Supplementary information for: Smellie et al. Northern Victoria Land Late Miocene glaciovolcanic sequences Bull.Volc.
Late Miocene volcanic sequences in northern Victoria Land, Antarctica: products of glaciovolcanic eruptions under different thermal regimes[Supplementary information]
J.L. Smellie*, S. Rocchi, P. Armienti
*Corresponding author.Affiliation: University of Leicester, Department of Geology, University Road, Leicester LS1 7RH, UK; E-mail:
Descriptions of the principal outcrops visited
In the following descriptions, each locality is described in terms of local sub-sequences (or local units; LU) each of which is separated by a prominent stratigraphical break. Each LU consists of the products of an individual eruption and in the following descriptions the LUs are numbered chronologically upward from the exposed base; subdivisions within LUs (corresponding to lithofacies associations; e.g. lobe-hyaloclastite; subaerial lava—autobreccia, etc) are identified by letter from base up (a, b, etc). Where appropriate, further subdivision is into beds, again numbered from base up. The system of numbering is for descriptive purposes only and not intended to imply correlations of sequences or beds between localities, which are typically separated by a few to several tens of km of unexposed ground. Summary sketch vertical sections of the individual sequences are shown in Figure 1, with the LUs and/or bed numbers indicated. The locations of the outcrops described are shown in Figure 1 of the main paper.
Shield volcanoes
Coulman Island
Coulman Island was examined by helicopter on its western side. The island is almost wholly inaccessible because of continuous serracced cliffs. Only the basal few tens of metres at a single unnamed locality (T5.1) were examined in detail, situated at the SW corner where McIntosh and Gamble (1991) began their section. The locality was illustrated by Hamilton (1972, fig. 26A). It consists of a basal, closely jointed, thick (> 55 m), flow-foliated fine-grained mafic lava (LU1a) with internal patches of coarse breccia overlain conformably by c. 40 m of thin (typically 0.5-1 m) grey sheet and lensoid highly vesicular lavas separated by similar thicknesses of maroon scoriaceous autobreccia surfaces (LU1b). The junction with the basal lava is marked by c. 50 cm of yellow-orange, zeolite cemented, coarse sandy volcanic breccia formed of scoriaceous and fine-grained lava clasts. The upper surface of LU1b is planar and seems uneroded. It is overlain by a new sequence (LU2a), possibly 50 m thick, composed of blocky-jointed mafic sheet lavas mainly 1-3 m thick (up to 8 m) encased in khaki-yellow-coloured glassy and crystalline pebble-grade monomict volcanic breccia. The lavas are fine-grained to aphanitic and predominantly non-vesicular, with irregular shapes and closely spaced sheet-like joints, and they disintegrate into breccia around their margins. The new sequence appears to dip slightly more steeply than the basal surface and apparently oversteps it. It changes up conformably into an inaccessible sequence composed of thin sheet lavas and pink autobreccia (LU2b) that is about 30 m thick, which extends virtually to the crest of the spur (c. 200 m), but then changes up, across a gently undulating sharp surface, into a conspicuous thin yellow layer, which is probably the base of a new sequence (LU3a) with features very similar to LU2a. LU3 appears to extend to the top of the ice-covered backing cliffs here. A very large block lying at the base of the cliffs may be derived from LU3a. It is composed of highly porphyritic mafic lavas (units 1 and 2 are aphyric), interbedded with fines-free breccia composed of granule to block-sized angular to slightly abraded clasts, glassy chilled pillows and possible grading.
Daniell Peninsula
Basal exposures at Cape Jones (T5.16) comprise c. 20 m of poorly exposed lavas with reddened autobreccia and local maroon agglutinate (LU1). The same(?) unit is better exposed in the western half of the cliffs where it forms a basal unit of subaerial lavas and autobreccias with a locally eroded upper surface (Figure 2). LU1 is overlain by khaki brown stratified indurated polymict lapilli tuff (LU2) that thickens eastward from c. 10 m to form much of the cliff at the eastern cape. The top surface of LU2 is uneven, sharply defined and eroded. Bedding is crude and on a dm scale, with rare dune bedforms and sag structures beneath blocks. The upper metre commonly shows slumping and small-scale faulting and parts are wavy laminated. About 2 % of the lapilli are oxidised and many of the blocks show evidence of abrasion. LU2 is extensively intruded by grey jointed lava encased in maroon-coloured baked lapilli tuff. The intrusion forms most of the section at the eastern cape itself. LU3 (c. 200 m thick) overlies LU2 and is mainly composed of very gently east-dipping thin sheets and irregular lenses of blocky jointed lava and “megapillows” encased in cogenetic, massive, fines-free, glassy coarse breccia (lobe-hyaloclastite; LU3a). At its base, the breccia contains lenses of well sorted coarse to fine sandstone showing discontinuous wavy laminations. It also locally contains numerous highly vesicular lava clasts, some up to 4 m across, sparse oxidised scoria and vesicular bombs(?). The lava dips decrease up-section to horizontal and they are succeeded by a thinner (up to c. 50 m) cogenetic sequence of subaerial horizontal sheet-lava flows (LU3b). LU4 is very similar to LU3 but is thinner (c.100 m), inaccessible and extends to the top of the exposed section. The surface between LU3 and LU4 is subhorizontal and looks uneroded.
A single prominent bluff was examined on the north side of Mandible Cirque. The basal crags are c. 600 m high and are composed of at least two different felsic sequences separated by a conspicuous erosional unconformity and there may be additional unconformities higher in the section. The sequence below the prominent unconformity is exposed toward the east end of the cliffs (T5.14). It extends laterally about 500 m and is cut out by the unconformity at both ends, before reappearing briefly at the eastern extremity. The unit comprises, from base up, a grey foliated fine-grained lava dome (bed 1a) that becomes pervasively fractured upward, forming jigsaw breccia that changes up into 2-8 m of massive crystalline breccia formed of angular lava blocks (bed 1b). The breccia is variably blocky to sandy, and pale grey to maroon-brown in patches, whilst the topmost 1.5 m is discoloured brown-orange along a highly uneven upper contact. It is sharply overlain by conspicuous bright yellow, crudely stratified very coarse sandy diamictite 8 m thick (bed 2) with numerous abraded polymict felsic lava blocks, up to 2 m in diameter, dispersed in the basal 1-2 m; cobbles up to 20 cm in diameter are scattered throughout and may become smaller upward. Bed 2 is succeeded across a sharp horizontal locally erosive surface by 8 m of crudely planar stratified, buff yellow, very coarse sandstone varying to granule-grade breccia (bed 3). It contains pebble-rich trails up to 30 cm thick traceable c. 10 m, and dispersed polymict subangular—angular pebble-sized clasts, including prominent yellow fiammé-like flattened pumice. The stratification dips gently eastward and progrades on internal non-erosive surfaces. The stratification often looks cyclic, with each of the sediment packages 15-45 cm thick becoming more thinly stratified upward. Small-displacement (dm scale) normal and reversed faults are common. Beds 2 and 3 seem to mantle the underlying dome topography and they show a progressive upward colour change from pale to darker yellow or buff. Bed 3 is overlain by a monomict glassy to aphanitic blocky breccia 0.5 to 1 m thick (bed 4a) that is the base of a deep brown closely jointed lava dome (bed 4b). The breccia locally penetrates the massive dome rock up to 8 m and it wraps over the top of the dome, where the clasts are crystalline rather than glassy. Wedges of the hyaloclastite breccia also intrude downward and locally mingle with bed 3. The dome is at least 30-35 m thick. It is overlain sharply and unconformably across an eroded surface by crudely stratified yellow sandy gravel (bed 5) that contains numerous dispersed fragments reworked from the underlying brown dome. Similar rock also locally intrudes down into the dome along fractures. The contact is the prominent angular unconformity that separates the lower and upper sequences. Where accessible, its surface is not exposed. Bed 5 is 2 m thick where examined but swells to nearly 20 m further west before thinning markedly again. It is overlain by 2-4 m of massive, coarse, felsic hyaloclastite breccia with glassy blocks up to 1 m in diameter (bed 6a) that shows conspicuous yellow coloration within 0.5 m of its base and top. The overlying dome (bed 6b), at least a few tens of metres thick, is pale red-brown and finely foliated. It is formed of pervasive jigsaw breccia affecting at least the basal 15 m.
The entire western cliff face at the Mandible Cirque locality (T5.2) is composed of numerous spectacularly exposed, superimposed pale grey sheets of felsic lava that together form the upper sequence. Its basal surface is the prominent unconformity described above, that can be traced from the centre to the east end of the cliffs, and which contains a prominent deep “U”-shaped trough. The lowest exposures at T5.2 consist of bright green-yellow coarse glassy hyaloclastite breccia similar to “bed 6a” (described above). The hyaloclastite breccia is 8-15 m thick and it wraps around a grey and red-brown finely foliated dome 22 m tall by 60 m in length similar-looking to “bed 6b”. The dome and breccia are gradational and the breccia is discoloured orange or yellow within 0.5 m of the dome lithology. Many of the breccia clasts retain foliation and fold structures also seen in the massive dome. At its west end, the breccia is in steep (c. 70°) contact with a similar hyaloclastite breccia that extends up and overlies the first breccia. Traced further west, the new breccia shows a very coarse crude shallow east-dipping discontinuous stratification 2-6 m thick. The breccia is reddish-brown locally and it laps onto and overrides a new dome to the west that is at least 50 m thick, off-white and sugary textured. The dome is finely foliated and contains irregular zones of fractured and brecciated rock. It is intruded by at least two thin (30 cm) dykes composed of bright yellow pumiceous lapilli tuff and tuff. The dome continues west for c. 150-200 m but was not examined to its limit. It is capped by 2 m of brown coarse breccia and the overlying inaccessible cliff section is composed of alternating, foliated, off-white to pale grey felsic sheets c. 8-10 m thick and rusty brown carapace breccias. The upper quarter of the cliff face contains spectacular coarse glassy breccias similar to those at the base of the section, and pale grey felsic lava sheets thicker than the breccias.
The ridge-forming sequence directly above the Mandible Cirque cliffs (T5.13) appears to be continuous with underlying exposures. It is badly exposed, mainly scree and snow covered, and few field relations were observed. The ridge is formed of further felsic domes/lavas and sugary-textured coarse breccias except at the north end where they are draped by a sequence of thin dark grey mafic sheet lavas. Similar relations (i.e. lower pale–coloured felsic domes/lavas overlain by thin mafic lavas) were observed by binoculars on the adjacent ridge 4 km to the northwest. There is no evidence for a caldera fault between the felsic and mafic sequences.
The > 400 m mafic section exposed at Cape Phillips (T5.32; Figure 3) consists of at least eight discrete local units, each comprising a distinctive association of contrasting lithofacies. The upper, usually thinner lithofacies association in each local unit is composed of continuous, even-bedded, subhorizontal crystalline grey lavas with maroon-coloured autobreccias. It gradationally overlies a thicker lithofacies association of aphanitic sheet, lensoid and irregularly shaped lavas encased in abundant massive, chaotic or crudely stratified coarse lithic and glassy breccia (probably the “marble cake palagonite breccia and lava” described by Hamilton, 1972). The LUs are individually 50-100 m thick and are mainly bounded by sharp, planar, apparently non-erosional surfaces on which any bedding in the breccia-dominated lithofacies is either parallel to or gently onlaps the underlying subaerial lava-dominated sequence. Although the LUs appear to be sheet-like, several wedge out laterally. The Cape Phillips sequence also contains a conspicuous thick stratum of bright yellow felsic lapilli tuff intruded by very irregular sheets of aphanitic grey, closely jointed lava (LU6a), and there is also a well-exposed section cut across a crudely stratified maroon-coloured cinder cone at the base of the cliffs (LU1).
The predominantly mafic sequence at Cape Daniell (T5.29) is constructed from at least five LUs in the basal 600 m of the 800 m section exposed there (the upper 200 m was unexamined). The lithofacies associations are very similar to those at Cape Phillips and Cape Jones but they dip homoclinally at c. 15° to the WNW. The individual LUs are variably 65-230 m thick. The junction between the two lithofacies associations in each LU (i.e. the passage zone) is gradational and usually planar (dipping), parallel to the enclosing sub-sequence boundaries, but in a few instances it is uneven on a scale of several metres. The Cape Daniell section also contains a conspicuous stratum of crudely stratified gravelly hyaloclastite breccia (LU4a). It is bright yellow-coloured within several metres of numerous dark grey, highly irregular intrusive masses of closely jointed aphanitic to fine grained lava with poorly-formed pillows locally. The lava is intensely fractured marginally and disintegrates into the enclosing breccia (see Hamilton, 1972, fig. 28g).
Hallett Peninsula
Localities were examined at Redcastle Ridge (T5.26, T5.30), eastern Edisto Inlet (T5.25, T5.28) and northern Cotter Cliffs (T5.19, T5.20), together with helicopter observations around almost the entire peninsula. Mount Harcourt is well exposed in tall cliffs on its south side but it is inaccessible except for a mafic section exposed at Redcastle Ridge c. 12 km to the north (Figure 4). There, the basal unit (T5.26), up to an elevation of c. 70 m a.s.l., at least, comprises chaotic to poorly stratified yellow lapilli tuff. The stratification is highly deformed and heavily intruded by multiple grey mafic lava sheets that show spectacular evidence for peperitic mingling at their upper and lower margins. The lapilli tuff changes upward cryptically into poorly exposed blocky-, platy- and finely columnar-jointed (entablature) fine-grained lava with highly irregular sheet-like shapes alternating with bright yellow gravelly hyaloclastite (T5.30). Lapilli in the breccia are not uncommonly highly vesicular. The breccia passes up across an unexposed planar gradational contact (passage zone?) that dips south at c. 10° into irregular alternating grey crystalline platy lava sheets and maroon scoria and autobreccia. The lava—scoria—autobreccia section is onlapped, across an apparently uneroded surface, by a younger sequence of platy-jointed crystalline lava sheets individually a few tens of metres thick and much thinner grey and reddened autobreccias that dip at < c. 8° to the north and which also rest on a much thicker lithofacies association of chaotic lavas and hyaloclastite breccia (lobe-hyaloclastite), most of which is inaccessible. The junction between the sheet lavas and underlying lobe-hyaloclastite also dips gently northward initially but then bends round to dip very gently to the south and the section is onlapped by further (younger) platy jointed crystalline lavas and reddened autobreccias that are almost flat to gently north-dipping which appear to be continuous with the sequence at Mt Harcourt. The impression given is of at least three successively erupted volcanic centres, the oldest situated in the north (with a vent in Edisto Inlet) and the youngest at Mt Harcourt. The basal and middle sequences may be > 300 m (including basal lapilli tuffs) and > 200 m thick, respectively (estimated). Field relations between Mt Harcourt and Cape Wheatstone, and inferred from patchy outcrops at the head of Arneb Glacier suggest that the Mt Harcourt—Recastle Ridge centres are all older than Hallett Peninsula.
At locality T5.19, on the east coast of Hallett Peninsula, > 200 m of felsic pumice-rich lapilli tuff is intruded by irregular very thick mafic sheets. Conversely, at T5.20, about 8 km north of T5.19 (northern Cotter Cliffs), three mafic sub-sequences were examined in a section c. 250 m in height. The basal sub-sequence (LU1) is well stratified and mainly formed of clastic rocks. It was only examined briefly. The lowest beds comprise a pale grey felsic dome/lava (bed 1a) and overlying coarse lithic breccia (bed 1b). The breccia is overlain by a sequence (beds 2-6) of conformable felsic lava-sourced beds, beginning with c. 2 m of bright yellow lapilli tuff or conglomeratic sandstone (bed 2) showing crude planar stratification to base but massive above. The lapilli tuff/sandstone also infills cracks between blocks in the underlying breccia. It is formed of finely pilotaxitic non-vesicular grains of felsic lava, together with angular and abraded accidental grains of mafic lava, obsidian(?) and rare fine-grained quartz-biotite-muscovite tectonite, which are much more common in the base of the bed. Bed 2 is overlain conformably by c. 1 m of dark green-grey monomict felsic tuff-breccia formed of angular finely pilotaxitic (aphanitic) non-vesicular blocks (bed 3); c. 1 m of yellow and rust-coloured, planar stratified, polymict pebbly breccio-conglomerate (bed 4); and c. 1 m of dark grey-green tuff breccia (bed 5) similar to bed 3 but with a 10 cm-thick weakly welded(?) base. LU1 is capped by 20 m of monomict (felsic) tuff-breccia (bed 6; cf. bed 3) that is rust coloured in the basal 0.5-1.0 m, then variably grey-green, grey-brown and (forming most of the deposit) cream-coloured to top. The coloration reflects a crude faint planar stratification. Clasts in bed 6 are predominantly non-vesicular and pilotaxitic (aphanitic) and the deposit contains a variable proportion of ash matrix. Bed 6 is composed of clast-supported aphanitic breccia to top. The upper surface of bed 6 is sharp and planar.