Abstract

The Sibley Group is a Neohelikian (1339 ± 33 Ma, RbSr, 87Rb = 1.42 × 10−11 year−1) red bed sequence located in an elongate basin extending northward for 150 km from Nipigon, Ontario. The lowest unit, the Pass Lake Formation (new name), is 50 m thick and consists of quartz arenite, deposited primarily in a shallow, quiet lacustrine environment. It is overlain by the Rossport Formation (new name), a unit of 135 m thickness consisting of a lower arenaceous red dolomite member, central chert–carbonate and stromatolite member, and an upper argillaceous red dolomite member. The Rossport Formation was deposited in a shallow, highly saline environment, in a basin of fluctuating size. The Kama Hill Formation (new name) is 50 m thick, and consists of purple shale composed of smectite, authigenic microcline, and quartz. It was deposited in a periodically dry mud flat, and is characterized by desiccation cracks, evaporite casts, and mud-chip microbreccias.Both the Rossport and Kama Hill Formations have an increased arenite content near the basin margins. Breccias cut the Rossport Formation and consist of stoped blocks of Rossport and Kama Hill rocks; these breccias are cut by sandstone dykes. The Sibley Group is situated in, and was possibly deposited in, a "failed arm" which radiates from a paleo-plume in the Keweenawan rift valley.

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