The ca 2.74 Ga Mopoke Member, Kylena Formation

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The ca 2.74 Ga Mopoke Member, Kylena Formation: a marine
incursion into the northern Fortescue Group?
ab
ac
ac
D. T. Flannery , M. J. Van Kranendonk , R. Mazumder
a
& M. r. Walter
ab
Australian Centre for Astrobiology, University of New South Wales, NSW 2052, Australia
b
School of Biotechnology and Biomolecular Sciences, University of New South Wales, NSW 2052,
Australia
c
School of Biological, Earth and Environmental Sciences, University of New South Wales, NSW 2052,
Australia
Published online: 10 Dec 2014.
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To cite this article: D. T. Flannery, M. J. Van Kranendonk, R. Mazumder & M. r. Walter (2014) The ca 2.74 Ga Mopoke Member, Kylena
Formation: a marine incursion into the northern Fortescue Group?, Australian Journal of Earth Sciences: An International Geoscience
Journal of the Geological Society of Australia, 61:8, 1095-1108, DOI: 10.1080/08120099.2014.960898
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Australian Journal of Earth Sciences (2014)
61, 1095 1108, http://dx.doi.org/10.1080/08120099.2014.960898
The ca 2.74 Ga Mopoke Member, Kylena Formation: a
marine incursion into the northern Fortescue Group?
D. T. FLANNERY1,2*, M. J. VAN KRANENDONK1,3, R. MAZUMDER1,3 AND M. R. WALTER1,2
1
Australian Centre for Astrobiology, University of New South Wales, NSW 2052, Australia.
School of Biotechnology and Biomolecular Sciences, University of New South Wales, NSW 2052, Australia.
3
School of Biological, Earth and Environmental Sciences, University of New South Wales, NSW 2052, Australia.
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2
The northern part of the Fortescue Group consists of interbedded flood basalts and sedimentary rocks that
were deposited on the southern margin of the Pilbara Craton, Western Australia, during one or more
periods of continental rifting between ca 2.78 and ca 2.63 Ga. Well-preserved sedimentary intervals within
the group have yielded stable carbon and sulfur isotope data that have been used to infer changes in
geobiological processes in the Neoarchean. However, the Fortescue Group is notable for being a
predominantly subaerial succession, and it remains unclear whether data obtained from these intervals
should be interpreted in the context of deposition in marine environments, possibly recording changes in
the global ocean/atmosphere system, or in local and restricted lacustrine settings. Here, we describe the
sedimentology, stratigraphy, stromatolites and stable carbon isotope geochemistry of the ca 2.74 Ga
Mopoke Member, Kylena Formation, the oldest stromatolitic horizon in the Fortescue Group. This unit
differs in terms of internal stratigraphic relationships, sedimentology, carbonate mineralogy and stable
isotope geochemistry when compared with intervals of probable lacustrine origin in the overlying
Tumbiana and Maddina formations. In contrast, we suggest that parts of the Mopoke Member may have
been deposited under open marine conditions, or alternatively, in a lacustrine environment characterised
by differing water chemistry and basement topography. Stromatolitic microfabrics of the Mopoke
Member are dominated by spar, dolospar and vertically aligned calcitic crusts, rather than the micritic
microfabrics described from other Fortescue Group stromatolites. Mud-draped ripples are common
sedimentary features in the Mopoke Member, suggesting a tidal influence. Mopoke Member d13Ccarb
values are generally slightly positive, but also include some significantly depleted values, which may relate
to the reoxidation of 13C-depleted organic matter. d13Corg values average 36.7%, consistent with
Neoarchean marine units reported from elsewhere, but significantly less 13C-depleted than values
reported from overlying lacustrine intervals in the Fortescue Group. We conclude that some features of
Fortescue Group datasets relevant to the field of geobiology may be facies dependent, and that more
work focusing on the overall depositional environments of the Fortescue Group is needed in order to
appropriately interpret geobiological data reported from that group.
KEYWORDS: Neoarchean, tidalites, methanotrophy, microbially induced sedimentary structures,
stromatolites, lacustrine.
INTRODUCTION
Well-preserved Neoarchean sedimentary rocks of the Fortescue Group, Western Australia, have been used to investigate the evolution of the early biosphere. The Fortescue
Group is known, among other things, for providing
insights into the composition of the Archean atmosphere
based on stable carbon and multiple sulfur isotopes (e.g.
Pavlov et al. 2001; Ono et al. 2003; Eigenbrode & Freeman
2006; Thomazo et al. 2009), for insights into Archean microbial communities from stromatolites (e.g. Walter 1972,
1983; Packer 1990; Buick 1992; Awramik & Buchheim 2009),
for reports of molecular biomarkers (e.g. Brocks et al. 1999;
Rasmussen et al. 2008) and for cyanobacteria-like microfossils (Schopf & Walter 1983). Extremely low d13Corg values
*
Corresponding author: flannery@jpl.nasa.gov
Ó 2014 Geological Society of Australia
reported from the group are generally interpreted as
reflecting the incorporation of biomass derived from methane-cycling communities during a period of enhanced
global methanotrophy in the Neoarchean (Hayes 1994).
Previous studies have focused on the ca 2720 Ma
Tumbiana Formation. However, additional stromatolitic
intervals are present in the Kylena, Maddina and Jeerinah formations. Whereas a lacustrine interpretation has
been proposed for the stromatolitic Meentheena Member of the Tumbiana Formation (Lipple 1975; Walter
1983; Bolhar & Van Kranendonk 2007; Awramik & Buchheim 2009; Coffey et al. 2013: but see Thorne & Trendall
2001 and Sakurai et al. 2005 for differing interpretations),
the depositional setting of stromatolitic intervals within
the Kylena, Maddina and Jeerinah formations remain
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poorly constrained, and it is unclear whether data
reported from these units should be interpreted in the
context of deposition within global oceans, or in local
and restricted lacustrine settings.
Here, we examine the Mopoke Member, a laterally
extensive stromatolitic carbonate horizon within the ca
2740 Ma Kylena Formation, the oldest stromatolitic horizon in the Fortescue Group. We compare it with demonstrably marine units described from other Archean
formations and with intervals within overlying formations of the northern Fortescue Group, focussing on differences in mineralogy, sedimentology, stratigraphy and
stable isotope geochemistry. The Mopoke Member is distinct from superficially similar Fortescue Group stromatolitic horizons in the Tumbiana, Maddina and Jeerinah
formations, which are likely of lacustrine origin, and
instead may have been deposited during a marine incursion into the northern Fortescue Group. We conclude that
many of the observed differences between northern Fortescue Group units probably relate to the prevalence of
locally variable lacustrine environments in this group,
rather than to secular biological and/or environmental
change during the Neoarchean.
of rifting associated with extensive volcanism (Arndt
et al. 1991; Blake 1993; Thorne & Trendall 2001; Blake
et al. 2004). The Fortescue Group unconformably overlies
Paleo- to Mesoarchean granite greenstone basement
and reaches a maximum local thickness of about 6 km
(Thorne & Trendall 2001). Subaerial tholeiitic flood
basalts are the predominant lithology, but the group also
includes subordinate pillow basalt and basaltic tuff, felsic volcanic rocks, volcaniclastic rocks, clastic sedimentary rocks and stromatolitic carbonate.
Thorne & Trendall (2001) divided the Fortescue Group
into four sub-basins for descriptive purposes (Figure 1).
The northeast, northwest and Marble Bar sub-basins
comprise the northern Fortescue Group, which is composed predominantly of subaerial basalt and siliciclastic
rocks. The southern sub-basin lies to the south of the
Fortescue River, has been more severely folded and
metamorphosed, and consists mostly of pillow basalt
and offshore sedimentary rocks: the supposed deeperwater lateral equivalents of Fortescue Group rocks preserved in the northern sub-basins. Metamorphism of the
northwest and northeast sub-basins was limited to prehnite pumpellyite facies (Smith et al. 1982).
Geological setting
The Kylena Formation and the Mopoke Member
The Fortescue Group is a succession of flood basalt and
sedimentary rocks deposited on the southern margin of
the Pilbara Craton, Western Australia (Figures 1, 2),
from ca 2.78 Ga to ca 2.63 Ga, during one or more periods
The Kylena Formation (Smithies 1998; formerly the
Kylena Basalt) consists of subaerial lava flows and thin
units of carbonate, volcaniclastic and siliciclastic sedimentary rocks. Where present, the Kylena Formation
Figure 1 Map showing the extent of Fortescue Group outcrop in Western Australia and the Fortescue Group sub-basins of
Thorne & Trendall (2001). Modified after Thorne & Trendall (2001).
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A marine incursion into the northern Fortescue Group?
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Figure 2 Simplified Fortescue
Group stratigraphy in the northeast Pilbara sub-basin. Je, Jeerinah Formation; T, Tumbiana
Formation;
Ky,
Kylena
Formation.
disconformably overlies fluvial sandstone of the ca 2750
Ma (Blake et al. 2004) Hardey Formation, or unconformably overlies Paleo- to Mesoarchean basement rocks.
The Kylena Formation is in turn disconformably
overlain by pyroclastic and lacustrine deposits of the ca
2720 Ma (Blake et al. 2004) Tumbiana Formation.
The Mopoke Member (Williams 2007) is a thin horizon
of carbonate, siliciclastic and volcaniclastic rocks
Figure 3 The Mopoke Member seen in outcrop in the Meentheena Centrocline.
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Figure 4 Geological map of
Mopoke Member outcrop showing the localities referred to here
and the main roads providing
access to the northeast sub-basin.
(Figures 3, 4) separating two compositionally distinct lava
flows of the Kylena Formation in the northeast sub-basin.
Columnar jointing and pillow structures are locally developed in basaltic lava flows below the Mopoke Member in
the Meentheena area (Figure 5). Upper Kylena Formation
flows are generally thicker, consist of basalt and basaltic
andesite, and were erupted under subaerial conditions
(Williams 1999). Flows of both the upper and lower Kylena
Formation are typically massive with highly vesicular,
amygdaloidal and commonly scoriaceous flow tops. In the
northeast sub-basin, the total stratigraphic thickness of
the Kylena Formation ranges between 0 and ~600 m
(Thorne & Trendall 2001) and the Mopoke Member is
<20 m thick. No radiometric dates are available for the
Mopoke Member, but it is younger than ca 2740 Ma, the
average of three dates obtained for the lower Kylena Formation (2749 § 5, Nelson et al. 2006; 2741 § 3, Blake et al.
2004; 2735 § 6, Bodorkos et al. 2006), and older than the
base of the overlying, 2727 2721 Ma Tumbiana Formation
(Blake et al. 2004).
Previous investigations
Previous investigation of the Mopoke Member has been
limited to brief lithological descriptions published in the
explanatory notes accompanying Geological Survey of
Figure 5 (a) Columnar jointing in a Kylena Formation basaltic lava flow, Meentheena Centrocline. Scale is provided by Y.
Hoshino. (b) Pillows developed in a lower Kylena Formation lava flow, Meentheena Centrocline.
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Western Australia map sheets and reports (Williams 1999,
2007; Thorne & Trendall 2001; Farrel 2006; Williams &
Bagas 2007), and a single sample of the Mopoke Member
was analysed by Bolhar & Van Kranendonk (2007) as part
of their trace elemental analysis of Fortescue Group carbonate rocks. This sample returned a trace-element distribution interpreted as a lacustrine signature, but there are
no data on what stratigraphic level the analysed sample
comes from within the member or the degree of alteration
of this sample. Writing in explanatory notes, Williams &
Bagas (2007) also considered the Mopoke Member to be
lacustrine. In contrast, Thorne & Trendall (2001) proposed
a marine origin for the Mopoke Member and described
transgressive and regressive marine facies.
METHODS
Fieldwork was undertaken during seasons of 1 to 2
months duration in 2011 and 2013. Sections were measured with a Jacob’s staff, a measuring tape and an
Abney level. Hand samples were taken from outcrop in
order to investigate lithology, microbialite microstructure and isotope geochemistry. Care was taken when
sampling to avoid fractures, weathering rinds and recent
cements. Geological thin-sections were prepared using
standard petrographic techniques. Optical microscopy
was performed using a Carl Zeiss Photomicroscope III
and a Leica DM2500 running LAS 3.6 software. LA-ICPMS, Raman and XRD analyses were performed at the
UNSW Analytical Centre. Stable isotope analyses were
performed by Environmental Isotopes Pty Ltd and the
West Australian Biogeochemistry Centre.
LA-ICP-MS
Elemental information for each region/spot was collected using a New Wave NWR213 laser ablation unit
coupled to a Perkin Elmer NexION 300D ICP-MS. Spot
ablation of the geological thin-sections was performed
using a frequency-quintupled Nd:YAG laser emitting a
final pulse wavelength of 213 nm focused on the region of
interest, with ablated material transported to the ICPMS via a mixture of helium and argon carrier gas. Spot
size was 50 mm for all analyses. The international NIST
61x series of standards was used for calibration. The following isotopes were analysed: 23Na, 24Mg, 27Al, 28Si, 32S,
39
K, 43Ca, 55Mn, 57Fe and 88Sr. The experimental parameters were optimised to ensure an appropriate signal-tonoise ratio for each isotope and a reproducible signal.
Raman spectroscopy
Raman spectra were obtained from standard geological
thin-sections using a Renishaw inVia Raman Microscope
running Renishaw WiRE 3.2 software. Spot size was ~1.5
mm at 50x magnification using a 514.5 nm Argon ion
laser with an 1800 gr/mm grating for all analyses. Spectra were identified using the RRUFF Project online database (http://rruff.info/).
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XRD
Samples for bulk powder XRD analysis were selected on
the basis of minimal alteration and fracturing. Samples
were crushed and milled according to standard techniques before XRD analyses were performed using a PANalytical Xpert Multipurpose X-ray Diffraction System
operating at 45 kV and 45 mA.
Stable isotopes
C and O stable isotope analyses of carbonates were performed using a GasBench II coupled with a Delta XL
Mass Spectrometer (Thermo-Fisher Scientific). Threepoint normalisation was used in order to reduce raw values to the international scale (Paul & Skrzypek 2007).
Normalisation was performed based on international
standards provided by IAEA: L-SVEC, NBS19 and
NBS18. Values for international standards of carbon
(d13C) are according to Coplen et al. (2006). The external
error of all d13Ccarb and d18Ocarb analyses is ~0.10%.
Organic matter C isotope analyses were performed on
samples that were first washed and had weathering
rinds removed by sawing. They were then crushed using
a hammer and placed in a rock mill. Powdered samples
were digested in hydrochloric acid to remove carbonate.
Approximately 500 mg of powdered rock sample was
reacted with 10% HCl at room temperature overnight
and then rinsed and dried overnight at 50o C. The high
carbonate content and low organic C content required
this step to be repeated up to three times. Dried rock
power was weighed in tin cups for analysis of d13C values
and C concentrations using a modified Europa Roboprep
CN Elemental Analyser (EA) attached to a Finnigan Mat
Conflo III and Finnigan 252. Samples were then analysed
relative to internal gas standards that were calibrated
using international carbon isotope standards NBS-22
(d13C D 30.03% VPDB; Coplen et al. 2006) and IAEA-CH6
(d13C D 10.45% VPDB). Microanalysis standard B2105
was run to calibrate for concentration and as an
unknown. Values are reported in permil (%) relative to
the international standard VPDB, which is defined by
the International Atomic Energy Agency standard
IAEA-NBS19 (d13C D +1.95%; Coplen et al. 1995).
THE MOPOKE MEMBER
Lithology
The Mopoke Member consists primarily of dark grey-coloured stromatolitic limestone and dolomite (Figure 3) and
thinner units of calcilutite/calcisiltite, calcareous mudstone, siltstone, sandstone and black chert. Four lithofacies are defined here for descriptive purposes (L1 to L4,
Figure 6). L1 consists of flat to wavy-laminated carbonate
hosting large domical stromatolites (Figure 7a c). It is
typically ~1 m thick, but varies in thickness over distances of several hundred metres. It reaches a maximum
thickness of 3 m at locality 5, where it hosts large domical
stromatolites with up to 20 cm of synoptic relief. In the
Meentheena area (Figure 1) L1 may be as little as ~10 cm
thick. Coarsely laminated carbonate of L1 has encrusted
pre-existing topography, including basalt cobbles derived
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Figure 6 Section through the Mopoke Member in the Meentheena area showing lithology as determined by XRD and stable
carbon and oxygen isotope values obtained from hand samples. Modified after Flannery et al. (2012).
from lower Kylena Formation lava flows (Figure 7a). Bulk
powder XRD spectra indicate L1 is predominantly calcite
and 5 10 wt% silica. The microfabric of both domical and
stratiform laminated carbonate in L1 consists of coarsely
recrystallised spar and dolospar intersected by stylolites
and/or multiple generations of radiating blades of radiaxial calcite (Figure 7d f). The radiaxial calcite commonly
occurs in conspicuous, isopachous laminae visible in outcrop (Figure 7d). Some occurrences are associated with
dissolution breccia (Figure 7f). LA-ICP-MS spot analyses
of radiaxial calcite yielded values of ~10 000 15 000 ppm
Mg, ~800 1000 ppm Fe and ~100 1000 ppm Sr. Edgewise
tabular carbonate clasts, in places reworked into rosettes,
also occur within L1 (Figure 8a). The first few centimetres
of L1 are locally pervasively silicified and rich in siliciclastic grains. Dolomite inclusions visible in thin-section
indicate some of the silica has replaced a carbonate lithology. This style of silicification in the base of L1 has occasionally resulted in the high-fidelity preservation of
detrital and kerogenous laminae in microstromatolites
(Figures 8b, 9).
L2 is generally less than 30 cm thick and consists of
planar-laminated calcareous siltstone/shale (Figure 8c).
L3 consists of irregularly silicified microbial laminations (Figure 8d, e) in recrystallised dolostone and limestone, and a minor siltstone and sandstone component
preserving microbially induced sedimentary structures
(Figure 8f). In thin-section, the replacive silica of L3 surrounds islands of relict spar and dolospar, resulting in a
clotted texture that is visible at low magnification
(Figure 10a, b). Zones where silicification has proceeded
further have the appearance of massive black chert in
outcrop and scattered dolomite rhombs floating in a
microcrystalline
silica
matrix
in
thin-section
(Figure 10c). Silicification occasionally appears to conform to bedding (Figure 10d), but for the most part is discordant. Kerogen visible in thin-sections occurs in
black-coloured streaks and blobs in both carbonate stromatolites and in replacive silica zones.
L4 is composed predominantly of laminated calcareous mudstone and siltstone featuring desiccation cracks,
symmetrical ripple cross-lamination and mud drapes
(Figure 10e, f). Ripple crests are straight to slightly sinuous, peaked to slightly rounded and occasionally bifurcate, with wavelengths 1 to 3 cm and amplitudes <8 mm.
The predominant carbonate mineral in L3 is micritic
calcite. L3 and L4 commonly repeat up to three times
before the end of outcrop. Tuff, reported in the explanatory notes of multiple GSWA map sheets (e.g. Hickman
1983; Williams 2007; Williams & Bagas 2007), was not
found to be a significant component of the Mopoke Member at the localities investigated, although a basaltic
agglomerate is present at locality 1.
Stratigraphy
A series of sections were measured along the roughly
north south-trending extent of Mopoke Member outcrop in the northeast sub-basin (Figures 4, 11). The
minimum measured thickness of the Mopoke Member
ranges between 3 and 15 m and a maximum thickness
of ~20 m can be estimated for sections in the Meentheena area. There is a slight thickening of units away
from the Yilgalong Granitic Complex in a northerly
direction. A southerly thickening away from the same
paleotopographic high is not apparent in the southernmost sections measured. The southernmost locality
logged for this study is located approximately 67 km
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Figure 7 (a) Stromatolites in L1 nucleated on basalt cobbles weathered from subaerial lava flows of the lower Kylena Formation. (b) Domical stromatolites in L1 exposed in plan view. (c) Domical stromatolite in L1 exposed in horizontal cross-section.
(d) Radiaxial calcite associated with dissolution breccia in L1. Increments on Jacob’s staff are 10 cm. (e) Radiaxial calcite in L1
seen in a thin-section photomicrograph under crossed polars. Scale bar D 1 mm. (f) Thin-section photomicrograph showing
the convergent C-axes and curved cleavages of radiaxial calcite in L1 under crossed polars. Scale bar D 1 mm.
northwest of Nullagine, where the Mopoke Member is
of similar composition and stratigraphy to the other
localities. The northernmost locality logged is in the
hinge of the Oakover Syncline, where the basal portion
of the Mopoke Member consists of several metres of
planar-laminated siltstone/shale. Overlying units are
consistent with the stratigraphy of sections logged to
the south. At other localities, the lowermost unit of the
Mopoke Member is either a coarsely laminated stromatolitic carbonate or a thinly bedded intraformational conglomerate or lithic sandstone deposited in
the topographic lows of lava flows. Where not preceded
by shale or conglomerate, the lowermost bed of L1 lies
conformably upon vesicular basaltic flow tops that
show minimal reworking. With the exception of locality 1 (where there is an agglomerate), the uppermost
unit of the Mopoke Member is L3 or L4 at all localities
investigated. The internal stratigraphy of the Mopoke
Member is laterally consistent, with centimetre-thick
beds (for example, the calcilutite layers in L2) traceable
laterally over distances of more than 50 km. The dip of
Mopoke Member units is consistently less than 15
across the entire northeast sub-basin and relates to the
gentle folding responsible for large-scale geological
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Figure 8 (a) Edgewise conglomerate clasts in L1. (b) Thin-section photomicrograph showing a silicified microstromatolite in a
siliciclastic lens near the base of L1. Scale bar D 1 mm. (c) Laminated siltstone/mudstone (L2) with white-coloured calcilutite
banding. (d) Partially silicified microbial laminations in L3. (e) Partially silicified stromatolites in L3. (f) Bedding-plane exposure of L3 with microbially induced sedimentary structures.
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Figure 9 (a) Pervasively silicified base of L1 in the Meentheena Centrocline. (b) Thin-section photomicrograph showing darkcoloured kerogen preserved in a silicified stromatolite collected from the L1 outcrop shown in (a). (c) Raman spectra showing
‘D’ and ‘G’ bands indicating the dark-coloured material shown in (b) contains disordered organic matter.
structures such as the Oakover Syncline and Meentheena Centrocline.
Stable isotopes
Limited stable carbon and oxygen isotope data are shown
in Figure 6 and Table 1. d13Ccarb values range from 2.8%
to 2.0% VPDB with a single outlier at 5.6%. The spread
in values correlates to lithological variation (Figure 6):
values obtained from L1 cluster between 1.2% and 1.8%
(mean D 1.6%); those from L2 between 2.8% and 2.6%
(mean D 2.7%); those from L3 between 5.6% and
2.0% (mean D 0.7%); and a single value of 1.5% was
obtained for L4.
d18Ocarb values range from 21.2 to 17.4% VPDB
(mean D 19.3%) (Figure 6). Samples that appeared in
outcrop to be more severely altered were observed to
yield the most 18O depleted values. There is no significant correlation between d18Ocarb and d13Ccarb values
(coefficient of correlation D 0.13).
d13Corg values for carbonate samples range from
40.7% to 29.8% VPDB (mean D 36.3%). There is a
trend of increasing 13C-depletion upsection from L1
towards L4 (Figure 6). A single d13Corg value of 46.8%
was obtained from a sample of pervasively silicified kerogenous stromatolitic material collected from the base
of L1 (Figure 9).
INTERPRETATIONS
Sedimentary features of the Mopoke Member suggest an
upwards-shallowing, shallow marine depositional
environment.
L1
L1 probably represents a marine transgression over the
basaltic lava plain of the lower Kylena Formation. The
stromatolites suggest this facies was deposited in the
photic zone, and in a minimum of 20 cm of water as
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Figure 10 (a) Clotted fabric resulting from the replacement of spar (left of image) by silica in L3, shown in transmitted light.
Scale bar D 1 mm. (b) Transmitted light thin-section photomicrograph in cross polarised light showing silica replacing dolospar in L3, an intermediate stage in the advancement of the clotted fabric shown in (a) and (c). Scale bar D 1 mm. (c) Thin-section photomicrograph montage in reflected light showing dolomite rhombs floating in a clotted fabric related to the nearcomplete diagenetic silicification of microbial laminae in L3. Scale bar D 1 mm. (d) Partially silicified microbial laminations
in L3. Note the concordance of silica with bedding. (e) Mud drapes on small-scale ripple cross stratification in L4. (f) Small
scale, bifurcating symmetrical ripple cross-stratification in L4. Increments on Jacob’s staff are 10 cm.
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A marine incursion into the northern Fortescue Group?
1105
Figure 11 Stratigraphic sections logged through the Mopoke Member, Kylena Formation.
constrained by the synoptic relief of stromatolitic laminae (Donaldson 1976). ‘Rosettes’ of reworked edgewise
conglomerate clasts imply deposition above storm wave
base (Kazmierczak & Goldring 1978). Pervasive silicification seen in the basal portion of L1 at some localities
appears to have been relatively early, as it has resulted
in the preservation of kerogen and microbial fabrics
(Figure 8b) that have elsewhere been obliterated by
recrystallisation. The fabric of L1 carbonate, which is
dominated by spar, dolospar and isopachous calcite
crusts, is comparable with previously reported shallow
water Neoarchean marine units (e.g. Sumner & Grotzinger 2004), but distinct from the exclusively micritic
fabrics reported from other Fortescue Group stromatolitic carbonates (e.g. Walter 1972; Coffey et al. 2013).
Much of the radiaxial calcite in L1 (Figure 7d f), which
has not been reported from other Fortescue Group units,
appears to be primary, or at least a replacement of a
phase growing at the sediment water interface in that
it has nucleated on areas of pre-existing topography and
has formed laterally extensive, isopachous laminae that
are onlapped by detrital grains (cf. Kendall 1985; Wilson
& Dickson 1996). Where radiaxial calcite in the rock
record has been interpreted as a primary marine precipitate (e.g. Schroeder & Purser 1986), it is commonly considered a proxy for generally warm, high CaCO3
saturation and high pCO2 conditions consistent with
proposed models of the Neoarchean environment (eg.
Grotzinger & Knoll 2003; Dauphas & Kasting 2011). There
are no definitive sedimentological features in L1 discriminating between marine and lacustrine settings, but
the lateral continuity of facies is unlike the regular lateral facies changes reported from many modern lakes
and from the Meentheena Member in the overlying
Table 1 Stable isotope values for Mopoke Member samples.
Sample
Description
d13Ccarb % VPDB
d18Ocarb % VPDB
YOS1
Stromatolitic carbonate (L1)
1.23
19.82
2.5.11.2
Irregularly silicified stromatolitic carbonate (L3)
5.68
19.90
3.5.11.2
Irregularly silicified stromatolitic carbonate (L3)
2.08
19.81
2.5.11.3A
Green shale (L2)
2.81
19.51
2.62
18.14
2.5.11.3B
Green shale (L2)
2.5.11.3C
Green shale (L2)
2.5.11.3D
Green shale (L2)
3.5.11.3
Irregularly silicified stromatolitic carbonate (L3)
d13Corg % VPDB
38.6
31.1
29.8
0.79
20.71
30.5.11.3
Laminated calcareous siltstone (L4)
1.58
19.40
2.6.11.3
Stromatolitic carbonate (L1)
0.67
21.23
2.5.11.4
Stromatolitic carbonate (L1)
1.47
19.39
2.6.11.4A
Stromatolitic carbonate (L1)
1.84
17.89
30.5.11.5
Irregularly silicified stromatolitic carbonate (L3)
0.06
19.52
2.5.11.5
Laminated calcareous siltstone (L4)
2.6.11.4.6
Stromatolitic carbonate (L1)
1.71
17.48
30.5.11.7
Stromatolitic carbonate (L1)
1.89
18.46
37.2
40.7
32.5
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Tumbiana Formation, which is probably lacustrine
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L2
L2 may have been deposited in a deeper-water environment below the zone of carbonate precipitation (cf. Murphy & Wilkinson 1980; Treese & Wilkinson 1982). A
deeper, lower-energy environment for L2 is supported by
the absence of current-generated sedimentary features
and evidence for subaerial exposure, such as desiccation
cracks. The distinctive pair of white-coloured calcilutite
layers in the thinly bedded unit of L2 that interrupts L1
(Figure 8c), may represent a brief return to carbonate
precipitating conditions, perhaps in events similar to
the ‘whitings’ observed in some modern settings (e.g.
Hodell et al. 1998). The thickening of L2 towards locality
1 (Figure 11) suggests a basin that deepened towards the
northeast, in contrast to a previously proposed southward thickening of the Kylena Formation into its supposed southerly equivalent, the Boongal Formation
(Thorne & Trendall 2001).
L3
The decreasing synoptic relief of stromatolites in L3
implies a shallower minimum water depth than in L1.
The small-scale domical and conical stromatolites and
microbially induced sedimentary structures in L3 are
commonly associated with shallow-water facies (cf. Porada & Bouougri 2007). The laminated, reticulate surfaces
preserved in marly sediment (Figure 8f) are similar to
reticulate structures reported from both ancient and
modern bacterial mats (e.g. Shepard & Sumner 2010;
Flannery & Walter 2011). Most of the silica in this unit is
interpreted to be of late diagenetic origin, given the textural relationships seen in thin-section, which indicate
it post-dates the coarse recrystallisation of the carbonate
comprising stromatolitic laminae.
L4
Desiccation cracks and small scale, bifurcating, symmetrical (wind-generated) ripple cross-lamination suggest a
very shallow water to periodically exposed depositional
setting for L4. The double mud drapes deposited on ripples may reflect a regime of semi-diurnal tides (cf. De
Boer et al. 1989). Horizontal sections showing a gradation
into thicker mud drapes (visible in Figure 10e) may represent deposition during neap tides, when current velocities were lower and the transport of sand did not take
place (cf. Eriksson & Simpson 2000; Williams 2000;
Mazumder & Arima 2005).
Stable isotopes
Archean marine d13Ccarb values are generally near zero
(Veizer et al. 1989, 1990). Values from L1 in the Mopoke
Member are also near zero. However, some values from
L2, L3 and L4 are significantly 13C depleted ( 5.6 to
2.0%). The depleted values may relate to diagenetic alteration and the early or late remineralisation of associated
organic carbon, which is significantly depleted ( 40.7 to
29.8%). This hypothesis is supported by the visibly
most recrystallised sample yielding the lowest value
( 5.6%). Alternatively, some of the observed d13Ccarb variation in L2, L3 and L4, especially towards positive values, may be primary. The positive values may represent
a previously undocumented marine excursion, or if this
part of the Mopoke Member represents a periodically
restricted (e.g. lagoonal) setting, the values could be
related to fluctuating water levels and/or rates of
organic matter burial. More data are needed. d18Ocarb values are consistently depleted ( 21.2 to 17.4%), suggesting alteration via 18O depleted meteoric waters (Walls
et al. 1979). The decreasing d13Corg values upsection, from
29.8% in L1 to 40.7% in L4 (Figure 6) may be related to
the increased burial of methanotrophic biomass in an
increasingly restricted setting, an interpretation consistent with the shallowing upwards environment suggested by sedimentological features. L1 yielded the most
enriched d13Corg values, which are comparable with average marine values reported from throughout the geological record, whereas L2, L3 and L4 yielded lower values
that are closer to those reported from overlying, lacustrine units of the Fortescue Group (e.g. Awramik & Buchheim 2009; Coffey et al. 2013).
Overall depositional environment
With the exception of L2 (planar-laminated calcareous
siltstone/shale), all lithofacies appear to have been
deposited under shallow water conditions, with sedimentary features in lithofacies overlying L2 suggestive
of upwards shallowing. Karst features affecting all carbonate facies are the result of periods of prolonged subaerial exposure during the deposition of L3 and L4.
Some parts of the Mopoke Member somewhat resemble a less extensive version of the Meentheena Member
(Tumbiana Formation), and some of the same arguments
that have been employed to infer a lacustrine setting for
the Tumbiana Formation may be used to suggest a similar setting for these parts of the Mopoke Member, including deposition within a largely subaerial succession, the
prevalence of symmetrical ripple cross-lamination and
the absence of assuredly marine sedimentary features.
However, much of the Mopoke Member is distinct from
other stromatolitic units of the Fortescue Group (including the Meentheena Member) in terms of carbonate mineralogy, stratigraphy and sedimentology. The balance of
probabilities currently favours a marine interpretation
for at least part of the Mopoke Member based on the following observations:
The succession of centimetre-scale lithofacies may
be traced laterally across the entire extent of outcrop (100+ km). This is not a characteristic generally associated with shallow water lacustrine
deposits, which typically exhibit frequent lateral
and vertical facies changes (Platt & Wright 2009).
Frequent lateral facies changes are observed in the
Meentheena Member (Tumbiana Formation),
which is considerably thicker yet consists of lenticular units that for the most part cannot be traced
laterally over distances of more than a few kilometres (cf. Awramik & Buchheim 2009).
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A marine incursion into the northern Fortescue Group?
Spar, dolospar and radiaxial calcite (Figure 7d f)
are conspicuous features of the Mopoke Member
and distinguish it from micritic limestone members of the overlying Tumbiana and Maddina formations (cf. Walter 1983; Awramik & Buchheim
2009). In this respect, the Mopoke Member bears a
closer resemblance to demonstrably marine early
Precambrian formations from which isopachous
sea-floor crusts, fans and dolomite are commonly
reported, and where micrite is rare (e.g. Beukes
1987; Grotzinger & Knoll 1995, 2003; Sumner 1995;
Sumner & Grotzinger 2004).
Although lava flows overlying the Mopoke Member
are subaerial and in some places thick enough to
form columnar joints, there is some pillow basalt
in the lower Kylena Formation. In contrast, the
lava flows that underlie fluvial and lacustrine sedimentary rocks of the Tumbiana, Maddina and
Jeerinah formations are exclusively subaerial.
The double mud drapes observed in L4 are best
accounted for by tidal influences.
Comparatively high d13Corg values obtained from
the Mopoke Member are consistent with values
reported from Neoarchean carbonate units deposited in shallow water marine environments (e.g.
Fischer et al. 2009). These values contrast with the
extremely low values (down to 60%) reported
from overlying lacustrine units in the Fortescue
Group.
The trace-elemental analyses of Bolhar & Van
Kranendonk (2007), interpreted as suggestive of a
lacustrine depositional environment, are stratigraphically unconstrained. Trace element analyses may have been compromised by alteration of
the selected samples by meteoric waters, which is
suggested by the consistently depleted stable oxygen isotope data reported here.
CONCLUDING REMARKS
A periodically restricted lacustrine environment, which
differed in terms of basement topography and water
chemistry when compared with other lacustrine units of
the Fortescue Group, is one possible depositional environment for the Mopoke Member. However, the mineralogy, sedimentology, stratigraphy and stable carbon
isotope geochemistry is most consistent with a marine
depositional environment. d13Corg values for the Mopoke
Member are more enriched in 13C than those reported
from overlying lacustrine intervals in the Fortescue
Group and closer to Archean units known to be of
marine origin, suggesting a facies control on the 13Cdepletion of organic matter in the Fortescue Group. We
conclude that differences in carbonate mineralogy and
stable isotope values reported from the northern Fortescue Group may represent local variations in restricted
settings rather than globally significant biological and/
or environmental change during the Neoarchean. Paleobiological data obtained from the northern Fortescue
Group should thus be carefully considered in the context
of depositional environments.
1107
ACKNOWLEDGEMENTS
We thank the Geological Survey of Western Australia for
generous logistical support in the field and Tamsyn
Garby, Yosuke Hoshino and Kee Wenyu for their contribution to fieldwork. We also acknowledge Anne Rich and
Karen Privat at the UNSW Analytical Centre, Anita
Andrew at Environmental Isotopes and the West Australian Biogeochemistry Centre for assistance with analytical work. DTF was supported by a Commonwealth
Government of Australia Australian Postgraduate
Award. MRW was supported by the University of New
South Wales and a grant from the Australian Research
Council. MVK acknowledges funding from the University
of New South Wales and the Agouron Institute. RM is
grateful to the University of New South Wales for a postdoctoral fellowship. We thank Aivo Lepland, Kath Grey
and David Martin for their comments on the manuscript.
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Received 15 May 2014; accepted 16 August 2014