Breccia pipe and sinkhole linked fluidized beds and debris flows in the Athabasca Oil Sands: dynamics of evaporite karst collapse-induced fault block collisions

Abstract The Middle Devonian hypogene evaporite dissolution, karst collapse resulted in the fragmentation of the Upper Devonian strata into a mosaic of differentially subsided fault blocks underlying the northern area of the Athabasca Oil Sands Deposit. Regional salt dissolution collapse developed structural troughs up to 50 km long on the sub-Cretaceous unconformity. The structural depressions formed above collapsed collinearly aligned, brine-filled, evaporite dissolution chimneys extending up to 100 m high within the Prairie Evaporite salt beds. Individual and coalesced arrays of dissolution chimneys developed along fault lineament dissolution trends that dissected the 10 km wide Prairie Evaporite salt scarp. 3D seismic images of chimney collapse structures in the Middle Devonian evaporite basin depocenter across central Saskatchewan are analogues used to interpret collapse structures evident in the Upper Devonian succession and the overlying Lower Cretaceous McMurray Formation. The Devonian-Cretaceous fault bound blocks located above the dissolution chimneys were gravity driven collapse structures. These structures included oblique rotational trajectories during the descents towards and into underlying dissolution voids. Breccia pipes were impinged along the deeper reaches of the inter-block fault planes as adjacent blocks obliquely rotated apart and towards each other, resulting in zones with compressional and extensional bed deformations during the variable but often rapid rates of vertical descent. The breccia pipes pass upward, along fault planes, into oblique shear zones with twisted beds and zigzag suture welding between adjacent blocks. Upper reaches of the collision zone, between adjacent blocks, were dominated by compressional deformations that resulted in sinkhole development bound on one side by the fault plane. Continued bed compression deformed and fragmented many sinkhole structures. Some of these collapse-induced fault block collisions suggest cataclysmic events, and caused seismicity triggered fluidization in the upper intervals of breccia pipes with mobilization as debris flows that spread across adjacent fault block surfaces.