Late Neoproterozoic cap carbonates: Mackenzie Mountains, northwestern Canada: precipitation and global glacial meltdown

Abstract

The 3-27 m-thick cap carbonate overlying "Marinoan" Ice Brook Formation glacigene sediments and Keele Formation carbonate and terrigenous clastic rocks consists of two distinctive stratigraphic units. A lower, splintery, buff-weathering, microcrystalline dolostone of extensive lateral uniformity comprises mm-laminated peloidal sediment with local, low-angle, hummocky-like cross-stratification, micro-ridges, and synsedimentary tepees, all elongated perpendicular to depositional strike. This dolostone is unconformably overlain by an upper limestone that exhibits pronounced facies variation from inboard peloidal lime grainstone and mudstone to shelf-edge cementstone to outboard lime wackestone and mudstone. Calcite cementstones range from isolated crystal fans in laminated limestone to huge, decimetre-scale crystal arrays, to hemispherical and elongate crystal stromatolites wholly composed of acicular crystals that form decametre-scale reeflike structures. Crystal stromatolites are precipitates and replaced microbiolites that constructed biostromes and bioherms, like those on many flat-topped, reef-rimmed platforms. The calcite crystals have all the physical and chemical attributes of neomorphosed aragonite. This aragonite extensively replaced sediment and microbiolite just below the sea floor and grew upward into the overlying water column. Such interpreted massive synsedimentary replacement is rare in geological history and attests to the highly saturated state of the immediate postglacial ocean. All sediment is interpreted to have been CaCO3 originally. Low and constant δ18O values reflect diagenetic modification of these carbonates, although chemical attributes, such as Sr and C isotopes in some lithologies, are near pristine. Lower dolostones, virtually identical to most other coeval Marinoan caps worldwide, were part of a global precipitation event of remarkable similarity. Upper limestones are a more local phenomenon, deposited during sea-level rise in an aragonitic sea returning to equilibrium after global glaciation. Low 87Sr/86Sr ratios and varying δ13C values with carbonate sedimentary facies indicate that both units must have formed relatively rapidly, prior to significant fluvioglacial runoff, or that the influence of this runoff on the chemistry of seawater along continental shelves was minimal. The cap carbonate is thus interpreted to have formed in two steps: (1) during initial marine ice melting accompanied by oceanic overturn and upwelling, preceding continental margin rebound, and (2) during initial stages of sea-level rise accompanying continental deglaciation. While confirming brief, but extensive, carbonate precipitation from an ocean highly perturbed by global glaciation, the rocks also suggest that this event did not permanently affect either late Neoproterozoic ocean chemistry or the contained marine biosphere.