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Logging Tasks:
1. At the published coordinates describe this rock in terms of size, colour and weathering and how is this rock put to good use here?
2. At WP1 S32 13.605 E025 28.036 have a look at the pictures of dolerite (under weathering) in this listing and assign a ‘type’ of formation to what you see on either side of the road.
3. At WP2 S32 13.713 E25 27.919 you will see some freshly exposed dolerite. There are eight images below "Composition of Dolerite". From these images, listed 1 to 8 starting from the left, assign one that best fits the look of this fresh exposure.
4. At WP3 S32 13.494 E25 27.963 you will find some sabunga in the cutting. a) Describe the colour and texture of what you see and b) whether there are some bigger corestones amongst it. Quantify your answer for b).
Geology and Soils Overview of the Mountain Zebra Park
The geology of the region is dominated by sedimentary rock types such as sandstones, siltstones and mudstones of the Beaufort Series of the Karoo System with post-Karoo dolomitic intrusions being prevalent in some areas. The Beaufort Series contains a range of yellow-grey to dark-grey and greenish fine to medium grained sandstones which interchange with thick black to black-green and purple bands of mudstone and unstable shales (Van der Walt 1980).
The areas to the south and west of the existing Mountain Zebra NP are covered by extensive dolerite sheets and a number of dykes. Soils are generally shallow except along sloping pediments and large parts of the park are rocky with little or no soil.
Topography and Hydrology
The mountainous terrain of the Mountain Zebra NP is part of the south quarter of the Karoo Mountain Veld Complex which forms part of the Great Escarpment separating the Great Karoo and Upper Karoo. The southern boundary of the park follows the summit of the Bankberg, the highest point of which is 1957 m above sea level. To the north the Mountain Zebra NP extends across open flats to include the distinctive inselberg known as Salpeterkop. At 1000 to 1200 m the flats to the north-east form the lowest part of the park. The Wilgerboom River, running in a north-north-easterly direction through the park, only flows strongly after good rain but generally contains pools throughout the year.
Dolerite of the Karoo
About 182 million years ago, from the latitude of Beaufort West in the central Karoo to southern Malawi, from East London to the Okavango and beyond, the earth began to crack open and molten basalt poured out. It oozed upwards from a hundred kilometres or more below the surface through all the Karoo’s layered sediments. It filled cracks, crevices, hollows and faults, creating level sills, saucer-shaped rings and vertical dykes.
As the molten magma came into contact with wet sediments or groundwater, hydrothermal explosions punched upwards, creating thousands of holes in the Earth’s crust – through which hydrocarbon gases poured. Nearly two million cubic kilometres of molten rock flowed out of the Earth. This catastrophic episode of the Karoo’s geological life coincided with the tearing apart of the Gondwana supercontinent into its jigsaw parts of Africa, Antarctica, Australia and South America.
Dolerite Sheets, Sills and Dykes
Tensional cracks heralding the break-up of Gondwana permitted the ascent of pressurised basaltic magma from great depths. Up through the thick cake of Karoo sediments it came – long after the Ecca shale through which it cuts had been deposited – to flow over the surface as sheet after sheet of lava; where it didn’t make it to the surface, it squeezed between layers of sediments to form sills.
Less visible are the underground cracks left in the adjoining sediment as the superheated dolerite rose. It is these fractures that are so useful to Karoo people today. Groundwater collects in the crevices around dolerite pipes, sills and dykes. Anyone looking to drill for water in the Karoo will do so in the vicinity of dolerite.
It’s still uncertain how long this ancient fiery apocalypse took to play out. In geological terms, it was brief, but intense. The time frame has not been determined. It could have happened over a few decades or up to a few hundred thousand years. But the volumes were incredible, amounting to at least 1.5 million cubic kilometres of molten rock.
The first recorded instance was 250 million years ago, coinciding with the Permian extinction, which killed off more than 90% of all life on the planet.
Then came the Karoo igneous event 182.5 million years ago.
The geological maps of South Africa that are pinned to walls in most geologists’ offices show the Karoo basin thickly laced with pink. Pink marks the dolerite – the magmatic rock that cooled and hardened underground. Dolerite dykes and sills outcrop over two thirds of South Africa (Chevallier and Woodford 1999).
Wikipedia lists the Composition of Dolerite as:
Freshly exposed dolerite normally has a fine, but visible texture of euhedral lath-shaped plagioclase crystals (62%) set in a finer matrix of clinopyroxene, typically augite (20–29%), with minor olivine (3% up to 12% in olivine diabase), magnetite (2%), and ilmenite (2%) Accessory and alteration minerals include hornblende, biotite, apatite, pyrrhotite, chalcopyrite, serpentine, chlorite, and calcite. The texture is termed diabasic and is typical of diabases. This dolerite texture is also termed interstitial. The feldspar is high in anorthite (as opposed to albite), the calcium endmember of the plagioclase anorthite-albite solid solution series, most commonly labradorite.
Images of various igneous rocks -
1st row, left to right = Nos. 1 to 4
2nd row, left to right = Nos. 5 to 8
Weathering
Sills of dolerite often cap Karoo koppies, giving rise to their characteristic flat tops. The intrusions weather to form “castle koppies” of piles up dolerite boulders and “woolsack” weathering. Woolsack weathering is when the rocks look like rows of stacked up wool sacks.
Then there is the blocky weathering of the dolerite sills. As dolerite cooled and solidified after its molten intrusion, joints formed, approximately at right angles to one another. When these rocks were exposed at the surface, more than a hundred million years later, the rainwater seeped down and along the joints, and the rocks began to weather inwards from the joints. As the process continues and material washes away, the formerly sharply angular block-edges become rounded.
Dolerite has a strongly rectangular jointing pattern but its weathering is of the spheroidal type and results in rounded boulders with sometimes conspicuous weathering skins, so called “onion skin weathering”. The rounded cobbles and rocks with their flaky outer skins are difficult to use as there are always spaces between the stones, and rows are almost impossible to pack.
"Sabunga" is a South Africa colloquial name for a type or variety of sand and / or gravel, from weatherized dolerite. The weathered rock most widely used for road construction in South Africa, yet also notorious for its variability and propensity to marginal behaviour, is dolerite.
References
www.sanparks.co.za/assets/docs/conservation/park_man/mznp_approved_plans.pdf
http://karoospace.co.za/dolerite-karoos-fracking-game-changer/
Nick Norman 2013 book Geology Off the Beaten Track
Julian ‘Goonie’ Marsh, Professor Emeritus of Geology at Rhodes University in the Eastern Cape
http://www.vassa.org.za/wp-content/uploads/2011/01/VASSAJrnl19-compressed.pdf
http://www.wikipedia.org/wiki/Diabase