- © 2005 Geological Society of South Africa
The discussion by Turner and Thomson raises a number of interesting points which we feel required greater clarity, and presents certain statements which in truth are not hard and fast facts. A number of the criticisms levelled at the basin development paper of Bordy et al. (2004a) are based on Turner’s belief in a transient hypothetical plume model for the development of the main Karoo Basin during the Triassic to Early Jurassic (Turner, 1999). While space precludes a full discussion on the flaws of this model as addressed by Hancox et al. (2002), and which will be documented elsewhere, a number of the arguments that follow are equally as applicable to Turner (1999) as they are to the discussion put forward by Turner and Thomson.
Turner and Thomson (1998) and Turner (1999) questioned the applicability of the reciprocal flexural model of Catuneanu et al. (1998) to the Triassic-Jurassic succession of the Karoo Basin, and highlighted the lack of reliable radiometric dates for the “upper Karoo”, as well as the lack of stratigraphic data for the upper Molteno and lower Elliot formations. It was in fact these papers and presentations that led to the initiation of the current spate of research into the Molteno and Elliot formations (Hancox, 1998a; b; 2000; Lucas and Hancox, 2001; Bordy et al., 2004a to d 349–362), all of which have markedly increased our knowledge and understanding of this part of the Karoo Basin fill.
One of Turner and Thomson’s major concerns with the flexural model is the use of the Hälbich et al. (1983) dates for orogenic events in the Cape Fold Belt (CFB). Although Turner and Thomson do not provide references for their concerns with the 40Ar/39Ar system, we recognise that problems do exist with …