Fig. 1. (a) Map showing the distribution of the Beacon Supergroup, Ferrar Dolerite, and Kirkpatrick Basalt in the Transantarctic Mountains (TAM). Mafic volcaniclastic deposits, including the Mawson Formation, are found underlying the Kirkpatrick Basalt in North Victoria Land (NVL), South Victoria Land (SVL) and the Central Transantarctic Mountains (CTM). Modified from Hanson and Elliot (1996). (b) Hypothetical former distribution of the Mawson Formation (including the Carapace Sandstone at the nunatak of the same name). The illustrated belt is 195 km-long, with a mean width of 21 km. Exposed thicknesses of mafic volcaniclastics are generally over 60 m. Base map modified from Stump (1995).

Fig. 2. Summary of Beacon Supergroup stratigraphy in South Victoria Land (SVL), using information from Barrett and Kohn (1975), Barrett (1991), Bradshaw (1991), and other sources cited by McClintock (2001). CTM = Central Transantarctic Mountains. Local thickness (Allan Hills) of uppermost Beacon Supergroup in bracketed italics.

Fig. 3. Geologic map of Coombs Hills, modified from McClintock (2001). Thin dashed lines are 200 m contours. Numbered stars denote locations discussed in the text: (1) the ‘basalt’ plug shown in Fig. 4; and (2) the East Ridge tuff ring. Sections 96-48 and 96-49 were studied by Elliot (2000) and Elliot and Hanson (2001); 96-49 displays 370 m of unbedded coarse volcaniclastic rocks overlain by 22 m of poorly exposed layered rocks, whereas the Mawson at 96-48 is under 200 m-thick (base not exposed) and consists of finer-grained layers.

Fig. 4. ‘Basalt’ plug cut by volcaniclastic dike on the West Ridge of Coombs Hills (location 1 on Fig. 3). (a) Pace and compass map; (b) interpretative section X–X′; (c) photo of the plug (marked by a dashed line) looking southward, with Mt Brooke in the background; (d) view of the plug's interior, showing preferentially eroded volcaniclastic dike cutting the plug. For facies codes (LT_h, TB_j) see Table 1.

Fig. 5. Geological map and cross-sections of southern Allan Hills, modified from Ross (2005) and Ross and White (2005b). Bold lines marked AH-1 to AH-4 indicate prominent block-rich layers. LT = lapilli-tuff; TB = tuff-breccia.

Fig. 6. Geological map and vertical sections for central Allan Hills, modified from Ross (2005) and Reubi et al. (2005). “TB_j” on section B–B′ stands for poorly sorted, ‘basalt’-rich tuff-breccia zones cross-cutting m₁.

Fig. 7. Features of poorly sorted, Beacon-rich lapilli-tuff (LT_a) zones invading the m₁ member of the Mawson Formation in central Allan Hills. (a) View of the abrupt, steep to vertical contact (dashed line) between a LT_a zone and host m₁. Hammer for scale. (b) Summary of ten field clast counts from LT_a zones in Feistmantel Valley, after Ross (2005). (c) A fragment of incipiently to poorly vesicular ‘basalt’ (arrow) in a LT_a zone (photo courtesy of O. Reubi).

Fig. 8. Aerial photograph of Carapace Nunatak with superimposed longitude/latitude grid and geology. The grid appears slightly tilted because of the imperfect photo registration, based on six GPS points. There is also some distortion in the western margin of the photograph, as shown by the bold arrow, the two ends of which correspond to the same geographical point. See also the geological map by Bradshaw (1987). Numbers on the map correspond to localities discussed in the text: (1) and (2) outcrops of Carapace Sandstone; (3) and (4) volcaniclastic intervals in the Kirkpatrick sequence.

Fig. 9. Plane-polarized, transmitted light photomicrographs of Carapace Sandstone samples: (a) OU74526, from the lower part of the formation; (b) OU74528, from the middle part. Constituents: B, dense, formerly glassy ‘basalt’ (now altered to clays and palagonite); FG, altered, fine-grained sedimentary? rock; Mi, mica; Q, quartz; Z, zeolite cement. Circles on (b) due to glue bubbles.

Fig. 10. Kirkpatrick lavas at Carapace Nunatak. (a) Pillow-palagonite breccias showing dismembered ‘basalt’ pillows “floating” in hyaloclastite (angular fragments of black glassy ‘basalt’) with abundant palagonite (orange in outcrop, grey on the photo) and sparse white cement. (b) Local pillow lavas. Ice axe handle is 70 cm-long. (c) Photomicrograph of the first lava, showing the intersertal texture. (d) Photomicrograph of a pillow interior, showing the glomeroporphyritic texture. (e) Photomicrograph of a lava above the volcaniclastic interval, with an intersertal to intergranular texture. Last three images in plane-polarized transmitted light. Note the scale change between (c)–(d) and (e), making the crystal size much smaller in (c)–(d). Abbreviations: AG, altered glass, CPX, clinopyroxene; G, fresh glass or tachylite; O, opaque minerals; P, plagioclase.

Fig. 11. Plane-polarized, transmitted light photomicrographs of fine lapilli-tuffs and coarse tuffs containing accretionary lapilli (a.l.) in the volcaniclastic intervals of the Kirkpatrick sequence at Carapace Nunatak: (a)–(c), sample OU74524; (d), sample OU74525. Overall, the most common constituents in these rocks are formerly glassy ‘basalt’, either dense or vesicular (Bd, Bv) and quartz (Q) (the photos do not show this), but crystalline ‘basalt’ clasts (Bc) are more frequent than in all Mawson samples; OPX, orthopyroxene. Note different scale for (b).

Fig. 12. Variation diagrams for selected major and trace elements versus Mg-number. Major elements were re-calculated to 100% on a volatile-free basis for these plots. The bold lozenges represent the samples listed in Table 9 and Table 10. Grey symbols are published analyses for the Kirkpatrick Basalt (MFCT, SPCT: see text) and Ferrar Dolerite sills, throughout the Transantarctic Mountains, after Elliot (1972), Mensing et al. (1984), Siders and Elliot (1985), Ford and Kistler (1980), Hergt et al. (1989), (Fleming et al., 1992) and (Fleming et al., 1995), Wihelm and Wörner (1996), Antonini et al. (1999), Elliot et al. (1999), Demarchi et al. (2001) and P.R. Kyle, pers. commun. (2003) [major and trace element data for MFCT flows at Gorgon Peak, Prince Albert Mountains, referred to in Kyle et al., 1983]. Sample OU73163 is discussed in a footnote of Table 10.

Fig. 13. Geochemical discrimination diagrams: (a) alkalis versus silica after Le Maître (1989); (b) silica versus Zr/TiO₂ after Winchester and Floyd (1977); (c) tholeiitic versus calc-alkaline after Irvine and Baragar (1971). Relevant fields for (a) are B: basalt, O1: basaltic andesite, O2: andesite, O3: dacite. Symbols and sources of data as in Fig. 12; sample OU73163 is discussed in a footnote of Table 10.

Fig. 14. Geological and geomorphological evolution of the Coombs–Allan Hills area from the Permian to the early Jurassic. Thickness of rock units is schematic only. Cartoons (a) to (b) show a slight tilt of the strata to the northwest because of the overall direction of stream flow during deposition of the Victoria Group. This tilt is removed arbitrarily in (c) (could have occurred earlier) because of the debris avalanche flowing north-eastward (also, there is stream flow to the NE in d). The limit of the topographic depression for the Allan Hills area in (c) and subsequent cartoons is speculative, except for the northeast end: the Lashly Formation is still exposed in the Roscollyn Tor area, marked by a ‘X’. The emplacement of sills is not shown. See text for discussion.

Facies descriptions and interpretations for the non-bedded, poorly sorted volcaniclastic rocks in the Mawson Formation north of Mt Brooke at Coombs Hills, after Ross (2005) and Ross and White (2006)

Facies descriptions and interpretations for intrusive rocks in the vent complex north of Mt Brooke at Coombs Hills, after Ross (2005)

Calculation of the mass flow of basaltic magma in a cylindrical conduit based on the Wilson and Head (1981) model

Observations and interpretations for the Kirkpatrick Basalt between the East Ridge and the summit of Mt Brooke at Coombs Hills, after Ross (2005)

Observations and interpretations for cross-cutting zones of Beacon-rich lapilli-tuff (LT_a) at central Allan Hills, after Ross (2005)^a