The architecture and variability of valley-fill deposits within the Cretaceous McMurray Formation, Shell Albian Sands Lease, northeast Alberta

Abstract This study focuses on the stratigraphy and internal architecture within the middle/upper portion of the McMurray Formation as seen at the Shell Albian Sands Lease (Rge 10W4M and Rge 9W4M along Twp 95), NE Alberta. Based on facies patterns and stratal relationships mapped from borehole data and mine exposures a number of conclusions can be made regarding architecture of the middle-to-upper McMurray including: 1) the informal subdivision of the McMurray into middle and upper units, 2) general environments of deposition, 3) the character of the stratigraphic framework, 4) the inferred accommodation setting. Key findings of this work include the following: Subdivision of the McMurray Formation into middle and upper units based on upward change from inclined estuarine strata to flat bedded nearshore marine and coastal plain strata is feasible through much of the study area. However, the boundary displays considerable complexity and is not characterized by a single stratigraphic surface or contact that can be correlated across the study area. Facies and bedding characteristics indicate a significant portion of the IHS was deposited by tidal bars rather than tidal-fluvial point bars as most previous studies assigned the facies to. The middle-to-upper McMurray section is subdivided by a series of high relief unconformities. The morphology of the unconformities suggests they are erosional valleys filled with a transgressive succession of facies that ranges from fluvial or inner estuarine at the base to outer estuarine or nearshore marine at the top. Estuarine and nearshore marine strata within the valley fills show considerable variation in the amount of tide versus wave influence. Initial valley fills are tide-influenced whereas later valley fills show a progressive increase in wave influence. The progressive change in tide versus wave influence is interpreted to reflect a change in shoreline morphology from an initial shoreline setting that was highly embayed to a final shoreline setting that was linear to weakly embayed. The middle-to-upper McMurray largely fills in accommodation space created by changes in relative sea level. Structural subsidence associated with the dissolution of the Prairie Evaporite was likely occurring but to a lesser extent than with the lower McMurray. As a result, stratal relationships within the middle-to-upper McMurray are relatively flat and not strongly correlated with relief on the Pre-Cretaceous unconformity. The stratigraphic architecture of the contrasts sharply with the lower McMurray where widespread unconformities are not observed and facies patterns are controlled by subsidence patterns related to dissolution and collapse of the underlying Devonian Prairie Evaporite.