Ediacaran proximal glaciomarine sedimentation in the Bonavista Peninsula, Avalon Zone (Newfoundland)

Four ice ages characterize the Neoproterozoic icehouse Earth period. The two youngest, Gaskiers and Mortensnes glaciations, correspond to relatively short duration (i.e., <1 Myr) localized glaciations limited to >30° paleo-latitude, but significantly were followed by rapid Ediacaran biota evolution. However, the recognition of their glaciogenic origin is confounded by the coeval intensification of Late Neoproterozoic rifting and breakup of Rodinia that conceivably produced coarse-grained strata with characteristics similar to glacial diamictites. The Gaskiers glaciation (ca. 580 Ma) is best characterized in sedimentary successions exposed on the Avalon Peninsula of southeast Newfoundland, where the Gaskiers Formation records deepwater glaciomarine sedimentation over- and underlain by deep-marine turbidites. However, these deep glaciomarine strata do not provide a direct record of terrestrial glaciation, nor sea level changes that would have coincided with glacial advances and retreats, and thus, their glacial origin has been questioned.

Here, we present a sedimentological and stratigraphic analysis of the correlative shallow glaciomarine Trinity diamictite in the Bonavista Peninsula and associated shallow marine strata of the Rocky Harbour Formation. The Monk Bay Member underlies the Trinity diamictite and consists of stacked 6-to-10 m-thick upward-coarsening, shallow-marine clastic parasequences with rare dropstones, each capped by a gravel transgressive lag. The overlying Trinity diamictite consists of a massive clast-rich diamictite with abundant faceted, flat-iron, stoss-and-lee, or bullet-shaped clasts and rare striated clasts and an overlain by a clast-poor laminated diamictite with dropstones, dump and grounding structures. The overlying King’s Cove North Member comprises a distinctively pistachio-color silicified tuff, >50m thick mudstone succession, and uppermost stratal unit of thick-bedded turbidites.

Parasequences of the Monk Bay and Trinity diamictite units are interpreted as deposits of glacial advance and retreat cycles controlled by glacio-eustatic and glacio-isostatic forces. The gravel-rich transgressive lag capping each parasequence records wave reworking of ice-rafted material. These cycles culminated in the deposition of the Trinity diamictite, marked initially by a continuous rainout, followed by finer-grained fall-out of ice-rafted debris, and finally, glacial grounding. Overlying this, the Kings Cove North facies records glacio-eustatic flooding of the shelf, driven by a glacial retreat that marks the end of the Gaskiers glaciation on the Bonavista Peninsula and coincident glacio-eustatic sea-level rise, which evidently outpaced glacial-isostatic rebound.