# Early Paleoproterozoic warming and oxidation of the Planet recorded by sedimentology and rare earth elements of two Huronian carbonate deposits

Brittany Ramsay, Philip Fralick, Martin Homann, Stefan Lalonde, Robert Riding

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## To cite this version:

Brittany Ramsay, Philip Fralick, Martin Homann, Stefan Lalonde, Robert Riding. Early Paleoproterozoic warming and oxidation of the Planet recorded by sedimentology and rare earth elements of two Huronian carbonate deposits. Precambrian Research, 2025, 427, pp.107886. 10.1016/j.precamres.2025.107886. hal-05412336

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Submitted on 11 Dec 2025

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# Early Paleoproterozoic warming and oxidation of the Planet recorded by sedimentology and rare earth elements of two Huronian carbonate deposits

Brittany Ramsay<sup>a,b</sup>, Philip Fralick<sup>a,*</sup> ◆, Martin Homann<sup>c,d</sup>, Stefan Lalonde<sup>c</sup>, Robert Riding<sup>e</sup>

## ARTICLE INFO

Keywords:

Stromatolites
Precambrian Carbonates
Great Oxidation Event
Huronian Supergroup
Kona Dolomite
Gordon Lake Formation
Rare Earth Elements

## ABSTRACT

One of the most important transitions the Earth experienced is the appearance of significant free oxygen in its atmosphere at approximately 2.4 Ga. This drove the Earth into a protracted series of glaciations well recorded in the Huronian Supergroup of central Canada. Greenhouse conditions developed as the Earth emerged from this event. Carbonate units in the 2.31 Ga Gordon Lake Formation and 2.17 Ga Kona Dolomite, of the Huronian Supergroup and Chocolay Group respectively, record deposition in sabkha to shallow nearshore environments. Nearshore seawater chemistry reflected by these early and middle Lomagundi Event carbonates differs from other time periods. All carbonates contain abundant hematite as particulate matter scattered throughout, as accumulations in depressions between small pseudocolumnar stromatolites, and forming layers separating them. Rare earth elements (REEs) in the carbonates have patterns that differ from those of other time periods. Sabkha carbonates have pronounced middle REE (MREE) and heavy REE (HREE) enrichments. Clays preferentially binding light REEs in the detritus during weathering on the craton probably led to the groundwater flow to the sabkha having those REE characteristics. Intertidal to subtidal carbonates contain rare earth element patterns with only excess middle REEs. Some of the iron oxyhydroxides deposited in the carbonates of the tidal flats and nearshore would have released their REEs, which were enriched in MREEs, during early diagenetic reduction by degrading organic matter. These REEs combined with those provided by seawater were then incorporated into the carbonate cements. The combination of abundant glacially created debris covering the craton and acidic drainage created the distinct REE characteristics in these carbonates.

## 1. Introduction

Despite our general understanding, many unanswered questions remain regarding the evolution of Earth's early atmosphere, oceans, and life. Investigations of these questions commonly use chemical sediments, particularly carbonates, since they contain sensitive proxies capable of recording details of past ocean-atmosphere systems. Combined with an understanding of the environments in which they formed and their diagenetic histories, samples of these sediments can be a powerful tool in reconstructing physical and chemical conditions of the ancient hydro-sphere. This is of particular importance for the stratigraphic sedimentary record of the ~ 2.4 to 2.3 Ga Huronian Supergroup and ~ 2.2 Ga Chocolay Group in central North America, since they record

development of free atmospheric oxygen and Earth's earliest sustained glacial episodes (Bennett et al., 1991; Prasad and Roscoe, 1996; Bekker et al., 2005; Bennett, 2006; Bekker and Kaufman, 2007; Holland, 2009; Konhauser et al., 2009, 2011; Ketchum et al., 2013; Lyons et al., 2014; Shawwa et al., 2024). Carbonates in the Gordon Lake Formation, of the Huronian Supergroup, and Kona Dolomite, of the Chocolay Group (Fig. 1A,B,C), were sampled and the sedimentology and diagenetic history of each sample investigated. This information allows differences in the REE patterns that exist between samples to be more fully understood. The net result is the ability to comment on the hydrology that existed in these coastal areas following the Gowganda/Duitschland and Enchantment Lake/Timeball Hill glaciations.

We use rare earth element geochemistry, in conjunction with

<footnote><sup>a</sup> Department of Geology, Lakehead University, Thunder Bay, Ontario, Canada</footnote>

<footnote><sup>b</sup> Department of Geological Sciences and Geological Engineering, Queens University, Kingston, Canada</footnote>

<footnote><sup>c</sup> Laboratoire Geo-Ocean Ocean, Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, 29280 Plouzané, France</footnote>

<footnote><sup>d</sup> Department of Earth Sciences, University College London, UK</footnote>

<footnote><sup>e</sup> Department of Earth, Environmental and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USA</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail addresses: brittany.ramsay@queensu.ca (B. Ramsay), philip.fralick@lakeheadu.ca (P. Fralick), martin.homann@ucl.ac.uk (M. Homann), stefan.lalonde@univ-brest.fr (S. Lalonde), rriding@utk.edu (R. Riding).</footnote>
sedimentological investigation of each sample, to explore the effects of an oxidizing atmosphere on deposition of the dolostones of the Gordon Lake Formation, Huronian Supergroup (Fig. 1C), and Kona Dolomite, Chocolay Group (Fig. 1B), in Ontario, Canada and Michigan, USA. The detailed sedimentology presented here not only allows samples to be assigned to specific depositional environments, but also enhances knowledge of the environments that were present in the studied areas. Combining this with data on REE and redox sensitive element abundances permits the development, for the first time, of models defining hydrologic pathways in the shoreline and near shore sediments deposited at 2.31 Ga (Rasmussen et al., 2013) and 2.17 Ga (Rasmussen et al., 2024).

## 2. Huronian supergroup

The over ten km thick Huronian Supergroup forms a broad area of early Paleoproterozoic sedimentary rocks cropping out for 250 km north and north-east of Lake Huron, Ontario, Canada (Fig. 1A). Huronian strata were formally divided into four groups and twelve formations by Robertson et al. (1969). Immature sandstones and conglomerates of the Livingstone Creek Formation and the overlying basalts and rhyolites of the Thessalon Volcanics occupy basins with limited areas conformably underlying the much more laterally extensive Huronian formations.

At $2452.5 \pm 6.2$ Ma rift volcanism slowed and terminated with the eruption of the Thessalon Formation underlying the lowermost Huronian Supergroup, though it may have started 22 my prior to that (Ketchum et al., 2013). The rift valley fill, of the volcanic succession and underlying locally derived sandstones and fanconglomerates of the Livingston Creek Formation (Bennet, 2006), is overlain by the Matinenda and McKim Formations. The base of these represents the transition from cratonic rifting to divergent margin development (Fralick and Miall, 1981; Fralick, 1984; Miall, 1985; Fralick and Miall, 1989;

Bennett et al., 1991; Hoffman, 2013; Kurucz et al., 2021), although Young and others (Young and Nesbit, 1985; Young et al., 2001; Young, 2014) believed that the transition occurred higher in the stratigraphy at the base of the third glacial unit, the Gowganda Formation. In particular Fralick and Miall (1989), Hoffman (2013) and Kurucz et al. (2021) provide persuasive arguments for the earlier timing. During this interval falling temperatures resulted in the onset of multiple global glacial and interglacial events. Evidence of these is preserved in sedimentary basin deposits (Prasad and Roscoe, 1996; Hoffman, 2013; Rasmussen et al., 2024).

The first Huronian glacial advance overrode fluvial sands and gravels of the Matinenda Formation, incorporating uraniferous sands into the basal diamictite of the Ramsay Lake Formation (Fralick and Miall, 1989). The diamictites are relatively thin and not present in all areas of the basin (Fralick and Miall, 1989) indicating that, though they may correlate with the Makganyene glaciation of South Africa (Gumsley et al., 2017), they probably do not represent a 'snowball Earth' event (Kurucz et al., 2021). These sediments are followed by thick, coarsening upwards deltaic to fluvial deposits of the Pecors and Mississagi Formations (Fralick and Miall, 1989; Long, 1978), which are overlain by glacial diamictites of the Bruce Formation and its cap carbonate, the Espanola Formation. The diamictites of this middle Huronian glacial event probably correlate with the glacial deposits at the base of the Duitshland Formation in the eastern Transvaal Basin of South Africa (Bekker et al., 2001, 2006; Rasmussen et al., 2013). The geochemistry of the cap carbonates broadly resembles Neoproterozoic cap carbonates and arguments that the Bruce glaciation represents snowball Earth conditions have been made (Bekker and Karhu, 1996; Bekker et al., 1999, 2001, 2005; Kurucz et al., 2021). The carbonates transition into shallow water marine sandstones of the Serpent Formation, which in turn are overlain by diamictites of the Gowganda Formation. Two glacial advances have been recognized in the Gowganda (Coleman,

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Locations and stratigraphy of the Upper Huronian Supergroup and Chocolay Group. A) Locations in central North America. Location of B) and C) is indicated by the red squares in A). B) Location of the Kona Dolomite (Chocolay Group) in northern Michigan, USA. This includes two sample localities, the large Kona stromatolites (UTM 16, E472038.56, N5150210.46) and the Lindberg Quarry (UTM 16, E464690.81, N5147537.82). C) Location of the Huronian Supergroup in central Ontario, Canada, with three sample sites: the Plummer Occurrence (Hofmann et al., 1998) (UTM 17, T E284767.00, N5142209.00), another site also lies within Plummer Township (UTM 17, E273361.00, 5150244.00), and the dolomite knob (Bennett, 2006) (UTM 16, 694103.00, 5182064.00).</figcaption></figure>
1905; Ovenshine, 1970; Casshyap, 2003) separated by coarsening upward deltaic sediments (Junnila and Young, 1995). A coarsening upward succession overlying the upper diamictite is gradual into tidally influenced, quartz-rich sandstones of the Lorrain Formation (Rice, 1991). Sandstones of the Lorrain grade upward into mudstones, siltstones, and lesser sandstone and dolomite beds of the Gordon Lake Formation, and overlying sandstones of the Bar River Formation (Bennett 2006). Bi-directional paleocurrents, gypsum and anhydrite pseudomorphs, together with marine and fluvial sedimentary features, indicate a tidally influenced sabkha coastal shelf, deposited during episodic marine transgression and regression (Wood, 1973; Chandler, 1988; Hill et al., 2016; Hill and Corcoran, 2018).

Evidence in the Huronian Supergroup for a rise in atmospheric oxygen was first commented on by Roscoe (1969, 1973). He recognized that common detrital pyrite and uraninite in conglomerates of the Livingstone Creek and Matinenda Formations required low levels of oxygen in the atmosphere. Additionally, all deposits below the Gowganda are grey to cream coloured with no indication of oxidized iron (Roscoe, 1969; Frarey and Roscoe, 1971; Prasad and Roscoe, 1996). This is in marked contrast to the distinctive iron oxides forming red-beds in the overlying Gowganda and stratigraphically higher Formations (Roscoe, 1973; Wood, 1979; Young et al., 2001). Roscoe (1973) termed this apparent transition in atmospheric oxygen levels the 'Oxy-atmoversion'. Sulfur isotopic studies may indicate where this transition generally occurs. The end of mass-independent fractionation could be present in the Espanola Formation cap carbonate, deposited during the interglacial preceding the Gowganda glaciation (Papineau et al., 2005; Papineau et al., 2007). Alternatively, metamorphic overprinting may have erased evidence of this transition (Cui et al., 2018). Work in the Fennoscandian Shield places the GOE between 2501.5 ± 1.7 and 2434 ± 6.6 Ma (Warke et al., 2020), in disagreement with evidence from the Huronian Supergroup (Roscoe, 1973; Papineau et al., 2005; Papineau et al., 2007). This might reflect fluctuating atmospheric oxygen levels from ~ 2430 to 2220 Ma (Gumsley et al., 2017; Poulton et al., 2021). The presence of gypsum, pseudomorphs after sulfates and molds of hopper halite crystals in the Gordon Lake sediments suggests considerable increase in surface temperatures following the Gowganda glaciation (Chandler, 1988).

The Huronian Supergroup was deposited between 2452 ± 6 Ma, the age of the underlying Copper Cliff Rhyolite (Ketchum et al., 2013), and 2217 ± 9 Ma, the age of cross-cutting mafic intrusions (Corfu and Andrews, 1986), and, more recently, the 2258 ± 0.5 Ma age of a sill intruding the lower Huronian (Bleeker et al., 2023). The depositional ages of the lower and middle portions of the Supergroup are poorly constrained. The Gordon Lake Formation in the upper Huronian succession (Fig. 1C) has U-Pb zircon age determinations of 2308 ± 8 Ma, 2308 ± 10 Ma and 2311 ± 11 Ma on zircons in tuff layers from three different locations (Rasmussen, et al., 2013), and more recently a 2318 ± 8 Ma age from a tuff layer in drill-core (Rasmussen et al., 2024). Hill et al. (2018) published an age of 2315 ± 10 Ma for long prismatic detrital zircons from the Gordon Lake Formation, and 2302 ± 19 Ma for a youngest zircon population from the overlying Bar River Formation. They inferred that the youngest zircons in their study were deposited sometime after 2315 ± 5 Ma. The presence of carbon isotope values ranging from δ¹³C + 5.0 to + 8.2 ‰ in Gordon Lake carbonates indicate that it was deposited during the Lomagundi Event (Bekker et al., 2006).

### 3. Chocolay group

The Paleoproterozoic Chocolay Group outcrops in the United States south of Lake Superior (Fig. 1A). It overlies 2.7 Ga Archean basement forming the lowest unit of the Marquette Range Supergroup. Glacial diamictite (Ojakangas et al., 2001) of the Enchanted Lake Formation is the basal unit of the Chocolay Group in the east; however to the west it is absent and mature sandstones, which otherwise conformable overly the diamictite, form the basal strata. The sandstones represent development of a tide-dominated shallow marine setting after waning glacial

conditions (Larue, 1981). They have various Formation names depending on location, with the one underlying the carbonates sampled for this study being termed the Mesnard Quartzite (Fig. 1B). It is conformably overlain by the more than 1 km thick Kona Dolomite, which is in turn succeeded by the deeper water Wewe Slate (Larue, 1981).

Evidence of significant oxygen in the atmosphere appears as red-beds in the Chocolay quartzites (Larue, 1981; Bekker et al., 2006). This is further emphasized by the presence of pseudomorphs of gypsum crystals and nodules in the Kona Dolomite, indicating that abundant sulfate was being formed (Larue, 1981). The warming trend after Enchanted Lake glaciation is similar to the trend following that of the Gowganda ice sheet.

The Chocolay Group was thought to be correlative with the upper Huronian Supergroup (Young, 1973, 1983; Gair, 1981; Ojakangas et al., 2001). This was due to a similar progression of lithologies, from glacial diamictite; to mature, marine sandstones deposited in shallow water; to dolostone (Kona Dolomite) and then dolostone in a siliciclastic succession (Gordon Lake Formation). Also, the carbonate rocks of both successions contain evaporites and dolomite with similar heavy carbon isotopes (Bekker, 2006). Geochronology performed by Vallini et al. (2006) on detrital zircons from the Enchanted Lake Formation established that this unit is younger than 2317 ± 6 Ma Diagenetic xenotime from the same rocks provided a minimum age of 2133 ± 11 Ma. Vallini et al. (2006) also produced an age bracket for the Sunday Quartzite of between 2647 ± 5 Ma (youngest detrital zircon population) and 2207 ± 5 Ma. (diagenetic xenotime), in addition to the Sturgeon Quartzite of 2306 ± 9 Ma (youngest detrital zircon population) and 2115 ± 5 Ma (diagenetic xenotime). These ages are compatible with the Kona Dolomite being correlative with the 2310 Ma Gordon Lake Formation. However, Rasmussen et al. (2024) were able to obtain ²⁰⁷Pb/²⁰⁶Pb zircon ages on tuffaceous layers above the Kona-Wewe Slate contact. These ²⁰⁷Pb-²⁰⁶Pb ages of 2174 ± 9 Ma and 2172 ± 6 Ma indicate that the Chocolay Group is younger than the Huronian succession (Rasmussen et al., 2024). Interestingly, the diagenetic xenotime age of 2207 ± 5 Ma (Vallini et al., 2006) for the Sunday Quartzite indicates that it probably is not correlated with the Mesnard quartzite underlying the Kona. This is in agreement with the δ¹³C values of Bad River Dolomite that overlies the Sunday Quartzite being close to 0, whereas those for the Kona are heavier. This also implies that the Sunday Quartzite and overlying Bad River Dolomite do not correlate with the heavy δ¹³C in the Lorraine-Gordon Lake (Bekker et al., 2006) and the stratigraphic position of the former remains problematic.

### 4. Methodology

Sedimentological and stratigraphic aspects of the Gordon Lake Formation and Kona Dolomite were noted in the field and 36 samples of various lithofacies were collected. All of these samples preserve primary textures, although they have been affected by deformation and varying degrees of recrystallization. Samples were cut and polished, standard and polished thin sections were made, and sub-samples were powdered using an agate mill at the lapidary facility in the Department of Geology at Lakehead University.

Petrographic and binocular examination was conducted on 44 thin sections, and 14 carbon coated polished thin sections were examined with a Hitachi SU-70 Schottky field emission scanning electron microscope with spectrographic analytical capacity (FE-SEM-EDX).

Whole-rock powders were prepared for analyses using 5 ml hydrofluoric and 10 ml nitric acids in open Teflon beaker digestions, with samples evaporated to dryness and repeated three times. Final dissolution was in two percent nitric acid with a 250 times dissolution for ICP-AES analysis and 1000 times dissolution for ICP-MS analyses. Sample splits of the 36 powders run for whole-rock analysis were used to extract only the carbonate fraction. This was necessary as more than ~ 1 percent siliciclastics in a sample cause the REE pattern to reflect the
siliciclastic phase. Carbonate was dissolved in 20 ml of five percent acetic acid. After 24 h the acidic acid was removed using an eye-dropper, a technique that, in our experience, provides a more colloid-free solution than using a centrifuge. This solution was evaporated and redissolved in two percent nitric acid. Dissolutions were the same as for whole-rock. Three blanks were prepared with the samples, along with a USGS (BHVO-2) and one in-house carbonate standard replicated three times. ICP-AES analysis was performed at the Instrumentation Laboratory, Lakehead University, using a Vista Pro Radial atomic emission spectrometer and ICP-MS was performed using a Perkin-Elmer Elan DRC-e quadrupole mass spectrometer. The internal standards of the MS were: 5 ppm REEs, Y, Th, U; 20 ppm Mo, Nb, Sb, Sn, Ta, W; 100 ppm Ba, Pb, Rb. The errors in accuracy of elements used in this study are (for whole rock analyses): Al₂O₃ = 3 %, Ba = 1 %, Cr = 2 %, U = 8 %, V = 4 %. The errors in precision (based on analyzing two samples of BHVO-2) are Al₂O₃ = 11 %, Ba = 7 %, Cr = 2 %, V = 2 %.

Precision and accuracy cannot be ascertained for the partial dissolutions as differing amounts of fluid extracted during multiple processing of the same standard material result in differing results. However, as it is the shape of the REE patterns that is of interest, what is important is that the shapes of the REE patterns produced by the multiple samples of the same standard are similar. Post Archean Australian Shale (PAAS), also abbreviated as *sn*, normalized patterns for the two standards used, in-house standard SR-16B used for evaluating accuracy and precision of partial dissolution data and U.S.G.S. standard BHVO-2 used for evaluating whole rock digestion data, are very similar to the patterns obtained for these standards (see *Supplemental Material*).

As BaO passing into the mass spectrometer will be counted as Eu, this can create a false, extended Eu anomaly. This potential problem was investigated for the partial digestion data, which were used for the REE plots, by plotting Ba vs Eu anomaly. Problem with BaO survival will cause a positive correlation on the plot. No correlation was observed. The plot Ba/Sm vs. Eu/Sm was also used to evaluate Ba contamination, as advised by *Barrat and Bayon* (2024). No correlation was present on this plot either. Thus, the Eu anomalies and middle REE (MREE) enrichment present in the samples was not caused by Ba. (See *Supplemental Material* for these plots.).

Carbonate carbon and oxygen isotopes were measured from whole-rock powders at G.G. Hatch Isotope Laboratories, University of Ottawa, by methods described in *Revez et al.*, (2001). Accuracy for $\delta^{13}$C is $\pm$ 0.02 (‰, PDB). Accuracy for $\delta^{18}$O is $\pm$ 0.08 (‰, PDB).

Slabbed and polished rock samples were scanned using an XRF scanner table at the European Institute for Marine Studies in Brest, France. False colour maps were produced from these data. This data is semi-empirical. Thus, accuracy is not given.

For the geochemical data used in this paper see the *Supplemental Material* File.

# 5. Sedimentology of the samples

Precambrian carbonate sedimentary rocks can have been deposited in a wide variety of environments that represent exposure to differing water chemistries during both deposition and diagenesis. This is particularly important for carbonate sediments since they reflect both the fluids that precipitated them and those that they had diagenetic interaction with. It is therefore necessary to understand, as much as possible their depositional environments and the diagenetic pathway that they followed.

Based on morphological, textural, and mineralogical differences, nine major lithofacies associations (LA) were identified. Both microbialite and non-microbialite fabrics are present within the carbonate and associated siliciclastic-rich samples of the Gordon Lake Formation (GL) and Kona Dolomite (KD). Microbialite lithofacies include stromatolites and fenestral fabric. Descriptive terminology follows that defined by *Grey & Awramik* (2020). Non-microbialite lithofacies include: ripple laminated carbonate; rhythmic planar laminated carbonate, laminated

siltstones and mudstones with clasts, massive siltstone containing gypsum pseudomorphs, halite pseudomorphs, and hummocky cross-stratified grainstone.

For outcrop descriptions and interpretations of the depositional environment of the Gordon Lake Formation see: *Hofmann et al.* (1980), *Chandler* (1986, 1988), *Bennett et al.* (1989, 1991), *Young* (1991) and *Bekker et al.* (2006). For the Kona Dolomite see: *Taylor* (1972), *Puffett* (1974), *Clark* (1974), *Gair* (1975), *Wohlabaugh* (1980), *Wohlabaugh and Mancuso* (1990) and *Bekker et al.* (2006).

## 5.1. Descriptions – Kona dolomite

### 5.1.1. Stromatolites

Various stromatolites occur within the Kona Dolomite near Marquette, Michigan, proximal to the underlying Mesnard Quartzite (Fig. 1B). An outstanding outcrop shows cross-sections of large laterally linked, walled columnar stromatolites, joined by deep narrow depressions. The depressions gradually shallow upward, resulting in wide domal rather than columnar stromatolites. This 4 to 5 m high outcrop (Fig. 2A,B,C) was described by *Twenhofel* (1919), and the unit was termed the Big Cusp Algal Dolomite member (Taylor, 1972; Wohlabaugh, 1980; Wohlabaugh and Mancuso, 1990). Here we refer to them as Large Kona stromatolites. The columns have good inheritance and range in size from 10–15 cm wide and up to 3 m tall (Fig. 2A), after which they become more domal (Fig. 2BC). They begin as slender, fairly uniform, and erect steep sided columns with high relief (stubbyslender). They widen slightly upwards, become laterally linked (crustose) and show less relief (Fig. 2C).

Macroscopic isopachous and continuous laminated layers can be traced laterally and average 1.5 cm thick or less (Fig. 2B,C). At this scale the laminae are relatively smooth, with high degrees of inheritance and synoptic relief of up to several tens of centimeters. These morphotypes are distinct in size and shape. In lamination and inheritance they broadly resemble domal stromatolites from the 3.45 Ga Strelley Pool Formation in Western Australia (Figs. 2, 3 of Allwood et al., 2009), and linked domes (Fig. 6, Wilks and Nisbet, 1985) and pseudocolumns at Steep Rock (Fig. 8, Fralick and Riding, 2015). In size, some Kona examples resemble flat topped domes in the Cambellrand-Malmani carbonate platform (Sumner and Grotzinger, 2004; Fig. 2 of Riding, 2011). Overall, Kona stromatolites commonly show high relief and relatively even, laterally continuous layering that can be traced across the domes, with narrow steep-sided depressions between the columns. At mesoscale, these large and mainly pseudocolumnar stromatolites are composed of highly wrinkled laminae that are frequently associated with narrow mini-pseudocolumnar stromatolites (Fig. 2D,E) that maintain broadly erect orientations while following the overall steep-sided curvature (Fig. 2D) of the main large domes and columns. The ‘mini-pseudocolumnary’ stromatolites do not exceed 11 mm in height and 4 mm in width (Fig. 2E). They locally diverge epinastically, slightly opposite from the dome’s overall concave curvature (Fig. 2D). The fabric of these large domes is composed of variably silicified gray, light pink to purple carbonate, together with very fine-grained to aphanitic dark purple iron oxide-rich dolomitic laminae whose microfabrics are moderately recrystallized. Iron oxides are locally concentrated in depressions between the mini-pseudocolumns.

A variety of stratiform, undulose and low domal stromatolites (Fig. 3A,B) occur ~ 10 km west of the large stromatolites at Site 1. Well preserved centimetric domal stromatolites 1–4 cm high are present between crinkly stratiform stromatolites. The low domes are composed of light-colored pink to reddish carbonate with darker gray (silicified) to reddish (Fe oxide-rich) laminae. The lamination is wavy to smooth, with moderate inheritance and relief, and generally laterally continuous but can be convolute (Fig. 3A,B). Occasional intraformational clasts that locally overlie the low domes (Fig. 3B) only represent a small portion of the generally domal-stratiform samples. The lamination is accentuated by variations in iron oxide content that highlight the overall relatively
<figure><img src="image_5.png" /><figcaption>**Fig. 2.** Large Kona stromatolites, Marquette, Michigan. The outcrop is approximately 4 to 5 m high. Large columns laterally linked by narrow depressions pass gradually upward into large laterally linked domes. A) Columnar stromatolites near the base. B) Domal stromatolites near the top of the outcrop have thicker lamination and lower synoptic relief than the stratigraphically lower columns. Hammer is 38 cm long. C) On the left, columns have flanks that descend steeply into intervening depressions. Towards the right, laterally continuous laminae transition from columnar to domal and stratiform. D) Polished slab of large Kona stromatolites. Small narrow pseudocolumns maintain subvertical sinuous orientation on the concave flank of a large column defined by hematitic dolomicrite layers (upper half of image). E) Details of small pseudocolumns showing delicate lamination and the presence of hematite in some dolomite layers. Arrow shows interrupted growth over intraclasts. Millimeter-centimeter scale.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 3.** Kona Dolomite A) Cut slab of small domal to stratiform stromatolites composed of hematitic dolomite and locally draped by pale fine-grained carbonate. B) Layered carbonate with small discrete 2 cm wide domes and scattered intraclasts (arrow). C) Parallel laminated carbonate. Layers ~ 0.5 cm thick resemble the parallel laminated layers in the wavy bedded lithofacies shown in D. D. Wavy bedded, laminated dolomite with climbing current ripples below (lower arrow) separated by parallel laminated light gray fine-grained sediment from climbing wave ripples (upper arrow). E) Load structures defined by underlying carbonaceous laminae. The arrow indicates a soft-sediment deformed layer overlying a dome composed of two layers with carbon-rich tops.</figcaption></figure>
isopachous lamination. The lightest pink carbonate laminae are 1–15 mm thick, average 5 mm. This light pink carbonate occupies depressions between the domes rather than forming the stromatolites.

### 5.1.2. Ripples and low domes

The ripples and low domes L.A. is composed of centimetric rippled, planar to wavy and slightly domed laminae of interlayered pink and gray dolomite (Fig. 3C,D). Laminae range in thickness from 2–10 mm. The gray laminae are more regular and locally truncated; they either drape over or conform to the pink carbonate laminae (Fig. 3D). Lighter gray layers commonly have upper surfaces of darker gray, carbon-rich dolomite. Flame and load structures can occur within these layers (Fig. 3E). The relatively well-defined, irregular millimetric laminae of fine-grained carbonate are highlighted by variations in iron oxide content. Laminae are irregularly undulose with numerous minor truncations. Current and wave ripples are both present, with climbing versions of both forms (Fig. 3D).

### 5.1.3. Hummocky Cross-Stratified (HCS) carbonate

This fabric underlying the large stromatolites is characterized by packages of undulose cross-stratification with low angle truncated surfaces (Fig. 4A). Small to medium-scale, wavy laminae of light gray, partly silicified carbonate outlined by light pink carbonate range 0.4–1 cm in thickness, with wavelengths commonly between 10 and 20 cm.

### 5.1.4. Gypsum pseudomorphs

Red dolomite and chert filled pseudomorphs after gypsum occur within very fine-grained, dark burgundy-brown carbonate matrix (Fig. 5A). The pseudomorphs are elongate and diamond shaped, with very straight edges. Depending on orientation they can appear cubic or rhombohedral in cross-section. They range in size from 2x3 to 10x9 mm, average 8x6 mm, and appear randomly oriented within the matrix, although they commonly occur in close proximity to one another, forming rosettes joined at one point and radiating outwards (Fig. 5B). Residual sulfur is present in the pseudomorphs (Fig. 5C) and in places the outlines of small swallow-tail gypsum-like crystals can be observed (Fig. 5D). The surrounding matrix is massive micritic dolomite with randomly scattered silt sized quartz grains (Fig. 5E).

<figure><img src="image_7.png" /><figcaption>**Fig. 4.** Kona Dolomite. A. Parallel laminated, fine-grained sand at the very base of the photograph, overlain by hummocky cross-stratification with low angle truncation surfaces (arrows). Gordon Lake Formation. B) Pseudomorphs after chevron-textured halite. Centimeter-millimeter scale; C) Chevron textured halite replaced by dolomite with some layers partially replaced by quartz. Millimeter scale.</figcaption></figure>

### 5.1.5. Parallel laminae

Smooth, millimetric, and isopachous carbonate laminae in various shades of pink alternate with semi-continuous dark gray chert layers that range from < 1 to 8 mm thick (Fig. 3C). Pink laminae tend to be continuous and planar, whereas dark laminae are more discontinuous, wispy, and in places are brecciated and contain small domes. Subangular dolomite crystals occur sporadically throughout the pink micritic layers. Other examples of parallel lamination are more subtle, with slightly crinkled but generally planar laminae.

## 5.2. Descriptions – Gordon Lake Formation

### 5.2.1. Fenestral fabric

Samples of fenestral fabric from the Gordon Lake Formation are similar to those obtained by Hofmann et al. (1980) from sparse carbonate layers in the Bar River Formation (Fig. 6). In contrast to the other microbial-bearing lithofacies, the fenestral fabric consists of both dolomitic carbonate and siliciclastic sediment. The laminae are primarily dolomite, but can also contain sub-angular to sub-rounded, silt-sized quartz and feldspar grains floating in them. Millimeter-scale desiccation cracks occur rarely.

### 5.2.2. Siliciclastic siltstone and mudstone

This assemblage of interlayered parallel laminated reddish to burgundy brown mudstones and siltstones, commonly ranges 0.5 to 10 cm in thickness, with very fine-grained beds of light cream coloured dolomite mud, 0.1 to 1 cm thick (Fig. 7A). Rectangular granule and pebble-size clasts of pinkish cream carbonate and siliciclastics are locally present. The larger clasts are commonly composed of dark gray or maroon carbonate, with rims of pinkish, cream or white carbonate (Fig. 7A), and contain remnant patches of calcium sulfate (Fig. 7C). Carbonate clasts that are white, cubic-rectangular in shape and $≤$ 2 mm in size, may be pseudomorphs after halite (Fig. 7B). Rare, deep red sandstone to siltstone layers locally contain abundant millimetric carbonate fragments (Fig. 7A, layer 4).

### 5.2.3. Chevron halite pseudomorphs

In addition to displacive halite pseudomorphs within dolomite clasts (Fig. 7B), pseudomorphs also occur as layered deposits (Fig. 4B,C). These exhibit the typical form of chevron halite in layers 1–5 mm thick. Pseudomorphs after clear halite filling former dissolution cavities are uncommon. In these examples, the halite is replaced by dolomite or, in places, quartz.

## 6. Depositional system

Both the Kona Dolomite and the Gordon Lake Formation have been broadly recognized as tidal flat deposits (Taylor, 1972; Wood, 1973; Robertson 1973; Wohlabaugh, 1980; Ams and Holm 1999). Wright and Rust (1985) described bi-directional cross-laminated sandstone associated with fenestral fabric, and bedded siltstone and mudstone with clasts suggesting tidal influence. Below, the depositional systems are discussed sequentially from offshore to onshore supratidal flat environments.

### 6.1. Interpretation – Kona dolomite

#### 6.1.1. Large columnar to domal stromatolites

Large Kona Stromatolites are laterally linked pseudocolumns with intervening steep-sided narrow depressions (Fig. 2A) likely indicative of a high energy environment, much like the hummocky cross-stratified lithofacies Association (L.A.) upon which they rest. The large columns transition upward into domal stromatolites (Fig. 2B,C). The deep intervals between these columnar stromatolites resemble those of ‘encrusting/domical stromatolites’ in the Strelley Pool Chert (Allwood et al., 2009; Figs. 3, 4). However, Kona columnar to domal stromatolites generally are much larger with steeper sides and developed atop wavy to
<figure><img src="image_8.png" /><figcaption>**Fig. 5.** Kona Dolomite. A) Chert-dolomite pseudomorphs after gypsum in reddish brown dolomicrite. B) Rosette pseudomorphs after gypsum. C) XRF scanning table false colour image indicating the presence of sulfur (green) in the pseudomorphs. D) Plane-polarized light photomicrograph of the pointed end of a pseudomorph (above) with smaller gypsum pseudomorphs (below). E) Crossed-polar photomicrograph of the edge of a pseudomorph (bottom right) and adjacent groundmass (top left) showing quartz grains (arrow) sporadically scattered through the fine-grained dolomite.</figcaption></figure>
<figure><img src="image_9.png" /><figcaption>**Fig. 6.** Gorgon Lake Formation. Fenestral microbialite and associated layers: Layer 1, faint undulating and self-intruding layers of light creamy tan carbonate grading into dark reddish brown micrite with abundant iron oxides, sharply overlain by Layer 2, a transition zone of laminated and irregularly clotted fabric with irregular dark red fenestrae and thin disjunct layers of lighter carbonate spots distributed among slightly lighter reddish-brown carbonate, irregularly overlain by Layer 3, light tan-coloured fenestral carbonate with well-defined but laterally discontinuous wavy laminae that exhibit good inheritance and low synoptic relief. The fenestrae are elongate, dark reddish brown, filled with blocky cement, 1 to 18 mm long and commonly 1 mm thick. This fabric closely resembles the laminoid fenestral dolostone of Hofmann et al. (1980, Fig. 4b). Underlying material from Layer 2 appears to extrude through Layer 3 (arrowed), into Layer 4. The lower part of Layer 5, which consists of pale homogeneous micrite, is broken. Layer 6 consists of a fluidized mixture of lithotypes resembling those of Layers 2–5 and contains blotchy fenestrae.</figcaption></figure>

planar laminated sediment, rather than inheriting the morphology of
underlying debris. This supports the view that their morphology reflects
changing environmental or biological factors. Narrow columnar and
branching forms of stromatolites are known to develop in shallower
water environments, whereas large conical forms and massive columns
form in deeper environments (Grotzinger, 1989; Kah and Knoll, 1996;
Bartley et al., 2000). The lateral continuity of the large Kona columns,
their regularity, and gradual transition from moderate high relief to low,
flat topped, domal stromatolites (Fig. 2C) are suggestive of a shallowing

response, reflecting diminishing accommodation space.

Present-day marine stromatolites of this size are rare; however, they
do occur locally in tidal channels in the Exuma Cays, Bahamas (Dravis,
1983; Dill et al., 1986; Riding et al., 1991). There the high energy water
movement through tidal channels between sand shoals carry clays and
sand in suspension and create ripples along the channel floor. This is
similar to Kona columnar stromatolites, which likely grew atop the
sandy cross-stratification as groups of slender columns (Fig. 2A) with
sub-vertical sides (high angle of repose) that create deep depressions
between neighboring columnar stromatolites. The high angle was likely
established in a high energy environment or a higher Re* (Boundary
Reynolds Number) that did not allow the mat to develop outward.
Subsequently, the stromatolite continued to build upward rather than
outward. As accommodation decreased, columnar stromatolites transi-
tioned to lower relief domal stromatolites. Features with similar size and
substrate, though different formational mechanisms, are present in the
Bahamas (Dravis, 1983; Dill et al., 1986; Shinn, 1983), where domal
stromatolites are composed of columnar stromatolites that occur in
clusters within tidal channels.

Studies of branching and closely adjacent stromatolite column
development (e.g., Dupraz et al., 2006) provide clues to the develop-
ment of large Kona columns and their ‘mini-pseudocolumnar’ stromat-
olites (Fig. 2D). Microbial mats depend on nutrient availability, and this
factor is limited by diffusion through the material to the mat surface
(Dupraz et al., 2006). Stromatolite surfaces have an activation zone
where particles are attracted to the mat; the activation zone for two
neighboring stromatolite columns overlap and create what is considered
a protected zone (Dupraz et al., 2006). The particles will generally
adhere to the higher surfaces rather than the protected zone and low
lying areas are more susceptible to collecting detritus which will also
deter mat development (Dupraz et al., 2006; Grotzinger and Knoll,
1999). This appears to have been a factor in the growth of the large
columnar and domal stromatolites, as iron oxyhydroxides accumulated
in the small depressions between micro-stromatolites impeding accre-
tion in these areas, but it was also incorporated into their crests (Fig. 2D,
E). In some cases, the small stromatolites were completely inundated by
very iron oxyhydroxide-rich siliciclastic material (Fig. 2D), indicating
that the water mass sporadically had large amounts of iron oxy-
hydroxide in suspension, possibly entrained by storm waves.

The balance between sedimentation and mat growth or degradation
are key factors in stromatolite development (Grotzinger and Knoll,
1999). Microbial mat accumulation can be modeled as a competition
between sedimentation, precipitation and erosion (Lewandowski and
Walser 1991; Van Loosdrecht et al., 2002). Narrow interspaces linking
the columns (Fig. 2A) indicate that mats continued between the col-
ums, in contrast to the ripple and low dome L.A. where mats generally
did not develop between domes (Fig. 3B). In Kona stromatolites, the
transition from large columns to domes occurs where the steeply dipping
narrow intervening depressions shallow to the point where adjacent
columns become one dome (Fig. 2B,C). This is comparable to encrusting
domical stromatolites in the Strelley Pool Formation (Allwood et al.,
2009). They both show a change in laminar architecture that gradually
reduces the depressions between each stromatolite, except that those in
the Strelley Pool Formation change shape from domal to flat lying rather
than columnar to domal. Allwood et al. (2009) attributed the change to
increase in precipitation, because the stromatolites have discrete,
organic-rich, wrinkly laminae between sediment layers, suggesting
distinct episodes of organic layer formation, rather than trapping and
binding. Furthermore, in large Kona stromatolites laminae do not
thicken in the lows, but rather follow the steep contours of each column
which have slopes greater than the angle of repose for particulate
sediment (~30°; Bagnold, 1941; Allwood et al., 2009). The apparent
absence of detrital sediment in the carbonate makes it difficult to argue
that sediment trapping significantly contributed to Kona column ac-
cretion. In polished section (Fig. 2D), iron oxide-rich sediment occurs
between the smaller columns, which may represent sediment
<figure><img src="image_10.png" /><figcaption>**Fig. 7.** Gordon Lake Formation. A) Typical layers of siliciclastic sandstone, siltstone and mudstone with subrounded dolomite clasts. 1) Massive, tan/beige micrite; 2) parallel to wavy laminated, hematite-rich, fine-grained sandstone loading into tan/beige micrite overlain by 3) medium-grained, hematite-rich sandstone with scattered carbonate intraclasts at its base; 4) layer grading from coarse-grained sand, granules and small pebble sized imbricated carbonate intraclasts to medium-grained sand; eroded by 5) pebble-size carbonate intraclasts, overlain by 6), fine-grained carbonate, 7) additional pebble layer, and 8) parallel laminated siltstone, locally graded and with coarse sand grains throughout. B) Tan dolomite clast containing displacive white halite pseudomorphs. C) Photomicrograph of relic radial clusters interpreted to be sulfate, in a dolomite clast.</figcaption></figure>

incorporation. However, it seems unlikely that particle agglutination made any significant contribution to the accretion of these meter-scale columnar domes. The available evidence suggests that precipitation processes (either microbially mediated or on a microbial template) dominated the accretion of these large Kona stromatolites.

Overall, stromatolite size, lateral continuity, vertical transition from columnar to domal morphotypes, together with their development atop wavy to planar sediments (HCS LA), all suggest a relatively shallow offshore depositional environment.

### 6.1.2. *Hummocky Cross-Stratification*

It is widely accepted that hummocky cross stratification is produced by the combined influence of oscillatory motion under large waves and geostrophic flows produced by storm surge (Hamblin and Walker, 1979; Dott and Bourgeois, 1982; Hunter and Clifton, 1982; Walker 1984; Swift and Nummedal, 1987; Scott, 1992). The bedform is thought to indicate deposition between storm wave base and fairweather wave base, since oscillatory flow produced by wave orbitals is needed to form the bed-form (shoaling waves; Clifton et al., 1971), and currents active during fairweather (longshore currents; McCubbin, 1982) that are commonly capable of reworking the storm deposits. Yang et al. (2006) described HCS from a present-day broad macrotidal flat in Korea, but this appears to be a special case due to the frequency of large storms and water depth during high tides. Interestingly, they used changes in HCS wavelength to define water depth, showing that wavelength increases as waves shoal and then decreases after they break, reform and continue on into shallower areas. Accordingly, the small HCS wavelengths observed in this study (Fig. 4A) could indicate formation either offshore in deeper water or nearshore after the open ocean waves had broken and reformed, but still at depths below fairweather wave base.

### 6.1.3. *Ripples and low domes*

This lithological association is composed of wave ripples, current ripples, climbing examples of both, low domes, and convolute lamina-tion in dolomitic carbonate with varying iron oxide contents (Fig. 3A,B, D). The climbing ripples and overlying draping laminae represent

alternating timeframes of rapid accretion from suspension that exceeds both rate of ripple migration and the typical rate of accretion. Ripple structures are common on tidal flats and in shallow offshore settings. Internal layering in the domes, together with their shape and size, is similar to swell and pinch structures and incipient lenses described by De Raaf et al. (1977) from Carboniferous shallow marine deposits. Both the Kona domes described here, and those of de Raff et al. (1977), are associated with current and wave ripples with overlying draping and parallel laminae. The carbon-rich layers at tops of carbonate laminae (Fig. 3E) indicate microbial mat development associated with these ripples. However, it appears that there was too much energy for the mat development to prosper. Higher energy environments cause high mat friction resulting in low mat stability and therefore, limited stromatolite formation (Grotzinger, 1986a; Grotzinger, 1986b; Hofmann et al., 2004; Tice et al., 2011). Whether the ripples and low domes formed in shallow offshore or lower intertidal zones, or both, is difficult to ascertain.

### 6.1.4. *Small domal to stratiform stromatolite beds*

Beds of undulating and low domal stromatolite are typically 1–3 cm thick (Fig. 3A,B) and well preserved. They alternate with semi-continuous layers that may not be microbial, and areas with intraformational clasts. Some semi-continuous layers that fill depressions between stromatolites appear to have been deposited as fine-grained sediment (allochthonous), whereas others that thicken over the tops of domes appear to be microbial. Some low domes contain intraformational clasts (Fig. 3B), with crinkly mat features and internally clotted layers. These grade into smooth stratiform stromatolites suggesting erosion, or a higher Re*, and therefore, periodically higher energy. Their association with intraformational clasts may indicate storm events. Present-day intertidal sediment surfaces at Shark Bay are dominated by pustular and tufted microbial mats (Jahnert and Collins, 2012), whereas the upper subtidal contains flat mat or colloform stromatolite varieties, among others. Low domal Kona stromatolites, which have some clotted and poorly defined lamination (Fig. 3B), may have been situated within a similar intertidal flat environment. Nonetheless, the association with smooth stratiform microbialite layers is also consistent with an upper
subtidal environment such as those seen today in Shark Bay (Jahnert and Collins, 2012).

### 6.1.5. Subaerial gypsum pseudomorphs

Subaerial pseudomorphed crystals closely resemble gypsum in Abu Dhabi sabkha sediments (Bush, 1973; Butler et al., 1982), and exhibit the discoidal habit (Fig. 5A), crystal rosettes (Fig. 5B) and clusters typical of gypsum precipitated in upper tidal flat and sabkha environments (Kendall and Harwood, 1996).

The dense matrix supporting the gypsum pseudomorphs lacks sedimentary features and is mainly fine-grained carbonate (Fig. 5A). Silt-sized siliciclastic grains, mostly quartz, that occur sporadically throughout the very fine-grained groundmass (Fig. 5E) probably represent silt blown onto the sabkha. In addition to storm-induced flooding, sabkhas experience subaerial exposure and periods of wind driven sedimentation and deflation controlled by water table positioning (Shinn, 1983; Kendall and Harwood, 1996), as in examples at Abu-Dhabi (Patterson and Kinsman, 1981). The gypsum crystals precipitate within these sediments by evaporation of capillary brines. Kona gypsum crystals were oxidized and altered by iron oxyhydroxides prior to dissolution (Fig. 5D), possibly during storm events when the sabkha was flooded. The voids were later infilled by dolomite, which was then partially replaced by quartz (Fig. 5D).

### 6.1.6. Parallel laminae

Smooth, millimetric-centimetric isopachous dolomite laminae of the Parallel Lamination L.A. are present in both the Gordon Lake Formation and Kona Dolomite. The layers are similar to individual laterally continuous laminae in the Ripples and Low Domes L.A. (Fig. 3D). They both represent low energy periods necessary for these sediments to be deposited, possibly from water column precipitation. However, as opposed to the Ripples and Low Domes LA the Parallel Laminae L.A. could have formed in environments shielded from storm activity, such as hypersaline lagoons (cf. Fig. 9 of Kendall, 1979a). Alternatively, this L.A. may represent microbially enhanced precipitation that formed flat-mat stromatolites.

## 6.2. Gordon Lake Formation

### 6.2.1. Fenestral fabric

Our samples of fenestral fabric from the Gordon Lake Formation are similar to those that Hofmann et al. (1980) also obtained from the sparse carbonate layers in this formation. They occur within dolostone units in assemblages dominated by quartz-rich silt and mud (Fig. 6). These show good banding in low domes, interlayered with parallel laminated, fine-grained dolomite defined by the presence or absence of randomly distributed, over-sized, coarse silt quartz and feldspar grains. These have the appearance of wind-blown siliciclastic sediment deposited in a carbonate environment (cf. Shinn, 1983; Warren, 1991; Gomez and Astini, 2015). Soft sediment deformation features (Fig. 6, arrow) caused by upward movement of fluids imply a water table near the surface, though periods of drying and desiccation crack formation also affected the surface. These carbonate features are typical of microbial mat sediment deposited by episodic aeolian processes on upper tidal flats to sabkhas intermittently flooded by seawater, but predominantly subaerially exposed (Shinn, 1983; James, 1983; 1984; Alsharhan and Kendall, 2003; James and Jones, 2015); present-day upper intertidal sabkha environments (Shinn, 1983; James 1983, 1984; James and Jones, 2015). The layering in places closely resembles that of low domes with fenestral fabrics and discontinuous layering interpreted as intertidal deposits by Hofmann et al. (1980).

### 6.2.2. Chevron halite pseudomorphs

Chevron halite forms from brines in shallow marine (Gornitz and Schreiber, 1981; Handford, 1991; Losey and Benison, 2000; Pomoni-Papaioannou et al., 2004) and saline lacustrine (Logan, 1987; Sun

et al., 2017) settings (Kendall, 1979b). The lack of common evidence for dissolution, such as pseudomorphs of clear, non-layered cavity fills and presence of truncation surfaces (Figure B,C) indicate that episodes of emergence were not common (cf. Lowenstein and Hardie, 1985). Layering in chevron halite is caused by changes in brine concentration, with rapid alternations indicating precipitation in shallow water (Kendall, 1979b; Lowenstein and Hardie, 1985; Brodylo and Spencer, 1987). Thus, Kona chevron halite (Fig. 4B,C) likely precipitated as seafloor crystal crusts in shoreline hypersaline lagoons or ponds in which exchange with offshore water was limited.

### 6.2.3. Siliciclastic siltstone and mudstone

Parallel laminated burgundy red to tan mudstones and siltstones with intraformational clasts of dolomicrite (Fig. 7A) are the siliciclastics and sulphate nodules described by Chandler (1988) who deduced that these represent near shore-sabkha sediments. Unlike present-day progradational coastal sabkhas, such as those in the Persian Gulf (Kinsman, 1969; Evans et al., 1973; Purser and Evans, 1973; Shinn, 1983), Chandler (1988) described the Gordon Lake Formation as transgressive upward-fining, peritidal sabkha cycles, rather than coarsening-upward regressive cycles (Kendall, 1984). These ~ 15 cm thick fining-up cycles consist of mudstone breccia layers that transition to fine-grained, parallel laminated to rippled sandstones and siltstones, capped in plates by desiccated mudstone breccia layers described as reddened nodule zones that commonly contain mud-chip breccia layers at the base of upward fining cycles, interpreted as storm beds by Chandler (1986). These nodules and mud-chip breccias have a variety of compositions, including anhydrite, chert, and gypsum. The intraformational clasts in this study (Fig. 7B,C) correspond to the silicified carbonate nodules that contain anhydrite in Chandler's (1988) study.

The intraclasts were probably generated during storm events when water levels were higher (e.g., Shinn, 1983). Massive dolomite clasts could have been eroded by storm waves breaking on the lower sabkha and transported further on the sabkha. Interstitial clumps and pseudomorphs of gypsum after displacive halite in the massive dolomitic clasts support this interpretation. It is also possible that the massive carbonate clasts with displacive halite (Fig. 7B) were eroded from carbonate precipitates in nearshore salinas and hypersaline ponds (e.g., Lowenstein, 1989), as halite pseudomorphs are common in carbonates formed in these settings (Kendall and Harwood, 1996).

## 6.3. Summary of sample sedimentology

The overall depositional environment in which the large Kona stromatolites formed was probably a shallow shelf subject to storm events capable of depositing HCS which the large Kona stromatolites formed upon (Fig. 8). In the shallow subtidal and on the lower tidal flat, low domal stromatolitic beds with ripples formed. Shoreward on the tidal flats, stratiform or undulating stromatolites dominated due to the lack of accommodation space. Bladed gypsum crystals precipitated in massive, mixed carbonate and siliciclastic matrixes similar to sabkha deposits in the Persian Gulf where the rising water table localizes deposition of wind-blown sediments and synsedimentary evaporites (Fryberger et al., 1983) (Fig. 8).

No carbonate from the off-shore was obtained from the Gordon Lake Formation. The parallel laminated carbonate may have formed in lagoons or the upper tidal flat. More restricted lagoons or sabkha ponds would have accommodated the chevron halite. In the uppermost tidal flat and sabkha areas fenestral fabric and storm eroded carbonates were deposited as intraformational conglomerates, in places with siliciclastic matrix material, probably produced during storm runoff (cf. clastic outwash sheets prograding onto upper sabkha, Butler et al., 1982).
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** Paleogeographic reconstruction of the Kona depositional area represented by the samples. The total succession of over 500 m of Kona sedimentary rocks is undoubtedly much more complex. The Gordon Lake Formation carbonate samples were mostly deposited in sabkha environments.</figcaption></figure>

## 6.4. Authigenesis and diagenesis of samples

### 6.4.1. Description

Given the age of these rocks, primary fabrics are well preserved. Gypsum pseudomorphs and fenestrae within dolomite are replaced, and predominantly infilled, by blocky low-Mg calcite cements toward the center. Around the edges of gypsum pseudomorphs, ghostly remnants of bladed dolomite rhombs remain locally, highlighted by microcrystalline hematite zonations and rims (Fig. 5D,E). The remaining lithofacies are predominantly very fine-grained dolomite averaging ~ 25 µm with larger neomorphic spar in places. The dolomite probably mimetically replaced calcite, based on strontium concentrations lower than 300 ppm and mostly lower than 100 ppm. Dolomite crystals are commonly unimodal in size with either planar-e crystal boundaries or, more commonly, planar-s boundaries (planar-euhedral and -subhedral respectively, defined by Sibley and Gregg, 1987). Unimodal planar-s microcrystalline dolomite entirely replaces carbonate clasts within the siliciclastic siltstone and mudstone lithofacies (Fig. 7B,C), and most micritic intergranular cement within each stromatolitic and laminated lithofacies. Dolomite crystals are segregated into clots or layers of uniformly sized subhedral crystals, and elsewhere are sutured and anhedral. Crystal size may be partially controlled by the original grain size and textures. This is especially evident where primary stromatolitic lamination is highlighted by variable silica replacement and alternation between two dolomite crystal sizes. Areas of sutured and anhedral dolomite commonly cross cut and obscure original fabrics.

### 6.4.2. Interpretation

Slightly younger Paleoproterozoic dolostones of the Denault Formation (Labrador Trough) contains exquisitely preserved primary fabrics in which microcrystalline mimetic dolomite replacement makes up 80 % of the total rock volume and records four stages of deposition from

marine and authigenic deposition, shallow burial and burial diagenesis (Zentmyer et al., 2011). These well preserved, microcrystalline mimetic textures are generally accepted as the product of early mimetic replacement of aragonite and calcite precipitates by very finely crystalline dolomite (Zentmyer et al., 2011 and references therein).

Similarly, the very fine crystal sizes dominating Kona Dolomite and Gordon Lake carbonate lithofacies, together with planar dolomite crystal morphology, suggest that dolomitization was also synsedimentary (Gregg and Sibley 1984; Zentmyer et al., 2011). Nucleation and growth kinetics determine grainsize distribution within a sample (Spry, 1969). The rate of crystal growth is determined by surface attachment of atoms or molecules to a crystal or nucleation site (Human, 1981; Sunagawa, 1992). A surface-roughness factor relates the main kinetic factors responsible for crystal growth rate in systems driven by precipitation rather than transportation. Planar crystal interfaces are produced under low supersaturation and/or low temperatures, while nonplanar interfaces form above those normal critical roughening factors (Gregg and Sibley, 1984). Uniform grain-size distributions result from single nucleation events on homogeneous nucleation sites (Gregg and Sibley, 1984). The uniform microcrystalline and planar dolomite suggests that replacement diagenesis occurred as one event within sediment at the sediment-water interface.

Early dolomitization was likely promoted by evaporitic conditions. Both the Kona Dolomite and Gordon Lake Formation contain evidence for authigenic gypsum precipitation within the sediment. Under evaporitic conditions authigenic gypsum precipitation lowers the amount of SO₄²⁻ in the water column and increases the Mg/Ca ratio. With less of the kinetic SO₄²⁻ inhibitor (Baker et al., 1981; Morrow and Ricketts, 1988; Morrow and Abercrombie, 1994; Warren, 2000) and more Mg available, dolomite replacement rates would be enhanced. Overall, dolomitization of the Kona and Gordon Lake samples was likely synsedimentary considering the fine crystal size, planar dolomite crystals, and
favourable environmental conditions.

Calcium carbonate likely precipitated first, with contemporaneous authigenic gypsum growth in the sabkha and marine cementation in the intertidal flat, potentially with some silicification. Soon after, mimetic dolomite replaced micritic intergranular cement, gypsum precipitates, marine cements and grains. Subsequent burial diagenesis formed fabric destructive, sutured and anhedral dolomite crystals.

# 7. Geochemistry results

## 7.1. Results

### 7.1.1. Carbonate partial dissolution

Partial dissolution methods were designed to dissolve only the carbonates, and extract elements contained within them, as opposed to the siliciclastic and iron oxide components. This provides information on the fluid the carbonate fraction precipitated from. Plotting Al and Fe against total concentration of REEs in the reacted partial dissolution fluid gives an indication of how successful this separation was in extracting the REEs from only the carbonates. As some Fe in the dolomite was also liberated, the REE plot was normalized with division of Fe by $Ca + Mg$, which represents the amount of carbonate dissolved from each sample. If this normalization is not conducted a false positive correlation will be produced, as the differing amount of carbonate containing Ca, Mg and Fe in each sample will produce a plot where samples with more carbonate will plot with more Fe and REEs and ones with less will plot with less. The normalization eliminates this problem. A lack of correlation indicates that REEs contained in siliciclastics or Fe oxides were not added in significant amounts to the fluid that was analyzed.

The Fe concentration in the whole rock dissolution data was also plotted against the REE + Y content of the samples. This also produced no correlation indicating the REE patterns were not significantly altered by derivation from Fe oxides in the samples. (See Supplemental Materials for graphs.).

Also of interest is a comparison of the amount of Fe in the ferroan dolomite with the amount in the whole rock. Fifty SEM-EDX analyses of ferroan dolomite samples from all lithologies resulted in an average of Fe in the carbonate of 1.06 wt% with a median of 0.91 wt% and a range of 0.10 to 3.83 wt%. The whole rock analyses produced iron concentrations averaging 1.0, with a median of 0.9 and a range of 0.17 to 4.53. The amount of iron in the partial analyses is far lower averaging 0.11 wt %, with a mean of 0.09 wt% and a range of 0.01 wt% to 0.33 wt%. With this low an amount of iron in the partial dissolution it is likely that it came from the easily dissolved carbonate.

### 7.1.2. PAAS normalized REE trends

Carbonate deposits incorporate REEs from the surrounding seawater without fractionation (Webb and Kamber, 2000; Webb et al., 2009), and thus can provide information on the geochemistry of that water (Nothdurft et al., 2004; Bolhar et al., 2004; Kamber et al., 2004). REE patterns of samples can be divided into two groups. The Kona Dolomite large columnar to domal stromatolites, hummocky cross-stratification, ripples and low domes, small domal to stratiform stromatolites, parallel laminates (Fig. 9A to E), and the Gordon Lake Formation parallel laminates and subaqueous halite pseudomorphs (Fig. 10B,E), are middle REE (MREE) enriched and show moderate, heavy rare earth element (HREE) depletion. In contrast, the Gordon Lake fenestral fabric, dolomite clasts in siltstone and mudstone, cement in siltstone and mudstone (Fig. 10 A,C,D), and Kona Dolomite matrix surrounding gypsum pseudomorphs (Fig. 9F) all show moderate to extreme MREE and HREE enrichment.

### 7.1.3. Redox sensitive trace elements

Vanadium, chromium and uranium behave in similar manners in reducing and oxidizing environments. The solubility of vanadium

greatly increases if V in the plus 3 state is oxidized forming vanadate (5 + ) or vanadyl (4 + ) (Sugiyama, 1989; Shiller and Mao, 1999, 2000). Solubility can be further increased if it forms complexes with sulfate (Wanty and Goldhaber, 1992). If an oxidized solution becomes reduced some V will precipitate (Wehril and Stumm, 1989; Shiller and Mao, 2000). Chromium behaves in a similar manner, with the caveat that if enough oxygen is present in the atmosphere to oxidize manganese, the resulting manganese oxide can oxidize Cr to the 6 + state (Eary and Rai, 1987; Oze et al., 2007). Once oxidized the Cr can become mobile, but if it encounters a reducing environment it can revert back to the 3 + state and precipitate. Uranium also behaves during changing oxidation states in a manner similar to V during redox transformations (Brown, 2003). After precipitation of these elements into the sediment their mobility depends on the mineralogy of the sediment, water/rock ratios and water compositions. Wang et al. (2021) note that chromium isotopes can be altered in limestones that have high Cr/Al ratios. The inference can be made that Cr concentrations also would be vulnerable. However, the whole rock samples in this study have one to many orders of magnitude lower Cr/Al ratios. This is important as the preferred sites for Cr and V to occupy during diagenesis are not carbonates, but phyllosilicates, preferably smectites and chlorites (Northrop and Goldhaber, 1990) where they substitutes for Al in the octahedral sites (Breit and Wanty, 1991). This configuration should be far less susceptible to later diagenetic alteration than in purer carbonates where Cr and V do not fit readily into the lattice. In this study whole rock analyses were used to obtain the concentrations of these elements as the samples have abundant siliciclastics compared to the amount of Cr and V they contain. Also, the amount of Cr and V in relatively pure carbonates are commonly below detection with our MS. Uranium behaves differently as it can fit into the carbonate lattice and thus is difficult to alter diagenetically.

### 7.1.4. Carbon and oxygen isotopes

A cross-plot of carbon and oxygen isotopic values from both formations shows a lack of correlation (Fig. 11), indicating that the carbon isotopes were not diagenetically modified in the same manner as the oxygen isotopes. Carbonate minerals equilibrate with fluid $\delta^{18}O$ at a much lower water-rock ratio than fluid $\delta^{13}C$ (Banner and Hanson, 1990) which means that, by comparison, the $\delta^{18}O$ are more easily reset. Negative $\delta^{18}O$ in dolomites are commonly associated with diagenesis at depth (Swart, 2015). Since the dolomite is likely very early diagenetic, it probably contains a primary carbon isotopic signature. These results are similar to those obtained by Bekker et al. (2006) from outcrops in the same areas.

## 7.2. Geochemistry discussion

### 7.2.1. REE Systematics

REE's have unique chemical and physical properties that enable them to be used as powerful tracers. Their identical trivalent charge and systematic decrease in ionic radius with consecutive atomic numbers allow them to be proxies for geochemical processes in oceans, rivers, estuaries, lakes and groundwater (Johannesson and Zhuo 1999; Shields and Stille 2001; Haley et al., 2004; Kamber at al., 2004; Bolhar et al., 2005; Bolhar and Van Kranendonk, 2007; Allwood et al., 2010; Planavsky et al., 2010; Fralick and Riding 2015; Zhu et al 2016; Fralick et al., 2017). They have been used as proxies for interpreting both modern and ancient sediments as they are stable elements that are commonly not mobilized by diagenesis (McConchie 1987; Banner et al., 1998; Webb and Kamber, 2000; Frimmel, 2009; Webb et al., 2009).

In order to better display the pattern shape of sedimentary rocks, REEs are commonly normalized to PAAS (Post Archean Australian Shale; Taylor and McLennan, 1985) and graphically represented. Formulas for calculating anomalies are those of Bau and Dulski (1996) who, because of positive La anomalies in Precambrian carbonates, suggest using Pr and Nd to express Ce anomalies, i.e. $Ce$ anomaly = $Pr/(0.5xPr + 0.5xD)$.$^{13}$ For a fuller discussion of the use of REEs as proxies for water
<figure><img src="image_12.png" /><figcaption>Fig. 9. PAAS normalized REE patterns for 5% acetic acid treated samples from the Kona Dolomite. The subtidal to intertidal samples (A to E) are similar and show pronounced MREE enrichment. In F, the pattern for the diagenetic carbonate forming the gypsum pseudomorphs is similar to those from the sub-tidal to intertidal, except for the large negative Ce anomaly, which may have been formed during diagenetic alteration of the gypsum. The REE pattern for the dolomite matrix surrounding the gypsum pseudomorphs is similar to other sabkha samples (see Fig. 10 A,C,D).</figcaption></figure>
<figure><img src="image_13.png" /><figcaption>**Fig. 10.** PAAS normalized REE patterns for 5% acetic acid treated samples from the Gordon Lake Formation. Sabkha samples (A,C,D) are similar, showing pronounced LREE depletion. Samples B and E are similar to the near-shore and tidal flat samples of Fig. 9.</figcaption></figure>

chemistry see the **Supplemental Material**.

### 7.2.2. REE implications for seawater and depositional environments

The Kona Dolomite and Gordon Lake Formation REE<sub>SN</sub> patterns are not typical Proterozoic or Archean REE<sub>SN</sub> patterns for carbonate platforms (e.g. Kamber and Webb, 2001; Fralick and Riding, 2015; Sarangi et al., 2017). Neoarchean and Paleoproterozoic carbonates tend to display positive Eu, La, and Gd anomalies, no Ce anomalies, and have high Y/Ho ratios (cf. Kamber and Webb, 2001, Kamber et al., 2004; Bolhar et al., 2004; Bolhar et al., 2005; Planavsky et al., 2010; Allwood et al., 2010). Some of these features are not present in the 2.31 Ga Gordon Lake Formation or 2.17 Ga Kona Dolomite (Figs. 9 and 10),

which are instead characterized by pronounced MREE enrichment and mostly lack strong elemental anomalies (Ce and Eu. The dolomitic carbonate samples deposited surrounding the gypsum pseudomorphs and the fenestral fabric samples from the sabkha have chondritic Y/Ho values, whereas offshore to tidal flat samples and clasts in siltstones and mudstones have supra-chondritic values, but only from 30 to 33 (See Supplemental Material for graph.).

Only two samples have a prominent negative Ce anomaly (Fig. 9F, 10A): one is from a subaerial gypsum pseudomorph and probably represents later stage fluids that caused the replacement reaction; the other is from fenestral fabric. The vast majority of the samples display no evidence of a negative Ce anomaly, even though by this time, 2310 Ma,
<figure><img src="image_14.png" /><figcaption>**Fig. 11.** δ¹³C (‰, PDB) vs δ¹⁸O (‰, PDB) cross-plot. No correlation exists between the C and O isotopes.</figcaption></figure>

atmospheric oxygen was becoming well established (Farquhar et al., 2000; Farquhar and Wing, 2003; Bekker et al., 2004; Holland, 2006; Lyons and Gill, 2010). This is also exemplified by the presence of non-zero I/Ca + Mg ratios in some of the carbonate samples used here (Fang et al., 2024), and other Gordon Lake Formation samples (Hardisty et al., 2014), which provides evidence for the presence of free oxygen in the water column. Although negative Ce anomalies preserved in carbonate chemical sediments inarguably indicate that the fluid the sample precipitated from was exposed to oxic conditions, the absence of a Ce anomaly does not necessarily suggest the opposite (Tostevin et al., 2016). Some modern samples, such as those from sabkha environments along the restricted coastline of Abu-Dhabi (Lokier and Steuber, 2008; Clarkson et al., 2014), display no sign of a Ce anomaly despite forming in oxic waters (Tostevin et al., 2016). Ocean margin and near-shore settings or enclosed bays, may also exhibit reduced negative Ce anomalies (Johannesson et al., 2006; Nozaki et al., 1997; Zhang and Nozaki, 1998). Tostevin et al., (2016) suggest that this may be the result of their formation in restricted settings since modern open seawaters consistently display negative Ce anomalies and restricted settings may not. The HCS carbonate would have probably been deposited in open marine conditions; however, the carbonate sand itself could have been derived from more restricted deposits. This is not the case for the large columnar stromatolites, which lack negative Ce anomalies and formed off-shore, as denoted by their association with HCS bearing grainstones. It is therefore possible that the REE patterns in the carbonates reflect the seawater or groundwater derived pore fluids they precipitated from, but these fluids may have been very different from the further offshore world ocean.

Interestingly, samples of the subtidal and intertidal carbonates (i.e. samples with HCS, stromatolites, ripples and parallel lamination) commonly have MREE enrichment and positive Eu anomalies (Fig. 9A to E; 10B, E), whereas those from sabkha environments (fenestral microbialite, carbonate clasts in siltstone and mudstone, and gypsum

pseudomorphs) (Fig. 9F; 10A,C,D) are MREE and HREE enriched, and have negative or no Eu anomaly. However, caution must be exercised here as the MREE enrichments may add to the appearance of Eu anomalies.

Chert beds underlying the Australian Strelley Pool Formation exhibit REE trends (Allwood et al., 2010) that are similar to the MREE and HREE enriched Huronian sabkha carbonates. Allwood et al. (2010) attributed what they termed LREE depletion mostly to hydrothermal input. This is unlikely the case for the sabkha samples, as they do not have positive Eu anomalies and there is no evidence of igneous activity on the Huronian passive margin. Instead, the MREE and HREE enrichment may be the result of light REEs (LREEs) being preferentially left behind in the parent rock and its derivative weathering products due to their preferential adsorption onto clays (Balashov et al., 1964; Ronov et al., 1967; Nesbitt, 1979; Duddy, 1980; Schau and Henderson, 1983; Braun et al., 1993). This may have been combined with weathering by acidic water on the craton. MREE enrichments are typical for acidic terrestrial waters. Johannesson and Zhuo (1999) review the origins of MREE enrichment in mildly acidic surface waters (Hoyle et al., 1984; Elderfield et al., 1990; Sholkovitz, 1995). MREE enrichments are also present in acid brines (Fee et al., 1992; Gosselin et al., 1992), acid lakes (Johannesson and Lyons, 1995), and dilute acid groundwater associated with acid mine drainage (Nordstrom et al., 1995). In these examples the MREE enrichment is probable due to acidic waters preferentially leaching MREEs from iron and manganese hydroxides and oxides in the catchment areas (Johannesson and Zhou, 1999). This is similar to the processes that probably occurred to create the MREE enriched patterns in terrestrial carbonates lying between the late Paleoproterozoic Gunflint and Rove Formations (Fralick et al., 2017). This interval is marked by the development of a euxinic wedge of coastal seawater in the overlying Rove Formation that represents a pulse of oxygen produced sulfuric acid generation during weathering of the craton (Poulton et al., 2004, 2010). In addition, Ashok and Absar (2025) note that some of their Paleoproterozoic stromatolitic carbonates acquired MREE enriched patterns due to Fe oxides being reduced in the sediment by degrading organic matter. This is similar to MREE enrichment in Late Cryogenian carbonates caused by iron hydroxide dissolution below the redoxocline (van Smeerdijk Hood and Wallace; 2015). Similar processes have been recorded in the recent, such as those that occurred in a modern wetland area in France that caused MREE enrichment of its groundwaters due to degrading organics reducing iron oxyhydroxides (Gruau et al., 2004).

It is reasonable that groundwater moving from the craton to the sabkha would be depleted in LREEs during weathering production of clays. This may have been accentuated in the sabkha as siliciclastics there weathered, as HREEs preferentially complex with CO₃²⁻ under elevated pH conditions (Quinn et al., 2006). This would imply that the upper intertidal flat to sabkha environment (where the fenestral fabric, massive carbonate with gypsum, and bedded siltstone and mudstone with carbonate clasts likely were deposited) developed higher pH waters with a plentiful supply of carbonate in solution. This is reasonable as sabkha environments are, by definition, supersaline and evaporative and thus have higher, more basic pH values. Regardless of the source of the MREE and HREE enrichment, the lack of Eu anomaly and chondritic Y/Ho ratios in the sabkha carbonate sediments compared to the positive Eu anomalies of the open marine and intertidal sediments strongly indicates that the sabkha received groundwater from the craton, similar to the landward portion of the modern, extensive Abu Dhabi sabkha (Patterson and Kinsman, 1981).

In contrast to the sabkha the most prominent feature of the REE<sub>SN</sub> trends for the intertidal to subtidal carbonate is MREE enrichment with HREE depletion (Fig. 9A to E; 10B,E). As discussed above weathering by acidic ground and surface water will preferentially result in the release of MREEs and their adsorption onto iron oxyhydroxides (Johannesson and Zhou, 1999, and references therein). On land 2500 Ma ago, prior to the GOE, oxidative weathering of sulfide minerals led to a relatively short-lived increase in chalcophile trace elements and sulfate to the
oceans (Anbar et al., 2007; Reinhard et al., 2009). Konhauser et al. (2011) called attention to this and highlighted how the reactions associated with oxidative weathering of sulfide minerals at approximately 2.3 Ga generated significant acidity. Furthermore, it has been speculated that in the late Neoarchean, increased atmospheric partial pressure of CO₂ may have produced acidic surface waters with pH values of approximately 4.7 (Reinhard et al., 2009). The development of increased concentrations of CO₂ following the Gowganda and Enchantment Lake glaciations would also have lowered the pH of surface waters. Sulfide oxidation driven by the GOE, and possibly a further rise in atmospheric oxygen levels following the glaciations (Sekine et al., 2011), should have added to the already acidic surface waters (Konhauser et al., 2011), and might thus have provided the acidic conditions necessary to deliver dissolved Fe and associated MREE enrichment to the near-shore of the ocean. This may have been augmented by the delivery of iron hydroxides to the ocean by overland flow. The presence of iron oxides filling depressions between small pseudocolumnar stromatolites forming the larger buildups and their abundance in the rocks of this study testifies to the delivery of iron hydroxides in suspension. The early diagenetic degradation of the organics in the stromatolites would have reduced the iron hydroxides releasing the MREE enriched REEs associated with them and made the rare earths available for incorporation into the carbonate cements (e.g. Gruau et al., 2004; Ashok and Absar, 2025). A similar diagenetic process has been documented from the South Korea Plateau in the Sea of Japan where reductive dissolution of Fe oxyhydroxides in the sediment column liberated MREEs that were then incorporated into high-Mg calcite (Kang et al., 2014).

The original calcite would have sampled the REEs in the waters from which they precipitated without fractionating them (Nothdurft et al., 2004; Kamber et al., 2004; Webb et al., 2009). However, REE patterns can be susceptible to modification, through element mobility, during dolomitization (e.g. Nothdurft et al. 2004; Fralick et al., 2024). As the dolomitization that altered Kona Dolomite and Gordon Lake Formation Ca-carbonates was early it would have probably been driven by local pore fluids. This is in contrast to modification of REE patterns caused by dolomitization reported by Nothdurft et al. (2004) where the fluids were probably deep basinal and travelled upwards through faults (Wallace et al., 1991), and that of Fralick et al. (2024) where the dolomitizing fluid was offshore seawater with a different chemistry. In a situation where the dolomitizing fluids were not exotic Banner et al. (1988) found that the dolomitizing event did not affect REE patterns. As the dolomitizing fluids that affected the Kona and Gordon Lake carbonates were

early diagenetic, it is unlikely that they were exotic, and thus, it is unlikely they would have modified the REE patterns. These patterns probably reflect the sabkha groundwaters, and coastal seawater and derived pore water from which the carbonate precipitated.

### 7.2.3. Redox-Sensitive metals and their implications

The elements Cr, V and U are sensitive to redox conditions. When oxidized to a higher valence state their solubility greatly increases (Calvert and Pendersen, 1993; Russell and Morford, 1999). Consequently, enrichments of redox-sensitive trace metals in ancient marine sedimentary rocks have been used to constrain the timing of oxygenation on Earth (Anbar et al., 2007; Willie et al., 2007). Konhauser et al. (2011) utilized Cr to deduce implications regarding the duration and sequence of events associated with the GOE. At 2.3 Ga there were peak Cr concentrations in oceanic sediments (Konhauser et al., 2011). In our dataset for whole-rock dissolution, the sabkha carbonate samples have similar V/Al₂O₃ and Cr/Al₂O₃ values to the sabkha siliciclastics indicating that the fluids these carbonates precipitated from were not enriched in V or Cr relative to siliciclastics. However, they are enriched in U; and this is probably due to the ability of U to substitute into the calcite lattice, as opposed to V and Cr, which cannot. Samples of subtidal to intertidal carbonates are enriched in V and Cr compared to siliciclastics (Fig. 12), indicating that a fluid with a composition different than that from which the sabkha carbonates precipitated was responsible. This strongly agrees with Konhauser et al.'s (2011) findings of Cr enrichment in marine sediment deposited at this time and is also aligned with the vast amount of iron oxyhydroxides that must have been delivered to form the abundant hematite associated with the carbonates. However, the sabkha groundwater does not exhibit such enrichment. In this case, the water may have been delivered to the previously oxidized sabkha sediment by groundwater derived from the craton, similar to present-day Abu Dhabi sabkha (Patterson and Kinsman, 1981). The acidic groundwater (Konhauser et al., 2011) would have flowed through the sabkha sediments until being diluted by seawater forming the pore water in the intertidal flats to shallow offshore. The rise in pH there would have promoted Fe, Cr and V to come out of solution.

### 7.2.4. Carbon isotopic implications for the onset of the Lomagundi Event

Carbon isotopes in marine carbonate rocks are most widely used as proxies for carbon cycling through time, tracking the balance between organic carbon burial and carbonate production vs. weathering of carbonate rock and volcanic exhalations (Schidlowski, 2001). Throughout Earth history the variations in carbon isotope ratios have typically been

<figure><img src="image_15.png" /><figcaption>**Fig. 12.** Relative V, Cr and U (ppm) enrichment in samples that underwent whole-rock dissolution. Normalization by Al₂O₃ (%) adjusts for varying amounts of siliciclastics in the samples. Lines roughly separate the various fields. The open squares and crosses are siliciclastic dominated samples of sediment deposited on sabkhas. They plot in the lower left denoting lower amounts of U relative to their siliciclastic content compared to other samples. The coloured data points are carbonate dominated samples and have up to 20 times higher U/Al₂O₃ ratios than the siliciclastics. The excess U is probably in the carbonate phase as it will fit into the calcite lattice. Samples deposited in the nearshore to intertidal flats (blue and red circles, stars, black triangles and grey and green diamonds) are enriched in V/Al₂O₃, Cr/Al₂O₃ and U/Al₂O₃ relative to the siliciclastics. These samples have up to 40 times higher V/Al₂O₃ and 10 times higher Cr/Al₂O₃ ratios than the siliciclastics, whereas supratidal sabkha carbonates have ratios of these elements similar to the siliciclastics. (See Fig. 11 for symbol legend).</figcaption></figure>
small ($|\delta^{13}\text{C}| = \pm 5 \text{‰}$ PDB) and short lived ($<20$ Ma).

The Huronian Supergroup contains three glacial diamicictite units. The middle unit is overlain by a distinct cap carbonate, the Espanola Formation. The Espanola Formation and its correlative cap carbonates in North America (Vagner and Bottle Creek Formations in the Medicine Bow Mountains and Sierra Madre respectively) exhibit negative $\delta^{13}\text{C}$ values ranging from -10.0 to -0.8 ‰ (Bekker et al., 2006; Kurucz et al., 2021). In contrast to the negative $\delta^{13}\text{C}$‰ post-glacial event, the Kona Dolomite and Gordon Lake carbonates exhibit positive values (Fig. 11) (Bekker et al., 2006). This coincides with the onset of markedly positive values from 2.3 to 2.1 Ga (Bachan and Kump, 2015) of the Lomagundi Event (Schidlowski et al., 1975, 1976; Karhu and Holland 1996). This event is recorded in limestones and dolostones that precipitated globally and preserve markedly positive $\delta^{13}\text{C}$ values, which reach upwards of +16 ‰. These unusually positive $\delta^{13}\text{C}$ values could suggest that a larger fraction of carbon was fixed and buried as isotopically light organic carbon (Planavsky et al., 2012) rather than as carbonate carbon (Bachan and Kump, 2015), though contrasting views exist (Yudovich et al., 1991; Hayes and Waldbauer, 2006; Purohit et al., 2010; Mayika et al., 2020; Prave et al., 2022). The Gordon Lake Formation was deposited after the Gowganda-Duitschland glacial episodes (Rasmussen et al., 2013) and prior to the Rietfontein Dam glaciation, which is probably correlative with the glacial deposits forming the Enchantment Lake Formation underlying the Kona Dolomite (Rasmussen et al., 2024). Therefore, the Lomagundi Event would have spanned the time period starting in the interglacial between the Gowganda and Rietfontein Dam and continued following Rietfontein Dam deglaciation.

## 8. Conclusions and summary

The Gordon Lake Formation and Kona Dolomite were deposited following the GOE at approximately 2310 Ma (Rasmussen et al., 2013) and 2170 Ma (Rasmussen et al., 2024) respectively. Prior to this, with rising temperatures and oxygen (Bekker et al., 2001; Bekker et al., 2004), the Gowganda/Duitschland glaciation waned and organic productivity flourished (Bekker and Holland, 2012; Rasmussen et al., 2013) resulting in the Lomagundi Event that is recorded in Gordon Lake carbonates (Bekker et al., 2006). This interval was followed by the ~2256 ± 6 Ma glacial unit in the upper Timeball Hill Formation, and probably the diamicitites of the Enchantment Lake Formation (Rasmussen et al., 2013). The 2172 ± 6 Ma (Rasmussen et al., 2024) Kona Dolomite, with $\delta^{13}\text{C}$ values of ~-6 to -8, representing the Lomagundi Event (Bekker et al., 2006), was deposited after this glaciation.

Kona and Gordon Lake carbonates were deposited during the transition from Archean-type ocean chemistry to a more oxygenated world (Murakami et al., 2011). The marine oxygenation that had already commenced in the mid-Archean (Riding et al., 2014, 2022; Patry et al., in press) spread rapidly to the atmosphere during the G.O.E., significantly altering terrestrial dissolved fluxes to the world ocean. This not only resulted in increased flux of sulfur from the oxidation of sulfides, but these reactions also promoted acidic conditions which subsequently increased delivery of Cr (Konhauser et al., 2011), and probably elements with related oxidation-reduction behavior such as V, and U. The low pH of meteoric water would have also promoted the dissolution and delivery of iron to the coastal areas of the world ocean (Konhauser et al., 2011; Bekker and Holland, 2012). This would have augmented the flux of particulate Fe-oxyhydroxide. Fine-grained particulate iron oxide in the carbonate sediment of the Gordon Lake Formation and Kona Dolomite attest to oxidized iron becoming a major component of nearshore sediments.

We suggest a likely scenario as follows. During weathering on the craton LREEs would have preferentially been removed from solution by the clays that were forming (e.g. Balashov et al., 1964; Ronov et al., 1967; Nesbitt, 1979; Duddy, 1980; Schau and Henderson, 1983; Braun et al., 1993). The lack of Eu anomalies and near chondritic Y/Ho ratios of sabkha REE patterns indicates that it was these groundwaters flowing

to the sabkha that resulted in the carbonates precipitating there developing MREE and HREE enrichment compared to LREEs. The acidic weathering conditions at 2.31 Ga and 2.17 Ga also delivered abundant dissolved Fe, and MREEs leached from them (e.g. Johannesson and Zhou, 1999), to the ocean. The buffering effect of the seawater raised pH causing Fe oxyhydroxides to precipitate, represented by abundant hematite in the dolostones, and also formed layers draping the microstromatolites in the large columnar stromatolites. In addition detrital iron oxyhydroxides, formed during weathering, would have been carried to the ocean by overland flow and deposited with the carbonates on tidal flats and the shallow nearshore. During early burial diagenesis degradation of organic matter resulted in reduction of the iron in some oxyhydroxides freeing the associated rare earths enriched in MREEs (Palmer, 1985; Palmer and Elderfield, 1986; Banner et al., 1988) to the pore waters, where they could be incorporated into cements. This led to the MREE enrichments of carbonates precipitated in nearshore and tidal flat environments. These processes and products occur today in areas with restricted water supply (e.g. Fee et al., 1992; Gosselin et al., 1992; Nordstrom et al., 1995; Johannesson and Zhou, 1999; Gruau et al., 2004) but are rare in modern oceans (Kang et al., 2014). This highlights the abnormal coastal water chemistry that could develop following the Paleoproterozoic glaciations. The combination of abundant fresh sedimentary debris with acidic ground and surface water leaching it and flowing, carrying entrained iron oxyhydroxides, into the coastal sediments and ocean produced a geochemical environment different from what came before or after.

## CRediT authorship contribution statement

**Brittany Ramsay:** Writing – original draft, Visualization, Methodology, Investigation, Formal analysis. **Philip Fralick:** Writing – original draft, Visualization, Supervision, Resources, Project administration, Methodology, Funding acquisition, Data curation, Conceptualization. **Martin Homann:** Writing – review & editing, Validation, Formal analysis. **Stefan Lalonde:** Supervision, Funding acquisition, Formal analysis. **Robert Riding:** Writing – review & editing, Supervision, Methodology, Formal analysis.

## Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Philip Fralick reports financial support was provided by Natural Sciences and Engineering Research Council of Canada. Stefan Lalonde reports financial support was provided by European Research Council. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

## Acknowledgments

We wish to thank Luke Beranek, the journal assistant editor who guided this manuscript through, and reviewers Andrey Bekker, Malcolm Wallace and Jamie Robbins whose extensive, useful comments helped to craft this research into a much better manuscript. The investigation was supported by a Natural Science and Engineering Discovery Grant to PWF. Sophie Kurucz and Amy Cleaver helped with fieldwork. Drafting was done by Jordan Heroux.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.precamres.2025.107886.
# Data availability

Data will be made available on request.

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