Arafura Sea

The Arafura Sea is a shallow cratonic sea that lies between Irian Jaya, the Indonesian (western) end of New Guinea, and Australia. The western border of the sea is the Indian Ocean, and on the east, it is separated from the Gulf of Carpentaria by the Arafura Sill. Although a minor rise on the sea floor, the sill separates a clastic (Gulf of Carpentaria) from the carbonate province of the Arafura Sea. Unlike the flat floor of the Gulf of Carpentaria, the floor of the sea is cut by the Arafura Channel, a major channel system that leads to the Indian Ocean at the shelf edge (Grim and Edgar, 1998). Some of these tributary channels are filled with sediment, but the main channel is sediment free, exposing older rocks on the channel walls. The main channel was apparently the conduit for water draining the Gulf of Carpentaria during low stands.
 

Studies in the Arafura Sea have been concentrated in the southern part and our understanding of sedimentation in this area comes from sparce surveys and information from industry wells. From these data it is clear that carbonate sedimentation prevails at very low rates of accumulation. At least four major unconformities have been identified, Pleistocene, Pliocene, late Miocene-early Pliocene, and early Miocene or early Tertiary (Veevers, 1971; Jongsma, 1974). These four do not correlate well with the 17 unconformities identified in the Gulf of Carpentaria based of seismic data (Edgar and others, in prep). Interpretation of a single northerly seismic line in the northern part of the Arafura Sea indicates little siliciclastic sediment offshore Irian Jaya in the Timika region. Here the coastal plain is very narrow in contrast to the extensive coastal plain of southern Papua New Guinea. The narrow coastal plain, development of estuaries, and lack of sediment off shore is puzzling in view of the proximity of the coast to the high Central Range, where remnants of glaciers still exist, rainfall is high, and erosion of the mountains is equivalent to the erosion in Papua New Guinea (Weiland and Closs, 1996).
 

Gulf of Carpentaria

The 900- by 600-km diameter Gulf of Carpentaria is a shallow, flat-floored epicontinental sea that lies on the Australian plate between northern Australia and the island of New Guinea. The gulf is separated from the Coral Sea on the northeast by the 12-m-deep Torres Strait and from the Arafura Sea on the northwest by the Arafura Sill, which lies about 53 m below the sea surface. The maximum water depth is about 70 m, and below 50 m, the bottom gradient is about 1:13,000 (Fig. 1). In general, the floor of the gulf is smooth except for a few isolated features with only a few meters of relief. The climate grades from the humid tropics in the north (8S) to the dry tropics in the south (18S). Dry southeasterly winds are interrupted for three austral summer months of the year by a northwesterly monsoon and associated rain. Rivers are predominantly ephemeral, although several perennial rivers flow mostly from the south and southeast. The gulf is underlain by two basins, the Jurassic/Cretaceous Carpentaria basin and the overlying Cenozoic Karumba basin. The Karumba basin is sediment starved; subsidence exceeds the rate of sediment accumulation.
 

The siliciclastic sedimentary section is divided into two units, a lower Miocene-Pliocene(?) and upper unit probably of Pleistocene/Holocene age. The most outstanding feature of the upper sedimentary unit is the basin-wide reflections that are cut by buried channels, inferring at least 14, possibly 17, major transgressive/regressive events (Fig. 3) within the resolution of the seismic system (~ 3 m). Channels in the upper few meters of sediment that were not recorded by the seismic system were identified on yet unpublished 3.5 kHz records. The depth of the channels recorded on the seismic records varies from a few meters to about 40 m, indicating that channels cut to these depths at a low-gradient were formed under subaerial conditions.. Basin-wide interfluve surfaces are interpreted to be paleosols capping and underlying deposits that are about 4 to 6 m thick. Flat paleosol surfaces are good time markers, even on a continental scale, having been formed following rapid regressions over large areas and largely preserved by rapid low-energy transgressions. Transgressions over a very flat surface, where shallow water (up to 20 cm depth) may extend for kilometers or ten's of kilometers, will be very low energy and are unlikely to erode the underlying paleosol. Paleosols can be easily traced to the west margin of the gulf, but they converge at least within the resolution of the seismic system near the east margin, possibly forming a compound paleosol.