Palynological Facies Fossils in Nonmarine Environments in the Paleogene of the Bighorn Basin

Abstract

Palynomorphs occur as lithofacies fossils among depositional environments of the uppermost Paleocene to lowermost Eocene sediments of the central Bighorn Basin in Wyoming. Carbonaceous sediments occur within the predominantly red-bedded sequences and they contain lithofacies interpretable as deposition in ponds, crevasse splays, reduced distal floodplains, swamp margins, and clastic swamps. The palynomorph content of these lithofacies, primarily from a limited stratigraphic interval, is shown by canonical variates analysis to be made up of distinct palynologic assemblages. These assemblages differ primarily in abundance of taxa present rather than their presence or absence. The ponds are particularly distinguished by high numbers of reworked marine dinoflagellates, algal zygospores (Tetraporina), bisaccate gymnosperm pollen, and low numbers of Alnus and Taxodiaceae. The levee/crevasse splay contains high numbers of some bisaccate pollen types, pteridophyte spores, and locally high quantities of Alnus. The reduced floodplain contains abundant bisaccate pollen and pteridophyte spores, Platycaryapollenites, and Taxodiaceae. Swamp margin samples are distinguished by high numbers of Cycadopites and locally high numbers of Plicatopollis and Alnus. The swamp is dominated by pollen of Taxodiaceae and Alnus as well as high numbers of Sparganiaceaepollenites, with locally high numbers of Pistillipollenites. Pond deposits are generally more numerous and contain a richer palynoflora, however, the occurrence of the important Paleocene-Eocene boundary taxon Platycaryapollenites occurs in swamp and neighboring facies about 200 m below its first occurrence in pond facies. Thus, lithofacies distribution effects can affect the accuracy of biostratigraphic interpretations. INTRODUCTION