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Trace fossils and palaeoenvironments in the Ecca group of the Nongoma Graben, northern Zululand, South Africa
Palaeogeography, Palaeoclimatology, Palaeoecology, 36 ( 1981 ) : 113--123 Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
113
TRACE FOSSILS AND PALAEOENVIRONMENTS IN THE ECCA GROUP OF THE NONGOMA GRABEN, NORTHERN ZULULAND, SOUTH AFRICA
B. R. TURNER, I. G. STANISTREET and M. K. G. WHATELEY
Department of Geology, The University, Newcastle upon Tyne NE1 7RU (Great Britain) Department of Geology, University of the Witwatersrand, Johannesburg 2001 (South Africa) Southern Sphere Mining and Development Company (Pry) Ltd., P.O. Box 50065, Randburg 2125 (South Africa) (Received January 16, 1981 ; revised version accepted June 17, 1981 )
ABSTRACT Turner, B. R., Stanistreet, I. G. and Whateley, M. K. G., 1981. Trace fossils and palaeoenvironments in the Ecca Group of the Nongoma Graben, northern Zululand, South Africa. Palaeogeogr., Palaeoclimatol., Palaeoecoh, 36: 113--123. Because of the lack of b o d y fossils and the generally poor exposure of the Ecca Group in the Nongoma Graben, northern Zululand, trace fossils provide useful evidence of the environment and conditions of deposition in addition to that derived from sedimentological criteria. Trace fossils occur repeatedly at specific levels within coarsening-upward deltaic sequences within the lower and upper parts of the succession, but unlike previous studies their distribution appears to be largely independent of bathymetry. On the lower delta plain interdistributary bay-fill sequences appear to contain the greatest abundance and diversity of trace fossils, and in terms of Seilacher's (1967)bathymetrically controlled ichnofacies include both shallow-water (Skolithos, Diplocraterion) and deep-water (Helminthopsis) forms. Scolicia occurs at lower levels in the sequence within the prodelta deposits and Planolites in the deep-water offshore shales. Both forms, however, are "facies crossing" ichnogenera and would not be expected to be restricted to any particular ichnofacies or depositional environment. The scarcity of trace fossils in the shales probably reflects the generally inhospitable conditions within the basin which restricted faunal development. INTRODUCTION T h e N o n g o m a G r a b e n in n o r t h e r n Z u l u l a n d ( F i g . 1) d e v e l o p e d d u r i n g t h e early stages of extensional disruption and crustal thinning prior to continental break-up and separation of east and west Gondwanaland. Vertical displacement contemporaneous w i t h E c c a ( P e r m i a n ) s e d i m e n t a t i o n r e s u l t e d in t h e deposition of a series of deltaic and fluvial complexes which prograded mainly s o u t h w a r d a c r o s s a s h a l l o w o p e n s h e l f f a c i e s o f silt a n d m u d . R e p e t i t i o n o f d e l t a i c a n d f l u v i a l d e p o s i t i o n a l s e q u e n c e s is a t t r i b u t e d t o e p i s o d e s o f t e n s i o n a l 0031-0182/81/0000--0000/$02.75
© 1981 Elsevier Scientific Publishing Company
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115 build-up and release within the crust (Whateley, 1980), not to factors inherent in the depositional system as on the flanking craton to the north (Hobday, 1973, 1978). Fractures active during basin subsidence and sedimentation were reactivated in post-Karoo times, resulting in block faulting and preservation of the Ecca Group within the Nongoma Graben. Trace fossils occur within the Ecca Group sediments of the Nongoma Graben where there appears to be a strong relationship between trace fossil distribution, lithofacies and palaeoenvironments of known sequential development. Most traces are associated with the coarsening-upward deltaic sequences in the succession where certain traces appear repeatedly at specific levels in the sequences. They are absent in all but the lowermost fluvial deposits which consist mainly of interdistributary bay sequences transitional between the delta plain and alluvial plain. Because of the lack of body fossils in the succession, and the generally poor exposure and heavy reliance on cores, the trace fossils, together with other sedimentary criteria, provide evidence of position within the sequence as well as the environment and conditions of deposition. DEPOSITIONAL HISTORY The Ecca Group in the Nongoma Graben comprises a lower progradational deltaic sequence, a middle fluviatile sequence and an upper transgressive deltaic sequence (Fig. 2). Detailed descriptions of these sequences and their lithofacies components are presented elsewhere (Whateley, 1980) and only a brief outline is presented here. Deposition was initiated following isostatic uplift of deglaciated highlands to the north, south and east which shed arkosic detritus into a shallow intracratonic basin fed by high-gradient streams. Because of the low energy of the water body, beaches, barriers and other features indicative of active coastal and shallow-water marine environments were not developed and fine-grained detritus was carried basinward, acting as a base for fluvially induced progradation. Repeated uplift and/or subsidence of the source and depositional site generated a series of coarsening-upward (regressive) deltaic sequences comprising alternating thin beds of siltstone and sandstone of the prodelta environment gradationally overlain by coarse-grained, crossbedded and locally bioturbated sandstones of the distributary m o u t h bar. Within the prodelta sediments are grain-flow deposits and turbidites which developed in response to flood discharge down the gentle prodelta slope, or through growth faulting and loading of water-saturated muds and silts by sand and gravel. The delta plain consists predominantly of interdistributary bay deposits which include: (1) small-scale, coarsening-upward bay-fill and minor mouth-bar sequences of locally bioturbated and rippled siltstone overlain by coarse, trough cross-bedded sandstone; (2) erosively based, often lenticular, coarse crevasse-splay sandstone; and (3) carbonaceous shale and siltstone (with rare coal) of the marsh and restricted bay environ-
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117
ments. Cut into the interdistributary bay deposits are erosively based, rippled and cross-bedded, coarse distributary channel sandstones. The delta plain is succeeded laterally (upstream) and vertically by alluvial plain deposits dominated by fining-upward sequences of erosively based sandstone overlain by siltstone, carbonaceous shale and coal. T w o types of fining-upward sequence occur. The first is poorly defined and consists of coarse, tabular, sheet-like, low-sinuosity channel sand bodies characterized by trough crossbedding and overlain by minor amounts of overbank fines. The second consists of lenticular, stacked high-sinuosity channel sand bodies (with occasional lateral accretion surfaces) showing a distinct upward decrease in grain size and concomitant variation in internal stratification from planar to trough cross-bedding through flat-bedding into ripple cross-lamination, capped by a considerable thickness of overbank fines. The distribution of these sequences within the succession suggests that deposition initially resulted from high-gradient, low-sinuosity {non-braided) bed-load channels controlled by repetitive source area uplift and/or basinal subsidence. Following this, tectonic events were spaced further apart, and although low-sinuosity channels developed at first, stabilization of source and depositional site p r o m o t e d lower gradients, increased production of fines and high-sinuosity channels in response to source area denudation and possibly early drifting. These conditions favoured the development of thick floodplain deposits and the accumulation of economically important coals which are mainly located at the top of fining-upward sequences. Fluvial conditions were terminated by transgression and renewed tectonic activity leading to a further phase of deltaic sedimentation and the development of coarsening-upward sequences. However, tectonism and relief differences were more subdued than in early Ecca times with the result that the sequences are less well defined and as transgression proceeded (rate of basin subsidence exceeded rate of sedimentation) they became overlain by progressively finer grained deposits of the prodelta and shelf environments. TRACE FOSSILS
Ichnogenus Diplocraterion Torell 1870 These are vertical U-shaped burrows, sand infilled within sandstone beds, and containing retrusive spreiten (Fig.3). They have only been found in borehole core of the upper transgressive deltaic sequence and are greater than 2.5 cm long (terminology of Goldring, 1962) and preserve at least 4 cm height of stacked spreiten. The burrows represent living and feeding structures and the spreiten develop in response to erosion or deposition of the substrate under fairly high-energy marine conditions.
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Ichnogenus Helminthopsis Heer emend. Sacco 1888 These consist of simple, freely winding horizontal burrows on the upper surface of fine- to medium-grained sandstone beds (Fig.3). The burrows are approximately 2 mm wide and 15 cm long with a relief of up to 2 mm.
Ichnogenus Planolites Nicholson 18 73 These are simple, smooth, straight to slightly sinuous, unbranched traces of uniform width which wind discretely across the top of shale beds. The traces are paler in colour than the enclosing shales, approximately 10 mm wide, and show a slightly positive relief and elliptical cross-section (Fig.3). Planolites is generally interpreted to be the active infilled burrow of infaunal worms (H~ntzschel, 1975). The colour contrast between the burrow and the darker oxygen-deficient muds indicates aeration of the burrow by the respiratory, water pumping or digestive activity of the ichnofauna (Hallam, 1975).
Ichnogenus Scolicia De Quatrefages 1849 These comprise two types of simple trails on the upper surface of interbedded ripple cross-laminated sandstone and siltstone which probably belong to the Scolicia group (Fig.3). The first type is thin, irregularly meandering, intersecting V-shaped grooves, 1--3 mm wide and 1 m m deep. The second type comprises a trail of imbricated stacked laminae, possibly produced by the peristaltic m o v e m e n t of an animal's " f o o t " . Scolicia is generally interpreted to be the trace of a sediment or suspensionfeeding bivalve or gastropod ploughing through an unconsolidated substrate within a marine environment (Crimes, 1975). It has also been attributed to echinoids (Ward and Lewis, 1975), and comparable traces have been reported from a non-marine fluvial environment (Turner, 1978).
Ichnogenus Gen. et sp. indet.? nov. Vertical tubes with a central core, usually structureless, and an outer halo which is laminated, with laminae sloping downward and outward from the centre (Fig.3). Previous workers have described this form as Skolithos, but the internal structure displayed does n o t fit this identification. Stanistreet (1979) suggested that the structure was similar to traces made by the bivalve Mya arenaria on the tidal flats of the North Sea (Reineck, 1958) and he related the trace fossil to a b a n d o n m e n t phases in delta development. The trace fossil is to be described and diagnosed in a forthcoming paper (Stanistreet, in prep.).
120
Ichnogenus Skolithos Haldeman 1840 Traces included in this ichnogenus consist of unbranched, vertical, cylindrical dwelling burrows infilled with sand and occurring within medium to coarse sandstone beds (Fig.3). The burrows are of uniform diameter, closely spaced and with a cylindrical surface expression. Skolithos is normally found in high-energy, shallow-water marine environments where sediment is constantly reworked by marine processes (Crimes, 1977). Because of this, the animal retreats into its burrow to withstand current activity and rapid changes in salinity and temperature (Crimes, 1975).
Oblique burrows These were f o u n d in borehole core and are occasionally defined by coarser, light-coloured, sandstone (Fig.3) within a finer-grained matrix.
Bioturbation Bioturbation is c o m m o n in the interdistributary bay sequences of the delta plain. Under l o w ~ n e r g y conditions where sedimentation is slow biogenic activity was extensive and the original sediments have been completely reworked and colour mottled with few recognizable traces left. With increasing grain size, the amounts of organic matter, fine-grained matrix and mottled textures decrease and individual burrows become more identifiable. The increased energy of the environment and faster rate of deposition p r o m o t e d burrowing of the upper few centimetres of the bed because of the inability of the burrowing organism to escape through vast thicknesses of sediment. Most intensely burrowed zones seldom exceed 30 cm in thickness. DISTRIBUTION OF TRACE FOSSILS
The distribution of trace fossils in the succession is shown in Fig.3. Deep vertical burrows (Skolithos, Diplocraterion) and meandering trails (Helminthopsis, Scolicia) are associated with sandstones at the top of coarseningupward deltaic sequences in the lower progradational and upper transgressive deltaic deposits. Scolicia also occurs at lower levels within the mixed sand and silt facies of the prodelta environment, whilst Planolites, which is a notorious "facies crossing" form, is confined to the shelf shales defining the top and b o t t o m of the succession. Bioturbation is most c o m m o n in the interdistributary bay deposits of the delta plain/alluvial plain. The general pattern of trace fossil distribution in the succession shows that the interdistributary bay-fill sands contain the greatest abundance and diversity of trace fossils with mixing of shallow-water and deep-water forms. In the delta front deposits are found shallow-water forms such as Skolithos and Diplocraterion. In passing from the delta front basinward into the
121 thinner-bedded, lower-energy sands and shales of the prodelta and shelf environments the abundance and diversity decreases markedly but with no well-defined change in the ichnofauna indicative of increasing water depth. INTERPRETATION Trace fossils can be useful environmental indicators (Crimes, 1970; Frey, 1971) and according to Seilacher (1964, 1967) they tend to occur in depthcontrolled communities more correctly referred to as ichnofacies. He associated the Glossifungites and Skolithos ichnofacies with the shallow-water littoral zone, the Cruziana ichnofacies with the sublittoral zone and the Z o o p h y c o s and Nereites ichnofacies with progressively deeper water. There is frequent overlap between different depth ichnofacies and, because they represent the behaviour of the producer organism, similar traces may be made by different animals whose zoological affinities are often unknown. The deep vertical burrows of Skolithos and Diplocraterion are generally indicative of high-energy, shallow-water (littoral) marine conditions although Diplocraterion can occur in the deeper-water Cruziana ichnofacies. The position of Skolithos at the top of coarsening-upward deltaic sequences supports the interpretation that the organism was inhabiting distributary m o u t h bars during t e m p o r a r y abandonment. Diplocraterion tends to be found at the gradation between prodelta and distributary m o u t h bar deposits. The retrusive spreiten of the latter indicates active deposition characteristic of this environment, and that progradation was taking place. The meandering trails such as Hehninthopsis and Scolicia cannot be equated with Seilacher's (1967) deep-water ichnofacies as suggested by H o b d a y and Tavener-Smith (1975). Since the behavioural response of the trace-making organism depends largely on energy conditions, substrate t y p e and the nature of the f o o d supply (Crimes, 1970), exceptions to the generalized depth-dependent relationships of Seilacher (1967) are to be expected. For example, the centre of a sheltered nutrient-rich interdistributary bay open to the sea may reproduce conditions similar to the deeperwater environments favouring a similar ethological approach by the ichofauna. Gen. et sp. indet. ? nov., the trace of a deposit-feeding organism, could then inhabit a higher-energy environment at the edge of the bay. The occurrence of Scolicia within relatively shallow prodelta sediments was considered unusual by H o b d a y and Tavener-Smith (1975), mainly because of its reported occurrence in deep-water Eocene flysch sediments in northern Spain (Crimes, in H o b d a y and Tavener-Smith, 1975). In fact Scolicia is a facies-crossing form and frequently occurs in shallow.water sediments which may be marine or non-marine as pointed o u t by Turner (1978), who describes Scolicia from some Upper Triassic braided stream deposits in Lesotho. Since the traces occur within prodelta sediments relatively undisturbed by strong traction currents, the sediment or suspension-feeding gastropod or bivalve probably occupied the substrate during quiet interfluvial periods.
122
No representative of Seilacher's (1967) deep-water Nereites ichnofacies was found. However, estimates based on sediment thickness suggest a maxim u m water depth for the Ecca basin of a b o u t 500 m (Visser and Loock, 1978), a value which is considerably less than that normally occupied by this ichnofacies. Planolites is the only trace fossil found in the shelf shales although farther north Helminthopsis and Taphrhehninthopsis occur sporadically and Planolites is absent ( H o b d a y and Tavener-Smith, 1975). The infrequent occurrence of trace fossils in the shales may be attributed to the existence of euxinic conditions within the deeper areas of the basin which were generally inhospitable to animal life, especially during early Ecca times, when water temperatures following deglaciation were very cold and a stratified water b o d y m a y have developed. Also because of the shallow depths of the basin, periods of high fluvial influx m a y have reduced salinities, increased water turbulence and restricted faunal development. There is a lack of trace fossil evidence of the transgression terminating fluvial deposition in the area. It has been suggested (Le Blanc-Smith, in Whateley, 1980} that the Ecca Group in the Nongoma Graben is completely deltaic in origin rather than of mixed fluvio-deltaic origin, with the lower progradation phase succeeded by a transgressive phase and then another progradational phase. The coals according to this model would be associated with the transgressive deltaic phase. Bioturbation is c o m m o n and locally abundant in the coarseningupward sequences of the lower progradational deltaic phase and the immediately overlying deposits of interdistributary bay origin. The presence of Skolithos further indicates a shallow-water environment which was probably deltaic and marine rather than brackish (cf. H o b d a y , 1978). Above this the succession is dominated by fining-upward fluvial sequences which lack bioturbation and trace fossils (cf. H o b d a y and Tavener-Smith, 1975); these only appear again in the coarsening-upward deltaic sequences at the t o p of the succession. Thus, the pattern and distribution of bioturbation and trace fossils support a mixed fluvio-deltaic origin for the succession rather than a completely deltaic one. ACKNOWLEDGEMENTS
We should like to thank M. J. Benton for his criticism of an earlier version of the manuscript. The manuscript was t y p e d by Mrs. K. Otto. REFERENCES Chamberlain, C.K., 1971. Morphology and ethology of trace fossils from the Quachita Mountains, southeast Oklahoma. J. Paleontol., 45: 212--246. Crimes, T. P., 1970. The significance of trace fossils in sedimentology, stratigraphy and palaeoecology with examples from Lower Palaeozoic strata. In: T. P. Crimes and J. C. Harper (Editors), Trace Fossils. Geol. J., Spec. Iss., 3: 101--126. Crimes, T. P., 1975. The stratigraphical significance of trace fossils. In: R. W. Frey (Editor), The Study of Trace Fossils. Springer-Verlag, New York, N.Y., pp. 109--130.
123 Crimes, T.P., 1977. Trace fossils of an Eocene deep sea sand fan, northern Spain. In: T. P. Crimes and J. C. Harper (Editors), Trace Fossils, 2. Geol. J., Spec. Iss., 9: 71--90. Frey, R. W., 1971. Ichnology -- the study of fossil and recent Lebenspuren. School Geosci., State Univ. Misc. Publ., 71-I: 91--126. Goldring, R., 1962. The trace fossils of the Baggy Beds (Upper Devonian) of North Devon, England. Pal~ontol. Z., 36: 232--251. Hallam, A., 1975. Preservation of trace fossils. In: R. W. Frey (Editor), The Study of Trace Fossils. Springer-Verlag, New York, N. Y., pp. 55--64. H~intzschel, W., 1975. Trace fossils and problematica. In: C. Teichert (Editor), Treatise ou Invertebrate Paleontology W. Miscellanea, Supp. 1. University of Kansas Press, Lawrence, Kansas, pp. 3--269. Hobday, D. K., 1973. Middle Ecca deltaic deposits in the Muden--Tugela Ferry area of Natal. Trans. Geol. Soc. S. Afr., 76: 309--318. Hobday, D. K., 1978. Fluvial deposits of the Ecca and Beaufort Groups in the eastern Karoo Basin, southern Africa. In: A. D. Miall (Editor), Fluvial Sedimentology. Can. Soc. Pet. Geol., Mem., 5: 413--430. Hobday, D. K. and Tavener-Smith, R., 1975. Trace fossils in the Ecca of northern Natal and their palaeoenvironmental significance. Palaeontol. Afr., 18 : 47--51. Reineck, H.-E., 1958. Wiihlbau -- Gefiige in Abhiingigkeit von Sediment-Umlagerungen. Senckenb. Leth., 39: 1--23. Seilacher, A., 1964. Biogenic sedimentary structures. In: J. Imbrie and N. D. Newell (Editors), Approaches to Paleoecology. John Wiley, New York. N.Y., pp. 296--316. Seilacher, A., 1967. Bathymetry of trace fossils. Mar. Geol., 5: 413--428. Stanistreet, I. G., 1979. Trace fossils as sedimentological and palaeoenvironmental indices in the Ecca Group of the Transvaal. Abstr. 18th Congr. Geol. Soc. S. Afr., Part 2: 84--87. Stanistreet, I. G., in prep. Trace fossils as sedimentological and palaeoenvironmental indices in the Ecca Group (Lower Permian) of the Transvaal. Turner, B. R., 1978. Trace fossils from the Upper Triassic fluviatile Molteno Formation of the Karoo (Gondwana) Supergroup, Lesotho. J. Paleontol., 52: 959--963. Visser, J. N. J. and Loock, J. C., 1978. Water depth in the main Karoo Basin, South Africa, during Ecca (Permian) sedimentation. Trans. Geol. Soc. S. Afr., 81: 185--192. Ward, D. M. and Lewis, D. W., 1975. Palaeoenvironmental implications of storm-scoured, ichnofossiliferous Mid-Tertiary Limestones, Waihao district, South Canterbury, New Zealand. N . Z . J . Geol. Geophys., 11 : 881--908. Whateley, M. K. G., 1980. Structural Controls of Sedimentation in the Ecca Group in Northern Zululand. Thesis, University of the Witwatersrand, Johannesburg, 149 pp. (unpublished).
113
TRACE FOSSILS AND PALAEOENVIRONMENTS IN THE ECCA GROUP OF THE NONGOMA GRABEN, NORTHERN ZULULAND, SOUTH AFRICA
B. R. TURNER, I. G. STANISTREET and M. K. G. WHATELEY
Department of Geology, The University, Newcastle upon Tyne NE1 7RU (Great Britain) Department of Geology, University of the Witwatersrand, Johannesburg 2001 (South Africa) Southern Sphere Mining and Development Company (Pry) Ltd., P.O. Box 50065, Randburg 2125 (South Africa) (Received January 16, 1981 ; revised version accepted June 17, 1981 )
ABSTRACT Turner, B. R., Stanistreet, I. G. and Whateley, M. K. G., 1981. Trace fossils and palaeoenvironments in the Ecca Group of the Nongoma Graben, northern Zululand, South Africa. Palaeogeogr., Palaeoclimatol., Palaeoecoh, 36: 113--123. Because of the lack of b o d y fossils and the generally poor exposure of the Ecca Group in the Nongoma Graben, northern Zululand, trace fossils provide useful evidence of the environment and conditions of deposition in addition to that derived from sedimentological criteria. Trace fossils occur repeatedly at specific levels within coarsening-upward deltaic sequences within the lower and upper parts of the succession, but unlike previous studies their distribution appears to be largely independent of bathymetry. On the lower delta plain interdistributary bay-fill sequences appear to contain the greatest abundance and diversity of trace fossils, and in terms of Seilacher's (1967)bathymetrically controlled ichnofacies include both shallow-water (Skolithos, Diplocraterion) and deep-water (Helminthopsis) forms. Scolicia occurs at lower levels in the sequence within the prodelta deposits and Planolites in the deep-water offshore shales. Both forms, however, are "facies crossing" ichnogenera and would not be expected to be restricted to any particular ichnofacies or depositional environment. The scarcity of trace fossils in the shales probably reflects the generally inhospitable conditions within the basin which restricted faunal development. INTRODUCTION T h e N o n g o m a G r a b e n in n o r t h e r n Z u l u l a n d ( F i g . 1) d e v e l o p e d d u r i n g t h e early stages of extensional disruption and crustal thinning prior to continental break-up and separation of east and west Gondwanaland. Vertical displacement contemporaneous w i t h E c c a ( P e r m i a n ) s e d i m e n t a t i o n r e s u l t e d in t h e deposition of a series of deltaic and fluvial complexes which prograded mainly s o u t h w a r d a c r o s s a s h a l l o w o p e n s h e l f f a c i e s o f silt a n d m u d . R e p e t i t i o n o f d e l t a i c a n d f l u v i a l d e p o s i t i o n a l s e q u e n c e s is a t t r i b u t e d t o e p i s o d e s o f t e n s i o n a l 0031-0182/81/0000--0000/$02.75
© 1981 Elsevier Scientific Publishing Company
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115 build-up and release within the crust (Whateley, 1980), not to factors inherent in the depositional system as on the flanking craton to the north (Hobday, 1973, 1978). Fractures active during basin subsidence and sedimentation were reactivated in post-Karoo times, resulting in block faulting and preservation of the Ecca Group within the Nongoma Graben. Trace fossils occur within the Ecca Group sediments of the Nongoma Graben where there appears to be a strong relationship between trace fossil distribution, lithofacies and palaeoenvironments of known sequential development. Most traces are associated with the coarsening-upward deltaic sequences in the succession where certain traces appear repeatedly at specific levels in the sequences. They are absent in all but the lowermost fluvial deposits which consist mainly of interdistributary bay sequences transitional between the delta plain and alluvial plain. Because of the lack of body fossils in the succession, and the generally poor exposure and heavy reliance on cores, the trace fossils, together with other sedimentary criteria, provide evidence of position within the sequence as well as the environment and conditions of deposition. DEPOSITIONAL HISTORY The Ecca Group in the Nongoma Graben comprises a lower progradational deltaic sequence, a middle fluviatile sequence and an upper transgressive deltaic sequence (Fig. 2). Detailed descriptions of these sequences and their lithofacies components are presented elsewhere (Whateley, 1980) and only a brief outline is presented here. Deposition was initiated following isostatic uplift of deglaciated highlands to the north, south and east which shed arkosic detritus into a shallow intracratonic basin fed by high-gradient streams. Because of the low energy of the water body, beaches, barriers and other features indicative of active coastal and shallow-water marine environments were not developed and fine-grained detritus was carried basinward, acting as a base for fluvially induced progradation. Repeated uplift and/or subsidence of the source and depositional site generated a series of coarsening-upward (regressive) deltaic sequences comprising alternating thin beds of siltstone and sandstone of the prodelta environment gradationally overlain by coarse-grained, crossbedded and locally bioturbated sandstones of the distributary m o u t h bar. Within the prodelta sediments are grain-flow deposits and turbidites which developed in response to flood discharge down the gentle prodelta slope, or through growth faulting and loading of water-saturated muds and silts by sand and gravel. The delta plain consists predominantly of interdistributary bay deposits which include: (1) small-scale, coarsening-upward bay-fill and minor mouth-bar sequences of locally bioturbated and rippled siltstone overlain by coarse, trough cross-bedded sandstone; (2) erosively based, often lenticular, coarse crevasse-splay sandstone; and (3) carbonaceous shale and siltstone (with rare coal) of the marsh and restricted bay environ-
116
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Fig.2. Generalized stratigraphic column of the Ecca Group in the Nongoma Graben, northern Zululand, showing fining-upward fluvial sequence and coarsening-upwarddeltaic sequences with location of trace fossils.
117
ments. Cut into the interdistributary bay deposits are erosively based, rippled and cross-bedded, coarse distributary channel sandstones. The delta plain is succeeded laterally (upstream) and vertically by alluvial plain deposits dominated by fining-upward sequences of erosively based sandstone overlain by siltstone, carbonaceous shale and coal. T w o types of fining-upward sequence occur. The first is poorly defined and consists of coarse, tabular, sheet-like, low-sinuosity channel sand bodies characterized by trough crossbedding and overlain by minor amounts of overbank fines. The second consists of lenticular, stacked high-sinuosity channel sand bodies (with occasional lateral accretion surfaces) showing a distinct upward decrease in grain size and concomitant variation in internal stratification from planar to trough cross-bedding through flat-bedding into ripple cross-lamination, capped by a considerable thickness of overbank fines. The distribution of these sequences within the succession suggests that deposition initially resulted from high-gradient, low-sinuosity {non-braided) bed-load channels controlled by repetitive source area uplift and/or basinal subsidence. Following this, tectonic events were spaced further apart, and although low-sinuosity channels developed at first, stabilization of source and depositional site p r o m o t e d lower gradients, increased production of fines and high-sinuosity channels in response to source area denudation and possibly early drifting. These conditions favoured the development of thick floodplain deposits and the accumulation of economically important coals which are mainly located at the top of fining-upward sequences. Fluvial conditions were terminated by transgression and renewed tectonic activity leading to a further phase of deltaic sedimentation and the development of coarsening-upward sequences. However, tectonism and relief differences were more subdued than in early Ecca times with the result that the sequences are less well defined and as transgression proceeded (rate of basin subsidence exceeded rate of sedimentation) they became overlain by progressively finer grained deposits of the prodelta and shelf environments. TRACE FOSSILS
Ichnogenus Diplocraterion Torell 1870 These are vertical U-shaped burrows, sand infilled within sandstone beds, and containing retrusive spreiten (Fig.3). They have only been found in borehole core of the upper transgressive deltaic sequence and are greater than 2.5 cm long (terminology of Goldring, 1962) and preserve at least 4 cm height of stacked spreiten. The burrows represent living and feeding structures and the spreiten develop in response to erosion or deposition of the substrate under fairly high-energy marine conditions.
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Fig.3. Depositional model for the Ecca Group in the Nongoma Graben, northern Zululand, showing trace fossil distribution.
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119
Ichnogenus Helminthopsis Heer emend. Sacco 1888 These consist of simple, freely winding horizontal burrows on the upper surface of fine- to medium-grained sandstone beds (Fig.3). The burrows are approximately 2 mm wide and 15 cm long with a relief of up to 2 mm.
Ichnogenus Planolites Nicholson 18 73 These are simple, smooth, straight to slightly sinuous, unbranched traces of uniform width which wind discretely across the top of shale beds. The traces are paler in colour than the enclosing shales, approximately 10 mm wide, and show a slightly positive relief and elliptical cross-section (Fig.3). Planolites is generally interpreted to be the active infilled burrow of infaunal worms (H~ntzschel, 1975). The colour contrast between the burrow and the darker oxygen-deficient muds indicates aeration of the burrow by the respiratory, water pumping or digestive activity of the ichnofauna (Hallam, 1975).
Ichnogenus Scolicia De Quatrefages 1849 These comprise two types of simple trails on the upper surface of interbedded ripple cross-laminated sandstone and siltstone which probably belong to the Scolicia group (Fig.3). The first type is thin, irregularly meandering, intersecting V-shaped grooves, 1--3 mm wide and 1 m m deep. The second type comprises a trail of imbricated stacked laminae, possibly produced by the peristaltic m o v e m e n t of an animal's " f o o t " . Scolicia is generally interpreted to be the trace of a sediment or suspensionfeeding bivalve or gastropod ploughing through an unconsolidated substrate within a marine environment (Crimes, 1975). It has also been attributed to echinoids (Ward and Lewis, 1975), and comparable traces have been reported from a non-marine fluvial environment (Turner, 1978).
Ichnogenus Gen. et sp. indet.? nov. Vertical tubes with a central core, usually structureless, and an outer halo which is laminated, with laminae sloping downward and outward from the centre (Fig.3). Previous workers have described this form as Skolithos, but the internal structure displayed does n o t fit this identification. Stanistreet (1979) suggested that the structure was similar to traces made by the bivalve Mya arenaria on the tidal flats of the North Sea (Reineck, 1958) and he related the trace fossil to a b a n d o n m e n t phases in delta development. The trace fossil is to be described and diagnosed in a forthcoming paper (Stanistreet, in prep.).
120
Ichnogenus Skolithos Haldeman 1840 Traces included in this ichnogenus consist of unbranched, vertical, cylindrical dwelling burrows infilled with sand and occurring within medium to coarse sandstone beds (Fig.3). The burrows are of uniform diameter, closely spaced and with a cylindrical surface expression. Skolithos is normally found in high-energy, shallow-water marine environments where sediment is constantly reworked by marine processes (Crimes, 1977). Because of this, the animal retreats into its burrow to withstand current activity and rapid changes in salinity and temperature (Crimes, 1975).
Oblique burrows These were f o u n d in borehole core and are occasionally defined by coarser, light-coloured, sandstone (Fig.3) within a finer-grained matrix.
Bioturbation Bioturbation is c o m m o n in the interdistributary bay sequences of the delta plain. Under l o w ~ n e r g y conditions where sedimentation is slow biogenic activity was extensive and the original sediments have been completely reworked and colour mottled with few recognizable traces left. With increasing grain size, the amounts of organic matter, fine-grained matrix and mottled textures decrease and individual burrows become more identifiable. The increased energy of the environment and faster rate of deposition p r o m o t e d burrowing of the upper few centimetres of the bed because of the inability of the burrowing organism to escape through vast thicknesses of sediment. Most intensely burrowed zones seldom exceed 30 cm in thickness. DISTRIBUTION OF TRACE FOSSILS
The distribution of trace fossils in the succession is shown in Fig.3. Deep vertical burrows (Skolithos, Diplocraterion) and meandering trails (Helminthopsis, Scolicia) are associated with sandstones at the top of coarseningupward deltaic sequences in the lower progradational and upper transgressive deltaic deposits. Scolicia also occurs at lower levels within the mixed sand and silt facies of the prodelta environment, whilst Planolites, which is a notorious "facies crossing" form, is confined to the shelf shales defining the top and b o t t o m of the succession. Bioturbation is most c o m m o n in the interdistributary bay deposits of the delta plain/alluvial plain. The general pattern of trace fossil distribution in the succession shows that the interdistributary bay-fill sands contain the greatest abundance and diversity of trace fossils with mixing of shallow-water and deep-water forms. In the delta front deposits are found shallow-water forms such as Skolithos and Diplocraterion. In passing from the delta front basinward into the
121 thinner-bedded, lower-energy sands and shales of the prodelta and shelf environments the abundance and diversity decreases markedly but with no well-defined change in the ichnofauna indicative of increasing water depth. INTERPRETATION Trace fossils can be useful environmental indicators (Crimes, 1970; Frey, 1971) and according to Seilacher (1964, 1967) they tend to occur in depthcontrolled communities more correctly referred to as ichnofacies. He associated the Glossifungites and Skolithos ichnofacies with the shallow-water littoral zone, the Cruziana ichnofacies with the sublittoral zone and the Z o o p h y c o s and Nereites ichnofacies with progressively deeper water. There is frequent overlap between different depth ichnofacies and, because they represent the behaviour of the producer organism, similar traces may be made by different animals whose zoological affinities are often unknown. The deep vertical burrows of Skolithos and Diplocraterion are generally indicative of high-energy, shallow-water (littoral) marine conditions although Diplocraterion can occur in the deeper-water Cruziana ichnofacies. The position of Skolithos at the top of coarsening-upward deltaic sequences supports the interpretation that the organism was inhabiting distributary m o u t h bars during t e m p o r a r y abandonment. Diplocraterion tends to be found at the gradation between prodelta and distributary m o u t h bar deposits. The retrusive spreiten of the latter indicates active deposition characteristic of this environment, and that progradation was taking place. The meandering trails such as Hehninthopsis and Scolicia cannot be equated with Seilacher's (1967) deep-water ichnofacies as suggested by H o b d a y and Tavener-Smith (1975). Since the behavioural response of the trace-making organism depends largely on energy conditions, substrate t y p e and the nature of the f o o d supply (Crimes, 1970), exceptions to the generalized depth-dependent relationships of Seilacher (1967) are to be expected. For example, the centre of a sheltered nutrient-rich interdistributary bay open to the sea may reproduce conditions similar to the deeperwater environments favouring a similar ethological approach by the ichofauna. Gen. et sp. indet. ? nov., the trace of a deposit-feeding organism, could then inhabit a higher-energy environment at the edge of the bay. The occurrence of Scolicia within relatively shallow prodelta sediments was considered unusual by H o b d a y and Tavener-Smith (1975), mainly because of its reported occurrence in deep-water Eocene flysch sediments in northern Spain (Crimes, in H o b d a y and Tavener-Smith, 1975). In fact Scolicia is a facies-crossing form and frequently occurs in shallow.water sediments which may be marine or non-marine as pointed o u t by Turner (1978), who describes Scolicia from some Upper Triassic braided stream deposits in Lesotho. Since the traces occur within prodelta sediments relatively undisturbed by strong traction currents, the sediment or suspension-feeding gastropod or bivalve probably occupied the substrate during quiet interfluvial periods.
122
No representative of Seilacher's (1967) deep-water Nereites ichnofacies was found. However, estimates based on sediment thickness suggest a maxim u m water depth for the Ecca basin of a b o u t 500 m (Visser and Loock, 1978), a value which is considerably less than that normally occupied by this ichnofacies. Planolites is the only trace fossil found in the shelf shales although farther north Helminthopsis and Taphrhehninthopsis occur sporadically and Planolites is absent ( H o b d a y and Tavener-Smith, 1975). The infrequent occurrence of trace fossils in the shales may be attributed to the existence of euxinic conditions within the deeper areas of the basin which were generally inhospitable to animal life, especially during early Ecca times, when water temperatures following deglaciation were very cold and a stratified water b o d y m a y have developed. Also because of the shallow depths of the basin, periods of high fluvial influx m a y have reduced salinities, increased water turbulence and restricted faunal development. There is a lack of trace fossil evidence of the transgression terminating fluvial deposition in the area. It has been suggested (Le Blanc-Smith, in Whateley, 1980} that the Ecca Group in the Nongoma Graben is completely deltaic in origin rather than of mixed fluvio-deltaic origin, with the lower progradation phase succeeded by a transgressive phase and then another progradational phase. The coals according to this model would be associated with the transgressive deltaic phase. Bioturbation is c o m m o n and locally abundant in the coarseningupward sequences of the lower progradational deltaic phase and the immediately overlying deposits of interdistributary bay origin. The presence of Skolithos further indicates a shallow-water environment which was probably deltaic and marine rather than brackish (cf. H o b d a y , 1978). Above this the succession is dominated by fining-upward fluvial sequences which lack bioturbation and trace fossils (cf. H o b d a y and Tavener-Smith, 1975); these only appear again in the coarsening-upward deltaic sequences at the t o p of the succession. Thus, the pattern and distribution of bioturbation and trace fossils support a mixed fluvio-deltaic origin for the succession rather than a completely deltaic one. ACKNOWLEDGEMENTS
We should like to thank M. J. Benton for his criticism of an earlier version of the manuscript. The manuscript was t y p e d by Mrs. K. Otto. REFERENCES Chamberlain, C.K., 1971. Morphology and ethology of trace fossils from the Quachita Mountains, southeast Oklahoma. J. Paleontol., 45: 212--246. Crimes, T. P., 1970. The significance of trace fossils in sedimentology, stratigraphy and palaeoecology with examples from Lower Palaeozoic strata. In: T. P. Crimes and J. C. Harper (Editors), Trace Fossils. Geol. J., Spec. Iss., 3: 101--126. Crimes, T. P., 1975. The stratigraphical significance of trace fossils. In: R. W. Frey (Editor), The Study of Trace Fossils. Springer-Verlag, New York, N.Y., pp. 109--130.
123 Crimes, T.P., 1977. Trace fossils of an Eocene deep sea sand fan, northern Spain. In: T. P. Crimes and J. C. Harper (Editors), Trace Fossils, 2. Geol. J., Spec. Iss., 9: 71--90. Frey, R. W., 1971. Ichnology -- the study of fossil and recent Lebenspuren. School Geosci., State Univ. Misc. Publ., 71-I: 91--126. Goldring, R., 1962. The trace fossils of the Baggy Beds (Upper Devonian) of North Devon, England. Pal~ontol. Z., 36: 232--251. Hallam, A., 1975. Preservation of trace fossils. In: R. W. Frey (Editor), The Study of Trace Fossils. Springer-Verlag, New York, N. Y., pp. 55--64. H~intzschel, W., 1975. Trace fossils and problematica. In: C. Teichert (Editor), Treatise ou Invertebrate Paleontology W. Miscellanea, Supp. 1. University of Kansas Press, Lawrence, Kansas, pp. 3--269. Hobday, D. K., 1973. Middle Ecca deltaic deposits in the Muden--Tugela Ferry area of Natal. Trans. Geol. Soc. S. Afr., 76: 309--318. Hobday, D. K., 1978. Fluvial deposits of the Ecca and Beaufort Groups in the eastern Karoo Basin, southern Africa. In: A. D. Miall (Editor), Fluvial Sedimentology. Can. Soc. Pet. Geol., Mem., 5: 413--430. Hobday, D. K. and Tavener-Smith, R., 1975. Trace fossils in the Ecca of northern Natal and their palaeoenvironmental significance. Palaeontol. Afr., 18 : 47--51. Reineck, H.-E., 1958. Wiihlbau -- Gefiige in Abhiingigkeit von Sediment-Umlagerungen. Senckenb. Leth., 39: 1--23. Seilacher, A., 1964. Biogenic sedimentary structures. In: J. Imbrie and N. D. Newell (Editors), Approaches to Paleoecology. John Wiley, New York. N.Y., pp. 296--316. Seilacher, A., 1967. Bathymetry of trace fossils. Mar. Geol., 5: 413--428. Stanistreet, I. G., 1979. Trace fossils as sedimentological and palaeoenvironmental indices in the Ecca Group of the Transvaal. Abstr. 18th Congr. Geol. Soc. S. Afr., Part 2: 84--87. Stanistreet, I. G., in prep. Trace fossils as sedimentological and palaeoenvironmental indices in the Ecca Group (Lower Permian) of the Transvaal. Turner, B. R., 1978. Trace fossils from the Upper Triassic fluviatile Molteno Formation of the Karoo (Gondwana) Supergroup, Lesotho. J. Paleontol., 52: 959--963. Visser, J. N. J. and Loock, J. C., 1978. Water depth in the main Karoo Basin, South Africa, during Ecca (Permian) sedimentation. Trans. Geol. Soc. S. Afr., 81: 185--192. Ward, D. M. and Lewis, D. W., 1975. Palaeoenvironmental implications of storm-scoured, ichnofossiliferous Mid-Tertiary Limestones, Waihao district, South Canterbury, New Zealand. N . Z . J . Geol. Geophys., 11 : 881--908. Whateley, M. K. G., 1980. Structural Controls of Sedimentation in the Ecca Group in Northern Zululand. Thesis, University of the Witwatersrand, Johannesburg, 149 pp. (unpublished).