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

The 3-27 m-thick cap carbonate overlying "Marinoan" Ice Brook Formation gla cigene sediments and Keele Formation carbonate and terrigenous clastic rock s consists of two distinctive stratigraphic units. A lower, splintery, buff -weathering, microcrystalline dolostone of extensive lateral uniformity com prises mm-laminated peloidal sediment with local, low-angle, hummocky-like cross-stratification, micro-ridges, and synsedimentary tepees, all elongate d perpendicular to depositional strike. This dolostone is unconformably ove rlain 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 isol ated crystal fans in laminated limestone to huge, decimetre-scale crystal a rrays, 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 platf orms. The calcite crystals have all the physical and chemical attributes of neomorphosed aragonite. This aragonite extensively replaced sediment and m icrobiolite just below the sea floor and grew upward into the overlying wat er column. Such interpreted massive synsedimentary replacement is rare in g eological history and attests to the highly saturated state of the immediat e postglacial ocean. All sediment is interpreted to have been CaCO3 origina lly. Low and constant delta O-18 values reflect diagenetic modification of these carbonates, although chemical attributes, such as Sr and C isotopes i n some lithologies, are near pristine. Lower dolostones, virtually identica l to most other coeval Marinoan caps worldwide, were part of a global preci pitation 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 Sr-87/Sr-86 ratios and varying delta C-13 values with carbonate sedimentary facies indicate that both unit s must have formed relatively rapidly, prior to significant fluvioglacial r unoff, or that the influence of this runoff on the chemistry of seawater al ong continental shelves was minimal. The cap carbonate is thus interpreted to have formed in two steps: (1) during initial marine ice melting accompan ied 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 precipitatio n from an ocean highly perturbed by global glaciation, the rocks also sugge st that this event did not permanently affect either late Neoproterozoic oc ean chemistry or the contained marine biosphere.