麻豆传媒

The Late Archean Burwash Formation is a supracrustal package that overlies volcanics of the Yellowknife Supergroup in the southern Slave Province. The Formation has been deformed by three generations of folds, and has been metamorphosed to at least greenschist facies. The Formation comprises predominantly turbidites, with minor sandy debris flows that are often associated with felsic volcanic tuff layers.

The base of the formation is exposed along contacts with the underlying volcanics and also along the southern margin of the Sleepy Dragon basement massif. Here, the Raquette Lake Formation comprises a coarse clastic unit that overlies regolith of the basement massif. Occasional carbonate-rich fissures have been observed that extend into the basal clastic conglomerate, and less frequently into the underlying regolith. This suggests that the unit was deposited during extensional tectonism. Overlying the unit are black, siliceous shales, interbedded with fine grained felsic tuffs. These are overlain by the basal turbiditic sandstones of the Burwash Formation. This fine-grained package likely records a transgression into a deeper water environment.

Zircon grains from a felsic tuff layer low in the stratigraphy of the Burwash Formation were dated at ca. 2663.5 Ma providing the earliest estimate turbiditic sedimentation along the Sleepy Dragon Complex. Higher in the stratigraphy, an age estimate of 2661.6+2.32/-2.15 Ma was obtained. Used in conjunction with a previously reported age for a lower tuff, a sedimentation rate of 0.6m to 1.6 m/1000 years is estimated. This sedimentation rate estimate is comparable to other modern turbidite systems in both active and passive tectonic settings.

Further from the basement and higher in the stratigraphy, medium to coarse grained, sand filled channels are partially exposed, and represent deposition by high concentration turbidity currents. Thinner bedded, mud-dominated intervals that are compacted and often poorly exposed, represent levees to these channels. In other locations, laterally continuous sandstone beds adjacent to and overlying a mud-dominated interval represent sheet sandstones and levee systems. This is analogous to high amplitude reflection packages (HARPS) from some modern fan systems, where avulsion of a channel results in sheet sandstone deposition on levee banks and adjacent areas. These observations suggest that Archean deep-water turbidite systems developed in similar fashion to modern day turbidite systems.

The early evolution of the Burwash Formation basin comprised an early extensional tectonic setting that resulted in deposition of the coarse clastic unit, and regolithic development along the basement massif margin. These observations are consistent with development of a back-arc basin on thinned or stretched continental crust, similar to the southern Japan Sea.

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Supervisor: John Waldron