Glacially influenced provenance and Sturtian affinity revealed by detrital zircon U–Pb ages from sandstones in the Port Askaig Formation, Dalradian Supergroup

Abstract

The Cryogenian ‘Sturtian’ snowball Earth glaciation ( c. 717–658 Ma) likely had a major role in shaping continental landscapes and biotic radiations of the late Neoproterozoic Era. However, an incomplete sedimentary record and inadequate syn-glacial age constraints make Cryogenian studies challenging. We present detrital zircon U–Pb ages for >2000 zircons from 11 sandstone samples taken at <200 m stratigraphic resolution throughout the Port Askaig Formation, a c. 1.1 km thick glaciogenic succession within the Dalradian Supergroup of Scotland and Ireland. Eight new maximum depositional age constraints, including a key constraint on deglaciation (<662.7 ± 7.8 Ma), support lithostratigraphic and stable isotope evidence that suggests the Port Askaig Formation preserves a relatively complete record of the global ‘Sturtian’ glaciation. An increasing contribution from Archean and Paleoproterozoic detritus to the sandstones through the lower c. 500 m of the Port Askaig Formation likely reflects the progressive glacial unroofing of the previously buried Lewisian Gneiss terrane. Archean and Paleoproterozoic grains then become scarce in the uppermost c. 300 m of the formation, which we attribute to glacial modification of the Laurentian continental margin landscape during the waning stage of ‘Sturtian’ glaciation. The disruption to sediment transport pathways caused by this modification, and evidenced by the detrital zircon data, points to partially warm-based ‘Sturtian’ ice sheets that were, to some degree, dynamic.