477
S. Afr.1. Geol., 1988,91 (4) ,477-489
Sedimentology of the Rooihoogte Formation, Transvaal Sequence
P .G. Eriksson
Department of Geology, University of Pretoria, Pretoria 0002, Republic of South Africa
Accepted 23 September 1988
The 2300-2200 Ma Rooihoogte Formation, the basal unit of the Pretoria Group, generally overlies a
palaeokarst surface developed on the underlying Malmani Subgroup dolomites. Basal chert breccias and
conglomerates of the Bevets Member are interpreted as proximal to medial alluvial fan deposits, reflecting
streamflow, mass flow, and landslide sedimentation. Three major fans, draining gently uplifted Malmani
dolomite source terrains, entered the basin to the southeast of Pretoria, south of Koster and from the far
northwestern Transvaal. Smaller fans formed in the eastern Transvaal and adjacent to a palaeohigh in the
central portion of the basin. Quartzose sandstones overlying the conglomerates in the eastern and southern
parts of the depository represent distal fan sediments, best developed on the major fan southeast of Pretoria.
Source material probably included Black Reef Formation quartzites. Reduction of provenance area relief
terminated alluvial sedimentation in the eastern Transvaal and led to distal lacustrine fan deltas developing in
the west. These mudrocks are succeeded by the thin Polo Ground Quarztite Member, interpreted as a distal fan
deposit laid down by renewed, but short-lived uplift in the western alluvial source regions.
Die Rooihoogteformasie (2300-2200 Ma) wat die basale eenheid van die Pretoriagroep is, oorle oor die
algemeen 'n paleokarstoppervlakte wat op die onderliggende dolomiete van die Malmani-subgroep ontwikkel
het. Basale chertbreksies en konglomerate van die Bevetslid word gei"nterpreteer as proksimale tot mediale
alluviale waaierafsettings wat stroomvloei-, massavloei- en grondverskuiwingsedimentasie vertoon. Drie
hoofwaaiers, wat effens opgehefde brongebiede dreineer, het die kom na die suidooste van Pretoria, suid van
Koster en vanaf die verre noordwestelike Transvaal binnegegaan. Kleiner waaiers het in die oostelike
Transvaal en grensend aan 'n paleohoog in die sentrale gedeelte van die kom gevorm. Kwartssandstene wat die
konglomerate in die oostelike en suidelike dele van die afsettingskom oorle verteenwoordig distale
waaiersedimente wat die beste ontwikkel is op die hoofwaaier suidoos van Pretoria. Bronmateriaal het
waarskynlik kwartsiete van die Swartrifformasie ingesluit. Reduksie van die relief van die brongebied het
alluviale sedimentasie in die oostelike Transvaal beeindig en gelei tot die ontwikkeling van distale lakustriene
waaierdeltas in die weste. Hierdie modderstene word gevolg deur die dun Polo Ground-kwartsietlid wat
gelnterpreteer word as 'n distale waaierafsetting. Die is afgeset deur hernieude opheffing in die westelike
alluviale brongebiede. Die opheffing was kort van duur.
Introduction
The Rooihoogte Formation is the basal unit of the
2300-2200 Ma (Button, 1986) Pretoria Group, Transvaal
Sequence. These rocks rest unconformably on the
dolomites of the Malmani Subgroup and the ironstones
of the succeeding Penge Formation (Button, 1973). In
the southeast and northwest Transvaal, as well as in
Botswana, they unconformably overlie older rocks
(Visser, 1969). The Rooihoogte Formation outcrops
sporadically within the Transvaal Basin and generally
overlies a palaeokarst surface developed on the
carbonate rocks of the Malmani Subgroup (Button,
1986). The underlying Malmani dolomites and Penge
iron formations were first tilted and eroded before
development of the karstic surface (Button, 1973; Ryan,
1986).
The Rooihoogte Formation has previously been
investigated as part of regional studies of the Pretoria
Group or Transvaal Sequence (Visser, 1969; Button,
1973; Klop, 1978; Engelbrecht, 1986). Economic
investigations of these rocks have also been carried out
in the western Transvaal (for example, Ryan, 1986;
Steyn et al., 1986). Previous workers proposed
transgressive marine or basinal deposition for the
Rooihoogte sedimentary rocks (Visser, 1969; Button,
1973, 1986). Published sedimentary profiles through the
formation are scarce (for example, Klop, 1978; Steyn et
al., 1986). This paper examines lithofacies in four
detailed profiles measured in the Pretoria-Koster area
(Figure 1) and proposes an alluvial depositional model
for the Rooihoogte Formation.
The Rooihoogte Formation comprises a basal chert
breccia which grades, both laterally and vertically, into a
chert conglomerate. This zone is commonly known as
the Bevets Conglomerate and has member status. The
basal breccia is well developed in the eastern Transvaal
and around Pretoria (Visser, 1969; Button, 1973),
whereas conglomerates predominate in the western
Transvaal (Engelbrecht, 1986). A thin quartzite
succeeds these rocks in the eastern Transvaal (Button,
1973) and thickens towards Pretoria (Visser, 1969). In
the western part of the Transvaal Basin the chertconglomerate member is overlain by a shaly zone. This
interval is, in turn, succeeded by the Polo Ground
Quartzite Member, which is also restricted to the
western part of the depository. The latter unit
encompasses localized dolomite and chert beds. The
Rooihoogte Formation is conformably overlain by the
Timeball
Hill
Formation,
an
assemblage of
carbonaceous and ferruginous shales with subordinate
quartzites and diamictites (SACS, 1980).
Lithology of the Rooihoogte Formation
Four lithofacies are identified in the profiles (Figure 2)
measured in the study area: chert breccia, chert
conglomerate, mudrocks and cross-bedded quartz
S.-Afr. Tydskr.GeoI.1988,91 (4)
478
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Note the location of the study area: profiles A, Band C are located to the west of Pretoria and profile D south
of Koster.
479
S.Afr.J . Geol. ,1988,91 (4)
A
Timeball Hill Formation - quartzites and shales
Facies
Fine to very fine quartz wacke. Troug..h cross-bedding
Basal laminated and graded mudstone lenses.
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Silty mudstone.Horizontal/wavy stratification,climbing ripple
cross-lamination, graded bedding
Chert conglomerate with 12 cm laminated mudstone bed. Irregular
upper and lower bounding surfaces to conglomerate.
Alternating very fine sandstone, siltstone and mudstone.
Horizontal stratification, cOllITDnly topped by current and
wave ripple narks, mi.nor flaser bedding, flat-topped ripple
narks, double-crested ripple narks and climbing ripple
- - - - - - cross-lamination
(m)
10
Silty very fine sandstone with mi.nor thin (few mn - 2cm) siltstonemudstone interbeds. Horizontal stratification, COOJlX)nly topped by
current ripple narks; mi.nor climbing ripple cross-lamination
6
PoorI y exposed interval
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Chert breccia - elongated angular clasts, 1 mn - W mn in sizesiliceous, carbonate-bearing & ferruginous ITBtrix
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Basal karst surface - filled with chert breccia
Malmani Subgroup - dolomites
Figure 2 Profiles A-D measured through the Rooihoogte Formation west of Pretoria. Location of profiles given in Figure 1.
S.-Afr.Tydskr.GeoI.1988,91 (4)
480
B
Unexposed Rooihoogte Formation
Facies
Silty mudstone and mudstone. Horizontal stratification
.
'D.
b
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._Chert breccia with 'thin (1 -3 cm) chert interbeds
Chert and cherty siltstone. Horizontal and convolute stratification
Horizontal stratification and
Silty very fine sandstone.
graded bedding
--
10
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graded bedding
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t>
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Malmani Subgroup - dolomites
Figure 2 Continued
wackes. Profiles A and D are complete, representing the
entire Rooihoogte succession from basal breccias and
conglomerates, through mudrocks to the uppermost
Polo Ground Quartzites. Profiles Band C comprise
mainly the argillaceous portion of the formation (Figure
2).
The basal unconformable contact with the underlying
chert-bearing dolomites of the Malmani Subgroup is
very irregular, compnsmg a deeply weathered
palaeokarst surface. Sinkholes and depressions within
this surface vary from 1 to 2 m deep and 1 to 3 m across
to depths of 5 to 15 m and widths between 25 and 40 m.
481
S.Afr.J . Geol. ,1988,91 (4)
c
Unexposed Rooihoogte Formation.
Silty very fine sandstone with some chert beds and lenses.
Horizontal stratification and some graded bedding
Facies
--4--------. - - -
Siltstone. Horizontal stratification and some graded bedding
(m)
10
Silty mudstone. Horizontal stratification, some soft sediment defortion structures and minor graded bedding
Silty mudstone with interbedded wavy chert beds and lenses.
Horizontal stratification and minor channel-fill trough cross-beds.
Erosively-based chert conglomerate lens (35 cm thick) with irregular mudstone-filled upper contact
Silty mudstone and mudstone. Horizontal stratification and some
current ripple marks; minor flat- topped current ripple marks
Not exposed
7.
Malmani Subgroup - dolomites
Figure 2 Continued
The larger depressions are found in the Koster area.
These irregularities are filled by chert breccia, or, in the
case of profile D, by breccia grading up into chert
conglomerate. The resultant karst-fill surface is
succeeded by a breccia layer 10 to 18 m in thickness,
except at Koster, where 100 m of conglomerate overlies
this surface (Figure 2).
Chert breccia
This rock type is characteristically chaotic, lacking any
structures or bedding (Figure 3). Sorting is extremely
poor with grain sizes varying between about 1 and 500
mm. Clasts are mostly elongated and angular with
subordinate more equidimensional, subrounded shapes.
Most of the grains are of chert with a few of dolomite.
The matrix comprises secondary silica and lesser
carbonate. Locally the matrix is made up of ferruginousmanganiferous material, thought to represent a
palaeowad or manganese-rich weathering residuum.
Chert conglomerate
The conglomerates mostly exhibit neither bedding nor
preferred clast orientation, except for the upper few
metres at Koster, where a vague bedding is visible due to
orientation of elongated pebbles parallel to the regional
dip and strike. The clasts are predominantly composed
of chert with subordinate dolomite and vary in size from
about 2 to 70 mm. The most common sizes are 1 - 1,5
cm. Shapes comprise approximately equal proportions
of elongated, subrounded forms and equidimensional
rounded pebbles (Figure 4); angular shapes are
uncommon. The matrix consists predominantly of sandsized chert grains. At Koster the proportion of matrix
increases from about 10-15% of the rock in the basal
conglomerates to about 30-40% In the upper
conglomerates. Both matrix-supported and clastsupported conglomerates occur.
Mudrocks
This zone is very variable in thickness, from about 18 m
up to 250 m (Figure 2). Predominant rock types include
mudstone and silty mudstone, with subordinate siltstone
and silty, very fine sandstones. There is commonly an
alternation of the different lihtologies and the rocks are
mostly ferruginous, with lesser cherty varieties and
uncommon carbonaceous mudrocks. Chert interbeds
S.-Afr.Tydskr.GeoI.1988,91(4)
482
o
Timeball Hill Formation - quartzites and shales
\
JvEciilllTI,planar cross-bedded quartz wacke. Commn lenses of v8ry
coarse pebbly sandstone with planar cross-beds
r
Facies
Croos-bedded
quartz wacke
(Polo-Ground
~)
(m)
100
Silty,mudstones and subordinate, interbedded thin «1-2 m)
fine silty quartzose sandstone beds. Horizontal stratification
and saTE graded bedding. Poor1y exposed
80
60
40
20
Chert conglomerate - matures upwards and uppermost few metres
vaguel y bedded. lDwer clasts 2mn - 7G-rrn, IIDstly angular to
subangular + elongated; upper clasts 2mn - 4Grrn & subrounded
& equidimensional. Sandy siliceous matrix throughout, but
proportion of matrix increases upwards from 10-15% to 30-40%
Basal sinkholes, up to 15 m deep, in dolomites - filled by chert
breccia which grades up into chert conglomerate
Malmani Subgroup - dolomites.
Figure 2 Continued
and lenses, 4--60 mm thick, occur and generally display
irregular upper and lower bounding surfaces. A thinly
developed. chert pebble conglomerate is present in
profiles A, Band C; the mudrocks outcrop too poorly in
profile D to establish whether the conglomerate is
present at Koster as well. This thin conglomerate is very
similar to that in the Bevets Member, with chert clasts
set in a sandy matrix. Elongated clasts are generally
orientated parallel to bedding and interbedded
mudstones and chert are present. The thickness of the
conglomerate varies from 30 cm to 1 m. In profile C the
conglomerate occurs as a lens 5 m wide and 0,35 m thick;
the basal contact is erosive and the irregular upper
surface of the conglomerate is filled by silty mudstones.
483
S.Afr.J.Geol.,1988,91(4)
Figure 3 Poorly sorted, angular, chaotic chert breccia, typical
of the basal portion of the Rooihoogte Formation.
quartz wacke at Koster. The latter includes abundant
lenses of very coarse pebbly lithic wacke, 10--50 cm thick
and 1-5 m wide. The pebbles consist of siltstone and
silty, very fine sandstone, indicating erosion of the
underlying lithofacies. These coarse clasts are angular to
subrounded in shape and are set in a medium to coarse
sandy matrix. Both the pebbly lenses and the medium
quartz wackes at Koster exhibit common planar crossbeOs. Set thicknesses vary from 10 to 50 cm and angles of
inclination are between 10° and 25°.
The finer sandstones of profile A display predominant
trough crossbeds, 1-8 m wide and 10--70 cm thick, minor
interbedded laminated mudstones and thin horizontal
stratification.
Figure 4 Clast-supported subrounded to rounded chert pebble
conglomerate, which generally overlies the basal chert
breccias .
This latter occurrence probably represents a channel-fill.
The conglomerate lenses and beds strongly resemble the
'dakpan' conglomerate described by Visser (1969) from
the Pretoria area.
The predominant mudrocks exhibit characteristic
horizontal stratification, 1 mm to about 60 mm thick,
locally reaching 12-25 cm. The laminae are commonly
graded. Ripples occur, particularly in profiles A and C.
Ripple heights vary between 2 mm and 20 mm and
wavelengths from 1,5 cm to 10 cm. They comprise
mostly slightly sinuous to sinuous asymmetrical forms,
with fewer straight-crested ripples. A few flat-topped
and double-crested ripples are found. Minor climbing
ripple cross-lamination and flaser beds occur in profile A
(Figure 2). Uncommon soft sediment deformation
structures are present in profile C, and profile B exhibits
a channel-fill trough cross-bed immediately below a
chert interbed; the trough is about 1 m wide and 0,4 m
thick.
Cross-bedded quartz wackes
This lithofacies is thin, varying from 6 to 10 m in
thickness. It comprises ferruginous fine to very fine
quartz wacke in profile A, and ferruginous medium
D
Ripple lIBTks (n=34) (mudrocks - profiles A & C)
Cross-lamination asscx::iated with flaser beds
(n=lO) (mudrocks - profile A)
em
L1J
Trough and planar cross-beds (n=12)
(Polo Ground Quartzite - profiles A & D)
(All data are directional and directions are
SllI1TIeCI, not superimposed)
Figure 5 Palaeocurrent directions measured in the Rooihoogte
Formation of the study area. Location of profiles given in
Figure 1.
484
Palaeocurrents
Palaeocurrent directions were measured from ripples
and from cross-lamination associated with the flaser
bedding in the mudrocks of profiles A and C, as well as
from the cross-bedding in the uppermost quartz wackes
in profiles A and D (Figure 5). The directional
orientation of the clasts in the conglomerates was not
determined due to poor outcrops and a general lack of
preferred clast alignment.
The ripples are generally directed towards the
northeast and southwest, as are the cross-laminae
associated with the flaser bedding. Both trough and
planar cross-beds are aligned mostly towards the
northwest, approximately perpendicular to the ripple
directions (Figure 5).
Comparison with previous workers
The research of Visser (1969) in the Pretoria area,
Button (1973) in the eastern Transvaal, and Engelbrecht
(1986) in the western Transvaal make an analysis of
basinal trends in the Rooihoogte Formation possible.
The four lithofacies outlined above can also be identified
in the work of these authors.
The Rooihoogte Formation in the eastern Transvaal
comprises chert breccias overlying a palaeokarst surface,
which commonly grade upwards into a thin chert
conglomerate. Both lithologies are absent locally. The
breccia fills irregularities in the karstic surface and
becomes finer, better rounded, and acquires a more
sandy matrix upwards. Very large chert fragments, up to
1 m in length, point to a local origin for this material
(Button, 1973). The chert breccia grades upwards into a
chert conglomerate-quartzite lithology, with a maximum
thickness of about 7 m. Breccias typically overlie chertrich dolomites and the conglomerate-quartzite
association normally succeeds chert-poor material.
Dolomite domes in highly karsted areas are overlain by a
thin cover of this reworked material, adjacent to
sinkholes filled by breccia (Button, 1973). The
occurrence of wad-like breccia overlying Penge
Formation iron formations again supports local
derivation of material. Clasts in the breccia are
commonly platy in shape and are generally orientated in
the plane of the local stratification (Button, 1973).
Conglomerates typically include chert, quartzite, and
shale pebbles with angular to subrounded shapes, also
commonly orientated in the plane of stratification. A
vague imbrication occurs locally. The matrix of the
conglomerates comprises mostly medium quartzose
sandy material (Button, 1973). There is a lateral and
vertical gradation of the conglomerate into a medium
quartzose sandstone which exhibits some chert clasts,
pebbly layers, sporadically developed planar and trough
cross-beds, and vague ripple marking (Button, 1973).
The mudrocks and Polo Ground Quartzite Member
found in the present study area are absent in the eastern
Transvaal. Within the Marble Hall Fragment (Figure 1)
the Rooihoogte Formation comprises about 10 m of
conglomerate, consisting of subrounded to rounded
S.-Afr. Tydskr.GeoI.1988,91 (4)
quartzite pebbles and fewe~ chert p~bbles in a
ferruginous
quartz-tourmahne-muscovlte
matrix
. ,
(Snyman, 1956).
The Rooihoogte succession found m the PretorIa area
resembles that in the eastern Transvaal and that
described in this study." An irregular palaeokarst surface
is filled by about equal proportions of basal chert. breccia
and overlying chert pebble conglomerate; there IS ~lso a
lateral gradation of breccia into conglomerate (Vls.ser,
1969). The breccias gene.rally .have recryst~lhzed
siliceous intergranular materIal which grades up mto a
sandy matrix in the overlying conglomer.ates. Maxim~m
thickness of this breccia-conglomerate IS 128 m which
occurs to the southeast of Pretoria (Visser, 1969). The
coarsest breccia fragments, up to 1 m long, also occur in
the same region and again indicate local derivation of
material. This very coarse interval is succeeded by a
medium lithic wacke which contains well-rounded chert,
quartz, and shale clasts. The basal wackes include
claystones and the uppermost sandstones are finer
quartz wackes (Visser, 1969). Average thickness is about
30 m with a maximum of 150 m occurring to the
south~ast of Pretoria, overlying the thickest
development of the breccia-conglomerate (Visser,
1969); these thick sandstones are very mature.
Structures in the sandstones include thin and massive
bedding, ripple marks, graded bedding, trough crossbeds and parting lineations. The troughs vary from small
to large, with widths up to several metres. Ripples
include lunate, interference, asymmetrical and
symmetrical types (Visser, 1969).
Between 12 and 15 m of mudrock overlies the
quartzose sandstones near Pretoria. Interbedded chert
and cherty shales characterize the uppermost few metres
of this zone. In general the mudrocks tend to be silty and
pyritic. Rhythmic layering with basal sole marks occurs
(Visser, 1969). The mudrocks are succeeded by the Polo
Ground Quartzite, comprising two layers with a total
thickness between 3 and 6 m (Visser, 1969).
The chert breccia phase of the Rooihoogte Formation
is absent in the western Transvaal (Klop, 1978;
Engelbrecht, 1986). Poorly sorted, well-rounded chert
pebble conglomerates overlie the dolomitic rocks in this
part of the basin. The matrix comprises siliceous to
ferruginous material and clasts are commonly bladed in
form and may be orientated parallel to bedding (Klop,
1978). Thicknesses in the south of this region vary
between 0 and 50 m and towards the Botswana border
the conglomerate grades up into about 5 m of chloritic
quartzite (Klop, 1978; Engelbrecht, 1986). Maximum
thicknesses, up to 250 m, occur in the extreme
northwestern part of the basin and the chert pebbles in
the conglomerate here are poorly rounded, indicating a
less mature nature (Engelbrecht, 1986).
Metamorphosed mudrocks, 10-150 m thick, overlie
the conglomerate in the western part of the Rooihoogte
depository. The maximum thickness occurs in the
extreme northwest, above the very thick conglomerate.
Some fine laminae with reversed grading are preserved
(Klop, 1978). The succeeding Polo Ground Quartzite
Member is between 2 and 10 m thick and occurs
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base of Pretoria Group
chert breccia
chert conglomerate and associated sandstones (- - - )
mudrocks
Polo Ground Quartzite
Thicknesses
10m
(10m)
[ 10m J
10 m
chert breccia
chert conglomerate
sandstones
mudrocks
Postulated major fan systems
I
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486
throughout the western Transvaal Basin. This medium
quartz wacke contains minor chert grains and, in the
southwest, includes interbedded slate, chert and partly
clastic, stromatolitic dolomite (Klop, 1978; Engelbrecht,
1986). The quartz wackes themselves exhibit some crossbedding, slump structures, shale rafts, shale interbeds,
and chert pebble layers (Klop, 1978). The Rooihoogte
formation within the Crocodile River Fragment (Figure
1) consists of 2,5 m of chert conglomerate with rounded
pebbles, imbrication, and horizontal bedding, overlain
by 8 m of coarse feldspathic sandstone characterized by
trough cross-stratification and wavy bedding (F.1 .
Hartzer, 1987, pers. comm.).
The regional distribution of the four lithofacies in the
Rooihoogte Formation is summarized in Figure 6. The
chert breccia is well developed in the eastern Transvaal,
with thicknesses reaching tens of metres (Button, 1973).
Thicknesses increase towards Pretoria, particularly
southeast of the city, and the breccia wedges out towards
the west of Pretoria. The chert conglomerate is thinly
developed in the eastern Transvaal and Marble Hall
areas and thickens towards the west, reaching
maximums of 100 m at Koster and 250 m in the far
northwest of the basin (Figure 6). Thin quartzose
sandstones are associated with the conglomerates in the
eastern Transvaal and thicken towards Pretoria; a
maximum thickness of 150 m is achieved southeast of
Pretoria where the rocks also exhibit much greater
maturity. To the west of Pretoria these sandstones are
absent and shaly lithologies overlie the conglomerates.
Maximum mudrock development is at Koster and in the
northwestern Transvaal. The overlying Polo Ground
Quartzite Member is restricted to the western part of the
basin, being thinly developed in all outcrops.
Detailed sedimentological data are not available for
the Rooihoogte Formation in the Potchefstroom
Synclinorium, a sub-basin developed to the south of the
main depository (Figure 1).
Depositional model
The thick breccias and conglomerates of the Rooihoogte
Formation are not compatible with a transgressive
marine or basinal setting, as proposed by previous
workers. In addition, the rapid lateral facies changes,
general poor sorting and common angular fragments in
these rocks mitigate against such an interpretation. The
chert breccias which grade vertically and laterally into
more mature chert conglomerates can more plausibly be
viewed as alluvial deposits, representing the products of
landslide, streamflow, and mass gravity sedimentation in
the proximal reaches of fan systems. The succeeding
mudrocks in the western Transvaal may represent a
lacustrine or a distal fan-delta facies, which transgressed
over proximal, coarse detritus. The Polo Ground
Quartzites possibly formed from distal alluvial fan
sediments which spread out as thin sheets over a filled
lacustrine basin.
In situ subaerial weathering of the underlying
dolomites and iron formations is indicated by local
residual wad occurrences and by the very large chert
S.-Afr.Tydskr.GeoI.1988,91(4)
clasts, up to 1 m long, found in the eastern ~ransvaal and
around Pretoria. The resultant chert resIduum would
have been thickest over upraised areas (Button, 1969),
thereby providing relief and the potential for alluvial fan
development.
.
Although the thick conglomerates .and br~cclas f.ound
in this study are supportive of fan sedImentatIon (NIlsen,
1982), they may also represent accum.ulated residuum in
erosional depressions in the underlymg unconformable
surface. However, the lateral and vertical grading of the
breccias into increasingly mature conglomerates and
sandstones indicates widespread and relatively
continuous reworking of residual detritus, with only a
restricted and localized preservation of weathering
residuum. The chert breccias of the lower Rooihoogte
Formation are compatible with landslide and mass flow
processes, and the more mature conglomer~tes and
sandstones with streamflow and lesser gravIty flow
sedimentation. The gradations among these rock types
indicate that streamflow, mass flow, and landslide
operated
together
within
the
deposition
palaeoenvironment, a characteristic feature of m~st
alluvial fans (Bull, 1972; Nilsen, 1982), The breccIas
were probably cemented I in situ by siliceous and
carbonate-bearing groundwaters; groundwater flow
through fan deposits is normally well developed (Bull,
1972).
The apparently debris flow-dominated Rooihoogte
alluvial fans reflect either a proximal setting or an arid
palaeoclimate (Nilsen, 1982). Such fans are normally
small, with dimensions of tens of kilometers, and
generally exhibit steep gradients (McGowen, 1979).
With the exception of three postulated major fans
(Figure 6), similar dimensions are envisaged within the
Rooihoogte basin. The larger fans could represent
coallescence of smaller deposits, to form fan systems.
The Rooihoogte fans are thought to have had low-angle
slopes, deduced from the nature of the preserved mass
flow deposits. The thickness of these rocks in the
formation, and their general lack of a muddy matrix,
mitigate against grain flow or true debris flow deposition
(Lash, 1984). However, thick, relatively mud-poor mass
flow deposits can develop through density-modified
grain flow processes (Lowe, 1982). Alignment of clasts
parallel to bedding, observed in the breccias and
cong10merates under consideration here, is common in
such sediments, and they generally develop on low-angle
slopes (Lash, 1984). Their lack of mud provides them
with a low-matrix strength, leading to their widespread
development, as noted in this study. Such low-angle
slopes would indicate deposition on the flanks of uplifted
source areas rather than an association with faulting or
graben formation (Collinson, 1986).
The deep weathering of the pre-Rooihoogte dolomite
is consistent with such uplift. Residual chert detritus was
presumably fed into the basin margins by landslide and
density-modified grain flow processes, where it largely
filled up the karstic surface. Streamflow processes
reworked much of the detritus and also contributed
significantly to filling karst features. The generally
chaotic, poorly sorted conglomerate, with its weakly
487
S.Afr.J. Geol., 1988,91 (4)
developed local bedding and imbrication, indicates a
proximal fan setting. In the eastern and southern
portions of the basin there was an intimate association of
gravity-slide, mass flow and streamflow processes. Very
large chert clasts and palaeowad occurrences indicate in
situ residua which were apparently not transported. In
the western Transvaal streamflow processes probably
reworked the landslide and mass flow deposits,
suggesting a more distal fan setting (Nilsen, 1982). From
thickness data it appears that three large fan systems,
located to the southeast of Pretoria, in the Koster area
and in the northwestern Transvaal (Figure 6) entered the
predominated
elsewhere,
basin.
Smaller
fans
particularly in the eastern Transvaal.
The more mature quartzose sandstones which succeed
the breccia-conglomerate interval in the eastern
Transvaal, around Pretoria and in the extreme
southwestern Transvaal, are probably distal fan
deposits. The pebble layers, sporadic trough cross-beds
and horizontal stratification in these rocks are
compatible with distal sheetflood sediments described by
Bull (1972) and Nilsen (1982). The combination of chert,
quartz, and mudrock clasts in these sandstones indicates
erosion of dolomitic rocks and underlying material,
probably including rocks of the Black Reef Formation.
The quartz-bearing conglomerates at Marble Hall and
the conglomerates and partly feldspathic sandstones in
the Crocodile River Fragment probably indicate similar
distal sheetflood deposits, or, alternatively, braided
stream fan sediments analogous to those described by
Steel (1974). Detritus in these two regions was also,
apparently, partly derived from rocks underlying the
dolomites. The region between Marble Hall and the
Crocodile River Fragment may have been a slightly
elevated source area during Rooihoogte times (Eriksson
et al., in press) with small fans developed adjacent to this
area. The upward-fining succession of breccia grading
into conglomerate, succeeded by more mature
sandstones, particularly in the eastern portion of the
basin, supports fan retreat and the lowering of gently
upraised source areas (Heward, 1978).
The lithic wackes, which gradationally overlie the
conglomerates in the Pretoria region, contain basal
mudrock interbeds and muddy sandstones in their upper
few metres. This, allied to the presence of thin, locally
graded beds, current and wave ripple marks with lunate
and interference forms, points to basinal influences in
the distal portions of the alluvial fans. In view of the
mudrocks which succeed the conglomerates in the
western part of the basin, the fans are presumed to have
terminated in lacustrine fan-deltas. Distal fans
commonly grade into such lacustrine settings (Nilsen,
1982).
The alternation of mudstones, siltstones and silty very
fine sandstones, and the presence of graded bedding in
the mudrock interval point to a fluctuating supply of
sediment, suspension settling, and dilute density current
activity (Collinson & Thompson, 1982), processes
common within lacustrine palaeoenvironments. Thin
chert beds, reflecting localized chemical sedimentation
in areas removed from active sediment inflow, also
support a lacustrine fan-delta setting (Nilsen, 1982). The
interbedded chert conglomerates found in the present
study area (Figure 2) probably represent mass flow
reworking of coarse material from proximal fan reaches.
, Weak lacustrine current and tidal activity is presumed to
have formed the sinuous current ripples, double-crested
and flat-topped forms observed in these profiles; minor
flaser beds and climbing ripple cross-lamination also
support weak tidal currents (Reineck & Singh, 1975).
Tides up to 6-8 cm are found in lakes with dimensions
between 400 and 1000 km (Schopf, 1980). The
approximately 500 km long Pretoria Group basin would
thus have been characterized by microtidal conditions.
Pyritic shales around Pretoria are compatible with
reducing
conditions
within
a
lacustrine
palaeoenvironment; the sole marks in these mudrocks
indicate erosion where fine sediment was transported
into the basin.
Gentle and continuous subsidence of the western
Transvaal Basin was necessary to accommodate up to
250 m of lacustrine sediment. This was allied to low relief
within the previous alluvial source areas and,
presumably, low gradients on the fans themselves. Fine
detritus from the distal portions of the fans was
reworked and redistributed by low energy current and
tidal activity within a lacustrine basin that gradually
transgressed over the coarse alluvial deposits.
Palaeocurrent data from the study area indicate ripple
migration towards both the northeast and southwest,
approximately parallel to the preserved basin margin
(Figures 1 and 5). Maximum subsidence occurred along
an axis running from Koster towards the far
northwestern Transvaal (Figure 6). The lacustrine fandelta palaeoenvironment was restricted to the western
half of the Rooihoogte basin, sedimentation in the
eastern portion having terminated with the coarse fan
deposits.
Renewed, but short-lived uplift in the western alluvial
source regions is assumed to have led to the formation of
the Polo Ground Quartzites. Thin sheets of fine to
coarse, sandy material were shed, possibly by distal fan
sheetflow processes over the western portion of the
basin, overlying the lacustrine mudrocks. Cross-bedding
indicates transport towards the northwest (Figure 5),
approximately perpendicular to the proposed basin
margin. Pebble layers around Koster and breccia
occurrences in the far northwestern Transvaal support
reworking of coarse proximal fan material; mudrock
clasts show that underlying lacustrine deposits were also
reworked. Dolomites and cherts, preserved in the
southwest of the basin, point to localized lacustrine
deposition.
Conclusions
The coarse chert breccias and conglomerates of the
Rooihoogte Formation which outcrop around the
Transvaal basin are interpreted as proximal to medial
alluvial fan deposits. A combination of streamflow,
density-modified grain flow and landslide deposition are
envisaged, adjacent to gently uplifted source terrains.
488
Three major fans are postulated to have entered the
basin, deriving detritus from weathered chert-bearing
Malmani Subgroup dolomites to the south-southeast of
Pretoria, south of Koster, and from the far northwestern
Transvaal (Figure 6). Smaller fans were active in other
areas, particularly the eastern Transvaal. Quartzose
sandstones which overlie the coarse rocks in the eastern
and southern portions of the basin are presumed to
represent distal sheetflood sediments. A maximum
thickness of 150 m of very mature sandstones deposited
on the large fan southeast of Pretoria suggests
weathering and erosion of Black Reef Formation
quartzites. Pre-Malmani source rocks are also envisaged
for small fans which developed along the margins of a
slightly positive palaeohigh between the Crocodile River
and Marble Hall Fragments (Figure 6).
The present-day exposures of basement granitoid
rocks to the south and southeast of Pretoria, to the south
of Koster, and in the northwestern Transvaal (Figure 6)
are thought to represent the major provenance regions
for this alluvial sedimentation. Associated exposures of
Witwatersrand and Ventersdorp Supergroup rocks in
these areas support uplift and removal of lower
Transvaal Sequence strata, Chert-rich detritus from the
Malmani dolomites, quartzitic sands from the Black
Reef Formation, and possibly material from pre-existing
rocks as well, could have been supplied to the
Rooihoogte basin from these regions.
As alluvial fan sedimentation terminated in the
eastern part of the basin with reduction of source area
relief, a distal lacustrine fan-delta palaeoenvironment
developed in the west, allied to similar provenance
region reduction. Fine sediments, partly derived from
the alluvial deposits, gradually transgressed over the
western fans. Renewed uplift of short duration around
the western part of the basin led to the thin quartz
wackes of the Polo Ground Member being laid down by
distal fan processes.
Acknowledgements
The author thanks the University of Pretoria for
research funds, and Mrs. 1. Cornelius and Mrs. M.
Potgieter for draughting the figures. In addition, various
staff members of the Department of Geology, University
of Pretoria, are acknowledged for stimulating
discussions. The constructive criticism of an anonymous
reviewer and Mr. C. C. Callaghan is appreciated.
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