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The Cadotte Member of Northwestern Alberta: A High-Energy Barred Shoreline
The Cadotte Member (Albian) of northwestern Alberta is a clastic shoreline to offshore progradational sequence, similar to many such sequences common in the Cretaceous North American Interior Seaway. What is unusual about the Cadotte sequence is the presence of a prominent erosional scour surface that subdivides this progradational sequence into lower and upper units. The two units can be easily recognized in the subsurface on gamma-ray logs.
The lower unit is finer grained and consists of a coarsening-upward offshore to middle shoreface shale, siltstone and sandstone sequence. The offshore facies (Facies 1) consists of interbedded shale and wave-rippled to graded-bedded siltstone and very fine grained sandstone, representing deposition below storm-weather wave base. The overlying lower shoreface facies (Facies 2) consists of interbedded graded-bedded and hummocky cross-stratified sandstone and shale with a moderate amount of bioturbation. This bioturbation occurs in the upper portion of sandy layers, giving rise to laminated-to-burrowed structure. Deposition took place below the average storm-weather wave base. The middle shoreface facies (Facies 3) gradationally overlies Facies 2 and consists of very fine to fine grained, horizontal to low-angle cross-stratified sandstone, interpreted as amalgamated hummocky cross-stratification. Bioturbation in this facies is restricted to a thin argillaceous sandstone zone near its top, containing such burrows as Skolithos, Palaeophycus and Terebellina. This facies was deposited below fairweather wave base where severe storms reworked bottom sediments.
The upper unit, erosionally overlying the lower unit in over 80 per cent of the wells studied, comprises the following facies: the upper shoreface (Facies 4) consists of cross- and horizontally stratified, medium to coarse grained sandstone, pebbly sandstone and conglomerate. Macaronkhnus segregatis occurs at the top of this facies and into the base of the overlying facies. Deposition of Facies 4 occurred in current swept longshore troughs and rip channels of a barred high-energy shoreline. Gradationally overlying Facies 4, and at times difficult to separate from it, is the foreshore (Facies 5) consisting of horizontally to low-angle cross-stratified, medium grained sandstone deposited by the action of swash and backwash on high-energy beaches. A rooted and highly homogenized fine to medium grained sandstone representing the backshore (Facies 6) gradationally overlies the foreshore facies.
This sequence resulted from the seaward progradation of a high-energy barred coastline. The seaward migration of the longshore trough and rip channel created the erosional scour surface and eroded the longshore bar.
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