STRATIGRAPHIC CONTROL OF THE GEOCHEMISTRY OF HOLOCENE INNER-SHELF FACIES, GREAT-BARRIER-REEF

Cleveland Bay and Halifax Bay are adjacent embayments, situated on the inner-shelf region (0-20 m) of the central Great Barrier Reef shelf o ff Townsville, northeast Australia. These bays contain Holocene sedime nts up to 5 m in thickness, deposited during the last stages of the po st-glacial sea-level rise. X-ray diffraction and X-ray fluorescence te chniques were used to examine the mineralogy and geochemistry of the m ain Holocene facies, sampled in 40 vibrocores. The sediments of southe rn Halifax Bay and Cleveland Bay consist mainly of quartz (ca. 50%), a lkali 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 comprise s, in order of increasing age, modern bay, shoreline and mangrove sedi ments. The oxides of the major components (SiO2, Al2O3, Fe2O3, MgO, Ca O, K2O, Na2O, P2O5) in Halifax Bay show the predicted linear correlati ons 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 Ho locene facies. However, in Cleveland Bay these elements are poorly cor related 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 migratio n of elements between adjacent facies. A similar oxic zone is poorly d eveloped in Halifax Bay because the shoreline facies there is either t hin or absent. That the chemical signatures of Holocene inner-shelf fa cies are spatially-variable, and are strongly influenced by the strati graphy, has wide implications for geochemical studies of marine sedime nts.