Therniokarst - a mechanism of de-icing ice-cored moraines TERRY R. HEALY

Flealy, T. R. 1975 03 01 : Thermokarrt - a mechanism of de-icing ice-corcd moixiiics. Borms, Val. 4, pp. 19-23. Oslo. ISSN 0300-9483.

Thcrmokai-st features occur widely in the younger i orcd moi-aincs and kame ~ C I I ; I C C S i n I-y Valleys region of Victoi ia Land, Antarct Common form.; at-e mcll dolines, natui-al bridges, and :,uhsut-lacc. flow. Such fcalut-cs typically dc\elop i n stagnant ice with a low material exchange.

The developmciit of ii thermokartt cavc system near Llic 'raylot. Glacici- duting thc southci-11 summer of 1973-74 is dc\ci-ihed, and thc role of' the thei~mokarst prow<.; i n land- form devclopmeiit i i i the DIy Valleys area is discusscd.

' / 'wry R . I l d y . I)c.purlineti/ of Ertrfli Scirticrs. Uciivwsity ~j Waiktrro. H o m i l f o r r , New' Z(>ulatril, l4r/1 Novrtiihrr, 1974.

BOREAS

Pseudokarst features generally develop in stag- nant ice conditions (Clayton 1964; Embleton & King 1968). In the Dry Valleys region of Victoria Land, Antarctica, glacier economies are very inactive and stagnant ice conditions abound (PCwC 1960). The stagnant ice normal- l y occurs as ice-cored moraine and is now known to be extensive over a large area of the Dry Valleys (Calkin & Nichols 1972; Rains & Selby 1972).

Thrmokiirst is the term used to denote the process whereby the melting of stagnant glacial ice, or of ground ice in unconsolidated sedi- ments, creates pseudokarst landforms (Jennings 197 1 ; Sweeting 1972). Common glacial pseudo- karst forms evident in Antarctica are melt dolines, tunnels, caves, sinking streams and dry gullies, subsurface flow, springs and natural bridges, linear valleyside finger gullies, and knob and kettle topography.

The following account reviews the extent of ice-cored moraines and outlines the thermo- karst process operating as a mechanism for de-icing the ice-cored features, which also re- sults in distinctive landforms.

Extent of ice-cored features in the McMurdo Dry Valleys The areal extent of ice-cored moraine and kame terraces in the McMurdo Dry Valleys has not always been readily appreciated. l t was early noted in isolated instances by geologists of Scott's expeditions. David & Priestley (1914) recognised the Strand inoraines as ice-cored, while Debenham (1914) reported ice-cored lateral moraine of the lower Koettlitz. Taylor (1914, 1922) reported that the low moraine promontory at Cape Chocolate 'was built of primeval ice' (Taylor 79 14:456). However, Taylor did not mention that the moraines of the snouts of the Hobbs and Taylor Glaciers which he describes in detail were also ice- cored.

A number of later workers have likewise not emphasised that many if not most of the young moraines in the Dry Valleys are ice-cored (Webb & McKelvey 3959; Nichols 1966a; McCraw 1967; Skinner & Kicker :1968). Re- ports of their location have recently been given in Calkin & Nichols (1972) and listed in Rains & Selby (1972).

20 Terry R . Heirly BOREAS 4 (1975)

M C M U R D O S O U N D

Fig . 1. Rcportcd and k n o w n ice-col-ed features, McMui-do Sound, Antatclica.

The 1973-74 Antarctic summer field season was remarkable for its warmth. Temperatures at Vanda Station exceeded +10 C on several occasions. On one day a phenomenal +15"C was recorded. As a result the intensity of thaw was much greater than normal so that even deep-seated ice began to melt. The presence of ice-core became evident from the thermoltarst features and meltwater patches.

Ice-cored features were observed by the writer along the margin of the Koettlitz Glacier from Trough Lake to the Miers Valley, and in the Taylor Valley around Lakes Bonney, Chad, and Fryxcll. Other reported locations around McMurdo Sound are illustrated in Fig. 1.

Ceomorphology of ice-cored features The geomorphology of some of the ice-cored

moraines and kame terraces has been discussed generally by Nichols (1966b), Calkin & Nichols (1972), and more particularly by Rains & Selby (1972) for the Barwick Valley. Usually the overlying moraine is less than 2 m thick. Often the ice is so well concealed by the blanket of moraine that existence of the ice-core is ap- parent only during excessive thaw activity.

Near the present glacier margins fresh look- ing ice-cored moraines commonly exhibit a well-defined linear form with steep sides and a very thin mantle of silty debris blanketing the ice-core, as for example at the Taylor Glacier snout (Fig. 2), or near the terminus of the Webb Glacier (Rains & Selby 1972).

In contrast the ice-cored moraines around Lakes Chad and Fryxell and the Lower Taylor Valley are covered by a greater thickness of moraine of about 2 m. The terrain here is

UORI-.AS 4 (1975) Thevrnokavst 21

PSEUDOKARST FEATURES NEAR TAYLOR GLACIER SNOUT,

LAKE BONNEY, ANTARCTICA. JANUARY, 1974

Stream f low from

characterised by knobs and mounds, small dry abandoned meltwater gullies, and by kettles and hollows.

Certain kame terraces are distinctive ice- cored features. Ice-cored kame terraces border the Koettlitz Glacier on the north side of Wal- cott Bay in the vicinity of the Alph River. Characteristic features include steep-sided gullies deeply dissecting the kame terrace. Oc- casionally ice outcrops in the sides of these gullies. In addition there are kettles, dry kettle depressions, and some large ponds. Another distinctive kame terrace, at least partly ice- cored, occurs at 60 m above the western lobe of Lake Bonney, while kame moraines have been recorded at the snout of the Lower Wright Glacier (Nichols 1971).

Johnson (1 97 I ) notes five processes effecting degradation of ice-cored moraines in Yukon Territory, Canada. These are the development of large cracks in the ice, slips along shear planes, mudflows of the silty unconsolidated moraine veneer, direct melting of exposed ice, and development of kettles. Only the last can be considered as a thermokarst process.

Some of these processes are also operating on certain Antarctic ice-cored moraines. For example, degradation on the very thin moraine- veneered ice at the Taylor Glacier snout results

from a combination of development of cracks, slips, mudflows, and direct melting of exposed ice.

However, the most potent mechanism of ice removal during the very warm summer field season of 1973-74 was the thermokarst process. This particularly applied where exogenic melt- water streams flow through ice-cored features blanketed by fairly thick glacial drift deposits.

A classic example of thermokarst processes related to a sinking stream and resulting in pseudokarst landforms was observed near the snout of the Taylor Glacier during the height of the 1973-74 thaw. A small meltwater stream from the Calkin Glacier flowed directly down- slope until it reached a prominent kame terrace about 60 m above Lake Bonney. In previous seasons this stream had incised a v-notch 2-3 m deep into the kame terrace. On 4 January, 1974, it was first noticed that this stream dis- appeared into a small melt doline a few metres in diameter and re-emerged about 20 m down- stream.

The stream was revisited ten days later. The melt doline had considerably enlarged, and the stream disappeared into a cave some 15 m long and 3 m high (Figs. 2, 3). It was possible to walk into this cave, though foolhardy due to the frequency of falling rocks which were

22 Tcrry R. H e ~ i l y BOREAS 4 (1975)

Fig. 3. Gully fotmation by thermokarst processes. In the foreground is the doline and cave entrance. Note the sunken \tt-eam and na tura l hi-idges. The Taylor Glacier is in the background.

melting out from the roof of the cave. At this time the process of cave enlargemcnt and col- lapse of the walls was extremely active.

The cavc was not formed within pure ice. Rather the walls of thc cave appeared to be stagnant ice with large lenscs of glaciofluvial outwash delta-fan material, somewhat similar to the kame moraine described by Nichols (Ighhb, 1971) for the Lower Wright Glacier. Generally the sediment consistcd of well-sorted gravels of 1 cm diameter set in a finer sandy matrix, and interspersed with few large boul- ders.

After passing through the cave the stream disappeared beneath a mass of white ice. The stream ‘sunk’ for the next 80 m below the original v-shaped gully floor, but its position could bc traced from the collapse of the drift material above the subsurface stream. Surpris- ingly the stream did not cut vertically down- wards beneath thc gully floor, but cut ‘diago-

nally’, i.e. both laterally and vertically through the stagnant ice.

The resulting gully form was essentially v- shaped with gully sides comprised of large moraine boulders. Presumably the finer sedi- ments had been washed out prior to the stream sinking below the surface. Distinct natural bridges had formed from collapse of the large boulders off the gully side.

Comparison with fluvially inactive gullies nearby suggests that the sharpness of the gully profile is masked by a blanket of sand and smail pebbles covered by lag gravels. This would occur during the winter periods of strong Antarctic winds.

Other thermokarst features abundant in the moraines around McMurdo Sound are kettles. Debenham (1914) recorded the occurrence of water-filled depressions 7 m deep in the moraines bordering the Koettlitz Glacier. In this area kettles seem to be related to the

BOREAS 4 (1975) Theumokaust 23

ponding of subsurface melt and drainage along major fractures in the ice-core. The develop- ment of kettles in stagnant ice of the Lower Wright Valley has also been discussed by Nichols (1971).

Implications and conclusion The likelihood of finding an ice-core in the 60 m kame terrace above the western lobe of Lake Bonney, higher than the snout of the present Taylor Glacier, would not normally be anticipated. Yet the thermokarst observations and the occurrence of numerous parallel finger- like meltwater gullies cutting through this ter- race indicates that it is indeed ice-cored. More- over, the occurrence of many pseudokarst features in the younger moraines and kame terraces suggests that ice-cored features are probably of greater extent in the Dry Valleys than previously conceded.

The origin of such ice-cored moraines, kame moraines, and kame terraces is also problemati- cal (0strem 1959, 1964; Nichols 1966b; Souchez 1967; Qstrem & Arnold 1970; John- son 1971), but discussion is not entered into here.

The main mechanism for removal of this stagnant ice is by thermokarst processes result- ing from subsurface flow of meltwater. Land- forms produced are melt doline, kettles, gullies with natural bridges, and subsurface drainage.

nckrroM,ledgen?erits. - The New Zealand University GI-ants Committee provided financial support for the Wnikato Univcrsity Antarctic Research Unit expedi- tion. Thanks are also expressed to the N.Z.D.S.I.R. and US. Navy for logistic suppoi-t and transport in the ficld.

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