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.
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