Planificación

6.1.1.Geology of the Noisy Complex

6.1.1.1 Stratigraphic Context

In 1969, Vijoen and Viljoen mapped and described the upper formations of the Onver- wacht Group, including the Hooggenoeg Formation, the Kromberg Formation and the Swatkoppie Formation [now interpreted to be structurally disturbed parts of other units within the Onverwacht Group, (Lowe and Byerly, 1999b)]. They observed that in the type area in the Western Limb of the Onverwacht Fold (OF), the main feature of the Hoogge- noeg Formation was the apparent cyclicity of the volcanism recorded in the sequence. They distinguished that the base of the Hooggenoeg formation is characterised by ul- tramafic basalts which can be massive and pillowed, whilst towards the top of the se- quence felsic volcanic materials interspersed with cherts become more predominant. The uppermost part of the formation is dominated by a broad zone of felsic volcanics which are capped by a thick chert horizon. Viljoen and Viljoen (1969) identified that this upper- most layer can be traced over the total strike distance (56 km) of the western limb of the Onverwacht Anticline. The felsic volcanics are described as being massive, structureless feldspar porphyries which fine upwards into dense light grey sericitic volcanics thought to be of pyroclastic origin. The unit is capped by 112m thick black and white cherts, with some banded iron formations too. Bedded felsic pyroclastic deposits, greywackes and iron rich shales are interbedded within the cherts. In the Eastern limb of the OF, the section is

characterised by layered tuffs with visible bedding and flame structures as well as thick agglomerates. Viljoen and Viljoen (1969) argue that the emplacement of the uppermost layer of the Hooggenoeg Formation was during a lull in explosive activity which emplaced the lower Hooggenoeg as near-surface intrusives and/or extrusives. The sedimentary structures observed in the tuffs are thought to indicate deposition of ash fall deposits into shallow waters.

Following the classic work of Viljoen and Viljoen in 1969, a number of authors continued to study the rocks of the upper Hooggenoeg Formation (de Ronde and de Wit, 1994; de Wit et al., 1982; de Wit, 1987; Lowe and Byerly, 1999a; Lowe et al., 1985). The interpreta- tions of Viljoen and Viljoen, (1969) remained true for the most part. Better understanding of the alteration caused by hydrothermal process that affected the rocks of the BGB led to the reinterpretation of the cyclic volcanic activity and the understanding that the felsic units are in fact silicified mafic and ultramafic volcanic deposits (de Wit et al., 1982; de Wit, 1987). In addition, the work of Lowe and Byerly, (1999b) proposed a new strati- graphic classification of the rocks within the Onverwacht Group. In this new classification the Hooggenoeg Formation was subdivided into six stratigraphic units (H1-H6). Unit H6 comprised the thick sequence of felsic volcaniclastic and volcanic rocks. The silicified sedi- mentary rocks of the uppermost Hooggenoeg Formation were named the Buck Reef Chert (BRC) after the work of Hall, (1918). However, Lowe and Byerly (1999) argued that the BRC belong at the base of the overlying Kromberg Formation, rather than at the top of the Hooggenoeg Formation. In the new stratigraphic regime, the BRC became unit K1 of the Kromberg Formation. The authors further divided the BRC into four subdivisions -for de- tails the reader is directed to Lowe and Byerly, (1999b) and Lowe and Worrel, (1999). The reassignment of the BRC to the Kromberg Formation was controversial and many au- thors did not adopt this reclassification amongst them Brandl and de Wit, (1997). An ex- tensive mapping project was undertaken by de Vries et al., (2006) to better understand the stratigraphic architecture of the BRC. The authors highlight that the contact between the BRC and underlying stratigraphy is gradational (as previously identified by Lowe and Worrel, (1999); Viljoen and Viljoen, (1969b)), and there is no regional unconformity (despite there being some local unconformities) at the base of the BRC which would war- rant stratigraphic separation from the Hooggenoeg. Furthermore, de Vries et al., (2006) consider that the felsic volcanics and the BRC are closely related and therefore introduce the Buck Ridge volcano-sedimentary complex (BR-vsc) as a new stratigraphic unit. The

work of de Vries et al., (2006) focused on the north-western limb of the OF fold. Detailed logging and mapping in the south-eastern limb (Etimambeni Section) of the fold by de Wit et al., (2011) shows that the base of the BR-vsc has an unconformable contact with the Hooggenoeg Formation. The geochemical signature of the volcanics within the Hoogge- Figure 6.1 Geological map showing sampling localities for this study (white star) and sampling site of previ- ous works. CG1, CG2 and CGM correspond to sites sampled by Biggin et al., (2011); NfD-T corresponds to sites sampled by Tarduno et al., (2010); NfT-U correspond to localities sampled by Usui et al., (2009). Map modified from de Wit et al., (2011).

noegg Formation and the BR-vsc is also very different (Furnes et al., 2011). Therefore, de Wit et al., (2011) propose the creation of a new geological unit within the Onverwacht Group: The Noisy Complex. In the study area the Noisy Complex is presented as a pre- dominantly clastic sedimentary sequence with no lavas, although there is a tuff in the up- permost part of the section studied. Concordant U/Pb zircon dating gives an age of 3455.2 ± 7.5 Ma (Biggin et al., 2011) for the tuff. This is consistent with previously published ages for the BR-vsc: 3451±5 Ma (De Vries, 2004), 3445 ± 6 Ma (de Wit et al. 1987, Kamo and Davis, 1994) and 3451±5 Ma (de Vries et al., 2006). De Wit et al., (2011) argue this is fur- ther evidence that the new Etimambeni Section should be merged with the BR-vsc to form the Noisy Complex. For further details on dating of the Noisy Complex, the reader is directed to de Wit et al., (2011) and references therein.

The newly created Noisy Complex nomenclature is not adopted by Grosch et al., (2011). The drilling of three boreholes in the mid-upper Onverwacht Group ( Kromberg, Noisy and Hooggenoeg Formations) in 2008 by the Barberton Scientific Drilling Program (BSDP) al- lowed further detailed study of the Noisy Formation. The drill core was obtained along the south-eastern limb of the OF fold; for full details of the drilling operations the reader is directed towards Grosch et al., (2009a).The recovered core allowed Grosch et al., (2011) to carry out a detailed chronostratigraphic study of the Noisy Formation. Detrial zircon ages from the study argue for a maximum depositional age for the Noisy Formation at ca.3432Ma and also represent the maximum age of the unconformity identified both in the new drill core and by de Wit et al., (2011) in the Etimambeni Section. Further detrial zircon ages from the study suggest the source of the clasts (see section 1.1.2, this chapter for stratigraphy details) within the conglomerate units is diverse and includes the BR-vsc, the Hooggenoeg pillow lavas and cherts, the Stoltzburg and Theesrpuit Plutons, as well as rocks from the Ancient Gneiss Complex (AGC, see Chapter 2 for further detail). The au- thors argue, given the chronological and stratigraphic evidence, the Noisy Formation on the south-eastern limb of the OF cannot be correlated with the BR-vsc in the north- western limb of the OF, as proposed by de Wit et al., (2011). Grosch et al., (2011) agree that the unconformable contact, the difference in ages of the two units, as well as the contrasting lithologies mean that the Noisy Formation is distinct from unit H6 (which in- cludes the BR-vsc) of the Hooggenoeg Formation. Despite the evidence presented by Grosch et al. (2011), the authors do not consider the lack of outcrop of the Noisy Forma- tion on the north-western limb of the OF.

The newly created stratigraphic of the Onverwacht Group, which includes the Noisy Com- plex, is adopted within the work presented in this chapter and thesis, as is shown in the geological map in Figure 6.1.