Abstract

We evaluate the definitions of palynomorph-based biozones (the ‘P-zone’ system) as used to chronicle lower Tertiary (especially Paleocene) strata in Rocky Mountain basins. We also consider reliability of those Wyoming-based biozones for inter-basinal correlation. Lower zones of the original (1978) reference sections (near Waltman, on the Casper Arch) and their replacements of 2006 (at Castle Gardens, in the Wind River Basin) probably involve Cretaceous strata. Upper parts of the same sections (Fort Union Fm.) were eroded during the Laramide orogeny, making it improbable that preserved elements (including zone P6) represent latest Paleocene time. Taxonomy supporting original P-zone definitions was unacceptably typological and has not since been substantively improved. Palynomorphs from Waltman were lost, although new specimens were collected (but not described) from Castle Gardens. The taxonomy was not renovated, however, and geological application of new biozonal definitions from Castle Gardens remains largely dependent upon unavailable palynomorphs from Waltman. Published palynological characteristics inadequately justify supposed phylogenetic lineages underpinning species-level, P-zone definitions. Conflicting geographic documentation of measured sections at Castle Gardens adds to confusion about biozonal definitions. We question how, if components of original biozone-defining fossils came from Cretaceous strata, claims of effectiveness could be true for application of Cretaceous fossils to correlation of Paleocene rocks. Importance of the originally sampled sections near Waltman therefore continues unabated. Contrary to presumed regularity in superpositional occurrences of biozones P1 through P6, we report sequences that violate orderliness. Probably certain P-zones temporally overlapped, and out-of-sequence occurrences reflect geographically varying floral responses to regional paleoenvironmental changes. Also, evidence exists that boundaries between specific P-zones may be diachronous (in magnitudes of millions of years) among basins. These issues weaken the presumed equation of individual P-zones with defined intervals of geologic history. The best start for improving understanding of Paleogene palynostratigraphy would involve studying sections that intersperse well-known mammalian records with closely spaced, palynomorph-rich strata.