A GLIMPSE OF THE CANADIAN CORDILLERA DURING THE “BORING BILLION”: STRESS AND STRATA DURING EVOLUTION OF THE LATE PALEOPROTEROZOIC (STRATHERIAN) MUSKWA ASSEMBLAGE, NORTHERN BRITISH COLUMBIA

The configuration of the western margin of ancestral North America during the late Paleoproterozoic is problematic. Strata within the Muskwa Assemblage is interpreted by some to represent an early rift-drift sequence, developed within the Columbia Supercontinent; others suggest that early rifting, perhaps represented by the Wernecke Group, was followed by thermal relaxation and sag-basin development, and that the Muskwa Assemblage was developed entirely on continental crust, with no major continental separation. The presence of six (exposed) to fifteen (inferred) kilometres of predominantly Paleoproterozoic strata in the Muskwa Ranges is suggestive of extensional conditions, although there is no clear evidence if this is rift- or sag-related, or might involve transtensional strike-slip mechanisms. All parts of the stratigraphic assemblage have indicators of extension in the form of slumps, incised channels and seismites.

The base of the Muskwa Assemblage may be conformable with unexposed underlying stata interpreted as part of the Wernecke Supergroup in seismic profiles. The lower exposed parts of the sequence appear to have been deposited in shelf and distally steepened ramp settings, which appear to have prograded and deepened to the west. Structures within the carbonate and siliciclastic sand dominated units indicate water depths from shallow subtidal to slightly below storm wave-base while mudstone-dominated sequences may have been deposited in water depths in excess of 120 m. The sudden transition from shallow-shelf sandstones in the Tuchodi Formation, to mudstones and wackes in the overlying Aida and Gataga formations suggests a transition to significantly deeper water depths in early Mesoproterozoic times.

The depositional style of exposed strata is consistent with the basin forming by intracontinental rifting or transtensional wrenching; processes that may have been generated by long-range (1000–5000 km) transmission of stresses from plate-margin collisional areas. There is no evidence for direct influence of NE–SW directed transform faults during deposition of the Muskwa Assemblage. Inferred NW–SE paleo-shorelines roughly parallel the strike of the modern Cordillera and are consistent with both extensional and transtensional models.