The CGS Field School: Field School 11 Field Guide

1 Introduction

The Cape Supergroup appears along the southern coast of South Africa and generally outcrops within the fold-thrust Cape Fold Belt mountains. The rocks of the Cape Supergroup were deposited into a large basin that extended across southern Gondwana. These rocks are easily correlated, e.g. in Argentina.

The rocks of the Cape Supergroup largely consist of sandstone/quartzite and shale/phyllite. These rocks are spectacularly exposed within the Cape Fold Belt, which will be the aim of our investigation here. The Cape Fold Belt highlights accretion along the southern margin of Gondwana with much of the deformational features preserved due to the reaction-style of the mostly rheologically-competent siliciclastic Cape Supergroup rocks.

2 Stratigraphy

The Cape Supergroup is subdivided into three Groups, from bottom to top, the largely quartzite of the Table Mountain Group, the interbedded mudstone, shale and sandstone of the Bokkeveld Group and the topmost interbedded sandstone and shale of the Witteberg Group. Folding and duplication by thrust repetition throughout the Cape Fold Belt has resulted in much of these rocks being tectonically thickened.

Extent of the Cape Supergroup and the correlated units in KZN (Shone and Booth, 2005)

The Table Mountain Group can be subdivided into, from bottom to top, the basal conglomerate layer of the Piekenierskloof Formation, the finely laminated siltstone and interbedded sandstone of the Graafwater Formation, the mostly quartz-arenite and quartzite of the Peninsula Formation, the glacially-derived Pakhuis diamictite, the black shale and interbedded mudrock, silstone and sandstone of the Cedarberg Formation and the uppermost sandstone/quartzite of the Nardouw Formation.

These formations contain abundant and varied sedimentary features, most notably planar and trough cross bedding features, which generally display a bimodal sedimentary transport direction, with a prevalence of south westerly flow direction. Within these units there are also an abundance of trace and body fossils, most notably within the finer Graafwater and Cedarberg Formations.

The Bokkeveld Group consists of interbedded dark mudrock, siltstone and fine-grained sandstone. The Bokkeveld rocks highlight at least five coarsening-up depositional cycles and also contain abundant crinoid, brachiopod, gastropod, bivalve and trilobite body fossils.

The Witteberg Group predominantly consists of interbedded sandstone and shale with lesser mudrock and siltstone also present.

Depositional settings are varied throughout the lithostragraphic profile of the Cape Supergroup, but there is a general consensus that the base of the Cape Supergroup represents coarse clastic sediment input from a high-energy erosional terrestrial setting.

Thereafter, deposition occurred within a shallow marine, tide dominated environment, e.g. as seen within the Graafwater Formation. The setting shifted sharply to a braided fluvial setting, of the Peninsula and then glaciation as seen by the Pakhuis. Melt waters derived from the glacier would have supplied much of the sediment for the Cedarberg deposition and finally, the Nardouw Formation probably represents another fluvial or marine setting.

3 The Cape Fold Belt

The c. 250 Ma Cape Fold Belt extends approximately 1200 km along the southern coast of South Africa and consists of an eastern and northern limb, joined within the syntaxis, where the two limbs merge in a region of high strain.

Location of the Cape Fold Belt in reference to Pangea (north) and Gondwana (south) at 270 Ma (Hansma et al., 2016)

Simplified map of the Cape Fold Belt, highlighting major faults (Hansma et al., 2016)

The style of deformation that resulted in the development of the Cape Fold Belt is hotly debated. This is largely due to the position of the Cape Fold Belt mountains almost 1000 km away from the African plate boundary. Far-field stresses have often been ascribed as a method for transmitting strain over such a great distance. Despite this, there is agreement that deformation ensued after shallow-angle subduction of the Paleo-pacific plate below the southern margin of Gondwana.

Several theories then postulate the mechanism for the Cape Fold Belt, and include; a compressional retro-arc foreland basin (e.g. Catuneanu et al., 1998); a transpressional retro-arc foreland basin (e.g. Johnson, 2000) or as the result of thin-skinned Jura style thrust propagation (e.g. Lindeque et al., 2011).

Example of a retro-arc foreland basin (Eriksson et al., 2008)

During the field school we will do at least two north-south traverses across the Cape Fold Belt, as well as east-west sections across the eastern limb, and a further north-south section along the western limb. In this time the participants will have an opportunity of investigating several examples of both ductile and brittle deformational features.

4 Rodinia to Gondwana

After all our discussions about how the early continental crust had formed and major secular changes of the Earth's ocean and atmospheres, we now fast forward some three billions years and can now start thinking about supercontinental growth and destruction. Along the southern coastline of South Africa, and extending into our mapping region of the Gariep Belt, we will investigate two major supercontinental cycles, namely, from the supercontinent Rodinia to Gondwana.

The shift from Rodinia to Gondwana can be summarised in the following diagrams, after Johnson, 2014: