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Applications of Structure-From-Motion Photogrammetry for Interpreting Inclined Heterolithic Strata of the Lower Cretaceous McMurray Formation, Steepbank River, NE Alberta, Canada

Abstract

Examples of Inclined Heterolithic Stratification (IHS) that locally display complex cross-cutting relationships are present in outcrops of the lower Cretaceous McMurray Formation in Northeastern Alberta. The McMurray Formation is the primary host of the Athabasca Oil Sands, and is informally subdivided into the lower, middle, and upper members. This study focuses on middle and upper McMurray Formation sections that are exposed along the Steepbank River. The outcrops have been previously interpreted to represent fluvial or estuary point-bar lateral accretion, with the upper section comprising vertically accreted marine deposits. The architecture and orientation of laterally accreted point-bar deposits - expressed as IHS - is difficult to assess in outcrop. This poster builds on earlier efforts by using an Unmanned Aerial Vehicle and structure-from-motion (SfM) photogrammetry. SfM photogrammetry is an efficient, inexpensive technique to characterize medium to large-scale bedding (e.g. IHS bedding), particularly when outcrop exposures would otherwise need to be abseiled to obtain sedimentological and structural data. The resulting georeferenced 3-D models showed several meters of basally situated, oil-saturated cross-bedded sandstone that is sharply overlain by finer sandstone and complex IHS. Four horizontal parasequences cap the outcrop models. Bedding measurements taken from the models showed that the IHS units can, in some locations be differentiated into different channel associations. A total of three different channel associations are identified in the dataset. Importantly, the upper parasequences are shown to form a broad and open antiform that cannot be discerned from 2-D outcrop mosaics alone. As such, the models must be flattened on the upper parasequences to correct the orientation data of the IHS. When this is done, some channel associations become more evident. We found this methodology to be both expedient and cost-effective. The photogrammetric model has several advantages: 1) bedding surfaces may be traced laterally instead of acquiring orientations from a section; 2) outcrop faces are locally flat, so numerical analyses of outcrop face - bedding intersections are required to assess true bedding dip (as with LiDAR datasets); and 3) the studied IHS gently dip, often less than fifteen degrees, so measurements based on long bedding-plane exposures are vastly superior to measurements taken with a compass on the outcrop.