THE EDIACARAN MALL BAY FORMATION, NEWFOUNDLAND, CANADA, AND THE PROTRACTED ONSET OF THE GASKIERS GLACIATION

The Gaskiers glaciation (580 Ma) was the first Neoproterozoic glaciation after the Cryogenian snowball Earth episodes and immediately preceded the appearance of the Earth’s first large, complex, soft-bodied, Ediacaran fossils of the Mistaken Point assemblage (575–560 Ma). The Gaskiers glaciation is generally regarded as being relatively short-lived and confined to the stratigraphic extent of the Gaskiers Formation. New research on the underlying km-thick, deep-water, siliciclastic Mall Bay Formation (ca. 600–580 Ma) on the Colinet Islands in Newfoundland, Canada, challenges this interpretation and shows evidence of a protracted onset to the Gaskiers glaciation. Dolomitic glendonites (pseudomorphs of ikaite) occur throughout the uppermost 500 m of the Mall Bay Formation and provide evidence for persistent near-freezing bottom waters. Interpreted iceberg-rafted debris in the form of dispersed outsized clasts, clast clusters (frozen aggregates), and coarse lags reworked by energetic contour currents also occurs through the 500 m below the base of the Gaskiers Formation, implying the existence of glacial ice somewhere at sea level. Cobble-sized glaciogenic dropstones in the uppermost 200 m of the Mall Bay Formation imply that the ice sheet reached the shelf edge by this time. Beginning 140 m below the base of the Gaskiers Formation, slope instability became significant, introducing glaciogenic diamictite debrites into this deep-water setting, perhaps signalling the arrival of the ice sheet margin at the shelf edge in proximity to the study area. These upward stratigraphic trends in the Mall Bay Formation imply a slow increase in the extent of glacial activity, peaking with the well-known Gaskiers Formation. The proposal of a protracted buildup to the Gaskiers glaciation has broader implications for understanding the evolution of the oldest-known complex macrofossils of the Ediacara Biota. A prolonged Ediacaran cold period before the widespread appearance of the Ediacara Biota implies that their metazoan predecessors may have evolved in deep water under stenothermal cold-water conditions. The sinking of cold water to the seafloor, as recorded by glendonites, may have increased ventilation of oxygen and nutrients, favoring conditions conducive to benthic metazoan habitation.