Palynofacies, coal petrographic facies and depositional environments: Amphitheatre formation (Eocene to Oligocene) and Ravenscrag Formation (Maastrichtian to Paleocene), Canada

Abstract

Low rank coals from two disparate geological settings have been subjected to petrographic and palynological analysis. The stratigraphic units studied and their locations are the Amphitheatre Formation, St Elias Mountains, Yukon Territory, of Eocene to Oligocene age; and the base of the Ravenscrag Formation, south-central Saskatchewan, of latest Cretaceous to earliest Paleocene age. The depositional setting of the Amphitheatre Formation ranges from distal sand-dominated braided stream and lacustrine environments to proximal gravel-dominated fluvial environments. The coals are low in inertinite (< 6%) and mostly high in huminite (> 85%). In localities with a dominance of angiosperm pollen the relative abundance of eu-ulminite B and densinite is greatest whereas in the ones with a dominantly coniferous pollen assemblage eu-ulminite A is the most prominent maceral. These observations suggest as an immediate cause/effect relationship, an at least partial dependance of present maceral content on floral precursors. This in turn probably reflects a certain combination of depositional environment conditions (pH, Eh, temperature, etc.), that likely controlled the plant community and the preservation of vegetal matter. The depositional environment of coals from the basal part of the Ravenscrag Formation contrasts sharply with that of the Amphitheatre. The Ravenscrag coals formed within a low energy, stable, floodplain environment. Palynological and coal maceral profiles for the basal Ravenscrag Formation coal, which spans the Cretaceous-Tertiary boundary, indicates that the environment of deposition progressed from an open canopied swamp forest with areas of open water, through a phase of low-lying to open water swamps with herbaceous, pterophytic vegetation to, at least locally, raised bogs. The change in coal petrography across the Cretaceous-Tertiary boundary is shown to be relatively minor in comparison to subsequent changes in coal petrography in the earliest Paleocene and to changes that occur in the palynological assemblages across the boundary interval.