ABSTRACT

The 350-m-thick Spirit River Formation (Lower Cretaceous) of west-central Alberta comprises a clastic wedge which prograded into an epeiric sea. Within the clastic wedge, eight major transgressive and regressive sequences of sedimentation from 30 to 50 m thick have been identified. Cores from four sequences in the central pan of the study area reveal that the sediments consist of fine-grained, well-sorted sandstone, with mostly low angle to horizontal laminations, and some Palaeophycus burrows. Conglomerate units up to 10 m thick occur locally. The sequences are capped by carbonaceous mudstone and/or coal. Each sequence is interpreted as consisting of shoreface-beach deposits (with conglomeratic fluvial channel and beach deposits) overlain by backswamp-lagoon sediments. These s quences have sharp, sandy bases, rather than shaly, gradational ones because of (a) shallow depths and high wave energies, and (b) preservation of basal transgressive deposits.

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 pans 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 ea bottom.