Abstract

The 950 m thick Sibley Group is a relatively flat-lying assemblage of siliciclastic and chemical sedimentary rocks exposed from the northwest shore of Lake Superior to the Lake Nipigon region of Ontario. Remnants of the Sibley Group occur in an ovoid area that sagged at ∼1.5 Ga, creating accommodation space for braided fluvial sediments, derived as either first or multicycle detritus from the Trans-Hudson Orogen. This was followed by a transgressive episode and deposition of lacustrine siliciclastics and evaporites. An influx of sediment from the south occurred immediately prior to final contraction of the lacustrine system and deposition of strandline, stromatolite-bearing carbonates. The change in paleoslope was accompanied by development of a north–south-oriented half-graben. Overlying subaerial deposits represent deposition on a sabkha or saline mud flat. This assemblage is abruptly succeeded upward by flooding of the basin and major deltaic progradation and capped by a delta-top fluvial system with extensive preservation of floodplain deposits. The majority of the deltaic sediment was derived from Proterozoic sources to the south. An unconformity separates this assemblage from a thick succession of sandstone deposited as an aeolian dune field, with detritus probably coming from as far as the New Quebec Orogen to Baltica region. The geochemistry of medium-grained sandstone denotes that sediment became more mature and quartz-rich upsection and that the source areas evolved to more felsic and less alkalic compositions. Paleomagnetically correlated units in the Belt Supergroup, Apache Group, and Troy Quartzite in western North America indicate that the broad climatic fluctuations recorded in the Sibley Group may represent continent-wide events.

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