Tuatara: Volume 15, Issue 3, December 1967
The McMurdo Oasis
The McMurdo Oasis
Antarctica is perhaps the most stable of the continents. Some earthquakes occur in shield areas such as Australia and North America, but the only major shocks recorded from Antarctica originate from outside the Antarctic Circle. This lack of earthquakes is, perhaps, only a feature of the present, but it is distinct. Antarctica has, however, undergone great past tectonic upheavals which have given rise to complexly folded strata, and to mountains which now rise to heights of the order of 15,000 feet above sea level.
Antarctica can be divided into two provinces, eastern and western Antarctica, each with their characteristic physiography and geology. The western province includes the Antarctic Peninsula, Marie Byrd Land, and the western coast of the Weddell Sea. It is geologically related to the fold mountain chains of the Andes and New Zealand. The eastern Antarctic geological province includes most of the remainder of the continent (Map 1) and is related to the African, Indian and Australian shields. Little is known about the transitional zone which separates these two provinces, and extends from the Ross to the Weddell Sea.
The eastern Antarctic province resembles a number of other ancient and highly stable landmasses, which are thought to have once been parts of a single continent — Gondwanaland. South Africa, India, Australia, parts of South America and eastern Antarctica, all have broadly similar geology, especially in respect of the Devonian to Jurassic rocks. These resemblances have supported the idea that a single southern continental mass was broken apart at some time in the late Mesozoic and the pieces have since drifted into the positions they now occupy.
Antarctica is a continent which is almost completely covered by a vast ice cap. Around the margins of the ice cap the peaks of the coastal mountains protrude to form nunataks, while further toward the sea the ice cap is split by the mountains into numerous glaciers which descend through steep sided valleys to the coast. Geology in such a country is thus a study of fragments of observations on isolated nunataks and steep sided valley walls, with many inferred page 118 relationships. However, in a few places around the Antarctic there are localised ice free areas which provide a ‘window’ through which the geology may be examined in detail. The largest and most well known of these ‘windows’ is the McMurdo Oasis, on the western coast of McMurdo Sound. To the south there is a further ice free area in the foothills of the Royal Society Range. During the last ten years, field parties from Victoria University have mapped most of the McMurdo Oasis on a reconnaissance scale, and it is largely the results of these expeditions which this article summarises.
The McMurdo Oasis, an ice-free valley system covering 2,500km2, lies about 100 miles to the north-west of Scott Base and is bounded by the Polar Plateau to the west, and the Wilson Piedmont Glacier to the east. Three major valleys extend from Lat. 77°10'S to Lat. 77°45'S. The central, deep, Wright Valley has an altitude not greater than 1,200 ft, yet is bordered on both sides by mountains rising to 6,000 ft. To the north lies the Victoria Valley with Lakes Vida and Vashka, and to the south, the Taylor Valley, in which are situated Lakes Bonney and Fryxell.
The rocks in these ‘Dry Valleys’ can be divided into two broad categories, a basement complex and a younger overlying sequence of rocks. The former has a complex history of folding, metamorphism and intrusion, while the latter is of simple structure and has suffered little disorder since formation. The approach adopted in this article is to present the geology of the Dry Valleys as a series of events which over a great period of time have built up the geology as seen today.
All of the rocks which are older than Lower Devonian can be conveniently grouped under the heading, basement rocks. The oldest rocks are marbles, schists and gneisses, originally laid down as impure limestones, mudstones and greywackes, in a Precambrian geosyncline which lay where the Victoria Land coastline now exists. Deposition of sediments in this geosyncline is inferred to have taken place more or less continuously during the late Precambrian and Cambrian. The Cambrian fossil Archaeocyathus (or a closely allied genus) has been found in correlated rocks outside the Dry Valley region. These metamorphosed sediments are called the Skelton Group of Metasediments and in the Wright and Victoria Valleys they have been named the Asgard Formation of the Skelton Group.
Diagrammatic Representation of the Geology in the Dry Valleys, McMurdo Sound, Antarctica.
FIG. 1: Deposition of Skelton Group sediments in a geosyncline of Precambrian to Cambrian age, with subsequent igneous intrusion.
FIG. 2: Ordovician Orogeny — folding and metamorphism of the sediments with contemporaneous intrusion of a syntectonic granodiorite batholith (crosses a). Post-tectonic intrusion of a granite of sheet-like form (carets b), with lamprophyre and porphyry dykes of two generations (black lines).
A period of erosion of unknown length followed this phase in the McMurdo Oasis. During the erosional period a peneplain was formed with a relief in the order of a few hundred feet. It is called the Kukri Peneplain after the Kukri Hills in which it was first recognised. The Kukri Peneplain marks the boundary between the basement rocks and the overlying, near horizontal sediments described below. These sediments, at least Devonian in age, date the development of the Kukri Peneplain as pre-Lower Devonian.
Devonian to Jurassic Geology
From Devonian times until Late Triassic times there was a period of slow sedimentation during which a thick series of continental type sediments were deposited. They include siltstones, sandstones, shales, arkoses and a pebble subgreywacke, overlain by at least 2,500 ft of well sorted, massive and cross-bedded quartzites. Interbedded thin coal seams are common in the upper parts of the sequence. Sedimentary structures in the rocks include graded bedding, cross laminations, cross-bedding, ripple marks, dessication cracks and concretions. Upper Permian plant spores and various ‘fossil problematica’ such as worm casts and the tracks of invertebrates have been found within these sediments. In areas adjacent to the Dry Valleys, freshwater placoderm fish fossils, Devonian in age, and plants typical of the Upper Paleozoic Glossopteris floral assemblage, have been found. The Devonian to Triassic sedimentary sequence is termed the Beacon Group. It was probably slowly deposited in shallow seas, estuaries and lakes from a granitic terrain similar to that which forms the basement complex in the Dry Valleys at present.
During Jurassic times there was a further period of igneous intrusion during which thick extensive dolerite sills were emplaced in the basement rocks and Beacon Group. Three main horizons were intruded:
1. within the post-tectonic granite,page 123
FIG. 3: Development of the Kukri Peneplain Surface (c) on the Post-tectonic Granite, with subsequent deposition of the Beacon Group (horizontal lines) unconformably on the granite.
FIG. 4: The basement geology cf the Dry Valleys today, in a section from east to west. Note the intrusion of dolerite sills (oblique shadings) at three horizons — within the Post-tectonic Grantie, and at the base and middle of the Beacon Group.
2. along the Kukri Peneplain Surface,
and 3. in the lower parts of the Beacon Group.
The first two horizons were each intruded by a single sill, while the upper horizon was intruded by a number of sills. The two lower ones are between 900 and 1,300 ft thick. The uppermost sills, within the Beacon Group, were intruded as horizontal sheets roughly parallel to the bedding of the sediments. They are thinner and less regular than the lower sills and tend to bifurcate and pass via steeply dipping dykes, from one level to another. The whole sequence of dolerite sills and dykes is called the Ferrar Dolerite Formation after H. T. Ferrar, the geologist to Scott's Antarctic Expedition of 1901.
A possible Tertiary deposit exists at Marble Point, but no sediments have yet been definitely recorded in the McMurdo Oasis between Upper Mesozoic and Quaternary times. The Quaternary geological history of the area is represented by a series of moraines deposited by glaciers, and by basaltic volcanics and a series of raised beaches along the coast.
The McMurdo Volcanics occur as numerous small scoria cones and lava flows typically on the upper slopes of the valleys. They are common in the Taylor Valley and in the area at the foot of the Royal Society Range, but are not present in the Wright and Victoria Valleys. These volcanics are a part of the episode which gave rise to the large volcanoes of Erebus, Terra Nova and Terror, which make up Ross Island, and to Mt. Discovery with the line of recent volcanoes extending northwards along the Victoria Land coast.
The moraines are a product of Quarternary and Recent fluctuations of the Polar Ice Cap, the Ross Ice Shelf and local névés. In areas to the south of the Dry Valleys, moraine loops cross the valley floors in large numbers. In the McMurdo Oasis the separate moraines are distinct, some younger and others older than the scoria cones and lava flows of the McMurdo Volcanics.
The most important event of recent times was the Victoria Orogeny during which the gross features of present day Victoria Land were formed. This orogeny was characterised by block faulting with considerable vertical uplift, but there was negligible folding of the Beacon sediments, which together with the Dolerite sills now dip at about 7° towards the west.
Beach deposits are widespread throughout the Ross Dependency, and may be found at the entrance of the Taylor Valley, and, on numerous islands to the north. It has been estimated that the oldest raised beach is 5,000 years old.page 125
Recent Geological Processes
Meteorological data from this region are scarce, and only a few summer records have been obtained by Victoria University Antarctic Expeditions. While in the Taylor Valley, the authors noted the predominant easterly wind direction recorded by Bull in the Wright Valley. For 25 days at Nussbaum Riegel, Taylor Valley, in December, 1966, westerly wind was recorded on only six occasions. The mean annual precipitation is probably within the range 10 inches to less than 1 inch water equivalents. Snowfalls do occur in summer, for between 8th-10th December, 1966, at Nussbaum Riegel (height 2,100 ft), four inches of snow were recorded. Even at Lake Fryxell (125 ft ASL.) snow was observed. However, this snow is particularly light and ‘dry’, and on melting yields little water. The snow very quickly ablates or melts into the moraine. Permafrost levels varied from depths of 12 inches, in moraine, to 4-5 ft in loess deposits.
The origin of the McMurdo Oasis, has been long debated. The only satisfactory theory yet proposed, is that of McKelvey and Webb, who suggested that there was a decrease in Plateau ice, resulting in a reduced ice flowage through the valleys. Wilson calculated from lake studies that the minimum age glaciers ceased flowing through the valleys was about 50,000 years ago. It should be emphasised that the present arid climate is not a cause of deglaciation.
At Scott Base the snowline is at sea-level, but suddenly to the west, in the McMurdo Oasis, it rises to 7,000 ft. Beneath the snowline, conditions are cold but dry, and it is surprising to find that the central Wright Valley with an annual rainfall of probably less than 10 inches, is one of the driest places in the world. Although the origin of the valleys is undoubtedly a result of glacial action, the present climate has grossly modified the features now exposed. The cold, dry climate affects the weathering processes, and erosional features of temperate climates are absent. (see Fig. 5).
FIG. 5: The Taylor Valley, viewed to the west, showing (1) the Kukri Hills to the left, and the Asgard Range to the right; (2) the Taylor Glacier, centre, flowing from the Polar Plateau into Lake Bonney; (3) Nussbaum Riegel, foreground, with numerous lamprophyre and porphyry dykes; (4) small basalt scoria cones along the southern (left) side of the Taylor Valley; (5) a large dolerite sill to the northern side of the Valley, dipping to the west. (Etched in black).
—Official U.S. Navy Photograph
From December to the beginning of February, meltwaters from the glaciers begin to flow as small streams. In small depressions of the valley floors the meltwaters accumulate to form lakes, the surfaces of which are frozen in winter. In late spring they melt around the perimeters and by mid-summer a 50 ft wide meltwater rim may be present. The River Onyx, in the Wright Valley, is one of the largest rivers flowing 18 miles inland to Lake Vanda (410 ft ASL.). Smaller streams flow into the other lakes, and at Lake Bonney one stream has carved an 8 ft deep channel in the moraine. However, in relation to other processes, water erosion is negligible. For the rest of the year, the dominant transporting agent is wind. High velocity, katabatic winds descend from the higher Polar Plateau through these valleys to the coast. Sand grains are blown across the valley floors modifying fractured dolerite blocks to a ventifact appearance so typical of the desert regions of the world. These blocks vary in size from a few inches to greater than 7 ft in diameter. Sand-dunes have accumulated in exposed areas, and drifting barchans are a feature in the Victoria Valley. Most of the valley floors are covered in moraine derived from Polar Plateau and local glaciers. On steeper slopes, coarse scree debris accumulates on top of the bedrock. An unusual but effective method of boulder movement downslope, apart from rolling, appears to be by sliding of rocks over snow patches.
In the valley bottoms the graves and sands are frozen solid with ice to a great depth. In winter the intense cold causes the ground to contract to form shrinkage cracks 10 - 20 ft across. The resultant polygonal pattern which is found in most low lying areas is highly characteristic of this region.
The authors wish to thank Professor J. Bradley for assistance in the preparation of this paper.
(See appended VUWAE List on Page 114.)
Gunn, B. M., Warren G., 1962. Geology of Victoria Land between the Mawson and Mulock Glaciers, Antarctica. N.Z. Geol. Surv. Bull. 71.
* Now at the Volcanological Observatory, Rabaul, New Guinea.