Ghost-rock karstification of Devonian limestone flooring the Athabasca Oil Sands in western Canada

Abstract

This study interprets a new style of ghost-rock karstification at a site in northeast Alberta, western Canada. The karstic area of the Upper Devonian paleotopography was localized at the western end of the Bitumount Trough, a 50 km-long collapse structure that resulted from removal of a 120 m thick interval of salt beds of the Middle Devonian Prairie Evaporite only 200 m below. Ghost-rock karstification developed in massive light grey limestone beds that alternate with chlorite-rich argillaceous intervals, of the Moberly Member, Devonian Waterways Formation. These weathered rock profiles expanded latterly and were deepened by slowly moving hydraulic gradient-driven phreatic groundwater descending along clusters of closely spaced 2–10 cm-wide joints that cross-cut the fracture-shattered limestone beds. The uppermost beds of greyish green chlorite-rich argillaceous limestone were decalcified as the flows along the joint clusters carried insoluble residues consisting of quartz silt mixed with illite-chlorite clays into the substrate. These chlorite-rich insoluble residues accumulated as replacement fabrics within decalcified porous zones of the altered limestone in the substrate. As the joint opening plugged with insoluble residues within the deepened decalcification weathered rock intervals, the alterite zones expanded laterally to form vertical pod-shaped structures. These 5–20 m in diameter and 10–20 m-deep pseudo-sinkhole zones consisting of alterite retained ghost-rock traces of the original limestone strata. This area was subsequently covered by the Lower Cretaceous Athabasca Oil Sands.