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The ca 2.74 Ga Mopoke Member, Kylena Formation: a marineincursion into the northern Fortescue Group?D. T. Flanneryab, M. J. Van Kranendonkac, R. Mazumderac & M. r. Walterab

a Australian Centre for Astrobiology, University of New South Wales, NSW 2052, Australiab School of Biotechnology and Biomolecular Sciences, University of New South Wales, NSW 2052,Australiac School of Biological, Earth and Environmental Sciences, University of New South Wales, NSW 2052,AustraliaPublished online: 10 Dec 2014.

To cite this article: D. T. Flannery, M. J. Van Kranendonk, R. Mazumder & M. r. Walter (2014) The ca 2.74 Ga Mopoke Member, KylenaFormation: a marine incursion into the northern Fortescue Group?, Australian Journal of Earth Sciences: An International GeoscienceJournal of the Geological Society of Australia, 61:8, 1095-1108, DOI: 10.1080/08120099.2014.960898

To link to this article: http://dx.doi.org/10.1080/08120099.2014.960898

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The ca 2.74 Ga Mopoke Member, Kylena Formation: amarine incursion into the northern Fortescue Group?

D. T. FLANNERY1,2*, M. J. VAN KRANENDONK1,3, R. MAZUMDER1,3 AND M. R. WALTER1,2

1Australian Centre for Astrobiology, University of New South Wales, NSW 2052, Australia.2School of Biotechnology and Biomolecular Sciences, University of New South Wales, NSW 2052, Australia.3School of Biological, Earth and Environmental Sciences, University of New South Wales, NSW 2052, Australia.

The northern part of the Fortescue Group consists of interbedded flood basalts and sedimentary rocks thatwere deposited on the southern margin of the Pilbara Craton, Western Australia, during one or moreperiods of continental rifting between ca 2.78 and ca 2.63 Ga. Well-preserved sedimentary intervals withinthe group have yielded stable carbon and sulfur isotope data that have been used to infer changes ingeobiological processes in the Neoarchean. However, the Fortescue Group is notable for being apredominantly subaerial succession, and it remains unclear whether data obtained from these intervalsshould be interpreted in the context of deposition in marine environments, possibly recording changes inthe global ocean/atmosphere system, or in local and restricted lacustrine settings. Here, we describe thesedimentology, stratigraphy, stromatolites and stable carbon isotope geochemistry of the ca 2.74 GaMopoke Member, Kylena Formation, the oldest stromatolitic horizon in the Fortescue Group. This unitdiffers in terms of internal stratigraphic relationships, sedimentology, carbonate mineralogy and stableisotope geochemistry when compared with intervals of probable lacustrine origin in the overlyingTumbiana and Maddina formations. In contrast, we suggest that parts of the Mopoke Member may havebeen deposited under open marine conditions, or alternatively, in a lacustrine environment characterisedby differing water chemistry and basement topography. Stromatolitic microfabrics of the MopokeMember are dominated by spar, dolospar and vertically aligned calcitic crusts, rather than the micriticmicrofabrics described from other Fortescue Group stromatolites. Mud-draped ripples are commonsedimentary 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 relateto the reoxidation of 13C-depleted organic matter. d13Corg values average �36.7%, consistent withNeoarchean marine units reported from elsewhere, but significantly less 13C-depleted than valuesreported from overlying lacustrine intervals in the Fortescue Group. We conclude that some features ofFortescue Group datasets relevant to the field of geobiology may be facies dependent, and that morework focusing on the overall depositional environments of the Fortescue Group is needed in order toappropriately 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 For-tescue Group, Western Australia, have been used to inves-tigate the evolution of the early biosphere. The FortescueGroup is known, among other things, for providinginsights into the composition of the Archean atmospherebased on stable carbon and multiple sulfur isotopes (e.g.Pavlov et al. 2001; Ono et al. 2003; Eigenbrode & Freeman2006; Thomazo et al. 2009), for insights into Archean micro-bial 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 microfos-sils (Schopf & Walter 1983). Extremely low d13Corg values

reported from the group are generally interpreted asreflecting the incorporation of biomass derived frommeth-ane-cycling communities during a period of enhancedglobal methanotrophy in the Neoarchean (Hayes 1994).

Previous studies have focused on the ca 2720 MaTumbiana Formation. However, additional stromatoliticintervals are present in the Kylena, Maddina and Jeeri-nah formations. Whereas a lacustrine interpretation hasbeen proposed for the stromatolitic Meentheena Mem-ber of the Tumbiana Formation (Lipple 1975; Walter1983; Bolhar & Van Kranendonk 2007; Awramik & Buch-heim 2009; Coffey et al. 2013: but see Thorne & Trendall2001 and Sakurai et al. 2005 for differing interpretations),the depositional setting of stromatolitic intervals withinthe Kylena, Maddina and Jeerinah formations remain

*Corresponding author: flannery@jpl.nasa.gov� 2014 Geological Society of Australia

Australian Journal of Earth Sciences (2014)

61, 1095�1108, http://dx.doi.org/10.1080/08120099.2014.960898

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poorly constrained, and it is unclear whether datareported from these units should be interpreted in thecontext of deposition within global oceans, or in localand restricted lacustrine settings.

Here, we examine the Mopoke Member, a laterallyextensive stromatolitic carbonate horizon within the ca2740 Ma Kylena Formation, the oldest stromatolitic hori-zon in the Fortescue Group. We compare it with demon-strably marine units described from other Archeanformations and with intervals within overlying forma-tions of the northern Fortescue Group, focussing on dif-ferences in mineralogy, sedimentology, stratigraphy andstable isotope geochemistry. The Mopoke Member is dis-tinct from superficially similar Fortescue Group stromat-olitic horizons in the Tumbiana, Maddina and Jeerinahformations, which are likely of lacustrine origin, andinstead may have been deposited during a marine incur-sion into the northern Fortescue Group. We conclude thatmany of the observed differences between northern For-tescue Group units probably relate to the prevalence oflocally variable lacustrine environments in this group,rather than to secular biological and/or environmentalchange during the Neoarchean.

Geological setting

The Fortescue Group is a succession of flood basalt andsedimentary rocks deposited on the southern margin ofthe Pilbara Craton, Western Australia (Figures 1, 2),from ca 2.78 Ga to ca 2.63 Ga, during one or more periods

of rifting associated with extensive volcanism (Arndtet al. 1991; Blake 1993; Thorne & Trendall 2001; Blakeet al. 2004). The Fortescue Group unconformably overliesPaleo- to Mesoarchean granite�greenstone basementand reaches a maximum local thickness of about 6 km(Thorne & Trendall 2001). Subaerial tholeiitic floodbasalts are the predominant lithology, but the group alsoincludes subordinate pillow basalt and basaltic tuff, fel-sic volcanic rocks, volcaniclastic rocks, clastic sedimen-tary rocks and stromatolitic carbonate.

Thorne & Trendall (2001) divided the Fortescue Groupinto four sub-basins for descriptive purposes (Figure 1).The northeast, northwest and Marble Bar sub-basinscomprise the northern Fortescue Group, which is com-posed predominantly of subaerial basalt and siliciclasticrocks. The southern sub-basin lies to the south of theFortescue River, has been more severely folded andmetamorphosed, and consists mostly of pillow basaltand offshore sedimentary rocks: the supposed deeper-water lateral equivalents of Fortescue Group rocks pre-served in the northern sub-basins. Metamorphism of thenorthwest and northeast sub-basins was limited to pre-hnite�pumpellyite facies (Smith et al. 1982).

The Kylena Formation and the Mopoke Member

The Kylena Formation (Smithies 1998; formerly theKylena Basalt) consists of subaerial lava flows and thinunits of carbonate, volcaniclastic and siliciclastic sedi-mentary 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 ofThorne & Trendall (2001). Modified after Thorne & Trendall (2001).

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disconformably overlies fluvial sandstone of the ca 2750Ma (Blake et al. 2004) Hardey Formation, or unconform-ably overlies Paleo- to Mesoarchean basement rocks.The Kylena Formation is in turn disconformably

overlain by pyroclastic and lacustrine deposits of the ca2720 Ma (Blake et al. 2004) Tumbiana Formation.

The Mopoke Member (Williams 2007) is a thin horizonof carbonate, siliciclastic and volcaniclastic rocks

Figure 3 The Mopoke Member seen in outcrop in the Meentheena Centrocline.

Figure 2 Simplified FortescueGroup stratigraphy in the north-east Pilbara sub-basin. Je, Jeeri-nah Formation; T, TumbianaFormation; Ky, KylenaFormation.

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(Figures 3, 4) separating two compositionally distinct lavaflows of the Kylena Formation in the northeast sub-basin.Columnar jointing and pillow structures are locally devel-oped in basaltic lava flows below the Mopoke Member inthe Meentheena area (Figure 5). Upper Kylena Formationflows are generally thicker, consist of basalt and basalticandesite, and were erupted under subaerial conditions(Williams 1999). Flows of both the upper and lower KylenaFormation are typically massive with highly vesicular,amygdaloidal and commonly scoriaceous flow tops. In thenortheast sub-basin, the total stratigraphic thickness ofthe 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 theMopoke Member, but it is younger than ca 2740 Ma, theaverage of three dates obtained for the lower Kylena For-mation (2749 § 5, Nelson et al. 2006; 2741 § 3, Blake et al.2004; 2735 § 6, Bodorkos et al. 2006), and older than thebase of the overlying, 2727�2721 Ma Tumbiana Formation(Blake et al. 2004).

Previous investigations

Previous investigation of the Mopoke Member has beenlimited to brief lithological descriptions published in theexplanatory 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.

Figure 4 Geological map ofMopoke Member outcrop show-ing the localities referred to hereand the main roads providingaccess to the northeast sub-basin.

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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 Memberwas analysed by Bolhar & Van Kranendonk (2007) as partof their trace elemental analysis of Fortescue Group car-bonate rocks. This sample returned a trace-element distri-bution interpreted as a lacustrine signature, but there areno data on what stratigraphic level the analysed samplecomes from within the member or the degree of alterationof this sample. Writing in explanatory notes, Williams &Bagas (2007) also considered the Mopoke Member to belacustrine. In contrast, Thorne & Trendall (2001) proposeda marine origin for the Mopoke Member and describedtransgressive and regressive marine facies.

METHODS

Fieldwork was undertaken during seasons of 1 to 2months duration in 2011 and 2013. Sections were mea-sured with a Jacob’s staff, a measuring tape and anAbney level. Hand samples were taken from outcrop inorder to investigate lithology, microbialite microstruc-ture and isotope geochemistry. Care was taken whensampling to avoid fractures, weathering rinds and recentcements. Geological thin-sections were prepared usingstandard petrographic techniques. Optical microscopywas performed using a Carl Zeiss Photomicroscope IIIand a Leica DM2500 running LAS 3.6 software. LA-ICP-MS, Raman and XRD analyses were performed at theUNSW Analytical Centre. Stable isotope analyses wereperformed by Environmental Isotopes Pty Ltd and theWest Australian Biogeochemistry Centre.

LA-ICP-MS

Elemental information for each region/spot was col-lected using a New Wave NWR213 laser ablation unitcoupled to a Perkin Elmer NexION 300D ICP-MS. Spotablation of the geological thin-sections was performedusing a frequency-quintupled Nd:YAG laser emitting afinal pulse wavelength of 213 nm focused on the region ofinterest, with ablated material transported to the ICP-MS via a mixture of helium and argon carrier gas. Spotsize was 50 mm for all analyses. The international NIST61x series of standards was used for calibration. The fol-lowing isotopes were analysed: 23Na, 24Mg, 27Al, 28Si, 32S,39K, 43Ca, 55Mn, 57Fe and 88Sr. The experimental parame-ters were optimised to ensure an appropriate signal-to-noise ratio for each isotope and a reproducible signal.

Raman spectroscopy

Raman spectra were obtained from standard geologicalthin-sections using a Renishaw inVia RamanMicroscoperunning Renishaw WiRE 3.2 software. Spot size was ~1.5mm at 50x magnification using a 514.5 nm Argon ionlaser with an 1800 gr/mm grating for all analyses. Spec-tra were identified using the RRUFF Project online data-base (http://rruff.info/).

XRD

Samples for bulk powder XRD analysis were selected onthe basis of minimal alteration and fracturing. Sampleswere crushed and milled according to standard techni-ques before XRD analyses were performed using a PANa-lytical Xpert Multipurpose X-ray Diffraction Systemoperating at 45 kVand 45 mA.

Stable isotopes

C and O stable isotope analyses of carbonates were per-formed using a GasBench II coupled with a Delta XLMass Spectrometer (Thermo-Fisher Scientific). Three-point normalisation was used in order to reduce raw val-ues to the international scale (Paul & Skrzypek 2007).Normalisation was performed based on internationalstandards provided by IAEA: L-SVEC, NBS19 andNBS18. Values for international standards of carbon(d13C) are according to Coplen et al. (2006). The externalerror of all d13Ccarb and d18Ocarb analyses is ~0.10%.

Organic matter C isotope analyses were performed onsamples that were first washed and had weatheringrinds removed by sawing. They were then crushed usinga hammer and placed in a rock mill. Powdered sampleswere digested in hydrochloric acid to remove carbonate.Approximately 500 mg of powdered rock sample wasreacted with 10% HCl at room temperature overnightand then rinsed and dried overnight at 50o C. The highcarbonate content and low organic C content requiredthis step to be repeated up to three times. Dried rockpower was weighed in tin cups for analysis of d13C valuesand C concentrations using a modified Europa RoboprepCN Elemental Analyser (EA) attached to a Finnigan MatConflo III and Finnigan 252. Samples were then analysedrelative to internal gas standards that were calibratedusing 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 B2105was run to calibrate for concentration and as anunknown. Values are reported in permil (%) relative tothe international standard VPDB, which is defined bythe International Atomic Energy Agency standardIAEA-NBS19 (d13C D +1.95%; Coplen et al. 1995).

THE MOPOKE MEMBER

Lithology

The Mopoke Member consists primarily of dark grey-col-oured stromatolitic limestone and dolomite (Figure 3) andthinner units of calcilutite/calcisiltite, calcareous mud-stone, siltstone, sandstone and black chert. Four litho-facies are defined here for descriptive purposes (L1 to L4,Figure 6). L1 consists of flat to wavy-laminated carbonatehosting large domical stromatolites (Figure 7a�c). It istypically ~1 m thick, but varies in thickness over distan-ces of several hundred metres. It reaches a maximumthickness of 3 m at locality 5, where it hosts large domicalstromatolites with up to 20 cm of synoptic relief. In theMeentheena area (Figure 1) L1 may be as little as ~10 cmthick. Coarsely laminated carbonate of L1 has encrustedpre-existing topography, including basalt cobbles derived

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from lower Kylena Formation lava flows (Figure 7a). Bulkpowder XRD spectra indicate L1 is predominantly calciteand 5�10 wt% silica. The microfabric of both domical andstratiform laminated carbonate in L1 consists of coarselyrecrystallised spar and dolospar intersected by stylolitesand/or multiple generations of radiating blades of radiax-ial calcite (Figure 7d�f). The radiaxial calcite commonlyoccurs in conspicuous, isopachous laminae visible in out-crop (Figure 7d). Some occurrences are associated withdissolution breccia (Figure 7f). LA-ICP-MS spot analysesof radiaxial calcite yielded values of ~10 000�15 000 ppmMg, ~800�1000 ppm Fe and ~100�1000 ppm Sr. Edgewisetabular carbonate clasts, in places reworked into rosettes,also occur within L1 (Figure 8a). The first few centimetresof L1 are locally pervasively silicified and rich in silici-clastic grains. Dolomite inclusions visible in thin-sectionindicate some of the silica has replaced a carbonate lithol-ogy. This style of silicification in the base of L1 has occa-sionally resulted in the high-fidelity preservation ofdetrital and kerogenous laminae in microstromatolites(Figures 8b, 9).

L2 is generally less than 30 cm thick and consists ofplanar-laminated calcareous siltstone/shale (Figure 8c).L3 consists of irregularly silicified microbial lamina-tions (Figure 8d, e) in recrystallised dolostone and lime-stone, and a minor siltstone and sandstone componentpreserving microbially induced sedimentary structures(Figure 8f). In thin-section, the replacive silica of L3 sur-rounds islands of relict spar and dolospar, resulting in aclotted texture that is visible at low magnification(Figure 10a, b). Zones where silicification has proceededfurther have the appearance of massive black chert inoutcrop and scattered dolomite rhombs floating in amicrocrystalline silica matrix in thin-section

(Figure 10c). Silicification occasionally appears to con-form to bedding (Figure 10d), but for the most part is dis-cordant. Kerogen visible in thin-sections occurs inblack-coloured streaks and blobs in both carbonate stro-matolites and in replacive silica zones.

L4 is composed predominantly of laminated calcare-ous mudstone and siltstone featuring desiccation cracks,symmetrical ripple cross-lamination and mud drapes(Figure 10e, f). Ripple crests are straight to slightly sinu-ous, peaked to slightly rounded and occasionally bifur-cate, with wavelengths 1 to 3 cm and amplitudes <8 mm.The predominant carbonate mineral in L3 is micriticcalcite. L3 and L4 commonly repeat up to three timesbefore the end of outcrop. Tuff, reported in the explana-tory notes of multiple GSWA map sheets (e.g. Hickman1983; Williams 2007; Williams & Bagas 2007), was notfound to be a significant component of the Mopoke Mem-ber at the localities investigated, although a basalticagglomerate is present at locality 1.

Stratigraphy

A series of sections were measured along the roughlynorth�south-trending extent of Mopoke Member out-crop in the northeast sub-basin (Figures 4, 11). Theminimum measured thickness of the Mopoke Memberranges between 3 and 15 m and a maximum thicknessof ~20 m can be estimated for sections in the Meen-theena area. There is a slight thickening of units awayfrom the Yilgalong Granitic Complex in a northerlydirection. A southerly thickening away from the samepaleotopographic high is not apparent in the southern-most sections measured. The southernmost localitylogged for this study is located approximately 67 km

Figure 6 Section through the Mopoke Member in the Meentheena area showing lithology as determined by XRD and stablecarbon and oxygen isotope values obtained from hand samples. Modified after Flannery et al. (2012).

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northwest of Nullagine, where the Mopoke Member isof similar composition and stratigraphy to the otherlocalities. The northernmost locality logged is in thehinge of the Oakover Syncline, where the basal portionof the Mopoke Member consists of several metres ofplanar-laminated siltstone/shale. Overlying units areconsistent with the stratigraphy of sections logged tothe south. At other localities, the lowermost unit of theMopoke Member is either a coarsely laminated stro-matolitic carbonate or a thinly bedded intraforma-tional conglomerate or lithic sandstone deposited inthe topographic lows of lava flows. Where not preceded

by shale or conglomerate, the lowermost bed of L1 liesconformably upon vesicular basaltic flow tops thatshow minimal reworking. With the exception of local-ity 1 (where there is an agglomerate), the uppermostunit of the Mopoke Member is L3 or L4 at all localitiesinvestigated. The internal stratigraphy of the MopokeMember is laterally consistent, with centimetre-thickbeds (for example, the calcilutite layers in L2) traceablelaterally over distances of more than 50 km. The dip ofMopoke Member units is consistently less than 15�

across the entire northeast sub-basin and relates to thegentle folding responsible for large-scale geological

Figure 7 (a) Stromatolites in L1 nucleated on basalt cobbles weathered from subaerial lava flows of the lower Kylena Forma-tion. (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 L1seen in a thin-section photomicrograph under crossed polars. Scale bar D 1 mm. (f) Thin-section photomicrograph showingthe convergent C-axes and curved cleavages of radiaxial calcite in L1 under crossed polars. Scale bar D 1 mm.

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Figure 8 (a) Edgewise conglomerate clasts in L1. (b) Thin-section photomicrograph showing a silicified microstromatolite in asiliciclastic lens near the base of L1. Scale bar D 1 mm. (c) Laminated siltstone/mudstone (L2) with white-coloured calcilutitebanding. (d) Partially silicified microbial laminations in L3. (e) Partially silicified stromatolites in L3. (f) Bedding-plane expo-sure of L3 with microbially induced sedimentary structures.

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structures such as the Oakover Syncline and Meen-theena Centrocline.

Stable isotopes

Limited stable carbon and oxygen isotope data are shownin Figure 6 and Table 1. d13Ccarb values range from �2.8%to 2.0% VPDB with a single outlier at �5.6%. The spreadin 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% and2.0% (mean D �0.7%); and a single value of �1.5% wasobtained for L4.

d18Ocarb values range from �21.2 to �17.4% VPDB(mean D �19.3%) (Figure 6). Samples that appeared inoutcrop to be more severely altered were observed toyield the most 18O depleted values. There is no signifi-cant 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 atrend of increasing 13C-depletion upsection from L1towards L4 (Figure 6). A single d13Corg value of �46.8%was obtained from a sample of pervasively silicified ker-ogenous stromatolitic material collected from the baseof L1 (Figure 9).

INTERPRETATIONS

Sedimentary features of the Mopoke Member suggest anupwards-shallowing, shallow marine depositionalenvironment.

L1

L1 probably represents a marine transgression over thebasaltic lava plain of the lower Kylena Formation. Thestromatolites suggest this facies was deposited in thephotic zone, and in a minimum of 20 cm of water as

Figure 9 (a) Pervasively silicified base of L1 in the Meentheena Centrocline. (b) Thin-section photomicrograph showing dark-coloured 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.

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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 dolo-spar in L3, an intermediate stage in the advancement of the clotted fabric shown in (a) and (c). Scale bar D 1 mm. (c) Thin-sec-tion photomicrograph montage in reflected light showing dolomite rhombs floating in a clotted fabric related to the near-complete diagenetic silicification of microbial laminae in L3. Scale bar D 1 mm. (d) Partially silicified microbial laminationsin L3. Note the concordance of silica with bedding. (e) Mud drapes on small-scale ripple cross stratification in L4. (f) Smallscale, bifurcating symmetrical ripple cross-stratification in L4. Increments on Jacob’s staff are 10 cm.

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constrained by the synoptic relief of stromatolitic lami-nae (Donaldson 1976). ‘Rosettes’ of reworked edgewiseconglomerate clasts imply deposition above storm wavebase (Kazmierczak & Goldring 1978). Pervasive silicifica-tion seen in the basal portion of L1 at some localitiesappears to have been relatively early, as it has resultedin the preservation of kerogen and microbial fabrics(Figure 8b) that have elsewhere been obliterated byrecrystallisation. The fabric of L1 carbonate, which isdominated by spar, dolospar and isopachous calcitecrusts, is comparable with previously reported shallowwater Neoarchean marine units (e.g. Sumner & Grot-zinger 2004), but distinct from the exclusively micriticfabrics reported from other Fortescue Group stromato-litic carbonates (e.g. Walter 1972; Coffey et al. 2013).Much of the radiaxial calcite in L1 (Figure 7d�f), whichhas not been reported from other Fortescue Group units,

appears to be primary, or at least a replacement of aphase growing at the sediment�water interface in thatit has nucleated on areas of pre-existing topography andhas formed laterally extensive, isopachous laminae thatare onlapped by detrital grains (cf. Kendall 1985; Wilson& Dickson 1996). Where radiaxial calcite in the rockrecord has been interpreted as a primary marine precip-itate (e.g. Schroeder & Purser 1986), it is commonly con-sidered a proxy for generally warm, high CaCO3

saturation and high pCO2 conditions consistent withproposed models of the Neoarchean environment (eg.Grotzinger & Knoll 2003; Dauphas & Kasting 2011). Thereare no definitive sedimentological features in L1 dis-criminating between marine and lacustrine settings, butthe lateral continuity of facies is unlike the regular lat-eral facies changes reported from many modern lakesand from the Meentheena Member in the overlying

Figure 11 Stratigraphic sections logged through the Mopoke Member, Kylena Formation.

Table 1 Stable isotope values for Mopoke Member samples.

Sample Description d13Ccarb %VPDB d18Ocarb %VPDB d13Corg%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 �38.6

2.5.11.3A Green shale (L2) �2.81 �19.51

2.5.11.3B Green shale (L2) �2.62 �18.14

2.5.11.3C Green shale (L2) �31.1

2.5.11.3D Green shale (L2) �29.8

3.5.11.3 Irregularly silicified stromatolitic carbonate (L3) 0.79 �20.71 �37.2

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) �40.7

2.6.11.4.6 Stromatolitic carbonate (L1) 1.71 �17.48

30.5.11.7 Stromatolitic carbonate (L1) 1.89 �18.46 �32.5

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Tumbiana Formation, which is probably lacustrine(Awramik & Buchheim 2009).

L2

L2 may have been deposited in a deeper-water environ-ment below the zone of carbonate precipitation (cf. Mur-phy & Wilkinson 1980; Treese & Wilkinson 1982). Adeeper, lower-energy environment for L2 is supported bythe absence of current-generated sedimentary featuresand evidence for subaerial exposure, such as desiccationcracks. The distinctive pair of white-coloured calcilutitelayers in the thinly bedded unit of L2 that interrupts L1(Figure 8c), may represent a brief return to carbonateprecipitating conditions, perhaps in events similar tothe ‘whitings’ observed in some modern settings (e.g.Hodell et al. 1998). The thickening of L2 towards locality1 (Figure 11) suggests a basin that deepened towards thenortheast, in contrast to a previously proposed south-ward thickening of the Kylena Formation into its sup-posed southerly equivalent, the Boongal Formation(Thorne & Trendall 2001).

L3

The decreasing synoptic relief of stromatolites in L3implies a shallower minimum water depth than in L1.The small-scale domical and conical stromatolites andmicrobially induced sedimentary structures in L3 arecommonly associated with shallow-water facies (cf. Por-ada & Bouougri 2007). The laminated, reticulate surfacespreserved in marly sediment (Figure 8f) are similar toreticulate structures reported from both ancient andmodern bacterial mats (e.g. Shepard & Sumner 2010;Flannery & Walter 2011). Most of the silica in this unit isinterpreted to be of late diagenetic origin, given the tex-tural relationships seen in thin-section, which indicateit post-dates the coarse recrystallisation of the carbonatecomprising stromatolitic laminae.

L4

Desiccation cracks and small scale, bifurcating, symmet-rical (wind-generated) ripple cross-lamination suggest avery shallow water to periodically exposed depositionalsetting for L4. The double mud drapes deposited on rip-ples may reflect a regime of semi-diurnal tides (cf. DeBoer et al. 1989). Horizontal sections showing a gradationinto thicker mud drapes (visible in Figure 10e) may rep-resent deposition during neap tides, when current veloc-ities were lower and the transport of sand did not takeplace (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 MopokeMember are also near zero. However, some values fromL2, L3 and L4 are significantly 13C depleted (�5.6 to2.0%). The depleted values may relate to diagenetic alter-ation and the early or late remineralisation of associatedorganic carbon, which is significantly depleted (�40.7 to

�29.8%). This hypothesis is supported by the visiblymost recrystallised sample yielding the lowest value(�5.6%). Alternatively, some of the observed d13Ccarb vari-ation in L2, L3 and L4, especially towards positive val-ues, may be primary. The positive values may representa previously undocumented marine excursion, or if thispart of the Mopoke Member represents a periodicallyrestricted (e.g. lagoonal) setting, the values could berelated to fluctuating water levels and/or rates oforganic matter burial. More data are needed. d18Ocarb val-ues are consistently depleted (�21.2 to �17.4%), suggest-ing alteration via 18O depleted meteoric waters (Wallset al. 1979). The decreasing d13Corg values upsection, from�29.8% in L1 to �40.7% in L4 (Figure 6) may be related tothe increased burial of methanotrophic biomass in anincreasingly restricted setting, an interpretation consis-tent with the shallowing upwards environment sug-gested by sedimentological features. L1 yielded the mostenriched d13Corg values, which are comparable with aver-age marine values reported from throughout the geologi-cal record, whereas L2, L3 and L4 yielded lower valuesthat are closer to those reported from overlying, lacus-trine units of the Fortescue Group (e.g. Awramik & Buch-heim 2009; Coffey et al. 2013).

Overall depositional environment

With the exception of L2 (planar-laminated calcareoussiltstone/shale), all lithofacies appear to have beendeposited under shallow water conditions, with sedi-mentary features in lithofacies overlying L2 suggestiveof upwards shallowing. Karst features affecting all car-bonate facies are the result of periods of prolonged sub-aerial exposure during the deposition of L3 and L4.

Some parts of the Mopoke Member somewhat resem-ble a less extensive version of the Meentheena Member(Tumbiana Formation), and some of the same argumentsthat have been employed to infer a lacustrine setting forthe Tumbiana Formation may be used to suggest a simi-lar setting for these parts of the Mopoke Member, includ-ing deposition within a largely subaerial succession, theprevalence of symmetrical ripple cross-lamination andthe absence of assuredly marine sedimentary features.However, much of the Mopoke Member is distinct fromother stromatolitic units of the Fortescue Group (includ-ing the Meentheena Member) in terms of carbonate min-eralogy, stratigraphy and sedimentology. The balance ofprobabilities currently favours a marine interpretationfor at least part of the Mopoke Member based on the fol-lowing observations:

� The succession of centimetre-scale lithofacies maybe traced laterally across the entire extent of out-crop (100+ km). This is not a characteristic gener-ally associated with shallow water lacustrinedeposits, which typically exhibit frequent lateraland vertical facies changes (Platt & Wright 2009).Frequent lateral facies changes are observed in theMeentheena Member (Tumbiana Formation),which is considerably thicker yet consists of lentic-ular units that for the most part cannot be tracedlaterally over distances of more than a few kilo-metres (cf. Awramik & Buchheim 2009).

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� Spar, dolospar and radiaxial calcite (Figure 7d�f)are conspicuous features of the Mopoke Memberand distinguish it from micritic limestone mem-bers of the overlying Tumbiana and Maddina for-mations (cf. Walter 1983; Awramik & Buchheim2009). In this respect, the Mopoke Member bears acloser resemblance to demonstrably marine earlyPrecambrian formations from which isopachoussea-floor crusts, fans and dolomite are commonlyreported, and where micrite is rare (e.g. Beukes1987; Grotzinger & Knoll 1995, 2003; Sumner 1995;Sumner & Grotzinger 2004).

� Although lava flows overlying the Mopoke Memberare subaerial and in some places thick enough toform columnar joints, there is some pillow basaltin the lower Kylena Formation. In contrast, thelava flows that underlie fluvial and lacustrine sedi-mentary rocks of the Tumbiana, Maddina andJeerinah formations are exclusively subaerial.

� The double mud drapes observed in L4 are bestaccounted for by tidal influences.

� Comparatively high d13Corg values obtained fromthe Mopoke Member are consistent with valuesreported from Neoarchean carbonate units depos-ited in shallow water marine environments (e.g.Fischer et al. 2009). These values contrast with theextremely low values (down to �60%) reportedfrom overlying lacustrine units in the FortescueGroup.

� The trace-elemental analyses of Bolhar & VanKranendonk (2007), interpreted as suggestive of alacustrine depositional environment, are strati-graphically unconstrained. Trace element analy-ses may have been compromised by alteration ofthe selected samples by meteoric waters, which issuggested by the consistently depleted stable oxy-gen isotope data reported here.

CONCLUDING REMARKS

A periodically restricted lacustrine environment, whichdiffered in terms of basement topography and waterchemistry when compared with other lacustrine units ofthe Fortescue Group, is one possible depositional envi-ronment for the Mopoke Member. However, the mineral-ogy, sedimentology, stratigraphy and stable carbonisotope geochemistry is most consistent with a marinedepositional environment. d13Corg values for the MopokeMember are more enriched in 13C than those reportedfrom overlying lacustrine intervals in the FortescueGroup and closer to Archean units known to be ofmarine origin, suggesting a facies control on the 13C-depletion of organic matter in the Fortescue Group. Weconclude that differences in carbonate mineralogy andstable isotope values reported from the northern Fortes-cue Group may represent local variations in restrictedsettings rather than globally significant biological and/or environmental change during the Neoarchean. Paleo-biological data obtained from the northern FortescueGroup should thus be carefully considered in the contextof depositional environments.

ACKNOWLEDGEMENTS

We thank the Geological Survey of Western Australia forgenerous logistical support in the field and TamsynGarby, Yosuke Hoshino and Kee Wenyu for their contri-bution to fieldwork. We also acknowledge Anne Rich andKaren Privat at the UNSW Analytical Centre, AnitaAndrew at Environmental Isotopes and the West Austra-lian Biogeochemistry Centre for assistance with analyti-cal work. DTF was supported by a CommonwealthGovernment of Australia Australian PostgraduateAward. MRW was supported by the University of NewSouth Wales and a grant from the Australian ResearchCouncil. MVK acknowledges funding from the Universityof New South Wales and the Agouron Institute. RM isgrateful to the University of New South Wales for a post-doctoral fellowship. We thank Aivo Lepland, Kath Greyand David Martin for their comments on the manuscript.

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Received 15 May 2014; accepted 16 August 2014

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