A.J. Bumby

Muddy Roll-up Structures in Siliciclastic Interdune Beds of the c. 1.8 Ga Waterberg Group, South Africa

Abstract Concentrically rolled-up silty mudrock laminae, 1–2 mm thick, are found in the uppermost of four, 40–110 cm thick interdune beds within a thick aeolian succession of the c. 1.8 Ga Makgabeng Formation, Waterberg Group, South Africa. These curved laminae are analogous to previously described “roll-ups,” biogenic structures generally ascribed to soft-sediment deformation or desiccation of microbial mats overlying either carbonate or siliciclastic sediments, within shallow- to deep-marine paleoenvironments. The Makgabeng roll-ups are thought to reflect desiccation of a microbial mat, followed by resedimentation of cohesive, discrete, curled mat fragments. Their alignment is considered to be the result of an extreme precipitation event. The significance of the South African example of roll-ups is that they appear to have formed within a fully terrestrial paleoenvironment, in one of the oldest known Precambrian deserts. Thus, they represent the oldest evidence for microbial colonisation of a terrestrial setting.

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.

Eolian Dune Degradation and Generation of Massive Sandstone Bodies in the Paleoproterozoic Makgabeng Formation, Waterberg Group, South Africa

ABSTRACT Massive sandstone bodies in the 1.85 Ga Makgabeng Formation, South Africa, are located within the middle and upper part of the formation and are associated with the oldest reported deposits of barchan and barchanoid-ridge dunes. Bases of massive sandstone bodies are channelized or planar. Channel-based bodies typically overlie low-angle or up to 26° inclined wind-ripple strata. Channel margins vary from low angle to vertical. Rarely, massive sandstones at channel margins display vague horizontal stratification and/or contain ripped-up fragments of wind-ripple strata. Planar-based massive sandstone bodies are lenticular and characteristically are interbedded with low-angle to horizontal dune toesets. These lobate massive sandstone bodies vary from 5 cm to 6 m in thickness and from 3 m to possibly over 50 m in width; some of the sandstone bodies onlap dune reactivation surfaces. Rare dewatering structures, parting lineations, and adhesion structures occur at tops of the massive sandstone bodies. Lower boundaries of massive sandstone bodies change from channelized to planar-based down the dune foresets. Massive sandstones were generated by two different mechanisms, which are inferred to have been triggered by significant precipitation events. The presence of steep margins, channeling, rip-up fragments, and dewatering features, and a lack of tractional structures, indicate that most massive sandstones were deposited from hyperconcentrated flows down the dune lee face. Flows were initially turbulent, analogous to hyperconcentrated flows, resulting in scouring of wind-ripple strata. As hyperconcentrated flows migrated onto the dune plinth, rapid deposition produced lobate massive sandstone deposits with planar bases. This sedimentation is linked to development of a hydraulic jump at the slipface-plinth intersection, which increased flow depth and reduced flow velocity. Some flows maintained their turbulence onto the dune plinth, resulting in erosion of the plinth and scouring into the underlying preserved dune deposits. Massive sandstone bodies linked to reactivation surfaces are more likely a result of partial lee-face collapse, which generated translational slides.

Advanced Early Jurassic Termite (Insecta: Isoptera) Nests: Evidence From the Clarens Formation in the Tuli Basin, Southern Africa

Abstract Sandstone pillars in the Lower Jurassic eolian strata of the Clarens Formation are concentrated in clusters, with up to four pillars within 25 m2 in two localities in the Tuli Basin of northern South Africa and southern Zimbabwe. The pillars are generally vertical, have a preserved height of up to 3.3 m, and are elliptical in plan view. Pillars are grouped into two styles of architecture: those with oriented elliptical shapes and side buttresses, and those less well oriented with a smooth outer wall, internal open spaces, and vertical shafts cutting the pillar. The long axes of the elliptical pillars are generally oriented to the north. Northwards-oriented side buttresses also are associated with some of the pillars. The internal architecture of the pillars is characterized by intense bioturbation with two different burrowing styles. Type 1 burrows are composed of a network of randomly oriented, anastomosing sandstone-filled tubes, 0.3 to 0.8 cm in diameter. Type 2 burrows are rare, north-south oriented, and have a smaller diameter. Other associated features are back-filled tubes, open, vertical shafts, and open spaces between the interior and exterior of the pillars. The pillars are interpreted as fossilized termite nests. Type 1 burrows are interpreted as termite passageways within the nest. Type 2 burrows may be related to invading ants. Back-filled burrows may be a result of either beetle predation on resident termites or backfilling by termites themselves. The strong north-south orientations are comparable with modern-day nest architecture of magnetic termites in northern Australia, where nest-orientation is related to cooling. The orientations and features reported here are interpreted to be modified for the high latitudes proposed for the Lower Jurassic Clarens desert. Complex nest architecture preserved in the Clarens Formation suggests that advanced eusocial behavior and ability to construct large nests had appeared in African termites by the Early Jurassic.

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.

Palaeoproterozoic sedimentation on the Singhbhum Craton: global context and comparison with Kaapvaal

Abstract The Singhbhum Craton has a limited Palaeoproterozoic supracrustal record, which suggests a three-part history, comprising: a long period of high freeboard and palaeosol formation on granitoids; subsequent rift-related mafic–ultramafic volcanism and subordinate sedimentation (c. 2.25–2.1 Ga: Dhanjori and Jagannathpur basin-fills; possibly also Simlipal, Malangtoli and Ongarbira basin-fills), which overlapped locally with mafic soil formation; and a major regression at around 2.0 Ga. Following a long hiatus, the approximately 1.6 Ga Dhalbhum–Dalma succession was laid down, probably under continental conditions. This rather truncated record stands in contrast to the chronologically and geographically much more widespread supracrustal basin-fills of the Kaapvaal Craton, and there appears to be an overall poor comparison between these two early Precambrian crustal blocks. However, on Kaapvaal, three analogous events are identified: widespread approximately 2.2 Ga mafic volcanism, followed by a well-developed palaeosol and a major transgression prior to 2.05 Ga. The three shared events between the two cratons are compatible with the postulate of a global, approximate 2.45–2.2 Ga shutdown of magmatic and tectonic geodynamics, with the origin of the triumvirate directly reflecting its resumption again after about 2.2 Ga. We recognize here that a large diversity of views on Singhbhums geodynamic history exists, predicated on a lack of precise geochronology and commonly poor outcrops, and the current hypotheses are presented with these factors in mind.