EVOLVING FLUVIAL LANDSCAPES OF THE DEVONIAN: THE CO-EVOLUTION OF LANDFORMS, PLANTS, AND ANIMALS

The Paleozoic “greening” of the terrestrial surface was among the most profound changes in Earth history. Cambro-Ordovician fluvial deposits were largely braided-river sand sheets. After land plants appeared in the mid-Ordovician, the global spectrum of alluvial styles and landforms changed radically, especially in the Devonian. Although a link to plant evolution is circumstantial, the onset and persistence of this profound Paleozoic facies shift implies a biological cause, probably through bank stabilization by rooted vegetation, increased mud from plant-induced weathering, and enhanced mud retention in channels. Through the Devonian, landscapes, plants, and animals co-evolved with positive feedback loops.

In Lochkovian strata of Wales, small meandering channels with point bars (among the first in the geological record) formed heterolithic lateral-accretion deposits, although trunk channels lack these features. The prominence of carbonate-rich soils suggests a change in groundwater / atmosphere interaction, likely biologically mediated. In Pragian to Emsian strata of the Campbellton and Gaspé regions of eastern Canada, abundant comminuted plant material in channel sandstones, well preserved plants, charcoal, and the world’s earliest thin coals indicate widespread colonization of alluvial plains and greatly increased biomass. Short rooting structures and modestly developed paleosols are present. Braided-river deposits show increased mud and a lenticular, less sheet-like architecture that may reflect increased cohesion from mixed-load sediments. The gigantic fungus Prototaxites provided the first logs to channels and may have had large roots. However, the rise of meandering rivers with sustained lateral accretion appears to predate most rooting systems.

By the Middle Devonian in eastern Canada, meandering channels, levees, crevasse splays, and long-lived floodplains were prominent, the latter promoting prolonged soil development and diversification of soil organisms. Large trees and rooting systems are present at Gilboa in NY State. By the Late Devonian, ~40% of fluvial formations worldwide were laid down by meandering rivers. Anastomosing and island-braided rivers completed the facies spectrum during the Pennsylvanian as drylands and disturbed habitats were colonized.