Friday, August 28, 2009

Table Mountain stratigraphy update

During my recent holiday in the Cape I visited my alma mater, the University of Cape Town and its Department of Geosciences. At his visit John Rogers of the geology department told me about his recent work on Table Mountain and later showed me some outcrops along the northern and western face of the mountain.

The stratigraphy of Tale Mountain (Western Cape Province, South Africa) always seemed to be a perfect example of "layered cake" stratigraphy: the base is Malmesbury Shale and Cape Granite with the famous Sea Point Contact between the two. Both are truncated at the top by an unconformity. The succession continues upwards with some basal conglomerate in places, the tidal or fluvial Graafwaater Formation, followed by the fluvial quartzites of the Peninsula Formation and topped by some glacial till, remnants of the Pakhuis Formation. This package of Graafwater Fm. and Peninsula Fm. can be found in the Cape Mountains where it is highly deformed.

The contact between the Graafwater Fm. and the Peninsula Fm. had always been interpreted as a conformable contact. By coincidence John Rogers noticed an angle between the Graafwater Fm. and the overlying Peninsula Fm., prompting the question, whether the contact between these two formations is actually conformable. Dating on zircons by Rogers et al. points to a significant hiatus between the two formations.

The picture above shows Cape Point and Rooikrans at the southern tip of the Cape Peninsula. It is one of the prime tourism and field-trip stops. I had been here a few times before in the past twenty years, but neither me, nor seemingly anybody else, had noticed the prominent syncline/anticline in the Graafwater Fm. beneath an undeformed Peninsula Fm.

My guess is that the folding is due to compression along a north-south axis because all observed folds dip along this axis. The quartzites of the Peninsula Fm. seem to be more competent. Instead of ductile deformation they show brittle deformation, e.g. a prominent reverse fault under Constantiaberg, the fault plain dipping to the south.

This puzzles me and I will have to do more reading on the regional geology of the Western Cape. Why is the principal stress from the south if the nearby Cape Mountains were folded by a main stress from the west? How far did the Graafwater Fm. extend? Is there an angular unconformity at Lekkranskloof (near Heerenlogement and Graafwater)? Interesting new detail about the assemby of Gondwana.

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