Fluvial to tidal transition zone facies in the McMurray Formation (Christina River, Alberta, Canada), with emphasis on the reflection of flow intensity in bottomset architecture

Abstract

An outcrop of the McMurray Formation along the Christina River (Alberta, Canada) has been investigated to better understand depositional processes and setting. The succession is formed by large-scale tabular sets of unidirectional trough cross-stratification. Many of these sets are characterized by profusely ripple-laminated and thick, laterally persistent bottomset intervals at their base. Additionally, reactivation surfaces and infrequent set climbers occur in the foresets. The bottomsets almost entirely consist of backflow cross-lamination. Available knowledge indicates that this points to a rather strong vortex circulation and related strong and persistent main flow velocity. The observed bottomset succession is discussed within the range of variation in bottomset architecture that results from the structure and strength of the flow in the wake behind dunes and related strength of the main flow. Sets descend along a gentle slope, suggesting that dunes filled a preexisting depression, thus representing conditions of a vertically expanding and decelerating flow. This means that aggradation rate was high, which is in accordance with the thickness of the preserved sets. Systematic changes in flow strength are documented by downstream cyclic variations in organic debris, bottomset thickness, and foreset dip. The periodic increase of flow velocity is interpreted as being produced by the increased strength of the river flow during the ebbing tide on the days around spring tide. Apart from these subtle variations, the area experienced large changes in flow strength due to seasonal differences in fluvial discharge. The turbidity maximum zone was located downstream of the study site since thick slackwater mud drapes that characterize the seaward part of the fluvial to tidal transition zone are not present; only a few thin mud drapes are found at the study locality. Therefore, it is concluded that deposition took place in the most landward part of this zone. This new interpretation of this facies in the Christina River area is in line with the inferred depositional setting of the transition to the overlying thick point bar units formed by inclined heterolithic stratification.