Derrick Midwinter

The Bjorne Formation is a predominantly sandstone unit in the Sverdrup Basin in the northern part of the Canadian Arctic Islands. The Lower Triassic succession of the Bjorne Formation and coeval Blind Fiord Formation nears 2,000 m thickness in two depocentres in the basin, and 1,000 m along the margins, a significant portion of the 13,000 m of Carboniferous to Tertiary basin fill. The Early Triassic represents a period of active subsidence and major infilling of the Sverdrup Basin. The study location is on Fosheim Peninsula, Ellesmere Island with the outcrop representing the eastern margin of the basin during three pulses of Early Triassic sedimentation. The sandstone members, Cape Butler, Pell Point and Cape O'Brien, are separated by thin marine shale units, which represent basin-wide transgression.

Each member comprises several hundred metres of stacked sandstones with some members having red siltstone interbeds. While each member comprises of successions composed primarily of upper flow regime, to the transition to lower flow regime beds, the lithofacies proportions are not identical. The Cape Butler Member is composed principally of upper flow regime bedforms with scarce dune stratification, whereas the Pell Point and Cape O'Brien members are predominantly cross-stratified with parallel stratification. Siltstone beds have climbing ripples, shrinkage cracks, and bioturbation. The architecture of the sandstone units represents sand sheets as they appear laterally extensive and multistory with little suggestion of major channel forms. The presence of trace fossils provides evidence of sub-aqueous conditions, although the degree of marine influence is not clear. The thick fluvial sandstones and marine shale tongues suggest the braided fluvial system passed directly basinward into a marine-influenced environment. These associations of sedimentary features suggest episodic and rapid sedimentation within the spectrum of a braid-delta.

Deposition took place immediately following the Permo/Triassic boundary. Extinction of many vegetational taxa at that boundary could have influenced fluvial styles because vegetation helps stabilize banks. The extinction event and the proximity to the Siberian Traps affected the depositional regime and sediment flux across the boundary.

Petrographic analysis of the sandstone identifies it as a quartzarenite to sublitharenite with early pore-filling calcite cement and late grain-replacive, pore-filling dolomite-ankerite rhombohedra. The calcite cement occludes primary porosity, and the rhombohedra occlude secondary porosity created by grain dissolution. Analysis of the provenance is indicative of a craton interior and recycled orogen source. Based on thermal maturation within the oil window, thickness of the sandstones, high permeability of some core samples, shale seals, and possible organic-rich source rocks, the Bjorne Formation is a prospective hydrocarbon reservoir.

Keywords: Bjorne Formation, Sverdrup Basin, Early Triassic, Sandstone, Braid-Delta, Ephemeral, Late Permian Extinction, Upper Flow Regime, Carbonate Cementation

Pages: 89
Supervisor: Martin Gibling