THE INTERACTION OF SEDIMENT AND VEGETATION–SOME EXAMPLES FROM THE CARBONIFEROUS OF NOVA SCOTIA

The rapid radiation of land plants in the Devonian forever changed clastic depositional systems. It is widely accepted that terrestrial vegetation acts as the physical embodiment of climate by controlling the hydrology, weathering rates, and surface stability of the landscape. Despite the obvious relationship between sedimentation and vegetation, direct evidence of their interaction has been only sporadically described in the ancient record. The Joggins section (a candidate UNESCO World Heritage Site) and sites in the Sydney Basin provide an excellent setting to investigate the influence of standing vegetation on alluvial sedimentation.

The arborescent lycopsids and Calamites of these coal-bearing Pennsylvanian floodplains may have localized significant accumulations of sediment during flood events. In rapidly aggrading, poorly-drained floodplain facies, erosional scours up to 4 m in diameter and 1.5 m in depth formed around standing vegetation. Many of these sandy, centroclinally-filled scours are the product of successive flood events around a single plant. Slightly upturned beds (sediment shadows) represent the accumulation around standing vegetation during more gentle flow conditions.

Well-drained floodplain facies contain a considerably different set of sedimentary structures. Although overbank floods also produced sand-filled scours around standing vegetation, the oxidizing conditions often completely degraded the host plant. These more episodic (and perhaps more vigorous?) floods occasionally inundated whole stands of vegetation forming large (~25m3) coalesced scour fills elongate parallel to flow direction. Occasionally, partially entombed plants would decay and collapse, allowing the formation of down-turned beds and/or mud-filled hollows. Unless vegetation is preserved or obvious, scours and sediment shadows formed around standing plants can be mistaken for purely hydrodynamic features such as sediment bars, small channels, antidune forms, or bedforms associated with dune-plane bed transitions. Many such vegetation-related features may have been overlooked or misinterpreted in the past.