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Mechanisms for the Redistribution of Mud Across a Low-Gradient Basin: Colorado Group Shale, west-central Alberta, Canada


The processes that redistribute muddy sediments large distances across low angle epicontinental seas are poorly understood. Mudstone-dominated rocks of the Colorado Group span over 200km across the low-gradient Western Canada foreland basin during the Late Cenomanian to Middle Turonian. The slope of the basin is too low for this distance of mud distribution to be explained by turbidites, hyperpycnal flows, wave enhanced sediment gravity flows, or tempestites. An alternative hypothesis is that these mudstones are remnant shoreface deposits where the shallower water fraction has been eroded by subsequent transgressions. Wave- and combined-flow ripples, graded beds, scour-fills, and starved ripple laminations were described from outcrop and core. These bedforms are produced by processes that occur in all of the above mentioned environments; however, their distributions will vary. Microfacies are described at a cm-scale to determine the vertical and lateral proportions of bedforms and therefore characterize the variation in depositional processes across the interval of interest. Outcrop, core and well log data are integrated in this study to determine the environment of deposition of these deposits. Thin, discontinuous conglomerate beds within the interval of interest are interpreted as transgressive lags of reworked, coarse-grained shoreface deposits. Closely spaced well logs were correlated to determine the stratigraphic architecture of the study area. Discontinuous bentonites within the interval of interest suggest that sediment packages are not as laterally continuous as previously thought. Similar depositional processes act on shelfal ramp and shoreface deposits but the lateral connectivity of facies varies. This has implications for predicting the distribution of reservoir fairways. Despite being proven hydrocarbon sources and reservoirs, the Belle Fourche and Second White Specks formations of the Colorado Group are often dismissed as unpredictable plays because facies distributions are poorly understood. Determining the environment of deposition is essential for characterizing subtle facies variations and for mapping fairway trends. These results also provide a solution for how mud can be redistributed hundreds of kilometers across a low-gradient basin.