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Abstract
Journal of Sedimentary Research, Section
B: Stratigraphy and Global Studies
Vol. 67 (1997)No.
6. (November), Pages 1083-1096
Stratigraphic Patterns of Deep-water Dolomite, Northeast Australia
George R. Dix
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 ~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. d13C values of dolomite
reflect incorporation of at least two sources of bicarbonate mixed with
seawater: (1) in more carbonate-rich sediment, d13C
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, d13C values
(-4 to -6%) reflect contribution from organic-matter diagenesis. The majority
of d18O 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,
~
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 ocean c volume
transport linked to eustatic change in sea level.
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