Seasonality and Lithologic Variation Along an Ancient Channelized Estuary: Examples and Implications from the Aptian McMurray Formation, Athabasca Oil Sand Deposit, Northeast Alberta

Curtis D. Lettley and S. George Pemberton
University of Alberta, Edmonton, AB

Lateral accretion deposits of estuarine channels form a volumetrically significant, yet poorly understood facies within the McMurray Formation. These channel deposits, manifested as repetitious interbeds of clean sand and silty mud with primary inclination, are classified as Inclined Heterolithic Strata (IHS). Significant sedimentologic and ichnologic variation between IHS packages coupled with complex cut-and-fill relationships make correlation of channel/valley fill packages problematic.

Seasonal trends in fluvial discharge affect the dynamics of water circulation and surficial sediment distribution in estuarine environments. Consequently, the nature and pattern of biologic stresses evolve seasonally, dramatically affecting patterns of benthic colonization. The application of these concepts offers an elegant means of explaining both the heterolithic nature of the McMurray channel deposits, as well as the great variability of character observed between IHS packages.

By comparing the sedimentologic and ichnologic signature of numerous packages of IHS, their relative position along an estuarine gradient can be assigned. The development of a model linking IHS character to longitudinal position has numerous implications including: the discrimination of individual channel/valley fill packages, the recognition of progradational, retrogradational, and aggradational stacking patterns, and prospecting for high grade reservoirs which may exhibit a consistent paleogeographic relationship with the channelized IHS deposits.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005