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Fluvial sedimentation and paleogeography of an early paleozoic failed rift, southeastern margin of Africa
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Palaeogeography, Palaeoclimatology, Palaeoecology, 28(1979): 169--184 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
FLUVIAL SEDIMENTATION AND PALEOGEOGRAPHY PALEOZOIC FAILED RIFT, SOUTHEASTERN MARGIN
OF AN EARLY OF AFRICA 1
DAVID K. HOBDAY and VICTOR VON BRUNN
Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas (U.S.A.) Department of Geology, University of Natal, Pietermaritzburg (South Africa) (Received November 16, 1978; revised version accepted May 17, 1979) ABSTRACT Hobday, D. K. and Von Brunn, V., 1979. Fluvial sedimentation and paleogeography of an early Paleozoic failed rift, southeastern margin of Africa. Palaeogeogr., Palaeoclimatol., Palaeoecol., 28: 169--184. An early Paleozoic failed rift, a local precursor of Gondwana fragmentation, was characterized by a steep, rugged northern terminus, with depositional slope decreasing longitudinally southwestward, where the trough opened toward a major ocean basin. There is a marked north--south zonation of the following facies: (1) boulder conglomerates showing crude horizontal stratification in the lower half, with gently inclined stratification and lenticular arrangements, including locally cross-bedded sandstones, in the upper half. These are interpreted to have been deposited in confined intermontane valleys which, following aggradation, developed into a gravelly braided stream complex. The conglomerates merge upward and southward into (2) red arkosic sandstones exhibiting weakly developed vertical arrangements of scour-based, massive pebbly sandstone overlain by cross-bedded sandstone, with thin red siltstones infrequently preserved at the top. Thick sets of planar cross-beds are attributed to deposition on the slipface of transverse bars; cosets of smaller-scale trough cross-beds were deposited in advance of' some bars and on top of others. There is evidence that bar dissection and current deflection occurred during river shallowing and bar emergence. An upward reduction in grain size through the fluvial succession is accompanied by a gradual change in channel geometry to narrow, steep-sided forms. A meandering tendency in these low-sinuosity mixed-load streams may have been inhibited by the lack of binding vegetation along the river margins. The fluvial arkoses interfinger southward with (3) a 1000 m succession of quartz arenites thought to have b e e n deposited on an embayed shelf where the outer margins of the braided fluvial complex were reworked by very high energy storm- and tide-dominated processes. INTRODUCTION D e p o s i t i o n o f t h e T a b l e M o u n t a i n G r o u p in t h e Z u l u l a n d - - N a t a l - - T r a n s k e i a r e a o c c u r r e d in a n e l o n g a t e b a s i n ( t h e N a t a l E m b a y m e n t ; V i s s e r , 1 9 7 4 ) w i t h its axis subparallel to the present-day coastline (Matthews, 1970). In Transkei and Natal the Table Mountain Group rests nonconformably on 1 Published with permission of the Director, Bureau of Economic Geology.
170 granitoid and associated rocks of the 1000 m.y. Namaqua--Natal belt (Nicolaysen and Burger, 1965); in Zululand it overlies older units of the Precambrian basement. The succession is correlated lithologically with the Table Mountain Group of the southern Cape Province, the upper part of which contains a marine fauna of Ashgillian (late Ordovician) age (Cocks et al., 1970). A substantially younger date assigned to the Transkei deposits by Lock (1973) appears to be unlikely. If correlation with the Cape Province succession is correct, then the upper two units of the Upper Cambrian to Upper Devonian Cape Supergroup, the Bokkeveld and Witteberg Groups, are not represented in Natal and adjoining states. The early Paleozoic trough had an axial gradient towards the south-southwest, and is thought to mark a line of incipient crustal rupture, behaving as a failed rift that was again active in late Paleozoic times during Lower Karoo sedimentation. Mesozoic breakup of this part of Gondwanaland occurred approximately along the trough axis. As a result, only the western flanks are preserved on the southern African subcontinent. Subsequent seaward warping, block faulting, and erosion of the coastal monocline have resulted in two parallel outcrop belts and a number of isolated exposures (Fig.l). Sediments of the Table Mountain Group deposited in the Natal Embayment show a marked north--south facies zonation. Coarse conglomerates are developed in the northern apical area of Zululand where they occupy paleovalleys in the highly dissected Precambrian basement. This rugged paleotopography terminates abruptly at about the latitude of the Tugela Fault (Du Toit, 1931; Rhodes and Leith, 1967), beyond which red arkosic sandstone facies overlie a peneplained basement surface. The arkoses interfinger southward with quartz arenites (Kingsley, 1975), which merge in southern Natal and thicken into Transkei where they make up a homogeneous, 1000 m thick succession of mature shallow marine sandstones (Hobday and Mathew, 1974). This paper focuses on the northern conglomerates and arkosic sandstones, which have been ascribed in general terms to an alluvial depositional system (Du Toit, 1931; Rhodes and Leith, 1967). Further, we will propose a reconstruction of the relationship between the fluvial facies and the thick quartzose sandstones to the south. Detailed study of the arkosic facies was facilitated by coastal exposures showing exhumed bedforms at Umdoni Park (Fig.l) and by three-dimensional views of sedimentary structures at a number of localities. PROXIMAL C O N G L O M E R A T E
DEPOSITS
The coarse-grained northern facies consists of conglomerates up to 60 m thick. Boulders as much as 2 m in diameter occur towards the base. There is a progressive upward decrease in maximum clast diameter to 25 cm near the top, and the uppermost conglomerates contain sandstone intercalations.
171
\
20~E
30~E
\
4,, ,
/
R
Durban
EXPLANATION I
(3
30-4i
J POSt Karoo
JT~T1
Upper Karoo
Umdoni Park Ecco Group Owyka
÷
m
Group
Table Mountain Group Precombricn Basement
\
20 I
40 I
60 Km I
Fig.1. Generalized geological map showing the distribution of the Table Mountain Group in Zululand, Natal and Transkei.
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The underlying Precambrian basement rocks include deeply incised quartzite, metavolcanics, gneiss, and schist. Local paleorelief of 300 m beneath the Table Mountain Group has been noted by Matthews (1970). Despite the lithologic heterogeneity of the basement, boulders resting directly on the unconformity are predominantly quartzites, attesting to their physical and chemical durability. Most clasts are moderately to well rounded, with the highest degree of rounding being manifest in the larger particles. Percussion marks are developed over the entire surface of even the largest boulders, suggesting that in-place abrasion was an important process in clast rounding and size reduction (Schumm and Stevens, 1973). The conglomerate is clast-supported through 95% of the examined succession. Towards the base there is a marked bimodality in grain size, with boulders of subequal dimensions, generally in the 60--120 cm size range, and a coarse feldspathic sandstone matrix. This indicates vigorous current activity, with suspended sand settling into the interstices of coarse bedload gravels following reduction in flow. The upward reduction in maximum clast size (Fig.2) to between 25 and 50 cm is accompanied by a decrease in sorting of the smaller gravel-sized clasts. Imbrication is well developed (Fig.3) and indicates southerly to southwesterly
Fig.2. Outcrop commencing about 10 m above base of Table Mountain Group showing upward reduction in m a x i m u m clast size and crude sub-horizontal layering. Thin sandstone intercalations are present at the top.
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Fig.3. Imbricated quartzite boulders in the proximal conglomerate deposits approximately 15 m above base.
paleocurrents with local variation around basement residuals. Clast long axes show a preferred orientation perpendicular to the imbrication direction (Fig.4), a disposition which is characteristic of large clasts that are rolled along a stream bed (Rust, 1972a). The conglomerate is generally massive with some crude subhorizontal stratification in the lower parts {Fig.2), giving way upward to irregular lenticular arrangements involving shallow scour surfaces, overlapping coarser and finer conglomerate pods, discontinuous sandstone partings, and gently inclined stratification. The upper sandstone interbeds show rare cross-bedding. A small proportion of the succession, varying with locality b u t nowhere more than 5%, is made up of very poorly sorted, matrix-supported conglc~merate in which no bedding, imbrication, or preferred clast alignment are apparent. These units suggest rapid dumping of sediment and are possibly debris flood deposits (Sharp and Nobles, 1953; Miall, 1970), the larger parts of which have been reworked by fluvial processes.
Paleoenvironmental interpretation The coarse basal conglomerate with abraded boulders up to 2 m in diameter is similar to the proximal fan facies of the Van Horn Sandstone, Texas,
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Fig.4. Clast long axis orientation and imbrication directions, proximal conglomerate deposits.
where the evidence of extremely high competence is explained as resulting from confinement of flow in narrow canyons (McGowen and Groat, 1971). Like the Van Horn Sandstone, conglomerates of the Table Mountain Group are thought to have accumulated in an intermontane environment favoring rapid physical weathering, with a landscape devoid of an effective vegetation cover. Preferential concentration of quartzite boulders, with the low proportion of locally derived clasts of other lithologies, indicates a humid climate that contributed to breakdown of nonresistant clasts during transport and storage (cf. Baker and Penteado-Orellana, 1978). As the valleys aggraded, upstream gradients were reduced, and flow was dispersed over a wider area; this environment possibly resembled the upper outwash fans of northeast Gulf of Alaska (Boothroyd, 1972; Boothroyd and Ashley, 1975), where longitudinal bars consist of crudely bedded, well-imbricated gravel, with little sand except as matrix. Smaller particle size and increased bed relief in the upper part of the conglomeratic interval is suggestive of an environment comparable to the midfans of Boothroyd and Ashley (1975), or the proximal braided reaches described by Williams and Rust (1969), Rust (1972b), and Smith (1974). These authors describe vertical aggradation giving rise to horizontal stratification, and low-angle bedding developing from lateral accretion. Crossbedded sand wedges along bar margins are similar to the lenticular sandstones
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within the conglomerates of the Table Mountain Group. Bars were low and shifting, differing from the more stable gravel bars described by Eynon and Walker (1974) which show laterally extensive cross-bed sets. The relatively poor sorting may have resulted from rapid introduction of material by debris floods on the steep, unvegetated valley margins. DISTAL A L L U V I A L SANDSTONE DEPOSITS
The arkosic sandstones of ]~at~A are up to 400 m thick and show an irregular upward and southward reduction in grain size from predominantly coarse to medium sand. They consist of a limited number of lithofacies -planar and trough cross-bedded sandstone, massive pebbly sandstone, and rare maroon siltstone, with numerous, closely spaced erosional discontinuities (Fig.5), some of which delineate channels of variable width-to-depth ratio. Markov analysis of vertical ordering in an outcrop area near Durban revealed a weak memory effect (Mathew, 1971) showing very poorly developed cyclicity. Our observations over a wider area of Natal show that erosional discontinuities bound vertical sequences with a preferred vertical arrangement of (1) sporadic, erosively based, massive pebbly sandstone,
Fig.5. A 20 m thick arkosic succession near Durban showing thick, solitary planar crossbed sets (p) with reactivation surfaces dotted in upper set, grouped planar cross-bed sets (gp) and channeling cosets of trough cross-beds (t).
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overlain by (2) solitary or grouped sets of planar cross-bedding, and/or multiple cosets of trough cross-bedding, with (3) siltstone terminating the sequence. Preservation potential appears to have decreased upward, so that in many cases the cross-bedded sandstones are truncated by the pebbly base of the next sequence. Pebbles comprise siltstone rip-up clasts, attesting to local derivation, together with variable proportions of extrabasinal quartzite and granitic clasts. In addition to these often incomplete sequences, there are numerous apparently random arrangements. As a result the degree of cyclicity is low. Preserved thic.kness of sequences, each representing a genetic package, is generally in the 20--200 cm range. The basal pebbly sandstone units occur as discontinuous thin sheets and lenses occupying shallow depressions in the scoured base. There is little or no pebble imbrication. The overlying planar cross-bed sets tend to be isolated, or to occur in groups of two or three sets. Thicknesses of individual sets range between 40 and 180 cm. Foresets typically show alternating coarse and fine textures, and have angular to slightly concave basal contacts. Reactivation surfaces, indicating nonuniform flow or fluctuating water levels, are c o m m o n (Fig.5), as is c o m p o u n d cross-bedding in grouped sets. Reverse cross-laminations (regressive ripples) are present in some examples (Fig.6); elsewhere we have observed small-scale cross-bed intrasets directed along the strike of the major foresets. These respective patterns suggest strong flow
Fig.6. Reverse cross-lamination intrasets in large-scale solitary set of planar cross-bedding, Umdoni Park. Scale in crn.
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Fig.7. Planar cross-bed set resting on exhumed surface of subjacent bar. Note textural variations in foresets and penecontemporaneous deformation near top. Overlying unit comprises smaller-scale trough cross-beds.
separation in front of large bedforms, and currents deflected laterally along the slipfaces. E x h u m e d form-sets of planar cross-bedding show a bar-type morphology, with horizontal to gently sloping tops (Fig.7). Traces of foresets in the flat upper surface indicate gradual changes in cross-bed dip direction of as much as 40 ° within a single, laterally persistent set. In addition there are abrupt differences in dip azimuths of up to 80 ° between superimposed sets. Trough cross-beds, in contrast, are present as thinner (6--50 cm), mutually truncating sets. They occur as cosets filling small channels {Fig.5) cutting through the thicker, planar cross-bedded units, and as lenticular units intercalcated with planar cross-beds. In a few instances they make up the entire sequence. Trough azimuths, as measured in horizontal exposures, are remarkably constant within a given coset, b u t show marked paleocurrent variation from one coset to another, both in vertical succession and laterally. In some cases this variation can be seen to result from differing trends of subordinate channels or scour-fill units containing grouped sets of uniform trough cross-bedding. Consequently, detailed measurements from a narrow stratigraphic interval show considerable dispersion of both planar and trough azimuths (Fig.8). Some small channels show low-angle lateral accretion bedding on the channel margin (Fig.9). Penecontemporaneous deformation structures range from minor corrugations along foresets (Fig.7) to complex recumbent overfolding. These
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Planar n=83
N I
Trough n:82
N I
Fig.8. Distribution of planar and trough cross-bed azimuths, arkosic sandstone, Umdoni Park.
structures are best developed in the larger planar cross-bedded sets. Small-scale cross-lamination, mainly microtroughs, but including rippledrift cross-lamination without stoss preservation, is frequently observed in the upward transition into horizontally laminated or structureless reddishbrown siltstone at the top of a sequence.
Fig.9. Shallow channel showing lateral accretion structures, incised into planar cross-beds. Note marked difference in foreset dip direction between planar cross-beds to left of channel and above channel.
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Paleoenvironmental interpretation The oxidized, arkosic composition and consistent southwesterly paleocurrent pattern favor a sandy alluvial floodplain origin for the Table Mountain Group in Natal. Smith (1970, 1972) has shown that a high proportion of planar cross-beds in an alluvial setting is characteristic of sandy transverse braid bars. The basal pebbly sandstone was probably developed as a coarse lag, locally spread out as a veneer, and accumulating to greater thickness in deeper parts of the channel. Rapid dumping of sediment by bank collapse (Coleman, 1969) may account for the absence of statification or clast imbrication in the basal layer. Like the exhumed bedforms or form-sets of the Table Mountain Group, transverse bars are tabular features with downstream margins that vary from irregular (Smith, 1971) to symmetrically lobate (Collinson, 1970). Slipface accretion generates steep, planar foresets of variable direction as different parts of the bar margin are activated (Smith, 1971). Cant (1978) has shown that foreset dip directions can diverge by as much as 180 °. In a study of the Tana River bars, Collinson (1970) demonstrated that at high river stage the slipface profiles change from straight to curved. This process may account for the coexistence of angularly based, tabular foresets and curved foresets in the Table Mountain Group. Furthermore, the reverse cross-lamination intrasets associated with the curved foresets are compatible with the strong reverse flow eddy documented by Collinson (1970) during flood. Those intrasets that are inclined approximately at right angles to the dip direction of the planar foresets are probably accounted for by Collinson's observation that during stages of low discharge, flow is diverted around the emergent bar margins and converges ahead of the bar. Alternatively they may reflect flow down the channel axis adjacent to the slipface of bars accreting sideways across the channel. Thick sets of planar cross-beds are thus interpreted as the deposits of transverse bars which were modified by processes associated with fluctuating river level. Smith (1971, 1974) has shown that low-water dissection produces a complex pattern of minor channels such as is observed in the Table Mountain Group. Alternatively, bar dissection of this type can occur without reduction in discharge if the surface area of the bar increases above a critical value (Smith, 1971). Superimposed planar cross-bed sets may have resulted from a rapid rise in river level causing a second bar to form on top of a preexisting bar (Smith, 1971). The compound cross-bedding in the Table Mountain Group may have been produced by the upper bar moving more rapidly and avalanching down the slipface of the lower bar. The relationship between planar and trough cross-beds can also be explained by analogy with modern sandy braided streams. Smith (1971) has suggested that during high river discharge dunefields are developed that merge into large bars as flow diminishes. This would explain the observed relationship between cosets of trough cross-beds truncated by thicker sets of planar cross-beds. Cant and Walker (1976) have described similar patterns in
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Devonian braided stream deposits. During high river stage trough crossbedded sands are commonly deposited over the flat bar top (Collinson, 1970; Smith, 1970), possibly accounting for that arrangement in the Table Mountain Group (Fig.7). Channel-filling trough cross-bed cosets incised into planar cross-bed sets may record deposition on a dissected, low-water bar surface. The lateral accretion structures on channel margins possibly developed on small side bars, features that are not uncommon in braided environments (Coleman, 1969; Collinson, 1970). The tendency for siltstone to overlie sandstone gradationally, with rippledrift cross-lamination indicating rapid sediment fallout (Jopling and Walker, 1968), suggests overbank deposition on inactive areas of the braided complex (Miall, 1977). This is a widespread phenomenon in the Bengal Basin (Coleman, 1969). Fining-upward patterns of the type that are locally preserved in the Table Mountain Group have been documented in the braided Donjek (Williams and Rust, 1969) and South Saskatchewan Rivers (Cant, 1978), and in ancient rocks by Cant and Walker (1976). In most cases though these fining upward cycles are both thicker and more orderly than those observed in the present study. The relatively poorly developed sequences in the Table Mountain Group may have resulted from constant switching of channel positions (Allen, 1965) or from greater destructive modification associated with changes in river level. OTHER DEPOSITS OF THE NATAL EMBAYMENT The arkosic sandstones described above merge vertically into fine-grained micaceous sandstones and interfinger southward with a thick quartz arenite succession.
Fine-grained micaceous sandstones The fine-grained sandstones and siltstones with rare pebbly beds that comprise the upper 300 m of the Table Mountain Group are poorly exposed and frequently highly deformed by slumping (Matthews, 1961). By comparison with the braided arkoses, channel forms are more commonly preserved and are narrow, well defined, and up to 3 m deep. Trough cross-bedding predominates, in sets up to 250 cm thick. Channel-fill units truncate one another laterally, and in places are stacked vertically. Laterally extensive red sandy siltstones up to 6 m thick intervene at irregular intervals, showing abrupt but nonerosive contacts with the underlying sandstone units. As a result there is no gradual upward-fining pattern of meandering channel type (Allen, 1965). The deposits resemble more closely the sequences ascribed by Moody-Stuart (1966) to low-sinuosity streams. Although the clearly recognizable channel forms, as distinct from the sheet-like braided stream deposits, may be attributed to finer-grained, more cohesive bank material
181 (Schumm, 1960), a meandering pattern was possibly inhibited in these mixed-load streams by the absence of binding vegetation.
Quartz arenites In southernmost Natal and Transkei, the maturity of the sandstones, the trace fossils, and the association of primary structures have together been taken as evidence for a shallow marine origin (Hobday and Mathew, 1974). Superposed en echelon sandbar units show complex internal stratification, with bioturbated and oxidized upper surfaces, and large-scale, offshoredipping planar cross-beds at the seaward ends. Some bars are b o u n d e d on their landward sides by rippled and small-scale cross-bedded, fine-grained sandstone, which is highly burrowed in part. Trace fossils include large subhorizontal Rhizocoralliurn. Cutting t~:rough the sand! ,ars i,~_ a rlirection perpendicular to the inferred shoreline trend are channels as much as 6 m deep containing seaward-directed cross-beds with numerous reactivation surfaces and occasional reversals. We envisage a tide- and storm-dominated shelf environment with sand shoals cut by channels fashioned by the combined effects of tidal flow and storm surge ebb. There are no structures characteristic of beach processes (Clifton et al., 1971). It is possible that deposition occurred in a macrotidal environment of the t y p e outlined by Hayes (1975) b u t with considerable modification by storms. The northward-tapering tongues of quartz arenite within the arkosic succession are thought to record episodes of marine incursion. PALEOGEOGRAPHIC SYNTHESIS The Natal E m b a y m e n t extended into eastern Gondwanaland from a major ocean basin to the southwest. High basement relief and boulder-sized fluvial deposits at the northern trough terminus indicate steep gradients, possibly arising from epeirogenic uplift of the adjoining Kaapvaal Craton, or from basinal downfaulting. Maud (1961) has mapped a series of arcuate faults parallel to the basin margins; most represent post-Karoo displacements, b u t m a y have been reactivated along older fault trends. Fluvial dispersal patterns were d o w n the plunging trough axis towards the southwest. Confined intermontane environments were characterized by flood deposition of boulder sheets, and subsequently by coarse gravel accumulation on longitudinal bars. Downstream reduction in gradient and particle size was accompanied by a change in bar morphology to transverse types (cf. the Platte River; Smith, 1970). A reddish coloration was imparted by postdepositional weathering, such as occurs in areas of seasonal rainfall fluctuation (McBride, 1974). In terms of both paleoclimate and paleogeographic extent, the sandy alluvial floodplain may have resembled the vast Kosi River fan, an area characterized by torrential annual floods and rapid sedimentation (Gole and Chitale, 1966). Aggradation, coupled with erosional
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reduction of the elevated northern source, caused northward overlap of the conglomerates by sandy braided stream deposits. Stream competence continued to decline, with the deposition of progressively finer-grained sandstones and siltstones by low-sinuosity, mixed-load streams. Quartz arenites accumulated at the seaward end of the Natal Embayment (Fig.10), the funneled configuration being conducive to a high tidal range. Both the Ord and Colorado (Gulf of California) Rivers debouch into elongate, structurally controlled basins (Wright et al., 1973; Meckel, 1975). However, the fluvial system of the Table Mountain Group was very much larger and coarser grained, and in some respects may have resembled the Ganges-Brahmaputra complex. Like the Ganges--Brahmaputra, the seaward margins were reworked by strong tidal currents, possibly augmented by storm processes, generating a series of sandbars cut by channels perpendicular to the shoreline. Gradual basement subsidence was balanced by sediment influx, and water depths remained shallow throughout the accumulation of 1000 m of quartz sand. The absence of finer sediments, particularly in the lower part of the succession, is possibly explained by highly effective winnowing processes that deposited silts and muds farther offshore. Development of high tidal mudflats may have been inhibited by the paucity of silt and clay; alternatively, the upper part of the succession containing argillaceous tidal flat deposits
Fig. 10. Schematic paleogeographic reconstruction.
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may have been removed by erosion associated with the late Paleozoic Dwyka ice-lobe that moved southwestward over the area (Matthews, 1970). ACKNOWLEDGEMENTS
Norman Smith (University of Illinois at Chicago Circle) and Lawrie Minter (Anglo American Corporation) accompanied us to selected outcrops in Natal and gave of their valuable experience. We gratefully acknowledge comments on the preliminary manuscript by Norman Smith, Tom Gustavson, and Tony Tankard. Figures were drafted under the direction of James Macon; the manuscript was typed by Lucille Harrell. REFERENCES Allen, J. R. L., 1965. A review of the origin and characteristics of recent alluvial sediments. Sedimentology, 5: 89--191. Baker, V. R. and Penteado-Orellana, M. M., 1978. Fluvial sedimentation conditioned by Quaternary climatic change in central Texas. J. Sediment. Petrol., 48: 433--451. Boothroyd, J. C., 1972. Coarse-grained sedimentation on a braided outwash fan, northeast Gulf of Alaska. Tech. Rep. 6-CRD, Coastal Research Division, University of South Carolina, Columbia, S. C., 127 pp. Boothroyd, J. C. and Ashley, G. M., 1975. Processes, bar morphology, and sedimentary structures on braided outwash fans, northeastern Gulf of Alaska. In : A. V. Jopling and B. C. McDonald (Editors), Glaciofluvial and Glaciolacustrine Sedimentation. Soc. Econ. Paleontol. Mineral., Spec. Publ., 23: 193--222. Cant, D. J., 1978. Development of a facies model for sandy braided river sedimentation: Comparison of the South Saskatchewan River and the Battery. Point Formation. In: A. D. Miall (Editor), Fluvial Sedimentology. Can. Soc. Pet. Geol. Mere., 5: 627--639. Cant, D. J. and Walker, R. G., 1976. Development of a braided-fluvial facies model for the Devonian Battery Point Sandstone, Quebec. Can. J. Earth Sci., 13: 102--119. Clifton, H. E., Hunter, R. E. and Phillips, R. L., 1971. Depositional structures and processes in the non-barred high-energy nearshore. J. Sediment. Petrol., 41: 651--670. Cocks, L. R. M., Brunton, C. R. C., Rowell, A. J. and Rust, I. C., 1970. The first lower Palaeozoic fauna proved from South Africa. J. Geol. Soc. Lond., 125: 583--603. Coleman, J. M., 1969. Brahmaputra River: channel processes and sedimentation. Sediment. Geol., 3: 131--239. Collinson, J. D., 1970. Bedforms in the Tana River, Norway. Geogr. Ann., 52: 31--56. Du Toit, A. L., 1931, Explanation of Sheet 109 (Nkandhla). Geol. Surv. S. Aft., 57 pp. Eynon, G. and Walker, R. G., 1974. Facies relationships in Pleistocene outwash gravels, southern Ontario: a model for bar growth in braided rivers. Sedimentology, 21 : 43--70. Gole, C. V. and Chitale, S. V., 1966. Inland delta building activity of the Kosi River. Am. Soc. Civil Eng. Proc., J. Hydraul. Div., 92: 111--126. Hayes, M. O., 1975. Morphology of sand accumulation in estuaries: an introduction to the symposium. In: L. E. Cronin (Editor), Estuarine Research, 2. Academic Press, New York, pp. 3--22. Hobday, D. K. and Mathew, D., 1974. Depositional environment of the Cape Supergroup in the Transkei. Trans. Geol. Soc. S. Afr., 77: 223--227. Jopling, A. V. and Walker, R. G., 1968. Morphology and origin of ripple-drift crosslamination, with examples from the Pleistocene of Massachusetts. J. Sediment. Petrol., 38: 971--984. Kingsley, C. S., 1975. A new stratigraphic classification implying a lithofacies change in the Table Mountain Sandstone in southern Natal. Trans. Geol. Soc. S. Afr., 78: 43--55.
184 Lock, B. E., 1973. The Cape Supergroup in Natal and northern Transkei. Geol. Mag., 110: 485--486. Mathew, D., 1971. A palaeoenvironmental study of the basal zone of the Table Mountain Sandstone at Kloof Gorge. Petros, 3: 6--17. Matthews, P. E., 1961. Slump structures in the Table Mountain Series of Natal. Trans. Geol. Soc. S. Afr., 64: 55--69. Matthews, P. E., 1970. Paleorelief and Dwyka glaciation in the eastern region of South Africa. In: Second Gondwana Symposium Proceedings and Papers, Cape Town, July, 1970, pp. 491--499. Maud, R. R., 1961. A preliminary review of the structure of coastal Natal. Trans. Geol. Soc. S. Aft., 64: 247--256. McBride, E. F., 1974. Significance of color in red, green, purple, olive, brown, and gray beds of the Difunta Group, north-eastern Mexico. J. Sediment. Petrol., 44: 760--773. McGowen, J. H. and Groat, C. G., 1971. Van Horn Sandstone, West Texas: an alluvial fan model for mineral exploration. Bureau of Economic Geology, Report of Investigations No. 72, The University of Texas at Austin, 57 pp. Meckel, L. D., 1975. Holocene sand bodies in the Colorado Delta, Salton Sea, Imperial County California. In: M. L. Broussard (Editor), Deltas, Models for Exploration. Houston Geological Society, Houston, Texas, pp. 239--265. Miall, A. D., 1970. Devonian alluvial fans, Prince of Wales Island, Arctic Canada. J. Sediment. Petrol., 40: 556--571. Miall, A. D., 1977. A review of the braided stream depositional environment. Earth-Sci. Rev., 13: 1--62. Moody-Stuart, M., 1966. High- and low-sinuosity stream deposits, with examples from the Devonian of Spitzbergen. J. Sediment. Petrol., 36 : 1102--1117. Nicolaysen, L. O. and Burger, A. J., 1965. Note on an extensive zone of 1000 million year old metamorphic and igneous rocks in southern Africa. Sci. Terre. 10: 497--516. Rhodes, R. C. and Leith, M., 1967. Lithostratigraphic zones in the Table Mountain Series of Natai. Trans. Geol. Soc. S. Afr., 70: 15--28. Rust, B. R., 1972a. Pebble orientation in fluvial sediments. J. Sediment. Petrol., 42 : 384--388. Rust, B. R., 1972b. Structure and process in a braided river. Sedimentology, 18: 221--245. Schumm, S. A., 1960. The effect of sediment type on the shape and stratification of some modern fluvial deposits. Am. J. Sci., 258: 177--184. Schumm, S. A. and Stevens, M. A., 1973. Abrasion in place: A mechanism for rounding and size reduction of coarser sediments in rivers. Geology, 1 : 37--40. Sharp, R. P. and Nobles, L. H., 1953. Mudflow at Wrightwood, southern California. Geol. Soc. Am. Bull., 46: 547--560. Smith, N. D., 1970. The braided stream depositional environment: comparison of the Platte River with some Silurian clastic rocks, north-central Appalachians. Geol. Soc. Am. Bull., 81: 2993--3014. Smith, N. D., 1971. Transverse bars and braiding in the lower Platte River, Nebraska. Geol. Soc. Am. Bull., 82: 3407--3420. Smith, N. D., 1972. Some sedimentological aspects of planar cross-stratification in a sandy braided river. J. Sediment. Petrol., 42: 624--634. Smith, N. D., 1974. Sedimentology and bar formation in the upper Kicking Horse River, a braided outwash stream. J. Geol., 82: 205--224. Visser, J. N. J., 1974. The Table ~Iountain Group: a study in the deposition of quartz arenites on a stable shelf. Trans. Geol. Soc. S. Afr., 77: 229--237. Williams, P. R. and Rust, B. R., 1969. The sedimentology of a braided river. J. Sediment. Petrol., 39: 649--679. Wright, L. D., Coleman, J. M. and Thorn, B. G., 1973. Processes of channel development in a high-tide-range environment: Cambridge Gulf-Ord River, Western Australia. J. Geol., 81: 15--41.
Palaeogeography, Palaeoclimatology, Palaeoecology, 28(1979): 169--184 © Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
FLUVIAL SEDIMENTATION AND PALEOGEOGRAPHY PALEOZOIC FAILED RIFT, SOUTHEASTERN MARGIN
OF AN EARLY OF AFRICA 1
DAVID K. HOBDAY and VICTOR VON BRUNN
Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas (U.S.A.) Department of Geology, University of Natal, Pietermaritzburg (South Africa) (Received November 16, 1978; revised version accepted May 17, 1979) ABSTRACT Hobday, D. K. and Von Brunn, V., 1979. Fluvial sedimentation and paleogeography of an early Paleozoic failed rift, southeastern margin of Africa. Palaeogeogr., Palaeoclimatol., Palaeoecol., 28: 169--184. An early Paleozoic failed rift, a local precursor of Gondwana fragmentation, was characterized by a steep, rugged northern terminus, with depositional slope decreasing longitudinally southwestward, where the trough opened toward a major ocean basin. There is a marked north--south zonation of the following facies: (1) boulder conglomerates showing crude horizontal stratification in the lower half, with gently inclined stratification and lenticular arrangements, including locally cross-bedded sandstones, in the upper half. These are interpreted to have been deposited in confined intermontane valleys which, following aggradation, developed into a gravelly braided stream complex. The conglomerates merge upward and southward into (2) red arkosic sandstones exhibiting weakly developed vertical arrangements of scour-based, massive pebbly sandstone overlain by cross-bedded sandstone, with thin red siltstones infrequently preserved at the top. Thick sets of planar cross-beds are attributed to deposition on the slipface of transverse bars; cosets of smaller-scale trough cross-beds were deposited in advance of' some bars and on top of others. There is evidence that bar dissection and current deflection occurred during river shallowing and bar emergence. An upward reduction in grain size through the fluvial succession is accompanied by a gradual change in channel geometry to narrow, steep-sided forms. A meandering tendency in these low-sinuosity mixed-load streams may have been inhibited by the lack of binding vegetation along the river margins. The fluvial arkoses interfinger southward with (3) a 1000 m succession of quartz arenites thought to have b e e n deposited on an embayed shelf where the outer margins of the braided fluvial complex were reworked by very high energy storm- and tide-dominated processes. INTRODUCTION D e p o s i t i o n o f t h e T a b l e M o u n t a i n G r o u p in t h e Z u l u l a n d - - N a t a l - - T r a n s k e i a r e a o c c u r r e d in a n e l o n g a t e b a s i n ( t h e N a t a l E m b a y m e n t ; V i s s e r , 1 9 7 4 ) w i t h its axis subparallel to the present-day coastline (Matthews, 1970). In Transkei and Natal the Table Mountain Group rests nonconformably on 1 Published with permission of the Director, Bureau of Economic Geology.
170 granitoid and associated rocks of the 1000 m.y. Namaqua--Natal belt (Nicolaysen and Burger, 1965); in Zululand it overlies older units of the Precambrian basement. The succession is correlated lithologically with the Table Mountain Group of the southern Cape Province, the upper part of which contains a marine fauna of Ashgillian (late Ordovician) age (Cocks et al., 1970). A substantially younger date assigned to the Transkei deposits by Lock (1973) appears to be unlikely. If correlation with the Cape Province succession is correct, then the upper two units of the Upper Cambrian to Upper Devonian Cape Supergroup, the Bokkeveld and Witteberg Groups, are not represented in Natal and adjoining states. The early Paleozoic trough had an axial gradient towards the south-southwest, and is thought to mark a line of incipient crustal rupture, behaving as a failed rift that was again active in late Paleozoic times during Lower Karoo sedimentation. Mesozoic breakup of this part of Gondwanaland occurred approximately along the trough axis. As a result, only the western flanks are preserved on the southern African subcontinent. Subsequent seaward warping, block faulting, and erosion of the coastal monocline have resulted in two parallel outcrop belts and a number of isolated exposures (Fig.l). Sediments of the Table Mountain Group deposited in the Natal Embayment show a marked north--south facies zonation. Coarse conglomerates are developed in the northern apical area of Zululand where they occupy paleovalleys in the highly dissected Precambrian basement. This rugged paleotopography terminates abruptly at about the latitude of the Tugela Fault (Du Toit, 1931; Rhodes and Leith, 1967), beyond which red arkosic sandstone facies overlie a peneplained basement surface. The arkoses interfinger southward with quartz arenites (Kingsley, 1975), which merge in southern Natal and thicken into Transkei where they make up a homogeneous, 1000 m thick succession of mature shallow marine sandstones (Hobday and Mathew, 1974). This paper focuses on the northern conglomerates and arkosic sandstones, which have been ascribed in general terms to an alluvial depositional system (Du Toit, 1931; Rhodes and Leith, 1967). Further, we will propose a reconstruction of the relationship between the fluvial facies and the thick quartzose sandstones to the south. Detailed study of the arkosic facies was facilitated by coastal exposures showing exhumed bedforms at Umdoni Park (Fig.l) and by three-dimensional views of sedimentary structures at a number of localities. PROXIMAL C O N G L O M E R A T E
DEPOSITS
The coarse-grained northern facies consists of conglomerates up to 60 m thick. Boulders as much as 2 m in diameter occur towards the base. There is a progressive upward decrease in maximum clast diameter to 25 cm near the top, and the uppermost conglomerates contain sandstone intercalations.
171
\
20~E
30~E
\
4,, ,
/
R
Durban
EXPLANATION I
(3
30-4i
J POSt Karoo
JT~T1
Upper Karoo
Umdoni Park Ecco Group Owyka
÷
m
Group
Table Mountain Group Precombricn Basement
\
20 I
40 I
60 Km I
Fig.1. Generalized geological map showing the distribution of the Table Mountain Group in Zululand, Natal and Transkei.
172
The underlying Precambrian basement rocks include deeply incised quartzite, metavolcanics, gneiss, and schist. Local paleorelief of 300 m beneath the Table Mountain Group has been noted by Matthews (1970). Despite the lithologic heterogeneity of the basement, boulders resting directly on the unconformity are predominantly quartzites, attesting to their physical and chemical durability. Most clasts are moderately to well rounded, with the highest degree of rounding being manifest in the larger particles. Percussion marks are developed over the entire surface of even the largest boulders, suggesting that in-place abrasion was an important process in clast rounding and size reduction (Schumm and Stevens, 1973). The conglomerate is clast-supported through 95% of the examined succession. Towards the base there is a marked bimodality in grain size, with boulders of subequal dimensions, generally in the 60--120 cm size range, and a coarse feldspathic sandstone matrix. This indicates vigorous current activity, with suspended sand settling into the interstices of coarse bedload gravels following reduction in flow. The upward reduction in maximum clast size (Fig.2) to between 25 and 50 cm is accompanied by a decrease in sorting of the smaller gravel-sized clasts. Imbrication is well developed (Fig.3) and indicates southerly to southwesterly
Fig.2. Outcrop commencing about 10 m above base of Table Mountain Group showing upward reduction in m a x i m u m clast size and crude sub-horizontal layering. Thin sandstone intercalations are present at the top.
173
Fig.3. Imbricated quartzite boulders in the proximal conglomerate deposits approximately 15 m above base.
paleocurrents with local variation around basement residuals. Clast long axes show a preferred orientation perpendicular to the imbrication direction (Fig.4), a disposition which is characteristic of large clasts that are rolled along a stream bed (Rust, 1972a). The conglomerate is generally massive with some crude subhorizontal stratification in the lower parts {Fig.2), giving way upward to irregular lenticular arrangements involving shallow scour surfaces, overlapping coarser and finer conglomerate pods, discontinuous sandstone partings, and gently inclined stratification. The upper sandstone interbeds show rare cross-bedding. A small proportion of the succession, varying with locality b u t nowhere more than 5%, is made up of very poorly sorted, matrix-supported conglc~merate in which no bedding, imbrication, or preferred clast alignment are apparent. These units suggest rapid dumping of sediment and are possibly debris flood deposits (Sharp and Nobles, 1953; Miall, 1970), the larger parts of which have been reworked by fluvial processes.
Paleoenvironmental interpretation The coarse basal conglomerate with abraded boulders up to 2 m in diameter is similar to the proximal fan facies of the Van Horn Sandstone, Texas,
174
Fig.4. Clast long axis orientation and imbrication directions, proximal conglomerate deposits.
where the evidence of extremely high competence is explained as resulting from confinement of flow in narrow canyons (McGowen and Groat, 1971). Like the Van Horn Sandstone, conglomerates of the Table Mountain Group are thought to have accumulated in an intermontane environment favoring rapid physical weathering, with a landscape devoid of an effective vegetation cover. Preferential concentration of quartzite boulders, with the low proportion of locally derived clasts of other lithologies, indicates a humid climate that contributed to breakdown of nonresistant clasts during transport and storage (cf. Baker and Penteado-Orellana, 1978). As the valleys aggraded, upstream gradients were reduced, and flow was dispersed over a wider area; this environment possibly resembled the upper outwash fans of northeast Gulf of Alaska (Boothroyd, 1972; Boothroyd and Ashley, 1975), where longitudinal bars consist of crudely bedded, well-imbricated gravel, with little sand except as matrix. Smaller particle size and increased bed relief in the upper part of the conglomeratic interval is suggestive of an environment comparable to the midfans of Boothroyd and Ashley (1975), or the proximal braided reaches described by Williams and Rust (1969), Rust (1972b), and Smith (1974). These authors describe vertical aggradation giving rise to horizontal stratification, and low-angle bedding developing from lateral accretion. Crossbedded sand wedges along bar margins are similar to the lenticular sandstones
175
within the conglomerates of the Table Mountain Group. Bars were low and shifting, differing from the more stable gravel bars described by Eynon and Walker (1974) which show laterally extensive cross-bed sets. The relatively poor sorting may have resulted from rapid introduction of material by debris floods on the steep, unvegetated valley margins. DISTAL A L L U V I A L SANDSTONE DEPOSITS
The arkosic sandstones of ]~at~A are up to 400 m thick and show an irregular upward and southward reduction in grain size from predominantly coarse to medium sand. They consist of a limited number of lithofacies -planar and trough cross-bedded sandstone, massive pebbly sandstone, and rare maroon siltstone, with numerous, closely spaced erosional discontinuities (Fig.5), some of which delineate channels of variable width-to-depth ratio. Markov analysis of vertical ordering in an outcrop area near Durban revealed a weak memory effect (Mathew, 1971) showing very poorly developed cyclicity. Our observations over a wider area of Natal show that erosional discontinuities bound vertical sequences with a preferred vertical arrangement of (1) sporadic, erosively based, massive pebbly sandstone,
Fig.5. A 20 m thick arkosic succession near Durban showing thick, solitary planar crossbed sets (p) with reactivation surfaces dotted in upper set, grouped planar cross-bed sets (gp) and channeling cosets of trough cross-beds (t).
176
overlain by (2) solitary or grouped sets of planar cross-bedding, and/or multiple cosets of trough cross-bedding, with (3) siltstone terminating the sequence. Preservation potential appears to have decreased upward, so that in many cases the cross-bedded sandstones are truncated by the pebbly base of the next sequence. Pebbles comprise siltstone rip-up clasts, attesting to local derivation, together with variable proportions of extrabasinal quartzite and granitic clasts. In addition to these often incomplete sequences, there are numerous apparently random arrangements. As a result the degree of cyclicity is low. Preserved thic.kness of sequences, each representing a genetic package, is generally in the 20--200 cm range. The basal pebbly sandstone units occur as discontinuous thin sheets and lenses occupying shallow depressions in the scoured base. There is little or no pebble imbrication. The overlying planar cross-bed sets tend to be isolated, or to occur in groups of two or three sets. Thicknesses of individual sets range between 40 and 180 cm. Foresets typically show alternating coarse and fine textures, and have angular to slightly concave basal contacts. Reactivation surfaces, indicating nonuniform flow or fluctuating water levels, are c o m m o n (Fig.5), as is c o m p o u n d cross-bedding in grouped sets. Reverse cross-laminations (regressive ripples) are present in some examples (Fig.6); elsewhere we have observed small-scale cross-bed intrasets directed along the strike of the major foresets. These respective patterns suggest strong flow
Fig.6. Reverse cross-lamination intrasets in large-scale solitary set of planar cross-bedding, Umdoni Park. Scale in crn.
177
Fig.7. Planar cross-bed set resting on exhumed surface of subjacent bar. Note textural variations in foresets and penecontemporaneous deformation near top. Overlying unit comprises smaller-scale trough cross-beds.
separation in front of large bedforms, and currents deflected laterally along the slipfaces. E x h u m e d form-sets of planar cross-bedding show a bar-type morphology, with horizontal to gently sloping tops (Fig.7). Traces of foresets in the flat upper surface indicate gradual changes in cross-bed dip direction of as much as 40 ° within a single, laterally persistent set. In addition there are abrupt differences in dip azimuths of up to 80 ° between superimposed sets. Trough cross-beds, in contrast, are present as thinner (6--50 cm), mutually truncating sets. They occur as cosets filling small channels {Fig.5) cutting through the thicker, planar cross-bedded units, and as lenticular units intercalcated with planar cross-beds. In a few instances they make up the entire sequence. Trough azimuths, as measured in horizontal exposures, are remarkably constant within a given coset, b u t show marked paleocurrent variation from one coset to another, both in vertical succession and laterally. In some cases this variation can be seen to result from differing trends of subordinate channels or scour-fill units containing grouped sets of uniform trough cross-bedding. Consequently, detailed measurements from a narrow stratigraphic interval show considerable dispersion of both planar and trough azimuths (Fig.8). Some small channels show low-angle lateral accretion bedding on the channel margin (Fig.9). Penecontemporaneous deformation structures range from minor corrugations along foresets (Fig.7) to complex recumbent overfolding. These
178
Planar n=83
N I
Trough n:82
N I
Fig.8. Distribution of planar and trough cross-bed azimuths, arkosic sandstone, Umdoni Park.
structures are best developed in the larger planar cross-bedded sets. Small-scale cross-lamination, mainly microtroughs, but including rippledrift cross-lamination without stoss preservation, is frequently observed in the upward transition into horizontally laminated or structureless reddishbrown siltstone at the top of a sequence.
Fig.9. Shallow channel showing lateral accretion structures, incised into planar cross-beds. Note marked difference in foreset dip direction between planar cross-beds to left of channel and above channel.
179
Paleoenvironmental interpretation The oxidized, arkosic composition and consistent southwesterly paleocurrent pattern favor a sandy alluvial floodplain origin for the Table Mountain Group in Natal. Smith (1970, 1972) has shown that a high proportion of planar cross-beds in an alluvial setting is characteristic of sandy transverse braid bars. The basal pebbly sandstone was probably developed as a coarse lag, locally spread out as a veneer, and accumulating to greater thickness in deeper parts of the channel. Rapid dumping of sediment by bank collapse (Coleman, 1969) may account for the absence of statification or clast imbrication in the basal layer. Like the exhumed bedforms or form-sets of the Table Mountain Group, transverse bars are tabular features with downstream margins that vary from irregular (Smith, 1971) to symmetrically lobate (Collinson, 1970). Slipface accretion generates steep, planar foresets of variable direction as different parts of the bar margin are activated (Smith, 1971). Cant (1978) has shown that foreset dip directions can diverge by as much as 180 °. In a study of the Tana River bars, Collinson (1970) demonstrated that at high river stage the slipface profiles change from straight to curved. This process may account for the coexistence of angularly based, tabular foresets and curved foresets in the Table Mountain Group. Furthermore, the reverse cross-lamination intrasets associated with the curved foresets are compatible with the strong reverse flow eddy documented by Collinson (1970) during flood. Those intrasets that are inclined approximately at right angles to the dip direction of the planar foresets are probably accounted for by Collinson's observation that during stages of low discharge, flow is diverted around the emergent bar margins and converges ahead of the bar. Alternatively they may reflect flow down the channel axis adjacent to the slipface of bars accreting sideways across the channel. Thick sets of planar cross-beds are thus interpreted as the deposits of transverse bars which were modified by processes associated with fluctuating river level. Smith (1971, 1974) has shown that low-water dissection produces a complex pattern of minor channels such as is observed in the Table Mountain Group. Alternatively, bar dissection of this type can occur without reduction in discharge if the surface area of the bar increases above a critical value (Smith, 1971). Superimposed planar cross-bed sets may have resulted from a rapid rise in river level causing a second bar to form on top of a preexisting bar (Smith, 1971). The compound cross-bedding in the Table Mountain Group may have been produced by the upper bar moving more rapidly and avalanching down the slipface of the lower bar. The relationship between planar and trough cross-beds can also be explained by analogy with modern sandy braided streams. Smith (1971) has suggested that during high river discharge dunefields are developed that merge into large bars as flow diminishes. This would explain the observed relationship between cosets of trough cross-beds truncated by thicker sets of planar cross-beds. Cant and Walker (1976) have described similar patterns in
180
Devonian braided stream deposits. During high river stage trough crossbedded sands are commonly deposited over the flat bar top (Collinson, 1970; Smith, 1970), possibly accounting for that arrangement in the Table Mountain Group (Fig.7). Channel-filling trough cross-bed cosets incised into planar cross-bed sets may record deposition on a dissected, low-water bar surface. The lateral accretion structures on channel margins possibly developed on small side bars, features that are not uncommon in braided environments (Coleman, 1969; Collinson, 1970). The tendency for siltstone to overlie sandstone gradationally, with rippledrift cross-lamination indicating rapid sediment fallout (Jopling and Walker, 1968), suggests overbank deposition on inactive areas of the braided complex (Miall, 1977). This is a widespread phenomenon in the Bengal Basin (Coleman, 1969). Fining-upward patterns of the type that are locally preserved in the Table Mountain Group have been documented in the braided Donjek (Williams and Rust, 1969) and South Saskatchewan Rivers (Cant, 1978), and in ancient rocks by Cant and Walker (1976). In most cases though these fining upward cycles are both thicker and more orderly than those observed in the present study. The relatively poorly developed sequences in the Table Mountain Group may have resulted from constant switching of channel positions (Allen, 1965) or from greater destructive modification associated with changes in river level. OTHER DEPOSITS OF THE NATAL EMBAYMENT The arkosic sandstones described above merge vertically into fine-grained micaceous sandstones and interfinger southward with a thick quartz arenite succession.
Fine-grained micaceous sandstones The fine-grained sandstones and siltstones with rare pebbly beds that comprise the upper 300 m of the Table Mountain Group are poorly exposed and frequently highly deformed by slumping (Matthews, 1961). By comparison with the braided arkoses, channel forms are more commonly preserved and are narrow, well defined, and up to 3 m deep. Trough cross-bedding predominates, in sets up to 250 cm thick. Channel-fill units truncate one another laterally, and in places are stacked vertically. Laterally extensive red sandy siltstones up to 6 m thick intervene at irregular intervals, showing abrupt but nonerosive contacts with the underlying sandstone units. As a result there is no gradual upward-fining pattern of meandering channel type (Allen, 1965). The deposits resemble more closely the sequences ascribed by Moody-Stuart (1966) to low-sinuosity streams. Although the clearly recognizable channel forms, as distinct from the sheet-like braided stream deposits, may be attributed to finer-grained, more cohesive bank material
181 (Schumm, 1960), a meandering pattern was possibly inhibited in these mixed-load streams by the absence of binding vegetation.
Quartz arenites In southernmost Natal and Transkei, the maturity of the sandstones, the trace fossils, and the association of primary structures have together been taken as evidence for a shallow marine origin (Hobday and Mathew, 1974). Superposed en echelon sandbar units show complex internal stratification, with bioturbated and oxidized upper surfaces, and large-scale, offshoredipping planar cross-beds at the seaward ends. Some bars are b o u n d e d on their landward sides by rippled and small-scale cross-bedded, fine-grained sandstone, which is highly burrowed in part. Trace fossils include large subhorizontal Rhizocoralliurn. Cutting t~:rough the sand! ,ars i,~_ a rlirection perpendicular to the inferred shoreline trend are channels as much as 6 m deep containing seaward-directed cross-beds with numerous reactivation surfaces and occasional reversals. We envisage a tide- and storm-dominated shelf environment with sand shoals cut by channels fashioned by the combined effects of tidal flow and storm surge ebb. There are no structures characteristic of beach processes (Clifton et al., 1971). It is possible that deposition occurred in a macrotidal environment of the t y p e outlined by Hayes (1975) b u t with considerable modification by storms. The northward-tapering tongues of quartz arenite within the arkosic succession are thought to record episodes of marine incursion. PALEOGEOGRAPHIC SYNTHESIS The Natal E m b a y m e n t extended into eastern Gondwanaland from a major ocean basin to the southwest. High basement relief and boulder-sized fluvial deposits at the northern trough terminus indicate steep gradients, possibly arising from epeirogenic uplift of the adjoining Kaapvaal Craton, or from basinal downfaulting. Maud (1961) has mapped a series of arcuate faults parallel to the basin margins; most represent post-Karoo displacements, b u t m a y have been reactivated along older fault trends. Fluvial dispersal patterns were d o w n the plunging trough axis towards the southwest. Confined intermontane environments were characterized by flood deposition of boulder sheets, and subsequently by coarse gravel accumulation on longitudinal bars. Downstream reduction in gradient and particle size was accompanied by a change in bar morphology to transverse types (cf. the Platte River; Smith, 1970). A reddish coloration was imparted by postdepositional weathering, such as occurs in areas of seasonal rainfall fluctuation (McBride, 1974). In terms of both paleoclimate and paleogeographic extent, the sandy alluvial floodplain may have resembled the vast Kosi River fan, an area characterized by torrential annual floods and rapid sedimentation (Gole and Chitale, 1966). Aggradation, coupled with erosional
182
reduction of the elevated northern source, caused northward overlap of the conglomerates by sandy braided stream deposits. Stream competence continued to decline, with the deposition of progressively finer-grained sandstones and siltstones by low-sinuosity, mixed-load streams. Quartz arenites accumulated at the seaward end of the Natal Embayment (Fig.10), the funneled configuration being conducive to a high tidal range. Both the Ord and Colorado (Gulf of California) Rivers debouch into elongate, structurally controlled basins (Wright et al., 1973; Meckel, 1975). However, the fluvial system of the Table Mountain Group was very much larger and coarser grained, and in some respects may have resembled the Ganges-Brahmaputra complex. Like the Ganges--Brahmaputra, the seaward margins were reworked by strong tidal currents, possibly augmented by storm processes, generating a series of sandbars cut by channels perpendicular to the shoreline. Gradual basement subsidence was balanced by sediment influx, and water depths remained shallow throughout the accumulation of 1000 m of quartz sand. The absence of finer sediments, particularly in the lower part of the succession, is possibly explained by highly effective winnowing processes that deposited silts and muds farther offshore. Development of high tidal mudflats may have been inhibited by the paucity of silt and clay; alternatively, the upper part of the succession containing argillaceous tidal flat deposits
Fig. 10. Schematic paleogeographic reconstruction.
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may have been removed by erosion associated with the late Paleozoic Dwyka ice-lobe that moved southwestward over the area (Matthews, 1970). ACKNOWLEDGEMENTS
Norman Smith (University of Illinois at Chicago Circle) and Lawrie Minter (Anglo American Corporation) accompanied us to selected outcrops in Natal and gave of their valuable experience. We gratefully acknowledge comments on the preliminary manuscript by Norman Smith, Tom Gustavson, and Tony Tankard. Figures were drafted under the direction of James Macon; the manuscript was typed by Lucille Harrell. REFERENCES Allen, J. R. L., 1965. A review of the origin and characteristics of recent alluvial sediments. Sedimentology, 5: 89--191. Baker, V. R. and Penteado-Orellana, M. M., 1978. Fluvial sedimentation conditioned by Quaternary climatic change in central Texas. J. Sediment. Petrol., 48: 433--451. Boothroyd, J. C., 1972. Coarse-grained sedimentation on a braided outwash fan, northeast Gulf of Alaska. Tech. Rep. 6-CRD, Coastal Research Division, University of South Carolina, Columbia, S. C., 127 pp. Boothroyd, J. C. and Ashley, G. M., 1975. Processes, bar morphology, and sedimentary structures on braided outwash fans, northeastern Gulf of Alaska. In : A. V. Jopling and B. C. McDonald (Editors), Glaciofluvial and Glaciolacustrine Sedimentation. Soc. Econ. Paleontol. Mineral., Spec. Publ., 23: 193--222. Cant, D. J., 1978. Development of a facies model for sandy braided river sedimentation: Comparison of the South Saskatchewan River and the Battery. Point Formation. In: A. D. Miall (Editor), Fluvial Sedimentology. Can. Soc. Pet. Geol. Mere., 5: 627--639. Cant, D. J. and Walker, R. G., 1976. Development of a braided-fluvial facies model for the Devonian Battery Point Sandstone, Quebec. Can. J. Earth Sci., 13: 102--119. Clifton, H. E., Hunter, R. E. and Phillips, R. L., 1971. Depositional structures and processes in the non-barred high-energy nearshore. J. Sediment. Petrol., 41: 651--670. Cocks, L. R. M., Brunton, C. R. C., Rowell, A. J. and Rust, I. C., 1970. The first lower Palaeozoic fauna proved from South Africa. J. Geol. Soc. Lond., 125: 583--603. Coleman, J. M., 1969. Brahmaputra River: channel processes and sedimentation. Sediment. Geol., 3: 131--239. Collinson, J. D., 1970. Bedforms in the Tana River, Norway. Geogr. Ann., 52: 31--56. Du Toit, A. L., 1931, Explanation of Sheet 109 (Nkandhla). Geol. Surv. S. Aft., 57 pp. Eynon, G. and Walker, R. G., 1974. Facies relationships in Pleistocene outwash gravels, southern Ontario: a model for bar growth in braided rivers. Sedimentology, 21 : 43--70. Gole, C. V. and Chitale, S. V., 1966. Inland delta building activity of the Kosi River. Am. Soc. Civil Eng. Proc., J. Hydraul. Div., 92: 111--126. Hayes, M. O., 1975. Morphology of sand accumulation in estuaries: an introduction to the symposium. In: L. E. Cronin (Editor), Estuarine Research, 2. Academic Press, New York, pp. 3--22. Hobday, D. K. and Mathew, D., 1974. Depositional environment of the Cape Supergroup in the Transkei. Trans. Geol. Soc. S. Afr., 77: 223--227. Jopling, A. V. and Walker, R. G., 1968. Morphology and origin of ripple-drift crosslamination, with examples from the Pleistocene of Massachusetts. J. Sediment. Petrol., 38: 971--984. Kingsley, C. S., 1975. A new stratigraphic classification implying a lithofacies change in the Table Mountain Sandstone in southern Natal. Trans. Geol. Soc. S. Afr., 78: 43--55.
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