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Sirius Formation Basal Contacts in the Beardmore Glacier Region

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Sirius Formation Basal Contacts in the Beardmore Glacier Region We wish to express our sincere appreciation to the other Edzwald, J.K., and C.R. OMelia. 1975. Clay distributions in recent members of the Vanderbilt/Ohio State University research estuarine sediments. Clays and Clay Mineralogy, 23, 39-44. team, our fellow scientists from the Beardmore South camp, Fan, P.F. 1976. Recent silts of the Santa Clara River drainage basin, and the civilian and military support personnel for their efforts southern California: A mineralogical investigation of their origin and on our behalf. evolution. Journal of Sedimentary Petrology, 46, 802-812. This work is supported by National Science Foundation grant Keller, W.D. 1970. Environmental aspects of clay minerals. Journal of Sedimentary Petrology. 40, 788-813. DPP 84-18354. Lindsay, J.F. 1969. Stratigraphy and sedimentation of lower Beacon References rocks in the central Transantarctic Mountains. Institute of Polar Studies Report, Vol. 33. Columbus: Ohio State University Press. Miller, M.F. 1984. Distribution of biogenic structures in Paleozoic non- Barrett, P.J. 1969. Stratigraphy and petrology of the mainly fluviatile marine and marine margin sequences: An actualistic model. Journal of Permian and Triassic Beacon rocks, Beardmore Glacier area, Ant- Paleontology, 58, 550-570. arctica. institute of Polar Studies Report, Vol. 34. Columbus: Ohio State Miller, M.F., and R.S. Frisch. 1986. Depositional setting of the (Permian) University Press. Mackellar Formation, Beardmore Glacier area. Antarctic Journal of the Berner, R.A., and R. Raiswell. 1984. C/S method for distinguishing U. S., 21(5). freshwater from marine sedimentary rocks. Geology, 12, 365-368. Miller, J.M.G., and B.J. Waugh. 1986. Sedimentology of the Pagoda Cody, R.D. 1971. Adsorption and the reliability of trace elements as Formation (Permian), Beardmore Glacier area. Antarctic Journal of the environment indicators for shales. Journal of Sedimentary Petrology, 41, U. S., 21(5). 461-471. Nesbitt, H.W., and G.M. Young. 1982. Early Proterozoic climates and Collinson, J.W., and J.L. Isbell. 1986. Permian-Triassic sedimentology of plate motions inferred from major element chemistry of lutites. the Beardmore Glacier region. Antarctic Journal of the U.S., 21(5). Nature, 299, 715-717. first at ito 5 meters deep and 20 to 30 meters apart. These forms Sirius Formation basal contacts in the probably represent glacial whaleback features. Overall, the plat- Beardmore Glacier region form dips approximately 2° to the northwest. Where naturally exposed, it is weathered, in places into small, irregularly shaped tors up to 40 centimeters high and 25 centimeters across. Re- moval of the overlying compacted diamictite reveals a fresh, M.C.G. MABIN heavily striated surface. Striation directions measured at six localities along the eastern side of the platform trended between Institute of Polar Studies 22° and 105°, with the predominant ice-flow direction being to Ohio State University the east-northeast, as reported by McKelvey et al. (1984) and Columbus, Ohio 43210 Mercer (1972). On the western side of the platform, striations are similarly oriented, and there are crescentic gouges up to 35 centimeters across, convex in an east-northeast direction (Har- wood personal communication). Also exposed on this western Sirius Formation outcrops examined in the Beardmore Glacier edge is a P-form feature 2 meters across and 0.5 meter deep, region during the 1985-1986 season are described by Webb et al. eroded into the dolerite (Harwood personal communication). (in preparation). The disconformity between the basal Sirius This meandering channel is exposed for 5 meters and is ori- Formation and underlying rocks was examined at five localities. ented southeastward. It indicates the presence of subglacial The morphology of the underlying surface exhibits varying meltwater beneath the ice that overrode the platform. degrees of ice moulding on a variety of rock types, as reported Sirius Formation deposits are well exposed in the Dominion from other sites (Mayewski and Goldthwait 1985). It is referred Range in cliffs on the east side of the Beardmore Glacier, be- to by Webb et al. (Antarctic Journal, this issue; in preparation) as tween 2.2 and 3.9 kilometers southwest of Plunket Point the "Dominion erosion surface." At two locations, Mount Sirius (85°06S 166°56E) (Oliver 1964; Mercer 1972). The diamictite and Plunket Point, it is extensively exposed and easily accessi- rests on an undulating surface of columnar-jointed dolerite at ble. Surveys of the contact were made using a theodolite and about 1,750 meters above sea level and 15 to 95 meters above the electronic distance meter. glacier. The undulations vary from 3 to 36 meters high and 80 to At the type locality, Mount Sirius (84°08S 163°15E), 85 300 meters apart and are interpreted as whaleback features. meters of diamictite rest on a platform of columnar jointed Striations on the dolerite trend between 355° and 25°, the varia- dolerite, which is approximately 2,200 meters above sea level tions representing diverging and converging or north-north- and 400 meters above the surrounding Bowden Névé. In plan east flowing ice over and around the whalebacks. Other glacial view, the platform is L-shaped, one limb extending north for 1.3 erosion features include minor plucking on the downglacier kilometers, the other east for 0.9 kilometer. Width varies from (northern) sides of the whalebacks, lunate fractures up to 8 125 to 500 meters, and it covers an area of 4 hectares. The centimeters across, and small grooves. A fault has displaced platform edge varies between 2,179 and 2,226 meters above sea both the dolerite and overlying Sirius Formation diamictite. The level. It has two sets of undulations: one set is 20 meters deep northern side is upthrown 55 meters, and the fault trace can be and 300 meters apart, and the other set is superimposed on the followed south for 2.1 kilometers. 32 ANTARCTIC JOURNAL Extensive exposures of Sirius Formation occur in the up- planation surfaces (Grindley 1967). Deposition of the Sirius thrown face of a major fault scarp that runs north-south along Formation probably pre-dates the cutting of these surfaces, the Dominion Range, 7 kilometers west of the Mill Glacier because they would be unlikely to have survived the scale of (Elliot, Barrett, and Mayewski 1974). The basal contact is inter- weathering implied by the deposits underlying the diamictite at mittently exposed between 5.5 and 11.2 kilometers south of the Orr peak. Beardmore Glacier, at between 1,850 and 2,500 meters above sea The basal contacts of the Sirius Formation described here level. In the south, two small exposures of diamictite rest in range from those showing extensive glacial erosion to surfaces shallow depressions on dolerite at about 800 and 620 meters largely unmodified by overriding ice. They range in elevation above the glacier. North of these, a 3.6-kilometer succession of through some 1,500 meters, and indicate glaciation of a rela- Sirius Formation rests in a broad depression at 350 to 400 meters tively high-relief landscape in a configuration similar to the above the glacier. It is cut mainly in sandstone and mudstone of present day. It is also likely that they are of different ages, and the Triassic Fremouw Formation (Elliot et al. 1974) and is but- some may have been formed during several different ice ad- tressed at either end by dolerite. The northernmost basal con- vances prior to the commencement of deposition of the Sirius tact exposed here is much lower, at only 50 meters above the Formation. glacier, and is a sloping platform cut across dolerite and Fre- I thank Dan Greene, Chuck McGrosky, and Steve Munsell for mouw Formation sediments (Mayewski and Goldthwait 1985, willing assistance in the field. The Cloudmaker exposure was figure 5). In all these exposures, the basal contact is difficult to kindly shown to me by George Denton. reach due to precipitous slopes. However, four sited were This work was supported by National Science Foundation reached and striations were observed trending between 25° and grant DPP 84-20622 to the Institute of Polar Studies, and an Ohio 65°. State University postdoctoral fellowship. A thick exposure of Sirius Formation deposits is located in the lower Beardmore Valley, near The Cloudmaker (84°23S References 169014E). Diamictite rests on a steeply sloping platform at about 1,060 meters above sea level and 80 meters above the Elliot, DEL, P.J. Barrett, and P.A. Mayewski. 1974. Reconnaissance geo- Beardmore Glacier. It is cut in nearly vertically dipping meta- logic map of the Plunket Point Quadrangle, Transantarctic Mountains, sediments of the Goldie Formation (Grindley, McGregor, and Antarctica. (U.S. Geological Survey Map A-4,1:250,000.) Washington, Walcott 1964), and dips 15° to 20° to the south. In profile, it D.C.: U.S. Government Printing Office. appears to represent the lower part of a glacial valley-side, the Grindley, G.W. 1967. The geomorphology of the Miller Range, Transan- floor of which would have been near the present glacier surface. tarctic Mountains, with notes on, the glacial history and neotectonics These cross-cutting sets of striations are visible on the platform of East Antarctica. New Zealand Journal of Geology and Geophysics, 10, beneath the diamictite. They trend at 325°, 40°, and 80°. The 557-598. Grindley, G.W., V.R. McGregor, and R.I. Walcott. 1964. Outline of the former two sets are consistent with ice-flow trending in the geology of the Nimrod-Beardmore-Axel Heiberg Glaciers region, general direction of the present valley. The 80° set is oriented Ross Dependency. In R.J. Adie (Ed.), Antarctic geology. Amsterdam: perpendicular to the present valley system and probably re- North-Holland Publishing House. flects local perturbations in the basal ice-flow regime.
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