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During austral summer 1995-1996, Grunow, Encarnacion, and Paulsen, plus Mike Roberts, were put-in by LC-130 on 20 November to a field camp just north of Cape Surprise (figure 1, camp 1). The objective of our field programs was to collect paleomagnetic, geochronologic, paleontologic, and structural samples from basement granitoids, sedimentary, and volcanic rocks to improve understanding of the Early Paleozoic tectonic evolution of the Transantarctic Mountains. We established a Ski-doo route between Cape Surprise and the Bravo Hills for our second base camp in early December (figure 1). We encountered many large sastrugi and crevasses in the Gabbro and Bravo Hills areas making Ski-doo travel quite slow. From 9 December, our fieldwork was done by helicopter from the MacGregor camp where Bert Rowell joined us for the remainder of our season. The weather was excellent until 10 December whereafter, on most days, cloud cover obscured many of the basement exposures between Lubbock Ridge and the Ross Ice Shelf. The localities visited by Ski-doo, Twin Otter, or helicopter are shown on figure 1.
Prior knowledge of the age of basement rocks in the Shackleton Glacier area did not allow good geologic correlation with events elsewhere in Antarctica. In the field area, a thick succession of silicic volcaniclastic rocks, lava flows, and some limestones forms the Taylor Formation. It is widely correlated with the Fairweather Formation of Liv Glacier region to the east. The Henson marble forms the Fairweather Formation's upper member and was commonly regarded as the equivalent of the limestones in the Taylor Formation (Wade 1974). On lithological grounds, Wade (1974) correlated the Henson Marble with the Lower Cambrian Shackleton Limestone. One of our samples from the Henson Marble at Mount Fairweather contains what appears to be recrystallized solitary, cone-shaped, double-walled archaeocyath (figure 2).
Well-preserved trilobites from the upper part of the succession of the Taylor Formation at Taylor Nunatak reveal that the limestones containing them are late Middle Cambrian and much younger than the Henson Marble. Seemingly, the Taylor Formation is not correlative with the Fairweather Formation. The trilobites include Amphoton sp. cf. A. oatesi Palmer and Gatehouse (1972) (figure 3) and Nelsonia sp., cf. N. schesis Palmer and Gatehouse, which can be tied to Middle Cambrian successions. Nelsonia is endemic to Antarctica, but N. schesis has been reported from northern Victoria Land (Cooper and Shergold 1991, pp. 20-62), where it occurs with cosmopolitan late Middle Cambrian trilobites. This age is compatible with a 515±6-million-year uranium-lead zircon date from Taylor Formation metarhyolites on Lubbock Ridge (Van Schmus et al. in press) and suggests that the enigmatic Cloudina? tubes from Taylor Nunatak (see Stump 1995) may have limited stratigraphic value.
Structurally, the Lower Cambrian? Fairweather Formation is tightly folded, foliated, and metamorphosed to greenschist/lower amphibolite facies, whereas the late Middle Cambrian Taylor Formation is relatively unmetamorphosed and largely only tilted with no penetrative deformation. Several north-south trending subvertical shear zones, including mylonites, cut probable correlatives of the Taylor Formation along the Shackleton Glacier. These shear zones have downdip stretching lineations and may be associated with tilting of the Taylor Formation. We believe that the structural differences between the Taylor and Fairweather formations reflect structural level such that both formations were deformed during a single event. It is possible, however, that a late Early to early Middle Cambrian deformation event may have caused tight folding of the Fairweather Formation, and a second deformation event in post-late Middle Cambrian time resulted in ductile shearing and tilting of the Taylor Formation. At O'Brien Peak, a granite that intrudes deformed marbles and clastics has an S-C fabric indicating sinistral shear parallel to the mountain front. High-grade metamorphic rocks were observed at the small Sage Nunatak, Bravo Hills, Mount Woodall, and Fallone Nunataks. Most of the granitoids between the Shackleton and Liv Glaciers are undeformed except at their margins. Approximately 500 paleomagnetic drill cores and approximately 40 samples for isotopic dating were collected at the locations shown on figure 1.
We thank Mike Roberts for his excellent mountaineering assistance and A.R. Palmer for confirming the trilobite identifications. This work was supported by National Science Foundation grant OPP 93-17673 to Grunow; paleontological analyses were supported from grant OPP 91-17444 to Rowell.
References
Cooper, R.A., and J.H. Shergold. 1991. Palaeozoic invertebrates of Antarctica. In R.J. Tingey (Ed.), The geology of Antarctica. Oxford: Blackwell.
Palmer, A.R., and C.G. Gatehouse. 1972. Early and Middle Cambrian trilobites from Antarctica (U.S. Geological Survey, Professional Paper 456-D). Washington, D.C.: U.S. Government Printing Office.
Stump, E. 1995. The Ross Orogen of the Transantarctic Mountains. Cambridge: Cambridge University Press.
Van Schmus, W.R., L.W. McKenna, D.A. Gonzales, A.H. Fetter, and A.J. Rowell. In press. U-Pb geochronology of parts of the Pensacola, Thiel, and Queen Maud Mountains, Antarctica. Proceedings volume VII ISARS, Siena, Italy.
Wade, F.A. 1974. Geological surveys of Marie Byrd Land and the central Queen Maud Range. Antarctic Journal of the U.S., 9(5), 241-242.