Fluvial character and architecture of an outcrop using sedimentology combined with UAV-based modeling, Cretaceous McMurray Formation, NE Alberta, Canada

Abstract

It is widely accepted that most occurrences of inclined heterolithic stratification (IHS) in the rock record form by laterally accreting point bars in freshwater fluvial, tidally influenced fluvial, or tidally dominated estuary channels. Despite the widespread distribution of IHS in the subsurface and outcropping strata of the lower Cretaceous McMurray Formation, the large-scale depositional architecture and lateral facies variability of these deposits remains controversial. The relatively limited lateral extent of many of the outcrops is a challenge, particularly when point-bar deposits on the scale of hundreds of meters to kilometers are interpreted in outcrops spanning anywhere from 100 to 300 meters laterally. This has in turn led researchers to leverage other datasets such as 3-D seismic to analyze the large-scale depositional architecture of the IHS, leading to two main interpretations for the IHS in the McMurray Formation: 1) a fluvially dominated environment owing to geomorphological features comparable to those in large modern fluvial systems, or 2) an estuarine environment owing to the presence of trace fossils characteristic of marine-derived faunal colonization in brackish-water settings and strong evidence of tidal modulation. The purpose of this study is to investigate the sedimentology and depositional architecture of IHS in a unique, kilometer-wide outcrop exposure of McMurray Formation strata and compare it to IHS observed at other McMurray Formation outcrops previously interpreted as estuarine channels. This is achieved by combining traditional field-based methods with Unmanned Aerial Vehicle-based outcrop modeling to create a 3-D outcrop model to visualize and analyze large point-bar geobodies deposited in a channel upwards of 25 meters deep and 750 meters wide exposed in outcrop at Crooked Rapids of the Athabasca River, west of the City of Fort McMurray. Importantly, this methodology uses bed orientation trends, paleocurrent data, and sedimentological observations to identify and map architectural elements, which constitute an eastward-accreting point bar crosscut by a southwestward-accreting counter point bar in the outcrop. The results strongly suggest that the IHS at Crooked Rapids was deposited in a freshwater fluvial environment. When compared to IHS deposited in estuarine depositional environments, fluvial IHS is driven by seasonal river discharge as opposed to the interplay between river discharge and the extent of the tidal prism. Therefore, fluvial IHS is: 1) dominantly sandstone with very minor waning-flow siltstone interbeds resulting from erosion by the succeeding freshet phase, and 2) completely devoid of bioturbation until flat-lying bar top or overbank strata is encountered. Using 3-D outcrop modeling to supplement sedimentological and ichnological observations strengthens the interpretation of complex fluvial geobodies and increases the overall understanding of the large-scale depositional architecture of point bars across the tidal–fluvial transition zone.