Paleogeographic significance of the oldest known oolite pebbles in the Archaean Swaziland Supergroup (South Africa)

Sedimentary Geology, 14 (1975) 123--133 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

PALEOGEOGRAPHIC SIGNIFICANCE OF THE OLDEST KNOWN OOLITE PEBBLES IN THE ARCHAEAN SWAZILAND SUPERGROUP (SOUTH AFRICA)

T.O. REIMER Anglo Alpha Cement (Pry) Ltd., Johannesburg (South Africa) (Received April 24, 1975)

ABSTRACT Reimer, T.O., 1975. Paleogeographic significance of the oldest known oolite pebbles in the Archaean Swaziland Supergroup (South Africa). Sediment. Geol., 14: 123--133. In conglomerates of the Archaean Swaziland Supergroup (>3200 m.y.) pebbles of a characteristic oolite were found, the source rock of which is a well-defined marker horizon at the base of the sedimentary succession. From the pebble finds it can be concluded that this oolite was deposited over a belt several km wide and up to 100 km long. This is comparable in size to recent conditions of oolite formation on the Great Bahama Bank.

INTRODUCTION In r e c e n t y e a r s o u r u n d e r s t a n d i n g o f t h e s e d i m e n t s o f the Swaziland Superg r o u p ( > 3 2 0 0 m . y . ) i n t h e B a r b e r t o n M o u n t a i n l a n d (Transvaal, S o u t h Africa; Fig. 1) has m a d e c o n s i d e r a b l e progress t h r o u g h detailed investigations o f subareas b y J o n e s (1963), Bell (1967), R e i m e r (1967, 1 9 7 3 ) , T o m l i n s o n (1967), A n h a e u s s e r ( 1 9 6 9 ) , Heinrichs (1969), V a n d e n Berg ( 1 9 6 9 ) a n d Heinrichs and R e i m e r ( 1 9 7 1 , in p r e p a r a t i o n ) . T h e stratigraphic c o l u m n o f Fig. 2 is b a s e d o n these r e p o r t s . A facies m o d e l f o r p a r t o f t h e Fig T r e e G r o u p c o u l d be d e v e l o p e d ( R e i m e r , 1973; Heinrichs a n d R e i m e r , in p r e p a r a t i o n ) a n d a p r e l i m i n a r y p a l e o g r a p h i c m a p was d r a w n u p f o r t h e M o o d i e s G r o u p ( R e i m e r , 1971). In b o t h cases t h e s o u r c e area o f t h e s e d i m e n t s lay s o u t h o f t h e present-day mountainland. T h e s o u r c e area o f t h e g r a y w a c k e s o f t h e S h e b a F o r m a t i o n ( l o w e r p a r t o f t h e Fig T r e e G r o u p , Fig. 2) c o u l d be r e c o n s t r u c t e d t h r o u g h detailed p e t r o graphic a n d g e o c h e m i c a l investigations ( C o n d i e et al., 1 9 7 0 ; R e i m e r , 1973). It c o n s i s t e d m a i n l y o f volcanic r o c k s (55%) a n d cherts (15%) o f the underlying O n v e r w a c h t G r o u p or its lateral equivalents. T h e r e m a i n i n g 30% w e r e p o t a s s i u m - r i c h granites a n d p e g m a t i t e s o f a laterally fairly extensive sialic crust. This sialic m a t e r i a l b e c a m e t h e p r e d o m i n a n t s o u r c e c o m p o n e n t during

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125

the deposition of the arenaceous sediments of the Moodies Group. In addition to crustal rocks Onverwacht volcanics and reworked Fig Tree sediments contributed to the Moodies sediments, the latter material in increasing

Fig.3. Msauli Oolite. a. Graded unit. b. poorly sorted layer. {Both from western part of Barberton Mountainland).

126

amounts in the higher parts of the Moodies. This is indicated by the upward decrease of the quartz/chert ratio within the Moodies sandstones evident from the modal analyses of Anhaeusser (1969). Intraformational erosion and reworking most probably occurred in the marginal parts of the Moodies depository. However, the lack of characteristic rocks made it so far extremely difficult to demonstrate such processes. In contrast to this the Fig Tree Group, especially in its southern facies, contains a number of sediments which in addition to their conspicuous appearance are of a highly siliceous nature. If eroded, such rocks should furnish ideal indicator pebbles. The most characteristic o f these siliceous rocks is an oolite-bearing horizon, the "Msauli Oolite", which forms part of the basal rocks of the Fig Tree Group in the southern facies (Heinrichs, 1969). It was first mentioned by Visser (1956) and the westernmost part of its outcrop area was described in detail by Reimer (1967). It is presently investigated over its whole extent by T. Heinrichs of the University of GSttingen. The origin of the siliceous oolite is still a matter of discussion but diagenetic silicification appears to have been an important factor. The zone attains a maximum thickness of about 20 m and consists of numerous graded beds up to 0,7 m thick. Individual units start with well-sorted large ooids of up to 1 cm in diameter gradually decreasing in size to the top where they grade into fine-clastic siliceous sediments (Fig. 3a). A unit usually is topped by chert of an originally particulate nature as demonstrated by frequently observable cross-bedding. Poorly sorted beds occur as well (Fig. 3b). The colour of the rock ~varies from gray over light turquoise to bright green. The latter two colours allow the rock to be identified even in fairly sheared condition and individual units could be traced laterally over several kilometers (Heinrichs, 1969 and personal communication, 1973). The total known extent of this distinctive marker horizon between the Onverwacht and the Fig Tree Group is shown in Fig. 1. To date 21 pebbles of this oolite and its associated rocks have been found at various places within the mountainland at four stratigraphic levels (Figs. 1 and 2). The main characteristics of these pebbles are presented in Table I. LOWER PART OF FIG TREE GROUP

In a graywacke from the base of the Sheba Formation near the Sheba Gold Mine a badly preserved fragment of a siliceous ooid was found by Reimer (1973) (sample 1, Table I). It could have been derived from outcrops of the Msauli oolite in the main source area to the south. However, a derivation from a narrow intrabasinal ridge which will be described below, appears more likely. Heinrichs and Reimer (1973) suggested that the ooids occurring in a band of the "Zwartkoppie Chert" below the Sheba Formation graywackes in the Sheba Mine (Ramsay, 1963) might have been supplied through reworking and redeposition from this ridge as well (Fig. 2).

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lightgray c r o s s - b e d d e d c h e r t as 6 as 6 as 6 d e n s e l y p a c k e d oolite as 16 slightly w e a t h e r e d o o l i t e w i t h c o r e o f c h e r t fragments w h i t e w e a t h e r e d oolite, m a t r i x p a r t l y f e r r u g i n o u s

o o i d s n o t c o m p l e t e l y silicified, m a t r i x darkgreen c h l o r i t i c c o m p l e t e l y silicified, m a t r i x d a r k g r e e n chloritic. as 2; m a t r i x w i t h p y r i t e c u b e s u p t o 0.25 m m as 4 reworked oolite material with isolated ooids o o i d s w i t h m a r g i n a l vein o f d i a g e n e t i c c h e r t as 6 green c r o s s - b e d d e d c h e r t w i t h r e w o r k e d Oolite material o o i d s a n d r e w o r k e d m a t e r i a l in clear m a t r i x as 6

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128 MIDDLE PART OF FIG TREE GROUP

Pebbles from this stratigraphic position {samples 2--11, Table I) were found in a conglomerate occurring below a sedimentary barite horizon on the farm Schoonoord 380 JU in the central part of the mountainland. The conglomerate was deposited on a local unconformity which developed over a shallow submarine ridge called "Proto-Inyoka Z o n e " by Heinrichs and Reimer (in preparation). Between lower and middle Fig Tree times erosion here progressed down to the basal cherts of this Group and thus could have reached the Msauli oolite as well. The position of this zone approximately coincides with the present day position of the Inyoka Fault in the central part of the mountainland {Fig. 1). The matrix of this conglomerate was originally dolomitic, but was later completely silicified (Fig. 4). The oolite pebbles are well rounded and their size distribution is presented in Fig. 5. It is similar to that of the pebbles as a whole which could indicate that they were all derived from the same locality. The specimens found in this conglomerate account for the 50% of all oolite pebbles found. Some parts of the conglomerate contain up to 4% oolite pebbles. The pebbles consist of slightly greenish-gray ooids in a dark-green matrix which is frequently less siliceous than the ooids. This nature of the matrix could have facilitated the breaking up of the rock into seperate ooids which then might have been redeposited as a problematic oolite occurring below sedimentary barite and the conglomerate mentioned (Heinrichs and Reimer,

1 cm Fig. 4. Pebble of Msauli Oolite in conglomerate of middle Fig Tree Group (Schoonoord

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in preparation). Disintegrated material of the Msauli Oolite could have been swept into pockets together with fragments of volcanic rocks and chert pebbles. Its present appearance could then have been caused by diagenetic processes which led to the partial replacement of the ooids by barite. The pebbles found in this conglomerate were derived from outcrops of the Msauli Oolite on the Proto-Inyoka Zone and were not transported over long distances. U P P E R P A R T OF FIG T R E E G R O U P

The pebbles from this stratigraphic position (samples 12--19, Table I) account for 40% of all oolite pebbles and were found in conglomerates on the farm Loenen 381 JU close to the Barberton--Havelock road (Fig. 1). They occur over a vertical extent of about 100 m, samples 18 and 19 coming from the highest outcrops. The Fig Tree Group consists at this locality of numerous thick bands of conglomerates alternating with shales, feldspathic sandstones, and in places even with cherts (Tomlinson, 1967). The matrix of these conglomerates is siliceous, argillaceous, or arenaceous. The size distribution of the pebbles larger than 0,5 cm is shown in Fig. 5. They are less well sorted than the pebbles in the conglomerate from the middle part of the Fig Tree Group which is not surprising in view of the nature of the surrounding sediments. The oolite pebbles found are coarser than the median grain size of the other pebbles (Fig. 5). This difference in size might indicate that the oolite pebbles were derived from the proximal parts of the source area. They account only for trace amounts, i.e. less than 1% of the whole conglomerate. The specimens (samples 12--19, Table I) represent all components of a graded oolite unit from coarse ooids to the cross-bedded chert usually terminating a unit. The pebbles are on average

130 larger than those found in the middle part of the Fig Tree Group. Despite their weathered state they appear to have originally been well silicified. The pebbles are usually fairly rounded, slightly flat in shape and frequently show indentations by other pebbles. Despite the small number of pebbles found, it appears that the size of a pebble is inversely proportional to the average size of its ooids. This was most probably caused by the fact that the material with larger ooids had more planes of inhomogeneity possibly due to incomplete silicification facilitating its disintegration into smaller units. These pebbles were derived from outcrops of the Msauli Oolite in the main source area to the south. In these conglomerates an additional oolite pebble was found which was not derived from the Msauli Oolite. It is characterised by a slightly argillaceous matrix which makes it similar to an oolite observed in argillaceous sediments of the lower part of the Fig Tree Group in the centre of the Barberton mountainland (Heinrichs and Reimer, 1971). MOODIES GROUP In conglomerates of this group so far only two oolite pebbles (10% of the total) have been found, one each in the northern and the southern facies (Figs. I and 2, Table I). The pebbles from the southern facies (sample 20 in Table I) was found in a fairly siliceous conglomerate in the lower part of the Moodies Group below Emlembe Mountain on the farm Josefsdal 382 JU close to the Swaziland border. It overlies the conglomerates of the upper part of the Fig Tree Group described above and was derived from the same general source area. The pebble from the northern facies (sample 21, Table I) was found in coarse-grained pebble-bearing sandstones on Schoongezicht 713 JT in the western part of the mountainland. It represents the westernmost known occurrence of such pebbles and occurs close to the westernmost outcrops of the Msauli Oolite (Fig. 1). The enclosing sandstones were derived from an area south of their present outcrop belt (Reimer, 1971). DISCUSSION OF OBSERVATIONS The concentration of the oolite pebbles in conglomerates of the upper Fig Tree Group, their scarcity in those of the lower Moodies Group, and their apparent absence in higher conglomerates is noteworthy. Conglomerates occur throughout the Fig Tree and Moodies Group in the southern facies and, consequently, the occurrence of the oolite pebbles within a certain stratigraphic interval is not solely attributable to this lithologic effect, i.e. to the presence or absence of conglomerates. It can rather be explained by the level of erosion in the proximal parts of the source area reaching the Msauli Oolite during the deposition of the upper sediments of the Fig Tree Group, but progressing below this level in Moodies times.

131 The occurrence of oolite pebbles in the conglomerate of the middle part of the Fig Tree Group appears to be rather localized phenomenon. These pebbles were eroded from a shallow submarine ridge and were only transported over a short distance. Some of this material could have been concentrated in a re-deposited oolite occurring below the conglomerate with the oolite pebbles. During early Fig Tree time this ridge might have been the source for the ooids observed in and below the graywackes of the Sheba Formation (Figs. 1 and 2). In Fig. 6 the occurrences of the pebbles are shown in relation to the general paleography of the depository of the Swaziland Supergroup, the strata having been reorientated into their approximate original positions. During early Fig Tree time the depositional area consisted of a turbidite basin in the north and was covered by shallower water to the south (l~eimer, 1973; Heinrichs and Reimer, in preparation). They were separated by the ridge of the Proto-Inyoka zone along which the Msauli Oolite was actively eroded (Fig. 6, block A). During middle and late Fig Tree time the zone lost its importance and oolite pebbles were supplied from the proximal parts of the source area (Fig. 6, block B). The areal distribution of the Msauli Oolite as reconstructed from the

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132 r e - o r i e n t a t i o n o f the preserved o u t c r o p s is s h o w n in relation t o the p r o b a b l e position o f the shore line during Fig Tree time in Fig. 6. A l t h o u g h it e x t e n d s at right angles the s e d i m e n t a r y strike c a n n o t be c o m p l e t e l y r e c o n s t r u c t e d ; the oolite belt m u s t have been at least 20 k m wide. Its laterally considerable e x t e n t w o u l d be a l m o s t d o u b l e d if the t w o o c c u r r e n c e s d e d u c e d f r o m the oolite pebbles described ( b l o c k B in Fig. 6) are t a k e n into consideration. Thus the oolite o c c u r r e d along a several k m wide belt r u n n i n g a p p r o x i m a t e l y E---W for a b o u t 100 km. This d i s t r i b u t i o n is c o m p a r a b l e to t h a t o f the r e c e n t oolites o c c u r r i n g along the Great B a h a m a Bank. Here oolites f o r m at a d e p t h o f 2 m in a belt o f up to 5 km in width over a length o f 90 km while Pleistocene oolites on A n d r o s Island cover a belt up to 22 km wide and 180 k m long (Ginsburg, in Blatt et al., 1973}. F r o m the d a t a p r e s e n t e d it can be concluded t h a t similar e n v i r o n m e n t a l c o n d i t i o n s existed during the earliest A r c h a e a n in s o u t h e r n Africa. This p a p e r illustrates the i m p o r t a n c e o f c o n g l o m e r a t e s as aids in determining the p a l e o g e o g r a p h y o f s e d i m e n t a r y f o r m a t i o n s even in intensely folded A r c h a e a n g r e e n s t o n e belts. ACKNOWLEDGEMENTS T h a n k s are due to Mrs. B.M. R e i m e r w h o f o u n d several o f the oolite pebbles and Mr. T.K. Heinrichs for s u p p l y i n g i n f o r m a t i o n on the e x t e n t o f the Msauli Oolite. Dr. H. Geldsetzer critically read the m a n u s c r i p t and m a d e suggestions for its i m p r o v e m e n t . The final version was kindly t y p e d by Mrs. P. Taylor. The p h o t o g r a p h s were t a k e n b y P. Nagel.

REFERENCES Anhaeusser, C.R., 1969. The Stratigraphy, Structure, and Gold Mineralization of the Jamestown and Sheba Hills Areas of the Barberton Mountainland. Thesis, Witwatersrand University, Johannesburg, unpublished. Bell, C.M, 1967. The Geology of the Farm Heemstede 33 and Portions of the Farms Schultzenhorst 31 and Mendon 32 in the Barberton Mountain Land. Thesis, Natal University, Durban, unpublished. Blatt, H., Middleton, G. and Murray, R., 1973. Origin of Sedimentary Rocks. Prentice Hall, Englewood Cliffs, N.J., 643 pp. Condie, K.C., Maeke, J. and Reimer, T.O., 1970. Petrology and geochemistry of early Preeambrian graywaekes from the Fig Tree Group, South Africa. Geol. Soc. Am. Bull., 81: 2759--2776. Heinriehs. T.K., 1969. Report on mapping carried out in search of chrysotile asbestos in part of the Barberton Mountainland between Geluk and Skokohla. Unpublished report, ETC mines, Barberton. Heinrichs, T.K. and Reimer, T.O., 1971. Geological map of portion of the central part of the Barberton Mountainland, unpublished. Heinrichs, T.K. and Reimer, T.O., 1973. Das Barberton Bergland, geringmetamorphes Relikt eines altpraekambrisehen Sedimentationsraumes. Paper presented at Annual Congress of Deutsche Geologische Gesellschaft at Frankfurt, unpublished.

133 Heinrichs, T.K. and Reimer, T.O., 1975. A sedimentary barite deposit from the Archaean Fig Tree Group of the Barberton Mountain land, in preparation. Jones, D.J., 1963. Geology of the Malolotsha Valley. Bull. Geol. Surv. Swaziland, 3: 17--27. Ramsay, J.G., 1963. Structural investigations in the Barberton Mountainland, Eastern Transvaal. Trans. Geol. Soc. S. Aft., 66: 352--398. Reimer, T.O., 1967. Die Geologie der Stolzburg Synklinale im Barberton Bergland (Transvaal-Stidafrika). Thesis, Goethe University, Frankfurt, unpublished. Reimer, T.O., 1971. Kanu-Faulten -- Ein ungewShnlicher Faltentyp aus geringmetamorphen praekambrischen Sedimenten. Neues Jahrb. Geol. Palaeontol. Monatsh., ] 971: 489---495. Reimer, T.O., 1973. Untersuchungen fiber Abtragung, Sedimentation und Diagenese in frfihen Praekambrium, am Beispiel der Sheba Formation (Siidafrika). Thesis, Goethe University, Frankfurt, unpublished. Tomlinson, R.G., 1967. The geology of an area between the Staircase Ridge and the Emlembe range, Barberton Mountainland. Thesis, Natal University, Durban, unpublished. Van den Berg, M.L., 1969. Report on the geology of the Josefdal area (Barberton Mountainland). Report to ETC mines, Barberton, unpublished. Visser, D.J.L., 1956. The geology of the Barberton Area. Geol. Surv. S. Afr. Spec. Publ., 1 5 : 2 5 3 pp.