The discontinuous nature of Paleocene coal beds raises questions about the relative timing of peat accumulation. Using fossil pollen and spores as guides, the relative ages of these coal beds, intervening carbonaceous shales, and surrounding sandstones, siltstones, and mudstones can be determined. Age determinations are important for interpreting structural and stratigraphic evolution of individual Paleocene coal basins. In turn, types of peat vegetation, which can be determined from pollen and spores, influenced the composition of coal in individual deposits.
It is significant to note that no other fossils are as appropriate as pollen and spores for determining relative ages of these rocks. Plant microfossils are intimately associated with peat and coal. Pollen and spores are spread over large areas in extremely short periods of time, and they are easily preserved. Following wide-spread extinctions of the flora at the end of the Cretaceous, angiosperms (flowering plants) began to occupy ecological niches left vacant by extinct plant species. The resulting rapid speciation of angiosperms permits fine characterization of time-lines in the Tertiary. In the Paleocene, pollen from the walnut family, including ancestral hickory and pecan, can be found within all coal basins in the Rocky Mountains and Northern Great Plains regions. Successive time intervals recognizable by different pollen species can be traced from New Mexico to Alberta, Canada. This is helpful for determin-ing the geologic ages of the coals and interpreting the detailed evolution of the coal basins.
Pollen and spores are also distinctive indicators of the nature of the mires in which coal-forming peats accumulated. For example, evidence that some mires were domed is provided by the presence in coals of spores from sphagnum moss, a plant found in modern domed mires. Furthermore, the thickness and areal extent of some Paleocene coals may be as much controlled by types of vegetation as by tectonic subsidence. Many thick coals are predominantly composed of woody material. Certain pollen indicates that much of the woody material found in the late Paleocene coals was contributed by gymnosperms related to the bald cypress found today in many mires.