Subglacial Meltwater System, Sessrumir Valley, Western Asgard Range, Antarctica

Subglacial meltwater system, Sessrumir Valley, western Asgard Range, Antarctica

DAVID E. SUGDEN

Department of Geography University of Edinburgh Edinburgh, Scotland

DAVID R. MARCHANT

Department of Geography University of Edinburgh Edinburgh, Scotland and Department of Geological Sciences and Institute of Quaternary Studies University of Maine Orono, Maine 04469 Figure 1. Field-based geomorphological map of the central part of the channel. GEORGE H. DENTON

Department of Geological Sciences and and sharply incised into a smooth bedrock slope with an over- Institute of Quaternary Studies all gradient of 10-28°. The long profiles of the tributary chan- University of Maine nels are complex. Some fall continuously downslope to the Orono, Maine 04469 main channel. Others are discontinuous and the long profile is interrupted by potholes and rock bars. Commonly, confluences are ungraded, and for example, tributaries may join each Channels and potholes occur in the mountains flanking the other and the main channel by means of a cliff. The overall McMurdo Dry Valleys. They have been described in the Quar- pattern of the tributary channels is dendritic in that they tend termain Mountains south of Taylor Glacier, in the Asgard and to merge into fewer, larger channels. There are, however, in- Olympus Range on either side of Wright Valley, and in the stances of bifurcation which lead to an anastomosing pattern. Convoy Range (Denton, Kellogg, and Prentice 1983; Denton In certain cases, rock channels 5-10 meters deep split and et al. 1984). These features are one of several lines of evidence rejoin at some point down-channel. used to infer that an ice sheet overrode the McMurdo Dry The channel system is associated with potholes tens of me- Valleys area from the southwest to the northeast in late Tertiary ters across and tens of meters deep. Typically the potholes are time. This view has been challenged by Selby (1986) and Au- circular in plan and have vertical or undercut sides. Cliffs may gustinus and Selby (1990), who suggested that there are al- surround the potholes on all sides or may be breached in one ternative explanations for the various line of evidence and that or two locations. Ice pools occupy the bottom of many poth- these need to be scrutinized further. For example, they sug- oles. Potholes commonly occur in three locations: at the con- gested that the channels and potholes were associated with fluence of two or more channels, at sharp corners along a local ice. Here, we examine the morphology of one channel channel, and on the gentle upper slope between the channels. system in Sessrumir Valley in the western Asgard Range (Ack- The morphology of the channel system is characterized in ert 1990) and conclude that it reflects subglacial meltwater ac- detail by forms associated with geomorphological processes tivity and is likely to have been associated with overriding ice. that have occurred since their formation. These include sand The channel system, which is 2.5 kilometers long and about dunes in the lee of north-facing cliffs as well as fretted and 600 meters wide, is cut into rock slopes along the flanks of an pitted rock surfaces. The floors of many channels are covered S-shaped butte marking the eastern wall of Sessrumir Valley. by an armor of pebbles with ventifacted facets and desert var- It begins at an elevation of 1,710 meters in the south and falls nish. These are commonly arranged into the form of ripples. to 1,510 meters in the north. Figures 1 and 2 show a field- The continuity, sinuosity, and confluent pattern of the chan- based geomorphological map of the central part of the channel nels and their association with potholes all point to the role of system and an oblique aerial photograph. The main channel water in their formation. Several other features point to the consists of five segments, each of which starts with a cliffed role of meltwater flowing hydrostatically in a subglacial en- "plunged pool" of 5-20 meters and runs for several hundred vironment. For example, these include the irregular long pro- meters before petering out at the next plunge pool. The chan- files, reverse gradients, anastomosing patterns cut into bedrock, nel segments increase in size to a maximum width of 145 me- and ungraded confluences. Such features are well known from ters and a depth of 25-50 meters. Tributary channels run into mid-latitude areas of former glaciation (Sugden and John 1976) the main channel from the slopes of the butte. They are cuffed and are explicable in terms of theories of subglacial hydrology

56 ANTARCTIC JOURNAL Figure 2. Oblique aerial photograph of the central part of the channel system.

(Shreve 1972: Sharp, Gemmell, and Tison 1989). Large potholes to regional ice flow rather than local ice flow confined to sep- and their association with irregularities in a channel are also arate valleys. most easily explained by the high velocity streams, heavily Three points were made clear by our study: loaded with debris, which are characteristic of subglacial re- • The pattern and morphology of channels and potholes has gimes. revealed the existence of subglacial meltwater systems in the The channel system has been modified by wind and weath- Asgard Range. The pattern of the channels is consistent with ering since its formation. Fretting of the rock walls, salt weath- the hypothesis of overriding ice from the southwest. ering, sand dunes, and ventifacts arranged into ripples all add • The relationship of the channels to the underlying topo- detail to the morphology; however, wind action cannot explain graphy implies that the main features, such as the butte, the characteristic features of water action listed above. were present before the overriding glaciation. The overall pattern and site of the channels is consistent • Wind action accounts for the detailed morphology within with an origin beneath overriding ice. Firstly, the channels are the channel system, including fretting, sand dunes, and in the lee of a butte and their pattern suggests that meltwater ripples. originated on the butte. The most likely explanation of this This work was funded National Science Foundation grant DPP pattern is that the meltwater was derived from a zone of pres- 86-13842. Helicopter support was provided by the U.S. Navy. sure melting on the upstream side of the butte beneath overriding ice. Secondly, similar channels are also incised into the saddles elsewhere in the main divide of the Asgard Range; References for example, the saddle in the headwall of the adjacent valley at an altitude of 1,750 meters is cut by a rock channel tens of Ackert, R.P., Jr. 1990. Surficial geology and stratigraphy in Njord Valley, meters deep which commences in a cliff at the actual divide. western Asgard Range, Antarctica: implications for late Tertiary glacial history. This implies movement of meltwater across the mountains. A (Master of science thesis, University of Maine, Orono, Maine.) Augustinus, P.C., and M.J. Selby. 1990. Rock slope development in third argument concerns the links between Sessrumir Valley McMurdo Oasis, Antarctica, and implications for interpretations of and the valley immediately to the east. The channel system glacial history. Geografiska Annaler, 72A, 1, 55-62. crosses from the former to the latter. This is a pattern pointing Denton, G.H., T.B. Kellogg, and M.L. Prentice. 1983. Ice sheet ov-

1990 REVIEW 57 erriding of the Transantarctic Mountains. Antarctic Journal of the U.S., Sugden, D.E., and B.S. John. 1976. Glaciers and landscape. New York: 18, 93-94. Arnold. Denton, G.H., M.L. Prentice, D.E. Kellogg, and T.B. Kellogg. 1984. Sharp, M., C. Gemmell, and J-L Tison. 1989. Structure and stability Late Tertiary history of the Antarctic ice sheet: Evidence from the of the former subglacial drainage system of the Glacier de Tsan- Dry Valleys. Geology, 12, 263-267. fleuron, Switzerland. Earth Surface Processes and Landforms, 14, 119- Selby, M.J. 1986. The evidence of Miocene ice sheet overriding of the 134. Transantarctic Mountains. An agnostic view. New Zealand Antarctic Shreve, R.L. 1972. Movement of water in glaciers. Journal of Glaciology, Record, 11(62), 205-214. 11, 205-214.

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