Discussion on 'Basin development during deposition of the Elliot Formation (Late Triassic - Early Jurassic), Karoo Supergroup, South Africa' (South African Journal of Geology, 107, 397-12)

Abstract

The paper by Bordy et al. (2004) represents a useful contribution to one of the most neglected parts of the Karoo succession, despite its importance in terms of biological and climate change, and evolving basin tectonics. However, the paper also raises a number of issues that warrant further discussion. The paper highlights one of the major problems with the Karoo Basin: the lack of reliable radiometric dates for the age of the basin fill. Bordy et al. (2004) divide the Upper Karoo succession on perceived unconformities which they link to more than twenty year old Ar/Ar dates for synchronous orogenic events in the Cape Fold Belt (Halbich, et al. , 1983). Unfortunately, most geochronologists do not consider such dates reliable because the Ar-Ar system, being based on the K-Ar system, is rather prone to the effects of alteration/diagenesis, which may induce a non systematic loss or gain of radiogenic Ar. In addition, with improvements in mass spectrometry techniques, there is now significant argument about fundamental issues relating to the methodology, more specifically: uncertainty in the half life of 40K; lack of agreement on the precise age of neutron flux monitors used to calibrate ages for unknown samples; the inherent variability of the neutron flux in a given reactor used to irradiate samples, leading sometimes to significant uncertainty in the “J”-factor; and differences in approach to criteria used for accepting data for use in age calculation. These uncertainties combine to mean that the absolute error on a given 40Ar-39Ar is significantly greater than the “in-run” uncertainties quoted in most publications. This can lead to errors of several percent in the absolute age of a Mesozoic rock. The age of the upper Karoo succession is poorly constrained (see discussion in Turner, 1999), and apart from the base of …