Magda Nunatak Complex (abandoned stratigraphical unit)

Magda Nunatak Complex is the name used for an association of lava, lava breccia ('hyaloclastite') and fossiliferous marine sedimentary strata that crop out only at Magda Nunatak in eastern King George Island (Fig. 1). Despite an early prescient study, which attributed the sedimentary rocks to the Polonez Cove Formation, but without citing supporting evidence (Tokarski et al. Reference Tokarski, Paolo and Rubinowski1981), the capping lava was correlated with the supposedly long-lived (Late Cretaceous-Oligocene) Mazurek Point Formation (Birkenmajer Reference Birkenmajer1982). Subsequent K-Ar dating of the capping lava yielded an Eocene age (49.5 ± 5 Ma), thus apparently confirming the correlation, and the age was used to suggest the existence of an Eocene glacial episode called the Kraków Glaciation (Birkenmajer et al. Reference Birkenmajer, Delitala, Narębski, Nicoletti and Petrucciani1986). However, ⁸⁷Sr/⁸⁶Sr dating of shelly material in the Magda Nunatak sediments yielded an age of 28.5 ± 0.64 Ma (Dingle & Lavelle Reference Dingle and Lavelle1998). Although ⁸⁷Sr/⁸⁶Sr ages are determined with reference to the Cenozoic seawater ⁸⁷Sr/⁸⁶Sr curve and are therefore subject to variable errors depending on the slope of that curve (for methodology, see McArthur et al. Reference McArthur, Howarth and Bailey2001), the ⁸⁷Sr/⁸⁶Sr ages obtained at Magda Nunatak make an Eocene age for the sequence unlikely. Moreover, Troedson & Smellie (Reference Troedson and Smellie2002) pointed to the strong lithological similarity with the Polonez Cove Formation and provided a preliminary ⁴⁰Ar/³⁹Ar age (lacking supporting data) slightly younger than the Sr age. Here we re-emphasize the remarkable similarity in lithofacies between the two outcrops, which is essentially on a bed-by-bed basis (Fig. 2; cf. Troedson & Smellie Reference Troedson and Smellie2002). Together with a new ⁴⁰Ar/³⁹Ar age reported here (see later), all doubt is removed that the Eocene age for the Magda Nunatak Complex is incorrect and it is indistinguishable from the Polonez Cove Formation, with which it is unequivocally correlated. Thus, there is no justification for retaining the Magda Nunatak Complex name for the outcrop and it is suggested that it should be removed from the stratigraphical lexicon.

Fig. 2.

View of Magda Nunatak, looking west, with the geology indicated. There is a remarkable and conspicuous similarity with the internal stratigraphy of the Polonez Cove Formation, exposed just 3 km to the south-east, and with which the outcrop is now correlated. An outcrop of the Bayview Member is also present within the sequence, out of sight at right, and a tiny outlier of the Chlamys Ledge Member occurs at the summit (also out of sight; Troedson & Smellie Reference Troedson and Smellie2002). The locations of the two dated samples shown are approximate. The rock face is c. 60 m high (estimated).

Hervé Cove tillite (name not recommended)

Additional evidence for Eocene ice conditions on King George Island was cited by Birkenmajer et al. (Reference Birkenmajer, Gaździcki and Krajewski2005), who suggested that a volcanic diamictite with supposedly exotic erratics on the south coast of Ezcurra Inlet (Fig. 1) was also a glacial deposit (tillite). With an inferred age of c. 45–41 Ma (by K-Ar), overlapping within the error of the K-Ar age for the Magda Nunatak Complex, the deposit was called the Hervé Cove tillite, and it was believed to be associated with volcanoes with ice-capped summits. The age of the deposit was subsequently shown to be c. 49 Ma (by ⁴⁰Ar/³⁹Ar and U-Pb dating; Nawrocki et al. Reference Nawrocki, Pańczyk and Williams2011). However, alpine glaciers are unlikely to be a source of exotic (i.e. non-local) erratics. The erratic lithologies were not described, but volcanic diamictites similar in lithofacies to the ‘Hervé Cove tillite’ are common in Eocene sequences elsewhere on King George Island and lack exotic erratics (Smellie et al. Reference Smellie, Hunt, McIntosh and Esser2021). The evidence for glacial deposition and alpine glaciers on King George Island during the Eocene is thus currently unproven (Mozer Reference Mozer2013, Smellie et al. Reference Smellie, Hunt, McIntosh and Esser2021; see also Nawrocki et al. Reference Nawrocki, Pańczyk and Williams2011), and it is recommended that the use of the name ‘Hervé Cove tillite’ should be discontinued unless a glacial origin can be much better demonstrated.

Polonez Cove Formation

Because of its association with an important glacial episode (the Polonez Glaciation), the Polonez Cove Formation is one of the most intensively investigated geological units in the South Shetland Islands (e.g. Barton Reference Barton1965, Birkenmajer Reference Birkenmajer1980, Reference Birkenmajer1982, Reference Birkenmajer1987, Reference Birkenmajer1995a, Paolo & Tokarski 1982, Gazdzicki & Pugaczewska Reference Gaździcki and Pugaczewska1984, Birkenmajer & Wieser Reference Birkenmajer and Weiser1985, Porebski & Gradzinski Reference Porębski and Gradziński1987, Reference Porębski and Gradziński1990, dos Santos et al. Reference dos Santos, Rocha-Campos, Trompette, Uhlein, Gipp and Simões1990, Smellie & Rex Reference Smellie and Rex1993, Smellie et al. Reference Smellie, Millar, Rex and Butterworth1998, Troedson & Smellie Reference Troedson and Smellie2002, Quaglio et al. Reference Quaglio, Anelli, dos Santos, de Perinotto and Rocha-Campos2008, Reference Quaglio, Warren, Anelli, dos Santos, Rocha-Campos and Gaździcki2014, Nawrocki et al. Reference Nawrocki, Pańczyk, Wójcik and Tatur2021b). The formation is continuously exposed in a 2.0 km-long cliff section between Low Head and Lions Rump, but it also includes substantial outcrops at Godwin Cliffs and Magda Nunatak and small outliers at Vauréal Peak and Three Sisters Point (Figs 1–4). The maximum thickness of the formation varies from 58 m at Godwin Cliffs to 59 m at Mazurek Point and c. 63 m (estimated) at Magda Nunatak (Troedson & Smellie Reference Troedson and Smellie2002). It rests unconformably on an uneven surface eroded into andesite lavas of the Middle Eocene Hennequin Formation (also called the Mazurek Point Formation; Pańczyk & Nawrocki Reference Pańczyk and Nawrocki2011, Smellie et al. Reference Smellie, Hunt, McIntosh and Esser2021). The basal surface has a general relief of c. 15–20 m, with local hollows or channels up to 10 m deep (e.g. at Magda Nunatak and Mazurek Point). The lava is sometimes sheared and brecciated in the vicinity of the contact, with diamict injected into fractures between clasts (Birkenmajer Reference Birkenmajer1995a, Troedson & Smellie Reference Troedson and Smellie2002, and unpublished observations by the authors). Glacial striations and roche moutonée landforms have also been described at Three Sisters Point (Paulo & Tokarski Reference Paulo and Tokarski1982, Birkenmajer Reference Birkenmajer1995a). The boundary with the overlying Polonez Cove Formation was regarded as an angular unconformity by Birkenmajer (Reference Birkenmajer1995a). However, it is probably better regarded as a disconformity. The contact is planar and sub-horizontal. Despite the abundance of fossils in the Polonez Cove Formation, particularly in its type area, many have been recycled from older strata (see Supplemental Information S3 & S4). Even contemporary fossils have relatively long age ranges, and it has only proven possible to determine an imprecise Oligocene palaeontological age (Gazdzicki & Pugaczewska Reference Gaździcki and Pugaczewska1984, Gaździcka & Gaździcki Reference Gaździcka and Gaździcki1985, Birkenmajer & Gaździcki Reference Birkenmajer and Gaździcki1986, and see later discussion).

Fig. 3.

Geological map of the Low Head-Lions Rump-Godwin Cliffs area, eastern King George Island. The ‘upper Oberek Cliff Member’ depicted at Battke Point and Godwin Cliffs is a fossiliferous sandy-gravelly deposit (broadly comparable to subunit ‘OC3’ of Troedson & Smellie Reference Troedson and Smellie2002) with a remarkable resemblance to similarly fossiliferous sedimentary rocks of the Low Head Member (especially subunit ‘L2’ of Troedson & Smellie Reference Troedson and Smellie2002).

Fig. 4.

Geological map of the Vauréal Peak area, eastern King George Island. See Fig. 3 for explanation of additional symbols.

The most complete sections of the Polonez Cove Formation occur between Low Head and Lions Rump and at Magda Nunatak (Figs 1, 2 & 5). The formation has been divided into six members, with further division of the members into multiple subunits on a sedimentological basis (Figs 2 & 6; Troedson & Smellie Reference Troedson and Smellie2002; cf. Birkenmajer Reference Birkenmajer1982, Porebski & Gradzinski Reference Porębski and Gradziński1987). The earliest-formed deposit is the diamictite-dominated Krakowiak Glacier Member (Fig. 7a). It contains basal lenses of lodgement tillite but is largely formed of debris flow deposits and minor sandstones laid down in an ice-front environment and from icebergs. It is spectacularly polymict, with abundant erratics derived from the South Shetland Islands, Antarctic Peninsula and East Antarctica (Birkenmajer & Wieser Reference Birkenmajer and Weiser1985, Troedson & Smellie Reference Troedson and Smellie2002). Multiple sources probably situated in the southern Antarctic Peninsula, Haag Nunataks, Ellsworth Mountains, Transantarctic Mountains and, speculatively, Gamburtsev Mountains were demonstrated by Nawrocki et al. (Reference Nawrocki, Pańczyk, Wójcik and Tatur2021b) on the basis of U-Pb isotopic ages of zircons obtained from a small selection of the erratics. The distal provenance of many erratics led Birkenmajer & Weiser (Reference Birkenmajer and Weiser1985) to suggest an association with a grounded pan-Antarctic regional ice sheet, but Troedson & Smellie (Reference Troedson and Smellie2002) suggested that the distal erratics were probably reworked (i.e. dropped by far-travelled icebergs, at least some of which were sourced in East Antarctica, and subsequently reincorporated in marine-based ice sourced locally in the South Shetland Islands).

Fig. 5.

View looking south from Lions Rump to Battke Point and Krakowiak Crag, showing the sequence of formations. The view illustrates clearly how the Wesele Cove Member locally intervenes between the Polonez Cove and Boy Point formations, contradicting the previously published stratigraphy (i.e. Birkenmajer Reference Birkenmajer1982, Reference Birkenmajer2001). The amount of relief shown is c. 200 m (estimated). The dramatic reduction in the extent of Krakowiak Glacier is evident when this image (taken in January 1996) is compared with that taken in 1976 by Smellie et al. (Reference Smellie, Pankhurst, Thomson and Davies1984, fig. 40).

Fig. 6.

View of the Polonez Cove Formation showing its constituent members, looking south from Mazurek Point. Approximately 45 m of section is shown in the foreground.

Fig. 7.

Compendium of photographs showing representative lithofacies in the Oligocene volcanic and sedimentary outcrops on eastern King George Island. a. Massive diamict; Krakowiak Glacier Member (Polonez Cove Formation) at Godwin Cliffs. The eraser is 5 cm long. b. Thinly stratified strata of the Bayview Member sandwiched between the Krakowiak Glacier and Low Head members (Polonez Cove Formation); beach crag c. 350 m south of Chopin Dyke; the field notebook is 16 cm long. c. Unusually well-defined graded beds of monomict fine lava breccia within a lava-fed delta at Godwin Cliffs; Oberek Cliff Member (Polonez Cove Formation); the mapping case is 30 cm in width. d. Crude massive beds of breccio conglomerate with dispersed white-coloured chlamys fragments, part of a lava-sourced fan delta at Battke Point; Low Head Member (Polonez Cove Formation); the hammer is 35 cm in length. e. Pervasively fractured and brecciated lava of the Boy Point Formation at Linton Knoll; the field notebook is 16 cm long. f. Well-stratified, pale-coloured sedimentary beds of sequence type 1 at Stamp Buttress (Cinder Spur Formation); the hammer is c. 70 cm in length. g. Crudely bedded gravelly breccio conglomerates of sequence type 2 at Martins Head (Cinder Spur Formation); the hammer is c. 70 cm long. h. Massive diamict of the Vauréal Peak Formation unconformably draping sedimentary rocks of the Cinder Spur Formation (sequence type 1) at the summit of Cape Syrezol; note how the diamict adheres to an almost vertical face cut in the older rocks; the large boulder in the foreground at right is c. 30 cm in length.

All of the members overlying the Krakowiak Glacier Member have a predominantly local basalt source, and some contain subunits that are entirely volcanic (Fig. 7b–d). The members alternate between sparsely fossiliferous, planar-bedded, generally fine-grained sediments (mainly sandstones) formed in relatively deep water (Bayview, Siklawa and Chlamys Ledge members); locally fossiliferous coarse epiclastic fan deltas sourced in recently active volcanism (subunits of the Low Head and Oberek Cliff members); and gravelly conglomeratic sediments with prominent channels and cross-stratification, typically highly fossiliferous and characteristic of deposition under relatively shallow-water high-energy conditions (other (upper) subunits of the Low Head and Oberek Cliff members). The entirely volcanic subunits consist of lava-fed deltas (in the Low Head and Oberek Cliff members; Porebski & Gradzinski Reference Porębski and Gradziński1990) and a co-genetic association of breccia with a central plug-like mass that were collectively formed during submarine extrusion of a highly crystalline lava dome (Smellie et al. Reference Smellie, Millar, Rex and Butterworth1998).

The extent of the Bayview and Siklawa members mapped during our study mirrors that shown by Troedson & Smellie (Reference Troedson and Smellie2002) but differs slightly in detail, as follows: 1) it is uncertain that the Siklawa Member continues to the south of the Chopin Dyke, its structural position there being represented by similar-looking thin-bedded, monomict bottom-set beds of an Oberek Cliff Member fan delta; and 2) the Bayview Member, best developed at Magda Nunatak (Troedson & Smellie Reference Troedson and Smellie2002), is everywhere else represented by small, erosionally isolated lenses (locations marked on Fig. 3), one of which (south of Chopin Dyke) was previously interpreted as part of the Low Head Member (Fig. 7b; Porebski & Gradzinski Reference Porębski and Gradziński1987, plate 7(2); cf. Quaglio et al. Reference Quaglio, Warren, Anelli, dos Santos, Rocha-Campos and Gaździcki2014).

The outcrop at Three Sisters Point is a small relict, measuring just 100 m long by c. 70 m wide (Birkenmajer Reference Birkenmajer1995a). It consists of diamictite and fossiliferous marine conglomerate correlated with the Krakowiak Glacier and Low Head members, respectively. The presence of striations and roche moutonée landforms cut in the basement lava noted by Paulo & Tokarski (Reference Paulo and Tokarski1982; see also Birkenmajer Reference Birkenmajer1995a) have not been observed in other outcrops of the formation. Moreover, similar features also occur on top of the overlying Polonez Cove Formation at Three Sisters Point, and their orientations are consistent with modern topography. Their origin is therefore ambiguous and they thus might all have been caused by recent glacial action (observations of AT). Two even smaller outcrops were mapped at Vauréal Peak by Birkenmajer (Reference Birkenmajer1982). Polonez Cove Formation diamictite (Krakowiak Glacier Member?), which was shown close to the summit of Vauréal Peak at c. 180 m above sea level, was not observed during our investigation. However, a small outlier is present midway between Cape Vauréal and Vauréal Peak (Fig. 4). Two lithofacies are present and, as at Three Sisters Point, have been correlated with the Krakowiak Glacier and Low Head members (Birkenmajer Reference Birkenmajer1982, Birkenmajer et al. Reference Birkenmajer, Soliani and Kawashita1989, Quaglio et al. Reference Quaglio, Anelli, dos Santos, de Perinotto and Rocha-Campos2008). At both localities, the Krakowiak Glacier Member comprises mainly massive diamictite with abundant erratic clasts. The Low Head Member at Three Sisters Point has not been described (cf. Birkenmajer Reference Birkenmajer1995a, fig. 4), but at Vauréal Peak it is a crudely stratified sequence c. 10 m thick, dominated by massive pebbly diamictite alternating with sandy fine conglomerate and minor sandstone. It is sparsely fossiliferous (Quaglio et al. Reference Quaglio, Anelli, dos Santos, de Perinotto and Rocha-Campos2008).

Boy Point Formation (redefined stratigraphical unit)

Previous studies divided the strata above the Polonez Cove Formation into two formations: a lower Boy Point Formation, composed of felsic-intermediate lavas, and an upper Wesele Cove Formation, formed of agglomerates, conglomerates and sandstones (Birkenmajer Reference Birkenmajer1982, Reference Birkenmajer2001, fig. 43). In fact, the published stratigraphy is inverted and the Wesele Cove Formation, as defined by Birkenmajer (Reference Birkenmajer1982), mainly overlies the Polonez Cove Formation directly across a sub-horizontal planar erosional unconformity, although in Oberek Cliff it is sandwiched between lavas of the Boy Point Formation (Figs 3, 5 & 8). In its form, attitude and extent, the basal unconformity resembles other planar erosional surfaces observed within the Polonez Cove Formation, and they are present within the overlying Boy Point Formation too. Thus, the Wesele Cove Formation is essentially a sedimentary sequence contained within a thicker lava sequence. Its provenance is also andesitic-dacitic, similar to the associated lavas. Therefore, it is suggested that the Wesele Cove Formation is more appropriately reclassified as a member of the lava-dominated Boy Point Formation and it is here renamed the Wesele Cove Member (Table II).

Fig. 8.

Sketched view of Oberek Cliff (c. 200 m high) showing the stratigraphical units present, including the large Chopin Dyke. Note how lava L2 of the Boy Point Formation appears to plough into sediments of the Wesele Cove Member at right. Note also that S6 probably underlies L6 (view affected by foreshortening). Because exposures are discontinuous, it cannot be proven that sedimentary sequences S4 (not in view), S5 and S6 are different units. They occupy a similar stratigraphical position. Colours used are explained in Fig. 3. CLM = Chlamys Ledge Member; KGM = Krakowiak Glacier Member; LHM = Low Head Member; OCM = Oberek Cliff Member; SM = Siklawa Member.

Table II.

Stratigraphy of eastern King George Island (this paper).

Note that the ‘Mazurek Point Formation’ (now Hennequin Formation) is omitted from the Oligocene stratigraphy; its age has been shown to be Eocene and unrelated geologically to the overlying formations of the Chopin Ridge Group (Pańczyk & Nawrocki Reference Pańczyk and Nawrocki2011, Smellie et al. Reference Smellie, Hunt, McIntosh and Esser2021).

^a Excludes the Sherratt Bay Formation (probable dolerite sill, part of the Cape Syrezol Group?).

^b Probably a distal marine equivalent of the Cinder Spur Formation.

^c New stratigraphical unit.

^d Redefined stratigraphical unit.

^e The Polonez Glaciation strictly only applies to the basal unit (Krakowiak Glacier Member), but conventional usage applies it to the entire formation.

The Boy Point Formation is dominated by andesite-dacite lavas (Birkenmajer Reference Birkenmajer1982). Although an outcrop at Stamp Buttress (close to Boy Point; Fig. 9) was also assigned to the formation, it is compositionally and physically dissimilar (massive basaltic andesite lava and autoclastic breccia indistinguishable from lavas at Boy Point nearby), and the original study by Birkenmajer (Reference Birkenmajer1982), which correlated unconformably overlying strata at Stamp Buttress with the Wesele Cove Formation, is incorrect. The supposed Wesele Cove Formation outcrop at Stamp Buttress is an integral part of the lava-dominated sequence that forms the Cinder Spur outcrop, and it is here assigned to the new Cinder Spur Formation (see below; Figs 9 & 10). The exposed upper surface of the Boy Point Formation everywhere corresponds to the present-day erosion surface, but a steep cross-cutting unconformity can be inferred from the distribution of outcrops on the west side of Wesele Cove, where the Boy Point Formation and most of the Polonez Cove Formation are cut out (Fig. 9). The total thickness of the Boy Point Formation is difficult to measure, but it is probably c. 100–150 m (Birkenmajer Reference Birkenmajer1982, estimated up to 120 m). Despite the formation being absent at Boy Point, the name is embedded in the literature and is retained here. However, the type section is at Oberek Cliff (Figs 3 & 8).

Fig. 9.

Geological map of the Cinder Spur area, eastern King George Island. See Fig. 3 for explanation of additional symbols.

Fig. 10.

View of Stamp Buttress, looking up Dunikowski Ridge, showing selected geological units (cf. Fig. 9). All of the rocks shown belong to the Cinder Spur Formation. Note how S(1), whilst occupying a channel cut in the lower lava sequence at right, is a conformable part of the sequence and not an outlier of the Wesele Cove Formation, as interpreted by Birkenmajer (Reference Birkenmajer1982) and which led to the lavas of the lower sequence being mapped incorrectly as part of the Boy Point Formation. Approximately 300 m of relief is shown. Red lines are prominent unconformities. L = lava; S(1), S(2) = sedimentary rocks of type 1 and type 2 sequences, respectively; see text for further explanation.

Four lavas (L1–L4; Fig. 3) are present within the Boy Point Formation at Oberek Cliff. They are distinctive highly porphyritic andesites and dacites in which largely opaque oxide-altered hornblende phenocrysts are common and conspicuous (Table III). Elsewhere in the South Shetland Islands, amphibole is a rare phenocryst phase in lavas (Smellie et al. Reference Smellie, Pankhurst, Thomson and Davies1984). The lavas are brightly coloured (mainly pale green, pink, pale grey and khaki-brown), non-vesicular and characteristically pervasively fractured to brecciated (Fig. 7e). In Oberek Cliffs, the lowest stratum included in the formation comprises c. 25 m of purplish grey to pale green lava (L1; Fig. 8), which varies irregularly between coarse lava breccia and highly fractured but otherwise intact lava. Lava L2, pale pinkish grey in colour and composed predominantly of lava breccia, is only 8 m thick south of Chopin Dyke but thickens northwards to c. > 40 m, across the fault occupied by the Chopin Dyke, before wedging out within the Wesele Cove Formation c. 400 m south of Battke Point. Lavas L3 and L4 are similar to L2 but with different colours.

Table III.

Major oxide analyses of Oligocene lavas and intrusions, eastern King George Island.

LOI = loss on ignition.

Within the lavas above the Wesele Cove Member, granule conglomerate and sandstone (S4) occupy a prominent hollow c. 8 m deep overlying L2 in Krakowiak Crag. S4 is pale green-grey and mainly conglomeratic. It contains large chlamys fossil moulds, which are locally abundant. A sequence of polymict, green granule and pebble conglomerates (S5), c. 4 m thick, also overlies L2 on the south flank of Chopin Dyke. It is apparently unfossiliferous and comprises lensoid beds and pebble trails a few centimetres to 1.5 m thick, which are crudely interstratified with rare thin sandstone lenses, with some channelized trough cross-stratification and (?)tabular cross-bedded sets. The stratigraphical position of S5 is uncertain. It overlies L2 similar to S4, but it is also overlain by L4 (L3 is absent south of Chopin Dyke), similar to S6 north of Chopin Dyke. Lithologically, S5 is more like S4. The characteristics of S6 are very uncertain as it is preserved solely as a felsenmeer of fissile platy debris composed of feldspathic sandstone and rare pebble conglomerate. It crops out on the undulating plateau above Krakowiak Crag.

A hitherto-unrecognized likely internal erosion surface truncates L2, S4 and S1 (S1 described below). It is planar and sub-horizontal and best seen in the east-facing Oberek Cliffs north of Chopin Dyke (Fig. 8). The surface is overlain by lavas L3 and L4. Lava L3, pale green to brown in colour, is intruded by and intimately intermixed with massive, mainly pale grey polymict diamictite, which appears to have originally covered parts of the underlying erosion surface. The youngest unit in the formation is pale grey to pink lava (L4). Its basal surface is uneven on a large scale, draping L3 and probably S6.

Wesele Cove Member: The lavas of the Boy Point Formation are interbedded with volcanic-sourced sediments. The most prominent deposit is identified here as the Wesele Cove Member, which forms a sequence > 80 m thick (see also Birkenmajer Reference Birkenmajer1982). It overlies lava L1 in Oberek Cliffs (Fig. 8), and it is dominated by distinctive pale grey or pale khaki-coloured feldspathic sandstones. The sandstones have a gently north-east-dipping homoclinal stratification that oversteps the basal unconformity with the Polonez Cove Formation. The Wesele Cove Member has been divided informally into three subunits (S1–S3) separated by planar, slightly erosive surfaces (Fig. 8). The separating surfaces are only obvious, as shallow notches, in Oberek Cliff on both sides of Chopin Dyke. Although some of the subunits presumably extend further north into Krakowiak Crag, they have not been mapped owing to the poor exposures and lithological similarities between the sediment packages. In addition, correlation of specific subunits across Chopin Dyke (which occupies a fault) is uncertain. The upper contact is a gently north-dipping, planar to locally very uneven surface overlain by lava L2, which cuts out the highest sedimentary package. The surface may be erosional or lava L2 may have sunk in unconsolidated sandstones of the Wesele Cove Member (Fig. 8). The member is described from the base up in Chopin Ridge.

The basal strata of the Wesele Cove Member, locally overlying L1 in southern Oberek Cliff, comprise up to 10 m of green (?)monomict gravelly conglomerate and laminated coarse sandstone, some with isolated outsize lava blocks, and there are lenses of blocky volcanic breccia. These beds are overstepped northwards by c. 25 m of very thick (up to 10 m) beds of massive, monomict lava breccia, alternating with thinner (few metres) units of pale green, thinly laminated sandstone. This sequence (S1) fines upward, from basal breccias, through finer breccias, up into sandstones. South of Chopin Dyke, S2 and S3 consist of a total of c. 28 m of white feldspathic sandstone and minor gravelly sandstone. Beds are typically hard to distinguish. The sequence also contains pebbly conglomerate and coarse breccio-conglomerate in beds up to 8 m thick, which become thinner northwards.

Other outcrops of the Boy Point Formation: Lavas of the Boy Point Formation also crop out at Linton Knoll (Fig. 3). At least five different lavas are present (Li–Lv), ranging from 2 to 20 m thick with thinner lavas towards the top; they are thinner than lavas in the outcrop above Krakowiak Crag but otherwise indistinguishable in appearance. The lavas are interbedded with a poorly seen, thin (≤ 6 m) deposit of pebble and cobble conglomerate and matrix-rich volcanic breccio-conglomerate (Sii). A thin wedge of pale grey feldspathic coarse sandstone and granule to pebble conglomerate also crops out in situ and as morainic debris at Godwin Cliff, directly beneath the lava sequence just described (Fig. 3). It may either be an outlying outcrop of the Wesele Cove Member or it is a sediment wedge above that member within the lava sequence, similar to sediment subunits S4 and S5 (described above).

Although the outcrop of the Boy Point Formation is apparently confined to the Lions Rump-Low Head-Linton Knoll area, rare clasts of lithologically identical hornblende-phyric andesite occur in sedimentary rocks at Martins Head and Harnasie Hill, in outcrops assigned to the Cinder Spur Formation (see below). The presence of the clasts may suggest that either the Boy Point Formation crops out more widely and its lavas were reworked into the younger formation or else hornblende andesite lavas may also have erupted within the Cinder Spur Formation but are not currently exposed.

Cinder Spur Formation (new stratigraphical unit)

The Cinder Spur Formation is the most extensive Oligocene stratigraphical unit on eastern King George Island. The type locality is at Cinder Spur, where the range of lithofacies and their relationships are best exposed (Fig. 9). Other outcrops occur on the west side of Legru Bay, at Retreat Bluffs, Stanczyk Hill, Martins Head and Harnasie Hill (Fig. 11). The only published maps of Cinder Spur are on a very small scale (Barton Reference Barton1965, Birkenmajer Reference Birkenmajer1982). Birkenmajer (Reference Birkenmajer1982) divided the sequence into five parts: a basal basalt lava sequence (Boy Point Formation, at its type locality; reassigned here to the Cinder Spur Formation), unconformably overlain by agglomerates, conglomerates and sandstones of the Wesele Cove Formation, itself unconformably overlain by lavas of the Dunikowski Ridge Formation. To the west, across a fault, the Dunikowski Ridge Formation (lava, conglomerate, agglomerate) was overlain by the Harnassie Hill Formation (agglomerate and tuff of a basaltic cinder cone), itself overlain by the Martins Head Formation (lava, agglomerate). Unfortunately, many of the published details of the geology of Cinder Spur are incorrect (e.g. see Figs 9 & 10).

Fig. 11.

View of outcrops of the Cinder Spur Formation on the west side of Legru Bay. The varied dip orientations seen in the outcrops suggest that they were formed in more than one volcanic edifice; that responsible for Martins Head, in which strata dip to the right (approximately to the north-west), must have been situated in Bransfield Strait but has been considerably eroded.

The Cinder Spur Formation is composed of alternating lavas and sedimentary rocks (Fig. 9). Its base is unexposed, but at Harnasie Hill it overlies Eocene strata of the Cape Vauréal Formation, although the nature of the contact is uncertain (Fig. 12; Smellie et al. Reference Smellie, Hunt, McIntosh and Esser2021). The top is mainly the present-day erosion surface except where it is erosively overlain by the Vauréal Peak Formation, of uncertain age (see later). The relationship with the Boy Point Formation is unknown as no contacts are exposed, but our interpretation of the outcrops on the west side of Wesele Cove suggest a strong erosional unconformity is present that is probably responsible for restricting the western outcrop extent of the Boy Point Formation. Because of faulting and large ice-covered gaps in exposure, the total thickness of the Cinder Spur Formation is unknown, but it must exceed 500 m (see also Birkenmajer Reference Birkenmajer1982).

Fig. 12.

Geological map of the Harnasie Hill area. See Fig. 3 for explanation of additional symbols.

At the type locality at Cinder Spur, the succession is formed of at least three unconformity-bounded sequences, as follows: 1) a lower sequence formed of two lavas, which Birkenmajer (Reference Birkenmajer1982) assigned to his Boy Point Formation (reassigned here to the Cinder Spur Formation), 2) a middle sequence at least 200–250 m thick, composed of alternating lavas and lithologically variable volcaniclastic sedimentary rocks and 3) an upper, lava-dominated sequence. The sequences dip at low to moderate angles (10–38°, typically 20–25°) to the west-south-west, swinging round to more southerly dips in the western outcrops. Intraformational unconformities were observed rarely within the middle sequence but are probably quite common (e.g. that mapped at the base of the upper sedimentary sequence shown in Fig. 9). Fossils are absent, although possible plant stem impressions were observed on a bedding plane at a single locality. The variable dip orientations at the different localities further west on King George Island (Figs 12–14) suggest that the sequences were erupted from more than one volcanic centre, and that responsible for the sequence at Martins Head was formerly situated in Bransfield Strait (Fig. 11).

Lavas in the Cinder Spur Formation are conspicuously feldspar-pyroxene-phyric basalts and silica-poor basaltic andesites (Table III). Many contain minor, mainly iron oxide-altered amphibole phenocrysts. Olive green and red-brown amphibole crystals are also ubiquitous and conspicuous in the associated sedimentary strata, in lava clasts and as phenoclasts. The abundance of amphibole in the sedimentary rocks suggests that hornblende andesite or dacite lavas crop out in situ within the Cinder Spur Formation rather than the debris being derived solely by erosion and reworking of the Boy Point Formation. Thus, the lavas in both formations may share a similar petrogenesis. They consist of massive centres encased in autobreccia characteristic of subaerial ‘a’ā effusion. Fresh clinopyroxene is particularly characteristic and conspicuous, occurring as abundant, often large phenocrysts (and phenoclasts in the interbedded sediments) up to several millimetres in length. Brown juvenile clast-rich muddy diamictites (slurry-flow deposits) occur directly beneath many of the individual lavas in the type section. Lavas at Cinder Spur are typically c. 10 m thick, rising to c. 25 m. Elsewhere, lava thicknesses are more variable, with massive portions ranging from c. 5 m to 30 m encased in relatively thin (few metres), pale grey to pink, scoriaceous autobreccia.

Sedimentary sequences interfinger with lavas throughout the Cinder Spur Formation. Two distinctive types of sedimentary sequences are present and are distinguished by stratification, colour and the dominant lithofacies. Type 1, which is less common except at Cinder Spur, consists of well-stratified, relatively thin-bedded, multi-coloured (mainly pale khaki-grey and red) fine conglomerate, gravelly sandstone and diamictite (Fig. 7f). Type 2 comprises poorly stratified, thick-bedded, mainly grey green coarse and fine conglomerates and breccio-conglomerates (Fig. 7g). The commonest and most widespread sedimentary lithofacies in the type 1 sedimentary sequences are pale-coloured, khaki-grey, green and red gravelly sandstones, granule and pebble conglomerates and purplish-brown pebbly mudstone diamictites. Although the colours often appear to be stratigraphically confined, they are also commonly vagrant and follow fractures that cross the local sequence irregularly. The lithologies are planar bedded and remarkably continuous on an exposure scale, most beds being restricted only by erosive channels. Channels are conspicuous and characteristic, usually several tens of metres wide and a few metres to sometimes tens of metres deep. Pale khaki-coloured juvenile lava lapilli with a wide range of sizes are common. However, the clast population is polymict, formed of locally derived feldspar-clinopyroxene-phyric lavas, with or without hornblende phenocrysts. Most beds are massive.

The more common type 2 sedimentary sequences are dominated by grey green, poorly sorted granule to small-pebble conglomerates, breccio-conglomerates and diamictites with abundant sandstone matrix and numerous dispersed sub-rounded-angular pebbles to boulders. As in the type 1 deposits, many clasts ≤ 2 cm are essentially monomict, formed of yellow juvenile lapilli, whereas clasts > 2 cm are polymict, dominated by locally derived lavas. Bedding is often difficult to discern but is planar and mainly up to 1 m thick. Most beds are massive. Some beds of coarse breccio-conglomerate are up to 10 m thick, with abundant fine sandstone or mudstone matrix; many display a strong bed-parallel fissile jointing. A particularly thick deposit of diamictite and conglomerate underlies the upper lava sequence at Cinder Spur, and it occupies a channel at least several tens of metres deep cut in the middle sequence at that locality.

As at Cinder Spur, lavas are subordinate or sub-equal to sedimentary strata in outcrops at Retreat Bluffs, Stanczyk Hill and Bremner Bluff, but they are dominant at Martins Head and Malczewski Point (Figs 13 & 14). By contrast, the Harnasie Hill outcrop is entirely sedimentary (Fig. 12). The exposed sequences are c. 250 m thick at each of the three main outcrops (Stanczyk Hill, Martins Head, Harnasie Hill). The bedding dips generally to the west or south-west, but it is more variable at Harnasie Hill. Interbedded sedimentary sequences individually up to 25 m in thickness are common in the summit succession at Martins Head and at Malczewski Point and Stanczyk Hill. They are mainly type 2 sequences of khaki to greenish-coloured, thick-bedded (> 1–2 m, up to 6 m) sandy coarse breccio-conglomerates or polymict volcanic breccias, in very thick (up to > 14 m) massive beds, together with brown-coloured muddy diamictites. In contrast, a 9 m-thick type 1 sequence of planar-bedded, multi-coloured muddy fine diamictites, less common pebbly conglomerates and coarse sandstones with large-scale channel scars crops out near the base of Stanczyk Hill, and binocular observations suggest that similar rocks are present in inaccessible Retreat Bluffs (Fig. 13). Type 1 and type 2 sedimentary sequences are also present at Harnasie Hill, where the thickest accumulation of type 2 sequence is present (≥ 200 m thick; Fig. 12). It is the type area for the Harnasie Hill Formation of Birkenmajer (Reference Birkenmajer1982, p. 45), described as ‘Basaltic lapilli tuff and breccia … alternating with agglomerate, tuff-sandstone and tuff-shale, … porphyritic basalt lava flow and lava-agglomerate-conglomerate intercalations‘. The whole sequence was interpreted as a basaltic cinder cone deposit. However, despite the presence of yellow to pale green (?)juvenile granule-size lapilli, which may be abundant locally, the deposits are overwhelmingly sedimentary. The strata at each locality are polymict except at Cape Syrezol, where the basal 25 m of the seaward-facing cliffs are formed by massive to crudely planar stratified, dark green, fine vitroclastic deposits, at least some of which are probably pyroclastic (coarse tuffs). Whilst the clasts characteristically appear to be locally derived lavas of the Cinder Spur Formation, a large block a few metres in diameter, composed of pale purple fissile lava with prismatic amphibole, was observed locally; the clast has a remarkable resemblance to lavas in the Boy Point Formation, and a similar hornblende andesite cobble was observed in muddy diamictite at Martins Head. The Harnasie Hill sedimentary strata are in faulted contact with lavas just west of Martins Head (Fig. 14).

Fig. 13.

Geological map of the Stanczyk Hill-Malczewski Point area. See Fig. 3 for explanation of additional symbols.

Lastly, Conglomerate Nunatak (Fig. 1) was mapped previously as green basaltic conglomerate of the Polonez Cove Formation (Low Head Member), probably on the basis of the presence of blocks of fossiliferous Polonez Cove Formation lying nearby (not observed during our study; cf. Birkenmajer Reference Birkenmajer1982; see also Tokarski et al. Reference Tokarski, Paolo and Rubinowski1981). The conglomerate was said to be overlain by tuff, agglomerate and lava of the Boy Point Formation. However, the outcrop is formed entirely of unfossiliferous, clast-supported breccio-conglomerate and conglomerate. The deposits are khaki or grey-coloured, rich in pale grey juvenile clasts and crudely stratified in the south-west exposures, changing south-eastwards into massive green strata dominated by basalt lava clasts. They are pervasively and prominently red-stained in places. The closest overall resemblance is to sedimentary strata in the Cinder Spur Formation, possibly both type 1 (south-west) and type 2 (south-east) sequences, with which the outcrop is provisionally correlated.

Vauréal Peak Formation

The study area contains the type section for the Vauréal Peak Formation (also informally called the Vauréal Peak tillite; Birkenmajer Reference Birkenmajer1982) at Vauréal Peak (Fig. 4). Its age is unknown but inferred to be c. 25 Ma (Birkenmajer et al. Reference Birkenmajer, Soliani and Kawashita1989). The formation has a minimum thickness of c. 70 m. It is formed of a khaki-brown to brownish-green massive coarse diamictite dominated by brown siltstone-fine sandstone matrix (30–70%) in which are dispersed angular-subangular (rarely well-rounded), coarse sand to cobble-sized polymict lava clasts. Similar to sedimentary rocks in the underlying Cinder Spur Formation, well-formed phenoclasts of clinopyroxene are common, plus lesser amphibole, and practically all of the lithic clasts are of feldspar-pyroxene-phyric basalt lavas with minor hornblende andesite or dacite. Large blocks of sandstone and granule conglomerate several metres in length similar to type 2 sequence rocks of the Cinder Spur Formation were also observed in basal diamictite at one locality (the so-called ‘fossil ice wedge’ locality described by Birkenmajer Reference Birkenmajer1982, figs 22 & 23). K-Ar whole-rock isotopic dating of a lava block in the outcrop near Vauréal Peak yielded an Eocene age (47.0 ± 1.1 Ma; Birkenmajer et al. Reference Birkenmajer, Soliani and Kawashita1989), indicating that some Eocene outcrops also contributed clasts to the Vauréal Peak Formation. A small proportion of outsize blocks is also present, up to 2.3 m in diameter (generally < 50 cm), but with at least one likely megaclast 3–4 m across, composed of Cinder Spur Formation lava breccia (Smellie et al. Reference Smellie, Hunt, McIntosh and Esser2021). Several of the larger blocks are fractured, and fine matrix from the diamictite infills the fractures and extends as thin veinlets a short distance into the surrounding diamictite (sketched in Birkenmajer Reference Birkenmajer1982, fig. 22). Stratification occurs very rarely within the Puchalski Peak outcrop (Fig. 4), comprising faintly (often wispy) stratified gravelly conglomerate, pebble trails and wavy (disturbed) planar laminated sandstone. The stratification is typically 10–20 cm thick and extends laterally only a few metres before fading out into massive diamictite. This is not the large-scale (?)stratification-like ‘plastic folds-undulations’ [sic] alluded to by Birkenmajer (Reference Birkenmajer1982, fig. 14), which were not observed by us.

The Vauréal Peak Formation occupies a V-shaped palaeovalley with steep to sub-vertical margins in the type locality outcrop between Vauréal Peak and Cape Vauréal (Fig. 4). The margins are well exposed at two localities, and features interpreted as a fossil ice wedge at one were described by Birkenmajer (Reference Birkenmajer1982). At the latter, lava of the adjacent outcrop of the Eocene Hennequin Formation is brecciated and contains scarce admixed fragments of possible Cape Vauréal Formation lava (also Eocene) and a few of dolerite (for a description of the Eocene stratigraphy, see Smellie et al. Reference Smellie, Hunt, McIntosh and Esser2021). The breccia grades laterally downwards through jigsaw-fractured lava into massive Hennequin Formation lava and up into Vauréal Peak Formation diamictite. Birkenmajer (Reference Birkenmajer1982) also recorded glacial striations on a surface of Eocene (Hennequin Formation) lava beneath the lava breccia just described, but they were not observed by us.

At Puchalski Peak, the Cape Vauréal Formation unconformably drapes a narrow west-south-west-trending ridge composed of steep-dipping pyroxene-feldspar-phyric lavas (correlated with the Eocene Carruthers Cliff Formation of Vauréal Peak; Smellie et al. Reference Smellie, Hunt, McIntosh and Esser2021) and dolerite (Fig. 4). Both the lavas and dolerite are heavily fractured, with the intensity of fracturing increasing close to the contact with the Cape Vauréal Formation. At Stanczyk Hill, Cape Syrezol summit and west of Martins Head, diamictite forms a spectacular exposure draped on and over the Cinder Spur Formation (Figs 13–15). The basal erosional surface cuts down at least 250 m, and diamictite adheres to almost vertical rock faces at three places in the Cliffs of Gloom (Fig. 7h).

Fig. 14.

Geological map of Martins Head. See Fig. 3 for explanation of additional symbols. The geological details shown in the Cliffs of Gloom and in the inset are based on binocular observations.

Fig. 15.

View of Stanczyk Hill from Cinder Spur, showing outcrops of Vauréal Peak Formation diamict (grey) draped over lavas and clastic rocks of the Cinder Spur Formation. The diamict drapes an eroded surface cut c. 250 m down into the volcanic sequence, which is being exhumed as the diamict is eroded away.

Outcrops of the Vauréal Peak Formation at Harnasie Hill (Fig. 12) rest on an uneven undulating palaeotopography developed on the Cinder Spur Formation. Additionally, at one locality, sedimentary strata in the underlying Cinder Spur Formation are very disturbed, with bedding dips varying irregularly from 8° to 20° in a zone a few metres wide close to the Vauréal Peak Formation.

Intrusions

Hypabyssal intrusions, including dykes, sills and irregular sheets of basalt and dolerite, are generally minor in the areas studied except at Harnasie Hill, where they are common and conspicuous, although mainly intruding Eocene strata (Fig. 12). The largest individual examples are ‘mega-dykes’ at Cinder Spur and the Chopin Dyke north of Low Head (130 and 75 m wide, respectively); a plug-like mass c. 125 m in diameter at Cape Syrezol; and a thick (several tens of metres) northerly inclined sheet at Sukiennice Hills (Figs 3, 9 & 12). The latter was interpreted as a sequence of dipping thin lavas by Birkenmajer (Reference Birkenmajer1994), and together with adjacent poorly exposed volcaniclastic rocks, they were combined in his Sukiennice Hills Formation, undated but with an estimated age speculatively thought to be close to the Eocene-Oligocene boundary (but see Smellie et al. Reference Smellie, Hunt, McIntosh and Esser2021). Although no contacts are exposed, the relatively coarse dolerite textures and presence of pervasive crude ‘layer-like’ structures dipping at 22–44° to the north, contrasting with the shallower (mainly c. 20°) southerly dips of the adjacent sedimentary strata, are distinctive features that are also seen in dolerite sills elsewhere in the South Shetland Islands (e.g. at Hall Peninsula (Snow Island), Williams Point (Livingston Island), Duff Point (northern Greenwich Island) and Fort William (Robert Island; Smellie et al. Reference Smellie, Pankhurst, Thomson and Davies1984, figs 21 & 22). Additionally, although the Sherratt Bay Formation at Cape Melville (Fig. 1) is currently interpreted as a ‘terrestrial plateau-basalt sheet’ > 60 m thick, and it is included in the Moby Dick Group (Table I; Birkenmajer Reference Birkenmajer1987), it is also doleritic, with pervasive columnar jointing and coarse (1–2 cm diameter) nodular textures on weathered surfaces caused by zeolite alteration (cf. Birkenmajer Reference Birkenmajer1987, and unpublished information of JLS). A thick dyke on the north flank of the eastern tip of Cape Melville intrudes the Cape Melville Formation and appears to be continuous with the underlying Sherratt Bay Formation (observation of AT). The nodular surface was interpreted as a hydrothermal and weathering feature created prior to deposition of the Destruction Bay Formation, implying an age older than the Destruction Bay Formation (Birkenmajer et al. Reference Birkenmajer, Soliani and Kawashita1988b). However, similar distinctive nodular textures occur in several of the dolerites listed above and were caused by deuteric alteration rather than weathering. The sum of the features we have described, particularly if the connection with a dyke is verified, is more characteristic of an intrusion (mainly a sill), albeit possibly a large one, or more than one, with a possible outcrop extending across Destruction Bay to Trowbridge Island (Fig. 1). The sill has not been accurately dated, but it yielded a poor K-Ar age of > 18 Ma (Birkenmajer et al. Reference Birkenmajer, Gaździcki, Kreuzer and Müller1985). Finally, the columnar-jointed oval outcrop at Low Head, regarded as a sub-volcanic plug by Birkenmajer (Reference Birkenmajer1982; see also Smellie et al. Reference Smellie, Pankhurst, Thomson and Davies1984), is not an intrusion. It is a small lava dome with associated carapace breccia extruded in a submarine setting coeval with deposition of the Oligocene Polonez Cove Formation, in which it is grouped within the Low Head Member (subunit L1 of Troedson & Smellie Reference Troedson and Smellie2002; see Smellie et al. Reference Smellie, Millar, Rex and Butterworth1998).

Summary of main stratigraphical results

As a result of our investigation, significant changes that we propose to the lithostratigraphy of eastern King George Island are summarized as follows (cf. Tables I & II):

1. 1) The Magda Nunatak Complex, formerly thought to be Eocene in age, is reassigned to the Oligocene Polonez Cove Formation, as originally suggested by Tokarski et al. (Reference Tokarski, Paolo and Rubinowski1981).

2. 2) The Wesele Cove Formation is shown to underlie the Boy Point Formation, a reverse of the published stratigraphy. Moreover, it is now known to be sandwiched between andesite-dacite lavas of the Boy Point Formation; as a result, the Wesele Cove Formation is reclassified here as a member of the Boy Point Formation.

3. 3) The Harnasie Hill Formation, previously interpreted as a discrete stratum formed of agglomerate of a ‘cinder cone‘ that intervenes between the Dunikowski Ridge and Martins Head formations, is neither formed of agglomerate nor does it form a single mappable datum. It occurs as multiple packages of sedimentary rock interbedded throughout the volcanic sequence. Moreover, the overlying and underlying Martins Head and Dunikowski Ridge formations are lithologically indistinguishable. Thus, the three formations are combined into a single unified volcano-sedimentary unit here named the Cinder Spur Formation.

4. 4) The Vauréal Peak Formation, a glacial diamict (tillite), is removed from the Legru Bay Group. It was previously linked stratigraphically with the underlying Martins Head, Harnasie Hill and Dunikowski Ridge volcanic formations. However, there is no obvious unifying characteristic that might be used to group together the different formations in the Legru Bay Group. Moreover, there is evidence for a substantial age gap between the Vauréal Peak Formation and the underlying units.