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Turner, E.C.


Volume NWT Open Report 2009-002, p.26 (2009)


The Gypsum formation (Little Dal Group; <1083 Ma; >779 Ma) consists of ~500 m of white weathering, intermittently cross-laminated gypsum. Regionally valid stratigraphic subdivisions of this thick and monotonous unit are defined by red, clay-bearing units, minor amounts of interbedded carbonate, and dark, organic-rich gypsum intervals. Near the top of the evaporite succession is a conspicuous carbonate marker unit (~10-15 m) consisting of nodular dolomitic<br/>lime mudstone with local molar-tooth structure. Stable isotopic signatures for the gypsum and associated carbonates appear to be typical for the early Neoproterozoic. The Gypsum formation records evaporative deposition in a regional, moderately deep, lagoonal basin. Abrupt, tectonically triggered onset of restriction produced a tectonic sill, and was accompanied by sudden subsidence of a supratidal carbonate system (upper Grainstone formation) to a bathymetric level below or roughly equivalent to storm wave-base. After a time, during which subsidence was matched by sediment accumulation, sudden basin freshening produced a carbonate marker unit that was of normal-marine, above storm-wave-base aspect; this was just as abruptly ended with a return to restricted conditions. Evaporite deposition terminated abruptly, likely as the result of a tectonic event. Overlying lithofacies of the basal Rusty Shale formation<br/>indicate slightly restricted subtidal carbonate deposition. Deposition of the Gypsum formation in a basin that experienced intermittent, ongoing, basin-wide tectonic adjustment, as recorded by its stratigraphy, supports the idea that the Mackenzie Mountains Supergroup was deposited in an extensional, syndepositionally tectonically active epicratonic basin, rather than in a passivemargin-<br/>like setting.