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

摘要

The Joffre Bridge Roadcut locality (PaskapooFormation) in south-central Alberta yields plant, mammal, fish,and insect fossils. A Late Paleocene (Tiffanian) age is indicatedby mammalian fossils, supported by magnetostratigraphy andpalynostratigraphy. This paper summarizes the flora (28 taxahave been identified to date) and describes the sedimentology toprovide a paleoenvironmental context. Outcrops at the site arelimited, but seven stratigraphic units are recognized and areinterpreted to represent five environments of deposition: floodplain, fluvial channel, abandoned channel, swamp, and crevassesplay. The flood-plain mudstones lack identifiable plant materialdue 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 channelabandonment sediments, rests directly on the paleosol andincludes remains of riparian trees. Carbonaceous mudstone,interpreted as a swamp facies, includes remains of only five taxa(taxodiaceous conifers and riparian trees). Light-colouredmudstones on top of the swamp facies include a more diverseassemblage (aquatic and understory plants, taxodiaceousconifers, and riparian trees). Those beds are interpreted as thedistal margin of an encroaching crevasse splay. Overlyingsediments coarsen upward and are unfossiliferous, except forone occurrence of articulated fish skeletons from a mass-deathevent. The most productive beds for plant fossils are the top ofthe channel-abandonment sequence, the swamp horizon, and thebase of the crevasse splay. Those beds have also yielded someinsect, fish, and mammal remains.