Rocks of the Sekwi Formation and underlying siltstones of G.S.C. units 10A and 13 exposed in the western Mackenzie Mountains define a continental terrace wedge that evolved during the Early Cambrian on the northwestern flanks of the North American continent. Growth of the continental terrace wedge can be categorized by three phases of platform up-, out- and in-building. Phase 1--Continuous out- and up-building, and evolution from a ramp to a platform configuration. Growth culminated in middle and upper Nevadella zone rocks with prominent karst surfaces, abundant intraclast pebbles in oolite shoal deposits and a switch in composition of slope and fan deposits from carbonates to terrigenous clastics. Phase 2--Coincided with transgression of the shelf near the Nevadella and onnia-Olenellus zone boundary. Maximum platform out-building occurred at this time and climaxed with subaerial exposure of mid-Bonnia-Olenellus zone rock. Slope deposits changed from carbonate to terrigenous sedimentation. Phase 3--Renewed slow up- and in-building that led to eventual drowning of the platform by latest Bonnia-Olenellus zone time. Sedimentation patterns on this platform are reciprocal. Periods of sea-level highstand are characterized by abundant limestones, whereas periods of sea-level lowstand are marked by terrigenous influxes. These two stages of sea-level stand are highlighted in compositional differences of carbonate breccia deposits. During sea-level highstands, breccias contain predominantly ribbon and nodular limestone clasts. They reflect conditions of early lith fication of the sea floor and non-equilibrium compaction of these deposits with subsequent sliding and downslope transport. In contrast during sea-level lowstands breccias consist preferentially of shelf and shelf-margin derived clasts.

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