STRATIGRAPHIC AND DEPOSITIONAL FRAMEWORK OF AN UPPER TRIASSIC CARBONATE RAMP SUCCESSION AT WILLISTON LAKE, BRITISH COLUMBIA, CANADA: IMPLICATIONS FOR SELECTION OF THE BASE-NORIAN GSSP

The Baldonnel, Ludington & Pardonet formations on Williston Lake, northeastern British Columbia, represent a complex, mixed siliciclastic-carbonate succession deposited in a distally-steepened, westward-dipping, carbonate-dominated ramp setting on the northwestern margin of Pangaea. These strata comprise several transgressive-regressive cycles within a single large-scale, overall deepening upwards trend that spanned the entire Carnian & Norian in the study area. These units are exceptionally fossiliferous. Most significantly conodont, ammonoid & bivalve collections from these strata have provided a template & framework for global biostratigraphic correlation. In the eastern part of the study area shallow marine (medial to proximal ramp) strata of the Baldonnel Formation are conformably but sharply overlain by deeper marine (distal ramp) strata of the Pardonet Formation. The Carnian-Norian (C-N) boundary occurs near, but not coincident with, the lithostratigraphic boundary. In the western part of the study area the C-N succession is represented by the Ludington & Pardonet formations. The entire succession is interpreted to reflect deposition in a deep marine (distal ramp) setting. A north-south-oriented hingeline separates the shallow marine Baldonnel Formation in the east from the deep marine Ludington Formation in the west. Regional isopach mapping & stratigraphic correlations show that chronologically constrained depositional units thicken considerably across this line supporting the interpretation that it represents the presence of a growth fault-bound western sub-basin. The presence of moderate-scale, olistolith-bearing debris flow deposits (debrites) as well as small-scale slumps/slides in the Ludington Formation at localities adjacent to the inferred scarp margin support the interpretation that this scarp was active during the Upper Triassic. Black Bear Ridge, a candidate locality for the base-Norian Global Boundary Stratotype Section and Point (GSSP) occurs on the western side of the hingeline within the depositional sub-basin. Evidence for continuous & relatively rapid sedimentation through the C-N boundary interval at this locality as well as minimal alteration by tectonic disturbances & lack of metamorphism make this locality an excellent GSSP candidate.