Sirius Formation
Glacial geology The Sirius Formation: Further considerations B. C. MCKELVEY Department of Geology and Geophysics University of New England Armidale, NSW 2351, Australia L$ J. H. MERCER, D. M.
HARWOOD,
and L. D. STOTT
Institute of Polar Studies
and
Department of Geology and Mineralogy Ohio State University Columbus, Ohio 43210
It has been recently suggested that the Pliocene Sirius Formation of Mercer (1972, 1981) and its equivalents in the Transantarctic Mountains reflect overriding of the mountain chain by a greatly thickened east antarctic ice sheet (Webb et al. 1984). The presence of recycled Cretaceous and Cenozoic marine microfossils is attributed to erosion and transport by the thickened ice sheet of marine strata occupying the Wilkes and Pensacola basins, now hidden beneath the east antarctic ice sheet (Webb et al. 1984). Precise timing of the overriding is uncertain but it must post-date the youngest microfossils present within the Sirius Formation. These are a diatom microflora determined to be between 3.1 and 2.2 million years old (Harwood 1984). To collect more field data and samples for paleontological analysis a team from the Institute of Polar Studies at Ohio State University in the 1983-1984 antarctic field season examined outcrops of the Sirius Formation at Mount Sirius (84°08'S 163°15'E) in the Central Transantarctic Mountains; and at Tillite Spur (85°59'S 126°36'W) and Metavolcanic Mountain (86°13'S 126°15'W), both near the head of the Reedy Glacier. Mercer (1968) describes the two latter localities. In addition, in southern Victoria Land west of McMurdo Sound, Sirius Formation outcrops were examined and collected at Table Mountain (77°56'S 161°59'E), Mount Feather (77°58'S 160°20'E), Mount Fleming (77°33'S 160°08'E), and near Shapeless Mountain (77°26'S 160°26'E). The geology of the Table Mountain and Mount Feather 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 indurated clasts in the diamictites were compatible with the known regional geology. No indurated exotic clasts were noted. The microfossjls faunas and floras are contained within semilithified and often deformed sediment fragments that are virtually indistinguishable from the diamictite matrices. 42
•
'
.,' 4 - - 'I,..
\ p . 1
Figure 1. Subhorizontal striated pavement developed on columnar Ferrar Dolerite at Mount Sirius. The pavement has been revealed by excavation of the overlying Sirius Formation (foreground). In the background the slightly weathered dolerite surface has lost all trace of striations. Pencil (15 centimeters) is aligned approximately parallel to the striations. Mount Sirius. At Mount Sirius an incomplete 84-meter-thick sequence of the Sirius Formation rests disconformably on a gently undulating striated pavement of Ferrar Dolerite. Direction of striations at four (excavated) pavement localities vary between 022° and 105°, with the predominant transport direction being toward 075° (figure 1). The freshness of the pavement beneath the tillite suggests that deposition followed immediately after erosion and scouring of the Ferrar Dolerite. The Sirius Formation consists predominantly of indistinctly stratified diamictites (maximum clast size 4 meters) and interbedded subordinate lensoidal pebble breccias or conglomerates, and very minor laminated siltstones (figure 2). The conglomerates, breccias, and silistones are most abundant in the uppermost 30 meters where soft sediment deformation is common and some beds locally are steeply (greater than 45°) inclined. Reedy Glacier (Tillite Spur). At Tillite Spur, semi-lithified diamictite of the Sirius Formation covers granitic rubble resting on shattered and fragmented bedrock of the Beacon Supergroup. Close inspection of the indistinctly stratified sequence reveals widespread gentle (about 5°) soft sediment deformation which precludes confident determination of imbrication and hence transport direction. Long axis orientation of clasts in the youngANTARCTIC JOURNAL
est horizons preserved indicates an average line of transport direction trending 175°1355°. At least one interval approximately 1.5 meters thick showed subvertical clast fabric, suggesting a period of periglacial weathering during deposition of the sequence (Derbyshire, Gregory, and Hails 1979). Southern Victoria Land. Of the southern Victoria Land localities visited, those near Shapeless Mountain and Mount Fleming abut glacier ice and appear to have been recently overridden. The relationship of the Sirius Formation to the subglacial topography cannot be determined. However, at Mount Feather and Table Mountain (and similarly at Mount Sirius and Tillite Spur) the Sirius Formation outcrops are perched far above present glacier ice. At Mount Feather the glacigene strata outcrop atop the steep-sided northeastern ridge of the mountain (at approximate 77°56'S 160°24E) as an erosion residual surmounting two subsequent scarps 700 meters and 1,000 meters high respectively (Brady and McKelvey 1979). On geomorphic evidence, deposition of the glacial strata appears to predate at least much of the downcutting of the present-day valley profiles. Paleontological evidence from Dry Valley Drilling Project site 11 in nearby Taylor Valley indicates that cutting of the existing ice drainage system is older than latest Miocene (Webb and Wrenn 1982). For this reason Brady and McKelvey (1979) and Barrett and Powell (1982) considered deposition of the Sirius Formation in southern Victoria Land to be at least a late Miocene and probably even older event. This interpretation is apparently in error, because a diatom microflora including a taxon restricted to early late Pliocene through earliest Pleistocene in the southern ocean (Webb et al. 1984) has been identified in the Sirius Formation at Mount Feather. The observed rates of erosion in southern Victoria Land make it inconceivable that the two erosion scarps could be younger than the Sirius Formation. The Formation (carving) of the precipitous northeastern spur of Mount Feather (on which the Sirius Formation rests) is therefore
an older event than the deposition of the formation itself. We suggest that at the time of deposition of the Sirius Formation both 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 was deposited from plateau-derived ice onto a Miocene or older landscape largely buried beneath locally derived, relatively stagnant ice. Subsequent flow of the mantling ice down the drainage system toward the Ross Sea removed much of the Sirius Formation. The only Sirius Formation sediments exposed today were deposited on exposed rock, such as the crest of the northeastern ridge of Mount Feather; the glacial material that was deposited upon the ice filling the valleys of the McMurdo oasis has subsequently been transported and reworked into deposits mantling the now ice-free valley floors. This interpretation of the depositional setting of the Sirius Formation at Mount Feather may well hold for all the Sirius Formation throughout the Transantarctic Mountains, so explaining its occurrence as scattered outcrops at relatively high altitudes. Finally, we would confirm the general southeastward (toward 145°) ice-flow direction determined for the Mount Feather Sirius Formation (Brady and McKelvey 1979) and emphasize that this direction is approximately at right angles to that of the overriding ice episode proposed recently for the same region by Denton et al. (1984). In other words, this latter overriding is a younger event than the deposition of the Sirius Formation. Acknowledgments. We thank James Leide and Carl Thompson for their invaluable assistance in the field. This work was supported by National Science Foundation grants DPP 81-17889A01 and DPP 80-23458A01 to the Institute of Polar Studies, Ohio State University, and by a research grant from the University of New England, Armidale, New South Wales, Australia. References Barrett, P.J., and R.D. Powell. 1982. Middle Cenozoic glacial beds at Table Mountain, southern Victoria Land. In C. Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press. Brady, H.T., and B.C. McKelvey. 1979. The interpretation of a Tertiary tillite at Mt. Feather, southern Victoria Land, Antarctica. Journal of Glaciology, 22(86), 189-193. Hrady, H.T., and B.C. McKelvey. 1983. Some aspects of the Cenozoic glaciation of southern Victoria Land. Journal of Glaciology, 29(102), 343-349.
-I tow
:
wq^wnw_'" '' Figure 2. Poorly sorted breccia and conglomerate (center left) overlain with irregular contact by massive diamictite. The largest clast visible (top center) is approximately 1 meter across. Older diamictites (foregound) underlie the breccia and conglomerate with indistinct contact. Southwestern spur of Mount Sirius.
1984 REVIEW
)enton, G. H., M. L. Prentice, D. E. Kellogg, and T. B. Kellogg. 1984. Late Tertiary history of the Antarctic ice sheet: Evidence from the Dry Valleys. Geology, 23, 263-267. erbyshire, E., K.J. Gregory, and J.R. Hails. 1979. Geomorphological processes. Folkestone Kent.: Dawson Westview Press. arwood, D.M. 1984. Diatoms from the Sirius Formation, Transantarctic Mountains. Antarctic Journal of the U.S., 18(5), 98-100. Mercer, J.H. 1968. Glacial geology of the Reedy Glacier area. Geological Society of America Bulletin, 79, 471-486. \lercer, J.H. 1972. Some observations on the glacial geology of the Beardmore Glacier area. In R.J. Adie (Ed.), Antarctic geology and geophysics. 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 prePleistocene glacial record. Cambridge, England: Cambridge University Press. \Vebb, RN., D.M. Harwood, B.C. McKelvey, J.H. Mercer, and L.D. Stott. 1984. Cenozoic marine sedimentation and ice volume variation on the East Antarctic craton. Geology, 12, 287-291. Webb, P. and J. H. Wrenn. 1982. Upper Cenozoic micropaleontology and biostratigraphy of eastern Taylor Valley, Antarctica. In C. Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press.
43
HARWOOD,
and L. D. STOTT
Institute of Polar Studies
and
Department of Geology and Mineralogy Ohio State University Columbus, Ohio 43210
It has been recently suggested that the Pliocene Sirius Formation of Mercer (1972, 1981) and its equivalents in the Transantarctic Mountains reflect overriding of the mountain chain by a greatly thickened east antarctic ice sheet (Webb et al. 1984). The presence of recycled Cretaceous and Cenozoic marine microfossils is attributed to erosion and transport by the thickened ice sheet of marine strata occupying the Wilkes and Pensacola basins, now hidden beneath the east antarctic ice sheet (Webb et al. 1984). Precise timing of the overriding is uncertain but it must post-date the youngest microfossils present within the Sirius Formation. These are a diatom microflora determined to be between 3.1 and 2.2 million years old (Harwood 1984). To collect more field data and samples for paleontological analysis a team from the Institute of Polar Studies at Ohio State University in the 1983-1984 antarctic field season examined outcrops of the Sirius Formation at Mount Sirius (84°08'S 163°15'E) in the Central Transantarctic Mountains; and at Tillite Spur (85°59'S 126°36'W) and Metavolcanic Mountain (86°13'S 126°15'W), both near the head of the Reedy Glacier. Mercer (1968) describes the two latter localities. In addition, in southern Victoria Land west of McMurdo Sound, Sirius Formation outcrops were examined and collected at Table Mountain (77°56'S 161°59'E), Mount Feather (77°58'S 160°20'E), Mount Fleming (77°33'S 160°08'E), and near Shapeless Mountain (77°26'S 160°26'E). The geology of the Table Mountain and Mount Feather 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 indurated clasts in the diamictites were compatible with the known regional geology. No indurated exotic clasts were noted. The microfossjls faunas and floras are contained within semilithified and often deformed sediment fragments that are virtually indistinguishable from the diamictite matrices. 42
•
'
.,' 4 - - 'I,..
\ p . 1
Figure 1. Subhorizontal striated pavement developed on columnar Ferrar Dolerite at Mount Sirius. The pavement has been revealed by excavation of the overlying Sirius Formation (foreground). In the background the slightly weathered dolerite surface has lost all trace of striations. Pencil (15 centimeters) is aligned approximately parallel to the striations. Mount Sirius. At Mount Sirius an incomplete 84-meter-thick sequence of the Sirius Formation rests disconformably on a gently undulating striated pavement of Ferrar Dolerite. Direction of striations at four (excavated) pavement localities vary between 022° and 105°, with the predominant transport direction being toward 075° (figure 1). The freshness of the pavement beneath the tillite suggests that deposition followed immediately after erosion and scouring of the Ferrar Dolerite. The Sirius Formation consists predominantly of indistinctly stratified diamictites (maximum clast size 4 meters) and interbedded subordinate lensoidal pebble breccias or conglomerates, and very minor laminated siltstones (figure 2). The conglomerates, breccias, and silistones are most abundant in the uppermost 30 meters where soft sediment deformation is common and some beds locally are steeply (greater than 45°) inclined. Reedy Glacier (Tillite Spur). At Tillite Spur, semi-lithified diamictite of the Sirius Formation covers granitic rubble resting on shattered and fragmented bedrock of the Beacon Supergroup. Close inspection of the indistinctly stratified sequence reveals widespread gentle (about 5°) soft sediment deformation which precludes confident determination of imbrication and hence transport direction. Long axis orientation of clasts in the youngANTARCTIC JOURNAL
est horizons preserved indicates an average line of transport direction trending 175°1355°. At least one interval approximately 1.5 meters thick showed subvertical clast fabric, suggesting a period of periglacial weathering during deposition of the sequence (Derbyshire, Gregory, and Hails 1979). Southern Victoria Land. Of the southern Victoria Land localities visited, those near Shapeless Mountain and Mount Fleming abut glacier ice and appear to have been recently overridden. The relationship of the Sirius Formation to the subglacial topography cannot be determined. However, at Mount Feather and Table Mountain (and similarly at Mount Sirius and Tillite Spur) the Sirius Formation outcrops are perched far above present glacier ice. At Mount Feather the glacigene strata outcrop atop the steep-sided northeastern ridge of the mountain (at approximate 77°56'S 160°24E) as an erosion residual surmounting two subsequent scarps 700 meters and 1,000 meters high respectively (Brady and McKelvey 1979). On geomorphic evidence, deposition of the glacial strata appears to predate at least much of the downcutting of the present-day valley profiles. Paleontological evidence from Dry Valley Drilling Project site 11 in nearby Taylor Valley indicates that cutting of the existing ice drainage system is older than latest Miocene (Webb and Wrenn 1982). For this reason Brady and McKelvey (1979) and Barrett and Powell (1982) considered deposition of the Sirius Formation in southern Victoria Land to be at least a late Miocene and probably even older event. This interpretation is apparently in error, because a diatom microflora including a taxon restricted to early late Pliocene through earliest Pleistocene in the southern ocean (Webb et al. 1984) has been identified in the Sirius Formation at Mount Feather. The observed rates of erosion in southern Victoria Land make it inconceivable that the two erosion scarps could be younger than the Sirius Formation. The Formation (carving) of the precipitous northeastern spur of Mount Feather (on which the Sirius Formation rests) is therefore
an older event than the deposition of the formation itself. We suggest that at the time of deposition of the Sirius Formation both 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 was deposited from plateau-derived ice onto a Miocene or older landscape largely buried beneath locally derived, relatively stagnant ice. Subsequent flow of the mantling ice down the drainage system toward the Ross Sea removed much of the Sirius Formation. The only Sirius Formation sediments exposed today were deposited on exposed rock, such as the crest of the northeastern ridge of Mount Feather; the glacial material that was deposited upon the ice filling the valleys of the McMurdo oasis has subsequently been transported and reworked into deposits mantling the now ice-free valley floors. This interpretation of the depositional setting of the Sirius Formation at Mount Feather may well hold for all the Sirius Formation throughout the Transantarctic Mountains, so explaining its occurrence as scattered outcrops at relatively high altitudes. Finally, we would confirm the general southeastward (toward 145°) ice-flow direction determined for the Mount Feather Sirius Formation (Brady and McKelvey 1979) and emphasize that this direction is approximately at right angles to that of the overriding ice episode proposed recently for the same region by Denton et al. (1984). In other words, this latter overriding is a younger event than the deposition of the Sirius Formation. Acknowledgments. We thank James Leide and Carl Thompson for their invaluable assistance in the field. This work was supported by National Science Foundation grants DPP 81-17889A01 and DPP 80-23458A01 to the Institute of Polar Studies, Ohio State University, and by a research grant from the University of New England, Armidale, New South Wales, Australia. References Barrett, P.J., and R.D. Powell. 1982. Middle Cenozoic glacial beds at Table Mountain, southern Victoria Land. In C. Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press. Brady, H.T., and B.C. McKelvey. 1979. The interpretation of a Tertiary tillite at Mt. Feather, southern Victoria Land, Antarctica. Journal of Glaciology, 22(86), 189-193. Hrady, H.T., and B.C. McKelvey. 1983. Some aspects of the Cenozoic glaciation of southern Victoria Land. Journal of Glaciology, 29(102), 343-349.
-I tow
:
wq^wnw_'" '' Figure 2. Poorly sorted breccia and conglomerate (center left) overlain with irregular contact by massive diamictite. The largest clast visible (top center) is approximately 1 meter across. Older diamictites (foregound) underlie the breccia and conglomerate with indistinct contact. Southwestern spur of Mount Sirius.
1984 REVIEW
)enton, G. H., M. L. Prentice, D. E. Kellogg, and T. B. Kellogg. 1984. Late Tertiary history of the Antarctic ice sheet: Evidence from the Dry Valleys. Geology, 23, 263-267. erbyshire, E., K.J. Gregory, and J.R. Hails. 1979. Geomorphological processes. Folkestone Kent.: Dawson Westview Press. arwood, D.M. 1984. Diatoms from the Sirius Formation, Transantarctic Mountains. Antarctic Journal of the U.S., 18(5), 98-100. Mercer, J.H. 1968. Glacial geology of the Reedy Glacier area. Geological Society of America Bulletin, 79, 471-486. \lercer, J.H. 1972. Some observations on the glacial geology of the Beardmore Glacier area. In R.J. Adie (Ed.), Antarctic geology and geophysics. 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 prePleistocene glacial record. Cambridge, England: Cambridge University Press. \Vebb, RN., D.M. Harwood, B.C. McKelvey, J.H. Mercer, and L.D. Stott. 1984. Cenozoic marine sedimentation and ice volume variation on the East Antarctic craton. Geology, 12, 287-291. Webb, P. and J. H. Wrenn. 1982. Upper Cenozoic micropaleontology and biostratigraphy of eastern Taylor Valley, Antarctica. In C. Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press.
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