Tectonic Relations of South Georgia Island to the Southernmost Andes

I.W.D DALZIEL Lamortt-Doherty Geological Observatory and Department of Geological Sciences, Columbia University, Palisades, New York 10964 R H DOTT IR) R d' WINN' JR ) Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 R. L. BRUHN Lamont-Doherty Geological Observatory and Department of Geological Sciences, Columbia University, Palisades, New York 10964

ABSTRACT 1, 2). In Tierra del Fuego a comparable time Andean-West Antarctic Cordillera and the was spent by Winn and Bruhn on the north Scotia Arc. Rocks on South Georgia Island at the side of the Beagle Channel, later in 1973, eastern end of the North Scotia Ridge are and Dott, Winn, and Bruhn spent six weeks GENERAL GEOLOGY no older than late Mesozoic. The Cumber- south of the channel in May and June 1974. land Bay and Sandebugten graywacke and The field work formed part of a United South Georgia Island, although more mudstone sequences there are comparable States Antarctic Research Program project than 1,500 km distant from the South in general lithology and structural style to to study the tectonic evolution of the American continent (Fig. 1), consists of the the Lower Cretaceous Yahgan Formation of the Beagle Channel area in southernmost South America. The Cumberland Bay South TO'" America rocks, which form most of South Georgia Island, were thrust northeastward over the Sandebugten sequence. The Cumberland Falkland Bay and Yahgan sequences contain Cre- Islands taceous fossils, whereas the Sandebugten rocks are unfossiliferous. The dominant dispersal of Cumberland Bay detritus was toward the northwest. The Sandebugten dispersal pattern was more complex but was dominated by a south- directed component. In Early Cretaceous Black and time, however, the South Georgia mic- Shag Rocks rocontinent apparently was attached to South America along the present southern Scotia Sea South margin of the Burdwood Bank. The Cum- Georgia berland Bay, Sandebugten, and farther westward along strike, the Yahgan, apparently were deposited in a marginal small ocean basin between a calc-alkalic volcanic arc built on a sliver of old South American S * è continental crust and the main part of the South m South American continent from which the Sandwich * Islands sliver moved away. According to this in- terpretation, deformation of the sediments occurred when the arc moved back toward the continent in middle Cretaceous time,

and the basin was closed and uplifted with 20°W the arc. Key words: areal geology, conti- V//A Andean - West Antarctic Cordillera nental drift, Cretaceous, paleogeography, 60°S I \ Extra -Cordilleron sedimentary basins sedimentary petrology, tectonics. Hinge line of folds in Lower Cretaceous sedimentary rocks $ Fold vergence INTRODUCTION * Active and recent volcanoes —- 1000-meter submarine contour Detailed geologic observations were made during a 6-wk period in January and February 1973, in the Cumberland Bay-Stromness Bay area of the island of South Georgia, and limited data were col- lected at Bird Island and Bay of Isles (Figs. Figure 1. Tectonic subdivisions of the northern limb of the Scotia Arc.

Geological Society of America Bulletin, v. 86, p. 1034-1040, 9 figs., July 1975, Doc. no. 50720

1034

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/7/1034/3433433/i0016-7606-86-7-1034.pdf by guest on 02 October 2021 TECTONIC RELATIONS OF SOUTH GEORGIA ISLAND TO SOUTHERNMOST ANDES 1035

same type of tightly folded upper Mesozoic Rocks platform are an eastward continua- quartz-rich, probably late Paleozoic Trinity flyschlike sedimentary rocks as are found tion of part of the Andean orogen and ap- Peninsula "Series" of the Antarctic Penin- on the nearest part of the continent. Seismic pear to represent a part of the North Scotia sula (Tyrrell, 1945; Adie, 1964). The rela- velocities across the North Scotia Ridge are Ridge — a microcontinent now missing tion between the Sandebugten and the similar to those of the mainland (Ewing and south of the Burdwood Bank (Dalziel and Cumberland Bay sequences has been inter- others, 1971). There is a prominent and ex- Elliot, 1971, 1973; see Fig. 1). preted as "probably unconformable" but ceptionally penetrative slaty cleavage in all The Sandebugten and the Cumberland with "sub-parallel thrusting in the contact sedimentary rocks, except in the southwest- Bay sequences of flyschlike strata might bet- zone" (Adie, 1964, p. 122; see also Skid- ernmost part of the island (Trendall, 1959). ter be thought of as members of one forma- more, 1972). The sedimentary rocks at the southeastern tion (Trendall, 1959). Their field distinction Correlation of the Sandebugten rocks end of the island are metamorphosed and is one of diametrically opposed fold ver- with the Trinity Peninsula "Series" implied intruded by dioritic igneous rocks (Fig. 2). gence, with the Sandebugten sequence verg- correlation also with the lithologically iden- The South Georgia sedimentary rocks ing southwest and the Cumberland Bay tical Miers Bluff Formation of the South closely resemble upper Mesozoic strata on rocks verging northeast away from the Shetland Islands, the Graywacke-Shale the Pacific side of the Andean high Cordil- Scotia Sea like the Yahgan Formation in Formation of the South Orkney Islands, lera in southernmost Argentina and Chile South America (Katz and Watters, 1966; and the (fossiliferous) upper Paleozoic and are of similar age (Wilckens, 1933; Dalziel, 1972; see Fig. 3). The Sandebugten Madre de Dios rocks at about lat 50° S. on Katz and Watters, 1966). There too, graywacke is richer in andesitic detritus. Al- the west coast of Chile (Dalziel, 1969, flyschlike sequences, particularly the Lower tered ash layers and a few flow rocks also 1970, 1971, 1972) and has important con- Cretaceous Yahgan Formation of the occur in the Cumberland Bay sequence sequences for reconstruction of Gondwana- Beagle Channel area (Fig. 1), were depos- (Trendall, 1953, 1959). The Cumberland land (Dalziel and Elliot, 1971, 1973). ited near a volcanic arc terrane and then Bay rocks of Annenkov Island off the severely deformed during the middle Cre- southwest coast (Fig. 2) have yielded well- STRUCTURAL GEOLOGY AND taceous Andean orogeny. As on South preserved Lower Cretaceous (Aptian) fos- FIELD RELATIONS Georgia Island (Fig. 3), the dominant ver- sils (Wilckens, 1947), but the Sandebugten gence (direction of overturning) of the folds rocks have yielded only trace fossils. The The Cumberland Bay and Sandebugten was away from the present Scotia Sea and lithology of the latter and its structural po- sequences on the northeast side of South Drake passage (Dalziel, 1972). South Geor- sition beneath the Cumberland Bay (Fig. 3) Georgia Island are deformed by tight gia and the neighboring Black and Shag led to the tentative correlation with the asymmetric folds with planar limbs and

Bird Island / Bay of Isles

Stromness Bay

Cumberland Bay / .1 oD NE

•DP\ % * Mt. ••: ••; Paget; g Annenkov

Undine South Harbour Cumberland Bay sequence of graywacke and shale with interbedded basaltic and spilitic lava (v); andesitic breccia on Annenkov Island (4). Sandebugten sequence of graywacke and shale. 1 Igneous complex 'I - .;>

SW NE Line of section 5 1015 20 25 V-'/rNv DP Dartmouth Point Kilometers BP Borff Point OH Ocean Harbour

Figure 2. Geologic sketch map of South Georgia Island (after Trendall, 1953). Line of section shown for Figure 3.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/7/1034/3433433/i0016-7606-86-7-1034.pdf by guest on 02 October 2021 1036 DALZIEL AND OTHERS

narrow hinge zones (Fig. 3). Large-scale (1972). Regional tectonic relations suggest northeast-directed thrust sheet overrode folds with limb separations of hundreds of that, if it were, a pre-Cretaceous (Gondwa- the south-southwest—directed structures meters predominate. Even thick sandstone nian) fabric should be present (Dalziel and (Fig. 3). layers are cut by strong axial-planar slaty Elliot, 1973). cleavage in which clasts have been flattened Additional reasons for our conclusion are SEDIMENTOLOGY (Dott, 1974, Fig. 1). Hinge lines and axial as follows: planes trend west-northwest. As reported 1. The trend of mineral and clast elonga- The Sandebugten and Cumberland Bay by Trendali (1953, 1959), the folds in the tion lineation within the slaty cleavages is sequences have characteristics typical of Cumberland Bay rocks verge north- nearly identical in the two sequences (Fig. classical European flysch. They are com- northeast, while those in the Sandebugten 4). The trend of dominant crenulation posed of very evenly stratified alternations verge dominantly south-southwest (Fig. 3), cleavages is also identical; only the dip di- of sandstone and slaty mudstone. Graded so that the axial planes and slaty cleavages rection of the axial planes and slaty cleav- bedding occurs in most of the sandstone dip respectively south-southwest and age and the plunge direction of the linea- beds. The average ratio of sandstone to north-northeast (Figs. 3, 4). Rocks on Barff tions differ. mudstone varies widely, but overall, sand- Point (Figs. 2, 4) have a north-northeast 2. At the thrust contact, the south- stone composes about 50 to 70 percent of vergence and have been considered by southwest-dipping slaty cleavage in the total volume. Sandstone beds range in geologists of the British Antarctic Survey to the Cumberland Bay rocks becomes thickness from a few centimeters to nearly 2 belong to the Cumberland Bay sequence mylonitic in character and clearly truncates m. Some fine-pebble conglomerate beds (Skidmore, 1972). Farther south from the north-northeast—dipping slaty cleavage are present in the Cumberland Bay. Barff Point, however, the axial planes and in the Sandebugten sequence (Fig. 3, inset). Micro—cross-lamination, convolute lamina- cleavage in rocks that are undoubtedly However, the cleavage in the Cumberland tion, and sole marks (chiefly groove marks) Sandebugten swing through the vertical so Bay near the contact is refolded by open are also common. We interpret most of the as to dip south-southwest as do the axial asymmetric folds with the same vergence sandstone beds to be turbidites, which ac- planes of the folds in the Cumberland Bay (south-southwest) as the dominant folds in cumulated rapidly near an active volcanic sequence (Fig. 3). Therefore, we see neither the Sandebugten. arc. structural nor lithologic reasons to corre- 3. Apparent interstratification of some The Cumberland Bay rocks are coarser late rocks on Barff Point with the Cumber- Cumberland Bay—like graywacke occurs and richer in volcanic detritus than the land Bay sequence. The slaty cleavages in within the Sandebugten, and nothing sug- Sandebugten rocks, but in terms of both sequences are deformed by anas- gestive of a basal conglomerate was found. sedimentary structures and deformation, tomosing and, commonly, conjugate crenu- 4. Rocks lithically comparable to the the two are nearly identical. Our prelimi- lation cleavages that also strike west- Sandebugten are interbedded with Cumber- nary petrographic analyses, however, sup- northwest. Associated folds are rare. land Bay-like sedimentary rocks in the bot- port Tyrrell's (1945) conclusion that the A well-exposed and previously unde- tom of the Lower Cretaceous Yahgan For- two sequences differ markedly in composi- scribed contact indicates that the Cumber- mation of southernmost South America. tion. The Sandebugten sequence contains land Bay rocks were thrust over Sandebug- Although the age of deposition of the an average of 44 percent quartz, 20 percent ten ones at Dartmouth Point (Figs. 3, 5). Sandebugten strata is not proven, it seems feldspar, and 36 percent lithic fragments, However, even without considering the pet- to us that both sequences were deformed at mostly fine-grained siliceous volcanic rocks rographic evidence, it is clear that the San- the same time, even though they were over- (Fig. 6). The Cumberland Bay sequence debugten rocks were not deformed before folded in different directions. The slaty contains 5 percent quartz, 20 percent the Cumberland Bay sequence. Hence, it is cleavage in the Sandebugten rocks also be- feldspar, and 75 percent lithic fragments, unlikely that the Sandebugten sequence comes mylonitic near the contact, thus which are predominantly andesitic in com- could be as old as late Paleozoic, as sug- perhaps conjugate shear zones (overthrusts) position (Fig. 6). Most point counts for gested by Adie (1964) and Skidmore developed before finally the north- Sandebugten sandstone plot completely

Allardyce Range Mt. Paget

Undine South Harbour Dartmouth Point Barff Point Annenkov Island

^^—NE

CUMBERLAND BAY SEQUENCE "" SANDEBUGTEN SEQUENCE

ENLARGEMENT OF DARTMOUTH POINT CONTACT ZONE

Cumberland Bay Sequence Mi -Crenulation cleavage Sandebugten Sequence Thin (2-5cm.) layer Kilometers Basaltic and spilitic lava of secondary Andesitic breccia 0 12 3 4 5 Kilometers mylonite (•—) Igneous complex 1-0 g^ Mylonitic zone ^ Facing direction Thrust with movement direction Mylonitic- Mylonitic

Figure 3. Structural cross section of South Georgia Island. The data are based on observations between Barff Point and the southwest end of Dartmouth Point, distant observations of the area between Dartmouth Point and Mount Paget, a photograph by Trendall (1953, Pl. H, fig. 1) of the coast of Undine South Harbour, and reports from Annenkov Island by Trendall (1953, 1959) and Timothy Pettigrew (British Antarctic Survey, 1973, written commun.). Structure shown between Dartmouth Point and Barff Point is extrapolated along strike from the west.

Location map

Ocean Harbour

pò Glacie" ¡SM

{ ] Cumberland Bay sequence of graywackes and shales. Denser ornament indicates areas mapped by authors. 2V Dip and strike of slaty cleavage and axial | -| Sandebugten sequence of graywackes and shales. surfaces of main phase asymmetric folds. Thrust contact - teeth on upper plate. ^r" Plunge of bedding/slaty cleavage intersection and hinge line of main phase folds. Plunge of lineation (mineral elongation). Figure 4. Geologic map of the Cumberland Bay area of South Georgia Island.

apart from the Cumberland Bay specimens; Paleocurrent analysis of the South Geor- well exposed, the average trend of grooves thus, the sequences were derived from dif- gia sedimentary rocks was hampered by is at an angle of 40° or 50° to the average ferent sources. Rare Cumberland Bay-like weathered outcrops, lichen cover, and the bedding and cleavage intersection. So- andesitic sandstone (Fig. 6), as well as a very penetrative cleavage that obscured called flame structures were not used be- bimodal paleocurrent pattern in the San- current features. Most troublesome was the cause they are predominantly gravity-load, debugten, leads us to conclude that the two widespread development of elusive rather than current, features. Moreover, units are closely similar in age and may rep- psuedo—sole marks produced by small off- most of the structures on South Georgia Is- resent partly intertonguing lithofacies. sets of sandstone and mudstone contacts at land are the result of bedding and cleavage Conversely, sandstone of the Trinity Penin- bedding and cleavage intersections. Many intersections. As seen in cross section, these sula "Series" does not overlap composi- of the resulting structures look like groove are a series of en echelon offsets of sand- tionally sandstone of the Sandebugten (Fig. or flute marks on bedding surfaces, and stone and mudstone contacts with flame- 6). As in many volcaniclastic sedimentary careful comparison of orientation of the like streaks of dark mudstone sheared up- rocks of intermediate to mafic composition, marks with those of local bedding and ward along cleavage planes (see Dott, prehnite is prevalent in the Cumberland cleavage intersections is necessary to dis- 1974, Fig. 1). Bay, but it is negligible in the much more criminate between the two. In the Strom- Statistical analysis of the orientation data siliceous Sandebugten graywacke. ness Bay area, where true sole marks are for sole marks and micro—cross-laminae shows clearly that dominant sediment dispersal of Cumberland Bay detritus was to- TABLE SUMMARY STATISTICS FOR ORIENTATIONS OF CURRENT STRUCTURES ward the north-northwest in the region flf Mean vector Vector Angular Rayleigh . readings azimuth magnitude deviation* significance TABLE 2. SUGGESTED STRATIGRAPHIC CORRELATIONS BETWEEN (r = 0-1) TIERRA DEL FUEG0 AND SOUTH GEORGIA ISLAND

Sandebugten rooks Tierra del Fuego South Georgia (all cross sets) Island South mode 136 200° 0.81 ±35° 10-20 North mode 78 16° 0.93 ±21° 10-2. , South of Cumberland Bay* I Beagle* I (mainly) Cumberland Bay rooks Lower Yahgan Cross sets 513 332° 0.25 ±70° 10"15 Cretaceous Formation 2 Groove marks 225 300°/120° 0.77 ±38° 10" ' ? 1 North of Sandebugten V Beagle* * Analogous to linear standard deviation, but calculated for a circular distribution by the equation e B Z(l-i*) radians. a Upper Silicic volcanic complex* ? The value for a uniform or nonoriented circular distribution is ±81.03°. Jurassic t The probability of a greater magnitude (r) due to chance alone (see Curray, 1956). The statistical F test also shows all four mean azimuths to be significant even at the 0.005 level. * Index fossils present.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/7/1034/3433433/i0016-7606-86-7-1034.pdf by guest on 02 October 2021 1038 DALZIEL AND OTHERS

contain Lower Cretaceous fossils. The structural style and metamorphism of the Yahgan on northern Navarino Island are virtually identical to those of the Cumber- land Bay rocks on northeastern South Georgia, and in both areas the folding and metamorphism diminish southward. Con- spicuous andesitic breccia and some flow rocks also occur in the higher parts of both sequences. Finally, we found that the dominant current transport of the andesitic Yahgan clastic material south of the Beagle Channel clearly was from south to north (Fig. 8). Thus the reconstruction of the northern limb of the Scotia Arc (Dalziel and Elliot, 1971, 1973) with the South Georgia microcontinent situated immediately east of the Beagle Channel and attached to the southern margin of the Burdwood Bank (Fig. 8) is supported. The trend of fold- hinge lines in the Yahgan Formation and the South Georgia sedimentary rocks suggests that essentially no rotation of the microcontinent has occurred relative to the Figure 5. Thrust contact between the overlying Cumberland Bay rocks above and the Sandebugten Burdwood Bank. rocks below looking west, Dartmouth Point, Cumberland East Bay, South Georgia Island. Paleocurrent data show sediment dispersal from the southeast in the Cumberland studied by us (Fig. 7; Table 1). Sandebugten Lithologic as well as age equivalence of Bay sequence and dominantly from the dispersal was more complex, showing a the Cumberland Bay sequence and at least north in the Sandebugten sequence (Fig. 7). southerly as well as weak northerly compo- part of the Yahgan Formation of the Beagle Most Sandebugten graywacke is relatively nent (Fig. 7). Our results reinforce quantita- Channel area of South America has long rich in quartz yet contains significant tively the paleocurrent inferences of Tren- been suspected. Our field observations feldspar and abundant felsic volcanic frag- dall (1959) for ten localities in Cumberland strengthen this view (Table 2). The Yahgan ments. The siliceous composition — cou- Bay rocks (Fig. 7, solid arrows). The results appears similar to the Cumberland Bay se- pled with dominant current transport from do not, however, agree very closely with quence in the field, even to the presence of the north — cannot be readily explained in Frakes' (1966) small number of observa- distinctive white quartz-prehnite layers, the island's present position because no tions from two localities. Because of the which appear to be recrystallized volcanic source for this distinctive sandstone occurs strong deformation of the rocks (Dott, ash. The lowest Yahgan on the north side of near the present South Georgia Island. Our 1974), only the general conclusion that the the Beagle Channel overlies Upper Jurassic proposed restoration, however, is sup- two sequences were influenced by two dis- silicic volcanic rocks (Table 2). The Yahgan ported by the presence of widespread Upper tinctly different current regimes seems differs in that it contains thinner and finer Jurassic rhyolite-quartz latite rocks in justified. Postulation of geostrophic, tidal, graywacke beds, which are identical pet- southern Tierra del Fuego and on Staten or other secondary current influences does rographically with quartz-rich Sandebugten Island (Fig. 8). Petrographic comparison not seem warranted at this time. graywacke (Fig. 6). The higher part of the suggests that practically every grain type in Yahgan south of the Beagle Channel, on the the Sandebugten graywacke has a counter- SUMMARY AND DISCUSSION other hand, is characterized by andesitic part in the silicic volcanic rocks, and the graywacke like that of the Cumberland Bay latter would have lain just north of South Previous suggestions (Trendall, 1953, sequence (Katz and Watters, 1966); both Georgia in its restored Lower Cretaceous 1959; Adie, 1964; Skidmore, 1972) that the Sandebugten sequence is correlative with the probable upper Paleozoic Trinity Peninsula "Series" on the Antarctic Penin- YAHGAN FORMATION sula implied that the Cumberland Bay TRINITY SIDE OF Figure 6. Triangular plot com- graywacke and shale and intercalated vol- PENINSU BEAGLE CANAL paring sandstone compositions of canic rocks were likely deposited on conti- "SERIES' the Cumberland Bay (19 of our slides and 28 from Skidmore, nental crust. Moreover, if rocks equivalent 1972), Sandebugten (21 slides), to the Trinity Peninsula "Series" were pres- lower Yahgan Formation on the ent, then South Georgia Island had to be north side of the Beagle Channel, considered in reconstructing the early SANDEBUGTEN Argentina (7 slides), and Trinity Mesozoic Gondwanide fold belt of Du Toit Peninsula sandstone (four data (1937), incorporating the Sierra de la Ven- points as averaged from point tana fold belt of Argentina, the Cape fold counts of 33 slides) at Cape Le- ou on belt of South Africa, and the fold belts in LAND 8 P'' the Antarctic Peninsula the Ellsworth Mountains, the Pensacola (after Halpern, 1965). The single BAY - circle within the Cumberland field Mountains, and the Antarctic Peninsula. THIS STUDY represents a sample of andesitic Hence, our recognition that the Cumber- sandstone from the Sandebugten land Bay and Sandebugten sequences are sequence. (At least 300 grain probably part of a single upper Mesozoic CUMBERLAND BAY points were counted per thin succession is important. -SKIDMORE (1972) section.)

position. Moreover, preliminary micro- scopic examination of Lower Cretaceous Yahgan sandstone interstratified with thin silicic volcanic layers near the base of the formation on the north shore of the Beagle Channel (Fig. 8) indicates compositional identity with Sandebugten graywacke (Fig. 6). Although age equivalence is not proved, the available evidence suggests that the Sandebugten is latest Jurassic or earliest Cretaceous in age, or both (see Table 2). Both the Cumberland Bay and most of the Yahgan graywackes are rich in andesitic detritus. In the Yahgan this detritus in- creases in abundance southward in rocks at higher stratigraphic levels now exposed near the Patagonian batholith. This late Mesozoic batholith was emplaced in sialic rocks forming part of the South American continent. The Yahgan is seen to rest upon Paleozoic metamorphic basement and Upper Jurassic silicic volcanic rocks along the Beagle Channel, but elsewhere the Yahgan overlies an ophiolitic complex sandwiched between the batholith and the Figure 7. Summary of paleocurrent data. Trendall's observations (1959) were apparently qualita- tive; our data were restored structurally with a stereonet and then analzyed by vector summation main part of the continent (Dalziel and (Curray, 1956; see also Dott, 1974). The polar histograms show number frequency; numerals indicate others, 1973, 1974). Hence, apparently total readings (compare Table 1).

Figure 8. Diagram showing the tectonic setting of South Georgia Island in Early Cretaceous time; the map is based on present geography and does not consider tectonic shortening across strike or possible oroclinal bending. Arrows indicate the general dispersal of sandstone types (Tierra del Fuego arrow represents 446 cross-lamina readings from 13 localities; see Fig. 7 and Table 1 for South Georgia data).

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/7/1034/3433433/i0016-7606-86-7-1034.pdf by guest on 02 October 2021 1040 DALZIEL AND OTHERS

SILICIC VOLCANIC ROCKS

CONTINENTAL BASEMENT

BATHOLITH

MAFIC CRUST

PACIFIC CRUST ISLAND ARC LOCAL CONTINENTAL BLOCK STABLE SOUTH AMERICA (DIAGRAMMATIC) Figure 9. Restored diagrammatic cross section showing probable derivation of the Cumberland Bay and Sandebugten sedimentary sequences. The small "continental block" in the marginal basin is diagrammatic; a number of blocks of this type are present.

these sediments, together with the Cumber- Hero for transportation to and from South Sed. Petrology, p. 1166-1173. land Bay and Sandebugten sequences of Georgia Island and in Tierra del Fuego, Du Toit, A. L., 1937, Our wandering continents: South Georgia, were deposited during the respectively. Edinburgh, Oliver and Boyd, 366 p. Late Jurassic(P) and Early Cretaceous in a Ewing, J. I., Ludwig, W. J., Ewing, M., and Eit- geotectonic environment similar to (but treim, S. L., 1971, Structure of the Scotia REFERENCES CITED Sea and Falkland Plateau: Jour. Geophys. probably much smaller than) the present Research, v. 76, p. 7118-7137. Japan Sea; namely, between an island arc Adie, R. J., 1964, Geological history, in Priestley, Frakes, L. A., 1966, Geologic setting of South system overlying the batholith to the south R., Adie, R. J., and Robin, G. de Q., eds., Georgia Island: Geol. Soc. America Bull., v. (Dalziel and others, 1973, 1974) and the Antarctic research: London, Butterworths, 77, p. 1463-1468. continent itself to the north (Fig. 9). The p. 118-162. Halpern, M., 1965, The geology of the General intense deformation visible in all three units Curray, J. R., 1956, The analysis of two- Bernardo O'Higgins area, northwest An- apparently took place when the initial uplift dimensional orientation data: Jour. Geol- tarctic Peninsula, in Hadley, J. B., ed., of the Andean Cordillera occurred as the ogy, v. 64, p. 117-131. Geology and paleontology of the Antarctic: Dalziel, I.W.D., 1969, Structural studies in the island arc moved toward the continent in Am. Geophys. Union, Antarctic Research Scotia Arc: Livingston Island: Antarctic Ser., v. 6, p. 177-209. middle Cretaceous time (Dalziel and Jour. U.S., v. 4, p. 137. Palmer, 1974). The South Georgia mi- Katz, H. R., and Watters, W. A., 1966, Geologi- 1970, Structural studies in the Scotia Arc: cal investigation of the Yahgan Formation crocontinental block was torn from the The Patagonian and Fuegian Andes: An- and associated igneous rocks of Navarino continent and carried eastward at some tarctic Jour. U.S., v. 5, p. 99-100. Island, southern Chile: New Zealand Jour. later time. 1971, Structural studies in the Scotia Arc: Geology and Geophysics, v. 9, p. 323-359. The South Orkney Islands: Antarctic Jour. Skidmore, M. J., 1972, The geology of South ACKNOWLEDGMENTS U.S., v. 6, p. 124-126. Georgia III. Prince Olav Harbour and 1972, Large-scale folding in the Scotia Arc, Stromness Bay areas: British Antarctic Sur- in Adie, R. J., ed., Antarctic geology and The study was principally supported by vey Sei. Repts., no. 73, 50 p. geophysics: Olso, Universitets Forlaget, p. Trendall, A. F., 1953, The geology of South the Office of Polar Programs, National Sci- 47-55. Georgia I: Falkland Islands Dependencies ence Foundation, under Grants GV-19543, Dalziel, I.W.D., and Elliot, D. H., 1971, Evolu- Survey Sei. Rept., no. 7, 26 p. GV-36656, and OPP 21415. We thank tion of the Scotia Arc: Nature, v. 233, p. 1959, The geology of South Georgia Island Philip M. Smith, Mort D. Turner, Robert L. 246-252. II: Falkland Islands Dependencies Survey Dale, and Robert Elder for their assistance. 1973, The Scotia Arc and Antarctic margin, Sei. Rept., no. 19, 148 p. Partial support as part of the International in Nairn, A.E.M., and Stehli, F., eds., The Tyrrell, G. W., 1945, Report on rocks from West Decade of Oceanographic Exploration was ocean basins and their margins. I. The Antarctica and the Scotia Arc: "Discovery" South Atlantic: New York, Plenum Press, p. provided under National Science Founda- Repts., v. 23, p. 37-102. 171-245. tion Grant GX-34410. The opportunity to Wilckens, O., 1933, Der Südantillen-Bogen: Dalziel, I.W.D., and Palmer, K. F., 1974, Pene- Geol. Rundschau, v. 24, p. 320-335. work on South Georgia Island was made by trative deformation and orogenic uplift at 1947, Paläontologische und geologische Sir Vivian Fuchs, former Director of the the southern extremity of the Andes: Geol. Ergebrisse der Reise von Kohl-Larsen British Antarctic Survey. We are indebted Soc. America Abs. with Programs, v. 6, p. (1928-29) nach Süd-Georgian: Abhand- to Dr. R. J. Adie, Deputy Director, and Mr. 705. lungen Senckenbergischen Naturf. Gesell., Derek Gipps, Senior Logistics Officer, for Dalziel, I.W.D., de Wit, M. J., and Palmer, K. F., bd. 474, p. 1-66. their advice and assistance. Mr. Rikky 1973, A fossil marginal basin in the south- Chinn, former Base Commander at South ern Andes: Geol. Soc. America Abs. with Georgia Island, and all the survey personnel Programs, v. 5, p. 589. MANUSCRIPT RECEIVED BY THE SOCIETY MARCH 1974, A fossil marginal basin in the south- 29, 1974 on the island extended a warm welcome. ern Andes: Nature, v. 250, p. 291-294. REVISED MANUSCRIPT RECEIVED SEPTEMBER 10, We are grateful to Captain T. Woodfield Dott, R. H., Jr., 1974, Paleocurrent analysis of 1974 and the crew of RRS Bransfield and to Cap- severely deformed flsych-type strata — A LAMONT-DOHERTY GEOLOGICAL OBSERVATORY tain Norman Deniston and the crew of R/V case study from South Georgia Island: Jour. CONTRIBUTION No. 2204

Printed in U.S.A.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/86/7/1034/3433433/i0016-7606-86-7-1034.pdf by guest on 02 October 2021