Response to G. J. Retallack's ‘Discussion’ of Jago et al. (2013)

The major purpose of Jago et al. (2013) was to discuss the implications of changes in current direction, as indicated by cross-bedding data, within the upper part of the Cambrian succession of the Flinders Ranges, South Australia. Retallack (2013) does not comment on this but has queried our interpretation of the age and depositional environment of the two top units of the Cambrian succession, the Pantapinna Sandstone and the Grindstone Range Sandstone. The age of the Lake Frome Group was discussed in detail by Jago et al. (2010), who noted that there are no reliable radiometric ages within the Cambrian succession of the Flinders Ranges. As indicated by Jago et al. (2013), the only reliable paleontological age within the Lake Frome Group is the presence of the trilobite Onaraspis rubra from the top part of the Moodlatana Formation (Jell in Bengtson et al. 1990); correlation with the latest Toyonian of Siberia was suggested by Geyer & Shergold (2000) and Paterson & Brock (2007). All that can be stated with certainty about the age of the overlying Balcoracana, Pantapinna and Grindstone Range Formations is that they are late early Cambrian or younger. The correlation charts used by Jago et al. (2012, 2013) show the Coobowie Limestone (containing probable early middle Cambrian fossils) of the Yorke Peninsula as equivalent to the Balcoracana Formation (Figure 1); this in turn suggests that the overlying Pantapinna Sandstone and Grindstone Range Sandstone are most likely to be of middle Cambrian age. Retallack (2008, 2009, 2013) suggests a late Cambrian to early Ordovician age for the Pantapinna Sandstone and Grindstone Range Sandstone. As suggested above, this is a possible age, but it is derived essentially by assuming a constant rate of deposition throughout the Cambrian succession of the Flinders Ranges. The relationship between thickness and time depends on many factors including eustatic sea-level changes, the tectonics of both source and depositional areas, the climate and the depositional environment. As noted by Jago et al. (2010) a uniform rate of sedimentation is highly unlikely given the range of depositional environments represented within the Cambrian rocks of the Flinders Ranges. These include tidal flat, deltaic, littoral and fluvial sediments plus shelf carbonates, reef carbonates and below wave base mudstones (see Gravestock 1995 and Zang et al. 2004 for details). Retallack (2013) states that our paper is ‘flawed by unsubstantiated claims of mainly marine environments for the upper Lake Frome Group,’ i.e. the Pantapinna Sandstone and the Grindstone Range Sandstone. What we actually suggested for the Pantapinna Sandstone was that it has ‘a marginal marine to fluviatile origin, but with some open marine horizons’ (Jago et al. 2013, p. 236). The Grindstone Range Sandstone was described in some detail by Jago et al. (2010). They indicated that the lower 368 m of the type section is probably of shallow marine origin; trilobite tracks and other trace fossils are recorded from low in the Grindstone Range Formation by Gravestock (1995) who suggested a low intertidal depositional regime. The upper part of the Grindstone Range Sandstone was described as the Dawson Hill Member (190 m thick in the type section) by Jago et al. (2010), who suggested rapid deposition under fluvial conditions with the possible exception of the top 10 m, which may have a littoral or shallow marine origin. Hence, despite Retallack’s statement, we regard substantial parts of both the Pantapinna Sandstone and the Grindstone Range Sandstone as being of non-marine origin as was stated clearly in Jago et al. (2013).