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Antarctic Geology and Gondwanaland

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Antarctic Geology and Gondwanaland Ii Antarctic Geology and Gondwanaland CAMPBELL CRADDOCK Department of Geology & Geophysics University of Wisconsin, Madison Although ideas of drifting continents had been advanced earlier, it was not until late in the nine- teenth century that geologists first suggested the existence of a large ancestral continent in the South- ern Hemisphere in the geologic past. This hypo- t etical protocontinent—termed Gondwanaland—in- Figure 1. Reassembly of Gondwanaland (after du Tot, 1937). luded present-day South America, Africa, Arabia, Madagascar, Ceylon, peninsular India, Antarctica, and Australia (see Fig. 1). Because of its position in surprisingly like Australia, only larger—around t e reassembly, Antarctica must clearly play an im- which, with wonderful correspondences in outline, portant role in determining whether Gondwanaland the remaining puzzle-pieces of Gondwanaland can actually ever existed. Despite its great importance with remarkable precision be fitted." tD the concept, however, the geology of Antarctica Because so little was then known about the geol- as almost unknown when Gondwanaland was first ogy of Antarctica, du Toits Gondwanaland reas- postulated. Only in the past few years has our knowl- sembly in effect predicted the geologic patterns to be dge of Antarctica, the last fragment of the supposed expected in the antarctic interior. It is a tribute to protocontinent to be explored, advanced to a point the genius and foresight of du Toit that subsequent ihere meaningful geologic tests of the Gondwana- geologic work in Antarctica has largely confirmed land hypothesis are possible. these predictions. Antarctica was first sighted 150 years ago from Preparations for the International Geophysical ships sailing near the tip of the Antarctic Peninsula. Year (1957-1958) marked the beginning of the cur- Later expeditions collected continental rocks such rent phase of scientific exploration. Twelve nations as granite and sandstone from icebergs, while the established more than 40 bases in Antarctica to sup- first geologic specimens obtained on the continent port their IGY programs, and scientific activity has were collected in 1894 at Cape Adare, south of New continued at a high level since that time. Ski-equip- Zealand. The ensuing 20 years saw vigorous explora- ped aircraft and tracked vehicles have opened the tion by expeditions from several nations, and the entire continent to geologic study, and only a few broad pattern of antarctic geology began to emerge. areas of significant rock exposure remain unvisited Private and national expeditions before and after by geologists today. It is safe to say that our knowl- World War II added further geologic information, edge of the geology of this remote continent has but large areas of the continent still remained unex- more than doubled during the past 15 years. plored and unknown up to 1955. Most of these early ologic observations were confined to the coastal tinge of Antarctica; except for the areas covered by Outline of Antarctic Geology he South Pole parties of Shackleton, Amundsen, and cott, and the Gould party in support of Byrds polar Early in this century, after geologic data became flight, the vast interior of the continent remained available from the Antarctic Peninsula, the Ross Sea eologically unknown. region, and other coastal localities, it was recognized In 1937, the great South African geologist Alex- that Antarctica could be divided into two major ánder du Toit published a book which must be rated geologic provinces. The first comprises the larger part among the most important ever written in the field of the continent that faces mainly upon the Atlantic of geology. In this brilliant synthesis, he set forth and Indian Oceans; since most of this province lies in detail the geologic evidence then available for in the area of east longitudes, it is commonly known continental drift and the existence of Gondwanaland. as East Antarctica (see Fig. 2). The second province Regarding Antarctica, he stated: consists of the smaller part of the continent that faces "The role of the Antarctic is a vital one. As will mainly upon the Pacific Ocean and is commonly be observed from Fig. 7 [present Fig. 1], the shield of known as West Antarctica. East Antarctica is a typi- East Antarctica constitutes the key-piece--shaped cal continental shield or stable platform, consisting May-June 1970 53 0• Era I Period I Epoch I Age in my. Quaternary -- 1-2 Cenozoic Pliocene - I Miocene - 25 Tertiary Oligocene - 40 Eocene -- 60 1. Paleocene --70- In Cretaceous - 135 EAST Mesozoic Jurassic 180 2 ANTARCTICA triassic 225- SOUth POlO 90°E - Premise 270 Carbooiferoos - 350 P Devonian WEST I Paleozoic 400 ANTARCTICA Silurian 440 Ordovician - 500 Cambrian --600- Precambrian pa S8 EA •Jco Figure 3. Geologic time scale. 180° brian and Cambrian strata, generally folded and somewhat metamorphosed, that occur in the Trans. Figure 2. Geologic provinces of Antarctica antarctic Mountains and at a few coastal localities. Overlying the East Antarctic basement complex i of a foundation of older igneous and metamorphic a succession of mainly flat-lying sedimentary an rocks overlain by a sequence of younger, flat-lying volcanic rocks. -These beds have been called the stratified rocks. By contrast, West Antarctica is Beacon Group (sedimentary) and the Ferrar Group composed of generally younger rocks that are widely (igneous), and they represent the Gondwana se deformed and metamorphosed. Intrusive and extru- quence—as described from the other southern con sive igneous rocks are abundant, and volcanic activ- tinents—in Antarctica. The Beacon Group contain ity continues there today. rocks as old as Devonian, and the Ferrar Group Rock exposures comprise less than 5 percent of rocks as young as Jurassic. These groups are exten- the area of the continent. Those in East Antarctica sively exposed in the Transantarctic Mountains, but occur in an oval belt consisting of the coastal region they have been found in place at only a few localities and the Transantarctic Mountains. The nunataks along the coast of East Antarctica. and mountain ranges within this belt reveal a base- The geologic history of West Antarctica is complex ment complex composed mainly of high-grade meta- and poorly known. All rocks whose ages are known morphic rocks and intrusive igneous rocks. Gneisses appear to have formed during the last 600 million of the granulite facies are the most abundant rocks, years, and no definitely Precambrian rocks have yet but lower-grade metamorphic rocks of the amphi- been discovered. In much of West Antarctica, the bolite and green schist facies also occur. A wide oldest rocks are igneous intrusive and metamorphic range of igneous rocks has been reported, but felsic varieties that form a basement complex believed to varieties such as granite are most common. These be Paleozoic in age. Sedimentary and volcanic se- crystalline basement rocks record a complex geologic quences of probable Paleozoic and Mesozoic age are history involving several cycles of deformation, meta- widely distributed, and most of these rocks art, morphism, and emplacement of igneous intrusives. strongly folded and somewhat metamorphosed. In In contrast to other continents, Antarctica has so trusive igneous rocks were emplaced across much o far revealed no rocks with apparent ages greater than West Antarctica during the Mesozoic, and perhaps 2 billion years, and indeed few older than 1.5 by. the early Cenozoic. Upper Cenozoic volcanic an This fact may imply that 1) the oldest rocks in the sedimentary layers on the Antarctic Peninsula ar basement of East Antarctica are truly younger than flat-lying and undisturbed. Volcanism which begax those of other continental shields, or 2) insufficient in the Miocene has continued into recent times in sampling has yet taken place to discover the oldest much of coastal West Antarctica. rocks present, or 3) later metamorphic events, es- pecially in the early Paleozoic (see Fig. 3), have reset the mineral clocks, causing apparent radio- Major Discoveries Bearing on Gondwanaland metric ages younger than the true ages of the rocks. The youngest known rocks in the East Antarctic base- In fitting Antarctica into his Gondwanaland re- ment complex are the fossiliferous Upper Precam- assembly, du Toit had to depend largely on the shape 54 ANTARCTIC JOURNAL of the continent because so little was then known suppose that Permian and modern geography are about its geology. Since that time, progress in ant- identical. Can a reasonable dispersal mechanism, arctic geology has yielded a number of discoveries such as wind or water currents, be found to connect that bear on the Gondwanaland problem. Eight lines these distant lands? And, is it possible that these two of evidence that appear significant are discussed widely separated lands, one polar and the other 1elow: tropical, could support nearly identical floras when 1. The basement rocks of coastal East Antarctica their climates would have been so different? re similar, both in a general way and in some de- Thus, the fossil animal and plant record in Ant- $ails, to those along the matching coasts of the other arctica strongly suggests that the present geographic Iondwanaland fragments in du Toits reassembly. isolation of the continent did not exist during Paleo- Work by geologists of several nations has shown that zoic and at least early Mesozoic time. he structural grain in these ancient antarctic rocks 4. The Ellsworth Mountains fold belt formed in s compatible with that in like rocks of the suggested early Mesozoic time and probably represents the ri atching coast. The basement rocks of all these continuation of the Cape fold belt of South Africa areas are compositionally similar, consisting of high- and a part of du Toits Samfrau geosyncline. Be- rade metamorphic rocks such as granulite gneisses, tween the Transantarctic Mountains of East Antarc- long with igneous intrusives such as granite. In tica and the coastal belt of West Antarctica lies a particular, an unusual hypersthene-bearing granitic large region that has been explored only during the rock, termed charnockite, is widespread in eastern last 15 years.
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