Abstract

In ODP Hole 815A, located along the edge of the Townsville Trough and marginal to the Marion Plateau of northeast Australia, dolomite forms about 5-20 wt % of carbonate within a distal part of a muddy contourite succession of Pliocene (2.6-4.0 Ma) age. Integration of seismic, lithic, and biostratigraphic data identifies a history of current-controlled episodic deposition subsequently shut down during rapid subsidence by nearly equal 2.6 Ma. Overlying pelagic-periplatform sediment contains no or trace amounts of dolomite. High-order, rhythmic (1-5 m thick) variation in dolomite, quartz, and carbonate content within the contourite succession defines a past, high-order stratigraphic variation in diagenetic potential for dolomite. delta 13 C values of dolomite reflect incorporation of at least two sources of bicarbonate mixed with seawater: (1) in more carbonate-rich sediment, delta 13 C values (1 to -2.0%) indicate likely contribution from dissolution of metastable carbonate; (2) in more siliciclastic-rich sediment, with locally elevated total organic content, delta 13 C values (-4 to -6%) reflect contribution from organic-matter diagenesis. The majority of delta 18 O values of dolomite over the entire contourite succession vary between 3.2 and 2%, and are interpreted to reflect near-surface to shallow (tens of meters) subsurface, marine-derived diagenesis. A few dolomites with lower (2 to 0%) values are restricted to two stratigraphic intervals, and reflect one or some combination of the following processes: a second stage of dolomitization at greater depths; near-surface diagenesis with much warmer bottom waters; or burial diagenesis with mixture of meteoric- and marine-derived pore fluids. Magnesian calcite is absent today in the contourite succession. However, the depositional and diagenetic frameworks of the contourite succession suggest that metastability of aragonite and magnesian calcite, variable current activity, and seawater diffusion across the sediment-water interface all promoted dolomitization. Compilation of dolomite stratigraphy along slopes off northeast Australia illustrates that dolomite distribution in Hole 815A is one of four intervals (D1-D4) in strata younger than 4 Ma: D1, 0.3-0.8 Ma; D2, 1-1.3 Ma; D3 (D3a, nearly equal 1.8 and D3b, 2.0-2.4 Ma); and D4, 2.6-4.0 Ma. The stratigraphy is linked to temporal variation in supply of shelf- and pelagic-derived metastable carbonate regulated by changes in paleoceanography, eustasy, and tectonics. D1, D3b, and D4 are in sediments with ages similar to previously documented dolomitization stages in shallow-water platforms elsewhere in the world, and suggest a coincidence between elevated diagenetic potential for deep-water dolomitization in platform margins and substantive changes in oceanic volume transport linked to eustatic change in sea level.

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