Big Bank Shoals of the Timor Sea
An environmental resource atlas

The Timor Sea, located off the coast of north-western Australia, broadly separates Australia from the Island of Timor, and covers the Sahul Shelf, the Shelf edge and the Timor Trough (Figure 1). These features are the surface expression of the complex geology of the area, with a history of rifting at various times from the late Devonian to the Jurassic and more recent modification by the collision between the Australian and Eurasian plates. The tectonic history of the area has caused the shoreline to change significantly over geological time. Evidence of this is clearly seen in the present morphology of the sea floor.

This Atlas focuses on a group of submerged banks, known as the Big Bank Shoals. These lie between Karmt Shoals and Echo Shoals, stretching for approximately 50 km in a NE-SW direction along the edge of the Sahul Shelf. They comprise some 13 significant submerged banks, ranging in size from 0.05 km² to 40 km². They emerge from a water-depth of 200 to 300 metres and rise steeply to within 16 to 30 metres below the water surface (Figure 2).

Figure 2: Cross section of the shoals region showing the Continental Shelf and two banks
(vertical exaggeration = 300)

The Sahul Shelf - geomorphology and palaeogeography

The Sahul Shelf is a broad, shallow platform off the northern Australian coast, ranging from 300 to 500 km in width, and is considered to be a recently drowned part of the Australian continent. It has been established that as recently as 18,000 years ago, most of the Sahul Shelf was exposed. Shoreline positions have been identified in locations which now lie 100 to 140 metres below sea level. Extensive palaeo-river channels, some up to 150 km long and 5 km wide, connect the Bonaparte Depression to the old shoreline. These channels are up to 240 metres deep (Figure 4). This suggests that extensive sub-aerial erosional processes, probably dominated by fluvial systems, formed as a result of uplift of the Shelf due to the plate collision to the north. The Bonaparte Depression, located to the southeast of the Big Bank Shoals, is believed to have formed an estuarine embayment or a shallow lake with depths of 18 to 28 metres (Lavering, 1993). Between 15,000 and 13,000 years before the present day, a rapid rise in the sea level inundated most of the Sahul Shelf.

Presently, water-depth on the Sahul Shelf ranges from 50 to 120 metres. It drops sharply to 3,000 metres in the Timor Trough, which runs parallel to the Island of Timor. At the outer edge of the Sahul Shelf lies a series of submerged carbonate banks. They are thought to represent drowned carbonate formations which formed a string of islands seaward of the palaeo-coastline, and which have been unable to keep pace with sea level rise in the past 20,000 years (Lavering, 1993).

Various theories exist for the origin and evolution of these Shoals. One possible origin is that they represent drowned platforms, after subsidence or sea level rise exceeded upbuilding. The platforms were submerged below the euphotic zone, terminating rapid production and accumulation of carbonate by photosynthetic organisms (Kendall and Schlager, 1981). The euphotic zone extends down to 100 metres, but may be as little as 30 metres in basins where fine grained carbonates or clastics are abundant. During drowning, numerous isolated build-ups may have developed on the deeply submerged platform. These consist of narrow pinnacle reefs to broad shelf-rimmed banks or 'shelf atolls' and down-slope banks (Klovan, 1974; Read, 1985).

During rapid sea level rise, these banks may have undergone three phases of accumulation:

• a lag phase, during which the accumulation lagged behind the rising sea level, and deeper water biotas were established, such as sponges and branching corals. (e.g. Sneezy, Wicked and South Bank).

• a catch-up phase, when the upward growth exceeded the sea level
  rise, and a shallow sequence developed. (e.g. hard and soft corals at Kepah, Kepting and Tiram).

• a keep-up or tracking phase, when the build-up kept pace with relative sea level rise, and the substrate was colonised by communities best adapted to their relative static water-depth. (e.g. the Halimeda banks such as Big Bank, Snow White, Happy, Grumpy and Udang).

Figure 3: The bathymetry of the Timor Sea region showing the location of the Big Bank Shoals (colour gridded bathymetric image provided by the Australian Geological Survey Organisation from it's 30^o arc second bathymetric database).

Another possible origin for these banks is carbonate accumulation associated with hydrocarbon seeps. Fault systems, up which hydrocarbons (oil and gas) may have seeped, are known to occur along the shelf edge in the vicinity of the Shoals. Research indicates that microbial utilisation of hydrocarbons creates by-products (carbon dioxide and bicarbonate) that catalyse precipitation of authigenic carbonates, such as aragonite and dolomite (Roberts, 1992). These products form the basis of the hard substrate required for colonisation by reef-building and bioherm-building organisms. Elevated hard substrates provide ideal habitats as they provide substrate to which organisms can adhere and expose filter-feeders to the maximum amount of passing nutrients.

Coring at Big Bank to a depth of 55 metres showed an extremely high Halimeda rubble content and a marked absence of terrigenous sediments. This indicates the in situ growth of these banks by accumulation of Halimeda skeletons, which has continued since the late Pleistocene. is Atlas represents a compilation of the various studies and reports resulting from the individual environmental study programs conducted during 1995 and 1996, as well as a literature research on the various biological communities encountered. It is a joint effort by BHP Petroleum and AIMS to further analyse the results of field investigations and to attempt to draw some conclusions based on the present state of knowledge of the environment. The principal aim of this project is to assist in the development of strategies to minimise the environmental impact of hydrocarbon exploration and production activities. The Atlas also serves to make these findings more accessible to the wider community, to demonstrate BHP's commitment to further understand the environment in which the company operates and its willingness to cooperate with Governments and the community to foster environmental protection.

Figure 4: Map of Continental Shelf showing palaeo-river channels.

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Last updated - 30 August, 2001

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