SIGNIFICANCE OF QUARTZ-RICH DEPOSITS OF THE PALEOPROTEROZOIC GORDON LAKE AND BAR RIVER FORMATIONS, UPPER HURONIAN SUPERGROUP, CANADA

The up to 12 km thick, Paleoproterozoic Huronian Supergroup is composed of lower, rift-related and upper, passive margin-related successions that developed following break-up of the late Archean supercontinent Kenorland. Basal volcanic units, syn-deformational structures, fluvial deposits, and a limited lateral extent for the lower Huronian Supergroup support the rift basin interpretation. In contrast, significant lateral extent of the upper glaciogenic deposits and overlying quartz arenite units containing wave ripples, herringbone cross strata and dessication cracks, is more akin to deposition along a passive margin. Similar stratigraphic transitions from coarse alluvial continental sedimentation to quartz-dominated, shallow marine deposition are found on cratons worldwide. Although the majority of the Huronian formations have been studied extensively, relatively little is known about the uppermost Gordon Lake and Bar River formations. These formations are significant because i) they are inferred to have been deposited immediately following the rise of atmospheric oxygen, and ii) they represent the last period of deposition prior to the Great Stratigraphic Gap. Near Elliot Lake, Canada, the Gordon Lake Formation is composed mainly of chert, quartz-rich siltstone and fine grained sandstone, whereas the Bar River Formation contains quartz arenite, minor siltstone and quartz-pebble conglomerate. Sedimentary facies analysis reveals an abundance and great variety of mud cracks and ripples between successive beds, indicating periodic alternation between shallow marine and subaerial conditions. The Bar River Formation also contains abundant trough cross-beds at various scales that are locally comparable to fluvial structures. This is in marked contrast to Bar River quartz arenites 100 km to the east that mainly contain small-scale planar and trough cross-beds, and wave ripples consistent with an upper shoreface setting. The complexity of the structures and facies across such an expansive area indicates that assigning the deposits to only one depositional environment, as previous workers have attempted, may be short-sighted. The abundance of quartz in both formations is probably not a function of climate, but rather of source composition, recycling and depositional environment.