The Polonez Cove Formation of King George Island, Antarctica: stratigraphy, facies and implications...

The Polonez Cove Formation of King George Island, Antarctica:stratigraphy, facies and implications for mid-Cenozoiccryosphere development

ALEXA L. TROEDSON and JOHN L. SMELLIEBritish Antarctic Survey, Natural Environment Research Council, High Cross, Madingley Road,Cambridge CB3 0ET, UK (E-mail: [email protected])

ABSTRACT

The middle to late Oligocene Polonez Cove Formation, exposed on south-

eastern King George Island, South Shetland Islands, provides rare evidence of

mid-Cenozoic West Antarctic cryosphere evolution. A revised lithostratigraphy

and facies analysis and a review of the palaeoenvironmental significance of the

formation are presented here. The diamictite-dominated basal member of the

formation (Krakowiak Glacier Member) records the presence and retreat of

marine-based ice on a shallow continental shelf. Five overlying members are

recognized. These consist of basaltic-sourced sedimentary rocks and lavas and

represent a variety of shoreface and shallow continental shelf environments in

an active volcanic setting. These units contain diverse reworked and ice-rafted

exotic clasts that become sparse towards the top of the formation, suggesting

a continuing but waning glacial influence. New 40Ar/39Ar dates from

interbedded lava flows indicate a late Oligocene age (25Æ6–27Æ2 Ma) for the

Polonez Cove Formation, but are slightly younger than skeletal carbonate Sr-

isotope ages obtained previously (28Æ5–29Æ8 Ma). There is evidence for wet-

based subice conditions at the base of the Polonez Cove Formation, but no

sedimentary facies to suggest substantial meltwater. This may reflect a subpolar

setting or may result from lack of preservation or a high-energy depositional

environment. A northern Antarctic Peninsula/South Shetland Islands

provenance is probable for most non-basaltic clasts, but certain lithologies

with possible origins in the Transantarctic and Ellsworth Mountains also occur

sparsely throughout the formation. There is evidence to suggest that the

presence of such far-travelled clasts within subglacially deposited facies at the

base of the formation reflects the advance of a local ice cap across marine

sediments containing the clasts as ice-rafted material. The presence of these

clasts suggests that extensive marine-based ice drained into the southern

Weddell Sea region and that a strong Weddell Sea surface current operated

both before and during deposition of the Polonez Cove Formation.

Keywords Antarctica, facies, glacial sedimentation, Oligocene, shallowmarine, volcanism.

INTRODUCTION

Past variations in Antarctic ice extent can beclosely linked to global sea level and other majorenvironmental changes. Better constraints onAntarctic cryosphere history are needed if these

links are to be thoroughly understood. Animportant approach to the problem involvesobtaining direct evidence from the analysis ofcontinental margin sediments at key sites (Barkeret al., 1998). Evidence from continental shelfdrilling has confirmed that major ice sheets have

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existed on East Antarctica since at least theearliest Oligocene, with many subsequent fluctu-ations (e.g. Hambrey et al., 1991; Cape RobertsScience Team, 1999; Shipboard Scientific Party,2001). However, the early glacial history of WestAntarctica remains less clear. The modern cryo-sphere in this region includes the mainly marine-based West Antarctic Ice Sheet and the smallerice caps, piedmont and tidewater glaciers andfringing ice shelves of the mountainous AntarcticPeninsula region. Based on modern conditions, ithas been inferred that the West Antarctic IceSheet and Antarctic Peninsula ice, in particular,is more dynamic and sensitive to external chan-ges than the East Antarctic Ice Sheet (e.g. Pudseyet al., 1994; Bart & Anderson, 2000).

It has been inferred by some that the growth ofsubstantial ice in the West Antarctic regionoccurred substantially later than in East Antarc-tica, and possibly not until the latest Miocene toearly Pliocene (e.g. Larter & Barker, 1989; Kennett& Barker, 1990). However, there is little directevidence, as no Oligocene to early Miocenecontinental margin sequences have been drilledin the region and the mid-Cenozoic terrestrialrecord is very sparse. The latter consists ofglacigenic and marine formations exposed onsouth-eastern King George Island, South ShetlandIslands. These formations provide a unique ter-restrial record of middle Oligocene to earliestMiocene glacial–interglacial cyclicity in the nor-thernmost Antarctic Peninsula region (Birkenma-jer et al., 1989; Dingle & Lavelle, 1998). The focusof this paper is the middle to late OligocenePolonez Cove Formation (PCF), the oldest glaci-genic unit known regionally (Dingle et al., 1997;Dingle & Lavelle, 1998).

The PCF consists of a basal unit dominated byglacigenic diamictite, overlain by a series ofbasalt-sourced sedimentary and dacitic volcanicunits. The presence of diverse pebble- to boulder-sized clasts, a minor proportion of which cannotbe sourced within the Antarctic Peninsula region,is a unifying characteristic of the formation. Manyclasts are faceted and striated, providing evidenceof glacial influence throughout. The formation isexposed most extensively in the vicinity of LionsRump, south-eastern King George Island (Fig. 1).

Birkenmajer (1980, 1982) was the first todescribe and recognize the potential significanceof the PCF for regional glacial history, whilePorebski & Gradzinski (1987) interpreted PCFlithofacies. This paper presents a detailedreappraisal of the lithofacies and palaeoenviron-mental significance of the formation, following

fieldwork by the authors and using a recentlyimproved chronostratigraphy. The study wasundertaken in response to continued uncertaintysurrounding aspects of the sedimentological and,to a lesser extent, stratigraphical interpretation ofthe PCF. In particular, the nature and extent ofglaciation implied by the deposits is conten-tious. Reports of exotic clasts with a proposedorigin in the Transantarctic and EllsworthMountains in a unit interpreted as lodgementtillite led to the suggestion that transport of suchclasts occurred as bedload in major ice streamssourced in the Transantarctic and EllsworthMountains (Birkenmajer, 1982). The distantprovenance of some clasts is supported by thisstudy. However, the concept of a major pan-Antarctic ice sheet capable of travelling along

Fig. 1. Maps showing: (A) the location of King GeorgeIsland in the northern Antarctic Peninsula region andin Antarctica (inset map also shows east–west divisionof the continent by the Transantarctic Mountains); (B)location of King George Island localities mentioned inthe text. An enlarged view of the boxed area aroundLions Rump is shown in Fig. 2.

278 A. L. Troedson and J. L. Smellie

� 2002 International Association of Sedimentologists, Sedimentology, 49, 277–301

the entire western flank of the broad WeddellSea embayment and crossing the northernAntarctic Peninsula is intuitively difficult toaccept, and an alternative hypothesis is presen-ted here. The nature of glacigenic influence andassociated palaeoenvironments in overlyingunits is also investigated. Finally, the signifi-cance of the findings for regional mid-Cenozoiccryosphere history is discussed.

REGIONAL SETTING AND PREVIOUSWORK

King George Island, the largest of the SouthShetland Islands, is situated on the periphery ofAntarctica and separated from the northernmostAntarctic Peninsula by Bransfield Strait (Fig. 1A).The South Shetland Islands formed on the lead-ing edge of the Antarctic Peninsula Mesozoic–Cenozoic magmatic arc, during eastward-dippingsubduction of proto-Pacific oceanic crust belowthe Gondwana margin (Pankhurst, 1982; Storey &Garrett, 1985). The opening of Drake Passagebetween South America and the Antarctic Penin-sula occurred during the Oligocene, between 22and 32 Ma (Barker & Burrell, 1977; Lawver et al.,1992). More recently, at »4 Ma, the South Shet-land Islands separated from the northern Antarc-tic Peninsula as a result of roll-back of DrakePassage oceanic crust and concomitant back-arcspreading in what became Bransfield Strait(Barker, 1982; Barker & Dalziel, 1983). Early tomid-Cenozoic volcanic and sedimentary strata areexposed along the southern edge of King GeorgeIsland (Smellie et al., 1984; Birkenmajer et al.,1986). The strata broadly become younger to theeast. Those west of the PCF exposures are mainlyterrestrial volcanic sequences, whereas someslightly younger marine strata occur further tothe east. The latter include the Cape MelvilleFormation, which consists of glaciomarine strataattributable to an earliest Miocene glacial episode(Birkenmajer et al., 1989; Dingle & Lavelle, 1998;Troedson & Riding, 2001).

The PCF was described and divided into fourmembers by Birkenmajer (1980, 1982) and is partof his Chopin Ridge Group. Porebski & Gradzinski(1987) described in detail the sedimentology anddepositional history of the formation as exposedon the coast south of Lions Rump. Porebski &Gradzinski (1990) and Smellie et al. (1998)investigated the genesis of volcanic units withinthe formation. A number of publications havedescribed the fossil fauna of the Low Head

Member of the formation (e.g. Gazdzicki &Pugaczewska, 1984). The presence and petrologyof diverse glacially transported exotic clastswithin the PCF, including some clasts with apossible provenance in the Transantarctic andEllsworth Mountains, were described by Bir-kenmajer (1980, 1982) and Birkenmajer & Wieser(1985). There have been numerous attempts todetermine the age of the PCF. K-Ar dating ofvolcanic rocks from surrounding and interbed-ded strata and dykes (e.g. Birkenmajer et al.,1985, 1989) has provided some erroneous results,with certain K-Ar dates conflicting with recentSr-isotope and 40Ar/39Ar dating results, as dis-cussed further below (Dingle et al., 1997; Dingle& Lavelle, 1998; Smellie et al., 1998).

LITHOSTRATIGRAPHY

Distribution, thickness, boundaries

The PCF is continuously exposed in 2Æ5-km-longcoastal cliffs and ledges between Lions Rump andLow Head (Fig. 2), which were the focus of allprevious accounts of PCF lithostratigraphy (e.g.Birkenmajer, 1980, 1982, 1994; Porebski & Grad-zinski, 1987). Further extensive exposures of thePCF occur inland at Godwin Cliffs, 2–3 km westof Lions Rump, and at Magda Nunatak, anisolated buttress »2 km north-west of GodwinCliffs. Exposures at Magda Nunatak were origin-ally assigned to a separate formation after anEocene K-Ar isotopic age was obtained from anoverlying basalt flow (Birkenmajer et al., 1985,1986). However, based on a striking stratigraph-ical similarity of the Magda Nunatak sequence tothe PCF in its type area (this paper), supported bySr-isotope dating (Dingle & Lavelle, 1998) andnew 40Ar/39Ar dating, the Magda Nunataksequence is interpreted to be part of the PCF.Small outliers of the basal PCF also occur at ThreeSisters Point, »10 km north-east of Lions Rump(Paulo & Tokarski, 1982), and at Vaureal Peak,10 km to the south-west (Birkenmajer, 1982)(Fig. 1B). Maximum thicknesses of the formationare 59 m at Mazurek Point, 58 m at Godwin Cliffsand 63 m (estimated) at Magda Nunatak.

Detailed vertical sections were measured at sixlocalities (Fig. 2). They form the basis for thepresent stratigraphical and sedimentologicalinterpretations (Figs 3–6). The PCF was previ-ously divided into four members with a simplehomoclinal stratigraphy (Birkenmajer, 1982;Porebski & Gradzinski, 1987). In this paper,

Polonez Cove Formation sedimentology 279


two new members are added to the previouslypublished stratigraphy, and the extent of someunits is redefined (Figs 3 and 4). In the previous

stratigraphy, the formation was separated into thefollowing four members: the Krakowiak GlacierMember (KGM; polymictic diamictite and sand-

Fig. 2. Map of Lions Rump areashowing approximate extent ofexposures of the Polonez CoveFormation and associated units andlocalities mentioned in the text.Stratigraphic sections were meas-ured at the six numbered sites.

Fig. 3. Diagram of generalized Pol-onez Cove Formation lithostratigra-phy, slightly modified from that ofPorebski & Gradzinski (1987), andsummary of 40Ar/39Ar and Sr-iso-tope age data for the formation. Errorbars represent 2r. Sr-isotope datesare from Dingle et al. (1997) andDingle & Lavelle (1998). 40Ar/39Ardates are new with the exception ofthe youngest Low Head Memberdate (Smellie et al., 1998). Age ofthe Oligocene–Miocene boundaryafter Shackleton et al. (2000).



stone); the Low Head Member (LHM; basalt lava,lava breccia, coarse basaltic sediments); the Sikl-awa Member (SM; planar-bedded sandstones,minor mudstone, granule conglomerate); and theOberek Cliff Member (OCM; basalt lava, breccia,conglomerate and gravelly sandstone). However,a previously unrecognized mudstone-dominatedunit (informally named the Bayview Member) islocally intercalated between the KGM and LHM.

Also, the heterogeneous facies grouped previouslywith the Oberek Cliff Member are separated hereinto two distinct members. The OCM is limited tothick exposures of basaltic breccia with interca-lated lava flows and associated coarse basalticsediments. Mainly planar beds of medium-grained to gravelly sandstone with sparse exoticclasts and occasional marine macrofauna inclu-ding Chlamys were previously included with the

Fig. 4. Schematic cross-sections of Polonez Cove Formation exposures, showing main lithofacies. Localities are:(A) coastal cliffs from Low Head to Battke Point; (B) Godwin Cliffs; (C) Magda Nunatak, north-east face (same viewas Fig. 8A). Note different scales for each. Numbered arrows indicate the locations of measured sections.



upper OCM. These beds are here reassigned to thenew, informally named Chlamys Ledge Member(CLM). Schematic cross-sections of the main PCFexposures are illustrated in Fig. 4.

The PCF is bounded by erosional unconformi-ties. It mainly overlies andesitic volcanic rocksassigned to the Mazurek Point Formation byBirkenmajer (1980). K-Ar dating of the MazurekPoint Formation suggested an unusually wide-ranging age (74–34 Ma; Birkenmajer et al., 1989),but lithology, geochemistry, other K-Ar ages andnew unpublished 40Ar/39Ar ages from associatedoutcrops indicate that the Mazurek Point Forma-tion is equivalent to the Eocene HennequinFormation of Smellie et al. (1984). The uppermostvolcanic basement is commonly porphyritic

andesite lava. Where exposed, the lava surfaceis irregularly jointed and Fe oxide coated. Itshows up to 15 m of relief. Striations on theunconformity have been reported previously(Paulo & Tokarski, 1982; Santos et al., 1990). Noconvincing striations were found in the presentstudy, and well-exposed bedrock surfaces wererare. At Conglomerate Bluff, the PCF overliesmoderately well-sorted, massive, boulder to cob-ble conglomerate forming a bed at least 10 mthick of uncertain age. The clasts are rounded tosubrounded and andesitic and were clearly de-rived from the Hennequin Formation. Thin beds(typically 5–30 cm thick) of andesitic pebbleconglomerate, breccio-conglomerate and brecciaalso occur between Polonez Cove Formation

Fig. 5. Stratigraphic logs throughthe Krakowiak Glacier Member at(A) western Godwin Cliffs (site 2)and (B) central Mazurek Point(site 4). Metre scale bars on left.



diamictite and volcanic basement at a few sites,including Battke Point and Three Sisters Point. Atthe latter site, Paulo & Tokarski (1982) described apossible roche moutonee on basement with adja-cent ‘leeside’ bedrock till.

The PCF exposures are consistently subhori-zontal to shallow dipping, and the formation isessentially unaffected by tectonic deformation.There are rare normal faults. The height of thebasal unconformity is variable, reflecting bothfaulting and basement relief. In the coastalexposures, the unconformity has a maximumelevation of c. 20 m a.s.l. at Battke Point, and itis below sea level at Low Head. A major fault ispresent between the coastal exposures and PCFexposures to the west, and the basal unconformity

is exposed at over 100 m a.s.l. at Godwin Cliffsand Magda Nunatak.

Arkosic sandstones and dacitic volcanic rocks,assigned to the Boy Point Formation by Bir-kenmajer et al. (1986), overlie the PCF in thecoastal exposures and at Godwin Cliffs. AroundPolonez Cove and at Godwin Cliffs, the upperPCF is truncated by an erosional unconformity.Exotic clasts are absent from the Boy PointFormation. Pectinid-bearing sandstones occurwithin the basal Boy Point Formation, indicatinga shallow-marine environment for at least thelower part of the formation. The Boy PointFormation is latest Oligocene in age and appar-ently only slightly younger than the PCF (Smellieet al. 1998).

Fig. 6. KGM photographs. (A) View of Polonez Cove Formation exposures at northern Mazurek Point (between sites3 and 4). The slightly undulating basal unconformity (2–6 m a.s.l), Krakowiak Glacier Member units K2 and K3,thickly bedded basaltic conglomerate of the Low Head Member and thinly bedded sheets of the Siklawa Member areidentified. The typically planar upper contact of the Krakowiak Glacier Member (interpreted as a marine erosionalsurface) is »9 m a.s.l. The section shown is »23 m thick. (B) Faceted, fractured andesitic (basement-derived) boulderlodged against basement within lodgement tillite of unit K1, basal Krakowiak Glacier Member, western Godwin Cliffs(site 2). The hammer, 33 cm in length, rests on the boulder. (C) Subrounded faceted boulders within slightly muddygravelly sandstone of unit K3, upper Krakowiak Glacier Member, north of Mazurek Point (near site 3). The largestboulder, 1Æ2 m long, is formed of gneiss, whereas the other two boulders are composed of basement-derived andesiteand are »0Æ6 m in diameter. Bryozoa encrust parts of the large boulder. (D) Cross-bedded sandstone in channel (»1 mthick) surrounded by muddy gravelly sandstone to diamictite, unit K3, Krakowiak Glacier Member, western GodwinCliffs (site 2).



Age

A middle to late Oligocene age for the PCF isindicated by Sr-isotope dating of fossil shells(Dingle et al., 1997; Dingle & Lavelle, 1998) and40Ar/39Ar dating of interbedded basaltic lavaflows (Smellie et al., 1998; this paper), althoughsomewhat older ages were obtained by the Sr-isotope method. Figure 3 illustrates the age dataobtained by the two methods. Other ages obtainedpreviously by the K/Ar method have provenunreliable and are not discussed further here.Ages of 26Æ4 ± 1Æ2 Ma and 27Æ2 ± 0Æ7 Ma wereobtained from basalt lava of the Low HeadMember at Low Head and southern MazurekPoint respectively. Oberek Cliff Member basaltsfrom southern Mazurek Point and MagdaNunatak gave ages of 26Æ0 ± 2Æ59 Ma and25Æ6 ± 1Æ3 Ma respectively. Sr-isotope agesobtained from bivalve and brachiopod fossilstaken from the KGM and LHM lie within therange 28Æ5 – 29Æ8 ± 0Æ8 Ma (Dingle et al., 1997;Dingle & Lavelle, 1998). Reasons for the discrep-ancy in ages determined by the two isotopicsystems are unclear at present, but a middle tolate Oligocene age for the PCF is certain.

SEDIMENTOLOGY

Krakowiak Glacier Member (KGM)

Description

The KGM consists largely of diamictite andgravelly sandstone, with minor interbeds andlenses of granule to pebble conglomerate andmedium sandstone. Sandy diamictite is the dom-inant diamictite type (classification after Mon-crieff, 1989). Thin mud stringers occur rarely.Gravel occurs predominantly as pebbles andcobbles, but boulders ranging up to »1Æ2 m indiameter occur in certain beds. Pebble- to boul-der-sized clasts are most commonly subangular orsubrounded. Faceting is very common, someclasts are bullet-nosed, and striations occur on asignificant minority of sedimentary clasts. Marinemacrofossils are sparse but widely distributed.

The KGM is composed of locally derivedandesitic material and a diverse collection oflithic grains, clasts and associated minerals. Thelatter include sedimentary, igneous and meta-morphic lithologies, but sedimentary clasts aremost common. The range of clast lithologies andtheir overall proportions resemble present-dayoutcrop proportions in the northern Antarctic

Peninsula and South Shetland Islands. However,as noted by previous authors, the mountainranges rimming the southernmost Weddell Seaare the nearest known provenance of certain clastlithologies. Examples of apparently far-travelledlithologies are red quartzose sandstone andarchaeocyathid-bearing and other coarse bio-clastic limestones.

The KGM is an extensive unit, with exposuresknown from localities up to 14 km apart. It wasobserved everywhere that the base of the PCF isexposed, infilling basal palaeorelief. The memberusually has a sharp and planar upper contact (e.g.Fig. 6A) but, in a few places, beds of basalticbreccia of the overlying Low Head Member cutnarrow erosional channels up to 2 m into theKGM. Additionally, the contact appears abruptlygradational into a diamictic facies of the LowHead Member (L4) in a few exposures. Measuredthicknesses generally range from 3 to 15 m, butrise to »24 m in a basement depression at GodwinCliffs (site 2). Three main units (K1–K3) arerecognized within the KGM, arranged in a tabularfashion, although K1 is generally thin and locallyabsent, and K3 is also absent at a few sites.Detailed sections are illustrated for two sites: awell-exposed section at western Godwin Cliffswhere all three KGM units are present (site 2);and a section typical of KGM coastal exposures atMazurek Point (site 4). The KGM is critical tounderstanding PCF glacial history, and individ-ual units are described in more detail below.Unit K1. This is well-indurated, intermediate tomuddy diamictite. It commonly has a red-brownsilty matrix that appears to consist largely ofcrushed basement-derived material. Mud contentranges from 20% to 40%. K1 is variably clast-rich to clast-poor. Gravel varies from granule toboulder-grade and is derived mostly (80–95%)from local andesitic basement, although diverseexotic clasts also occur. Large basement-derivedclasts are typically equant and subrounded,whereas exotic clasts are variably subroundedto subangular. Faceting is common on both clasttypes. K1 is typically <0Æ3 m thick, but is up to1 m thick in rare cases within basement lows. Insome localities, there is a degree of clast align-ment or imbrication, but no consistent direc-tional pattern was recognized across the studyarea. Ploughed clasts occur at a few sites (e.g.Fig. 6B). The upper contact of K1 is typicallysharp.

Fossils are generally uncommon within K1 but,at two localities, marine invertebrate fossils areabundant. On basement highs at central Godwin



Cliffs and at southern Mazurek Point, smalllenses of grey muddy diamictite a few metreslong and <1 m thick occur with concentrations ofmacrofossils. These include both intact andfragmented solitary corals, pectinid bivalves,brachiopods and benthic foraminifera. Alignedor imbricated shells parallel to sedimentarylaminations occur within one such pod.

Features indicative of deformation occur inassociation with K1 at a number of sites. Theseinclude ductilely deformed sedimentary clastsand brecciated or fractured clasts and grains.Many macrofossils in the sediment pods men-tioned above appear to be ductilely deformed orfractured. At some localities, notably parts ofGodwin Cliffs and Magda Nunatak, the upper-most metre of andesite lava is sheared andbrecciated, with matrix and rare exotic pebblesidentical to the basal PCF injected along joints inthe andesite. Iron oxide-coated, striated shearplanes extended from K1 into the uppermostbasement at these sites.Unit K2. This consists of moderately well-indu-rated, sandy to intermediate diamictite, withsome thin sandstone and granule to pebbleconglomerate beds and rare mudstone stringers.Bedding is typically lenticular and discontinu-ous. Diamictite occurs in unstratified beds, com-monly 1–1Æ5 m thick, with boundaries defined bytextural variations in the diamictite and by lensesof fine to medium sandstone or (rarely) thinmudstone. Sandstone lenses are typically <0Æ3 mthick, and are locally graded, laminated or cross-laminated. K2 has a brown-grey to grey matrixand generally low mud content. Gravel content ishighly variable. Diamictite beds are typicallyclast-rich, with up to 45% gravel. Gravel is mostlygranule to small pebble grade, although there areoccasional boulders and cobbles up to a maxi-mum diameter of c. 0Æ5 m. Large clasts arechaotically distributed, although maximum clastsize may decrease upsection from boulder tosmall cobble grade. Typically, over 50% of clastsare composed of basement-derived andesiticmaterial. Imbrication of elongate clasts and grainswas observed rarely. In a few localities, sandstonebeds appear faulted, folded or form sand dykes.Some sandstone beds contain fragments of marineinvertebrate fossils. Other observed features with-in a few diamictite beds include steeply inclined,brecciated marly lenses and brecciated or shearedpebbles and cobbles. K2 is generally 2–4 m thickbut, at western Godwin Cliffs, reaches a maxi-mum of »7 m thick where the KGM fills abasement low (Fig. 5A).

Unit K3. This consists of moderately to poorlyindurated intermediate to sandy diamictite andgravelly sandstone. Thin beds of pebble to gran-ule conglomerate and thin planar to channelized,sorted sandstone beds also occur in upper K3. K3is generally weakly to well stratified and, overall,the unit has a tabular morphology. Gravel modeand abundance commonly define stratificationwithin gravelly sandstone and diamictite beds.K3 has a grey to green-grey matrix, which ismuddy and fissile in patches. The diamictite isvariably clast-rich to clast-poor, with gravel con-tent varying from 1% to 50%. Clasts are mainlygranules to pebbles, but larger clasts occur dis-persed or in clusters and trains (e.g. Fig. 6C).

Upsection trends within K3 include an increas-ing dominance of sandstone over diamictite, adecrease in basement-derived clasts to <20% andincreased incidence of cobble- to boulder-gradeclasts. Thick-bedded diamictite occurs only with-in lower K3, whereas sandstone channels con-taining cross-beds were observed within theupper section of K3 at two localities. At south-western Godwin Cliffs, foresets occupy two chan-nels that are 1Æ5–3 m thick, 10 m wide and haveclearly erosional bases (e.g. Fig. 6D). In a numberof localities, thin (1–10 cm thick), planar, occa-sionally graded and moderately sorted sandstonebeds occur in upper K3. Macrofossils occursparsely in lower K3 and more commonly inupper parts of K3. Those observed include intact,ductilely deformed bivalves within sandstonebeds and bryozoans and worm tubes encrustingfaceted, subrounded cobbles and boulders. Ingeneral, K3 appears undeformed, but featuresindicative of deformation occur at a few sites.These include boudinaged sandstone beds inupper K3 north of Mazurek Point and inclinedbeds and shear planes associated with an opensynform structure at southern Mazurek Point(described by Porebski & Gradzinski, 1987). K3occurs at the top of the KGM and varies between 6and »16 m thick. It can be absent where the KGMis <5 m thick.

Interpretation

The ubiquitous presence of diverse lithic clastswith characteristics associated with glacial trans-port, such as facets, striations and bullet-nosedshapes, indicates that the KGM has a glacigenicorigin, as recognized by previous workers (Bir-kenmajer, 1982; Porebski & Gradzinski, 1987).There are a number of indications that thevolcanic basement surface was subject to



subglacial erosion. Evidence includes the (rarelyexposed) highly polished lava surface, fossilstriations observed by previous workers, thenature of basement relief, the locally brecciatednature of the contact with the overlying diamic-tite and the presence of diamictite intrusions intobedrock fractures. The latter two features resem-ble local till and ‘injection wedges’ described in aCanadian terrestrial glacial sequence (Brosteret al., 1979) and attributed to glacial deformation,erosion and deposition. The KGM marine fossilfauna is mostly reworked, but it is clear that theunit was deposited in association with marine-based ice. This interpretation differs from that ofprevious workers, who attributed the basal KGMto terrestrial glaciation.

K1 is subglacial tillite, deposited by a variety ofprocesses including lodgement, deformation andcomminution. Features of K1 characteristic ofsubglacial deposition include a high proportionof basement-derived clasts, rock matrix appar-ently consisting largely of finely crushed base-ment-derived material and occasional ploughedclasts associated with basement. The highlyindurated nature of this facies may reflect strongsubglacial compaction and subsequent loss ofporosity and comminution of grains. In places, K1sediments have been subject to deformation thatcan best be attributed to glaciotectonics, assurrounding units are undeformed. Polished andstriated joints that extend into basement at a fewsites are former shear planes (e.g. Boulton et al.,1974).

The evidence for glacial abrasion of volcanicbasement and the high proportion of basement-derived material in K1 suggests that preglacialsediment cover was minimal. However, preglacialdeposits such as the cobble to boulder andesiticconglomerate underlying the KGM at Conglomer-ate Bluff may have supplied the rounded tosubrounded andesite clasts common in the lowerKGM. Also, lenses of muddy diamictite contain-ing concentrations of marine macrofossils weremost likely marine sediment rafts entrainedduring ice advance. Their presence can be attrib-uted to a thin to patchy cover of preglacialadvance marine sediment on local basement.The rafts contain clear indications of deformationand reflect erosion and incorporation of marinesediment into a thin layer of deforming subglacialtill.

K2 is interpreted to be a subglacial to ice-marginal deposit. Major processes include melt-out and glacigenic debris flow. The occasionalmud drapes and thin sandstone and conglomerate

interbeds, some with shell fragments, indicateperiods of current reworking, possibly in subgla-cial cavities and channels by small-dischargesubglacial meltwater streams. The relatively highproportion of basement-derived clasts indicates apredominantly subglacial sediment source.Localized glaciotectonic structures reflect defor-mation in the vicinity of an unstable groundingline. The mud content of K2 is typically low,possibly because of flushing of fines by highporewater pressures (Lawson, 1981). High pore-water pressure may also have contributed topatchy sediment deformation.

K3 consists mainly of reworked and redepo-sited submarine glacial outwash and ice-rafteddebris. The common stratification suggests anice-proximal glaciomarine rather than subglacialorigin. Basement-derived clasts are much lessabundant than in the underlying units, suggest-ing that local sediment sources were swampedby abundant further travelled englacial sedimentreleased during grounding line retreat and byincoming icebergs from more distant sources.Thick diamictite beds with variable gravel con-tents within lower K3 are most likely debris flowdeposits. Channelized cross-bedded sandstonebeds exposed within upper K3 may be associatedwith proglacial or subglacial meltwater channels.Thin planar beds of moderately sorted, fine tomedium sandstone in upper K3, locally contain-ing intact bivalves, may be proglacial tractioncurrent deposits. Encrusting bryozoa and barna-cles on some cobbles and boulders confirm thatthe environment was glaciomarine. Outsizedclasts within sandstone, some of which occuras cobble and boulder lags, were deposited byice rafting during periods of current winnowingor low sediment supply. The lags become com-mon upsection, whereas thick debris flow dia-mictite beds become uncommon, suggesting arelative increase in the contribution from icerafting and distance from the grounding line (cf.Boulton & Deynoux, 1981). Unlike boulder lagsdescribed by Eyles (1988) for the YakatagaFormation (Alaska), the PCF examples do nothave planar tops and are of limited areal extent,and there is no suggestion of an associatedsignificant glacial readvance. Deformation fea-tures within ice-proximal deposits such as K3may relate to ice thrust, iceberg grounding orgravitational collapse (Eyles & McCabe, 1989).Localized folding in K3 has been attributedpreviously to iceberg grounding processes(Porebski & Gradzinski, 1987), but may equallyindicate a transient, local readvance.



Basaltic members of the Polonez CoveFormation

A detailed description and interpretation of thebasaltic (i.e. basalt-sourced sedimentary and vol-canic) members of the PCF was provided byPorebski & Gradzinski (1987, 1990), based onexposures between Mazurek Point and Low Head.Their main conclusions regarding the genesis ofthese members are supported here. However,based on field studies in surrounding areas, twoadditional members are identified, and a morecomplete assessment of regional facies variability

and associated palaeoenvironments is provided.Below is outlined where the present work aug-ments or differs from that of Porebski & Gradzin-ski (1987, 1990).

Five basaltic members are identified. All fiveoccur together only at Magda Nunatak, andthe stratigraphy at that site is presented inFig. 7 and illustrated in Fig. 8A. In manylocalities, complexities arise from the lateraldiscontinuity of units resulting from erosion orlateral facies changes and from the recurrence ofsimilar facies at different stratigraphical levels.Diverse extraformational sedimentary, igneous

Fig. 7. Stratigraphic log of PolonezCove Formation basalt-sourcedlithofacies at Magda Nunatak (site1). Generalized lithofacies descrip-tions and interpretations are alsogiven. All members of the PolonezCove Formation occur at this site.Metre scale bars on left.



and metamorphic clasts occur in varying abun-dance through these units and are referred to hereas exotic clasts. Preliminary studies indicate thatthe range and proportions of clast lithologiesresembles that of the KGM, although far-travelledclasts appear more common. Details of faciesrecognized within each member including extentof exposure, exotic clast occurrence and biota aresummarized in Table 1.

Bayview Member (BM)

The Bayview Member (BM) is described here forthe first time. It consists mainly of dark-brown,planar-bedded, medium to fine sandstone andmudstone and has a stratigraphical positionbetween the KGM and Low Head Member. Themember is most extensively developed at MagdaNunatak, where it is up to 6 m thick (Fig. 7). Itwas not recognized previously within the PCFstratigraphy, presumably because it does notoccur in exposures south of Mazurek Point wherethe PCF type section was defined. Macrofossilsobserved in one BM medium sandstone bedindicate a shallow-marine setting. The generallyscarce biota suggests that deposition was rapidand/or that the sea-floor was poorly oxygenated.Sediment was supplied dominantly from a localbasaltic source and deposited mainly from sus-pension and traction currents during normalmarine conditions. The rarity of outsized exoticclasts suggests that floating debris-laden ice wasuncommon. Deformation of macrofossils andsedimentary structures directly underneath LowHead Member breccio-conglomerate beds andcommon erosional incision by these beds suggeststhat the BM was poorly consolidated duringdeposition of the Low Head Member, and thatthe intermittent occurrence of the BM is related topost-depositional erosion.

Low Head Member (LHM)

The Low Head Member consists of laterallydiscontinuous bodies of coarse basaltic sediment,basaltic lava and lava breccia. It contains ashallow-marine faunal assemblage (Gazdzicki &Pugaczewska, 1984). These occur as intact, in situspecimens, debris lenses and isolated fragments.In a few places, the basal surface of the LHM is

Fig. 8. Photographs of basalt-sourced lithofacies. (A)View of the Polonez Cove Formation, north-east face ofMagda Nunatak. See Fig. 4C for identity of units. Cliffheight is »60 m. (B) Exotic ice-rafted clasts in coarsebasaltic sediments of the Low Head Member, GodwinCliffs. Pale clast below hammer handle is red sand-stone, a lithology unknown in the Antarctic Peninsularegion and inferred to have been transported over1800 km by icebergs calving in the southern WeddellSea. The hammer is 33 cm long. (C) Granitic boulderpavement within coarse basaltic sediments of the LowHead Member north of Mazurek Point (site 3). Theclasts are angular, and some appear brecciated. Thegranitic clasts occur over a horizontal distance of»10 m. Their presence is attributed to iceberg ground-ing.



Table

1.

Ch

ara

cte

rist

ics

of

basa

lt-s

ou

rced

PC

Fm

em

bers

(MN

,M

agd

aN

un

ata

k;

MP

,M

azu

rek

Poin

t;B

P,

Batt

ke

Poin

t;L

H,

Low

Head

).

Mem

ber

Un

its/

lith

olo

gie

sE

xte

nt

Textu

re,

fabri

cB

ed

din

gst

yle

Sed

imen

tary

stru

ctu

res

Exoti

ccla

stoccu

rren

ce

Bio

ta

Bayvie

wM

em

ber

Mu

dst

on

e,

san

dst

on

eN

ot

wid

esp

read

;gen

era

lly

<2

mth

ick,

6m

thic

kat

MN

Fis

sile

mu

dst

on

e;

med

ium

tofi

ne-g

rain

ed

mod

era

tely

well

-sort

ed

san

dst

on

e

Pla

nar-

bed

ded

sheets

;m

ed

ium

toth

inly

bed

ded

Lam

inati

on

s;cu

rren

tri

pp

les;

gra

din

gin

con

glo

mera

te,

soft

-sed

imen

td

efo

rmati

on

featu

res

belo

wL

HM

bre

ccia

Wit

hin

on

ebed

at

MN

;p

ebble

s,ra

recobble

s,m

ax.

0Æ1

5m

Wit

hin

on

ebed

at

MN

on

ly;

dis

pers

ed

;in

tact

biv

alv

es,

bra

ch

iop

od

s,so

lita

rycora

ls

Low

Head

Mem

ber

L1:

basa

ltla

va,

au

tocla

stic

lava

bre

ccia

(Sm

ell

ieet

al.

,1998)

Main

occu

rren

ces

at

LH

,m

ax.

25

mth

ick,

an

dso

uth

of

BP

,m

ax.

12

mth

ick

–P

lug-l

ike

lava

occu

rren

ce

(LH

);la

va

ton

-gu

e,3

mth

ick

by

10

mlo

ng

(BP

);bre

ccia

lobes

3–4

mth

ick

els

ew

here

NA

NA

NA

L2:

pebble

–cobble

con

glo

mera

te,

bre

ccio

-con

glo

mera

te,

pebbly

san

dst

on

e

Wid

esp

read

;tr

an

siti

on

al

from

L1

wh

ere

itoccu

rs;

5–15

mth

ick;

Most

lyp

ara

-con

glo

mera

te,

som

eort

ho-

an

dbre

ccio

-con

-glo

mera

ten

ear

base

Wed

ge-s

hap

ed

tole

nti

cu

lar

bed

s,so

me

ch

an

neli

zed

;2–3

mth

ick

gen

era

lly,

10–15

mth

ick

wed

ge

at

MN

Main

lym

ass

ive

tow

eakly

gra

ded

bed

din

g;

larg

e-s

cale

pla

nar

cro

ss-b

ed

fore

sets

incoars

econ

glo

mera

tean

dbre

ccia

at

MN

Rare

tosp

ars

en

ear

un

itbase

,m

ore

abu

nd

an

tu

p-u

nit

;m

ain

lyp

ebble

sto

cobble

s,ra

rebou

lders

Locall

ycom

mon

mari

ne

macro

-fo

ssil

s(b

ivalv

es,

bry

ozoan

s,sc

ap

hop

od

s,ost

racod

s,gast

rop

od

s,ben

thic

fora

min

ifers

,w

orm

tubes,

cri

noid

s);

most

lyfr

agm

en

ted

,in

len

ses

or

dis

pers

ed

;in

tact

pecte

nid

shell

sin

imbri

cate

dle

nse

sin

som

ebed

sL

3:

gra

vell

ysa

nd

ston

eW

idesp

read

;com

mon

lyoverl

ies

or

gra

des

late

rall

yin

toL

2;

max.

8m

thic

k

2–10%

gra

vel,

main

lygra

nu

leto

small

pebble

-gra

de

Len

ticu

lar

tota

bu

lar;

vari

able

bed

thic

kn

ess

,ty

pic

all

y<

3m

Weakly

tow

ell

-d

efi

ned

stra

tifi

-cati

on

defi

ned

by

cla

sttr

ail

s,gra

vel

con

ten

t;th

insa

nd

or

mu

dle

nse

s

Sp

ars

eto

locall

ycon

cen

trate

d;

main

lyp

ebble

sto

small

cobble

s,so

me

bou

lders



Table

1.

(Con

tin

ued

).

Mem

ber

Un

its/

lith

olo

gie

sE

xte

nt

Textu

re,

fabri

cB

ed

din

gst

yle

Sed

imen

tary

stru

ctu

res

Exoti

ccla

stoccu

rren

ce

Bio

ta

L4:

dia

mic

tite

,con

glo

mera

tean

dgra

vell

ysa

nd

ston

e

Lim

ited

occu

rren

ces;

max.

5m

thic

kin

coast

al

exp

osu

res,

5–15

mat

MN

<10%

mu

d,

gra

vel

con

ten

tvari

able

,<

20%

Len

ticu

lar

toch

an

nel-

shap

ed

;ty

pic

all

y1–3

mth

ick

Tro

ugh

cro

ss-

bed

din

g;

ch

an

nels

wit

hgra

vel

lags

at

base

s

Dis

pers

ed

com

mon

pebble

sto

small

cobble

s(<

0Æ1

m),

som

ebou

lders

;si

ze

an

dabu

nd

an

ce

may

incre

ase

up

-un

it

Rare

shell

fragm

en

ts

Sik

law

aM

em

ber

SM

1:

san

dst

on

e,

silt

ston

e,

cla

yst

on

e

Main

lyaro

un

dM

P,

typ

icall

y<

6m

thic

k,

max.

15

m

Vari

able

,in

clu

des

poorl

yso

rted

gra

vell

ysa

nd

ston

e,

mod

era

tely

sort

ed

med

ium

tocoars

esa

nd

ston

e,

min

or

fin

esa

nd

ston

e,

fin

esi

ltst

on

eto

cla

yst

on

e

Pla

nar-

bed

ded

sheets

;0Æ1

–1Æ5

mth

ick

Lam

inati

on

sin

mu

dst

on

e;

occasi

on

al

gra

din

g;

thin

cla

yst

on

ed

rap

es

on

san

dst

on

ebed

s,so

me

wit

hcon

volu

teor

bre

ccia

ted

bed

din

gan

dcu

rren

tri

pp

les

Rare

ap

art

from

SM

1a

(see

belo

w)

Rare

pla

nt

stem

cast

sw

ith

incla

yst

on

ed

rap

es

SM

1a:

pro

min

en

tw

ell

-in

du

rate

dgra

vell

ysa

nd

ston

e

Sou

thof

MP

on

ly,

bed

str

aceable

for

severa

l100

m

Typ

icall

y10–20%

gra

vel

Tw

oto

thre

ep

lan

ar

sheets

;m

ost

ly0Æ2

–0Æ5

mth

ick,

locall

yam

alg

am

ate

dan

d1–1Æ5

mth

ick

Weakly

gra

ded

inp

art

s;so

me

cla

stim

bri

cati

on

Com

mon

inp

art

s;m

ain

lyp

ebble

s,so

me

cobble

san

dbou

lders

;oft

en

elo

ngate

an

dta

bu

lar

Locall

ycon

cen

trate

d;

inta

ct

biv

alv

es,

soli

tary

cora

ls,

bra

ch

iop

od

s;P

enit

ella

biv

alv

ebu

rrow

mou

lds

(Gazd

zic

ki

etal.

,1982)

SM

2:

med

ium

tofi

ne

san

dst

on

e,

min

or

gra

nu

leto

pebble

con

glo

mera

te

MN

on

lyM

od

era

tely

sort

ed

con

glo

mera

teP

lan

ar-

bed

ded

sheets

;<

0Æ2

mth

ick

Sym

metr

ical

cu

rren

tri

pp

les;

revers

ean

dn

orm

all

ygra

ded

bed

s

Sp

ars

e;

main

lyp

ebble

s,m

ax.

0Æ1

5m

Biv

alv

econ

cen

trati

on

sin

on

efi

ne

san

dst

on

ebed

Obere

kC

liff

Mem

ber

OC

1:

poly

mic

tbre

ccia

,m

inor

gra

vell

ysa

nd

ston

e,

con

glo

mera

te

Inte

rmit

ten

toccu

rren

ce;

max.

4m

thic

k

Poorl

yin

du

rate

d;

bre

ccia

of

poorl

yso

rted

an

gu

lar

cobble

san

dp

ebble

sin

aw

eath

ere

dm

atr

ix

Pla

nar-

bed

ded

;m

ax.

1Æ5

mth

ick

–R

are

;m

ax.

1m

Non

e



Table

1.

(Con

tin

ued

).

Mem

ber

Un

its/

lith

olo

gie

sE

xte

nt

Textu

re,

fabri

cB

ed

din

gst

yle

Sed

imen

tary

stru

ctu

res

Exoti

ccla

stoccu

rren

ce

Bio

ta

OC

2:

coars

ela

va

bre

ccia

,ass

ocia

ted

lava

flow

s

Occu

rsat

MP

an

dM

N;

max.

»22

mth

ick

Fin

es-

poor;

colu

mn

ar

join

tin

gin

lavas

Pro

gra

din

gw

ed

ges;

lava

flow

soccu

rin

up

per

part

of

un

it;

thic

kly

tom

ass

ively

bed

ded

Occasi

on

al

gra

din

g,

steep

tota

ngen

tial

fore

set

bed

son

lap

ped

by

subh

ori

zon

tall

yst

rati

fied

bre

ccia

at

MP

Sp

ars

eN

A

OC

3:

san

dst

on

e,

con

glo

mera

te,

bre

ccio

-con

glo

mera

te

Wid

esp

read

;la

tera

lequ

i-vale

nt

of

OC

2;

max.

10

mth

ick

An

gu

lar

lava

lap

illi

,p

ebble

s

Pla

nar-

bed

ded

;vari

able

bed

thic

kn

ess

Weakly

tow

ell

-d

efi

ned

stra

tifi

cati

on

;so

me

gra

ded

san

dst

on

e,

cro

ss-b

ed

fore

sets

Rare

Rare

trace

foss

ils;

spars

esh

ell

san

dsh

ell

fragm

en

ts

Ch

lam

ys

Led

ge

Mem

ber

CL

1:

san

dst

on

e,

min

or

mu

dst

on

e,

con

glo

mera

te

Wid

esp

read

;m

ax.

10

mth

ick

Vari

able

gra

vel

con

ten

tan

dsa

nd

ston

egra

des;

bett

er

sort

ed

up

secti

on

Len

ticu

lar,

ch

an

nel-

shap

ed

or

pla

nar

bed

din

g;

main

lym

ed

ium

toth

ickly

bed

ded

Cm

-scale

stra

tifi

cati

on

;n

orm

al

an

dre

vers

egra

din

g;

pla

nar

cro

ss-

lam

inae

an

dcro

ss-b

ed

din

g;

subh

ori

zon

tal

thin

mu

dle

nse

san

dlo

ad

cast

son

bed

bou

nd

ari

es;

som

ecla

sttr

ail

sin

basa

lp

art

Sp

ars

eto

com

mon

;m

ost

abu

nd

an

tn

ear

som

ebed

top

s;m

ain

lyp

ebble

sto

small

cobble

s,m

inor

larg

ecobble

san

dbou

lders

Min

or

macro

foss

ils

(biv

alv

es,

bra

ch

iop

od

s,gast

rop

od

s);

most

lyfr

agm

en

ted

;ra

reim

bri

cate

dp

ecte

nid

len

ses

CL

2:

med

ium

san

dst

on

e,

min

or

silt

ston

e

Wid

esp

read

;m

ax.

»11

mth

ick

–P

lan

ar;

san

d-

ston

ebed

sty

pic

all

y0Æ3

–1

mth

ick,

silt

ston

ebed

sty

pic

all

y0Æ1

–0Æ3

mth

ick

Lam

inati

on

sin

silt

ston

eM

ost

lysp

ars

e,

com

mon

inso

me

bed

sor

alo

ng

bed

din

gp

lan

es

Most

lysp

ars

ebu

tcom

mon

ina

few

bed

s;m

ain

lybra

ch

iop

od

s,m

inor

biv

alv

es,

bu

rrow

stru

ctu

res



clearly erosional and cuts channels up to 2 m intounderlying KGM beds.

The LHM was deposited in a shallow-marineenvironment above wavebase and in close prox-imity to active basaltic volcanism, which providedthe bulk of the sediment. Porebski & Gradzinski(1987) noted that the biota and indications of high-energy sedimentation suggest a near-shore, storm-dominated depositional environment. Four mainunits were recognized within the LHM, termed L1to L4 here, one more than was recognized byPorebski & Gradzinski (1987). The numbers reflectthe order of occurrence in the section. Units L1 toL4 represent a continuum from primary basalticlava and autoclastic breccia to increasinglyreworked volcanic-derived sediments with avariable admixture of exotic material.

A lava ‘plug’ and adjacent monomict lavabreccia at Low Head (L1) is the only clearlyexposed example of the type of volcanic sourceproducing the basalt. The outcrop has beeninterpreted as a subaqueous lava dome and coevalcarapace breccia with an original diameter ofabout 500 m, emplaced at water depths of poss-ibly 30–80 m (Smellie et al., 1998). A similarorigin may apply to lava and lava breccia exposedsouth of Battke Point. Deposition of L2 brecciaand breccio-conglomerate beds was rapid andepisodic and occurred mainly by slumping,debris flow and current reworking to form len-ticular or lobate beds. Intact and locally imbri-cated concentrations of pectinid bivalve fossils(‘coquinas’ of Porebski & Gradzinski, 1987) occurwithin L2 and may represent periods of lowersedimentation rates. Some such lenses are con-voluted, indicating post-depositional slumping.Fragmented granule to sand-grade shell debrissuggests reworking of shallow sea-floor depositsduring storm events. A thick wedge-shaped L2conglomerate bed at Magda Nunatak containslarge-scale cross-bed foresets and is best inter-preted as the prograding front of a marginalmarine fan delta (Gilbert-type sedimentary delta).The almost monogenetic (basalt) clast populationin the foresets suggests that the deposits accumu-lated within a relatively short transport distancefrom a basaltic volcanic source. An incisedshoreface may be preserved adjacent to thesebeds, which are incised by gravelly sandstonesand diamictites of units L3 to L4. The uppermostL4 beds are clearly channelized and may beshallow deltaic to marine topset beds. Elsewhere(e.g. south of Mazurek Point), L4 directly overliesKGM diamictite with a sharp to abruptly grada-tional contact, and the two units appear to be

texturally identical. Shallow-marine reworking ofthe uppermost KGM after initiation of localbasaltic volcanism is inferred.

Exotic clasts occur in varying abundancewithin the sedimentary units of the Low HeadMember. The clasts generally range from pebbleto cobble sized, but rare boulders also occur.Such clasts are often faceted and are typicallysubangular or subrounded. The clasts areinferred to be ice rafted. A trend of increasingexotic clast abundance towards bed tops noted ata number of sites most likely reflects waningbasaltic sediment supply rather than increasedice rafting. In some beds, the size range of exoticclasts is relatively restricted, suggesting thatthese are somewhat sorted and reworked. How-ever, within certain beds, exotic clasts occur in awide range of sizes and sometimes occur inclusters, suggesting direct deposition from ice(e.g. Fig. 8B). The presence of a granitic boulderpavement at one site (Fig. 8C) is also consistentwith former grounded ice. There is no other localevidence for ice-contact deposits, and the pave-ment was most likely deposited from a groundediceberg.

Siklawa Member (SM)

The SM comprises planar-bedded sheets ofbasaltic sediment. The unit is sharply boundedby subhorizontal surfaces, and truncation ofunderlying beds is locally evident. The mostextensive exposures occur between northern Pol-onez Cove and Mazurek Point, where the memberis typically 4–6 m thick but ranges up to 15 m.Porebski & Gradzinski (1987) provided a compre-hensive description of these strata, here termedSM1. A distinctive facies variant (SM1a) formsprominent well-indurated beds and is distin-guished by the presence of common exotic clastsand macrofossils, which are otherwise sparse torare in SM1. At Magda Nunatak, a planar-beddedunit correlates with the SM that is defined here asSM2. It consists mainly of thin-bedded, brown todark-grey, medium to fine sandstone and is <2 mthick.

Based on the faunal assemblage, whichincludes brachiopods, bivalves and solitary cor-als, the Siklawa Member is a shallow-marine unit.Sedimentary character suggests that depositionoccurred in the offshore transition zone. Sedi-ment was supplied mainly from the reworkedproducts of local basaltic volcanism. Gravel-freemedium to coarse sandstone in SM1, which isgenerally poorly exposed with no sedimentary



structures evident, was most likely depositedfrom traction currents. Laminated fine sedimentswere deposited from suspension and by weaktraction currents. The distinctive SM1a beds weredeposited by gravity flows during storm or vol-canic events (Porebski & Gradzinski, 1987). Thinclaystone drapes are locally preserved at the topof SM1a beds. A number of plant stem casts wereobserved in such a claystone at Mazurek Point(site 4). These are the first plant fossils reportedfrom the PCF and indicate that terrestrial veget-ation survived nearby. The exotic clasts commonwithin facies SM1a are redeposited rather thandirectly ice rafted. However, outsized exoticclasts occur sparsely through the SM, suggestinglimited contemporaneous ice rafting.

Oberek Cliff Member (OCM)

Porebski & Gradzinski (1987, 1990) have des-cribed the OCM in detail. It is redefined here toinclude only basaltic lava, lava breccia andassociated coarse basaltic sediments occurringabove the Siklawa Member. Three OCM lithofa-cies are identified: OC1 is mainly polymicticvolcanic breccia; OC2 is coarse lava breccia andassociated lava flows; and OC3 consists of coarsebasaltic sediment. The OCM is exposed from LowHead to Battke Point, at western Godwin Cliffsand at Magda Nunatak. It is thickest where thickbasaltic breccia units with lava flows (facies OC2)are developed at southern Mazurek Point (£27 m)and Magda Nunatak (c. 25 m). The OCM has asharp, planar basal contact. However, where OC1and OC2 are absent, the contact can be difficult toidentify because of the similarity of OC3 to LowHead Member units.

Unlike the volcanic parts of the Low HeadMember (L1; see above and Smellie et al., 1998),the OCM contains a series of lava deltas compar-able in lithofacies architecture to sedimentaryGilbert-type deltas (Porebski & Gradzinski, 1990).Porebski & Gradzinski (1987, 1990) suggested thatfacies OC1 (also OC1 in their scheme) wasdeposited as laharic cohesive debris flows. These,together with the Siklawa Member, are the shal-low-marine substrate across which the deltaprograded and form the delta bottomsets (Poreb-ski & Gradzinski, 1987, 1990). Massive cross-bedswithin facies OC2 at Mazurek Point represent thevolcanic delta foresets from which the originallava ‘topset’ unit has been largely removed byerosion. Lava in the approximate structural posi-tion of a ‘topset’ is now only preserved locally inthe cliffs above Mazurek Point, where it is

subhorizontal and up to 8 m thick, comprising athin (2–3 m) basal colonnade and a thicker (4–5 m) overlying entablature. The margins of thelava are gradational with surrounding breccia,which is atypically massive at that locality andcontains patches of mosaic breccia and a few lavapillows. The columnar jointing with a suppressedcolonnade is characteristic of lava flooded andcooled by water (Long & Wood, 1986; de Grafet al., 1989), consistent with the presence ofmassive lava breccia enclosing the lava. Lava atthe summit of Magda Nunatak has a sinuous,southerly dipping base. It is about 15 m thick andhas a thin basal colonnade and much thickerentablature (Figs 4C and 8A). The underlying lavabreccia appears massive, but there are faint tracesof apparently north-dipping coarse stratification.A similar interpretation is offered for this lava,but the precise relationship between lava and theunderlying lava breccia, which is essentiallymassive, is less clear. Both lavas may be ‘pooledflows’ that accumulated in local depressionscaused by subsidence at the delta front (cf.Skilling, 1994).

Facies OC3 may represent the more distallateral equivalent of the volcanic delta sequenceseen in northern Oberek Cliff. At some localities(Battke Point, Godwin Cliffs), OC3 occurs asforeset deposits interpreted as prograding frontsof epiclastic marginal marine fan deltas linked toa proximal subaerial basalt lava source (Porebski& Gradzinski, 1987, 1990), similar to unit L2 atMagda Nunatak.

Exotic clasts are rare in the OCM. This reflectsthe dominantly volcanic depositional mode andthe rapid supply rate of basaltic material. Thepresence of rare single pebbles, cobbles andboulders indicates limited contemporaneous icerafting.

Chlamys Ledge Member

The Chlamys Ledge Member (CLM) is a litholog-ically varied unit consisting of brown to blackconglomerate and gravelly sandstone in its lowerpart (CL1) and pale-green to brown sandstone,gravelly sandstone and minor mudstone higherup (CL2). The CLM overlies the Oberek CliffMember along a planar, but poorly exposederosional surface, interpreted as a marine hiatalsurface. It is most thickly developed at westernGodwin Cliffs (20 m) and above Mazurek Point(»18 m). A thin exposure of the CLM occurs onthe upper western surface of Magda Nunatak,truncated by modern erosion.



The CLM was deposited in a shallow-marineenvironment mainly by turbidites, debris flowsand traction currents, but with minor depositionfrom suspension and ice rafting. Sediment wassupplied mainly by reworking of the products oflocal basaltic volcanism. Concentrated occurren-ces of exotic clasts and marine fossils occur insome beds and may be related to periods of lowbasaltic sediment supply. The transition fromCL1 to CL2 represents a decrease in depositionalenergy and probable deepening. Exotic clastabundance clearly decreases upsection, and thisis inferred to represent a decrease in ice rafting.

FACIES ASSOCIATIONS

Figures 9 and 10 illustrate schematically theinterpreted palaeoenvironments for the Krak-owiak Glacier Member and the overlying basalt-sourced units respectively. Two main faciesassociation representing distinct palaeoenviron-ments can be identified within the PCF, asdescribed below.

Glacial grounding line facies association

The Krakowiak Glacier Member is interpreted as aglacial grounding line facies association. Depos-

ition of thin lodgement to deformation tilliteacross a glacially eroded substrate (unit K1),followed by melt-out tillite (?), diamictic gravityflow and minor sandy channels (unit K2) indicatethat the basal Krakowiak Glacier Member wasassociated with glacial advance followed byinitial retreat. The minor incidence of reworkedmarine fossil fragments and deformed lenses ofglaciomarine sediment suggest that ice advancewas across a shallow sea-floor. K3, the thickeruppermost unit, consists of marine outwash andice-rafted sediment deposited in a shallow-waterice-proximal environment. The greater incidenceof stratification and thicker, more common, sortedsandy beds are evidence for discharge from smallsubglacial meltwater streams and/or shallow-marine reworking by traction currents.

Such a facies succession is typical of subaqu-eous moraine complexes formed at glacialgrounding lines (e.g. Benn, 1996). The apparentlysheet-like morphology of K3 may indicate rapidgrounding line retreat (Powell, 1981, 1983). Theabsence of mud-rich facies indicates that K3 wasice proximal, as meltwater plumes are buoyant insea water and carry fine-grained sediment awayfrom the grounding line (Powell, 1990). It mayalso reflect high depositional energy related to anopen coastal shallow-water setting. The incidenceof ice-rafted boulders in some K3 beds suggests

Fig. 9. Schematic illustration of inferred depositional processes and settings for Krakowiak Glacier Member times.Deposition occurred in subglacial and ice-proximal marine environments in association with a glacier grounded on ashallow shelf. A single advance–retreat cycle is recorded.



that the glacier terminated in a tidewater cliffrather than an ice shelf. The absence of freshbasaltic material suggests that local basaltic vol-canism had not yet commenced. The mostlyplanar upper surface of the Krakowiak GlacierMember is a significant unconformity. It isdirectly overlain by sediments containing shal-low-marine macrofossils and is interpreted as ashallow submarine erosional surface formedshortly after glacier retreat.

Volcanic-dominated, high-energyshallow-marine facies association

The major change in sediment type and supplyacross the unconformity signalled an end to theglacigenic sedimentation represented by theKrakowiak Glacier Member (including ice-contactdeposits) and coincided with the onset of basalticvolcanism from several small submarine centres.Domes and lavas from the basalt centres subse-quently formed the main source of detritus to thedeveloping sedimentary basin (Fig. 10).

At least two significantly different modes ofbasalt eruption and sediment supply are indica-ted. In Low Head Member times, eruption centreswere submarine and comprised small domes ofdegassed highly crystallized basalt lava (Smellie

et al., 1998). Lava breccia, formed by mechanicaldeformation of chilled dome surfaces, was shedby avalanching at the dome margins and over-ridden by the advancing dome cores. Aftervolcanism ceased, the relatively high-standingsubaqueous domes formed local secondary sour-ces of coarse basalt detritus, mobilized by slump-ing, debris flows and current and wave reworking(unit L2). In addition, virtually monogenetic,basaltic foreset-bedded deposits were formed inmarginal marine fan deltas probably at shortdistances from source, although there is noevidence for Gilbert-type lava deltas during LowHead Member times. Lenses of imbricated pecti-nids and other shell debris are common only inthe Low Head Member, and this may reflectepisodic deposition in relatively shallow waterdepths.

In contrast, volcanic input to the basin duringOberek Cliff Member times was directly by lava-fed volcanic deltas (primary volcanic facies OC2)and, secondarily, by lava-sourced proximal fandeltas (sedimentary facies OC3). In the former,the upward change from foreset basalt breccia totopset lava (although mostly eroded in the PCF)corresponds to a subaqueous to subaerial trans-ition (the ‘passage zone’ of Jones, 1969), and it isa very good guide to contemporaneous water

Fig. 10. Schematic illustration of inferred depositional processes and environments for basalt-sourced Polonez CoveFormation facies. Deposition occurred in near-shore marine to shallow-marine shelf environments on a volcaniccoast, influenced by occasional ice rafting and reworking of glacially transported material. Features include lavahyaloclastite deltas (e.g. facies OC2 around Mazurek Point–Polonez Cove and at Magda Nunatak) and volcanic-sourced (Gilbert-type) fan deltas (e.g. facies L2–L3, Magda Nunatak; OC3, Battke Point). The submarine lava domeenvironment recorded by facies L1 that apparently predated the local construction of subaerial volcanoes is notshown.



levels (Jones & Nelson, 1970). A shallow-marineenvironment is indicated (probably »20–30 mdepth, based on the thickness of foreset-beddedand columnar lava units at both Magda Nunatakand Mazurek Point). Porebski & Gradzinski(1987) reached a similar conclusion based onthe Mazurek Point locality. Foreset beds show avariety of orientations within both lava deltaand sedimentary fan delta deposits. This indi-cates either that the coastline was indented orthat there were multiple small basaltic islandcentres.

Shallow continental–shelf facies association

The Chlamys Ledge Member overlies the OberekCliff Member along a planar hiatal surface, whichrepresents a period of enhanced shallow-marineerosion that was responsible for removing mostof the Oberek Cliff Member topset lava. TheChlamys Ledge Member consists mainly of re-worked basaltic sediment, with no interbeddedprimary volcanic units, suggesting that volcan-ism was more distal and/or had ceased. Thedominance of deposits from sediment gravityflows, minor occurrence of mudstone, absence ofwave-formed erosional or depositional featuresand upward fining from facies CL1 to CL2suggest that contemporaneous water depths hadincreased relative to Oberek Cliff Member timesand continued to increase to the top of theformation.

PALAEOENVIRONMENTS

Glacier type and palaeoclimate

There are indications within the KGM that coevalglaciers were wet based. The evidence includesthe presence within units K2 and K3 of sporadicsorted sandstone lenses and channels with aprobable meltwater origin. Additionally, glacialmoulding of the Eocene andesitic basement andinjection of unit K1 matrix into basement jointsmay have been associated with the presence ofwater-saturated basal till. However, recession oftemperate tidewater glaciers is usually marked bydeposition of thick mud deposits over proximalmorainal bank facies because of the large volumesof meltwater released (Powell, 1984). Laminatedclay and silt beds characteristic of meltwatersediment plume deposits are absent in the KGM,either because concentrated turbid meltwaterdischarge was not a significant depositional

mechanism or from lack of preservation. Thelatter is a strong possibility, as the unit terminatesin an erosional surface. However, the presence ofin situ marine fossils in the upper KGM suggeststhat water turbidity was not significant and,possibly, that there was not a substantial amountof proglacial meltwater dilution. Alternatively, ifdeposition was occurring on a relatively open,storm-influenced shelf, suspended meltwater-derived fine sediment may have been dispersedrapidly offshore.

The late Cenozoic Yakataga Formation inAlaska has been proposed as a possible analoguefor the PCF (Santos et al., 1990). It comprises awell-exposed, 5-km-thick sequence of glaciallyinfluenced marine sediments deposited in atemperate, tectonically active setting (Eyles et al.,1991). Continental-shelf deposits preserved in theupper Yakataga Formation include ‘rain-out’ dia-mictite units up to 65 m thick and thinner unitsof tabular mudstone, channelized graded sand-stone and conglomerate. However, inner-shelfdeposits including ice-marginal facies have notbeen described, so a direct comparison with thePCF setting is not problematic. Nevertheless, themuch greater scale of the Yakataga Formationdeposits compared with the PCF and the greaterincidence of mud-dominated facies reflect, inpart, greater importance of meltwater processes inthe temperate and high-relief Alaskan setting.Overall, it is suggested that the relatively thinsequence of tillites, debris flow diamictites andminor sandstones forming the Krakowiak GlacierMember is more closely comparable with agrounding line system associated with polyther-mal ice in a subpolar environment. In suchsettings, direct melt-out from the ice front andfrom icebergs dominates sedimentation, andmeltwater processes can be less significant, asituation somewhat similar to some modernglacial settings along the Antarctic Peninsula(e.g. Ashley & Smith, 2000). However, moreinformation on modern deposits from well-defined depositional settings is required beforereconstruction of ice properties from a singlegrounding line deposit, such as the KrakowiakGlacier Member, can be attempted with confid-ence.

Within the basaltic-sourced facies, there is noevidence for significant, local marine-based ice.Some proportion of the exotic clasts present mayhave been reworked by wave and current action,from the underlying glacial deposits, or suppliedto the coast via streams from a glaciated hinter-land or sea-ice rafting. Together with the occur-



rence of rare plant debris, reported here for thefirst time, this suggests that cool temperateconditions prevailed after Krakowiak GlacierMember times, with the local cryosphere reducedto terrestrial glaciers (Fig. 10). However, becauseof the presence of large exotic clasts (e.g. graniteboulder pavement in L3), clast clusters and someapparently far-travelled clasts, continued icerafting is also required. This indicates thatmarine-based ice was present elsewhere, presum-ably further south. In general, variations in exoticclast abundance between the basalt-sourced sedi-mentary units may reflect differences in deposi-tional processes and basaltic sediment supply.However, exotic ice-rafted clasts are increasinglyrare upsection within the shelfal sandstones ofthe Chlamys Ledge Member and are absent inmarine sediments within the overlying Boy PointFormation, suggesting that climatic improvementcontinued into latest Oligocene times.

Although this study concurs with many inter-pretations of depositional mechanisms for indi-vidual sediment types within the KrakowiakGlacier Member (KGM) made by Porebski &Gradzinski (1987), those authors proposed asignificantly different environmental history fromthat envisaged here. Porebski & Gradzinski (1987)adopted the glacial model of Birkenmajer (1982).They linked the basal KGM deposits with majorcontinental ice streams rooted in the Ellsworthand Transantarctic Mountains, which flowedalong the western flank of the Weddell Sea andacross the northern Antarctic Peninsula. Themodel thus attempted to explain far-travelledexotic clasts within the lower KGM by invoking asubglacial source at least 1800 km distant fromKing George Island. The late Oligocene palaeog-eography of the Antarctic Peninsula is not wellknown. If the peninsula formed an importanttopographical barrier similar to today, subglacialtransport of clasts the length of the peninsulawould be improbable.

Alternatively, it is suggested that the smallproportion of far-travelled clasts were suppliedoriginally by iceberg rafting. Although exoticclasts within the PCF are lithologically diverse,the range of lithologies and their proportionsresemble present-day exposures in the northernAntarctic Peninsula region and South ShetlandIslands. It is suggested that, in the late Oligocene,these regions were covered by an ice cap withsome tidewater glaciers and were the majorsource of glacier bedload and ice-rafted debrisduring the ‘Polonez’ glacial event. The sporadicpresence of far-travelled exotic clasts in subgla-

cially deposited facies of the lower KGM can beexplained most simply by reincorporation of ice-rafted material on the sea-floor during glacialadvance. The exotic clasts are thus envisaged asglacially rafted debris subsequently incorporatedas bedload in the local Polonez glacier(s). Evi-dence for this mechanism is provided by thepresence of deformed rafts of glaciomarine sedi-ment in the basal KGM. The pervasive occurrenceof far-travelled exotic clasts in the upper KGMand in overlying PCF basaltic facies indicates thatice rafting from the southern Weddell Sea regioncontinued throughout the depositional period ofthe PCF.

Sea level

A shoreface to offshore marine shelf setting isinferred for the entire PCF. This is reflected in theabsence of appreciable mudstone deposits andthe occurrence of shallow-water fauna in allmembers. Possibly, the hiatal upper surface ofthe Krakowiak Glacier Member may be related toshallowing associated with glacio-isostatic re-bound. An upper unconformity formed by waveerosion after regression resulting from isostasyhas been recognized in comparable groundingline sequences (e.g. Domack, 1983). A series offluctuations between offshore transitional (Bay-view Member, Siklawa Member, upper ChlamysLedge Member) and shoreface conditions (LowHead Member, Oberek Cliff Member) are recordedthrough the basalt-sourced PCF members. Suchfluctuations may have been driven mainly byvariations in sediment supply associated withpulses of local volcanism. The relative contribu-tions of glacio-eustatic, glacio-isostatic and localtectonic effects are unclear. The preservation of apassage zone yields an unambiguous contem-poraneous sea level and estimate of water depthsat the top of the Oberek Cliff Member (»20–30 m).Subsequent gradual relative sea-level rise can beinferred from the formation of a marine-erodedhiatal surface at the base of the Chlamys LedgeMember and from facies changes within thatmember.

IMPLICATIONS FOR WEST ANTARCTICCRYOSPHERE HISTORY

Based on King George Island exposures, Bir-kenmajer et al. (1989) proposed a local glacialstratigraphy consisting of four mid-Cenozoicglacial episodes, in middle Eocene, middle



Oligocene, latest Oligocene and earliest Miocenetimes, a scheme that has been widely referencedin reviews of Antarctic glacial history. However,two of these episodes are not tenable. The Eocene‘Krakow’ event was based on an erroneous K/Arage from lava atop Magda Nunatak, an outlier ofthe PCF (Dingle & Lavelle, 1998; this paper). Thelatest Oligocene ‘Legru’ glaciation related to atillite exposed near Vaureal Peak (Fig. 1B), com-posed of local clasts overlying an incised volcanicbasement. The proposed age of the tillite is basedon the flawed assumption that it is age equivalentwith basement. Therefore, only two glacial epi-sodes can be inferred from King George Islandexposures, these being the middle to late Oligo-cene ‘Polonez’ event and the early Miocene‘Melville’ event (Dingle & Lavelle, 1998; Troedson& Riding, 2001; this paper).

The PCF is the earliest known exposure indi-cating significant cryosphere development andthe only proven Oligocene glacigenic deposit inthe region. It provides evidence for a singleepisode of grounded ice extension into theshallow-marine environment during the middleto late Oligocene (c. 26–30 Ma). Evidence of localglacial expansion onto the outer continental shelfin earliest Miocene times is provided by the CapeMelville Formation, a mainly glaciomarine sequ-ence exposed at Cape Melville, south-easternKing George Island (Fig. 1B) (Birkenmajer, 1984;Troedson & Riding, 2001). Like the PCF, the CapeMelville Formation contains ice-rafted debrisfrom as far afield as the Transantarctic Mountainsand diverse clasts with a likely northern Penin-sula origin, similarly reflecting extensive contem-poraneous marine-based glaciation around themargins of the Weddell Sea. The clasts alsoprovide evidence for a strong Weddell Sea gyre,and this cold current may be implicated in frigidconditions in the King George Island region. LateOligocene shallow-marine sedimentary forma-tions containing no evidence of glacigenic influ-ence directly overlie the PCF and underlie theCape Melville Formation, indicating an inter-vening latest Oligocene interglacial period (»26–24Æ5 Ma) (Birkenmajer et al., 1989; Dingle &Lavelle, 1998).

Few other Oligocene exposures have beenidentified regionally. Pyroclastic depositsexposed at Mount Petras in Marie Byrd Landprovide the only other terrestrial evidence forOligocene ice known from West Antarctica. Thesedeposits show characteristics of volcanic erup-tion beneath ice and have been 40Ar/39Ar dated atc. 29–27 Ma (Wilch & McIntosh, 2000), suggesting

deposition in a similar period to the PCF. LeMasurier & Rex (1983) suggested that the depositswere indicative of a thick ice sheet. However,Wilch & McIntosh (2000) suggested that theyprovide evidence of local alpine glaciation only,rather than significant Oligocene ice.

Little is known of the potentially more exten-sive offshore Oligocene record in the AntarcticPeninsula region. No drilling has been underta-ken on the Weddell Sea margin of the peninsula,and recent ODP drilling of the Pacific margin ofthe northern Antarctic Peninsula did not extendbeyond late Miocene strata (Shipboard ScientificParty, 1999). There have been a number ofstudies, based on seismic records, of late Ceno-zoic marine to glaciomarine sediment packagesand unconformities on continental shelves abut-ting the northernmost Peninsula and South Shet-land Islands. Conflicting views have emergedfrom these studies as to the age of initial wide-spread marine-based glaciation in the region. Ithas been argued that substantive ice did not erodethe northern Antarctic Peninsula margin untillatest Miocene to Pliocene times (Larter & Barker,1989, 1991; Larter et al., 1997). Bart & Anderson(1995) proposed instead that ice expanded ontothe continental shelf from the middle Miocene,based on an alternative age interpretation forsignificant unconformities. The PCF cannot settlethis controversy. As outlined here, the formationprovides local evidence only for grounded ice in ashallow-water, most likely nearshore, environ-ment during middle to late Oligocene times.Grounded ice may have been more widespreadfurther south in the Weddell Sea region of WestAntarctica, but the extent of such ice remainsunclear.

Composite deep-sea benthic foraminiferal d18Otrends indicate intermittent episodes of Antarcticice expansion superimposed on a long-termwarming trend during the mid-Oligocene tomid-Miocene (Miller et al., 1991). Distinct mid-to late Oligocene (Oi-2) and earliest Miocene(Mi-1) glacial events have been recognized. Oi-2extended from »29Æ7 to 27 Ma (Miller et al.,1998), while the Mi-1 peak occurred close tothe Oligocene–Miocene boundary, now dated at22Æ9 Ma (Shackleton et al., 2000). The Polonez(between 30 and 26 Ma) and Melville (c. 22–23 Ma) glacial episodes may be correlated withthese events. As such, the two King GeorgeIsland glacigenic formations may record extremepalaeoclimatic events that were neither long-lived nor typical of mid-Cenozoic conditions inthe region.



CONCLUSIONS

The Polonez Cove Formation contains a uniquerecord of mid-Cenozoic glacial activity in thenorthern Antarctic Peninsula region. Dating byboth 40Ar/39Ar and Sr-isotope methods confirmsthat the formation is of middle to late Oligoceneage but has only constrained deposition tobetween » 30 and 26 Ma. The genesis of theformation may be considered in two parts. Thebasal diamictite-dominated unit (Krakowiak Gla-cier Member) was formed during a single majorepisode of glacial grounding line advance andretreat on a shallow inner continental shelf.Overlying basalt-sourced members of the PolonezCove Formation were deposited in a nearshoremarine to offshore-transitional shelf environmenton a volcanic coast influenced by ice rafting.Glacially transported clasts are lithologicallydiverse and include both locally and distallysourced lithologies. It is inferred that ice coverwas widespread regionally and included signifi-cant marine-based ice for a period in the middleto late Oligocene.

ACKNOWLEDGEMENTS

The authors gratefully thank their field assistants,Andy Teesdale and Carl Haberl, for their unstint-ing help during the several months of arduousfieldwork required during this investigation. Weare also very grateful to the Captains, officers andcrews of HMS Endurance and RRS Bransfield fortheir invaluable logistical support, without whichthe fieldwork could not have taken place. Wethank S. Porebski and J. B. Anderson for usefulcomments on the original manuscript.

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Manuscript received 4 September 2000;revision accepted 19 September 2001.



The Polonez Cove Formation of King George Island, Antarctica: stratigraphy, facies and implications...

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