Abstract

Drs W. J. McCourt and J. A. Aspden write: The geological arguments used by Millward et al. 1984 to dismiss an island-arc origin for the Diabase Group (DG) are not convincing. The statement ‘the intimate association with basinal sediments necessitates the location of such an arc almost on top of the trench’ is unjustified; or are the authors suggesting that basinal (? deep water) sediments are restricted to trench environments? Similarily, ‘the insignificant amount of pyroclastic material’ is applicable only within the Cali area, whilst sizeable volumes of such rocks are present both to the north and south. Equally the objection to ‘the abundance of dolerite’ is curious; would one not expect to find coarser grained equivalents within the lower levels of a volcanic pile?

‘The presence of continentally derived clasts’ within the interbedded Dagua Group indicates that the sequence was deposited sufficiently close to a continentaledge to receive such material; it definitely does not ‘preclude the origin of arc as an allochthonous terrain’. In fact, Millward et al. point out that the sequence ‘could have been translated considerable distances along the S. American continent’, if this is so, then why is it not allochthonous?.

Millward et al. correctly point out that ‘the area described is small compared with the total Mesozoic belt of Colombia and Ecuador’. In fact, within Colombia alone it accounts for only 2.5% of the Western Cordillera. Of greater significance, however, is the fact that the area sampled forms part of a major elongate tectonic

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