Cyclicity of Inclined Heterolithic Stratification in the McMurray Formation, NE Alberta, Canada

Abstract

The Cretaceous McMurray Formation, located in northeastern Alberta, Canada, is the primary host of the Athabasca Oil Sands. The McMurray Formation is generally interpreted to represent the fluvial to estuarine deposits of the boreal sea transgression over a karsted Devonian landscape during the Aptian to the Albian. Commonly observed in the McMurray Formation is Inclined Heterolithic Stratification (IHS), which comprises dipping, regularly interbedded sandstone and mudstone. IHS units are most commonly taken to represent the deposits of laterally accreting point bars. Sandy IHS (defined herein as >70% sandstone) is potentially a reservoir-quality facies, whereas muddy IHS (i.e. <70% sandstone) is typically a non-reservoir facies. The mud-sand content of IHS is directly related to the tidal and fluvial forces that modulate deposition. An in depth understanding of the degree of the paleo-fluvial or -tidal energy acting on IHS deposition is one of the vital parameters needed to understand IHS mud-sand distributions. One way to consider the relative influence of tides, which result from lunar cyclicity, and fluvial flux, which reflects seasonal, annual or decadal variations in streamflow, is to document and interpret rhythmic bedding in IHS successions. Moreover there are basic observations that can be made, that constrain the temporal character of the observed sand-mud pairs. Herein, we focus on two basic and informative observations: (1) the nature and distribution of bioturbation, which reflects that sedimentation rates were low enough to permit infaunal colonization of the sediment; and (2) bed thickness observations that are consistent with tidal versus fluvial sedimentation. Using these observations, we identify IHS units, wherein bioturbation and sediment thickness are consistent with tidal sedimentation: for these the quantitative study of IHS cyclicity, using wavelet transforms, represents a method for determining the degree of tidal influence affecting IHS deposition. Incorporating quantitative and qualitative information, the degree of tidal or fluvial influence modulating IHS deposition is determined and used to better constrain the paleogeographic distribution of IHS.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015