THE MYSTERY OF THE SAHUL SHOALS: A DIFFERENT CARBONATE DEPOSITIONAL ENVIRONMENT

The Sahul Shoals are located between the Kimberly region in Western Australia and the island of Timor in the Timor Sea. South of the Shoals is a broad continental shelf and to the north is the Timor trough, a tectonically active basin which has subsided to 3000m as a result of the collision between the Australian Plate and the Indonesian Archipelago. This deformation was initiated between 5 and 6 million years ago. The Sahul Shoals rise up very steeply from between 200 and 500m water depth, and form a shallow water carbonate depositional environment that includes numerous platforms with a total area of over 10,000 square kilometers. The area is in the Indo-Pacific Warm Pool and surface sea water seasonally reaches temperatures in excess of 30 degrees C. What makes these carbonate platforms so unique however, is the fact that they don't appear to have ever grown to sea level during the Holocene. They are uniformly planed off down to approximately 20m below sea level. Normal coral reef communities struggle to survive on these Shoals, while Porites sp. and Halimeda are locally abundant. Some of the Porites heads are large, up to 6m high and 10m across. Trace element geochemistry and stable isotopes of carbon and oxygen were measured from a series of short Porites cores from several sites and give important clues as to why no other coral species are thriving in this environment and perhaps why these mysterious carbonate platforms have not grown to sea level during the Holocene.