ing paleontologic, microstructural kinematic analysis, and Spettstoesser (Eds.), Geology and paleontology of the Ellsworth argon-40/argon-39 and uranium! lead geochronologic inves- Mountains, West Antarctica (Memoir 170). Boulder, Colorado: tigations will constrain more closely the timing and kinemat- Geological Society of America. ics of this deformational event. If deformation resulted from Sporli, K.B. 1992. Stratigraphy of the Crashsite Group, Ellsworth Mountains, West Antarctica. In G.F. Webers, C. Craddock, and J.F. the Ross Orogeny as we suspect, then the structural history of Spettstoesser (Eds.), Geology and paleontology of the Ellsworth the Ellsworth Mountains is much more similar to that of the Mountains, West Antarctica (Memoir 170). Boulder, Colorado: Transantarctic Mountains than previously proposed. Geological Society of America. This research was supported by National Science Foun- Sporli, K.B., and C. Craddock. 1992. Structure of the Heritage Range, dation grants OPP 92-20395 and OPP 93-12040. Ellsworth Mountains, West Antarctica. In G.F. Webers, C. Crad- dock, and J.F. Spettstoesser (Eds.), Geology and paleontology of the Ellsworth Mountains, WestAntarctica (Memoir 170). Boulder, Col- References orado: Geological Society of America. Vennum, W.R., P. Gizycki, V.V. Samsonov, A.G. Markovich, and R.J. Craddock, C. 1983. The East Antarctica-West Antarctic boundary Pankhurst. 1992. Igneous Petrology and Geochemistry of the between the ice shelves: A review. In R.L. Oliver, P.R. James, and southern Heritage Range, Ellsworth Mountains, West Antarctica. J.B. Jago (Eds.), Antarctic earth science. New York: Cambridge Uni- In G.F. Webers, C. Craddock, and J.F. Spettstoesser (Eds.), Geology versity Press. and paleontology of the Ellsworth Mountains, West Antarctica Craddock, C., G.F. Webers, R.H. Rutford, K.B. Sporli, and J.J. Ander- (Memoir 170). Boulder, Colorado: Geological Society of America. son. 1986. Geologic map of the Ellsworth Mountains, Antarctica. Webers, G.F., R.L. Bauer, J.M. Anderson, W. Buggisch, R.W. Ojakan- Geological Society ofAmerica, Map and Chart Series, MC-57. gas, and K.B. Sporli. 1992. In G.F. Webers, C. Craddock, and J.F. Goldstrand, P.M., P.G. Fitzgerald, T.F. Redfield, E. Stump, and C. Spettstoesser (eds.), Geology and paleontology of the Ellsworth Hobbs. 1994. Stratigraphic evidence for the Ross orogeny in the Mountains, West Antarctica (Memoir 170). Boulder, Colorado: Ellsworth Mountains, West Antarctica: Implication for the evolution Geological Society of America. of the paleo-Pacific margin of Gondwana. Geology, 22(5), 427-430. Webers, G.F., C. Craddock, and J.F. Splettstoesser. 1992. Geologic his- Jago, J.B., and G.F. Webers. 1992. Middle Cambrian trilobites from the tory of the Ellsworth Mountains, West Antarctica. In G.F. Webers, Ellsworth Mountains, West Antarctica. In G.F. Webers, C. Crad- C. Craddock, and J.F. Splettstoesser (eds.), Geology and paleontol- dock, and J.F. Spettstoesser (Eds.), Geology and paleontology of the ogy of the Ellsworth Mountains, West Antarctica (Memoir 170). Ellsworth Mountains, WestAntarctjca (Memoir 170). Boulder, Col- Boulder, Colorado: Geological Society of America. orado: Geological Society of America. Webers, G.F., and K.B. Sporli. 1983. Palaeontological and stratigraphic Shergold, J.H., and G.F. Webers. 1992. Late Dresbachian (Idamean) investigations in the Ellsworth Mountains, West Antarctica. In R.L. and other trilobite faunas from the Heritage Range, Ellsworth Oliver, P.R. James, and J.B. Jago (Eds.) Antarctic earth science. New Mountains, West Antarctica. In G.F. Webers, C. Craddock, and J.F. York: Cambridge University Press, New York.
Geologic investigations in the Shackleton Range and Coats Land nunataks, Antarctica IAN W.D. DALZIEL, MARK A. HELPER, FREDERICK E. HUTSON, and STEPHEN W. GRIMES, Department of Geological Sciences and Institute for Geophysics, University of Texas, Austin, Texas 78712
ecent global plate reconstructions for the Neoproterozoic the hypothesis thoroughly. To facilitate a more detailed com- R(Daiziel 1991; Hoffman 1991; Moores 1991) suggest that parison, we spent 6 weeks examining and sampling the rocks prior to the latest Precambrian to Cambrian amalgamation of of these regions. A major goal of the work at both locations was Gondwana, the east antarctic craton was the core of an earlier the collection of well-characterized suites of samples suitable supercontinent, Rondinia, whose break-up gave rise to North for petrologic, isotopic, and paleomagnetic study. America. Our research tests this hypothesis by comparing the Fieldwork during the 1993-1994 field season was con- Precambrian geology of crustal provinces in North America ducted from an LC-130-placed tent camp on Recovery Glac- and East Antarctica that the plate reconstructions suggest ier, approximately 3 kilometers south of Watts Needle in the were once contiguous. The reconstructions start from the Read Mountains of the Shackleton Range (figure). The field premise that two, distinct, Precambrian crustal provinces of party consisted of Ian W.D. Dalziel, Mark A. Helper, Frederick the southwestern United States, the 1.8-1.6 billion-year-old E. Hutson, and Stephen W. Grimes and mountaineers Andy Yavapai-Mazatzal and the 1.3-1.0 billion-year-old Grenville Harris and John Roberts. The camp was supported for 10 days orogens, have equivalents in the Weddell Sea region of the east early in the season by Twin Otter, which provided transport to antarctic craton. Although previous work by British, German, the Coats Land nunataks and allowed 5 days of reconnais- and Russian geologists (cited below) has identified rocks of the sance work in the Shackleton Range in areas remote from the former age range in the Shackleton Range and of the latter in camp. These included reconnaissance study and sampling of nunataks along the Weddell Sea coast in Coats Land (Bertrab sites near the eastern end of the Pioneers Escarpment and the and Littlewood Nunataks), existing data are insufficient to test northern side of the Read Mountains, a site in the central Her-
ANTARCTIC JOURNAL - REVIEW 1994 4 RCOATS C LAG 1 LAND LBN 25 S LESSOR am km MS SR FIF FILCHNER BI ICE RONNE SHELF ICE SHELF ,LL(;GN / MP-P 9O MG FUCHS READ M I0 oi DOME of WN-Ap M) i W4,ov. ? (
A E C OVE R V G L A C I E R
Generalized outcrop map of the Shackleton Range (asterisk shows location of base camp) and its geographic setting (inset, with area of main map outlined). BI—Berkner Island; FIS—Filchner Ice Shelf; HM—Herbert Mountains; LBN—Littlewood and Bertrab Nunataks; MG—Mount Gass; MP—Mount Provender; MS—Mount Sheffield; MW—Mount Wegener; OH—Otter Highlands; PE—Pioneers Escarpment; SB—Stephen- son Bastion; SR—Shackleton Range; WN—Watts Needle; LGN—La Grange Nunataks. bert Mountains, and sites at the northern ends of the Otter In the Shackleton Range, a major focus of study was the and Haskard Highlands and at Mount Sheffield in the La Paleo- to Mesoproterozoic basement gneisses and granitoids Grange Nunataks. Fieldwork during the remainder of the sea- of the Read Mountains and their Neoproterozoic to Lower son was by Ski-Doo and sledge-supported ground traverses Cambrian sedimentary cover (Clarkson 1982; Marsh 1983a, from the Recovery Glacier camp. Ancillary camps established pp. 190-193; Pankhurst, Marsh, and Clarkson 1983, pp. during extended trips included a temporary camp near 176-182; Buggisch et al. 1990). Basement rocks in the Read Mount Wegener in the Read Mountains and a succession of Mountains have previously yielded rubidium-strontium temporary camps established during a 2-week traverse to the whole-rock ages within the range of those of the Yavapai- La Grange Nunataks, Haskard Highlands, and Stephenson Mazatzai province in the southwestern United States and, Bastion. although in part at higher grade, appear lithologically similar. Prior work on the Grenville-age igneous rocks of the The fabric of the gneisses throughout the Read Mountains is Bertrab and Littlewood Nunataks (Eastin and Faure 1971; dominated by a steeply north-dipping mylonitic foliation Marsh and Thomson 1984) suggests that these rocks have not with kinematic indicators consistently indicative of a reverse been affected by a Ross-age (approximately 500 million years) shear sense. This fabric least affects migmatites in the north- thermal overprint and, thus, that they may preserve a primary ernmost exposures and is most strongly developed in ortho- magnetization. Comparison with extensive North American and paragneisses in central portions of the range. The fabric is paleomagnetic reference data for the same time period could, locally cut by at least two generations of shallow to steep brit- thus, provide a test of the proposed reconstructions. To tie faults and by discordant felsic and mafic dikes and veins. examine this possibility, we collected hand specimens and Attitudes of these features were measured throughout the approximately 90 oriented rock cores of granophyre and rhy- range and samples of basement units representing the spec- olite from both groups of nunataks for paleomagnetic analysis trum of cross-cutting relations and compositions were col- and uranium-lead zircon geochronometry. Recently com- lected for uranium-lead geochronometry and neodymium, pleted paleomagnetic analyses of these samples indicates an strontium, and lead isotopic characterization. exceptionally well-defined primary magnetization that yields The sedimentary cover sequence of the Read Mountains a pole position that, within the framework of the plate includes the Neoproterozoic Watts Needle Formation. The restoration suggested by the Dalziel (1991), is not far from the Watts Needle Formation rests depositionally on Read Moun- North American Grenville apparent polar wander path for this tain basement and, at two localities along the southern time period (see Gose et al. 1994). These data are thus consis- mountain front, is not appreciably deformed or metamor- tent with, and lend support to, the suggested Neoproterozoic phosed (Clarkson 1984; Buggisch et al. 1990; Paech, Hahne, plate reconstructions. and Maass 1991). Stratigraphic sections at both of these loca-
ANTARCTIC JOURNAL - REVIEW 1994 5 tions were measured and sampled in detail. Hand specimens References and oriented rock cores, keyed to the measured sections, will be used in an integrated paleomagnetic, stable isotopic and Buggisch, W., G. Kleinschmidt, H. Kreuzer, and S. Krumm. 1990. geochemical study of this sequence in order to establish a Stratigraphy, metamorphism and nappe tectonics in the Shackle- more precise age and provide a basis for comparison with ton Range (Antarctica). Geodatische und Geophysikalische Verof- similar sequences elsewhere. Additional work in the cover fentlichungen, Reihel, 15, 64-86. sequence of the Read Mountains and elsewhere in the Shack- Clarkson, P.D. 1982. Geology of the Shackleton Range I. The Shackle- ton Metamorphic Complex. British Antarctic Survey Bulletin, 51, leton Range included 257-283. • sampling, clast counts, and analysis of sedimentary struc- Clarkson, P.D. 1984. Geology of the Shackleton Range II. The Turn- tures in the overlying Turnpike Bluff and upper Blaikiock pike Bluff Group. British Antarctic Survey Bulletin, 52, 109-124. Glacier Groups; Daiziel, I.W.D. 1991. Pacific margin of Larentia and East • study of cleavage/ bedding relationships and folding in the Antarctica/Australia as a conjugate rift pair: Evidence and impli- cations for an Eocambrian supercontinent. Geology, 19(6), Turnpike Bluff Group; and 598-601. • measurement of stratigraphic sections and collection of Eastin, R., and G. Faure. 1971. The age of the Littlewood Volcanics of oriented samples for paleomagnetic analysis in the lower Coats Land, Antarctica. Journal of Geology. 79(2), 241-245. Blaiklock Glacier Group at Mount Gass and Mount Proven- Gose, W.A., I.W.D. Dalziel, M.A. Helper, F. Hutson, and S. Grimes. der and in conglomerate of the upper Blaikiock Group at 1994. A positive test of the SWEAT hypothesis: New paleomagnetic data from the "Grenvillian" rocks of Coats Land, Antarctica. Trey Peaks, Otter Highlands. Geo- logical Society of America Abstracts with Programs, 26, 503-504. In the northern Shackleton Range, tectonically inter- [Abstract] leaved basement and high-grade metamorphic cover of the Hoffman, P.F. 1991. Did the breakout of Laurentia turn Gondwana- Shackleton Range Metamorphic Complex (Clarkson 1982; land inside out? Science, 252(5011), 1409-1412. Marsh 1983a, 190-193) were examined and sampled dur- Marsh, P.D. 1983a. The late Precambrian and early Paleozoic history pp. of the Shackleton Range, Coats Land. In R.L. Oliver, P.R. James, ing a 2-week traverse through the La Grange Nunataks and and J.H. Jago (Eds.), Antarctic earth science. Canberra: Australian Haskard Highlands. Most of the basement and supracrustal Academy of Science. metamorphic units recognized by Marsh (1983b, 1984) were Marsh, P.D. 1983b. The stratigraphy and structure of the Haskard sampled for uranium-lead geochronometry to establish pro- Highlands and Otter Highlands of the Shackleton Range. British tolith and metamorphic ages for the complex and to assess Antarctic Survey Bulletin, 60, 23-43. Marsh, P.D. 1984. The stratigraphy and structure of the La Grange the ages and relationships between fabric formation, thrust- Nunataks, northern Fuchs Dome and Herbert Mountains of the ing, and metamorphism. Shackleton Range. British Antarctic Survey Bulletin, 63, 19-40. This research is supported by National Science Founda- Marsh, P.D., and J.W. Thomson. 1984. Location and geology of tion grant OPP 91-17996. We wish to thank Antarctic Support nunataks in northwest Coats Land. British Antarctic Survey Bul- Associates, U.S. Navy personnel, and Captain Harry Hanlan letin, 65, 33-39. Moores, E.M. 1991. The southwest U.S.-East Antarctic (SWEAT) con- and his Twin Otter crew of Ken Borek Air Ltd. for their efforts nection: A hypothesis. Geology, 19(5), 425-428. in support of our work. Special thanks go to the personnel of Paech, H.-J., K. Hahne, and I. Maass. 1991. Sedimentological and tec- the Instituto Antartico Argentino station Belgrano II for their tonical results on sedimentary rocks outcropping at the southern gracious hospitality, to Werner Buggisch, who kindly provided flank of the Shackleton Range, Antarctica. Zeitschrift für Geologie a geologic map of the Read Mountains resulting from his work Wissenschaft, 19,159-167. Pankhurst, R.J., P.D. Marsh, and P.D. Clarkson. 1983. A geochronolog- with the German Antarctic program, and to Peter Clarkson of ical investigation of the Shackleton Range. In R.L. Oliver, P.R. the Scientific Committee on Antarctic Research for advice on James, and J.H. Jago (Eds.), Antarctic earth science. Canberra: Aus- LC-130 landing sites and logistics. tralian Academy of Science.
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