Geological setting and paleobotany of the Joffre Bridge Roadcut fossil locality (Late Paleocene), Red Deer Valley, Alberta

Abstract

The Joffre Bridge Roadcut locality (Paskapoo Formation) in south-central Alberta yields plant, mammal, fish, and insect fossils. A Late Paleocene (Tiffanian) age is indicated by mammalian fossils, supported by magnetostratigraphy and palynostratigraphy. This paper summarizes the flora (28 taxa have been identified to date) and describes the sedimentology to provide a paleoenvironmental context. Outcrops at the site are limited, but seven stratigraphic units are recognized and are interpreted to represent five environments of deposition: flood plain, fluvial channel, abandoned channel, swamp, and crevasse splay. The flood-plain mudstones lack identifiable plant material due to bioturbation and pedogenesis. They are capped by a thin, clay-rich paleosol with scattered vertebrate bones. An upward-fining sequence, interpreted as fluvial channel and channel abandonment sediments, rests directly on the paleosol and includes remains of riparian trees. Carbonaceous mudstone, interpreted as a swamp facies, includes remains of only five taxa (taxodiaceous conifers and riparian trees). Light-coloured mudstones on top of the swamp facies include a more diverse assemblage (aquatic and understory plants, taxodiaceous conifers, and riparian trees). Those beds are interpreted as the distal margin of an encroaching crevasse splay. Overlying sediments coarsen upward and are unfossiliferous, except for one occurrence of articulated fish skeletons from a mass-death event. The most productive beds for plant fossils are the top of the channel-abandonment sequence, the swamp horizon, and the base of the crevasse splay. Those beds have also yielded some insect, fish, and mammal remains.