The Neoproterozoic Dalradian Supergroup of Scotland: an alternative hypothesis

The Dalradian Supergroup is interpreted traditionally as recording c. 300 m .y. of 'episodic Neoproterozoic rifting. However, lower Dalradian (pre-Easd ale Subgroup) facies architecture is incompatible with rift-basin fill, and no unambiguous Neoproterozoic extensional structures are present in those rocks. Consequently, no objective evidence exists to infer that Dalradian s edimentation was initiated during extensional tectonism. That, combined wit h the accumulating data for contractile deformation in Scotland at c. 870-8 00 Ma, the Knoydartian orogeny, permits the proposal of an alternative tect onostratigraphic evolution for the Dalradian. I propose that Dalradian basi n genesis was initiated as a foredeep in response to Knoydartian orogenesis . The coarsening- and shallowing-upward, 6-8 km-thick Grampian Group-lower Lochaber Subgroup succession arguably represents a flysch to molasse Knoyda rtian foredeep overlain by a moderately stable post-orogenic shelfal sequen ce recorded by the relatively uniform thinner (c. 4 km) and compositionally more mature rocks of the upper Lochaber through Islay subgroups. Lithosphe ric-scale extensional tectonism and rifting did not occur until the late Ne oproterozoic, as marked by the laterally variable, volcanic- and igneous-be aring Easdale Subgroup, and was followed by the late Neoproterozoic-early P alaeozoic Iapetan rift-to-drift transition through the Southern Highland Gr oup.