A newly discovered cap carbonate above Varanger-age glacial deposits in Newfoundland, Canada

Abstract

ABSTRACT Discontinuous outcrops of diamictite-bearing units exposed along the southern coast of Conception Bay, Newfoundland, are shown to be Neoproterozoic glacial deposits. These are equivalents of the Gaskiers Formation, a well-documented glaciogenic diamictite exposed on the southern Avalon Peninsula. A newly discovered 50-cm-thick cap limestone bed directly overlies these Varanger-age glacials and is the only bedded carbonate unit in a nearly 15-km-thick Neoproterozoic succession of volcaniclastic flysch and molasse deposits. The cap limestone contains a strongly negative carbon-isotope signature, which is believed to be primary and is similar to many cap carbonate beds deposited globally. Deposition of cap carbonate is a response to a sudden increase in shallow-seawater alkalinity. This postglacial isotopic signature has been interpreted as either (1) the result of oceanic turnover and mixing of alkaline, 13C-depleted deep water with surface water, (2) the sudden equilibration of a CO2-charged atmosphere after meltback of a snowball Earth, or (3) the near cessation of primary productivity in the oceans. The Newfoundland cap bed is a rare deep-sea example that may be the only known example from a volcanic terrane of an active orogenic belt. Carbonate clasts in the underlying diamictite also have highly negative isotopic signatures. Samples from one exposure of the cap bed show a stratigraphic shift in 13C from -6o/oo at the base to -1.5o/oo at the top. The initial shift from moderately to highly negative values recognized in Neoproterozoic cap carbonate beds from Australia and Namibia is not preserved in the Newfoundland cap bed. The minor thickness of the bed and its incomplete isotopic signature is likely due to deposition in an active tectonic setting among island arcs. This setting would have been characterized by high rates of subsidence, high accumulation rates of siliciclastic sediment, high equilibrium bottom slopes, and deep-water depositional environments. All of these factors may have contributed to a delayed initiation and short duration of carbonate deposition relative to other cap carbonate deposits worldwide. A high paleolatitude for Avalonia at this time may have also caused a shortened and delayed development of the cap carbonate bed.