Discussion and reply tectonic control on development of a Frasnian – Famennian (Late Devonian) palaeokarst surface, Canning Basin reef complexes, northwestern Australia

Chow et al. (2004) claim to have found evidence in the Napier Range to show that the Frasnian – Famennian (F – F) boundary in Devonian reef complexes of the Canning Basin is an angular unconformity. They have also cast doubt on the hypothesis that the regression associated with this boundary is a world-wide (eustatic) event. The Napier Range locality was identified by Chow et al. (2004 figure 3) as being at AMG 51K 6733(00)E 80970(00)N. The karst surface that they describe is readily recognised in the field from photographs shown as figure 3a in Chow et al. (2004) and figure 6A in George et al. (2002). The locality can be accessed at the head of a creek that flows from the Napier Range at AMG 51K 671700E 8097300N, near the track between Barker Gorge and the Windjingayr Community settlement. The Chow et al. paper repeats and expands on an earlier description and interpretation of this locality by the same authors (George et al. 2002). They state that Famennian back-reef limestones at this place overlie karstified Frasnian ‘proximal forereef-slope’ limestones, with observed dips of about 25 – 308, reduced to 15 – 208 after removing 108 of post-Famennian basinward tectonic tilting. The authors maintain that such dips are too low to be depositional and that they must have been diminished by post-depositional landward tectonic tilting (said to be 5108), prior to truncation by erosion at the F – F boundary. Chow et al. maintain that limestones below the karstified surface include ‘very thickly bedded breccias featuring abundant microbial limestone clasts’, with ‘steep (subvertical) fabrics which suggests they are not in situ reef margin’. However, our visit to the locality showed that those claims have no factual basis. The limestone below the karstified F – F boundary surface consists of unbedded microbial – stromatoporoid reef limestone (Pillara Limestone, reef-margin subfacies), not bedded marginal-slope deposits. The reef limestone, which outcrops at the locality over many hundreds of square metres, has well-developed contemporary geopetal cavity fillings of red pelloidal micrite and spar. They show that the reef has been tilted about 158 west but is otherwise in situ. The dip of the geopetal structures is the same as that in the overlying Famennian limestones (Nullara Limestone, back-reef subfacies), showing that there is no angular discordance between them. Chow et al.’s ‘steep (subvertical) fabrics’, which they interpreted as evidence for allochthonous blocks, are in situ sheets of stromatoporoids (Actinostroma) in microbial – stromatoporoid reef limestone. They mark successive fronts of the prograding Frasnian platform. Similar near-horizontal progradation of the latest Frasnian platforms has also been observed in other places on the Lennard Shelf (Playford et al. 1989; Playford 2002). Figures 1 and 2 are photographs of a vertical face of this reef limestone, at AMG 51K 673160E 8096840N. This locality is 30 m south of the outcrop shown as figure 3a by Chow et al. (2004). It should be noted that even if ‘proximal forereef’ deposits with relatively low dips had been present below the F – F boundary, that would not in itself have been sound evidence for a tectonic episode at that boundary. Certainly, steep depositional slopes (up to almost vertical) are often seen in upper marginal-slope deposits, but slopes as low as 15 – 208 are also common in those deposits, for example at Windjana Gorge (see dips shown by Playford & Hocking 1998b). In conclusion, there is no factual basis for the claims by Chow et al. (2004) and George et al. (2002) that the F – F