween SBFand CBF might be 7 expected in theRoss Glacierregion. XFW analyses of a representative selection ofsamples from that area show considerable overlap in composition of CBF and SBF rocks m in all major elements. Calcium is generally higher in CBF Waipapa Terrane thanSBF (Fig. 6, Table 2).This distinction presumably reflects the more mafic provenance (also reflected by more m calcic plagioclase) of the CBF, although the sodium data, A m expected to be higher in the SBF, are equivocal (Fig. 6). A better indication of relative chemistry can be obtained using discriminantfunction analysis, whereby linear combinations of majorelement proportions (variably weighted)can be used todiscriminate between composi- 3 tional fields. Discriminantfunctions fordistinguishing different sedimentary provenances have been developed by Roser & Korsch (1987). They have shown that two functions can be used to display almost all variations in composition of sandstonesfrom circum-Pacific orogenicbelts, principally forrocks like theSBF and CBF. These functionswere applied to South Georgia analyses (Fig. 7). Rocks from the CaO 7 Royal Bay areaand SE Georgiaare relatively basic 1 3 (confirming detrital pyroxene data, Fig. 3). Clayton's (1982) analysesindicate very broadlytrenda from a basic- Fig. 6. Na,O-CaO composition (wt%) of SBF (triangles) and CBF intermediatesource in samples progressively structurally (squares) greywackes. Open symbols from Clayton(1982); closed deeper in the CBF pile (i.e. towards the north). The Barff symbols are samples from Royal Bay area (Table2). Range of Point Member fallsin this intermediate igneoussource field. compositions for some similar New Zealand greywacke terranes are TheSBF, by comparison, has morea acid to shown for comparison(see text for details and references).

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Table 2. X-ray fluorescence analyses of CBF and SBF metasedimentary rocks, Royal Bay area, South Georgia. Sample numbers, prefixed OU576,etc., are curated by the Geology Department, Universig of Otago. Analyses were by J. Moore. AI1 iron calculated as ferric; L01 =loss on ignition.

C umberland Bay Formation SandebugtenFormation Bay Cumberland

4549 48 55 59 60 66 63 52

SiO, 71.55 55.32 54.24 54.17 51.49 57.26 55.67 53.67 51.85 TiO, 0.53 1.21 1.01 0.94 1.15 1.06 1.24 1.16 1.15 13.34 16.21 16.39 16.98 17.66 16.70 17.59 20.05 17.78 4.44 9.76 9.19 9.09 11.05 8.49 9.93 9.50 10.98 MnO 0.02 0.14 0.14 0.17 0.20 0.20 0.13 0.21 0.20 MgO 0.94 3.21 4.02 3.18 4.08 2.82 2.43 2.95 4.03 CaO 1.25 5.59 5.39 6.91 5.87 3.44 3.18 2.34 5.95 Na,O 3.47 5.03 3.16 4.53 4.09 5.74 4.98 7.12 4.47 KZ0 1.96 0.93 2.28 0.58 0.58 0.91 1.41 0.39 0.60 P205 0.12 0.51 3.21 0.53 0.23 0.21 0.36 0.20 0.25 L01 -3.45 -1.86 3.86 -3.20 -3.27 2.02 3.34 --2.66 2.97 Total 101.07 99.77 102.89 100.28 99.67 98.85 100.26 --100.25 100.23

Structure 1986). Thedominant deformation involves open to tight Summaries of the structure of sedimentary rocks from South upright folding with variabledegrees of axial surface Georgia have been presented by Bell et al. (1977), Dalziel et cleavage development. In the Ross Glacier area, this folding al. (197S), Stone (1980, 1982), Tanner et al. (1981), Tanner has some development of axial surface cleavage in siltstones (1982) and Tanner & Macdonald (1982), based on restricted in fold hinges. Locally, an earlier cleavage formed by work in smaller areas (Fig. 8). incipient mica recrystallization and re-orientation is found sub-parallel to bedding (Craw & Turnbull 1986). Immediately to the north across the Ross Glacier Fault Cumberland Bay Formation (Fig. l), a different, deeper level of theCBF is exposed There is considerable variation in the degree of deformation (Craw & Turnbull 1986). Here, the upright folds are still of CBF rocks over South Georgia. In general, onedominant present with axial surface cleavage of varying intensity, but phase of folding is mapped (e.g. Skidmore 1972; Dalziel et the dominant deformation pre-dates these folds. The earlier al. 1975; Stone 1980; Tanner 1982) although polyphase deformationproduced tight to isoclinal folds with a deformationdoes occur (Tanner 1982; Craw & Turnbull pervasively developed axial surface cleavage. These two phases of deformation can be traced northwardsacross the island tothe north coast (Tanner 1982; Tanner & Macdonald 1982). The relationship between the incipient early cleavage south of the Ross Glacier and the early schistosity to the north is not known. The latter

I

Fig. 7. Discriminant function diagram showing separated sedimen- 500 tary provenances (after Roser & Korsch 1987). F1 and F2 are discriminant functions: involving TiO,, A1,0,, Fe,03, MgO, CaO, CBD 1 , m .l900 Na,O and K,O. Approximate composition fields for SBF and CBF (symbols as for Fig. 6) are indicated. Closed circle = Cooper Bay metasediments (Clayton 1982). Range for Caples terrane, New m.8. Generalized structural cross-sections of South Georgia. For Zealand, is enclosed in the curly line field. General trend for an locations refer to Fig. 1. CBD, Cooper Bay Dislocation. Sections ensialic calc-alkaline volcanic complex (TVZ, Taupo Volcanic are modified from Tanner (1982), BAS (1984) and Craw & Turnbull Zone, New Zealand) is shown for comparison. (1986).

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/4/591/4889414/gsjgs.145.4.0591.pdf by guest on 29 September 2021 may represent further development of theformer due to Fluid inclusions in 0.5-1 mm quartz grains from the veins tectonicprocesses, with increasedductile strain deeper in are relatively common. Primary two-phase (liquid-vapour) the sediment pile. Alternatively, the early incipient cleavage fluid inclusions (cf. Roedder 1984) are1-10pm across. may represent essentially non-tectonic mica orientation Secondary inclusions are smaller, occur astrains along developed within a soft sedimentsequence (cf. Moore & partially-healed fractures, and were not investigated. Data Geigle 1974; Stewart 1977). from thethree samples are reasonably consistent. An These observations suggest that the CBF sequence shows approximatehomogenization temperature range of170- a progressive increase in deformation with depth(and/or 200°C is implied, with amode near 180°C. Only eight regional structural position), from single-phase open folding inclusions from thethree samples werelarge enough for with steep axial surfaces in the south topolyphase, relatively melting phenomena to be observed (T', = 0 "C), and these ductile deformation with shallow-dipping axial surfaces data imply that fluid salinities were very low. towards thenorth of the island (Fig. 8; Tanner & Apressure correction to the homogenization tempera- Macdonald 1982). ture can be applied from stratigraphic data. Estimates of the thickness of the CBF sediment thickness range between 8 Sandebugten Formation and 10 km (e.g. Trendall 1959; Tanner 1982). Rocks from the study area were from the structurally highest part of this Most SBF rockshave suffered one ductile phase of pile SO pressures less than 2 kbarare implied. A2 kbar deformation, with development of a fully-recrystallized, maximum pressureestimate yields a fluid inclusion filling pervasive fold axial surface cleavage (Dott 1974; Stone 1980; temperature of about 280°C (maximum). The presence of Tanner 1982). Folds are tight or isoclinal and hinges can be metamorphic epidote in these rocks (Craw & Turnbull 1986) highly attenuated. This pervasive foliation has been folded, implies temperatures greater than 200-225 "C (Kristmans- locally tightly, by asecond semi-ductile phase with little dottir 1982; Mehegan et al. 1982), suggesting a minimum associated recrystallization. Second-phase, fold-axial-surface pressureestimate of 0.5 kbar.These estimates confirm cleavage is incipiently developed, with some local cleavage intuitive estimates of prehnite-pumpellyite facies conditions differentiation zones on a millimetre scale. A third, brittle (e.g. Stone 1980; Tanner 1982; Cho et al. 1986). phase of deformation has developed in some areas (Stone 1980). Some areas of SBF rocks on show only SBF P-T conditions one strong phase of deformation, with tight folds in bedding The more thoroughly recrystallized SBF is even harder to with incipient axial surface cleavage. It is not yet certain quantify in terms of metamorphic pressure and temperature. whether this phasecorrelates with the first phase of SBF Winn (1978) reported pumpellyite in the SBF;Craw & deformationelsewhere (cf. Stone 1980) orto the second Turnbull (1986) reported actinolitebut could not confirm phase (as (unpublished)observations by theauthors the presence of pumpellyite. Prehnite is notpresent (cf. suggest). Winn 1978; Tanner 1982). Iron-magnesium distribution The structural characteristics of the structurally lowest coefficients in actinolite-chlorite from two samples in the parts of theCBF are very similar tothe more-deformed SBF (KD values of 1.72, 1.51) are consistent with et portions of the SBF. This similarity, also noted by Dalziel pumpellyite-actinolite or greenschist facies. KD values (1.72, al. (1975) is consistent with the two units having been folded Coombs et al. 1976a). under the same deformation regime, with higher strain in Fluid inclusions are rare in SBF rocks. Only one sample the more ductile deeper portions. we examinedcontains usable inclusions, and even these were so small (1-3 pm)that exact temperatures of Metamorphism homogenization wereindeterminable. Temperatures were bracketed on 35 inclusions by combined heating and cooling CBF P-T conditions cycling, giving homogenization between 90 and 110 "C. The Numerous authors have reportedprehnite, and less upper limit is thought to be the best estimate. commonly pumpellyite,from CBF rocks (Trendall 1959; A chertsample from near St Andrews Bay (Craw & Skidmore 1972; Dalziel et al. 1975; Suarez & Pettigrew Turnbull 1986) contains the assemblage pyrite-sphalerite- 1976; Winn 1978; Stone 1980; Tanner 1982), implying that monoclinic pyrrhotite (Jamieson & Craw 1987). According the rocks have beenmetamorphosed to prehnite- to experimental work by Scott & Kissen (1973) this pumpellyite facies. Other metamorphic mineral assemblages assemblage canequilibrate only below 250°C. The iron are consistent with this grade of metamorphism. content of the sphalerite (11 mole% FeS) is consistent with Little work hasbeen doneon the quantification of equilibration (Scott & Kissen 1973). On the other hand, the pressure andtemperature of metamorphism,probably presence of actinolitein several SBF rocks implies a because of a lack of diagnostic minerals and assemblages. minimum temperature of about 280"C (Kristmansdottir The data presentedbelow are not well constrained, although 1982) or even higher (Hallner & Schurmann 1966). Clearly useful for comparative purposes. there are either minor inconsistencies-in experimental work, Three quartz veins in widely separated (2-5 km) orthe sulphide assemblage has re-equilibrated to lower mudstone horizons from the least deformed part of the CBF temperatures during cooling. A temperature estimate for the south of the Ross Glacier were collected for fluid inclusion SBF of 250 "C (minimum) to 300 "C seems likely from the work. These veins cross-cut bedding andappear to be above data. Using the250°C minimum,a minimum related to development of slaty cleavage in shear zones near pressure estimate of about 2.8 kbar is obtained from fluid fold hinges. The veins contain prehnite, epidote and chlorite inclusion data. These data are consistent with pumpellyite- and are presumed to be synmetamorphic though they may actinolite or greenschist facies metamorphism (Turner not have formed at the (thermal) peak of metamorphism. 1981).

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Comparison between SBF and CBF P-T conditions Cooper Bay Formation, e.g. BAS 1984) as rift zone deposits formedearly in the development of the back-arc basin On the basis of mineralogy alone, the SBF has apparently (Suarez & Pettigrew 1976; Bruhn & Dalziel 1977; Bruhn et suffereda higher grade of metamorphism thanthe CBF. al. 1978; Storey1983a). The generally accepted model for This is confirmed by fluid inclusion and petrological studies SBF deposition hasbeen that it is aturbidite sequence, which, on limited data, imply pressureand temperature north-easterlyderived from a largely continentalsource differences between the two units. These data suggest that including the TobiferaFormation of Jurassic siliceous there is less than 100 "C difference in temperature,and et et about 1 kbar (minimum) pressure difference. volcanics (Dalziel al. 1975; Winn 1978; Dott al. 1982), This difference in grade is also shown by K-Ar dating with interfingering of the SBF and CBF facies in the centre of the marginal basin (Dott et al. 1982; cf. Macdonald & (Tanner & Rex 1980; Thomson et al. 1982). Apparently youngerages in theSBF almostcertainly represent a Tanner 1983). However, some authors have suggested that relatively later post-metamorphic unroofing of theSBF fragments of thiscontinental material (including Tobifera Formation) were also presentbeneath the volcanic arc compared to the CBF (Thomson et al. 1982). In addition, source of theCBF and within the marginal basin (e.g. the relative homogeneity of SBF ages compared to the range of ages, influenced by combinationa of detritaland Tanner et al. 1981; Dott et al. 1982). The sedimentation mechanisms responsible for the CBF metamorphic minerals, in the CBF (Thomson et al. 1982) suggests thatSBF rockshave been subjectedto tempera- are now well tunderstood (Macdonald & Tanner 1983; tures higher thanthe mica-Ar 'closing' temperature of Macdonald 1986). Themore distalsediments onthe around 250 "C. north-eastside of the island weretransported toward the north-west by axially-directed tubidity currents, while those The data described above quantify a small but significant difference in temperatureand pressure of metamorphism in the south are moreproximal and deposited from currents between the SBF and CBF, due to the SBF having been moving both north-eastward andtheto north-west buried at least 4 km deeper. (Macdonald & Tanner 1983). In contrast, the sedimentology of the SBF is less well understood.The sediments aremore distal in character Metasomatism (Stone1980), and although thereare someindicators of Spectacularprehnite-rich alteration zones are common in more proximal deposition (Stone 1980, table 111), the theCBF, generally ator near the boundarybetween thick-bedded sandstone/pebble conglomerate facies com- differenttypes,rocke.g. sandstone/siltstone,or mon in the south of the CBF (Macdonald & Tanner 1983; siltstone/tuff (Trendall 1959; Skidmore 1972; Stone 1980, Craw & Turnbull 1986)is not developed.However, the plates IV andVI). Prehnite and fine-grained muscovite Coastal Member of the Ducloz Head Formation, a possible dominatethe assemblage, with minor pools and veins of correlative of the SBF, does have proximal characteristics quartz. (Tanner et al. 1981; Storey 1983a). Similar alteration zones are found in the SBF, although Palaeocurrentdata from the SBF are equivocal, in more rarely. As for the CBF, the zones are best developed contrast tothe CBF. Stone (1980) described flutes and at the margins of tuff layers. The higher grade of the SBF is groove casts directed tothe north-west; Tanner (1982) reflected in the mineralogy of the zones. Prehnite is no recorded similar, non-polar, NW-SE grooves. Dott (1974), longer present,and epidote is the dominantphase. Dalziel et al. (1975) and Winn (1978) also presented Muscovite is again a constituent but coarser grained than in palaeocurrent data fromripple foresets which suggested theCBF and strongly oriented parallel tothe pervasive NE-SW and N-S currents, with southwardtransport foliation. Quartz pools and veins are also common. The predominating.Although Frakes (1966) considered that similarity between these zones in both units indicates that structuraldisturbance of many SouthGeorgia rocks theyhave had similar metasomatichistories, with the introducedconsiderable uncertainty, Dalziel et al. (1975) slightly different mineralogy and texture of the SBF zones were confident in their restored palaeocurrent data. In turn, reflecting the higher strain and grade of metamorphism in Macdonald & Tanner (1983) rejected data from areas that unit. affected by F2 folding. In addition, cross-bedding may not always coincide with sole markings in giving true Discussion palaeocurrent directions and data from the SBF are thus of debatable value (see also discussion by Tanner 1982). SBF-CBF sources Inferencesregarding the source(s) of theSBF must Recent reconstructions of Cretaceous SouthGeorgia (e.g. thereforedepend largely on petrographicevidence. As Bell et al. 1977; Tanner et al. 1981; Tanner 1982; Storey & discussed and summarized above, there is evidence that at Macdonald 1984) show the Annenkov Island Formation as least part of the SBF source area was the same as that of the being an island arc-shelf deposit,the CBF as being a CBF. This is in contrast to most earlier conclusions, (e.g. back-arc basin or marginal basin turbidite deposit (Dott et Winn 1978; Tanner 1982), although Tanner et al. (1981) also al. 1982) derived fromthe same arc, and the SBF as an suggested thatpart of theSBF did comefrom the same olderto partly contemporaneous, largely continental- direction as the CBF, the south-west. derived unit beneath the CBF (e.g. Tanner et al. 1981, figs 24 & 25; Macdonald & Tanner 1983) although there is no The Barff Point Member age control on the SBF sediments. In these discussions, the role of the Barff Point Member is Possible sources of non-volcanic clasts in the CBF have critical. It includes detrital modes intermediate between the seldom been mentioned. The presence of continental debris SBFand CBF, and also has'intermediate' sedimentologic in theSBF is explained by having this unit (and, by characteristics. Its metamorphic and structuralposition, implication, the Ducloz HeadFormation and possibly the thickness and palaeocurrent orientation clearly need further

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study to confirm that this unit is in fact the link between the has occurred (e.g. Spencer 1984). A corollary of this is that two main units. the CBF has since been back-tilted down to the south-west, resulting in folding of the thrustplane and divergence of CBF and SBF sediments-a single source area? axial plane attitudes in the CBF and SBF as demonstrated by Stone (1980) and Tanner & Macdonald (1982). More Previous authors have consistently emphasized the detailed study of the fault zone is required to substantiate differences between the two units, and have played down this speculation. any similarities. While the end-memberpetrographies are undoubtedly different (Fig. 2) the publishedpetrographic data (with somereservations) and our own observations Comparisons with other regions show thatthere is overlap in composition and apparent Assuming that the existing models for South Georgia as an continuity in structural and metamorphic history. The data island arc-back arc basin or marginal basin sequence are presentedabove are consistent with an interpretation that correct, then the problem of SBF-CBF relationships can be the SBF represents a deeper portion of a single sedimentary consideredin the light of other marginal or arc-related pile made up largely of CBF lithologies. sequences aroundthe Pacific margin of Gondwanaand Most clastic lithologies are common to both units; the elsewhere. Similar comparisons have beendrawn (for compositional contrasts could result from metamorphic sedimentological aspects) by Macdonald & Tanner (1983), accentuation of differing proportions of the clastic e.g. with the Grenada Basin (Sigurdsson et al. 1980). constituents, plus variations in geology within a single but In New Zealand, arc-derived sedimentsform an complex source terrane. The overall compositional trend, if importantpart of a Permian-Mesozoic Orogen.Arc and it is a single pile, is from relatively silicic sediments at the marginal basin sediments (Brook Street terrane, Coombs et base (SBF)through progressively less silicic sediments al. 1976b), afore-arc basin (the Maitai Terrane, Landis (north-west South Georgia; Barff Point Member of CBF) to 1974,1980) and an associated trench slope/trench deposit the little-deformed and most mafic CBF in the south (e.g. (the Caples terrane)are preserved (Carter etal. 1978; Cape Darnley area; Tanner et al. 1981). Note, however, that Howell 1980; Bishop et al. 1982). In the Maitai and Caples acid volcanic, coarse-grainedgranitic, and metamorphic terranes, which were probablyderived from the adjacent debris is still present at the structurally highest part of the island arc, there are distinctive petrographic and sedimen- CBF (Fig. 4), and that white mica and mica-bearing clastic tologic units which can be likened to the SBF-CBF couplet. debris is present throughout (Thomson et al. 1982). In particular, a late Mesozoic arc-derived sequence in the This sequence can be explained by erosion of a complex North Island of New Zealand, the Waipapa terrane (Mayer sialic source and overlying evolved calc-alkaline volcanic 1969; Skinner 1972; Sporli 1978), shows remarkable arc. Similar progressive unroofing of ensialic arc terranes is similarity to SouthGeorgia sediments. In most of these described by Korsch (1984), Stewart (1977) and MacKinnon examples, geochemical variation of most elements between (1983). This process can be seen in erosion of many units is atleast as wide as forthe CBF-SBF couplet. circum-Pacific arcs today (e.g. Japan). Compositional range with respect to sodium and calcium is shown for some of these terranes in Fig. 6. The range of calcium content in the New Zealand examples is comparable The CBF-SBF contact to the combined spread of CBF and SBF data,noted above Juxtaposition of the two unitshas occurred along a as one of the principal geochemical distinctions of CBF and low-angle (5"-25") fault (Aitkenhead & Nelson 1962; SBF. Likewise the variation in Caples compositions as Dalziel etal. 1975; Stone 1980; BAS 1984). The fault is presented on the discriminant function diagram (Fig. 7) is as marked by abrupt changes in foliation or bedding attitude in greatas theSBF and CBF combined.Petrographic most places (Dalziel etal. 1975; Stone 1980), although in variations between some of the formations of the Maitai and some areas it is harder to detect (Craw & Turnbull 1986). Caples terranes,and inparticular within the Waipapa The fault has apparently been folded as it dips to the south, terrane, are similar to those within and betweenthe SBF south of Cumberland Bay, and to the north at the endof the and CBF (Fig. 9, cf. Fig. 2). Although these examples are Barff Peninsula (Stone 1980). Structures in the SBF beneath from fore-arc settings, they serve to illustrate the range of are also divergent(e.g. Tanner 1982; see Fig. 8). As the compositions that can bederived from a complex arc fault post-dates the two earliest phases of deformation in the terrane. SBF and CBF,it may well havebeen folded during the In all cases, the units are made up of a single sequence period which affected the enclosing structures. (as far as is known) and have been derived from a single, Stone (1980) suggests that movement on this fault was in although complex, island arc terrane. Other examples from a reverse sense, towards the NE, although this requires that the Pacific rim arethe New England area of Australia movement on the fault was ina normal sense near Barff (Cawood 1983; Korsch 1984) and Neogene sediments of the Point. Observations by the authors can neither confirm nor western Bering Sea (Stewart 1977; see Fig. 9). All of these deny this movementsense. Whether or not the fault is a areas reflect progressive unroofing of complex evolved thrust or a normal fault depends on how the fault plane is ensialic volcanics arcs (Sporli 1978; Cawood 1983; unfolded. Discussions above imply that the SBFrepresents a MacKinnon 1983; Korsch 1984). deeper portion of the sedimentary sequence which has the CBF at the top.If this is so, then the fault between the units hasyounger rocks structurally overlying olderrocks. A Tectonic implications normal sense of movement could be inferred, therefore. If the SBF and CBF, and the Ducloz Head Formation, the Similar structures in other orogenic belts are currently being Barff Point Memberand possibly theCooper Bay interpreted as normal faults along which tectonic unroofing Formation, are acceptedas having originally been in

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zone' of Suarez & Pettigrew 1976) or ina wide basin, analogous to the Weddell Sea. Deformation of the sequence probably occurred in response to overthrusting of the upper portions of the sequence onto the lower portions of the pile during compressional tectonicsassociated with subduction to the east of the arc. In such a deformation regime, open folds with moderately steep axial surfaces would be expected in the structurally higher parts,and tight folds with

WARAPA TERRANE ,

Fig. 9. Summary QFL detrital modes from New Zealand metasedimentary terranes,and from the Bering Sea. Compare with Fig. 2, and with similar trends shown in Fig. 7. Waipapaterrane modes from Mayer (1969) and Skinner (1972); Maitai data from Craw (1979); MacKinnon (1983); and unpublished data from D. D. Ritchie and D. H. Pillai; Caples data from Turnbull (1979), MacKinnon (1983) and unpublished data from K. Pound and N. Becker; Bering Sea modes from Stewart (1977). (Note: alteration and deformation effects have probably induceddata scatter, and data from Turnbull (1979) may over-estimate quartz content (MacKinnon 1983). Vitric-ash modesare omitted from Stewart's (1977) data field.)

sequence within aback-arc basin, thenfurther inferences can be drawn about thesource area. It was probably a single geochemically evolved volcanic island arc sequence which formed on a remnant, or remnants, or continental crust split -0 off the Gondwana margin during development of a marginal 9 basin (Dott et al. 1982; Storey & Macdonald 1984). This 0 remnant may have included TobiferaFormation, thus c providing a local source for the silicic volcanic clasts in the Tl SBFand that component of theCBF; e.g. Tanner et al. c (1981, figs24 & 25). It could also haveformed on a 0 basementcomposed of earlierarcs and arc-derived sediments such as is now seen on the Antarctic Peninsula 0 (Suarez 1976; Saunders et al. 1982). Remnants of such 0 crustal material are seen on South Georgia as the Drygalski m FiordComplex (Storey et al. 1977; Tanner & Rex 1979; Tanner 1982; Storey 19836). 9 We suggest, therefore,that acontinuous sedimentary z sequence,from the SBF through to the CBF, formed by erosion of an evolving ensialic arc. The presence of tuffs in both units suggests thatthe arc was active throughout deposition of the sequence. The source may well have been a continuous or near-continuous belt of rocks joining South America and Antarctica during Mesozoic subduction of the Pacific Plate in this region (Fig. 10; cf. Suarez 1976; Tanner Fig. 10. Pacific margin of Gondwanaland in the Cretaceous, & Rex 1979; Cooper et al. 1982). A further implication of modified after Suarez (1976) and Cooper et al. (1982). Speculative this is that South Georgia cannot be used as evidence for or palaeogeographic relationships between South Georgia andsome against the presence of a wide or narrow marginal basin to other, similar, arc-derived sedimentary sequences mentionedin the the (present) north-east of the island. The single sequence text are shown. Present-day outlinesof Pacific oceanic crust and proposed here could equally well have formed in a narrow Antarctic-South American crustal blocksare shown by long and elongate trough (e.g. Macdonald & Tanner 1983) (the 'rift short dashes, respectively.

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low-dipping axial surfaces would form in the more ductile, CLAYTON,R. A. S. 1982. A preliminary investigation of the geochemistry of higher-strained, deeper portions of the sequence (Tanner & the greywackes from South Georgia. British Antarctic Survey Bulletin, Macdonald 1982). Later uplift, with associated normal or 51, 89-109. Cooms,D. S., NAKAMURA,Y. & VUAGNAT,M. 197Q. Pumpellyite- thrustfaulting was followed by tilting and folding of the actinolite facies schistsof the Taveyanne Formation near Loeche, Valais, deformed metamorphic pile. Uplift and minor late-stage Switzerland. Journal of Petrology, 17, 440-71. deformation may haveresulted from the extensional -, LANDIS,C. A., NORRIS,R. J., SINTON,J. M., BORNS,D. J. & CRAW,D. 19766. The Dun Mountainophiolite belt, New Zealand, itstectonic tectonics associated with final break-up of Gondwana in this setting,constitution and origin, with special reference to the southern region in the Tertiary (e.g. Dott et al. 1982). portion. American Journal of Science, 276, 561-603. Finally, it should benoted that all the units discussed COOPER,R. A., LANDIS,C. A., LEMESURIER,W. E. & SPEDEN,I. G. 1982. from South Georgia are assumed to have formed adjacent to Geologichistory and regional patterns inNew Zealand and West each other,and the possibility of large(hundreds of Antarctic-theirpaleotectonic and paleogeographicsignificance. In: CRADDOCK,C. (ed.) Third Symposium on Antarctic Geology and kilometres) lateral movements between these ‘terranes’ has Geophysics. Madison, Wis., 43-54. notbeen considered, although lateral movement onthe CRAW, D. 1979.Melanges and associated rocks, Livingstone Mountains, Cooper Bay Dislocation was mentioned by Tanner et al. Southland, New Zealand. New Zealand Journal of Geology and (1981). Such lateralmovements may be proven in the Geophysics, 22, 443-54. future, when palaeomagnetic data become available. - & TURNBULL,I. M. 1986. Geological observations in the Ross Glacier area, South Georgia. British Antarctic Survey Bulletin, 71, 1-10, However, we believe that the CBF and SBF are sufficiently DALZIEL,I. W. D., Don, R. H., WINN,R. D. & BRUHN,R. L. 1975. similar to be considered part of the same ‘terrane’. Tectonic relations of South Georgia Island to the southernmost Andes. Geological Society of America Bulletin, 86, 1034-40. DICKINSON,W. R. 1970. Interpreting detrital modes of graywacke and arkose. We are grateful to the following individuals and institutions which Journal of Sedimentary Petrology, 40, 695-707. aidedthe New Zealand South Georgia Expedition, during which Don, R. H. 1974.Paleocurrent analysis of severelydeformed flysch-type the field work for this study was done: University of Otago; N.Z. strata4 case study from South Georgia Island. Journal of Sedimentary UniversityGrants Committee; N.Z. Department of Scientific & Petrology, 44, 116673. Industrial Research; N.Z. Lotteries Board; British Forces Falkland -, WINN,R. D. & SMITH, C. H. L. 1982. Relationship of late Mesozoic andEarly Cenozoic Sedimentation to the tectonicevolution of the Islands; Trans-Antarctic Association; Mt Everest Foundation; N.Z. southernmost Andes andScotia arc. In: CRADDOCK,C. (ed.) Third Alpine Club. The other members of the expedition are thanked for Symposium on Antarctic Geology and Geophysics. Madison,Wis., their help and N. Becker, K. Pound, D. H. Pillai and D. D. Ritchie 193-202. generouslyprovided data from their unpublished theses. Discus- FERGUSON,D. 1915. Geological observations in South Georgia. Transactions sions with, and draft criticisms from, C. A. Landis, D. G. Bishop, of the Royal Society of Edinburgh, 50, 797-814. W. A. Watters, D. I. M.Macdonald, J. L. Smellie,B. C. Storey FRAKES,L. A. 1966.Geologic setting of South GeorgiaIsland. Geological Society of American Bulletin, 1463-8. and I. W. D. Dalziel greatly improved the presentationof ideas and 77, HALLNER,E. & SCHURMANN, K. 1966. Stabilityof metamorphic amphiboles: clarity of themanuscript, asdid two thorough reviews by the tremolite-ferroactinolite series. Journal of Geology, 74, 322-31. anonymous referees. HOWELL,D. G. 1980. Mesozoic accretion of exotic terranes along the New Zealand segment of Gondwanaland. Geology, 8, 4m-91. References JAMIESON,R. A. & CRAW,D. 1987. Sphalerite geobarometry in metamorphic terranes: an appraisal with implications for metamorphic pressure in the AITKENHEAD, N.& NELSON,P. H. H. 1962. The geology of the area between Otago Schist. Journal of Metamorphic Geology, 5, 87-100. CumberlandWest Bay and Cape George, South Georgia. British KORSCH.R. J. 1984. Sandstone compositions from the New England Orogen, Antarctic Survey Preliminary Geology Report, l5 (unpublished). eastern Australia:lmplications for tectonicsetting. Journal of BELL,C. M., MAIR,B. F. & STOREY, B.C. 1977. The geology of part of an Sedimentary Petrology, 54, 192-211. islandarc-marginal basin system in southern South Georgia. British KRISTMANNSDOTTIR,H. 1982. Alteration in the IRDP drill hole compared to Antarctic Survey Bulletin, 46, 101-27. other drillholes in Iceland. Journal of Geophysical Research, 87, BISHOP,D. G., BRADSHAW,J. D. & LANDIS,C. A. 1982. Provisional terrane 6525-31. map of South Island, New Zealand. In: HOWELL,D. G., JONES, D. L., LANDIS,C. A. 1974. Paleocurrents and composition of lower Bryneira Strata COX, A. & NUR,A. (eds) Proceedings ofthe Circum-Pacific Terrane (Permian) at Barrington Peak, northwest Otago, New Zealand. New Conference. School of Earth Science, Stanford University, 515-21. Zealand Journal of Geology and Geophysics, 17, 799-806. BRITISHANTARCTIC SURVEY (BAS) 1984. South Orkney Islandswith South -1980. Little Ben Sandstone, Maitai Group (Permian): nature and extent Georgia and South Sandwich Islands. British Antarctic Survey Geologica[ in the Hollyford-Eglintonregions, South Island, New Zealand. New Map 1:5oOoW. British Antarctic Survey, Cambridge. Zealand Journal of Geology and Geophysics, 23, 551-68. - 1987. Geological Map of South Georgia I :250000. BritishAntarctic LETERRIER,J., MAURY,R. C., THOMSON,P,, GERARD,D. & MARCHAL, M. Survey, Cambridge (in press). 1982.Clinopyroxene composition as a method of identification of the BRUHN,R. L. & DALZIEL,I. W. D. 1977. Destruction of the early Cretaceous magmatic affinities of paleo-volcanic series. Earth and Planetary Science marginalbasin in the Andes of Tierra del Fuego. In: TALWANI, M.& Letters, 59, 139-54. PITMAN,W. J. (eds) Island Arcs,Deep-sea Trenches and Back-Arc MACDONALD,D. I. M. 1986. Proximal to distal sedimentological variation in a Bains. American Geophysical Union, Washington, D.C., 395-405. linear turbidite trough: implications for the fan model. Sedimentology, -, STERN,C. R. & DE WIT, M. 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Received 29 June 1987; revised typescript accepted 16 November 1987.

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