OUTCROP STUDIES OF THE LOWER MCMURRAY IN THE NORTHERN PART OF THE ATHABASCA OILSANDS, ALBERTA, CANADA, EVIDENCE FOR BRACKISH-WATER AND TIDAL INFLUENCE

The Cretaceous McMurray Formation located in Northeastern Alberta, Canada, is informally subdivided into the lower, middle and upper McMurray. The middle and upper McMurray are typically interpreted as estuarine and marine respectively, although many exceptions are known and debated regarding these generalizations. The lower McMurray is universally interpreted to be a fluvial deposit. South of Township 94, in the southern portion of the McMurray Formation’s extent, sedimentological and ichnological data support fluvial interpretations of the lower McMurray. The focus of this work, outcrop studies north of Township 94, document tidal signatures and Cylindrichnus dominated ichnological assemblages. Additionally, these tidal features and brackish water indicators increase in intensity northwards. The Lower McMurray should therefore be regarded as an estuarine deposit towards the north, rather than an entirely fluvial unit. The following evidence demonstrates that fluvial strata grades into more estuarine strata northward, following the McMurray Formation paleobasin direction, which suggests that a transgression reached as far south as the study area and is preserved in the lower McMurray. Using sedimentological and ichnological data as criteria to distinguish fluvial from tidally-influenced fluvial and estuarine strata therefore has important implications for determining high-resolution paleogeographic evolution in the rock record.