EMESE M. BORDY AND OCTAVIAN CATUNEANU
51
Sedimentology of the Beaufort-Molteno Karoo fluvial strata
in the Tuli Basin, South Africa
Emese M. Bordy
Rhodes University, Department of Geology, Grahamstown 6140, South Africa.
e-mail: [email protected]
Octavian Catuneanu
University of Alberta, Department of Earth and Atmospheric Sciences,
1-26 Earth Sciences Building, Edmonton, Alberta T6G 2E3, Canada.
e-mail: [email protected]
ABSTRACT
The sedimentary rocks of the Karoo Supergroup in the Tuli Basin (South Africa) may be grouped into four
stratigraphic units: the Basal, Middle and Upper units, and the Clarens Formation. This paper presents the findings of
the sedimentological investigation of the fluvial clastic deposits of the Middle Unit. The lack of bio- and
chronostratigraphic control hampers an unequivocal correlation of the Middle Unit with the formations of the main
Karoo Basin. Existing constraints place the Middle Unit in a stratigraphic position older than the Elliot Formation and
younger than the Ecca Group. Between these limits, the Middle Unit may correlate with either the Beaufort Group
or the Molteno Formation of the main Karoo Basin (Table 1).
The Middle Unit may be subdivided into three parts, based on lithological differences. The lower part includes
rudites and arenites (lithofacies Gh, Sp, Sh, and Sm); the middle part is dominated by siltstones and very fine
sandstones (lithofacies Fl); and the upper part consists of mudstones (lithofacies Fsm). This succession accumulated
in a perennial braided fluvial system, with well developed inter-channel flood plains. The observed lithofacies and
architectural elements are partly coeval, with the overall upward-fining profile related to gradual denudation and
peneplanation. Petrographic analyses indicate multiple source areas, as well as reworking of the underlying Basal
Unit. The accumulation of the Middle Unit in the Tuli Basin corresponds to a distinct stage of tectonic development
in the region, when the topographic gradient was consistently dipping from south-east to north-west. In contrast with
this, the Basal and Upper units were accumulated on topographic slopes dipping from east-northeast to westsouthwest, and from north-northwest to south-southeast respectively.
Introduction
The transfrontier Tuli Basin is situated around the triple
junction of the Zimbabwean, Botswanan and South
African borders (Figure 1). The basin is filled with the
sedimentary and igneous rocks of the Karoo Supergroup
(Figure 2). The sedimentary sequence is thinner
(estimated max. ~450-500m) and less continuous than
the correlative Late Carboniferous-Middle Jurassic
sedimentary rocks of the Karoo Supergroup in the main
Karoo Basin (~ 6000m - Catuneanu et al., 1998)
(Figure 1). In the South African part of the Tuli Basin,
the Karoo sedimentary rocks occupy an area of about
2
1000km (Figure 3) and consist of various terrigenous
clastic
and
chemical
deposits
(parabreccias,
conglomerate-breccias, conglomerates, sandstones, finegrained sediments, calcretes and silcretes). Four
distinctive lithostratigraphic units are separated in the
study area, namely the Basal, Middle and Upper units,
and Clarens Formation (Bordy, 2000). These units are
not recognized by the SACS (South African Committee of
Stratigraphy), and therefore the use of their names here
must be regarded as informal. Biostratigraphic data
constrains the correlation between the Upper Unit and
the Elliot Formation, as well as between the Basal Unit
and the Dwyka and Ecca Groups (Bordy, 2000; Table 1).
Beyond these constrains, no relative or absolute age
data are available from the Middle Unit. It may be the
correlative of either the Beaufort Group or the Molteno
Formation in the main Karoo Basin (Table 1).
The purpose of this paper is to present the results of
the sedimentological investigation of the Middle Unit
and its potential relation to the Karoo Supergroup strata
in the main Karoo Basin. To date, there is only one brief,
unpublished report by Chidley (1985) regarding the
description of the Karoo Supergroup in the South
African part of the Tuli Basin. The significance of this
paper is threefold: first of all, it contributes to our
knowledge of regional geology, secondly it highlights
the correlation difficulties to the main Karoo Basin and
lastly it helps to clarify the relationship with the Tshipise
Basin and other Karoo Supergroup satellite basins from
the northern part of South Africa. The results of this
paper are based on the analysis of the stratigraphic
relations, primary sedimentary structures, borehole
record, petrographic studies and, palaeo-flow measurements.
Geological background
Tectonic setting
In southern Africa, the deposition of the Karoo
Supergroup occurred in two broadly different tectonic
settings. According to Catuneanu et al. (1998), the
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SEDIMENTOLOGY OF THE BEAUFORT-MOLTENO KAROO FLUVIAL STRATA
Figure 1. Various basins and the distribution of the Karoo Supergroup in southern Africa (modified after Johnson et al., 1996).
sedimentary rocks of the main Karoo Basin are retroarc
foreland fills. North of the main Karoo Basin, the
formations are preserved in separate, fault-bounded
depositories which are interpreted either as rift basins or
intracratonic thermal sag basins (Rust, 1975; Watkeys
and Sweeney, 1988; Groenewald et al., 1991; Johnson
et al., 1996). It is not known to what extent the basins
were physically connected prior to post-Karoo erosion,
thus some of the present basins could be products of
erosion that occurred in response to post-Karoo crustal
movements (Visser, 1984; Johnson et al., 1996).
The Tuli Basin, together with the Tshipise (South
Africa, partly Zimbabwe) and Nuanetsi (Zimbabwe)
basins represent the so-called Limpopo area of Karooage basins. It has been proposed that the Limpopo area
forms the western arm of a failed rift triple-junction,
which later extended in a north-south direction, from the
Save Basin (Zimbabwe) to the Lebombo ‘Monocline’
(South Africa, Mozambique) (Figure 1) (Vail et al., 1969;
Burke and Dewey, 1973).The genesis of this rift system
is assumed to be associated with the Gondwana breakup (Burke and Dewey, 1973; Duguid, 1975). This midJurassic extensional episode took place towards the end
of the evolution of the Karoo-age basins, so it is unlikely
that it controlled the accommodation in the Tuli and
correlative depozones in the earlier stages of the Karoo
history. Alternatively, it has been proposed that early
subsidence in the Tuli Basin was primarily controlled by
flexural tectonism in the back-bulge setting of the
Karoo foreland system (Catuneanu et al., 1999; Bordy,
2000).
Stratigraphy
The investigated stratigraphic unit has a ribbon-like
outcrop belt running in a roughly east to west direction
through the southern-central part of the study area
(Figure 3). The unit is defined by sharp lower and
transitional upper boundaries (Figures 4, 5, 6 and 7).
The lower boundary is clearly detectable in several
outcrops where the Middle Unit is separated from the
underlying carbonaceous mudstones and siltstones
of the Basal Unit by a sharp, widespread surface (Plate
1A, B). Figures 5, 6 and 7 not only indicate the erosion
surface between the Basal and Middle units, but since
the sections were taken in different parts of the study
area, they also show that this erosional surface is
traceable throughout the basin. It is documented that
this lower boundary represents an unconformity which
is traceable throughout the Limpopo valley basins, Save
Basin (Stagman, 1978) and the Lebombo ‘Monocline’
(northwest Swaziland) (Turner and Minter, 1985).
Hence, this erosional surface is a regional unconformity.
There are a few outcrops along the southern margin of
the Breslau farm, as well as along the border between
the Vergenoeg and Sommerville farms (Figure 3), where
the conglomerates of the lowermost Middle Unit directly
rest on Archaean basement rocks, forming inselbergs in
the recent landscape.
The upper boundary of the Middle Unit was not seen
in any of the outcrops investigated because of the
lithological similarities between the argillaceous upper
part of the Middle Unit and the overlying mudstones of
the Upper Unit. Therefore, this upper boundary has
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EMESE M. BORDY AND OCTAVIAN CATUNEANU
53
Plate 1. A. Erosional surface (dashed line) separating the carbonaceous mudstones of the Basal Unit from the overlying Middle Unit
(20 cm conglomerate and 30cm massive, medium grained sandstone) (Somerville farm). Camera cap for scale. B. Erosional surface
separating the laminated, carbonaceous mudstones of the Basal Unit from the overlying Middle Unit (20cm conglomerate and 25cm massive,
medium grained sandstone) (Lizzulea farm). C. Reddish-brown sandstones and conglomerates of the lower part of the Middle Unit
(Eendvogelpan farm). Notebook for scale. D. Soft sediment deformation (convolute bedding) in otherwise laminated (Sh) sandstone (lower
part of the Middle Unit, Over Vlakte farm). Pen (circled) for scale. E. Horizontally bedded conglomerates (Gh) (lower part of the Middle
Unit, Eendvogelpan farm). Notebook for scale. F. Subrounded pebbles of quartzite and vein quartz (lower part of the Middle Unit, Bismarck
farm).
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SEDIMENTOLOGY OF THE BEAUFORT-MOLTENO KAROO FLUVIAL STRATA
A
B
C
D
Plate 2. A. Laminated (Fl) and unidirectional ripple-cross laminated (Sr) silty, very fine grained sandstone (Breslau farm). B. Ripple-cross
laminated (Sr) siltstone (asymmetrical ripples) (Breslau farm). C. Ripple-cross laminated (Sr) very fine grained sandstone (climbing ripples)
(Halcyon farm). D. Well- bedded, thin (12-20 cm) layer of silty, very fine grained sandstone (Breslau farm).
been arbitrarily established in the outcrop and borehole
descriptions at the base of the first occurrence of the
uniformly red-maroon mudstones and/or the reddish
mudstones with carbonate glaebules, which are typical
for the Upper Unit. The drill core descriptions did not
suggest any evident unconformity between the two
units, the transition seems to be gradual, therefore the
boundary is assumed to be non-erosional.
Outcrop sections
The Middle Unit comprises of reddish-brown, white and
mauve conglomerates, sandstones and siltstones, as well
as varicoloured, red-green-purple-grey non-calcareous
mudstones (due to its low resistance to weathering, the
latter occur mostly in boreholes). These lithosomes are
organized in an overall fining-upward succession
(Figure 4). Based on differences in grain size, this
relatively conformable succession may be subdivided
into three parts. The lower part includes conglomerates
and sandstones (lithofacies Gh, Sp, Sh, and Sm in
Figure 4; Table 2), with an average thickness of 4m. The
middle part is dominated by siltstones and very fine
sandstones (lithofacies Fl in Figure 4), with thicknesses
of 1-2m. The upper part consists of mudstones
(lithofacies Fsm in Figure 4), with an average thickness
of 4m. This brings the average thickness of the Middle
Unit to a total of 9m, which is valid for the outcrop
sections along the southern margin of the Tuli Basin.
The thickness of the Middle Unit does however increase
towards the north, into the subsurface, as revealed by
the borehole data (see Section 4).
The thinly bedded (0.2-0.5m) conglomerates, pebbly
sandstones and sandstones are developed in the lower
part of the Middle Unit (Plate 1C). They are maximum
5-7m thick in the exposures, the best of which are found
in the extreme east (Over Vlakte farm) and extreme west
(Breslau farm) of the study area. The pinkish, pale-red
and white sandstones are medium- to coarse-grained,
and consist of mainly subrounded, subangular and wellsorted grains. Based on macro- and microscopic
determinations, the lower part of the Middle Unit
contains very small amounts of feldspar, and therefore
shows great compositional maturity. The sandstones are
strongly planar cross-stratified (Sp), and there are also a
few massive and horizontally stratified sandstones
(Sm, Sh). Convolute bedding interpreted as softsediment deformation is exposed in one of the Sh beds
(Plate 1D). The conglomerates are usually horizontally
bedded (Gh) and frequently form stringers at the base of
the sandstone units (Plate 1E). The conglomerates
predominantly consist of white, rose, green and black
quartzite and vein quartz pebbles (diameter: 1-2cm)
(Plate 1F). In places, rip-up siltstone and mudstone
clasts were also observed (Halcyon farm, Figure 3). In
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A
B
C
D
Plate 3. Convolute lamination (A) and water escape structures (B) in silty, very fine grained sandstone (Eendvogelpan farm). Hammer for
scale. C. Lower bedding plane of a siltstone bed showing the remains of the underlying conglomerate bed with rip-up clasts (Halcyon
farm). Hammer for scale. D. Transitional zone of intraformational breccia (Gcm) containing small pebble, granule-sized rip-up clasts. The
lower part of the overlying siltstone bed is ripple-cross laminated (Fl), the upper part is colour mottled, massive (Fsm) (Halcyon farm).
comparison to the subrounded quartzite and quartz
pebbles, the rip-up clasts are very angular or angular.
The lower part of the Middle Unit is very well exposed
along the southern part of the outcrop belt (Figures. 5,
6, 7), at the contact with the underlying Basal Unit.
The siltstones and very fine-grained sandstones of
the medial part of the Middle Unit are found in thirteen
outcrops, on top of the lower Middle Unit deposits.
These strata consist of light pink, white or indurated
greenish, friable, predominantly ripple-cross laminated
siltstones (Plate 2A, B) or very fine- grained sandstones
(Sr) (Plate 2C). The deposits display prominent bedding
that separates layers 10- 20cm thick (Plate 2D), and
contain water escape structures (Plate 3B) and convolute
bedding (Plate 3A). The best outcrops of the medial part
of the Middle Unit are on the Eendvogelpan and Breslau
farms (Figure 3). The physical boundary between
conglomerates (lower Middle Unit) and the siltstones of
the middle Middle Unit was captured in only one
outcrop (Halcyon farm), where the conglomerates
progressively grade into siltstones and muddy siltstones
through a transitional zone of intraformational breccias
made of rip-up siltstone and silty mudstone clasts
(Plate 3C, D). Because the material of these rip-up clasts
is lithologically identical to the siltstones, silty
mudstones and mudstones of the medial and upper
parts of the Middle Unit, it is assumed that these clasts
are intraformational. The three parts of the unit are
therefore, at least partially, synchronous facies
equivalents.
The variegated, red-green-purple-grey mudstones of
the upper part of the Middle Unit are rarely exposed in
outcrops (e.g. along the Halcyon/Little Bess farms
boundary), but they have been encountered in
boreholes.
Borehole data
Because the Karoo Supergroup was affected by postKaroo denudation events, the precise lateral thickness
variation of the Unit could not be determined in the
outcrops. The 59 drill hole sections served to bridge this
information gap. Borehole records have been obtained
from the Coal Division of Anglo Operations Ltd. (50) and
De Beers Consolidated Mines Ltd. (9). According to the
borehole records, the Middle Unit consists of an
upward-fining sequence. The lower part of the Middle
Unit comprises of conglomerates, pebbly sandstones
and sandstones. These are overlain by either grey
mudstones with reddish stains (upper part of the Middle
Unit) or by ripple-cross laminated siltstones (medial part
of the Middle Unit) that grade up into the variegated,
red-green-purple-grey mudstones of the upper part of
the Middle Unit.
The analysis of the borehole data also reveals that
the average thickness of the Middle Unit is ~17m (max.
~46m in the Vergenoeg area). The average thickness of
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56
Table 1. Summary of the lithostratigraphic nomenclature and tentative correlation of the Karoo Supergroup strata of the transfrontier Tuli
Basin and main Karoo Basin (* there are two possible correlations for the Middle Unit).
main Karoo Basin
(Johnson, 1994)
Clarens
Formation
Tuli Basin (South African
part) (Bordy, 2000
Tuli Basin (South African
part) (Chidley, 1985)
Tuli Basin (Zimbabwean
part) (Thompson, 1975)
Clarens Formation
Clarens
Formation
“Forest Sandstone”
“Stormberg
Group”
Elliot
Formation
Tshipise
Sandstone
Member
Tuli Basin (Botswanan
part) (Smith, 1984)
Tsheung
Sandstone
Formation
Lebung
Group
Red Rock
Member
Upper Unit
Korebo
Formation
Bosbokpoort Formation
Molteno
Formation
Beaufort Group
Middle
Unit*
Middle Unit*
Klopperfontein Formation
Thyne
Formation
“Escarpment Grit”
Solitude Formation
Fripp Formation
Ecca Group
Mikambeni
Formation
Basal Unit (undifferentiated)
Seswe Formation
Fulton’s Drift Mudstones
Basal Beds
Madzar
Indwe
Formation
diamictites
Dwyka Group
the arenaceous deposits is ~7m, whereas the
argillaceous strata are ~10m thick on average. Figure 8
shows the sand:mud ratio of the Middle Unit, which is
~ 2:3. According to the thickness map of the Middle
Unit (Figure 9), two main depocentres may be
inferred in the central-western and central-eastern parts
of the study area. The location of these depocentres
Mofdiahogolo Formation
Basal Beds
Undifferentiated)
??
chiefly correlates with the two areas of highest
borehole density, which makes it likely that these
depocentres are more apparent than real. It seems most
likely that deposition took place on a northerlynorthwesterly dipping paleo-surface (Figure 10), as
suggested by the paleo-current data (see section on
paleo-currents).
Figure 2. The distribution of the igneous and sedimentary domains in the Tuli Basin (modified after Brandl, 1992).
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Table 2. The applied fluvial lithofacies (modified after Miall, 1996).
Facies code and syn Facies
Sedimentary structures
Petrographic studies
Petrographic studies were undertaken in order to gain a
better understanding of the paleo- environment, more
specifically the dynamics of sedimentation and the
probable source rocks. The petrographic studies are
comprised of detailed thin-section (15) descriptions, and
graphical and statistical evaluations of the petrographic
data by means of ternary diagrams and bar charts. The
thin-section descriptions were based on qualitative
observations and semi-quantitative visual estimates
using composition charts for mineral percentage, grain
size, sorting, roundness and sphericity.
Of the 15 Middle Unit samples, 11 quartz arenites
and one quartz wacke (HAL6: Halcyon farm) were
identified. Texturally, three samples are muddy
siltstones, while the other 12 samples consist of
sandstones and pebbly sandstones (EEN1, HAL7, LIZ3:
Eendvogelpan, Halcyon, and Lizzulea farms). Sample
HAL6 was taken from the transition between the lower
and medial part of the Middle Unit.
Microtexture analysis of the samples showed the
framework grains to be usually coarse-grained,
moderately-sorted, mostly subrounded-rounded and
randomly orientated. There were no grain size changes
across the sampling area. Cross-lamination was observed
in the samples from the medial part of the Unit (HAL1,
HAL2, HAL3: Halcyon farm), and sample HAL2 (Halcyon
farm) is flaser bedded. Most of the samples are
supermature (lower part) or submature (medial part).
Interpretation
Where grain supported, the particles are closely packed,
showing point and concavo-convex contacts. Sutured
grain contacts are rare, whereas authigenic overgrowths
are common on the quartz grains. The porosity ranges
from 0 to 40%.
The matrix, consisting of a homogenous mixture of
clay and quartz, is present in all samples from the
medial part of the Unit, and in one sample (REG7:
Regina farm) from the lower part. The quartz wacke
sample (HAL6: Halcyon farm) has a heterogenous
matrix. Among the closely packed grains, it is
exclusively clean clay, whereas among the loosely
packed grains it consists of coarse silty mud and muddy
coarse silt. Sample HAL6 also contains distinct lumps of
slightly deformed siltstone clasts (0.3-0.5cm) which seem
to have a similar texture to the matrix between the
loosely packed grains.
The modal proportions of the framework quartz
grains range from 98 to 100%. The quartz grains consist
of ~90% monocrystalline and ~10 % polycrystalline
grains. About 70% of the monocrystalline quartz grains
show undulatory extinction. Occasionally, the larger
monocrystalline quartz grains contain needle-like
(rutile?) mineral inclusions and subparallel lines or
randomly orientated very small fluid inclusions. There
are a few quartz grains with zircon and fluid inclusions
as well. The composite quartz grains consist of
numerous, irregular, tiny crystals. Detrital feldspar
(plagioclase) grains constitute 0 to 1% of the samples,
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SEDIMENTOLOGY OF THE BEAUFORT-MOLTENO KAROO FLUVIAL STRATA
and they are twinned on the albite law and slightly
altered (sericitisation). The proportion of lithic fragments
(lumps of slightly deformed muddy coarse siltstone)
ranges from 0 to 2%. About ~40% of the samples are
heavy mineral free; the rest of the samples contain more
than 2-3 grains of well-rounded heavy minerals (zircon,
brown tourmaline, opaque minerals, hornblende).
The following microscopic observations suggest that
the sedimentary rocks of the Middle Unit are multicycle
deposits: dominance of quartz grains, moderate to good
sorting, high roundness and sphericity, high proportions
of monocrystalline undulatory quartz grains, wellrounded heavy mineral grains.
The matrix inhomogeneity feature is interpreted as
the result of heterogeneity in the grain packing and
pene-contemporaneous matrix filtering processes;
where the grains were closely packed, only the very-fine
matrix fraction (clay) could filter through the narrow
pore throats between framework grains. Softsedimentary deformation in the muddy, coarse-siltstone
clasts indicates that the clasts were ripped up from a
proximal unlithified bed. Additionally, the silty-muddy
matrix, the rip-up clasts and the flaser-bedding indicate
significant energy fluctuation on the part of the
transporting agent.
As monocrystalline grains with mineral and fluid
inclusions tend to be eliminated through recycling and
weathering, their presence in the Unit indicates proximal
derivation, presumably from quartz veins and/or
plutonic igneous rocks (granite, pegmatite). Likewise,
the large (>1mm) monocrystalline quartz grains are
likely to have originated from quartz veins, massive
plutonic rocks or pre-existing sedimentary rocks. In
comparison to the other units of the Karoo Supergroup
in the Tuli Basin, the quartz grains of the Middle Unit are
far better rounded (Bordy, 2000). Therefore it is likely
that most of the quartz grains were recycled, although
the high proportions of undulatory grains may indicate
the presence of igneous plutonic rocks in the catchment
area.
The finely-crystalline composite quartz grains, with
their irregular crystal shapes and high crystal number,
indicate a metamorphic provenance, but the low
proportion of feldspars, micas and the absence of
metamorphic rock fragments reveal the dominance of a
(composite?) quartz-rich source area (e.g., the
underlying Basal Unit).
Despite the fact that the well-rounded heavy mineral
grains may imply long transportation distances, their
coarse (0.25-0.5mm) grain sizes, relatively high
concentrations and accompaniment by coarser
(0.5-2 mm), highly rounded monocrystalline quartz
grains, indicate that the heavy minerals were reworked
from a proximal, alluvial source via highly abrasive
processes. In addition to this sedimentary source,
subangular polycrystalline quartz and less-stable
hornblende grains point to a multiple source area
containing metamorphic and igneous rocks as well.
Brown tourmaline in the rocks also indicates a
metamorphic source (Pettijohn et al., 1987), but its
Figure 3. Geological map of the South African portion of the Tuli Basin (modified after Geological Map of the Beit Bridge area, 1:250000,
1957). The farm network is also illustrated, for further reference. A - A’, B - B’, and C - C’ represent the cross-sectional profiles in Figures
5, 6, and 7 respectively.
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EMESE M. BORDY AND OCTAVIAN CATUNEANU
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Figure 4. Generalized vertical profile of the Middle Unit in the southern Tuli Basin, South Africa. For lithofacies codes refer to Table 2.
Thicknesses may vary significantly along the basin (from <1 to 46m). The mean paleo-current direction is based on 26 measurements.
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SEDIMENTOLOGY OF THE BEAUFORT-MOLTENO KAROO FLUVIAL STRATA
Figure 5. Cross-sectional profile (A - A’ in Fig. 3) showing the contact between the carbonaceous mudstones of the Basal Unit and the
coarser, lower part of the Middle Unit. For lithofacies codes see Table 2.
Table 3. Lithological comparison between the Middle Unit (Tuli Basin) and the lower Beaufort strata (main Karoo Basin). Data derived
from Johnson et al., 1997 and Rubidge et al., 2000.
Lithological comparison between the Middle Unit (Tuli Basin) and lower Beaufort (main Karoo Basin):
1. Erosively-based channel lags of upward-fining cyclothemes consist of pebble- to cobble sized clasts of intraformational clay pebbles,
carbonate nodules and fossilized bone fragments in the lower Beaufort. The upward-fining cyclothem in the Middle Unit commences
with medium to well-sorted, subrounded-subangular, predominantly quartz pebble conglomerate. Intraformational mud clasts were
observed in only one sample.
2. Point-bar sequences characterized by lateral accretion surfaces common in the lower Beaufort, absent in the Middle Unit
3. The massive argillaceous mudstones are dark grey, green and no maroon rocks are present in the lower Beaufort. The mudstones and
siltstones of the Middle Unit are grey to brownish grey, yellow with reddish blotches scattered throughout. Purplish-grey coloured
become more dominant towards the top.
4. Evidence of subaerial exposure (e.g. raindrop impressions, wrinkle marks, mudcracks) is present in the lower Beaufort, absent in the
Middle Unit.
5. Calcareous nodules are present in the lower Beaufort, absent in the Middle Unit.
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Figure 6. Cross-sectional profile (B - B’ in Fig. 3) showing the contact between the carbonaceous mudstones of the Basal Unit and the
coarser, lower part of the Middle Unit. For lithofacies codes see Table 2.
Table 4. Lithological comparisons between the Middle Unit (Tuli Basin) and the upper Beaufort strata (main Karoo Basin). Data derived
from Johnson, 1976, Stavrakis, 1980 and Johnson et al., 1997.
Lithological comparison between the Middle Unit (Tuli Basin) and upper Beaufort (main Karoo Basin):
1. In some places, the erosively-based, upward-fining cyclothemes commence with intraformational conglomerates of mudball and shale
flake clasts in the upper Beaufort. The upward-fining cyclothem in the Middle Unit commences with medium to well-sorted,
subrounded-subangular, predominantly quartz pebble conglomerate. Intraformational mud clasts were observed in only one sample.
2. The greenish grey sandstones are characterized by upper flow regime structures (horizontal bedding, streaming lineation) in the upper
Beaufort. The arenaceous deposits of the Middle Unit are grey, white, pink, light purple or red. The sandstones are mainly planar crossstratified, massive and horizontally stratified sandstones are less common.
3. Ripple cross-lamination is scanty in the upper Beaufort (e.g. Katberg F.), abundant in the Middle Unit.
4. The massive argillaceous mudstones are greyish red or greenish grey in the upper Beaufort. The mudstones and siltstones of the Middle
Unit are grey to brownish grey, yellow with reddish blotches scattered throughout. Purplish- grey coloured become more dominant
towards the top.
5. Sand-wedge polygons, calcareous duricrusts, mud draped surfaces and mudcracked surface are present in the upper Beaufort, absent
in the Middle Unit.
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SEDIMENTOLOGY OF THE BEAUFORT-MOLTENO KAROO FLUVIAL STRATA
Figure 7. Cross-sectional profile (C - C’ in Figure 3) showing the contact between the Basal Unit and the overlying coarser, lower part of
the Middle Unit. For lithofacies codes see Table 2. The contact between the Basal and Middle units is taken as the datum. Note that the
Basal Unit fills the irregularities of the paleo-topography (Eendvogelpan farm, Figure 3).
coarse (0.25-0.5mm) grain size, and highly spherical and
rounded shapes, again indicate rather abrasive
transportation processes.
Paleo-current data
The collection of paleo-current data was carried out in
order to determine possible denudation trends and
sediment dispersal patters within the Unit. The low
quality outcrops allowed the measurement of a very
limited number (26) of paleo-current indicators. The
calculated statistical values and generated paleo-current
rose diagrams were based on the orientation data
derived from the foreset dip direction of the planar
cross-stratified beds. The measurements reveal
unidirectional current patterns and a northwesterly
(~320) pointing regional mean current direction
(Figure 11).
Facies interpretation
The outcrop exposures allow the recognition of the
following architectural elements: gravel bars (GB) and
sandy bedforms (SB) in the arenaceous rocks, and
overbank fines (FF) in the argillaceous deposits. The
conglomerates are believed to have formed as channel
lag deposits. The planar cross-bedded sandstones and
ripple cross-laminated very fine sandstones are
Figure 8. Sand: mud ratio of the Middle Unit.
Middle Unit
Cumulative thickness in m Percentage %
Argillaceous deposits
495.56
58.17
Arenaceous deposits
356.35
41.83
Total
851.91
100.00
SOUTH AFRICAN JOURNAL OF GEOLOGY
EMESE M. BORDY AND OCTAVIAN CATUNEANU
63
Figure 9. Thickness map of the Middle Unit for the South African part of the Tuli Basin.
interpreted here as intrachannel migrating, straight
crested sand dunes and ripples. The abrupt passage
from arenaceous into argillaceous deposits (mostly
mudstones, less commonly siltstones) could represent
rapid channel shifting and abandonment. The
intraformational, rip-up, siltstone and silty mudstone
clasts indicate the coexistence of high- and low-energy
processes and intense fluctuation in the energy
levels. These characteristics, as well as the regular
occurrence of both arenaceous and argillaceous
deposits, lack of point-bar successions, crevasse-splay or
levee deposits, indicate a braided perennial river system
rather than an anastomosing or meandering fluvial
regime.
The unidirectional paleo-current directions (Figure 11)
and low paleo-channel sinuosity (see calculation in
Bordy, 2000) are also consistent with the braided fluvial
system interpretation. The draining direction of the
channels, determined from the foreset orientation of the
planar cross-bedded strata, was from southeast to
northwest. This direction is markedly different from the
paleo-flow measurements recorded from the underlying
Basal Unit (eastnortheast to westsouthwest), and from
the overlying Upper Unit (northnorthwest to
southsoutheast) (Bordy, 2000; Bordy and Catuneanu, in
press). These differences suggest that significant
episodes of tectonic reorganization and/or changes in
fluvial regimes occurred in the basin between the timing
of deposition of the various units of the Karoo
Supergroup in the Tuli Basin, which led to changes in
the direction of topographic tilt (Bordy, 2000). The
overall fining-upward trend of the Unit (Figure 4) may
reflect a weakening intensity of the initial tectonic uplift
combined with denudation processes in the source area.
This decline resulted in a decrease in the regional slope
gradients and related coarse sediment input.
Based on field evidence, the Middle Unit was
generated in a bed-load dominated, braided river
system. This unit is non-fossiliferous and overlies a
regional unconformity. The conglomerates, sandstones,
siltstones and mudstone were derived mainly from
weathered, quartz-rich metamorphic and - to a lesser
extent - felsic plutonic and sedimentary source rocks
situated SSE of the study area.
Discussion
A period of considerable uplift and erosion is marked by
the regional unconformity at the conglomeratic base of
the Middle Unit and its lithological equivalents in the
north-eastern part of the southern African region (i.e., at
the base of the Escarpment Grit and Angwa Sandstone
Formation in Zimbabwe: Raath, 1972; Stagman, 1978;
Oesterlen, 1991). The intensity of this erosional period
in the study area is demonstrated by the fact that the
succeeding Middle Unit beds may occasionally rest
directly upon the pre-Karoo Archaean basement,
suggesting that the older Karoo beds were removed.
Similar situations have been reported from other
northeastern
Karoo
Supergroup
depositories
(e.g., Zimbabwe: Visser, 1984).
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64
SEDIMENTOLOGY OF THE BEAUFORT-MOLTENO KAROO FLUVIAL STRATA
Figure 10. Relief of the pre-Middle Unit surface.
As interpreted in the previous sections, the nonfossiliferous deposits of the Middle Unit were likely
formed in a braided river system with inter-channel
flood plains. The lack of bio- and chronostratigraphic
control hampers a unequivocal correlation of the Middle
Unit with the formations of the main Karoo Basin.
Existing constraints place the Middle Unit in a
stratigraphic position older than the Elliot Formation (as
the Upper Unit correlates with the Elliot Formation,
based on fossil content), and younger than the Ecca
Group (as the Basal Unit in the Tuli Basin correlates with
the Dwyka and Ecca groups in the main Karoo Basin;
Bordy, 2000). There are several upward-fining units of
former braided river systems both in the Beaufort Group
and in the Molteno Formation of the main Karoo Basin,
so precise correlations are uncertain (Table 1).
Tables 3 and 4 present lithological comparisons of
the Middle Unit to the braided river deposits of the
lower (e.g., Canape Formation) and upper (e.g., Katberg
Formation) beds of the Beaufort Group. The
paleontological differences between the lower and
upper beds of the Beaufort Group (i.e., Glossopteris sp.,
Schizoneura sp., Equisetum sp. plant, fish and therapsid
reptile remains of the lower Beaufort, and Dicroidium
sp., Schizoneura sp., Dadoxylon sp. plant, fish, amphibia
and therapsid reptile remains of the upper Beaufort)
were not useful for the correlation to the nonfossiliferous Middle Unit. In the other northeastern
Karoo depositories, strata equivalent to the Beaufort
Group are usually not reported (Tshipise and Nuatetsi
basins, and the northern Lebombo ‘Monocline’) (Visser,
1984; MacRae, 1988), or are found in the form of
condensed, non-fossiliferous successions (Save,
Springbok Flats and Ellisras basins) (Stagman, 1978;
MacRae, 1988; Faure et al., 1996).
There is a somewhat stronger lithological
resemblance between the upward-fining units of the
Middle Unit (Tuli Basin) and the braided river deposits
of the Indwe Member of the Molteno Formation in the
main Karoo Basin. According to the description in the
lexicon of South African stratigraphy (Johnson, 1994:
p.18), the Indwe Member commences with the Kolo
Pebble Bed which is a “coarse pebbly unit” (Christie,
1986: p.2068). The lithology of this basal succession of
the Indwe Member is pictured as a conglomerate or
pebbly sandstone layer by the following authors:
Johnson (1976: p.276 “relatively thin coarse-grained
layer containing numerous clasts eroded from
underlying material, as well as scattered quartzite
pebbles and boulders”), Turner (1983: p. 79), Eriksson
(1984: p.239 “lenses and beds of apparently massive
conglomeratic sandstone”), Cairncross et al. (1995: p.455
“basal conglomeratic lag”), Johnson et al. (1997: p.301
“erosively-based pebble and cobble horizon”). Based on
Turner’s (1983: p.79) and Cairncross et al. (1995: p.455)
descriptions, the clasts comprise mainly quartzite, vein
quartz, chert and rare rock fragments and are wellrounded and spheroidal. The basal conglomerate unit
SOUTH AFRICAN JOURNAL OF GEOLOGY
EMESE M. BORDY AND OCTAVIAN CATUNEANU
65
Figure 11. Paleo-current rose diagrams for planar cross-bedded sandstones in the Middle Unit.
fines upward to coarse- and medium-grained, crossbedded sandstones, and then into fine sandstones,
siltstones, shales and rare coal seams. The Indwe
Member is reported to be basin-wide (i.e., found
throughout in the present outcrop area of Molteno
Formation) by Turner (1977: p.242; 1983: p.79), and this
distribution pattern is accepted by Eriksson (1984:
p.240), Cairncross et al. (1995: p.453) and Johnson et al.
(1997: p.301).
In addition, there are noticeable similarities between
the lithofacies associations of the Middle Unit and the
Molteno Formation described from northeast Swaziland.
According to Turner and Minter (1985), the Molteno
Formation in the Hlane area rests on an erosion surface
and consists of two fining-upward cyclothems. The
conglomerates contain subangular and subrounded
pebbles, predominantly of quartzite and vein quartz.
The middle part of these fining-upward cyclothemes
(Hlane), consisting of sandstones and ripple-cross
laminated siltstones, clearly resemble the medial part of
the Middle Unit. However, it should be pointed out that
the argillaceous deposits in the Middle Unit are not
carbonaceous and lack coal seams.
The non-fossiliferous Middle Unit in the Tuli Basin
shows a strong lithological and stratigraphic similarity to
the Dicroidium-bearing strata both in the Tshipise Basin
(Fripp and Joan Formations - van den Berg, 1980: p.55)
and Zimbabwe (Escarpment Grit, Angwa Sandstone
Formation, Ripple Marked Flagstones - Oesterlen, 1991).
These Dicroidium-bearing Zimbabwean strata have
been correlated with the Molteno Formation (Wilson,
1970; Cooper, 1982). All these stratigraphic units are
underlain by an unconformity surface which is assumed
to be regional in extent.
Conclusions
1. The accumulation of the Middle Unit in the Tuli Basin
corresponds to a distinct stage of tectonic development
in the region, when the topographic gradient was
consistently dipping from south-east to north-west. In
contrast with this, the Basal and Upper units were
accumulated on topographic slopes dipping from eastnortheast to west-southwest, and from north-northwest
to south-southeast respectively (Bordy, 2000; Bordy and
Catuneanu, in press).
2. The lower boundary of the Middle Unit is represented
by a prominent regional unconformity that can be
mapped throughout the Tuli Basin. There are no
constraints regarding the stratigraphic hiatus associated
with this surface. The character (conformable or
unconformable) of the upper boundary of the Middle
Unit is yet difficult to assess due to the lithological
similarity between the upper part of the Middle Unit,
and the overlying mudstones of the Upper Unit. This
contact is currently taken at the base of the red-purple
mudstones that are typical of the Upper Unit.
3. The Middle Unit may be subdivided into three parts,
based on lithological differences. The lower part
includes rudites and arenites (lithofacies Gh, Sp, Sh, and
Sm); the middle part is dominated by siltstones and very
fine sandstones (lithofacies Fl); and the upper part
consists of mudstones (lithofacies Fsm; Figure 4).
Generally, the thickness of the Middle Unit increases
from south to north, up to about 30m.
4. The Middle Unit accumulated in a perennial gravel
bed braided system with well developed flood plain
areas. The overall fining-upward profile may be related
to the gradual denudation of source areas and
topographic peneplanation, as suggested by the gradual
upward decrease in grain size and fluvial energy.
Petrographic analyses indicate multiple source areas, as
well as reworking of the underlying Basal Unit deposits.
5. The lack of bio- and chronostratigraphic control
hampers a unequivocal correlation of the Middle Unit
with the formations of the main Karoo Basin. Existing
constrains place the Middle Unit in a stratigraphic
position older than the Elliot Formation and younger
than the Ecca Group. Between these limits, the Middle
Unit may correlate with either the Beaufort Group or the
Molteno Formation of the main Karoo Basin (Table 1).
Acknowledgments
This paper stems from the PhD research of EMB in the
Tuli Basin of South Africa. Financial support was
provided by De Beers Consolidated Mines Ltd.
(operating grant to OC), Rhodes University, NRF South
SOUTH AFRICAN JOURNAL OF GEOLOGY
66
SEDIMENTOLOGY OF THE BEAUFORT-MOLTENO KAROO FLUVIAL STRATA
Africa, and G. Soros. OC acknowledges additional
support from the University of Alberta and NSERC
Canada. We thank R.E. Jacob, J.S. Marsh (Rhodes
University), and B.R. Rubidge (University of the
Witwatersrand) for their comments and assistance during
the completion of this work. Support during the field
work was provided by numerous De Beers employees,
as well as private individuals (A. Stander, and many
farmers in the Tuli Basin).
We thank J. Neveling and D. Zolebay (reviewers) as
well as J.M. Barton and L.D. Ashwal (the editors) for
their helpful comments and effective handling of the
manuscript.
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