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

Morphometric convergence between Proterozoic and post-vegetation rivers

Proterozoic rivers flowed through barren landscapes, and lacked interactions with macroscopic organisms. It is widely held that, in the absence of vegetation, fluvial systems featured barely entrenched channels that promptly widened over floodplains during floods. This hypothesis has never been tested because of an enduring lack of Precambrian fluvial-channel morphometric data. Here we show, through remote sensing and outcrop sedimentology, that deep rivers were developed in the Proterozoic, and that morphometric parameters for large fluvial channels might have remained within a narrow range over almost 2 billion years. Our data set comprises fluvial-channel forms deposited a few tens to thousands of kilometres from their headwaters, likely the record of basin- to craton-scale systems. Large Proterozoic channel forms present width:thickness ranges matching those of Phanerozoic counterparts, suggesting closer parallels between their fluvial dynamics. This outcome may better inform analyses of extraterrestrial planetary surfaces and related comparisons with pre-vegetation Earth landscapes

1600-1500 Ma hotspot track in eastern Australia: implications for Mesoproterozoic continental reconstructions

The definitive version is available at www.blackwell-synergy.comMesoproterozoic A-type magmatic rocks in the Gawler Craton, Curnamona Province and eastern Mount Isa Inlier, form a palaeo-curvilinear belt for reconstructed plate orientations. The oldest igneous rocks in the Gawler Craton are the Hiltaba Granite Suite: c. 1600–1575 Ma. The youngest in the Mount Isa Inlier are the Williams-Naraku Batholiths: c. 1545–1500 Ma. The belt is interpreted as a segment of a hotspot track that evolved between c. 1600 and 1500 Ma. This hotspot track may define a quasilinear part of Australia’s motion between 1636 and 1500 Ma, and suggests that Australia drifted to high latitudes. An implication of this interpretation is that Australia and Laurentia may not have been fellow travellers leading to the formation of Rodinia. A hotspot model for A-type magmatism in Australia differs from geodynamic models for this style of magmatism on other continents. This suggests that multiple geologic processes may be responsible for the genesis of Proterozoic A-type magmas.Peter G. Betts, David Giles, Bruce F. Schaefer and Geordie Mar