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Foredeep submarine fans and forebulge deltas:orogenic off-loading in the underfilled Karoo Basin
O. Catuneanu a,*, P.J. Hancox b, B. Cairncross c, B.S. Rubidge d
a Department of Earth and Atmospheric Sciences, University of Alberta, 1-26 Earth Sciences Building, Edmonton, Alta., Canada T6G 2E3b Department of Geology, University of Witwatersrand, PO Wits, Johannesburg 2050, South Africa
c Department of Geology, Rand Afrikaans University, P.O. Box 524, Auckland Park, Johannesburg 2006, South Africad Bernard Price Institute for Palaeontological Research, University of Witwatersrand, PO Wits, Johannesburg 2050, South Africa
Received 5 February 2002; accepted 28 October 2002
Abstract
Third-order sequence stratigraphic analysis of the Early Permian marine to continental facies of the Karoo Basin provides a case
study for the sedimentation patterns which may develop in an underfilled foreland system that is controlled by a combination of
supra- and sublithospheric loads. The tectonic regime during the accumulation of the studied section was dominated by the flexural
rebound of the foreland system in response to orogenic quiescence in the Cape Fold Belt, which resulted in foredeep uplift and
forebulge subsidence. Coupled with flexural tectonics, additional accommodation was created by dynamic loading related to the
process of subduction underneath the basin. The long-wavelength dynamic loading led to the subsidence of the peripheral bulge
below base level, which allowed for sediment accumulation across the entire foreland system.
A succession of five basinwide regressive systems tracts accumulated during the Artinskian (�5 My), consisting of foredeep
submarine fans and correlative forebulge deltas. The progradation of submarine fans and deltaic systems was controlled by coeval
forced and normal regressions of the proximal and distal shorelines of the Ecca interior seaway respectively. The deposition of each
regressive systems tract was terminated by basinwide transgressive episodes, that may be related to periodic increases in the rates of
long-wavelength dynamic subsidence.
� 2003 Elsevier Science Ltd. All rights reserved.
Keywords: Underfilled foredeep; Forebulge; Flexural tectonics; Dynamic subsidence; Systems tracts
1. Introduction
This paper provides a case study for the sedimenta-
tion patterns that may develop in an underfilled retroarc
foreland system, under a unique combination of factorsthat control the dynamics of the basin and the amounts
of available accommodation. These factors refer to
flexural compensation in response to orogenic unload-
ing, dynamic subsidence related to subduction pro-
cesses, and eustatic fluctuations. The example is from
the Karoo Basin of South Africa (Fig. 1), and deals with
the nature and correlation of the marine to continental
facies of Early Permian age that are associated with theevolution of the Ecca interior seaway. This research
builds on previous work that already established the
large-scale (second-order) sequence stratigraphic
framework of the Karoo Supergroup, and the relation-
ship between the foreland stratigraphy and the orogenic
cycles of loading and unloading in the adjacent Cape
Fold Belt (Catuneanu et al., 1998). The objective of thispaper is to increase the resolution of sequence strati-
graphic analysis to the third-order level of cyclicity for
the Early Permian stage of orogenic unloading that af-
fected the evolution of the Ecca seaway in the Karoo
Basin. This stage of orogenic unloading followed the
second major paroxysm recorded in the Cape Fold Belt
(P2 in Fig. 2) (H€aalbich, 1992).
1.1. Geological background
The Late Carboniferous to Middle Jurassic KarooBasin forms one of the most complete stratigraphic
successions in the world that span this time interval
(Veevers et al., 1994). Along a dip-oriented profile, the
Journal of African Earth Sciences 35 (2002) 489–502
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* Corresponding author. Tel.: +1-780-492-6569; fax: +1-780-492-
7598.
E-mail address: octavian@ualberta.ca (O. Catuneanu).
0899-5362/03/$ - see front matter � 2003 Elsevier Science Ltd. All rights reserved.
PII: S0899-5362 (02 )00154-9
Karoo sedimentary fill displays a wedge-shaped geo-
metry, typical for foreland successions, with a maximum
preserved thickness in excess of 6 km adjacent to the
Cape Fold Belt (Rubidge, 1995). The sedimentary por-
tion of the basin fill includes glacial (Dwyka Group),
marine (Ecca Group), and nonmarine (Beaufort and
Stormberg groups) deposits, which are capped by the
volcanic Drakensberg Group that relates to the break-
Fig. 1. Geological map of the preserved Karoo Basin, showing the outcrop distribution of the main lithostratigraphic units of the Karoo Super-
group. The Adelaide and Tarkastad subgroups together form the Beaufort Group.
Fig. 2. Lithostratigraphy of the Ecca Group along a north–south transect through the basin (compiled information from Oelofsen, 1987; Millsteed,
1994; Rubidge, 1995; Visser, 1995; Scott, 1997; Berthold et al., 1999). The stratigraphic hinge line that separates the foredeep from the forebulge has
been previously mapped for consecutive time slices in the evolution of the basin (Catuneanu et al., 1998). All formations shown in this chart have
regional development, excepting for the Ripon, Fort Brown, and Waterford, which have other correlative formations to the west (see Fig. 5). P2, P3,
and P4 represent the second, third, and fourth tectonic paroxysms in the Cape Fold Belt (H€aalbich, 1983, 1992; H€aalbich et al., 1983; Gresse et al.,
1992; see text for details).
490 O. Catuneanu et al. / Journal of African Earth Sciences 35 (2002) 489–502
up of Gondwana (Smith et al., 1998; Fig. 1). The KarooBasin was transgressed by an interior seaway during the
deposition of the Dwyka and Ecca groups, which com-
pletely regressed from the limits of the preserved basin at
the end of Ecca time. The bathymetric conditions of this
interior seaway changed from deep marine (102 m),
during the Dwyka–lower Ecca interval, to shallow ma-
rine (101 m) during the upper Ecca time (Visser and
Loock, 1978).The stratigraphy of the Karoo Supergroup is mar-
kedly different between the southern (proximal) and
northern (distal) regions of the basin. These differences
reflect contrasting tectonic histories across the flexural
hinge line of the foreland system (Catuneanu et al.,
1998). A deep marine environment controlled the sedi-
mentation processes in the south, leading to the accu-
mulation of marine till with dropstones (Dwyka Group),basin floor pelagic sediments (Prince Albert and
Whitehill formations), and submarine fans (Collingham
and Ripon formations) (Fig. 2). Sedimentation in the
southern Karoo continued with shallow marine deposits
including shelf and marginal marine facies (Fort Brown
and Waterford formations, respectively; Fig. 2). Con-
comitant with the deposition in the south, sediment
aggradation took place in the northern region of theKaroo Basin as well, in nonmarine to shallow marine
environments. The Dwyka–Ecca distal stratigraphy is
represented by continental tillites (Dwyka Group),
shallow marine facies (Pietermaritzburg and Volkrust
formations), and coal-bearing fluvial-deltaic strata
(Vryheid formation; Fig. 2). The regression of the Ecca
seaway led to the establishment of a fully nonmarine
environment within the limits of the preserved KarooBasin, which resulted in the accumulation of the fluvio-
lacustrine Beaufort Group and the subsequent aggra-
dation of the fluvial and aeolian Stormberg strata
(Smith, 1990; Smith et al., 1993).
1.2. Tectonic setting
The Karoo Basin is a retroarc foreland system (de
Wit et al., 1988; Johnson, 1991; Catuneanu et al., 1998)
formed in front of the Cape Fold Belt in response to
crustal shortening brought about by the subduction of
the paleo-Pacific plate beneath the Gondwana plate(Lock, 1978, 1980; de Wit and Ransome, 1992; Py-
sklywec and Mitrovica, 1989). Catuneanu et al. (1998)
modeled the changes in accommodation in the Karoo
Basin as being controlled by the flexural response of the
lithosphere to orogenic cycles of loading and unloading.
They further showed that the out of phase history of
base level changes between the foredeep and the fore-
bulge flexural provinces generated contrasting stratig-raphies with a timing that matches the dated
compressional events in the Cape Fold Belt (H€aalbich,1983, 1992; H€aalbich et al., 1983; Gresse et al., 1992).
Eight tectonic paroxysms have been documented in theCape Fold Belt, and dated using radiometric techniques
(H€aalbich, 1983, 1992; H€aalbich et al., 1983; Gresse et al.,
1992), three of which (P2–P4) are indicated in Fig. 2.
The ages of the orogenic paroxysms have been obtained
by dating the final phases of compression and meta-
morphism associated with each pulse of orogenic ac-
tivity (cooling age of metamorphic minerals), which
means that the dates indicate the end of active stages oftectonism. Orogenic paroxysms were followed by stages
of orogenic quiescence (unloading), which define eight
cycles of orogenic loading and unloading, during the
evolution of the Cape Fold Belt. Within each cycle, the
relative duration between the stages of loading and un-
loading was inferred based on the stratigraphic patterns
in the Karoo Basin (Catuneanu et al., 1998).
Pysklywec and Mitrovica (1989) propose that a sig-nificant component (up to 30%) of the proximal subsi-
dence, and all the distal subsidence, could be accounted
for by dynamic subsidence caused by the deflection of
the lithosphere due to mantle flow coupled to adjacent
subduction. Such a shallow dipping subducting slab
(30–40�) is also able to reconcile the long wavelength of
the basin subsidence as previously proposed by Lock
(1980).
1.3. Accommodation in retroarc foreland systems
Retroarc foreland systems form through the flexuraldeflection of the lithosphere in response to a combina-
tion of supra- and sublithospheric loads (Beaumont,
1981; Jordan, 1981; Mitrovica et al., 1989; Sinclair and
Allen, 1992; Beaumont et al., 1993; DeCelles and Giles,
1996; Catuneanu et al., 1999; Fig. 3). Supracrustal
loading by orogens leads to the partitioning of foreland
systems into flexural provinces, i.e., the foredeep, fore-
bulge, and back-bulge. Renewed thrusting (addition ofload) in the orogenic belt results in foredeep subsidence
and forebulge uplift, and the reverse occurs as the oro-
genic load is removed by erosion or extension. This
pattern of opposite vertical tectonics modifies the rela-
tive amounts of available accommodation in the two
flexural provinces, and may generate out of phase (re-
ciprocal) proximal to distal stratigraphies (Catuneanu
et al., 1997b, 1999, 2000; Catuneanu and Sweet, 1999).Coupled with flexural tectonics, additional accommo-
dation may be created or destroyed by the superimposed
effects of eustasy and dynamic (sublithospheric) loading.
The latter mechanism operates at regional scales, and
depends on the dynamics and geometry of the subduc-
tion processes underneath the basin (Mitrovica et al.,
1989; Gurnis, 1992; Holt and Stern, 1994; Catuneanu
et al., 1997a). The eustatic and tectonic controls on ac-commodation may generate sedimentary sequences and
unconformities over a wide range of timescales, both
over and under 106 yr (Peper et al., 1992; Burgess et al.,
O. Catuneanu et al. / Journal of African Earth Sciences 35 (2002) 489–502 491
1997; Miall, 1997). Stratigraphic evidence for the Karoo
Basin suggests that both flexural tectonics and dynamic
loading have contributed to the total amount of subsi-
dence (Catuneanu et al., 1998; Pysklywec and Mitrovica,
1989). The partitioning of the Karoo foreland system
into foredeep and forebulge settings was documented byCatuneanu et al. (1998) and the hinge line separating the
two flexural provinces was mapped for consecutive time
slices. This work showed that the hinge line migrated
along the dip in relation to the redistribution of load in
the Cape Fold Belt from the end of the Carboniferous to
the Triassic (Catuneanu et al., 1998).
The interplay of base level changes and sediment
supply controls the degree in which the available ac-commodation is consumed by sedimentation (Miall,
1997). This defines the underfilled, filled, and overfilled
stages in the evolution of the foreland system, in which
the depositional processes are dominated by deep ma-
rine, shallow marine, or fluvial sedimentation respec-
tively (Sinclair and Allen, 1992). In the case of the
Karoo Basin, the underfilled, filled, and overfilled pha-
ses correspond to the accumulation of the Dwyka–lowerEcca (deep marine), upper Ecca (shallow marine), and
Beaufort-‘‘Stormberg’’ (nonmarine) successions (Fig. 4).
1.4. Aim of research
This paper focuses on the submarine fans of the lowerEcca succession in the south of the Karoo Basin (Ripon
and correlative formations), and their deltaic to fluvial
correlatives in the north (Vryheid formation) (Fig. 2).
These deposits accumulated during the underfilled phase
of the Karoo foredeep, when water depths in the region
of 500 m were recorded along the proximal rim of the
basin (Visser and Loock, 1978). The submarine fans are
related to the foredeep of the Karoo foreland system,
and have been intensively studied as deep marine reser-
voir analogues for the petroleum industry (Bouma andWickens, 1991, 1994; Kingsley, 1977, 1981; Wickens,
1994; Scott, 1997; Scott and Bouma, 1998). Their cor-
relative marginal marine and fluvial facies to the north
are related to the flexural forebulge of the Karoo fore-
land system (Catuneanu et al., 1998), and have also been
the object of numerous studies due to their included
economic coalfields (Cadle et al., 1982, 1993; Cairncross,
1989; Cairncross and Cadle, 1987, 1988; Le Blanc Smith,1980). A paleogeographic reconstruction showing the
distribution of submarine fans and fluvio-deltaic envi-
ronments in relation to the proximal and distal shore-
lines of the Ecca interior seaway respectively, is
illustrated in Fig. 5.
The accumulation of the Ripon formation and cor-
relative deposits took place after the second tectonic
paroxysm in the Cape Fold Belt (P2 in Fig. 2), during astage of orogenic quiescence (Wickens, 1994; Scott and
Bouma, 1998). Our objective is to analyse the relative
contributions of the external controls on sedimentation
which may have led to the accumulation of the observed
facies during this particular tectonic regime.
2. Lithostratigraphy
The formations that define the stratigraphic objective
of this paper are presented in turn for the foredeep and
the forebulge flexural provinces of the foreland system.
Fig. 3. Tectonic mechanisms controlling accommodation in retroarc
foreland systems. Flexural tectonics is related to supracrustal loading
by orogens (Beaumont, 1981; Jordan, 1981; Beaumont et al., 1993).
Dynamic subsidence is triggered by sublithospheric loading, which in
turn is controlled by the process of subduction underneath the basin
(Mitrovica, 1989; Gurnis, 1992; Burgess et al., 1997). The composite
lithospheric deflection may be modified by basement tectonics, not
shown. The horizontal and vertical scales vary with the amount and
distribution of loads, and the rheology and thickness of the litho-
sphere.
Fig. 4. The underfilled, filled, and overfilled phases of the Karoo
Basin, as reflected by the foredeep facies.
492 O. Catuneanu et al. / Journal of African Earth Sciences 35 (2002) 489–502
2.1. Flexural foredeep
The deposition of submarine fans in the Karoo
foredeep took place within three discrete depozones, or
subbasins, separated by structural highs (Fig. 5). Thewestern (Tanqua) and central (Laingsburg) subbasins
are separated by the Baviaanshoek and Hex River-
Bontberg anticlines (Wickens, 1994; Cole et al., 1998;
Scott and Bouma, 1998). One other structural high is
postulated along the meridian 24�E, to explain the
lithostratigraphic differences between the Laingsburg
(central) and southern subbasins (Rubidge, 1995).
2.1.1. Southern subbasin
The stratigraphic objective in the southern subbasin is
the Ripon formation. This succession was interpreted as
a submarine fan complex by Truswell and Ryan (1969)
and Kingsley (1977). The name Ripon formation was
proposed for these rocks by Johnson (1976). Kingsley
(1977, 1981) recognised three members which constitute
the Ripon formation in the Eastern Cape, namely thePluto�s Vale, Wonderfontein and Trumpeters members
(Fig. 6). The entire basal Pluto�s Vale Member consists
of rhythmic turbidite units of varying thickness. The
fine- to very fine-grained greywacke beds grade upwards
into siltstone and shale. Parts of Bouma sequences can
be recognised, but complete Bouma sequences are scarce
(Kingsley, 1977). The overlying Wonderfontein Member
consists mainly of olive-grey, massive to laminatedmudstones, which in places shows graded bedding. A 5
m thick greywacke sandstone unit is present halfway up
this mudstone, and in many places has slumping on its
upper surface. Overlying this unit is the Trumpeters
Member which forms another sequence of greywackes
and mudstones with turbidite features (Kingsley, 1977).
The average azimuthal paleocurrent direction for the
Pluto�s Vale Member is 343�, and that for the Won-derfontein and Trumpeters members is 339�.
2.1.2. Laingsburg subbasin
The sequence under study in the Laingsburg subbasin
is represented by the Vischkuil and Laingsburg forma-
tions (Fig. 6). The Vischkuil formation, which is be-
tween 200 and 400 m thick, overlies the Collingham
formation and consists of mudrocks alternating withsubordinate sandstones. The mudrocks represent basin-
plain suspension settling of clays, interrupted by spo-
radic influxes of muddy turbidity flows. The sandstones,
Fig. 5. Paleogeographic reconstruction of the environments established in relation to the Ecca interior seaway of the Karoo Basin during the
Artinskian (compiled information from Kingsley, 1977; Cadle et al., 1993; Wickens, 1994; Rubidge, 1995). The position of the stratigraphic hinge line
is mapped by Catuneanu et al. (1998). The distribution of the northern deltaic and fluvial facies is restricted to the forebulge region. The foredeep
accumulated submarine fan systems along the southern rim of the basin, and almost exclusively pelagic facies to the north. The average paleocurrent
directions of the gravity flows are towards north-northeast (Southern subbasin), east (Laingsburg subbasin), and northeast (Tanqua subbasin).
O. Catuneanu et al. / Journal of African Earth Sciences 35 (2002) 489–502 493
which increase in thickness and abundance upwards,represent more proximal turbidites (Wickens, 1994). The
presence of minor tuff beds in the shale indicates con-
tinued volcanic activity in the southern magmatic arc
region (Cole et al., 1998). Conformably overlying the
Vischkuil formation are the sandstone packages of the
Laingsburg formation, which are organized as a suc-
cession of submarine fan systems (Scott, 1997). Greater
compression on the southern branch of the Cape FoldBelt caused the Laingsburg subbasin to evolve into a
deeper, narrower depozone of a more typical foredeep
style (Scott and Bouma, 1998). The submarine fan sys-
tems all have paleocurrent directions toward the east
(Scott, 1997; Fig. 5).
2.1.3. Tanqua subbasin
The gravity flow deposits of the Tanqua subbasin are
represented by the Tierberg and Skoorsteenberg for-
mations (Wickens, 1994) (Fig. 6). The Tierberg forma-
tion consists of grey shale and subordinate thin siltstonelayers. Very fine-grained sandstone beds occur in its
uppermost part (Cole et al., 1998). Deposition of mud
from suspension was the dominant sedimentary process,and water depths most likely did not exceed 500 m
(Visser and Loock, 1978). This formation is overlain by
the approximately 400 m thick Skoorsteenberg forma-
tion, which is composed of five sandstone packages
separated by mudrock units (Wickens, 1994). The Sko-
orsteenberg formation represents a basin floor complex
with the sandstone packages having been deposited by
submarine fan systems. Palaeocurrent directions forfour of the five fans range from the south-southwest to
south, and from west to west-northwest for the other
(Scott and Bouma, 1998).
2.1.4. Regional correlation
The correlation of submarine fan systems across the
three foredeep subbasins (Fig. 6) is based on the work of
Kingsley (1977), Wickens (1994), Scott (1997), Cole et al.
(1998) and Scott and Bouma (1998). Five discrete
gravity flow events can be inferred to have manifested
across the entire Karoo foredeep, resulting in the suc-cession of submarine fan systems observed in each of the
three subbasins. The correlation of gravity flow deposits
Fig. 6. Lithostratigraphic correlation of the post-Collingham Ecca formations along the southern rim of the basin (compiled information from
Kingsley, 1977; Wickens, 1994; Scott, 1977; Scott and Bouma, 1998). The vertical profile for the Southern subbasin is the Peddie section of Kingsley
(1977). Note the inferred coeval progradation of the five submarine fan systems in the three separate subbasins, suggesting that sedimentation
processes were controlled by external factors at the regional scale of the basin.
494 O. Catuneanu et al. / Journal of African Earth Sciences 35 (2002) 489–502
is further constrained by the presence of the 30 m thick,low-density turbidites of the Collingham formation at
the base of the submarine fan complexes in all subba-
sins, which can therefore be used as a stratigraphic da-
tum. The Collingham formation is underlain by the
pelagic facies of the Whitehill formation (Fig. 2), which
can also be traced across the entire Karoo foredeep.
Furthermore, the three submarine fan complexes are
overlain by the age-equivalent shallow marine facies ofthe Fort Brown and Kookfontein formations (Cole
et al., 1998; Fig. 6). The coeval gravity flow deposits of
the Tanqua, Laingsburg, and Southern subbasins (i.e.,
Ripon and correlative formations; Fig. 6) are dated as
Artinskian, with possible extension into the Kungurian
(Visser, 1995; Scott, 1997; Fig. 2). This age is further
supported by the independent dating of the underlying
Whitehill pelagic facies as Late Sakmarian (Oelofsen,1987; Oelofsen and Araujo, 1987 in Wickens, 1994), and
also by the recent radiometric dating of the lower
Collingham formation from ash beds as 270� 1 My,
which confirms the Sakmarian age of the pre-Ripon
facies (M. de Wit and S. Bowring, pers. comm.; Fig. 2).
The contemporaneous formation and filling of the
foredeep subbasins was also recognized by Scott and
Bouma (1998). The correlation of submarine fan sys-tems along the strike of the foredeep (Fig. 6) suggests
that the gravity flow events were driven by basin-scale
allogenic mechanisms rather than local controls at the
subbasin scale.
Further to the north relative to the submarine fan
depozones (i.e., the area between the submarine fans and
the flexural hinge line in Fig. 5), the depositional envi-
ronment of the foredeep was less dynamic, being domi-nated by basin floor pelagic sedimentation (Zawada and
Cadle, 1988).
2.2. Flexural forebulge
In contrast with the deep marine conditions esta-
blished along the proximal rim of the foredeep during
the Artinskian, the forebulge region was dominated by
deltaic to fluvial sedimentation (Fig. 5). Van Vuuren and
Cole (1979) were amongst the first to refer to the cyclical
nature of the upward-fining and upward-coarsening
successions that typify the Vryheid formation, charac-teristics that are well-documented by Hobday (1973),
Cadle (1974) and Mathew (1974). These three authors
ascribed the cyclicity to stacked depositional sequences
originating from deltaic and fluvial processes (Hobday,
1978). The formation is thickest in Kwazulu–Natal
(Blignaut and Furter, 1940), and thins progressively
towards the west and south until pinching-out (Cadle
et al., 1993; Fig. 7).The distribution of the strata forming the Vryheid
formation resulted from a combination of influence of
the source areas, sedimentation patterns, basement geo-
logy, basin tectonics, and eustatic processes (Cadle et al.,
1982). Maximum subsidence took place in the east of the
preserved Karoo Basin (Whateley, 1980; Stavrakis,
1989; Cadle et al., 1982; Fig. 7), as shown by the isopachmap. The clastic influx into the basin was driven by
bedload dominated river systems, which provided sedi-
ment to the shoreline delta systems positioned further
basinward (Cadle and Cairncross, 1993).
The stratigraphy of the Vryheid formation is des-
cribed by a succession of five coarsening-upward se-
quences which display a remarkable lateral continuity
across the entire distal region of the Karoo Basin (Cadleet al., 1982; Figs. 7 and 8). In a complete succession each
of the five coarsening-upward sequences starts with fine-
grained marine facies, which grade upwards into coarser
delta front and delta plain-fluvial facies. Several coal
seams occur in the Vryheid formation, and are associ-
ated predominantly with the coarser-grained fluvial fa-
cies at the top of each sequence (Fig. 8). These coal
seams can be traced laterally across the entire area ofoccurrence of the Vryheid formation. Research on
the Karoo coalfields (Le Blanc Smith, 1980; Cairn-
cross, 1986; Winter et al., 1987; Taverner-Smith et al.,
1988; Cadle, 1995) recognize the repetitive process
Fig. 7. Isopach map of the Vryheid formation, suggesting the patterns
of subsidence of the forebulge region (modified from Cadle et al.,
1982). The depocenter of forebulge sedimentation appears to be lo-
cated in the north-eastern part of the preserved basin. The generalized
vertical profile of the Vryheid formation is illustrated on the top-left
side of the diagram, showing a succession of five coarsening-upward
deltaic sequences. This profile is representative for the Vryheid for-
mation across the entire area of occurrence due to the remarkable
lateral continuity of the five progradational sequences. The thickness
of this succession varies according to the isopach map. The southern
limit of the Vryheid facies coincides with the location of the strati-
graphic hinge line (Fig. 5).
O. Catuneanu et al. / Journal of African Earth Sciences 35 (2002) 489–502 495
of basinward migration of coarse-grained (gravel-bed
braided) fluvial systems feeding into deltaic depositionalsystems. These periods were followed by stages of rising
water tables, favorable for peat accumulation. Further
deepening of the basin waters led to the flooding of the
peat swamps, which became covered with fine, trans-
gressive clastic material. A shifting balance between
sedimentation and the rates of base level rise is therefore
most likely to explain the cyclic nature of the Vryheid
formation. The transgressive units which occur at thebase of each coarsening-upward sequence are some of
the most widespread and laterally continuous beds in
the Vryheid formation in this northern part of the basin.
Such units form good markers for stratigraphic corre-
lation, and may be exemplified by the bioturbatedmudstones and siltstones above the no. 2 coal seam and
the bioturbated, glauconite-bearing argillites above the
no. 4 and no. 5 coal seams (Cadle et al., 1982, 1993;
Cairncross, 1986) (Fig. 8). The very good lateral conti-
nuity of the Vryheid sequences and coal beds suggests
uniform depositional conditions across the entire fore-
bulge region, with no partitioning into subbasins.
The Vryheid formation is dated as Artinskian, withpossible extension into the Kungurian (McLachlan and
Anderson, 1973; Loock and Visser, 1985; MacRae,
1988; Visser, 1990; Aitken, 1994, 1998; Millsteed, 1994).
3. Sequence stratigraphy
The sequence stratigraphic nomenclature used in this
paper is illustrated in Fig. 9. The various types of se-
quences, systems tracts and bounding surfaces are de-
fined in relation to the relative sea-level (combined effect
of tectonics and eustasy) and transgressive–regressive(T–R) (combined effect of relative sea-level and sediment
supply) curves.
3.1. Cyclicity of foredeep facies
The Ripon and correlative formations are built by a
relatively conformable succession of five submarine fansystems that may be correlated along the strike of the
basin (Figs. 5 and 6). Each submarine fan system dis-
plays a coarsening-upward trend that reflects the pro-
gradation of the system during times of base level fall
(Posamentier et al., 1992; Hunt and Tucker, 1992). The
lifespan of each submarine fan complex was terminated
by stages of abrupt water deepening and transgression
across the foredeep, which inhibited the manifestation ofmajor gravity flow events (Galloway, 1989; Embry,
1995). During such stages, the depositional regime
changed from gravity flow to fine-grained pelagic sedi-
mentation. The sharp contacts between turbidites and
the overlying pelagic facies suggest abrupt shifts from
forced regressions to subsequent transgressions, which
did not allow for the accumulation of any significant
normal regressive lowstand deposits. At the other end ofthe cycle, the contacts between pelagic and overlying
submarine fan facies are generally marked by scour
surfaces cut by the earliest gravity flows of each fan
complex (basal surface of forced regression, Hunt and
Tucker, 1992; Fig. 9). Below each basal surface of forced
regression, the pelagic deposits are undifferentiated and
attributed here to the transgressive systems tracts. The
lack of any recognizable normal regressive deposits(lowstand and highstand systems tracts; Fig. 9) allows to
interpret the submarine fan complexes as regressive
systems tracts.
Fig. 8. Stratigraphic column for the north-eastern part of the Vryheid
formation (modified from Cadle et al., 1982). The succession consists
of five T–R sequences, each of them including a transgressive and a
regressive systems tract. The transgressive systems tracts are related to
the marine flooding of the forebulge area, whereas the regressive sys-
tems tracts reflect normal regressions of the distal Ecca shoreline in
relation to fluvial-deltaic progradation. The marine facies are mainly
represented by fine-grained shelf sediments. The deltaic facies consist
of cross-laminated and cross-bedded sandstones. The fluvial facies
occur at the top of the T–R sequences, being represented by cross-
stratified gravels. Abbreviations: Fl––horizontally-laminated fine-
grained sediments; Sr––ripple cross-laminated sandstone; Sp––planar
cross-bedded sandstone; St––trough cross-bedded sandstone; Gx––
cross-bedded gravel; b––bioturbation; TST––transgressive systems
tract; RST––regressive systems tract; MFS––maximum flooding sur-
face; MRS––maximum regressive surface.
496 O. Catuneanu et al. / Journal of African Earth Sciences 35 (2002) 489–502
In this interpretation, the Ripon and correlative
successions are composed of five T–R sequences, each
comprising a relatively thin basal transgressive systemstract overlain by a much thicker regressive systems tract.
The former includes deeper water, fine-grained pelagic
sediments, whereas the latter includes the actual sub-
marine fans. The two systems tracts are bounded by the
maximum regressive surface (top of submarine fan fa-
cies, which replaces the correlative conformity in the
absence of a lowstand systems tract; Fig. 9) and by the
maximum flooding surface (top of pelagic facies, whichis reworked by the basal surface of forced regression in
the absence of a highstand systems tract; Fig. 9).
3.2. Cyclicity of forebulge facies
The Vryheid formation is also built by a succession of
five T–R sequences, as illustrated in Fig. 8. A number of
marker horizons, including glauconitic sandstones and
coal seams, can be traced across the entire forebulge
region (Cadle et al., 1982, 1993). The vertical profile isdominated by coarsening-upward trends which relate to
the gradual progradation of fluvio-deltaic systems. Each
T–R sequence includes a generally thinner (less than
10 m thick) transgressive systems tract overlain by a
much thicker (tens of meters range) regressive systems
tract. The transgressive systems tracts represent periodic
floodings of the forebulge region, when the entire
northern part of the Karoo Basin was transgressed fromsouth to north by a shallow marine environment. Each
transgressive episode was followed by normal regres-
sions of the shoreline in the northern part of the basin,
in a southerly direction, during which the progradation
and aggradation of fluvio-deltaic systems took place.
Fig. 5 illustrates one of the regressive stages in theevolution of the Vryheid formation. The periodic change
in the direction of the shoreline shift between the
transgressive and regressive stages is likely related to the
interplay between sediment supply and varying rates of
base level rise.
The key stratigraphic surfaces that delineate the
transgressive and regressive systems tracts are the maxi-
mum regressive surfaces and the maximum floodingsurfaces (Fig. 8). The maximum regressive surfaces
bound the T–R sequences, marking the timing of the
maximum regressions of the distal shoreline of the Ecca
seaway. These surfaces are found at the top of the
coarsening-upward prograding facies. The maximum
flooding surfaces correspond to the timing of maximum
marine transgressions, and are found at the top of fine-
grained shelf facies.
3.3. Regional correlation
The resolution of the available time control only al-
lows for stratigraphic correlations at the formation level
(Fig. 2). The only two lines of correlation constrained by
biostratigraphic evidence are shown in Fig. 10 at the
base and top of the studied successions. The position of
these horizons indicate coeval transgressions for the
earliest foredeep and forebulge T–R sequences, as wellas coeval deposition of the latest foredeep submarine
fans and forebulge deltaic deposits. This suggests that
the proximal and distal shorelines of the Ecca seaway
Fig. 9. Types of sequences, bounding surfaces and systems tracts defined in relation to the base level and T–R curves (from Catuneanu, 2002).
Abbreviations: TST––transgressive systems tract; RST––regressive systems tract; LST––lowstand systems tract; HST––highstand systems tract;
FSST––falling stage systems tract; SU––subaerial unconformity; c.c.––correlative conformity; MRS––maximum regressive surface; MFS––maxi-
mum flooding surface; BSFR––basal surface of forced regression; (A)––positive accommodation; NR––normal (sediment supply-driven) regression;
FR––forced (base level fall-driven) regression.
O. Catuneanu et al. / Journal of African Earth Sciences 35 (2002) 489–502 497
(Fig. 5) may have experienced similar types of shore-
line shifts, i.e., coeval transgressions and coeval regres-sions.
Higher-frequency sequence stratigraphic correlations
can be further inferred within the studied interval. The
Artinskian (�Kungurian) foredeep and forebulge suc-
cessions both include five sequences of stacked subma-
rine fans and deltas respectively. We suggest a direct
correlation of these five sequences along the dip of the
Karoo foreland system, as illustrated in Fig. 10. Thisinterpretation assumes periodic episodes of basinwide
flooding of the Karoo Basin, separated by periods of
time of coeval forced and normal regressions of the
proximal and distal shorelines of the Ecca interior sea-
way respectively. The proposed correlation is based on
the following arguments:
(i) Chemical, petrographic, and microprobe analyses
of the sandstones in the submarine fan systems indicatethey were derived from the same distant source areas,
during times of tectonic quiescence and denudation in
the Cape Fold Belt (Wickens, 1994; Scott, 1997; Scott
and Bouma, 1998). Such times of tectonic quiescence
correspond to a stage of erosional orogenic unloading,
which is expected to result in the isostatic rebound of the
foredeep compensated by flexural subsidence of the
forebulge (Beaumont et al., 1993). This tectonic regimetranslates into proximal base level fall coeval with distal
base level rise, which supports the correlation between
proximal forced regressions (gravity flows) and distal
normal regressions (deltaic facies). The normal regres-
sions require sedimentation rates higher than the rates
of base level rise, which is a likely scenario given the low
rates of forebulge flexural subsidence.(ii) Basinwide transgressions require the manifesta-
tion of long wavelength controls on accommodation,
such as global (eustatic) sea-level rise, or regional scale
dynamic subsidence. Both these mechanisms are known
to have been operative during Ecca times (Cadle et al.,
1993; Pysklywec and Mitrovica, 1989), which allows for
the correlation of proximal and distal transgressive
systems tracts.An alternative interpretation would be to correlate
the proximal transgressive facies (pelagic sediments)
with the distal normal regressive deposits (deltaic facies),
and implicitly the proximal forced regressive deposits
(submarine fans) with the distal transgressive facies.
This is an unlikely scenario for a stage of tectonic qui-
escence, which involves a flexural regime of foredeep
uplift and forebulge subsidence. In order to have proxi-mal transgressions, the rates of flexural uplift must be
outpaced by the rates of long-wavelength base level rise.
In turn this infers even higher rates of distal relative sea-
level rise, as the long-wavelength base level rise is com-
plemented by flexural subsidence. The interplay of
flexural and long-wavelength controls results in more
accommodation being created in the distal region of the
foreland system, which implies that the distal shorelineis more likely to be the subject of transgressive, rather
than normal regressive, shifts.
The end members of the five regional sequences in
Fig. 10 are the foredeep submarine fans and the fore-
bulge deltas. Each of these sequences represent higher-
Fig. 10. Regional correlation between the foredeep submarine fan systems and the forebulge deltaic sequences. Both successions are dated as
Artinskian, with possible extension into the Kungurian. No higher resolution time control is available, but a one-to-one correlation between the five
submarine fan systems and the five deltaic sequences is proposed on theoretical grounds (see text for details).
498 O. Catuneanu et al. / Journal of African Earth Sciences 35 (2002) 489–502
frequency subdivisions of the second-order cyclothemsidentified in previous research (Catuneanu et al., 1998),
and are therefore interpreted here as third-order se-
quences. Considering the 5 My duration of the Artins-
kian (Fig. 2), the average timespan of these third-order
sequences is 1 My.
4. Discussion
The key element of this case study is the background
regime of tectonic quiescence during which the accu-
mulation of submarine fans and their correlative Vry-
heid deposits took place (Wickens, 1994; Scott, 1997;
Scott and Bouma, 1998). This fits well into the succes-
sion of tectonic paroxysms of H€aalbich (1983, 1992),
H€aalbich et al. (1983) and Gresse et al. (1992). The ac-cumulation of the submarine fan-deltaic sequences took
place during the quiescence time which separated the
second and third tectonic paroxysms in the Cape Fold
Belt (Fig. 2). This means that the duration of the third
tectonic paroxysm was of 5 My or less, which is inagreement with the fact that stages of orogenic thrusting
and loading tend to be much shorter relative to the
stages of orogenic quiescence (Catuneanu et al., 1997b).
In our case, considering the 20 My interval that sepa-
rates the tectonic paroxysms P2 and P3 (Fig. 2), the
inferred ratio between the post-P2 quiescence stage and
the pre-P3 loading stage is at least 3:1 (15 My or more
versus 5 My or less respectively).Superimposed on this background flexural regime of
foredeep rebound and forebulge subsidence, eustatic
fluctuations in sea-level and/or fluctuations in the rates
of long-wavelength dynamic subsidence modified the
amounts of available accommodation across the basin.
Both eustasy and dynamic subsidence may undergo
changes in rates and magnitudes at the observed time-
scales of 105–106 yr (Gurnis et al., 1996; Burgess et al.,1997; Miall, 1997), which could explain the interpreted
basinwide transgressive facies. We thus infer that the
coeval proximal forced regressions and distal normal
regressions are the result of long-term flexural tectonics,
Fig. 11. Conceptual model for the deposition of the observed transgressive and regressive facies. The two block diagrams illustrate a full cycle
leading to the sedimentation of each one of the five third-order sequences. The same flexural background of orogenic unloading operated in both
cases, but fluctuations in the rates of long-wavelength base level rise (dynamic subsidence and/or eustasy) led to cyclic changes in the relative im-
portance between the two controls on accommodation. Diagram (1) shows a foreland system dominated by flexural tectonics. The coeval forced and
normal regressions of the proximal and distal shorelines of the Ecca seaway account for the accumulation of gravity flow and deltaic facies. Diagram
(2) illustrates the periods of time when flexural tectonics was outpaced by increased rates of long-wavelength base level rise. The basinwide rise in base
level inhibited the major gravity flow events, leading to pelagic sedimentation in the foredeep, and triggered the flooding of the forebulge region.
O. Catuneanu et al. / Journal of African Earth Sciences 35 (2002) 489–502 499
and that this regime was periodically interrupted bypulses of basinwide drowning and transgressions in re-
lation to the higher-frequency fluctuations of eustasy
and/or dynamic loading. Such fluctuations in the
amount of dynamic loading are related to changes in the
angle and velocity of subduction underneath the retro-
arc foreland system (Mitrovica et al., 1989; Gurnis,
1992; Holt and Stern, 1994).
The block diagrams in Fig. 11 illustrate palaeogeo-graphic reconstructions of the foredeep and forebulge
environments during the regressive and transgressive
phases of each sequence. Fig. 11(1) shows the coeval
deposition of submarine fans (forced regressive shore-
line) and deltaic systems (normal regressive shoreline)
during the flexural regime of orogenic unloading that
dominated the Artinskian. Fig. 11(2) accounts for the
periods of time in which the rates of long-wavelengthbase level rise outpaced the background effects of flex-
ural tectonics. Such stages resulted in the drowning of
the foreland system, with the development of pelagic
facies in the foredeep and shallow marine transgressive
facies over the forebulge. It is important to note that
sediment aggradation took place across the entire fore-
land system during both regressive and transgressive
phases due to the fact that the flexural forebulge had notopographic expression, being placed below the base
level. Such a ‘‘missing’’ peripheral bulge has also been
documented in the Western Canada Basin, and is an
evidence for the manifestation of long-wavelength dy-
namic subsidence (Catuneanu et al., 1997a).
The importance of the external controls on sedi-
mentation is further supported by the in-phase accu-
mulation of submarine fans across the syn-depositionalstructural highs, within the three separate subbasins
(Fig. 6). The five inferred third-order sequences can be
traced across the flexural hinge line of the foreland
system, within both the foredeep and the forebulge
provinces (Fig. 10), which also supports the importance
of the external controls on accommodation and sedi-
mentation.
The accumulation of submarine fans in the Karooforedeep represents the last stage of the underfilled
phase in the evolution of the Karoo Basin. The succes-
sion of foredeep Ecca facies started with the basin floor
pelagic sediments of the Prince Albert and Whitehill
formations, which were gradually prograded by the
submarine fan systems of the Collingham, Ripon and
correlative formations. The contact between the Ripon
and Fort Brown formations, and their correlatives to thewest, marks the debut of the filled phase of the Karoo
Basin, which was dominated by the shelf and shallower
marine facies of the Fort Brown/Kookfontein and
Waterford formations. The final regression of the Ecca
seaway, represented in the stratigraphy by the Ecca-
Beaufort contact, marks the debut of the overfilled
phase in the evolution of the Karoo Basin (Fig. 4).
5. Conclusions
(1) Five third-order T–R sequences of Artinskian age
accumulated during a period of time of �5 My within
the Karoo foreland system. These sequences can be
mapped across the flexural hinge line of the basin, both
within the foredeep and the forebulge regions. Each T–
R sequence is composed of one transgressive and one
regressive systems tract.(2) The regressive systems tracts include foredeep
submarine fans and forebulge deltas. They accumulated
in relation to the flexural regime of orogenic unloading
in the Cape Fold Belt, which resulted in foredeep uplift
and forebulge subsidence. This tectonic regime deter-
mined the manifestation of coeval forced and normal
regressions of the proximal and distal shorelines of the
Ecca interior seaway respectively.(3) Coupled with flexural tectonics, additional ac-
commodation for sediment accumulation was created by
long-wavelength dynamic subsidence. As a consequence
of dynamic loading, the peripheral bulge of the foreland
system subsided below the base level, allowing for sedi-
ment accumulation across the entire foreland system.
(4) The transgressive systems tracts of each T–R se-
quence are related to periods of time of overall drown-ing of the foreland system in relation to increased rates
of long-wavelength base level rise. The transgressive
facies are represented by pelagic sediments in the fore-
deep, and shallow marine strata in the forebulge region.
(5) This case study may illustrate a predictable asso-
ciation of facies for the underfilled phase of any retroarc
foreland system in which accommodation is controlled
by both orogenic and sublithospheric loading. Themiddle portions of all studied T–R sequences are almost
exclusively composed of pelagic facies (potential source
rocks), which grade laterally into foredeep and forebulge
reservoirs (turbidites and deltaic facies respectively).
Acknowledgements
OC acknowledges financial support from the Uni-
versity of Alberta and NSERC Canada, while PJH and
BSR acknowledge support from the NRF and the
University of the Witwatersrand. We wish to thank P.G.
Eriksson and J.J. Veevers for their comments which
helped to improve earlier versions of this manuscript.
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