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Our observations of Sirius Group stratigraphy and paleogeographic settings in the upper Shackleton Glacier region are based on outcrops in two largely ice-free areas on the western (Dismal Buttress-Bennett Platform-Matador Mountain; 250 square kilometers) and eastern (Roberts Massif; 250 square kilometers) sides of the Shackleton Glacier (Webb et al., Antarctic Journal, in this issue-a, -b).
The Sirius Group crops out in the northern half of Roberts Massif, at its southern margins with the inland ice plateau, and at its western margin along the Shackelton Glacier. Outcrops also occur at three localities on the western margin of the Shackleton Glacier, at Dismal Buttress, Bennett Platform, and Matador Mountain. The 3-kilometer (km) long cliffs fronting Bennett Platform provide the most complete stratigraphy of the Sirius Group observed in the upper Shackleton Glacier area. The relationship between the sub-Sirius erosion surface and basal Sirius Group sediments is best exposed at Roberts Massif (Webb et al., Antarctic Journal, in this issue-b), although this erosion surface is also well preserved at all localities along the western margins of the Shackleton Glacier. At Roberts Massif, this contact was observed between about 2,000 meters (m) and about 2,500 m, whereas 25 km north at Bennett Platform the contact at the base of measured sections is slightly lower at about 1,800 to about 2,000 m. Structural relationships across the 6- to 10-km-wide Shackleton Glacier have not been resolved and elevation differences for the base of Sirius Group successions might result from natural topographic contrasts, differential faulting on separate structural blocks, or some combination of the two factors.
Two distinctive glacigene stratigraphic units are recognized. We refer to these informally as formations 1 and 2. An assemblage of rocks, which may also be part of the Sirius Group, is also noted here. These are found only as erratic boulders between 1,800 and 2,000 m on the surface of Quaternary moraines around the northern flanks of Roberts Massif and along the foot of the Bennett Platform cliffs, all within a kilometer or two of the Shackleton Glacier. The boulders are up to 4 m in diameter and consist of well to moderately lithified diamictite, sandstone, and conglomerate. Bedding planes are discernible in some sandstone and conglomerate boulders. One boulder from the moraine field below Bennett Platform contained abundant wood stems and striated clasts. Clasts of these lithologies occur in formations 1 and 2 and so are older. Similar lithologies occur as large boulders on the surface of Meyer Desert in the Dominion Range. We tentatively assign these rocks to the Sirius Group. They have not been seen in situ anywhere in the nunataks that border the inland ice plateau and are presumed to have been derived from sub-ice-sheet regions to the south of Shackleton and Beardmore Glaciers.
Formations 1 and 2 are well exposed in cliffs below Bennett Platform. A lower unit (formation 1), which is at least 100 m thick, rests unconformably on a striated erosion surface of Ferrar Dolerite and makes up the greater part of the cliff exposure at Bennett Platform. Formation 1 includes at least eight members. Individual lithofacies present in these members include the following: massive diamictite; massive diamictites with stratified conglomerate and breccia, some of which is slumped; weakly stratified diamictites and conglomerate; stratified conglomerate and dropstone laminite; and stratified and massive diamictite, conglomerate, and laminite with common slump structures. Modes of deposition interpreted for formation 1 include the following: lodgement till, glacigenic debris flows, subaqueous bottom current reworking, glaciofluvial sedimentation, and proximal and distal glaciolacustrine deposition associated with ice-rafting and laminite sedimentation. Disruption of internal stratigraphy is also evident, especially where facies are mixed, indicating remobilization by subaqueous gravity flows. Disruption of some sequences by brittle failure is probably related to synsedimentary and/or post-Sirius Group tectonic movements.
Formation 1 and overlying formation 2 are separated by a gently sloping disconformity. Formation 2 thickens from 6 to 46 m north to south along the Bennett Platform cliff front exposures and cuts into underlying formation 1. Formation 2 contains two members, a lower massive diamictite and an overlying more friable unit of diamictite and conglomerate. This formation is not as lithified as the underlying cliff-forming formation 1 and is capped by a lag or deflation surface of dolerite boulders along the upper surface of the platform. Deep weathering, rudimentary paleosols, salt lenses, ice-wedge pseudomorphs, and rock pedestal horizons are characteristic of the upper few meters of formation 2; all phenomena were probably produced by post-Sirius Group climatic events (Webb et al., Antarctic Journal, in this issue-a). It is likely that the original thickness of this unit exceeded the 46 m measured at Bennett Platform. Ten kilometers to the north of Bennett Platform, near Matador Mountain, a poorly exposed succession approximately 90 m thick, is located just above glacier level and is thought to be equivalent to formation 2 at Bennett Platform. This succession suggests that relief on the intra-Sirius erosion surface between formations 1 and 2 is at least 400 m. Some part of this figure might also be ascribed to subsequent fault dislocation. The succession at Dismal Buttress is also tentatively correlated with formation 2 at Bennett Platform.
Stratigraphic correlation over the approximately 25 km separating sections at Bennett Platform and Roberts Massif cannot be made with confidence at this time. Many of the lithofacies encountered in formation 1 at Bennett Platform, including deformed and undeformed massive diamictites and laminites, are also present at several localities on Roberts Massif. At the latter locality, some diamictite successions near the base of the Sirius Group exhibit a co-occurrence of severe deformation with infra-Sirius clastic dike injection and local fault and sheared-clast zones. Formation 2 is not known at Roberts Massif.
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
Carlquist, S. 1987. Upper Pliocene-lower Pleistocene Nothofagus wood from the Transantarctic Mountains. Aliso, 11, 571-583.
Francis, J.E., and R.S. Hill. In press. Fossil plants from the Pliocene Sirius Group, Transantarctic Mountains: Evidence for climate from growth rings and fossil leaves. Palaios.
Hill, R.S., and E.M. Truswell. 1993. Nothofagus fossils in the Sirius Group, Transantarctic Mountains: Leaves and pollen and their climatic significance. In J.P. Kennett and D.A. Warnke (Eds.), The antarctic paleoenvironment: A perspective on global change, part II (Antarctic Research Series, Vol. 60). Washington, D.C.: American Geophysical Union.
Hill, R.S., D.M. Harwood and P.-N. Webb. 1996. Nothofagus beardmorensis (Nothofagaceae), a new species based on leaves from the Pliocene Sirius Group, Transantarctic Mountains, Antarctica. Review of Paleobotany and Palynology, 94(1996), 11-24.
McKelvey, B.C., P.-N. Webb, D.M. Harwood, and M.C.G. Mabin. 1991. The Dominion Range Sirius Group: A record of the late Pliocene-early Pleistocene Beardmore Glacier. In M.R.A. Thomson, J.A. Crame, and J.W. Thomson (Eds.), Geological evolution of Antarctica. Cambridge: Cambridge University Press.
Mayewski, P.A. 1975. Glacial geology and late Cenozoic history of the Transantarctic Mountains, Antarctica (Report 56). Columbus: Ohio State University.
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, Geology of the Transantarctic Mountains (Antarctic Research Series, Vol. 36). Washington, D.C.: American Geophysical Union.
Webb, P.-N., and D.M. Harwood. 1987. The terrestrial flora of the Sirius Formation: Its significance in interpreting late Cenozoic glacial history. Antarctic Journal of the U.S., 22(4), 7-11.
Webb, P.-N., and D.M. Harwood. 1991. Late Cenozoic glacial history of the Ross embayment, Antarctica. Quaternary Science Reviews, 10(2/3), 215-223.
Webb, P.-N., and D.M. Harwood. 1993. Pliocene fossil Nothofagus (southern beech) from Antarctica: Phytogeography, dispersal strategies and survival in high latitude glacial-deglacial environments. In J.N. Alden, S. Odum, and J.L. Mastrantonio (Eds.), Forest development in cold climates. New York: Plenum.
Webb, P.-N., D.M. Harwood, B.C. McKelvey, M.C.G. Mabin, and J.H. Mercer. 1987. Sirius Formation of the Beardmore Glacier region. Antarctic Journal of the U.S., 22(1), 8-13.
Webb, P-N., D.M. Harwood, M.J. Hambrey, L.A. Krissek, A.C. Ashworth, M.C.G. Mabin, and F.G. Fabel. 1996a. The Late Cenozoic Sirius Group of the 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. 1996b. The sub-Sirius Group erosion surface at Roberts Massif, upper Shackleton Glacier region, Transantarctic Mountains. Antarctic Journal of the U.S., 31(2).