Facies distribution and stratigraphic architecture of the Lower Cretaceous McMurray Formation, Lewis Property, northeastern Alberta

Abstract

Abstract Within the Lewis study area (Townships 91–92, and Ranges 6-8W4), McMurray Formation strata comprise four facies associations that form a depositional continuum of braided-fluvial (FA1), tidally-influenced braided- or low-accommodation meandering-fluvial and meandering-tidal channel-fills (FA2), associated overbank (FA3), and open-estuarine tidal flat deposits (FA4). The primary reservoir occurs in transgressive FA2 deposits. Lower-FA2 channels incise older water-saturated FA1 sand and consist of medium to locally-coarse, bitumen-saturated sand with rare to locally common pin-stripe laminated and/or 1-5 cm thick mud beds. Channels were initially confined by steep valley walls formed along the sub-Cretaceous unconformity. As a consequence, coeval interchannel sediment (FA3) was cannibalized by lateral channel migration and occurs only as common mud-clasts. The net result was the accumulation of a sand-rich, sheet-like deposit with locally-preserved fine-grained interchannel deposits, suggesting a high rate of lateral versus vertical accretion. With the filling and elimination of the irregular paleotopography along the unconformity, upper-FA2 channels became unconfined and formed thick, areally extensive inclined heterolithic stratification (IHS) deposits. Similarly, coeval interchannel deposits are more widely-distributed and thickly preserved compared to underlying strata. The position of upper-FA2 channels was indirectly influenced by paleotopography along the unconformity because IHS strata are thickest and sandiest along trends that overlie earlier channel systems, suggesting that syn-depositional salt dissolution controlled channel position and infilling history. Stratigraphically-upward there is a gradational change to tidal-flat deposits (FA4), suggesting abandonment of the main north-south channel system and a condition of coarse-grained bedload sediment starvation. Although most point-bar deposits within the Lewis study area consist of 10-30 m thick vertically-stacked successions of incomplete IHS deposits, several complete successions from channel base to overbank deposits are preserved. The geometric similarity between meandering-fluvial channels of different scales, climatic regimes, and substrate conditions suggest that the geometry of meanders is governed by mechanical principles. Although the applicability of these relationships to meandering-tidal channels is equivocal, using these formulas on IHS deposits of the McMurray Formation can provide a first order approximation of channel width (W=1.5h/tanβ), depth (0.595h), meander wavelength (λ=10.9w1.01), and point-bar length (λ). For two examples of complete point-bar deposits, channel widths were estimated to be at 148 m and 336 m; channel depth 12.6 m and 18.7 m; meander wavelength 1718 m and 3930 m; and point-bar length 859 m and 1965 m.