11th CGS Field School - The Karoo

Day one

The group actually began investigating the Karoo rocks near Vryheid and Denny Dalton, in the middle of our Pongola day. However, the Karoo proper began as we departed Pongola and drove west toward the Drakensburg mountains. We had some really interesting weather on this day, with extremely heavy rainfalls and massive electrical storms. These conditions resulted in major damages in several parts of northern South Africa. Fortunately, we were able to avoid the worst of these conditions and only had to endure getting soaked while looking at the Mozaan-Dwyka unconformity.

View of the Clarens sandstone landscape in the Golden Gate National Park

Anyway, our Karoo transect began with a brief introduction of the main Karoo Basin and then we promptly began overlooking the basaltic (dolerite) rocks making up the Drakensburg Mountains. From here we then entered the Stormberg Group, particularly spending some time looking at the mostly desert-dune deposits of the Clarens Formation. These magnificent sandstone cliffs are wonderfully exposed within the Golden Gate National Park.

Golden (gate) girls

Spot the dyke

Day two

We then spent most of the second day driving across the upper parts of the Beaufort Group, particularly discussing the Permo-Triassic boundary, which is exposed within the Tarkastad Subgroup. This boundary is especially important as it marks the extinction of the Dicynodonts in favour of Lystrosaurus.

The !Gariep Dam, currently below 50 % capacity

We also took some time visiting the largest dam in South Africa, i.e. the !Gariep Dam. The effects of the drought were clearly visible with the dam less than 50 % capacity. This is a scary reminder for us to become more water wise and efficient.

On the Gariep dam wall, with our Japanese visitors

Day three

Our third day saw the group doing a transect from the escarpment, through the Cape Fold Belt mountains and into the underlying Pre-Cape Cango inlier. On the escarpment we began by looking at the geometry of the various dolerite (basaltic) dykes and sills. We also investigated the effect of the dolerite intrusion on the surrounding Beaufort sandstones. The dolerite intruded at a temperature of approximately 600-800 degrees and resulted in the formation of hornfels. These hornfels are also defined by the presence of andalusite.

L: "Dinosaur Egg/Onion-skin weathering" fracture-controlled preferential weathering of dolerite; R: Hornfels between dolerite sills

Taking out frustrations on that dyke

Through the Beaufort Group, the team also made a visit to the Karoo National Park to walk the famous Karoo fossil trail. The fossil trail provides a great overview of the various dinosaur fossils that were found in this region.

TL: Bradysaurus (Pareiasaur); TR: Dicynodont; BL: Bradysaurus head; BR: Diictodon

The team on the Karoo fossil trail

We then continued further south and intersected the lowermost group of the Karoo, the Ecca. Here we saw these mostly black mudrock and fine-grained sandstone highly folded and faulted due the effects of the c. 250 Ma Cape Orogeny.

T: (L-R) Ripon sandstone, White Hill shales, Collingham interbedded shale and tuff, Matjiesfontein chert bed; B: Swartberg Pass

We also discussed the merits and controls on the potential shale gas resources of the Karoo and discussed the various implications of harnessing (or attempting to harness) this gas from the Karoo.

The Whitehill Formation - all degassed

Ma Connie: "Forces were this big.."

Field School 11 Field Guide - Karoo Supergroup

1 Introduction

Here we continue further with the building of Gondwana and the formation of the Cape Fold Belt. Following the formation of Gondwana a period of regional subsidence presided and allowed for the formation of several basins of various sizes throughout Gondwana. These various basins were infilled from the late Carboniferous to early Jurassic, during a time that records a protracted period of environmentally-controlled sedimentation. The various sedimentary environments that existed during this period may be summarised below:

Sedimentary depositional environments (Jones, 2011)

The various Karoo depositional basins across much of Gondwana formed in response to various tectonic settings, particularly, extensional/subsidence. The Main Karoo Basin of South Africa, which will be the focus during the Field School, likely formed as a foreland basin, developing in response to shallow-angle subduction along the southern margin of Gondwana. This subduction would further result in the formation of the Cape Fold Belt.

Karoo basins throughout Africa (Catuneanu et al., 2005)

2 Deposition and Stratigraphy

The deposition of the Karoo rocks began when the South Pole was located over Gondwana, covering much of the supercontinent with a large ice sheet. This allowed for the formation of glacier-lakes into which varved mudrock were deposited. Furthermore, as Gondwana continued to drift, much of the melt water derived from this glacier resulted in the deposition of the characteristic Dwyka diamictite. Several exotic clasts found within these till-deposits highlight the aerial extent reached by this ice sheet.

Much of the glacial melt water further contributed to the infilling of deep basins. These basins reached depths where reducing conditions prevailed. These were the conditions where much of the Ecca carbonaceous shale and sandstone were deposited. Within the Ecca, interlayered tuff and volcanic ash layers are also prominent, i.e. within the Collingham Formation. 

It was at a time coeval to the deposition of the upper Ecca where the Cape Fold Belt mountains formed. Consequentially, sediment being derived from these high mountains resulted in the development of prograding deltas and fluvial environments that typify the upper Ecca. These comprise of many upward-fining turbidite sequences. 

Continued basin infilling and progradation would eventually result in the formation of a dominant fluvial depositional environment and the consequential deposition of the Beaufort Group of sandstone and mudrock. The mudrock sequences were especially ideal in preserving much of the fauna that existed during this period, and on the field school, we should have plenty of opportunity to investigate this.

A climatic shift toward a semi-arid, desert-type environment allowed for the deposition of the clastic Stormberg Group. These depositional conditions are spectacularly preserved by the large dune-type deposits of the Clarens, which we will see in the Golden Gate National Park.

The deposition of the Karoo Supergroup was completed as Gondwana began to breakup and the Karoo Large Igneous Province was emplaced.   

Overview of the Main Karoo Basin (Johnson et al., 1996)

The following figures summarise most of the lithostratigraphy of the Karoo Supergroup; from upper, middle and lower Karoo sequences:

Overview of the Stormberg Group - upper Karoo (Smith and Kitching, 1997)

Overview of the Beaufort Group - middle Karoo (Catuneanu et al., 2005)

Overview of the Ecca-Cape transition; lower Karoo (Flint et al., 2011)

3 Tectonic Evolution

There is much contention regarding the style of formation for the Karoo. The Karoo is generally regarded as being a retro-arc foreland basin that formed behind a fold-thrust belt, i.e. forming due to the shallow-angle subduction below the southern margin of Gondwana. Much of the contention stems from the exact timing of the Cape Orogeny (i.e. c. 500-250 Ma) and the basement architecture. Much of the subsidence and faulting seen throughout the Karoo would have used and reactivated existing crustal discontinuities formed during the formation of Gondwana.

Geophysical information regarding the basement architecture and crustal discontinuities can be summarised as follows:

Basement architecture below South Africa (A)-magnetic; (B)-gravity (Tankard et al., 2009)

An overview of Karoo deposition, related to the tectonic history can be summarised as follows:

Overview of the tectonic evolution and depositional setting of the Cape and Karoo Basins (Tankard et al., 2009)

The Cape Fold Belt is a north-vergent fold and thrust belt with a significant strike-slip component. This affects much of the southern region of the Karoo. During the field school we will conduct a section across this southern region of the Karoo and we can expect the following:

Cross section across the Cape Fold Belt and into the Main Karoo Basin (Tankard et al., 2009)

4 Shale Gas

A study conducted by the US Energy Information Administration suggested that the Main Karoo Basin has copious quantities of shale gas (methane), up to c. 390 Tcf. While this figure is likely a gross miscalculation, more reasonable estimates suggest that the Lower Karoo rocks could have c. 10-20 Tcf of natural gas. Despite this large estimated reserve range difference, even the lesser estimate suggests that these reserves could represent a potential Game Changer for the South African Energy and Minerals landscape.

Overview of the lower Ecca shale gas-bearing units (Geel et al., 2013)

Chemostratigraphy across the shale gas-bearing units of the lower Ecca (Geel et al., 2013)

Van Krevelan diagram for the lower Ecca (Geel et al., 2013)

Effect of igneous intrusive near hydrocarbon-bearing shale (Quaderer et al., 2016)

Factors potentially affecting the preservation and maturation of the shale gas reserves in the Karoo include the intrusion of the Karoo dolerite and the deformational extent of the Cape Fold Belt. These together result in the shale gas being overmatured and/or lost through degassing via brittle features. An ideal potential region can be shown as follows:

Recoverable shale gas from the Whitehill Formation (Cole, 2014)

There are still many factors that will be considered before any shale gas extraction occurs. However, the green light has been given for companies to continue with shale gas exploration. This is toward establishing exactly how much shale gas reserves are present in the Karoo. This remains a highly contentious issue that has implications toward the economy, energy production, climate change regulations and natural environment, all of which we will discuss in great detail.

2014 Diary: Day 3 - Great Karoo

Our team continued in their journey across South Africa today entering the Western Cape and also making a symbolic stop in the Eastern Cape. The night was short for most, but all remained very eager to get on the road and learn some geology. Our overnight stop was at the Gariep Dam, near the source of the Orange River. This is the largest water reservoir in South Africa, capable of storing more than 5 billion litres of water and covering an aerial extent of more than 370 cubic kilometres. Water from the Gariep Dam is also released into the Vanderkloof dam. The latter has 4 hydroelectricity generators capable of producing up to 360 MW of electricity when needed (which is most of the time these days).

 The Gariep-Vanderkloof dam and hydroelectricity plant

Today, the geology was completely dominated by the Beaufort Group. From the Gariep Dam, to Fraserburg the team was treated to various mudrock and sandstones of the Adelaide Subgroup. An extra treat was the witnessing of a Karoo flash-flood that caught the team by surprise. In an instant, all dry river beds were awash with raging torrents that devoured roads and everything in its path. In another instant, it was all over and blue skies and sunshine prevailed. It is worth noting here that the present day Karoo is very much an arid landscape, however more than 200 million years ago, this was a lush and fertile land similar to the today’s tropical regions.

Flash flood in the central Karoo

The highlight of the day was a visit to Fraserburg and the Gansfontein Paleosurface. This represents a pool-like environment where various creatures gathered and left remarkably many trace fossils for the team to ponder over. This paleosurface is located in the Abrahamskraal Formation within the Beaufort Group, a unit that covers the infamous Permian-Triassic (PT) boundary. Infamous of course, because this period in geological history relates to a major extinction event that resulted in approximately 90 % of life to be erased! Question: What are the possible causes for this extinction? Luckily, the Paleosurface at Gansfontein provides a memoir of some of these creatures. This most notably includes a mammal-like reptile, or tetrapod, known as a Bradysaurus, part of the Pareiasaur group. This is also thought to be an ancient ancestor to the modern day tortoise. This is based on similar morphology of the skull and evidence of bony plates that could be precursors of the tortoise shell. This was a herbivore that reached up to 3 metres long and weighed more than half a ton! For more information about the Gansfontein Paleosurface, click here to download a short report and detailed description: 1986_deBeer

Bradysaurus tracks on the Gansfontein Paleosurface

Reconstruction of Bradysaurus (http://en.m.wikipedia.org/wiki/Bradysaurus)

There are many other spectacularly exposed sedimentary features and arthropod trace fossils on this Paleosurface. This makes is something that is most definitely worth visiting that next time you're anywhere near Fraserburg. To visit the paleosurface, simply go to the Fraserburg Museum and ask and a guide will accompany you and show you around! Tomorrow the team can look forward to a trip across the Cape Fold Belt and consider elements of continental building and structural geology.

View the days route here: Day 3

2014 Diary: Day 2 - Solid Gold

As flashes of lightning and rumbles of thunder roars in the background, I can happily report that the team survived one of the longest days in field school history. The day began with unforeseen technical difficulties resulting in the need of a locksmith and a newly cut set of keys for one of our vehicles, the rest can be left to the imagination. Thereafter, we departed the overnight stop in Ermelo and continued over rolling hills of the Karoo (Stormberg Group) before entering Kwa-Zulu Natal and being warmly welcomed by numerous speed-cops (welcomed; nothing more I should add). Our stay in KZN was a short one, as we continued into the Free State and headed toward the Golden Gate Highlands National Park (www.sanparks.co.za/parks/golden_gate/).

The Golden Gate National Park is renowned for its majestic golden cliffs of the Clarens aeolian sandstone formations (upper Stormberg Group within the Karoo Supergroup). The team was left awestruck by the spectacular views and numerous photo opportunities.  The Clarens sandstone was deposited in a desert environment where large quantities of wind-blown sediments accumulated to form the iconic cliffs of this region. In addition, oxidation processes were common and resulted in an array of rust-stained colours spread over the cliffs. A close association was the emplacement of the Karoo Igneous Province. This was of course associated with the breakup of the Gondwana Supercontinent. This igneous province covered an area of more than 100,000 square kilometres and its start overlapped with the final stages of Clarens deposition. Because of this, the Clarens is often interlayered with layers of volcanic units, as well as cut by later dolerite intrusions.

Scenic views within the Golden Gate Highlands National Park

As lightning now strikes my abode and kills the electricity, I will sign off and look forward to a palaeontological-rich day 2. No further spoilers will be given, so be sure to check soon!  

To download the days route (kml) click here: Day 2