Pre-vegetation alluvium: Geological evidence for river behaviour in the absence of land plants
Pre-vegetation alluvium is unique; at the present day, plants affect multiple aspects of river functioning and deposition and so those rivers that operated before the evolution of land plants largely lack modern sedimentological analogue. However, such rivers were the norm for the first 90% of Earth history and so a better understanding of their sedimentary product enables insight into both the fundamental underlying mechanisms of river behaviour and the ways in which fluvial processes operated on ancient Earth and other rocky planets. This study presents five original fieldwork based case studies and an analysis of a holistic database of all of Earth’s pre-vegetation alluvium. Together these research strands offer perspectives on the sedimentological characteristics and stratigraphic trends of pre-vegetation alluvium and the behaviour and functioning of pre-vegetation rivers. Results show that, in pre-vegetation alluvial settings: 1) a variety of fluvial styles are represented, but diminished in comparison with syn-vegetation alluvium; 2) ‘sheet-braided’ architectures are common but may record a variety of fluvial planforms; 3) meandering planforms were less frequent, particularly in small- to moderate-sized river systems; 4) mudrock is on average 1.4 orders of magnitude less common than it is in syn-vegetation alluvium; and 5) microbial matgrounds were present, but had negligible effect on preserved architecture and facies. This thesis demonstrates that whilst the physical laws governing fluvial fluid-sediment interaction have not changed, the theatre in which they operated irrevocably evolved with the greening of the continents.