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Depositional Rec. 2019;5:247–271.
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wileyonlinelibrary.com/journal/dep2
1
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INTRODUCTION
The Permo‐Carboniferous glaciogenic deposits of southern
Africa, known as the Dwyka Group, have been the focus of
scientific investigations for over a century (Sutherland, 1870;
Du Toit, 1921; Gravenor et al., 1984; Visser, 1987, 1990,
1997; von Brunn, 1994, 1996; Bangert et al., 1999; Haldorsen
et al., 2001; Bordy and Catuneanu, 2002; Catuneanu, 2004;
Isbell et al., 2008; Stollhofen et al., 2008; Andersen et al.,
2016). These strata played a pivotal role in developing ideas
Received: 1 November 2018
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Revised: 17 April 2019
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Accepted: 26 April 2019
DOI: 10.1002/dep2.74
ORIGINAL RESEARCH ARTICLE
Ice‐margin fluctuation sequences and grounding zone wedges:
The record of the Late Palaeozoic Ice Age in the eastern Karoo
Basin (Dwyka Group, South Africa)
PierreDietrich
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AxelHofmann
This is an open access article under the terms of the Creat ive Commo ns Attri bution License, which permits use, distribution and reproduction in any medium, provided the original
work is properly cited.
© 2019 The Authors. The Depositional Record published by John Wiley & Sons Ltd on behalf of International Association of Sedimentologists.
Department of Geology,University of
Johannesburg, Johannesburg, South Africa
Correspondence
Pierre Dietrich, Department of Geology,
Auckland Park Kingsway Campus,
University of Johannesburg, Johannesburg,
South Africa.
Email: pierre.dietrich@univ‐rennes1.fr
Funding information
International Association of
Sedimentologists, Grant/Award Number:
postdoctoral research funding scheme
Abstract
In the eastern part of the Karoo Basin of South Africa, the sedimentary record of the
Late Palaeozoic Ice Age, the Dwyka Group, consists of an up to 200m thick accu-
mulation of massive to crudely stratified diamictite occasionally interstratified with
siltstone, sandstone and conglomerate horizons. Three distinct sedimentary units,
separated by intervening glacial erosion surfaces, are viewed as ice‐margin fluctu-
ation sequences. The lowermost one, resting on highly uneven, glacially abraded
Archaean basement, has been interpreted as a grounding zone wedge deposited after
the retreat and stabilization of the ice margin after the inundation of the Karoo Basin.
The grounding zone wedge interpretation is based on its thickness (up to 100m), the
dominance of diamictite, and its facies assemblage and inferred depositional pro-
cesses (rain‐out of debris, dropstone dumping, mass and debris flow, till). Overlying
the grounding zone wedge, deposits are sedimentary units interpreted as glacioflu-
vial or ice‐contact delta and grounding zone wedges, respectively. By analogy with
Quaternary sedimentary sequences, deposition of the Dwyka Group in the study area
might have been very rapid (tens to hundreds of thousand years) and may hence
correlate with the ultimate deglacial sequence of the Western Karoo Basin, as both
successions are covered by the postglacial Ecca Group. Although commonly ob-
served and imaged on modern, high‐latitude continental shelves, grounding zone
wedges have never been interpreted in the ancient geological record. This paper
therefore outlines a model defining criteria necessary for their identification.
KEYWORDS
Dwyka, exhumed glacial landscape, grounding zone wedge, Karoo, Late Palaeozoic Ice Age, South
Africa
248
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DIETRICH anD HOFMann
about the Late Palaeozoic Ice Age (LPIA), the longest of
all glacial periods experienced by our planet during the
Phanerozoic (Eyles, 2008; Fielding et al., 2008a). In addi-
tion, the Dwyka Group played a seminal role in the elabora-
tion of the plate tectonic concept (Wegener, 1915; Du Toit,
1937). Since these early studies, many others, encompassing
sedimentology, stratigraphy, palaeontology and geochemis-
try, have been conducted on the Dwyka glaciogenic depos-
its. This interest derives partly from the fact that these strata
are widespread throughout southern Gondwana and, as such,
have been used to infer growth and recession of ice sheets or
caps, climate changes during Gondwana times, and palaeo-
geography. It is now widely accepted that the deposition of
LPIA‐related strata across southern Gondwana resulted from
successive phases of growth and decay of ice sheets of some-
what restricted extent rather than a single massive ice sheet
covering most of the land masses (Fielding et al., 2008a;
Horton and Poulsen, 2009; López‐Gamundi and Buatois,
2010; Huuse et al., 2012; Isbell et al., 2012; Montañez and
Poulsen, 2013). These successive ice sheet waxing and wan-
ing events probably depended on parameters such as the
wander of the South Pole (Opdyke et al., 2001), variable con-
centration of atmospheric CO2 (Frank et al., 2008; Montañez
et al., 2016; Myers, 2016), and/or the erection of mountain
belts (Isbell et al., 2012; Goddéris et al., 2017). Investigations
on correlative glaciogenic deposits across Gondwana have
been carried out in South America (Dykstra et al., 2012;
Vesely et al., 2015; Tedesco et al., 2016; Fallgatter and Paim,
2017; Valdez Buso et al., 2017; Assine et al., 2018), the
Middle East (Martin et al., 2008; Stephenson et al., 2013),
India (Wopfner and Jin, 2009), Madagascar (Wescott and
Diggens, 1997), Australia (Fielding et al., 2008b), Antarctica
(Isbell, 2010; Cornamusini et al., 2017) and North Africa (Le
Heron et al., 2009; Bussert, 2010; Le Heron, 2018), where
spectacular, glacially related palaeoreliefs (fjords, ice stream
corridors) have been discovered.
In southern Africa, research on the Dwyka Group has
predominantly focused on the well‐exposed western part
of the South African Main Karoo Basin and the Namibian
Kalahari‐Karoo Basin, where the LPIA is now relatively
well‐constrained both in time and space (Visser, 1983, 1987,
1990, 1993, 1997; Grill, 1997; Bangert et al., 1999, 2000;
Stollhofen et al., 2000,2008; Himmler et al., 2008; Andrews
et al., 2019). Here, four phases of ice growth and decay
have been proposed on the basis of interstratifed glacio-
genic (diamictites) and non‐glaciogenic (Ice Rafted Debris
(IRD)‐free mudstones) deposits. Chronological inferences
are in addition well‐established due to abundant and datable
ash layers distributed throughout the deglacial sedimentary
successions (Bangert et al., 2000; Stollhofen et al., 2008;
Isbell et al., 2008). A smaller number of comparable stud-
ies have been conducted in the eastern Karoo Basin where
no formal interglacial periods are recognized and where age
constraints are essentially lacking (Matthews, 1970; von
Brunn, 1994, 1996; Haldorsen et al., 2001; Bangert and von
Brunn, 2001). Palaeogeography, bathymetries, sedimentary
dynamics and depositional conditions thus remain poorly
understood in the eastern Karoo Basin. This part of the
South African Karoo Basin is, however, crucial as it may
correspond to the central part of the ice sheet (Haldorsen et
al., 2001) and to the transition between an emerged domain
to the north (the Cargonian Highlands) and a deeper basinal
setting to the south (Figure 1; Von Brunn, 1994).
In an attempt to bridge this gap, new sedimentological
and palaeo‐geomorphological data from the eastern Karoo
Basin (Figure 1) are presented in this paper. This paper de-
scribes glacial surfaces (GS), provide information on palaeo-
landscapes and interprets sedimentological facies in terms of
depositional environments in order to provides a deglaciation
time framework, including ice‐margin fluctuations and asso-
ciated sedimentation processes. Also discussed are the spatial
and temporal correlation of the glacial deposits with coeval
strata found elsewhere in the Karoo Basin as well as time
inferences on relative sea‐level (RSL) changes, most prob-
ably forced by glacio‐isostatic adjustment (GIA). Although
the GIA is a process barely constrained in the deep time re-
cord, its unravelling remains crucial to disentangle ice‐mar-
gin advance‐retreat cycles and palaeo‐geographic inferences
(Dietrich et al., 2018). Particular emphasis will be given to
grounding zone wedges (GZWs) discovered here, which are
believed to constitute the first outcrop example of this kind
of sedimentary depocenter in the deep geological record. A
conceptual model that accounts for the characteristics of such
depocenters will be proposed.
2
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GEOLOGICAL SETTING
In South Africa, the glaciogenic deposits of the Dwyka
Group recording the LPIA constitute the lowermost strati-
graphic unit of the world‐famous Karoo Supergroup. The
evolution of the Karoo Basin, which has a present‐day
extent of 600,000 km2 and a cumulative thickness of up
to 6 km, started in the Carboniferous by the deposition
of glacially related strata, followed by the deposition of
marine, lacustrine and continental successions through
FIGURE 1 (A) Location of the study area in the South African Karoo Basin. The glaciogenic Dwyka Group lies at the base of the Karoo
Supergroup. The three palaeogeomorphic regions mentioned in the main text (highland palaeoplain, coastal‐platform domain and deep basin)
and characterizing the eastern part of the Karoo Basin are after von Brunn (1994). Map modified from Catuneanu et al. (1998). (B) Simplified
geological map of the study area. Locations of Figures 2, 3, 5 and logs of Figure 4 are displayed.
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249
DIETRICH anD HOFMann
Tugela Ferry
10 km
N
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R
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Elandskraal
Helpmekaar
Belungwana
Log A
Log C
Log B
log D
Log E
Log F
Fig. 5c
Figs 3, 5A,B
Fig. 2C Fig. 2C
KAROO BASIN
Drakensberg Group
Stormberg Group
Beaufort Group
Ecca Group
Dwyka Group
0 100 200 300 km
CAPE FOLD BELT
KAROO SUPERGROUP
Lesotho
Botswana
South Africa
Namibia
Swaziland
31°S
32°S
34°S
24°E22°E18°E 26°E 28°E
Johannesburg
East
London
Durban
Port
Elisabeth
AFRICA
Study area
N
C
A
R
G
O
N
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A
N
H
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G
H
L
A
N
D
S
Study area
Cape
Town
H
i
g
h
l
a
n
d
p
a
l
a
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p
l
a
i
n
s
C
o
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s
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a
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d
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m
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i
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D
e
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p
b
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A
B
Greenstone belt
Granitoids (Mpuluzi)
Pongola Supergroup
Natal metamorphic province
Natal Group
Dwyka Group
Younger Karoo deposits or
intrusions
Kaapvaal
Craton
Legend
GS0
Hard bed (bedrock)
Soft bed (sediment)
N
Figs 3, 5A,B
250
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DIETRICH anD HOFMann
Permo‐Triassic time and ended in the early Jurassic with
the extrusion of continental flood basalt related to the
break‐up of Gondwana (Smith et al., 1993; Johnson et
al., 2006; Isbell et al., 2008; Tankard et al., 2009). At its
initiation, the Karoo Basin formed an E–W striking de-
pocenter deepening southward. The tectonic setting that
permitted the initiation of the Karoo Basin and deposi-
tion of the lowermost Dwyka and overlying Ecca groups
has either been interpreted as a retro‐arc foreland basin
tied to the Cape orogeny (Catuneanu et al., 1998, 2005;
Catuneanu, 2004) or alternatively, if the onset of this
orogeny occurred only later (Linol and De Wit, 2016),
as a lithospheric deflection produced by far‐field stresses
related to subduction (Pysklywec and Mitrovica, 1999;
Tankard et al., 2009). At present, the sedimentary Dwyka,
Ecca, Beaufort and Stormberg groups and the volcanic
Drakensberg Group that constitute the Karoo Supergroup
from base to top crop out concentrically due to basin in-
version (Figure 1A).
In the eastern part of the Karoo Basin (KwaZulu‐Natal
Province, Figure 1B), the Dwyka Group unconformably
overlies Archaean and Proterozoic basement. In the north
of the study area (Figure 1B), the basement is formed by the
south‐eastern margin of the Kaapvaal Craton, which con-
sists of an assemblage of Archaean lithologies. The ~2.9Ga
Pongola Supergroup, which forms the dominant substrate
for the Dwyka Group, is made up of tilted and folded sedi-
mentary and volcanic rocks metamorphosed at greenschist
facies grade (Luskin et al., 2019). Locally, in the central
part of the study area, fragments of 3.3–3.4Ga greenstone
belts are exposed. These fragments consist mainly of tightly
folded mafic to ultramafic volcanic rocks and cherts sub-
jected to amphibolite facies grade (Hofmann et al., 2019).
Further north‐east, the basement is formed by 3.1Ga granit-
oids of the Mpuluzi suite (Kröner et al., 2019). In the south-
ern part of the study area, the Natal Suture Zone (Figure
1B) dated to 1.20–1.08Ga is characterized by an array of
major east‐west striking thrust faults that formed during the
accretion of terranes to the Kaapvaal Craton (Spencer et
al., 2015). Mainly south and east of the Natal Suture Zone,
Siluro‐Ordovician clastic sedimentary rocks of the Natal
Group cover the older basement and underlie the Dwyka
Group (Marshall, 2006).
In south‐eastern South Africa, Dwyka Group strata are
usually divided into three palaeogeomorphic regions ac-
cording to syn‐depositional bathymetries (Figure 1A). The
northern region is ascribed to a levelled highland palaeo-
plain, the central one being the coastal‐platform domain,
while the southern region is commonly interpreted as the
deep part of the Karoo Basin (Matthews, 1970; von Brunn,
1994, 1996; Haldorsen et al., 2001). The boundary be-
tween the central and the southern regions may correspond
to the Natal Suture Zone, which had a profound influence
on sedimentation modes (Figure 1B; von Brunn, 1994;
Tankard et al., 2009). The present study only focuses on
the central region (Figure 1; von Brunn, 1996). Here, the
Dwyka Group ranges in thickness between 0 and 200m
as a result of palaeorelief, and predominantly consists of
massive to faintly bedded diamictites, interstratified with
sandstones and conglomerates. Contrary to glacial de-
posits of the western Karoo Basin where four deglacial
successions have been recognized, only a single deglacial
event has been proposed, within which discrete, high‐fre-
quency ice fluctuation sequences might be recognized
(von Brunn, 1996; Haldorsen et al., 2001). In the study
area, although ash layers are occasionally interstratified
with the glaciogenic deposits, no formal age inferences
have been obtained for the Dwyka Group (Bangert and
von Brunn, 2001). Ice flow throughout the eastern Karoo
Basin have been confidently assessed on the basis of gla-
cial striae and streamlined landforms carved into the bed-
rock: while mainly flowing S to SSE in the northern and
central regions, the ice flow was directed towards the SW
in the southern domain underlain by post‐Archaean rocks,
indicating a strong basement control on ice flow (Du Toit,
1954; Matthews, 1970; Versfeld, 1988; von Brunn, 1994).
In the study area (Figure 1), the contact of the basement
with the overlying glaciogenic Dwyka Group is character-
ized by a highly uneven relief that consists of palaeotopo-
graphic lows and highs that differ in elevation by up to
200m (von Brunn, 1994). Hard‐bed polished, striated and
grooved surfaces indicating a SSE‐directed ice flow are
commonly found on the basement rocks (Du Toit, 1954;
von Brunn and Talbot, 1986; Versfeld, 1988; von Brunn,
1994, 1996; Haldorsen et al., 2001). This relief was subse-
quently almost entirely infilled by the glaciogenic depos-
its that filled local depressions, onlapped on slopes and
sealed or fringed most palaeotopographic highs. Directly
lying on the glaciogenic Dwyka Group, the lower Ecca
Group is subdivided into the lower mudstone‐dominated
Pietermaritzburg Formation (Bordy et al., 2017), which
bears rare dropstones (Tankard et al., 2009) and the upper
sandstone‐dominated Vryheid Formation. Interpreted as
postglacial sediments deposited after the retreat of the ice
margin from the basin, this succession might have been
fed at times by the ice sheet that remained on the northern
highland, the so‐called Cargonian Highland (Figure 1),
long after its disappearance from the basin (Visser, 1990,
1993, 1997, 1995; von Brunn, 1994, 1996; Isbell et al.,
2008; Buatois et al., 2010).
3
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MATERIAL AND METHODS
Fieldwork was carried out in 2017 and 2018 in KwaZulu‐
Natal Province of South Africa (Figure 1). While attention
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DIETRICH anD HOFMann
was paid to preglacial landforms throughout the study area
(Figure 2), representative high‐resolution stratigraphic sec-
tions (scale 1:100) were logged at outcrops and facies along
these sections were systematically noted and grouped into
sedimentary units (Figures 3 and 4). Facies analyses included
observation of grain size, sorting, sedimentary structures,
stratigraphic architecture and stacking pattern, geometries of
the sedimentary bodies and stratigraphic relationships with
underlying, overlying and adjacent units wherever feasible
(Figure 5). Samples of representative facies were collected
and thin sections prepared at the University of Johannesburg
for petrographic studies (Figure 6). Characteristic features
of both hard and soft‐bed GS and landforms, such as pol-
ish, striation (direction, shape, size, etc.), asymmetry, nature
and lithology, were systematically investigated and mapped
(Figures 1 through 10) when encountered.
4
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RESULTS
4.1
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The pre‐Dwyka palaeosurface: a hard‐
bed GS
A marked palaeorelief characterizes the Archaean base-
ment onto which the Dwyka Group lies in the study area.
The mostly gentle slopes can, however, locally dip at up to
35–40° and vertical walls against which glaciogenic strata
onlap have also been observed sporadically (fig. 4.5 in von
Brunn, 1994, see also Versfeld, 1988). Polished, striated
and grooved surfaces as well as trains of medium‐scale
streamlined, asymmetrical erosional landforms have been
recurrently observed carved into the bedrock throughout
the study area (Figure2). Asymmetrical landforms display
gently sloping polished and striated NNW‐facing sides
FIGURE 2 Hard‐bed streamlined landforms developed on Archaean basement rocks that underlie the Dwyka Group in the study area.
(A) Roches moutonnées developed in Archaean mafic volcanic rocks. Ice flow was towards the south (towards the observer) as indicated by the
plucked, lee side of the form. (B) Conspicuously asymmetric Roches moutonnées developed in Archaean quartzites of the Pongola Supergroup.
Ice flow towards the south (left) is clearly evidenced by the asymmetry of the form, the up‐glacier side (right) being shallow and striated while
the down‐glacier (left) side is devoid of striae (plucked) and steeper. Note the faintly stratified diamictite covering the striated pavement (white
arrows) (C) A U‐shaped trough, 800m wide and 100m deep, carved into Archaean basement rocks of the Pongola Supergroup, filled with Dwyka
sediments (dark strata, indicated by white arrows) and subsequently exhumed by sub‐modern erosion. This trough, oriented in a NNW‐SSE
direction, is thought to have been carved by flowing ice during Dwyka times. Figure 1B for location.
252
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DIETRICH anD HOFMann
FIGURE 3 Synthetic stratigraphic
log of the Dwyka Group in the study area.
The three sedimentary units (SU1 to SU3)
delineated by glacial erosion surfaces (GS)
are shown. Note that the measured section is
located in the southward continuation of the
glacially carved U‐shaped trough (Figure
2C). See Figures 1B and 6A for location.
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DIETRICH anD HOFMann
FIGURE 4 Detailed sedimentary logs from the study area. See Figure 1B for location.
csfcg
vf mvc
0
5
10
15 GS2
csfcg
vf mvc
40
45
50
55
60
65
70
GS3
csfcg
vf mvc
80
85
90
95
Log A
Log C
Log F
csfcg
vf mvc
0
5
10
15
20
25
30
35
Log B
GS3
GS2
csfcg
vf mvc
0
5
10
15
GES 1
csfcg
vf mvc
40
45
50
55
60
GS3
Ecca Group
GS2
GS2
csfcg
vf mvc
0
5
10
15
20
25
30
Log D
csfcg
vf mvc
35
40
45
csfcg
vf mvc
0
5
10
15
20
25
30
35
Log E
GS1
GS2
csfcg
vf mvc
0
5
10
15
20
25
30
35
Bedrock
GS0
Fig. 7 h
Fig. 8A
Fig. 9E
Fig. 8B
Fig. 8E
Fig. 9F
Fig.9H
Fig. 8C
Fig. 10A
Fig. 10B
Fig. 10D
Fig. 10C
Fig. 10F
Bedding punctured
by dropstones
Coarse/conglo. sandstone
Sandstone
Massive diamictite
Deformed diamictite
Fine-grained diamictite
Mudrocks with dropstones
Wave ripples
Climbing current ripples
Trough cross-bedding
Downstepping
extensional fractures
Concretions
Clasts-lonestones
Glacial Erosion Surface
Ice flow direction
Gravel lag
Sandstone raft
Lithofacies
Sedimentary structures
Black shale
Sedimentary units
SU1: grounding zone wedge
SU2: ice-contact fan
SU3: mixed-influenced
(GZW-ice-contact fan)
Ecca Group
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DIETRICH anD HOFMann
and steeper SSE‐facing sides devoid of polishing or stria-
tion (Figure 2A and B). Of particular interest is an out-
standing exhumed U‐shaped trough, 100 m deep, 800 m
wide and at least 2.3 km long, carved into strata of the
Pongola Supergroup that has been discovered in the study
area (Figure 2C). This trough’s long profile is oriented
NNW‐SSE (Figure 1) i.e. parallel to inferred ice flow.
Collectively, these glacial erosion features, as they under-
lie the studied glaciogenic sedimentary succession, define
the lowermost GS which is here named GS0.
4.2
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Facies analysis, soft‐bed GS and
stratigraphic architecture
In the study area, the Dwyka Group ranges in thickness
from 0 to 200m, which is mainly controlled by the pre‐gla-
cial relief, the greatest sedimentary thicknesses being found
in palaeo‐depressions (Figures 3 through 5). In palaeo‐de-
pressions, ~ 80–90% of the total thickness of the Dwyka is
made up of massive to crudely stratified diamictite, with
the remainder consisting of conglomerate and sandstone.
Within each of these three fundamental lithofacies (diam-
ictite, conglomerate, sandstone), variations in grain size
and sedimentary structures were observed (Figures 3 and
4, see also von Brunn, 1994). Photomicrographs of some
peculiar facies are displayed in Figure 6. Beside the above‐
described basal hard‐bed glacial erosion surface character-
izing the base of the sedimentary succession (GS0), three
other soft‐bed GS (GS1–GS3) have been unravelled at dif-
ferent stratigraphic levels in the Dwyka succession where
the thickness is greatest and hence the sedimentary record
the most complete. In areas of reduced thickness, only one
or two GS are commonly found. These soft‐bed GS are
associated with striated and grooved pavements, glacial
lineations and fluting or glaciotectonic deformation, as de-
tailed below. These GS were used to delineate three super-
imposed sedimentary units (SU1–SU3) characterized by
particular assemblages of diamictites, conglomerates and
sandstones. These three sedimentary units are interpreted
as ice fluctuation sequences. Note that other subordinate
FIGURE 5 Panoramic views showing the stacking patterns of three fluctuation sequences and the filling by Dwyka sediments of preglacial
topography developed in the underlying basement. (A) Southward continuation of the U‐shaped trough (GS0 highlighted by the wavy white line)
filled by Dwyka strata. Note the presence of the four glacial surfaces in the landscape. Location of the log in Figure 3 is shown. (B) Same area
observed from the other side of the Buffalo River. The glacial surface topping the first sedimentary unit (SU1), and forming a horizontal surface
well visible in the landscape, might portray the upstream face of a GZW (see text for details). Glacial grooves displayed in Figure 7G have been
found on this surface. (C) SU3 made up of 40m thick stacked crudely stratified to massive diamictite topped by a glacial surface. See Figure 1B for
location.
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DIETRICH anD HOFMann
GS observed throughout the sedimentary succession are in-
terpreted as within‐trend, lower‐rank fluctuation events, as
they are not distributed regionally and/or do not delineate
fundamentally different facies assemblages. Interestingly,
the log displayed in Figure 3, flanked by basement slopes,
and representing one of the most complete sedimentary
section found in the study area, lies in the direct projected
south‐eastward continuation of the cross‐shelf trough de-
scribed above (Figure 1).
4.2.1
|
Sedimentary unit 1
The lowermost sedimentary unit ranges from 0 to 70m in
thickness and exclusively occurs in topographic depres-
sions of the palaeo relief – and notably in the southward
continuation of the above‐described cross‐shelf trough
(Figures 5A, 6A and 7), the slopes of which are onlapped
or conformably draped by glaciogenic deposits (Figure
7A). This sequence is made up almost entirely of a grey-
ish to bluish diamictite bearing abundant clasts wrapped
in a muddy to sandy matrix (Figures 6A and 7A,B,C).
The diamictite may either be massive or crudely bed-
ded, exhibiting both normal and inverse grading (Figure
7C). The massive variety under the microscope reveals a
fabric devoid either of clear stratification or deformation
patterns (Figure 6A). The clasts occur in a muddy ma-
trix, range in size from sand to boulder, show the whole
range of degrees of roundness and consist predominantly
of granitoids, but also of komatiite, gneiss, basalt, con-
glomerate, quartzite, banded iron formation and (banded)
chert, representing both local (Archaean basement) and
exotic lithologies brought from at least 250km away (Du
Toit, 1954). Clasts are occasionally striated and/or fac-
eted. Some diamictite intervals are conspicuously devoid
of small clasts (Figure 7D), and locally interstratified
by well‐sorted, hummocky cross‐stratified, fine‐grained
sandstones (Figure 7E). In some rare occasions, lone-
stones are found puncturing and downwarping the faint
bedding developed in some finer‐grained diamictite.
Laminated, decimetre‐thick and very poorly sorted pol-
ymictic conglomeratic sandstone beds and lenses that
generally wedge out over a few metres are occasionally
incorporated within the diamictite (Figure 7F). Clasts
incorporated in these conglomeratic lenses are generally
well‐rounded and are wrapped in a sandy matrix exhibit-
ing faint horizontal to undulating bedding.
The uppermost 10m of SU1 is formed by the same grey-
ish to bluish diamictite bearing abundant clasts but bedding
is significantly better developed (Figures 3 and 8). The
planar to undulating top surface of this bedded diamictite,
which also forms the top surface of this first sedimentary
unit, is prominently marked by soft‐bed glacial erosion
forms such as striae, grooves and flutes, corresponding
to glacial surface 1 (GS1, Figures 5A,B and 7G). These
FIGURE 6 Photomicrographs (crossed polarized light) of some of the facies detailed in the text. (A) Massive diamictite forming the bulk
of the lowermost sedimentary unit; (B) and (C) climbing current ripple lamination that occurs in the second sedimentary unit (SU2). Note the
rhythmic lamination pattern, outstanding inverse grading and the outsized clasts (dropstones) puncturing and downwarping the lamination; (D) An
outsized sand grain in a matrix of muddy fine sand. Note the grain‐band structure (white arrow).
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glacial erosion features are commonly reworked by wave
and interference ripples (Figure 7H) or are occasionally
covered by pebble and cobble lags. Many basement highs
protrude from this GS against which the wave‐rippled unit
reworking the GS wedges out (Figure 5A). As sealed by
finer‐grained deposits (see below, sedimentary unit 2), GS1
capping this first fluctuation sequence hence appears in
the landscape as a horizontal surface highlighted by recent
erosion (Figure 5A and B).
In some places, a more complex facies assemblage up to
10m thick characterizes the top of SU1, displaying abrupt
vertical and lateral facies change (log C in Figures 4 and
FIGURE 7 Facies forming the bulk of the lowermost sedimentary unit (SU1) of the Dwyka Group in the study area. (A) Steep‐flanked
palaeotopography developed in basement rocks (left; quartzites of the Pongola Supergroup) and draped by crudely stratified diamictite (centre,
right). Bedding is steep on the palaeoslope and becomes horizontal away from it. (B) Close‐up view of massive to crudely stratified diamictite.
Note the dispersion tail on the left (lee) size of the boulder. A photomicrograph of this facies is displayed in Figure 6A. (C) Crudely stratified
diamictite facies. Beds that are decimetre‐thick and show inverse grading are interpreted as highly concentrated gravity flows (Visser, 1983; Sohn,
1997). Circled hammer for scale. (D) Diamictic facies devoid of small clasts but encompassing large, angular clasts. Note the isotropic nature of the
matrix that shows spheroidal weathering. (E) Hummocky cross‐bedded sandstones interpreted as storm deposit and interstratified within diamictite
facies. (F) Conglomeratic sandstone lens showing crude internal lamination and pinching‐out over a few metres. This lens is interstratified within
massive diamictite (B) bearing abundant clasts up to boulder in size. Circled hammer for scale. (G) Glacial grooves/flutes characterizing the top of
SU1 and developed in crudely stratified diamictite. Ice flow was towards the observer. Compass for scale. (H) Interference ripples reworking the
glacial surfaces (GS1). See Figures 3 and 4 for locations.
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DIETRICH anD HOFMann
8, see also von Brunn and Talbot, 1986). This assemblage
stacks from base to top, or laterally juxtaposes: (a) a fine‐
grained, dark grey, fissile argillaceous and weakly lam-
inated diamictite with clasts up to boulder size (Figure
8A and E); (b) a coarse‐grained, massive, arenaceous and
clast‐bearing diamictite (Figure 8A); (c) trough cross‐
stratified, poorly sorted and occasionally conglomeratic
sandstones (Figure 8B) that rework the underlying coarse‐
grained diamictite and incorporate some of its biggest
clasts; (d) normally graded, decimetre‐thick and laminated
sandstone beds interstratified with laminated siltstones
(Figure 8C); and (e) a 1m thick sheet of massive diamictite
bearing angular rock fragments conspicuously character-
ized by soft sediment deformation (Figure 8D). All these
facies are disrupted, sometimes intensely, by clastic dykes
and sheet intrusions, small‐scale downstepping extensional
fractures, conventional and intraformational striae and ev-
idence of horizontal shearing (Figure 8A and E), features
which collectively indicate the presence of a composite
GS (see also von Brunn and Talbot, 1986). At one locality,
large‐scale elongated ridges, 5–10m in width, with a relief
of up to 1m and at least 100m in length, and moulded
in a greenish, massive and fine‐grained diamictic mate-
rial characterized by abundant angular pebble‐sized clasts
have been observed on top of this particular assemblage of
facies (Figure 8F). Small‐scale soft sediment deformation
such as folds and shear bands is frequently observed within
this diamictite (Figure 8D).
4.2.2
|
Sedimentary Unit 2
The second sedimentary unit (SU2), floored by GS1, also
shows thickness variations (10–50 m), partly owing to
underlying basement highs not completely sealed by SU1
and protruding from GS1 (Figures 3 and 5). The facies as-
sociation (Figure 9) characterizing this unit differs from the
FIGURE 8 Facies that characterize the top of the first sedimentary unit (SU1) and interpreted as the effective grounding line of the
grounding zone wedge (GZW). (A) Fine‐grained, dark grey fissile diamictite overlain by massive, coarser‐grained diamictite. Note the presence of
clastic dykes and sheet intrusions in the lower diamictite interpreted as evidences of subglacial shearing. (B) Conglomeratic sandstones reworking
the massive and coarse‐grained diamictite. Note the presence of a large boulder (2m across) incorporated in the sandstone layer. (C) Normally
graded sandstone beds. (D) Greenish fine‐grained diamictite with abundant small‐scale, generally angular clasts and well‐developed folds, and
interpreted as subglacial till. (E) Clastic dykes developed in fine‐grained dark grey diamictite. Shearing is evident from the fissile character of this
fine‐grained diamictite. (F) Fine‐grained, greenish matrix forming elongated ridges interpreted as flutes. See Figures 3 and 4 for locations.
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DIETRICH anD HOFMann
underlying one as it consists of an interbedding of somewhat
finer‐grained diamictic material and sandstone beds that
overall show an upward increase in the sandstone proportion
along with a general grain size coarsening and bed thicken-
ing trend (Figure 3). The base of the sequence consists of
a faintly bedded yellowish‐greenish diamictite made up of
faintly laminated argillaceous siltstones with granule‐sized
lonestones (Figure 9A) and showing occasional arthropod
trackways (von Brunn and Talbot, 1986). This facies coars-
ens upward and becomes incrementally interstratified by
thin, well‐laminated, well‐sorted and normally graded yel-
lowish fine‐grained sandstone beds (Figure 9B and C) that
themselves coarsen and thicken upward. Interestingly, on
rare occasions, rhythmites formed by supercritical to in‐phase
climbing current ripples (sensu Hunter, 1977) and developed
in fine‐grained sandstones have been observed (Figures 6B,C
and 9D). Microfacies analysis reveals conspicuous inverse
grading and the presence of sand‐sized clasts puncturing the
bedding, and against which further laminae onlap (Figure 6B
and C). Above, the sandstone beds thicken and coarsen. Some
reddish scour‐based conglomeratic sandstone beds display
normal and inverse grading as well as soft sediment deforma-
tions (Figure 9E and F). In addition, the above‐described in-
terstratified diamictite grades into a greyish to bluish massive
to crudely stratified, sandy diamictite similar to the one mak-
ing up the bulk of SU1, and hosting abundant lonestones up
to boulder‐sized. The upper 5m of the sedimentary sequence
is made up of stacked conglomeratic sandstone beds that
commonly display horizontal, trough and planar cross‐bed-
ding (Figure 9G), climbing current and wave ripples, gravel
FIGURE 9 Facies that make up the
second sedimentary unit (SU2) (A) Faintly
laminated argillaceous siltstone with
granule‐sized lonestones. (B) Alternation
between facies shown in (A) and planar‐
laminated, and occasionally normally
graded sandstone beds. (C) Close‐up view
of planar‐laminated sandstone beds. (D)
Supercritical to in‐phase climbing current
ripples displaying rhythmic modulation
of the angle of climb, see Figure 6B and
C for photomicrographs. (E) Alternation
between reddish coarse‐grained, normally
graded sandstone and diamictite bearing
abundant clasts. (F) Close‐up view of the
coarse‐grained, sometimes conglomeratic,
normally graded sandstone beds. (G) Trough
cross‐bedded, poorly sorted, occasionally
conglomeratic sandstones. (H) Boulder
pavement with planar upper surfaces
observed on top of SU2.
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DIETRICH anD HOFMann
lags, soft sediment deformations and fluid escape structures.
Some segments of the uppermost sandstone beds are folded
at a large wavelength of hundreds of metres forming large
depressions, while grain‐band structures (sensu Busfield and
Le Heron, 2018) have been observed microscopically (Figure
6D). A 3m thick, boulder‐bearing, clast‐supported, massive
to faintly laminated conglomeratic sheet capped by a boulder
pavement with planar upper surfaces has been observed at
the top of this coarsening‐upward succession at one locality
(Figure 9H and log A in Figure 4).
4.2.3
|
Sedimentary Unit 3
The third and uppermost sedimentary unit (SU3) consists
of an alternation of diamictite and sandy‐conglomeratic
deposits and is commonly 60–70 m thick but reaches
100m in places (Figures 4, 5C and 10). The base of the
sequence ubiquitously consists of the greyish to bluish
massive to crudely stratified diamictite that bears abun-
dant clasts up to boulder size, similar to the one making
up the first sedimentary unit. Further upward, interlay-
ered with diamictite beds, thick (1 m), poorly sorted,
faintly laminated and mostly clast‐supported conglomer-
atic layers occur in association with deformed sandstone
rafts, clastic dykes and occasional sandstone beds with
climbing ripple cross‐lamination. On a few occasions,
elongated meso‐scale ridges 5–10m in width and 1m in
depth and moulded in a greenish massive diamictite that
show soft sediment deformations and pebble‐sized clasts
(Figure 10A) have been observed. The top surface of these
meso‐scale streamlined features bears striae and grooves
FIGURE 10 Sedimentary facies
that compose the third sedimentary unit
(SU3). (A) Elongated ridges (section
view) 5–10m in width and with a relief
of 1m and made of greenish massive
diamictite are interpreted as flutes. (B)
Superimposed on flutes (A) are striae and
grooves oriented parallel to the long axis
of the ridges as well as rill marks. (C)
Alternation between massive to faintly
laminated diamictite bearing abundant clasts
and clast‐rich conglomerate. (D) Granular
sandstone beds showing well‐developed
planar cross‐bedding and conglomeratic
layers. (E) Lonestone puncturing the
bedding developed in mudstones. (F) Soft
sediment glacial pavement (note the lateral
bulges on both sides of the groove) locally
characterizing GS3. (G) Complex folding
developed in dropstone‐bearing mudstone
and interpreted as subglacial glaciotectonic
deformation. (H) Downstepping extensional
fractures shown in plan‐view and developed
in dropstone‐bearing mudstone (black
arrows).
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DIETRICH anD HOFMann
oriented parallel to the long axis of these ridges, and
occasional rill marks (Figure 10B). Rare boulder pave-
ments with faceted upper faces occur in association with
the elongated ridges and are sealed by finely laminated,
poorly sorted siltstones with abundant IRD up to cob-
ble size. The top of this sedimentary unit is either made
up of a conspicuously stratified diamictite, which differs
from the basal diamictite in possessing a coarser‐grained
matrix, in a similar way to the lowermost fluctuation
sequence, or by a stack of poorly sorted conglomeratic
sandstone beds displaying planar and undulating lami-
nations, planar, trough and climbing‐dune cross lamina-
tions and climbing current ripples (Figure 10C and D).
A striated pavement displaying striae and small‐scale
grooves was observed at the top of this sandstone succes-
sion at one locality (Figure 10F), where it was reworked
by small‐scale, curve‐crested dunes. Alternatively, sand-
stones and diamictites are capped by a thin (1–3 m)
layer of well‐laminated IRD‐bearing greenish mud-
stones (Figure 10E), intensively deformed by pervasive
soft sediment deformation such as folding and faulting
(downstepping extensional fractures; Figure 10G and H).
The top surface of this unit, which also constitutes the top
of the Dwyka Group in this region, is highly undulating
and uneven, forming a relief that can attain 40 m over
kilometre‐scale wavelengths. The contact with the
overlying black shales of the lower Ecca Group
(Pietermaritzburg Fm) is either abrupt above deformed
IRD‐bearing mudstones or transitional where undeformed
mudstones grade upward into the black shales (Figure 3
and logs E and F in Figure 4).
5
|
INTERPRETATIONS:
DEPOSITIONAL ENVIRONMENTS,
ICE‐MARGIN FLUCTUATIONS
AND ASSOCIATED MODE OF
SEDIMENTATION
5.1
|
The pre‐Dwyka palaeosurface: an
exhumed glacial landscape
Polished and striated surfaces undoubtedly indicate that
flowing ice carved the bedrock prior to deposition of the
Dwyka Group in this region. Asymmetrical bedforms are
interpreted as Roches moutonnées and confirm that ice was
flowing towards the SSE. Oriented parallel to the inferred
deepening axis of the basin, and eroded into the central pal-
aeobathymetric domain interpreted as a coastal‐platform do-
main (see above and Figure 1A), the U‐shaped trough might
be regarded as a small‐scale cross‐shelf trough carved by
subglacial processes probably linked to flowing ice, as also
evidenced by streamlined forms and polished pavements
(Krabbendam et al., 2016; Newton et al., 2018; see also
Andrews et al., 2019).
The palaeosurface onto which the Dwyka Group lies
thus corresponds to a glacial landscape scoured by the
SSE‐ward moving glaciers and subsequently sealed by gla-
ciogenic sediments. In recent times, preferential erosion of
the Dwyka Group over the more resistant Archaean lith-
ologies of the basement has exhumed this pre‐Dwyka pa-
laeosurface. The path of the modern Tugela and Buffalo
rivers (Figures 1 and 2C) closely follows the network of
pre‐Dwyka fluvial and/or glacial valleys that had been
completely filled up with glaciogenic sediments during
the glacial period and subsequently re‐excavated during
Cenozoic and Recent times. Hence, in KwaZulu‐Natal
Province, in areas where glaciogenic deposits have been
entirely eroded away, the modern topography exhib-
its Dwyka‐aged polished and striated surfaces as well as
roches moutonnées carved into the ancient basement and
hence corresponds to the exhumed Permo‐Carboniferous
glacial landscape that prevailed at the onset of the depo-
sition of the Dwyka sediments (von Brunn, 1994, 1996;
see also Guillocheau et al., 2018). Such an exhumed LPIA
glacial landscape echoes those of the same age found in
Namibia (Andrews et al., 2019), Chad (Le Heron, 2018)
and in South America (Assine et al., 2018).
Although it appears evident that glacial erosion was
at least partly responsible for carving and shaping the
pre‐Dwyka palaeosurface (striated and polished surfaces,
Roches moutonnées), it seems, however, probable that
other processes also contributed to the arrangement of
this surface prior to the deposition of the Dwyka Group.
Indeed, as stated by von Brunn (1994, 1996), the complex
structural grain of the Archaean basement rocks probably
contributed significantly to the uneven configuration of
the palaeosurface which was later modified and accen-
tuated by glacial scouring that exploited existing weak-
nesses (Newton et al., 2018). The presence of vertical
walls undoubtedly indicates that the structural heritage
had a preponderant influence in the shaping of the base-
ment. Moreover, a pre‐existing network of fluvial valleys
was potentially reworked and amplified by direct glacial
abrasion (von Brunn, 1994, 1996), as is the case for some
Quaternary high‐latitude valleys that originated from the
combination of both pre‐glacial fluvial and glacial erosion
processes (Lajeunesse, 2014; Livingstone et al., 2017).
Indeed, the major hiatus that separates the basement from
the Dwyka Group suggests that the study area constituted
an emerged domain during pre‐Dwyka times as a result of
crustal uplift preceding the inception of the Karoo Basin
and the onset of glaciation (Visser, 1990; Tankard et al.,
2009) and onto which a fluvial system may well have
developed.
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DIETRICH anD HOFMann
5.2
|
Depositional environments
5.2.1
|
SU1: Grounding zone wedges
Von Brunn (1994, 1996) and Haldorsen et al. (2001) inter-
preted the diamictite‐dominated unit SU1 as having resulted
from the settling of suspended material and dropstones
dumped from drifting icebergs close to the ice front as well as
resedimentation of glaciogenic material through debris flows
(see also Visser, 1997). Indeed, the large array of predomi-
nantly diamictic facies supplemented by intervening conglom-
erate and sandstone beds observed throughout this sequence
indicates a variety of highly energetic depositional processes.
Indeed, resedimentation of (subglacial) glaciogenic mate-
rial through sediment gravity flows (mass and debris flow,
turbidites) is thought to have originated in the deposition of
massive to faintly bedded diamictite, conglomerate lenses as
well as normally graded sandstone beds (Talling et al., 2012;
Le Heron et al., 2017 and references therein), while massive
rain‐out of debris and dumping of debris from either a float-
ing ice shelf or abundant drifting icebergs are being expressed
by IRD‐bearing diamictite (e.g. Figure 6D) possibly supplied
by a meltwater plume (Dowdeswell et al., 2015). Hummocky
cross‐stratified deposits (Figure 6E) are interpreted as origi-
nating from storm events. So the build‐up of this thick
sequence thought to have occurred in a subaqueous, ice‐prox-
imal setting continuously supplied by glaciogenic materials,
as evidenced by exotic lithologies, and the presence of inter-
vening GS. Furthermore, the occurrence of the fine‐grained,
greenish diamictite material bearing soft sediment deforma-
tion (Figure 8D) and interpreted as subglacial till (Evans et
al., 2006), striae and grooves forming a composite GS (log C
in Figure 4 and Figures 7 and 8) capping SU1 points towards
the presence of a nearby grounded, fluctuating ice front prone
to override its own proximal sedimentary products. Finally,
elongated ridges moulded into subglacial till represent meso‐
scale streamlined landforms interpreted as flutes (Ely et al.,
2016) may indicate they were implemented in a subglacial
environment by overriding flowing ice (Schoof and Clarke,
2008). Collectively, the association of these deposits (Figures
7 and 8) capped by a composite GS (Figure 8) enables this
succession to be interpreted as a GZW (see also Visser,
1997). The GZW correspond to asymmetrical, wedge‐shaped
depocenters emplaced at the submarine grounding zone of
fast‐flowing or streaming ice by the continuous delivery and
resedimentation through a variety of sedimentary processes
of subglacial material (till) from up‐glacier (von Brunn and
Talbot, 1986; Powell and Domack, 2002; Dowdeswell and
Fugelli, 2012; Bell et al., 2016; Dowdeswell et al., 2016a,
2016b; Lajeunesse, ; Bart et al., 2017; Prothro et al., 2018;
Lajeunesse et al., 2018; Batchelor et al., 2018; see also Demet
et al., 2019 for some facies). Although generally interpreted
as being deposited at the grounding line of a floating ice shelf
(Batchelor and Dowdeswell, 2015), GZWs may also form at
tidewater termini, depending on the availability of meltwa-
ter and/or deformability of subglacial till (Powell and Alley,
1997). These authors, however, indicate that a tidewater ter-
minus would promote the deposition of ice‐contact fan and
hemipelagic sediments in proximal and distal positions, re-
spectively, in which case IRD would be exhausted within a
few hundred metres of the grounding zone. Hence, the virtual
absence of hemipelagics as well as the wealth of clasts (drop-
stones) throughout this unit suggests that a floating ice shelf
probably existed during the deposition of this lowermost unit.
Such an interpretation would account for the diversity of
facies and associated processes observed in this sedimentary
unit as well as its significant thickness throughout the study
area. In such a context, the complex facies assemblage at the
top of SU1 and displaying abrupt vertical and lateral facies
change (Figure 8), clastic dykes and sheets and superim-
posed striated surfaces (composite GS) might correspond to
the effective grounding zone of the GZW prone to multiple
ice‐marginal fluctuation and episodic grounding (von Brunn
and Talbot, 1986). Wave and interference ripples reworking
in place the GS indicate that the ice retreated after the con-
struction of GZW. It is thought that the GZW was then aban-
doned and subsequently winnowed and washed‐out of the
fine fraction of the diamictite by nearshore processes, either
immediately or after a fall of RSL that exposed the summit of
the GZW – the effective GS – to wave action (Dietrich et al.,
2017, 2018; Demet et al., 2019). Furthermore, the wedging
of wave‐rippled sediments or the pebble/cobble lag against
the basement slopes indicate that by the time basement highs
situated above this horizon emerged only palaeotopographic
lows were inundated. An outstanding modern analogue of
such a GZW reworked in a shallow‐marine domain and which
correlative shore‐related deposits onlap basement slopes is
given in Lajeunesse et al. (2018, their fig. 12).
5.2.2
|
SU2: Ice‐contact fan
On the one hand, the upward coarsening and thickening of the
sandstone beds is indicative of a progradational sequence of
a subaqueous sedimentary system over which high‐density
sediment gravity flows such as turbidites and debris flows
expressed by normally graded sandstone beds and conglom-
eratic layers (Talling et al., 2012) spread. On the other hand,
the diamictite interstratified in between these sandstone and
conglomeratic beds indicates the proximity of an ice margin.
The general coarsening‐upward trend, as well as the increasing
proportion of IRD, suggests that the ice margin was advancing
towards the depositional area. The coarse grain size of the up-
permost sandstone beds, the rapid coarsening‐upward as well
as soft sediment deformation and sedimentary structures wit-
nessing traction flows indicate that the deposition was rapid
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DIETRICH anD HOFMann
and sustained in a dynamic, proximal setting. In such a glacial
context, this sedimentary sequence is therefore interpreted as
a glaciofluvial or ice‐contact delta or fan (sensu Lønne, 1995,
see also Lønne et al., 2001; Dowdeswell et al., 2015). Delta
topset beds might be represented by the uppermost cross‐bed-
ded and rippled sandstone beds. The boulder conglomerate
capping this sedimentary sequence is interpreted as the most
dynamic deposit, most probably deposited in an ice‐contact
environment by the deconfinement of meltwater flows exiting
subglacial tunnels (Russell et al., 2006; Alexander and Cooker,
2016; Aquino et al., 2016). Altogether, the boulder pavement,
the undulating geometry of the sandbeds making up the top
surface of SU2, as well as the microscopic grain bands that
could be interpreted as boudins are evidence of overriding ice
(Visser and Hall, 1985; Buechi et al., 2017; Busfield and Le
Heron, 2018 and references therein). The rhythmic climbing
ripples seem to indicate a tidal influence on deposition and
might have resulted from the interaction of tides with glacioflu-
vial inputs to generate and/or support tide‐influenced sediment
gravity flows (Smith et al., 1990; Cowan et al., 1998; Dietrich
et al., 2017). Observed inverse grading (Figure 6B and C) may
represent waxing flow of hyperpycnal events (Mulder et al.,
2001), whereas sand clasts puncturing the laminae are inter-
preted as dropstones derived from floating ice (sea ice, drifting
icebergs or ice shelf; Dowdeswell et al., 2015).
In contrast to the underlying sedimentary succession ei-
ther emplaced at the grounding zone of a floating ice shelf
or at a tidewater terminus, ice‐contact fans are exclusively
associated with tidewater ice margins (Powell and Alley,
1997; Batchelor and Dowdeswell, 2015). The presence of a
floating ice shelf nearby cannot, however, be conclusively
ruled out, as both the diamictite and sandstone horizons bear
IRD. Lonestones could also originate from drifting icebergs
or sea ice or, alternatively, from resedimentation of till ma-
terial through debris flow (Vesely et al., 2018). Hence, the
upward disappearance of the diamictite in favour of the sand-
stones might either be interpreted as the increasing influence
of the progradation of the ice‐contact fan on sedimentation
or, alternatively, as the emergence of an initially subaqueous
ice‐contact fan that would have grown up into an ice‐contact
or glaciofluvial delta. Such a transition would have promoted
the demise of a glaciomarine system and permitted the transi-
tion from a marine to a continental ice margin (Powell, 1990;
Lønne et al., 2001; Dowdeswell et al., 2015).
5.2.3
|
SU3: Mixed‐influenced GZW‐ice‐
contact fan
This third and uppermost fluctuation sequence appears as an
amalgamation of deposits resulting from GZW sedimentation
(diamictite) and from ice‐contact fans and deltas (coarse‐grained
sandstones). It is therefore suggested that SU3 corresponds to
a glaciomarine setting comprising and interdigitating ice‐con-
tact fans and GZWs, similar to the setting shown in fig. 20b
of Dowdeswell et al. (2016b). Such a combination arguably
results from the availability of basal meltwater emerging at
the grounding zone and permitting the deposition of ice‐con-
tact fans in a setting otherwise dominated by the continuous
delivery of subglacial material along a line source (GZW). The
meso‐scale elongated ridges made up of greenish diamictic
material showing soft sediment deformation are thus also in-
terpreted as flutes and hence suggest the presence of flowing
ice. Finally, the intensively deformed and folded horizons of
IRD‐bearing mudstones, sometimes associated with downstep-
ping extensional fractures, and lying in the same stratigraphic
position as striated and grooved pavement (Figure 9F) are in-
terpreted as the effect of subglacial glaciotectonism (Evans et
al., 2006).
6
|
DISCUSSION
6.1
|
Ice margin fluctuations and
grounding‐line sedimentation
As each sedimentary unit is bounded at its bottom and top
by GS, they are interpreted as recording ice‐margin fluc-
tuations. In such a context, the lower part of each sequence
records the retreat of the ice margin from the position it
occupied at the implementation of the underlying GS while
the upper part corresponds to the glacial advance culmi-
nating in the overriding of ice (development of the over-
lying GS). Note, however, that the turnaround between
the ice‐margin retreat and the subsequent advance may
be largely undecipherable in the sedimentary record; each
fluctuation sequence can be highly asymmetric and facies
dislocation does not necessarily represent any ice‐margin
retreat‐advance trend reversal but rather reflect prograda-
tion of related sedimentary systems (Normandeau et al., in
press; Powell and Cooper, 2002; Normandeau et al., 2017;
Dietrich et al., 2018).
Glacial maxima conditions during Dwyka times are
thought to be represented by the lowermost GS carved into
the bedrock (GS0) when the ice margin was located in south‐
west South Africa and KwaZulu‐Natal was entirely covered
by ice (Visser, 1997; Haldorsen et al., 2001; Tankard et al.,
2009). The ice‐margin then retreated throughout the Karoo
Basin and stabilized over the study area. This stabilization
was probably permitted by the grounding of the ice margin
on the coastal‐platform domain of the eastern Karoo Basin
(Figure 1A), where abundant bedrock pinning points may
have acted as a zone of reduced water depth that permit-
ted the ice anchorage. The first fluctuation sequence (SU1)
was deposited during ice‐margin stabilization in front of
the grounding line by the deposition of the GZW (SU1;
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DIETRICH anD HOFMann
Figure11). The GS (GS1) capping this first sedimentary
sequence is interpreted as a subsequent re‐advance of the
ice margin over its own proglacial deposits. This glacial
advance may either have been climate‐triggered (cooling)
or, alternatively, originated from a reduction of the water
depth. Such a shallowing could have either been due to a
RSL fall forced by a GIA consecutive to the ice‐margin
retreat and/or to the accumulation of sediments in front of
the grounding zone and possibly underneath the floating
ice shelf that would have permitted the anchoring of the ice
and then its advance (Alley et al., 2007; Anandakrishnan
et al., 2007; Brinkerhoff et al., 2017; Batchelor et al.,
2018). It is here thought that positive feedback existed
between pre‐glacial topography, ice‐margin stabilization
and deposition of sediments: ice anchorage and position of
the grounding zone immediately after the initial ice‐mar-
gin retreat were at least partly controlled by bedrock pin-
ning points, which in turn also controlled the position of
the GZWs themselves, acting to further stabilize the ice
margin and even permitting its autogenic re‐advance. This
first fluctuation sequence was hence mainly deposited in
troughs of the preglacial topography (Figure 11; see also
Fallgatter and Paim, 2017), while highs may have pinned
and maintained a more or less perennial ice shelf.
The onset of deposition of the second ice‐margin fluctu-
ation sequence (SU2) corresponds to the retreat of the ice
margin from its stillstand position (GS1) and its reworking
by shallow‐water currents, giving rise to wave and interfer-
ence ripples. Hence, the initial ice‐margin retreat may be only
virtually marked by the abandonment and reworking of GS1
(Figure 11); the overlying coarsening‐upward succession
(SU2) marking the subsequent ice‐contact fan and/or deltaic
progradation possibly tied to the glacial advance culminating
in the overriding of its own proglacial deposits (GS2, see also
Powell and Cooper, 2002). As may have been the case for
the underlying GZW, the progradation to aggradation of the
ice‐contact depositional system could have permitted an au-
togenic glacial advance (Boulton, 1990). A similar scenario
is envisioned for the third, uppermost fluctuation sequence
(SU3). It should be noted, however, that although amplitudes
of ice‐margin advance‐retreat fluctuations are not constrained
here, the virtual absence of prominent deposits lacking gla-
cial features, like the interglacial black shales in the western
Karoo Basin (Visser, 1997; Isbell et al., 2008) suggests that
FIGURE 11 Synthetic depositional model for the Dwyka Group in the study area, depicting the three fluctuations of the ice‐margin and their
associated deposits. The last cartoon shows the modern‐day situation where Cenozoic (Guillocheau et al., 2018) and sub‐modern erosion exhumed
glacial landscapes carved by the Dwyka ice sheets.
264
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DIETRICH anD HOFMann
retreat phases of the three fluctuations were probably of re-
stricted amplitude and of short term. The final retreat of the
ice‐margin from the study area, marked by the facies disloca-
tion characterizing the transition from the Dwyka to the Ecca,
may have abandoned the subaqueous glaciogenic depocen-
ters (GZW, ice‐contact fan) over which the Pietermaritzburg
and Vryheid delta subsequently prograded (Figure 11; see
discussion in Dietrich et al., 2018 as well as their fig. 7). Such
a deltaic progradation that would have sealed abandoned gla-
ciogenic depocenters may have permitted the preservation of
the described sedimentary succession.
It has been shown above that depositional environments
evolved throughout the Dwyka Group from GZW through
ice‐contact fan to a bimodal setting. The occurrence of these
environments is thought to be controlled by the availability
of basal meltwater possibly reflecting palaeoclimate rather
than the morphology of the ice‐margin terminus (ice shelf vs.
tidewater glacier, Powell and Alley, 1997; Dowdeswell et al.,
2016b). Hence, it is suggested that the Dwyka ice sheet re-
sponsible for the deposition of the lower fluctuation sequence
was characteristic of a high‐latitude, arguably cold polar set-
ting which permitted the implementation of GZWs owing to
the lack of flowing basal meltwater. Above, the occurrence of
ice‐contact fans (SU2) and mixed‐influenced fan‐GZW (SU3)
indicates an increase in basal meltwater availability possibly
tied to climate amelioration (Bjarnadóttir and Andreassen,
2016; Dowdeswell et al., 2016b). The total lack of eskers and
subglacial channels as well as the predominance of stream-
lined features (flutes, striated pavements, U‐shaped trough)
throughout the glaciogenic succession indicate, however, that
sedimentary processes continued to be dominated by flowing
ice, subglacial deformation and erosion and delivery of plas-
tered material (subglacial till) to the ice‐contact systems.
6.2
|
A Quaternary‐style deglaciation timing
for the Dwyka Group
Although sparse tuffaceous beds have been described from
the Dwyka Group in the vicinity of the study area, no age
inference has been extracted from them (Bangert and von
Brunn, 2001). Hence, the precise age of Dwyka sedimenta-
tion in the eastern Karoo Basin remains unknown. Modes of
sedimentation and volume of the sedimentary units can, how-
ever, provide valuable insights into the duration of their depo-
sition as they are thought to correlate with the duration of the
stillstanding ice margin that permitted their deposition (Bart
et al., 2017; Prothro et al., 2018). Despite their significant ex-
tent, several tens to hundreds of cubic kilometres, GZWs are
typically deposited in a few hundreds to thousands of years,
while duration of deposition of grounding‐line fans and deltas
ranges from years to hundreds of years (Dowdeswell etal.,
2016b, fig. 13; see also Demet et al., 2019). In fact, one of the
largest GZW of the outer Antarctica shelf (eastern Ross Sea)
emplaced during the Last Glacial Maximum (LGM) being
more than 100km in length and 200m in thickness and rep-
resenting a volume of ~ 500 km3 was deposited in less than
4,000years (Bart et al., 2017). Although no volume estimates
were deduced in the present study considering that the spatial
extent of the depocenters is unknown, thicknesses occupied
by these GZWs may be viewed as comparable or even less,
indicating that the timing of deposition was in the same order
of magnitude as the sub‐modern examples.
Then, each fluctuation sequence taken separately only
represents a few thousands to tens of thousands of years at
most. The virtual absence of hemipelagic sediments, as well
as condensed or highly bioturbated horizons separating the
fluctuation sequences, furthermore indicates rapid deposition
of, and no significant time gaps within each fluctuation se-
quence. The time span encompassed within each glacial ero-
sion surfaces is, however, not constrained but as ice‐margin
retreat phases and the deposition of associated sedimentary
suites were rapid, correlative advances were arguably of the
same frequency. In Quaternary series, GS stacked in a sedi-
mentary succession ascribed to a deglacial trend and record-
ing glacial stillstand or re‐advance represent short intervals
of time (lower‐rank glacial retreat surface sensu Zecchin et
al., 2015, see also Occhietti, 2007; Lajeunesse et al., 2018;
Dietrich et al., 2018). Hence, the duration of the deposition
of the whole Dwyka Group in this area may be viewed as
having taken place in a time range similar to Quaternary de-
glacial sequences, as already proposed by Haldorsen et al.
(2001), and being thus comparable to other ancient deglacial
sequences (Girard et al., 2015; Dietrich et al., 2018).
6.3
|
RSL change and associated GIA
As outlined above, the duration of the deposition of the whole
Dwyka Group in the study area may have been short, on the
order of a few thousands to tens of thousands of years. In a
stable epicratonic tectonic setting, RSL variations would have
only been controlled, in such a deglacial context, by glacio‐
eustatic and glacio‐isostatic fluctuations; crustal subsidence
being arguably too slow to have a notable influence over such
short periods of time (Boulton, 1990; Girard et al., 2015;
Dietrich et al., 2017, 2018). Because of minor amplitudes,
ice‐margin advance‐retreat phases recorded in the study area
are thought to have been largely uncoupled to the pattern of
continental‐scale ice sheet fluctuations (Isbell et al., 2008,
and see below), and hence unaccompanied by important
glacio‐eustatic sea‐level change. The pattern of RSL change
throughout the studied sedimentary section is hence thought
to solely reflect glacio‐isostatic processes tied to ice‐margin
fluctuations. In terms of vertical glacio‐isostatic movement,
glacial retreat must be accompanied by RSL fall forced by
the GIA (Boulton, 1990; Dietrich et al., 2018) while glacial
|
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DIETRICH anD HOFMann
advances are preceded by RSL rises induced by glacio‐iso-
static deflection (Storms et al., 2012).
Although arguably (glacio)marine in origin, depositional
bathymetries of SU1 are largely unknown. The reworking of
the overlying GS (GS1) by nearshore processes, however,
undoubtedly indicates that deposition took place in a very
shallow to sub‐emerged domain. This emersion most prob-
ably implies that RSL fell after the retreat of the ice‐margin
responsible for the implementation of the GS due to the GIA.
Furthermore, if the lowermost GS carved into the bedrock
(GS0) corresponds to glacial maxima conditions, then the ac-
companying GIA must have been the most important one of
the whole Dwyka sequence. The second and third fluctuation
sequences bear no formal diagnostic criteria to constrain any
RSL changes; general trends have then to be inferred from the
presence of GS: RSL rise being inferred from glacial advance
culminating in the implementation of a GS while RSL falls
follow the GS (glacial retreat).
6.4
|
Relationship with coeval
glaciogenic deposits
The presence of GZWs and associated ice‐contact fans and/
or deltas have been inferred based on facies associations and
associated features. As similar deposits cover a large part of
the intermediate platform domain (Du Toit, 1954; von Brunn,
1994, 1996; Haldorsen et al., 2001), it is suggested here that
they might belong to a compound GZW or an assemblage
of backstepping/overlapping GZWs, a setting commonly
encountered on formerly glaciated margins (Dowdeswell
et al., 2016a; Bjarnadóttir and Andreassen, 2016; O’Brien
et al., 2016; Bart et al., 2017). Their overall architecture
would, however, in the absence of any regional data (outcrops
extending over tens of kilometres, onland seismic reflection
surveys, Decalf et al., 2016), remain largely undecipherable.
Glacial deposits from KwaZulu‐Natal interpreted as lacus-
trine varvites (Savage, 1971; von Brunn, 1994; Haldorsen
et al., 2001) may have formed in depositional environments
confined to topographic depressions of the pre‐Dwyka GS
(glacial valleys) and/or inherited from the ice‐proximal gla-
ciogenic features (von Brunn, 1994; Dietrich et al., 2017).
In the absence of any absolute ages from the study area,
correlation of Dwyka strata with those from the western
Karoo Basin remains speculative. As the ice‐margin fluc-
tuations inferred here are most probably of high frequency,
they must be discrete at the scale of sequences deciphered in
the Western Karoo Basin (Visser, 1990; Isbell et al., 2008).
This suggests that the deposition of the entire deglacial suc-
cession described here and formed by three higher‐frequency
fluctuation sequences corresponds to one of the single gla-
cial–deglacial cycles observed in the western Karoo Basin,
and most probably the last one—at least in term of litho-
stratigraphic unit as the deglaciation of the Karoo Basin may
FIGURE 12 Outline for a conceptual
model defining identifying criteria for
grounding zone wedges (GZW) including
the four indicators (geomorphological,
architectural, sedimentological and
biological). The three inner arrowed circles
represent the likelihood that the different
indicators will be deciphered by different
investigative methods (outcrop and core
studies, seismic and bathymetric surveys.)
266
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DIETRICH anD HOFMann
have been diachronous (Griffis et al., 2019)—since they are
both covered by Ecca Group strata. In such a perspective, the
three lower glacial–deglacial cycles present in the western
Karoo would be virtually not recorded in the KwaZulu‐Natal
Province of South Africa that was still covered by the ice
masse at that time.
6.5
|
Criteria to identify GZWs
Although extensively imaged on modern high to mid‐latitude
shelves (Dowdeswell et al., 2016a), this paper reports on what
is thought to be the first described outcrop of GZW in the
pre‐Quaternary record (see Demet et al., 2019 for a post‐LGM
example), even though Ordovician buried fossil GZWs have
been imaged onland by seismic methods (Decalf et al., 2016).
This lack of known ancient examples partly arises from the fact
that this type of depocenter has been defined from the study
of Quaternary glaciogenic successions, the term ‘ground-
ing zone wedge’ itself referring to a geometry (asymmetric
sedimentary depocenters; Bell et al., 2016; Batchelor et al.,
2018) largely missing in the ancient geological record, erased
by post‐depositional processes. In addition, the dimensions
(hundreds to tens of thousands of metres in length and several
metres to hundreds of metres in thickness) and aspect ratio of
GZWs may far exceed the size of available outcrops which
makes their identification even more problematic. Similarly,
even if GZWs are characteristically wedge‐shaped and asym-
metrical, with a steep up‐glacier face and a smoother down‐
glacier face, slopes generally do not exceed a few degrees for
the steepest face and less than 1° for the shallowest face and
are thus hardly decipherable at outcrop or in the landscape, es-
pecially if affected by regional tectonic deformation. Besides,
facies models for GZWs are scarce, which may explain how
few have been reported in the stratigraphic record (Prothro et
al., 2018; Demet et al., 2019). Indeed, although the definition
of GZW implies depositional processes (‘till emerging from
beneath the glacier along a line source is redistributed by sub-
aqueous debris flows’: Bell et al., 2016), associated facies are
not necessarily straightforward.
Advantage is therefore taken of the described GZW out-
crops in the eastern Karoo Basin of South Africa combined
with parameters inferred from bathymetric, seismic surveys,
outcrop and core studies to propose an outline for a concep-
tual model defining criteria to identify this type of depocen-
ter. Even if the original morphology of the depocenters, as
well as depositional processes often directly observed or
inferred in modern glaciomarine settings, are intrinsically
lacking in ancient successions, the latter offer access to facies
and stratigraphic architecture of subglacial to ice‐marginal
settings unreachable in modern context, or at a spatial res-
olution that cannot be attained for Quaternary depocenters
which are hardly penetrated by high‐frequency seismic waves
and piston cores (Dowdeswell et al., 2019).
Four indicators (geomorphological, architectural, sed-
imentological and biological) are proposed to account for
the diversity of depositional processes and stratigraphic
architecture and geomorphologic expression character-
izing GZW depositional environments, as detailed below
and summarized in Figure 12. It is highly unlikely that all
these indicators would be ticked for most of the studies
performed on either modern, Quaternary or ancient set-
tings, yet it is envisioned that the presence of only some of
these parameters would permit the confident identification
of a GZW.
• Geomorphological indicators encompass the archetypical
asymmetric, wedge shape of the GZWs, ranging from 2 to
200m in height and from 0.2 to 200km in length (see fig.
8 in Demet et al., 2019) as well as the presence of charac-
teristic glacial features and landforms superimposed on or
encompassed within the GZW (megaflutes, iceberg plough-
marks, mega‐scale glacial lineations, moraines, etc.). The
association or juxtaposition of recessional curve‐crested
GZWs may strengthen such an interpretation. Bathymetric
and seismic surveys are particularly well‐suited for the rec-
ognition of the geomorphological indicators (Dowdeswell
et al., 2016a).
• Architectural indicators, which can be unravelled by both
seismic and outcrop studies, comprise the identification
of the stacking pattern and the stratigraphic architecture
of the GZW pointing to the active progradation of the
GZWs (internal architecture showing seaward‐dipping
clinoforms topped by horizontal sheets, truncation of
horizons by erosion surfaces, landward‐dipping back-
sets; Dowdeswell and Fugelli, 2012; Lajeunesse et al.,
2018).
• Sedimentological indicators are best emphasized by out-
crop investigations and consist, on the one hand, of bed-
ding and lamination patterns that show a predominance
of thick, massive to crudely stratified strata, interstratified
with cross‐stratified conglomeratic sandstones in which
the presence of contorted beds and glaciotectonically de-
formed horizons is common (Demet et al., 2019). On the
other hand, a wide range of grain sizes is supposed to char-
acterize the largely unsorted material making up the bulk
of GZWs (diamictite) while the abundance of exotic, out-
sized, faceted and/or striated clasts should be regarded as
characteristic.
• Finally, biological indicators can aid in the interpretation
of GZW as specific diatom and foraminifera assemblages
have been found in the diamictic material forming GZWs
(Prothro et al., 2018). It is also envisioned that after their
abandonment subsequent to glacial retreat, ice‐contact de-
posits such as GZWs may be colonialized by marine fauna
and flora that can leave behind bioturbation (O’Brien et al.,
2016).
|
267
DIETRICH anD HOFMann
7
|
CONCLUSIONS
The glaciogenic Dwyka Group of the eastern Karoo Basin in
South Africa consists of an up to 200m thick succession of
diamictite facies interstratified with sandstone and conglom-
erate. Three distinct sedimentary units have been recognized.
The lower one, lying directly on a highly uneven surface on
basement rock carved by glacial action, is thought to represent
GZW deposits. Such rocks are common to modern high‐lati-
tude continental shelves but have so far remained unrecognized
in the ancient geological record. In that sense, the present study
advances the comprehension of glacial depositional environ-
ments. The second and third units are represented by ice‐con-
tact delta and mixed‐influenced GZW deposits, respectively.
The three depositional units are separated by GS (striated
pavements, glaciotectonic complex) and represent ice‐margin
fluctuation sequences. Their deposition is interpreted, by anal-
ogy with Quaternary depositional systems, to have been very
rapid (tens to hundreds of thousand years) during retreat of the
Gondwana ice sheet. The RSL variations are thought to only
represent glacio‐isostatic signals, forced by the demise of the
ice sheets. Deposition of glaciogenic Dwyka strata was prob-
ably very rapid on a geological time scale.
ACKNOWLEDGEMENTS
The first author is grateful to the IAS (International Association
of Sedimentologists)—which is here warmly thanked and ac-
knowledged—as he has been granted and benefited from the
financial support of a postdoctoral research funding scheme
(spring 2017 session). Associate editor Peter Swart, Daniel
Le Heron and an anonymous reviewer are thanked for their
critical yet constructive and encouraging remarks that greatly
improved the manuscript. Thomas Gyomlai, Trishya Owen‐
Smith, Christophe Ballouard, Marlina Elburg and Fabien
Humbert are also thanked for their support and assistance
during various field missions. The authors declare no conflict
of interest associated with this manuscript.
REFERENCES
Alexander, J. and Cooker, M.J. (2016) Moving boulders in flash floods
and estimating flow conditions using boulders in ancient deposits.
Sedimentology, 63, 1582–1595
Alley, R.B., Anandakrishnan, S., Dupont, T.K., Parizek, B.R. and
Pollard, D. (2007) Effect of sedimentation on grounding‐line stabil-
ity. Science, 315, 1838–1841
Anandakrishnan, S., Catania, G.A., Alley, R.B. and Horgan, H.J. (2007)
Discovery of till deposition at the grounding line of Whillans ICE
STREAM. Science, 315, 1835–1838
Andersen, T., Kristoffersen, M. and Elburg, M.A. (2016) How far can
we trust provenance and crustal evolution information from detri-
tal zircons? A South African case study. Gondwana Research, 34,
129–148
Andrews, G.D., McGrady, A.T., Brown, S.R. and Maynard, S.M.
(2019) First description of subglacial megalineations from the late
Paleozoic ice age in southern Africa. PLoS ONE, 14, e0210673.
https ://doi.org/10.1371/journ al.pone.0210673
Aquino, C.D., Buso, V.V., Faccini, U.F., Milana, J.P. and Paim, P.S.G.
(2016) Facies and depositional architecture according to a jet ef-
flux model of a late Paleozoic tidewater grounding‐line system from
the Itararé Group (Paraná Basin), southern Brazil. Journal of South
American Earth Sciences, 67, 180–200
Assine, M.L., de Santa Ana, H., Veroslavsky, G. and Vesely, F.F. (2018)
Exhumed subglacial landscape in Uruguay: Erosional landforms,
depositional environments, and paleo‐ice flow in the context of the
late Paleozoic Gondwanan glaciation. Sedimentary Geology, 369,
1–12
Bangert, B. and von Brunn, V. (2001) Tuffaceous beds in glaciogenic
argillites of the Late Palæozoic Dwyka Group of KwaZulu‐Natal,
South Africa. Journal of African Earth Sciences, 32, 133–140
Bangert, B., Stollhofen, H., Lorenz, V. and Armstrong, R. (1999) The
geochronology and significance of ash‐fall tuffs in the glaciogenic
Carboniferous‐Permian Dwyka Group of Namibia and South Africa.
Journal of African Earth Sciences, 29, 33–49
Bangert, B., Stollhofen, H., Geiger, M. and Lorenz, V. (2000) Fossil record
and high‐resolution tephrostratigraphy of Carboniferous glaciomarine
mudstones, Dwyka Group, southern Namibia. Communications of the
Geological Survey of South West Africa/Namibia, 12, 235–245
Bart, P.J., Krogmeier, B.J., Bart, M.P. and Tulaczyk, S. (2017) The par-
adox of a long grounding during West Antarctic Ice Sheet retreat
in Ross Sea. Scientific Reports, 7, 1–8. https ://doi.org/10.1038/
s41598-017-01329-8
Batchelor, C.L. and Dowdeswell, J.A. (2015) Ice‐sheet grounding‐zone
wedges (GZWs) on high‐latitude continental margins. Marine
Geology, 363, 65–92
Batchelor, C.L., Dowdeswell, J.A. and Ottesen, D. (2018) Submarine
glacial landforms. In: Micallef, A., Krastel, S. and Savini, A. (Eds.)
Submarine Geomophology. Amsterdam: Springer, pp. 207–234
Bell, T., Cooper, A.K., Solheim, A., Todd, B.J., Dowdeswell, J.A.,
Canals, M.et al. (2016) Glossary of glaciated continental margins
and related geoscience methods. In: Dowdeswell, J.A., Canals, M.,
Jakobson, M., Todd, B.J., Dowdeswell, E.K. and Hogan, K.A. (Eds.)
Atlas of Submarine Glacial Landforms: Modern, Quaternary and
Ancient. Geological Society, London, Memoirs, 46, 555–574.
Bjarnadóttir, L.R. and Andreassen, K. (2016) Ice‐stream landform as-
semblage in Kveithola, western Barents Sea Margin. In: Dowdeswell,
J.A., Canals, M., Jakobson, M., Todd, B.J., Dowdeswell, E.K. and
Hogan, K.A. (Eds.) Atlas of Submarine Glacial Landforms: Modern,
Quaternary and Ancient. Geological Society, London, Memoirs, 46,
325–328.
Bordy, E.M. and Catuneanu, O. (2002) Sedimentology of the Beaufort‐
Molteno Karoo fluvial strata in the Tuli Basin, South Africa. South
African Journal of Geology, 105, 51–66
Bordy, E.M., Spelman, S., Cole, D.I. and Mthembi, P. (2017)
Lithostratigraphy of the Pietermaritzburg Formation (Ecca Group,
Karoo Supergroup), South Africa. South African Journal of
Geology, 120, 293–302
Boulton, G.S. (1990) Sedimentary and sea level changes during
glacial cycles and their control on glacimarine facies architec-
ture. In: Dowdeswell, J.A.and Scourse, J.D. (Eds.) Glacimarine
Environments: Processes and Sediments. Geological Society,
London, Special Publications, 53, 15–52
268
|
DIETRICH anD HOFMann
Brinkerhoff, D., Truffer, M. and Aschwanden, A. (2017) Sediment trans-
port drives tidewater glacier periodicity. Nature Communications, 8, 90.
https ://doi.org/10.1038/s41467-017-00095-5
von Brunn, V. (1994) Glaciogene deposits of the Permo‐Carboniferous
Dwyka Group in the eastern region of the Karoo Basin, South
Africa. In: Deynoux, M., Miller, J.M.G., Domack, E.W., Eyles,
N., Fairchild, I.J. and Young, G.M. (Eds.) Earth’s Glacial Record.
Cambridge, UK: Cambridge University Press, pp. 60–69
von Brunn, V. (1996) The Dwyka Group in the northern part of
Kwazulu/Natal, South Africa: Sedimentation during late palaeozoic
deglaciation. Palaeogeography, Palaeoclimatology, Palaeoecology,
125, 141–163
von Brunn, V. and Talbot, C.J. (1986) Formation and deformation
of subglacial intrusive clastic sheets in the Dwyka Formation of
Northern Natal, South Africa. Journal of Sedimentary Research, 56,
35–44
Buatois, L.A., Netto, R.G. and Mangano, M.G. (2010) Ichnology of
late Paleozoic postglacial transgressive deposits in Gondwana:
Reconstructing salinity conditions in coastal ecosystems affected by
strong meltwater discharge. In: Lopez‐Gamundi, O.R. and Buatois,
L.A. (Eds.) Late Paleozoic glacial events and postglacial transgres-
sions in Gondwana. Boulder, CO. pp. 149–174
Buechi, M.W., Frank, S.M., Graf, H.R., Menzies, J. and Anselmetti, F.
(2017) Subglacial emplacement of tills and meltwater deposits at
the base of the overdeepened bedrock troughs. Sedimentology, 64,
658–685
Busfield, M.E. and Le Heron, D.P. (2018) Snowball Earth under the
microscope. Journal of Sedimentary Research, 88, 659–677
Bussert, R. (2010) Exhumed erosional landforms of the Late Palaeozoic
glaciation in northern Ethiopia: Indicators of ice‐flow direction, pa-
laeolandscape and regional ice dynamics. Gondwana Research, 18,
356–369
Catuneanu, O. (2004) Basement control on flexural profiles and the dis-
tribution of foreland facies: The Dwyka Group of the Karoo Basin,
South Africa. Geology, 32, 517–520
Catuneanu, O., Hancox, P.J. and Rubidge, B.S. (1998) Reciprocal flex-
ural behaviour and contrasting stratigraphies: A new basin develop-
ment model for the Karoo retroarc foreland system, South Africa.
Basin Research, 10, 417–439
Catuneanu, O., Wopfner, H., Eriksson, P.G., Cairncross, B., Rubidge,
B.S., Smith, R. M. H. et al. (2005) The Karoo basins of south‐cen-
tral Africa. Journal of African Earth Sciences, 43, 211–253
Cornamusini, G., Talarico, F.M., Cirilli, S., Spina, A., Olivetti, V. and
Woo, J. (2017) Upper Paleozoic glacigenic deposits of Gondwana:
Stratigraphy and paleoenvironmental significance of a tillite succes-
sion in Northern Victoria Land (Antarctica). Sedimentary Geology,
358, 51–69
Cowan, E.A., Cai, J., Powell, R.D., Seramur, K.C. and Spurgeon, V.L.
(1998) Modern tidal rhythmites deposited in a deep‐water estuary.
Geo‐Marine Letters, 18, 40–48
Decalf, C., Dowdeswell, J.A. and Fugelli, E.M.G. (2016) Seismic
character of possible buried grounding‐zone wedges in the
Late Ordovician glacial rocks of Algeria. In: Dowdeswell,
J.A., Canals, M., Jakobson, M., Todd, B.J., Dowdeswell, E.K.
and Hogan, K.A. (Eds.) Atlas of submarine glacial landforms:
Modern, Quaternar y and Ancient. Geological Society, London,
Memoirs, 46, 245–246.
Demet, B.P., Nittrouer, J.A., Anderson, J.B. and Simkins, L.M. (2019)
Sedimentary processes at ice sheet grounding‐zone wedges revealed
by outcrops, Washington State (USA). Earth Surface Processes and
Landforms, 44(6), 1209–1220. https ://doi.org/10.1002/esp.4550
Dietrich, P., Ghienne, J.‐F., Schuster, M., Lajeunesse, P., Nutz, A.,
Deschamps, R. et al. (2017) From outwash to coastal systems in the
Portneuf‐Forestville deltaic complex (Québec North Shore): Anatomy
of a forced regressive deglacial sequence. Sedimentology, 64, 1044–1078
Dietrich, P., Ghienne, J.‐F., Lajeunesse, P., Normandeau, A., Deschamps,
R. and Razin, P.(2018)Deglacial sequences and glacio‐isostatic ad-
justment: Quaternary compared with Ordovician glaciations. In: Le
Heron, D.P., Hogan, K.A., Phillips, E.R., Huuse, M., Busfield, M.
and Graham, A.G.C., (Eds.) Glaciated Margins: The Sedimentary
and Geophysical Archive. Geological Society, London, Special
Publications, 475, SP475.9.
Dowdeswell, J.A. and Fugelli, E.M.G. (2012) The seismic architecture
and geometry of grounding‐zone wedges formed at the marine mar-
gins of past ice sheets. Geological Society of America Bulletin, 124,
1750–1761
Dowdeswell, J.A., Hogan, K.A., Arnold, N.S., Mugford, R.I., Wells, M.,
Hirst, J.P.P. et al. (2015) Sediment‐rich meltwater plumes and ice‐
proximal fans at the margins of modern and ancient tidewater gla-
ciers: Observations and modeling. Sedimentology, 62, 1665–1692
Dowdeswell, J.A., Canals, M., Jakobsson, M., Todd, B.J., Dowdeswell,
E.K. and Hogan, K.A. (2016a) Atlas of submarine glacial land-
forms: modern, quaternary and ancient. Geological Society, London,
Memoirs, 46, 618.
Dowdeswell, J.A., Canals, M., Jakobsson, M., Todd, B.J., Dowdeswell,
E.K. and Hogan, K.A. (2016b) The variety and distribution of
submarine glacial landforms and implications for ice‐sheet recon-
struction. In: Dowdeswell, J.A., Canals, M., Jakobson, M., Todd,
B.J., Dowdeswell, E.K. and Hogan, K.A. (Eds.) Atlas of Submarine
Glacial Landforms: Modern, Quaternary and Ancient. Geological
Society, London, Memoirs, 46, 519–552.
Dowdeswell, J.A., Hogan, K.A. and Le Heron, D.P. (2019) The gla-
cier‐influenced marine record on high‐latitude continental margins:
synergies between modern, Quaternary and ancient evidence. In: Le
Heron, D.P., Hogan, K.A., Phillips, E.R., Huuse, M., Busfield, M.
and Graham, A.G.C. (Eds.) Glaciated Margins: The Sedimentary
and Geophysical Archive. Geological Society, London, Special
Publications, 475, SP475.13.
Du Toit, A.L. (1921) The Carboniferous glaciation of South Africa.
South African Journal of Geology, 24, 188–227
Du Toit, A.L. (1937) Our wandering continents: an hypothesis of conti-
nental drifting. Edinburgh: Oliver & Boyd, 366 pp.
Du Toit, A.L. (1954) The Geology of South Africa (S.H. Haughton Ed.).
London: Oliver and Boyd, 611 pp.
Dykstra, M., Kneller, B. and Milana, J.P. (2012) Bed‐thickness and
grain‐size trends in a small‐scale proglacial channel‐levée sys-
tem; the Carboniferous Jejenes Formation, Western Argentina:
Implications for turbidity current flow processes. Sedimentology,
59, 605–622
Ely, J.C., Clark, C.D., Spagnolo, M., Stokes, C.R., Greenwood, S.L.,
Hughes, A.L.C. et al. (2016) Do subglacial bedforms comprise a
size and shape continuum? Geomorphology, 257, 108–119
Evans, D.J.A., Phillips, E.R., Hiemstra, J.F. and Auton, C.A. (2006)
Subglacial till: Formation, sedimentary characteristics and classifi-
cation. Earth‐Science Reviews, 78, 115–176
Eyles, N. (2008) Glacio‐epochs and the supercontinent cycle after 3.0
Ga: Tectonic boundary conditions for glaciation. Palaeogeography,
Palaeoclimatology, Palaeoecology, 258, 89–129
|
269
DIETRICH anD HOFMann
Fallgatter, C. and Paim, P.S.G. (2017) On the origin of the Itararé
Group basal nonconformity and its implications for the Late
Paleozoic glaciation in the Paranà Basin, Brazil. Palaeogeography,
Palaeoclimatology, Palaeoecology, (In press).
Fielding, C.R., Frank, T.D. and Isbell, J.L. (2008a) Resolving the Late
Paleozoic ice age in time and space. Geological Society of America,
Special Paper, 441, 354.
Fielding, C.R., Frank, T.D., Birgenheier, L.P., Rygel, M.C., Jones, A.T.
and Roberts, J. (2008b) Stratigraphic imprint of the Late Palaeozoic
Ice Age in eastern Australia: a record of alternating glacial and nongla-
cial climate regime. Journal of the Geological Society, 165, 129–140
Frank, T., Birgenheier, L., Montañez, I., Fielding, C. and Rygel, M.
(2008) Late Paleozoic climate dynamics revealed by comparison
of ice‐proximal stratigraphic and ice‐distal isotopic records. In:
Fielding, C.R., Frank, T.D. and Isbell, J.L. (Eds.) Resolving the
Late Paleozoic Ice Age in Time and Space. Geological Society of
America, Special Paper, 441, 331–342.
Girard, F., Ghienne, J.‐F., Du‐Bernard, X. and Rubino, J.‐L. (2015)
Sedimentary imprints of former ice‐sheet margins: Insights from an
end‐Ordovician archive (SW Libya). Earth‐Science Reviews, 148,
259–289
Goddéris, Y., Donnadieu, Y., Carretier, S., Aretz, M., Dera, G.,
MacOuin, M. et al. (2017) Onset and ending of the late Palaeozoic
ice age triggered by tectonically paced rock weathering. Nature
Geoscience, 10, 382–386
Gravenor, C.P., von Brunn, V. and Dreimanis, A. (1984) Nature and
classification of waterlain glaciogenic sediments, exemplified by
Pleistocene, Late Paleozoic and Late Precambrian deposits. Earth‐
Science Reviews, 20, 105–166
Griffis, N.P., Mundil, R., Montañez, I.P., Isbell, J., Fedorchuk, N.D.,
Linol, B. et al. (2019) Deciphering the Timing of the LPIA in
Southern Gondwana Usin U‐Pb CA‐TIMS Geochronology. Vienna,
Austria: EGU General Assembly.
Grill, H. (1997) The Permo‐Carboniferous glacial to marine Karoo
record in southern Namibia: sedimentary facies and stratigraphy.
Beringeria, 19, 98. Würzburg
Guillocheau, F., Simon, B., Baby, G., Bessin, P., Robin, C. and Dauteuil,
O. (2018) Planation surfaces as a record of mantle dynamics: The
case example of Africa. Gondwana Research, 53, 82–98
Haldorsen, S., Von Brunn, V., Maud, R. and Truter, E.D. (2001) A
Weichselian deglaciation model applied to the Early Permian gla-
ciation in the northeast Karoo Basin, South Africa. Journal of
Quaternary Science, 16, 583–593
Himmler, T., Freiwald, A., Stollhofen, H. and Peckmann, J. (2008)
Late Carboniferous hydrocarbon‐seep carbonates from the gla-
ciomarine Dwyka Group, southern Namibia. Palaeogeography,
Palaeoclimatology, Palaeoecology, 257, 185–197
Hofmann, A., Anhaeusser, C.R., Dixon, J., Kröner, A., Saha, L.et al.
(2019) Archaean Granitoid‐Greenstone Geology of the Southeastern
Part of the Kaapvaal Craton. In: Kröner, A. and Hofmann, A. (Eds.)
The Archaean Geology of the Kaapvaal Craton, Southern Africa.
Amsterdam: Springer, pp. 33–54.
Horton, D.E. and Poulsen, C.J. (2009) Paradox of late Paleozoic glacio-
eustasy. Geology, 37, 715–718
Hunter, R.E. (1977) Terminology of Cross‐Stratified Sedimentary
Layers and Climbing‐Ripple Structures. Journal of Sedimentary
Research, 47, 697–706
Huuse, M., Le Heron, D.P., Dixon, R., Redfern, J., Moscariello, A. and
Craig, J. (2012) Glaciogenic reservoirs and hydrocarbon systems:
an introduction. In: Huuse, M., Redfern, J., Le Heron, D.P., Dixon,
R.J., Moscariello, A. and Craig, J. (Eds.) Glaciogenic reservoirs
and hydrocarbon systems. Geological Society, London, Special
Publications, 368, 1–28.
Isbell, J.L. (2010) Environmental and paleogeographic implications of
glaciotectonic deformation of glaciomarine deposits within Permian
strata of the Metschel Tillite, southern Victoria Land, Antarctica.
In: Lopez‐Gamundi, O.R. and Buatois, L.A. (Eds.) Late Paleozoic
Glacial Events and Postglacial Transgressions in Gondwana. The
Geological Society of America, Special Paper, 468, 81–100
Isbell, J.L., Cole, D.I. and Catuneanu, O. (2008) Carboniferous‐Permian
glaciation in the main Karoo Basin, South Africa: Stratigraphy, dep-
ositional controls, and glacial dynamics. In: Fielding, C.R., Frank,
T.D. and Isbell, J.D. (Eds.) Resolving the Late Paleozoic Ice Age in
Time and Space. The Geological Society of America, Special Paper
441, 71–82
Isbell, J.L., Henry, L.C., Gulbranson, E.L., Limarino, C.O., Fraiser,
M.L., Koch, Z.J. et al. (2012) Glacial paradoxes during the late
Paleozoic ice age: Evaluating the equilibrium line altitude as a con-
trol on glaciation. Gondwana Research, 22, 1–19
Johnson, M., van Vuuren, C.J., Visser, J.N.J., Cole, D.I., de Wickens,
H.V., Christie, A.D.M.et al. (2006) Sedimentary rocks of the Karoo
Supergroup. In: Johnson, M.R., Anhaeusser, C.R. and Thomas,
R.J. (Eds.) Geology of South Africa. Johannesburg: Council for
Geoscience, pp. 461–500
Krabbendam, M., Eyles, N., Putkinen, N., Bradwell, T. and Arbelaez‐
Moreno, L. (2016) Streamlined hard beds formed by palaeo‐ice
streams: A review. Sedimentary Geology, 338, 24–50
Kröner, A., Hoffmann, J.E., Wong, J.M., Geng, H.Y., Schneider, K.P.,
Xie, H. et al. (2019) Archaean crystalline rocks of the Eastern
Kaapvaal Craton. In: Kröner, A. and Hofmann, A. (Eds.) The
Archaean Geology of the Kaapvaal Craton, Southern Africa.
Amsterdam: Springer, pp. 1–32.
Lajeunesse, P. (2014) Buried preglacial fluvial gorges and valleys
preserved through Quaternary glaciations beneath the eastern
Laurentide Ice Sheet. Bulletin of the Geological Society of America,
126, 447–458
Lajeunesse, P. (2016) Late Wisconsinan grounding‐zone wedges, north-
western Gulf of St Lawrence, eastern Canada. In: Dowdeswell,
J.A., Canals, M., Jakobson, M., Todd, B.J., Dowdeswell, E.K. and
Hogan, K.A. (Eds.) Atlas of submarine glacial landforms: Modern,
Quaternary and Ancient. Geological Society, London, Memoirs, 46,
227–228.
Lajeunesse, P., Dietrich, P. and Ghienne, J.‐F. (2018) Late Wisconsinan
grounding zones of the Laurentide Ice Sheet margin off the Québec
North Shore (NW Gulf of St Lawrence). In: Le Heron, D.P., Hogan,
K.A., Phillips, E.R., Huuse, M., Busfield, M. and Graham, A.G.C.
(Eds.) Glaciated Margins: The Sedimentary and Geophysical
Archive. Geological Society, London, Special Publications, 475,
SP475.11.
Le Heron, D.P. (2018) An exhumed Paleozoic glacial landscape in
Chad. Geology, 46, 91–94
Le Heron, D.P., Graig, J. and Etienne, J.L. (2009) Ancient glaciations
and hydrocarbon accumulations in North Africa and the Middle
East. Earth‐Science Reviews, 93, 47–76
Le Heron, D.P., Tofaif, S., Vandyk, T. and Ali, D.O. (2017) A diam-
ictite dichotomie: Glacial conveyor belts and olistostromes in the
Neoproterozoic of Death Valley, California, USA. Geology, 45,
31–34
270
|
DIETRICH anD HOFMann
Linol, B. and De Wit, M.J. (2016) Origin and Evolution of the Cape
Mountains and Karoo Basin. Switzerland: Springer Nature
Livingstone, S.J., Chu, W., Ely, J.C. and Kingslake, J. (2017)
Paleofluvial and subglacial channel networks beneath Humboldt
Glacier, Greenland. Geology, 45, 551–554
Lønne, I. (1995) Sedimentary facies and depositional architecture of
ice‐contact glaciomarine systems. Sedimentary Geology, 98, 13–43
Lønne, I., Nemec, W., Blikra, L.H. and Lauritsen, T. (2001) Sedimentary
architecture and dynamic stratigraphy of a marine ice‐contact sys-
tem. Journal of Sedimentary Research, 71, 922–943
López‐Gamundí, O.R. and Buatois, L.A. (2010) Late Paleozoic gla-
cial events and postglacial transgressions in Gondwana. Geological
Society of America, Special Paper 468.Houston, TX, USA, 218 pp.
Luskin, C., Wilson, A., Gold, D. and Hofmann, A. (2019) The Pongola
Supergroup: Mesoarchaean Deposition Following Kaapvaal Craton
Stabilization. In: Kröner, A.and Hofmann, A. (Eds.) The Archaean
Geology of the Kaapvaal Craton, Southern Africa. Amsterdam:
Springer, pp. 225–254.
Marshall, C.G.A. (2006) The natal group. In: Johnson, M.R.,
Anhaeusser, C.R. and Thomas, R.J. (Eds.) Geology of South Africa.
Johannesburg: Council for Geoscience, pp. 433–460
Martin, J.R., Redfern, J. and Aitken, J.F. (2008) A regional overview
of the late Paleozoic glaciation in Oman. In: Fielding, C.R., Frank,
T.D. and Isbell, J.D. (Eds.) Resolving the Late Paleozoic Ice Age in
Time and Space. The Geological Society of America, Special Paper,
441, 175–186.
Matthews, P.E. (1970) Paleorelief and the Dwyka glaciation in the east-
ern region of South Africa. In second Gondwana Symposium, Cape
Town, 491–499.
Montañez, I.P. and Poulsen, C.J. (2013) The Late Paleozoic ice age: an
evolving paradigm. Annual Review of Earth and Planetary Sciences,
41, 629–656
Montañez, I.P., McElwain, J.C., Poulsen, C.J., White, J.D., DiMichele,
W.A., Wilson, J.P. et al. (2016) A Late Paleozoic climate window of
opportunity. Proceedings of the National Academy of Sciences of the
United States of America, 113, 2334–2336.
Mulder, T., Migeon, S., Savoye, B. and Faugères, J.‐C. (2001) Inversely
graded turbidite sequences in the deep Mediterranean: a record
of deposits from flood‐generated turbidity currents? Geo‐Marine
Letters, 21, 86–93.
Myers, T.S. (2016) Palaeoclimate: CO2 and late Palaeozoic glaciation.
Nature Geoscience, 9, 803–804
Newton, M., Evans, D.J.A., Roberts, D.H. and Stokes, C.R. (2018)
Bedrock mega‐grooves in glaciated terrains: a review. Earth‐Science
Reviews, 185, 57–79. https ://doi.org/10.1016/j.earsc irev.2018.03.007
Normandeau, A., Dietrich, P., Hughes Clark, J., Van Wychen, W.,
Lajeunesse, P. and Ghienne, J.‐F. (in press) The retreat pattern of
glaciers controls the occurrence of turbidity currents on eastern
Baffin Island, high‐latitude fjord deltas. Journal of Geophysical
Research: Solid Earth.
Normandeau, A., Dietrich, P., Lajeunesse, P., St‐Onge, G., Ghienne,
J.‐F., Francus, P. et al. (2017) Timing and controls on the delivery
of coarse sediment to deltas and submarine fans on a formerly gla-
ciated coast and shelf. Geological Society of America Bulletin, 129,
1424–1441
O’Brien, P.E., Beaman, R., De Santis, L., Domack, E.W., Escutia,
C., Harris, P.T.et al. (2016) Submarine glacial landforms on
the cold East Antarctic margin. In: Dowdeswell, J.A., Canals,
M., Jakobson, M., Todd, B.J., Dowdeswell, E.K. and Hogan,
K.A. (Eds.) Atlas of Submarine Glacial Landforms: Modern,
Quaternary and Ancient. Geological Society, London, Memoirs,
46, 501–508.
Occhietti, S. (2007) The Saint‐Narcisse morainic complex and early
Younger Dryas on the south‐eastern margin of the Laurentide Ice
Sheet. Géographie Physique Et Quaternaire, 61, 89–117
Opdyke, N.D., Mushayandebvu, M. and De Wit, M.J. (2001) A new
palæomagnetic pole for the Dwyka system and correlative sedi-
ments in sub‐Saharan Africa. Journal of African Earth Sciences, 33,
143–153
Powell, R.D. (1990), Glacimarine processes at grounding-line fans
and their growth to ice-contact deltas. In: Dowdeswell, J.A., &
Scourse, J.D. (eds) Glacimarine Environments: Processes and
Sediments. London : Geological Society Special Publication, 53,
53–73.
Powell, R.D. and Alley, R.B. (1997) Grounding‐Line Systems:
Processes, Glaciological Inferences and the Stratigraphic Record.
In: Barker, P.F. and Cooper, A.K. (Eds.) Geologic and Seismic
Stratigraphy of the Antarctic Margin 2. American Geophysical
Union, 71, 169–187.
Powell, R.D. and Cooper, J.M. (2002) A glacial sequence stratigraphic
model for temperate, glaciated continental shelves. In: Dowdeswell,
J.A. and O Cofaigh, C. (Eds.) Glacier‐Influenced Sedimentation on
High‐Latitude Continental Margins. Geological Society, London,
Special Publications, 203, 215–244.
Powell, R. and Domack, G.W. (2002) Modern Glaciomarine
Environments. In: Menzies, J. (Ed.). Oxford: PButterworth
Heinemann, pp. 361–389.
Prothro, L.O., Simkins, L.M., Majewski, W. and Anderson, J.B. (2018)
Glacial retreat patterns and processes determined from integrated
sedimentology and geomorphology records. Marine Geology, 395,
104–119
Pysklywec, R.N. and Mitrovica, J.X. (1999) The role of subduction – in-
duced subsidence in the evolution of the Karoo Basin. The Journal
of Geology, 107, 155–164
Russell, A.J., Roberts, M.J., Fay, H., Marren, P.M., Cassidy, N.J.,
Tweed, F.S. et al. (2006) Icelandic jokulhlaup impacts: Implications
for ice‐sheet hydrology, sediment transfer and geomorphology.
Geomorphology, 75, 33–64
Savage, N.M. (1971) A varvite ichnocoenosis from the Dwyka series of
Natal. Lethaia, 4, 217–233.
Schoof, C.G. and Clarke, G.K.C. (2008) A model for spiral flows in
basal ice and the formation of subglacial flutes based on a Reiner‐
Rivlin rheology for glacial ice. Journal of Geophysical Research,
113, B05204
Smith, N.D., Phillips, A.C. and Powell, R.D. (1990) Tidal drawdown: A
mechanism for producing cyclic sediment laminations in glacioma-
rine deltas. Geology, 18, 10–13
Smith, R.M.H., Eriksson, P.G. and Botha, W.J. (1993) A review of
the stratigraphy and sedimentary environments of the Karoo‐aged
basins of Southern Africa. Journal of African Earth Sciences, 16,
143–169.
Sohn, Y.K. (1997) On traction‐carpet sedimentation. Journal of
Sedimentary Research, 67, 502–509
Spencer, C.J., Thomas, R.J., Roberts, N.M.W., Cawood, P.A., Millar, I.
and Tapster, S. (2015) Crustal growth during island arc accretion
and transcurrent deformation, Natal Metamorphic Province, South
Africa: New isotopic constraints. Precambrian Research, 265,
203–217
|
271
DIETRICH anD HOFMann
Stephenson, M.H., Jan, I.U. and Al‐Mashaikie, S.Z.A.K. (2013)
Palynology and correlation of Carboniferous‐Permian glacigene rocks
in Oman, Yemen and Pakistan. Gondwana Research, 24, 203–211.
Stollhofen, H., Stanistreet, I.G., Bangert, B. and Grill, H. (2000) Tuffs,
tectonism and glacially related sea‐level changes, Carboniferous –
Permian, southern Namibia. Palaeogeography, Palaeoclimatology,
Palaeoecology, 161, 127–150
Stollhofen, H., Werner, M., Stanistreet, I.G. and Armstrong, R.A.
(2008) Single‐zircon U‐Pb dating of Carboniferous‐Permian tuffs,
Namibia, and the intercontinental deglaciation cycle framework. In:
Fielding, C.R., Frank, T.D. and Isbell, J.D. (Eds.) Resolving the Late
Paleozoic Ice Age in Time and Space. The Geological Society of
America, Special Paper, 441, 83–96.
Storms, J.E.A., de Winter, I.L., Overeem, I., Drijkoningen, G.G. and
Lykke‐Andersen, H. (2012) The Holocene sedimentary history of
the Kangerlussuaq Fjord‐valley fill, West Greenland. Quaternary
Science Reviews, 35, 29–50
Sutherland, P.C. (1870) Notes on an ancient boulder‐clay of Natal.
Geological Society of London Quaterly Journal, 26, 514–527
Talling, P.J., Masson, D.G., Sumner, E.J. and Malgesini, G. (2012)
Subaqueous sediment density flows: Depositional processes and de-
posit types. Sedimentology, 59, 1937–2003
Tankard, A., Welsink, H., Aukes, P., Newton, R. and Stettler, E. (2009)
Tectonic evolution of the Cape and Karoo basins of South Africa.
Marine and Petroleum Geology, 26, 1379–1412
Tedesco, J., Cagliari, J., dos Coitinho, J., da Cunha Lopes, R. and
Lavina, E.L.C. (2016) Late Paleozoic paleofjord in the southern-
most Parana Basin (Brazil): Geomorphology and sedimentary fill.
Geomorphology, 269, 203–214
Valdez Buso, V., Aquino, C.D., Paim, P.S.G., de Souza, P.A., Mori,
A.L., Fallgatter, C. et al. (2017) Late Palaeozoic glacial cycles
and subcycles in western Gondwana: Correlation of surface and
subsurface data of the Paraná Basin, Brazil. Palaeogeography,
Palaeoclimatology, Palaeoecology, (In press).
Versfeld, J.A. (1988) The Geology of the Nondweni Greenstone Belt.
Natal: University of Natal, 298 pp.
Vesely, F.F., Trzaskos, B., Kipper, F., Assine, M.L. and Souza, P.A.
(2015) Sedimentary record of a fluctuating ice margin from the
Pennsylvanian of western Gondwana: Paraná Basin, southern Brazil.
Sedimentary Geology, 326, 45–63.
Vesely, F.F., Rodrigues, M.C.N.L., da Rosa, E.L.M., Amato, J.A.,
Trzaskos, B., Isbell, J.L. et al. (2018) Recurrent emplacement of
non‐glacial diamictite during the late Paleozoic ice age. Geology,
46, 615–618
Visser, J.N.J. (1983) Submarine debris flow deposits from the Upper
Carboniferous Dwyka Tillite Formation in the Kalahari Basin,
South Africa. Sedimentology, 30, 511–523.
Visser, J.N.J. (1987) The palaeogeography of part of southwest-
ern Gondwana during the Permo‐Carboniferous glaciation.
Palaeogeography, Palaeoclimatology, Palaeoecology, 61, 205–219
Visser, J.N.J. (1990) The age of the late Palaeozoic glacigene deposits
in southern Africa. South African Journal of Geology, 93, 366–375
Visser, J.N.J. (1993) Sea‐level changes in a back‐arc‐foreland transi-
tion: the late Carboniferous‐Permian Karoo Basin of South Africa.
Sedimentary Geology, 83, 115–131.
Visser, J.N.J. (1995) Post‐glacial Permian stratigraphy and geography
of southern and central Africa: Boundary conditions for climatic
modeling. Palaeogeography, Palaeoclimatology, Palaeoecology,
118, 213–243.
Visser, J.N.J. (1997) Deglaciation sequences in the Permo‐Carboniferous
Karoo and Kalahari basins of southern Africa: a tool in the analysis
of cyclic glaciomarine basin fills. Sedimentology, 44, 507–521.
Visser, J.N.J. and Hall, K.J. (1985) A model for the deposition of the
Carboniferous Kruitfontein boulder pavement and associated beds,
Elandsvlei, South Africa. Transaction of the Geological Society of
South Africa, 87, 161–168.
Wegener, A.L. (1915) Die Entstehung der Kontinente und Ozeane.
Wiesbaden: Friedrich Vieweg & Sohn, 231 pp.
Wescott, W.A. and Diggens, J.N. (1997) Depositional history and strati-
graphical evolution of the Sakoa Group (Lower Karoo Supergroup)
in the southern Morondava Basin, Madagascar. Journal of African
Earth Sciences, 24, 585–601.
Wopfner, H. and Jin, H.C. (2009) Pangea megasequences of Tethyan
Gondwana‐margin reflect global changes of climate and tectonism
in Late Paleozoic and Early Triassic times–A Review. Paleoworld,
18, 169–192.
Zecchin, M., Catuneanu, O. and Rebesco, M. (2015) High‐resolution
sequence stratigraphy of clastic shelves IV: high‐latitude settings.
Marine and Petroleum Geology, 68, 427–437.
How to cite this article: Dietrich P, Hofmann A.
Ice‐margin fluctuation sequences and grounding zone
wedges: The record of the Late Palaeozoic Ice Age in
the eastern Karoo Basin (Dwyka Group, South Africa).
Depositional Rec. 2019;5:247–271. https ://doi.
org/10.1002/dep2.74
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