Abstract

The 350-m-thick Spirit River Formation of west-central Alberta comprises a clastic wedge which prograded into an epeiric sea. Eight major transgressive and regressive sequences of sedimentation from 30 to 50 m thick have been identified. On a large scale, the shoreline sequences can each be correlated to marine sequences which grade from shale at the bases to fine sandstone at the tops. The upper parts of each marine sequence form shoreline-attached sheet sandstones. Each coarsening-upward sequence is bounded by marine shale laid down during a transgression when coarser sediment was trapped in nonmarine or marginal marine areas. Some sequences break up seaward into two coarsening-upward units because of minor transgressions. Within the marine portions of some thicker sequences, 5- to 10-m-thick coarsening-upward units can be defined which are believed to reflect fluctuations in sediment supply. The correlation of these units is an application of a technique called "event correlation" which outlines approximate time surfaces. These correlations reveal a series of imbricated, seaward-sloping surfaces of deposition within each major sequence. Sedimentation in the epeiric seaway proceeded by seaward accretion of evenly sloping bodies of sediment, with no development of a shelf-slope-basin configuration. The gradual slope formed because of (a) shallow depths, controlled by the stable tectonic setting and rapid sediment input, and (b) high-wave-energy levels which distributed sediment almost uniformly over the sea bottom.--Modified journal abstract.

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