11th CGS Field School - Southern Cape

As we approach the half-way mark of the 11th CGS Field School, we continued along the Cape Fold Belt to look at more structural elements. Here are some of the day's highlights:

Overview of the Enon Conglomerates, highlighting extensive weathering and uplift during the Cretaceous

Late fracture in the Enon Conglomerates

Getting a closer look at the Enon

Yasmeen's adopted children 

The Huisrivier Thrust Zone

Language lessons within the group

Field School 11 Field Guide - Cape Supergroup

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:

900 Ma Rodinia reconstruction

850-750 Ma primary phase of Rodinia dispersal 

750-650 Ma secondary phase of Rodinia dispersal with block rotations

650-550 Ma primary phase of Gondwana formation

500 Ma final assembly of Gondwana

Finally, post 180 Ma breakup of Gondwana (Reeves, 2016)

The Geology of Table Mountain

If you are attending the 35th International Geological Congress meeting in Cape Town this year (or if you are ever in the Mother City), you will undoubtedly stand in awe of the city's most iconic feature - Table Mountain. You may then even decide to venture up the mountain, either by hiking up or by using the famous cable car. If you are so inclined, you may often gaze upon the immense beauty of the Fynbos vegetation. These characteristic shrubs are of world acclaim, accounting for more than 20% of endemic plant species on the African continent. No wonder that this region has been established as a World Heritage Site. You may also occasionally look upon the rocks and wonder about their history. Well, you're in luck: below is a brief overview of the geology of Table Mountain and a geological map for your pleasure!

 Some examples of the Fynbos on Table Mountain. Geophyte (Watsonia) top left, Ericas (Red Heath) right and Aster (bottom left).

The rocks making up Table Mountain fall largely within the Table Mountain Group, which further falls within the Cape Supergroup. The Table Mountain rocks are clastic sedimentary rocks that were deposited in an evolving braided stream - lagoon - beach environment. This evolution is defined by continuous fluctuations of sea-level and sedimentary transport styles over millions of years, with these rocks being deposited between ca. 500-450 million years ago. Look carefully at the variation in the rock texture, grain size and even evidence of depositional orientations.

Glacial deformation in the Pakhuis (top), Peninsula large quartzite boulders and cliff faces (bottom left), Graafwater fine-grained sandstone (bottom right).

The lowermost rocks that you will encounter on Table Mountain are thin reddish-maroon siltstone, fine-grained sandstone and light brown to purple sandstone of the Graafwater Formation. These were formed in a tidal flat estuarine environment and have an abundance of various sedimentary features and trace fossils - how many can you spot?

Overlying the Graafwater are the pebble-rich quartzite and quartz arenites of the Peninsula Formation. These well bedded grey rocks comprise much of the steep cliff faces of Table Mountain and formed as a braided fluvial system depositing along the subtidal continental shelf. 

If you still have some energy upon reaching the top of Table Mountain and would like to experience an ice-age proportional climatic shift, then do continue toward Maclear's Beacon. As you approach this famous landmark you may notice the large and rounded outcrops of the Pakhuis Tillite. These immature rocks consist of glacial diamictite with faceted and striated quartz pebbles, which were deposited during a 35 million year long ice-age during the upper Ordovician period. Can you spot any glacially-driven deformation in these rocks?

Geological map of Table Mountain - Special 35th IGC edition. Download high resolution version here.

RSA Geotour 2015: Day 3 - 5

Lets continue!

Overview of the route along the Cape Fold Belt

Day 3

On this third day we continue through the Natal Sector of the NNMB as we depart PMB and travel across the Mzumbe Terrain. As we continue along the coast we’ll cross the Mellville Thrust Zone and enter the granulite-facies Margate Terrain. Here we encounter more granites and exotic varieties of the charnockite of the Oribi Gorge Suite. Charnockites are granitoids that contain orthopyroxene. In order to allow for the stabilisation of orthopyroxene, the bulk rock composition must have low water content and exist at high temperatures, i.e. in granulite facies space. One of the charnockite we’ll be visiting is the Port Edward Enderbite. This is a charnockite that consists of quartz, antiperthite, orthopyroxene and magnetite. The name Enderbite is derived from the type locality of this rock, in Enderby Land, Antarctica. For now, this is as close as we can get – however we should have some Antarctic alumni on our team, so remember to ask them about Enderby Land!

We will then continue south and exit the NNMB, for now, and enter the Karoo Supergroup once again. This time we now travel through the Adelaide Subgroup of the Beaufort Group. We will have a chance to look at this more carefully once we cross the historical Kei River. Soon after entering the Eastern Cape we’ll find our way to the overnight destination of East London. 

Day 4

By now I’m sure everybody’s heads will be spinning (like mine, for example, while typing all this geology); and this will thus be the ideal time to travel west and into the Cape Fold Belt (CFB). The rocks encompassed within the CFB were deposited ca. 485 – 300 Ma under continental shelf to beach marine environments. The Cape Supergroup comprises of three distinct lithostratigraphic groups. These include: The quartzite and shale of the Table Mountain Group; shale of the Bokkeveld and sandstone and shale of the Witteberg Group.

These rocks were later deformed to form the CFB. This was in response to compression associated with the formation of Gondwana. The CFB can be separated into three tectonic domains, including; the western and eastern limbs, separated by the Syntaxis situated north of Cape Town. Our journey will take us from East London to Port Elizabeth, across the CFB and along the Zuurberg Pass. Here we will be able to investigate the intricate structural features defining the Western Limb of the CFB. Perhaps we may also get the compasses out and get everybody to measure some strikes and dips!

After crossing the CFB we join the Garden Route and travel along the vast quartzite of the Peninsula Formation. For our engineering geology friends, we’ll stop and have a look at some of the incredible bridges build over the mammoth gorges. Our overnight stop on this day is Knysna.

Day 5

This is the final leg of this phase of the journey. Today we travel from Knysna to the Council for Geoscience regional office in Bellville. First, our journey continues out of this picturesque town and continues down the Garden Route toward George. Before this, we may have a quick stop at the famous Knysna Estuary and potentially some calc-silicate rocks associated with the Cape Granite Suite. Upon arriving in George, we’ll cut back across the CFB once again and head toward Oudtshoorn. In Oudtshoorn we’ll note a Cango Precambrian inlier of ca. 900 Ma rocks and one of the large E-W faults that define much of the CFB. These faults are often synonymous for the Enon Formation of red beds residing along the scarp. Also, these faults are renowned for controlling the development of numerous hot springs. Perhaps one day we’ll harness this heat and generate energy from it!

From Oudtshoorn we’ll take the R62 and see lots of cool stuff! Basically many, many more structural features, e.g. the Huis River Thrust Zone and Cogmanskloof. Eventually we’ll near the, now even more famous, Syntaxis and we’ll get to see how much more intensely the CFB features are around here. The remainder of this trip will take us through most of the Table Mountain Group, and eventually onto the Malmesbury shale. Sadly, these shale mark the approach of Bellville and the end of this first leg of our South African geological tour.

Our group Our group will now get to learn some remote sensing, GIS and participate in several lectures and short courses in Bellville. In addition, we’ll have a tour of the Cape Peninsula before starting the next leg of our geological tour. This next leg will take us from Cape Town, up the western Limb of the CFB and into the Namaqualand-sector of the NNMB. Stay tuned for that!  

2014 Diary: Day 4 - Duplexing

The team completed the first section of the Field School today as they travelled from the overnight stop of Laingsburg to Cape Town. They will now swap the nomadic lifestyle of uncovering the geology of South Africa for a short course on remote sensing, further preparation for looming field work and last, but not least, a closer look at the fairest Cape’s geology.

Our team of young intrepid geologists aligning and scrambling like crazy to see the Matjiesfontein Chert Bed (in lower left corner)  in the Laingsburg region

After having looked at the top of the Karoo on day 3 (e.g. in the Golden Gate Highlands National Park), the team had an opportunity to now investigate its lowermost successions. This was of major interest because some of these units include the highly prospective black carbonaceous shale. After scrambling to the top of one of the many hills around the Laingsburg region, the team had a look at the Matjiesfontein Chert bed. This ca. 60 cm chert bed forms a prominent marker horizon separating the White Hill from the overlying Collingham Formation. Below the Whitehill we found the Prince Albert. Together these three units share relatively high total organic carbon content, with the lions share going to the White Hill. The team further considered various aspects of structural geology at play, with the fissile black shale showing peculiar pencil cleavage planes. The intersection of these planes results in the shale breaking apart into needle-like splinters. Tiny crystal of the mineral pyrite was common in most of the black shale. This signifies the effect of sulphur-releasing bacteria assisting in deconstructing of organic material (i.e. plant and animals reducing in anoxic deep marine environments). The release sulphur would ultimately react with iron in the sediment and form pyrite.

After three days, the final stop within the Karoo Supergroup was symbolically, to the bottom Dwyka Group. The team initially thought that it looked very much like a volcanic rock of some sort, similar ideas shared with some of the first geologists who had interpreted this rock many years ago. However, the team would soon realise that the Dwyka was not volcanic, nor the result of cataclysmic meteorite impact, but rather deposited by a glacier. Polar wander theory would show that the South Pole was located near the location where the Dwyka would have been deposited, with global-scale ice sheets advancing and retreated, picking up foreign clasts from several thousand kilometres away; eventually forming the famous Dwyka Tillite. This was also of particular interest because in the Gariep the team will be seeing a similar tillite of different age. Several ice-ages?

Exquisite folding of within the Cape Fold Belt 

When one door closes, another opens, and behind the second door was the Cape Supergroup. There was no respite for the team as they were thrown into the deep end of classical structural geology. Driving south, along the Seweweekspoort Pass, the team entered the Witteberg (i.e. the topmost of the Cape Supergroup) and were once again totally awestruck by the level of deformation shown. As we continued south, and west toward Cape Town we were really treated to some of the best Alpine-style continental building demarking thrusting and immense folding along the way. The Cape Fold Belt (CFB) was formed as a consequence to the amalgamation of Gondwana. Of interest is the fact that the CFB is located more than 1000 km away from the continental margin, however strain was able to accumulate substantially enough to form the exceptionally large mountains seen. How would this have happened?

Kogmanskloof, now geoheritaged in the Western Cape

Within the CFB, the team noted several hot springs aligned major thrust faults. This provides evidence suggesting that this mountain belt has high heat flows. Groundwater aquifers are recharged from the high mountains and circulated by an intricate fault network. The groundwater is heated thanks to the effect of the Cape Granite. Being relatively young ca. 550 million years old, the granite provided a rich abundance of heat producing elements (i.e. uranium, thorium and potassium). These elements would slowly undergo radiogenic decay and emit heat. Heat, now enjoyed by thousands visiting the various spas in the southern Cape!

The team will now focus on the mapping to come. Tomorrow will be spent performing remote sensing and trying to delineate different potential geological units in the Gariep Field area prior to arriving there on Sunday.

Check out todays route here: Day 4