Sedimentology of Surficial Cool­water Carbonates on the Continental Shelf of Tasmania, Australia

Abstract

The continental shelf of Tasmania and Bass Strait is veneered by surficial carbonate and terriginous sediment, which represents the coldest carbonate assemblage on the southern shelf of Australia. Three major facies are recognized based on dominant components, and are further separated into eight sub­facies. Quartzose sand is a nearshore facies, whereas Holocene biogenic sand and gravel is found outboard in deeper waters. Relict sand and gravel is located between these facies on the majority of the shelf, although it is rare on the narrow, eastern shelf. When Holocene biotic sediments are considered, five facies can be identified based on the abundance of bryozoans, molluscs, benthic foraminifer, sponge spicules, and coralline algae. Molluscs and bryozoans dominate the most widespread facies; molluscs are numerous nearshore, in large bays, and on granitic ridges, and bryozoans are abundant in deeper, or nutrient rich waters. Again, eastern and western shelves differ, in that molluscs, which are common in the east, only occur near rivers on the western shelf. The key factor controlling this distribution is the level of trophic resources available. Dominant bryozoan growth forms provide detailed resolution of the Holocene sediment and relative importance can be linked to water energy or substrate. In general, delicate branching forms are found in deeper water on the eastern and western shelves, as well as the eastern side of the Bass Strait, whereas articulated zooidal forms dominate higher energy waters and areas with unstable or fine substrate, such as the southern shelf and western Bass Strait.