FLUVIAL STYLE AND VEGETATION ABUNDANCE AS A PALEOCLIMATE PROXY IN THE PENNSYLVANIAN AND EARLY PERMIAN: AN EVALUATION OF PRESERVATIONAL ISSUES

Fluvial style is strongly influenced by the amount and seasonality of precipitation, and can be a useful paleoclimate proxy. In assessing paleoclimate, the relative abundance of vegetation is a key factor but also reflects plant preservation. We compare fluvial style and plant preservation for three widely exposed formations from equatorial Pangea.

The Early Pennsylvanian Tynemouth Creek Formation of Atlantic Canada contains the sandy and muddy fills of narrow fixed channels encased in floodplain deposits. Fragmentary vegetation was preserved in active and abandoned channels and sandy floodplain splays, with upright trees and rooted levels. The Boss Point Formation, an island-braided sandbed system, contains abundant logs in channel sands with some roots and upright trees. Both formations had abundant coarse sediment. Cordaitalean vegetation, an effective dryland colonizer, predominates in both formations, and upright sphenopsids (calamitaleans) are common in splays.

The middle Early Permian Clear Fork Formation of north-central Texas was deposited in fine-grained, meandering channels with lateral-accretion deposits and rill marks that imply near-complete flow cessation. Roots are present low in channel fills, but there are few logs and apparently no upright trees. Plant fossils, including walchian conifers and some sphenopsids, are sparse in active channels but are exquisitively preserved in fine-grained abandoned-channel fills, probably under anoxic conditions.

For the Early Pennsylvanian, abundant coarse bedload led to rapid deposition in channels and splays, enhancing the preservation of logs and upright trees. In contrast, rapid burial was rare in Early Permian channels, enhancing oxidation and reducing the preservation potential of logs and upright trees but enhancing preservation of delicate plants in abandoned channels. Nevertheless, although preservational issues were important, the profound contrast in vegetation abundance between Pennsylvanian and Permian sites suggests that paleoclimate played a crucial role. A combined analysis of fluvial style and vegetation abundance suggests Early Permian aridity, with periods of minimal or no flow in channels and a modest vegetation cover – conditions that also would have enhanced oxidation except in abandoned channels.