| dc.description.abstract | Vegetation plays a key role in fluvial and estuarine morphodynamics through promoted mud production, enhanced capture of muds and sediment stabilization by deep-rooting. Major stages in terrestrial flora evolution are co-eval with the radiation of morphodynamic features such as anastomosing river behaviour, point bar sedimentation and coal formation. In modern estuaries the effects of vegetation have been studied through modelling efforts, but their reflection in the rock record is poorly understood.
The present study aims to identify tangible evidence in a stratigraphic succession of the effects of vegetation on estuarine sedimentary processes through original fieldwork on an Early Viséan-aged (ca. 350 Ma, Early-Middle Carboniferous) case study which crops out in NW Ireland. The Carboniferous period is associated with its abundant, complex and tall-stature vegetation, familiar from museum dioramas and textbook illustrations of the ‘coal age’ equatorial wetlands. Within the stratigraphic succession of the Early Viséan case study, what sedimentary features are indicative of the sculpting role vegetation played at the time of deposition?
The study builds out from this individual effort to a comparison of estuarine strata through geological time, with a focus on the Early Palaeozoic rise and radiation of terrestrial flora. How have the sedimentary characteristics of estuarine successions changed over the course of the Early Palaeozoic under the influence of major developments in the evolution of terrestrial flora? What stratigraphic (dis)similarities between the Carboniferous case study and younger analogues may be attributed to post-Carboniferous evolutionary advances in terrestrial flora? To this extend, the fieldwork is bolstered by a literature survey of 19 studies on Cambrian to Cretaceous (541-66 Ma) estuarine strata.
Vegetation-dependent signatures in the Carboniferous case study are: rhizoliths, fusain, coal seams and plant impressions. Vegetation-influenced signatures include packages of inclined heterolithic stratification (IHS) and mudrock deposits. While packages of IHS and mudrock may form abiotically, computer and analogue modelling efforts demonstrate vegetation promotes the development of these signatures.
The impact of vegetation is intricate in estuaries as these environments host a complex variation of morphodynamic conditions over a limited spatial scale. This is reflected in estuarine successions in the diversity of facies and architectural styles, even within architectural elements formed by similar mechanisms. For example, packages of IHS may be dominantly coarse-grained, fine-grained, comprise distinct heterolithic couplets or display a trend in couplet grain size.
A review of 19 Cambrian to Cretaceous-aged successions demonstrate a marked increase in IHS occurrence frequency and an increase in mudrock abundance of 1.3 orders of magnitude in post-Devonian (359-66 Ma) successions relative to older successions (541-359 Ma). These trends are co-eval with major steps in the Early Palaeozoic evolution of terrestrial flora.
Key in shaping estuaries are vascular and deep-rooting plants, which have evolved by the Early Viséan. The Cretaceous (145-66 Ma) advent of herbaceous, grass-like and salt marsh vegetation introduces new powerful ecosystem engineers to estuaries. As a result, crevasse splays may occur less frequently in (post-)Cretaceous estuaries as closely spaced reeds, a grass-like herbaceous plant, introduced a greater degree of vegetation density in the geomorphic landscape. Additionally, since the advent of salt marsh vegetation, potent in retaining muds, lateral channel mobility in estuaries may have been restricted more effectively than previously.
This work demonstrates the vegetation-dependent and vegetation-influenced sedimentary signatures in an Early Viséan estuarine case study. The occurrence of vegetation-influenced sedimentary features, such as packages of IHS and mudrock deposits, is promoted by the above-ground baffling and below-ground binding properties of vegetation. A 1.3 orders of magnitude increase in the abundance of these sedimentary features is found to be co-eval with land plant evolution through a literature survey that may serve as a pilot for future research. | |
