Initiation and growth of the East Antarctic ice sheet DAVID JOHN DREWRY CONTENTS x. Initiation of the Antarctic ice sheet • 256 Direct evidence . • 256 Indirect evidence . • 259 Discussion • 260 2. Models for the growth of the E. Antarctic ice sheet . 263 A model for E. Antarctic ice sheet evolution • 265 Glacio-tectonic implications . • 268 3. References . • 27 t SUMMARY There appears to be little support for an Gamburtsev Mountains and other, smaller sub- initiation of continental glacierization in the glacial mountain massifs within continental Palaeogene. The palaeobiological evidence East Antarctica provided growth centres for the indicates warm-temperate climates in the ice sheet• Extensive glacial erosion took place Antarctic Peninsula and Wilkes Land coast of within these highland areas at this time. Many East Antarctica. Glacial marine sediments from glacial valleys in the Transantarctic Mountains JOIDES Leg 28, oxygen-isotope analyses from were subsequently utilized by outlet glaciers of Leg 29, global sea level changes and palaeonto- the ice sheet. logical investigations favour the development Tectonic implications of the growth model of full-bodied ice sheets from local, longer lived indicate that the Transantarctic Mountains icefields and glaciers only in the late Cenozoic were probably 15oo-2ooo m lower at the com- (after the lower Miocene) with a possible mencement of glaciation, and that the initial maximum about 5 m.y. BP. vertical movements of the Victoria Orogeny Recent geophysical exploration has enabled began in the Eocene in response to the crustal a model for the evolution of the East Antarctic separation of the Australian and Antarctic ice sheet to be developed. The Transantarctic lithospheric plates. Mountains, the north-eastern sector of the THE MOST RECENT, CENOZOIC, phase of Antarctic geological history has been dominated by the initiation and growth of ice sheets that now cover both East and West Antarctica. The age of the first ice accumulations and choice of model for the development of the ice mass are major questions of Antarctic glacial history and have important erosional and diastrophic implications. This paper, first critically examines recent contributions to dating the initial accumulation of snow and ice and attempts to present an integrated chronology. The second part explores, in the light of recent geophysical (especially radio echo sounding) investigations, various models for the evolution of the East Antarctic ice sheet and discusses their relationship to erosional and tectonic events• Although events in West Antarctica are considered from time to time in developing the central theme of this paper a detailed discussion of the history of glaciation of this part of Antarctica is not presented. Jl geol. Soc. I, ond• vol. x3x , x975, PP. 255-273, 6 figs. Printed in Northern Ireland. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/131/3/255/4885008/gsjgs.131.3.0255.pdf by guest on 24 September 2021 ~56 D. J. Drewry I. Initiation of the Antarctic ice sheet The results of Antarctic expeditions early in the 2oth century suggested to geologists and glaciologists that the Antarctic ice sheet may have been some millions of years older than those that once covered North America and Eurasia (Wright& Priestley i922 ; Priestley I923). Despite these suggestions later investigators maintained that the present Antarctic glacial commenced contem- poraneously with those of the Northern Hemisphere during the Pleistocene (Rudmose Brown i927; King I965). Since the early i97o's , however, the be- ginnings of the ice sheet have again been pushed into the Cenozoic, as far back as the Eocene (Margolis & Kennett i97I). Criteria for the recognition of glacial episodes are both numerous and equiv- ocal. Rarely do single features offer an unambiguous interpretation of glacial conditions. Some evidence relates directly to the action of immediately adjacent ice masses--glacial deposits (till, ice-contact stratified drift, rhythmites with dropstones, outwash sediments and hyaloclastites), features of glacial erosion (striae, stoss and lee topography, U-shaped valleys, cirques, etc.) or sedimentary structures deformed by flowing ice. Certain sedimentary accumulations (loess and glacial marine sediments) are formed at a much greater distance from glaciers themselves but still reflect the presence of ice masses. Other lines of evidence for glacial conditions are more indirect and they indicate the effect, often on a global scale, of ice sheets. Sea level, for example, can be substantially altered by the development of terrestrial ice caps whilst the initiation of prolonged and extensive glacial conditions is usually reflected in dramatic changes in global climatic and vegetational environments. Fig. I attempts to summarize these lines of evidence. In Antarctica much of the datable material for early glacial events lies hidden beneath the ice sheet which now covers ~95 per cent of the continent. Those criteria which have been investigated are boxed in Fig. I and many of them have been critically reviewed by Denton et al. (I 97 I) and Mercer (I973). In the discussion that follows only more recent evidence is presented in detail or where it is thought alternative interpretations of the data are possible. DIRECT EVIDENCE Glacial diamictite and associated ice-contact deposits In Antarctica tills and tillites, which provide the most direct and satisfactory demonstration of former glacial conditions, are virtually unknown. A tilloid deposit, dated to between 7 and 12 m.y. K-Ar BP, from the Jones Mountains, Ellsworth Land has been described and an ice-contact deposit with a maximum age of 7.4 m.y. BP from Coulman Island (Hamilton I969). It is still uncertain whether the former relates to a full-bodied ice sheet or a local, high altitude ice field. Undated semi-lithified tills of the Sirius Formation have been reported from the central Transantarctic Mountains by Mercer (i97i) and Mayewsld (i972) and are thought to be early glacial deposits. The discovery of a volcanic sequence of hyaloclastites from Marie Byrd Land (Le Masurier i97i , I972), dating back to the early Tertiary (42 ± 9 m.y. BP), Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/131/3/255/4885008/gsjgs.131.3.0255.pdf by guest on 24 September 2021 The East Antarctic ice sheet 257 and considered to have been erupted beneath a continental ice sheet has been critically assessed by Mercer (z973). It is possible that a small, local ice cap may have been present in Byrd Land at this time but there is little supplementary evidence. Glacial marine sediments Some evidence for Antarctic glacial history, found in sediments of the Southern Ocean, has been reported by Denton et al. (I97I) and Mercer (I973). Typical ice-rafted deposits containing large, exotic and often striated clasts have been described from shallow piston cores at least 5 m.y. old. Much new material has recently become available following the activities of the Deep Sea Drilling Project (DSDP) in Antarctic and sub-Antarctic waters (Drewry z973a). Cores obtained by Glomar Challenger off Victoria and Wilkes Lands (cores 267, 268, 27 ° and 274) show the first definite occurrence of ice- rafted, glacial marine sediments containing striated exotics, at an inferred palaeolatitude of 54°S in the upper Oligocene, 2o-25 m.y. BP (Hayes et al. I973). Such results appear unambigous and quite acceptable. Less so, however, is the more indirect evidence from marine deposits devoid of striated clasts, exotics or dropstones. A number of criteria have been developed to enable such sediments to be distinguished from 'normal' pelagic accumulations. Single quartz grains exhibit surface rnicrotextures that have been differentiated into groups which are thought to reflect their source environment (Krinsley & Takashi i962 , Krinsley & Doornkamp i973). A detailed study of sediments under PROXIMITY TO ICE MASS CONTACT PRO-GLACIAL G L 0 BA L BASAL TERMINAL] 10 100 1000 10000 km I • I , I , l [ ' .TILLI OUTWAS H l- L.... L O E S S ICE-CONTACT b. r'---------', STRATIFIED, ; ,,, u~ I HYALO- ; GLACIAL MARINE: ICLASTITE I DRIFT I I RHYTHMITES I .SEDIMENTS I U ,.. ..... 1 I I O- DEFORMED SEDIMENTS u I FEATURES z Lmmn~m6 I.LJ > IJ.l I--- CRUSTAL U WA R P I N G i SEA-LEVELC"ANGES i 1.1..I L tuna Q I .... L__. 1 I CLIMATIC AND I z I I L____VEGETATIONAL CHANGES || F zo. 1. Schema illustrating criteria for the recognition of glacial events. Lines of evidence applicable to Antarctica are boxed. Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/131/3/255/4885008/gsjgs.131.3.0255.pdf by guest on 24 September 2021 258 D. J. Drewry the scanning electron microscope by Margolis & Kennett (I97Z) enabled them to establish quantitative criteria for isolating ice-rafted debris. These have been widely accepted and applied to the recognition of the onset of glaciation. Accord- ing to Geitzenauer et al. (z968) and Margolis & Kennett (I97Z) core analyses from sub-Antarctic areas indicate the first presence of 'glacial' marine sediments in the early Eocene--recognized by the abundance and surface microtextures of quartz grains. Major cooling phases--evidenced by the changing quantities of this 'ice-rafted debris'--are attributed to the lower Eocene, upper-middle Eocene and the Oligocene. No glacial marine sediments occur in the early- middle Miocene sections but they are abundant and widely distributed from late •OI • "% /r % sP - ,~.~; e~ ANTARCTICA I'°E " V>, ,,, e ~)~ "0 ~ ICE SHELF ~,. ~ 2721. M c-'~ ~ _,<" .~ 271 • ~ ~ ¢ o 273~./~ / I , IO00KUl m°C 70 S i • 274 • .. • 269 i / i F;o. 2. Location map of places and regions in Antarctica mentioned in the text. B, Beardmore Glacier, BS, Byrd Station, C, Coulman Island, D, Mount Sidlcy, J, Jones Mountains, Me, McMurdo Sound, M, Miller Range, SI, Seymour Island, SP, South Pole, SS, South Shetland Islands. Solid, numbered circlesin the Southern Ocean and Ross Sea refer to JOIDES sites drilled on Leg 28 (Hayes eta/.