Sedimentological and Ichnological Analysis of the McMurray IHS (Kearl Area)

Alina Shchepetkina; Myers, Reed A.; Scott, Jennifer J.; Gingras, Murray K.; Pemberton, S. George

The Lower Cretaceous McMurray oil sands contain inclined heterolithic stratification (IHS), which formed due to lateral and locally vertical accretion within channels. Variable interpretations as to the depositional affinity of IHS have been put forth, and these range from fluvial to middle estuary. This paper attempts to integrate ichnological and sedimentological data to propose a refined paleogeographic interpretation for a local occurrence of the McMurray Formation IHS.

A detailed analysis of 5 wells from the Kearl area (57.21 N, 111.26 W) of the Athabasca oil sands was conducted. Research concentrated on establishing sedimentological and ichnological trends by: 1) core logging 2) facies analysis, and 3) analysis of the individual bed thicknesses, bioturbation indices, diversity and diameter of ichnofossils, and their distribution.

Two main facies associations were observed: 1) thickly bedded cross-stratified sandstone; and 2) IHS of variable bed thickness and sand-mud ratios. The thickly bedded sandstones are characterized by high-angle, trough cross-stratification and grain size striping. Overall, there is a lack of bioturbation, however rare thin mud beds and rip-up clasts can show low bioturbation intensities and an impoverished diversity. The cross-bedded units are the result of strong currents, and the sedimentary environment was brackish in its nature, which are consistent with the fluvio-tidal transition.

IHS units are characterized by interbedded sand- and mudstone, where the mud-to-sand ratio, bioturbation intensity, diversity and maximum trace diameter increase upwards. We interpret this trend to represent a subtidal channel grading to intertidal zone succession, wherein the lowered current energy upwards is associated with a drop in sedimentation rate, and promotes the accumulation of organic rich sediment, providing abundant food resources for deposit feeding organisms. Ichnological observations, including overall diminutive, low-diversity suites of trace fossils, and abrupt changes in the style and distribution of bioturbation, further indicate that the IHS units represent amalgamated geobodies and that sediment deposition occurred during changing salinity levels or within a brackish-water setting. In short, the presence of brackish-water trace-fossil assemblages, and evidence of shoaling into an intertidal zone are both strongly associated with sedimentation in tidally influenced channels of the inner estuary.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013