EARLY CAMBRIAN EXTENSION IN THE SELWYN BASIN, CANADA AND ELSEWHERE IN THE WESTERN LAURENTIAN CORDILLERA

Cryogenian-Early Cambrian volcanic rocks overlain unconformably by siliciclastic rocks are interpreted to indicate that continental separation of western Laurentia from continents presently unknown occurred in the Early Cambrian. The initial Paleozoic transgression (Sauk Sequence) onto the western margin of Laurentia is marked by siliciclastic rocks deposited in fluvial to deep marine settings. The conformably overlying Lower Cambrian carbonate-siliciclastic units record a relatively intact carbonate platform that developed during thermally-driven regional subsidence that extended from Sonora, Mexico to western Nevada and from northeastern Washington to southeastern Alaska.

The Lower Cambrian carbonate-siliciclastic units of western Laurentia commonly are subdivided into two 2^nd-order or composite 3^rd-order depositional sequences that are similar to grand cycles (sensu Aitken). The lowstand systems tracts (LST) to these sequences commonly are marine siliciclastic rocks that grade upward into transgressive systems tracts (TST) composed of interbedded shaly carbonate or carbonate-rich shale, which in turn grade upward into a carbonate-dominated highstand systems tract (HST).

Facies of the western Laurentian Lower Cambrian strata, arranged from land toward the basin were: fluvial-nearshore siliciclastics, carbonate tidal flats, mixed carbonate-siliciclastic lagoons, high-energy ooid grainstone shoals containing or surrounded by algal-archeocyathan mounds forming a barrier between restricted facies to the east and open marine facies to the west, downslope nodular facies and interbedded calcisiltite and shale deposited below fair-weather wave base. The carbonates were deposited on a gently sloping ramp, locally interrupted by scarps reflecting syndepositional extensional faulting.

Early Cambrian syn-depositional faulting in the Selwyn Basin corroborates Early-Middle Cambrian extension in Canada and Middle Cambrian extension in the Pacific Northwest and Nevada. Early-Middle Cambrian deep-water facies in the Selwyn Basin, the Northern Rocky Mountains, and western Great Basin all occur on lower plate extensional margins suggesting this setting may preferentially allow preservation of such deposits.