Variations in Inclined Heterolithic Stratification (IHS) across the Tidal-Fluvial Transition: The McMurray Formation of the Southern Athabasca Oil Sands, Alberta, Canada
The Lower Cretaceous middle McMurray Formation in the Athabasca Oil Sands region of Alberta, Canada was deposited, in part, as a series of laterally accreting tidal-fluvial estuarine point bars. These deposits consist of inclined heterolithic stratification (IHS), but show marked variations in facies successions. This complexity is a consequence of spatial and temporal fluctuations in a variety of processes, including the mixing of marine and fresh water, and variations in fluvial discharge and tidal flux. Differentiating between these controls can be challenging; however, sedimentology and ichnology have the potential to determine the presence and relative importance of each of these parameters. In this study, the sedimentology and ichnology of IHS at various positions along the longitudinal profile of a single valley trend were analyzed.
Preliminary analyses, based on 1800 meters of subsurface core, indicates that 3 broad zones of IHS style can be reliably identified in the study area as one moves seaward: fluvial-, mixed tidal-fluvial-, and tidal-dominated IHS. In the fluvial-dominated zone, IHS is sand-dominated with very little bioturbation. Sandstone bed thicknesses are of meter-scale and are unburrowed, whereas the mudstones are generally only a few centimeters thick, and display rare deposit-feeding trace fossils. Further seaward, in the mixed tidal-fluvial zone, mixed sandy and muddy IHS prevail, and display higher bioturbation intensities. Beds are decimeter to meter scale in thickness, with sporadically distributed, brackish-water trace fossils largely confined to the mudstones. These characteristics suggest stronger tides, the presence of fluid mud, and higher but variable salinities. In the tidal-dominated zone, the IHS is muddy and rhythmic sand-mud couplets are conspicuous. Recurring brackish-water burrows are associated with the mudstone fraction, but typically subtend through the sandstones. The resulting facies successions reflect heightened tidal energies and reduced freshwater input (i.e., more stable salinities).
High-resolution facies analyses such as this are essential for refining IHS models, and lead to greater fidelity in determining their paleogeographic position. Moreover, the work has important implications for predicting reservoir heterogeneities in channel/IHS successions, as the Athabasca Oil Sands region volumetrically comprises one of the largest hydrocarbon reservoirs on Earth.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California