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The role of fluid- and sediment-gravity flow processes during deposition of deltaic conglomerates (Cardium Formation, Upper Cretaceous), west-central Alberta

Dept. of Earth Sciences, and Ottawa-Carleton Geosciences Centre, University of Ottawa, Ottawa, ON K1N 6N5

Most previous studies on conglomerates of the Carrot Creek Member (Cardium Formation) in the Cyn-Pem/Carrot Creek area of west-central Alberta have focused on understanding the stratigraphic context of these informally termed conglomerate "bars". However, a detailed understanding of the depositional mechanisms that controlled the spatial distribution of conglomerate facies, which in turn control reservoir quality and related production characteristics within and between different pools, is presently lacking. As a result, a number of important questions still remain. However, many of these issues can be resolved by differentiating distributary-channel from delta-front conglomerates, and also appreciating the sedimentological importance of the steep depositional gradient (>15°) on the delta front. Distributary-channel and delta-front conglomerates commonly consist of interstratified, matrix-rich and matrix-poor conglomerate. In distributary-channel conglomerates, this bipartite texture is most likely related to the spatial variability of gravel and sand deposition on the leeside of gravel-bed, subaqueous dunes and/or bars. Reservoir quality in these conglomerates was significantly reduced by a later infiltrated sand matrix. Marine conglomerates, on the other hand, were deposited on a steep delta-front by sediment-gravity processes. Planar-based, ungraded, matrix-supported conglomerate overlain abruptly by better-sorted, very coarse sandstone or granule/fine pebble conglomerate represent, respectively, en masse deposition from high-concentration sediment-gravity-flow dispersions and grain-by-grain gravity-driven transport and deposition. Most dispersions collapsed on the delta slope, but others flowed to the base of the slope and deposited their sediment in a granular jump. Much of the sand-sized sediment deposited at the mouths of the distributary-channel network was winnowed and remobilized by waves and transported alongshore and away from the delta front and, as a consequence, preserved the high-reservoir quality that characterizes delta front conglomerates.

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