R. van der Merwe

Late Archaean superplume events: a Kaapvaal–Pilbara perspective

Abstract The 2714–2709 Ma Ventersdorp Supergroup overlies Mesoarchaean basement rocks and sedimentary strata of the Neoarchaean Witwatersrand Supergroup. The latter basin was inverted by tectonic shortening and suffered the loss of up to 1.5 km of stratigraphy prior to deposition of the Ventersdorp volcanics. Thermal uplift and fluvial incision prior to the basal Klipriviersberg Group flood basalts appear to have been limited, but this could also reflect a hot dry palaeoclimate acting on a peneplained plateau. Rapid ascent of ponded magma beneath thinned sub-Witwatersrand lithosphere, transported laterally from a mantle plume starting head possibly situated marginally to the Kaapvaal craton is inferred for this unit of up to 2 km of predominantly tholeiitic basalts with subordinate, basal komatiites. Crustal extension related to ascent of the ponded magma followed, leading to the formation of a set of graben and half-graben basins, in which immature clastic sedimentary, and felsic to mafic lavas and pyroclastics of the Platberg Group were laid down. The Platberg basins show no evidence for reactivation of pre-existing crustal structures. The Fortescue Group of the Pilbara craton has an analogous lower flood basaltic succession, followed by graben-fills similar to those of the Platberg Group. Differences in the Fortescue include evidence for significant thermal uplift prior to the onset of volcanism, subaqueous basalts in the south of the Pilbara craton, evidence for two episodes of flood basaltic volcanism, possibly related to two plumes at c. 2765 and 2715 Ma, and graben basins aligned along existing cratonic structures. Both Kaapvaal and Pilbara flood basalts and graben-related sedimentary-volcanic deposits are thought to have been part of a c. 2.7 Ga global superplume event. The plume inferred for the Fortescue Group flood basalts was probably related to rifting and the breakup of a plate larger than the preserved Pilbara craton. Uppermost Ventersdorp units (Bothaville Formation terrestrial clastic and Allanridge Formation tholeiitic rocks) suggest a combination of thermal subsidence, allied to continued plume (minor komatiites) and graben basin influences. In the Kaapvaal craton, the Transvaal Supergroup lies unconformably above the Ventersdorp. Basal “protobasinal” successions reflect discrete fault-bounded basin-fills, analogous to those of the Platberg Group; however, it is inferred that the former depositories were related to craton marginal plate tectonic influences, specifically the c. 2.6 Ga Limpopo orogeny. Thin fluvial sheet sandstones of the Black Reef Formation unconformably succeed the protobasinal rocks and reflect the transition to an epeiric drowning of much of the Kaapvaal craton. A shallow shelf carbonate-banded iron formation platform succession (Chuniespoort-Ghaap Groups) developed in two sub-basins on the Kaapvaal craton. They are mirrored by the approximately coeval Hamersley chemical epeiric sediments on the Pilbara craton, and both Kaapvaal and Pilbara transgressive successions are related here to a possible second, c. 2.5 Ga superplume event, which raised sea levels globally. Evidence for the younger superplume event is less clear than for the c. 2.7 Ga event.

Shear-zone controlled basins in the Blouberg area, Northern Province, South Africa: syn- and post-tectonic sedimentation relating to ca. 2.0 Ga reactivation of the Limpopo Belt

Abstract The extent of the deposition and of the preservation of the Blouberg Formation and Waterberg Group was at least partially controlled by brittle reactivation along the Palala Shear Zone. The Palala Shear Zone in the Blouberg area (Northern Province, South Africa) is characterised by granulite-grade gneiss, and formed by sinistral transpressional collision between the Southern Marginal Zone (Kaapvaal Craton) and the Central Zone of the Limpopo Belt. The Limpopo collision is thought to have occurred either at 2.0 Ga or at 2.7 Ga with reactivation at 2.0 Ga. Deposition of the Blouberg Formation was characterised by syn-sedimentary tectonism, which is reflected by a sudden upward coarsening in sedimentary rocks, and by the presence of a strongly folded and thrusted lower member. Bedding orientations and slickenside lineation orientations suggest that vergence was towards the south, and such a tectonism can be inferred to have produced a highland area to the north, bound on the southern margin by the southern strand of the Melinda Fault. The presence of an inferred northerly upland area is supported by palaeocurrent directions and the preservational extent of the Setlaole and Makgabeng Formations of the Waterberg Group (post-Blouberg Formation). The extent and stratigraphy of the overlying Mogalakwena Formation suggests that these strata onlapped northwards over the denuding highlands. Younger Sibasa basalts of the Soutpansberg Group have been dated at ca. 1.85 Ga. Blouberg and Waterberg strata can therefore be interpreted as syn- and post-tectonic sedimentary rocks, respectively, following a ca. 2.0 reactivation event along the Palala Shear Zone. It is difficult to reconcile the succession of geological events at Blouberg with a ca. 2.0 Ga Limpopo orogeny, and thus sedimentary strata in the study area support a 2.7 Ga date for Limpopo collision, with syn-Blouberg tectonism relating to ca. 2.0 reactivation within the previously assembled Limpopo Belt.

Compressive deformation in the floor rocks to the Bushveld Complex (South Africa): evidence from the Rustenburg Fault Zone

Abstract The north-northwest-south-southeast striking Rustenburg Fault Zone in the western Transvaal Basin, South Africa, has been extensively mapped in order to unravel its tectonic history. In post-Pretoria Group times, but before the intrusion of the Bushveld Complex at ∼2050 Ma, the area surrounding the fault zone was subjected to two compressive deformational events. The shortening direction of the first event was directed northeast-southwest, producing southeast-northwest trending folds, and the shortening direction of the second was directed north-northwest - south-southeast, producing east-northeast - west-southwest trending folds. The second set of folds refolded the first set to form typical transitional Type 1-Type 2 interference folding. This compression ultimately caused reactivation of the Rustenburg Fault, with dextral strike-slip movement displacing the Pretoria Group sediments by up to 10.6 km. The subsequent intrusion of the Bushveld Complex intensely recrystallised, and often ponded against the strata along the fault zone. The fault rocks within the fault zone were also recrystallised, destroying any pre-existing tectonic fabric. Locally, the fault zone may have been assimilated by the Bushveld Complex. After the intrusion of the Bushveld Complex, little movement has occurred along the fault, especially where the fault passes under areas occupied by the Bushveld Complex. It is thought that the crystallisation of the Bushveld Complex has rheologically strengthened the neighbouring strata, preventing them from being refaulted. This model is at variance with previous assumptions, which suggest that continuous regional extension during Pretoria Group sedimentation culminated in the intrusion of the Bushveld Complex.

The stratigraphic relationship between the Waterberg and Soutpansberg Groups in Northern Province, South Africa: Evidence from the Blouberg area

The relative stratigraphic position of the mid-Proterozoic Waterberg and Soutpansberg Groups in Northern Province has long been a subject of debate. These two major Proterozoic groups are juxtaposed in the Blouberg area, although the nature of the contact between the two sedimentary units is generally regarded as being faulted along the southern strand of the Melinda Fault. The Blouberg area is also the type locality for yet another small mid-Proterozoic succession, that of the Blouberg Formation, which locally outcrops beneath both the Waterberg and Soutpansberg strata. Existing maps of the Blouberg area show the Wyllie’s Poort Formation of the Soutpansberg Group to unconformably overlie rocks of the Blouberg Formation in the western foothills of Blouberg mountain. However, recent mapping suggests that these “Blouberg” strata themselves unconformably overlie the Blouberg Formation. This, therefore, raises important questions regarding the stratigraphic placement of these intermediate strata in the western foothills of Blouberg mountain, which outcrop unconformably above the Blouberg Formation and unconformably below the Wyllie’s Poort Formation. The intermediate strata are characterised by a thin basal conglomerate, consisting of cobbles of quartz, quartzite and B.I.F. This basal conglomerate grades vertically into trough cross-bedded sandstone and granulestone, which are characterised by heavy mineral concentrations on foresets. Although this facies compares poorly with strata of the Mogalakwena Formation in adjacent areas, which are generally more conglomeratic, they are identical with strata from more distal outcrops of the Mogalakwena Formation further to the south-west Palaeocurrent directions recorded from the intermediate strata are unimodal towards the west-south-west, in common with those recorded from the Mogalakwena Formation. This suggests that the intermediate strata, unconformably overlain by the Wyllie’s Poort Formation of the Soutpansberg Group, can be correlated with strata of the Mogalakwena Formation. The Waterberg Group thus appears to pre-date the Soutpansberg rocks. Additional evidence for the age relationships proposed here can be gained from an east-north-east-trending dyke swarm, which locally intrudes the basement, Blouberg and Waterberg rocks. This dyke swarm does not intrude Soutpansberg strata in adjacent areas. Patterns of spider diagrams of incompatible trace elements recorded from these dykes are very similar to those from the basaltic Sibasa Formation of the Soutpansberg Group. This suggests that the dyke swarm may have acted as feeders to the Sibasa lavas, and also suggests that the Soutpansberg Group post-dates the Waterberg Group.

A half-graben setting for the Proterozoic Soutpansberg Group (South Africa): evidence from the Blouberg area

Abstract The ca. 1.85 Ga Soutpanberg Group of the Kaapvaal Craton has in the past been interpreted as a volcano–sedimentary sequence deposited either within an aulacogen/rift basin, or as a more extensive cover sequence preserved only within a post-sedimentary graben. Evidence from the Blouberg area of Northern Province, South Africa, indicates that the Soutpansberg strata were laid down within a half-graben bound to the south by a northwards-dipping normal fault. Palaeocurrent directions within the Wyllies Poort Formation of the Soutpansberg Group suggest that planar-bedded, ripple-marked sandstones were deposited by transverse-flowing rivers, whilst small- to large-scale trough and planar cross-bedded sandstones indicate transport in large, low sinuosity longitudinal trunk rivers, flowing approximately parallel to the axis of the half-graben. The N–S oriented syn-Southpansberg extensional regime may reflect orogenic collapse after the cessation of N–S orientated compression developed at ca. 2.0 Ga by collision or reactivation tectonics in the Limpopo Belt.

The Palaeoproterozoic Woodlands Formation of eastern Botswana-northwestern South Africa: lithostratigraphy and relationship with Transvaal Basin inversion structures

Abstract The Woodlands Formation (uppermost Pretoria Group) of eastern Botswana overlies thick quartzites of the Sengoma Formation (Magaliesberg Formation) and comprises a lower unit of interbedded mudrocks and fine-grained recrystallised quartzitic sandstones, succeeded by chaotic and very coarse-grained inferred slump deposits. Within the adjacent western region of South Africa, interbedded mudrocks and quartzitic sandstones stratigraphically overlying the Magaliesberg Formation are now assigned to the lower Woodlands Formation. Within the entire region, interference folding produced by northeast-southwest (F 1 and F 3 ) and northwest-southeast (F 2 ) compression, and concomitant faulting characterised inversion of the Pretoria Group basin. This deformation is of pre-Bushveld age and affected all units in the Pretoria Group, including the uppermost Silverton, Magaliesberg and Woodlands Formations, and intrusive Marico Hypabyssal Suite (pre-Bushveld) mafic sills. The Nietverdiend lobe of the Bushveld Complex, intrusive into this succession, was not similarly deformed. Movement along the major Mannyelanong Fault in the northwest of the study area post-dated Transvaal Basin inversion, after which the “upper Woodlands” chaotic slump deposits were formed. The latter must thus belong to a younger stratigraphical unit and is possibly analogous to apparently syntectonic sedimentary rocks (Otse Group) in the Otse Basin of eastern Botswana.