Trans. geol. Soc. S. Afr., 84 (1981), 7-17
A PALAEOENVIRONMENTAL ANALYSIS OF THE CLARENS
FORMATION IN THE NATAL DRAKENSBERG*
by
P.G. ERIKSSON
ABSTRACT
The Jurassic Clarens Formation was studied in part of the Natal Drakensberg. The methods of
investigation included the measuring and analysis of sedimentary sections, and the petrographic (including staining and point-counting) and scanning electron microscopic analysis of the sediments.
Petrological analysis of the rock samples revealed these sediments to be fine-grained greywackes
with poor sorting, sub angular grain shapes and a matrix made up mainly of chloritic mud, which infrequently contains considerable quantities of calcite cement. Statistical evaluation of sand grain surface
textures observed under the scanning electron microscope revealed that three important factors contribute to the formation of these sediments: diagenesis, high-energy chemical activity, and sediment
source material. Aeolian and glacial textures are also present on sand grains. From the scanning electron microscopy it was concluded that these sediments were deposited in an environment dominated by
chemical and diagenetic processes. A wet desert margin would give the necessary combination of high
temperatures and the presence of water. Distal glacial source material would have been responsible for
much of the sediment formed, and this was to some extent reworked by desert margin and dry aeolian
conditions in the depositionary basin.
Four sedimentary facies were defined on the basis of lithology, sedimentary structures and fossils.
Facies One comprises fine-grained sediments structured by fine horizontal laminations, convolute la'mination and bedding, and thin horizontal bedding. Sedimentary structures of lesser importance include
planar and trough cross-bedding (lamination), mud clasts, desiccation cracks, and trace fossils (Planolites burrows and dinosaur footprints). This facies is interpreted as having been deposited in a wet desert environment. This environment experienced desert flooding which resulted in sediments being deposited by both sheetflow and wadi fluvial processes and in ephemeral (playa) lakes. Facies Two
comprises medium- to coarse-grained sandstones structured by planar and trough cross-beds and channels. The sediments are infrequently arranged in upward-fining cycles. Sediments of this facies are interpreted as having formed in braided wadi channels, mainly in times of flood. Two further facies are
recognized, Facies Three being characterized by massive fine-grained sandstones with occasional slump
structures, while Facies Four is dominated by fine-grained sandstones exhibiting large-scale aeolian
cross-bedding. These facies are interpreted as having been formed by mass flow and wind-blown processes respectively.
The vertical and lateral distribution of these facies points to an environmental change from south to
north along a direction consistent with the major palaeocurrent trend for the postulated wet desert and
fluvial facies. This change is from a wetter palaeoenvironment dominated by fluvial and wet desert processes (in the south) to a more arid one characterized by mass flow and aeolian processes (in the
north). The climate was probably semi-arid with a low rainfall which precipitated as short, very intense
storms, which provided the water necessary for desert margin processes to take place. Towards the
centre of the desert basin, dry aeolian conditions prevailed. This proposed desert margin and more arid
interior is defined from the regional work carried out by Beukes (1969, 1970) on the Clarens Formation. This basin was probably situated near the centre of the Gondwanaland supercontinent. Palaeowinds blew from the South American continent situated to the west, bringing in loessic material to the
desert basin where. it was reworked locally by both humid and dry desert processes. Carboniferous glacial deposits in central Brazil possibly provided much of the original source material for the Clarens
Formation.
CONTENTS
Page
I. INTRODUCTION .......................................................... ,"
8
II. GENERAL GEOLOGY. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
8
III. PETROLOGY AND SCANNING ELECTRON MICROSCOPy ......... . . . ,.....
9
A. Petrology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
B. Scanning Electron Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
9
IV. SEDIMENTARY FACIES......................................................
11
A. Facies One - Fine-grained, Finely Laminated, Convolutely Laminated, Thinly
Bedded Sandstones ..........................................................
11
1. Description ..............................................................
11
2. Interpretation ............................................................
12
B. Facies Two - Medium- to Coarse-grained Cross-bedded Sandstones. . . . . . . . . . . . . . . .
13
1. Description ..............................................................
13
2. Interpretation ............................................................
13
C. Facies Three - Massive, Fine-grained Sandstones. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
1. Description ..............................................................
14
2. Interpretation ............................................................
14
D. Facies Four - Very Fine-grained Sandstones Characterized by Large-scale Planar
Cross-bedding. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
14
1. Description ..............................................................
14
2. Interpretation ....................................... , . . . . . . . . . . . . . . . . . . . .
14
V. PALAEOENVIRONMENTAL SyNTHESIS.....................................
14
VI. REGIONAL AND GONDWANALAND PALAEOGEOGRAPHY AND ITS
RELATIONSHIP TO THE PALAEOENVIRONMENT OF THE STUDY
AREA........................................................................
15
ACKNOWLEDGMENTS. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
16
REFERENCES................................................................
16
*Presented at the Sedimentological Division's symposium on the geology
of the Karoo Basin held in Port Elizabeth in September 1979.
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
8
I. INTRODUCTION
The Clarens Formation crops out over a large area in
the central high-lying regions of all provinces of the
Republic of South Africa, and in Lesotho. The area of
study is situated along the western border of Natal with
Lesotho, and extends from the Sani Pass Border Post in
the south to the Cathedral Peak State Forest in the north
(Fig. 1). Previous workers studying aspects of the Clarens
Formation included Du Toit (1939), Haughton (1924),
and Beukes (1969, 1970).
Haughton (1924) outlines a gradational palaeoclimatic
transition in time, during the deposition of the Upper
Karoo Supergroup sediments, from more humid to more
arid climatic conditions. This is borne out by the change
in the dominant fossil vertebrate fauna from heavylimbed to lighter-limbed more cursorial types of dinosaurs
(Haughton, 1924). He considers the Clarens Formation to
have been deposited in an arid climate, largely by wind
and to a lesser extent by water processes. Du Toit (1939)
ascribes the deposition of the Clarens Formation to a
similar environment. The most significant work done on
the Clarens Formation is the regional study of Beukes
(1969, 1970) in which he reviews the stratigraphy, petrology and environments of deposition of the Clarens Formation. Beukes (1970) is the first to substantiate the
desert origin of these sediments and ascribes their
deposition to the migration of aeolian dune fields, sand
flows, ephemeral streams and sedimentation in playa
lakes. Thus he attributed the deposition of the Clarens
Formation sediments to both wet and dry desert sedimentary environments. More recently, investigations of
Clarens Formation outcrops in the Giant's Castle area of
the Natal Drakensberg by Van Dijk (1978) and Van Dijk
et ai. (1978) led to the conclusion that these sediments
o
30
40
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Nata 1- Lesotho border
Sections Studied
•
Drakensberg. Volcanics
Clarens Formation
~
Molteno and Elliot
Formations
D
Beaufort Group
Figure 1
Location map and general geology
were laid down by playa lake, sheetflow and ephemeral
stream processes in a basically aeolian dune field environment.
In summary, this paper presents the results of the thin
section and scanning electron microscopic analysis of
these sediments and defines and describes the palaeoenvironment of deposition of these rocks. To this end, six
widely spaced vertical sections, each 1-2 km in lateral extent, through the Clarens Formation were examined in
detail. Thicknesses were measured with a tape and altimeter, while all sedimentary structures, lithological variations and fossils were recorded and described .. Fortythree sediment samples were taken and these were subjected to detailed thin section (including staining and
point-counting) and scanning electron microscope studies.
Sedimentary facies were defined on the basis of sedimentary structures, compositional and textural parameters, and from facies arrangements a final palaeoenvironmental synthesis was reconstructed.
II. GENERAL GEOLOGY
In the Natal Drakensberg, the Clarens Formation is approximately 115-195 m thick, the average thickness being
145 m. The formation in general dips very gently towards
the south-west, which is thought to be related to the morphology of the depositional basin, and thickens from east
to west.
The contact between the Clarens and the underlying
Elliot Formation is generally gradational. This gradation
is characterized by the interfingering of lenses of Clarens
Formation sediments with Elliot Formation strata. Thus
the lower gradational contact of the Clarens Formation
suggests a gradual environmental change between the
Elliot and Clarens formations as postulated by Haughton
(1924). The contact of the Clarens Formation with the
overlying Drakensberg volcanics is sharp, but infrequently, small and irregularly-shaped extrusions of basalt
below the contact suggest that a minor amount of volcanism took place prior to the cessation of sedimentation.
There are three common types of weathering features
present in the study area, an anomalous "mudstone-like"
weathering, spheroidal weathering and "chemical-skin"
weathering, as well as infrequent plume structures.
Anomalous "mudstone-like" weathering affects the finegrained sandstone and siltstones, giving them the appearance of cracked and weathered mudstone layers. Spheroidal weathering is a rather unusual phenomenon in sedimentary rocks. Beukes (1969, 1970) reports its presence
and states that it might indicate the absence of structures
in a sedimentary unit. However, this type of weathering
does not necessarily indicate a massive sediment, as it is
present in both sedimentary units showing well-defined
structures, as well as in apparently massive units.
"Chemical-skin" weathering results in the formation of a
layer of chemically weathered material covering the exposed surface of the Clarens Formation rocks. This probably forms as a result of the chemical solution of these
sediments through the action of rain water. The solution
then flows over and covers the outcrops, before solidifying to form a patina on the rocks, creating the impression
that they are massive.
The Clarens Formation, in the study area, consists of
fine-grained sandstones, sandy siltstones and mudstones,
with subordinate coarser-grained sandstones. Though often apparently massive, the Clarens Formation does not
lack sedimentary structures and occasional trace fossils.
These structures, together with the different rock types,
point to deposition of sediments in playa lakes, and by
sheetflow, fluvial, and aeolian processes. Sedimentary
structures are often obscured by weathering effects, but
occasionally, as at Giant's Castle and Kamberg, they are
very well-preserved.
PALAEOENVIRONMENTAL ANALYSIS OF CLARENS FORMATION
TABLE I·
Stratigraphy of the Upper Karoo Supergroup in South Africa
(Eriksson, 1979a)
Formation
Description
Drakensberg Group
(Jurassic)
Basaltic lavas with interbedded sandstones at their base.
Clarens Formation
(Jurassic)
Sandstones, siltstones with subordinate mudstones.
Elliot Formation
(Triassic)
Red-coloured mudstone, shale and
siltstones with subordinate sandstones.
Molteno Formation
(Triassic)
9
Formation rock samples shows no regularity, are ally ,
stratigraphically and petrologically. Hence it must be presumed that the source of calcite was probably either
through the percolation of groundwater, or through the
existence of a source material characterized by the presence of small, irregular calcareous lenses or horizons.
1\
Orthoquartzite
Sandstones with interbedded shale
and mudstone.
The stratigraphy of the Upper Karoo Supergroup is
shown in Table I. The name Clarens Formation (Johnson
et ai., 1976) is among those new lithostratigraphic terms
coined for the Karoo Supergroup to replace old names
such as the Cave Sandstone (now named the Clarens Formation) and the Red Beds (now named the Elliot Formation).
Beukes (1969, 1970) defines three vertical intervals in
the Clarens Formation, each characterized by particular
combinations of lithology and sedimentary structures,
these being interpreted as different types of desert environments. In this study these intervals have not been
distinguished. However, there appears to be both a vertical and a lateral distribution of the different desert facies.
III. PETROLOGY AND SCANNING ELECTRON
MICROSCOPY
A total of 43 rock samples from the Clarens Formation
were subjected to thin section and scanning electron
microscope analyses. Difficulty was encountered in distinguishing between quartz and untwinned feldspar in the
thin sections, and thus staining was carried out to identify
the feldspar. Point;counting enabled the proportions of
quartz, feldspar and micas/chlorite to be accurately determined.
A. Petrology
Generally the Clarens Formation sediments are finegrained, rather poorly sorted low-rank greywackes. All 43
samples show the presence of diagenetic overgrowths of
quartz around the grains and even intergrowing between
the individual grains and groups of grains. The majority
of the rocks are well compacted, welllithified sediments,
with very few voids. Most of the rocks are characterized
by grain sizes in the fine-grained sandstone interval, or
finer intervals, but with a range of grain sizes in the fine
size intervals. Many of the samples have considerable
quantities of matrix (30 per cent and greater), which is
generally of mudstone-siltstone size. The shapes of the
grains, though sometimes modified by overgrowths, are
generally sub angular. Both grain and matrix support are
present in these sediments, but grain support dominates.
Figure 2 illustrates the composition of the 28 non-calcareous rock samples of the Clarens Formation. With the
exception of one high-rank greywacke and three arkoses,
the samples plot in the field of the low-rank greywackes
with approximately 50-60 per cent quartz, about 10 per
cent feldspar and 30-40 per cent matrix. Accessory minerals present include zircon, opaque iron minerals, garnet,
sphene, riebeckite, hornblende and spinel. The matrix is
mainly made up of ferruginous chloritic mud, and calcite
is sometimes present as cement. Apart from the mudstone matrix, several samples have mudstone clasts, composed of secondary calcite or calcite combined with chloritic mudstone. The distribution of calcite in the Clarens
•• composition of sample
Figure 2
Composition of 28 samples of the Clarens Formation, Natal Drakensberg.
A number of the petrographic characteristics of these
sediments are important in the reconstruction of the
palaeoenvironment. These are listed together with their
implications:
1. Fine-grained sediments - possibly loess deposits, or
derived from a sedimentary source.
2. Poorly rounded, sub angular grain shapes - accounted
for either by a short transport distance or deposition in
a low-energy subaqueous environment, or perhaps derived from the source material.
3. Poor sorting - suggests no regular sedimentary processes at work, i.e. no perennial rivers, no marine nor
continuous wind-blown desert conditions. Poor sorting
can imply that agents such as sheetflow processes,
playa lakes and ephemeral streams transported and deposited this poorly sorted sediment.
4. Presence of calcareous cement - this implies either
capillary movement of groundwater in the environment of deposition and in the' diagenesis of the sediments, or a source material which provides an irregular source of calcareous minerals.
B. Scanning Electron Microscopy
Scanning electron microscopy enabled surface textures
of the quartz grains to be identified, according to the descriptions and definitions given by Krinsley and Doornkamp (1973). Table II lists the frequency of occurrence of
surface textures (given as percentage of samples showing
a particular texture). As can be seen from this table, precipitated upturned (silica) plates are ubiquitous, whereas
deep surface solution, large-scale chemical decomposition
and adhering particles are very common textures. Other
textures which occur frequently include smooth precipitation surfaces, irregular solution-precipitation surfaces,
mechanically formed upturned plates, flat cleavage face
and cleavage planes. Some textures are absent altogether,
notably rounded grains and dish-shaped concavities.
These latter two textures are restricted to an aeolian en-
10
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
Figure 3
a. Scanning electron micrograph of part of a sand grain showing mechanically formed upturned plates, mechanical V-forms and precipitated upturned silica plates.
b. Scanning electron micrograph of part of a sand grain showing quartz crystal growth , solution pitting, and precipitated upturned silica
plates .
c. Scanning electron micrograph of a whole grain which appears to have been covered by a smooth precipitation surface texture. This
texture has subsequently been subject to large-scale chemical decomposition, leaving only remnants of the precipitation surface .
d. Scanning electron micrograph of a conchoidal fracture on a sand grain, which shows cleavage plates and planes, solution pitting and
adhering particles.
TABLE II
Frequency of Occurrence of Quartz Grain Surface Textures,
Clarens Formation, Natal Drakensberg
Quartz grain surface texture
Rounded grains
Adhering particles
Dish-shaped concavities
Cleavage planes
Flat cleavage face
Mechanical V-forms
Mechanically formed upturned plates
Precipitated upturned plates
Straight or slightly curved grooves
Conchoidal fracture
Quartz crystal growth
Silica plastering
Dulled solution surface
Chemically-etched V-forms
Capping layer
Smooth precipitation surface
Silica coating reproducing underlying structure
Disintegration by solution or salt crystal growth
Large-scale chemical decomposition
Deep surface solution
Irregular solution-precipitation surface
Occurrence
(% )
(n = 43)
o
86
o
65
53
23
53
100
o
7
9
7
5
o
5
58
9
33
98
95
51
vironment (Krinsley and Doornkamp, 1973). Figure 3
(a-d) shows some of the surface textures seen on the
quartz grain surfaces investigated.
The results obtained from the scanning electron microscopy were compared with a table from Krinsley and
Doornkamp's (1973) work on quartz sand grain surface
textures, which enabled the relative importance of different environmental and diagenetic factors to be established. By this comparison process, three "environments"
or factors can be identified as important: source material,
diagenesis and high-energy chemical (decomposition)
processes, while glacial and · aeolian "environments" appear as less significant factors.
The results of this study suggest that these factors have
a number of palaeoenvironmental implications:
1. The importance of source material, which implies
minor mechanical reworking before deposition.
2. Diagenesis and high-energy chemical (decomposition)
processes, which indicate a high temperature, wet environment of deposition. A playa lake or desert lake
would be an appropriate environment for these chemical changes to take place.
3. Fairly minor influence of either glacial or aeolian "environments", as these two factors are obviously mutually exclusive, one (glacial) surface texture probably
11
PALAEOENVIRONMENTAL ANALYSIS OF CLARENS FORMATION
results from the sedimenary source material, while the
other texture (aeolian) is the product of the palaeoenvironment of deposition, but is dominated by chemical influences.
IV. SEDIMENTARY FACIES
On the basis of the geological sections studied (Fig. 4
shows an example) , four sedimentary facies are defined in
the Natal Drakensberg, in terms of lithologies and sedimentary structures which include biogenic structures
(Eriksson, 1979a).
A. Facies One - Fine-grained, Finely Laminated,
Convolutely Laminated, Thinly Bedded Sandstones
1. Description
This facies is primarily characterized by very finegrained sandstones, interbedded with fine-grained sandstones, siltstones and mudstones. No units are coarser
than a fine-grained sandstone. These sediments generally
exhibit rather poor sorting, a matrix consisting of ferruginous chloritic mudstone, with some calcareous cement,
and mudstone clasts.
The most common sedimentary structures are horizontal and convolute laminations and thin horizontal bed-
GEOLOG I CAL CROSS -SECTION· AT
H IGHMOOR STATE FOREST
NATAL DRAKENSBERG
(metres)
50
LITHOLOGY
SEDIMENTARY STRUCTURES
~
fTI
4
SOMEWHAT FERRUG I NOUS
VERY FINE - GRAI NED
21
SANDSTONE - SILTSONE- MUDSTONE
30
TROUGH CROSS-BEDS AND CROSSLAMINATIONS FINE AND CONVOLUTE
LAMINATIONS. RUNZEL MARKS.
SCOUR MARKS. THIN BEDDING
PLANOLITES BURROWS.
-i
0
fTI
(J)
IT1
::0
-i
fTI
z
5
;0
~
l>
(J)
(J)
~2
SOMEWHAT POORLY SORTED
VERY FINE-GRA I NED
SANDSTONE- SILTSTONE-MUDSTONE
~
r
APPARENTLY MASSIVE.
0
~
IT1
z
5
;0
10
4
2~
4
17
THIN BEDDING. CONVOLUTE BEDDING AND
FERRUG I NOUS POORLY SORTED VERY
LAMINATION. FINE LAMI NATIONS SAND VOLCANOES
FINE-GRAINED MUDDY SANDSTONE
RUNZEL MARKS. MUDCRACKS. CHANNELS.
(WITH MUDCLASTS)
TROUGH CROSS - BEDS PLANOLITES BURROWS
WITH MUDSTONE LENSES.
PLANT FOSSILS (ROOT MARKS).
~
SOMEWHAT POORLY SORTED VERY FINETH fN BEDDI NG
IT1
-i
GRAI NED SANDSTONE- SILTSTONE- MUDSTONE
PLANT FOSSILS (ROOT MARKS)
0
THIN BEDDING. MUD CRACKS. SMALL SCALE TROUGH CROSS-BEDS
VERY FINE-GRAINED SANDSTONE
IT1
CHANNELS WITH BASAL LAGS Of' CALCITE CLASTS a CHERT (f)
VERY FINE-GRAINED SANDSTONEIT1
FRAGMENTS.
CONVOLUTE
BEDDING
FINE
LAMINATIONS.
SILTSTONE- MUDSTONE
::0
-i
FINE TO MEDIUM-GRAINED SANDSTONE
fTI
VERY FINE-GRAINED SANDY SILTSTONE UP
INTO SILTY MUDSTONE .
FINE LAMINATIONS. CROSS LAMINATIONS.
CONVOLUTE LAMINATIONS. THIN BEDDING.
FINE TO MEDIUM-GRAINED SANDSTONE
TROUGH CROSS BEDS
......-sIJ~Rt.06~~o<;:tINEO SANDY SILTSTONE UP INTO
FINE TO MEDIUM-GRAINED SANDSTONE
LTY MUDSTONE
-
z
<
;0
THIN BEDDING. FINE LAMINATIONS
SMALL SCALE TROUGH CROSS-BEDs
~I
SOMEWHAT FERRUGINOUS POORLY SORTED
FINE-GRAINED MUDDY SANDSTONE
TROUGH CROSS-BEDS
r
c
MEDIUM TO COARSE-GRAINED SANDSTONE
PLANAR CROSS-BEDS
r
4
MEDIUM-GRAINED SANDSTONE
THIN BEDDING.
6
FERRUGINOUS VERY POORLY SORTED FINE
TO MEDIUM-GRAINED MUDDY SANDSTONE
PLANAR CROSS- BEDS.
SCOUR-AND- FILL STRUCTURES
15
~
:::l>
thickness (m)
Figure 4
Geological cross-section at Highmoor State Forest, Natal Drakensberg.
fTI
z
5
;0
12
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
FigureS
Fine laminations and thin bedding, seen at Giant's Castle . Divisions on survey rod are 20 cm.
Figure 7
Sand-filled mudcracks on cave roof. Location, Highmoor State
Forest. Divisions on survey rod are 20 cm.
ding. The fine horizontal laminae (Fig. 5) are abundant in
this facies and vary in thickness from 0,5 mm to nearly 5
cm, though most are 1-4 mm thick, with 1 mm to 1 cm being a common thickness. Convolute lamination (0,5 mm
to 5 cm in thickness), convolute bedding (varying in thickness from 5 cm to 1,5 m) and compaction structures are
also frequent features. Small overturned or recumbent
folds (Fig. 6) some of which resemble miniature nappes,
are associated with the convolute structures. Water escape structures (or sand volcanoes) are also present. Thin
horizontal bedding (Fig. 5) is another feature of this facies
and varies in thickness from 5 cm to 75 cm, but is mostly
10-30 cm thick.
faces, and are featureless, rounded, flattened tubes 2,5
mm to 1 cm in width, which are sinuous and cross each
other at random. Such burrows can be identified from the
work of Kennedy (1975) as belonging to Planolites and resulting from vermiform animals. Vertebrate footprints are
present in this facies at Giant's Castle and Kamberg, and
appear to be dinosaur footprints (Fig. 8). Ellenberger
(1970) identifies the prints at Giant's Castle as belonging
to the dinosaurs Dinopentadiscus vandijki, D. lentus, Molapopentapodiscus supersaltator, and Vandijkopentapus
giantscastlensis. According to Van Dijk (1978) quadrupedal and bipedal walking and bipedal hopping tracks are
evident from the Giant's Castle site, and these dinosaurs
varied in size from sparrow to ostrich size (Van Dijk et
al., 1978). The tracks 'observed at Kamberg resemble
those present at Giant's Castle and were probably made
by dinosaurs of similar species (Van Dijk, pers. comm.).
Figure 6
Recumbent fold (indicated by arrow) associated with convolute
lamination present in Facies One sediments, Giant's Castle.
Hammer for scale.
Other, less common, features present in this facies include cross-bedding and lamination, mudstone clasts,
trace fossils and mudstone cracks. The stratification includes planar and trough cross-bedding and cross-lamination. The more abundant planar foresets form wedgeshaped sets of cross-bedding, and cross-IaJ11ination, with
low foreset angles of 5-120. Mudstone clasts occur at
the base of occasional channels or, more frequently, at
the base of thin sandstone beds. The mudstone clasts are
up to 5 cm in diameter and are either rounded, elongated
or wavy in shape, and display fine horizontal laminations.
Occasionally, mudstone clast conglomerate occurs. The
desiccation cracks (Fig. 7) are usually sand-filled polygonal features and sometimes two orders are present, the
smaller set being contained in the larger one.
Trace fossils found in this facies include horizontal burrows, vertebrate footprints, and coprolites. The horizontal burrows are generally present on bedding plane sur-
Figure 8
Casts of Dinosaur footprints on cave roof. Location, Giant's
Castle . These footprints have been pressed through a thin layer
of fine-grained sediment. Divisions on staff are 20 cm.
Other, less frequent, features which occur in this facies
are runzel marks, channels, scour and fill structures, and
scour marks. The runzel marks are generally closely associated with the burrows. Channels, 1-2 m wide and
10-60 cm deep, exhibit varied channel fills which include,
thinly bedded sandstones, massive sandstones with a basal
lag of mudstone clasts, and planar cross-bedded sandstones. Minor current ripples are also present.
2. Interpretation
The lithologies and various sedimentary structures
which characterize this facies, all point to a rather complex environment of deposition with a number of different, but related, sedimentary processes at work.
PALAEOENVIRONMENTAL ANALYSIS OF CLARENS FORMATION
The poorly sorted, fine-grained sandstones, siltstones
and mudstones with a chloritic matrix and calcareous
cement found in this facies, can be interpreted as being of
desert origin, with deposition mainly in playa lakes.
According to Glennie (1970), water-laid desert sediments
are poorly sorted, while Cooke and Warren (1973), state
that playa lakes are characterized by fine-grained clastic
sediments which include clay, silt and granular particles,
the coarser granular particles being present along the
nearshore areas of the lakes where influents enter
(Reeves, 1968). Chlorides and carbonates are common
minerals in playa lakes (Cooke and Warren, 1973). The
carbon dioxide to form the carbonate is brought to the
lakes by sheetwash and wadi flows (Reeves, 1968), probably having originally come from the erosion of semi-arid
desert margin soil horizons, which, according to Cooke
and Warren (1973), are often enriched in carbonates.
Thin and parallel laminations are one characteristic of
lacustrine sediments (Fisher and Brown, 1972). Desert
floodplain deposits also show horizontal laminations as a
common feature (Glennie, 1970). Many workers have reported an association between horizontal, cross- and convolute lamination (Allen, 1977). Picard and High (1972)
record this association for the Eocene lacustrine Green
River Formation, whereas McKee et al. (1967) associate
these structures with varying velocities from a desert
flood as it reaches its peak and then subsides. McKee et al.
(1967) propose that convolute lamination forms during
the waning stages of a desert flood, but Glennie (1970)
finds that convolute lamination is typical in sabkha deposits. Walker and Middleton (1978) note that ephemeral
lake-floors are often covered by convolute lamination, associated with desiccation 'cracks and ripple marks. Allen
(1977) finds an association between syndepositional folds
and convolute lamination. McKee (1962, in Glennie,
1970) suggests that contorted recumbent folds can be
caused by sediment-laden flood water flowing across and
over saturated sediment. This might well happen when
sheetflow sediments move over the surface of a playa
lake. Convolute lamination and associated folding generally form under the influence of gravity (Allen, 1977;
Woodcock, 1976). Sand volcanoes indicate loading of saturated sediments by overlying deposits, or the agitation of
a freshly deposited sediment (Gill and Kuenen, 1958).
Thus the convolute lamination (bedding) is thought to be
due to sheetflows from desert floods depositing saturated
sediments in playa lakes. Here gravity and loading may
affect "quick" sediment to produce sand volcanoes and
folds in the sediment. Thin bedding is characteristic of
both lacustrine clays (Glennie et al., 1978) and lacustrine
sandstone beds (Reeves, 1968).
Thus the combination of horizontal and convolute laminations and thin horizontal bedding in this facies point to
lacustrine deposition (and sometimes playa lacustrine deposition), higher energy flows and processes from desert
floods, as well as to the deposition of saturated sediment
which slumps and deforms under gravity and loading. All
these phenomena could take place simultaneously in a
playa-lacustrine depositional environment, affected and
modified by fluvial and sheetflow processes.
Planar cross-bedding and lamination is reported from
desert stream flood deposits (McKee et al., 1967) and
from lacustrine sediments (Picard and High, 1972). This
structure is found in desert environments associated with
fine and convolute laminations (McKee et al., 1967), and
associated with mudstone cracks (Donovan and Foster,
1972). Trough cross-beds occur in present-day braided
rivers (Smith, 1970). Glennie (1970) finds that mudstone
clasts can form in a desert due to initial desiccation, followed by wadi flooding. Mudstone cracks with a sandy infill indicate subaerial exposure and desiccation of waterlain clays in a desert (Glennie, 1970). Glennie (1970) also
13
found, in Wadi Dhaid on the Trucial coast, an association
between mudstone cracks and scour marks. This could be
interpreted as an association between desiccation within a
playa lake, and higher energy sheetflow processes.
Certain forms of Planolites trace fossils belong to the
Scoyenia Ichnofacies, which embraces, typically, non-marine clastics, particularly continental red beds and flood
plain deposits (Frey, 1975). The Clarens Formation is
underlain by the continental red beds of the Elliot Formation (Truswell, 1970). The Scoyenia assemblage also includes terrestrial species frequenting water holes (Frey,
1975) and this would agree with the presence of dinosaur
footprints, these creatures frequenting the sites of ephemeral lakes for drinking purposes. The dinosaurs found in
the Clarens Formation were light-limbed and probably
cursorial, and were all very likely the inhabitants of an
arid or semi-arid environment, having adapted themelves
for rapid movement from place to place (Haughton,
1924).
Less frequent structures, of relatively minor importance
in this facies, such as runzel marks, channels, scour and
fill structures, scour marks and ripples, reinforce the conclusion that these sediments were deposited by a combination of playa-lacustrine, wadi fluvial and sheetflow processes, probably in the wetter (semi-arid) margin of an
arid desert basin (Eriksson, 1979a). Sheetflows (sheetflooding) are an important sedimentary process in deserts
(Leopold, Emmett and Myrick, 1966, in Cooke and Warren, 1973, pp. 152-153) and transport much of the unconsolidated desert surface debris during times of flood, depositing this material in playa lakes. The association
between desert fluvial (or wadi) and playa-lacustrine deposition is reported by many workers, including Glennie
(1970) for the Djofra Graben in Libya, Eriksson (1977)
for Pleistocene sediments in N amaqualand and Krinsley
(1970) for the Sabzevar Playa in Iran. All these sedimentary processes interacted in this palaeoenvironmental
setting during sporadic, but intense, desert rainfall. The
effect of such rain is more important in deserts than
aeolian processes. "The wind is seldom as effective in denudation as running water" (Cooke and Warren, 1973).
Facies One is thus interpreted as a wet desert environment.
B. Facies Two - Medium- to Coarse-grained,
Cross-bedded Sandstones
1. Description
This facies is characterized by well-sorted medium- to
coarse-grained sandstones, with a matrix consisting of
chloritic mudstone, with no calcite. Sedimentary features
include planar and trough cross-beds, channels (some
erosively based) and thin bedding, with minor occurrences of herringbone cross-beds, convolute laminations,
scour and fill structures.
Planar cross-bedding is the most common sedimentary
structure. Set thicknesses vary from 30 cm to 3 m and
foreset angles are between 2 and 15° . Trough crossbeds display widths of 1-15 m, and depths of 25 cm to 2
m. Filled channels vary in width from about 2-10 m, and
in depth from about 0,5-5 m. A large nested channel system (individual channels 50-60 m wide, and 3-5 m deep)
is present at Kamberg. The channels often have a basal
lag and an upward-fining fill, formed by both vertical accretion and side-filling.
2. Interpretation
These sediments are interpreted as having been laid
down by desert flooding involving braided wadi channels
and sheetflows. Desert channels, or wadis, display planar
cross-bedding and tend to be braided (Glennie, 1970),
and braided streams are characterized by horizontal bedding, tabular cross-bedding, with minor trough cross-beds
14
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
and ripple cross-lamination (Rackley, 1976). Smith (1970)
describes the occurrence of trough cross-beds in braided
stream deposits. Flood waters break the banks of a desert
channel (Glennie, 1970) and this results in a sheet of sediments characterized by planar cross-bedding, climbing
ripples and horizontal laminations (thin bedding), spreading over the surrounding terrain (McKee et ai., 1967).
Channelling is common in desert streamflow deposits
(Glennie, 1970). The braiding of the channels enhances
side-filling and systems of channels developing. The sidefilling could also be due to channel shifting being brought
about through channels becoming blocked with flood deposits or even aeolian material (Eriksson, 1979b). The
nested channel system found at Kamberg could be the result of an intense desert storm. Such storms produce viscous sediment-water mixtures and this may give rise to
wide, shallow braided channels, or if the channels overflow, sheetflow deposits. The sediments of Facies Two
are thus considered to have been deposited by desert
flash-flood processes giving rise to braided wadi channels
and sheetflooding.
C. Facies Three - Massive, Fine-grained Sandstones
1. Description
This facies occurs infrequently throughout the stratigraphy of the study area and comprises poorly sorted finegrained sandstones. Some of these rocks show the presence of calcareous cement and the dominant matrix is
chloritic mudstone. The sandstones are massive.
2. Interpretation
The poor sorting and massive bedding of these sediments suggests deposition by mass movement. One of the
main features of debris flow deposits is very poor sorting
(Steel, 1974). Mass movement is rare in desert areas, but
some slumping and mass movement has been documented
by Cooke and Warren (1973), when intense desert storms
saturate unconsolidated sediments. Krinsley (1970) reports mass movements in the Sabzevar Basin in the Iranian Desert. Viscous mass flows can result from very
rapid runoff across unvegetated surface debris (Sharp and
Nobles, 1953, in Cooke and Warren, 1973, p. 26).
It is concluded that these massive fine-grained sandstones were deposited by mass movements of viscous debris flows which formed in an arid environment when fine
sediments became saturated during intense desert storms.
V. PALAEOENVIRONMENTAL SYNTHESIS
Palaeocurrent directions measured from the sediments
deposited in the fluvial and wet desert environments (Fig.
9) display a bimodal distribution, indicating transport of
material from the east and south-south-east to the west
and north-north-west. This contrasts with palaeocurrent
data from the sediments deposited in the aeolian environment which indicates that the dominant transport direction was from the west to the east (Fig. 9). The two wet
environments thus reflect significantly different palaeocurrent directions from those of the dry aeolian environment. However, the sediments in all the facies have
uniform petrographic characteristics and show similar
properties when examined by scanning electron microscopy. It is concluded, therefore, that these features can be
accounted for if the fluvial and wet desert sediments are
regarded as reworked, originally aeolian detritus, initially
transported into the basin from the west. This reworking
would have taken place in the wet margin of the desert
basin, while the sediments of aeolian origin were deposited in the more arid interior of the basin. The average
grain size of all the sediments (very fine-grained sandstone to siltstone) is nearly fine enough for these sediments to be termed loess. Reworking of aeolian sediments in a wet desert margin could account for the
subangular, fine-grained nature of the sediments found in
the study area. From the analyses of sediment grains
under the electron microscope evidence is provided that
the original wind-blown material transported from the
west was derived from a glacial source. This evidence
would reinforce the concept that these sediments were
originally laid down as loess, as loess is generally considered to be derived from a glacial environment.
FlUVial-Wet Desert
n=19
~
D. Facies Four - Very Fine-grained Sandstones
Characterized by Large-scale Planar Cross-bedding
1. Description
The sediments comprise very fine-grained sandstones,
which have a chloritic mudstone matrix and calcareous
cement. The sandstones are structured by large-scale
planar cross-beds. Set thicknesses vary from 60 cm to 4 m,
and average about 2 m, whereas foreset angles are high,
varying from 1-32°, but are frequently 22-28°.
Generally the cross-beds form interfingering wedgeshaped sets which can dip in opposite directions. Tabular
sets are also present.
Figure 9
Palaeocurrent directions, Clarens Formation, Natal Drakensberg.
2. Interpretation
Extremely large-scale cross-bedding with tabular or
wedge-shaped sets is the predominant structure of aeolian
dune deposits (Walker and Middleton, 1978). The main
feature of these structures is their great scale, with set
thicknesses varying from a few metres to about 35 m
(Walker and Middleton, 1978). Dips of cross-bedding in
palaeo dunes are generally between 20° and 29°
(Bigarella, 1972). The large set thicknesses and high foreset angles suggest that the sediments of Facies Four were
deposited by aeolian dunes in an arid or semi-arid desert
area.
A three-dimensional fence diagram (Fig. 10) was constructed to illustrate the lateral and vertical distribution of
environments for the Clarens Formation in the study
area. As can be seen from this figure, the southern part of
the study area is dominated by the wet desert, fluvial, and
mass flow environments, and the northern part by aeolian
and mass flow processes. Vertically there is a change from
fluvial through wet desert to mass flow facies which is also
present laterally from south to north. Thus, there is an
apparent change from a wetter to a more arid environment. The lateral change from south to north, which is in
a similar direction to the major fluvial and wet desert
Aeolian
n=13
15
PALAEOENVIRONMENTAL ANALYSIS OF CLARENS FORMAnON
o
~
WET DESERT
•
MASS FLOW
FLUVIAL
100
Komberg
(m)O
o!
2
4
!
!
6 8 10 (km)
!
I
!
Figure 10
Vertical and lateral facies variation.
The thickness distributions of these different zones, and
the grain size distribution for Zone 3 is given by Beukes
(1970), and this information is utilized to attempt to define the regional palaeogeography. Zones 1 and 2 were
taken to represent essentially dry aeolian conditions, and
Zone 3 to represent wetter desert conditions, such as
would be found on the margins of a large arid sedimentary basin. Two palaeogeographical domains were defined by utilizing thickness and textural parameters for
each zone. These particular parameters for each domain
are outlined below:
Domain 1 - Zone 1 greater than 25 m thick, Zone 2
greater than 35 m thick, Zone 3 less than 60
m thick and characterized by very finegrained sandstone.
Domain 2 - Zone 1 less than 25 m thick, Zone 2 less
than 35 m thick, Zone 3 greater than 60 m
thick and characterized by siltstone-sandstone.
From the distribution of these two domains (Fig. 12), it
appears that the wetter desert margin (Domain 2) lies to
palaeocurrent component (Fig. 9), is consistent with the
change from a wet desert margin to the drier interior of
an arid basin.
On the basis of all the available evidence derived from
petrography, scanning electron microscopy, sedimentary
facies and their relative distribution, and palaeocurrent
directions, the sediments of the Clarens Formation in the
study area are considered to have been deposited in the
wet margin of an arid sedimentary basin, by ephemeral
wadi flows, by sheetflows, by minor mass flows, by the
migration of aeolian dunes and in playa lakes (Fig. 11).
The source material for the Clarens Formation was an
aeolian loessic deposit blown from the west, which was reworked within the desert margin by fluvial and mass flow
processes and then transported into playa lakes thus giving rise to the distribution of sedimentary associations
which presently outcrop along the Natal Drakensberg escarpment.
VI. REGIONAL AND GONDWANALAND
PALAEOGEOGRAPHY AND ITS RELATIONSHIP TO
THE PALAEOENVIRONMENT OF THE STUDY AREA
Beukes (1970) studied the Clarens Formation in South
Africa on a regional basis and defined three vertical
zones, as follows:
Zone 1 - an environment of deposition characterized by
dunes, playa lakes and sandflows.
Zone 2 - a dominantly aeolian dune environment.
Zone 3 - a wetter environment of deposition with relatively more playa lakes than Zone 1, and some
sandflows.
Grain sizes within this zone vary between very
fine-grained sandstone and siltstone-sandstone.
~
DOMAIN1.
G
.1
Fluvial and Wet Desert
Palaeocurrent Directions
(n-19)
Figure 12
Palaeogeographical reconstruction of the Clarens Formation
(after Beukes, 1970).
Figure 11
Schematic palaeoenvironmental reconstruction of the Clarens Formation in the Natal Drakensberg.
GEOl8411-B
TRANSACTIONS OF THE GEOLOGICAL SOCIETY OF SOUTH AFRICA
16
the south and east of a drier central part (Domain 1) of a
large overall arid basin. The fluvial and wet desert palaeocurrent directions from the study area indicate transport
of material from the margin towards the centre of the basin. Thus the local palaeoenvironmental setting deduced
for the study area substantiates the larger scale palaeogeographic setting as defined by the writer from the work
of Beukes (1970).
The Triassic Clarens Form!ltion is inferred to have been
deposited by westerly palaeowinds at a time before the
breakup of Gondwanaland when both the South
American and African continents were juxtaposed, and
the palaeolatitude of Lesotho was approximately 50°
south at the time (Bigarella and Van Eeden, 1970). The
same authors propose the existence of a high pressure cell
over the large Gondwana landmass which would have
produced westerly palaeowinds to the south of this cell
(Fig. 13). They draw attention to the analogous situation
today where there is a high pressure cell situated over Siberia in winter, which gives rise to westerly winds south of
this cell. These winds are the predominant winds which
transport sediment in the Gobi Desert dunefields.
~
Position of Carboniferous glacials in the Brazilian
states of Mato Grosso and Goias.
@
Clarens Formation in South African Drakensberg
Ci?l
(tl) Hypothetica I palaeocirculatory
I
2
3
4
patterns
Antarctica
India
Africa
South America
Figure 13
Gondwana palaeogeography, possible palaeowind circulation
and possible source material for the Clarens Formation.
In South America there are Carboniferous glacial sediments in the states of Mato Grosso and Goias in Brazil
(Fig. 13) which, according to Sanford and Lange (1960),
provided much of the material deposited in the Botucatu
Desert which was formed soon after the Clarens Formation desert sediments. With the postulated Gondwana
palaeowind circulation during the Triassic (Fig. 13), it is
possible that these glacial deposits may have provided a
sedimentary source for the earlier deposits of the Clarens
Formation. This explanation could account for the glacial
textures of grains as seen under the electron microscope,
and the palaeowind directions determined for the aeolian
environment in the study area.
ACKNOWLEDGMENTS
This paper is a summary of an M.Sc. thesis carried out
with financial support from the CSIR and the University
of Natal, Pietermartizburg. During the preparation of the
thesis, Professor V. von Brunn and Professor D.R.
Hunter of the Department of Geology, University of
Natal, Pietermaritzburg, were of great assistance. Profes-
sor E.D. van Dijk of the Department of Zoology assisted
with the palaeontology. Preethum Seyambu carried out
the photographic work, and Mrs L.A. Ie Roux kindly did
the drafting.
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Department of Geology
University of Natal,
3201 Pietermaritzburg.
Accepted for Publication by the Society on 28.6.1981.