Arden Bashforth

Ph. D. Thesis

The Paleoecology of Vegetation on Pennsylvanian Basin Margins

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Most paleobotanical studies involving Pennsylvanian vegetation have focused on communities that inhabited mires and associated clastic substrates in poorly drained basinal lowlands. In contrast, this thesis provides a paleoecological evaluation of riparian floras on basin margins and inland settings. Such landscapes were characterized by steep gradients and high-energy regimes due to their proximity to uplands, and the prevalence of coarse-grained sediment enhanced soil drainage and hindered peat accumulation.

To help resolve the full spectrum of vegetation cover in tropical Euramerica, megafloral assemblages were documented from strata that accumulated on disturbance-prone fluvial tracts within or adjacent to Pennsylvanian mountain belts. Taphonomic indicators and multrivariate analyses reveal that riparian vegetation comprised a collage of monospecific to low-diversity communities, with patchiness occurring at local and regional scales. Habitat partitioning saw plant clades organized along ecological gradients controlled by the drainage and stability of substrates. In the Tynemouth Creek Formation (Lower Pennsylvanian, Cumberland Basin, New Brunswick), which records deposition on a fluvial megafan under strongly seasonal conditions, gigantic cordaitalean forests dominated the landscape, particularly alongside ephemeral channels. Floodplains were largely dry and degraded, although pteridosperms, ferns, and lycopsids persisted around poorly drained depressions. On the Ny'r~any Member braided-river plain (Middle Pennsylvanian, Central and Western Bohemian Basin, Czech Republic), raised channel margins supported long-lived site-occupiers, such as cordaitaleans and pteridosperms. Pioneering vegetation comprising fast-growing, opportunistic taxa, particularly ferns and sphenopsids, was common on wetter, shifting substrates of frequently flooded abandoned channels, low-lying floodplains, and lake margins. In La Magdalena Coalfield (Late Pennsylvanian, Spain), pteridosperms dominated marginal wetlands adjacent to steep basin margins. Opportunistic ferns were abundant in or adjacent to frequently disturbed braided channel belts, whereas communities on interfluve wetlands distant from channel influence comprised pteridosperm patches enclosed in a fern-dominated matrix.

In the South Bar Formation (Middle Pennsylvanian, Sydney Coalfield), sandstone successions comprising flood deposits are capped by log accumulations, many of which are overlain by abandoned channel mudstones. It is proposed that flood sediment buildup and log jam development prompted avulsion and channel abandonment, thus providing some of the earliest evidence of the effects of large woody debris on fluvial systems.

Pages: 312
Supervisors: Howard Falcon-Lang, Martin Gibling