Stratigraphic control of the geochemistry of Holocene inner-shelf facies, Great Barrier Reef

Abstract

Cleveland Bay and Halifax Bay are adjacent embayments, situated on the inner-shelf region (0–20 m) of the central Great Barrier Reef shelf off Townsville, northeast Australia. These bays contain Holocene sediments up to 5 m in thickness, deposited during the last stages of the post-glacial sea-level rise. X-ray diffraction and X-ray fluorescence techniques were used to examine the mineralogy and geochemistry of the main Holocene facies, sampled in 40 vibrocores. The sediments of southern Halifax Bay and Cleveland Bay consist mainly of quartz (ca. 50%), alkali feldspars (ca. 20%), clay minerals (ca. 20%) consisting of mixed-layer clays, smectite, kaolinite and illite, and carbonate (ca. 10%) including aragonite and calcite. The Holocene facies sequence comprises, in order of increasing age, modern bay, shoreline and mangrove sediments. The oxides of the major components (SiO 2, Al 2O 3, Fe 2O 3, MgO, CaO, K 2O, Na 2O, P 2O 5) in Halifax Bay show the predicted linear correlations with geochemically related minor elements (Pb, Rb, Sr, Y, Zr, Ga, Zn, Ni, Co, Mn, Cr, Ti, Sc, V, Ba), and can be used to discriminate Holocene facies. However, in Cleveland Bay these elements are poorly correlated within each facies and provide very poor facies discrimination. These findings probably result from the presence of sandy shoreline sediments up to 2 m in thickness in the Holocene sequence of Cleveland Bay, with the consequent development of a weak oxic zone and migration of elements between adjacent facies. A similar oxic zone is poorly developed in Halifax Bay because the shoreline facies there is either thin or absent. That the chemical signatures of Holocene inner-shelf facies are spatially-variable, and are strongly influenced by the stratigraphy, has wide implications for geochemical studies of marine sediments.