Abstract

Riparian vegetation profoundly influences modern fluvial channels in a variety of ways, depending on the life-history strategies of different plant types, disturbance frequency, and drainage conditions of available habitats. Direct evidence for these dynamic relationships is usually cryptic in ancient deposits. We report evidence for interactions between rivers and in situ vegetation for selected sites in the lower Pennsylvanian Joggins Formation of Atlantic Canada, encompassing fixed, meandering, and distributary channels originally up to 6 m deep. Channel bodies are associated with a suite of fossilized plant remains, specifically lycopsids that preferred stable wetland settings, disturbance-tolerant calamitaleans, and slow-growing, long-lived cordaitaleans.

Vegetation was effective in stabilizing banks and bars and promoting aggradation. Lycopsids and calamitalean groves colonized the channel bed during periods of reduced flow, drawing on the groundwater table, and mounds around upright trunks indicate that they formed bar nuclei after flow resumed. Bank-attached bars with lateral-accretion sets contain upright trees, which may have stabilized inclined sediment surfaces, and trees present between small distributary channels may have formed vegetated islands. Erect lycopsids rooted below the channel base project up into the channel fill, where they formed obstacles and nucleated sediment mounds in active channels. On channel cutbanks, upright lycopsids are tilted towards the channel, and early formed rhizoconcretions are associated with deep cordaitalean root systems in the tops of channel fills. These features imply that vegetation contributed to stabilization of sediment surfaces. The predominance of in situ over transported plant remains suggests that these low-flow-strength rivers had limited ability to erode and entrain large woody debris, especially for small channels with strengthened banks.

We infer that patterns of interaction between vegetation and rivers with a range of fluvial style broadly resembled those of today. By the early Pennsylvanian, rivers had moved from a geomorphic and biogeomorphic mode of operation into a fully ecological mode with prominent feedback loops between vegetation and fluvial processes. Vegetation is commonly poorly preserved in fluvial systems but should be incorporated into facies models for Pennsylvanian and younger strata, possibly also for some Devonian and Mississippian formations.