The Rapitan Group: relevance to the global association of late Proterozoic glaciation and iron-formation
The University of Western Ontario, London, (Canada)
2 v. (xxv, 603 pages), Illus. (some col.), Maps
The Rapitan Group (770-600 Ma) is a glacio-marine sedimentary sequence cropping out in Mackenzie and Wernecke Mountains. A marked basal unconformity marks the culmination of a regional extensional event, the Hayhook Orogeny. The lowest unit, the Mount Berg Formation, is a grey-green mixtite seen only in the southern Mackenzies. Conformably above this, the Sayunei Formation is dominated by maroon, silty rhythmites with abundant dropstones. Conformably overlying this, the Shezal Formation successively comprises maroon and greenish-grey mixtites. The Sayunei and Shezal Formations crop out nearly continuously throughout the southern and central Mackenzie Mountains. An homologous succession of rhythmites and mixtites is seen in the Upper Tindir Group of eastern Alaska. In the northern Mackenzies, however, only red and greenish-grey mixtites occur. Jasper-hematite iron-formulation of probable rift-related, hydrothermal origin is found in the red mixtites of the northern Mackenzies as well as in the upper Sayunei and Upper Tindir rhythmites. In Mackenzie Mountains the stratigraphic sequence is interpreted to represent an earlier, local(?) glacial advance (Mount Berg) separated from a later, more extensive one (Shezal) by an interstadial period (Sayunei). Submarine hydrothermal activity and possible volcanism are inferred to have taken place at the same time as the iron-formation was deposited. On nearly every continent late Proterozoic glacial and glacio-marine sequences, commonly bearing iron-formation of hydrothermal origin, were deposited following major episodes of faulting and extensional tectonism. Diachroneity of these sequences over some 400 Ma suggests that widespread glaciations accompanied a global episode of continental disintegration. The development of new seaways in combination with extensive continental uplift resulted in global(?) climatic changes, including glaciation. Rift related hydrothermal activity resulted in local deposition of iron-formation. Hence, both glacial deposits and iron-formation in the late Proterozoic are manifestations of a major tectonic episode in the earth's crustal development.
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