麻豆传媒

Channel stability has been linked to the emergence of vegetation in the Paleozoic resulting in the evolution of single, deeper channels that meandered slowly across floodplains and narrow, probably anastomosing systems. This study reports an investigation on vegetation influence in stabilizing fluvial channel banks visible in the Joggins Fossil Ciffs. This long-standing issue has been difficult to evaluate because in situ plants and root systems are rarely well preserved in the fossil record. Joggins Fossil Cliffs is one of the best places for this study because of its well preserved upright trees associated with channels. The ability of vegetation to stabilize channel banks depends on plant vigour, density of vegetation cover, and root depth. Methods such as photomosaic tracing, petrographic observation, X-ray diffraction and electron microprobe observation were used in investigating and analysing data relevant to the hypothesis.

Field observations at Joggins provide evidence of root architecture and root mat impact on channel-bank stability. Root systems of upright trees form the baseline and margin line of some channels, suggesting that root architecture had an impact on channel-bank form and stability. Upright trees rooted at the top of preserved meandering-channel banks, tilted towards the channel at one locality, show evidence of bank resistance to erosion. Presence of narrow distributary channels with upright trees at both margins and extending out as an apparently dense cover into the adjoining floodplain deposit also indicates vegetation stability of channel banks. The presence of trees projecting up into the base of channels and causing bars to form around them show evidence of vegetation influence on channels, although this probably did not have any impact on the channel bank. Field observations and results, together with past research, have shown evidence for the impact of root mats in stabilizing channel banks, and abundant root casts and long roots on channel tops at some of the localities suggest possible root mat impact on the channels. Past literature studies indicate that the binding power of root mats increases the cohesion of the soil, thereby increasing strength. This hypothesis forms the working hypothesis for research on root mats in this study. Results indicate that vegetation has significant effects on channel forms and stability. Vegetation increases channel stabilization and reduces cross-sectional area.

Thin section, X-ray diffraction and microprobe analysis has shown evidence of calcite and siderite influence in channel bank stability. Calcite crystallization around roots resulted in expansion and fragmentation of quartz, resulting in cementation of sediment as rhizoliths and compaction of the adjoining sediment. Although the timing of this cementation is difficult to constrain, such rhizoliths commonly form around growing roots in modern settings. Calcite acts as general cement in the sandstones and, if precipitated early, would have increased the strength of the bank, making it resistant to erosion. Siderite, which occurs as nodules around roots on some preserved channel banks, may have formed erosion-resistant armour. Thus, early calcite and siderite crystallisation may play an important role in the stabilization of channel banks. In general, vegetation works together with other factors such as mineral formation to create an effective impact on channel bank stability.

Pages: 146
Supervisor: Martin Gibling