--> Distribution of Bitumen Sand Complexes in the Northern Athabasca Deposit: Spatio-Temporal Impact of Hypogene Salt Karst Collapse on the Overlying Tectono-Stratigraphic Architecture

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Distribution of Bitumen Sand Complexes in the Northern Athabasca Deposit: Spatio-Temporal Impact of Hypogene Salt Karst Collapse on the Overlying Tectono-Stratigraphic Architecture

Abstract

Linear dissolution trends in the Prairie Evaporite (Middle Devonian) halite beds controlled the tectono-stratigraphic architecture of the McMurray (Aptian) sand complexes in the northern area of the Athabasca Oil Sands Deposit, northeastern Alberta. Collinear alignments of multi-km long sand bars, oriented NW or NE, resulted from this overarching structural control by multi-km long cross-cutting dissolution patterns in the hypogene salt karst only 200 m below concurrent with McMurray deposition. The 50 km long V-shaped Bitumount Trough, the largest of these collapse troughs, filled with lower McMurray braided river sand bars oriented NW-SE and NE-SW. This lattice-like channel network that overprinted the reticulate karst paleo-topography consisting of a mosaic of differentially subsided Upper Devonian fault blocks. For example, a 25 km long linear chain of Upper Devonian fault blocks collapsed along the northern margin of the western Bitumount Trough concurrent with lower McMurray deposition. This collapse controlled the overly thick deposition of a 25 km long lower McMurray sand trend. Accommodation space expanded during the deposition of the middle McMurray fluvio-estuarine deposits across the structurally unstable substrate, creating an estuarine funnel. During the middle interval, salt removal northward of the Bitumount Trough resulted in a 200 km2 central collapse depression at the V-join between the western and eastern Trough segments. Structural terraces along the southern rim of the central collapse developed as the Upper Devonian-lower McMurray fault block that staircase stepped down northward into the central collapse. The salt removal patterns controlled the distribution and alignment of large middle-upper McMurray sand complexes that aligned along tidal mega-channels at aggradation sites centered above the fault block staircase step downs. A few of the fluvio-tidal sand bars realigned NE-SW along the terraces. Westward of the central collapse, collinearly aligned tidal bar sands, oriented NW-SE, accumulated along multiple sub-parallel tidal channels, each 10-30 km long. Continued salt removal underlying the northern margin of the western Bitumount Trough during the upper interval resulted in a second 25 km long sand trend parallel to the lower interval trend. These salt collapse-induced giant sand complexes trapped oil migrations into the area that were subsequently biodegraded into bitumen.