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A review of geological constraints on the pre-break-up position of the
Ellsworth Mountains within Gondwana: implications for Weddell
Sea evolution
MICHAEL
L. C U R T I S
& BRYAN
C. S T O R E Y
British Antarctic Survey, Natural E n v i r o n m e n t Research Council, High Cross, Madingley
Road, Cambridge, CB3 0ET, U K
Abstract: It has long since been recognised that the Ellsworth-Whitmore mountains
(EWM) crustal block possesses an anomalous structural and stratigraphic history relative to
its neighbouring West Antarctic crustal blocks, and the Transantarctic Mountains. This has
led to uncertainties in the original pre-break-up position of the EWM within Gondwana.
Positions vary from along the East Antarctic margin, west of the Pensacola Mountains, to
within the Natal embayment region between Africa and Antarctica. The original position of
the EWM within Gondwana has important implications, as its subsequent transposition has
to be accounted for during the tectonic evolution of the Weddell Sea region.
Several geological features have been identified within the EWM as potential constraints
on Gondwana reconstructions. These include a Grenvillian age basement devoid of mineral
reset ages; an apparently continuous stratigraphic succession from Cambrian to Permian
times; Middle-Upper Cambrian extension-related volcanic rocks; no Ross age deformation; and a dextral transpressive component to the Early Mesozoic Gondwanide
deformation. Based on a consideration of these key geological features, and comparisons
between the Ellsworth Mountains and the palaeo-Pacific margins of Gondwana, we
conclude that the EWM displays geological affinities with both the Antarctic and South
African margins, and that it was located outboard of both. A prerequisite of this conclusion
is that rotation and translation of the EWM must be included in models of early Weddell Sea
tectonic evolution.
The Ellsworth-Whitmore Mountains (EWM),
one of five main crustal blocks forming West
Antarctica (Dalziel & Elliot 1982; Storey et al.
1988a), is a large terrane, approximately 875 km
long by 500 km wide, which includes the Ellsworth and Whitmore mountains, and a number
of scattered hills and nunataks to the north of the
Thiel Mountains (Fig. 1). These crustal blocks
are generally accepted to have assembled in
their approximate present day positions, adjacent to East Antarctica, by Late Cretaceous
time following break-up of the Gondwana
supercontinent, during the Mid- to Late Jurassic
(Grunow et al. 1991). Recent geophysical data
(Hiibscher et al. this volume) support previous
suggestions (e.g. de Wit et al. 1988) that an
additional extended crustal block, the Filchner
block, may reside within the Weddell Sea
E m b a y m e n t (WSE) and must be taken into
consideration in any reconstruction of the West
Antarctic microplates within Gondwana.
Several positions have been proposed for the
pre-break up location of the E W M within
Gondwana. Schopf (1969) aligned the Ellsworth
Mountains with the Cape Fold Belt, adjacent to
the Coats Land coast of East Antarctica, to
complete the Triassic Gondwanide fold belt
which is present in South America, South Africa
and the Pensacola Mountains in Antarctica. This
approximate position (Fig. 2a) has been accepted by many authors (Clarkson & Brook
1977; Storey & Macdonald 1987; Dalziel &
Grunow 1992; Lawver et al. 1992; Goldstrand et
al. 1994; Dalziel et al. 1994), and is supported by
palaeomagnetic data (Watts & Bramall 1981;
Grunow et al. 1991; Grunow 1993;). Analyses of
the palaeomagnetic data result in elaborate
break-up models involving anticlockwise rotation of the E W M , after Gondwanide deformation and prior to Gondwana break-up, and
lateral displacement along the Antarctic margin
during Gondwana break-up and opening of the
Weddell Sea. A prerequisite of such reconstructions is that any model describing the tectonic
From Storey, B. C., King, E. C. & Livermore, R. A. (eds), 1996, Weddell Sea Tectonicsand
Gondwana Break-up, Geological Society Special Publication No. 108, pp. 11-30.
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on September 12, 2016
12
M.L. CURTIS & B. C. STOREY
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Fig. 1. (a) Location map of West Antarctica. Areas
shaded grey are beneath the 1000m bathymetric
contour, areas of black fill are mountain ranges and
major rock exposures discussed within the text. SR,
Shackleton Range; BGI., Beardmore Glacier; P,
Pensacola Mountains; H, Haag Nunataks. (b)
Detailed location map of the Ellsworth-Whitmore
mountains crustal block. Dashed line delineates the
boundaries to the EWM block, and the Haag
Nunataks block (HNB). Areas of black fill as above.
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evolution of the Weddell Sea must take into
account the tectonic rotation and lateral translation of the E W M block within the Weddell Sea
region prior to, or during the formation of the
present Weddell Sea floor. The n e e d to provide
such complex seafloor spreading histories is
avoided by the models of Wilson et al. (1989),
and Schmidt & Rowley (1986), who proposed
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PRE-BREAK-UP POSITION OF ELLSWORTH MTS
13
!-
E I I ~
Fig. 2. End-member models for reconstructed positions of the EWM block: (a) Within the Natal Embayment
(Dalziel et al. 1994); (b) Along the East Antarctic margin, west of the Pensacola Mountains (Schmidt &
Rowley 1986).
the EWM to be part Antarctica (i.e. west of the
present day Weddell Sea) by at least 240 Ma, and
in the latter case proposed that the EWM was
located outboard and to the west of the
Pensacola Mountains throughout the Neoproterozoic and Palaeozoic (Fig. 2b).
This paper examines geological data that may
constrain the pre-break-up position of the
EWM. Structural, tectonic and stratigraphic
data will be reviewed, in addition to new field
data collected by M.L.C. from the Heritage
Range of the Ellsworth Mountains during the
1993-94 austral summer (Curtis 1994).
Preeambrian-Cambrian boundary
To standardize this discussion and avoid ambiguity, a Precambrian-Cambrian boundary of
540 Ma is used throughout this manuscript (Odin
et al. 1983; Compston et al. 1992; Cooper et al.
1992; Isachsen et al. 1994). The formally
approved IUGS division of the Proterozoic Eon
into Palaeo-, Meso- and Neoproterozoic eras are
also used throughout (Plumb 1991). Stratigraphic names based on fossil data are quoted as
published.
Geology of the Ellsworth-Whitmore
Mountains Crustal Block
The geology of the EWM is dominated by a
Cambrian to Permian sedimentary succession
that is best exposed in the Ellsworth Mountains
themselves (Webers et al. 1992a). The sedimen-
tary rocks were folded during the Gondwanide
Orogeny, and intruded by Mid-Jurassic breakup-related granites (Millar & Pankhurst 1987;
Storey et al. 1988b). Compared to stratigraphic
and tectonic histories along other parts of the
Gondwanian margin, the geology of the Ellsworth Mountains possesses a number of enigmatic features that are inconsistent with the rest
of the margin. These features will be outlined in
the following sections for critical comparisons
with possible pre-break-up positions for the
EWM block.
Although in most cases the boundaries of the
EWM are clear, topographically defined features, the precise location of the southern
boundary to the EWM block is not. Craddock
(1983) placed the boundary to the immediate
north of the Hart Hills aligning it with major
elongate subglacial troughs, thus forming a
structurally homogeneous block defined by
deformed Palaeozoic metasedimentary rocks
possessing a NNW-SSE structural grain, the
so-called Ellsworth domain of Storey & Dalziel
(1987). Jankowski et al. (1983) suggested a
position running between the Hart and Stewart
hills. However, the shared NE-SW structural
grain of the metasedimentary rocks within the
Hart and Stewart hills, which is cross cut at
Pagano Nunatak, close to the Hart Hills, by a
post-tectonic Middle Jurassic granite of the
EWM suite, suggests that the Hart and Stewart
hills are part of the EWM block, forming the
Marginal domain of Storey & Dalziel (1987).
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14
M.L. CURTIS & B. C. STOREY
Therefore, the southern boundary to the EWM
block must run between the Stewart Hills and
the Thiel Mountains (Fig.lb), as suggested by
Storey & Dalziel (1987). This interpretation,
however, requires a major swing in the structural
grain of the EWM block from NNW to NE, to
have existed prior to the initiation of Gondwana
break-up and the emplacement of the Ellsworth-Whitmore Mountains granites.
E l l s w o r t h M o u n t a i n s : stratigraphy
The most anomalous geological feature of the
Ellsworth Mountains is the apparently conformable Middle Cambrian to Permian stratigraphy, representing 13km of sedimentation
without any significant unconformities (Webers
et al. 1992a). This is in marked contrast to
elsewhere along the palaeo-Pacific margin of
Gondwana where a major tectonothermal
event, the Ross Orogeny in Antarctica and the
Saldanian/Pan-Africian tectonism in southern
Africa, took place during Late Cambrian to
Ordovician times. Although a significant change
in lithofacies occurred in the Ellsworth Mountains at this time, where shallow marine deltaic
and carbonate-platform deposits of the Upper
Heritage Group (Webers et al. 1992a, b), were
rapidly replaced by siliciclastic, quartzitedominated shallow marine to fluvial sequences
of the Crashsite Group (Sp6rli 1992), no major
unconformity exists. The 3km thick Crashsite
Group is dated at its base as Upper Cambrian
(Shergold & Webers 1992), with an Upper
Devonian age (Boucot et al. 1967; Webers et al.
1992c) for its upper boundary, and is considered
to be a continuous succession. Collinson et al.
(1994) speculate the presence of a disconformity
within the succession, although no stratigraphic
evidence exists to date. Erosion surfaces (Sp6rli
1992), and fluvial conglomerates (Goldstrand et
al. 1994) are locally present along the Heritage
Group-Crashsite Group contact, although, the
boundary is dominantly conformable and often
gradational especially in the NW Heritage
Range. These localized stratigraphic features
have been cited as evidence for the existence of a
disconformity related to the Ross Orogeny
(Goldstrand et al. 1994). Most workers agree
that only one phase of Early Mesozoic deformation exists (Craddock 1972; Webers et al.
1992a; Sp6rli & Craddock 1992; Curtis 1994).
Yoshida (1982) suggested that low grade metamorphism plus cleavage and fold formation
occurred in Mid- to Late Cambrian times, and
more recently, Thorstenson et al. (1994) described localised small-scale, pre-Gondwanian
folds of uncertain structural age and affinity
within the Late Cambrian Springer Peak Formation. However, the conformable relationship
between the Heritage and Crashsite groups,
would appear to preclude the presence of a
major Late Cambrian unconformity, and hence
the presence of significant Ross age deformation, supporting the view that the Ross
Orogeny had little effect in the Ellsworth
Mountains.
Lying conformably above the dominantly
fluvial Crashsite Group are glacial diamictites of
the Permo-Carboniferous Whiteout Conglomerate. Palaeogeographic reconstructions
for the Whiteout Conglomerate, and palaeo-ice
flow directions derived from the southern Heritage Range, indicate a mean ice flow direction
toward 310° , relative to the present day orientation of the EWM (Matsch & Oj akangas 1992).
Gradationally overlying the Whiteout Conglomerate, are 1 km of argillite and sandstone
forming the Polarstar Formation. Collinson et
al. (1992, 1994) suggest a back-arc basin setting
for deposition of the Polarstar Formation sandstones, based on the presence of calc-alkali
volcaniclastic rocks indicating a transitional to
dissected arc source (Matsch & Ojakangas
1992).
M i d - to L a t e C a m b r i a n e x t e n s i o n a l
tectonics within the Heritage R a n g e
Volcanic centres, now represented by pillow
lavas, tuffaceous diamictites, and ash flow tufts,
of bimodal composition (Vennum et al. 1992),
plus gabbroic, and doleritic dykes are present
throughout the Middle to Upper Cambrian
successions of the Heritage Range. Geochemical analysis of these rocks indicate a 'withinplate extensional' setting for their extrusion
(Vennum & Storey 1987; Vennum et al. 1992).
The association of rapid lateral facies changes
within the Heritage Group, the presence of
dominantly basic bimodal volcanism in both
source and accumulation areas (Webers et al.
1992a, b), and observations of syn-extrusive
extensional faults associated with the volcanic
centres (recent mapping by M.L.C.) support an
extensional tectonic setting for the Lower
Palaeozoic Heritage Group.
E l l s w o r t h M o u n t a i n s : structure
One of the most striking aspects of the Ellsworth
Mountains is their NNW-SSE structural grain,
which is almost orthogonal to the strike of the
Transantarctic Mountains (TAM) and the Antarctic Peninsula. Deformation effects all levels
of the stratigraphy, and has been related to the
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PRE-BREAK-UP POSITION OF ELLSWORTH MTS
Late Permian-Early Triassic Gondwanian
Orogeny (du Toit 1937). Within the Ellsworth
Mountains this Gondwanide deformation is
manifest as intense, close to tight, upright to
inclined folds, plunging gently about a N N W SSE horizontal axis. A well-developed, pervasive axial planar cleavage is present, associated with k-values of <1 (mean 0.6) (Sp6rli &
Craddock 1992; Curtis 1994). Contemporaneous mineral stretching lineations switch
orientation between dominantly down-dip, and
strike-parallel, often associated with reverse and
dextral shear sense indicators, respectively.
The presence of lineation switching, k-values
<1, steep fabrics and structures, and the
coexistence of contemporaneous dextral,
oblique, and reverse shear, led Curtis (1994) to
interpret the Gondwanian deformation of the
Heritage Range to be of a dextral transpressive
nature.
The southerly nunataks o f the E W M block
Scattered nunataks to the south of the Ellsworth
Mountains consist of predominantly Palaeozoic
shallow marine strata (Storey & Macdonald
1989) intruded by granitic plutons of Middle
Jurassic age (Millar & Pankhurst 1987). The
sedimentary rocks are deformed by northwesterly oriented folds with a weak to moderately developed axial planar cleavage (Storey &
Dalziel 1987). Their similarity in stratigraphy
and structural geometry to the Ellsworth Mountains themselves indicates that they form a
structurally homogeneous domain (Thiel 1961;
Craddock 1972, 1983; Webers et al. 1982, 1983;
Storey & Dalziel 1987).
Haag Nunataks and the basement o f the
E W M block
Haag Nunataks forms the only exposure of a
small crustal block to the NE of the Ellsworth
Mountains. They are composed of Proterozoic
basement gneisses with Grenvillian Rb-Sr
whole-rock ages (1176 __ 76 Ma), 1058 _+53 Ma
microgranite sheets, and 1003 + 18Ma aplogranites (MiUar & Pankhurst 1987). K-Ar
mineral dates of Clarkson & Brook (1977) are
essentially concordant with the aplogranite age
indicating no significant thermal activity has
taken place during the last 1 Ga. The gneisses at
Haag Nunataks have primitive mantle Nd
compositions, giving depleted mantle ages of
only l100-1300Ma, confirming the Grenvillian
crustal age of the gneisses (Storey et al. 1994).
Aeromagnetic data from the surrounding
15
region reveal that basement of similar magnetic
characteristics to that exposed at Haag Nunataks, extends beneath the southern and
western margins of the Weddell Sea, and the NE
margin of the Ellsworth Mountains (Maslanyj &
Storey 1990). The presence of a Proterozic
basement beneath the Palaeozoic succession
throughout the remainder of the EWM is
supported by inherited Nd isotopic systematics
in the Middle Jurassic granites in the Pirrit and
Nash hills, the Whitmore Mountains and Pagano
Nunatak. Tdm model ages from these Jurassic
granites range between 1048 and 1282 Ma, with a
mean of 1123 +__70 Ma (Storey et al. 1994).
Key geological features of the E W M crustal
block
The EWM crustal block possesses a number of
key geological characteristics which must be
accounted for in any reconstruction of the
palaeo-Pacific margin of Gondwana. They are as
follows.
(1) The basement to the EWM block is most
likely formed of high-grade metamorphic crust
of Grenvillian age. No mineral age resetting has
occurred.
(2) The Ellsworth Mountains appear to possess a continuous sedimentary succession from
Mid-Cambrian to Permian, without significant
unconformity.
(3) Mid to Late Cambrian 'within-plate extension' bimodal volcanic rocks, associated with
minor extensional faults, are present in the
Heritage Group.
(4) No Ross-age structures are present. A
minor disconformity along the Upper Heritage
Group boundary appears to be the only manifestation of the Ross Orogeny.
(5) The Ellsworth Mountains were deformed
during a single Early Mesozoic event, which
resulted in NNW-SSE-trending folds and associated cleavages.
(6) A significant component of contemporaneous dextral shear suggests dextral transpression.
Palaeo-Pacific Antarctic margin of
Gondwana: North Victoria Land to
Dronning Maud Land
Precambrian evolution
According to the SWEAT hypothesis (Moores
1991; Dalzie11991), the palaeo-Pacific margin of
Gondwana formed by the separation of Laurentia from a Neoproterozoic supercontinent
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on September 12, 2016
16
M. L. CURTIS & B. C. STOREY
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Crustal Ages
>2.0 Ga
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EWM
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Fig. 3. Gondwana reconstruction for 520 Ma, showing the distribution of crustal ages. CFB, Cape Fold Belt;
EM, Ellworth Mountains; PM, Pensacola Mountains; FP, Falkland Plateau; EWM, combined Ellsworth
Whitmore Mountains-Haag Nunataks crustal block. Reconstruction of main continents based on Dalziel et al.
(1994), and Falkland Plateau reconstruction of Marshall (1994).
(Stump 1992a). The hypothesis is based primarily on the matching of Precambrian cratonic
domains and Precambrian-Palaeozoic sedimentary successions (Moores 1991) between North
America and Antarctica, in particular the
boundary between Grenvillian and Palaeoproterozoic age crust, a crustal boundary referred to as the Grenville Front (Dalziel 1991).
The location of the Grenville Front in Antarctica
between the Palaeoproterozoic rocks in the
Shackleton Range and rocks of Grenvillian age
in Coats Land and Dronning Maud Land
(Moores 1991; Dalziel 1991) has subsequently
been modified by Storey et al. (1994) based on
geochronological, isotopic and aeromagnetic
data. Grenvillian age crust also borders the
Kaapvaal craton in southern Africa, forming the
crust of the Natal and Cape provinces, and is
continuous into Dronning Maud Land in East
Antarctica (Fig. 3) (Groenewald et al. 1991).
P a l a e o z o i c t e c t o n o t h e r m a l events, a n d
stratigraphy
During Neoproterozoic to Early Palaeozoic
times, the extensional or passive margin setting
of the TAM segment of Gondwana changed to
one of subduction and associated orogenesis
(Stump 1992a), termed the Beardmore Orogeny
(Grindley & McDougall 1969), Ross Orogeny
(Gunn & Warren 1962) and Shackleton Event
(Laird 1991). These events represent complex
periods of sedimentation, magmatism, metamorphism, and deformation, the precise temporal relationships between which, and their
tectonic interpretations, are not clearly established, but it is likely that they represent a
prolonged active margin setting for the Transantarctic Mountains during late Neoproterozoic
and Palaeozoic times. The Ross Orogeny, the
main orogenic event, is roughly contempor-
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PRE-BREAK-UP POSITION OF ELLSWORTH MTS
aneous with deformation events that affected
the entire length of the palaeo-Pacific margin of
Gondwana, including the Pan-African Orogeny
(Kennedy 1964), the Delamerian Orogeny of
Australia (Rutland et al. 1981), and the Brasiliano Orogeny of South America (Cordani et
al. 1973). As the present consensus of opinion
locates the EWM block within the region of the
Natal embayment adjacent to East Antarctica
(Clarkson & Brook 1977; Watts & Bramal11981;
Storey & Macdonald 1987; Dalziel & Grunow
1992; Lawver et al. 1992; Grunow 1993; Goldstrand et al. 1994; Dalziel et al. 1994), only the
Ross and Pan-African orogenies will be considered here.
The time of initiation of tectonism, and
magmatism within the TAM are not entirely
clear. An early deformation event, the Beardmore Orogeny (Grindley & McDougall 1969),
folded greywacke-shale successions of the
Beardmore Group and Patuxent Formation
prior to deposition of Lower and Middle
Cambrian carbonate successions, respectively
(Stump 1992b; Storey et al. 1992). Although,
Rowell et al. (1992) suggested that folding of the
Patuxent Formation in the Pensacola Mountains
maybe an early Middle Cambrian event, Stump
(1995) interprets the Patuxent Formation to be
the time equivalent and correlative of the Goldie
Formation (Beardmore Group), supporting the
conclusion of Storey et al. (1992) that its
deformation is synchronous with that of the
Beardmore Group, and is Neoproterozoic in
age.
Outboard of the Lower Cambrian carbonate
shelf deposits are an association of bimodal
volcanic and carbonate rocks, the 'Queen Maud
Terrane' of Rowell et al. (1992). In the Queen
Maud Mountains, this volcanic and carbonate
association (Liv Group) is of Early and MidCambrian age (Stump 1985; Yochelston &
Stump 1977; Rowell et al. 1993; Palmer &
Gatehouse 1972), whereas in the Pensacola
Mountains, a Mid-Cambrian Nelson Limestone
is conformably overlain by silicic volcanic rocks
of the Gambacorta Formation (Schmidt et al.
1965) of Early Ordovician age (501 + 3 Ma,
Millar & Storey 1995). The presence of bimodal
volcanic rocks within, and overlying Cambrian
carbonate successions, together with mafic
pillow lavas within the Pensacola Mountains,
have been used to imply an extensional tectonic
phase (Stump 1992a, b, 1995; Millar & Storey
1995) within at least part of the orogen during
Cambrian times, although compressional and
strike-slip deformation was also taking place at
this time in other parts of the orogen (Goodge et
al. 1993; Stump 1995).
17
The earliest known magmatic phase, which
has been linked to the Beardmore Orogeny
(Stump 1995), occurred during latest Neoproterozoic to Early Cambrian times, as indicated by U-Pb age dates of 551-521 Ma from
igneous and metamorphic rocks (Goodge et al.
1993), and of 551 ___4Ma from an unfoliated
quartz syenite (Rowell et al. 1993) from the
central TAM. The main, subduction-related,
magmatic episode, emplacement of the Granite
Harbour Intrusives (Borg et al. 1987) occurred
during the Late Cambrian, synchronous and
subsequent to the main deformation event
(Stump 1992a, b). Late Cambrian deformation
and plutonism is less pronounced within the
Thiel and Pensacola Mountains, towards the
Weddell Sea end of the TAM (Stump 1995).
Syn- or post-orogenic deposits are present in
the Pensacola Mountains (basal Neptune Group,
Macdonald et al. 1991), central TAM (Douglas
Conglomerate, Rowell et al. 1988), and the
Bowers Terrane of North Victoria Land (Leap
Year Group, Rowell et al. 1988). These successions lie upon folded Cambrian carbonates
with varying degrees of unconformity, and have
themselves been folded. The degree of both
deformation and unconformity lessens toward
the Pensacola Mountains where syntectonic
sedimentation at the base of the Neptune Group
has been described by Macdonald et al. (1991).
There is poor biostratigraphical constraint on
these successions, especially their upper boundaries, prompting Laird (1991) to suggest an Early
Devonian or Silurian age for their deformation,
named the Shackleton Event. However, Rowell
et al. (1988) suggest that these successions could
be Ordovician in age and that the main Ross
deformation continued for a protracted period
during the Ordovician, at least in certain parts of
the TAM.
The 'physical and temporal limit of the Ross
Orogen', is expressed along the entire length of
the TAM by the kukri peneplain, which is
overlain by the Devonian to Triassic Beacon
Supergroup (Stump 1992a, 1995). Along the
2500km mountain belt, the Kukri peneplain
forms an angular unconformity overlain by
sub-horizontal Beacon deposits. This relationship differs in the Pensacola Mountains where
the Kukri peneplain is represented by a disconformity (Schmidt & Ford 1969), that together
with the Beacon Supergroup equivalents are
folded by the Late Permian to Early Triassic
Weddellian (Ford 1972), or Gondwanide
Orogeny (du Toit 1937). This orogeny produced
upright folds and is also responsible for deformation within the Ellsworth Mountains and the
Cape Fold Belt of South Africa.
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18
M. L. CURTIS & B. C. STOREY
The Shackleton Range, on the Weddell Sea
side of the TAM, is unusual in that structures
formed during the Ross Orogeny are normal to
the trend of the TAM (Buggisch et al. 1990). In
Late Precambrian times, prior to formation of
these structures, limestones of the Watts Needle
Formation were deposited in a shallow marine,
mostly intertidal environment. During the Early
Cambrian, greywackes, feldspathic sandstones
and conglomerates (Turnpike Bluff Group)
were deposited in a basinal (?back-arc) environment and subsequently during the Ross Orogeny
thrust southwards over the Precambrian crystalline basement. Finally, the Ordovician Blaiklock Glacier Group (Clarkson 1972) formed as a
molasse deposit (Buggisch et al. 1990).
The tectonic evolution of the Dronning Maud
Land sector of the palaeo-Pacific margin of
Gondwana was also significantly different to that
of the TAM. Lower Palaeozoic sedimentary
rocks are entirely absent, and the main evidence
for early Palaeozoic tectonism is a regional
isotopic resetting event at c. 470Ma together
with scarce coeval magmatism and reactivation
of older structural lineaments (Moyes et al.
1993). These Pan-African events have been
related to crustal thickening through large-scale
underplating rather than subduction processes
(StiJwe & Sandiford 1993). However, sedimentation occurred during Late Palaeozoic times
and the Urfjell Group, a 1.6 km thick succession
of quartzite and conglomerate has been correlated with the Beacon Supergroup on lithological grounds (Barrett 1991). The rocks are
slightly folded, are in tectonic contact with
underlying basement and are overlain by Mesozoic sedimentary rocks and lavas.
M i d - to L a t e P a l a e o z o i c
During the Late Carboniferous to Early Permian Gondwanide glaciation, a trough-shaped
depression formed along the inner continental
shelf adjacent to the edge of the continental ice
sheet possibly as a result of isostatic flexural
loading (Collinson et al. 1994). Palaeo-ice flow
directions within the TAM are generally along
this linear margin-parallel basin, except for
those observed in southern Victoria Land. This
Late Carboniferous to Early Permian glaciation
is present throughout Gondwana and may
provide a good test of microplate positions prior
to Gondwanian break-up as ice flow directions
and ice cap reconstructions for the PermoCarboniferous are well constrained in South
Africa, unfortunately less precise information is
known for the Antarctic (Visser 1993; Collinson
et al. 1994).
Key features o f Transantarctic M o u n t a i n s
geology
(1) The TAM contain evidence for at least three
main deformation episodes that together represent periods of compressive deformation between Late Neoproterozoic and Devonian
times; the Beardmore and Ross (Late Cambrian) orogenies, and a possible Ordovician
event, the Shackleton Event.
(2) Early to Mid-Cambrian shelf carbonates
are common, with an outboard association of
carbonates and bimodal volcanic rocks suggestive of an extensional tectonic regime.
(3) Extensive arc magmatism, low to highgrade metamorphism, and thrust-related deformation characterise the period of the Ross
Orogeny (550-480 Ma).
(4) The TAM are unified by a single geological feature,
the mid-Palaeozoic
Kukri
peneplain, albeit less pronounced in the Pensacola Mountains.
(5) The intensity of Ross and Shackleton
deformation, the intensity of magmatism, and
the angularity of the end-Cambrian to Early
Ordovician unconformity and Kukri peneplain,
deminish toward the Weddell Sea sector of the
TAM.
Southern Africa and the Falkland Islands
The Cape and Natal provinces in southern
Africa, are formed of a Grenvillian age basement which surrounds the Archean to Mesoproterozic age, Kaapvaal Craton, and a
southern belt of deformed Neoproterozoic sedimentary rocks of the Saldanian Province. Both
are overlain by Phanerozoic supracrustal rocks
of the Cape and Karoo supergroups. The
Falkland Islands are included in this section as
they most likely formed an easterly extension of
the Cape Province along the palaeo-Pacific
margin prior to Gondwana break-up.
Grenvillian b a s e m e n t
In southern Africa, 1.1 Ga crust is present as the
Namaqua-Natal Province, a 400 km wide belt of
metamorphic and magmatic rocks around the
southern margin of the Kaapvaal Craton
(Thomas et al. 1994) (Fig. 3). No Pan-African
isotopic ages have been recorded and the
metamorphic basement is unconformably overlain in Natal by elastic fluviatile sedimentary
rocks of the Natal Group dated at 490Ma
(Thomas et al. 1992).
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on September 12, 2016
PRE-BREAK-UP POSITION OF ELLSWORTH MTS
19
Witteberg Group
N
Bokkeveld Group
A
s
Table MountainGroup
-
i
Pre-Cape metasediments
j
Subsurface
o,O,/
G~9e
Saldanian Front
Inlier
Town ~
""
lizabeth
Gamtoos Inlier
Malmesbury Group
0
I
300km
I
I
I
Fig. 4. Geological map of the Cape Supergroup, South Africa, with location of pre-Cape inliers.
Late N e o p r o t e r o z o i c to Cambrian preCape sedimentary rocks, and Saldanian
tectonism
Southern Cape Province is characterised by a
Late Neoproterozoic to (?)Cambrian basement
of highly deformed low-graJe metasedimentary
rocks and granites, unconformably overlain by
the Early Ordovician to Early Carboniferous
Cape Supergroup. Exposure of pre-Cape Supergroup rocks is restricted to the Malmesbury
Group of the southwestern Cape, and three
inliers in the southeastern Cape, the Kango,
Gamtoos, and Kaaimans groups (Fig. 4).
The Malmesbury Group is exposed in three
tectonic domains, the stratigraphic relationships
between which are unclear. Age constraints are
poor, with minimum and maximum relative ages
for the base of the Malmesbury Group having
been established at 600 Ma and 950 Ma, respectively. The top of the group may be as young as
Mid-Cambrian, based on the presence of clasts
derived from late-syntectonic Cape granites
within the uppermost formation of the Malmesbury Group, the Franschhoek Formation. These
pre-Cape successions were deformed into tight,
upright folds about a NW axis by the Saldanian
Orogeny; polyphase deformation is common
(Hartnady et al. 1974). High-level Neoproterozoic to Mid-Cambrian granite intrusions are also
present, with emplacement ages of 530 + 15 Ma,
6 0 0 + 2 0 M a (Schoch et al. 1975), and
5 2 2 _ 1 2 M a (Schoch & Burger 1976). These
granites display late-syntectonic relationships to
the Saldanian deformation with evidence for
some post-emplacement deformation that is
difficult to separate from younger Gondwanide
deformation (Tankard et al. 1982). Saldanian
deformation took place at shallow crustal levels
with metamorphic grade varying from unmetamorphosed, to a maximum grade of lower
greenschist facies (Hartnady et al. 1978; Tankard et al. 1982).
The Malmesbury Group was deposited in a
basin adjacent to the Kalahari Province (Tankard et al. 1982), with early deep-water turbidites
associated with volcanic tufts and brecciated
flows of intermediate composition (Hartnady et
al. 1974). The Upper Franschhoek Formation is
interpreted as a late stage infilling of the basin
(DuToit 1954), with derived clasts of Cape
Granite suggesting that it was a molasse-type
deposit. The molasse deposits heralded the end
of the Saldanian Orogeny, as demonstrated by
the post-Malmesbury Group, Klipheuwel Formation, a 2 km thick, northward thinning wedge
of intertonguing conglomerates, feldspathic
sandstones, mudstones and shales, containing
intrabasinal clasts of Malmesbury Group rocks
and granite (Visser 1967). It becomes progressively less deformed upward, recording the end of
the Saldanian deformation in Mid-(?)Late
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20
M.L. CURTIS & B. C. STOREY
Cambrian time (Tankard et al. 1982). The
Klipheuwel Formation is commonly unconformably overlain by the Cape Supergroup. However, the two sequences are locally conformable,
with the Upper Klipheuwel Formation considered to be the facies equivalent of the basal
Cape Supergroup.
In the southeastern Cape, the Saldanian
Orogeny was preceded and accompanied by the
deposition of thick successions of marginal and
rift basin related sedimentary rocks overlain by
molasse deposits, exposures of which are restricted to three elongate inliers. These litho/
tectonostratigraphic successions are formed of
terrigenous clastic and carbonate rocks, displaying polyphase deformation and low-grade metamorphism (Tankard et al. 1982). The Kango
Group contains a carbonate and shale succession
overlain by deep-water turbidites, and syntectonic alluvial and molasse sediments related to
basin deformation, uplift and erosion (Le Roux
1977). This late molasse succession, the
Schoemans Port Formation, may be analogous
to the Klipheuwel Formation of the southwestern Cape (Tankard et al. 1982). The Gamtoos
Group possesses a very similar stratigraphy, and
palaeoenvironmental and tectonic history to the
Kango Group, and is considered to be the more
distal part of the same basin (Stocken 1954). The
Gamtoos Group was interpreted as conformable
with the Cape Supergroup (Haughton et al.
1937; Stump 1976), however, Shone et al. (1990)
recognized evidence for deformation prior to the
Early Mesozoic Gondwanide Orogeny, and
suggested that the contact is unconformable.
The latter observations are consistent with a
metamorphic age date for dykes within the
Kango Group of 782 Ma (H~ilbich 1979),
suggesting these groups may be broadly equivalent in age to the Malmesbury Group
(Haughton 1969; Stump 1976).
A thermotectonic event, equivalent to the
Saldanian Orogeny, 600-450 Ma, is recognized
throughout the Pan-African belts of southern
Africa representing intercratonic readjustments
within Gondwana. The exception to this is the
Natal-Transkei coast of South Africa, where no
mineral reset ages are recorded in the Grenvillian basement (Groenewald et al. 1991).
widespread by Late Cambrian times (Le Roux
1977; Rust 1973; Vos & Tankard 1981; Tankard
et al. 1982). Unconformably deposited on these,
and older rocks, are sedimentary rocks of the
Table Mountain Group (TMG), the lowest
group in the Cape Supergroup succession.
Locally the Klipheuwel Formation and basal
TMG are conformable (Tankard et al. 1982).
The TMG contains 4km of quartz arenite,
conglomerate and mudstone of Late Cambrian/
Early Ordovician (Cocks et al. 1970) to Early
Devonian age (Tankard et al. 1982). The basal
Piekenierskloof Formation was deposited in a
widespread fluvial braid-plain, a facies equivalent of the Klipheuwel and associated formations. This was transgressively overlain by
quartz arenites of barrier island and tide dominated shallow shelf origin forming the Graafwater Formation (Rust 1973; Visser 1974;
Tankard & Hobday 1977). This, in turn, was
overlain by the Peninsula Formation, a succession of quartz arenite, 2 km thick, originally
thought to be a barrier-beach to shallow shelf
deposit (Visser 1974), but recently reinterpreted
as a dominantly alluvial plain deposit merging
into a tidal-flat and back-barrier lagoon toward
the SE, with repeated marine incursions (Broquet 1992). Palaeocurrents indicate a southerly
sediment transport direction.
Following deposition of the quartz arenites,
120m of massive basal tillite, striated pavements, and glaciolacustrine deposits of the
Ashgillian age Pakhuis Formation, formed at
the periphery of a major glacial ice sheet centred
on Africa (Rust 1973). Palaeo-ice flow directions
were to the south. A post-glacial marine transgression resulted in the deposition of 140m of
fine-grained sandstone, siltstone and mudstone
in a muddy shelf environment (Broquet 1992),
which are conformably overlain by l l 0 0 m of
quartz arenite of the Nardouw Formation. The
Nardouw Formation has a similar lithology and
depositional environment to the Peninsula Formation with alluvial plain deposits becoming
tidally reworked in a shallow shelf to shore zone
environment (Tankard et al. 1982).
U p p e r Cape S u p e r g r o u p : B o k k e v e l d a n d
Witteberg g r o u p s
L o w e r Cape S u p e r g r o u p : Table M o u n t a i n
Group
The Klipheuwel, Franschhoek and Schoemans
Port formations share similar stratigraphic relationships and relative ages, suggesting that
syn- to post-orogenic alluvial sedimentation was
The Devonian Bokkeveld Group represents a
tectonically dynamic period in the development
of the Cape basin. Rapid basin subsidence,
coupled with high rates of sedimentation, resuited in the deposition of up to 3.2 km of fluvial
to deltaic facies sediments, as part of a prograding delta which experienced periodic marine
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PRE-BREAK-UP POSITION OF ELLSWORTH MTS
reworking. A well-known Malvinokaffric invertebrate faunal assemblage suggests an Emsian age. Two depocentres, with different
subsidence rates were present, a westerly basin
containing 1.5 km of deltaic sediments, and an
easterly basin with 3.2 km (Tankard et al. 1982).
Lying above the Bokkeveld Group is a 2 km
succession of deltaic and glaciogenic sediments
of the Late Devonian to Early Carboniferous
Witteberg Group. The lower half of the group
has close affinities to strata of the Bokkeveld
Group, with progradational deltaic sediments
representing delta front, tidal fiat, and fluvial
environments. The top of the lower Witteberg
has witnessed marine transgressive reworking,
with a thick quartz arenite succession deposited
along an erosive contact. The locus of deposition
remained the elongate Cape basin, with both
east and west depocentres retaining tectonic
independence. A scarce Malvinokaffric invertebrate faunal assemblage is present (House 1979)
representing the final episode of marine deposition in the Cape Supergroup. More argillaceous sediments of the upper Witteberg
Group lie conformably above the quartz arenites
representing pro-glacial lacustrine environments, with entirely non marine biota present
(Rust et al. 1970). Interbedded shales, varvites,
and diamictites represent the initiation of continental glaciation represented by the overlying
Dwyka Formation which forms the base of the
Karoo Supergroup.
Permo-Carboniferous glaciation within the
basal Karoo Supergroup
The base of the Karoo Supergroup, the Dwyka
Formation, generally rests unconformably on
Early Palaeozoic to Precambrian strata. However, along the axis of the Cape depocentre, the
Dwyka Formation and Witteberg Group are
disconformable with the break representing most
of the Carboniferous Period (Dunleavey & Hiller
1979). The Dwyka Formation is characterized by
thick tillite/diamictite deposits controlled by the
underlying topography, with a general northward thinning of the sequence. A distinct
regional palaeo-ice flow pattern has been recognised for southern Africa, with southerly directed
ice flow within central southern Africa, swinging
rapidly to a westerly direction in the Cape
Province (Tankard et al. 1982).
The Falkland Islands: an extension o f the
Cape Province
Stratigraphic and structural similarities between
the Falkland Islands and the Cape Fold Belt
21
have been recognized for many years (Du Toit
1927), with palaeogeographical reconstructions
placing them off the coast of Natal, and rotating
them by 180° (Adie 1952). Palaeomagnetic data
(Mitchell et al. 1986) have confirmed the early
hypothesis of Adie (1952) and identified a 120°
tectonic rotation related to the early stages of
Gondwana break-up, and a further rotation of
60° associated with drift to its present position.
Stratigraphic correlations between the Fox Bay
Formation of the Gran Malvina Group, and the
Bokkeveld Group of South Africa, have been
demonstrated by Marshall (1994). A tentative
correlation is also made between possible tillites
in the basal Port Stephens Formation and the
Pakhuis Formation of the Cape Supergroup.
Identical stratigraphic and structural transitions
between the Dwyka and Whitehill Groups, and
Ecca Group along the northern margin of the
Cape Fold Belt, and the transition between the
Port Stanley Formation and Upper Lafonia
Group in East Falkland are also noted by
Marshall (1994).
There are no correlatives to the Lower TMG
or the pre-Cape successions within the Falkland
Islands. Instead the Port Stephens Formation
lies unconformably above basement gneisses at
Cape Meredith, which possess K-Ar ages of
953-977Ma (Rex & Tanner 1982). They are
inferred to be Grenvillian in age, and therefore,
comparable with adjacent basement gneisses in
Natal and Antarctica.
There appears little doubt in placing the
Falkland Islands off shore the present day South
African east coast. Stratigraphic and structural
correlations suggest that they are an extension of
the Cape Fold Belt although the lack of evidence
for Saldanian events and Pan-African reset ages
in the Cape Meredith Complex suggest that the
Falkland Islands must be on the northern side of
the Saldanian belt off the Natal-Transkei coast.
Overlap of crustal blocks within this reconstruction can be accounted for by closing the Falkland
Plateau Basin, and restoring crustal extension
within the Maurice Ewing Block as suggested by
Marshall (1994) (Fig. 3). However, one problem
with this reconstruction is a 535Ma (PanAfrican/Ross orogen) age for crust recovered
from the Maurice Ewing Bank (Beckinsale et al.
1977), indicating that a small block of younger
crust, or thermotectonically reset crust is surrounded by Grenvillian age crust of Natal, and
Cape Meredith, neither of which display mineral
reset ages post 1 Ga. This date may represent the
most westerly extent of the Ross age tectonothermal event in Dronning Maud Land. More
speculatively, this problem may be resolved by
rotating the Maurice Ewing Bank with the
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22
M.L. CURTIS & B. C. STOREY
Falkland Islands and placing it outboard of the
Saldanian Belt and to the south of the Falkland
Islands in a Gondwana reconstruction.
Key features o f Cape Fold Belt, and
Falkland Islands geology
(1) Both the Cape Fold Belt and Falkland
Islands are underlain by Grenvillian age crust,
however, their late Neoproterozoic to Early
Palaeozoic geological histories differ.
(2) In South Africa, the pre-Cape successions, of late Neoproterozoic to MidCambrian age were strongly deformed and
metamorphosed to low grades during the Saldanian Orogeny, and intruded by numerous synand post-tectonic granites between 630 and
500Ma.
(3) A prolonged period of thermotectonic
stability existed during Early Ordovician to
Early Carboniferous times with uninterrupted
deposition of up to 8 km of alluvial to marginal
marine sediments (Cape Supergroup).
(4) A Mid-Late Carboniferous disconformity
was followed by the deposition of extensive
continental glacial diamictites.
(5) The Falkland Islands have no record of a
600-500Ma thermotectonic event, and do not
possess sedimentary equivalents to the pre-Cape
successions and Lower Table Mountain Group.
(6) An excellent correlation exists between
post-Ordovician to Permian successions of the
Falkland Islands and South Africa, indicating a
close spatial relationship between the regions
during this time.
Geological correlations between the
Ellsworth Mountains and the Gondwanian
margin successions
Precambrian domains
The recognition of Precambrian domains along
the palaeo-Pacific margin of Gondwana provide
an initial constraint on the original position of
the EWM within Gondwana. The presence of
crust of Grenvillian age at Haag Nunataks and
most likely beneath the EWM, indicates that the
EWM must have been originally located off the
Coats Land or Dronning Maud Land coasts, of
Antarctica as noted by Storey et al. (1994), or
within the Natal embayment. A position to the
south or west of the Shackleton Range, would
place the EWM Grenvillian basement adjacent
to a much older part of the East Antarctic
craton, which appears an unlikely scenario.
However, Borg & DePaolo (1994) have invoked
sinistral translation of allochthanous terranes
during Laurentia separation to explain present
Antarctic crustal distributions along the TAM.
Although no geological evidence exists at
present to support these strike-slip faults, such a
hypothesis suggests that we must exercise care in
locating the EWM block within the Dronning
Maud Land-Coats Land Grenvillian belt based
solely on its crustal age.
Cambrian successions
Correlations of the Heritage Group with the
Gamtoos Group of the eastern Cape have been
made based on the presence of an argillite and
carbonate succession in apparent conformity
with the overlying Cape Supergroup (Goldstrand et al. 1994). However, this correlation
must be considered doubtful given the structural
observations of Shone et al. (1990), and the
strong correlation of the Gamtoos Group with
the adjacent Kango Group, the latter having a
minimum age of 782Ma. Although it is just
possible for the Ellsworth Mountains succession
to be stratigraphically younger than the Saldanian tectonism, the lack of evidence for such
an event or for any mineral resetting at Haag
Nunataks makes a position close to the Saldanian belt unlikely. Moreover, current published data suggest that within the southern
Cape Province, Cambrian age successions are
represented by the thick, Post-Saldanian, molasse deposits of the Klipheuwel, Schoemans
Port, and Franschhoek Formations. These terrestrial sedimentary rocks are in strong contrast
to the Middle and Late Cambrian shallow
marine, carbonate shelf and associated extensional volcanic rocks of the Ellsworth Mountains and preclude a close spatial association of
the EWM and the Saldanian belt. A position for
the EWM within the Natal embayment to the
north of the Saldanian belt is also unlikely due to
the complete absence of Cambrian strata in the
Natal Group, where the oldest sedimentary
rocks are Ordovician in age (Thomas et al.
1992).
In contrast to the above, Lower and Middle
Cambrian carbonate and volcanic successions
are present along the TAM (e.g. Liv Group in
the Queen Maud Mountains, and the Nelson
Limestone in the Pensacola Mountains). These
are potentially similar to Cambrian strata in the
EWM, and they could both form part of an
outboard lithostratigraphic terrane, the Queen
Maud Terrane of Rowell et al. (1992). However,
with the exception of the localised Ross age
disconformity at the base of the Crashsite
Group, which may represent the distal effects of
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PRE-BREAK-UP POSITION OF ELLSWORTH MTS
the Ross Orogeny (Goldstrand et al. 1994), the
absence of substantial Ross age deformation
within the Heritage Group rules out an original
location of the Ellsworth Mountains along the
TAM.
We, therefore, conclude that the above
relationships and correlations may best be
explained if the EWM block had been situated
outboard of both the Ross and Saldanian
orogenic fronts off the Dronning Maud LandCoats Land sector of Antarctica in Early
Palaeozoic times.
O r d o v i c i a n to P e r m i a n successions
Ordovician to (?)Silurian successions are locally
preserved along the TAM. They are typical
molasse sediments of a proximal nature, lying
unconformably on basement gneisses or deformed Cambrian successions, and are themselves deformed by the Shackleton Event,
resulting in uplift, erosion and folding. These
molasse deposits are overlain by flat-lying,
undeformed rocks of the Beacon Supergroup. In
contrast, the end Cambrian to Early Devonian
Crashsite Group of the Ellsworth Mountains, is
a locally disconformable, thick (3 kin),
quartzite-rich sandstone and argillite succession,
exhibiting increasing compositional and textural
maturity during the Ordovician and Silurian,
and interpreted to have been deposited in
shallow marine conditions prone to emergence
and fluvial influences. No depositional hiatus or
tectonic events equivalent to the Shackleton
Event have been identified. These tectonic,
stratigraphic and palaeoenvironmental contrasts
suggest the Ellsworth Mountains cannot be
closely correlated with post-Ross successions
along the Antarctic margin (Fig. 5a).
Du Toit (1937), Craddock (1970) and Sp6rli
(1992) have previously correlated the Crashsite
Group with the Table Mountain Group of South
Africa. Similarities in stratigraphic thickness (c.
3 km), alluvial/fluvial to shallow marine palaeoenvironments, and the absence of unconformities equivalent to the Shackleton Event,
suggest a strong correlation between these
sequences. The facies variations and changes in
palaeocurrent directions within these successions suggest that the Crashsite Group in the
Ellsworth Mountains must have been located
outboard and to the south of the Cape Province.
Although no glacigenic equivalent to the Ashgillian age Pakhuis Formation has been reported
within the Crashsite Group, the Pakhuis Formation is interpreted as peripheral deposits to an
African ice cap, the influence of which, may not
have extended outboard of the Cape Province.
23
The major difference in the Palaeozoic successions of the two regions are in the Devonian
rocks. In South Africa, the Bokkeveld and
Witteberg groups form up to 5.2km of dominantly deltaic sediments, with non-marine proglacial deposits present in the upper Witteberg
Group. In contrast, the Devonian age Mount
Wyatt Earp Formation of the Upper Crashsite
Group is only 300m thick and is formed of
impure, shallow marine sandstones, however;
rare ice-rafted pebbles are present near the top
of the formation. The Permo-Carboniferous,
glaciogenic Whiteout Conglomerate correlates
with the Dwyka Group, and Lafonia Diamictite
of South Africa, and the Falkland Islands
respectively. However, the contact with the
underlying Mount Wyatt Earp Formation has
been interpreted as being conformable, contrasting with the Carboniferous disconformity
observed between Upper Witteberg Group and
tillites of the Dwyka Formation in southern
Africa. It is possible, however, that the sharp
contact between the glacial diamictites and
underlying pro-glacial deposits of the Mount
Wyatt Earp Formation may represent a Carboniferous disconformity. This speculative interpretation would lessen the enigmatic nature of the
Upper Palaeozoic succession of the Ellsworth
Mountains, relative to its Gondwana margin
correlatives (Fig. 5a, b).
Conclusions: the EUsworth Mountains
within Gondwana
We conclude that no single stratigraphic succession along the former palaeo-Pacific margin
of Gondwan'a provides a satisfactory correlation
for the entire Middle Cambrian to Permian
geological evolution of the Ellsworth Mountains. Instead, the key stratigraphic characteristics appear to be explained by a combined
model where the geological influences transform
from an Antarctic margin influence during the
Cambrian, to that of a southern African influence between the Ordovician to Permian,
following cessation of Ross and Pan-African
orogenies. This interpretation suggests that the
EWM block was originally located in a position
adjacent to both the Antarctic and South
African margins. A position within the Natal
embayment is attractive and has been suggested
previously enabling continuity with the PanAfrican-free Grenvillian belt south of the Kalahari craton. However, when one considers the
Falkland plateau and associated crustal blocks,
as for example in Marshall (1994), there is
clearly insufficient space in the Natal embayment for all these blocks. Consequently we
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on September 12, 2016
Southern
Victoria I
Land
~'~
Northern
Victoria Land
mO
-
EIIsworth
Mountains
"~ ~
~n-
~ ,"
n . o=
~E
~. ~
E~
~
=~==
"~
'-
~'o
~,=,~
=
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-
~"
~
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'-
Beacon
=o~
.~
~-"
o
"~"
Beacon
........................................................................
Supergroup
Blaiklock
Supergroup
Neptune
Glacier
Group
Formation
Nelson Let.*
Mt. W a l c o t t
.............
Fm.
Liv
Group
*
Byrd Group
A
I
I
Polarstar
Fm.
Permian
Black Rock
Slates Fm.
2Me. . . . . . .
,. ,
Lafonian
Diamictite
iiiiil
................................
36c
.,,.
EoxBayEm.
Bokkeveld Group '
~" . . . .
(.~
...
._
Port Stephens
Fro.
Table
. ? ? ? ? ? ,
Mountain
¢o
.....
....
c~
Group
Ordovician
~iiiii~.i~i!:ii:;i:~:~i
sos
Cambrian
~:
i i::i:: ~ii i
ii:ili i!iii ii!i~iiiiiiiiiiiiii:;:.
i iiii':i!ii~,,~ ..........................................................................
Heritage
:!::::i:.:::::::::::::::
::::::::::::::::::::::::
!!::~!i::i
i::i~~::!i::ii::i::ii::ii::::i::
~i~::!i::!i:i!~i
!~i~!:i::ii::i:~'~i!ii:~:!~:~!i~i!~~~:.ii~iiiiiil
!iii~iii
i!ii~!~i~i~i !~:i :!~ii~~i:i~i:.~!i~i~i~
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ozoic
B
;
',
.......
. . . . . . .
Silurian
438
: i
Portl~tanley
.'~74!
4DA
Dwyka Group
~ to
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Early
Devon.
Ecca Group
. . . . . . .
,°°
Carboniferous
Late D e v
Cape province, South Africa
I
I
East
West
Falkland
Islands
EIIsworth
Mountains
:.:i3
~
;3
'~: ii !:~.i:.iiii!::ii~i:i:~i~:i~:iii~i~i:~i~i:~i:i::~:~:~i:~:~ii::~:~:i::~::~ii:i
,,0~:,::~~::.:~: ~,:!:~~~!,!:~:,ii'',i',iii!:,iili~,ii',',i',:~',:~ii',i
oos
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i',:,i!i~':~i :i'~,;:i~i~i~i:~i:~ii:~:~i
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Meredith
Natal
IMarie
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Land
I
I
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on September 12, 2016
PRE-BREAK-UP POSITION OF ELLSWORTH MTS
oo,,ero ? /
Africa
[/
[~'
Crustal blocks
- ~
Permo-Carboniferous
palaeo-ice flow directions
Table mountain Group &
Howard nunataks palaeocurrent directions
East
Antarctica
25
k~\
\!/
Block
- 2 8 0 Ma
Fig. 6. Permo-Carboniferous iceflowdirections, and Early Palaeozoic palaeocurrent directions for the EWM
block relative to Gondwana. (Based on Tankard et al. 1982, and Visser 1993.)
prefer to locate the Ellsworth Mountains in a
position south of Saldanian belt and outboard of
the Ross and Pan- African age tectonothermal
provinces. If one accepts that the Weddell Sea
embayment area may be underlain by extended
continental crust (Filchner crustal block), as in
some geophysical models (Htibscher et al. this
volume), and accepts a simplest case scenario of
a coupled Filchner, EWM and Haag Nunataks
block, then the Filchner component would lie
between the EWM and the Coats Land coast.
Therefore, the Ellsworth Mountains would
occupy a position outboard of the Falkland
Plateau, and South African coast (Fig. 3). This
position is intermediate between the two endmember models presented in the introduction of
this paper. Such a position for the EWM block is
consistent with the trend of decreased Ross
deformation of the Cambrian successions, and a
reduction in the angularity of the post-Ross
Ordovician unconformity, toward the Weddell
Sea sector of the TAM palaeo-Pacific margin.
Further constraints on the orientation of the
EWM block relative to the other Gondwana
continents can be made by aligning the Gondwanide structural trend within the EWM, with
that of the main Gondwana continents. Alignment of the Gondwanide structural grain of the
Ellsworth Mountains with the Cape Fold Belt, as
originally suggested by Schopf (1969), reveals an
approximate 90° clockwise rotation, supporting
the palaeomagnetic data of Watts & Bramall
(1981). However, the Ellsworth Mountains display a structural and kinematic style consistent
with dextral transpressive deformation in contrast to the classical fold and thrust belt geometry of the Cape fold belt (H~ilbich 1992).
Therefore, our reconstruction aligns the Ellsworth Mountains with the Pensacola Mountains, thereby introducing an element of
obliquity relative to the Cape Fold Belt that is
sympathetic with the dextral interpretation of
the EUsworth Mountains. Support for this position relative to the rest of Gondwana is provided by the coincidence of palaeocurrent
directions between the Table Mountain and
Crashsite groups, and the palaeo-ice flow directions within the Whiteout Conglomerate, corn-
Fig. 5. Chronostratigraphic charts comparing the Ellsworth Mountains (Webers et al. 1992a) with (a) the
Transantarctic Mountains (adapted from Laird 1991, with aditional sources as quoted in main text), and (b) the
Cape Province and the Falkland Islands (complied predominantly from Tankard et al. 1982, and Marshall 1994,
respectively, additional sources as quoted in main text).
The Cambrian timescale of Odin et al. (1983) has been adopted in both cases.
* Denotes association of volcanic and carbonate rocks. GV, Gallipolli Volcanic; LPV, Lawrence Peaks
Volcanics; BPV, Black Prince Volcanics; RCV, Rupert Coast Metavolcanics; HH, Haskard Highlands; SPF,
Schoemans Port Fm.
Downloaded from http://sp.lyellcollection.org/ at Pennsylvania State University on September 12, 2016
26
M.L. CURTIS & B. C. STOREY
pared with regional Gondwana directions (Fig.
6). We suggest that the E W M formed an extensional basin within a compressive margin
(?back arc) during Cambrian times, and
although it lay outboard of Pan-African sutures
it lay inboard of a palaeo-Pacific margin, remnants of which may now be preserved within the
Antarctic Peninsula crustal block (Harrison &
Loske 1988).
Implications for Weddell Sea history
The implications of the suggested position for
the E W M to the evolution of the Weddell Sea
region are that the movement history of the
E W M involved both rotation and translation of
the crustal block either prior to or during
break-up. According to palaeomagnetic data
from the Mid-Jurassic granites within the E W M ,
much of the rotation took place prior to 180 Ma
(the emplacement age of the granite suite;
Grunow et al. 1987), and before generation of
any known oceanic lithosphere. It is likely
therefore that this rotation took place either
during the l a t e s t phase of the Gondwanide
Orogeny or during the initial stages of Gondwana break-up, and must have involved a complex period of Late Triassic or Early Jurassic
crustal shearing. Cox (1992) has suggested that a
Stage 1 break-up phase involved sinistral motion
of Africa relative to Antarctica, and this could
have produced block rotations in the Weddeil
Sea region. Although much of the evidence for
this may be hidden beneath the WSE, there is
evidence for major crustal shearing within the
Gastre Fault system of southern South America
at this time (Rapela & Pankhurst 1992). The
major difference between the model presented
here and previous models (e.g. Marshall 1994) is
that the E W M could assume more or less its
present position by rotation and crustal extension without generating large volumes of oceanic
lithosphere prior to the known history of Late
Jurassic seafloor spreading in the Weddell Sea
(Bell et al. 1990).
We are grateful to E. Stump and D. Macdonald for
thoughtful reviews of an earlier version of this manuscript.
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