Migrations of Devonian brine up-section and the origin of calcite-cemented sand in the Lower Cretaceous McMurray Formation, Athabasca Oil Sands

Abstract

Calcite-cemented sand intervals are widely distributed in the lower interval of the McMurray Formation of the Lower Cretaceous Athabasca Oil Sands deposit, northeastern Alberta. Flows of Devonian brine were driven up-section upon mixture with meteoric-charged groundwater that came in contact with the Prairie Evaporite Formation only 200 m below. Voluminous migrations of carbonate-saturated Devonian formation water during lower McMurray Formation deposition account for the distribution of decimetre to multimetre thick calcite-cemented sand intervals and giant concretions within the syndepositional fill of the Bitumount Trough, a 100 km long salt dissolution–collapse structure that floored the northern Athabasca Oil Sands deposit. Precipitations of calcite cement concurrent with deposition of the lower McMurray Formation provide insight into the eventual disposition of the voluminous brine that would have resulted from the dissolution of as much as 130 m of Middle Devonian salt section below the Bitumount Trough in contrast to deep basin storage elsewhere. Breccia pipes developed in Middle–Upper Devonian carbonate beds flooring the Bitumount Trough provided migration pathways up-section for the carbonate-saturated Devonian brines. Giant concretions of calcite-cemented sand, 1–2 m across, developed in homogeneous sands proximal to collapse-breccia pipes that vent along the Trough floor, whereas brine flows directed along heterogeneous sands resulted in tabular calcite-cemented beds. Similar but volumetrically less significant Quaternary brine migrations diluted with glacial meltwater were discharged at the surface as saline springs along the modern river valleys, resulting in geochemical trends characterized by concentration of total dissolved solids but without associated precipitations of calcite cement.