LOWER PALEOZOIC EVOLUTION OF THE CANADIAN CORDILLERAN MARGIN OF LAURENTIA: CONSTRAINTS FROM REGIONAL STRATIGRAPHIC AND BIOSTRATIGRAPHIC CORRELATIONS

The ancient Laurentian margin rifted in the latest Neoproterozoic to early Cambrian and experienced repeated phases of extension with minor faulting, volcanism, and platform flooding. Over the last decade, the stratigraphic and conodont biostratigraphic information from four platform-to-basin transects across the ancient Laurentian margin have greatly advanced our knowledge of the evolution of the Cordilleran margin during the early Paleozoic. In total, 26 stratigraphic sections were studied, from which over 25 km of strata were measured and described and over 1 200 conodont samples were collected (averaging 2.5-4 kg each) that yielded over 100 000 conodont elements. The conodont microfossils include key zonal species used for regional correlation of uppermost Cambrian to Middle Devonian strata along the Cordillera. Transect one lies in the southern Rocky Mountains of southwestern Alberta and southeastern British Columbia, from the Bow Platform to White River Trough. Transects two and three lie in the northern Rocky Mountains, northeastern British Columbia and span the MacDonald Platform to Kechika Trough and Ospika Embayment. Autochthonous strata of these transects is correlated to strata of the fourth transect across the parauthochthonous Cassiar Terrane. The stratigraphic framework records an evolutionary history of a margin that is more complex than a simple passive margin. The detailed conodont biostratigraphy temporally constrains at least two periods of renewed extension along the margin, in the latest Cambrian and late Early Ordovician. A well-defined ancient shelfbreak persisted from the late Early Ordovician to Devonian, possibly due to tectonic steepening, and the succession contains several intervals of slope debris breccia deposits, distal turbidite flows and associated alkalic volcanics. Siliciclastics in the succession were sourced by a reactivation of tectonic highs such as the Peace River Arch. Prominent hiatuses punctuate the succession, including unconformities of latest Ordovician-early Silurian, early-middle Silurian and sub-Devonian age.