Mineralogical and Biogenic Composition of the Zanzibar Channel Sediments, Tanzania

• DOI: 10.1006/ecss.2000.0686
• Publication Date: 2002-09-16T15:46:35Z
• Authors: Y.W. Shaghude; K.O. Wannäs
• Keywords: 14. Life underwater; 01 natural sciences; 0105 earth and related environmental sciences

Abstract Sediments from the Zanzibar Channel have been analysed for mineralogical and biogenic composition. The main objective of the study was to describe the depositional environment of the sediments based on these parameters. The study was also conducted to give an insight into sediment provenance of the sediments. Quartz, feldspar (both microcline and plagioclase) and hornblende are the main mineralogical components, while zircon, muscovite, biotite and augite occur as accessory minerals. The quartz grains are generally angular to sub-angular and highly strained, indicative of immature sediments, probably derived from highly metamorphosed rocks. The presence of hornblende also supports the conclusion that the sediments are immature. A possible source is the metamorphic rocks of the Mozambique belt, located about 25–50 km from the coastline. Benthic foraminifera are the dominant biogenic constituents. Other common biogenic constituents are molluscs (pelecypods and gastropods) and corals. Bryozoans and ostracods are relatively less common, while sea urchins are accessory biogenic components. The composition and abundance of these biogenic components and morphological, sedimentological and mineralogical data permit construction of a simple model which distinguishes between three bio-physiographic settings in the Zanzibar Channel using depth contours: (1) the coastal zone (0–10 m depth); (2) the reef platform/patch reefs zone (10–20 m depth) and (3) the central channel zone (>20 cm depth). In the coastal zone, the sediments are dominated by fine sand, sized with a very prominent fine sand mode. The proportion of benthic foraminifera is lower than in the other two zones ( 25%). Two lithogenic mineral components (quartz and feldspar) occur in significant proportions. In the reef platform/patch reefs zone, the sediments are coarse to medium sand sized with no distinct mode. the proportion of benthic foraminifera is highest (80–85%) there, and so is the biogenic:quartz ratio (>10) but that of lithogenic components is lowest (