Browsing by Subject "Sedimentation and deposition"

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    Sediment redistribution in the Uilkraals estuary as a consequence of human disturbance
    (1988) Crowther, J; Meadows, Michael E
    The construction of bridge embankments and other physical obstacles in estuaries has often resulted in the redistribution of sediments, which ultimately leads to detrimental impacts in these environments. The aim of this study is to investigate how the sediment distribution and dynamics within a specific estuary, the Uilkraals estuary, have been affected by human disturbance; focusing on the impacts arising from construction of two temporary embankments and a permanent bridge and embankment. The Uilkraals estuary is situated in the south-western Cape, approximately 60 kilometres north-west of Cape Agulhas. Human impact has been extensive and engineering projects of various sizes and permanence have marked the recent history of this estuary. A bridge and embankment which cross the estuary 800 m from the mouth were built in 1973. In 1978 an embankment was built between the bridge and the estuary mouth. A second embankment which replaced the first in 1980, was removed before the end of that year. Extensive dune reclamation occurred on the left bank between 1938 and 1973. The techniques used in the study include: (i) a quantitative analysis of all available aerial photographs from 1938 to 1987 (ii) a ground survey and (iii) core and surface sediment sampling. The quantitative analysis reveals that the major changes in estuarine characteristics have been in response to human disturbance. The contour map and cross-sections drawn from the ground survey indicate a build-up of sediment downstream of the bridge and scouring of the channel upstream. The latter suggests the dominance of the flood-tidal current in the estuary. The core sediment analyses are unable to distinguish any real difference in the modes of sediment deposition on either side of the bridge embankment. The embankment has, however, affected deposition by acting as a "hydraulic shelter" to sediment accumulated downstream of the bridge during high run-off events and by initiating deposition of sediment upstream of the embankment. The surface sample analyses indicate that there has been an increase in flow velocities in the vicinity of the bridge since its construction and that the major agents which bring marine sediment into the estuary are flood-tidal currents and wind. It is concluded that the sediment distribution and dynamics of the Uilkraals estuary have been affected by human disturbance. Recommendations for future management of the estuary are that no further embankments should be constructed in the estuary and that the construction of a culvert or culverts under the existing bridge embankment would alleviate a number of problems presently experienced in the estuary.
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    Tectonic implications from the geochemistry of Mfongosi Group metasediments, Natal Metamorphic Province, South Africa
    (2003) Basson, I; Watkeys, M K
    The formation of the Mesoproterozoic Natal Metamorphic Province occurred during the closing stages of the assembly of the supercontinent Rodinia (∼1150 to ∼950 Ma), forming part of the ∼1190 to ∼950 Ma Grenvillian orogeny. This was preceded by the accretion, at 1210 Ma, of Mesoproterozoic island arcs along the southern margin of the Archaean Kaapvaal Craton. The Mfongosi Group represents an enigmatic sequence of rocks found between the Kaapvaal Craton and the partially ophiolitic Natal Nappe Zone to the north of these accreted island arcs. Mfongosi metagreywackes adjacent to the Kaapvaal Craton have ocean island arc major element geochemical signatures. These contrast with metagreywackes approximately two kilometres further south, near the contact zone between the Natal Thrust Front and Natal Nappe Zone, which have active continental margin major element geochemical signatures. A third type of metasediment is represented in both areas by geochemically distinct low-Ca+Na, high-K meta-arkoses to meta-lithic arkoses, which were formed by relatively minor sedimentation from a passive continental margin. The Mfongosi Group is a fore-arc complex, incorporating elements of a fore- arc basin and an accretionary prism deposited in a trench, which filled during final oblique collision between the Kaapvaal Craton and an oceanic island arc to the south. The inclusion of an active continental margin signature in the metasediments is enigmatic, although the geochemistry of the metagreywackes suggests that the margin of the Grunehogna Province is a probable source. Subsequent closure of this basin resulted in inversion and accretion of the metasedimentary sequence onto the southern margin of the Kaapvaal Craton. [ABSTRACT FROM AUTHOR]