Sedimentology of the Neogene to Modern glauconite-goethite-phosphate system : east australian continental margin between 29° and 32° south latitude. Le système sédimentaire glauconite-goethite-phosphate du Néogène à l'Actuel : la marge continentale orientale de l'Australie entre 29° et 32° de latitude sud
Authors: Riggs Stanley R. , Snyder Scott W. , O'Brien Geoffrey W. , Cook Peter. J. , Heggie David T. 
Published: June 2018

Outer shelf and upper slope environments (250 to 525 m) on the East Australian continental margin off northern New South Wales, represent an important iron-rich sediment province. The dominant (up to 30 9c) authigenic-diagenetic components are glauconite and associated goethite, with minor concentrations of carbonate fluorapatite (mean P₂O₅ content = 0.37 %) within a muddy foraminiferal fine sand. The outer shelf and shelf-slope break (250 to 425 m depth) contain a series of small scarp features interpreted to be outcrops of indurated subsurface units. Sedimentologic and biostratigraphic data from these features suggest that they are Miocene hardgrounds composed of ferruginous-phosphate pavements and gravels. Plank tonic foraminiferal assemblages in unconsolidated surface sediments overlying the hardgrounds range in age from late Pliocene through Holocene and are draped across the continental margin as a veneer covering older strata which only locally crop out.

Relative concentrations of glauconite and goethite change downslope and downsection as follows. (1 ) Surface sediments at depths from 250 to 340 m are dominated by goethite with subordinant amounts of weathered glauconite grains. (2) Sediments underlying (1) and constituting the surface sediments from 340 to 465 m are dominated by glossy dark green glauconite with subordinant amounts of goethite. (3) Sediments underlying both of the above and constituting the surface sediments from water depths of 465 to 525 m are dominated by glossy dark green glauconite with subordinant concentrations of light green glauconite within foraminiferal tests. Glauconite forms within foraminiferal tests as light green grains and is subsequently altered to glossy dark green grains with accompanying dissolution of the tests. Above the shelf-slope break, dark green glauconite is altered to goethite within the surface sediment. Goethite occurs at the top of multiple depositional cycles in some sediments between 375 and 465 m depth, suggesting either periods of upper slope oxidation or off-shelf sediment transport during sea-level lowstands.

Two types of phosphate occur within the sediments of outer shelf and upper slope. Type I (dark reddish-brown, ferruginous phosphate) is found in water depths from at least 200 to 425 m where it forms hardground pavements and associated nodules and lag gravels. Type I phosphates contain 7 9c to 12 9c P205, 10 % to 50 % Fe₂O₃, and are in radioactive equilibrium indicating uranium-series ages in excess of 800 ka (O'BRiENand Veeh, 1980 ; O'Brien et al., 1986). Type II (gray phosphate) occurs in water depths from 340 to 525 m as irregularly shaped nodules and internal molds within macrofossils in muddy sediments with unweathered light and dark green glauconite. Type II phosphates contain 4% to 19 9c P205, 2 9c to 4 9c Fe₂O₃, negligible goethite, and yield uranium-series ages that range from 250 ka to 2.5 ka (O'Brien and Veeh, 1980 ; O'Brien et ai, 1986).