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A Mid Cretaceous paleo-Karoo River valley across the Knersvlakte plain (northwestern coast of South Africa): Evidence from apatite fission-track analysis

Institute of Geology and Palaeontology, Basel University, 4056 Basel, Switzerland email: a.kounov{at}unibas.ch

G. Viola

Geological Survey of Norway, 7491 Trondheim, Norway email: giulio.viola{at}ngu.no

M.J. de Wit

AEON and Department of Geological Sciences, University of Cape Town, 7701 Rondebosch, South Africa email: maarten{at}cigces.uct.ac.za

M. Andreoli

South African Nuclear Energy Corporation, PO Box 582, 0001 Pretoria, South Africa; School of Geosciences, University of the Witwatersrand, P Bag 3, 2050 Wits, South Africa email: marco{at}necsa.co.za

Apatite fission track analysis of gneiss samples across the Knersvlakte plain in the coastal area of western South Africa reveals the existence of a relatively deeply incised paleovalley flanked by high ridges formed during the Mid Cretaceous. This paleovalley coincides with the present course of the Krom River through the Great Escarpment.

Modelling of the fission-track data suggests that the main stage of intense channel erosion, triggered by uplift of the catchment area, occurred between ~120 and ~110 Ma and was followed by a period of progressive hillslope erosion and interfluve degradation. It is suggested that the paleo-Karoo River and its tributaries were responsible for the formation of this relief. The present-day lower relief formed by continuing erosion starting at ~90 Ma, time when all the analysed samples had already been exhumed to a depth corresponding to temperatures below 60°C (~2 to 3 km).

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