PALEOBOTANICAL EVIDENCE FOR “PLUVIAL” INTERVALS IN THE WESTERN PANGEAN TROPICS DURING THE EARLY PERMIAN

The transition from the Pennsylvanian to the Permian in the tropics of western Pangea was marked by a general trend toward increased temperature and decreased soil moisture, based on physical and chemical indicators, such as paleosol morphologies and oxygen isotopes of authigenic minerals. Vegetation tracked these changes and there is a 1:1 correspondence of species pools with climate proxies: floras dominated by spore-producing plants and primitive seed plants characterize wetter-cooler conditions, with floras dominated by more derived seed plants characterizing drier-warmer conditions. Taxa characteristic of wet habitats, particularly tree ferns and sphenopsids, continued to appear sporadically during periods that physical and chemical indicators suggest were dry-warm, possibly reflecting persistent wet sites on otherwise more xeric landscapes.

During the middle Artinskian, parts of the Waggoner Ranch Formation of north-central Texas are characterized by physical and chemical indicators of relatively wetter conditions. Floristically, this interval consists of intercalated “wet”, tree-fern-dominated, and ”dry”, seed-plant dominated floras. Quantitative sampling indicates that there are only minor, but noteworthy, overlaps between the wetter-cooler and drier-warmer species pools. Furthermore, some of the tree-fern-dominated floras contain elements typical of much older floras from the earliest Permian. In several instances, the wet floras occur in channel-form deposits suggesting short, “pluvial” periods. Considering the close association of floral composition and climate, it can be inferred that there were fluctuations in soil moisture and possibly temperature that permitted the short-term spatial expansion of wetland vegetation during the Early Permian, possibly from populations persisting locally in sites marginal to water bodies. These intervals of climatic oscillation further suggest that glacial-interglacial cycles similar to those of the Pennsylvanian also characterized Permian glaciations.