Reply to the Discussion by Callow et al. on ''Were the Ediacaran siliciclastics of South Australia coastal or deep marine?'' by Gregory J. Retallack, Sedimentology, 59, 1208-1236

Callow et al. (2012) conclude with two contradictory statements: (i) ‘‘It seems entirely probable, while as yet unproven, that soils or soil-like processes could be present in intertidal to supratidal portions of the Ediacara Member’’; and (ii) ‘‘On the basis of sedimentological observations, deposition of the fossil-bearing strata within fully marine, tempestite sandstones is considered to be the most parsimonious interpretation’’. My paper (Retallack, 2012a) presents evidence from a variety of geochemical, petrographic, granulometric and palaeopedological analyses, as well as sedimentological observations, that would appear to falsify their second statement, while supporting their first statement. ‘As yet unproven’ can be taken as an opinion of a working hypothesis, following Popper’s (1968) view that mathematics may offer formal proof, but science can only falsify. Thus, Callow et al. (2012) agree that there are non-marine as well as marine parts of the Ediacara Member, as also argued by Mawson & Segnit (1949), Goldring & Curnow (1967) and Jenkins et al. (1983), but not the entirely deep marine interpretation of Mount (1989), and Gehling (2000, and for Fedonkin et al., 2008). Callow et al. (2012) are mistaken in asserting that marine facies and hummocky cross-stratification were not mentioned or illustrated by Retallack (2012a). Hummocky cross-stratification illustrated by Gehling (2000) and heterolithic intertidal facies illustrated by Jenkins et al. (1983) were both mentioned and incorporated in an explicit reinterpretation of sequence stratigraphy of the Ediacara Member, that included horizons of marine influence (Retallack, 2012a, p. 1214, figs 4D and 6). In their original description of hummocky cross-stratification, Dott & Bourgeois (1982) used examples from deltaic sandstones of the Eocene Coaledo Formation of Oregon, familiar to this author from many years of student excursions. Subsequent discoveries of hummocky stratification have been in shoreface to shallow marine shelf palaeoenvironments, not the deep ocean (Higgs, 2011). Callow et al. (2012) also note: ‘‘Here it is concluded that analysis of demonstrably subaqueous, and most probably marine, microbial mat fabrics has led Retallack (2012a) towards mistaken conclusions about his subaerial palaeosols’’. This does not follow as a conclusion, because demonstration that mat fabrics of the Ediacara Member are subaqueous cannot be found in their preceding discussion. The South Australian Ediacaran mat fabrics are detailed by Retallack (2012a), as well as another recent publication (Retallack, 2012b), which develops general criteria for discrimination of microbial mats (marine-lacustrine) and microbial earths (non-marine subaerial). In summary, microbial earths have vertically oriented organisms intimately admixed with minerals of the soil, whereas microbial mats are laminated, and detachable from their mineral substrate as flakes, skeins and rollups. Microbial earths have irregular relief, healed desiccation cracks and pressure ridges even in clay-poor sandstones, whereas microbial mats have flexuous, striated domes and tufts. Microbial earths form deep soil profiles with downward variation in oxidation, clay abundance and replacive nodular subsurface horizons, whereas microbial mats form caps to unweathered, chemically reduced sedimentary layers. Microbial earths develop increasingly differentiated soil profiles through time, whereas microbial mats build upward in laminar to domed increments. On the basis of these criteria, the healed cracks and ridges characteristic of the microbially induced sedimentary structure designated ‘old elephant skin’ and its underlying stratal homogenization, nodularization and geochemical differentiation in the Ediacara Member are interpreted as subaerial microbial earths, and not subaqueous microbial mats (Retallack, 2012a). Sedimentology (2013) 60, 628–630 doi: 10.1111/j.1365-3091.2012.01362.x