LEG 133

Northeast Australian Margin

Leg 133 occupied 16 sites along two transects over the platforms and basins of the northeast Australian margin; one transect extending eastward from the outer shelf and slope of the Great Barrier Reef (Site 819 to Site 822) into the Queensland Trough (Site 823) and onto the Queensland Plateau (Site 811, and Sites 824 and 825), the second transect extending northward from the edge of the Marion Plateau (Sites 815, 816, and 826) across the Townsville Trough to the Queensland Plateau slope (Site 812 to Site 814). The platforms of the Great Barrier Reef represent the largest areas of neritic carbonate deposition currently on earth, and their tectonic setting on a young passive margin proximal to major depocenters makes them of immense scientific interest as analogs for similar associations that have occurred repeatedly throughout geological time. Litho-, chemo-, magneto-, and biostratigraphic analyses established a Cenozoic record of environmental change which differentiates between the influences of sea-level fluctuation, tectonic subsidence, terrigenous flux, paleoclimate, and paleoceanography on carbonate platform development.

The initiation of carbonate sedimentation on the Queensland Plateau began in the early middle Eocene when the seas transgressed across the metasedimentary continental basement. Temperate faunas inhabited the local seas during the latest Oligocene but, by the latest early Miocene, tropical faunas dominated on the Queensland and Marion plateaus, reflecting the northward movement of the Australian Plate and the initiation of the southward flow of tropical waters from the equatorial Pacific. During the early middle Miocene, the tropical waters supported robust reefal growth that gradually declined during the late middle Miocene, possibly in conjunction with paleoenvironmental changes induced by the steady drop in eustatic sea level during this period.

Carbonate production on the shallow-water banks diminished dramatically in the late Miocene in response to global climatic deterioration and continued eustatic sea-level falls, accelerated by a seasonal influx of colder waters. Apparently the banks were unable to respond to climatic amelioration in the earliest Pliocene. A pulse of more rapid subsidence on the Queensland Plateau, in combination with rising eustatic sea level, may have essentially drowned the banks. Conditions stabilized in the late early Pliocene when the carbonate banks were rejuvenated, remaining more or less productive until the present but on a scale much reduced from that of the early to middle Miocene and barely able to keep up with continued subsidence. Carbonate banks of the Marion Plateau never recovered from being suffocated by increased terrigenous influx. Initiation of reef growth on the Great Barrier Reef is even younger, beginning about 1 Ma.