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The Dwyka Series in Rhodesia
The Dwyka Series in Rhodesia by GEOFFREY BOND Received 5 January 1970
CONTENTS 1. INTRODUCTION page 463 2. FACIES DISTRIBUTION IN THE BASAL KARROO OF THE MID-ZAMBEZI REGION 465 468 3. THE PROBLEM OF THE AGE OF THE RHODESIAN DWYKA
471
REFERENCES
ABSTRACT: Glacial relics in the mid-Zambezi Karroo basin of deposition include tillite, outwash gravels, varves, a glaciated pavement, dropstones, and pre-glacial valleys which still contain some sediment. The geographical distribution of these indications of glaciation suggests a limited ice-cap centred on high ground at the eastern end of the basin, and a small freshwater lake in its central part. Suggestions on the age of the Rhodesian Dwyka are discussed. The palaeomagnetic and the palynological ages are in conflict. The difficulties of precise correlation of non-marine facies, such as coal measures with a Glossopteris flora but few animal fossils, and glacial sediments almost devoid of macrofossils, are stressed . The probability that on a drifting continent such facies will be diachronous, is pointed out.
1. INTRODUCTION
a title for this paper it might have been better to omit the term 'Dwyka' because it immediately implies a correlation with the wellknown Dwyka Series of South Africa. There is, however, little doubt that the rocks to be discussed are equivalent to some part of the rather long glacial succession of South Africa. The object of this paper is to put together the evidence from this glacial episode as it is expressed in Rhodesia in a palaeogeographic picture, and to see how this fits into the wider context of Southern Africa as a whole. In terms of published papers the amount of information available is small, and the Rhodesian Dwyka could not be said at present to be well documented. However, a great deal of information has been gathered in the last few years. Some of this is to be found in Ph.D. theses already presented to London University, still more will be found in theses which are still in preparation. Some of it is in my own field notes. In writing this paper I have had access to much of the new evidence. I am particularly grateful to Mrs. Rosemary Falcon for palynological data, and to A. G. Mann and G. Harper for information on their thesis areas in the eastern part of the mid-Zambezi Karroo basin. I have also seen their evidence in the field. I have visited the new exposures found by Humphreys and Chappell in the central part of the basin, and had the benefit of reading their theses (Humphreys, 1969; Chappell, 1969). While
IN CHOOSING
463
464
GEOFFREY BOND
I have drawn on the work of all these colleagues for factual information, the synthesis is my own responsibility, but I express to all of them my warmest thanks for much stimulating discussion. The first description of a Rhodesian diamictite was given by Molyneux (1922). He found tillites and associated fluvioglacial shales, which he assigned to the Lomagundi System, on the Msukwi River (Fig. 1) while on a foot traverse through the area. When Stagman (1962) mapped the area in more detail, he transferred them to the Karroo System as Dwyka tillite. Recent work by Mann and by Harper (Ph.D. theses in preparation) has shown that these rocks were involved in the violent pre-Karroo tectonism of the area, and are probably therefore of Katangan age. It is also worth noting that Molyneux (1909) had earlier described diamictites from the base of the Karroo System on the north flank of the Zambezi Valley, in the country which is now zambia. He took great pains to find a non-glacial origin for them, though they would probably now be thought to show some glacial imprint. Within the boundaries of Rhodesia the Dwyka was said to be absent as recently as the third edition of du Toil's Geology of South Africa (1954). Haughton (1969) writes: 'The absence of widespread continuous deposits of glacial origin at the base of the succession is presumed to be evidence of the existence ofa continental ice-cap over much of the area during Dwyka time. Only towards the end of the epoch and on the waning of the ice front did valley glaciers deposit morainic material.' The last part of this statement is based on the first account of Dwyka-like sediments in Rhodesia (Bond, 1952), which was soon followed by descriptions of similar rocks in Zambia (Taverner-Smith, 1960; Gair, 1959). A later account (Bond & Stocklmayer, 1967), based on a single borehole core, recorded apparent oscillations in the intensity of glaciation in the Zambezi Valley. The succession in that borehole includes three cycles of diamictites each followed by fluvioglacial shales, some of which are varved. Each cycle is thought to record the advance and retreat of a local ice-front. Since there is an apparent absence of similar sediments in the Limpopo Valley, the account in this paper is confined to the Karroo sedimentary basin of the mid-Zambezi Valley. Bearing in mind the late discovery of the Dwyka in the Zambezi Valley, the absence of similar rocks in the Limpopo basin may turn out to be more apparent than real. The detailed facies studies of the whole of the glaciated region of Gondwanaland by Frakes & Crowell (for example, 1967, 1969) have stimulated much interest in the late Palaeozoic glaciation of the southern hemisphere. They have made it possible to produce regional syntheses from each of the fragments of the old supercontinent. The discrimination between different facies of deposition of diamictites has recently been discussed in great detail by Landim & Frakes (1968).
THE DWYKA SERIES IN RHODESIA
465
Their sophisticated methods have not yet been applied in Rhodesia and the interpretation of facies based on less exact criteria, which have been used in previous accounts, must suffice for the time being.
2. FACIES DISTRIBUTION IN THE BASAL KARROO OF THE MID-ZAMBEZI REGION The base of the Karroo System has been seen in natural exposures and in borehole cores at many places within the mid-Zambezi basin. The contact is well-defined, as befits an unconformity of this magnitude, and the underlying rocks are fresh and unweathered. In the west, the Lower Karroo rests on Pre-Cambrian crystalline rocks of great age, but in the central and eastern parts of the basin the underlying rocks are often late Pre-Cambrian sediments, the Sijarira 'Series'. Outside the limits of the basin, various beds of the Upper Karroo rest directly on the old rocks of the Rhodesian basement, which are generally deeply weathered. The oldest member of the Lower Karroo System which can be recognised consistently over nearly the whole of the mid-Zambezi basin is the upper part of the Lower Wankie Sandstone of Lightfoot (1914; 1929). In a few places, irregularities of the Karroo floor were sufficiently pronounced for the basement to protrude through it. Below this horizon the basal beds are very variable iii thickness and lithology. The greatest thickness known is 330 ft. (about 100 m.), in the Matabola Flats borehole (Fig. 1) in the central part of the basin (Bond & Stocklmayer, 1967), but thicknesses fluctuate widely in nearby exposures. The basal beds can be divided into distinct facies, which show varying degrees of glacial imprint. On Fig. 1 the following features have been distinguished: tillite, outwash gravels, varves, striated floors, Karroo valleys with outwash gravels, direction of movement of erratics, number of glacial cycles present, absence of glacial indications, and Upper Karroo rocks resting directly on older formations. In general there is little difficulty in distinguishing these features, such things as varves and striated floors presenting no problem. The diamictites, however, are much more difficult to divide with assurance; it is not always easy, for example, to distinguish between true tillites and outwash gravels. Since the criteria suggested by Landim & Frakes (1968) have not yet been applied, the less sophisticated approach of Bond & Stocklmayer (1967) has in some cases been used. This attempts to distinguish between them by using the ratio of clasts to matrix expressed as a simple percentage. In other instances, field criteria only have been applied, as consistently as possible. Lightfoot (1914; 1929), working in the Wankie coalfield at the western end of the basin, was impressed by the absence of anything resembling
GEOFFREY BOND
466
the Dwyka. Watson (1960) remapped the area and confirmed his early observations. The same lack of glacial imprint was also noted by Watson (1962) in the Lubimbi coalfield.
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Fig. 1. Distribution of basal facies of the Karroo System in Rhodesia
In the central and eastern parts of the basin some sort of evidence of glaciation is commonly present at the base of the Karroo, and there is even some evidence of multiple glaciation. Two diamictites with intervening
THE DWYKA SERIES IN RHODESIA
467
varved shales occur in the locality from which the Dwyka was first described (Bond, 1952). There are three phases of diamictites, with fluvioglacial phases between them, in the Matabola Flats borehole (Bond & Stocklmayer, 1967).There are also three cycles several kilometres to the north-east, in natural exposures mapped by Chappell (1969). Farther east again, in boreholes at Bari and Renji, only one episode can be seen. In the extreme east of the region the Lower Karroo rocks are banked against a buried scarp, which present erosion has partly exhumed. The only evidence of glaciation in the vicinity of the scarp and to the east of it, is provided by deeply incised valleys still floored with coarse gravels, which can be traced westward until they are overlain by Lower Wankie Sandstone (Mann and Harper; Ph.D. theses in preparation). The gravels are cemented by coarse crystalline calcite and are jointed and faulted. They are massively current bedded. The present drainage has re-excavated the valleys in which they lie, and current-bedding directions indicate flow down one major river and up one of its tributaries, all in a general westerly direction. The gravels are poorly sorted and extremely coarse. They contain dolomite boulders of several tons, and occasional large soled and striated boulders of hard quartzite. It is also worth noting here that only in the eastern portion of the basin does the Lower Wankie Sandstone contain nests of dropstones and isolated boulders. Although these could conceivably have been rafted by vegetation, their large size is more consistent with ice-rafting. They have not been seen elsewhere in the basin. Most of the clasts in the diamictites are of rocks which are either common types in the crystalline basement, or in Sijarira quartzitic sandstones. A few types, however, are more restricted in occurrence and are consistent with derivation from an easterly source (Bond, 1952; Bond & Stocklmayer, 1967). All the features described above are summarised graphically on Fig. 1, from which some palaeogeographical reconstructions can be suggested. The most obvious point to be seen on Fig. 1 is the absence of glacial effects in the west, their development in the central and eastern part, and the sharp cut-off in the extreme east. A topographical high in Karroo times is known west of Wankie (Bond, 1953) and the floor under the Karroo is slightly irregular in the Wankie coalfield. The evidence, however, indicates that the Karroo landscape had a much greater relief at the eastern end of the basin. Sedimentation begins in the west with the Lower Wankie Sandstone, and there is only one doubtful record of a thin varved sediment below it. The abundance of fluvioglacial shales in the central part of the basin implies that this had a lower floor. At times a freshwater lake could have occurred here, into which meltwater streams poured seasonal loads of sediment with the development of varves. Evidence of more than one glacial cycle is also confined to this part of the basin, suggesting that it was PROC. GEOL. ASS., VOL. 81, PART 3, 1970
30
468
GEOFFREY BOND
near the maximum extension of an ice-lobe, and therefore the area most sensitive to the effects of minor advances and retreats. The balance of the evidence from events to be seen in the geological record of this time and the geographical distribution of facies, points not to a major continental ice-sheet but to a local mountain gathering-ground on the east of the basin, the 'Lomagundi Highlands'. Ice may have been present locally on the higher ground west of Wankie, but it has left no evidence of its activity. There is some palaeontological evidence to support the notion that considerable areas were, at least seasonally, free from snow or ice. Even the lowest Dwyka shale in the rnid-Zarnbezi Valley contains plant spores. The later shales yield considerable numbers, with a greater diversity of types. An insect wing, as yet undescribed, has been found only recently in the last fiuvioglacial shale, just below the base of the Lower Wankie Sandstone. Plant populations, insects, and open water could not occur under completely frigid conditions, but they are consistent with the short summers in quite high latitudes where the climatic regime is comparable with that of many Arctic regions today. In Natal and the Cape Province of South Africa the Dwyka sediments are thick and continuous. North of the Transvaal glacial sediments become thin and sporadic, as though they were locally generated and depended on high ground of limited extent to form glacial 'outliers', marginal to the main centres. The evidence of the mid-Zambezi Valley fits this pattern geographically and in the nature of the sediments.
3. THE PROBLEM OF THE AGE OF THE RHODESIAN DWYKA In the first description of the mid-Zambezi Dwyka (Bond, 1952) it was tentatively suggested that the deposits were formed in late Dwyka time. It was argued that the deposits represented the waning phases of glaciation of local mountain type. rather than those of a continental ice-sheet. Waning glaciation is more likely to leave a record of sediment, because early deposits of waxing ice-sheets are probably rubbed out during the advance. Since the Upper Dwyka Shales of South Africa were palaeontologically regarded as Upper Carboniferous. the Rhodesian glaciation was also labelled Upper (even uppermost) Carboniferous. Furthermore. the wellknown flora from a thin shale band in the Upper Wankie Sandstone is stratigraphically only about 300 ft. (91 m.) above the glacial beds. This flora has been regarded as Middle Ecca in the South African sequence and Lower Permian by virtue of the 'northern floral element' which it contains. On the rather flimsy grounds of lithological similarity with South Africa,
THE DWYKA SERIES IN RHODESIA
469
the underlying Black Shale and Coal of Lightfoot's succession has also been regarded as Middle Ecca,leaving only the Lower Wankie Sandstone to represent the Lower Ecca. Chappell (1969) discovered a well-preserved flora in the Black Shale and Coal member in the central part of the Zambezi basin. The relative abundance of Gangamopteris indicates Lower Ecca affinities (Bond, 1968), a hint that the Wankie coal may be a little older than was previously thought likely; Gangamopterists first known from the Upper Carboniferous (Alvin and others, 1967, 252). There is no sign in the field of a major break in sedimentation in this part of the succession. The palynological evidence (R. Falcon, personal communication) shows no discontinuity, the spore assemblage merely shows continuing diversification. Although none of the geological evidence from the Zambezi Valley is conclusive, on balance it suggests a late Upper Carboniferous age for this glacial episode. The palynological evidence so far gathered is consistent with an even younger age, in the Lower Permian. McElhinney (1967), however, has suggested that the palaeomagnetic evidence, from varved shales within the Rhodesian Dwyka, is much more compatible with a Lower than with an Upper Carboniferous age. This poses some intriguing questions. The South African Middle Ecca is regarded as Lower Permian. If the floral remains from the Upper Wankie Sandstone are correctly correlated with those of the Middle Ecca, this must also be Lower Permian. If the palaeomagnetic 'age' is accepted, what has happened to the Upper Carboniferous? It could hardly be represented by the Lower Wankie Sandstone, which in any case has all the signs of rapid accumulation, and not of a condensed succession. Should the Rhodesian 'Middle Ecca', in spite of its 'northern elements', be put down into the Upper Carboniferous? This merely produces the complication of missing time higher in the sequence. The Tapinocephalus Zone of the Lower Beaufort has been identified with considerable assurance in the middle part of the Madumabisa Mudstone member of the Rhodesian Karroo. Internal evidence from an insect (Zeuner, 1955), and from fish remains, phyllopods and non-marine bivalves of Russian aspect (Bond, 1955b) all support an Upper Permian age for this faunal horizon. If this is so, then the Lower Permian might be missing, but the field evidence again fails to show any break. Borehole cores reveal no sign of erosion, conglomerates or even a weathering horizon which could signify a major discontinuity in sedimentation. This kind of reasoning could be used to chase missing time right up the stratigraphical column of the Rhodesian Karroo. Another alternative which is implied by Haughton (1969) and by Swift (1962) is that the whole region was covered during the Upper Carboniferous by a stagnant cake of continental ice, so that the land surface was in a state of suspended
470
GEOFFREY BOND
animation, being neither eroded, weathered nor receiving sediment. During its drift across the polar regions, Rhodesia may at some time have been covered by such an ice-mass, but, as pointed out above, it has left no sediments to record its presence, and can only remain a hypothetical possibility. It has also been shown that the Zambezi Dwyka is more likely to have been formed in the late stages of an ice age than in the early stages, and that sedimentation was continuous, but the glacial episode brief. What really emerges from this kind of discussion is the great difficulty of equating thick non-marine successions in Gondwanaland with a stratigraphical scheme based so predominantly on marine invertebrate faunas. There are places in Gondwanaland, notably in Australia and Madagascar, where marine and non-marine facies are interbedded to provide starting points for such correlations, which must be carried vast distances by means of non-marine faunas and floras, and by lithological similarities. Fine correlations may well get blurred at the edges in the process. Lithological facies may be diachronous over the large distances involved, especially if fairly rapid rates of continental drift have operated (Briden, 1967). To illustrate this point it is worth digressing up the stratigraphical column to the Upper Triassic Stormberg Series of the Karroo. On the grounds of lithological similarity the Cave Sandstone of South Africa was for a time correlated with the Forest Sandstone of Rhodesia. Both formations indicate rather arid conditions just before the onset of the Stormberg vulcanicity. The vertebrate remains, however, show that the Forest Sandstone is of approximately the same age as the Upper Red Beds of the Cape (Bond, 1955a) and the arid facies is diachronous. Palaeomagnetism indicates that the base of the basalts is older in the Drakensberg than it is in Rhodesia, and isotopic dating has demonstrated that there is 20 m.y, difference in time between the start of vulcanicity in the east and west. There could be an analogy here with Ecca times, coal-forming conditions being possible earlier in the north than in the Transvaal and Natal, The Glossopteris flora, which provides the main evidence for correlation, may not be a sensitive enough indicator to diagnose the time difference. The coal-bearing facies of the Ecca may be at least as diachronous as the arid facies of the Stormberg. The Dwyka facies is poor in macrofossils and is even more difficult to correlate on a finely divided time-scale. Syntheses which attempt to put the time-sequence of glaciations in order as Gondwanaland drifted across the pole depend on very exact correlations. It is perhaps time that the fundamental evidence on which the generally accepted correlations are based, was critically reviewed. Until this is done the dating of the various members of the Rhodesian Lower Karroo sequence will remain in some doubt. It is hoped that detailed palynological work may provide an answer. Preliminary results from this line of enquiry seem to indicate that even the thickest Rhodesian Dwyka only records a
THE DWYKA SERIES IN RHODESIA
471
brief glacial episode, that this was near the end of the very long ice age which occurred in South Africa, that it has no Lower Carboniferous affinities and may therefore have occurred as late as the beginning of the Permian Period. REFERENCES ALVIN, K. L. and others. 1967. Gymnospermophyta. In Harland, W. B. and others (Eds.): The Fossil Record. London (Geological Society), 247-68. BOND, G. 1952. Evidence of Glaciation in the Lower Part of the Karroo System in Southern Rhodesia. Trans. geol. Soc. S. Afr., 15, I-II. - - - . 1953. Notes on the Geology of the Headwaters of the Deka River. Occ. Pap. natn. Mus. Sth. Rhod., No. 18, 521-8. - - - . 1955a. A Note on Dinosaur Remains from the Forest Sandstone (Upper Karroo). Occ. Pap. natn, Mus. Sth. Rhod., No. 20, 795-800. - - - . 1955b. The Madumabisa (Karroo) Shales of the Middle Zambezi Region. Trans. geol. Soc. S. Afr., 58, 71-101. - - - . 1968. The Flora of the Black Shale and Coal Group (K2) in the MidZambezi Basin. Arnoldia, 3, No. 38, 1-2. - - - & V. R. C. STOCKLMAYER. 1967. Possible Ice Margin Fluctuations in the Dwyka Series in Rhodesia. Palaeogeog, Palaeoclim. and Palaeoecol., 3, 433-46. BRIDEN, J. C. 1967. Recurrent Continental Drift of Gondwanaland. Nature, Lond., 215, 1334---9. CHAPPELL, J. 1969. The Geology of the Eastern Portion of the Chisarira Game Reserve and Adjacent Country, Rhodesia. Ph.D. thesis. University of London. DUTOIT, A. L. 1954. The Geology of South Africa. Oliver & Boyd (3rd Ed.). FRAKES, L. A. & J. C. CROWELL. 1967. Facies and Palaeogeography of Late Lafonian Diamictite, Falkland Islands. Bull. Geol. Soc. Am., 78, 37-58. - - - . 1969. Late Palaeozoic Glaciation: I. South America. Bull. geol. Soc. Am., 80, 1007-42. GAIR, H. S. 1959. The Karroo System and Coal Resources of the Gwembe District, North-East Section. Bull. geol. Sun. Nth. Rhod., No.1. HARPER, G. Ph.D. thesis (in preparation). HAUGHTON, S. H. 1969. The Geological History of South Africa. Geological Soc. of S. Africa. Cape Town. HUMPHREYS, M. 1969. The Geology of the Western Portion of the Chisarira Game Reserve, and Adjacent Country, Rhodesia. Ph.D. thesis. University of London. LANDIM, P. M. B. & L. A. FRAKES. 1968. Distinction between Tills and Other Diamictons based on Textural Characteristics. I. sedim. Petrol., 38, 1213-23. LIGHTFOOT, B. 1914. The Geology of the NW. part of the Wankie Coalfield. Bull. geol. Sun. Sth. Rhod., No.4. - - - . 1929. The Geology of the Central Part of the Wankie Coalfield. Bull. geol. Surv. Sth, Rhod., No. 15. MANN, A. G. Ph.D. thesis (in preparation). McELHINNEY, M. W. 1967. The Mid-Carboniferous Continental Drift Episode. Some Evidence from Australia and Africa. U.N.E.S.C.O. Symp. on Continental Drift. Montevideo. MOLYNEUX, A. J. C. 1909. The Karroo System in Northern Rhodesia. Q. II geol. Soc. Lond., 65, 408-39. - - - . 1922. Geological Reconnaissance in the Sanyati Valley. Bull. geol, Sun. Sth. Rhod., Short Rep. No. 13. STAGMAN, J. G. 1962. The Geology of the Southern Urungwe District. Bull. geol. Sun. Sth. Rhod., No. 55. SWIFT, W. H.1962. Outline of the Geological History of Southern Rhodesia. Bull. geol. Sun. Sth. Rhod., No. 50. TAVERNER-SMITH, R. 1960. The Karroo System and Coal Resources of the Gwembi District. South-Western Section. Bull. geol. Sun. Nth. Rhod., No.4.
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WATSON, R. L. A. 1960. The Geology and Coal Resources of the Country around Wankie, S. Rhodesia. Bull. geol. Surv. Sth. Rhod., No. 48. - - - . 1962. The Geology of the Kamativi and Lubimbi Areas. Bull. geol. Surv. Sth. Rhod., No. 57. ZEUNER, F. E. 1955. A Fossil Blastoid from the Permian of Rhodesia. Ann. Mag. nat. Hist., Ser. 12,8,685-8.
Geoffrey Bond Department of Geology University College of Rhodesia Box MP 167, Mount Pleasant Salisbury, Rhodesia
CONTENTS 1. INTRODUCTION page 463 2. FACIES DISTRIBUTION IN THE BASAL KARROO OF THE MID-ZAMBEZI REGION 465 468 3. THE PROBLEM OF THE AGE OF THE RHODESIAN DWYKA
471
REFERENCES
ABSTRACT: Glacial relics in the mid-Zambezi Karroo basin of deposition include tillite, outwash gravels, varves, a glaciated pavement, dropstones, and pre-glacial valleys which still contain some sediment. The geographical distribution of these indications of glaciation suggests a limited ice-cap centred on high ground at the eastern end of the basin, and a small freshwater lake in its central part. Suggestions on the age of the Rhodesian Dwyka are discussed. The palaeomagnetic and the palynological ages are in conflict. The difficulties of precise correlation of non-marine facies, such as coal measures with a Glossopteris flora but few animal fossils, and glacial sediments almost devoid of macrofossils, are stressed . The probability that on a drifting continent such facies will be diachronous, is pointed out.
1. INTRODUCTION
a title for this paper it might have been better to omit the term 'Dwyka' because it immediately implies a correlation with the wellknown Dwyka Series of South Africa. There is, however, little doubt that the rocks to be discussed are equivalent to some part of the rather long glacial succession of South Africa. The object of this paper is to put together the evidence from this glacial episode as it is expressed in Rhodesia in a palaeogeographic picture, and to see how this fits into the wider context of Southern Africa as a whole. In terms of published papers the amount of information available is small, and the Rhodesian Dwyka could not be said at present to be well documented. However, a great deal of information has been gathered in the last few years. Some of this is to be found in Ph.D. theses already presented to London University, still more will be found in theses which are still in preparation. Some of it is in my own field notes. In writing this paper I have had access to much of the new evidence. I am particularly grateful to Mrs. Rosemary Falcon for palynological data, and to A. G. Mann and G. Harper for information on their thesis areas in the eastern part of the mid-Zambezi Karroo basin. I have also seen their evidence in the field. I have visited the new exposures found by Humphreys and Chappell in the central part of the basin, and had the benefit of reading their theses (Humphreys, 1969; Chappell, 1969). While
IN CHOOSING
463
464
GEOFFREY BOND
I have drawn on the work of all these colleagues for factual information, the synthesis is my own responsibility, but I express to all of them my warmest thanks for much stimulating discussion. The first description of a Rhodesian diamictite was given by Molyneux (1922). He found tillites and associated fluvioglacial shales, which he assigned to the Lomagundi System, on the Msukwi River (Fig. 1) while on a foot traverse through the area. When Stagman (1962) mapped the area in more detail, he transferred them to the Karroo System as Dwyka tillite. Recent work by Mann and by Harper (Ph.D. theses in preparation) has shown that these rocks were involved in the violent pre-Karroo tectonism of the area, and are probably therefore of Katangan age. It is also worth noting that Molyneux (1909) had earlier described diamictites from the base of the Karroo System on the north flank of the Zambezi Valley, in the country which is now zambia. He took great pains to find a non-glacial origin for them, though they would probably now be thought to show some glacial imprint. Within the boundaries of Rhodesia the Dwyka was said to be absent as recently as the third edition of du Toil's Geology of South Africa (1954). Haughton (1969) writes: 'The absence of widespread continuous deposits of glacial origin at the base of the succession is presumed to be evidence of the existence ofa continental ice-cap over much of the area during Dwyka time. Only towards the end of the epoch and on the waning of the ice front did valley glaciers deposit morainic material.' The last part of this statement is based on the first account of Dwyka-like sediments in Rhodesia (Bond, 1952), which was soon followed by descriptions of similar rocks in Zambia (Taverner-Smith, 1960; Gair, 1959). A later account (Bond & Stocklmayer, 1967), based on a single borehole core, recorded apparent oscillations in the intensity of glaciation in the Zambezi Valley. The succession in that borehole includes three cycles of diamictites each followed by fluvioglacial shales, some of which are varved. Each cycle is thought to record the advance and retreat of a local ice-front. Since there is an apparent absence of similar sediments in the Limpopo Valley, the account in this paper is confined to the Karroo sedimentary basin of the mid-Zambezi Valley. Bearing in mind the late discovery of the Dwyka in the Zambezi Valley, the absence of similar rocks in the Limpopo basin may turn out to be more apparent than real. The detailed facies studies of the whole of the glaciated region of Gondwanaland by Frakes & Crowell (for example, 1967, 1969) have stimulated much interest in the late Palaeozoic glaciation of the southern hemisphere. They have made it possible to produce regional syntheses from each of the fragments of the old supercontinent. The discrimination between different facies of deposition of diamictites has recently been discussed in great detail by Landim & Frakes (1968).
THE DWYKA SERIES IN RHODESIA
465
Their sophisticated methods have not yet been applied in Rhodesia and the interpretation of facies based on less exact criteria, which have been used in previous accounts, must suffice for the time being.
2. FACIES DISTRIBUTION IN THE BASAL KARROO OF THE MID-ZAMBEZI REGION The base of the Karroo System has been seen in natural exposures and in borehole cores at many places within the mid-Zambezi basin. The contact is well-defined, as befits an unconformity of this magnitude, and the underlying rocks are fresh and unweathered. In the west, the Lower Karroo rests on Pre-Cambrian crystalline rocks of great age, but in the central and eastern parts of the basin the underlying rocks are often late Pre-Cambrian sediments, the Sijarira 'Series'. Outside the limits of the basin, various beds of the Upper Karroo rest directly on the old rocks of the Rhodesian basement, which are generally deeply weathered. The oldest member of the Lower Karroo System which can be recognised consistently over nearly the whole of the mid-Zambezi basin is the upper part of the Lower Wankie Sandstone of Lightfoot (1914; 1929). In a few places, irregularities of the Karroo floor were sufficiently pronounced for the basement to protrude through it. Below this horizon the basal beds are very variable iii thickness and lithology. The greatest thickness known is 330 ft. (about 100 m.), in the Matabola Flats borehole (Fig. 1) in the central part of the basin (Bond & Stocklmayer, 1967), but thicknesses fluctuate widely in nearby exposures. The basal beds can be divided into distinct facies, which show varying degrees of glacial imprint. On Fig. 1 the following features have been distinguished: tillite, outwash gravels, varves, striated floors, Karroo valleys with outwash gravels, direction of movement of erratics, number of glacial cycles present, absence of glacial indications, and Upper Karroo rocks resting directly on older formations. In general there is little difficulty in distinguishing these features, such things as varves and striated floors presenting no problem. The diamictites, however, are much more difficult to divide with assurance; it is not always easy, for example, to distinguish between true tillites and outwash gravels. Since the criteria suggested by Landim & Frakes (1968) have not yet been applied, the less sophisticated approach of Bond & Stocklmayer (1967) has in some cases been used. This attempts to distinguish between them by using the ratio of clasts to matrix expressed as a simple percentage. In other instances, field criteria only have been applied, as consistently as possible. Lightfoot (1914; 1929), working in the Wankie coalfield at the western end of the basin, was impressed by the absence of anything resembling
GEOFFREY BOND
466
the Dwyka. Watson (1960) remapped the area and confirmed his early observations. The same lack of glacial imprint was also noted by Watson (1962) in the Lubimbi coalfield.
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Fig. 1. Distribution of basal facies of the Karroo System in Rhodesia
In the central and eastern parts of the basin some sort of evidence of glaciation is commonly present at the base of the Karroo, and there is even some evidence of multiple glaciation. Two diamictites with intervening
THE DWYKA SERIES IN RHODESIA
467
varved shales occur in the locality from which the Dwyka was first described (Bond, 1952). There are three phases of diamictites, with fluvioglacial phases between them, in the Matabola Flats borehole (Bond & Stocklmayer, 1967).There are also three cycles several kilometres to the north-east, in natural exposures mapped by Chappell (1969). Farther east again, in boreholes at Bari and Renji, only one episode can be seen. In the extreme east of the region the Lower Karroo rocks are banked against a buried scarp, which present erosion has partly exhumed. The only evidence of glaciation in the vicinity of the scarp and to the east of it, is provided by deeply incised valleys still floored with coarse gravels, which can be traced westward until they are overlain by Lower Wankie Sandstone (Mann and Harper; Ph.D. theses in preparation). The gravels are cemented by coarse crystalline calcite and are jointed and faulted. They are massively current bedded. The present drainage has re-excavated the valleys in which they lie, and current-bedding directions indicate flow down one major river and up one of its tributaries, all in a general westerly direction. The gravels are poorly sorted and extremely coarse. They contain dolomite boulders of several tons, and occasional large soled and striated boulders of hard quartzite. It is also worth noting here that only in the eastern portion of the basin does the Lower Wankie Sandstone contain nests of dropstones and isolated boulders. Although these could conceivably have been rafted by vegetation, their large size is more consistent with ice-rafting. They have not been seen elsewhere in the basin. Most of the clasts in the diamictites are of rocks which are either common types in the crystalline basement, or in Sijarira quartzitic sandstones. A few types, however, are more restricted in occurrence and are consistent with derivation from an easterly source (Bond, 1952; Bond & Stocklmayer, 1967). All the features described above are summarised graphically on Fig. 1, from which some palaeogeographical reconstructions can be suggested. The most obvious point to be seen on Fig. 1 is the absence of glacial effects in the west, their development in the central and eastern part, and the sharp cut-off in the extreme east. A topographical high in Karroo times is known west of Wankie (Bond, 1953) and the floor under the Karroo is slightly irregular in the Wankie coalfield. The evidence, however, indicates that the Karroo landscape had a much greater relief at the eastern end of the basin. Sedimentation begins in the west with the Lower Wankie Sandstone, and there is only one doubtful record of a thin varved sediment below it. The abundance of fluvioglacial shales in the central part of the basin implies that this had a lower floor. At times a freshwater lake could have occurred here, into which meltwater streams poured seasonal loads of sediment with the development of varves. Evidence of more than one glacial cycle is also confined to this part of the basin, suggesting that it was PROC. GEOL. ASS., VOL. 81, PART 3, 1970
30
468
GEOFFREY BOND
near the maximum extension of an ice-lobe, and therefore the area most sensitive to the effects of minor advances and retreats. The balance of the evidence from events to be seen in the geological record of this time and the geographical distribution of facies, points not to a major continental ice-sheet but to a local mountain gathering-ground on the east of the basin, the 'Lomagundi Highlands'. Ice may have been present locally on the higher ground west of Wankie, but it has left no evidence of its activity. There is some palaeontological evidence to support the notion that considerable areas were, at least seasonally, free from snow or ice. Even the lowest Dwyka shale in the rnid-Zarnbezi Valley contains plant spores. The later shales yield considerable numbers, with a greater diversity of types. An insect wing, as yet undescribed, has been found only recently in the last fiuvioglacial shale, just below the base of the Lower Wankie Sandstone. Plant populations, insects, and open water could not occur under completely frigid conditions, but they are consistent with the short summers in quite high latitudes where the climatic regime is comparable with that of many Arctic regions today. In Natal and the Cape Province of South Africa the Dwyka sediments are thick and continuous. North of the Transvaal glacial sediments become thin and sporadic, as though they were locally generated and depended on high ground of limited extent to form glacial 'outliers', marginal to the main centres. The evidence of the mid-Zambezi Valley fits this pattern geographically and in the nature of the sediments.
3. THE PROBLEM OF THE AGE OF THE RHODESIAN DWYKA In the first description of the mid-Zambezi Dwyka (Bond, 1952) it was tentatively suggested that the deposits were formed in late Dwyka time. It was argued that the deposits represented the waning phases of glaciation of local mountain type. rather than those of a continental ice-sheet. Waning glaciation is more likely to leave a record of sediment, because early deposits of waxing ice-sheets are probably rubbed out during the advance. Since the Upper Dwyka Shales of South Africa were palaeontologically regarded as Upper Carboniferous. the Rhodesian glaciation was also labelled Upper (even uppermost) Carboniferous. Furthermore. the wellknown flora from a thin shale band in the Upper Wankie Sandstone is stratigraphically only about 300 ft. (91 m.) above the glacial beds. This flora has been regarded as Middle Ecca in the South African sequence and Lower Permian by virtue of the 'northern floral element' which it contains. On the rather flimsy grounds of lithological similarity with South Africa,
THE DWYKA SERIES IN RHODESIA
469
the underlying Black Shale and Coal of Lightfoot's succession has also been regarded as Middle Ecca,leaving only the Lower Wankie Sandstone to represent the Lower Ecca. Chappell (1969) discovered a well-preserved flora in the Black Shale and Coal member in the central part of the Zambezi basin. The relative abundance of Gangamopteris indicates Lower Ecca affinities (Bond, 1968), a hint that the Wankie coal may be a little older than was previously thought likely; Gangamopterists first known from the Upper Carboniferous (Alvin and others, 1967, 252). There is no sign in the field of a major break in sedimentation in this part of the succession. The palynological evidence (R. Falcon, personal communication) shows no discontinuity, the spore assemblage merely shows continuing diversification. Although none of the geological evidence from the Zambezi Valley is conclusive, on balance it suggests a late Upper Carboniferous age for this glacial episode. The palynological evidence so far gathered is consistent with an even younger age, in the Lower Permian. McElhinney (1967), however, has suggested that the palaeomagnetic evidence, from varved shales within the Rhodesian Dwyka, is much more compatible with a Lower than with an Upper Carboniferous age. This poses some intriguing questions. The South African Middle Ecca is regarded as Lower Permian. If the floral remains from the Upper Wankie Sandstone are correctly correlated with those of the Middle Ecca, this must also be Lower Permian. If the palaeomagnetic 'age' is accepted, what has happened to the Upper Carboniferous? It could hardly be represented by the Lower Wankie Sandstone, which in any case has all the signs of rapid accumulation, and not of a condensed succession. Should the Rhodesian 'Middle Ecca', in spite of its 'northern elements', be put down into the Upper Carboniferous? This merely produces the complication of missing time higher in the sequence. The Tapinocephalus Zone of the Lower Beaufort has been identified with considerable assurance in the middle part of the Madumabisa Mudstone member of the Rhodesian Karroo. Internal evidence from an insect (Zeuner, 1955), and from fish remains, phyllopods and non-marine bivalves of Russian aspect (Bond, 1955b) all support an Upper Permian age for this faunal horizon. If this is so, then the Lower Permian might be missing, but the field evidence again fails to show any break. Borehole cores reveal no sign of erosion, conglomerates or even a weathering horizon which could signify a major discontinuity in sedimentation. This kind of reasoning could be used to chase missing time right up the stratigraphical column of the Rhodesian Karroo. Another alternative which is implied by Haughton (1969) and by Swift (1962) is that the whole region was covered during the Upper Carboniferous by a stagnant cake of continental ice, so that the land surface was in a state of suspended
470
GEOFFREY BOND
animation, being neither eroded, weathered nor receiving sediment. During its drift across the polar regions, Rhodesia may at some time have been covered by such an ice-mass, but, as pointed out above, it has left no sediments to record its presence, and can only remain a hypothetical possibility. It has also been shown that the Zambezi Dwyka is more likely to have been formed in the late stages of an ice age than in the early stages, and that sedimentation was continuous, but the glacial episode brief. What really emerges from this kind of discussion is the great difficulty of equating thick non-marine successions in Gondwanaland with a stratigraphical scheme based so predominantly on marine invertebrate faunas. There are places in Gondwanaland, notably in Australia and Madagascar, where marine and non-marine facies are interbedded to provide starting points for such correlations, which must be carried vast distances by means of non-marine faunas and floras, and by lithological similarities. Fine correlations may well get blurred at the edges in the process. Lithological facies may be diachronous over the large distances involved, especially if fairly rapid rates of continental drift have operated (Briden, 1967). To illustrate this point it is worth digressing up the stratigraphical column to the Upper Triassic Stormberg Series of the Karroo. On the grounds of lithological similarity the Cave Sandstone of South Africa was for a time correlated with the Forest Sandstone of Rhodesia. Both formations indicate rather arid conditions just before the onset of the Stormberg vulcanicity. The vertebrate remains, however, show that the Forest Sandstone is of approximately the same age as the Upper Red Beds of the Cape (Bond, 1955a) and the arid facies is diachronous. Palaeomagnetism indicates that the base of the basalts is older in the Drakensberg than it is in Rhodesia, and isotopic dating has demonstrated that there is 20 m.y, difference in time between the start of vulcanicity in the east and west. There could be an analogy here with Ecca times, coal-forming conditions being possible earlier in the north than in the Transvaal and Natal, The Glossopteris flora, which provides the main evidence for correlation, may not be a sensitive enough indicator to diagnose the time difference. The coal-bearing facies of the Ecca may be at least as diachronous as the arid facies of the Stormberg. The Dwyka facies is poor in macrofossils and is even more difficult to correlate on a finely divided time-scale. Syntheses which attempt to put the time-sequence of glaciations in order as Gondwanaland drifted across the pole depend on very exact correlations. It is perhaps time that the fundamental evidence on which the generally accepted correlations are based, was critically reviewed. Until this is done the dating of the various members of the Rhodesian Lower Karroo sequence will remain in some doubt. It is hoped that detailed palynological work may provide an answer. Preliminary results from this line of enquiry seem to indicate that even the thickest Rhodesian Dwyka only records a
THE DWYKA SERIES IN RHODESIA
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brief glacial episode, that this was near the end of the very long ice age which occurred in South Africa, that it has no Lower Carboniferous affinities and may therefore have occurred as late as the beginning of the Permian Period. REFERENCES ALVIN, K. L. and others. 1967. Gymnospermophyta. In Harland, W. B. and others (Eds.): The Fossil Record. London (Geological Society), 247-68. BOND, G. 1952. Evidence of Glaciation in the Lower Part of the Karroo System in Southern Rhodesia. Trans. geol. Soc. S. Afr., 15, I-II. - - - . 1953. Notes on the Geology of the Headwaters of the Deka River. Occ. Pap. natn. Mus. Sth. Rhod., No. 18, 521-8. - - - . 1955a. A Note on Dinosaur Remains from the Forest Sandstone (Upper Karroo). Occ. Pap. natn, Mus. Sth. Rhod., No. 20, 795-800. - - - . 1955b. The Madumabisa (Karroo) Shales of the Middle Zambezi Region. Trans. geol. Soc. S. Afr., 58, 71-101. - - - . 1968. The Flora of the Black Shale and Coal Group (K2) in the MidZambezi Basin. Arnoldia, 3, No. 38, 1-2. - - - & V. R. C. STOCKLMAYER. 1967. Possible Ice Margin Fluctuations in the Dwyka Series in Rhodesia. Palaeogeog, Palaeoclim. and Palaeoecol., 3, 433-46. BRIDEN, J. C. 1967. Recurrent Continental Drift of Gondwanaland. Nature, Lond., 215, 1334---9. CHAPPELL, J. 1969. The Geology of the Eastern Portion of the Chisarira Game Reserve and Adjacent Country, Rhodesia. Ph.D. thesis. University of London. DUTOIT, A. L. 1954. The Geology of South Africa. Oliver & Boyd (3rd Ed.). FRAKES, L. A. & J. C. CROWELL. 1967. Facies and Palaeogeography of Late Lafonian Diamictite, Falkland Islands. Bull. Geol. Soc. Am., 78, 37-58. - - - . 1969. Late Palaeozoic Glaciation: I. South America. Bull. geol. Soc. Am., 80, 1007-42. GAIR, H. S. 1959. The Karroo System and Coal Resources of the Gwembe District, North-East Section. Bull. geol. Sun. Nth. Rhod., No.1. HARPER, G. Ph.D. thesis (in preparation). HAUGHTON, S. H. 1969. The Geological History of South Africa. Geological Soc. of S. Africa. Cape Town. HUMPHREYS, M. 1969. The Geology of the Western Portion of the Chisarira Game Reserve, and Adjacent Country, Rhodesia. Ph.D. thesis. University of London. LANDIM, P. M. B. & L. A. FRAKES. 1968. Distinction between Tills and Other Diamictons based on Textural Characteristics. I. sedim. Petrol., 38, 1213-23. LIGHTFOOT, B. 1914. The Geology of the NW. part of the Wankie Coalfield. Bull. geol. Sun. Sth. Rhod., No.4. - - - . 1929. The Geology of the Central Part of the Wankie Coalfield. Bull. geol. Surv. Sth, Rhod., No. 15. MANN, A. G. Ph.D. thesis (in preparation). McELHINNEY, M. W. 1967. The Mid-Carboniferous Continental Drift Episode. Some Evidence from Australia and Africa. U.N.E.S.C.O. Symp. on Continental Drift. Montevideo. MOLYNEUX, A. J. C. 1909. The Karroo System in Northern Rhodesia. Q. II geol. Soc. Lond., 65, 408-39. - - - . 1922. Geological Reconnaissance in the Sanyati Valley. Bull. geol, Sun. Sth. Rhod., Short Rep. No. 13. STAGMAN, J. G. 1962. The Geology of the Southern Urungwe District. Bull. geol. Sun. Sth. Rhod., No. 55. SWIFT, W. H.1962. Outline of the Geological History of Southern Rhodesia. Bull. geol. Sun. Sth. Rhod., No. 50. TAVERNER-SMITH, R. 1960. The Karroo System and Coal Resources of the Gwembi District. South-Western Section. Bull. geol. Sun. Nth. Rhod., No.4.
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WATSON, R. L. A. 1960. The Geology and Coal Resources of the Country around Wankie, S. Rhodesia. Bull. geol. Surv. Sth. Rhod., No. 48. - - - . 1962. The Geology of the Kamativi and Lubimbi Areas. Bull. geol. Surv. Sth. Rhod., No. 57. ZEUNER, F. E. 1955. A Fossil Blastoid from the Permian of Rhodesia. Ann. Mag. nat. Hist., Ser. 12,8,685-8.
Geoffrey Bond Department of Geology University College of Rhodesia Box MP 167, Mount Pleasant Salisbury, Rhodesia