Mineralogy and geochemistry of sediments on the far northern Great Barrier Reef shelf

The sea-floor sediments of the Far North Section of the Great Barrier Reef Lagoon have been investigated by seismic traverse, and by detailed analysis of surface grab-samples and short (3-5m) cores. Sample collection was along three transects; two from coast to reef, the third parallel to and near the shore from Cape York to Weymouth Bay. In total, 195 samples have been investigated, using binocular microscopy, size analysis, X-ray diffraction, bulk and trace geochemistry, scanning electron microscopy and electron microprobe analysis.\ud \ud The seismic data reveal an erosional unconformity ranging from at the sea bed to ~12 m below the sediment surface, interpreted as the Holocene-Pleistocene unconformity, resulting from lowering of sea-level during the Pleistocene. Marine transgression since the last glacial maximum has allowed the deposition of terrigenous and marine sediment across the Lagoon. The sediment character differs above and below the unconformity. Above, the sediments are seismically transparent, dominated by Halimeda bioherms, whereas those below are almost seismically opaque, non-marine sediments. Areas of the lagoon north of Raine Island are devoid of Halimeda because of poor nutrient supply. From the reef to shore, the Halimeda bioherms decrease in thickness and character, forming thick mounds in the east, but becoming flatter and thinner westward, and eventually disappearing toward the inner shelf.\ud \ud The land bordering this section of the GBR Lagoon provides a narrow catchment between the Great Dividing Range and the coast. The area is subdued, erosion is minor, and both sediment and fresh water inputs to the lagoon are low. The source rocks for sedimentation into the Lagoon include quartzose sand dunes, lateritic regolith, Jurassic sediments and volcanics. Facies analysis shows the near shore inner shelf sediments are dominantly terrigenous, made up of quartz and clays with minor carbonate. Their detailed composition closely reflects the local sources. The clays include both kaolinite and smectite, derived from different sources; the kaolinite from the lower, more deeply weathered parts of the landscape, and smectite from the less mature high-lands and coastal volcanics. Sediments of the mangrove swamps are more clay-rich than others, and show the effects of post-depositional change as a result of changing physico-chemical conditions (redox). These changes include quartz grain fragmentation, growth of pyrite framboids, precipitation of secondary magnesian calcite and the conversion of kaolinite to smectite.\ud \ud \ud Geochemical analysis allowed four factors to be developed from which a classification of the sediments of the lagoon was erected:\ud \ud Factor 1: samples dominated by terrigenous sediment, Ga, Zn, Ni, Mn, Cr, Ti, Ba, Rb, Y, Nb, \ud Al, NaO, SiO₂, K₂O and V. \ud Factor 2: carbonate factor: Ca, Mg, Sr, P₂O₂, Pb, Cr, Ni and Zn. \ud Factor 3: transition elements Ti, Fe, Ga, Mn, V, As,Y and Nb. \ud Factor 4: S and Fe\ud \ud These factors, plus the mineralogy, were used to establish sedimentary facies for the area and to map their distribution. Facies analysis delineate five facies:\ud \ud Facies 1: Near shore terrigenous sandy mud facies, characteristic of swamp and mangrove intertidal zones. \ud Facies 2: Inner shelf shore sand, consisting largely of coastal dune sand. \ud Facies 3: Shelly beach sand, derived from both Pleistocene and Holocene \ud beach ridges, mostly found in Newcastle Bay. \ud Facies 4: Halimeda generated carbonate Facies, occupying the mid-outer shelf. \ud Facies 5: Pure carbonate coral reef Facies, characteristically occurring in the outer shelf and parts of the middle shelf.\ud \ud Beyond the terrigenous dominated near shore region, carbonate sediments gradually increase in proportion, becoming very dominant as Halimeda and coralline algae Facies toward the outer shelf. The Halimeda Facies is shown by increased aragonite and higher Fe and Sr values. Coralline algae Facies has higher Mg-calcite. The distribution of these facies results from variations in nutrient supply for the Halimeda, and position relative to the reef itself and to the shore