Mudstone Clast Breccia in the Cretaceous Athabasca Oil Sands, Western Canada: Fluvial Debris-Flow Transitions into Traction Carpets

Abstract

AbstractThe origin and distribution of mudstone clast breccia bed deposits of the middle McMurray Formation responded to a range of current flow conditions associated with downchannel migrations by kilometers-long point bars, which accumulated as reservoirs of the Cretaceous Athabasca oil sands in northern Alberta. Fabrics consisting of dispersed clast entrainments within centimeters- to decimeters-thick stratified sand beds accumulated as grain flows and small-scale debris flows that filled shallow, flat-bottomed troughs during bed erosion on the point bar. Most low-density clast dispersals were sourced by eroded mudstone beds exposed along the bar bed surfaces. Clast-laden debris flows, 1–2 m thick, filled deeper troughs on the point bar, often as overlying flow-surge deposits, resulting in internal erosional surfaces within the breccia bed. Elsewhere, voluminous debris flows were sourced by undercut older channel walls that eroded mud-dominated counter point bar beds or exhumed lower McMurray floodplai...