Geological Society of London
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Chapter 36 Neoproterozoic glacial record in the Mackenzie Mountains, northern Canadian Cordillera

posted on 2016-06-21, 10:43 authored by P. F. Hoffman, G. P. Halverson

In the Mackenzie Mountains, an arcuate foreland thrust-fold belt of Late Cretaceous–Paleocene age in the northern Canadian Cordillera, two discrete glacial–periglacial sequences of Cryogenian age (the Rapitan Group and the Stelfox Member of the Ice Brook Fm.) are separated by c. 1.0 km of non-glacial strata. The older Rapitan diamictite occurs in an amagmatic rift basin; the younger Stelfox diamictite occurs on a passive-margin continental slope.

The Rapitan Group consists of three formations. The lower Mount Berg Fm. is a complex of diamictites and conglomerates of limited extent. The middle Sayunei Fm. is a thick sequence of maroon-coloured mudrocks hosting innumerable graded layers of silt- and fine-grained sandstone. It lacks wave- or traction current-generated bedforms, and is lightly sprinkled with granule aggregates (‘till pellets’) and lonestones of dolostone and rare extrabasinal granitoids. It is capped by a hematitic Fe-formation that was reworked into the disconformably overlying Shezal diamictite. The Shezal Fm. is a complex of olive-green coloured boulder diamictites with subordinate, dark-grey shales, siltstones and parallel-sided sandstones. Some of the boulders are faceted and striated, and include dolostone, quartzite, siltstone and gabbro in declining order of abundance. Diamictite terminates abruptly at the top of the Shezal Fm., which is sharply overlain by dark shales or by <52 m of fetid, dark-grey, 13C-depleted limestone with graded bedding.

The Stelfox Member is dominated by non-stratified, carbonate-clast diamictite with faceted and striated clasts, locally associated with subordinate, well-laminated shales containing till pellets and ice-rafted dropstones. It is thin or absent on the palaeocontinental shelf, but thickens seaward (southwestward) on the palaeocontinental slope. A thin clay drape separates it from a laterally continuous post-glacial ‘cap’ dolostone, which is a very pale coloured, micro- to macropeloidal dolostone with low-angle cross-laminae, giant wave ripples and local bioherms of corrugated stomatolites. In the NW, the dolostone is followed by reddish and greenish marls, followed by black shale of the Sheepbed Fm. In the SE, the dolostone is overlain by pink or grey limestones with well-developed sea-floor cements pseudomorphic after aragonite. In this area, the top of the dolostone is ferruginous and contains digitate rosettes of sea-floor barite cement, variably calcitized. The dolostone–limestone contact is perfectly conformable, and synclinal structures previously intepreted as karst features are tectonic in origin.

The grand mean palaeomagnetic pole for the well-studied Franklin Large Igneous Province (c. 718 Ma) of Arctic Laurentia, coeval with the basal Rapitan Group in the Mount Harper area, Yukon Territory, places the Mackenzie Mountains firmly in the tropics, at 18±3°N palaeolatitude, at the onset of the Rapitan glaciation.

Carbon (δ13C), oxygen (δ18O) and strontium (87Sr/86Sr) isotopes have been measured in carbonates bracketing the Rapitan and Stelfox diamictites. Sulphur isotope data (δ34S) have been obtained from carbonate-associated sulphate and barite above the younger diamictite, and calcium isotope data (δ44Ca) from the younger carbonate itself. The results are broadly consistent with data from other areas. Iron isotope (δ57Fe) and cerium anomaly (Ce/Ce*) values increase systematically upwards through the Sayunei Fe-formation, supporting an interpretation that deposition occurred within a redox chemocline through which the basin floor descended as a consequence of isostatic loading by the advancing Shezal ice sheet.