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Sedimentology and diagenesis of late palaeoproterozoic carbonates, Southern McArthur Basin, northern Australia

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Winefield, PR (1999) Sedimentology and diagenesis of late palaeoproterozoic carbonates, Southern McArthur Basin, northern Australia. PhD thesis, University of Tasmania.

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Abstract

The late Palaeoproterozoic (Statherian) McArthur Group comprises a thick (5
km) sequence of platformal stromatolitic dolostone and clastic sediment with local pyritic,
carbonaceous siltstone units. Exposure of the McArthur Group is largely confined to the
elongate, - N-S trending Batten Fault Zone, southern McArthur Basin. The Barney Creek
Formation and Reward Dolomite in particular, are characterised by rapid thickness
changes that reflect the onset of a phase of tectonically controlled subsidence. Detailed
study of the 1ciddle McArthur Group sequence that includes these units has recognised
three distinct depositional sequences, each of which provides important insights into the
prevailing basin architecture and palaeoenvironmental conditions during sedimentation.
The Emmerugga Depositional Sequence (EDS) represents a phase of increasing
accommodation (from Myrtle Shale to Emmerugga Dolomite) during which carbonate
platform lithofacies accumulated in a series of shallowing-upward cycles that thicken and
deepen upsection. An abrupt basinward shift in lithofacies at the top of the EDS marks
the onset of tectonically-induced basin subsidence recorded by the deposition of the
Barney Creek Depositional Sequence (BCDS). The BCDS comprises the Teena Dolomite,
Barney Creek Formation and Reward Dolomite, and is characterised by rapid lateral
lithofacies variation and the development of numerous sub-basins adjacent to pre-existing
major -N-S trending structures (i.e. Emu, Tawallah and Hot Spring Faults).
Facies architecture within the BCDS demonstrates that the thickest sections of
Basinal lithofacies (or depocentre maxima) occur adjacent to NNW-NW trending
segments of the major fault systems, while condensed BCDS sections developed adjacent
to NNE-NE segments. This facies architecture is consistent with differential strike-slip
movement along sinuous ~N-S faults resulting in the formation of transtensional (or
releasing bend) sub-basins adjacent to NNW-NW fault segments, while 'transpressive' subbasins
or BCDS platform margins developed along NNE-NE trending fault sections.
Adjacent to sub-basin margins, gravitationally unstable slopes were formed that are
characterised by coarse-grained clastic Slope lithofacies and abundant slope-related
synsedimentary deformation. This caused the development of neptunian dykes;
liquefaction breccias, megabreccias and soft-sediment intrafolial folds.
The initial stages of BCDS deposition were characterised by abundant submarine
cementation, including the widespread precipitation of radiating fan-like seafloor cements
(or Coxco needles). Coxco fan-like cements are recognised throughout a number of
lithofacies and are interpreted to be associated with the upwelling of anoxic, HC03
charged bottom water. This upwelling event is thought to have been triggered by changes in the bathymetry of the basin during the onset of differential subsidence. Isotopic analysis
of the basal BCDS sequence records a negative excursion in 013C values that is consistent
with the interpretation of upwelling of anoxic bottom water and the widespread
precipitation of carbonate.
The Lynott Depositional Sequence (IDS) overlies the BCDS and is interpreted to
represent a phase of renewed basin subsidence. The spatial distribution of the LDS
lithofacies is distinctly different to that of the BCDS, suggesting a subtle but important
shift in the controlling structures on LDS deposition.
Diagenesis of the middle McArthur Group ts dominated by abundant
synsedimentary cements and precipitates which include: fibrous dolomite cements; micritic
cements; fan-like seafloor cements; and spheroidal dolomite. Depositional components
and early synsedimentary cements are exclusively preserved as dolomite, with average 013C
and 0180 values ranging from O%o to -2%o PDB and -4%o to -10%o PDB respectively.
Textural relationships and excellent fabric preservation support early dolomitisation of the
sequence. Pervasive dolomitisation of dully luminescent depositional and early diagenetic
components must have occurred relatively early during diagenesis, as overlying burialrelated
dolospar cements are zoned under cathodoluminescence, indicating that they
precipitated as primary dolomite.
Widespread synsedimentary cementation of the middle McArthur Group
effectively controlled the pattern of later diagenesis. Primary porosity is commonly
rimmed by several generations of fibrous dolomite cement, with only a relatively minor
component of post-depositional dolospar cement occluding porosity. The petrography
and geochemistry of dolospar cements is consistent with a burial origin with 0180, and to a
lesser extent 013C, showing a trend toward lighter values. Several distinct dolospar
generations are recognised that form a regionally recognisable pattern (Do1~Do2~Do3)
reflecting progressive burial of the middle McArthur Group sequence. The development
of a burial cement stratigraphy allows some constraints to be placed on the timing of
discordant MVT -style sulphide precipitation and hydrocarbon migration. Base-metal
sulphides commonly directly overlie brightly luminescent dolospar (Do2b), while bitumen
post-dates Do2b. Considerations of the burial history of the middle McArthur Group
sequence indicate that the onset of hydrocarbon migration most likely occurred prior to
maximum burial (-3 km) of the sequence (-1600 Ma). This suggests that precipitation of
pre-bitumen dolospar cements (i.e. Dot, Do2a.& Do2b) and sulphides probably occurred
prior to -1600 Ma.

Item Type: Thesis (PhD)
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Date Deposited: 30 Aug 2011 05:45
Last Modified: 11 Mar 2016 05:54
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