Carbonate sedimentation on subtropical shelves around Lord Howe Island and Balls Pyramid, Southwest Pacific

Lord Howe Island and Balls Pyramid are mid-oceanic basaltic islands, 20 km apart, in the Tasman Sea. Subaerial carbonates dating back at least 350 ka and a Holocene fringing reef occur on Lord Howe Island. No reefs or subaerial carbonate deposits occur on Balls Pyramid. Both islands sit near the centre of wide shelves that are on average 40–50 m deep with a distinct break of slope, between 70 and 100 m depth. The Lord Howe shelf is characterised by a discontinuous drowned ridge, which rises to 30 m depth and is located between the mid-shelf and shelf edge. It is composed of limestone and is interpreted as a fossil reef being veneered by Holocene coralline algae. Early to Middle Holocene branching-coral gravel is found in the lee of the fossil reef, indicating limited give-up reef growth during the Postglacial transgression. The surface sediments across the shelf are calcareous, except in close proximity to the island where volcanic content is significant. Coralline algae represent the dominant grain type, with minor amounts of coral. Bryozoans, Halimeda and foraminifera are common; however, they are not volumetrically important. Rhodoliths and molluscs occur near the shelf edge, and appear to have accumulated at times of lower sea level. These subtropical shelves are mid-oceanic examples of the transition between tropical and temperate carbonate sedimentation. They indicate the potential for carbonate production on broad planated shelves outside reef-forming seas.