# Metamorphic origin of anastomosing and wavy laminas overprinting putative microbial deposits from the 3.22 Ga Moodies Group (Barberton Greenstone Belt)

Masafumi Saitoh, Nicolas Olivier, Marion Garçon, Maud Boyet, Christophe

Thomazo, Julien Alleon, Jean-François Moyen, Vincent Motto-Ros, Johanna

Marin-Carbonne

► To cite this version:

Masafumi Saitoh, Nicolas Olivier, Marion Garçon, Maud Boyet, Christophe Thomazo, et al.. Meta-morphic origin of anastomosing and wavy laminas overprinting putative microbial deposits from the 3.22 Ga Moodies Group (Barberton Greenstone Belt). Precambrian Research, 2021, 362, pp.106306. 10.1016/j.precamres.2021.106306. hal-03283638

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1 **Metamorphic origin of anastomosing and wavy laminas overprinting putative microbial**

2 **deposits from the 3.22 Ga Moodies Group (Barberton Greenstone Belt)**

3

4 Masafumi Saitoh¹,²,*,**, Nicolas Olivier¹, Marion Garçon¹, Maud Boyet¹, Christophe Thomazo³,⁴,

5 Julien Alleon², Jean-François Moyen⁵, Vincent Motto-Ros⁶, and Johanna Marin-Carbonne²

6

7 1. Université Clermont Auvergne, CNRS, IRD, Laboratoire Magmas et Volcans, F-63000 Clermont-

8 Ferrand, France

9 2. Institut des Sciences de la Terre, Faculté des géosciences et de l'environnement, Université de

10 Lausanne, 1015 Lausanne, Switzerland

11 3. Laboratoire Biogéosciences, Université Bourgogne Franche-Comté, Dijon, France

12 4. Institut Universitaire de France, 1 rue Descartes, 75231 Paris CEDEX 05, France

13 5. Laboratoire de Géologie de Lyon : Terre, Planètes et Environnement, UJM-UCLB-ENSL-CNRS,

14 42023 Saint Etienne, France

15 6. Université Lyon, Université Claude Bernard Lyon 1, CNRS, Institut Lumière Matière, UMR

16 5306, F-69622, Villeurbanne, France
17 *Correspondence: msaitoh@se.kanazawa-u.ac.jp; Tel.: +81-76-264-6515

18 **Present address: School of Geosciences and Civil Engineering, Kanazawa University, Kakuma,

19 Kanazawa 920-1192 JAPAN

20

21 **Keywords**

22 Paleoarchean; Barberton Greenstone Belt; Moodies Group; anastomosing and wavy laminae;

23 metasomatism

24

25 **Abstract**

26

27 Anastomosing branching and wavy laminae in quartz-rich sandstones of the 3.22 Ga Moodies

28 Group in the Barberton Greenstone Belt (BGB), South Africa, have been extensively described as

29 fossilized microbial mats developed in a terrestrial to marine transitional environment. Petrological

30 and geochemical characteristics of a ~350 m thick Moodies succession in the Saddleback Syncline

31 in the central BGB were analyzed to reconstruct the post-depositional history of the sediments and to

32 better constrain the origin of the laminae. The studied sandstones are composed mainly of quartz,
potassium feldspar, and chert clastic grains with various proportions of microquartz and sericite

33

34 cements. In coastal floodplain and inter- to supra-tidal environments, quiescent periods with low
current velocity allowed the repeated deposition of thin (<5 mm thick) and fine-grained laminae with
35

36 clay matrix and potentially organic matter. In contrast, subtidal settings under consistently high-
energy conditions led to the deposition of coarse- to medium-sized sands without fine-grained
37

38 lamina. Intergrain areas in those coarse sediments were infilled with a microquartz cement during
burial diagenesis. Due to the presence of clay matrix, the fine-grained laminae had a reduced
39

40 porosity that prevented the microquartz cementation within the laminae. This original clay matrix
was later replaced with sericite cements during metamorphism, forming the anastomosing and wavy
41

42 laminae observed at the hand-specimen scale in the studied sandstones. Microscopic dark areas
within the laminae are composed mostly of opaque minerals with scarce carbonaceous material. The
43

44 scarcity of carbonaceous material and the fluctuating energy settings, potentially inauspicious for the
continuous development and preservation of biomats, are not in favor of a flourishing Paleoarchean
45

46 microbial life on the Moodies sediments.

47

48

**1. Introduction**
49

50

Among the various clues to understand the record of traces of life on early Earth, ancient microbial biofilms and microbially induced sedimentary structures (MISS) have been a subject in numerous studies over the last 15 years (e.g., Noffke et al., 2003, 2008; Noffke, 2007; Wacey, 2009; Hickman-Lewis et al., 2018; Lepot, 2020). The development of these structures depends largely on the depositional setting through colonization of biofilms and their potential physical interactions with clastic grains (Noffke et al., 2001). Some Archean rocks are composed of siliciclastic sediments deposited in tidal and shallow shelf settings that may have been favorable to the development of biofilms (e.g., Noffke, 2007, 2010; Noffke et al., 2013; Westall, 2008; Westall et al., 2011).

51

52

53

54

55

56

57

58

However, the preservation of Archean biofilms and/or MISS required specific and complex taphonomic processes and their biogenicity has often been debated (e.g., Bower, 2011; Mariotti et al., 2014; Davies et al., 2016, 2018; Noffke, 2018; Westall et al., 2018; Hickman-Lewis et al., 2019).

59

60

61

Noffke et al. (2006) first proposed that wrinkle and roll-up sedimentary structures in the ~3.22 Ga Moodies Group in the Barberton Greenstone Belt (BGB), South Africa, were MISS. Heubeck (2009) subsequently interpreted that ubiquitous green anastomosing branching and wavy laminae in the Moodies sandstones were remnants of microbial mats. Homann et al. (2015) classified the

62

63

64
laminae into three morphotypes and suggested a relationship between the mat morphologies and

their associated depositional environments. A systematic difference in carbon and nitrogen isotopic

composition between the terrestrial and marine deposits was also observed and interpreted to reflect

the oldest evidence for the co-existence of distinctive terrestrial and marine ecosystems (Homann et

al., 2018; Homann, 2019; Thomazo et al., 2020).

The BGB has undergone lower greenschist-facies regional metamorphism (< 350°C) and has a

complex thermal history (e.g., de Ronde and de Wit, 1994), such as the emplacement of several

plutonic domains around the BGB at 3.23-3.21 Ga (Fig. 1a) (e.g., de Ronde and de Wit, 1994; Cutts

et al., 2014; Moyen et al., 2019), followed by multiple tectono-thermal events during the

Mesoarchean to Paleoproterozoic (Weis and Wasserburg, 1987; de Ronde et al., 1991a; Lécuyer et

al., 1994; Toulkeridis et al., 1994, 1998). The Moodies sediments in the BGB were presumably

affected by complex post-depositional processes, including polymetamorphism and more recent

alteration via fluid circulations (e.g., Heubeck, 2019; Bonnand et al., 2020); however, the influence

of post-depositional processes on the Moodies rocks has not yet been examined in detail.

In this study, we analyzed the microfacies and major and trace element composition of the

lamina-bearing Moodies sandstones. Based on petrological and geochemical results, we
reconstructed multiple post-depositional processes of the analyzed sandstones in association with

their depositional settings and with the regional metamorphic history of the BGB. They enabled us

to discuss the origin of the anastomosing and wavy laminae in the Moodies rocks in the framework

of their post-depositional history.

85

## 2. Geological setting

87

The BGB at the Makhonjwa Mountains is located in northeastern South Africa and northern

Eswatini (Swaziland) near the eastern margin of the Kaapvaal Craton (Fig. 1a). It is one of the best-

preserved Paleoarchean greenstone belts consisting of a >10 km thick succession of interlayered

volcanic and sedimentary rocks, termed the Barberton Supergroup (e.g., Lowe et al., 1999;

Hofmann, 2005). The Barberton Supergroup (ca. 3.55 to 3.22 Ga) is subdivided (in ascending order)

into the Onverwacht, the Fig Tree, and the Moodies groups. The Onverwacht Group (ca. 3.55 to 3.26

Ga) is composed mainly of mafic to ultramafic volcanic rocks (basalts and komatiites) with

interlayered cherts and rare felsic volcanics as well as very few clastic units. The lithologically

diverse Fig Tree Group (ca. 3.26 to 3.23 Ga) includes clastic sedimentary rocks (shales, sandstones
and conglomerates), immature volcaniclastics, intermediate to felsic lavas, banded iron formations, 97

barite, and cherts. The Moodies Group (ca. 3.22 Ga) is ~3.7 km thick and consists mainly of quartz- 98

rich sandstones and siltstones with subordinate conglomerates, felsic volcanics, shales and jaspilites 99
(Fig. 1b) (e.g., Eriksson, 1977, 1979, 1980; Heubeck and Lowe, 1994a, b, 1999; Homann et al., 100

2015; Heubeck, 2019). The BGB is tightly folded; the central belt consists of a series of synclines, 101
mostly cored by Moodies Group strata and separated by strike faults replacing tight anticlines 102

(Anhaeusser et al., 1981; Heubeck and Lowe, 1994a, b). 103

The Moodies Group has been suggested to be the oldest terrestrial to marine transitional 104
sedimentary succession (e.g., Eriksson, 1977, 1979, 1980; Heubeck and Lowe, 1994a, b; Eriksson 105

and Simpson, 2000; Eriksson et al., 2006; Simpson et al., 2012; Homann et al., 2015; Heubeck, 106
2019). Heubeck et al. (2013) indicated that the entire Moodies Group accumulated very rapidly 107

(within <1–14 million years) at ~3.22 Ga, based on the dating of zircons in intercalated tuff beds of 108
the sedimentary succession. North of the Inyoka Fault, a major divide in the BGB, the Moodies 109

Group is exposed in several tectonic units, including (from northeast to southwest) the Eureka 110
Syncline, Dycedale Syncline, Saddleback Syncline, Moodies Hills Block, and Stolzburg Syncline 111

(e.g., Homann et al., 2015; Heubeck et al., 2016). 112
In the present study, we focused on a Moodies sedimentary succession in the Saddleback
Syncline in the central BGB (Fig. 1). The ~3000 m thick Moodies Group is exposed continuously
along the NE-SW trending syncline (Homann et al., 2015; Heubeck, 2019). In the Saddleback
Syncline, the bottom 50-80 m thick part of the Moodies Group is composed mainly of interbedded
cobble conglomerates and sandstones (Fig. 1b). These basal conglomerates are correlated to the
MdB unit of the group in the northern Eureka Syncline (Table 1) (Anhaeusser, 1976; Homann et al.,
2015), and are thought to have been deposited in a fluvial to alluvial setting (Eriksson, 1980;
Heubeck and Lowe, 1994a). This conglomerate unit is overlain by ~1000 m thick quartz-rich
sandstones in the lower part of the Saddleback succession. These sandstones are correlated to the
MdQ1 unit of the Eureka succession (Anhaeusser, 1976; Homann et al., 2015), and would have been
deposited on coastal floodplain to subtidal settings (Homann et al., 2015). Anastomosing branching
and wavy laminae are frequently observed in these sandstones and are interpreted to be remnants of
original microbial mats (Noffke et al., 2006; Heubeck, 2009; Homann et al., 2015).

The overlying 400-1000 m thick interval is composed mainly of fine-grained sandstones and
siltstones and is correlated to the MdS1 unit of the Moodies Group (Fig. 1b) (Anhaeusser, 1976).
This interval was deposited on a prodelta slope below the wave base (Heubeck, 2019). The MdS1
sandstones and siltstones are overlain by up to 150 m thick quartzarenites, which is correlated to the

129

MdQ2 unit in the Eureka Syncline (Anhaeusser, 1976). The MdQ2 unit is interpreted as reflecting

130

subtidal and intertidal settings (Eriksson, 1977), or an offshore subaqueous dune field (Heubeck and

131

Lowe, 1994a). The MdQ2 arenites are overlain by a basaltic lava (MdL2 in Anhaeusser, 1976). This

132

characteristic MdL2 lava is then overlain by banded iron formation (BIF), jaspilite, shale and

133

siltstone (MdI2 in Anhaeusser, 1976). The MdI2 unit is overlain by ~300 m thick sandstones with

134

conglomerates and siltstones, and these conglomerates and siltstones are correlated to the MdS2 unit

135

(Anhaeusser, 1976). The uppermost Moodies succession in the Saddleback Syncline is characterized

136

by up to 100 m thick quartzarenites and conglomerates (MdQ3).

137

138

138

3. Analyzed sedimentary succession and its depositional settings
139

140

In the present study, we analyzed a ~350 m thick succession in the upper part of the MdQ1

141

unit of the lower Moodies Group (25°50'19.9" S, 31°05'01.1" E) (Figs. 1c and 2a) (Anhaeusser,

142

1976; Homann et al., 2015). The MdQ1 unit is ~1000 m thick and is composed mostly of quartz-rich

143

sandstones. This MdQ1 unit corresponds to the MD1 unit of alluvial plain facies in Eriksson (1978).

144

144
Heubeck et al. (2013) interpreted that the MdQ1 sediments were deposited in a braided fluvial system. Heubeck (2009) subdivided the MdQ1 unit into four subunits based on the lithofacies and reconstructed their depositional settings: a coastal floodplain (subunit 1), a tidal coast (subunit 2), and subtidal and shoreface settings (subunits 3 and 4) (Table S1). Gamper et al. (2011) substantially changed the interpretation of their depositional settings: a terrestrial coastal zone (subunit 1), a subtidal to intertidal zone (subunit 2), and a nearshore zone (subunits 3 and 4). Homann et al. (2015) later revised these depositional settings based on macroscopic lithofacies description: a coastal floodplain (subunit 1), an intertidal zone (subunit 2), a subtidal zone (subunits 3), and an upper inter-to supra-tidal zone (subunit 4) (Fig. 1c). The analyzed succession in the present study includes the four subunits of the MdQ1 unit and corresponds to stratigraphic Log 9 in Homann et al. (2015) in the central Saddleback Syncline. In this study, we follow the interpretation of depositional setting of the analyzed four subunits in Homann et al. (2015) (Figs. 1c and 2a).

The lowermost subunit 1 is ~200 m thick and is composed of coarse- to fine-grained sandstones. Cross-bedding, herringbone structures, and channels with erosive base are observed on the outcrop. Pebbles are most commonly observed in the lower part of the subunit (Fig. 2b).

Anastomosing branching and wavy laminations are frequently recognized at the hand-specimen
scale (Fig. 2c). Homann et al. (2015) interpreted the environment of the subunit 1 as a coastal

floodplain, based notably on low-angle planar and trough cross-bedding and lenticular

conglomerates with desiccation cracks.

The overlying subunit 2 is ~40 m thick and consists of medium-grained sandstones (Figs. 1c

and 2d). Abundant asymmetric ripples, common herringbone structures, and some small and local

pebbles are observed. Green anastomosing branching and wavy laminations at the hand-specimen

scale are recognized. Based on the presence of herringbone cross-bedding, sigmoidal foresets, and

mud drapes, Homann et al. (2015) suggested that the subunit 2 accumulated in an intertidal setting.

The ~80 m thick subunit 3 is composed mainly of medium- to fine-grained sandstones (Fig.

1c). Large cross-beddings are frequently observed through the subunit (Fig. 2e). As observed in

Homann et al. (2015), this subunit is characterized by the lack of anastomosing branching and wavy

laminations. The authors constrained its depositional environment to a subtidal setting, based on

large-scale and low-angle planar foresets and putative gutter casts.

The uppermost subunit 4 is ~40 m thick and consists of medium-grained sandstones (Fig. 1c).

Cross-beddings and subvertical gas-/fluid-escape structures are frequently observed (Figs. 2f and g).

Some anastomosing branching and wavy laminae are recognized. This subunit 4 corresponds to the
basal part of the MD3 unit in Eriksson (1979), in which this part was interpreted to be a delta-front
sediment. Homann et al. (2015) reinterpreted the depositional setting of the subunit 4 as an upper
inter- to supra-tidal setting, based on abundant desiccation cracks in the subunit. A slight difference
in thickness of the subunits 2 and 3 is observed between Log 9 in Homann et al. (2015) and the
present log (Fig. 1c). This could be explained by substantial lateral changes in thicknesses of
sedimentary bodies (megaripples and dunes) that form these subunits.

183

4. Materials and analytical methods

185

Forty-two rock samples were collected from the analyzed Saddleback succession. A part of
these samples has been previously analyzed for quadruple sulfur isotope geochemistry (Saitoh et al.,
2020). The lithofacies of all the collected rocks was observed in detail via optical microscopy. The
proportion of grain to matrix/cement in the rocks was estimated according to Baccelle and Bosellini
(1965). In addition to the microfacies description, multiple geochemical analyses (Raman
spectroscopy, EPMA, SEM-EDS, LIBS, ICP-OES, ICP-MS, and EA) were conducted on a selected
subset of the collected rock series.
193

194 4.1. Raman spectroscopy

195 Eight samples in the analyzed succession (SAD4, 6, 20, 21, 26, 34, 39, and 40) were analyzed by Raman spectroscopy at the Laboratoire Magmas et Volcans (LMV), Clermont-Ferrand, France.

197 Raman spectra were collected using a Renishaw InVia confocal Raman micro-spectrometer

198 equipped with a 532 nm laser source, a Peltier-cooled CCD detector, an edge filter, a motorized

199 XYZ stage and a Leica DM 2500M optical microscope. Scattered light was collected by a back-scattered geometry. Laser power on the sample was reduced by filters in order to operate at powers

201 <1 mW. We used a 1200 grooves/mm grating, which results in a spectral resolution better than 1 cm⁻¹. The slit aperture was set either to 20 µm (high confocality setting) or 65 µm (standard

203 confocality setting) depending on grain size. A 100× microscope objective was used. These analytical conditions result in lateral and vertical spatial resolutions of <1 and 2 µm, respectively.

205 Daily calibration of the spectrometer was performed based on a Si 520.7 cm⁻¹ peak. All spectra were recorded in the 95-2695 cm⁻¹ wavenumber range and from ~1050 to 3400 cm⁻¹ in presence of carbonaceous matter, using WIRE™ 4.4 software. The number of acquisitions on a single spot

208 varied from three to fifteen, and the duration of each acquisition ranged between 5 and 30 s. Ninety-
eight spots were measured in total in the selected eight samples. 2D maps were acquired on several

areas using a 0.8 µm or a 2 µm step between points. Mineral identification was performed by

comparison with standard spectra from the WIRE and RRUFF databases.

212

4.2. Electron Probe Micro Analyzer (EPMA)

Selected nine samples were analyzed by EPMA at the University of Lausanne, Switzerland.

Quantitative spot analyses for Si, Mg, Na, K, Al, Ti, Fe, Ca, and Cr were conducted for five samples

(SAD5, 22, 32, 37, and 39) with a JEOL 8230F microprobe, using a beam current of 400 nA and an

accelerating voltage of 15 kV, with a dwell time of 100 ms. Lab standard minerals (albite for Si and

Na, forsterite for Mg, orthoclase for K and Al, TiO₂ for Ti, fayalite for Fe, anorthite for Ca, and

chromite for Cr) were used for spot analyses. Moreover, X-ray compositional maps for Si, Mg, Na,

K, Al, Ti, and Fe were obtained for five samples (SAD20, 23, 26, 34, and 39) with the same

analytical conditions.

222

4.3. Scanning Electron Microscopy (SEM) and Energy Dispersive Spectroscopy (EDS)

224
SEM-EDS measurements were conducted for five samples in the subunits 1 and 2 at the

University of Lausanne, using a Tescan Mira LMU SEM equipped with the Oxford Instruments

AZtec 3.4 software package, to identify mineral phases in the sandstones. The measurements were

operated at 20 kV acceleration voltage, ~6 pA probe current, and 21 mm working distance.

Backscattered electron (BSE) images were taken on finely polished thin sections coated with a 15

nm carbon layer, and the EDS analyses were carried out on them to determine the chemical

composition of minerals in the analyzed areas. We conducted both spot analyses and semi

quantitative elemental mapping, and the spatial distribution of Si, K, Al, Na, Ti, Fe, and Mg were

visualized in the maps.

234

4.4. Laser-induced breakdown spectroscopy (LIBS) imaging

236

LIBS imaging analyses were conducted for one sample from the subunit 2 at the Institute of

Light and Matter, Lyon, France. In LIBS imaging, a series of laser-induced plasma is generated at

different positions on the sample. Such plasma sources allow specific optical responses resulting

from the electronic relaxation of atoms and ions excited by the high plasma temperature to be

240
elicited from the elements constituting the sample. The light emitted by the plasma is collected and

analyzed using an optical spectrometer equipped with an intensified charge-coupled device (ICCD)

camera. The elemental "signal" (atomic and ionic emissions) is then extracted from the recorded

spectra, and elemental maps can be obtained in a pixel-by-pixel manner (Sancey et al., 2014; Fabre

et al., 2018; Motto-Ros et al., 2020). The sandstone sample was analyzed using Nd:YAG laser with

pulse energy of about 600 µJ operating at 100 Hz, and a lateral resolution (i.e. distance between two

consecutive laser shots) of 25 µm. Two different Czerny-Turner spectrometers were used in order to

detect intense lines of Al (309.2 nm nm), B (208.9 nm), Cr (267.7 nm), Fe (302.0 nm) and Ti (323.4

nm), among others.

250

4.5. Major and trace element geochemistry

252

Fresh parts of the rock samples were carefully selected for major and trace element

measurements and cut to remove weathered surfaces. The rock pieces were crushed into small chips

(<1 cm in diameter) and were ultrasonically cleaned with distilled water and subsequently with pure

acetone. The cleaned chips were then powdered using an agate mill. Up to 25 g of powder was
prepared for each sample. Twenty-nine samples from the four subunits were selected for major element analyses at the LMV in Clermont-Ferrand. Sample powder (100 mg) was mixed with LiBO₂ flux (300 mg) and fused at 1100 °C. The solutions were analyzed using the LMV HORIBA-Jobin-Yvon ULTIMA-C Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES). The precision and accuracy were determined from repeated measurements of the BHVO-1 reference material. Results are consistent within 3% of the certified data except for K₂O (4%). The total volatile content (loss on ignition) was determined from 1g of sample powder.

Trace elements were measured on twelve samples from the four subunits at the LMV. About 50 mg of sample powder were digested using ammonium bifluoride (Zhang et al., 2012). Closed savillex Teflon vials were placed into an oven at 220°C for 24 hours. After cooling, samples were dissolved in concentrated HNO₃ and evaporated to dryness several times. All measurements were performed using the LMV Agilent Inductively Coupled Plasma Mass Spectrometer (ICP-MS) in Clermont-Ferrand, France. Samples and certified rock standard were measured in 0.5M-HNO₃-0.05M-HF with a dilution factor of about 3,500. Counts per second were calibrated using certified rock standards BHVO-2. The blank and reference materials (BE-N, BIR-1 and BHVO-2) were measured every three samples. Measurements were corrected from oxide interferences and
instrumental drift if significant. The precision and accuracy were determined from four repeated

measurements of the BE-N and BIR-1. Results are consistent within 4% of published data, except

for U, Th, and Pb (<10%).

Trace element concentrations of the analyzed sandstones were normalized to upper continental

crust (UCC) composition (Rudnick and Gao, 2014), and their rare-earth element (REE)

concentrations were normalized to Chondrite and to Mud from Queensland (MuQ) (McDonough

and Sun, 1995; Kamber et al., 2005).

280

4.6. Elemental Analyzer (EA)

282

For three samples from the subunits 1, 3, and 4, total organic carbon (TOC) contents were

analyzed by EA. Powdered samples (~0.5 g) were reacted with 6N HCl at room temperature for >12

h to remove carbonate at the LMV. The residue was cleaned by repeated centrifugation with

ultrapure water and was then reacted with 46M HF at 55°C for >48 h to dissolve silicate. The

residue was cleaned by repeated centrifugation with ultrapure water and was dried at 55°C for >12 h.

Aliquots of the dried residue (1–20 mg) were weighed in a tin capsule and their TOC contents were
measured by a vario MICRO cube elemental analyzer (Elementar GmbH, Hanau, Germany) at

Laboratoire Biogéosciences, Université Bourgogne Franche-Comté, Dijon, France. USGS40-

certified reference material (C = 40.8 wt.%) was used for calibration and the TOC content is shown

as a dry weight percentage of the total fraction. The analytical reproducibility, based on duplicate

analyses of USGS40, is better than 0.1 wt.% (1σ) for the residue.

294

## 5. Results

296

### 5.1. Petrological description

298

#### 5.1.1. Clastic grains

The analyzed Moodies succession is composed mostly of sandstones with some conglomerates

(Figs. 1-3). The sandstones contain abundant coarse to very fine sand-size clastic grains. The grains

range in size mostly from 100 to 1000 µm. Those clastic grains are mainly quartz (>50%) (Figs. 3a

and 3b). Potassium (K) feldspar (microcline and orthoclase) (<25%) and chert grains (<15%) are

also frequently observed and the latter consists mostly of microquartz crystals with intracrystalline
305 fine sericite (Figs. 3c and d). As some chert grains include substantial amounts (>5%) of sericite,

306 Heubeck and Lowe (1999) described them originally as “quartz-sericite (QS) grains”. A small

307 amount of albite (<2%) is also contained in the sandstones. The roundness and sphericity of quartz

308 and K-feldspar grains are variable but larger grains (>400 µm in diameter) show lower roundness

309 and sphericity (<0.3 and <0.4, respectively). In contrast, the roundness and sphericity of chert grains

310 are relatively higher (mostly >0.7 and >0.7, respectively). The sandstones are poorly sorted through

311 the analyzed succession.

312 Many clastic grains show long to concavo-convex intergranular pressure solution (IPS)

313 textures (arrows in Fig. 3d). Undulose extinction is often recognized in monocrystalline quartz

314 grains (Fig. 3b). Some quartz grains contain apatite (Ca₅(PO₄)₃Cl) crystals as inclusion (Figs. 3e and

315 g). These apatite inclusions are monocrystalline and elongated (length close to 30 µm). Some K-

316 feldspar grains are altered patchily by albite (NaAlSi₃O₈) (Supplementary Fig. S1). Rounded detrital

317 zircon grains (<200 µm in diameter) are present in the sandstones. The analyzed sandstones also

318 contain small monazite-(Ce) grains (Figs. 3f and h). The grain size of the monazite is mostly <10

319 µm, and its particle shape is generally irregular. Monazite grains sometimes aggregate within the

320 intergrain areas. An overgrowth texture is not recognized within a monazite crystal. Our description
321 of the Moodies clastic grains is generally consistent with that in previous studies (Heubeck and

322 Lowe, 1999; Hessler and Lowe, 2006; Heubeck, 2019), in which quartz (70-90%) and K-feldspar

323 (<20%) are major clastic grains in the MdQ1 sandstones in the eastern Saddleback Syncline.

324

325 5.1.2. *Intergrain areas*

326 Most of the intergrain areas are filled with microquartz and sericite (KAl₂AlSi₃O₁₀(OH)₂) (Fig.

327 4), although micropores (mostly <200 µm in diameter) are also observed in the sandstones

328 throughout the succession. These microquartz and sericite were classified originally into

329 “pseudomatrix” in Heubeck and Lowe (1999). However, we describe them as a cement in this study

330 because their fabrics suggest that they recrystallized from an original matrix (see below and Sections

331 6 and 7). No original clay matrix is identified clearly in the present sandstones. This observation is

332 consistent with the previous interpretation that the original matrix in the Moodies sandstones has

333 been recrystallized totally to a mosaic of fine-grained sericite and microquartz (Heubeck and Lowe,

334 1999). The microquartz cements fill irregular-shaped spaces between the clastic grains. The sericite

335 cements are also observed around the detrital grains. The sericite crystals are normally elongated up

336 to 30 µm (Fig. 4d). Muscovite is also recognized in the sandstones commonly filling spaces between
337 the clastic grains (Fig. 3e). The size of muscovite crystals is up to 200 µm and their shapes are

338 usually irregular but sometimes subhedral. Some veins filled with sericite cut the microquartz

339 cements and clastic grains (Figs. 4e and f).

340 Through the analyzed succession, the microquartz cements are generally dominant in the

341 coarser sandstones. In contrast, fine-grained laminae in the subunits 1, 2 and 4 (Fig. 1c), which are

342 frequently intercalated into the coarse sediments, are enriched mostly in sericite (Figs. 4b and c).

343 The subunit 3 lacks such fine-grained laminae and thus the intergrain areas are filled mainly with

344 microquartz cements (Fig. 4a). In the subunits 1 and 2, the relative proportion of clastic grains to the

345 microquartz and sericite cements is mostly between 40 and 50%. The proportion is higher in the

346 overlying subunit 3 (up to 60%), reflecting the lack of sericite-bearing fine-grained lamina in this

347 subunit. In the uppermost subunit 4, the proportion is between 35 and 50%.

348

349 **5.1.3. Accessory minerals**

350 Various types of oxide mineral are observed in the analyzed sandstones (Fig. 5). Titanium (Ti)

351 oxide particles are common throughout the succession (Fig. 5a). Ti oxides are sometimes observed

352 as aggregates of small (<20 µm) particles. These aggregates are mostly irregular-shaped and
sometimes coat clastic grains or fill intergrain areas (Fig. 5b). The Raman results show that the Ti oxides are rutile and anatase. Some veins bearing Ti oxides cut both of the microquartz and sericite cements and clastic grains (Figs. 5c-f and 7). Iron (Fe) oxide grains are also recognized in the sandstones. The Fe oxides are irregular shaped and their sizes are mostly <50 µm. The Fe oxides are observed within the cements along grain boundaries. Some Fe oxides are also observed along a micropore (Fig. 5g). The Raman results show that some Fe oxides are goethite. The macroscopic LIBS images demonstrate that Fe is mainly concentrated in the secondary altered parts of the sandstones (Supplementary Figs. S2 and S3). The common occurrence of accessory Fe-oxides in the Moodies sandstones is consistent with the previous description in Heubeck and Lowe (1999). Small sulfide grains are also contained in the analyzed sandstones (Fig. 5h) (Saitoh et al., 2020). The sulfides are exclusively pyrite and range in size mostly from 5 to 100 µm. The shape of the pyrite crystals is mainly irregular but some are euhedral to subhedral. The pyrite crystals are mostly within the cements and along grain boundaries.

### 5.1.4. Anastomosing and wavy laminae

On the outcrop, anastomosing branching and wavy laminae are frequently observed in the
subunits 1, 2 and 4 of the analyzed succession (Figs. 1c, 2c, and 6) (also see log N°9 in Homann et al., 2015). The laminae are dark greenish colored at the hand-specimen scale when observed with the naked eye and are continuous laterally over several to tens of meters. Microscopic observations show that the laminae at the hand-specimen scale correspond to thin (mostly <5 mm thick) and fine-grained laminae intercalated into coarse-grained sandstones (Figs. 2c and 6). Homann et al. (2015) classified these laminae into three morphotypes. They suggested that planar-type laminae are a characteristic of coastal floodplain (subunit 1), whereas wavy-types were typically formed in an intertidal setting (subunit 2). Tufted-types characterized upper inter- to supra-tidal settings (subunit 4). However, we observed frequent planar- and wavy-type laminae in the subunits 1, 2, and 4, and no tufted-type lamina was observed in the subunit 4. The subunit 3 is characterized by the lack of the anastomosing branching and wavy lamina, as previously observed in Gamper et al. (2011) and Homann et al. (2015).

The laminae are composed of finely fragmented quartz and K-feldspar grains (<250 µm in diameter) (Figs. 6 and 7). The intergrain areas in these laminae are composed dominantly of sericite with minor microquartz cements (Figs. 6c-f). Opaque minerals are commonly observed within the laminae and are composed mostly of rutile and anatase and more occasionally of goethite (Fig. 8).
Some Ti oxides show an anastomosing microtexture (Figs. 8a and d). In some cases, Ti oxide particles are observed at the boundary between the fine-grained lamina and coarse-grained sediments (Fig. 6e). The Ti concentration within the laminae is confirmed by the EPMA (Fig. 7) and LIBS (Supplementary Figs. S2) images. The LIBS images also show the presence of chromium and boron within the laminae, which are likely incorporated into sericite.

Carbonaceous material occurs scarcely within the anastomosing and wavy laminae in the subunits 1, 2, and 4 (Fig. 1c), as shown by Raman analyses (Figs. 8 and 9). The carbonaceous material was rarely detected, even in laminae, for the seven samples investigated. Only some micrometric regions in one lamina in SAD39 were enriched in carbonaceous material (Fig. 9). Note that the Raman maps are visual representations of a binary matrix, i.e., colored pixels rely on the presence of a given organic/mineral phase, but do not inform on its concentration. In accordance with the previous Raman data in Homann et al. (2018), the detected carbonaceous materials show a Raman signature typical of disordered carbonaceous material that has been submitted to greenschist-facies metamorphism, with an intense and narrow D1 band, a less intense G band, and a D2 band appearing as a shoulder (Figs. 8c and 9f). In the lamina in SAD39, carbonaceous materials occur commonly in association with Ti oxide (rutile and anatase) (Figs. 9c-e): both of carbonaceous
401 material and Ti oxide are frequently detected at the same spot (Fig. 9f).

402

403 5.2. Major and trace element geochemistry

404

405 Major element diagrams of the analyzed Moodies sandstones are shown in Fig. 10 and

406 Supplementary Fig. S4. All the data are reported in Table 2. The results demonstrate that Si, Al, and

407 K are the three major elements of the analyzed sediments. This is consistent with the microscopic

408 observation that quartz, K-feldspar, and chert clastic grains and microquartz and sericite cements are

409 the major components of the rocks. The SiO₂ contents of the analyzed rocks are mostly between 75

410 and 95 wt.%. The SiO₂ contents of the subunit 3 are higher (>85 wt.%) than of other subunits (Fig.

411 1c), which is consistent with the higher proportion of quartz cement in the intergrain area and the

412 lack of laminae with terrigenous matrix in the subunit (Fig. 10a). Other than that, no major chemical

413 difference is observed in the different subunits. The Na₂O content of the sample (SAD18) in the

414 subunit 1 is substantially high (> 5 wt.%, see Table 2) and it is consistent with its extensive

415 albitization observed under microscope. The Al₂O₃ contents of the sandstones are correlated with the

416 SiO₂ (R²=0.97), TiO₂ (R²=0.75), K₂O (R²=0.48), and Fe₂O₃ contents (R²=0.44) (Figs. 9a and b and
417 Supplementary Fig. S5) (Table 2). All the subunits share similarly low loss of ignition (LOI)

418 between 0.47 and 2.55 wt.% (Fig. 10d). The major element composition of the analyzed sandstones

419 is similar to that of previously analyzed Moodies sandstones (orange circles in Fig. 10). In particular,

420 the present sandstones share the low Fe₂O₃, MnO, and MgO contents with the only one sandstone

421 sample from the Saddleback Syncline available in the literature (orange dashed arrows in Fig. 10)

422 (Hessler and Lowe, 2006).

423 The trace element concentration of the analyzed Saddleback sandstones is reported in Table 3,

424 and their patterns normalized to the upper continental crust (UCC) are shown in Supplementary Fig.

425 S6. Chondrite- and MuQ-normalized REE patterns are shown in Fig. 11. The trace element patterns

426 of the analyzed sandstones are similar to the modern UCC pattern and show slight depletion in Cs,

427 Nb, Ta, Pb, Sr, V, Co, and Zn and enrichment in Li, Sc, and Cr. The MuQ-normalized patterns show

428 no positive Eu anomaly (Eu/Eu* < 1.12). No clear difference is observed between the different

429 subunits in the analyzed succession. The present trace element data are consistent with the previous

430 data of the Moodies sandstones (Hessler and Lowe, 2006).

431

432 **6. Chronology of depositional and post-depositional processes**
433

434

Based on the petrological descriptions and the geochemical results, we propose a scenario of
several processes that have affected the studied Moodies sediments sequentially during and after
their deposition (Fig. 12).

435

436

437

438

## 6.1. Stage 1: Sedimentation

439

440

The first stage corresponds to the deposition of detrital grains, mainly quartz, K-feldspars, and chert fragments (Fig. 12a). Quartz and feldspar grains derived mostly from plutonic and felsic volcanic rocks (e.g., Heubeck and Lowe, 1999; Hessler and Lowe, 2006), whereas chert grains were presumably derived from older sedimentary rocks of the Onverwacht and Fig Tree groups (e.g., Heubeck and Lowe, 1999; Saitoh et al., 2020).

441

442

443

444

445

The analyzed sediments are subdivided into two types according to their depositional settings.
The sediments of the subunit 3 deposited in a subtidal environment (Fig. 1c), in which a constant and
high current velocity led to the deposition of coarse grains through large sand bodies (dunes and
megaripples, also see Homann et al., 2015). In such a high energy sandy setting, most of fine-

446

447

448
grained clay and potential organic matter may have been winnowed and not have accumulated (e.g., Noffke et al., 1997). Accordingly, the SiO₂ and Al₂O₃ contents of samples from the subunit 3 are higher and lower, respectively, than those from other subunits (Fig. 10a). In marked contrast, the sediments of the subunits 1, 2, and 4 deposited in coastal floodplain and inter- to supra-tidal settings (Fig. 1c) (Homann et al., 2015). These depositional settings were likely characterized by periodical changes in current velocity under the influence of river flush or tides. When the current velocity was temporarily low, relatively fine detrital grains accumulated possibly with a matrix (clay minerals and organic matter), contributing to the formation of a fine-grained lamina intercalated into coarser sediments (Fig. 12a). Although such original clay matrix is not petrographically observed in the present rocks (Figs. 4 and 6), It is commonly recognized in other Moodies localities (e.g., Eriksson and Simpson, 2000; Homann et al., 2018).

In the present sandstones, the supposed presence of the original clay matrix is consistent with the major element geochemistry of the rocks. The Al₂O₃ contents are well-correlated with the SiO₂, K₂O, TiO₂, and Fe₂O₃ contents (Figs. 10a and b and Supplementary Fig. S5), along a trend with other Moodies shales and sandstones (McLennan and Taylor, 1983; Hessler and Lowe, 2006; Toulkeridis et al., 2015), and igneous clasts in Moodies conglomerates (Sanchez-Garrido et al.,
465 2011; Agangi et al., 2018). These correlations suggest that the major element composition of the initial Moodies sediments was controlled mainly by the relative proportion of quartz and clay matrix, because a similar correlation is commonly observed in modern terrigenous sediments and typically reflects the effect of hydrodynamic sorting of Al₂O₃-, K₂O-, Fe₂O₃-, and TiO₂-rich clay from coarse quartz particles in distal and proximal environments (e.g., Singh, 2009). In the subunits 1, 2, and 4 of the present Saddleback succession, more quiescent periods leading to the deposition of fine grains may also have been favorable for potential biofilm installation (e.g., Noffke et al., 1997; Homann et al., 2015).

473

474 6.2. Stage 2: Microquartz cementation

475

476 After the initial sedimentation phase, an interstitial fluid enriched in silica led to microquartz precipitation and cementation between detrital grains in the analyzed Moodies sediments (Fig. 12b).
477
478 Three potential processes can be considered for the microquartz cementation: 1) low-temperature sediment-water interaction at or below the surface (e.g., Knauth and Lowe, 1978; Lowe, 1999), 2)
479
480 microquartz precipitation from a hydrothermal fluid (e.g., Knauth and Lowe, 1978; de Wit et al.,
1982; Hofmann, 2005; Van Kranendonk, 2006), and 3) IPS and subsequent microquartz cementation
during burial (e.g., McBride, 1989; Tada and Siever, 1989).

Our observation suggests that IPS was the main mechanism for the microquartz cementation in
the analyzed sediments, for the following reasons. Firstly, the gas-/fluid-escape structures are
frequently observed in the analyzed Moodies sandstones (Figs. 2f and g). These structures should
have formed in an early diagenetic stage before sediment consolidation (e.g., Lowe, 1975; Homann
et al., 2015). The well-preserved and silicified gas-/fluid-escape structures suggest that the
microquartz cementation and sediment silicification occurred during burial and are not concomitant
with sedimentation (Heubeck, 2019). Microquartz precipitation around the sediment-water interface
is therefore unlikely. Secondly, the trace element patterns are similar in all the subunits and show no
substantial depletion in mobile elements (Supplementary Fig. S6), implying that the microquartz
cementation from an additional hydrothermal fluid was not significant.

Finally, the frequent IPS textures in the analyzed sandstones (Fig. 3d) suggest that the pressure
solution during burial contributed substantially to the microquartz cementation. The microquartz
cement is commonly observed in the coarser sediments throughout the analyzed succession. It is
likely that these coarse sediments were characterized by high intergrain porosity during their
497 deposition in a relatively high-energy setting (Stage 1), and were favorable to IPS and silicification

498 during burial diagenesis. The high SiO₂ contents of coarse sandstones in the subunit 3 (>85 wt.%;

499 Fig. 10a) may partly reflect the intense post-depositional silicification, because mineral sorting alone

500 rarely leads to such substantially high SiO₂ contents (e.g., Singh, 2009). In contrast, the microquartz

501 cement is less present within the fine-grained laminae in the subunits 1, 2, and 4 (Figs. 1c, 4b and c).

502 The original clay matrix with low porosity likely prevented pervasive microquartz cementation

503 within the laminae.

504

505 6.3. Stage 3: Sericite cementation

506

507 The sericite cements are frequently observed in the analyzed Moodies sediments and are

508 particularly common within the fine-grained laminae in the subunits 1, 2, and 4 (Figs. 1c, 4, and 6c-

509 f). Some sericite veins clearly cut micro-quartz cements (Figs. 4e and f). This geometric relationship

510 implies that the sericite cementation occurred after the sediment silicification and consolidation

511 (Stage 2). We interpret that the original clay matrix within the fine-grained laminae (Stage 1) was

512 replaced with sericite by a fluid circulation along the bedding during later metasomatism (Fig. 12c).
The major and trace element geochemistry of the analyzed Moodies sandstones does not show a typical feature of sediments being flushed at high temperature with high water/rock ratios (Figs. 10 and 11 and Supplementary Fig. S6) (e.g., bell-shape in the REE pattern and depletion in moderately immobile elements; McLennan, 1989). This suggests that the sericite cementation was not due to a hydrothermal fluid circulation. Rather, the major and trace element composition of the analyzed sandstones is similar to that of their potential sources (i.e., igneous clasts in the Moodies; Agangi et al., 2018). It indicates that secondary processes were not intense enough to leach out less-mobile elements and to erase the geochemical information about the provenance of the sediments (Hessler and Lowe, 2006). The analyzed Moodies sandstones have not been altered substantially by a hydrothermal fluid. We thus infer that the sericite cementation occurred during metasomatism, with a relatively low-temperature fluid, that has been commonly observed in the entire BGB rocks (e.g., Condie et al., 1977; de Ronde and de Wit, 1994; Hofmann, 2005), including the Moodies Group (e.g., Heubeck and Lowe, 1999; Heubeck, 2019). The present geochemical data suggest that the original clay matrix in the laminae was replaced with sericite during low-grade and fluid mediated metamorphism (Dunoyer de Segonzac, 1970; Heubeck 2019).

528
529 6.4. Stage 4: Ti oxide formation

530

531 Ti oxides (rutile and anatase) are common in the analyzed sandstones (Fig. 5a). They form
532 aggregates of small (<20 µm) particles, coat clastic grains, fill irregular-shaped intergrain areas, and
533 represent anastomosing textures (Figs. 5b and 8). The dark and microscopic Ti-oxide laminae follow
534 parallel to the bedding and are present mainly in the fine-grained sericite-rich laminae. Also, some
535 veins bearing Ti oxides cut both of clastic grains and microquartz and sericite cements (Figs. 5c-f).

536 All these textures indicate that the Ti oxides formed secondarily after the microquartz and sericite
cementation (Stage 2 and Stage 3, respectively) (Fig. 12d). The Ti oxides can commonly form
537 during diagenesis and low-grade metamorphism via recrystallization of Ti-bearing minerals like

538 ilmenite, biotite and feldspars (e.g., Richards et al., 1988; Smith et al., 2009; Bower, 2011; Galvez et
539 al. 2012). Ti is highly stable and insoluble in a fluid (e.g., Brookins, 1988), though it can be more
540 mobile in the presence of phosphate ions (Pe-Piper et al., 2011). Our observations suggest that the Ti
541 oxides formed via the reaction of Ti-bearing detrital grains during metamorphism (Figs. 12a and d).
542

543 A close association of carbonaceous material and Ti oxide in sedimentary rocks has been
544 reported (Foucher et al., 2012; Galvez et al., 2012; Lekele Baghekema et al., 2017; Sirantoine et al.,
2020), even in fossilized microbial mats (Noffke et al., 2008). Although the carbonaceous material has been observed only in the laminae that contain Ti oxides in the present sandstones (Figs. 8 and 9), the general scarcity of carbonaceous material through the analyzed succession does not allow us to conclude firmly on the systematic association of carbonaceous material with Ti oxide. The origin of this apparent association is still unknown and various processes have been suggested for it from biological accumulation to post-depositional and diagenetic mobilization (Glamoclija et al., 2009; Sirantoine et al., 2020).

6.5. Stage 5: Rainwater infiltration

Finally, Fe oxides precipitated during a recent oxic rainwater infiltration into the analyzed Moodies sediments (Fig. 12e) (Heubeck, 2019). The Fe oxides are irregular shaped and observed within the cements along grain boundaries and rarely within a fine-grained lamina. Some Fe oxides are observed along a micropore (Fig. 5g). The occurrence of Fe oxide along a micropore is consistent with the rainwater infiltration scenario into the sediments. The macroscopic LIBS images also suggest that Fe is concentrated mainly in the reddish-brown superficially altered parts and that
561 its distribution is unrelated to the bedding in the analyzed sandstones (Supplementary Figs. S2 and

562 S3). A plausible Fe source for the oxides is pyrite crystals in the sandstones (Saitoh et al., 2020).

563 Although the timing of the rainwater infiltration and Fe oxide formation is poorly constrained,

564 Heubeck (2019) suggested that they occurred very recently. It is apparently consistent with the work

565 by Bonnand et al. (2020), showing that the emplacement of the negative Ce anomalies of the

566 Moodies BIF samples occurred in the last 100 Ma.

567

568 6.6. Correspondence to the regional thermal history

569

570 During the Moodies deposition, the emplacement of several plutonic domains occurred around

571 the BGB at 3.23-3.21 Ga (Fig. 1a) (e.g., de Ronde and de Wit, 1994), including the Kaap Valley

572 Pluton at ~3.21 Ga (e.g., Moyen et al., 2019 and references therein) and the Archean Gneiss

573 Complex (AGC) at 3.24-3.07 Ga (e.g., Taylor et al., 2012; Kröner et al., 2018; Wang et al., 2020),

574 associated with the major D2 deformation event in the BGB (e.g., de Ronde and de Wit, 1994). The

575 3.23-3.21 Ga emplacement was followed by major tectono-thermal events at ~3.1, ~2.7, and ~2.1 Ga

576 (Weis and Wasserburg, 1987; de Ronde et al., 1991a; Lécuyer et al., 1994; Toulkeridis et al., 1994,
577 1998). The extensive emplacement of the large plutons of the granodiorite-monzogranite-syenite
578 (GMS) occurred at ~3.1 Ga (beige colored areas in Fig. 1a) (e.g., Anhaeusser et al., 1981). The wide
distribution of this event indicates that the BGB sediments, including the Moodies, were
579
substantially affected by associated heating. There are a number of gold deposits in the northern
580
central BGB, some in close proximity (<10 km) to the analyzed Saddleback section (Munyai et al.,
581
2011; Selvaraja et al., 2017). These gold deposits formed at ~3.1-3.0 Ga based on rutile and titanite
582
U-Pb ages (de Ronde et al., 1991a, 1992; Agangi et al., 2019), presumably in association with the
583
GMS event. This age is identical to the Sm-Nd age of carbonate-derived clay minerals in the Fig
584
Tree Group in the central BGB (Toulkeridis et al., 1994), supporting the large influence of the GMS
585
emplacement on the BGB sediments. Another major thermal event occurred widely in the BGB at
586
~2.7 Ga (e.g., de Ronde et al., 1991b; Toulkeridis et al., 1998). Toulkeridis et al. (1998) suggested
587
extensive silicification and sericitization of carbonates in the Onverwacht and Fig Tree groups by
588
low-temperature fluids (~200°C) at ~2.7 Ga, resetting the Pb-Pb, Sm-Nd, and Rb-Sr isotopic
589
systems. The ~2.7 Ga thermal event was followed by a later one widespread in the BGB at ~2.1 Ga,
590
based on Rb-Sr, Sm-Nd, and ⁴⁰Ar/³⁹Ar dating of the Onverwacht and Fig Tree sediments (e.g., Weis
591
and Wasserburg, 1987, de Ronde et al., 1991b).
592
The present Moodies sandstones in the central BGB were presumably affected by all these polymetamorphism/tectono-thermal events (e.g., Heubeck and Lowe, 1999). Previously published Raman temperature estimates on Moodies carbonaceous materials provided temperature ranging between 350 and 375°C (Nabhan et al. 2017; Homann et al. 2018; Kohler and Heubeck, 2019). If the microquartz cements formed during burial diagenesis, the sericite cementation (Stage 3; Fig. 12c) and Ti oxide formation (Stage 4; Fig. 12d) in the analyzed Moodies sandstones could correspond to any of the multiple tectono-thermal events that occurred at a regional scale between ~3.1 and ~2.1 Ga.

601

## 7. Origin of the analyzed Moodies anastomosing and wavy laminae

603

Noffke et al. (2006) were the first to propose the presence of various types of MISS (e.g., wrinkle and roll-up structures) in the Moodies sediments from the Dycedale and Saddleback synclines. Later studies generalized the presence of fossilized microbial mats in the Moodies sediments as anastomosing and wavy laminae (Heubeck, 2009, 2019; Homann et al., 2015, 2018; Homann, 2019; Köhler and Heubeck, 2019). Specific ecological and taphonomic conditions are
609 generally required for the formation and preservation of microbial mats in ancient sandstones
610 (Noffke et al., 2001). Along the different Moodies successions in the Saddleback Syncline, Homann
611 et al. (2015) argued in favor of morphological adaptation of microbial mats (planar-, wavy-, and
tufted-types) to the different paleoenvironments (coastal, intertidal, and inter- to supratidal settings,
612 respectively). Unfortunately, we have not been able to reproduce such a correlation between mat
613 morphology and depositional setting along the studied section. The identification of structures
614 morphologically similar to microbial mats in Moodies fluviatile conglomerates also led some
615 authors to conclude the existence of a Paleoarchean terrestrial biosphere (Homann et al., 2018). The
616 presence and preservation of biofilms or anastomosing and wavy laminae in the Moodies sediments
617 thus seem to occur in various depositional settings (Homann, 2019).

618
619 Some Moodies environments are nevertheless characterized by the absence of such laminae as
620 seen in the subunit 3 of the Saddleback Syncline succession. The sediments of this subunit
621 accumulated in a relatively shallow subtidal setting, characterized by strong tidal currents and storms
622 (Homann et al., 2015). Under such consistently high-energy conditions, putative biofilms or a matrix
623 with organic matter would be likely reworked and vanished (Gerdes et al., 2000; Noffke et al.,
624 2003). In contrast, coastal floodplain and supra- to inter-tidal settings under fluctuating energy
conditions, corresponding to the present subunits 1, 2, and 4, are known to be more suitable for the
biomat formation and preservation (e.g., Noffke et al., 1997; Noffke, 2010; Newman et al., 2017). In
these latter subunits, the laminae are relatively continuous laterally over several to tens of meters and
are present stratigraphically almost every centimeter over a >200-m-thick succession. Archean to
modern examples of MISS that developed in tidal environments indicate that, even when microbial
biofilms can tolerate high energy events, their installation and development require a period of no
sediment accumulation under quiet conditions (e.g., Gerdes and Klenke, 2007; Eriksson and Wilde,
2010; Noffke, 2010; Cuadrado et al., 2013). Such ecologic and taphonomic windows of microbial
mat development and preservation (also see Noffke, 2009) appear questionable in the light of the
rather tight and continuous record of anastomosing and wavy laminae, with little evidence for
tearing and transportation of biofilm fragment, along the analyzed Moodies succession.

Based on the present results, we reinterpret the formation of the anastomosing and wavy
laminae observed along the studied succession in the Saddleback Syncline, previously suggested to
be ancient microbial mats (Homann et al., 2015) (Fig. 12). Our petrological observations show that
these laminae are composed mainly of fine grains with secondary sericite and Ti oxide (Figs. 6, 8,
and 12), and are aligned with the bedding and sometimes crosscut detrital grains and microquartz
cements that formed during diagenetic burial (Fig. 7). These observations suggest a strong

contribution of post-burial metasomatic fluid flow to the lamina during Mesoarchean to

Paleoproterozoic tectono-thermal events (Fig. 12c). Moreover, the multiple sulfur isotope records of

pyrite indicate the secondary addition of hydrothermal sulfur to the analyzed sandstones possibly via

fluid injections (Saitoh et al., 2020). Nonetheless, the major and trace element compositions of these

sandstones are not depleted in moderately immobile elements, and the MuQ-normalized REE

patterns show no positive Eu anomaly (Fig. 11). These geochemical data do not support an extensive

hydrothermal fluid circulation, but suggests that the fluids were rather endogenous and circulated

only within the sedimentary unit. The observed Moodies anastomosing and wavy laminae are thus

closely associated with low-grade fluid-mediated metamorphic events (Fig. 12). Similar dark

laminated textures with a mineral assemblage including Ti oxide were previously described in

younger 2.9 Ga sandstones of the Pongola Supergroup in Bower (2011), and were interpreted to be

formed as a result of the circulation of low-grade metamorphic fluids. The author precisely warned

the community about the risk to misinterpret lamination with such mineral assemblage as ancient

microbial mats.

Microscopic dark laminae from the same Moodies succession were previously referred as
"kerogenous" (Homann et al., 2015). However, carbonaceous matter was rarely identified in our Raman analyses (Figs. 8 and 9). Our results demonstrate that not all wavy anastomosing laminae in the Moodies rocks are kerogenous. Raman spectroscopy does not provide quantitative estimation of the amount of carbonaceous matter and sufficient chemical information to identify disordered carbonaceous matter as kerogen (Pasteris and Wopenka, 2003). However, both of the present and previous studies reported the substantially low LOI (<2.55 wt.%) and TOC contents (<1000 ppm) in the Moodies sandstones (Tables 2 and S2; Homann et al. 2018). It is worth noting that the lamina-bearing subunits 1 and 4 do not show any TOC enrichment compared to the subunit 3 (devoid of anastomosing and wavy lamination), suggesting that carbonaceous material is homogeneously scarce in the studied Moodies sandstones (Table S2). Nonetheless, we emphasize that the present results do not demonstrate that organic matter was ordinally scarce in the Moodies laminae in the subunits 1, 2, and 4 during the sediment accumulation. Rather, it is likely that the observed fine-grained laminae primarily contained some organic matter in the analyzed Moodies sediments (Stage 1) (Fig. 12a). However, due to the substantial secondary overprints during the long post-depositional history (Fig. 12), it is difficult to determine whether the rarely detected carbonaceous materials in the laminae are a remnant of original microbial mats or of sedimentary organic matter, which had been incorporated
673 into the fine-grained laminae during the sediment accumulation. In the analyzed Moodies
674 sandstones, intense silica precipitation that occurred mainly in the coarse-grained sediments after
675 burial (Stage 2) may have prevented, at least in part, organic remains from escaping from the
676 sediments during burial (Fig. 12b). Indeed, the entombment of microorganisms into silica
677 significantly limits their molecular degradation during experimental diagenesis (Alleon et al., 2016;
678 Igisu et al., 2018). Spatially resolved studies of organic microstructures in similarly metamorphosed
679 Palearchean cherts have consistently documented their chemical preservation with both
680 geochemical and petrographic evidence against organic migration from the microstructures (e.g., van
681 Zuilen et al., 2007; Westall et al., 2011; Alleon et al., 2018, 2021; Hickman-Lewis et al., 2020). On
682 the other hand, later metasomatism (Stage 3) and a recent oxidative weathering event (Stage 5) may
683 have participated to leach carbonaceous matter from the Moodies sediments (Petsch et al., 2000).

684 The occurrence of carbonaceous material in the Moodies sandstones has been reported in
685 several other localities in the central Saddleback Syncline (Gamper et al., 2011) and in the Stolzburg
686 and Dycedale synclines (Noffke et al. 2006; Nabhan et al., 2017; Homann et al., 2018; Köhler and
687 Heubeck, 2019), and has been associated with the morphology of anastomosing and wavy laminae
688 interpreted as a remnant of original microbial mats. However, based on the present petrological and
689 geochemical observations, the anastomosing and wavy laminations frequently observed in the
690 studied Moodies sandstones are not necessarily consistent with the existence of original microbial
691 mats. Our results also emphasize that the Moodies sediments are substantially different from cherts
692 from the older Onverwacht and Fig Tree groups that contain substantial amounts of organic matter
693 (e.g., Hofmann and Bolhar, 2007; Van Zuilen et al. 2007; Hofmann et al., 2013; Marin-Carbonne et
694 al. 2018; Alleon et al., 2021). In the future, research on new sedimentary successions and rock
695 samples would advance our understanding of traces of life preserved in the Moodies sediments,
696 especially in the framework of the upcoming BASE ICDP drilling project, although it will be
697 important to bear in mind the secondary overprint of metasomatism on the Moodies laminae.

698

699 **8. Conclusions**

700

701 The quartz-rich sandstones of the 3.22 Ga Moodies Group were collected from a ~350 m thick
702 succession in the Saddleback Syncline in the central Barberton Greenstone Belt, South Africa.
703 Petrological and multiple geochemical characteristics of the rocks were analyzed to reconstruct their
704 post-depositional history and to examine the origin of anastomosing and wavy laminae in the rocks,
which were previously suggested to be a remnant of microbial mats. The laminae are frequently observed in coastal floodplain and inter- to supra-tidal settings, but not in a consistently high-energy subtidal setting, in the analyzed succession. Our results show that, after an initial phase of sedimentation (Stage 1), the studied sandstones have been affected by complex post-depositional processes, including at least three main processes with four post-depositional stages: (i) intergranular pressure solution leading to the microquartz cementation during the diagenesis burial (Stage 2); (ii) formation of a sericite cement (Stage 3) and Ti oxide (Stage 4) during low-grade and fluid mediated metamorphism; and (iii) rainwater infiltration that triggered more recent precipitation of Fe oxide (Stage 5). This post-depositional history appears strongly dependent on the paleoenvironment and nature of the sediments. In high energy settings, coarse-grained sediments characterized by an intergrain porosity allowed the microquartz cementation, which prevented the coarse sediments from later fluid injections. In contrast, under fluctuating energy settings, periods of more quiescent hydrodynamic conditions allowed the repeated deposition of fine-grained laminae with a clay matrix. This original matrix, which prevented the fine-grained laminae from the microquartz cementation, was later replaced with sericite. Although we do not exclude that some of the observed Moodies laminae were originally microbial mats, the mineral assemblage of the studied
721 anastomosing and wavy laminae (i.e., mainly sericite associated with Ti oxide and rare carbonaceous

722 material) shows a substantial metamorphic overprint, implying that similar laminae cannot be

723 unambiguously associated with the concept of a flourishing Paleozoarchean microbial life at 3.22 Ga.

724

725 Acknowledgements

726

727 We thank Christoph Heubeck, Martin Homann, Phillip Andreas Zametzer and Deon Janse van

728 Rensburg for their assistance with locating the sampling section and for fruitful discussions.

729 Moritz Mühlberg, assisted with the sampling. Thanks to Anne-Sabine Grosjean who help for the

730 selection and thin section preparation. Federica Schiavi assisted with the Raman analyses. Martin

731 Robyr assisted with EPMA mappings. Marilyne Imbault, Mhammed Benbakkar and Krzysztof

732 Suchorski assisted with the ICP-AES and ICPMS analyses. We thank Bertus Smith for fruitful

733 discussion. Frances Westall and two anonymous reviewers gave us fruitful comments to improve the

734 manuscript. This research was supported by the Laboratory of Excellence Clercvolc, the INSU

735 Programme National de Planetologie and the European Union's Horizon 2020 research and

736 innovation program ERC (STROMATA, grant agreement 759289).
737

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1059 **Figure captions**

1060

1061 Fig. 1. Geological map (a) and stratigraphy (b, c) of the Moodies Group. (a) Simplified geological

1062 map of the Barberton Greenstone Belt (BGB) in northeast South Africa. The map is an enlarged

1063 square in the inset. A star shows the sampling locality in this study. (b) General stratigraphy of the

1064 Moodies Group in the Saddleback Syncline. (c) The analyzed sandstones in this study with

1065 sedimentary environments (according to Homman et al., 2015). Figures are modified from Homann

1066 et al. (2015).

1067

1068 Fig. 2. Outcrop photos of the analyzed Moodies sandstones. (a) A distant outcrop view of the

1069 sampling locality in the central Saddleback Syncline (the same hill of the stratigraphic Log 9 in

1070 Homann et al., 2015). The Moodies strata are dip subvertically and overturned. A person (circled) for

1071 scale. (b) Conglomerate (lower part) and megaripple (upper part) in the subunit 1. (c) Anastomosing

1072 and wavy laminae in the subunit 1. (d) Small channel, small megaripples, and flat laminae with
1073 ripples in the subunit 2. (e) Large-scale cross-bedding in the subunit 3. (f, g) gas-/fluid-escape

1074 structures in the subunit 4. A hummer (f) and pen (g) for scale are 28-cm- and 12-cm-long,
1075 respectively.

1076

1077 Fig. 3. Clastic grains of the analyzed sandstones. (a) Typical medium-grained quartz-rich sandstones

1078 (crossed nicols). (b) A quartz (Qz) grain with wavy extinction (crossed nicols). (c) A microcline

1079 (Mc) grain (crossed nicols). A chert (Ch) grain is also surrounded by quartz grains in the right side.

1080 (d) A rounded chert grain (crossed nicols). Arrows show intergranular pressure solution (IPS)

1081 texture. (e) Apatite inclusions (circled) in a quartz grain. Muscovite (Ms) is also shown. (f) BSE

1082 image of monazite-(Ce) (white grains). A partly albitized (Ab) K-feldspar (Kfs) is also shown. (g, h)

1083 EDS spectra of an apatite inclusion in quartz (g) and of monazite-(Ce) (h).

1084

1085 Fig. 4. Cements in the analyzed sandstones (crossed nicols except for d). (a) Coarse-grained

1086 sandstone with microquartz cements (arrows) in the subunit 3. (b) Fine-grained lamina enriched in

1087 sericite. (c) Boundary between lower coarse sediments and an upper fine-grained lamina enriched in

1088 sericite (intergranular brown crystal). (d) BSE image of elongated sericite (Ser) crystals between
1089 quartz (Qz) grains. (e, f) Sericite-bearing veins cutting irregular-shaped microquartz cements

1090 (arrows).

1091

1092 Fig. 5. Secondary minerals in the analyzed sandstones. (a) Reddish brown titanium (Ti) oxides

1093 (arrows) in grain boundaries (opened nicols). (b) BSE image of Ti oxides (arrow). (c-f) Ti oxide

1094 bearing vein (arrow) cutting a quartz grain (c, d) and microquartz-sericite cements (e, f). c and e:

1095 opened nicols, d and f: crossed nicols. (g) BSE image of iron oxides (arrows) along a micropore. (h)

1096 Pyrite particles (reflected light).

1097

1098 Fig. 6. Anastomosing and wavy laminae in the analyzed sandstones. (a, b) Outcrop (a) and slab (b)

1099 photos of the laminae. (c, d) A fine-grained microscopic lamina filled with sericite cements.

1100 Titanium oxide particles (black dots) are commonly observed within the lamina (arrows). c: opened

1101 nicols, d: crossed nicols. (e, f) Ti oxides (arrows) at the boundary between lower coarse-grained

1102 sediments and an upper fine-grained lamina. e: opened nicols, f: crossed nicols.

1103

1104 Fig. 7. Photomicrograph (opened nicols) and EPMA maps of a lamina in the subunit 4 of the
1105 analyzed succession. The lamina is composed mainly of fine-grained quartz and K-feldspar with

1106 Mg-bearing sericite and Fe- and Ti-oxides. Note that a larger chert grain at the lower left corner is

1107 cut by the lamina (arrows).

1108

1109 Fig. 8. Raman spectra of the analyzed laminae in SAD21 (a-c) and in SAD4 (d-f) in the subunit 1 of

1110 the analyzed succession. Flat/wavy laminae at the hand-specimen scale are composed mostly of Ti

1111 oxides (rutile and anatase). D1, G, and D2 bands of carbonaceous material (CM) was rarely detected

1112 (c) in the laminae for most of the samples investigated. Only some micrometric regions in one

1113 lamina in SAD39 are enriched sporadically in CM in the present data set (see Fig. 9).

1114

1115 Fig. 9. Raman maps on the CM-bearing lamina in SAD39. Note that this is the only lamina enriched

1116 sporadically in CM in the present dataset (also shown in Fig. 6c). (a) Microphotograph of the lamina

1117 (opened nicols). Lined and mapped regions are shown. On a line, a spot at which CM was detected is

1118 shown in red. Note that, even in this lamina, CM exists only sporadically. (b) Raman map 1. CM and

1119 K-feldspar are shown in red and yellow, respectively. (c-e) Raman map 2. Note an apparent

1120 correspondence between CM and Ti oxide. (f) Raman spectra showing the coexistence of CM and
1121 rutile on the map 1. Background fluorescence was subtracted.

1122

1123 Fig. 10. SiO₂-Al₂O₃ (a), K₂O-Al₂O₃ (b), Fe₂O₃-MgO (c), and SiO₂-LOI (d) cross plots of the

1124 analyzed Moodies succession. Previous sandstone data are from Hessler and Lowe (2006). Shale

1125 data are from McLennan and Taylor (1983), Hessler and Lowe (2006), and Toulkeridis et al. (2015).

1126 Igneous clast data are from Sanchez-Garrido et al. (2011) and Agangi et al. (2018). The present-day

1127 upper continental crust (UCC) composition is from Rudnick and Gao (2014).

1128

1129 Fig. 11. Chondrite- and MuQ-normalized REE patterns of the four subunits of the analyzed

1130 succession. Note that the REE patterns are generally consistent through the succession and no clear

1131 Eu anomaly is recognized on the MuQ-normalized patterns.

1132

1133 Fig. 12. Schematic model for the depositional and post-depositional history (in the five stages; a-e)

1134 of the analyzed Moodies sandstones. IPS: intergranular pressure solution.