Abstract:
Deep ocean drilling within a shelf-edge transect off northeast Australia in water depths of 210-570 m (Sites 819, 820, and 821, Ocean Drilling Program Leg 133) has penetrated 400 m of mixed carbonate and siliciclastic lithologies of pleistocene age (<1.48 Ma ). Mineralogic, geochemical, and petrographic analyses of 592 sediment samples form the database for this investigation of diagenetic patterns and processes under burial. Authigenic replacement dolomite forms at the expense of magnesian calcite; prolific dolomitization occurs during slow sedimentation rates near the sediment-water interface where diffusion from overlying seawater provides an additional source of Mg. Negative delta (super 13) C signatures (<-12 per mil PDB) indicate that dolomite precipitation is bacterially-mediated within the diagenetic zone of sulphate reduction. Palisade calcite and sparry calcite burial cements (10-25 mu m) are suspected to be of magnesian calcite mineralogy by palisade cement morphology, selective dolomite replacement, alteration to finer-grained, anhedral, microcrystalline calcite (5 mu m), and excess abundance of magnesian calcite (as detected by XRD) for which the amount of skeletal magnesian calcite observed petrographically is inadequate. Onset of lithification to chalk-equivalent strata occurs at greater burial depths in northeast Australia (below 93-118 m in comparison to 55-70 m in pure carbonate slopes). The deeper first occurrence of partly lithified strata is caused by more rapid burial rates that exceed the rate of diagenesis and by the possible insulating effect of clay minerals (kaolinite or chlorite and illite) within an intergranular muddy matrix (<2 mu m) that preserves initial metastable carbonate mineralogy. Lithification is enhanced by increased carbonate contents (>57%) that contain an original metastable mineralogy -- susceptible to dissolution and subsequent precipitation of stable calcite cements. Metastable aragonite persists consistently as skeletal fragments to a burial depth of at least 400 m (generally <20% of the carbonate fraction), whereas aragonite at depths below 250 m in pure carbonate slopes may occur only locally. Metastable aragonite and magnesian calcite and their constituent trace elements arc preserved in anomalously higher quantities at depth intervals of low carbonate content and increased terrigenous influx (high relative detrital quartz content). The carbonate mineral components, that include anhedral, microcrystalline calcite, form isolated, unconnected domains within a greenish, clay-bearing, muddy intergranular matrix that acts as a physical and chemical barrier to impede rapid burial diagenesis.