Revisiting the origin of the Ediacaran Loch na Cille Boulder Bed, SW Scotland

The Ediacaran Loch na Cille Boulder Bed (LCBB) on the Tayvallich Peninsula, SW Scotland, has been interpreted as a glaciogenic deposit and is widely incorporated into Neoproterozoic glaciostratigraphic frameworks. Support for a glacial origin has principally rested on the inferred presence of ‘exotic’ clasts and proposed correlation with the c . 580 Ma Gaskiers glaciation. However, unequivocal evidence for glacial deposition remains poorly documented, while uncertainty regarding the definition and extent of the LCBB relative to other clast-bearing lithologies on the Tayvallich Peninsula has hindered detailed reassessment.

Petrographic observations and a U–Pb isotope age of 605.5 ± 9.1 Ma from a supposed exotic feldspar-rich igneous clast within the LCBB show close similarities with an underlying 595 ± 4 Ma syenitic intrusion and are thus consistent with derivation from the Tayvallich Volcanic Formation rather than from an extrabasinal source terrane. Limestone and metasandstone clasts within the LCBB are likewise interpreted to have been derived from the underlying Tayvallich Limestone and Crinan Grit formations. No unequivocal indicators of glacial transport or ice-rafted deposition were identified in the LCBB which, taken together with evidence for local provenance of all recognised clast types, undermines a glaciogenic interpretation. Instead, the LCBB is more parsimoniously interpreted as the product of erosion and reworking of underlying lithologies within a submarine slope environment.