Sedimentology and stratigraphy of a continental terrace wedge: the Lower Cambrian Sekwi and June Lake formations (Godlin River Group), Mackenzie Mountains, Northwest Territories, Canada

Abstract

Lower Cambrian sedimentary rocks exposed in the Western Mackenzie Mountains are present over an area of approximately 50000 sq. km. They represent a continental terrace wedge that evolved during the Lower Cambrian on the northwestern flanks of the North American continent. The continental terrace wedge belongs to the Mackenzie Platform and Selwyn Basin and is located west of the Mackenzie Arch. Strata of the continental terrace wedge comprise two formations: June Lake (new) and Sekwi (redefined). The Lower Cambrian rocks can be further described in terms of five major depositional regimes: (1) Tidal and supratidal deposits - muddy and peloidal dolostones and limestones with cryptalgal fabrics, dessication cracks, karst, Runzel marks, intraclasts, fenestral textures, ripple marks, crossbeds, fossil coquinas and bioturbation; (2) Intertida, shallow subtidal, slope and rise deposits - micritic, peloidal and bioclastic limestones and dolostones with shaly interbeds. The deposits are characterized by bioturbation, early cementation, turbidites and breccia beds; (3) Oolite shoal deposits- oolitic, oncolitic, bioclastic packstones and grainstones which can be further subdivided into two Subfacies I and II. Subfacies I is characterized by plane parallel beds, is typically developed at the base of the oolite facies, and is interbedded with bioturbated horizons and bioclastic archeocyathid-Renalcis sp. boundstones. Interpreted as a product of relatively deep or low energy conditions of deposition, Subfacies I is generally succeeded by high angle crossbedded lithologies of Subfacies II which reflect higher energy conditions and constant agitation; (4) Submarine slope and fan deposits - dark brown shaly siltstones with fine grained sandstone interbeds. These deposits are characterized by plane parallel, wavy and ripple cross-laminations, load casts, lenticular bedding and graded layers. Breccia beds with shelf-derived clasts and isolated shelf clasts may also occur interbedded with these deposits; (5) Nearshore and coastal plain deposits - sandstones, siltstones and shales; sandstones characteristically possess trough and planar cross-stratification, and plane-parallel stratification. In addition heterolithic deposits and abundant traces of organic activity are preserved with these rocks. Growth of the continental terrace wedge is characterized by three prominent phases of platform up-, out- and in-buildings. Phase 1: Continuous out - and up-building, and evolution from a ramp to a platform configuration follows transgression of the Backbone Ranges Formation. Growth culminates 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 rise deposits from carbonates to terrigenous elastics. Phase 2: Coincides with transgression of the shelf near the Nevadella and Bonnia~Olenellus zones boundaries. Maximum platform out-building occurs at this time and climaxes with subaerial exposure of mid-Bonnia-Olenellus zone rocks. Again, slope deposits record this event as they change from carbonate to terrigenous sedimentation. Phase 3: Slow up- and in-building of the platform follows Phase 2 and leads to eventual drowning of the platform by latest Bonnia-Olenellus zone time. Striking carbonate breccia beds characterize slope and rise deposits of the continental terrace wedge. These beds are characterized by a basal clast interval with pebble to boulder sized clasts and a capping turbidite with A, B, C intervals of the Bouma sequence. The process of breccia bed formation is initiated by sliding, slumping and failure of deposits on or near a slope. Material disaggregates, remolds and flows as a liquefied slurry. Mixing with the overlying water mass produces a turbidity current on top of the flow. Furthermore, each basal breccia interval is characterized by one of the following clast fabrics: disorganized, normally graded, or inversely to normally graded. Textural characteristics of these deposits are a function of transport distance. With increasing transport there is a downslope transition from slumped deposits to disorganized flow deposits to normal and inverse-normal graded deposits. Breccia beds are not unique to the Lower Cambrian sequence of the Mackenzie Mountains and are instead widespread and commonly occurring slope and basin deposits around the world. Significant, however, is that breccia beds in Southeast Missouri, United States and the McArthur River District, Australia are associated with lead-zinc deposits. In addition, they are associated in Poza Rica and the Reforma Trend of Mexico with large oil accumulations.