Barrier island associated washover fan and flood tidal delta systems: A geomorphologic analysis and proposed classification scheme for modern washover fans and examination of a flood tidal delta complex in the Cretaceous upper McMurray Formation, Alberta, Canada
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A detailed study of modern washover fan (fan) morphologies will clarify common fan geometries, lead to a better model for a “typical” fan, identify the preservation potential and probable geometries of fan facies in subsurface datasets and outcrops, and ultimately improve hydrocarbon recovery in barrier island reservoir systems. This study uses satellite imagery to conduct a spatial analysis of 118 modern fans to quantify geomorphologic attributes of fans. A new classification scheme for fans is proposed that refines the current fan model, dividing fans into channelized and non-channelized fans. Channelized fans are subdivided according to the location of primary deposition: barrier depo-center or lagoonal depo-center. Non-channelized fans are subdivided based on morphology: dissipative, lobate, or apron-sourced. Quantitative cross-plots of morphologic relationships are analyzed to define trends in fan morphologies. The most common type of fan encountered in our study is a non-channelized, line-sourced, lobate washover fan with an area of less than 1 km2 that is fully contained on a barrier and exhibits no subaqueous deposition in back-barrier waters. The Lower Cretaceous McMurray Formation is the primary reservoir of the Athabasca Oil Sands in Alberta, Canada. The upper McMurray is commonly interpreted as deposits of embayed coastal systems. Our location is in an under-studied area located 80 km northwest of Fort McMurray. Lateral and vertical facies changes, sedimentary structures, key surfaces, trace fossils, and bitumen saturation were documented in eight cores located along a 20 km transect situated paleo-landward of a Devonian paleo-high acting as a bedrock-barrier. Our data indicate that a flood tidal delta complex prograded landward into a back-barrier embayment through the stable, bedrock-controlled inlet. This system overlies middle McMurray fluvial sands and Devonian basement and was transgressed by marine waters prior to deposition of the overlying Wabiskaw Formation. Flood tidal delta sandbodies are bitumen saturated and therefore make good reservoirs; however, heavily bioturbated tidal flats can act as a barrier to flow where they encase flood tidal deltas, as encased sands were devoid of bitumen. This complex coastal paleogeography produced back-barrier deposits that contain a slightly more diverse, marine trace fossil assemblage than might otherwise be expected.