The Lower Cambrian to Lower Ordovician Carbonate Platform and Shelf Margin, Canadian Arctic Islands

Abstract

Five stratigraphic successions are recognized in the Sauk megasequence in the Canadian Arctic Islands. The Lower Cambrian succession was deposited on a distally steepened carbonate ramp that overlaid thick Lower Cambrian siliciclastics. The second Middle Cambrian succession is composed of oolitic grainstone, microbial boundstone, lime mudstone, and mixed carbonate-siliciclastic facies assemblages that developed on a platform. The second and third successions are separated by an unconformity that spanned most of the Steptoean. The third succession includes mixed carbonate-siliciclastic strata and spans the Sunwaptan Prosaukia Biozone to the Cordylodus proavus Biozone. This succession was terminated by the Cambrian–Ordovician unconformity. The shelf had a ramplike configuration during this time. The fourth succession starts above the Cambrian-Ordovician unconformity with a widespread shelf sandstone that spans the C. proavus to C. lindstromi Zones. This was followed by a rapid deepening in the earliest Ordovician (Iopetognathus fluctivagus Zone) marked by the deposition of open-marine carbonates. A progressive shallowing culminated in evaporite units in the Stairsian. A marked change in basin architecture occured during this fourth succession. Distinctive shelf-margin units appeared consisting of fenestral mudstone, shoal deposits, and common karst breccias. The shelf margin during this interval was very steep, and carbonate was not transported into the deep water. The platform also changed configuration during this time, with the development of an intraplatformal basin. Evaporites accumulated in this silled basin. Strata in the intraplatformal basin are thicker than those at the shelf margin. The fifth succession (Tulean–Blackhillsian) consists of shallow subtidal carbonates.The first two sequences in the Arctic Islands correspond closely to Sauk I and II elsewhere in Laurentia. Strata in the Arctic that are equivalent to the standard Sauk III supersequence contain three unconformity-bounded stratigraphic assemblages. This reflects local tectonic conditions that resulted from the change from a passive-margin setting in the Early Cambrian–earliest Ordovician to convergence later in the Early Ordovician. The downgoing slab, interpreted to be dipping below Laurentia, affected carbonate sedimentation along northwestern Laurentia during this time.