A storm and tidally-influenced prograding shoreline—Upper Cretaceous Milk River Formation of Southern Alberta, Canada

The Santonian-Campanian Milk River Formation of Southern Alberta represents the transition from an open shelf, through a storm-dominated shoreface into a non-marine sequence of shales and sandstones, with coal. The open shelf deposits consist of interbedded bioturbated mudstones with sharp-based hummocky cross-stratified sandstones. There are no indications of fairweather reworking of the sandstones, which are therefore interpreted as having been deposited below fairweather wavebase. The shoreface sequence consists of a 28 m thick sandstone. It has a very sharp, loaded base, and is dominated by swaley cross-stratification, a close relative of hummocky cross-stratification. Angle of repose cross-bedding is preserved in scattered patches only in the top 5 m of the sand body. Channels up to 180 m wide and 7 m deep are cut into this sand body, with channel margins characterized by lateral accretion surfaces. Regional dispersal trends, as well as local palaeocurrent readings suggest flow toward the NW. Within the channels there is some herringbone cross-bedding and at least two examples of neap-spring bundle cycles, suggesting that the channels are tidally-influenced. Above the channels there is a sequence of carbonaceous shales with in situ root casts and lignitic coal seams. No marine, brackish or lagoonal fauna was identified, and the sequence appears to represent a distal floodplain. The sequence from interbedded hummocky cross-stratified sandstones and bioturbated mudstones into a 10–20 m thick, sharp-based shoreface sandstone characterized by swaley cross-stratification is uncommon. The scarcity or absence of angle of repose cross-bedding in the shoreface, and the dominance of swaley cross-stratification suggests that the shoreface was so storm-dominated that almost no fairweather record was preserved. Other examples of swaley cross-stratified shorefaces are reviewed in the paper.