Subsurface and outcrop characterization of large tidally influenced point bars of the Cretaceous McMurray Formation (Alberta, Canada)

Abstract

Outcrop and subsurface investigations highlight that the main reservoirs within the Cretaceous McMurray Formation, Alberta, Canada, were located in an ancient estuary and that the major economic targets are represented by large point bar deposits. The underlined morphological characteristics of the associated McMurray-valley drainage show morphometric similarity to modern river systems like the Mississippi River. Hydrological parameters estimated by some geomorphologic data suggest that a paleodischarge of about 15,000m3/s (including the tidal prism) and a paleomeander migration rate of between approximately 30m/year and 65m/year characterized the McMurray.However, tidal influence and marginal-marine affinity are evidenced by abundant erosion and reactivation surfaces, mud clast breccia deposits, cleaner channel sands and bioturbated heterolithic sands interfingered with cleaner channel sands. Thus, internal structure observed on the Steepbank River outcrops appears to be more complex than the usual bi-partite system composed of cross-stratified sands at the base and inclined heterolithic stratified sands upward. The highlighted internal structures of large tidally influenced point bars of the McMurray Formation are quite similar to compound dune deposits in which inclined heterolithic sands (IHS) are interfingered with clean sands. These deposits are also associated with ichnological associations characterizing a brackish environment for the McMurray-valley drainage.A modern analog of the tidally influenced point bar deposits of the McMurray Formation is the meandering tidally influenced channels of the Garonne River (located in the Aquitaine basin, SW France). Facies described in cores acquired in both areas are very similar, which allows a possible comparison of these point bars, in spite of the size difference between rivers. The Garonne river point bar is located 95km from estuary mouth near the city of Bordeaux. In this area, tidal currents are the dominant dynamic agent and can reach velocities up to 2m/s. In the point bar core, the facies of the IHS exhibit 3 typical sedimentary structures: mud clast breccias more abundant at the base of the channel, heterolithic stratified sands recording semi-lunar cycles (up to 28 tides) and thick clay layers at the top of the channel. In the overbank deposits, classic tidal bedding recording the semi-diurnal and semi-lunar cycles is observed. Levees and crevasse splays are absent.This study aims to provide a depositional model and geometric framework for tidally influenced point bar reservoirs.