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Glacial geology

The Sirius Formation: Furtherconsiderations

B. C. MCKELVEY

Department of Geology and GeophysicsUniversity of New England

Armidale, NSW 2351, Australia

J. H. MERCER, D. M. HARWOOD, and L. D. STOTT

Institute of Polar Studiesand

Department of Geology and MineralogyOhio State UniversityColumbus, Ohio 43210

It has been recently suggested that the Pliocene Sirius Forma-tion of Mercer (1972, 1981) and its equivalents in the Transan-tarctic Mountains reflect overriding of the mountain chain by agreatly thickened east antarctic ice sheet (Webb et al. 1984). Thepresence of recycled Cretaceous and Cenozoic marine micro-fossils is attributed to erosion and transport by the thickened icesheet of marine strata occupying the Wilkes and Pensacolabasins, now hidden beneath the east antarctic ice sheet (Webb etal. 1984). Precise timing of the overriding is uncertain but itmust post-date the youngest microfossils present within theSirius Formation. These are a diatom microflora determined tobe between 3.1 and 2.2 million years old (Harwood 1984).

To collect more field data and samples for paleontologicalanalysis a team from the Institute of Polar Studies at Ohio StateUniversity in the 1983-1984 antarctic field season examinedoutcrops of the Sirius Formation at Mount Sirius (84°08'S163°15'E) in the Central Transantarctic Mountains; and at TilliteSpur (85°59'S 126°36'W) and Metavolcanic Mountain (86°13'S126°15'W), both near the head of the Reedy Glacier. Mercer(1968) describes the two latter localities. In addition, in southernVictoria Land west of McMurdo Sound, Sirius Formation out-crops were examined and collected at Table Mountain (77°56'S161°59'E), Mount Feather (77°58'S 160°20'E), Mount Fleming(77°33'S 160°08'E), and near Shapeless Mountain (77°26'S160°26'E). The geology of the Table Mountain and MountFeather localities has been described by Barrett and Powell(1982) and Brady and McKelvey (1979, 1983) respectively.

In all the localities examined, the compositions of the indu-rated clasts in the diamictites were compatible with the knownregional geology. No indurated exotic clasts were noted. Themicrofossjls faunas and floras are contained within semilithifiedand often deformed sediment fragments that are virtually indis-tinguishable from the diamictite matrices.

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Figure 1. Subhorizontal striated pavement developed on columnarFerrar Dolerite at Mount Sirius. The pavement has been revealed byexcavation of the overlying Sirius Formation (foreground). In thebackground the slightly weathered dolerite surface has lost all traceof striations. Pencil (15 centimeters) is aligned approximately paral-lel to the striations.

Mount Sirius. At Mount Sirius an incomplete 84-meter-thicksequence of the Sirius Formation rests disconformably on agently undulating striated pavement of Ferrar Dolerite. Direc-tion of striations at four (excavated) pavement localities varybetween 022° and 105°, with the predominant transport direc-tion being toward 075° (figure 1). The freshness of the pavementbeneath the tillite suggests that deposition followed imme-diately after erosion and scouring of the Ferrar Dolerite. TheSirius Formation consists predominantly of indistinctly strat-ified diamictites (maximum clast size 4 meters) and interbeddedsubordinate lensoidal pebble breccias or conglomerates, andvery minor laminated siltstones (figure 2). The conglomerates,breccias, and silistones are most abundant in the uppermost 30meters where soft sediment deformation is common and somebeds locally are steeply (greater than 45°) inclined.

Reedy Glacier (Tillite Spur). At Tillite Spur, semi-lithified di-amictite of the Sirius Formation covers granitic rubble resting onshattered and fragmented bedrock of the Beacon Supergroup.Close inspection of the indistinctly stratified sequence revealswidespread gentle (about 5°) soft sediment deformation whichprecludes confident determination of imbrication and hencetransport direction. Long axis orientation of clasts in the young-

42 ANTARCTIC JOURNAL

est horizons preserved indicates an average line of transportdirection trending 175°1355°. At least one interval approx-imately 1.5 meters thick showed subvertical clast fabric, sug-gesting a period of periglacial weathering during deposition ofthe sequence (Derbyshire, Gregory, and Hails 1979).

Southern Victoria Land. Of the southern Victoria Land localitiesvisited, those near Shapeless Mountain and Mount Flemingabut glacier ice and appear to have been recently overridden.The relationship of the Sirius Formation to the subglacial to-pography cannot be determined. However, at Mount Featherand Table Mountain (and similarly at Mount Sirius and TilliteSpur) the Sirius Formation outcrops are perched far above pres-ent glacier ice. At Mount Feather the glacigene strata outcropatop the steep-sided northeastern ridge of the mountain (atapproximate 77°56'S 160°24E) as an erosion residual surmount-ing two subsequent scarps 700 meters and 1,000 meters highrespectively (Brady and McKelvey 1979). On geomorphic evi-dence, deposition of the glacial strata appears to predate at leastmuch of the downcutting of the present-day valley profiles.Paleontological evidence from Dry Valley Drilling Project site 11in nearby Taylor Valley indicates that cutting of the existing icedrainage system is older than latest Miocene (Webb and Wrenn1982). For this reason Brady and McKelvey (1979) and Barrettand Powell (1982) considered deposition of the Sirius Formationin southern Victoria Land to be at least a late Miocene andprobably even older event. This interpretation is apparently inerror, because a diatom microflora including a taxon restrictedto early late Pliocene through earliest Pleistocene in the south-ern ocean (Webb et al. 1984) has been identified in the SiriusFormation at Mount Feather. The observed rates of erosion insouthern Victoria Land make it inconceivable that the two ero-sion scarps could be younger than the Sirius Formation. TheFormation (carving) of the precipitous northeastern spur ofMount Feather (on which the Sirius Formation rests) is therefore

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wq^wnw_' " ''Figure 2. Poorly sorted breccia and conglomerate (center left) over-lain with irregular contact by massive diamictite. The largest clastvisible (top center) is approximately 1 meter across. Older diamic-tites (foregound) underlie the breccia and conglomerate with indis-tinct contact. Southwestern spur of Mount Sirius.

an older event than the deposition of the formation itself. Wesuggest that at the time of deposition of the Sirius Formationboth the Beacon Valley and the eastern cirque of Mount Feather(i.e., either side of the ridge), as well as other valleys in the area,were ice-filled. It follows that the Pliocene Sirius Formation wasdeposited from plateau-derived ice onto a Miocene or olderlandscape largely buried beneath locally derived, relativelystagnant ice. Subsequent flow of the mantling ice down thedrainage system toward the Ross Sea removed much of theSirius Formation. The only Sirius Formation sediments exposedtoday were deposited on exposed rock, such as the crest of thenortheastern ridge of Mount Feather; the glacial material thatwas deposited upon the ice filling the valleys of the McMurdooasis has subsequently been transported and reworked intodeposits mantling the now ice-free valley floors.

This interpretation of the depositional setting of the SiriusFormation at Mount Feather may well hold for all the SiriusFormation throughout the Transantarctic Mountains, so ex-plaining its occurrence as scattered outcrops at relatively highaltitudes.

Finally, we would confirm the general southeastward (toward145°) ice-flow direction determined for the Mount Feather SiriusFormation (Brady and McKelvey 1979) and emphasize that thisdirection is approximately at right angles to that of the overrid-ing ice episode proposed recently for the same region by Den-ton et al. (1984). In other words, this latter overriding is ayounger event than the deposition of the Sirius Formation.

Acknowledgments. We thank James Leide and Carl Thompsonfor their invaluable assistance in the field. This work was sup-ported by National Science Foundation grants DPP 81-17889A01and DPP 80-23458A01 to the Institute of Polar Studies, Ohio StateUniversity, and by a research grant from the University of NewEngland, Armidale, New South Wales, Australia.

ReferencesBarrett, P.J., and R.D. Powell. 1982. Middle Cenozoic glacial beds at

Table Mountain, southern Victoria Land. In C. Craddock (Ed.), Ant-arctic geoscience. Madison: University of Wisconsin Press.

Brady, H.T., and B.C. McKelvey. 1979. The interpretation of a Tertiarytillite at Mt. Feather, southern Victoria Land, Antarctica. Journal ofGlaciology, 22(86), 189-193.

Hrady, H.T., and B.C. McKelvey. 1983. Some aspects of the Cenozoicglaciation of southern Victoria Land. Journal of Glaciology, 29(102),343-349.

)enton, G. H., M. L. Prentice, D. E. Kellogg, and T. B. Kellogg. 1984. LateTertiary history of the Antarctic ice sheet: Evidence from the DryValleys. Geology, 23, 263-267.erbyshire, E., K.J. Gregory, and J.R. Hails. 1979. Geomorphologicalprocesses. Folkestone Kent.: Dawson Westview Press.arwood, D.M. 1984. Diatoms from the Sirius Formation, Transan-tarctic Mountains. Antarctic Journal of the U.S., 18(5), 98-100.

Mercer, J.H. 1968. Glacial geology of the Reedy Glacier area. GeologicalSociety of America Bulletin, 79, 471-486.

\lercer, J.H. 1972. Some observations on the glacial geology of theBeardmore Glacier area. In R.J. Adie (Ed.), Antarctic geology andgeophysics. Oslo: Universitetsforlaget.

\Iercer, J. H. 1981. Tertiary terrestrial deposits of the Ross Ice Shelf area,Antarctica. In M.J. Hambrey, and W.B. Harland (Eds.), Earth's pre-Pleistocene glacial record. Cambridge, England: Cambridge UniversityPress.

\Vebb, RN., D.M. Harwood, B.C. McKelvey, J.H. Mercer, and L.D.Stott. 1984. Cenozoic marine sedimentation and ice volume variationon the East Antarctic craton. Geology, 12, 287-291.

Webb, P. and J. H. Wrenn. 1982. Upper Cenozoic micropaleontologyand biostratigraphy of eastern Taylor Valley, Antarctica. In C. Crad-dock (Ed.), Antarctic geoscience. Madison: University of WisconsinPress.

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