MORPHOLOGICAL SIGNATURES OF TIDALLY-INFLUENCED POINT BARS AND THEIR APPLICATION TO THE MCMURRAY FORMATION

The McMurray Formation is composed of a significant proportion of inclined heterolithic stratification, traditionally interpreted to result from deposition within large scale (deep), accreting point bars in an estuarine setting. Recently many of these IHS successions have been interpreted to represent stacked, separate genetic units of accretionary channel deposits that accumulated within relatively shallow channels. Typically, these successions maintain relatively uniform dip directions and dip angles, a phenomenon yet to be directly addressed in the literature. Recent studies of modern point bars using aerial photographs however elucidate existing morphological differences between tidal point bars and their fluvial counterparts. These include sinuosity, width-to-depth ratios, and point-bar length. These differences remain poorly documented yet are believed to be crucial elements to subsurface modeling of IHS reservoirs.

Nine modern estuaries are examined from Canada, United States, northern Australia, and Papua New Guinea and several notable observations are made: tidally-influenced accretion deposits maintain lower sinuosity values than their fluvial counterparts indicating more uniform point-bar dip directions; point bar length in tidally influenced reaches can locally exceed 150m; and channel depth in tidal channels, in agreement with available literature, remains relatively shallow. Affirmation of low sinuosity deposits within the tidally influenced reaches of these estuaries is obtained from channel sinuosity profiles as well as the observation of only rare channel abandonment deposits, or oxbow lakes. In addition, some tidal channels that possess large (100’s meters in length) point bars appear to have a negligible fluvial input.

The implications regarding the understanding of ancient McMurray IHS successions may be that: 1) preservation of stacked IHS successions results in more uniform dip directions of tidal channel deposits; 2) the thickness of individual channel units is relatively thin (< 8-9m); and 3) significant fluvial input may not be needed to explain long, laterally continuous genetic channel units.