Overview of Lower Cambrian Mixed Carbonate-siliciclastic Deposition along the Western Laurentian Passive Margin
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Copyright 2012 by The American Association of Petroleum Geologists. The Lower Cambrian (Waucoban Series; Fallotaspis Olenellus Biozone) carbonate-siliciclastic units of western Laurentia outcrop from Sonora, Mexico, to western Nevada, and from northeastern Washington to east-central Alaska. The allochthonous Cassiar terrane in northwestern British Columbia was originally deposited between these two segments, forming a widespread large carbonate platform along this margin during the initial Paleozoic flooding (basal Sauk megasequence) of North America. The Lower Cambrian carbonate-siliciclastic units of western Laurentia commonly are subdivided into two second-order or composite third-order depositional sequences. The lowstand systems tracts to these sequences commonly are marine siliciclastics that grade upward into transgressive systems tracts composed of interbedded shaly carbonates or carbonate-rich shales that grade upward into carbonate-dominated highstand systems tracts. The carbonates commonly record deposition on a gently sloping ramp; however, areas of localized syndepositional extensional faulting created locally faulted carbonate margins. Large allochthonous blocks of shallow-water carbonates and siliciclastics were deposited basinward of these active fault zones. The facies of the western Laurentian Lower Cambrian carbonate ramps, arranged from land toward the basin, are fluvial nearshore siliciclastics, carbonate tidal flats, mixed carbonate-siliciclastic lagoons, high-energy ooid grainstone shoals containing or surrounded by algal-archaeocyathan mounds, downslope nodular facies, and interbedded calcisiltite and shale deposited below a fair-weather wave base. The algal-archaeocyathan mounds constructed small isolated biostromes ((90 m [(295 ft] thick), not large continuous reefs. The continuity of the ooid grainstone shoals along the western Laurentian margin indicates that they formed a barrier separating restricted facies to the east from open-marine facies to the west that stretched from present-day east-central Alaska to northwestern Mexico.
