VEGETATIONAL PATTERNS ACROSS THE PENNSYLVANIAN-PERMIAN BOUNDARY IN WESTERN EQUATORIAL PANGAEA, A CONTRAST WITH THE DUNKARD

Plant-bearing beds of latest Pennsylvanian and earliest Permian age crop out in dominantly nonmarine strata in New Mexico, Utah and Texas. These regions were in the western part of Pangaea. All contain paleofloras that are, on average, of mixed wetland and dryland composition. Detailed outcrop study, however, indicates spatial and/or temporal separation of these floras. This probably reflects a combination of moisture variation during glacial-interglacial cycles and overall drier climates in western parts of the supercontinent.

Both New Mexico and Utah essentially lack organic-rich deposits of Pennsylvanian-Permian transitional age. In New Mexico, mixed floras from floodplain deposits suggest landscape complexity, and give way to fully “Permian-type” assemblages (conifers, cordaitaleans, callipterids) during the latest Pennsylvanian. In Utah, slightly further north of the paleoequator, mixed floras, similar to those of New Mexico, are confined to floodplain complexes sandwiched between eolian dune sands. Further eastward in Texas peat swamps and associated wet floodplains persisted nearly to the end of the Pennsylvanian. These are intercalated with deposits bearing xeromorphic “Permian-type” flora that become dominant in Early Permian deposits, coincident with the disappearance of organic-rich beds.

By comparison, the approximately coeval Dunkard succession was more consistently ”wet” throughout this entire transition, reflecting a strong west-east climate gradient north of the central Pangaean mountain ranges. Causes of the west-to-east climate gradient appear to have a complex cause, presently incompletely understood.