DRYLAND VEGETATION IN THE PALEOZOIC: THE PENNSYLVANIAN RECORD FROM JOGGINS, NOVA SCOTIA

The advent of vascular plants in the Late Silurian profoundly affected the earth-atmosphere system. Carbon storage as peat and dispersed plant material in sediments reduced atmospheric CO₂ and promoted Gondwanan glaciation. Vegetation stabilized river banks, favoured the development of meandering and anastomosed channels, encouraged the storage of sediment on vegetated floodplains, and strongly influenced sediment yield. Although floodplain deposits are prominent in Devono-Carboniferous strata, dryland ecosystems are commonly enigmatic because vegetation is rarely preserved in these oxidizing settings, in contrast with the well-known lycopsid assemblages of coastal wetlands. We describe here Pennsylvanian dryland alluvial deposits and their floral assemblages from the Joggins section, with special emphasis on the charcoal record which is prominent in several channel bodies.

Thick Langsettian redbeds at Joggins were deposited in a strike-slip basin where anastomosed rivers and small valleys traversed dryland plains with levee-splay complexes and soils of humid seasonal type. Plants are preserved in all these facies as charcoal, impressions, compressions, and calcified permineralizations, with rare decorticated stumps. Dispersed cordaite (gymnosperm) material comprises 74% of the thanatomass, with pteridosperms (16%), sphenopsids (10%, mainly Calamites), and rare lycopsids. A low-diversity, fire-prone and ecologically stressed cordaitalean assemblage dominated the dryland plains, although sphenopsids are co-dominant in some levee-splay (riparian) strata. Channel fills with plants, tetrapods, land snails and large bivalves represent waterholes on the seasonally dry plains. Cordaitalean charcoal in coarse channel-base lags suggests that wildfires increased the sediment flux.

Marine deposits at Joggins are dominated by cordaitalean and progymnosperm material, rather than by wetland floral elements that might have been expected to border the ocean. Their predominance suggests transport from shorelines and uplands at times when the basin was completely flooded and the wetland flora was drowned out. Thus, cordaitalean vegetation probably dominated more rugged uplands beyond the basin-bounding faults, in addition to the alluvial plains.