Sedimentology and geochemistry of modern temperate bryozoan carbonates around Tasmania, Australia

• Main Author: Amini, ZZ
• Format: Thesis
• Language: English
• Published: 2002
• Subjects: Bryozoa; Marine sediments; Carbonates; Antarctic; Antarc*
• Online Access: https://eprints.utas.edu.au/19136/; https://eprints.utas.edu.au/19136/1/whole_AminiZahraZargar2002_thesis.pdf

This thesis describes various aspects of the modern temperate, shallow (<200m) shelf sediments around Tasmania (latitudes -38°-44° S, longitudes -144°- 149° E), Australia, and is structured into three different sections: eastern and western Tasmania, and Bass Strait. These are ideal open shelf environments for carbonate sedimentation due to availability of nutrients, presence of different water masses, limited terrigenous input and low variation in seawater temperature and salinity. These conditions maintain sufficient saturation of CaCO3 and thus preserve extensive temperate carbonates around Tasmania. In this study, sedimentological and geochemical properties are combined with present oceanographic features to obtain a better understanding of the origin, variation and distribution of sediments off Tasmania. In addition to bulk sediments, specimens of different bryozoan morphotypes have been selected to evaluate their reliability in recording ambient environmental conditions, mineralogy and geochemical characteristics. Cool temperate carbonates dominate over siliciclastics on the continental shelf around Tasmania. The dominant grain size in all sample studied is very coarse to medium sand, followed by fine sand, gravel fractions and minor fines (muds) and thus sediments are moderately to well sorted. The major biota in bulk sediments off eastern and western Tasmania are bryozoans, followed by foraminifera and molluscs, and the remaining fauna such as echinoderms, worm tubes, sponge spicules and crustaceans occurs in small amounts. The term bryoforamol assemblage is applied to this carbonate skeletal association. However, in Bass Strait, the dominant fauna consists of bryozoans, followed by molluscs, minor content of foraminifera, and thus the term bryomol assemblage is used. Skeletal fragments, siliciclastic grains and debris occur in significant quantities in all sample studied. Pellets and intraclasts comprise minor components. Biota varies with different size fractions. Bryozoans and molluscs occur mainly in gravel; mixtures of bryozoan, foraminifera and molluscs in the sand fraction; and mainly bryozoan and foraminifera in the fine sand fraction. Fines consist of debris and minor amounts of sponge spicules. Variation in grain size, and biotic and abiotic constituents in different size fractions is attributed to different factors such as growth size and susceptibility to breakage of the organisms, depth, latitude and water energy. Bryozoans are the major components in bulk sediments and in all different size fractions as a result of low productivity of the other organisms, normal salinity, presence of nutrient-rich upwelling sub-Antarctic water, minor amounts of fines (mud) and availability of hard to particulate substrates. Bryozoan contents increase with increasing water depth, bivalves are dominant in shallow-depths, foraminifera in middepth and gastropods are mostly located around 130 m. Some biotic and abiotic grains are bored and encrusted suggesting slow rates of sedimentation.