Lateral and vertical facies sequences in the Upper Cretaceous Chungo Member, Wapiabi Formation, southern Alberta

Abstract

The Chungo Member of the Wapiabi Formation (Campanian) shales out northward and (or) eastward from Lundbreck to the Bow Valley, southern Alberta. At Lundbreck, basinal mudstones are overlain by turbidite and hummocky cross-stratified sandstones in an overall coarsening-upward sequence. The marine part of the sequence is capped by dominantly swaley cross-stratified sandstones, interpreted as storm-dominated shoreface deposits. Nonmarine fluvial deposits overlie the shoreface. Essentially the same sequence persists northward to the area of the Highwood River, but in the Highwood River – Sheep River – Longview area, the swaley cross-stratified shoreface sandstones are progressively cut out by a north- and (or) east-ward-dipping erosion surface with up to 40 m of relief. The erosion surface is overlain by a lag of chert pebbles and then by a marine coarsening-upward sequence of offshore to shoreface deposits (the lower tongue of the Nomad Member). Nonmarine Chungo deposits then prograded north- and (or) eastward to overlie this marine tongue in the Highwood area. They are in turn abruptly overlain by a gravel-coated transgressive surface marking the upper marine tongue of the Nomad Member. Finally, marine shales of the Nomad Member are abruptly overlain by dominantly fluvial rocks of the Belly River Formation.We suggest that the main Chungo shoreface, stratigraphically equivalent to the Milk River sandstone of southeastern Alberta, prograded north- and (or) eastward to at least the Highwood River area. A rapid relative lowering of sea level followed by stillstand resulted in the formation of the erosion surface and the supply of gravel to the basin. The overlying coarsening-upward sequence indicates the resumption of shoreline progradation, but this appears to have been relatively short-lived; it was terminated by the transgression of the upper Nomad tongue.