Substantial exposures of a thick basal sedimentary sequence, the Aston Formation of Somerset Island, occur on eastern Prince of Wales Island and incorporate at least 2600 ft (793 m) of arkose, subarkose, and orthoquartzite, with minor dolostone, chert, and mudstone. The detrital sediments, derived from contemporary uplands on the site of the present Boothia Arch, accumulated in shallow marine to lagoonal environments. A thin, extensive, and distinctive stromatolitic dolostone in the two major exposures of the formation has an environmental and possibly a regional time–stratigraphic significance. Gabbro dikes and sills, some of them exceeding 1000 ft (305 m) in thickness, intrude the Aston Formation.

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