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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: jls55@le.ac.uk
Descriptions of the principal outcrops visited
In the following descriptions, each locality is described in terms of local subsequences (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 finegrained 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)
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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 marooncoloured 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 12 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 nonerosive surfaces. The stratification often looks cyclic, with each of the sediment packages 1545 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
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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, offwhite 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
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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 marooncoloured 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 65230 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 subsequence 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).
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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,
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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 pumicerich 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.
Above LU1 at T5.20 is a second sub-sequence (LU2) composed of 40-50 m of lobehyaloclastite, comprising irregular and crudely sheet- or lens-like non-vesicular lavas 2-4 m
thick and  20 m long and gravelly to blocky hyaloclastite breccia with numerous whole and
broken pillows up to 1.5 m in diameter (LU2a). The pillows have thick (to 8 mm) glassy
rims, and many of the larger ones are tadpole shaped. In places individual hyaloclastite layers
are up to 5 m thick, massive and intruded by very irregular lava masses extending vertically
over c. 20 m. The hyaloclastite also contains a small proportion of coarsely vesicular maroon
clasts and the finer, gravelly layers locally show crude, discontinuous planar stratification
with bed thicknesses of a few cm to c. 0.5 m. LU2a is overlain gradationally by about 30 m of
thin planar sheet lavas with maroon aa surfaces (LU2b) that are then overlain sharply by a
third sequence comprising > 50-70 m of lobe-hyaloclastite (LU3).
At least five local units are exposed on the east flank of Edisto Inlet between localities
T5.25 (Roberts Cliff) and T5.28 (2 km north of Salmon Cliff). Despite ice-covered gaps in
exposure, observations at the two localities examined directly, and by helicopter for the
intervening outcrop (Salmon Cliff), have enabled a correlation of the major local units over at
least 7 km of coastline and a unified notation is used in the descriptions here (LU1-LU5). The
basal outcrop at Roberts Cliff comprises c. 30 m of bright yellow stratified mafic lapilli tuffs
(LU1), which was described and illustrated by Harrington et al. (1967) and informally named
the Roberts Cliff tuffite formation. Although the outcrop is probably a very large, fractured in
situ block, with stratification dipping at 70° to slightly overturned, its upper surface appears
to be very little eroded. The overlying sequence (LU2a) consists of interbedded dark grey,
conspicuously feldspar phyric, non-vesicular fine-grained mafic lavas with grey and black,
rarely slightly oxidised aa surfaces and interbedded coarse breccias. Fine impersistent
stratification is present within a few metres of LU1, in gravel-grade monomict hyaloclastite
breccia with pillows showing tiny normal radial cooling joints. The breccia also contains
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conspicuous yellow lapilli tuff clasts derived from LU1 (cf. Harrington et al., 1967) together
with highly vesicular grey and less common maroon scoria. The breccias higher up in LU2a
are formed by a mixture of finely crystalline, aphanitic and glassy clasts, plus sparse maroon
scoria. They are thin (1-4 m), monomict deposits and are commonly khaki stained. Some
show crude planar stratification. At c. 70 m a.s.l. there is a much thicker lava (> 10 m),
bulbous and lens-shaped, with prominent subvertical coarse columnar joints that change up
into variably orientated thinner columns (cf. colonnade and entablature. The lava marks the
base of LU2b and the rest of the section, up to 140 m a.s.l., is dominated by further thick,
lensoid, colonnaded lavas with dark grey aa surfaces. At 140 m, there is a change up into
conspicuously planar, much thinner (1-2 m), sheet lavas, paler grey, coarse grained and
coarsely vesicular, with thin (few dm) grey and maroon aa surfaces (LU2c). Its top is at c.
170 m a.s.l. The surface is gently convex-up over a horizontal distance of several hundred
metres and is apparently uneroded. The overlying sequence (LU3a) comprises chaotic
massive hyaloclastite and irregular aphanitic and fine-grained lava sheets and lenses but was
not examined closely.
Salmon Cliff exposes sub-sequences LU3 and LU4 but was not visited. From
helicopter observations, the top surface of LU3b is very well exposed at Salmon Cliff and is
convex-up and apparently uneroded (it is parallel to the underlying lavas, which are
apparently not cross-cut). LU3 is at least 300 m thick in its thickest exposure (estimated; base
unseen) and is overlain by chaotic lavas and breccias (LU4a) that were examined at the next
locality to the north (T5.28). However, the local top of LU3b has dropped to 138 m a.s.l. at
T5.28. The mafic lavas in LU3 are sparsely feldpar-phyric. LU3b extends down to c. 100 m
a.s.l., where it changes gradationally down into relatively poorly exposed irregular lava
lenses/sheets and breccia (LU3a). The top surface of LU3b is quite well exposed in places at
T5.28. It is uneven on a scale of 1-2 m related to the presence of resistant massive lava and
less resistant autobreccia and looks very little eroded. The surface is overlain by LU4a,
initially comprising a few metres of clast-supported, polymict volcanic breccia that is pale
khaki brown, indurated, and massive to poorly stratified. The clasts in the basal breccia in
LU4a appear to be derived from mafic lavas in LU3b but they also include platy foliated pale
coloured tephriphonolite fragments characteristic of LU4, some of which are sugary textured,
plus rare black glass. The remainder of LU4a is formed by chaotic irregular lobes, lenses and
pillow-like masses of platy non-vesicular tephriphonolite lavas 2-20 m thick showing
prominent sheeting and blocky joints. They disintegrate around their margins into massive,
monomict, fines-free, finely crystalline, aphanitic and glassy breccia. The top 4 m of a
prominent 20 m thick lava in LU4a (c. 40 m above the basal unconformity) is frothy and
autobreccia clasts are oxidised (base of LU4b? c. 175 m a.s.l.). Above that is c. 65 m of thin
(2-6 m) sheet lavas with conspicuous autobreccia that is rarely oxidised or show rare khakicoated rims (LU4b). The top of LU4b is at 224 m a.s.l. but is unexposed. The succeeding 60
m comprises a new sequence (LU5a) of poorly exposed fine-grained irregular sheets or lobes
of platy lava similar-looking to those in LU4 (i.e. tephriphonolite or benmoreite?) but are 1015 m thick. They show well-developed sheeting- and blocky joints and glassy rims and are
interbedded with thick (commonly 10 m), massive sandy-gravelly hyaloclastite breccia
formed of blocky hyalopilitic lava fragments, dispersed pillows and sparse pumice and a few
oxidised vesicular lava clasts. Some clasts show slight abrasion but platy fragments dominate.
Adare Peninsula
Cliff-top exposures on the west side of Cape Adare (T5.31) were examined at an
elevation of c. 285 m. They consist of an 8 m-thick mafic lava, very fine grained to glassy,
with a well exposed 4 cm-thick glassy uneven base and well developed blocky joints. The
jointed lava forms upward-pointing apophyses separated by similarly sized screens (crude
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ramp structure or flow-top folds?) of black frothy lava that are capped by faintly maroon
coloured scoria. The lava may be dipping at c. 50° to the west. It rests on pale grey, frothy
microvesicular aphanitic mafic lava rubble. The plateau surface contains abundant maroon
scoriaceous autobreccia rubble in the regolith. Helicopter observations indicate that the cliffs
below are formed of multiple flat-lying sequences individually c. 100-200 m thick. Each is
composed of a lower chaotic unit of lava and breccia (likely lobe hyaloclastite) overlain by
thin sheet lavas and autobreccia, thus resembling the sub-sequences visited at several
localities further south (described above). The sheet lava units are typically < 30% the
thickness of the chaotic breccia/lava units. The plateau has numerous erratic boulders (first
noted by Hamilton, 1972) and very large blocks measuring up to 4 m in each dimension,
mainly pale green fine-grained quartz-rich metasandstone (Robertson Bay Group) but also
green platy felsic lava and some very coarse granite.
Inland Suite (principally small isolated monogenetic volcanic outcrops)
Outcrops in the Mt Finch area (T5.21-24) consist of highly degraded remnants of
subaerial cinder cones formed of black and red (oxidised) scoria and bombs, and subaerial
lavas and autoclastic breccias. Two outcrops (T5.21 & 22) rest unconformably on basement
granodiorite whilst the third (T5.24) comprises two tiny isolated exposures (< 50 m wide
each) in the middle of Gruendler Glacier. The Tucker Glacier outcrop (T5.33), situated c. 2
km northwest of Crater Cirque, comprises, from base up, (1) a polished and striated
granodioritic basement surface that generally dips at 10 to 30° toward Tucker Glacier (the
striations trend up and down glacier); (2) very poorly exposed diamict composed of abraded
and facetted Robertson Bay Group (RBG) metasandstones and rare granodioritic cobbles and
boulders in a white mud matrix; (3) c. 1 m of rust-coloured massive lapilli tuff or gravelly
volcanic sandstone; (4) up to 6 m of massive, lapilli-grade, fines-free lithic and glassy breccia
(hyaloclastite) containing chaotic lobes and pillowy aphanitic lava masses; and (5) > 40 m of
non-vesicular, dark grey fine-grained lava with prominent wavy and fanning small prismatic
columns (entablature). At one locality the small (15-25 cm wide) columns in the lava are
subhorizontal orientated and crop out on a vertical rock face overlooking Tucker Glacier.
The outcrop designated informally by Harrington et al. (1967) as the Herschel
Tuffaceous Moraine is a sedimentary sequence situated on the west flank of Edisto Inlet c. 3
km northeast of Luther Peak (T5.27). It crops out in a small discontinuous crag about 70 m
long that trends obliquely up the cliff slope in a southerly direction and is c. 20 m thick. The
upper surface of the underlying RBG bedrock is polished flat and shows prominent parallel
striations with chatter marks. At its type locality (Harrington et al., 1967, fig. 40), the
sequence has two major parts: a lower, thinner, white and brown diamict-dominated section
(beds 1-4, below), and a much thicker upper section dominated by rusty sandstone and blocky
gravel breccia. The two sedimentary sections are in gradational contact. From the base up, the
section comprises:
(1) A conspicuous layer of white, massive muddy boulder diamicton and diamictite 1->4
m thick, with polymict, abraded, facetted and rarely striated RBG metasandstone
clasts dispersed in 60-70 % muddy matrix that drapes the steep underlying basement
slope; where the basal white diamict is thickest, it shows crude boulder trains, some of
which are truncated upward along internal sharp joints that have striated surfaces;
(2) 0-1 m wedge of fawn-brown, massive gravelly—boulder diamicton with close-packed
to dispersed, angular RBG clasts up to 60 cm in diameter; dispersed rusty highly
vesicular glassy mafic lava lapilli are prominent in the upper half and up to 70 %
silty—sandy matrix that is dominated by cuspate and blocky poorly vesicular mafic
glass; some of the fine matrix shows faint wavy-deformed laminations; this bed
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reappears upslope to the south where it is c. 3 m thick, slope-parallel and changes
progressively up-dip into crudely stratified coarse angular breccias and thin rusty
coarse sandstones; clasts in the breccias are both imbricated and slope-parallel;
(3) 4 cm of impersistent, rusty laminated siltstone with very coarse sand grains, dispersed
angular RBG-derived pebbles, abundant mafic glass and rare felsic pumice;
(4) 1 m of grey-brown siltstone with prominent fissile jointing and thin beds of wavydeformed very coarse sandstone, and scattered angular cobbles and boulders that also
sometimes occur in silt-matrixed clusters; the deposit is dominated by poorly to
moderately vesicular mafic glass fragments; it becomes sandier and better stratified
upward, with laminations draping the cobbles and boulders in the top 30 cm; the
coarse clasts lack sag structures but they penetrate laminations; the stratification
onlaps bed 3 before bed 4 wedges out to the south; and
(5) c. 16-18 m of rust-coloured, thinly bedded very coarse volcanic sandstone and granule
conglomerate, in beds 1 cm to a few dm thick separated by mm to 1 cm-thick beds of
parallel laminated silt. Beds are laterally continuous on a scale of 30 m (outcrop
width) and some show amalgamation. Outsize RBG blocks up to 1 m in diameter are
common and conspicuous and lack impact structures, and there are nests of
imbricated platy RBG metasandstone. Higher up, stratification is coarser and less
obvious and there are more numerous lenses, up to 2 m thick and 10 m long, of closepacked angular RBG blocks. Traced laterally to the south, sandstone beds wedge out
by lapping onto the basal diamicton but others higher in the section are essentially
slope parallel. Slumping affecting several metres thickness of sediments, is
particularly prominent in the upper part of the section. The upper section outcrop rises
upslope to the south where it becomes dominated by sand-matrixed, coarse, platy,
blocky RBG gravel.
Lithofacies descriptions and interpretations
The volcanic outcrops in the HVP have been divided into eight primary volcanic
lithofacies and six sedimentary lithofacies. Two broad compositional groups are
distinguished. They are referred to here as mafic, which also includes basalts, basanites,
hawaiites, mugearites and tephriphonolites (collectively designated “m” in the lithofacies
codes used below), and felsic, which includes trachytes and (less common) rhyolites (“f” in
the lithofacies codes). Figure 1 illustrates the types and proportions of the major lithofacies
present in the major outcrops, and selected lithofacies are illustrated in Figure 5. Table 1
summarizes the lithofacies notations used, and indicates which lithofacies are illustrated.
Detailed descriptions and interpretations are provided below.
Volcanic lithofacies
Coherent mafic lava (lithofacies mL)
Two types are distinguished, both common in almost every outcrop visited or
observed. The first (designated mLs) is mid-grey fine to coarsely crystalline mafic lava with
coarsely spaced irregular or polygonal joints. The lithofacies forms relatively thin (typically
0.5-1 m, seldom exceeding c. 2 m) planar horizontal to gently dipping sheets and lenses that
alternate with a similar or lesser thickness of grey or maroon breccia (lithofacies mBm(cr);
Fig. 5a). The second lithofacies (mLj; Fig. 5b) is typically darker grey, fine-grained to
aphanitic and poorly to non-vesicular, often with glassy rinds 1 cm thick. The mLj lavas form
sheets up to 20 m long, or have very irregular shapes. Thicknesses of a few metres are
common (up to c. 50 m) and some are megapillows measuring 2-4 m in diameter. Joints are
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well developed and more conspicuous than in lithofacies mLs; they are closely spaced and
typically blocky in appearance. Some joint surfaces show up to 3 successive chilled (glassy)
zones (i.e. multiple chilled surfaces). Less commonly narrow curviplanar prismatic columns
of entablature type are present, rarely associated with a much thinner basal colonnade of
coarser-spaced columns. The lithofacies often transforms around its margins, through a zone
of intense jigsaw fracturing, into monomict aphanitic to glassy breccia (lithofacies mBm(gl)).
Lithofacies mLs, with its crystalline nature, coarsely developed jointing (cooling
fractures) and close association with relatively thick maroon (oxidised) autobreccia
(lithofacies mBm(cr)) consists of subaerially emplaced mafic lava flows of aa type. The lavas
show no evidence for rapid chilling (e.g. glass, blocky jointing, entablature) caused by
interaction with water, and essentially dry conditions and emplacement on gently dipping
volcano slopes are inferred. By contrast, the much finer grain sizes, glassy rims and patterns
of cooling fractures (blocky joints; entablature) in lithofacies mLj and association with
hyaloclastite lithofacies mBm(gl) & mBm(bl)) are unequivocal evidence for more rapid
cooling than lithofacies mLs. Wet conditions are inferred, with multiple chilled surfaces
forming by intermittent progressive fracturing along joint surfaces due to ingress of water (cf.
Kawachi and Pringle, 1988), analogous to chattermarks on polygonal cooling-joint surfaces
(cf. DeGraff et al., 1989; Grossenbacher and McDuffie, 1995).
Coherent felsic lava (lithofacies fL)
The coherent felsic lava lithofacies is uncommon and was only visited at Mandible
Cirque (T5.2 & 14), where it forms a spectacular succession at least 900 m thick, and at the
base of locality T5.20 (Cotter Cliffs, E Hallett Peninsula), although other examples were
observed during helicopter fly-pasts of E Hallett Peninsula. The lithofacies is invariably
encased in monomict felsic lava breccia (lithofacies fBm). It comprises high and moderateaspect ratio (i.e. height to width) platy-jointed sheets of felsic lava measuring a few tens of
metres in thickness and c. 50–200 m in length. The outcrops are variably grey, cream or
brown and are often thinly foliated, with foliations locally deformed into folds on a dm scale,
including isoclinal. The foliation wraps around the lava outcrops. In thin section, the lava
varies from aphanitic, with prominent spherulitic, snowflake (i.e. micropoikilitic) and
granophyric textures, to finely crystalline. The crystalline examples are also rarely associated
with foliated obsidian margins or layers.
Lithofacies fL forms a series of superimposed felsic lavas, which are interbedded with
felsic lava breccia (fBm(cr) & fBm(gl)) at locality T5.2 and by other lithofacies at localities
T5.14 and T5.20. The variable aspect ratios suggest that domes, lavas and/or coulées might
be present (collectively called sheets here) but they were observed in cross section only
(strike sections). The sugary textures are present in examples showing spherulitic, snowflake
or granophyric textures, and are indications of devitrification of felsic glass, probably at
relatively warm temperatures (> 200-300°C) and the perlite is consistent with a wet
environment (Lofgren 1971a,b; De Rosen-Spence et al. 1980; McPhie et al., 1993; Akay and
Erdogan 2001; Tuffen et al., 2008). The crystalline examples lack signs of any original glass,
consistent with slower cooling, although those examples are also sometimes associated with
obsidian margins or layers. The eruptive environment is not clearly defined by features of the
lithofacies itself but can be inferred from associated breccia lithofacies (fBm, below).
Massive mafic breccia, clast-supported (lithofacies mBm)
Two types of massive clast-supported mafic breccia are distinguished and are as
common as lithofacies mLs and mLj, which they accompany. In the first (mBm(cr)), the
angular clasts are grey and red (oxidised), mainly 1-5 cm in diameter but varying up to 20
cm, and highly vesicular to non-vesicular with a variable amount of very coarse sand-size
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matrix (Fig. 5a). The second breccia type (mBm(gl); Fig. 5b,c,d) consists mainly of darker
grey finely crystalline, aphanitic and glassy blocky-angular clasts a few mm to 2 cm in
diameter, mainly non-vesicular, and up to 25 % larger clasts (typically up to 15 cm, some up
to 50 cm); locally < 5 % of lapilli-sized clasts are red in colour and vesicular and may be
accompanied by more coarsely crystalline lava fragments (Fig. 5c). Pillows may also be
present. mBm(gl) is typically chaotic, associated with abundant irregular lava masses and rare
megapillows (lithofacies mLj) or else shows crude metres-thick large-scale planar
stratification defined by the parallel orientation of interbedded short lava sheets and lenses. In
strike sections, the stratification is horizontal, whilst in rarer dip sections, it is inclined gently
(< 10°) and oversteps onto a sharp basal surface. The third breccia type (mBm(bl)) comprises
angular glassy lapilli and blocks mingled with massive lapilli tuff in a prominent zone 1 m
wide surrounding mafic intrusions (lithofacies mLj) in stratified lapilli tuffs (lithofacies LTs).
The crystalline nature and often-prominent oxidation colours seen in lithofacies
mBm(cr), and gradational contacts with crystalline lava (mLs) indicate that they are
subaerially emplaced autobreccias associated with aa lava. By contrast, the predominantly
aphanitic to glassy fines-free breccias with dispersed pillows (lithofacies mBm(gl)) are
hyaloclastite breccias (sensu White and Houghton, 2006) associated with emplacement of
water-cooled lavas (lithofacies mLj), with which they are in gradational contact and from
which they formed by mechanical spalling and likely local steam explosions (cf. Kokelaar,
1986). The coarse scale of the crude homoclinal stratification seen in dip sections, with beds
extending several tens of metres and overstepping a sharp basal surface, is suggestive of the
large-scale foresets in hyaloclastite breccias of lava-fed deltas, although there are important
differences too (e.g. presence of oxidised and crystalline vesicular clasts, generally more
chaotic nature and greater proportion of associated lavas). The third breccia type (lithofacies
mBm(bl)) is clearly associated spatially and genetically with intrusive lava sheets in stratified
lapilli tuffs, and is interpreted as blocky peperite (cf. Skilling et al., 2002). Stratification in
the associated lapilli tuffs is absent within the peperitic zone, suggesting that it has been
destroyed, probably by associated highly localised hydrothermal activity caused by intrusion
of the lava into the lapilli tuffs while they were wet.
Stratified mafic lava breccia (lithofacies mBs)
This lithofacies is a less common, more obviously stratified version of lithofacies
mBm(gl), mostly comprising fines-free fine to coarse gravelly hyaloclastite breccia with
crude dm- to a few metres-thick beds that extend a few metres to a few tens of metres downdip and may show wavy deformation (Fig. 5e). Bed surfaces are rarely well defined and the
beds may show narrow reverse graded bases and/or normal-graded tops. Khaki-yellow
discoloration of clast rims is locally prominent and minor red (oxidised) and crystalline
vesicular lapilli are present similar to mBm(gl).
The crude stratification and grading in fines-poor gravelly breccia, and close
association with interbedded water-cooled lava and coarse hyaloclastite breccia (lithofacies
mLj and mBm(gl)), suggest a subaqueous setting, remobilisation of unconsolidated in situ
hyaloclastite breccia and transport as concentrated or hyperconcentrated density flows
(Mulder and Alexander, 2001). The finer breccias beds are often in a structural position
analogous to distal run-out deposits linked up-dip to coarser hyaloclastite breccia, similar to
bottomset beds in lava-fed deltas, despite the presence of crystalline and/or oxidised clasts.
Massive felsic lava breccia, clast-supported (lithofacies fBm)
Two types of felsic lava breccia are present. The first (fBm(cr)) is dominated by
angular blocks of non-vesicular massive and foliated crystalline lava up to 40 cm across and
minor coarse sandy-gravelly matrix. Some examples also contain fragments of foliated
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obsidian and pumice. The lithofacies wraps around or forms a cap on coherent lava
(lithofacies fL), and it also occurs as patches or irregular apophyses up to 8 m long within the
lava. Contacts with coherent lava are gradational. The breccia is rarely deep maroon-brown
or yellow-coloured to top (Fig. 5j) but more commonly is the same colour as the associated
lava (i.e. pale grey or brown). The second type (fBm(gl)) is bright yellow-green to pale purple
in colour, dominated by angular black, lilac and yellow-green non-vesicular perlitised
obsidian blocks up to 30 cm across (mainly 0.5-5 cm; Fig. 5f). Contacts with adjacent
coherent lava (fL) are gradational to relatively sharp. The coarse sand/granule matrix is finesfree and formed of angular non-vesicular obsidian, usually showing well-developed perlite
textures. Some obsidians show mm-scale foliation and the breccia is intruded by numerous
irregular apophyses of foliated sugary-textured rock sourced in the associated lava sheet (fL).
The yellow-green coloration is due to pervasive clay alteration of the perlitised gravelly
clasts. Rare crude discontinuous breccia beds dip steeply away from adjacent lava flow
fronts. Uncommon irregular layers of breccia mingled with sandstone are interstratified with
sandstones for a few metres beneath some lavas.
Both fBm(cr) and fBm(gl) are autobreccias related to associated felsic lava sheets.
However, the crystalline nature of the clasts, rare maroon coloration and pumice, sandy—
gravelly matrix and absence of evidence for water chilling in fBm(cr) indicate that it formed
subaerially. Pumice is scarce and might have been explosively erupted ahead of the degassed
lava and caught up in the sheet effusion rather than being a frothy lava top. By contrast,
evidence for water chilling is abundant in the glass-dominated fBm(gl), which is a felsic
hyaloclastite. The associated perlitisation suggests that the lava interacted with water whilst
still hot (McPhie et al., 1993; Davis and McPhie, 1996; Tuffen et al., 2008). The rare bedding
suggests avalanching of oversteepened piles of clasts at subaqueously-emplaced lava flow
fronts. The presence of layers of hyaloclastite breccia within stratified sandstone has two
possible origins: (1) as avalanches of basal autobreccia interbedded with coeval sandstones
by resedimentation; or (2) water trapped in unconsolidated sands beneath an advancing lava
sheet has flashed to steam, leading to intimate intermingling of glassy fragments and sand
grains. We favour the first explanation, i.e. coeval deposition/redeposition of breccias and
sediments ahead of an advancing lava, because: the breccia layers have a sandy matrix
identical to the enclosing sediments; relatively fine stratification in the latter is not disrupted
as would be expected by an origin by vigorous intrusion during water flashing to steam
within wet sediments); an origin linked to steam flashing and intrusion would also cause
breccias/sediment mixtures to penetrate up into the overlying lava (cf. bulk interaction
explosivity of Kokelaar (1986); see also Smellie et al., 1998, fig. 9), which was not observed;
and isolated clasts of glassy felsic lava are present “floating” in layers of the sediment
implying co-deposition, not intrusion.
Massive felsic breccia, matrix-supported (lithofacies fBmm)
White, cream and grey-green-coloured beds 1-20 m thick were examined at a single
locality: T5.20 (northern Cotter Cliffs, E Hallett Peninsula). The beds alternate with stratified
felsic lava-sourced volcanic sandstone and granule conglomerate (lithofacies Ss), have sharp
surfaces and are formed of abundant blocky, non-vesicular to poorly microvesicular,
aphanitic to finely crystalline lava clasts up to 35 cm in diameter and up to c. 20 % fine—
medium tuff matrix (Figs 5g,h). The thickest deposit, which may extend at least 150-200
metres along strike, locally shows variations in coarse-clast concentration that mimic bedparallel colour variations and are suggestive of a crude internal planar stratification. The
thinner beds have reverse-graded bases and normal-graded tops and are associated with local
reddish-brown discoloration extending up to 10 cm into the adjacent sedimentary beds
(lithofacies Ss; Fig. 5h).
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The association of monomict aphanitic felsic lava blocks and lesser fine matrix, and
discoloration of the tops of underlying beds suggest an origin as block and ash deposits
linked to collapses of local domes and/or lava flow fronts, whilst the reddish-brown
discoloration of adjacent bed surfaces possibly suggests deposition while still hot.
Stratified lapilli tuff (lithofacies LTs)
This lithofacies includes both mafic and felsic units. They frequently form steep conelike edifices, now exposed in cross-section (e.g. Hamilton, 1972, fig. 28D). The lithofacies is
composed of indurated grey-brown to yellow and khaki-yellow stratified lapilli tuff, in thick
deposits many tens to a few hundred metres thick (e.g. basal outcrop at Redcastle Ridge;
Cape Jones) and as much thinner (< few tens of metres) conspicuous pale layers heavily
intruded by dark grey mafic lava (lithofacies mLj; e.g. Cape Phillips; cf. Hamilton, 1972, figs
28E-H). Grain size is mainly 1 mm to 1.5 cm, comprising poorly to highly vesicular mafic or
felsic glass and tachylite, with c. 5 % larger lithic lapilli and blocks (polymict, angular, rarely
abraded fine-grained lavas, some oxidised) up to 80 cm in diameter rarely associated with sag
structures. Beds are c. 1 dm thick, massive or planar laminated and are crudely to well
defined, extending laterally up to 20 m. They are poorly to moderately sorted depending on a
variable proportion of fine tuff matrix (palagonite altered), often wavy-deformed with locally
numerous minor faults. Felsic counterparts were visited only at T5.19 and T5.20 (both
northern Cotter Cliffs) and Cape Phillips. At T5.20, the lapilli tuffs are interbedded with
massive felsic lava breccia (lithofacies fBmm).
The abundance of blocky-angular glass grains, their very variable vesicularity and
relatively fine grain size, palagonite alteration, variable sorting, planar fine stratification and
sag structures, are characteristic of products of phreatomagmatic explosions involving
variable water/magma ratios (e.g. Houghton et al., 1999). Interaction with groundwater or
surface water (i.e. seawater or lakes (pluvial or glacial)) is a prerequisite and the scarcity or
absence of non-juvenile clasts indicate that explosions took place high in the vent,
presumably in the tuff cone itself or shallow prevolcanic basement, consistent with a watersaturated edifice (e.g. Sohn 1996; White et al., 2003). The products are mainly fine-grained
pyroclastic density current deposits and probably some minor fall.
Stratified mafic scoria and bomb deposits (lithofacies mLP(b))
Monomict planar stratified deposits form a pale to dark pink-coloured bisected conelike outcrop at the base of the crags at Cape Phillips. The stratification is parallel to the
outward-dipping surfaces of the outcrop and is formed of alternating coarse and fine
moderate to highly vesicular scoria and 10-40 % bombs, some with sags. Many clasts are
maroon coloured. Clasts are mainly a few mm to 1 or 2 cm in diameter. Beds are crudely
defined, clast-supported, steep-dipping (44°), up to a few dm thick and extend down-dip < 10
m. Some beds up to 2 m thick are formed almost entirely of dense intact and fragmented
bombs. A few small granitoid clasts are present.
The steep-dipping clast-supported fines-free gravel-grade beds composed of oxidised
finely crystalline scoria and bombs probably formed by avalanching grain-flows during
construction of a subaerial cinder cone, which is exposed in cross section. The alternation
between scoria and bomb beds suggests switching between Hawaiian and Strombolian
eruptive conditions (cf. Houghton et al., 1999).
Sedimentary lithofacies
Massive diamicton and diamictite (lithofacies Dmm)
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Polymict diamicton and diamictite form basal massive beds 1-4 m thick overlying
local basement at two localities (W Edisto Inlet (Herschel Tuffaceous Moraine) and Tucker
Glacier; Fig. 5i). The basement surfaces show prominent striations, some with chattermarks,
and are glacially smoothed. The lithofacies is a chaotic assemblage of coarse clasts dispersed
in up to 60-70 % white clay matrix. The clast types reflect the local basement lithology and
comprise Palaeozoic Robertson Bay Group (RBG) metasediments and granodiorite (at W
Edisto Inlet and Tucker Glacier, respectively). The clasts are angular to subrounded (mainly
subangular), a few mm to 35 cm in diameter. They are commonly facetted and rarely striated.
In places, the diamict rarely contains ill-defined trains of coarse clasts that are sometimes
truncated by shear-like joints (Fig. 5i). A second distinctive type of massive diamicton, which
overlies massive white clay-matrixed diamicton at W Edisto Inlet, forms a discontinuous
brown layer 1-3 m thick at W Edisto Inlet, comprising angular pebbles, cobbles and blocks of
RBG metasediments up to 60 cm in diameter variably close-packed and dispersed in up to 70
% silty—fine sandy matrix. The matrix is mainly formed of cuspate and blocky poorly
vesicular mafic glass.
The white clay-matrixed massive diamict lithofacies, with facetted and striated
abraded clasts, internal shear planes, rare boulder trains and association with striated and
glacially moulded basement surfaces, is interpreted as basal tillite (lodgement), deposited
beneath wet-based (erosive) glaciers that were strongly coupled to their bed. Conversely, the
angularity of the coarse basement-derived clasts in the brown-coloured massive diamicton at
W Edisto Inlet does not suggest derivation and transport of clasts beneath a glacier and is
more consistent with a supraglacial depositional setting. However, the mafic glass in the
matrix occurs as individual shards. They were not derived by erosion of a lithified deposit
and presumably formed during coeval explosive phreatomagmatic eruptions. Therefore, we
suggest that a glacier surface was locally covered by angular blocky supraglacial moraine,
then draped by fresh ash from a coeval mafic eruption. Both deposits were subsequently
mixed together and redeposited in a meltwater lake (cf. overlying lithofacies Ssg) during one
or more debris flow events as the glacier surface decayed.
Stratified diamictite (lithofacies Dms)
Stratified polymict diamictite 8 m thick is prominent at Mandible Cirque (T5.14). It
sharply overlies maroon-brown massive felsic lava breccia (lithofacies fBm(cr); Fig. 5j). The
deposit is yellow in colour and contains clasts of grey and maroon-brown abraded felsic lavas
up to 2 m in diameter (mainly < 20 cm) that define a weak, crude, discontinuous planar
stratification. They are set in up to 70 % yellow silty—sandy glass-rich matrix with a wavy
fabric formed of flattened and undeformed felsic pumice and shards. The lithic clasts are
angular—subangular, and some protrude from the bed top.
The crude stratification and protruding bed-top clasts in this diamictite lithofacies
indicate multiple depositional events and significant matrix strength, respectively. It probably
formed by deposition from pulsing or multiple debris flows. The sequence immediately
above that with the diamict contains a prominent erosional unconformity with “U”-shaped
valleys and water-rich terrestrial conditions consistent with a glacial setting. The abraded
felsic lava-derived lithic debris is similar to material carried at the base of a wet-based glacier
and melted out, reworked and redeposited either in a subglacial or proglacial setting near an
ice front. The abundant glass in the matrix indicates that felsic eruptions were coeval with the
glacial environment.
Stratified volcanic sandstone and granule-pebble conglomerate (lithofacies Ss)
Eight metres of stratified, polymict, yellow-coloured gravelly sandstones and granule
conglomerate rich in fine sand matrix are also present at Mandible Cirque (T5.14) above the
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stratified diamict (lithofacies Dms; Fig. 5k), whereas they form much thinner, more sand-rich
and beds (c. 1 m thick) at northern Cotter Cliffs (T5.20). They are dominated by subangular
fragments of non-vesicular finely crystalline felsic lava, including pumice and obsidian, but
also contain a minor proportion of crystalline mafic lava and (at T5.14) metamorphic
basement clasts. Abrasion of clasts is common and ubiquitous. Fiamme-like flattened
pumices are also locally conspicuous at T5.14, up to 8 cm long and 1 cm thick and parallel to
bedding. Planar stratification is prominent but crude. At T5.14, the stratification also occurs
as dipping discontinuous pebble trails that prograde across internal non-erosive surfaces and
some of the sediment packages a few dm thick become more prominently planar stratified
upward and look cyclic (Fig. 5k).
The polymict nature, ubiquitous clast abrasion and stratification characteristics
suggest that the Mandible Cirque beds were transported by traction currents and/or
hyperconcentrated flows sourced in a coeval felsic volcanic provenance. It is unclear if the
beds are volcanic or sedimentary. The dipping stratification resembles foresets of sandwave
bedforms, driven either by dilute pyroclastic density currents or prograding in a fluvial
channel, although the pumices must have been waterlogged to be transported as fluvial
bedload. The presence of fiamme need not imply a pyroclastic origin for the host sediment.
Although fiamme are most commonly recognised in welded pyroclastic facies, including fall
deposits and those of pyroclastic density currents, fiamme may have a variety of origins
including diagenetic (e.g. Bull and McPhie, 2007). From the presence of associated
undeformed pumices, it is likely that fiamme formation was by local compaction rather than
hot welding, perhaps as a consequence of loading by overlying coeval felsic lava domes.
Laminated siltstone and sandstone with dropstones (lithofacies Ml(d))
Up to 1 m of grey-brown weakly laminated fissile siltstone with cm-thick medium to
coarse sandstone layers showing wavy deformation occurs in the W Edisto Inlet outcrop. It
also contains scattered angular pebble- to boulder-sized lonestones of RBG metasediment
that sometimes occur in mud-matrixed clusters. The outsize clasts penetrate the laminations,
although sag structures are poorly seen, and the laminated sediment drapes them in turn. The
deposit becomes sandier and better stratified upward and passes gradually up into planarbedded sandstones (lithofacies Ssg). The sandstones are formed mainly from poorly to
moderately vesicular mafic glass fragments with cuspate to blocky shapes, and some
tachylite.
The sandstones are dominantly vitroclastic, formed from fine mafic tephra generated
in phreatomagmatic eruptions and resedimented. The outsize clasts are angular and none
show striations, polish, ice moulding or other textural evidence for glacial erosion, suggesting
that they have not been transported as basal glacial debris (cf. Dowdeswell et al., 1985; Benn
and Ballantyne, 1994). The combination of thinly interbedded siltstone and sandstone layers,
dispersed lonestones and discrete mud-matrixed clusters of lonestones suggests a
combination of fine material dropping out from suspension, rain-out from floating ice
(including dropstones and lumps of till?) and sedimentation by sediment gravity flows
(turbidites). Similar deposits are found in ice-marginal lakes and glaciomarine settings,
although glaciomarine deposits typically contain biogenic material (especially diatomaceous
ooze) and show abundant bioturbation (e.g. Hambrey 1994; Benn and Evans, 1998). Both are
absent here and a (glacio-)lacustrine setting is favoured, consistent with interpretations of
other lithofacies (Dmm, Dms, Ssg and Bcp) in the same section.
Graded-stratified volcanic sandstone and granule conglomerate (lithofacies Ssg)
Rust-coloured, thinly bedded very coarse volcanic sandstone and granule
conglomerate form a sequence c. 18 m thick at W Edisto Inlet (Fig. 5l). The sequence has a
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wavy slightly erosional base on laminated siltstone (lithofacies Ml(d)) and is interbedded with
lenses of cobbly—blocky breccia (lithofacies Bcp). The beds are massive to less commonly
normal graded, often with sub-central pebble concentrations, and are 1 cm to a few dm thick
separated by mm to 1 cm-thick beds of parallel laminated fine sand—silt. Individual beds are
laterally continuous on a scale of 30 m (outcrop width) and amalgamation is common. The
grains are dominated by palagonite altered mafic glass and tachylite, mainly non- to
moderately vesicular, rarely highly vesicular, and showing slight abrasion. Minor subangular
basement clasts (fine-grained quartzo-feldspathic and phyllitic metasediments (RBG) are also
present. Large (metre-scale) overfolds verging downslope are locally present and are
depositionally overlain by other lithofacies (Bcp).
The continuous bedding, sharp erosive bases, normal grading, abraded clasts and
association with thin planar laminated siltstone caps are characteristics of sediment deposited
from cohesionless turbulent sediment gravity flows in ponded water (a likely meltwater lake).
The graded sandstone beds resemble sandy turbidites (Ta, Ta-b; Lowe, 1982), whilst the
presence of massive coarser sandy—gravelly beds, with or without laminated caps and
showing normal grading confined to their upper parts suggest that a sediment concentration
threshold was exceeded and they were deposited from hyperconcentrated flows (cf. Mulder
and Alexander, 2001). The folds observed are synsedimentary and attest to steep depositional
surfaces and occasional sediment slumping.
Clast-supported polymict breccia (lithofacies Bcp)
Seen at a single locality (W Edisto Inlet), this lithofacies comprises lenses and
discontinuous beds of clast-supported cobbly—bouldery breccia formed of non-volcanic
basement clasts (RBG) up to 80 cm across interbedded with beds of volcanic sandstone
(lithofacies Ssg) and with minor sandy matrix of the same. The deposits become more
abundant in higher parts of the outcrop, towards the underlying basement surface. Imbricated
and bed-parallel platy clasts are locally present. The bedding is steep and essentially parallel
to the underlying bedrock surface (c. 30-40°). Isolated outsize RBG blocks up to 1 m in
diameter are common and conspicuous in the associated sandstone beds (Ssg). They mainly
lack impact structures and are regarded as a single-clast end member of the lithofacies; a few
have sag structures (cf. Fig. 5l).
As in lithofacies, Ml(d), the coarse clasts lack any obvious evidence for glacial
erosion. They thus appear not to have been transported as basal glacial debris. The abundance
of angular clast-supported blocks of the local basement, in steep-dipping lenses and short
sheets, is consistent with formation as mass flow deposits transported by grain flow during
short-lived avalanching events on steep slopes (cf. Lowe, 1982; Mulder and Alexander,
2001). The breccia lenses are interbedded with sandstone turbidites (lithofacies Ssg) in a
sequence with basal tillite (lithofacies Dmm) and a glacially modified basement surface. They
are interpreted as subaqueously deposited scree deposits that accumulated in an ice-marginal
lake. The cobbly-bouldery isolated single clasts might be dropstones, although sag structures
are very rare and indistinct. However, the coarse grain size of the host sediment (very coarse
sand- and granule-grade) might have prevented recognisable sags developing more widely.
Moreover, because of the steep underlying slopes, they might also have tumbled and rolled
into position and simply represent runout of subaqueously emplaced far-travelled single
clasts related to the scree collapse events that formed the associated breccia lenses and
interbeds.
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Figure captions
Figure 1. Summary sketch vertical sections of the individual sequences examined at selected
localities in the Hallett Volcanic Province. The differentiation into local units, each
separated by a sequence boundary, and (in (b)) numbered beds, are also shown. A
proposed correlation between T5.25 and T5.28, NW Hallett Peninsula, is also shown and
the local units are numbered accordingly.
Figure 2. View of Cape Jones with photo-interpretation of the main geological local units
(LUs). Only LUs 1-3 were examined directly. The shaded unit within LU 2 is intrusive
mafic lava coeval(?) with LU2. The cliff face is c. 300 m high.
Figure 3. Photo-interpretive view of Cape Phillips, looking northwest, with the main
geological local units distinguished. Only the basal 3 LUs were examined directly. The
boundaries of most of the individual LUs are parallel and sub-horizontal but some wedge
out laterally. Most surfaces appear to lack any evidence for erosion but some (e.g. base of
LU5 and LU7) appear locally erosive. However, the sequences lack glacial and fluvial
deposits. Note the prominent cone-like shape of LU1 (a well-preserved cinder cone). The
cliff face is c. 450 m high.
Figure 4. Sketch section of Redcastle Ridge looking east, showing the principal geological
local units and their relationships. The section is about 3.5 km long and rises 500 m from
left to right. At least three eruptive centres are present and become younger in a southerly
direction. The youngest appears to be continuous with Mt Harcourt. Each centre is
composed of a lower chaotic section of water-cooled breccia and lava (lobe-hyaloclastite)
and an upper section of relatively continuous, gently convex dipping, planar subaerial
sheet lavas. The stratigraphically lowest outcrop, at the northern tip of the ridge (T5.26),
is a tuff cone sequence. It may represent the core of the lowest shield volcano or else is an
unrelated smaller and older centre.
Figure 5. Photographs showing selected lithofacies in the Hallett Volcanic Province. Mafic
lithofacies: (A) subaerial lava and oxidised scoriaceous autobreccia (lithofacies mLs &
mBm(cr); T5.28); notebook is 17 cm long; (B) typical massive association of irregular
water-cooled lava masses and hyaloclastite breccia (lithofacies mLj & mBm(gl); T5.1);
rock face is c. 25 m high; (C) massive hyaloclastite breccia (mBm(gl)) with admixed
vesicular oxidised clasts (above pencil; T5.20); pencil is c. 15 cm long; (D) massive
hyaloclastite breccia (mBm(gl)) lacking oxidised clasts (T5.28); pencil is c. 15 cm long;
(E) stratified fine gravelly hyaloclastite breccias (mBm(gl)), viewed looking
approximately toward source (i.e. strike section; T5.16); pencil is c. 15 cm long. Felsic
lithofacies: (F) blocky felsic hyaloclastite breccia (fBm(gl)) underlying felsic lava (not
seen) and overlying stratified sandstone (Ss; lower left side; T5.14); notebook is 17 cm
long; (G) coarse ash-poor felsic breccia (lithofacies fBmm); top of thick block and ash
deposit; notebook is 17 cm long (T5.20); (H) block and ash deposit (fBmm), showing
crude reverse and normal grading to base and top, respectively; discoloration of the
adjacent sandy conglomerate beds (lithofacies Ss) might indicate hot emplacement of the
block and ash bed; the hammer is 40 cm long (T5.20); Sedimentary lithofacies: (I)
polymictic basement clasts weathered out of white muddy diamictite, showing prominent
abrasion and facetting (lithofacies Dmm; T5.33); pencil is c. 15 cm long; (J) crudely
stratified polymict diamictite (lithofacies Dms) resting unconformably on reddish felsic
lava autobreccia (fBm(cr); T5.14); notebook is 17 cm long; (K) finely stratified
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sandstones prograding across a basal reddish-coloured conglomerate (lithofacies Ss;
T5.14); notebook is 17 cm long; (L) Stratified sandstones (lithofacies Ssg) deformed by
impact caused by large lonestone (dropstone; left side); pencil is 4 cm long (T5.27).
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Table 1. Lithofacies notation and summary lithofacies descriptions for late Miocene volcanic and sedimentary outcrops in the
Hallett Volcanic Province.
Lithofacies1
Primary volcanic lithofacies
(monomict, volcanogenic)
Mafic sheet lava
Felsic sheet lava
Massive mafic breccia, clast-supported
Stratified mafic breccia
Lithofacies
Code2
mL
fL
mBm
mBs
Characteristics
Illustration
Two types: mLs - fine to coarsely crystalline
lava sheets; coarse jointing;
mLs - Fig. 5A;
mLj - finely crystalline and/or aphanitic lava
sheets and irregular masses; glassy rims;
blocky & hackly jointing, entablature
Grey, cream or brown crystalline lava with
conspicuous sugary texture; relatively thick
(typically few tens of metres) with high to
moderate aspect ratio (thickness to width);
often finely foliated; foliation may show
strong deformation (tight folds, including
isoclinal); non-vesicular
Three types: mBm(cr) - fine to coarsely
crystalline clasts, grey or red (oxidised);
mLj - Fig. 5B
mBm(gl) - predominantly aphanitic and/or
glassy clasts, sometimes associated with intact
and/or broken lava pillows; crystalline and red
(oxidised) clasts also present, sparse to locally
abundant;
mBm(gl) - Fig. 5B,C,D
mBm(bl) - coarse fragments of blockyangular non-vesicular glass in lapilli tuff
matrix; found within 1.5 m of margins of
intrusive lava (lithofacies mLj)
As mBm(gl), but in crude planar beds dm to m
thick, massive or with reverse graded bases
and/or normal graded tops; laterally
discontinuous
mBm(cr) - Fig. 5A;
Fig. 5E
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Massive felsic breccia, clast-supported
Massive felsic breccia, matrix supported
Stratified lapilli tuff
Stratified mafic scoria & bomb deposits
Sedimentary lithofacies
(polymict; abraded clasts common)
Massive diamicton/diamictite
Stratified diamictite
Stratified volcanic sandstone and granule-
fBm
fBmm
LTs
mLP(b)
Dmm
Dms
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Northern Victoria Land Late Miocene glaciovolcanic sequences
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Two types: fBm(cr) - fine to coarsely
crystalline clasts; and fBm(gl) - predominantly
glassy clasts
Abundant blocky non-vesicular to poorly
vesicular aphanitic felsic clasts; up to c. 20%
fine-medium glassy tuff matrix; beds 1-20 m
thick; reverse graded bases and normal graded
tops; local discoloration of associated
sedimentary beds
Planar stratified yellow lapilli tuff; mafic and
felsic deposits; small proportion of accidental
and accessory clast; clasts mainly blocky
variably vesicular glass (mafic) or pumice
(felsic); variable fine tuff matrix; commonly
intruded by very irregular sheets of mafic lava
(lithofacies mLj), which may bake and
discolour (redden) adjacent tuffs
Monomict stratified pink—red lapilli
deposits; bisected cone outcrops with radially
outward-dipping beds; highly vesicular scoria
and bombs; rare small granitoid clasts (at
T5.32)
fBm(g) - Fig. 5F
Polymict, massive; two types Dmm - white
clay matrix; abraded (incl. striated & facetted)
non-volcanic clasts; crude trails of coarse
clasts; internal shear surfaces; overlies
striated/polished/moulded basement bedrock;
Dmm - Fig. 5I
Dmm(gl) - brown diamict with angular
basement clasts and abundant fine mafic glass
matrix
Polymict; weak stratification; abraded felsic
lava clasts; matrix rich in felsic pumice and
shards
Thin planar stratification; felsic; abraded
Fig. 5G,H
Fig. 3 (unit 1)
Fig. 5J
Fig. 5K
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pebble conglomerate
Laminated siltstone & sandstone with
dropstones
861
862
863
864
865
866
Ml(d)
Graded-stratified volcanic sandstone and
granule conglomerate
Ssg
Clast-supported polymict breccia
Bcp
Northern Victoria Land Late Miocene glaciovolcanic sequences
Bull.Volc.
clasts; rare basement clasts and fiamme-like
pumice (T5.14)
Weakly laminated fissile siltstone with cmthick coarse sandstone layers; angular
lonestones with rare sags
Massive coarse sandstone and lesser parallel
laminated fine sandstone; dominated by
vesicular blocky mafic glass; continuous
massive and graded beds; common
amalgamation
Cobbly and boulder non-volcanic breccias;
lenses and single clasts (no sags); interbedded
volcanic sandstone (lithofacies Ssg);
imbrication common; steep bedding
Fig. 5L
1 - “mafic” includes basalt, basanite/tephrite, hawaiite, mugearite, tephriphonolite; “felsic” includes trachyte, rhyolite
2 – lithofacies codes: L – lava; B – breccia; LT – lapilli tuff; LP – lapilli; D – diamicton/diamictite; S – sandstone; M – mudstone (siltstone);
compositional prefixes: m – mafic; f – felsic; descriptive suffixes: m – massive; s – stratified; m – matrix supported; l – laminated; g –
graded; c – clast supported; p - polymict; suffix modifiers: (cr) – crystalline; (gl) – glassy; (bl) – blocky; (b) – bombs; (d) – dropstones
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