THE NORTH-WESTERN FREE STATE
GOLDFIELD
G. fEElNGA
THE NORTH-WESTERN FREE STATE
GOLDFIELD
CONTRIBUTION TO THE GEOLOGICAL INTERPRETATION OF THE
RECENTLY DISCOVERED GOLDFIELD IN THE NORTH-WEST OF
THE ORANGE FREE STATE; WITH SPECIAL REFERENCE TO
1. STRUCTURE
2. KIMBERLEY SERIES
PROEFSCHRIFT
TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAP AAN DE TECHNISCHE HOGESCHOOL TE DELFT, OP GEZAG VAN DE RECTOR MAGNIFICUS Dr O. BOTTEMA, HOOGLERAAR IN DE AFDELING DER ALGEMENE WETENSCHAPPEN, VOOR EEN COMMISSIE UIT DE SENAAT TE VERDEDIGEN OP WOENSDAG 13 OCTOBER 1954, DES NAMIDDAGS
TE 2 UUR
DOOR
GERRIT FERINGA
MIJNINGENIEUR
GEBOREN TE ABCOUDE-BAAMBRUGGE
UITGEVERIJ EXCELSIOR — Oranjeplein 96 — 's-GRAVENHAGE
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Dit proefschrift is goedgekeurd door de
p r o m o t o r prof. dr i r F . J, F a b e r
AAN M I J N OUDERS AAN M I J N VROUW
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L E V E N S L O O P G E R R I T F E R I N G A w e r d g e b o r e n de 2 6 s t e Mei 1902 te A b c o u d e -B a a m b r u g g e ( N e d e r l a n d ) . Na h e t d o o r l o p e n v a n de L a g e r e S c h o o l en de 5 j a r i g e H o g e r e B u r -g e r S c h o o l te A m s t e r d a m s t u d e e r d e hij a a n de T e c h n i s c h e H o -g e s c h o o l t e Delftj w a a r hij i n J a n u a r i 1927 g e d i p l o m e e r d w e r d a l s M i j n i n g e n i e u r .
V a n 1927 tot e i n d e 1935 w a s hij i n d i e n s t van de Koninklijke S h e l l G r o e p , i n I n d o n e s i ë en R o e m e n i S , S e d e r t 1936 i s hij w e r k z a a n n bij de g o u d i n d u s t r i e i n Z u i d - A f r i k a , w a a r h i j v a n a i 1949 i n functie i s bij de F r e e S t a t e D e v e l o p m e n t a n d I n v e s t m e n t C o r p o r a t i o n Ltd. v o o r de e x -p l o r a t i e n a a r goud.
CONTENTS
P A R T I GEOLOGY AND STRUCTURE
p a g e I I N T R O D U C T I O N A H i s t o r i c a l R e s u m e 1 B G e o l o g i c a l K n o w l e d g e 2 C S t r a t i g r a p h i c a l D e s c r i p t i o n of t h e f o r n n a t i o n s o c c u r r i n g on t h e R a n d p r o p e r 4 D D i s c o n f o r m i t i e s a n d U n c o n f o r m i t i e s 7 II S T R A T I G R A P H Y O F THE N O R T H - W E S T F R E E S T A T E G O L D F I E L D A I n t r o d u c t i o n 8 B S t r a t i g r a p h i c a l D e s c r i p t i o n 10 C E c o n o m i c H o r i z o n s 17 III D R I L L I N G R E S U L T S A I n d i v i d u a l B o r e h o l e D a t a 19 B S t r u c t u r a l I n t e r p r e t a t i o n of D r i l l i n g R e s u l t s 19 C D i s c u s s i o n of t h e I n d i v i d u a l S t r u c t u r a l U n i t s 25 r v T E C T O N I C A L STUDY AND I N T E R P R E T A T I O N O F THE
N O R T H - W E S T E R N F R E E S T A T E G O L D F I E L D A G e o l o g i c a l H i s t o r y of the A r e a 28 B The V i r g i n i a F a u l t 32 V I N F L U E N C E O F T E C T O N I C S ON E C O N O M I C P O S S I B I L I T I E S 34 T A B L E S P A R T 2 THE K I M B E R L E Y S E R I E S I I N T R O D U C T I O N II P R E V I O U S WORK III D E S C R I P T I O N O F THE K I M B E R L E Y S E R I E S A E x i s t i n g c l a s s i f i c a t i o n 37 B D e f i c i e n c i e s of e x i s t i n g c l a s s i f i c a t i o n 38 C R e - z o n i n g of the K i m b e r l e y q u a r t z i t e - c o n g l o m e r a t e f o r m a t i o n 39 IV I N T E R P R E T A T I O N O F B O R E H O L E R E S U L T S A The o c c u r r e n c e of t h e A Reef H o r i z o n 42 B The m i n e r a l i s e d t o p c o n g l o m e r a t e s of Z o n e E , C. 2 45 C The b o t t o m c o n g l o m e r a t e s of Z o n e E . C, 2 45 D D i s c o r d a n t c h a r a c t e r of Z o n e E , C, 2 45 E S t r a t i g r a p h i c a l c o n s i d e r a t i o n s f o r Z o n e E . C, 2 47 F E v i d e n c e of e r o s i o n p r i o r t o the d e p o s i t i o n of the E l s b u r g S e r i e s 48
page
V CORRELATION AND PROPOSED REVISED SUBDIVISION 51 VI GEOLOGICAL HISTORY OF THE KIMBERLEY SERIES 52 VII ECONOMIC POSSIBILITIES OF THE KIMBERLEY SERIES 53 V n i THE KIMBERLEY SERIES IN THE SOUTHERN AREA
A V i r g i n i a - M e r r i e s p r u i t A r e a 54
B G. F , block 56 IX COMPARISON BETWEEN THE ODENDAALSRUS AREA 61
AND THE EAST RAND AREA SUMMARY SAMENVATTING REFERENCES • ENCLOSURES la, b, c, d LIST OF ILLUSTRATIONS fig. fig. fig. fig. fig. fig. fig-fig. fig. fig-fig. fig-fig. fig. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 Operational A r e a s opp. p. 1 Geological Column of the Witwatersrand p. 3 Geological Column of the N. W. F r e e State p. 9 Keyplan p. 18 Sections through the Odendaalsrus Goldfield p, 20 Borehole r e s u l t s in Unit V p. 22 Contourplan Basal Reef p. 23 Countourplan L e a d e r - B a s a l p. 24 Sketch p. 31 Geological Column Kimberley S e r i e s p. 39 Sections showing Disconformable C h a r a c t e r of the A Reef p. 43 Plan of A Reef Area n e a r Odendaalsrus p. 44 The A Reef Dis conformity in F r , N, Sh, 2 p. 45 Sections showing Disconformable C h a r a c t e r of Zone E. C. 2 p, 47 Sections showing Disconformable C h a r a c t e r of Zone V, S. 5 p. 49 Sketch showing
1 Incomplete peneplanation of the A Reef footwall 2 Abornnal development of Zone V, S. 5 in erosion
channel p . 4 9 fig. 17 Channel in Virginia and G, F . A r e a s p. 55
fig. 18 Sedimentation conditions in G, F . A r e a p. 57 fig. 19 Disconformities in G, F . A r e a p. 60 fig, 20 Kimberley S e r i e s in N. W. F r e e s t a t e and in E a s t Rand p. 62
O P E R A T I O N A L A R E A S of t h e G R E A T E R W I T W A T E R S R A N D
o lO JO 3 0 I 1 1 I
PART I GEOLOGY AND STRUCTURE
I . I N T R O D U C T I O N A. H i s t o r i c a l R e s u m e
The d i s c o v e r y of gold i n t h e l a t t e r half of t h e 19th C e n t u r y i n the f o r m a t i o n o u t -c r o p p i n g on t h e W i t w a t e r s r a n d , and the s u b s e q u e n t d e v e l o p m e n t of the gold m i n i n g i n d u s t r y in t h i s a r e a , r e v e é i l e d t h e e c o n o m i c i m p o r t a n c e of t h e s e f o r m a t i o n s , w h i c h b e c a m e known u n d e r t h e n a m e W i t w a t e r s r a n d B e d s , The b e d s w e r e t r a c e d at t h e s u r f a c e f r o m t h e o r i g i n a l d i s c o v e r y s i t e s n e a r J o h a n n e s b u r g to K r u g e r s -d o r p an-d b e y o n -d in the W e s t , an-d to B o k s b u r g in the E a s t (fig. 1), At t h e s e p o i n t s , y o u n g e r f o r m a t i o n s t a r t e d to c o v e r t h e W i t w a t e r s r a n d B e d s , and t h e c o n c l u s i o n w a s a r r i v e d at t h a t t h e W i t w a t e r s r a n d B e d s m i g h t p o s s i b l y e x t e n d f u r t h e r E a s t and W e s t t h a n w a s o r i g i n a l l y e x p e c t e d b y the p r o s p e c t o r s who d i s c o v e r e d t h e g o l d -field, T h i s p r o v e d to be c o r r e c t ; the F a r E a s t Rand w a s o p e n e d up, b r i n g i n g t h e known E a s t e r n l i m i t of t h e m i n i n g a r e a to S p r i n g s - Nigel, t h u s l i n k i n g up w i t h the H e i d e l b e r g a r e a w h e r e W i t w a t e r s r a n d B e d s a r e o u t c r o p p i n g a g a i n ; a n d i n t e n -s i v e e x p l o r a t o r y w o r k i n a d i r e c t i o n W e -s t f r o m R a n d f o n t e i n w a -s -s t a r t e d , in w h i c h m o d e r n g e o p h y s i c a l m e t h o d s w e r e u s e d : The f a c t t h a t s h a l e s c o n t a i n i n g m a g n e t i t e w e r e known to o c c u r in t h e L o w e r W i t w a t e r s r a n d B e d s l e d to the u s e of the r n a g -n e t o m e t e r i -n t h e i -n v e s t i g a t i o -n . T h i s w o r k w a s f i -n a l l y c r o w -n e d w i t h s u c c e s s ; a f t e r m a g n e t o m e t r i c a l p r o s p e c t i n g i n d i c a t e d the p o s s i b i l i t y of W i t s w a t e r s r a n d B e d s b e i n g p r e s e n t i n t h e s o - c a l l e d F a r W e s t Rand, s u b s e q u e n t d r i l l i n g p r o v e d t h i s to be the c a s e , a n d t h e We s t e r n l i m i t of t h e g o l d f i e l d w a s e x t e n d e d up t o B l y v o o r u i t -s i c h t . The f a v o u r a b l e r e s u l t s o b t a i n e d w e r e i n d u c i v e t o f u r t h e r w o r k ; m o r e o v e r i s o l a t -ed o c c u r r e n c e s of W i t w a t e r s r a n d B e d s s u r r o u n d e d by y o u n g e r f o r m a t i o n g a v e h o p e t h a t i t w o u l d be p o s s i b l e to find a d d i t i o n a l g r o u n d u n d e r l a i n by a u r i f e r o u s f o r m a t i o n , m a s k e d b y t h e o v e r l y i n g y o u n g e r S y s t e m s . Tlie u s e of g e o p h y s i c a l p r o s p e c t -ing m e t h o d s w a s t h e r e f o r e i n t e n s i f i e d , a n d e x p l o r a t o r y d r i l l i n g i n t h e K l e r k s d o r p d i s t r i c t p r o v e d t h e a s s u m p t i o n to be c o r r e c t . P r o s p e c t i n g w a s a l s o c a r r i e d on in t h e O r a n g e F r e e S t a t e , and t h e l o c a t i n g of the i m p o r t a n t new O d e n d a a l s r u s G o l d f i e l d i s d i r e c t l y a t t r i b u t a b l e to the u s e of g e o -p h y s i c a l m e t h o d s of i n v e s t i g a t i o n . Not o n l y m a g n e t o m e t r i c a l w o r k w a s done i n t h i s a r e a , but a l s o g r a v i m e t r i c a l a n d t o r s i o n b a l a n c e s u r v e y s w e r e c a r r i e d out, the l a t t e r m a k i n g u s e of t h e d i f f e r e n c e in s p e c i f i c g r a v i t y b e t w e e n t h e l i g h t e r U p p e r W i t w a t e r s r a n d B e d s a n d t h e h e a v i e r p o s t W i t w a t e r s r a n d f o r m a t i o n s . The f o l l o w -ing s p e c i f i c g r a v i t i e s a r e g i v e n by B o r c h e r s a n d White (1943) p, 131 and by F r o s t , M c j n t y r e , P a p e n f u s a n d W e i s s (1946) p, 8. K a r r o o s a n d s t o n e s K a r r o o s h a l e s K a r r o o d o l e r i t e V e n t e r s d o r p l a v a V e n t e r s d o r p l a v a V e n t e r s d o r p c o n g l o m e r a t e s V e n t e r s d o r p q u a r t z i t e s V e n t e r s d o r p s e d i m e n t s E l s b u r g q u a r t z i t e s W i t w a t e r s r a n d q u a r t z i t e s W i t w a t e r s r a n d s h a l e s B. & W. -2 , 9 8 2 . 8 6 -2 . 8 1 2 . 6 8 -2 . 6 5 -F r o s t et al 2 . 3 2 . 4 2 , 9 8 2. 81 ( b a s i c ) 2. 68 ( a c i d ) -2 . 7 6 ; -2 . 7 0 -2 . 6 5 2 . 8 0 The c o m b i n a t i o n of t h e two m e t h o d s w a s r e m a r k a b l y s u c c e s s f u l . S o m e of t h e r e -s u l t -s of g e o p h y -s i c a l p r o -s p e c t i n g in the N. W, F r e e S t a t e c a n be found in the above p u b l i c a t i o n s , i, e. B o r c h e r s a n d White (1943) P l a t e s XXIX a n d XXX; F r o s t et al (1946) P l a t e s III, IV a n d V, The r e s u l t s o b t a i n e d by p r o s p e c t i n g a n d s u b s e q u e n t e x p l o r a t o r y d r i l l i n g i n t h e N. W. O r a n g e F r e e S t a t e i n d u c e d t h e v a r i o u s m i n i n g h o u s e s t o o p e n t h i r t e e n l a r g e new m i n e s , t h e m a j o r i t y of w h i c h a r e at p r e s e n t in
B G e o l o g i c a l K n o w l e d g e
T h e f i r s t s y s t e m a t i c d e s c r i p t i o n a n d c l a s s i f i c a t i o n of the fornnations found on the W i t w a t e r s r a n d w a s given by M o l e n g r a a f f (1904),
A d d i t i o n a l f i e l d w o r k s u b s e q u e n t to h i s p u b l i c a t i o n , t o g e t h e r w i t h d a t a a c c u n n u l a t - •* ed in t h e c o u r s e of e x p l o i t a t i o n of t h e goldfield, h a s r e s u l t e d in a d e t a i l e d c l a s s i f i
-c a t i o n of t h e f o r m a t i o n s found in t h e W i t w a t e r s r a n d d i s t r i -c t s . T h i s -c l a s s i f i -c a t i o n , b a s e d on t h e w o r k of M e l l o r (1917), who nnapped the W i t w a t e r s r a n d p r o p e r , and of o t h e r s , is a t p r e s e n t in g e n e r a l u s e by the G o v e r n m e n t G e o l o g i c a l S u r v e y and by t h e g e o l o g i c a l d e p a r t m e n t s of t h e Gold m i n i n g i n d u s t r y .
T h e f o r m a t i o n s a r e c l a s s i f i e d i n t o the following S y s t e n n s , w h i c h a r e given below in a s c e n d i n g o r d e r , I. S w a z i l a n d S y s t e m . II. W i t w a t e r s r a n d S y s t e m , w h i c h a g a i n i s s u b d i v i d e d into a. The L o w e r W i t w a t e r s r a n d D i v i s i o n . b . The U p p e r W i t w a t e r s r a n d D i v i s i o n . III. V e n t e r s d o r p S y s t e m . IV. T r a n s v a a l S y s t e m .
V. K a r r o o S y s t e m (which c o v e r s a l a r g e p a r t of the Cape P r i v i n c e and of t h e T r a n s v a a l , and n e a r l y all the F r e e S t a t e ) .
A s u m m a r y of the s t r a t i g r a p h y of t h e R a n d and i t s e x t e n s i o n s w i l l b e given in t h e following, t o g e t h e r w i t h a s h o r t d e s c r i p t i o n of f o r n n a t i o n s not found on t h e R a n d p r o p e r o r in i t s i m m e d i a t e v i c i n i t y , but b e l o n g i n g to t h e S y s t e m s m e n t i o n e d and found in o t h e r p a r t s of t h e Union w h e r e d e v e l o p m e n t of the Systenns w a s m o r e c o m -p l e t e .
T h e o r i g i n of t h e g o l d h a s b e e n t h e s u b j e c t of s e v e r a l s t u d i e s , w h i c h r e -s u l t e d in t h e following t h e o r i e -s .
1. H y d r o t h e r m a l t h e o r y ,
T h e gold h a s b e e n i n t r o d u c e d i n t o the c o n g l o m e r a t e s a f t e r the d e p o s i t i o n * t h e r e o f , by h y d r o t h e r i n a l a c t i o n w h i c h i s b e l i e v e d to o r i g i n a t e f r o m a d e e p s e a t e d y o u n g e r g r a n i t e body w h i c h h a s not b e e n l o c a t e d , 2. P r e c i p i t a t i o n t h e o r y . T h e gold h a s b e e n p r e c i p i t a t e d t o g e t h e r w i t h the o t h e r m i n e r a l s f r o m t h e w a t e r s in w h i c h t h e c o n g l o n n e r a t e s w e r e d e p o s i t e d . T h i s t h e o r y h a s now b e e n d i s c a r d e d . 3 . P l a c e r t h e o r y .
T h e gold i s s y n g e n e t i c w i t h the c o n g l o m e r a t e in w h i c h it i s found, and w a s d e p o s i t e d a s d e t r i t a l gold d u r i n g t h e f o r m i n g of t h e c o n g l o m e r a t e s . S u b s e q u e n t l y , s u l p h u r e t t e d h y d r o t h e r m a l w a t e r s d i s s o l v e d the gold, c o n v e r t -e d t h -e i r o n o x i d -e s i n t o i r o n s u l p h i d -e s a n d r -e d -e p o s i t -e d b o t h t h -e gold a n d t h e s u l p h i d e s m o r e or l e s s in s i t u .
T h e t h e o r i e s g i v e n a b o v e h a v e b e e n c r i t i c i z e d a s n o t t a k i n g i n t o c o n s i d e r a t i o n a l l f a c t o r s found in s t u d y i n g the gold o c c u r r e n c e in the " b a n k e t s " or c o n g l o m e r a t -e s . M u c h w o r k h a s r -e c -e n t l y b -e -e n u n d -e r t a k -e n t o o b t a i n a b -e t t -e r i n s i g h t into t h -e p r o b l e m s w h i c h p r e s e n t t h e m s e l v e s w h e n s t u d y i n g t h e d i s t r i b u t i o n of the gold in t h e v a r i o u s h o r i z o n s . In t h i s c o n n e c t i o n 1 h a v e to m e n t i o n t h e w o r k done by S h a r p e (1949), w h o s e a p p l i c a t i o n of t h e " c y c l o t h e m " t h e o r y t h r o w s an e n t i r e l y new l i g h t on t h i s i n t e r e s t i n g s u b j e c t . It f a l l s b e y o n d t h e s c o p e of t h i s t h e s i s , h o w e v e r , to d i s c u s s t h e m e r i t s a n d d i s -a d v -a n t -a g e s of t h e t h e o r i e s d e -a l i n g w i t h t h e o r i g i n -a n d d i s t r i b u t i o n of t h e gold in t h e W i t w a t e r s r a n d . F o r t h o s e who w a n t to s t u d y t h i s q u e s t i o n m o r e fully 1 s h o u l d l i k e to r e f e r to t h e v a r i o u s p u b l i c a t i o n s of w h i c h t h e following m u s t b e m e n t i o n e d : G r e g o r y : T h e o r i g i n of the gold in the R a n d b a n k e t . T r a n s . I n s t . M i n . & M e t ,
V o l . 17, 1 9 0 8 . Young: T h e B a n k e t , 1917.
G r a t o n : H y d r o t h e r n n a l o r i g i n of the R a n d G o l d D e p o s i t s . Suppl. to No 3 . E c o n o m i c G e o l o g y , Vol. XXV, May 1930,
S h a r p e : T h e E c o n o m i c A u r i f e r o u s B a n k e t s of t h e U p p e r W i t w a t e r s r a n d B e d s a n d t h e i r R e l a t i o n s h i p to S e d i m e n t a t i o n , T r a n s . G e o l . S o c . S. Afr, , Vol, LII, 1949.
G E O L O G I C A L C O L U M N of ihe W I T W A T E R S R A N D . -i5 < u> 7 < a -2 UJ t -i / ) > w a (T O o Cfl o: U l 1 -z u > 2 UJ i -tf) > t n o z < dc a UJ 1 -< Ï h -Ï _ n o l o m i l ï Scri«i Block (tccf S c r i d « ^ » a 3 • K • -• • " a. -a ; i -E " -" ' a « ^ « oc ' U ) s '" a. - I E l i b u r g RE£f G r o u p C l i b u r g Quartiit« 20000L Croup KimMrlty • ~ r ~ .:::: " " • • • -... ^ H ' n m » * r - / | Shale* ^ 1 Uppir Croup LoW«r B i r d Group MolnMcf OuortiK* Group ISOOO-Upp«r ^ :::: .-.
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I 1 -^ Po rmot i 0 " Mogntiian Llm«ilon«i w i t h C h « r t b o B d l . J l w ü u i t A n d d l U c L o v o C o n g i o m c r o l . . i r . Q u o r t i . t t Q u o r u m O u a i - i i i i , with Congl. o m t r o t i i ond many O u a r t t i t « « i t h Pcbbk bondi , one » e « t U r « p t b b l t t S h o l d , and v«ry a r f l l l l a e f o J » O y o r t i i U -O u a r t l l t * . Conglornarolci O u o r t t i t t , w i t h Conglomerate* I n th« m i d d l i , C e n g l e m a r a t d . O u a r t i l K . WithZon« In the middle.
w i t h O u o r l i i t * b a n d i i n l c r v a n l n g
O u o r t i l t e
Argllrace«ui Ouartilta With Shall band!
Local Nome» U.K. I . f t . U.K. J. M K 1.3. L K. i . a . StrotlgraphlcQl Break) u..»,.™,., D t i l a n a l l o n ot Black Reef E l i b u r g R e i f i L I v l n g i t o n t . Johnttonc Group ^I:-'E{if"-' 1 JeppcMoim^^l S h o F O rma t lo n Amygdololdol Lava . »o_^H Q u a r t i l t e w i t h 5 h a l « ] o r t t i I w i t h Conglomtroti t o w o r d t [ h i F e r r u j i n o u t S h o l f moriOih Orange Grove O K I Pri-Wllwoterirant JcfTXttoWTi Amygaolold Coronoiion S h a l l L o w e r C o r TU Premlie Real. SOO lOOO = Ft.
C, S t r a t i g r a p h i c a l D e s c r i p t i o n of t h e f o r m a t i o n s o c c u r r i n g on the Rand p r o p e r
I. S w a z i l a n d S y s t e n n '
The s y s t e m c o n s i s t s of b a s i c i g n e o u s r o c k s , i n t e r c a l a t e d with and o v e r l a i n by fine g r a i n e d s e d i m e n t s . I n t e n s e m e t a m o r p h i s m h a s c o n v e r t e d t h e f o r m a t i o n i n t o s c h i s t o s e r o c k s . It h a s b e e n found t h a t t h e s e s c h i s t s c a r r y a c o n s i d e r a b l e a m o u n t of gold in q u a r t z v e i n s and i m p r e g n a t i o n s . The a g e of t h e S y s t e m i s c o n s i d e r e d to be A r c h a e a n ,
H e a v y e r o s i o n to w h i c h t h i s a n c i e n t S y s t e m h a s b e e n s u b j e c t e d h a s r e s u l t e d in a n a l m o s t c o m p l e t e r e m o v a l of t h e s c h i s t s i n t h e a r e a of t h e W i t w a t e r s r a n d , w h e r e o n l y o c c a s i o n a l p a t c h e s of the S w a z i l a n d S c h i s t s h a v e b e e n p r e s e r v e d . The S y s t e m i s confined to t h e p e r i p h e r y of g r a n i t e s , known a s t h e Old G r a n i t e , w h i c h i s b e l i e v e d to be i n t r u s i v e i n t o t h e S c h i s t s . The e m p l a c e m e n t of the Old G r a n i t e o c c u r r e d p r i o r to t h e d e p o s i t i o n of t h e W i t w a t e r s r a n d S y s t e m .
IL W i t w a t e r s r a n d S y s t e m '
The W i t w a t e r s r a n d b e d s c o n s i s t of a l t e r n a t i o n s of a r g i l l a c e o u s and a r e n a c e o u s s e d i m e n t s i n w h i c h c o n g l o m e r a t e s o c c u r . T h e s e c o n g l o m e r a t e s a r e a l l m o r e o r l e s s a u r i f e r o u s , a n d a r e l o c a l l y known a s , . r e e f s " , but only s o m e a r e r i c h enough t o b e of e c o n o m i c v a l u e . It m u s t b e s t a t e d t h a t c a l c a r e o u s r o c k s a r e c o n s p i c u o u s -l y a b s e n t ,
R e m a r k :
It d e p e n d s on s e v e r a l f a c t o r s w h e t h e r an a u r i f e r o u s c o n g l o m e r a t e c a n be c o n -s i d e r e d to be of e c o n o m i c v a l u e o r n o t . Gold c o n c e n t r a t i o n in t h e c o n g l o n n e r a t e , d e p t h , s t r u c t u r e , and d i m e n s i o n s of t h e reef, a s w e l l a s e x i s t i n g f a c i l i t i e s and t e r r a i n c h a r a c t e r i s t i c s , m u s t be t a k e n i n t o c o n s i d e r a t i o n .
It w o u l d a p p e a r , u n d e r p r e s e n t c i r c u m s t a n c e s , t h a t a m i n i m u m gold c o n c e n -t r a -t i o n of 3. 5 - 4, 0 dw-t p e r -t o n of 2000 l b s . o v e r a s -t o p e wid-th of 40 i n c h e s s h o u l d b e a v a i l a b l e ( 6 - 7 g r a m p e r m e t r i c t o n o v e r 1. - M s t o p e w i d t h ) b e f o r e t h e o t h e r f a c t o r s e n u m e r a t e d c a n be c o n s i d e r e d .
The b a s e of t h e S y s t e m r e s t s u n c o n f o r m a b l y on t h e Old G r a n i t e with o c c a -s i o n a l p a t c h e -s of S w a z i l a n d S c h i -s t -s , A. T h e L o w e r W i t w a t e r s r a n d D i v i s i o n c o n s i s t s of: 1. D o m i n i o n Reef S e r i e s a. D o m i n i o n Reef ( K l e r k s d o r p only). b . Q u a r t z i t e w i t h c o n g l o m e r a t e s ( K l e r k s d o r p only), c. A m y g d a l o i d a l l a v a ( K l e r k s d o r p a n d V r e d e f o r t ) , d. P o r p h y r i t i c l a v a ( K l e r k s d o r p ) , 2. H o s p i t a l Hill S e r i e s (fig. 2) a. O r a n g e G r o v e Q u a r t z i t e s , b . W a t e r T o w e r S l a t e s ( s t r o n g l y m a g n e t i c ) , c. R i p p l e m a r k Q u a r t z i t e ( C e n t r a l R a n d ) , followed by s h a l e s and s l a t e s . d. S p e c k l e d B e d s ( C e n t r a l R a n d & V r e d e f o r t ) . C o a r s e s p e c k l e d q u a r t z i t e s . e. C o n t o r t e d B e d s . Highly m a g n e t i c f e r r u g e n o u s s l a t e s . f. H o s p i t a l Hill Q u a r t z i t e s , i n w h i c h t h r e e s h a l e h o r i z o n s o c c u r . 3. G o v e r n m e n t Reef S e r i e s (fig, 2) a. P r o m i s e B e d s . T h e s e c o n s i s t of q u a r t z i t e , w i t h a t h i n c o n g l o m e r a t i c z o n e , known a s t h e P r o m i s e Reef, n e a r t h e b a s e . 1) S e e : M e l l o r (1917) p p . 19 & 24; M o l e n g r a a f f (1904) p p . 4 , 5. 2) S e e : M e l l o r (1917) p p . 2 4 - 3 1 ; M o l e n g r a a f f (1904) pp. 7 - 1 3 .
Shales have been deposited towards the middle and the top of the beds. b.' Coronation Beds
These consist of quartzite, with a strongly magnetic shale horizon in the lower portion of the upper half.
This shale horizon, the Coronation Shale, r e s t s on a tillite, the Lower Coronation Tillite.
A second tillite, the Upper Coronation Tillite, often occurs directly above the Coronation Shale.
A conglomerate, known as the Coronation Reef, occurs in the quartzite above the Coronation Shale, n e a r to the Upper Coronation Tillite. c. Government Beds
These consist of q u a r t z i t e s , with shales of considerable thickness near the top and the middle of the beds and at the base.
A conglomerate known as the Government Reef occurs in the quartzite some distance above the Middle Shale Horizon,
Another conglomeratic zone i s found some distance above the Top Shale Horizon and i s known as the Afrikander - Rietkuil Conglomerate Group. 4. Jeppestown S e r i e s (fig. 2)
These consist of argillaceous sediments with subordinated, mostly impure, q u a r t z i t e s . Three main shaly horizons a r e known, the lower one generally being magnetic, s e p a r a t e d by q u a r t z i t e s . A horizon of andesitic lava, vol-canic b r e c c i a and tuffs o c c u r s in the middle of the s e r i e s , known as the Jeppestown Amygdaloidal Lava. In the West and F a r West Rand, quartzites become m o r e pronounced; in the Klerksdorp a r e a these q u a r t z i t e s become predominant. The Upper Jeppestown Shales are overlain in the Central and West Rand by a q u a r t z i t e , which thins out and finally disappears towards the E a s t , and which is unknown on the F a r E a s t Rand,
This quartzite i s often included in the Upper Witwatersrand Division, but I am of the opinion that p r e f e r e n c e should be given to the inclusion of the q u a r t -zite in the Lower WitwatersrcUid Division, for r e a s o n s which will be given l a t e r .
B. The U p p e r W i t w a t e r s r a n d D i v i s i o n
This division c o n s i s t s almost exclusively of q u a r t z i t e s , containing g r i t s and conglomerate bands. Only one important and p e r s i s t e n t argillaceous horizon is known, the Kimberley Shale Horizon, The subdivision in g e n e r a l use i s as follows:
1, M a i n - B i r d S e r i e s (fig, 2)
Q u a r t z i t e s , containing the following conglomerate groups: (North Reef
a. Main Reef Group ^Main Reef
of conglomerates JMain Reef Leader (South Reef
b. Livingstone - Johnstone Reef Horizon. c. Bird Reef Horizon,
Immediately above the Bird Reef Horizon an amygdaloidal lava o c c u r s in the E a s t Rand and in the Heidelberg district, named Bird Amygdaloidal Lava. This igneous h o r i z o n i s r e p r e s e n t e d , m o r e towards the West, by a tuffaceous rock, named Bird Reef M a r k e r .
2. Kimberley - E l s b u r g S e r i e s (fig, 2)
The bottonn l i m b of this S e r i e s c o n s i s t s of an argillaceous zone, the division being:
a. Kimberley Shale.
b. Kimberley Q u a r t z i t e s , containing s e v e r a l conglomerates, known as the Kimberley Reefs.
c. E l s b u r g Quartzites with the E l s b u r g Conglomerates at the top. N o t e :
It i s c u s t o m a r y to include the Kimberley Shade in the K i m b e r l e y - E l s b u r g S e r i e s , However, I consider this to be i n c o r r e c t , and, in my opini'on, the Kimberley
Shale Horizon should be r e g a r d e d as the youngest fornnation of the M a i n - B i r d S e r i e s , Reasons for this i n t e r p r e t a t i o n will be given l a t e r (pages 12 and 63),
The geological column, fig. 2, h a s been drawn up in a c c o r d a n c e with this opinion.
i n . V e n t e r s d o r p S y s t e m or Pniel S e r i e s (fig. 2) '
Above the W i t w a t e r s r a n d S y s t e m , renewed volcardc activity has deposited a thick l a y e r of lava, with which the V e n t e r s d o r p System opens. This System is divided into
1. V e n t e r s d o r p Lower Lava o r K l i p r i v i e r Lava.
Mostly andesitic and amygdaloidal lava, but occasional p o r p h y r i t i c p h a s e s a r e known in the lower portion. The lava is often b r e c c i a t e d .
2. V e n t e r s d o r p Upper S e d i m e n t s .
These c o n s i s t of tuffaceous s h a l e s , q u a r t z i t e s containing pebble bands of v e r y mixed origin, conglonnerates, and i n t e r m i t t e n t lava flows. These lavas a r e also andesitic in c h a r a c t e r , and m a y contain amygdaloidal p h a s e s , The V. U. S. a r e generadly not p r e s e n t on the Rand p r o p e r , I have been in-fornned, however, that f o r m a t i o n s belonging to the V, U, S. have been found a s r e m n a n t s in m i n e s of the West Rand.
rv, T r a n s v a a l S y s t e m (fig. 2) '
This System, which c o v e r s the V e n t e r s d o r p System, c o n s i s t s of: 1, Black Reef S e r i e s .
Thin q u a r t z i t e s and c o n g l o m e r a t e s bands which a r e found to be a u r i f e r o u s . In s e v e r a l i n s t a n c e s the Black Reef i s of economic value. Towards the top of the S e r i e s s h a l e s nnaike t h e i r a p p e a r a n c e .
2, Doloinite S e r i e s , r e s t i n g on the Black Reef S e r i e s , and consisting of m a g -n e s i a -n l i m e s t o -n e with ba-nds of c h e r t s .
3, P r e t o r i a S e r i e s , consisting of q u a r t z i t e s and s h a l e s , r e p r e s e n t i n g the lower portion of the S e r i e s .
V. K a r r o o S y s t e m
This System is the l a s t one to be briefly mentioned. It c o v e r s m o s t of the
Orange F r e e State Goldfield, and the E a s t e r n Extensions of the Rand. The System opens with:
1, Dwyka S e r i e s .
This S e r i e s is of g l a c i a l origin. It is c h a r a c t e r i z e d by the well-known Dwyka Tillite, consisting of p e b b l e s , b o u l d e r s and fragments of widely varying composition; the pebbles a r e m o s t l y angular to subangular. Where the s e -r i e s is found to be st-rongly developed, shale bands and occasional thin coal l a y e r s m a y occur, the l a t t e r in the upper portions only.
2, E c c a S e r i e s ,
consisting of: Lower E c c a Shales, which a r e absent on the Rand, Middle E c c a Sandstones with shale bands, in which coal l a y e r s a r e often found, and Upper E c c a Shales,
3, Beaufort S e r i e s ,
consisting of: Sandstones with shale bands and p a r t i n g s . This S e r i e s f o r m s the youngest m e m b e r of the K a r r o o cover of the s o - c a l l e d W i t w a t e r s r a n d Basin.
1) See: Molengraaff (1904) pp. 19,20.
2) See: Mellor (1917) pp. 36, 37; Molengraaff (1904) pp. 24-39. 3) See: Mellor (1917) pp. 38, 39; Molengraaff (1904) pp. 6 1 , 6 2 , 75,
D. D i s c o n f o r m i t i e s a n d U n c o n f o r nni t i e s T h e t e r m s ,, Dis c o n f o r m i t y " a n d , , U n c o n f o r m i t y " i n t h i s t h e s i s c o r r e s p o n d w i t h t h e m e a n i n g i n w h i c h t h e y a r e u s e d in t h e f i e l d u n d e r c o n s i d e r a t i o n , a n d c a n be b e s t d e s c r i b e d a s f o l l o w s : U n c o n f o r m i t y a n d d i s c o n f o r m i t y both i n d i c a t e a b r e a k i n s e d i m e n t a t i o n , a s s o -c i a t e d with e r o s i o n . " U n -c o n f o r m i t y " i s u s e d w h e n t h e a r e a l e x t e n t of t h e b r e a k i s r e g i o n a l , and t h e a m o u n t of e r o s i o n of a c o n s i d e r a b l e m a g n i t u d e . " D i s c o n f o r -m i t y " i s u s e d w h e n t h e a r e a l e x t e n t of t h e e r o s i o n a d b r e a k i s l i -m i t e d , w h e r e a s t h e a m o u n t of e r o s i o n a s s o c i a t e d with t h e " d i s c o n f o r m i t y " n e e d not be of a c o n -s i d e r a b l e m a g n i t u d e . D i f f e r e n c e -s in dip of t h e f o r m a t i o n -s a b o v e a n d below t h e e r o s i o n a l p l a n e m a y o r m a y not o c c u r i n both i n s t a n c e s . In t h e a b o v e g i v e n s t r a t i g r a p h i c a l c l a s s i f i c a t i o n , no s p e c i a l m e n t i o n h a s b e e n m a d e of the o c c u r r e n c e of i i n c o n f o r m i t i e s a n d d i s c o n f o r m i t i e s , f o r w h i c h t h e r e i s a c o n s i d e r a b l e a m o u n t of e v i d e n c e . Well r e c o g n i s e d u n c o n f o r m i t i e s o c c u r at t h e b a s e of t h e K a r r o o S y s t e m ^), a t the b a s e of the T r a n s v a d l S y s t e m ', at the b a s e
of t h e V e n t e r s d o r p L o w e r L a v a 3 j a n d at t h e b a s e of t h e W i t w a t e r s r a n d S y s t e m . T h e r e i s e v i d e n c e of an u n c o n f o r m i t y at the b a s e of t h e V e n t e r s d o r p U p p e r S e d i -m e n t s . T h e r e i s a l s o e v i d e n c e of u n c o n f o r -m i t i e s l e s s r e g i o n a l i n c h a r a c t e r t h a n the a b o v e . As e x a m p l e s I w o u l d l i k e to m e n t i o n : a. An a n g u l a r d i s c o n f o r m i t y in the q u a r t z i t e u n d e r l y i n g t h e V e n t e r s d o r p L o w e r L a v a in t h e K l e r k s d o r p d i s t r i c t , at the b a s e of the s o - c a l l e d Big P e b b l e Z o n e ( K i m b e r l e y - E l s b u r g S e r i e s ) .
R e f e r e n c e to t h i s d i s c o n f o r m i t y i s m a d e by B a i n e s (1949) p . 306.
b . The u n c o n f o r m i t y at t h e b a s e of t h e M a i n Reef G r o u p , w h i c h t r u n c a t e s t h e q u a r t z i t e s above the J e p p e s t o w n Shades in an E a s t e r l y d i r e c t i o n on the C e n t r a l and E a s t R a n d . A t t e n t i o n to t h i s u n c o n f o r m i t y h a s b e e n d r a w n by S h a r p e (1949) p. 2 7 3 , R e m a r k : T h e l a t t e r u n c o n f o r m i t y I c o n s i d e r to be of sufficient i m p o r t a n c e to h a v e a b e a r i n g on the g e o l o g i c a l c l a s s i f i c a t i o n . A f t e r t h e J e p p e s t o w n S h a l e s w e r e l a i d down, d e p o s i t i o n c o n d i t i o n s g r a d u a l l y b e c a m e m o r e s h a l l o w , r e s u l t i n g i n q u a r t z i t e f o r m i n g s e d i m e n t s . This uplift c o n -t i n u e d and, p r i o r -to -the d e p o s i -t i o n of -the M a i n Reef G r o u p , b r o u g h -t -the s e d i m e n -t s above t h e w a t e r l e v e l . E r o s i o n r e m o v e d p o r t i o n of the q u a r t z i t e in the C e n t r a l a n d W e s t e r n p a r t of the Rand a n d e v e n u n c o v e r e d the J e p p e s t o w n S h a l e s in t h e E a s t R a n d .
It i s t h e r e f o r e c l e a r t h a t a s t r a t i g r a p h i c a l b r e a k o c c u r r e d b e f o r e the d e p o s i t i o n of t h e Main Reef g r o u p took p l a c e .
As t h e c h a n g e f r o m the U p p e r J e p p e s t o w n S h a l e s into t h e i r o v e r l y i n g q u a r t z i t e s i s a g r a d a t i o n a l one it i s a l s o e v i d e n t t h a t no s u c h s t r a t i g r a p h i c a l b r e a k o c c u r r e d at the top of the U p p e r J e p p e s t o w n S h a l e s ,
In c o n s e q u e n c e , I c a n n o t a g r e e with t h o s e g e o l o g i s t s who s t a r t the L o w e r Wit-w a t e r s r a n d D i v i s i o n Wit-with t h e J e p p e s t o Wit-w n S h a l e G r o u p , but I m u s t s t r o n g l y f a v o u r the i n t e r p r e t a t i o n which c o n s i d e r s the M a i n Reef F o o t w a l l q u a r t z i t e s to r e p r e s e n t the c l o s i n g s t a g e of the J e p p e s t o w n S e r i e s ,
T h e s e a r e not t h e o n l y o c c u r r e n c e s of u n c o n f o r m i t i e s i n t h e W i t w a t e r s r a n d f o r m a t i o n s , S e v e r a l p u b l i c a t i o n s of the S, A. G e o l o g i c a l S o c i e t y d e a l with t h e q u e s -t i o n of u n c o n f o r m i -t i e s and d i s c o n f o r m i -t i e s 4)_
The g e o l o g i c a l c o l u m n , fig. 2, giving t h e s e q u e n c e of f o r m a t i o n s f r o m the D o l o -m i t e S e r i e s d o w n w a r d s , -m u s t be c o n s i d e r e d i n c o n j u n c t i o n with w h a t h a s b e e n w r i t t e n a b o v e . It s h o w s , b e s i d e s the S y s t e m s and S e r i e s , t h o s e u n c o n f o r m i t i e s
and d i s c o n f o r m i t i e s w h i c h h a v e b e e n g e n e r a l l y a c c e p t e d , and the c o l u m n n e e d s no f u r t h e r e x p l a n a t i o n .
1) S e e : M o l e n g r a a f f (1904) p. 6 1 . 2) S e e : S h a r p e (1949), p. 2 7 9 .
3) S e e : M o l e n g r a a f f (1904) pp. 2 1 , 22.
I I S T R A T I G R A P H Y O F T H E F O R M A T I O N S I N T H E N O R T H W E S T F R E E S T A T E A I n t r o d u c t i o n T h e e x t e n s i v e d r i l l i n g in t h i s n e w l y d i s c o v e r e d goldfield h a s y i e l d e d a c o n s i d -e r a b l -e a m o u n t of g -e o l o g i c a l i n f o r m a t i o n . T h e a i m of t h e g e o l o g i s t s e n g a g e d in t h i s e x p l o r a t i o n p r o g r a m n n e w a s i n the f i r s t p l a c e t h e d e t e r m i n a t i o n of p r o b a b l e r e e f i n t e r s e c t i o n s on e v i d e n c e of f o r -m a t i o n d u r i n g the d r i l l i n g of any h o l e . In a d d i t i o n , it w a s felt t h a t it w o u l d be h i g h l y a d v i s a b l e to t r y and c o r r e l a t e the h o r i z o n s t r a v e r s e d w i t h the known s u c -c e s s i o n of t h e W i t w a t e r s r a n d p r o p e r .
The f i r s t o b j e c t w a s a c h i e v e d by i n t r o d u c i n g l o c a l n a m e s f o r v a r i o u s h o r i z o n s , ( B o r c h e r s a n d White (1943), fig. 1; pp 1 3 5 - 1 4 4 ; F r o s t et a l . (1946), P l a t e VU; pp 1 8 2 1 ) b a s e d on t h e c h a r a c t e r i s t i c s i n a r e a s d r i l l e d . Tliis m e t h o d , a l t h o u g h h i g h -ly s u c c e s s f u l in the c l o s e c o n t r o l on d r i l l i n g , h a d t h e d i s a d v a n t a g e of c r e a t i n g c o n f u s i o n , a s d i f f e r e n t n a m e s w e r e g i v e n to t h e s a m e h o r i z o n in v a r i o u s p a r t s of t h e field. T h e s e c o n d o b j e c t , h o w e v e r , h a d not b e e n s a t i s f a c t o r i l y r e a c h e d u p t o now. B a i n e s (1949); (pp. 313, 314; P l a t e VI) e m p h a s i z e d t h e f a c t t h a t t h e f o r m a t i o n s i n t h e a r e a of V i e r f o n t e i n in t h e K l e r k s d o r p d i s t r i c t show a r e m a r k a b l e s i m i l a r i t y to t h o s e r e v e a l e d by d r i l l i n g in t h e O d e n d a a l s r u s a r e a . He p r o v e d t h a t t h e V a a l
Reef in the f o r m e r c o r r e s p o n d s to the B a s a l Reef in the l a t t e r . U n f o r t u n a t e l y , no d i r e c t l i n k h a s so f a r b e e n found b e t w e e n the K l e r k s d o r p a r e a and t h e W e s t Rand, w i t h t h e r e s u l t t h a t t h e s t r a t i g r a p h i c a l p o s i t i o n of t h e B a s a l Reef H o r i z o n w a s , up to now, a s u b j e c t of doubt. In t h e p u b l i c a t i o n s d e a l i n g w i t h t h e c o r r e l a t i o n of t h i s m a i n e n o n o m i c h o r i z o n of t h e new g o l d f i e l d , two w i d e l y d i v e r g i n g o p i n i o n s h a v e b e e n f o r w a r d e d , v i z . :
a. Tlie B a s a l Reef H o r i z o n i s c o n s i d e r e d to be t h e e q u i v a l e n t of t h e Main Reef C o n g l o m e r a t e s ( B a i n e s ( l 9 4 9 ) , p- 314).
b. The B a s a l Reef H o r i z o n i s c o n s i d e r e d t o be p a r t of t h e B i r d Reef G r o u p ( B a i n e s (1949), p . 317).
When s t u d y i n g t h e s e p o s s i b l e c o r r e l a t i o n s , the n e e d of a d e t a i l e d s t r a t i g r a p h i -c a l -c o l u m n of the f o r m a t i o n s d r i l l e d t h r o u g h in the new g o l d f i e l d i s e v i d e n t . It a p p e a r e d , h o w e v e r , t h a t a d e t a i l e d g e o l o g i c a l c o l u m n only e x i s t e d f o r t h e f o r m a -t i o n s up -t o and i n c l u d i n g -t h e B a s a l Reef H o r i z o n a n d i -t s i m m e d i a -t e f o o -t w a l l , a n d t h a t n o d e t a i l e d c o l u m n b e l o w t h e m a i n e c o n o m i c h o r i z o n h a d y e t b e e n p u b l i s h e d . It s t r u c k m e t h a t the k e y to a s a t i s f a c t o r y s o l u t i o n m i g h t be found i n the d e t a i l e d s t u d y of the f o r m a t i o n s u n d e r l a y i n g t h e B a s a l Reef H o r i z o n . T h i s l e d m e to c o l l e c t a l l a v a i l a b l e d a t a , e s p e c i a l l y of t h o s e h o l e s t h a t p e n e t r a t e d d e e p l y i n t o t h e o l d e r f o r m a t i o n s , and to study t h e a v a i l a b l e c o r e s a s f a r a s p o s s i b l e . The r e s u l t s of t h i s s t u d y w e r e b e y o n d e x p e c t a t i o n s : It b e c a m e p o s s i b l e to e x -t e n d -the g e o l o g i c a l c o l u m n f r o m -t h e B a s a l Reef H o r i z o n , -t h r o u g h -t h e c o m p l e -t e M a i n - B i r d S e r i e s , to the C o r o n a t i o n B e d s of the G o v e r n m e n t S e r i e s . M o r e o v e r ,
the c o l u m n ( s e e fig. 3) s h o w s a s t r i k i n g r e s e n n b l a n c e to the c o l u m n of the R a n d p r o p e r .
No f o s s i l s h a v e e v e r b e e n found below the K a r r o o f o r m a t i o n s in t h e W i t w a t e r s r a n d G o l d f i e l d s , C o n s e q u e n t l y , t h e c o r r e l a t i o n i s b a s e d on t h e r e m a r k a b l e c o n -s i -s t e n c y of t h e f o r m a t i o n -s f r o m the K i m b e r l e y S h a l e -s to the J e p p e -s t o w n S h a l e -s , a n d on t h e m u t u a l r e l a t i o n s h i p of the p o s i t i o n of c o n g l o m e r a t i c h o r i z o n s in t h e s e f o r m a t i o n s . It s h o w s t h a t the B a s a l Reef m u s t be c o n s i d e r e d to r e p r e s e n t a c o n g l o m e r a t e of t h e U p p e r B i r d G r o u p , i. e. the M o n a r c h Reef of R a n d f o n t e i n E s -t a -t e s , T h i s c o r r e l a -t i o n i s c o r r o b o r a -t e d b y -t h e r e s u l -t s o b -t a i n e d i n r a d i o a c -t i v i -t y b o r e h o l e l o g g i n g , f o r U3 Og p o s s i b i l i t i e s , a s shown by Sinnpson (1952), pp. 141 a n d
151, in one of the p a p e r s r e a d f o r the S. A. G e o l o g i c a l S o c i e t y .
-G E O L O -G I C A L C O L U M N ot the N W. FREE S T A T E 2 to UT O ^ a: i£ > • ^ i? -z ^ 2 u 1-U l > U ) a r < (X tn CE U i K < * 1-» B e a u f o r t S e r . c i . ;2 • u u u Dw •= " -O c • ' > ._ 9 , • A ^ O ' ," . ' ' ' -» ^ • • -* ' / F 0 rma t io n S a n d i t o n e i w U h S h a l t I n t e r M t i a t i o n i Upper 1 ^ 1 Ecca • M i d d l e y k a S c r i e i , «Ï „ n a « Ü -^.
P
. :; • " > " a -a •: " • a • -> . ; « a -, E : » " o v e n u r i d U p p e r S e d t i n . L o w e r Kenb. Shale B i r d G r o u p L o W f r B i r d B M f O u a r U l U G r o u p . M a i n R e e f C o n g I G r o u p . Upper // f, (f ^ i " 77^ :;: ... -::•.: sssS ^ "ifra -.. _.. .'.'."' i n x n S a n d i t o n e i w i t h S h a l l b o n d ) , in l o w e r p o r t i o n * o m ( n a r r o w C o Q i l O y c r i . T l l l l t i A n i t e J I t l e L a * a T u f t o e w u i S h o l e i , O u o r t i i u s . C o n g l o m e r o t e » A n d e i l l l c Big Pebble C o n g l o m e r a t e . P e b b l e j o f wid«ly v o r ^ i n f l o r i g i n . C o n g i o m e r o l e a t a b o v e , w i t h l n l t r b t d d e d , t e n e r o l y d a r k . O u a r l i l l t * . L l g h t g r i y O u o r l Z l U . w i l h C o n g l o m e r o l t i . Impurs y e Ü o w É i h O u o r t t l t e , c o n t a i n i n g Pebble a n d Big Pebble C o n g l o t n e r o t e t o o d ' M i . e d ' C o r ^ l . t o w a r d ' V l r y o r g i l o c e o u i O u o r t t l l l . Ou o r t W t e i , becoming cleaner WTth d e p t h . P e b p l i b a n d i occur In l o w t r p o r t i o n . Q u a r t l l l i l . m o l t l y -• C o n g l o m e r a t i c Z o n . C o n g i o m e r a u j of ' m l » t d p e b b l e ' t y p t Q u o r l i ü t E i . w h l t i i h a n d impure g r e y , ytllowlif^ d a r k g r v y t o w o r d l b a n . C o n g l o m . r o l i c ï o n . . O u a r t i i t e i . w i t h t h i c k i l l i c i r a u i w n i U b a n d In G r i t t y eongi ome r a t e , in g r e y O u o r t z H e » O I U o in g r e i n i m - g r i y O u a r l i l t e . g r a d i n g Into a r g l l l a c e o u t q u a r t i i l e . S » » V e r . a r g i l l a c e o u i Q u o r l ï H i Jeppest. n o G f i l w i t h S h a l l b a n d » a n d M i d d l e J e p p e x . ^ n d a k > H B a n d i d O u o r t i i i i t > a r k t l l g h t O u o r t i l i g . A m y g d a l o i d a l t o » a W h i l e i l l l c e o u t O u a r E t l t e L o w e r ^ ^ H M a g n s t i c S h a l e -"PP*" J H H . 0 1 . ^ G o v e r n . Q w t i i t e G r o u p ffl A r g l l l a c e o u i O u o r t i l t e . White s l l l c c o u i O u o r i i l t e w i t h o c c a i i o n a l g r i t t y C o n g i o m e r o l e t o w o r d l bo»» S h a l l ond argril.OuortiJte. W h i t * O u a r t j l t i , w i t h ^ H G i > « « n m . ^ H ^ ^ 1 H i g h l y m a g n e t i c S h a l e° "' •
Upper Q u o r u i U .or. S h e l l Lower . « r o n a l k i n Q u a r t i l t i W G r e y i j h t o ilghUoloured O u o r t i i K w i t h g r c i n l i h l l n g i and g r l i i y C o n g l o m i r o l e ot b a i l M a g n . S h a i e . w n h m i l 11 k m w d . L i g h l c o l o u r e d g r c i n i t h Q u o r t z l l e . Local Names V * n i e r . d . V P P < r k o « < i V « n t « r i d . l ^ r S t d . V e n K r t d ^ o w i r t * " " Z o n e V S i l o n e V S 5. Z o n o T i . E C i Z o n e EC 2 . Z o n e i E C a . d . Z o n e E S i . U p p e r F o o t w a l l . U i d d l l F o o w o l i . L o w e r F o o t w a l l . & t r o t l a r « p h t e a l B r e o k i I J n c o n . „ , - „ . v P o i t l b l i p i l t o Q l o f f i i l y , D c i l g n a t l o n of Middle ktoinbow R M ( L o w e r J A R e e f . . B R e t t „ . L « o d < r R i c f . . . B O i O l R C ( ( U p p - r I n l , f t t e f ""' L I v . J o h n I t . G r o u p A d a M o y R e « ' A ( r . - R i c t k u i i G r o u p ^ o w i m m e n t R e < ( C o r o n a t i o n R i d = Ft.j o r i t y of t h e g e o l o g i c a l d e p a r t m e n t s i n the field, at t h e s a m e t i m e s h o w s the p l a c e of t h e v a r i o u s z o n e s in t e r m s of the S t a n d a r d C l a s s i f i c a t i o n of t h e Rand p r o p e r .
In t h i s c o l u m n the d i s c o n f o r n n i t i e s and m a j o r u n c o n f o r m i t i e s r e c o g n i s e d in the f i e l d h a v e b e e n shown a s w e l l . It m u s t be u n d e r s t o o d , h o w e v e r , t h a t d e t a i l e d s t u d y of the f o r m a t i o n p e n e t r a t e d by t h e d r i l l i s e s s e n t i a l f o r the e s t a b l i s h i n g of t h e s e a n d s i m i l a r b r e a k s in s e d i m e n t a t i o n . It i s p r o p o s e d t o go m o r e fully into t h i s p r o b l e m f o r one of t h e S e r i e s e n c o u n t e r e d in the field i. e, t h e K i m b e r l e y S e r i e s , w h i c h will be t h e s u b j e c t of a s e p a r a t e s t u d y .
B. S t r a t i g r a p h i c a l D e s c r i p t i o n
The a c c o n n p a n y i n g g e o l o g i c a l c o l u m n , fig, 3, g i v e s a p i c t u r e of the s t r a t i g r a p h y e n c o u n t e r e d i n t h e O d e n d a a l s r u s Goldfield.
As t h e d r i l l i n g r e s u l t s in the N. W. F r e e S t a t e h a v e not r e v e a l e d the c o m p l e t e s u c c e s s i o n f r o m t h e b a s e of t h e W i t w a t e r s r a n d S y s t e m u p w a r d s , i n f o r m a t i o n f o r t h e l o w e r p o r t i o n of t h e L o w e r W i t w a t e r s r a n d D i v i s i o n b e i n g i n c o m p l e t e , a n d a s , m o r e o v e r , d i f f e r e n c e s e x i s t e d in the c o r r e l a t i o n of t h e m a i n gold b e a r i n g reef i n t h i s a r e a , i t i s t h o u g h t a d v i s a b l e to adopt a d e s c e n d i n g o r d e r in the following d e -s c r i p t i o n .
L K a r r o o S y s t e m ' 1. B e a u f o r t S e r i e s .
The S e r i e s s t a r t s u n d e r the s o i l and i s e n c o u n t e r e d up to a depth of m a x . 300 ft,
2. E c c a S e r i e s .
R e p r e s e n t e d by the U p p e r E c c a S h a l e s and the Middle E c c a S a n d s t o n e s , w i t h s h a l e b a n d s c o n t a i n i n g c o a l l a y e r s , w h e r e a s t h e L o w e r E c c a S h a l e s a r e a b s e n t , T h i c k n e s s a v e r a g e s 900 ft.
3. Dwyka S e r i e s .
C o n s i s t i n g m o s t l y of t i l l i t e , with s h a l e b a n d s w h e r e t h e d e v e l o p m e n t i s t h i c k , i t s d e v e l o p m e n t v a r i e s f r o m a few i n c h e s to s e v e r a l h u n d r e d s of f e e t . I n -s t a n c e -s a r e known w h e r e the t i l l i t e i -s r e p r e -s e n t e d by a v e r y n a r r o w band, but e x a m p l e s of a t h i c k n e s s of m o r e t h a n 700 ft. a r e a l s o e n c o u n t e r e d . F o s -s i l -s h a v e not b e e n found in b o r e h o l e c o r e -s ,
n . V e n t e r s d o r p S y s t e m (fig. 3) '
D r i l l i n g did not r e v e a l t h e p r e s e n c e of t h e T r a n s v a a l Systenn in the a r e a u n d e r c o n s i d e r a t i o n , a n d t h e f o o t w a l l of t h e K a r r o o f o r m a t i o n s i s f o r m e d by fornnations-of the V e n t e r s d o r p S y s t e m , The p r e s e n c e fornnations-of the T r a n s v a a l S y s t e m i s , h o w e v e r , known m o r e t o w a r d s the E a s t in t h e v i c i n i t y of K r o o n s t a d , a n d m o r e t o w a r d s t h e N o r t h i n V i e r f o n t e i n , The a b s e n c e of the T r a n s v a a l Systenn in t h e O d e n d a a l s r u s A r e a i s b e l i e v e d to be the r e s u l t of the e r o s i o n , known to h a v e o c c u r r e d p r i o r to t h e d e p o s i t i o n of the K a r r o o S y s t e m ,
The V e n t e r s d o r p f o r m a t i o n s a r e , i n t h e g r e a t m a j o r i t y of c a s e s , found in the b o r e h o l e s d r i l l e d in t h e new g o l d f i e l d , and a r e r e p r e s e n t e d by both the V e n t e r s -d o r p U p p e r S e -d i m e n t s an-d t h e V e n t e r s -d o r p L o w e r L a v a . A. V e n t e r s d o r p U p p e r S e d i m e n t s (V. U . S . ). T h e s e h a v e b e e n s u b j e c t e d to e r o s i o n p r i o r to the d e p o s i t i o n of the K a r r o o S y s t e m , and a r e a b s e n t i n a c o n s i d e r a b l e n u m b e r of b o r e h o l e s . In o t h e r b o r e h o l e s , h o w e v e r , t h i c k n e s s e s of 1000 ft. and m o r e h a v e b e e n r e c o r d e d , As an e x t r e m e c a s e , b o r e h o l e M. 1. i n the N. W e s t of t h e f i e l d nnay be m e n -t i o n e d , w h e r e m o r e -t h a n 4700 f-t. of V, U, S. , w h i c h h e r e r e s -t u n c o n f o r m a b l y on L o w e r W i t w a t e r s r a n d B e d s , w e r e p e n e t r a t e d . 1) S e e : B o r c h e r s a n d White (1943) pp. 135, 136; F r o s t et a l . (1946) p p . 17, 18. 2) S e e : B o r c h e r s and White (1943) pp, 136, 137; F r o s t et a l , (1946) p. 18.
The i n t e r n n i t t e n t l a v a flows p r e v i o u s l y m e n t i o n e d a r e often w e l l d e v e l o p e d i n t h i s p a r t of t h e F r e e S t a t e . One of t h e s e b e c o m e s of l o c a l i m p o r t a n c e E a s t of t h e De B r o n fault, a n d i s known in the f i e l d a s V e n t e r s d o r p U p p e r L a v a . In c o n t r a d i s t i n c t i o n w i t h the K a r r o o S y s t e m , the V. U. S. a r e a l m o s t e x -c l u s i v e l y k n o w n f r o m d r i l l i n g r e s u l t s . Only t h r e e l o -c a l o u t -c r o p s of t h e s e s e d i m e n t s h a v e b e e n found i n t h e a r e a u n d e r c o n s i d e r a t i o n , a l l t h r e e a r e s i t u a t e d in the v i c i n i t y of O d e n d a a l s r u s . B, V e n t e r s d o r p L o w e r L a v a . The l a v a w a s s u b j e c t e d to e r o s i o n , a n d i s c o m p l e t e l y m i s s i n g in s o m e b o r e -h o l e s , In m o s t i n s t a n c e s , -h o w e v e r , d r i l l i n g r e v e a l e d t -h i s l a v a ; t-he p r o b a b l e m a x i m u m t h i c k n e s s in the s o - c a l l e d p o s i t i v e a r e a (which i s u n d e r l a i n by a u r i f e r o u s f o r m a t i o n ) i s in the v i c i n i t y of 3500 ft, ( B o r e h o l e s T. V. 2. a n d K, K. K. 1. ) B e y o n d the p o s i t i v e a r e a t h e r e a r e i n d i c a t i o n s f o r s t i l l t h i c k e r d e v e l o p m e n t i n a N. W e s t e r l y d i r e c t i o n ( b o r e h o l e s D. T, 1. and T. M, 2. ). III. W i t w a t e r s r a n d S y s t e m A. U p p e r W i t w a t e r s r a n d D i v i s i o n 1. E l s b u r g S e r i e s (fig. 3),
The u p p e r l i m b of the S e r i e s , known a s Z o n e V. S, 1, c o n s i s t s of a c o n g l o -m e r a t e of big p e b b l e s to b o u l d e r s of q u a r t z , q u a r t z i t e , s h a l e , c h e r t s a n d o c c a s i o n a l p o r p h y r i e s . The t h i c k n e s s of t h e c o n g l o m e r a t e c a n b e c o m e u p to 2000 ft. a s i s t h e c a s e i n the S. W e s t e r l y p a r t of t h e field, w h e r e a s in the W i n b u r g d i s t r i c t adjoining t h i s p a r t , t h e t h i c k n e s s a p p e a r s to be s t i l l i n -c r e a s i n g . H o w e v e r , in the O d e n d a a l s r u s A r e a a t h i -c k n e s s of 850 ft s e e m s to be n o r m a l ,
In a N o r t h and E a s t d i r e c t i o n , the c o n g l o m e r a t e b e c o m e s l e s s t h i c k , and p e b b l e s i z e d e c r e a s e s ; i n t e r c a l a t i o n s of q u a r t z i t e s t a r t c o m i n g i n and f i n a l l y the zone i s o n l y r e p r e s e n t e d by q u a r t z i t e w i t h an o c c a s i o n a l p e b b l e band, N o r m a l l y , a p r e d o m i n a n t l y c l e a n s i l i c e o u s q u a r t z i t i c f o r m a t i o n . Z o n e s V, S. 2 - 4 , i s found below t h i s c o n g l o m e r a t i c z o n e . It s o m e t i n n e s c o n t a i n s a few g r i t t y and c o n g l o m e r a t i c h o r i z o n s . T h e s e h o r i z o n s b e c o m e l o c a l l y of e c o n o m i c i m p o r t a n c e i n the V a n d e n H e e v e r s r u s t a r e a only, w h e r e t h e i r d e v e l o p m e n t i s nnuch s t r o n g e r . The q u a r t z i t e s of Z o n e s V . S . ' 2 4 b e c o m e p r e -d o m i n a n t l y c o n g l o m e r a t i c i n the S o u t h - W e s t e r n p a r t of the f i e l -d , but t h e g r o u p a s s u c h i s s t i l l c l e a r l y r e c o g n i s a b l e . The t h i c k n e s s of Z o n e s V, S. 2 - 4 a v e r a g e s a p p r o x i m a t e l y 600 ft.
The l o w e s t m e m b e r of the E l s b u r g S e r i e s i s f o r m e d by a c o n g l o m e r a t e of a c h a r a c t e r c o m p a r a b l e to Zone V. S. 1. P e b b l e s , h o w e v e r , a r e g e n e r a l l y s m a l l e r , the d i a n n e t e r not e x c e e d i n g 3 " ; and t h i c k n e s s i s c o n s i d e r a b l y l e s s t h a n f o r Z o n e V, S. 1. , v i z , v a r y i n g fronn a few i n c h e s to a m a x i m u m of 100 ft. T h i s c o n g l o m e r a t i c z o n e i s known a s Zone V, S, 5 o r a s the V e n t e r s d o r p
B a s a l C o n g l o m e r a t e , It s h o w s a t e n d e n c y , s i m i l a r to the one o b s e r v e d in Zone V. S, 1. , f o r r e d u c t i o n in p e b b l e s i z e i n a N o r t h and E a s t d i r e c t i o n , and f i n a l l y i s only r e p r e s e n t e d by a g r i t t y h o r i z o n in the e x t r e m e E a s t of the field,
T o t a l t h i c k n e s s of the E l s b u r g S e r i e s a v e r a g e s a p p r o x i m a t e l y 1500 ft. Although the above f o r m a t i o n s w e r e o r i g i n a l l y n a m e d V e n t e r s d o r p S e d i -n -n e -n t s , it i s -now g e -n e r a l l y a c c e p t e d t h a t t h i s -n a m e i s i -n c o r r e c t , a-nd t h a t t h e y r e p r e s e n t t h e E l s b u r g S e r i e s of the W i t w a t e r s r a n d p r o p e r .
2. K i m b e r l e y S e r i e s (fig. 3).
The K i m b e r l e y S e r i e s , which u n d e r l i e s the E l s b u r g S e r i e s , i s r e p r e s e n t e d by d i r t y , g e n e r a l l y y e l l o w i s h s o m e w h a t a r g i l l a c e o u s q u a r t z i t e s w h i c h c o n t a i n s e v e r a l p e b b l e h o r i z o n s . S o m e of t h e s e c o n g l o m e r a t e s a r e of e c o n o m i c inn-p o r t a n c e . The q u a r t z i t e - c o n g l o m e r a t e g r o u inn-p i s d e inn-p o s i t e d on t o inn-p of a v e r y a r g i l l a c e o u s h o r i z o n , the K i m b e r l e y S h a l e .
M e n t i o n s h o u l d be m a d e of the fact t h a t t h e r e i s c o n s i d e r a b l e e v i d e n c e f o r a t r u n c a t i o n of the K i m b e r l e y S h a l e by t h e o v e r l y i n g q u a r t z i t e s in a S o u t h e r l y d i r e c t i o n . B a i n e s (1949) P l a t e IX, h a s d r a w n a t t e n t i o n to t h i s d i s c o n f o r m i t y and i n d i c a t e d t h a t t h e e r o s i o n c a u s i n g i t h a s e v e n r e m o v e d t h e u n d e r l y i n g f o r m a t i o n s to below t h e B a s a l Reef H o r i z o n f u r t h e r t o w a r d s t h e South. A l -though it a p p e a r s t h a t t h i s d i s c o n f o r m i t y m a y p o s s i b l y b e c o u p l e d w i t h s o m e t h i n n i n g of t h e u n d e r l y i n g f o r n n a t i o n s , it s e e n n s to be b e y o n d doubt t h a t t h e q u a r t z i t e - c o n g l o m e r a t e g r o u p t r a n s g r e s s e s o v e r t h e K i m b e r l e y S h a l e a n d e v e n o l d e r f o r m a t i o n s . The t r a n s g r e s s i v e c h a r a c t e r of t h e q u a r t z i t e c o n -g l o m e r a t e -g r o u p o v e r the u n d e r l y i n -g s h a l e h a s b e e n c o n f i r m e d in u n d e r -g r o u n d d e v e l o p m e n t : i n at l e a s t one shaft s u n k in the f i e l d an a n g u l a r d i s c o n f o r m i t y of 2.J w a s o b s e r v e d . The q u a r t z i t e - c o n g l o m e r a t e g r o u p i s k n o w n a s Z o n e s T. 1-E. C. 1-4; t h e t o t a l t h i c k n e s s v a r i e s b e t w e e n 200 to 400 ft. No d e t a i l e d d e s c r i p t i o n of t h e s e z o n e s w i l l be g i v e n h e r e , a s the K i n n b e r l e y f o r m a t i o n s w i l l be d e a l t w i t h s e p a r a t e - ly-3. B i r d S e r i e s (fig. 3). a. U p p e r B i r d G r o u p .
The K i m b e r l e y S h a l e o r Zone E . S. 1, a l s o known a s t h e U p p e r S h a l e M a r k e r , c o n s i s t s of v e r y fine g r a i n e d v e r y a r g i l l a c e o u s q u a r t z i t e s w i t h s h a l e i n t e r c a l a t i o n s , and c a n d e v e l o p into a t r u e s h a l e in sonne p a r t s of t h e field. T h i c k n e s s v a r i e s f r o m 0 to 250 ft. Although u s u a l l y i n c l u d e d in t h e K i m b e r l e y S e r i e s , t h i s c o r r e l a t i o n i s , in m y o p i n i o n , i n c o r r e c t , f o r t h e following r e a s o n s .
1. Z o n e E . S. 1 g r a d u a l l y b e c o m e s c o a r s e r g r a i n e d a n d l e s s a r g i l l a c e o u s with d e p t h , a n d g r a d e s into t h e u n d e r l y i n g q u a r t z i t e s of the U p p e r B i r d G r o u p . O b v i o u s l y , no s t r a t i g r a p h i c a l b r e a k e x i s t s b e t w e e n t h e l a t t e r G r o u p and t h e K i m b e r l e y S h a l e .
2. The d i s c o n f o r m i t y at the top of the K i m b e r l e y S h a l e , d e s c r i b e d p r e -viously, c l e a r l y s h o w s an e i r o s i o n a l b r e a k at t h i s p o i n t .
It i s t h e r e f o r s u g g e s t e d to i n c l u d e t h e U p p e r S h a l e M a r k e r in t h e Upjaer B i r d G r o u p of t h e B i r d S e r i e s , a s s h o w n in fig. 3.
T h e q u a r t z i t e below the Shale H o r i z o n b e c o m e s c l e a n e r with d e p t h , a s i n d i c a t e d by t h e d i m i n i s h i n g c o n t e n t s of a r g i l l a c e o u s m a t e r i a l . S o m e s e r i c i t e and m u s c o v i t e i s s t i l l p r e s e n t in the q u a r t z i t e , which i s known a s Zone E . S. 2.
The o c c u r r e n c e of t h e f i r s t h i g h l y s i l i c e o u s b a n d i s t a k e n a s t h e d i v i s i o n b e t w e e n Zone E . S. 2 and E. S. 3. The l a t t e r i s m a i n l y c o m p o s e d of c l e a n s i l i c e o u s q u a r t z i t e and c o n t a i n s a c o n g l o m e r a t i c h o r i z o n at i t s b a s e . The b o t t o m c o n g l o m e r a t e s of t h i s Zone a r e known a s the L e a d e r Reef H o r i z o n ,
w h i c h i n t h e g r e a t m a j o r i t y of c a s e s i s not of e c o n o m i c v a l u e . A q u a r t -z i t e with o c c a s i o n a l g r i t b a n d s o c c u r s below the L e a d e r Reef H o r i -z o n , a n d i s known a s Z o n e E . L. 1. In a l a r g e p a r t of t h e f i e l d t h i s zone r e s t s •on a k h a k i c o l o u r e d s h a l e , which v a r i e s c o n s i d e r a b l y in t h i c k n e s s but
n e v e r a t t a i n s a developnnent of m o r e t h a n 2 0 ft. The s h a l e c a n a n d often does c o n t a i n q u a r t z i t e b a n d s , a n d i s known a s Zone E . L. 2. It i s f o l l o w e d by a c l e a n q u a r t z i t e w i t h g r i t and c o n g l o m e r a t e b a n d s : Zone E . L. 3. T h i s z o n e i s a l s o known as the B a s a l Reef H o r i z o n . A g r i t t y c o n g l o m e r a t e o r g r i t i s found at i t s b a s e , t h e B a s a l Reef, w h i c h i s the p r i n c i p a l g o l d b e a r e r of the N o r t h W e s t F r e e S t a t e Goldfield. N o t e : It i s c u s t o m a r y in the South A f r i c a n m i n i n g i n d u s t r y to u s e t h e t e r m " g r i t " f o r a s m a l l p e b b l e c o n g l o m e r a t e of w h i c h the s i z e of t h e c o n s t i t u e n t p e b b l e s r a n g e s f r o m 2 m m to 4 m m . The t e r m i s u s e d h e r e a n d in t h e f o l -lowing in a c c o r d a n c e with t h i s c u s t o m .
A t h i n n i n g of the f o r m a t i o n b e t w e e n t h e b a s e of Zone E . S. 3 and the B a s a l Reef h a s b e e n o b s e r v e d t o o c c u r in the South of t h e field. T h i s t h i n n i n g t a k e s p l a c e i n a S o u t h e r l y d i r e c t i o n , a n d t h e L e a d e r a n d B a s a l R e e f s h a v e m e r g e d in the V i r g i n i a a r e a into t h e L e a d e r - B a s a l H o r i z o n .
The B a s a l Reef i s a s s o c i a t e d with a b r e a k in s e d i m e n t a t i o n , and t h e r e i s a m p l e e v i d e n c e of a d i s c o n f o r m i t y at t h i s p o i n t .
The a v e r a g e t l i i c k n e s s fronn the b a s e of t h e K i m b e r l e y S h a l e s to the B a s a l Reef H o r i z o n announts to a p p r o x i n n a t e l y 350 ft.
B e f o r e d e s c r i b i n g the F o o t w a l l B e d s , the following r e m a r k s h o u l d be m a d e .
The i m p o r t a n c e of the f o r m a t i o n s below t h e B a s a l Reef H o r i z o n for c o r r e l a t i o n p u r p o s e s h a s b e e n m e n t i o n e d in the a b o v e . It i s t h e r e f o r e n e c e s -s a r y t o g i v e a d e t a i l e d d e -s c r i p t i o n of t h e -s e f o r m a t i o n -s , e -s p e c i a l l y f r o m t h e B a s a l Reef t o the U p p e r J e p p e s t o w n S h a l e .
As t h e d r i l l i n g c a m p a i g n in the a r e a h a d a s i t s p r i m e o b j e c t the l o c a t i n g a n d t e s t i n g of the B a s a l Reef, the d e s c r i p t i o n i s b a s e d on a l i m i t e d a m o u n t of b o r e h o l e s . The a v a i l a b l e i n f o r m a t i o n i s , h o w e v e r , ' quite sufficient to d e s c r i b e the c h a r a c t e r i s t i c s of the f o r m a t i o n e x p l o r e d .
It i s the p r a c t i s e in t h e field to d i v i d e the f o r m a t i o n s below the B a s a l Reef into t h r e e d i s t i n c t g r o u p s , i. e. the U p p e r F o o t w a l l , the Middle F o o t -w a l l a n d the L o -w e r F o o t -w a l l . As p r e v i o u s l y sho-wn, s e e p a g e 8, the B a s a l Reef m u s t be c o n s i d e r e d to be t h e e q u i v a l e n t of the M o n a r c h Reef ( U p p e r B i r d G r o u p ) . The f o r m a t i o n s below the U p p e r B i r d c o n g l o m e r a t e s on the R a n d a r e d i v i d e d i n t o t h r e e g r o u p s a s w e l l , v i z . the L o w e r B i r d C o n g l o m e r a t e G r o u p , the M a i n Reef Q u a r t z i t e G r o u p , and the Main Reef C o n g l o m e r a t e G r o u p . T h i s d i v i s i o n i s b a s e d on l i t h o l o g i c a l d i f f e r e n c e s , w h i c h find t h e i r p a r a l l e l in t h o s e on w h i c h the F o o t w a l l B e d s of the B a s a l Reef h a v e b e e n s u b d i v i d e d in the F r e e S t a t e . F o r t h i s r e a s o n I s u g g e s t to a d o p t the l o c a l d i v i s i o n in the a r e a a s the official o n e .
b. U p p e r F o o t w a l l ( L o w e r B i r d G r o u p ) , (fig. 3).
A fine t o m e d i u m g r a i n e d l i g h t g r e y s i l i c e o u s q u a r t z i t e c o n t a i n i n g c o l o u r -e d s p -e c k s u n d -e r l i -e s th-e B a s a l R-e-ef. T h i s q u a r t z i t -e i s known a s Z.on-e E . F . 1, a l s o a s t h e S p e c k l e d F o o t w a l l . It b e c o m e s d a r k e r in c o l o u r and c o a r s e r g r a i n e d with d e p t h , and at a d i s t a n c e v a r y i n g f r o m 380 ft. to 180 ft. ( r e s p . in the v i c i n i t y of O d e n d a a l s r u s and of V i r g i n i a ) f r o m the B a s a l Reef, s m a l l p e b b l e c o n g l o n n e r a t e s a n d g r i t s o c c u r . T h e s e m a r k the b e -g i n n i n -g of a c o n -g l o m e r a t i c z o n e , a n d a s t h e s e con-glomerates-^ a r e known a s t h e I n t e r m e d i a t e R e e f s , the n a m e I n t e r m e d i a t e Reef Zone h a s b e e n g i v e n to t h i s c o n g l o m e r a t i c h o r i z o n , w h i c h i s a p p r o x i m a t e l y 500 ft. t h i c k .
The I n t e r m e d i a t e Reef Z o n e s t a r t s w i t h a g r i t t y c o n g l o m e r a t e of q u a r t z a n d c h e r t p e b b l e s , o c c a s i o n a l s h a l e p e b b l e s , and a l m o s t i n v a r i a b l y s h a l e p e l l e t s . It i s w e l l m i n e r a l i s e d , but c o n t a i n s low gold v a l u e s only.
N o t e :
The t e r m s " m i n e r a l i s e d " and " m i n e r a l i s a t i o n " in t h i s w o r k h a v e b e e n u s e d in a c c o r d a n c e with the m e a n i n g a c c e p t e d in the S o u t h - A f r i c a n m i n i n g i n d u s t r y : T h e y m e r e l y i n d i c a t e the p r e s e n c e of s u l p h i d e s and v a l u a b l e m i n e r a l s in t h e c o n g l o m e r a t e s , a n d do not r e f e r to t h e o r i g i n of t h e s e s u l p h i d e s e t c . T h e n f o l l o w s a d a r k e r g r e y , c o a r s e to g r i t t y q u a r t z i t e , of a p p r o x i m a t e l y 120 ft. , w i t h n a r r o w g r i t s a n d s m a l l p e b b l e b a n d s c o m p a r a b l e to the c o n -g l o m e r a t e d e s c r i b e d a b o v e . A c o n -g l o m e r a t i c h o r i z o n of about 40 ft. t h i c k n e s s f o l l o w s , t h e c o n g l o m e r a t e s s h o w i n g the s a m e c o n s t i t u e n t s a n d p e b b l e s i z e a s d e s c r i b e d a b o v e . It r e s t s on a 50 ft. t h i c k l i g h t g r e y to w h i t e q u a r t z i t e , w h i c h c o n t a i n s d a r k e r a s w e l l a s y e l l o w i s h b a n d s . T h i s q u a r t z i t e i s d e p o s i t e d on a c o n g l o m e r a t i c h o r i z o n i d e n t i c a l w i t h the one a b o v e , of 30 ft. t h i c k n e s s . The f o r m a t i o n s d e s c r i b e d above f o r m the
U p p e r I n t e r m e d i a t e Reef G r o u p , w h i c h h a s a t o t a l t h i c k n e s s of a p p r o x i m a t e -ly 250 ft.
A y e l l o w i s h g r e y i m p u r e q u a r t z i t e , t h i c k n e s s 90 ft. , c o n t a i n i n g s o m e l i g h t b a n d s a n d a s i n g l e n a r r o w c o n g l o m e r a t e b a n d i n the m i d d l e , s e p a
