An early proterozoic braid-delta system in the Pretoria group, Transvaal sequence, south Africa

Jo~,,~

ofApi~anEarth Sciences, Vol. 15, No. 1, pp. 111-125, 1992.

0899-5362J92 $5.00 + 0.00 © 1992 Pergamon Press Ltd

Printed in Great Britain

An Early Proterozoic braid-delta system in the Pretoria Group, Transvaal Sequence, South Africa U. M. SCHREmERand E G. ERIKSSON Department of Geology, University of Pretoria, Pretoria 0002, South Africa (First received 5th November, 1991; revised form received 8th March, 1992) Abstract - The Dwaal Heuwel Formation of the ProterozoicPretoria Group in the eastern Transvaal, South

Africa, was deposited in a large-scale braid-delta system. Predominant immature to mature, cross-bedded sandstone probably represents fluvial braidplaln deposits, whereas minor conglomeratic sandstone and mudrock are interpreted as proximal alluvial fan deposits and shallow water deposits, respectively. Sandy

stream-flowdepositsdominated the braidplain succession,whereasmass flow sedimentationoccurredin the northernmost fan systems,the latter acting as sourceregions for the fluvialdetritus. A distal lacustrine basin is represented by a laminated mudrock facies which occurs in the southern portion of the study area. INTRODUCTION

F a n - d e l t a s are c o a r s e - g r a i n e d s e d i m e n t a t i o n s y s t e m s at t h e inter-face b e t w e e n a n alluvial plain a n d a d j a c e n t b a s i n , a n d have a t t r a c t e d m u c h a t t e n t i o n recently, b e c a u s e of their potential for m i n e r a l d e p o s i t s (e.g. N e m e c a n d Steel, 1988). A distinction is m a d e b e t w e e n the "classical" fandelta, w h e r e a n alluvial f a n p a s s e s directly into a n a d j a c e n t b a s i n , a n d b r a i d - d e l t a s , w h e r e a fluvial braidplain, n o t n e c e s s a r i l y related to a n alluvial fan, enters a b a s i n (McPherson et aL, 1987). Braided rivers p r o b a b l y were c o m m o n before the developm e n t Of l a n d v e g e t a t i o n in D e v o n i a n t i m e s ( S c h u m m , 1968), a n d very extensive b r a i d p l a i n s y s t e m s , a s well a s t h e i r m o r e distal equivalents, b r a i d - d e l t a s (Fuller, 1985) were able to develop. A r c h a e a n e x a m p l e s of f a n - d e l t a s from Africa include the famous Witwatersrand Supergroup (Kingsley, 1984; Minter, 1991) a n d the Moodies G r o u p of t h e B a r b e r t o n S e q u e n c e (Eriksson, 1978). Both e x a m p l e s are a s s o c i a t e d with active fault s y s t e m s (Stanistreet et aL, 1986), a n d a p p e a r to have h a d relatively small d i m e n s i o n s , of the order of a few t e n s o f k i l o m e t r e s a c r o s s (Minter a n d Loen, 1991). In c o n t r a s t , the Early Proterozoic Dwaal H e u w e l F o r m a t i o n of t h e T r a n s v a a l S e q u e n c e , w h i c h is well p r e s e r v e d a n d relatively u n d e f o r m e d , a p p e a r s to r e p r e s e n t a very large a n c i e n t braiddelta s y s t e m , with d i m e n s i o n s m o r e t h a n 1 0 0 - 2 0 0 k i l o m e t r e s across. The a r e n a c e o u s D w a a l H e u w e l F o r m a t i o n previously n a m e d D w a a h e u w e l Quartzite (Molengraaf, 1898) or spelt D w a a l H e u v e l F o r m a t i o n (Button, 1975) u n c o n f o r m a b l y overlies t h e 2 2 2 4 + 21 Ma ( R b / S r w h o l e rock) (Burger a n d Coertze, 1973-

1974) a n d e s i t e of t h e H e k p o o r t F o r m a t i o n , a n d i s overlain by the argillaceous Strubenkop F o r m a t i o n (Table I). It is developed only in t h e e a s t e r n p a r t of the Transvaal b a s i n (Fig. 1), wedging o u t t o w a r d s the Pretoria area. In t h e w e s t e r n Transvaal, t h e stratigraphic position b e t w e e n t h e H e k p o o r t volcanics a n d S t r u b e n k o p m u d r o c k s is o c c u p i e d b y the Droogedal F o r m a t i o n (Eriksson et al., 1989). The Dwaal H e u w e l F o r m a t i o n o c c u r s in a n a r r o w belt t h r o u g h o u t t h e e a s t e r n a n d n o r t h e a s t e r n T r a n s v a a l , a s well a s in t h e " f r a g m e n t s " of T r a n s v a a l - a g e r o c k s s u r r o u n d e d b y B u s h v e l d int r u s i v e s at Marble Hall a n d D e n n i l t o n (Fig. 1). F o u r t e e n lithostratigraphic profiles m e a s u r e d in the p r e s e n t s t u d y reveal a t h i c k e n i n g from app r o x i m a t e l y 4 5 m in t h e S o u t h of t h e m a i n o u t c r o p area to m o r e t h a n 100 m in t h e North. A m a x i m u m t h i c k n e s s of 120 m is r e c o r d e d n e a r Potgietersrus, w h e r e a s in the Marble Hall a n d D e n n f l t o n areas, t h i c k n e s s e s of 60 m a n d 80 m, respectively, are p r e s e n t (Fig. 1). The u n i t r e s t s d i s c o r d a n t l y o n a 25 m t h i c k sericitic m u d r o c k layer, developed at t h e top of the H e k p o o r t a n d e s i t e s , w h i c h is i n t e r p r e t e d a s a palaeosol (Button, 1973, 1979). The u p p e r contact with the argillaceous Strubenkop Formation is s h a r p to gradational over a few metres. C r o s s - b e d d e d s a n d s t o n e p r e d o m i n a t e s in the Dwaal H e u w e l F o r m a t i o n (Figs 2-6); l a m i n a t e d or massive mudrock and conglomeratic sandstone o c c u r locally. An overall u p w a r d - f i n i n g arrangem e n t oflithofacies c h a r a c t e r i z e s t h e s u c c e s s i o n in m o s t areas, a n d c o m p o s i t i o n a l a n d t e x t u r a l m a t u rity i n c r e a s e t o w a r d s t h e S o u t h . Detailed lithostratigraphic profiles, revealing the t h r e e - d l m e n s i o n a l g e o m e t r y of t h e Dwaal H e u w e l F o r m a t i o n a n d its

111

112

U.M. SCHREmERand P. G. ERIKSSON l

5os

tad I I• Lydenburg 8

Burger s for t ~ ' ~ ' ~ 5

~dodo

m 6O

el

-30OB

Chuniespoort

• Belfast oE.

•

?

covered by Bushveld

I

[ sandstone facies

i

mudrock facies

conglomerate

rocks

facies

Dennilton

0 i

i0 r

20 1

30 km I

25os Potgietersrus

I

Fig. I. Geometry of the Dwaal Heuwel Formation in the Eastern Transvaal. Numbers indicate profiles shown in Figs 2 to 6. Capital letters give locations of samples taken for geochemical analysis [Fable II).

Table I. Stratigraphy of the Pretoria Group. Transvaal Sequence.

Western Transvaal

Eastern Transvaal

Rooiberg Group

Rooiberg Group

Dullstroom Formation Houtanbek Formation

lithofacies, palaeocurrent m e a s u r e m e n t s , and petrographic and geochemical analyses are used in this paper to postulate an alternative palaeoenvironment of deposition to the shallow marine one suggested by Button (1973, 1975). LITHOLOGY OF THE DWAAL HEUWEL FORMATION

Steenkampsberg Formation Nederhorst Formation Lakenvlei Formation

Vermont Formation Magaliesberg

Formation

The Dwaal Heuwel Formation comprises three lithofacies: predominant immature to mature crossbedded sandstone, laminated or massive mudrock, and conglomeratic sandstone (Figs 2-6).

Magaliesberg Formation

Silverton Formation

Silverton Formation

Daspoort For~mation

Daspoort Formation

Strubenkop Formation

Strubenkop Formation

DROOGEDAL FORMATION

DWAAL HEUWEL FORMATION

Hekpoort Formation

Hekpoort Formation

Boshoek Formation

Boshoek Formation

Timeball Hill Formation

Timeball Hill Formation

Roolhoogte Formation

Rooihoogte Formation

Chuniespoort Group

Chuniespoort Group

S a n d s t o n e facies The Dwaal Heuwel sandstone is mostly fine- to medium-grained; thin (a few millimetres to 20 cm thick) very fine-grained and pebbly sandstone interbeds create localized upward-coarsening and, less commonly, upward -fining sedimentary cycles, reaching up to 10 m in thickness (e.g. Fig. 3). Clasts contained in the coarse intercalations are between 2 and 5 m m in diameter and are composed of chert, quartz, feldspar and limonite. Thin mudrock partings, with wavy laminations, are widespread. The petrology of the sandstones is variable,

An Early Proterozoic braid-della system in the Pretoria Group, Transvaal Sequence, South Africa

113

Lefend

I

]

a p p a r e n t l y massive

EEE~ p l a n a r b e d d i n g / l a m i n a t i o n

planar cross-bedding/lamination

trough c r o s s - b e d d i n g / l a m i n a t i o n

upward-coarsening c y c l e

channet-filt

upward-fining cycle lithofacies

a - mudrock

b - medium-grained sandstone

c - pebbty sandstone/ conglomerate

[m]

symmetrical ripple monks; thin (-- 1 cm) siltstone interbeds

b

a

thin ( < 1 cm) siltstone interbeds

b

thin (few cm) medium- to coarse-grained sandstone interbeds

0

c~

E

O

~-

C

e-

,.-

C

~

0

0

0

0

0

cm~E~m

0 ¢-.4=.-

u

u

sandstone

Pig. 2. LlthostraUgraphic profile t h r o u g h the Dwaal Heuwel ForrnaUon n e a r Machadodorp. Contacts between llthological units are gradaUonal.

114

U.M. SaIREIBERand P. G. ERIICSSOS

[m] EL

b EL

A

upward-finlng fine- to medium-grainecl sandstone

thin ( 4 5 m m ) slltstone Interbeds

thin (-- 5-20 ram) siltstoneinterbeds

b

thin (few c m ) coarse-grained sanclstone interbeds

thin (-- 5 c m ) very coarse g r a i n e d sandstone interlDed upward-fining fine- to m e d i u m - g r a i n e d s a n d s t o n e

upword-l~ning fine- to mediurn-grainecl sandstone [.

,o

thin (few ram) siltstone a n d (few c m ) fine-grained sandstone interbeds

I

thin (~< I c m ) very fine-groined sandstone inted0eds upworc~fining yew fine- to fine-grained sandstone

b

]

thin (few c m ) medium-grained sandstone interlc)ecls; thin ( < 5 c m ) upward-fining cycles

I

thin ( X I c m ) siltstone inter*oecls thin (several c m ) coarse- to ~,~ry c o a r s e - g r a i n e d sanclstone interbeds

b

thin (few c m ) p e b b l y sandstone intetl:)eds

I

l

I

! C m

•

•

.

•

•

•

|

p e b b l y sorK:Istone

,~'~'~'~'~' ~,

i4~ °~

sandstone

Fig. 3. Lithostratigraphlc profile through the Dwaal Heuwel Formation north of Machadodorp. Contacts between units are gradational. See Fig. 2 for legend.

An Early Proterozoic braid-delta system in the Pretoria Group, Transvaal Sequence, South Africa

115

[m]

thin (I-2 cm) fine-to medium-grained sandstone interbeds: asymmetrical npple marks

thin medium-groined sandstone (--Icm) ond siltstone (few ram) interbeds

b

asymmetrical ripple marks

thin (few m m

to c m ) siltstone interi0eds

thin (few m m ) siltstoneinterbeds

-Ethin (few ram) siltstone interbeds

thin (few mm) siltstone interioeds

no outcrop 42 m)

b

L

pebbles (~ 2 mm) thin siltstone (few ram) and cOarse<3rained to pebbly sandstone (few cm) interbeds

thin (few c m ) pebbly sandstone interbecls

thin (few turn to cm) siltstone interbeds 0

~

'

% 0 °~

~

E

e

sandstone

Fig. 4. Llthostratagraphlc profile through the Dwaal Heuwel Formation near Lydenburg. Contacts between units are gradattonal. See Flg. 9. for legend.

U. M. SCHREIBERand P. G. ERIKSSON

116

ranging from mature quartz arenltes to llthtc and, locally, arkoslc wackes, wlth up to 33 % matrix material (Fig. 7). Sublttharenltes, containing between 10 % and 25 % chert and minor mudrock fragments, are common. In the southern part of I~0

[m]

]

1oo rne~lum- to coorze-~oined sonastone intert3eds (~ 10 cm) r i p p l e m a r k s : pebbles (~ 2 r a m )

the m a i n outcrop area, increasing maturity from basal argillaceous and feldspathic/lithic sandstone to uppermost quartz arenite is observed. Poor sorting and moderate to good grain roundn e s s characterize this lithofacies, the latter being particularly p r o m i n e n t in the m a t r l x - p o o r , medium-grained arenites. The Dwaal Heuwel sandstones are ferrugineous, iron occurring in the form of limonite and hematite, the latter imparting a red stain to the matrix clay minerals. The rock commonly displays limonitic speckling, due to disseminated limonite p s e u d o m o r p h s after pyrite (<3 cm in size). Manganese minerals occur locally o n surfaces. [m]

asymmetrical

n o o u t c r o p (3 m)

(j

)

no outcrop (12 m)

medium- to coorse-gratned sandstone interbeds (10-20cm)

thin (few cm) coarse-groined sondstone

inlert~ls

n o o u t c r o p (5 m )

+

A

b

upwotd--llning cycles (several cm) few thln (~ I cm) s i l t s t o n e interl:)eds thin

:I

(I-2 cm) upv~ItoU-~ning cyctes

thin slltstone (5 cm) and (t0.20 cm) interbeds

b

very coorse-groined sondstone

tnln (1-3 cm) upwata~Ining cycles

m

b pebOles ( - 2 mm)

thlfl

+

(t 0*20cm) inlerbeds of conglomeratic sandstone

C

b a thtn

(-- 5 cm) interbecls of s a n d y $iltstone

thin

(2-5 cm) upward-fining cycles

C +

10

b

sandstone sandslone

Fig. 5. Lithostratlgraphie profile through the Dwaal Heuwel Formation near Ohrlgstad. Contacts between units are gradational. See Fig. 2 for legend

Fig. 6. Llthostratigmphic profile through the Dwaal Heuwel Formation south of Chuniespoort. Contacts between units are gradational. See Fig. 2 for legend.

An Early Proterozoic braid-delta system in the Pretoria Group, Transvaal Sequence, South Africa

elongated in form. Chaotic, matrlx-supported and poorly sorted conglomerates with a m e d i u m - to coarse-grained, hematite-stained, s a n d y matrix predominate.

Q

~Quartz arenite / ~

Arenites

117

Subarkose Su~itharenite

F'

\ Matrix

- 15%

NF

Q wacRes

\

F

Matrix 16 - 75%

\ ~RF

Fig. 7. Petrographic classification of Dwaal Heuwel Formation sandstones (after Pettljohn et al., 1972}.

Planar cross-bedding, with set thicknesses between 30 c m and 1 m, is c o m m o n in the Dwaal Heuwel sandstone. In the less m a t u r e rocks, an accentuation of foresets by heavy minerals, commonly hematite, is observed. Foreset inclination angles are between 15 ° and 20°; at one locality east of Lydenburg (Fig. 1), overturned planar foresets are encountered. Trough cross-beds, 10 to 30 cm deep and 30 to 50 c m wide, a n d thin planar bedding are present locally. Other, less c o m m o n sedimentary s t r u c t u r e s include symmetrical and asymmetrical ripple marks, channel-fills between 1 a n d 12 m wide a n d 10 to 70 cm deep, and mudcracks.

Conglomerate facies Conglomeratic interbeds, 0.5 to 3 m thick, occur in the northern and northeastern part of the outcrop area {Figs I-6}. The pebbles, which average 0.5 c m in diameter, but m a y reach 12 c m in the northernmost profiles,are composed of chert, vein quartz, quartzite and. locally,Jasper, limonitc and iron-formatlon. They range from well-rounded to subangular in shape, and are spherical to

Mudrock facies Mudstones and siltstones form a subordinate part of the Dwaal Heuwel Formation and are poorly exposed. Thick m u d r o c k units, with a n average thickness of 3.5 m a n d a m a x i m u m of 15 m, are present only in the South (Figs 2 to 5), w h e r e a s thin (5 - I0 c m thick) silty to very fine-grained sandstone i n t e r b e d s a n d millimetre to c e n t i m e t r e t h i c k m u d s t o n e and siltstone intercalations are comm o n t h r o u g h o u t the outcrop area. The thicker units often are c o n t i n u o u s for kilometres along strike, whereas the t h i n n e r ones have been followed only over a few m e t r e s to tens of metres; they appear to have a lenUcular geometry. Planar lamination and thin (5 - 15 cm thick} planar bedding are common; thin wavy bedding is observed locally. Colour-banding, due to the alternation of lighter and darker laminae, is common, as are desiccation cracks. Siltstone generally p r e d o m i n a t e s over m u d s t o n e in this facies. Lithofacies relationships Lithostratigraphic sections m e a s u r e d through the Dwaal Heuwel Formation display both overall upward-coarsening and more c o m m o n upwardfining a r r a n g e m e n t s of lithofacies (Figs 2 to 6}. In addition, upward-fining and upward-coarsening meso-cycles, m e a s u r i n g between a few m e t r e s and tens of metres in thickness, consist in t u r n of smaller, subordinate cycles, which m a y be as thin as 1 cm. The n u m b e r and thickness of these sedim e n t a r y cycles vary along strike, and they c a n only rarely be correlated between outcrops. In the s o u t h e r n part of the m a i n outcrop area, a 5 to 15 m thick m u d r o c k unit is present n e a r the middle of the formation (Fig. 2), dividing it into a lower upward-fining, and an u p p e r upward-coarsening half. Concomitant with a n o r t h w a r d increase in thickness, a n overall increase in grain size and a decrease in m a t u r i t y are observed. Mudrock interbeds, present in the South, become less frequent and t h i n n e r towards the North, w h e r e a s the opposite is true of the conglomeratic lithofacies (Fig. 1). Moreover, m a t u r e arenite is not e n c o u n t e r e d in the n o r t h e r n part of the outcrop area. Palaeocurrent directions Due to the apparent scarcity of trough crossbedding (Figs 2 to 6), which is a more reliable palaeocurrent indicator t h a n p l a n a r cross-bedding (High and Picard, 1974), m e a s u r e m e n t s were carried out chiefly on three-dimensionally exposed

U. M. SCHREIBERand P. G. ERIKSSON

118

planar cross-bed sets. These are predominantly oriented towards the south, southwest and west (Fig. 8), suggesting sediment transport generally from the Northeast to the Southwest. This is supported by the overall thinning of the formation to the Southwest, and by the greater a b u n d a n c e of finer and more m a t u r e rocks in this direction. At Marble Hall and Dermilton, planar cross-bed sets oriented to the Northeast are found.

I

30°E FSrU$

ChL~i espoor t

Penge

Ripple mark morphology

n=60

Ripple m a r k s in the Dwaal Heuwel Formation are c o m m o n l y symmetrical, pointing to generation through wave action. The c o m m o n r o u n d e d crests are straight to slightly sinuous, with localized c r e s t l i n e b i f u r c a t i o n , also s u g g e s t i n g wavegeneration (CoUinson andThompson, 1982). Smallscale ]adderback ripple m a r k s occur infrequently, a n d i n d i c a t e wave action d u r i n g e m e r g e n c e (Collinson and Thompson, 1982). Ripple indices generally vary little (ripple index : length/height - 4 to 28; ripple s y m m e t r y index : length stoss side /length lee side -1.2 to 12), and most of these s t r u c t u r e s indicate wave-activity, with m i n o r current-formed ripple m a r k s (Tanner, 1967).

t a r o t ir'u~

Fig. 8. Palaeocurrent directions in the Dwaal Heuwel Formation.

GEOCHEMISTRY

Major element compositions of eleven samples of Dwaal Heuwel Formation s a n d s t o n e s were deter- Heuwel Formation plot in the fields corresponding mined with s t a n d a r d X-ray fluorescence techni- to arkose, s u b a r k o s e a n d sublithic a r e n i t e s ques on a n ARL 8420 spectrometer (Table II). (Fig. 9). Average SiO 2 content (87.7 wt. %) is comparatively A positive correlation between a l u m i n a and low for a sandstone, a n d average Al203 (6.08 wt. %) potassium is shown in Fig. 10; both elements is the highest known among Pretoria Group sand- decrease in the more siliceous samples. In anAl203 stones in the eastern Transvaal (Schreiber, 1990). - CaO + Na20 - K20 diagram (Nesbitt and Young, The average FeO t content of 3.47 wt. % is exceeded 1989), Dwaal Heuwel s a n d s t o n e s with less t h a n 90 only by ironstone and ferrugineous sandstone of wt. % SiO 2 plot n e a r the composition of illite the Timeball Hill Formation of this group. Dwaal (Fig. 11), which m a k e s up most of the fine fraction Heuwel s a n d s t o n e s also show a relatively high of the rock_ High F e O t content is accounted for by average K20 content. In an Na20/K20 - SiO2/AI203 disseminated hematite, and secondary limonite. binary diagram, i m m a t u r e arenites of the Dwaal Minor biotite is observed in thin section. Table II. Major e l e m e n t c o m p o s i t i o n (given a s oxide weight percent) o f Dwaal Heuwel F o r m a t i o n s a n d s t o n e s (FeO t = total Fe as FeO). A SiO 2 83.40 TiO~__ 0.25 A[ ~3 7.89 FeO t 4.73 MnO 0.02 MgO 0.43 CaO 0.07 Na~O 0.03 K?6 1.74 P20~ 0.15 Cr~ 0.06 Ni6 3 b.d. 0.24 2.05 Tot

101.00

B

C

D

E

F

G

H

!

J

K

Mean

79.12 0.50 11.66 3.59 b.d. 0.24 0.11 0.14 3.41 0.21 0.07 0.01 0.22 1.82

84.11 0.25 5.86 7.51 0.01 0.03 0.01 0.05 1.56 0.07 0.06 b.d. 0.07 1.22

95.96 b.d. 2.70 0.85 0.01 b.d. b.d. 0.03 0.57 0.08 0.05 b.d. 0.07 0.64

97.84 b.d. 1.03 0.68 0.01 b.d. b.d. b.d. 0.25 0.08 0.06 b.d. 0.06 0.25

93.51 0.02 2.74 2.50 0.03 b.d. b.d. b.d. 0.62 0.02 0.05 b.d. 0.05 0.73

82.88 0.24 8.14 4.56 b.d. 0.01 0.07 0.11 2.16 0.07 0.05 b.d. 0.08 1.37

97.96 b.d. 1.64 0.75 0,01 b.d. 0.10 b.d. 0.41 0.07 0.06 0.01 0.09 0.51

71.08 0.33 15.51 6.47 b.d. 0.12 b.d. 0.26 4.67 b.d. 0.05 b.d. 0.09 2.15

87.46 0.11 5.33 4.14 0.01 0.57 0.06 b.d. 1.21 0.18 0.08 0.01 0.08 1.06

91.59 0.07 4.35 2.42 b.d. 0.02 0.10 0.60 0.70 0.09 0.07 b.d. 0.14 0.90

87.69 0.16 6.08 3.47 0.01 0.13 0.05 0.11 1.57 0.09 0.06 b.d. 0.11 1.15

101.10

100.81

100.65

100.27

100.27

99.75

101.59

100.24

101.03

100.68

100.73

An Early Proterozoic braid-delta system in the Pretoria Group, Transvaal Sequence, South Africa

/8:

K20

!

i

4.719

./

/ o

3.617 -

/ g,ey..,oc ke

119

i

Iog w t % N a 2 0

Quodz ofen~e

wt%K20

2.515 -i omose

,~

1.413

•

•

•

0.311

.148

8,450

15,752 A1203

3 log wt%

SiO 2

Fig. 10. Correlation: KaO by AI203 for Dwaal Heuwel Formation sandstones.

wt% AI203

AI203

Fig. 9. Na~O/K20 - SiOa/AI,O3binary diagram for Dwaa] Heuwel Formation sandstones (fields aRer Pett~ohn et al., 1972).

PALAEOENVIRONMENTAL SETTING

Conglomerate and conglomeratic sandstone lacking b e d d i n g or imbrication in the n o r t h e r n part of t h e Dwaal Heuwel outcrop area (Figs 1-6) are c o n s i s t e n t with gravity-flow deposits on proximal to medial r e a c h e s of alluvial fans (Bull, 1972; Nflsen, 1982), which m a y have fed a s a n d y b r a i d e d river s y s t e m to the South. Chaotic conglomerate also occurs in glacial deposits, b u t the lack of faceting or glacial s c r a t c h m a r k s on the pebbles, a n d the lobate geometry of the Dwaal Heuwel conglomerates mitigate against s u c h a n origin; m a r i n e conglomerates c o m m o n l y are matrix-poor and well-sorted, as well as widespread. Alternatively, the coarse lithofacies of t h e Dwaal Heuwel F o r m a t i o n m a y represent proximal braided river deposits, although coarse clastic sedim e n t s of this depositional environment are m o r e c o m m o n l y clast-supported, a n d elongated pebbles are oriented with their long axes transverse to flow direction (Cant, 1982). The rapid vertical facies c h a n g e s from proposed debris flow to water-laid deposits (Fig. 6) are characteristic of alluvial fans (Nilsen, 1982; F u e c h t b a u e r , 1988). Alternating t h i n b e d s of conglomeratic a n d fine- to m e d i u m grained c r o s s - b e d d e d s a n d s t o n e form upwardfining cycles of t h e order of several m e t r e s (Fig. 6), which is compatible with deposition adjacent to a source area with relatively low rates of uplift (Steel, 1977). Proximal Dwaal Heuwel alluvial fan deposits probably were eroded, a n d redeposited on a braid-

CaO+Na20

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Fig. I I. Al203 - CaO + Na,O - K20 diagram for immature Dwaal Heuwel Formation sandstones (after Nesbltt and Young, 1989).

plain a n d in a distal braid-delta. Sedimentary features of t h e Dwaal Heuwel F o r m a t i o n sandstones, s u c h as lateral a n d vertical facies variability, p r e d o m i n a n t p l a n a r a n d m i n o r t r o u g h crossstratification (Figs 2-6), small- to m e d i u m - s i z e d channel-fills (Fig. 4), a n d t h i n pebbly a n d m u d d y interbeds (Figs 2-6), are compatible with deposition by a n extensive braided river s y s t e m (Cant, 1982), or, alternatively, on a "wet" alluvial fan (Nilsen, 1982). The characteristic features of these two depositional e n v i r o n m e n t s are very similar, and, indeed, the latter is considered a braided alluvial plain by a n u m b e r of a u t h o r s (Rust, 1978, 1979; Rust a n d Koster, 1984; Fraser a n d S u t t n e r , 1986; Nemec

120

U. M. SCHREIBERand P. G. ERIKSSON

a n d Steel, 19881. On fluvial braidplains sheetsandstones and wedge-shaped channel and bar s a n d s t o n e s develop t h r o u g h lateral migration of t h e b r a i d e d river, w i t h t h i n , i m p e r s i s t e n t m u d r o c k s being laid down d u r i n g flood stages (Campbell, 1976); t h i n pebbly interbeds (Figs 3, 4) are c o m m o n l y deposited on longitudinal bars (Miall, 1977). During times of decreased discharge a n d subaerial exposure, desiccation s t r u c t u r e s form on braid bars. Planar a n d t r o u g h cross-beds, c o m m o n in t h e Dwaal Heuwel s a n d s t o n e s , are typical o f s a n d y b r a i d e d rivers (Miall, 1977); accent u a t i o n of foresets by heavy minerals probably reflects reworking by run-off, a n d m a y also occur on d i s t a l alluvial f a n s a n d in b r a i d - d e l t a s (McGowen, 1979). Foreset inclination angles of 15 - 20 °, p r e s e n t in the Dwaal Heuwel Formation, frequently are f o u n d where a river carrying abund a n t bedload d e b o u c h e s into quieter water to create a delta with relatively steep depositional slopes (Collinson a n d T h o m p s o n , 1982). The welldeveloped cyclicity of t h e Dwaal Heuwel sands t o n e s (Figs 2-6) is also compatible with deposition by low sinuosity s t r e a m s (Allen, 1965). The Dwaal Heuwel cross-strata c o m m o n l y show southerly to westerly orientations, b u t a rather wider s p r e a d of p a l a e o c u r r e n t s exists locally (Fig. 8); widespread p a l a e o c u r r e n t dispersal frequently occurs on b a r m a r g i n s (Rust, 1978), a n d bimodal directions, outside the tidal regime, are described from fluvial e n v i r o n m e n t s by Alam et al. ( 1985); t h e y m a y also occur at the d o w n s t r e a m end of s a n d b a r s (Fuechtbauer, 1988). Planar crossbedding, as prevalent in the Dwaal Heuwel sandstones, m a y show a high directional variation in braided rivers, a n d m a y be oriented transverse to flow direction on c r o s s - c h a n n e l b a r s (Cant a n d Walker, 1978). B u t t o n (1975) who m e a s u r e d over 800 cross-bed foresets t h r o u g h o u t the outcrop area of the Dwaal Heuwel F o r m a t i o n , also f o u n d p r e d o m i n a n t l y southerly to westerly orientations; his remarkably low variances (average 2400) are supportive of a fluvial/deltaic p a l a e o e n v i r o n m e n t (variance of 4000-6000) (Pettijohn a n d Potter, 1964) where the downslope c u r r e n t is prevalent. S e d i m e n t transport from the Northeast to the S o u t h w e s t d u r i n g the deposition of the Dwaal Heuwel Formation is also indicated by a n overall decrease in grain size in t h a t direction, as well as by a decrease in sedim e n t t h i c k n e s s (Figs 1-6). O v e r t u r n e d p l a n a r foresets, observed locally in the Dwaal Heuwel Formation, m a y reflect water flow in the s e d i m e n t after rapid deposition in braided deltas (Collinson a n d T h o m p s o n , 1982). Localized asymmetrical a n d symmetrical ripple m a r k s m a y have formed on t h e topographically higher b a r s a n d floodplains, where absence of high

velocity flows facilitated preservation (Cant, 1982). Relatively m a t u r e , fine- to m e d i u m - g r a i n e d s a n d s t o n e s at t h e top of t h e formation in t h e S o u t h of the s t u d y area, probably formed t h r o u g h reworking by c h a n n e l c u r r e n t s a n d waves in t h e distal braid-delta (Reineckand Singh, 1980; Fuechtbauer, 1988). Basinal reworking m a y also explain the a p p a r e n t d o m i n a n c e of wave-formed ripple m a r k s over current-formed ones. Small-scale ladderback ripple m a r k s in the t r o u g h s of larger sets probably formed during emergence (Collinson and T h o m p s o n , 1982) on the braid-delta. Over eighty per cent of t h e Dwaal Heuwel sands t o n e s classify as s u b l i t h a r e n i t e s , feldspathic wackes a n d lithic w a c k e s (Fig. 7), which is in accordance with s a n d y b r a i d e d river deposit (Miall, 1977). In the geochemical plot of m a j o r element ratios (Fig. 9), arkoses a n d s u b a r k o s e s a p p e a r to be more i m p o r t a n t t h a n lithic s a n d s t o n e s , possibly as a result of reworking: feldspar present in the original s e d i m e n t was b r o k e n down during s u b s e q u e n t t r a n s p o r t a n d resedimentation. As is characteristic of oxygenated deposits, reddish to brownish colours are c o m m o n in the Dwaal Heuwel Formation. Localized c o n c e n t r a t i o n s of pyrite are atypical of m o d e m fluvial e n v i r o n m e n t s , b u t m a y form at the base of a b a n d o n e d c h a n n e l s in both the braidplain a n d braid delta setting, where s t a g n a n t conditions prevail (Coleman a n d Prior, 1982). In addition, anoxic to weakly oxidizing conditions are t h o u g h t to have existed d u r i n g the Early Proterozoic (Fischer, 1965). The l a m i n a t e d to thinly b e d d e d m u d r o c k facies of the Dwaal Heuwel F o r m a t i o n points to low energy, s u b a q u e o u s deposition. The a p p a r e n t restriction of these m u d r o c k s to t h e s o u t h e r n , distal part of the p r o p o s e d braid-delta system, indicates s u s p e n s i o n settling in an adjacent basin. M u d c r a c k s suggest deposition n e a r the air-water interface a n d recurring subaerial exposure, whereas locally observed wavy b e d d i n g p l a n e s point to the presence of w e a k d i s p e r s a l - c u r r e n t s in t h e depository. Alternatively, t h e Dwaal Heuwel m u d rocks m a y r e p r e s e n t times of increased discharge a n d the development of extensive floodplains within the braid-delta. Extensive braid-delta systems, fed by proximal coalescing alluvial fans a n d entering a distal basin, are t h u s proposed here as t h e depositional env i r o n m e n t of the Early Proterozoic Dwaal Heuwel Formation. The proposed braidplains a n d distal braid-deltas a p p a r e n t l y h a d d i m e n s i o n s of 100200 k m at their widest p a r t s (Fig. 12). Detritus to form the proximal fans was supplied by an uplifted hinterland, situated to t h e n o r t h a n d n o r t h e a s t of the s t u d y area, possibly along the Limpopo Mobile Belt. C o m p a r e d to their great extent, the proposed b r a i d p l a i n / b r a i d - d e l t a deposits display relatively

An Early Proterozoic braid-delta system in the Pretoria Group, Transvaal Sequence, South Africa

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senting marine sheetsands, with upward-fining and upward-coarsening s e d i m e n t a r y cycles reflecting altemating periods of marine transgression and regression, or delta progradation. Sedimentary structures, such as p r e d o m i n a n t planar and minor trough cross-stratification, a n d the presence of desiccation features in the interbedded mudrocks, are compatible with this depositional setting, but a southerly thinning of the Dwaal Heuwel Formation by some 60 %, accompanied by significant lateral facies c h a n g e s (Figs 1-6), does not agree with the sheet-like n a t u r e of shallow marine sands. The thinning of both the Dwaal Heuwel Formation and the correlated Droogedal Formation towards the Pretoria region in the center of the Transvaal b a s i n , and their absence in the latter area (Fig. 12), also does not support widespread marine sedimentation. Moreover, conglomeratic interbeds in the North of the Dwaal Heuwel outcrop area (Fig. 6) often exceed the thickness of conglomerates typically found marking the base of marine transgressional cycles, while their poor sorting and high m u d content are not compatible with the textural and compositional m a t u r i t y of shallow marine deposits (Fuechtbauer, 1988). REGIONAL

RELATIONSHIPS

The Dwaal Heuwel a n d Droogedal Formations rest unconformably u p o n the Hekpoort volcanic rocks (Table I), which thin noticeably both to the North and to the East of the Transvaal basin

U.M. SCHREIBERand P. G. ERIKSSON

122

(Fig. 13). They are correlated with the Ongeluk lavas of the n o r t h w e s t e r n Cape Province, which are between 400 a n d 900 m thick (SACS, 1980). Considering the widespread o c c u r r e n c e of this volcanic unit over about 100.000 l~n 2, the conspicuous northeasterly decrease in thickness observed in the Transvaal basin is t h o u g h t to be of a secondary nature. Uplift a n d erosion of the n o r t h e r n and eastern basin m a r g i n s m a y have r e d u c e d the andesites to as little as one tenth of their original thickness. In support of this postulate is the presence of a thin 2 to 5 m thick palaeosol at the top of the lavas t h r o u g h o u t wide areas of the Transvaal (Von Backstroem, 1960; Liebenberg, 1961; Button, 1973). Uplift of the north e m and eastern margins of the T r a n s v a a l basin, in addition to eroding the Hekpoort volcanic rocks, m a y at the same time have created upraised provenance regions from which the succeeding Dwaal Heuwel and Droogedal braid-delta s y s t e m s were fed. An isopach map of the two correlated formations (Fig. 14) shows a southerly thinning, both in the eastern and the w e s t e m part of the Transvaal basin, t h u s supporting an overall source area situated to the North {Fig. 12), possibly related to the Limpopo Mobile Belt between n o r t h e r n South Africa and s o u t h e r n Zimbabwe. The Hekpoort andesite m a y also have c o n t r i b u t e d d e t r i t u s to the Dwaal Heuwel Formation, as biotite, a c o m m o n constit u e n t of volcanic rocks, is e n c o u n t e r e d locally in the sandstone.

The Dwaal Heuwel/Droogedal braidplain/braiddelta deposits are in t u r n overlain by Strubenkop Formation m u d r o c k s (Table I), which are thickest (70 m) in the South of the depository, and thin in the direction of the proposed braidplalns and braid-deltas. It is suggested t h a t they represent the basinal equivalent of the more proximal, predominantly a r e n a c e o u s Dwaal Heuwel/Droogedal FormaUons. SUMMARY The lateral a n d vertical facies variability of the Dwaal Heuwel Formation, c o m b i n e d with the textural and compositional i m m a t u r i t y of the predominant sandstone, and characteristic sedimentary s t r u c t u r e s (planar and m i n o r trough crossbedding, small- and medium-scale channel-fills) are compatible with an extensive braidplain/braiddelta system, with proximal alluvial fan deposits in the North of the s t u d y area and basinal m u d r o c k s in the South. Predominant cross-bedded sandstone probably was laid down on fluvial braidplains, which covered large parts of the eastern part of the Transvaal basin (Fig. 12). Reworking through waves and c h a n n e l c u r r e n t s in a distal braid-delta setting is t h o u g h t to have removed primary feldspar present in the sediment and produced m a t u r e arenites; reworking m a y also have generated wave ripple marks, not commonly a s s o c i a t e d w i t h fluvial e n v i r o n m e n t s . T h e Droogedal Formation, the equivalent of the Dwaal

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An Early Proterozoic braid-delta system in the Pretoria Group, Transvaal Sequence, South Africa DWAALHEUWEL/DROOGEDAL

123

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Council for Scientific and Industrial Research and the University of Pretoria for financial support. Mrs. M.

Geringer and Mrs. M. van Leeuwen are acknowledged for drafting the figures. REFERENCES

Alam, N. M., Crook, K. A. W. and Taylor, G. 1985. Fluvial herringbone cross-stratification in a modem tributary mouth-bar, Coonamble, New South Wales, Australia. Sedimentology 32, 235-244. Allen, J. R. L. 1965. Late Quaternary Niger delta and adjacent areas: Sedimentary environments and lithofacies. Bull. Am. Assoc. Petrol. Geol. 49, 547600. Bull, W. B. 1972. Recognition of alluvial fan deposits in the stratigraphic record. In: Recognition o f Anclent Sedimentary Environments (Edited by Rigby, J. K. and Hamblin, W. K.), 63-83. Soc. Econ. Palaeont. Mineral, Tulsa, Oklahoma, U.S.A. Burger, A. J. and Coertze, F. J. 1973-1974. Age determinations - April 1972 to March 1974. Ann. Geol. Surv. S. Afr. 10, 135-141. Button, A. 1973. A regional s t u d y of the stratigraphy and development o f the Transvaal Basin in the eastern and northeastern Transvaal. Ph.D. thesis (unpubl.), Univ. Witwatersrand, Johannesburg, South Africa. Button, A. 1975. A palaeocurrent study of the Dwaal Heuwel Formation, Transvaal Supergroup. Trans. Soc, S. Aft. 78, 173-178. Button, A. 1979. Early Proterozoic weathering profile on the 2200 M.Y. old Hekpoort Basalt, Pretoria Group, South Africa: Preliminary results. Inform. Circ. Econ. Geol. R e s Unit, 133, Univ. of Witwatersrand, Johannesburg, South Africa.

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