Glacial geology Recycled marine microfossils from basal debris-ice in ...
Glacial geology Recycled marine microfossils from basal debris-ice in ice-free valleys of southern Victoria Land D.M. HARWOOD and P-N. WEBB Institute of Polar Studies Ohio State University Columbus, Ohio 43210
Wright Upper Glacier in the ice-free valleys of southern Victoria Land, Antarctic extends from the Polar Plateau, over the Airdevronsix Icefalls, and into Wright Valley, a distance of about 7 kilometers and a drop in elevation from 1,200 meters at the base of the ice-falls to 1,000 meters at the terminus. Surface icelevel contours suggest Wright Upper Glacier is composed largely of ice from a local ice-dome inland of the Transantarctic Mountains (Drewry 1980), with only minor ice contribution from the east antarctic ice sheet. Basal debris-ice was collected from the terminus of Wright Upper Glacier in 1983. The ice was melted at McMurdo Station, and debris extracted from the ice was examined for microfossil and sediment content at Ohio State University. Cenozoic marine microfossils including marine diatoms, foraminifera, and sponge spicules were recovered from this basal debris. The diatoms are poorly preserved but can be identified as belonging to genra Actinocyclus, Cosinodiscus, Stephanopyxis, Thalassjonema and Thalassjosjra. No whole diatoms were re-
covered, preventing species identification. Numerous fragments that could possibly be from Thalassiosira torokina (upper Miocene through upper Pliocene) were encountered. Foraminifera are greatly compressed, deformed, and largely infil led, suggesting an origin in a thick sedimentary sequence of probable Tertiary age. Age-diagnostic taxa were not identified. The fauna consists of one internal cast of a clacareous foram (Family Anomalinidae) and four agglutinated taxa (Family Lituolidae). The largest foraminifera recovered was greater than 250 micrometers in great dimension. Sediments in the basal debris-ice consist primarily of clay and silt (mostly angular quartz and some feldspar). Minor amounts of brown chert and sedimentary carbonate were recognized. The paucity of rounded quartz grains and heavy minerals suggests a source area outside the ice-free valley region, where Beacon Supergroup sediments, Ferrar Dolerites, and basement complex granites are dominant. The absence of volcanic material (McMurdo Volcanic Group) also argues against a Ross Sea origin for this material. 1986 REVIEW
The microfossils recovered from Wright Upper Glacier are very similar to those recovered from the Sirius Formation (Harwood 1983, 1986, Antarctic Journal this issue; Webb et al. 1984, in preparation). The foraminiferal faunas are unlike all faunas reported from deposits in the ice-free valley region; agglutinated foraminifera are not known as part of the valley fauna (Webb 1974; Webb and Wrenn 1982; Ishman 1985) and appear to be exotic. The microfossils in the Sirius Formation originated in the Wilkes subglacial basin in East Antarctica (Webb et al. 1984) and were reworked into this glacial unit by east antarctic ice (figure). The nature of preservation and composition of the foraminifera and diatom assemblages are similar to Sirius Formation microfossil assemblages. The immediate sources for the microfossils recovered from the basal debris-ice are local deposits of the Sirius Formation beneath or near Wright Upper Glacial (figure). The Sirius Formation occurs near Wright Upper Glacier on Mount Fleming (near plateau ice levels), on Shapeless Mountain, and probably beneath the polar plateau as indicated by erratics of the Sirius Formation in the Elephant Moraine (76°11'S 157°10'E) which also contain marine diatoms (Harwood 1986). Other possible sources for the microfossils include: (1) the Wilkes subglacial basin, by direct transport from the east antarctic ice sheet and (2) a local marine basin in the western portion of Wright Valley which has been uplifted with the Transantarctic Mountains. The Sirius Formation appears to be the most likely source for the microfossils because an ice-dome presently blocks east antarctic ice from entering Wright Valley and because the fauna bears no resemblance to known ice-free valley faunas. Microfossils from East Antarctica are apparently being transported across the Transantarctic Mountain rock threshold and into the ice-free valley region. Given the suggested history of major increases in the size of the east antarctic ice sheet (Mayewski and Goldthwait 1985; Webb et al. 1984; Denton et al. 1984), marine microfossils in glacial deposits in the ice-free
Diagram illustrating transport of microfossils into the Sirius Formation (A) by the east antarctic ice sheet and erosion and transport of microfossils (B) by Wright Upper Glacier.
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valleys and other areas to the east of the Transantarctic Mountains divide may have a source in sedimentary basins in East Antarctica. Correlation of glacial deposits by their constituent microfossils should be approached with caution in light of evidence for reworking of marine sediments from East Antarctica. The presence of similar microfossil assemblages in glacial deposits does not imply coeval deposition but more likely reflects an earlier marine event "upstream" from these deposits. This research was supported by National Science Foundation grant DPP 83-15553 and DPP 84-20622.
References Denton, G.H., M.L. Prentice, D.E. Kellogg, andT.B. Kellogg. 1984. Late Tertiary history of the Antarctic ice sheet: Evidence from the Dry Valleys. Geology, 12, 263-267. Drewry, D.J. 1980. Pleistocene bimodal response of Antarctic ice. Nature, 287, 214-216. Harwood, D.M. 1983. Diatoms from the Sirius Formation, Transantarctic Mountains. Antarctic Journal of the U.S., 18(5), 98-100.
Glacial geology of Spike Cape, McMurdo Sound M.C.G. MABIN Institute of Polar Studies Ohio State University Columbus, Ohio 43210
Spike Cape is a small ice-free area on the western side of McMurdo Sound, 15 kilometers northwest of Marble Point. A promontory rises to about 150 meters above sea level and is connected by a tombolo with two low knolls rising to 25 meters above sea level. These form a small peninsula that extends northeast for 1.25 kilometers. Bedrock is microdiorite gneiss (Gunn and Warren 1962) that has been smoothed and streamlined by glacial action. A thin mantle of glacial deposits now covers most of the area and at lower elevations, the bedrock and till has been further modified by marine processes (Nichols 1968). The surficial geology is shown in the figure. Differences in composition and weathering allow three till sheets to be identified: knoll till, promontory till, and piedmont till. Knoll till caps the two low knolls near Spike Cape. It consists of poorly sorted sand to boulder-sized material. Most is locally derived gneiss, but erratic rock types are common including granodiorite, granite, and scoriaceous basalts of the McMurdo Volcanic Group. The till and exposed bedrock are noticeably weathered. Quartz-rich veins stand up to 8 centimeters above the gneiss country-rock, and cavernous weathering has re88
Harwood, D.M. 1986. Diatom biostratigraphy and paleoecology with a Cenozoic history of antarctic ice sheets. (Doctoral dissertation, Ohio State University, Columbus, Ohio.) Ishman, S.E. 1985. Foraminiferal hiostratigraphy and paleoecology of Dry Valley Drilling Project cores 10 and ii, Taylor Valley, Antarctic. (Masters Thesis, Ohio State University.) Mayewski, PA., 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.F. Splettstoesser (Eds.), Geology of the Transantarctic Mountains. (Antarctic Research Series, Vol. 36.) Washington, D.C.: American Geophysical Union. Webb, P.-N. 1974. Micropaleontology, paleoecology and correlation of the Pecten gravels, Wright Valley, Antarctica and a description of Trochoelphidiella onyxi n.gen., n.sp. Journal of Foraminiferal Research, 185-199. Webb, P.-N., 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.-N., B.C. McKelvey, D.M. Harwood, M.C.G. Mabin, and J.H. Mercer. In preparation. Sirius Formation of the Beardmore Glacier region. Antarctic Journal of the U.S.
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.
moved over 30 centimeters of material from some of the granodiorite and granite boulders. Many of the cobbles and boulders have had their exposed surfaces rounded by exfoliation and grain-disintegration processes. A soil profile has begun to develop in the till which is oxidized a red-brown color to a depth of 25 centimeters. Promotory till mantles the steep slopes near the Wilson Piedmont Glacier. It has similar characteristics as the knoll till, except that no McMurdo volcanic clasts were found. The piedmont till occurs on the generally flat area at the top of the promontory, in front of a recently retreated portion of the Wilson Piedmont Glacier. It consists of boulders and cobbles scattered over the gneiss bedrock. It is composed mainly of gneissic rocks with occasional granodiorite and granite erratics; however, no McMurdo volcanic clasts were seen. The rocks are all unweathered and some boulders can be seen precariously perched on smaller rock fragments. Striations were found on the surface of a dark fine-grained dyke that intrudes the gneiss (figure). The dyke is 2.5 meters wide and extends east-northeast for about 400 meters from the glacier margin. Two contrasting sets of striations commonly occur, and cross-cutting relationships allow relative age differentiation and sense of movement to be determined. The most common set trends between 65° and 80° (approximately eastnortheast) and is preserved on rock faces oriented toward the Wilson Piedmont Glacier. The second set trends at 320°-140° (northwest to southeast) and is preserved on small facets oriented away from the Wilson Piedmont Glacier. The formation of the east-northeast trending set has obliterated most of the northwest-to-southeast trending set, the latter being preserved only on protected lee-side rock facets. Thus, the east-northeast set is interpreted as the youngest, formed by an expanded Wilson Piedmont Glacier. The sense of ice-flow direction for the older set cannot be directly determined, although they are oriented perpendicular to the present general flow of the ANTARCTIC JOURNAL
Wright Upper Glacier in the ice-free valleys of southern Victoria Land, Antarctic extends from the Polar Plateau, over the Airdevronsix Icefalls, and into Wright Valley, a distance of about 7 kilometers and a drop in elevation from 1,200 meters at the base of the ice-falls to 1,000 meters at the terminus. Surface icelevel contours suggest Wright Upper Glacier is composed largely of ice from a local ice-dome inland of the Transantarctic Mountains (Drewry 1980), with only minor ice contribution from the east antarctic ice sheet. Basal debris-ice was collected from the terminus of Wright Upper Glacier in 1983. The ice was melted at McMurdo Station, and debris extracted from the ice was examined for microfossil and sediment content at Ohio State University. Cenozoic marine microfossils including marine diatoms, foraminifera, and sponge spicules were recovered from this basal debris. The diatoms are poorly preserved but can be identified as belonging to genra Actinocyclus, Cosinodiscus, Stephanopyxis, Thalassjonema and Thalassjosjra. No whole diatoms were re-
covered, preventing species identification. Numerous fragments that could possibly be from Thalassiosira torokina (upper Miocene through upper Pliocene) were encountered. Foraminifera are greatly compressed, deformed, and largely infil led, suggesting an origin in a thick sedimentary sequence of probable Tertiary age. Age-diagnostic taxa were not identified. The fauna consists of one internal cast of a clacareous foram (Family Anomalinidae) and four agglutinated taxa (Family Lituolidae). The largest foraminifera recovered was greater than 250 micrometers in great dimension. Sediments in the basal debris-ice consist primarily of clay and silt (mostly angular quartz and some feldspar). Minor amounts of brown chert and sedimentary carbonate were recognized. The paucity of rounded quartz grains and heavy minerals suggests a source area outside the ice-free valley region, where Beacon Supergroup sediments, Ferrar Dolerites, and basement complex granites are dominant. The absence of volcanic material (McMurdo Volcanic Group) also argues against a Ross Sea origin for this material. 1986 REVIEW
The microfossils recovered from Wright Upper Glacier are very similar to those recovered from the Sirius Formation (Harwood 1983, 1986, Antarctic Journal this issue; Webb et al. 1984, in preparation). The foraminiferal faunas are unlike all faunas reported from deposits in the ice-free valley region; agglutinated foraminifera are not known as part of the valley fauna (Webb 1974; Webb and Wrenn 1982; Ishman 1985) and appear to be exotic. The microfossils in the Sirius Formation originated in the Wilkes subglacial basin in East Antarctica (Webb et al. 1984) and were reworked into this glacial unit by east antarctic ice (figure). The nature of preservation and composition of the foraminifera and diatom assemblages are similar to Sirius Formation microfossil assemblages. The immediate sources for the microfossils recovered from the basal debris-ice are local deposits of the Sirius Formation beneath or near Wright Upper Glacial (figure). The Sirius Formation occurs near Wright Upper Glacier on Mount Fleming (near plateau ice levels), on Shapeless Mountain, and probably beneath the polar plateau as indicated by erratics of the Sirius Formation in the Elephant Moraine (76°11'S 157°10'E) which also contain marine diatoms (Harwood 1986). Other possible sources for the microfossils include: (1) the Wilkes subglacial basin, by direct transport from the east antarctic ice sheet and (2) a local marine basin in the western portion of Wright Valley which has been uplifted with the Transantarctic Mountains. The Sirius Formation appears to be the most likely source for the microfossils because an ice-dome presently blocks east antarctic ice from entering Wright Valley and because the fauna bears no resemblance to known ice-free valley faunas. Microfossils from East Antarctica are apparently being transported across the Transantarctic Mountain rock threshold and into the ice-free valley region. Given the suggested history of major increases in the size of the east antarctic ice sheet (Mayewski and Goldthwait 1985; Webb et al. 1984; Denton et al. 1984), marine microfossils in glacial deposits in the ice-free
Diagram illustrating transport of microfossils into the Sirius Formation (A) by the east antarctic ice sheet and erosion and transport of microfossils (B) by Wright Upper Glacier.
87
valleys and other areas to the east of the Transantarctic Mountains divide may have a source in sedimentary basins in East Antarctica. Correlation of glacial deposits by their constituent microfossils should be approached with caution in light of evidence for reworking of marine sediments from East Antarctica. The presence of similar microfossil assemblages in glacial deposits does not imply coeval deposition but more likely reflects an earlier marine event "upstream" from these deposits. This research was supported by National Science Foundation grant DPP 83-15553 and DPP 84-20622.
References Denton, G.H., M.L. Prentice, D.E. Kellogg, andT.B. Kellogg. 1984. Late Tertiary history of the Antarctic ice sheet: Evidence from the Dry Valleys. Geology, 12, 263-267. Drewry, D.J. 1980. Pleistocene bimodal response of Antarctic ice. Nature, 287, 214-216. Harwood, D.M. 1983. Diatoms from the Sirius Formation, Transantarctic Mountains. Antarctic Journal of the U.S., 18(5), 98-100.
Glacial geology of Spike Cape, McMurdo Sound M.C.G. MABIN Institute of Polar Studies Ohio State University Columbus, Ohio 43210
Spike Cape is a small ice-free area on the western side of McMurdo Sound, 15 kilometers northwest of Marble Point. A promontory rises to about 150 meters above sea level and is connected by a tombolo with two low knolls rising to 25 meters above sea level. These form a small peninsula that extends northeast for 1.25 kilometers. Bedrock is microdiorite gneiss (Gunn and Warren 1962) that has been smoothed and streamlined by glacial action. A thin mantle of glacial deposits now covers most of the area and at lower elevations, the bedrock and till has been further modified by marine processes (Nichols 1968). The surficial geology is shown in the figure. Differences in composition and weathering allow three till sheets to be identified: knoll till, promontory till, and piedmont till. Knoll till caps the two low knolls near Spike Cape. It consists of poorly sorted sand to boulder-sized material. Most is locally derived gneiss, but erratic rock types are common including granodiorite, granite, and scoriaceous basalts of the McMurdo Volcanic Group. The till and exposed bedrock are noticeably weathered. Quartz-rich veins stand up to 8 centimeters above the gneiss country-rock, and cavernous weathering has re88
Harwood, D.M. 1986. Diatom biostratigraphy and paleoecology with a Cenozoic history of antarctic ice sheets. (Doctoral dissertation, Ohio State University, Columbus, Ohio.) Ishman, S.E. 1985. Foraminiferal hiostratigraphy and paleoecology of Dry Valley Drilling Project cores 10 and ii, Taylor Valley, Antarctic. (Masters Thesis, Ohio State University.) Mayewski, PA., 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.F. Splettstoesser (Eds.), Geology of the Transantarctic Mountains. (Antarctic Research Series, Vol. 36.) Washington, D.C.: American Geophysical Union. Webb, P.-N. 1974. Micropaleontology, paleoecology and correlation of the Pecten gravels, Wright Valley, Antarctica and a description of Trochoelphidiella onyxi n.gen., n.sp. Journal of Foraminiferal Research, 185-199. Webb, P.-N., 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.-N., B.C. McKelvey, D.M. Harwood, M.C.G. Mabin, and J.H. Mercer. In preparation. Sirius Formation of the Beardmore Glacier region. Antarctic Journal of the U.S.
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.
moved over 30 centimeters of material from some of the granodiorite and granite boulders. Many of the cobbles and boulders have had their exposed surfaces rounded by exfoliation and grain-disintegration processes. A soil profile has begun to develop in the till which is oxidized a red-brown color to a depth of 25 centimeters. Promotory till mantles the steep slopes near the Wilson Piedmont Glacier. It has similar characteristics as the knoll till, except that no McMurdo volcanic clasts were found. The piedmont till occurs on the generally flat area at the top of the promontory, in front of a recently retreated portion of the Wilson Piedmont Glacier. It consists of boulders and cobbles scattered over the gneiss bedrock. It is composed mainly of gneissic rocks with occasional granodiorite and granite erratics; however, no McMurdo volcanic clasts were seen. The rocks are all unweathered and some boulders can be seen precariously perched on smaller rock fragments. Striations were found on the surface of a dark fine-grained dyke that intrudes the gneiss (figure). The dyke is 2.5 meters wide and extends east-northeast for about 400 meters from the glacier margin. Two contrasting sets of striations commonly occur, and cross-cutting relationships allow relative age differentiation and sense of movement to be determined. The most common set trends between 65° and 80° (approximately eastnortheast) and is preserved on rock faces oriented toward the Wilson Piedmont Glacier. The second set trends at 320°-140° (northwest to southeast) and is preserved on small facets oriented away from the Wilson Piedmont Glacier. The formation of the east-northeast trending set has obliterated most of the northwest-to-southeast trending set, the latter being preserved only on protected lee-side rock facets. Thus, the east-northeast set is interpreted as the youngest, formed by an expanded Wilson Piedmont Glacier. The sense of ice-flow direction for the older set cannot be directly determined, although they are oriented perpendicular to the present general flow of the ANTARCTIC JOURNAL
