Discussion : a unifying model for the Torridon Group (early Neoproterozoic), NW Scotland : product of post-Grenvillian extensional collapse, by GE Williams and J Foden

The Torridonian succession in NW Scotland is a well-exposed sequence of sedimentary rocks that record the early Neoproterozoic geological evolution of Laurentia. An excellent review by Stewart (2002) summarised the observations on the three main 'Torridonian' units: the Stoer, Sleat and Torridon groups. The Torridon Group, deposited between 1000 – 950 Ma, was initially interpreted as a rift basin ( [Stewart, 1982] and [Williams, 2001]), but a number of studies ( [Rainbird et al., 2001], [Kinnaird et al., 2007] and [Krabbendam et al., 2008]) have now concluded that it represents a foreland basin positioned in front of the Grenville Orogen (1200 – 960 Ma). In a recent paper in Earth Science Reviews, Williams and Foden (2011) propose, instead, that the Torridon Group was deposited in an ‘extensional collapse basin’. We question this conclusion and take this opportunity to highlight aspects of their new interpretation that we find equivocal.