Shallow water carbonate basin of the Zanzibar channel, Tanzania
Sammanfattning: The shallow water carbonate basin of the Zanzibar channel, Tanzania, has been investigated. This study discusses and describes the sea bottom topographic features, sediment composition, major characteristics of sediment distribution, provenance of sediments, and the depositional settings of the sediments . Zanzibar Channel consists of a Acentral [email protected], a feature which is about 30-40 m in depth and approximately in the middle of the channel. The central deep is flanked to the east by an irregular topography, and to the west by a smooth topography. The morphological differences between the two sides of the channel are interpreted as partly caused by the differences in the tectonic development and partly due to differences in the deposition of land derived sediments on either side of the channel. The observed physiographic and tectonic difference between the western and eastern side of the channel have to a large extent controlled the major trends found in the sediment composition and distributions. The surface sediment composition is dominantly of biogenic origin except adjacent to the mainland coastline where the flux of siliciclastic (terrigenous) material from Rivers Ruvu and Wami occur along an approximately 5 km coastal band, and a thin lobe which projects from River Ruvu to the middle of the channel. The minor contribution of siliciclastic sediments originating from Zanzibar Island are generally limited to the intertidal areas west of the island. The lack of significant siliciclastic flux from Zanzibar Island and the shallow nature of the basin are considered to be the leading factors which have contributed to the development of the carbonate basin. The biogenic composition of the sediments in the channel is dominated by benthic foraminifera, followed by molluscs (pelecypods and gastropods) and corals.The mineralogy of the siliciclastic sediment is dominated by quartz, feldspar and hornblende. A possible source for the terrigenous sediments in the western part of the channel is the metamorphic rocks of the Mozambique belt. The siliciclastic sediments on the eastern side of the channel are most probably reworked materials from the Masingini ridge sandstones north of Zanzibar Town and former beach ridges, but the possible origin from the western side of the channel cannot totally be ruled out. Three bio-physiographic zones of the sediments in the basin have been discriminated: 1- the coastal zone, 2- the reef platforms/patch reefs zone and 3- the central channel zone. The Tidal Dominated Reef Platform Environment (TDPRE) east of the Zanzibar channel is further subdivided into two subdivisions; northern and southern. The current pattern in the TDPRE sediments is influenced by the ebb-flood tidal phases and the local variability of the topography in the area. Satellite remote sensing data has been used to investigate the nearshore bathymetry in some parts of the TDPRE sediments and the preliminary results of the study demonstrated that remote sensing approach may potentially be used for the nearshore bathymetric mapping of the Tanzanian coast, but the approach used by the present study needs to be further tested using other vegetation types. Remote sensing has also been used to investigate the coastal sediment dynamics in an accreting coast proximal to the river. The current growth rate of the Ruvu delta is potentially threatening the future existence of the Zanzibar channel carbonate basin, with its associated ecosystem. Future studies are therefore focusing on further investigation on the most important significant factors which have influenced the recent growth of the delta. The satellite remote sensing approach is being proposed for the future studies in other coastal sedimentary environments of Tanzania, as it might be the best and most cost effective option for obtaining the lacking geological data in the Tanzania integrated coastal zone management studies. The present study also recommends for further grab sampling in the central and western parts of the channel, as well as further analysis of the carbonate sediments. The suggested studies would shed further light on the hydraulic regime of the carbonate sediments and its bio-physiographic setting.
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