Sedimentary development of the Upper Ecca and Lower Beaufort Groups (Karoo Supergroup) in the Laingsburg subbasin (SW Karoo Basin, Cape Province/South Africa) 

ADELMANN, D. and FIEDLER, K. (Institut für Geologie, Geophysik und Geoinformatik, Freie Universität Berlin, Malteserstr. 74 - 100, 12249 Berlin, Germany)

GEOLOGICAL SETTING

The Karoo Supergroup (SACS 1980) covers almost two thirds of the present land surface of Southern Africa. Its strata record an almost continuous glaciomarine to terrestrial sequence. Deposition began in the Permo-Carboniferous (280 Ma) and terminated 100 millions years later in the Early Jurassic. Its sediments attain a maximum cumulative thickness of 12 km. Overlying basaltic lavas are at least 1.4 km thick. Both were accumulated in a retroarc foreland basin (Cole 1992), termed Karoo Basin. Along its southern periphery the Karoo Basin is bordered by a complex fold-thrust belt, known as Cape Fold Belt (Fig. 1). It developed during a series of compressional pulses starting in the Late Carboniferous and terminating in the Late Triassic (Hälbich et al. 1983). The Late Palaeozoic development was initiated by plate convergence, subduction and accretion along the palaeo-Pazific margin in the southwestern part of Gondwana (Fig. 2).
The Cape Fold Belt consists of an E-W striking southern branch with north-verging folds and a N-S striking western branch of open, upright folds, which merge in a 100 km wide syntaxis area (Fig. 1). Its mountain ranges are composed of siliciclastic sediments with a cumulative thickness of 8 000 m belonging to the Ordovician-Carboniferous Cape Supergroup.
The deposition of the Karoo Supergroup started after a hiatus at the Cape/Karoo Supergroup boundary with the glacial Dwyka Group. After glaciation, an extensive sea remained over the gently subsiding shelf fed by meltwater. Clays and muds of the Lower Ecca Group were accumulated. Deformation of the southern rim of the basin resulted in uplift and erosion of mountain ranges far to the south. Rapid downwarping of the basin axis occurred as a result of loading by thrust sheets in the adjacent Cape Fold Belt (Cole 1992). The southwestern part of the Karoo basin was separated by the Cape Fold Belt syntaxis (Fig. 1) into the Laingsburg and Tanqua subbasins (Wickens 1992). Deltaic progradation entailed the filling of the subbasins by thick submarine fan and deltaic sediments of the Upper Ecca Group (Kingsley 1981). Gradual shallowing of the foredeep occurred during the Late Permian period due to the rate of sedimentation exceeding the rate of subsidence. The large-scale regressive sequence culminated in deposition of the fluvial-lacustrin Beaufort Group. Early Triassic pulses of uplift in the Cape Fold Belt brought deposition in wide areas of the Karoo Basin to a close. Only in the central part of the Karoo Basin the fluvial, alluvial and aeolian sediments of the Triassic Molteno, Elliot and Clarens Formations were deposited (Fig. 1).

Figure 1: The Karoo Basin in South Africa and the location of study area

Figure 2: Schematic diagram showing the ongoing accretion tectonics along the southern margin of Gondwana during the late Palaeozoic. Also shown are the extensive foreland basins, such as the Karoo and its correlatives, which formed in response to the accretions (from de Wit 1992).
 

SEDIMENTOLOGY

Tectonic processes predominantly controlled the depositional style of the Laingsburg subbasin (SW Karoo basin). Tectonic growth and uplift of the Cape Fold Belt associated with rapid subsidence of the foredeep and an increased input of clastic sediments produced a system of prograding submarine fans.
Sedimentological features indicate a non-channelized lower fan sedimentation for the lower part of the Upper Ecca Group while the middle part represents a channelized upper fan. Sand and siltstones of a fluvially-dominated delta followed the submarine fan sediments. Upper delta plain sedimentation commenced at the base of the Beaufort Group with the appearance of large-scaled channels grading into meandering fluvial systems. For the entire investigated sequence, which is described in detail below, a main sediment transport in northeastern directions can be observed.
Low sand/pelite ratio of submarine fan and delta sediments as well as a widespread spatial distribution of the distal outer fan sediments point to a slow uplift of the southern source area during the deposition of the entire sequence. A moderate relief and low sediment supply is typical. Corresponding to Shanmugan & Moila (1988) the sediments of the Karoo Basin may have settled down under relative stable tectonic conditions.

Figure 3: Stratigraphic columnar section of the study area.

Figure 4: Looking southward from the northern border of the study area. In front the Karoo Supergroup with moderate relief. In the background mountain-ridges of the Cape Fold Belt built up by the Early Palaeozoic Cape Supergroup.Collingham Formation
An abrupt lithological change from dark carbonaceous shales of the Lower Ecca Group (Whitehill Formation) to turbiditic siliciclastic rocks associated with thin tuffitic beds (Collingham Formation) marks a rapid change of tectonic conditions in the SW part of Gondwana (Fig.5).

The lower part of the Collingham Formation consists of an alternation thin-layered silt and mudstones. Sedimentological analysis point to both turbiditic and contouritic sedimentation in a hemipelagic environment. The upper part shows increasing thicknesses of the sand and siltstone layers (up to 30 cm), graded bedding, horizontal and ripple cross lamination. Corresponding to Wickens 1992 the sediments can be interpreted as products of low density turbidity currents. Based on the absence of characteristic features of submarine fans, which can be observed in the upper units, they may represent turbidites on a basin plain or distal products of the lower fan.

Figure 5: White-weathering shales of the Whitehill Formation (Pw), overlain by alternating siltstones and shales of the Collingham Formation (Pc). 5 km southeast of Laingsburg (Fig. 1).

Vischkuil Formation
Alternation of hemipelagic muds and fine to very fine-grained turbidites formed in a lower fan are characteristic for the Vischkuil Formation. The turbiditic siltstones and sandstones often show Tbcd- and Tab-Bouma cycles. Typical upward thickening units and gradual increase of grain size towards the upper boundary of the formation were observed. Gradual increase of turbiditic deposition was caused by the progradation of the submarine fan. The middle Vischkuil Formation shows a conspicuous, approximately 25,0 m thick zone of intensely folded and sheared sandstone beds (Fig. 6). These sandstones are interpreted as slide and slumping structures. Postdepositional and in place syndepositional soft sediment deformation may reflect a rapid deposition of high density turbidite deposits onto underconsolidated clays on a relatively low gradient basin floor (Wickens 1992). Another important feature are two transport directions which can be interpreted as amalgamation of fan lobes.

Figure 6: Middle part of the Vischkuil Formation showing an approximately 25 m thick zone of gravitationally distorted beds. 5 km southeast of Laingsburg (Fig. 1).Laingsburg Formation

The sediments of the Vischkuil Formation lead gradually to the turbidites of the Laingsburg Formation. Both units together form a major upward coarsening succession. The Laingsburg Formation consists of turbiditic sand/siltstones and hemipelagic mudstones containing plant rests and ichnofossils like arthropods tracks. Sedimentological features such as Tab- and Tabc-Bouma cycles indicate a lower fan sedimentation for the lower part of the Laingsburg Formation. The middle part shows the transition from non-channelized to channelized conditions within the submarine fan. Channel sandstones (Fig. 7) and subordinated levee and overbank deposits are the main features. The upper Laingsburg Formation with its massive sandstone bodies was accumulated in the upper fan.

Figure 7: A north-south-orientated channel filled with sand (ca. 20 m thick) within a fine-grained sandstone/pelite alternation. This entire unit represents a turbiditic sequence formed in a middle part of a submarine fan complex. Laingsburg Formation. 4 km south of Laingsburg (Fig. 1).

Fort Brown Formation
The basal Fort Brown Formation reflects a mixture of alternating deltaic and turbiditic conditions. The remaining part of the Fort Brown Formation is exclusively composed of deltaic sediments. It comprises two coarsening-upward-sequences. Both are several 100 m thick. Their basal parts consist of dark pelites and rhythmic alternating sand- and mudstones layers of a prodelta. They pass over into sandy distal bar and distributary mouth bar deposits of the deltafront.Waterford Formation
The Waterford Formation represents an arenaceous horizon overlying the Fort Brown Formation. A variety of sedimentary phenomena such as upward coarsening (thickening) cycles as well as distal sandbar, distributary mouth bar, crevasse splay and interdistributary bay deposits can be observed. The succession of the Waterford Formation reflects the transition from the fluvially-dominated upper delta front to the lower delta plain. Together with the prodelta/deltafront sediments of the Fort Brown Formation they illustrate a major progradational depositional event.Abrahamskraal Formation (Beaufort Group)
The Waterford Formation grades into the fluvially-dominated upper delta plain deposits of the basal Abrahamskraal Formation, consisting of alternating mud- and sandstones subordinated siltstones. Sedimentological features indicate a deposition in a channel, crevasse splay and floodplain environment. At the top of the investigated succession the upper delta plain sedimentation pass over into exclusively fluvial deposition with reddish pelites and yellow arenites associated with mudcracks, gypsum concretions and caliche horizons. The nature of cross stratification led toward an interpretation as a meandering fluvial system.
 

SANDSTONE PETROGRAPHY

Petrographic analyses of Upper Ecca and Lower Beaufort sandstones cover the lithic arkose and feldspathic litharenite fields of the QFL diagram (Fig. 8a)(after Folk 1974). Quartz, feldspar and fragments of sedimentary, magmatic and metamorphic rocks with some micas and stable heavy minerals are the constituents of the sand fraction. Within the entire investigated sequence no significant variations of the composition have been observed (Fig. 8a, b).
Mineralogical data and QmFLt (Fig. 8b), QtFL and QpLvLs diagrams (after Dickinson 1985) point to two different provenance types:

Johnson 1991 investigated the southeastern Karoo Basin and proposed an active magmatic arc as exclusive provenance area during the Permian. He postulated that neither the uplifted Cape Fold Belt nor a gneiss/granite basement constituted the main source for Ecca and Lower Beaufort strata. Since the Lower Triassic (Upper Beaufort Group) he noticed an increased influence of the Cape Fold Belt.
The differences between the southwestern and southeastern part of the Karoo Basin concerning the origin of the detritus may be explained as follows:
1. The distance between the magmatic source area and the present southwestern Karoo basin was significantly higher than in the southeastern part.
2. Only the western part of the Cape Fold Belt was uplifted during the early orogenic activities in the Early Permian (278 ± 5 Ma, Hälbich et al. 1983). Deposition of recycled sediments was restricted to this area. The uplift of the eastern Cape Fold Belt started first during the Upper Beaufort sedimentation.

Figure 8a: QFL-diagram after Folk 1974 (Q: mono- and polycrystalline quartz, F: feldspar, L: sedimentary, metasedimentary and volcanic lithic fragments including »chert«, A: arkose, L: litharenite, LA: lithic arkose, SA: subarkose, AL: feldspathic litharenite, SL: sublitharenite, Q: quartzarenite)

Figure 8b: QmFLt-diagram after Dickinson 1985 (Qm: monocrystalline quartz, F: feldspar, Lt: sedimentary, metasedimentary and volcanic, lithic fragments including polycrystalline quartz and »chert«)
 

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