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Home , Antarctandes, East Antarctic Ice Sheet, Jones Mountains

Initiation and growth of the East

DAVID JOHN DREWRY

CONTENTS x. Initiation of the • 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 provided growth centres for the indicates warm-temperate climates in the ice sheet• Extensive glacial erosion took place and coast of within these highland areas at this time. Many . Glacial marine sediments from glacial valleys in the 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 . 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 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.

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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 (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 , 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 (Le Masurier i97i , I972), dating back to the early Tertiary (42 ± 9 m.y. BP),

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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 , 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

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F zo. 1. Schema illustrating criteria for the recognition of glacial events. Lines of evidence applicable to Antarctica are boxed.

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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

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i / i F;o. 2. Location map of places and regions in Antarctica mentioned in the text. B, Beardmore Glacier, BS, , C, Coulman Island, D, Mount Sidlcy, J, Jones Mountains, Me, McMurdo Sound, M, Miller Range, SI, Seymour Island, SP, , SS, . Solid, numbered circlesin the Southern Ocean and refer to JOIDES sites drilled on Leg 28 (Hayes eta/. z973).

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Miocene times onward. Recent studies ofquartz grains by Fitzpatrick & Summer- son (I97I) and Brown (t973a), however, suggest that 'glacial' fractures and markings are not uniquely produced and may originate from non-glacial high- energy processes such as turbidity currents and sub-aerial sedimentation. There is, therefore active discussion surrounding the unique identification of deposi- tional environments by scanning electron microscopy (Margolis & Krinsley 1973; Brown 1973b) and it is probably necessary for further work to be undertaken before unambiguous or statistically significant interpretations can be made. Glacial marine sediments are also distinct from deep water pelagic and car- bonate deposits on the basis of their trace element constituents and lie close to the A1-Fe join of the Al-Fe-Mn + Ti ternary series (Angino I964, Angino & Andrews I968 ). Unfortunately this promising techni.que has yet to be applied to the recognition of the beginning of glacial marine accumulation in Antarctic deep sea cores. Glacial erosion features Datable glacial erosion events are rare and the few from Antarctica are dis- discussed thoroughly by Denton et al. (I97I). Minimum ages for the production of striated pavements on Mr. Sidley in West Antarctica and in the Jones Moun- tains, Ellsworth Land are 6.2 and 7-x2 m.y. BP respectively. A minimum date for the glacial excavation of the ice-free valleys in southern Victoria Land is given by Fleck et al. (I972) at 4.2 m.y. BP in the case of Wright Valley and 3.9 m.y. (corrected) K-Ar a BP for Taylor Valley.

INDIRECT EVIDENCE Palaeontolo gical--palaeodimatic inter~Oretations Palaeobiological criteria for possible glacial climatic conditions in the Cenozoic are important due to the lack of unambiguous, direct evidence. In situ fossili- ferous Tertiary sediments, restricted to the Antarctic Peninsula (Seymour and Cockburn Islands and South Shetland Islands) and pollen and microplanktonic remains discovered in moraines from McMurdo Sound have yielded a number of palaeoclimatic indicators, principally macrofloral assemblages. They have been reviewed by Mercer (I 973) and Cranwell (I 969). Although interpretations have been both difficult and conflicting the existence of the cold-temperate Southern Beech (Nothofagus) on King George Island, Seymour Island, and in McMurdo Sound, the vertebrate remains of giant whale (Zeuglodon), four species of penguin and a cool molluscan fauna suggest cold-temperate conditions since the lower to middle Miocene (x5-2o m.y. BP) and, as Mercer (I973) has suggested, provide some of the most convincing evidence available for temperate, non-glacial conditions during the Palaeogene. The paucity of terrestrial fossiliferous deposits from the Antarctic Cenozoic places considerable importance on the micropalaeontological and oxygen- isotope record of deep sea cores from the Southern Ocean, although few extend into the early Cenozoic. The majority of the microbiological studies suggest a warm southern ocean in the Palaeogene (Kemp I972 ) and an initial cooling

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phase commencing in the upper Miocene. Analysis and extrapolation of work on silicoflagellates from the S. Atlantic and S. New Zealand suggest warm-temperate climates (palaeotemperatures of I5°-25°C) in the Antarctic Peninsula and Wilkes Land coast in the late Eocene (Mandra & Mandra I97O , I97I , Mandra et al. I974; Hornibrook I968 , Dodonov & Markov 1966 ). The oxygen-isotope record of dated marine sediments provides a valuable gauge of oceanic temperatures and also of the isotopic composition of sea water which varies with the amount of fresh water accumulated in Polar ice masses (Shackleton i967). Only a limited number of oxygen-isotope results are avail- able which extend into the late Mesozoic and early Cenozoic and none, at present, are from Antarctica. The most recent and pertinent oxygen-isotopic investigation is that of Shackle- ton & Kennett (i974a, b , Shackelton, pers. comm.) for JOIDES sites 277, 279, 28I and 284 from DSDP Leg 29 south of New Zealand (Kennett & Houtz I973). Their results suggest that deep ocean temperatures around the Antarctic fell steadily through the early Cenozoic, reaching near to o°C by the early Oligocene. Their interpretation of these results implies that small, local glaciers could have reached down to sea level at this time. The development of a full- bodied ice sheet, however, is not indicated by the stable isotope ratios until mid-late Miocene. Thermoluminescence studies Estimates for the duration of cold climatic conditions have been calculated from the thermoluminescent activity of some Antarctic sedimentary rocks (Ronca I964, Ronca & Zeller i965). The resulting values (between 2.I and i4.o m.y. BP) are low and interpretation ambiguous, probably resulting from the serious limitations to the technique presented by a number of uncertain environmental assumptions. Sea level changes The partial but important effects on global sea level of changes in the volume of continental ice sheets is well known. The most useful compilation of data regarding the use of sea levels for interpretation of Tertiary glacial episodes is provided by Tanner (i968b). The main conclusion of these studies is that sea level has been falling, although in a fluctuating and discontinuous manner since the mid-Tertiary (2o-25 m.y. BP) suggesting the steady accumulation of ice in Antarctica and Greenland from this time. During the Plio-Plelstocene there was an increase in the rate of lowering of sea level commensurate with the growth of ice sheets over Eurasia and North America.

DISCUSSION The deposition of 'glacial' marine sediments in sub-Antarctic seas appears time- correlative with early Tertiary 'sub-glacial' eruptions of hyaloclastites in W. Antarctica. Studies of hyaloclastites and deep-sea sediment microtextures cannot, however, provide unambiguous or conclusive evidence for full-scale continental ice sheets at this time, especially in view of the necessity for cold,

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polar waters in W. Antarctica for the development sequence from ice shelves to ice sheets (Mercer I973). We may accept, however, that there may be suffi- cient grounds for supporting the existence of local, mountain glacial activity. Antarctic palaeobotanical and palaeozoological evidence is difficult to reconcile with these investigations from W. Antarctica. Studies of micro- and macro- fossils from Antarctica and adjacent land masses support warm-temperate climates during the Palaeogene; conditions only became colder towards the middle-late Miocene. Much of the evidence comes from the Antarctic Peninsula which may have been peripheral to the earliest centres of ice accumulation, The majority of marine sedimentary evidence at present favours the first occurrence of typically glacio-marine deposits around Antarctica in the period between the late Oligocene (JOIDES data) and the late Miocene (micropalaeontological data). Interpretation of composite sea level curves and the sedimentary and oxygen- isotope record of deep sea cores from Glomar Challenger both support the growth of large ice masses in E. Antarctica in late Tertiary time after about 25 m.y. BP. This is compatible with the growth of a in the late Miocene when ocean temperatures were more favourable for the development and maintenance of large ice shelves between the islands of the archipelago that form the continental, bedrock relief of Marie Byrd Land. The bulk of the data discussed above has been plotted in Fig. 3 which shows, against Cenozoic time, evidence for certain climatic interpretations or for the existence of ice in Antarctica. Taking Fig. 3 alone there appears to be con- siderable overlap although cold, glacial conditions are dominant since the middle Miocene and warm-temperate climates prior to the early Eocene. It is important to realize that during the Cenozoic there have been important changes in the distribution of continents and oceans due to continued ocean-floor spreading altering or intensifying the pattern of oceanic currents and atmospheric circulation (Kennett et al. I972 , Frakes & Kemp I973). The 'noise' introduced into Fig. 3 by using present day continental positions can be reduced by attempting palaeogeographic corrections to data locations plotted. Cenozoic drift curves for southern continents have been established in detail only for (WeUman et al. x969) although there is limited geomagnetic data available for the Antarctic Peninsula and southern S. America (Blundell I962 , Dalziel et al. i973). Based on ocean-floor spreading rates (Weissel & Hayes i97I ) and Australia's polar wandering curve (Wellman et al. I969) palaeolatitudes have been inferred for the data as shown in Fig. 4. Some confidence is attached to those calculated for Wilkes Land and Victoria Land. Some uncertainty may be present for data from Marie Byrd Land and Ellsworth Land due to a possible separate mode of drifting in the late Mesozoic (Scharn- berger & Scharon I97i ). Palaeolatitudes from the Antarctic Peninsula are decidedly uncertain. Nevertheless the data, replotted in Fig. 4, is much less ambiguous than Fig. 3 showing a reasonable separation of cold, 'glacial' and warm-temperate climatic indicators in both space and time. In conclusion, therefore, evidence for early Tertiary continental glaciation in Antarctica is ambiguous and unconvincing. Small, local highland ice masses

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in W. Antarctica (and elswhere ?) at this time are possible but require further and more direct substantiation. A late Cenozoic age (only after 25 m.y. BP) is favoured for the development of full-bodied ice sheets in first E. and then W. Antarctica. These would have developed from the expansion of local ice-fields and glaciers in mountainous areas by about the middle Miocene consequent upon fundamental changes to atmospheric and oceanic circulations following separation of the Antarctic, S. American and Australian crustal plates. Whether early ice masses waxed and waned prior to a stable late Cenozoic ice sheet is unknown at present but hopefully the results of the present DSDP and Dry Valley Drilling OB oll ~i~l ~_1 • ,o !l l DE

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• dl ~f, I I .o:-1 ;: t , ~I ut~ ~- 50- d ? Ol I mt I M i.~ I BC I'~,C o oo~°COLD 60- C COOL W WARM TEMR TEMPERATE 70- °i!, GLACIAL INDICATORS NON-GLACIAL 80 F Io. 3. Time-incidence of evidence for glacial and non-glacial environments and associated climatic inferences in and around Antarctica. A, Incidence of ice-rafted debris from cores of JOIDES Leg 28 (Hayes et al. 1973) , B,C, Occurrence of signifi- cant quantities ( > to %) of"ice-rafted" debris in Eltanin deep sea cores in SW. Pacific (Margolis & Kennett 197 t ; Geitzenauer et a/. t 968), D,E, Abundant cold-water fora- minifera (Hays x969, Bandy et al. 1971), F, Fossil penguins from Cockburn Island (Marples 1953) , G, Cool molluscan fauna from Seymour Island (Wilckens 1912), H, Glacio-volcanic events in ice-free valleys, McMurdo Sound (Denton et al. 197 x, Fleck et al. 1972), I, Incidence of tillites in Jones Mountains, Ellsworth Land (Rutford el al. 197i), J, Periods of "sub-glacially" erupted rocks (hyaloclastites) in Marie Byrd Land (LeMasurier 1972), K, Period of progressively falling sea levels (Tanner 1968a), L, Micro and macrofossil floral assemblages from Seymour Island (Dus6n 191o), M,N, Macroflora from S. Shetland Islands (Barton t964, Orlando z964), O.P. Microflora from McMurdo Sound (McIntyre & Wilson 1966 , Cranwell 1969), Q, Silicoflagellates from S. Island New Zealand (Mandra & Mandra 1971 , Mandra et al. 1974) , R, Silicoflagellates from S. Atlantic (Mandra & Mandra 197o), S, Lignite containing Araucaria from Iles Kerguelen (Nougier z97o).

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Project (DVDP) investigations will give much more detail of these events and clarify both the temporal intensity and spatial extent of Genozoic glaciation in Antarctica. 2. Models for the growth of the E. Antarctic ice sheet The mode of development of the ice sheet in E. Antarctica is important for the interpretation of the history of Antarctic glacial events. Two models have been proposed, which on evidence to date have appeared equally plausible.

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o. ~J. m ." so- T o lJ • "11" 40- t j J. \ 1. 0 50- R O. ii 6O I I 1 I I 9o ° 80 ° 70 ° 60 ° 50 ° 40 ° 30 ° S PALAEOLATITUDE FIO. 4. Data from Fig. 3 replotted in their inferred palaeolatitudinal positions based upon drift curves and ocean floor spreading data described in the text. Solid lines show glacial indicators, dashed lines non-glacial indicators (L should also be dashed). The dotted line for 'J' indicates uncertainty of interpretation. The lower shaded area attempts a suggested separation of non-glacial from mountain glacial conditions; the upper shaded area and line a postulated separation of local glacial conditions from full-scale continental ice sheets. Key as Fig. 3 except A, JOIDES core 270, A', JOIDES core 274, T, Climatic maximum in Otway Basin, Victoria, Australia (Gill I96t), U, Major fall in temperatures in Otway Basin (Dorman x966). Lines at the top of the diagram indicate salient features for present-day Antarctica: x & 2, Tree growth limits for southern and northern hemispheres respectively, 3 & 4, Equatorward extent of permanent glaciers at sea-level, northern and southern hemispheres respectively, 5, Maximum extent of Antarctica continental shelf (including Antarctic Peninsula). That surrounding E. Antarctica arrowed, 6, Maximum extent of Antarctic continental ice sheet (arrow for E. Antarctic ice sheet), 7, Maximum northward extent of Antarctic icebergs.

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Following ideas outlined for the development of the N. American, Greenland and Scandinavian ice sheets a number of investigations have favoured the initial accumulation of snow and ice within the Transantarctic Mountain belt and the consequent inland expansion from there of a continental ice sheet (Bull et al. x962 , Mercer I968 , Calkin & Nichols I97I). According to Denton et al. (I97I) the principal disadvantages of this model derive from the lack of evidence in the Transantarctic Mountains for any inland moving ice during the early build-up phase. An extensive network of glacial valleys, they argue, should have been eroded on the interior flank of the mountains similar to the situation in the mountains of southern Alaska. They do admit, however, that these valleys may lie buried beneath the overriding ice sheet or may have been destroyed by subse- quent glacial erosion. In an alternative model Denton et al. (I97I) consider that precipitation- bearing storms might have penetrated far inland to allow the accumulation of snow and the growth of glaciers in the interior mountain regions ofE. Antarcticam dominated by the Gamburtsev-Vernadskii chain. There are, however, equally serious objections to this model. The extensive, high 'plateau' that Denton claims to be the source of glaciation is poorly defined topographically, estimates being based on only two, poorly controlled geophysical ground traverses in this part of E. Antarctica (Drewry 1975) Much of the sub-glacial relief of the Gam- burtsev Mountains shown in Kapitsa's (i968) map is derived from indirect evidence. Following Denton's own argument for the Transantarctic Mountains, the sub-ice areas of the central E. Antarctica plateau should also exhibit signs of early mountain glacial activity-valley systems, presumably in this area, with a radiating pattern. At present the very widely spaced seismic and gravity soundings can provide no evidence for these features. Finally, there may be no definite proof for the existence of the Gamburtsev Mountains prior to glaciation. According to one source the mountains represent a neotectonic reactivation of the basement resulting from the loading of the crust by a growing ice sheet (ibid.). The major difficulties encountered by both these models arise principally from the lack of sufficiently detailed geophysical measurements. If, for example, systems of glacial valleys were detected beneath the ice sheet on the inland flank of the Transantarctic Mountains, or in parts of the Gamburtsev Mountains, their presence might constitute sufficient evidence to support a particular sequence of events or even provide data on the diastrophic history of these mountainous regions. There is a growing body of evidence from recent continuous airborne radio echo soundings undertaken by the Scott Polar Research Institute in Antarctica which indicates that there are extensive networks of valleys descending the inland flanks of the Transantarctic Mountains. The identification of these valleys and the interpretation of their specifically glacial origin has been discussed in detail elsewhere (Drewry i97i , x972 , x973b). Similar valleys, though less well defined and numerous, have been found in parts of the Gamburtsev Moun- tains. The location and trend of these known sub-glacial valleys are shown in Fig. 5.

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A MODEL FOR E. ANTARCTIC ICE SHEET EVOLUTION The existence of inland-oriented sub-glacial valley networks has several important implications affecting models of the evolution of the E. Antarctic ice sheet• Their presence indicates that both the southern portion of the Transantarctic Mountain belt and the north-eastern sector of the Gamburtsev Mountains may have provided growth centres for the E. Antarctic ice sheet. It is probable that the Gamburtsev Mountain glaciers developed after those in the Transan- tarctic Mountains due to their interior continental position being less favourable for the accumulation of large quantities of snowfall. It is also highly probable that other extensive mountain massifs that have been detected only recently by radio echo sounding within continental E. Antarctica (Fig. 5) may have nourished their own ice fields and contributed to ice sheet expansion. The situation in the poorly defined (topographically) areas of remains uncertain.

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FIO. 5" High mountainous regions of Antarctica inferred as growth centres for the Antarctic ice sheet. Lines indicate margins of highland areas at the 5oo m contour. Solid lines from radio echo sounding (Drewry I973b), dashed from Bentley (1972) and dotted from Kapitsa ( t 968). Arrows indicate major valleys and troughs utilized by ice in early accumulation phase as determined by radio echo sounding (see text). Dotted arrows show contemporary storm tracks in the Ross Sea embayment (U.S. Navy I965). M,C, Highland massifs in central East Antarctica (from radio echo sounding), E, , G, Gamburtsev Mountains.

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A proposed chronology The sequence of glacial events likely to have occurred in the Transantarctie Mountains and affecting and interacting with those in central East Antarctica is outlined below, based on evidence for glacial erosion from radio echo sounding. Following initial uplift of the Transantarctic Mountains sometime in the period between the end of the Jurassic (I6O m.y. BP) and the beginning of the Oligocene (26 m.y. BP), global air temperatures, falling progressively, may have led to the development of ice fields and glaciers on the windward eastern and steeper side of the mountain chain, where cyclonic storms entering the Ross Sea would have been forced over the coastal ranges (Fig. 5)- Since atmos- pheric circulation patterns around Antarctica are controlled to a large extent by land]sea distributions, the characteristics of oceanic circulation and contin- ental topography were probably analogous with those existing today. This pattern is thought to have been initiated after the crustal separation of the Australian and South American continental plates from Antarctica (Frakes & Kemp i972 , Kennett et al. i972 , Hughes i973, Foster i974). Expansion of mountain glaciers would have brought them rapidly into deep water of the Ross Sea where calving would be the major ice-loss factor. That calving of icebergs into the Ross Sea commenced at an early stage of glaciation is compat- ible with the first occurrence of ice-rafted debris in deep sea cores retrieved by Glomar Challenger west of the Pennell Bank and off (cores 273 , 274) in the late Oligocene and early Miocene (Hayes et al. 1973). Ice would also have flowed westward down the gentler slopes of the inland flank of the Transantarctic Mountains. Ablation here would have been mainly by melting and interior moving glaciers could have expanded into partially coalescent piedmont complexes. With continued growth the ice-divide, initially within the highest eastern zone of the mountains would have moved inland, in a fashion similar to the migration of the ice-divide in Scandinavia (Flint i97t ). Ice moving out of the Transantarctic Mountains would have coalesced with glaciers from other interior highland ice caps to form a proto-E. Antarctic ice sheet (Figs. 5,6). Once this growing ice sheet developed sufficiently steep surface gradients, the ice flow pattern would have been drastically modified to favour seaward moving inland ice. The net effect has been to eventually submerge inland-trending valleys in the Transantarctic Mountains. A schematic represen- tation of this growth model is shown in Fig. 6. Major erosive sequences in the Transantarctic Mountains In parts of McMurdo Sound region there are large ice-free troughs penetrating inland across the exposed portion of the mountains to the edge of the ice sheet. A number of investigators have attributed the major stage in the cutting of these valleys to a full-bodied ice sheet in E. Antarctica predating 4.2 m.y. BP. An upper limit for their age is, however, unknown (Bull et al. I962, Calkin & Nichols x97I , Denton et al. i97i ). This contention of erosion by an ice sheet expanding out of the central regions of East Antarctica is a necessary consequence of the argument of Denton et al. (I 97 x) that the major growth centre for the ice sheet was the interior Gamburtsev

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Mountain chain. The evolutionary model proposed in the previous section allows a different mechanism to be considered for the development of these valleys. It is argued that the exposed and sub-glacial extension of the Transantarctic

m ul A VALLEY 4~ GLACIERS EXPANDING PIEDMONTS

O, ~-~,~--..^-v-- B

.C

°10 I -- I I D 4000- ICE SHEET PRESENT ICE LEVEL MAXIMUM / PRESENT ] _,~._ ......

0 l i ~-~l[~r~'~.. 0 100 200 km Fxo. 6. Diagrammatic model for the development of the E. Antarctic ice sheet in the Transantarctic Mountains indicating the progressive inland shift of the ice divide and change in the ice-flow pattern (see text). Profiles are based upon radio echo sounding data and take into account progres- sive isostatic depression and erosion with ice accumulation. Tectonic uplift during late Cenozoic glacierization is estimated at x 5oo m and the profiles have been adjusted accordingly. Profile D indicates the elevation of the exposed and sub-ice mountains at the onset of full-scale continental glacial conditions (pecked line). Present day elevatiom are given by the solid line. The suggested maximum level of the E. Antarctic ice sheet is indicated (after Hollin t962 ). Sirius Formation deposits (semi-lithified till) at Mt. Feather (Mayewski t972) are also indicated on both profiles (open and solid triangles).

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Mountains and the few isolated inland blocks beneath the ice adjacent to the mountains nourished ice caps during the build-up phase of the continental ice sheet. Local, wet-based valley glaciers would have extended towards the Ross Sea carving glacial troughs prior to the development of a full-bodied ice sheet, and may have taken place in the late Oligocene and early Miocene. There is no necessity for this period to have been the single most important erosive episode for the valleys-just that some or many were initiated. With the development of the continental ice sheet in E. Antarctica the initial valleys may have been utilized and subsequently modified (enlarged by outlet glaciers) when the margin of the seaward flowing ice reached levels sufficiently high to spill over the bedrock thresholds that characterize many of the Transan- tarctic Mountain outlet glaciers, at present ice free or not. Due to spatial varia- tions in extent, and elevation of thresholds and altitude of the ice sheet margin, utilization of valleys as outlets of the ice sheet would not have been synchronous. If the surface slopes of outlet glaciers were sufficiently low and ice discharge high it would have been possible for pressure and frictional melting at the bed to continue effective erosion and modify the bedrock valleys. Even those valleys not utilized by the ice sheet could probably have supported local glaciers. How much erosion they may have achieved is uncertain. It may be, therefore, that the largest outlet glaciers today which show little evidence of headward thresholds may be some of the oldest in the Transantarctic Mountains. Those with prominent thresholds and at present deglaciated may only relate, as outlet glaciers, to the last and/or maximum stage of glacierization and hence may not be entirely typical of sequences of occupation in the Trans- antarctic Mountains generally. Preliminary results of core analyses from JOIDES Leg 28 in the Ross Sea suggest a slow rate of sedimentation in the Miocene up to about 5 m.y. BP followed by rapid sedimentation ofice-rafted debris at the Mio-Pliocene boundary with a source area probably in the Transantarctic Mountains. The sequence is interpreted as indicating a glacial maximum followed by melting, collapse and retreat of the ice sheet and implying extensive erosion. Whether these events can be correlated with the proposed glacial chronology of the ice-free valleys is still uncertain. It might appear that in the late Miocene the E. Antarctic ice sheet did achieve its maximum dimensions, finally utilizing the local glacier valleys in southern Victoria Land. Increased sedimentation may indeed reflect an ice retreat following this maximum. The collapse of a more extensive and grounded "ice-shelf" in the Ross Sea embayment is thought preferable to a short period of very intense erosion in the Transantarctic Mountains. Glacio-tectonic implications The presence of inland draining valleys beneath the ice sheet margin of E. Antarctica and the contention that early glacial deposits in the Reedy and Beardmore Glacier areas can only have been produced by local, wet-based ice caps (Mercer 1968 , 1971 ) lead us to believe that a substantial highland zone existed along the line of the present-day Transantarctic Mountains at the onset of glaciation. The major problems, however, confronting the dating of uplift

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sequences or the amount of pre- and inter-glacial uplift arise from the lack of geological evidence for tectonic events since the mid-Jurassic. Gunn & Warren (i962) proposed the name Victoria Orogeny for epeirogenic events sometime in the Tertiary (they favoured the late Tertiary) which produced the Transantarctic Mountains. Webb (I972) has suggested that the Victoria Orogeny should include all movements in the Cenozoic affecting the mountains and he speculated that uplift may have comprised a number of short vertical pulses. Grindley (I967) considers that the bulk of uplift dates from the inception of continental glaciation in E. Antarctica. He contends that in pre-glacial times the Transantarctic Mountains did not exist as a mountain barrier but as a low plateau over which an ice cap, expanding from interior E. Antarctica, subse- quently spread. The present elevated character of the mountains is attributed by Grindley to a response in the upper mantle to ice loading of the crust within E. Antarctica which caused an outward flow of mantle material, bulging up the continental margin. We reject this hypothesis on the basis of sub-glacial evidence of inland trending valleys on radio echo records. There are also reasonable theoretical grounds, however, for rejecting Grindley's suggestion. In their respective analyses of crustal loading and flexure Brotchie & Silvester (i969) and Walcott (I97O) show that due to the stiffness of the crust the zone of downward warping has a greater radius than that of the superimposed load. If we assume an ice sheet of equilibrium profile, 3000 m thick and with a radius of i5oo km (equivalent to the E. Antarctic ice sheet) the zone of downward displacement extends to IOO km or 25 ° km beyond the ice sheet edge (Brotchie & Silvester and Walcott models respectively). The maximum amount of down warping in this zone is probably about I OO m whilst the amplitude of the fore-bulge should only be in the order of 40-5 ° m under simple elastic flexure. The width of the exposed Transantarctic Mountains is rarely more than 150 km (except in the Beardmore Glacier area and in northern Victoria Land). Their overall structure is that of a series of differentially tilted fault-blocks with few known faults running parallel to the principal uplift axis just inland of the Ross Sea Coast. On these two grounds it would seem unlikely, assuming the crustal deformation models are reasonably realistic, that uplift could have taken place within the mountainous zone, although we accept that elastic and non-elastic displacements do take place at greater distances from the ice edge as a result of mantle displacements from ice loading. There is substantial support for the idea that the Transantarctic Mountains, although in existence well before the onset of glacial conditions, were nevertheless, considerably lower than today and that uplift continued for some time after the continent became glacierized. Thus, in the Beardmore Glacier region the faulting of presumed early till sequences (Sirius Formation of Mercer I97I ) suggests tectonic dislocation. Furthermore, if we assume that the advent of glacial conditions required temperatures in the warmest months to be No°O, elevations of the mountain chain equivalent to those at present, would give sea level temperatures far too high to be compatible with the palaeo-polar position of the continent some 20-25 m.y. BP (Smith et al. x973, Mercer i968 ).

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Following this argument it is possible to make a crude estimate of the elevation of the Transantarctic Mountains at the onset of glaciation. Taking present-day sea level temperatures for the warmest month from areas with characteristics analogous to those which may have prevailed in the Transantarctic Mountains (e.g. elevated area supporting small, permanent ice fields and glaciers; close proximity to open ocean waters and in a polar location) we find a range of between +4°C and +6°C for areas along the western margin of Davis Strait and Baffm Bay (i.e. Baffin and Ellesmere Islands), in West Spitzbergen and Novaya Zemlya. If we consider that winds tracking in from over the open Ross Sea would have been moisture-laden before rising over the proto-Transantarctic Mountains, the saturated adiabatic lapse-rate is applicable and is taken as ,-.,6 ° x IO-3cm -1. Calculations based on these rough-and-ready figures suggest that the mountains may have reached elevations between 500 m and I5oo m at the onset of glaciation. The values quoted above would imply subsequent uplift of about i5oo m to 2ooo m since the late Oligocene to bring the mountains to their present altitude (eustatic changes would appear to have only a low order affect on these figures, of about I OO m (Tanner i968b)). This figure is in close agreement with that derived independently by Grindley (i967) for the uplift of early glacial features, although he considered the time interval to be only of the order of 2 m.y. with an implied rate of uplift of 3.3 × Io-4 m a -1 (metres/annum). This is an order of magnitude greater than that envisaged here based on the figures given earlier (~7.5 × Io-6 m a -1) which is comparable with the rate of uplift over the past 3.4. m.y. of 7.7 x IO -5 m a -1 for Wright Valley as given by Webb (I972). If we assume the rate of uplift was constant and continuous (despite Webb's suggestion of distinctive pulses), taking into account the continued partial isostatic depression of the mountains and a reduction in mountain relief by late Cenozoic erosion, a suggested date for the initial vertical movements of the Victoria Orogeny in the Transantarctic Mountains is late Eocene. This is contemporaneous with estimates for the separation date of the Australian and Antarctic crustal plates with which major igneous and tectonic activity may be correlated. Deep sea drilling on JOIDES Legs 28 and 29 (Hayes et al. I973) has confirmed the geomagnetic and ocean-floor spreading separation age of 5 ° m.y. BP. Hence upwelling of mantle material, subsequent sea-floor spreading at a mid-ocean ridge migrating parallel to the coast of East Antarctica and the stress fields generated within the Antarctic plate may be related to neotectonic reactivation of the Ross Orogenic zone and uplift of the Transantarctic Mountains. Osmaston (I973) has also suggested that lateral heat flushing into older adjacent lithosphere resulting from continental break-up and sea-floor spreading may cause upwarping by as much as 3ooo m. Details of any relationship in Antarc- tica must await futher geophysical investigations.

AOKNOWLEDGEMElq'r$. Radio echo sounding in Antarctica was undertaken by Scott Polar Research Institute teams with the logistic support of the U.S. National Science Foundation and U.S. Navy and I wish to thank all those who assisted in the collection of data used here. R. J. Ache, G. de Q. Robin and N. J. Shackleton were kind enough to read and comment on drafts of the manuscript.

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Received so March x974; revised typescript received Io July I974; read 30 October x974. DAVID JOHN DR~.WRY BSC. Phi). Scott Polar Research Institute, Cambridge CBo xER.

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