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Post 11 Southern Cape Geology: Evolution of a rifted margin

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The Southern Cape Geology excursion combines the scenic beauty of South Africa’s South Cape with the geological evolution of a Gondwanian rifted margin and break-up coastline. The tectono-sedimentary history of the on-land equivalents of the offshore hydrocarbon producing Bredasdorp Basin will be the key focus along the route from Cape Town eastwards past Africa’s southern-most point Cape Agulhas, Mossel Bay to Wilderness, returning via the interior Cape Fold Belt (CFB) mountains and valleys.

The excursion traverses several of the Mesozoic Uitenhage Group basins along the Worcester/Pletmos basin line, which forms a discontinuous east/west line of half grabens for over 400km. The syn-rift basin fill consists of continental to transitional sediments, equivalent to the offshore pre-1At1 synrift sequence containing the shallow marine gas reservoirs of the F-A, E-M and F-O gas fields.

The sedimentary geology and the basin and tectonic setting of the Uitenhage Group formations will be viewed and deliberated. Stops and discussions will also be on the “basement rocks” of the Palaeozoic Cape Supergroup and the “cover rocks” of the Cenozoic Bredasdorp Group.

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Field Trip Leader: Jean Malan and Jurie Viljoen
Start/End: Cape Town International Convention Centre                              
Departs: 09:00AM
Date: 5 days, Saturday 3rd September to Wednesday 7th September 2016
Price: R 14 700 per person sharing with single supplement of R 2 245 per person

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Field Trip Post 11: Southern Cape Geology: Evolution of a rifted margin was led by Jean Malan (a previous employee at the Council - currently in the hydrocarbon industry) and myself to examine the tectono-sedimentary history of the on-land equivalents of the offshore hydrocarbon producing units. The excursion traverses several of these Mesozoic Uitenhage Group basins, which constitute a discontinuous east/west line of half grabens of some 400 km along the Worcester/Pletmos basin bounding line, with basin fill of continental to transitional sediments, equivalent to the offshore Early Cretaceous/Jurassic syn-rift sequence. Regular stops were also made to examine the sedimentary geology and tectonic setting of the basement rocks of the Palaeozoic Cape Supergroup, and cover rocks of the Cenozoic Bredasdorp Group (Figure 1).

The first day of the excursion followed the Southern Cape coastline from Cape Town past Gordon’s Bay and Cape Hangklip to Betty’s Bay, and included a visit to the African Penguin colony. At Hermanus we tried to spot some whales and from there to The Kelders, via the Strandveld wine area to Cape Agulhas, the southernmost tip of African. We overnight at the coastal village of Arniston. On route we looked at the Ordovician to Silurian Table Mountain Group rocks, the Mesozoic Uitenhage Group outcrops and we examined the overlying Cenozoic cover.

The morning of the second day focused on Cenozoic coastal outcrops and the proximal facies of the offshore oil and gas Mesozoic sediments to be seen in the De Hoop Nature Reserve where the Cape Fynbos vegetation also gave us spectacular views. We travelled inland and eastwards crossing the Breede River by pontoon, past Heidelberg, Riversdale and Herbertsdale, stopping at Mesozoic Uitenhage Group outcrops (Heidelberg and Mossel Bay Basins), on to our overnight stop at Gondwana game reserve near Mossel Bay.

An early morning big five game drive started day three after which we travelled to Mossel Bay to view the coastal outcrops at Cape St Blaize, the contact and relationship between the steeply dipping Table Mountain Group and undeformed overlying Jurassic Robberg Formation . We visited a middle Stone Age site and continued our route eastwards to Wildernis, our easterly turn around point. From there it was back to George and turning inland we crossed the Outeniqua Pass into the Little Karroo to the land-locked, fault-controlled, Mesozoic Oudtshoorn Basin. We overnight just outside the Ostrich Capital of the world, Oudtshoorn.

On day four we had several opportunities to examine the tectono-sedimentary history and sediments of the interior Oudtshoorn Basin and views of the spectacular landscape of red coloured hills formed by the younger conglomeratic facies. After a visit to an Ostrich farm we travel west along Route 62 through South Africa’s port producing area, with stops in Huisrivier Pass and Seweweekspoort where we saw spectacular views through the Cape Fold Belt. We overnight at the Montagu Springs resort.

The last day of the excursion took us to the Koo Valley from where we did a tractor trailer drive and some 600 m ascent up to a high mountain point from were panoramic views of the Western Cape stratigraphy are visible. It also overlooks the wine and fruit producing area of Robertson situated some 1,000m below it. Lunch was a traditional South African potjiekos before our return journey to Cape Town, with a last stop at the top of Du Toitskloof Pass.

Our tour group of 20 consists of participants from all the continents (except Antarctica) and they varied from non-geologists to engineering and environmental geologists to real hard rock Greenland and Australian geologists (Figure 2).

Jurie Viljoen

Figure 1 - Excursion route of the Southern Cape geology field trip.
Figure 2 - Participants on the Southern Cape geology excursion. In the background reddish coloured conglomerate of the Buffelskloof Formation (Uitenhage Group) dips northwards towards the southward dipping Cango Fault on the left.
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