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Multiple Phases of Early Paleozoic Deformation in the Central Transantarctic Mountains

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Multiple Phases of Early Paleozoic Deformation in the Central Transantarctic Mountains interest to see if this new community maintains the strong References correspondence evident for the lower Fremouw, or if Antarctica Colbert, E. H. 1982. Triassic vertebrates in the Transantarctic Mountains. developed a unique fauna during the later portion of the Early In M. Turner and J. Splettstoesser (Eds.), Geology of the Central Transan- Triassic. tarctic Mountains. Antarctic Research Series, 39(2), 11-35. A paleoecological analysis of the lower Fremouw Formation is Cosgriff, J.W., W.R. Hammer, J.M. Zawiskie, and N.R. Kemp. 1978. advancing concurrent with the description of this new fauna. New Triassic vertebrates from the Fremouw Formation of the Queen Extensive collection of the lower Fremouw over four field sea- Maud Mountains. Antarctic Journal of the U.S., 13(4), 23-24. Sons has led to a data base that may allow us to determine Hammer, W. R., and j.W. Cosgriff. 1981. Myosaurus gracilis, an anomo- preferred paleoenvironments for certain taxa. New specimens dont reptile from the Lower Triassic of Antarctica and South Africa. collected from the Coalsack Bluff and Graphite Peak localities Journal of Paleontology, 55(2), 410-424. Hammer, W.R., W.J. Ryan, J.W. Tamplin, and S.L. DeFauw. 1986. New during the 1985-1986 field season are under study, and a com- vertebrates from the Fremouw Formation (Triassic), Beardmore plete analysis of the collection made during the 1977- 1978 field Glacier region, Antarctica. Antarctic Journal of the U.S., 21(5), 24-26. season (Cosgriff et al. 1978) has recently been completed. 86Keyser, A. W., and R.M.H. Smith. 1978. Vertebrate biozonation of the This research is supported by National Science Foundation Beaufort Group with special reference to the western Karoo Basin. grants DPP 85-11334 and DPP 84-18354. Annals of the Geological Survey (South Africa), 12, 1-36. Multiple phases of early Paleozoic deformation in the central Transantarctic Mountains MARGARET N. REES Department of Geoscience University of Nevada Las Vegas, Nevada 89154 GARY H. GIRTY Department of Geological Sciences San Diego State University San Diego, California 92192 SUSAN K. PANTTAJA Department of Geoscience University of Nevada Las Vegas, Nevada 89154 PETER BRADDOCK Queen Charlotte College Picton, New Zealand New structural data from the lower Paleozoic Douglas Con- glomerate in the northern Churchill Mountains (figure 1) indi- cate that the conglomerate was deformed at least three times before the deposition of the overlying Devonian beds of the Beacon Supergroup. Recognition of these post-Lower Cambrian and pre-Devonian events makes all existing tectonic models for the area too simplistic. Figure 1. Location map showing position of area mapped within the Our geological investigation of the northern Churchill Moun- central Transantarctic Mountains and the extent of the Churchill Mountains between the Byrd and Nimrod glaciers. ("MH" denotes tains revealed extensive outcrops of Douglas Conglomerate that Mount Hamilton, "CC" denotes "Crackling Cwm," and "MD" de- were not observed or visited by previous field teams (Burgess notes Mount Dick.) and Lammerink 1979; Skinner 1964, 1965; Stump et al. 1979); consequently we produced a new geological map and propose new structural interpretations. In addition, south of the map- The field team (Rees, Girty, Panttaja, and Braddock) was put ped area at location 14 (figure 2) trilobites were collected from into the field by an LC-130 ski-equipped Hercules airplane on 24 the Shackleton Limestone and, at location 17, a previously unre- November 1986 and picked up from the same location on 5 corded, 1000-meter-long exposure of pillow lava was examined January 1987. The landing site was located on the Nicholson and sampled. Peninsula (80°42S 159°23E). Overland travel by snowmobile to 1987 REVIEW 33 15830 our understanding of the Early Paleozoic history of the region (Rees and Rowell 1987; Rees and Rowe!l in press.) Structural data were recorded from the Douglas Con- glomerate at ten locations (figure 2). At most locations, abun- dant small-scale, tight to isoclinal folds are deformed by north- east, or northeast and northwest striking asymmetric folds, or their associated axial plane cleavage. These two sets of struc- tures are in turn locally transected by a set of northeast-striking kink bands. This often repeated pattern suggests a hierarchy of structure-forming events that are designated D 1, D2, and D3 in 800 order of relative ages of formation. Major faults shown in figure 2 dip steeply to the northeast or northwest and are interpreted to be contractional or reverse faults developed during D 1 . Sim- 2 ilar types of faults also were observed at "Crackling Cwm," but mapping there is still incomplete. Folds and faults attributed to C D1 deformation record northwest-southeast directed con- CC - - traction. Significantly, the faults shown in figure 2 are folded, 0 1 2 3 along with S, by a large northwest plunging D2 synform. Con- I_ I I jugate D2 KILOMETERS sinistral and dextral verging folds and associated cleavages record northeast-southwest shortening. D 3 kink bands were produced locally by a northwest-southeast SHACKLETON & DOUGLAS XD2 SYNFORM contraction. INTERLEAVED The timing of these events is not well constrained nor is the DOUGLAS CONGLOMERATE Dl REVERSE FAULT age of the Douglas. Nevertheless, deposition of the con- glomerate post-dates folding and uplift of the Lower Cambrian Shackleton Limestone on which it rests with marked angular E.:...J5HACKLETON LIMESTONE 3 LOCATION NUMBER unconformity in the southern Churchill Mountains (Rees et al. 1985; Rowell et al. in press; Rowell, Rees, and Braddock 1986; Figure 2. Generalized geologic map illustrating distribution of Rowell and Rees in press). Furthermore, preliminary po- Shackleton Limestone and Douglas Conglomerate outcrops map- tassium/argon dates derived from clasts from the Douglas sug- ped during the 1986-1987 field season in the northernmost Church- ill Mountains. Note two Major D contractional faults that have been gest that their source area was uplifted during the Ross folded by a 02 synform. Outcrops at "Crackling Cwm" (CC) are Orogeny. It therefore is possible that all three deformational tectonically interleaved Shackleton and Douglas formations and all events recorded in the Douglas Conglomerate occurred after faults are not shown. Numbers indicate areas where detail structural the Ross Orogeny and before the deposition of Devonian data was collected. ("MH" denotes Mount Hamilton.) sediments. Sedimentological data and samples of the Douglas were col- the Mount Hamilton area (figure 3) was significantly hampered lected from ridges and nunataks numbered 1 through 12, ex- by a broad sastrugi field and numerous white-out days. cluding outcrop 7 (figure 3). Preliminary detrital mode analyses Throughout the field season, we commonly experienced an- of sandstone and rudstone units suggest both a recycled orogen noying, low to moderate velocity, katabatic winds. (sensu Dickinson and Suczek 1979) and basement uplift (sensu Our primary objective for this project was to collect from the Dickinson et al. 1983) provenance for the Douglas. Clasts are Douglas Conglomerate sufficient field data and laboratory data primarily limestone eroded from the Shackleton and several so that we may determine the age, tectonic setting, provenance, types of sandstone and quartzite probably derived from the and deformational history of the formation. A synthesis of field Proterozoic basement. Less common are granitoid, rhyolite, data and limited laborarory analyses have already enhanced basalt, chert, and dolomite clasts whose sources are not yet MD MH 17 14 13 7 6 Figure 3. A perspective sketch of the field area illustrating ridges and nunataks from which field data and samples were collected from the Shackleton Limestone, Douglas Conglomerate, and a pillow lava sequence. ("MD" denotes Mount Dick, and "MH" denotes Mount Hamilton.) 34 ANTARCTIC JOURNAL known. The framework modes and their sources suggest depo- sitional setting. Fifth International Syinposiu in on Antarctic Earth Sciences sition in a foreland basin but do not preclude accumulation in a Abstracts. pull-apart basin. Resolution of basin type is hindered because Rees, MN., and A.J. Rowell. In press. The pre-Devonian Paleozoic sequences are fragmentary, folded, and sheared, thus obscur- clastics of the central Transantarctic Mountains: Stratigraphy and ing stratigraphic relationships and thicknesses. In addition, depositional settings. Volume of the Fifth International Symposium on Antarctic Earth Sciences. basin type has not been clarified by lithofacies analysis because Cambridge, U.K. Rees, MN., A.J. Rowell, B.R. Pratt, and P. Braddock. 1985. The Byrd the lithofacies imply development of alluvial fans and fan deltas Group of the Holyoake Range, central Transantarctic Mountains. in lacustrine and marine environments (Rees and Rowell in Antarctic Journal of the U.S., 20(5), 3-5. press) and all such associations are common to both basin types. Rowell, A.J., and M.N. Rees. In press. Setting and significance of the This work was supported in part by National Science Founda- Shackleton Limestone, central Transantarctic Mountains. Volume of tion grant DPP 85-18157 to the University of Nevada Las Vegas. the Fifth International Symposium on Antarctic Earth Sciences. Cambridge, U.K. References Rowell, A.J., M.N. Rees, and P. Braddock. 1986. Pre-Devonian Pal- eozoic rocks of the central Transantarctic Mountains. Antarctic Journal of the U.S., 21(5), 48-50. Burgess, C.J., and W. Lammerink. 1979. Geology of the Shackleton Rowell, A.J., M.N. Rees, R.A. Cooper, and B.R. Pratt. In press. Early Limestone (Cambrian) in the Byrd Glacier area. New Zealand Antarctic Paleozoic history of the central Transantarctic Mountains: Evidence Record, 2, 12-16. from the Holyoake Range, Antarctica. New Zealand Journal of Geology Dickinson, W. R., and C. A. Suczek. 1979. Plate tectonics and sandstone and Geophysics. compositions. American Association of Pet roleu in Geologists Bulletin, 63, Skinner, D.N.B. 1964. A summary of the geology of the region between 2164-2182.
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