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It's About to Get a Lot Less Salty – Comparison of a Fluvial Outcrop to Estuarine Outcrops Using UAV-Based Outcrop Modelling in the Lower Cretaceous McMurray Formation, NE Alberta, Canada


Despite the relatively widespread distribution of fluvial/estuarine lateral accretion point-bar deposits – expressed as Inclined Heterolithic Stratification (IHS) – throughout the lower Cretaceous McMurray Formation, the depositional architecture and lateral variability of these deposits remains a topic of debate. To address this, an Unmanned Aerial Vehicle and Structure-from-Motion photogrammetry are used to characterize the Crooked Rapids outcrop and compare it to other McMurray Formation outcrops that show markedly different sedimentary facies and depositional architectures.

At the Crooked Rapids outcrop, one kilometer of continuously exposed McMurray Formation strata provides depositional-strike and -dip views of the key sedimentological units. The outcrop is approximately 60 meters high, comprised of four sharply bound units: (1) a lowermost limestone basement of the Waterways Formation; (2) an inclined sandstone to mudstone unit that is almost entirely sandstone to the north and dominantly mudstone to the south; (3) a sharp-based, horizontally bedded unburrowed sandy mudstone that locally scours the unit below; and (4) horizontally bioturbated sandy mudstone. This study focuses Unit 2. Within Unit 2, the abundance of mudstone and occurrence of mud-dominated deposits interfingering with the sandstone increases southwards. The resulting IHS is interpreted to represent the transition from lower to middle point-bar. Notably, the sandstone is dominated by trough cross stratification that are generally oriented NNW, while master bedding surfaces within this unit dip southwards. The sandstone is not bioturbated and terrestrial organic detritus is abundant. Likewise, the mudstone beds are unburrowed and contain coalified debris. Facies and architectural data support an interpretation of fluvial point-bar with abandonment-phase mudstones to the south.

This outcrop greatly differs from other McMurray Formation outcrops to the east and northeast. In addition to the lack of bioturbation, there are no other outcrops where IHS is observed to interdigitate with channel-thalweg sands. Additionally, some geobodies displayed in McMurray Formation outcrops are interpreted as forward accreted compound dunes. In most cases, owing to the presence of bioturbation and local evidence for tidal modulation, the other locales are collectively interpreted as estuary deposits. As such, this study provides a fluvial baseline for the other outcrops.