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The Late Cenozoic Sirius Group of the upper Shackleton Glacier region, Transantarctic Mountains

P.-N. Webb, Byrd Polar Research Center, Ohio State University, Columbus, Ohio 43210

D.M. Harwood, Department of Geology, University of Nebraska, Lincoln, Nebraska 68508

M.J. Hambrey, Institute of Geography and Earth Sciences, University of Wales, Aberystwyth, Ceridigion SY23 3DB, United Kingdom

L.A. Krissek, Byrd Polar Research Center, Ohio State University, Columbus, Ohio 43210

A.C. Ashworth, Department of Geosciences, North Dakota State University, Fargo, North Dakota 58105

M.C.G. Mabin, Department of Geography, James Cook University, Townsville, Queensland 4811, Australia

F.G. Fabel, School of Earth Sciences, La Trobe University, Melbourne, Victoria 3083, Australia

Examination of Late Cenozoic (Sirius Group) glacigene geology undertaken in the Beardmore Glacier region in 1985-1986 and 1990-1991 was extended to the adjacent Shackleton Glacier region during the 1995-1996 season. Our work in the latter area followed the investigations of McGregor (1965), Claridge and Campbell (1968), Mayewski (1975), and Mayewski and Goldthwait (1985) and emphasized the geographic distribution, stratigraphy, sedimentology, and structural relationships of the Sirius Group. Fieldwork was concentrated in two areas:

• the northern, or lower, Shackleton Glacier and coastal Queen Maud Mountains area, and
• the southern, or Roberts Massif-Bennett Platform area to the west and east of the upper Shackleton Glacier.

No outcrops of Sirius Group sediments were identified as a result of Twin Otter and helicopter surveys of the northern and coastal area. It had been hoped that low-elevation localities on the northern side of the Queen Maud Mountains between Shackleton and Liv Glaciers might provide glaciomarine Sirius Group successions similar to those recovered in the lower Beardmore Valley below The Cloudmaker (Webb et al. 1994, 1996a). Mayewski (1975) reported a Sirius Group locality at Mount Roth, but the outcrop was not located. Outcrops reported by previous workers at Roberts Massif and Bennett Platform were examined in detail. Minor deposits of the Sirius Group at Dismal Buttress and Half Century Nunatak were visited briefly. New localities were located at Matador Mountain, and at Schroeder Hill and Landry Bluff (Cumulus Hills). An area of approximately 1,750 square kilometers was examined in our survey of Sirius Group strata in the upper Shackleton Glacier region.

Summary of results

• The Sirius Group has been known from this area for 30 years. Our fieldwork demonstrated that its distribution is more extensive and its stratigraphy more complex than previously reported.
• The Sirius Group of the upper Shackleton Valley is interpreted mostly as the subglacial deposits of an ice stream or major trunk glacier. Fabrics from clasts in diamictites and directions from alignment of grooves, striations, and gouge marks on the underlying dolerite trend south-north. This orientation is subparallel to the modern Shackleton Glacier trunk valley (Webb et al., Antarctic Journal, in this issue, 1996b).
• Stratigraphic analyses indicate that deposition entailed numerous discrete events and included both ice-contact and subaqueous modes of sedimentation.
• Glacigene successions crop out within high-relief glacial paleo-valleys on the southern or inland ice plateau side of Roberts Massif, suggesting that the Sirius Group extends much further southward beneath the present east antarctic ice sheet. This assumption gains credence from the recovery of Sirius Group sediments from inland nunataks at Mount Wisting, Mount Block, and Otway Massif (Mayewski 1975; Elliot personal communication).
• Sirius Group strata of the upper Shackleton paleovalley were originally both thicker and much more extensive, filling much of the floor of the Shackleton paleodrainage system. Large volumes of sediment were removed, probably by latest Neogene glacial processes, to expose a pre-Sirius glacial topography. The rift basins of the Ross Sea are the likely repositories of these retransported Sirius Group sediments.
• Pre-Sirius topography (the sub-Sirius Group erosion surface of Webb et al., Antarctic Journal, in this issue, 1996b) was relatively subdued and located at a lower elevation than today. This terrain appears to have undergone post-Jurassic-pre-Sirius fault dislocation, an episode probably associated with the early horst-graben style uplift of the Transantarctic Mountains in this region. Remobilized Sirius Group sediments were later injected as clastic dykes into fault joints in Paleozoic-Mesozoic Beacon Supergroup and Ferrar Dolerite rocks.
• Two lithostratigraphic units (formations 1 and 2 of Webb et al., Antarctic Journal, in this issue, 1996c) were recognized in a series of sections that crop out at Roberts Massif and Bennett Platform. In terms of the investigations of Mayewski (1975) and Mayewski and Goldthwait (1985), the sub-Sirius Group erosion surface and formations 1 and 2 are an integral part of their Queen Maud Glaciation. The lower part of our formation 1 is equivalent to their till member; the upper part of formation 1 and all of formation 2, to their stratified member.
• A major episode of post-Sirius Group structural deformation is recognized at Roberts Massif, where formerly contiguous sub-Sirius Group erosion surfaces and overlying Sirius Group sediments are separated by normal faults, which in some instances exhibit vertical dislocations of up to about 300 meters (Webb et al., Antarctic Journal, in this issue, 1996b).
• Mayewski and Goldthwait (1985) recognized a post-Sirius event, the Gallup Interglacial. Features associated with this event included fluvial channels and potholes cut into the surface of the Sirius Group strata at Bennett Platform and at other localities in the Transantarctic Mountains (including Mount Feather in the western Quartermain Mountains). They interpreted the Gallup Interglacial as a period of climate amelioration, surficial weathering, and abundant surface water. The surficial ferruginous weathering, rudimentary paleosols, salt layers, surface aqueous transport, erosion, and pedestal development we observed in the uppermost sediments of formation 2 at Bennett Platform are probably associated with the Gallup Interglacial.
• The youngest episode of structural deformation we observed occurs at Bennett Platform where Sirius Group successions near the edge of the platform are broken into a number of blocks or slivers and downfaulted toward Shackleton Glacier. Mayewski and Goldthwait (1985) explained deep fissures at the foot of fault scarps as fluvial channels.
• Volcanism is not associated with the pre-, syn-, or post-Sirius Group tectonic episodes recognized in the Shackleton Valley area.

Samples collected during the field season will be examined for wood, seeds, and microfossil material, notably palynomorphs, diatoms, silicoflagellates, foraminifera, insects, and vertebrate fossil debris. Pending these laboratory studies, the age of Sirius Group sediments in this area is regarded as late Cenozoic.

This work was supported by National Science Foundation grants OPP 94-19056 (Peter Webb) and OPP 91-58075 (David Harwood). We thank Derek Fabel and John de Vries for assistance during our field activities.

References

Claridge, G.G.C., and I.B. Campbell. 1968. Soils of the Shackleton Glacier region, Queen Maud Range, Antarctica. New Zealand Journal of Science, 11(2), 171-218.

Elliot, D.H. 1996. Personal communication.

McGregor, V.R. 1965. Notes on the geology of the area between the heads of the Beardmore and Shackleton Glaciers. New Zealand Journal of Geology and Geophysics, 8(2), 278-291.

Mayewski, P.A. 1975. Glacial geology and late Cenozoic history of the Transantarctic Mountains, Antarctica (Report 56). Columbus: Ohio State University, Institute of Polar Studies.

Mayewski, P.A., and R.P. Goldthwait. 1985. Glacial events in the Transantarctic Mountains: A record of the east antarctic ice sheet. In M.D. Turner and J.R. Splettstoesser (Eds.), Geology of the Transantarctic Mountains (Antarctic Research Series, Vol. 36). Washington, D.C.: American Geophysical Union.

Webb, P.-N., D.M. Harwood, M.C.G. Mabin, and B.C. McKelvey. 1994. Late Neogene uplift of the Transantarctic Mountains in the Beardmore Glacier region. Terra Antartica, 1(2), 463-467.

Webb, P.-N., D.M. Harwood, M.G.C. Mabin, and B.C. McKelvey. 1996a. A marine and terrestrial Sirius Group succession, middle Beardmore Glacier-Queen Alexandra Range, Transantarctic Mountains, Antarctica. Marine Micropaleontology, 27(1996), 273-297.

Webb, P.-N., D.M. Harwood, M.J. Hambrey, L.A. Krissek, A.C. Ashworth, and M.C.G. Mabin. 1996b. The sub-Sirius Group erosion surface at Roberts Massif, upper Shackleton Glacier region, Transantarctic Mountains. Antarctic Journal of the U.S., 31(2).

Webb, P.-N., D.M. Harwood, M.J. Hambrey, L.A. Krissek, A.C. Ashworth, and M.C.G. Mabin. 1996c. Stratigraphy of the Sirius Group, upper Shackleton Glacier region, Transantarctic Mountains. Antarctic Journal of the U.S., 31(2).