ABSTRACT

The 60 m thick, Campanian-Maastrichtian, Bearpaw-Horseshoe Canyon transition is exposed along the Red Deer Valley, Drumheller, Alberta. These marine to marginal marine deposits can be divided into seven disconformity-bounded units (A to G). Each unit contains a transgressive and a regressive phase. Units are defined by marine flooding surfaces, which often occur above thin (0.3 to 2 m) coal seams. Erosional relief along these surfaces is minimal and depositional records of the transgressions are very thin or absent. Exceptions occur in Unit B, where an incised valley (up to 16 m deep and at least 20 km long) was infilled with fine to medium grained esturaine sand, sourced from the marine side during transgression, and in Units E and G where thin (1 to 4 m), bioturbated, transgressive sheet sands occur.

Most of the preserved deposits in the succession belong to highstand systems tracts. They are represented by prograding sandy shoreface systems (Units A, C and F) up to 18 m thick and at least 14 km long in the progradation direction. Shoreface systems are sealed up-dip by stratigraphic pinch-out into nonmarine shale and siltstone.

Vertical connectivity between sandbodies is generally poor except in the cases of Units A and B where the sand filled estuarine channel (B) erodes into shoreface sandstone (A), and Units E and F where shoreface sandstone (F) overlies a transgressive sandsheet (E).

In the marginal marine zone, sandbody geometry and location are determined by the relative interactions between tectonic subsidence, sea level change, sediment supply, and sediment point source during any particular disconformity-bounded depositional episode.

Each unit is estimated to represent about 140,000 years of deposition. This is an order of magnitude lower than the third order eustatic sea level cycles proposed for the Upper Cretaceous by Haq et al. (1.8 million years). The estimated periodicity suggests that autocyclic shifts in marginal marine depocentres, possibly with superimposed glacioeustatic sea level fluctuations, controlled sedimentation.

End_of_Record - Last_Page 205-------