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Wave Dominated Shorelines and Incised Channel Trends: Lower Cretaceous Glauconite Formation, West-Central Alberta
In west-central Alberta, the Lower Cretaceous Glauconite Formation (redefined) comprises a northwestward-thickening wedge of marine sandstone and shale, locally incised by fluvial channels, which was deposited at, or near, the southern margin of the Lower Albian Clearwater Sea. Three distinct transgressive/regressive successions can be correlated across the study area, and each consists of a basal shale overlain by a coarsening- and thickening-upward, coal-capped, regressive marine sandstone. The upper part of each of the three successions is locally dissected by deep, narrow channels which are infilled with fluvial, and possibly estuarine, sandstone and mudstone. These three successions record minor transgressive/regressive events, which are superimposed on the larger scale advance and subsequent retreat of the Clearwater Sea from western Alberta.
A depositional model is proposed which suggests that recurrent fluctuations in relative sea level exerted a strong control on the distribution of reservoir sandstones in the Glauconite Formation. The thin, brackish water to marine shales at the base of each succession were deposited during a transgression which records a rise in relative sea level. During a subsequent slow drop in relative sea level, wave-dominated shorelines prograded seaward, depositing thick bar or sheer-like marine sandstones which were capped by coal. An ensuing rapid drop in relative sea level initiated very rapid seaward progradation of the coastal zone, exposure of the inner shelf, and incision of fluvial channels into older marine successions. During the lowstand in sea level, large volumes of sand were supplied to a younger (lowstand) shoreline which prograded seaward. During the subsequent rise in relative sea level, the previously incised fluvial channels aggraded with sediment, and the lowstand shoreline-delta complex was transgressed and reworked by marine processes to form a shelf sand ridge complex.
Major fluctuations in relative sea level are generally interpreted as the result of eustatic sea-level fluctuations. However, the provenance of the detritus, and the “geometry” (paleoslope) of the basin changed dramatically during deposition of each of the three successions in the Glauconite Formation. These observations are cited as evidence that tectonic uplift and/or subsidence was at least partly responsible for initiating base-level fluctuations in the foreland basin of western Alberta.
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