Bidirectional fabric evolution in Hamelin Pool microbialites, Shark Bay, Western Australia

Hamelin Pool, Shark Bay, Western Australia hosts the world's largest and most extensive assemblages of living marine microbialites, comparable in size and shape to ancient structures found throughout the fossil record. Documented here are the internal fabrics of modern microbialites collected throughout Hamelin Pool. Mesoscale and microscale observations of microbialite polished slabs and thin section scans, optical microscopy and scanning electron microscopy coupled with energy‐dispersive X‐ray spectroscopy formed the basis for a fabric classification system that combines accretionary mat type with microfabric. Accretionary mat types included pustular, smooth, colloform, as well as ‘transitional’ mats that are a cross between pustular and smooth mats. Mapping of fabrics in 45 microbialite heads indicated bidirectional evolution. An upward progression of fabrics corresponded to changes in mat type as the head grew upward into shallower water. A downward evolution of microfabrics occurred as surface mats transitioned into the subsurface of the microbialite structure. Downward microfabric evolution occurred as a result of early taphonomic processes, and involved a progression from the original depositional architecture to subsequent stages of “Micritic Thickening”, and finally, “Cement Infilling”. The observed bidirectional evolution of microbialite microfabrics within Hamelin Pool offers a conceptual framework for the study of modern microbialites, not simply as the sole product of accretionary mat types but rather as the combined result of the activity of surface mats and their taphonomic evolution. Early taphonomic processes induce further lithification of the microbialites which may enhance preservation potential in the geological record.


| INTRODUCTION
Living microbialite systems offer an opportunity for first-hand investigation of processes involved in the formation of organosedimentary structures that dominated the planet for 80% of Earth history (Riding, 2011).Macroscale, mesoscale and microscale features of microbialites can provide clues regarding environmental and biological conditions associated with formation of these long-lived structures (Ginsburg, 1991;Trompette, 1982).The challenge to geobiologists is how to interpret the fossil record.
The formation and evolution of internal structure, or fabric, is a critical component of microbialite morphogenesis (Grey & Awramik, 2020) and has been used as a basis for microbialite classification (Burne & Moore, 1987).Previous authors have proposed that competing forces of biology and the environment control mesoscale and microscale fabric (Andres & Reid, 2006;Ginsburg, 1991;Suosaari et al., 2019a).Studies of modern systems have shown that microfabrics can be the result not only of the initial accretionary processes of the microbial community, but also of early taphonomic processes starting as soon as the surface mat is covered by the next generation of mat (Airo, 2010;Arp et al., 1998Arp et al., , 2012;;Défarge et al., 1996;Planavsky & Ginsburg, 2009;Sforna et al., 2017).
Detailed petrographic investigations of the internal fabrics of the iconic Hamelin Pool structures are rare, and these microbialites have commonly been portrayed as coarse grained structures accreted primarily by trapping and binding (Hoffman, 1976;Logan, 1961;Logan et al., 1974;Playford, 1990).The important contribution of micrite as a framework-building component in Hamelin Pool microbialites was first noted by Reid et al. (2003).More recently, a detailed petrographic study of microbial mats on the surfaces of Hamelin Pool microbialites documented the fundamental role of the coccoid cyanobacteria Entophysalis in the precipitation of microbial micrite in all structure-forming mats in Hamelin Pool (Vitek et al., 2022).The goal of the present study was to extend this initial work by Vitek et al. (2022) to (1) describe and classify the internal fabrics of Hamelin Pool microbialites, (2) use this fabric classification to map the distribution of fabrics in 45 heads collected from the pool between 2012 and 2014 and (3) develop a conceptual model that accounts for the observed distribution of fabrics.Results show a bidirectional evolution in fabrics as living surface mats progressed to fully formed microbialites.Fabrics evolved upwards, corresponding to changes in mat type that occurred as heads grew into shallower water; fabrics evolved downward due to early taphonomic changes as former surface mats were subsequently buried by new generations of mats.Documentation of this bidirectional evolution serves as a link between modern Earth surface processes and the rock record.

| BACKGROUND
As the first discovered example of modern microbialites equivalent in size and shape to ancient microbialites (Golubic & Hofmann, 1976;Logan, 1961;Logan et al., 1974) Hamelin Pool is an iconic site for the study of microbialite formation and evolution.Microbial mats and structures with diverse sizes and shapes dominate the 135 km shoreline.Here, Microbialite/Stromatolite Provinces (Figure 1) were previously defined by morphologically distinct microbialite structures paired with characteristic shelf physiography (Suosaari et al., 2016a(Suosaari et al., , 2019b)).Environmental conditions in Hamelin Pool are important controls on the presence and distribution of microbialites in this hypersaline embayment (Suosaari et al., 2016a(Suosaari et al., , 2016b(Suosaari et al., , 2019b)).Average salinity in Hamelin Pool ranges from 60 to 78 PSU (practical salinity units) and the average daily tidal range is ca 60 cm.Nevertheless, over the course of a year, the sea surface can vary by nearly 2 m (Suosaari et al., 2016b).
In all three of these previously described structurebuilding mat types, in situ precipitation of microbial micrite was recognised as an important accretion mechanism.Micrite comprises on average 36% to 81% of accreted products, with up to 99% micrite in some pustular mats and 98% micrite in some colloform mats (Figure S1; Vitek et al., 2022); energy-dispersive X-ray spectroscopy analyses indicated that this micrite is aragonite (Vitek F I G U R E 1 Hamelin Pool, Shark Bay, Western Australia.Boundaries of Stromatolite/Microbialite Provinces described by Suosaari et al. (2016aSuosaari et al. ( , 2016bSuosaari et al. ( , 2019aSuosaari et al. ( , 2019b) ) are indicated by red lines.Black dots indicate the locations of sampled microbialite heads; heads are grouped by transect.The table gives full sample names corresponding to the numbered localities within each transect.Base map: SIO, NOAA, US Navy, NGA, GEBCO, Image Landsat/Copernicus.et al., 2022).The remaining accreted products were trapped sediments, including peloids, ooids, Fragum erugatum bivalves, foraminifera, serpulid tubes, as well as detrital quartz and dolomite.Petrographic analysis showed that precipitation of micropeloidal micrite in the microbialite-building mat types was closely associated with cells of the coccoid cyanobacterium Entophysalis, and microtextures of the micrite reflected the size and distribution of Entophysalis colonies and associated extrapolymeric substances (EPS) (Vitek et al., 2022).In pustular surface mats, where large colonies of Entophysalis are abundant, highly-porous clots of micropeloidal micrite (<250 μm) were found to be distributed randomly throughout the mat (Figure S1A).In contrast, smooth and colloform mats contained smaller colonies of Entophysalis which were distributed along horizons, and peloidal micrite formed highly-porous fine laminae <1 mm thick (Figure S1B,C).In all surface mat types, micrite associated with Entophysalis had a characteristic sub-polygonal porosity resulting from cell and/or colony entombment (Vitek et al., 2022).
The present study extends the descriptions of initial accretion by the three surface mats as documented by Vitek et al. (2022).Presented here are new observations of a fourth accretionary mat type, transitional mats, as well as an evaluation of microbialite fabrics produced by all four mat types and their evolution in the subsurface of actively growing heads.The initial description and interpretation of fabrics were guided by earlier studies of Bahamian stromatolites, which showed that subsurface laminae are fingerprints of former surface mats (Reid et al., 2000).It was originally assumed that changes in fabrics of the Hamelin microbialites equated to changes in accretionary mat type.After detailed examination, the primary fabrics of each mat type were found to evolve with early taphonomic alteration as the former surface mats were subsequently buried by new generations of mats, forming modified fingerprints of former surface mats.

| Field observations, sample collection, mat types
Fieldwork was carried out over three 2 month field seasons (March and April 2012-2014) using small boat surveys.Forty five microbialite heads were collected throughout the Microbialite Provinces along 11 transects (Figure 1) and the mat type on the surface of each head (referred to as 'surface mat type') was recorded.
Microbial mats in Hamelin Pool have historically been classified on the basis of surface texture (see Logan et al., 1974;Playford, 1990), and this traditional terminology is retained in this study.In addition to the three commonly noted mats that form microbialite heads (pustular, smooth and colloform), a fourth structureforming mat, a transitional mat, that contained characteristics of both pustular and smooth mats, was also recognised.Field observations of this 'transitional' mat type indicated the presence of small dark pustules on smooth surfaces; percent micrite in transitional mats is intermediate between pustular and smooth mats (Figure 2).Similar transitional mats, described as 'morphological composites of two distinct mat types', such as composite mats of smooth and pustular, were identified by Morris et al. (2020).Additional mat types that have been described in Hamelin Pool, including pinnacle mats (Suosaari et al., 2018), tufted and blister mats (Logan et al., 1974) are not included in the present study because these mats do not form discrete buildups.Note also that smooth and pustular mats forming heads with topographic relief are distinct from intertidal sheet mats that also have smooth and pustular surface textures, but which do not lithify and do not form discrete structures (Babilonia et al., 2018;Suosaari et al., 2016aSuosaari et al., , 2019bSuosaari et al., , 2022a)).
Ten of the microbialite heads collected had pustular surface mats, five had transitional surface mats, six had smooth surface mats and six had colloform surface mats (Table S1).Surfaces of the remaining 18 heads, which were classified as 'other', include hard films (bronze, red or black), cauliflower caps, serpulid crusts and macroalgal assemblages dominated by Acetabularia and/or Hormophysa; discussion of the surfaces classified as 'other' is beyond the scope of this study.

| Sample preparation and petrographic analysis
Each of the 45 heads was slabbed into three 1 inch thick slices using a slab saw fitted with a continuous-rim diamond blade, using water as a coolant and lubricant.Slabs were impregnated with Akemi Akepok 1006 epoxy, and polished at Aradon in Malaga, Western Australia, using large (1 m in diameter) reciprocating flat laps (vibrolaps).Slabs were evenly weighted and placed face-down in an abrasive slurry of water and aluminium-oxide (160 grit) for ca 12 h, before being washed and dried.Two of the three slabs from each microbialite head were polished on felt with aluminium-oxide (1200 grit) and used for mesoscale fabric mapping.The remaining unpolished slabs were cut into multiple billets representative of mesoscale structure variability to be made into 2 × 3″ petrographic thin sections; 186 thin sections were prepared by National Petrographic Service, Inc., Rosenberg, Texas; 44 thin sections were prepared by Ray Lund at Quality Thin Sections, Tucson, Arizona.Images of both the slabs and thin section scans are available online (Suosaari et al., 2023).
Thin sections made from the impregnated slabs were examined using an Olympus BH-2 petrographic microscope with plane-polarised and cross-polarised transmitted light at various magnifications.Photomicrographs were collected using a Lumenara Infinity 3 digital microscopy camera (Infinity Analyze v 6.5.6).Thin section scans were produced at 4800 dpi using a Canon CanoScan8800F flatbed scanner (MP Navigator EX).

| Scanning electron microscopy with energy-dispersive X-ray spectroscopy
Multiple thin sections were examined by scanning electron microscopy (SEM) equipped with EDS at the University of Miami and Smithsonian Institution National Museum of Natural History.Images from seven of these thin sections, captured on the ThermoFisher Scientific Apreo Field Emission SEM at Smithsonian are presented in this study.Images were captured in directional F I G U R E 2 Hamelin Pool microbialite-building mat types and tidal distributions including 'transitional' mat type.(A) Field photographs showing pustular, transitional, smooth and colloform mats, modified from Suosaari et al. (2016aSuosaari et al. ( , 2016b; Figure 4).Transitional mats are characterised by small pustules, which appear as black dots on smooth mat surfaces (arrows).(B) Depth transect showing transitional mats at mean sea level, between pustular mats of the upper intertidal zone and smooth mats of the lower intertidal zone.(C) Box and whisker plot showing the mean (x), median (horizontal line) and standard deviation of % micrite in comparison to sediment grains within each surface mat type (modified from Vitek et al., 2022).
backscatter mode with a lens-mounted gaseous analytical detector (DBS-GAD) at an acceleration voltage of 15 kV, with a working distance of ca 7.5 mm.Elemental composition of mineral phases was determined using an octane energy dispersive X-ray spectrometer (EDS).Regions of interest (ROI) were boxed out with masking tape and copper tape was used to pinpoint specific cements identified using petrographic and reflective microscopes; EDS was performed on flat surfaces.For secondary imaging, thin section ROIs were etched with 2% hydrochloric acid for 10 s and imaged again using secondary electrons to observe mineral morphologies using an angular backscattered detector at an acceleration voltage of 15 kV, with a working distance of ca 10 mm.Mineralogy was inferred from EDS elemental analysis; aragonite and high-Mg calcite (C, O and Ca) were differentiated by peaks of Sr and Mg, respectively.

| Terminology
Microbialite refers to organosedimentary deposits that have accreted as a result of a benthic microbial community trapping and binding sediment and/or forming the locus of mineral precipitation, following Burne and Moore (1987).
Fabric refers to the structural architecture of microbialites and encompasses two scales of observation: mesoscale and microscale.Mesoscale fabrics are observable with the naked eye (i.e., centimetre to millimetre-scale features seen in hand samples or thin section scans); microscale fabrics, or microfabrics, are observed with a microscope (micron-scale).'Initial architecture' refers to the arrangement of products (trapped and bound grains, micrite and organic matter) accreted by microbial mats.
Taphonomy is used to reference the transition of organic remains produced by the biosphere into the lithosphere (Martin, 1999).More specifically, taphonomic processes can be grouped into two phases: biostratinomy, events that occur before final burial; and diagenesis, events that occur after final burial (Lyman, 2010).The term 'early taphonomy' is used to refer to biostratinomic processes involved in the decay of living microbial mats to form lithified sedimentary structures before these structures are subject to diagenetic processes associated with final burial or uplift that removes the structures from the environment of deposition.
Terminology for mesoscale and microscale features of clots and laminae, as shown in Figure 3, is adapted from Grey and Awramik (2020).Clot sizes are classified as miniclots (submillimetre), mesoclots (millimetre to centimetre) and maxiclots (>1 cm); clot shapes include subrounded and irregular; and clot alignment is described as random or aligned (Figure 3A).Laminae thicknesses is described as fine (submillimetre), medium (millimetre to centimetre) and coarse (>1 cm); laminae shapes include flat, gently convex or steeply convex; and laminae continuity is described as continuous (extending laterally across the structure), or discontinuous (forming a series of aligned lenses) (Figure 3B).Textural terms for carbonate minerals include nanocrystals, micrite, microspar and spar.Nanocrystal is used for crystals ≤1 μm in size (Jones, 2017).Micrite is used for crystals 1 to 4 μm in size (Folk, 1959).Microspar is used for crystals 4 to 30 μm in size (Folk, 1965).Spar is used for crystals greater than 30 μm in size (Folk, 1965).
Crystal habits include equant crystals, which have approximately the same side length in every direction; acicular crystals, which have a needle-like shape; bladed crystals, which are longer than they are wide with width exceeding depth; and nanobulbous crystals, which are rounded to anhedral nanocrystals, often with fuzzy margins.Botryoids are acicular crystal aggregates with a radial arrangement that typically nucleate on the walls of voids and grow in the direction of free pore space (Ginsburg & James, 1976).Granular crystals are rounded or equant anhedral crystals of approximately the same size.Dispersed micrite is used to describe diffuse accumulations of unconsolidated micrite crystals.

| FABRICS: OBSERVATIONS, EVOLUTIONARY PATHWAYS, CLASSIFICATION AND DISTRIBUTION
Described below are observed fabrics and evolutionary pathways for each of the four structure-forming mat types in Hamelin Pool.These pathways form the basis for a classification system that is then used to map the distribution of fabrics within the 45 heads collected from the various provinces (Suosaari et al., 2019a(Suosaari et al., , 2019b)).

| Mesoscale observations
Each mat type forms characteristic mesoscale fabrics, as observed on polished slabs and thin section scans.Fabrics formed by pustular mats lack lamination and are composed of randomly distributed micritic clots, which can be seen with the naked eye in thin section scans (Figure 4A).Fabrics formed by the newly recognised transitional mats vary in degree of lamination in polished slab, while in thin section scans, discontinuous laminae are visible, reflecting alignment of micritic clots (Figure 4B).Fabrics formed by smooth mats generally appear laminated in both polished slab and thin section scans; laminae are continuous with gently convex shapes (Figure 4C).Fabrics formed by colloform mats also appear laminated in both polished slab and thin section scan (Figure 4D); laminae are discontinuous with steeply convex shapes.In all fabrics, with increasing depth, clots become enlarged and laminae thicken (Figure 4).These general trends in mesoscale features can be quantified using microscale observations.

| Microscale observations
Most microfabrics found in the upper 1 to 2 cm of the heads (Figure 5) are similar to those in the living surface mats described by Vitek et al. (2022).These microfabrics are considered 'original' and are designated as Microfabric 0 (Figure 5).Pustular mats form unlaminated microfabrics of randomly distributed miniclots to mesoclots of micrite (approximately 2 mm in diameter).The micritic clots are subrounded to irregular in shape, and are often surrounded by dispersed micrite, which appears golden in plane-polarised light, as well as variable amounts of sediment (Figure 5A,B).Proportions of trapped and bound sediment in pustular mat samples ranged between ca 0 and 65%, and averaged ca 20% (n = 10) (Vitek et al., 2022).Transitional mats are characterised by aligned miniclots to mesoclots of micrite forming fine, discontinuous laminae.The micritic clots are subrounded to irregular in shape and are often surrounded by dispersed golden micrite (Figure 5C,D).Proportions of sediment in transitional mats ranged between ca 10 and 90%, and averaged ca 40% (n = 5; Figure S2).Smooth mats form fine, continuous horizons of micrite, typically less than 250 μm thick, with abundant sediment grains (Figure 5E,F).Proportions of sediment in smooth mats ranged between ca 30 and 95%, and averaged ca 65% (n = 10; Vitek et al., 2022).Colloform mats form finely laminated fabrics composed of gently to steeply convex micritic horizons, usually less than 500 μm thick, often stacked into columns.Continuity of laminations between columns in colloform mats is low, and sediment is often observed in the spaces between columns.Proportions of sediment in colloform mats ranged between ca 0 and 50%, and averaged ca 20% (n = 10; Vitek et al., 2022).
The characteristic honeycomb texture created by the sub-polygonal porosity in peloidal micrite in living surface mats described by Vitek et al. (2022) and shown in Figure S1A3 are commonly obscured in thin sections of dried samples collected from the tops of the microbialite heads as these pore spaces have been infilled with micrite or microspar.Due to this early infilling, the 'original' fabrics of the heads are often denser than the microfabrics illustrated in formalin-preserved surface mats illustrated by Vitek et al. (2022).High concentrations of the extracellular pigment scytonemin, which cyanobacteria use as a means of protection from harmful UV radiation (Ehling-Schulz & Scherer, 1999;Gao & Garcia-Pichel, 2011;Garcia-Pichel & Castenholz, 1991;Golubic & Hofmann, 1976), commonly give precipitated micrite in shallow mats a reddish tint (Figure 5A through D), whereas micrite in deeper mats appears grey (e.g. Figure 5G,H).
A downward evolution of original microfabrics is observed below the upper 1 to 2 cm of the heads.This evolution appears to have occurred in two stages.The first stage of evolution, termed 'Micritic Thickening', forms Microfabric A (Figure 6).During Micritic Thickening, pre-existing features, either clots or laminae, were enhanced through further precipitation of micrite and microspar.In pustular and transitional mats, micritic clots, which were sub-centimetre scale in the original fabrics (porous miniclots to mesoclots), have become enlarged to form dense mesoclots and maxiclots; this enlargement is associated with the addition of reddish brown to dark brown micrite creating a heterogenous mottled appearance in thin section (Figure 6A,B).In addition, a downward decrease in the abundance of dispersed micrite within pustular and transitional mats suggests that the crystals formed during accretion of initial architecture may act as nuclei for further precipitation and/or become incorporated in the larger dense clots (Figure 6C,D).Infrequently, precipitation of additional dispersed micrite at depth appears to be associated with subsurface colonies of Entophysalis (Figure S3).
In the laminated microfabrics of smooth and colloform mats, Microfabric A is characterised by the precipitation of micrite and microspar at the edges of laminae and within intralaminar pore spaces.This precipitation made laminae thicker (typically greater than 1 mm; fine to medium laminae) and denser (Fig- ure 6E,G,I).Subsurface micrite displays a variety of colours and textures.Granular to irregular patches of red-brown (Figure 6E,F) and grey micrite (Figure 6G,H) are common, often outlined by darker micrite, creating a mottled appearance.Dark to black micrite, which may have a micropeloidal texture, rims grains and fills pore spaces; this dark micrite is commonly associated with microspar fringe cement and may encompass granules of golden micrite (Figure 6I,J).
A second stage of subsurface evolution termed 'Cement Infilling' forms Microfabric AB (Figure 7), which often follows the Micritic Thickening of clots and laminae that characterises Microfabric A. In rare cases, Micritic Thickening is not observed prior to Cement Infilling, and thus these microfabrics probably evolved directly from Microfabric 0 to Microfabric B (Figure S4).During the Cement Infilling stage, precipitation of spar and, less commonly, peloidal micrite, fill or partially fill remaining open pore spaces.The filling of interlaminar pores often fuses adjacent laminae (Figure 7A), further thickening the structures until the original structural architecture is overwhelmed and obscured.Infilling spar occurs as botryoidal aragonite (Figure 7A,B) and fringes displaying isopachous (Figure 7C,D), fan-shaped, or irregular meshworks of aragonite crystals.In addition, micropeloidal micrite, ranging in appearance from black (Figure 7E,F) to red (Figure 7G,H) is sometimes observed infilling pores.Peloids with fuzzy to sharp borders are surrounded by sparry fringes.
The SEM/EDS analyses of micrite, microspar and spar in all fabrics described above show a variety of crystal morphologies, all composed of aragonite (Figure 8).Aragonite mineralogy is indicated by dominant peaks of Ca, C and O with a small strontium peak measured in EDS (Table S2).Aragonite present in original fabrics, whether clots or laminated, occurs mainly as nanobulbous, equant or acicular crystals less than 4 μm in size, as well as needles between 5 and 10 μm (Figure 8A,B,C).The mixed red and dark areas of Microfabric A contain mixed crystal sizes, with red areas comprised of equant crystals and needles up to 10 μm in size (Figure 8D), while dark areas commonly display acicular to equant crystals less than 4 μm long (Figure 8E).Spar in Microfabric AB occurs as botryoidal aragonite (Figure 8F) or acicular aragonite fringe cements (Figure 8G).Dark micropeloidal infill exhibits nanobulbous, equant or acicular crystals less than 4 μm in size (Figure 8H).The transition of all four types of surface mats into the subsurface is accompanied by distinct changes in microfabric, in general evolving from (1) original architecture (Microfabric 0), to (2) Micritic Thickening (Microfabric A) and (3) cement infilling (Microfabrics AB and B).Microfabric AB has not been observed in pustular mats (Figure 9A, Figure S5) and in some cases smooth mat fabrics evolve directly from Microfabric 0 to B without a stage of Micritic Thickening.It should be noted that boundaries between microfabrics are not absolute, and transitions between defined microfabrics are gradational.For example, a fabric with occasional pores infilled with botryoidal cements may be classified as Microfabric A; the fabric would be classified as AB when the botryoidal cement fused layers to thicken laminae, or the structure was so thoroughly infilled with cement that lamination is no longer evident.Mat type combined with microfabric is the basis for a classification system presented below.

| Fabric classification
Based on a combination of mesoscale and microscale observations described above, a classification scheme was developed that combines accretionary mat type with stage of microfabric evolution.Recognition of accretionary mat type relies on the integration of mesoscale and microscale features (Table 1).Recognition of microfabric evolution relies on microscale observations that distinguish between Microfabrics 0, A, B and AB.
The new classification scheme that combines mat type and microfabric is shown in Figure 10.This scheme denotes the microbial mat responsible for the fabric type by colour and the microfabric evolution by pattern.Fabrics interpreted to be formed by pustular mats are coloured green, transitional mats are orange, smooth mats are red and colloform mats are blue.Patterns superimposed on the colours show the degree of microfabric evolution for each fabric.Original microfabrics with clotted textures are denoted by dots and laminated structures are denoted by lines.Fabrics that have undergone Micritic Thickening to form Microfabric A are represented by a thickening of the original pattern, which may be dots or lines.Fabrics that display cement infilling between laminae, forming Microfabric AB, are represented by a cross-hatched pattern.

| Fabric distribution within microbialite heads
The classification scheme presented in Figure 10 was used to map fabrics in the 45 microbialite heads collected in Hamelin Pool, and cartoons showing mapped fabrics for a polished slab of each head are presented in Figure 11 with regional context in Figure S9.The heads were coded by colours representing the microbial mat responsible for the fabric type and patterns representing the degree of microfabric evolution.Mapping focussed on the four main accretionary mat types (pustular, transitional, smooth and colloform).Fabrics that could not be assigned to one of these four mat types, most of which were associated with intermittently accreting surface films and/or macroalgal assemblages, were colour-coded grey.Two heads were selected for a more detailed description in this paper.One head from the Goat Transect (Goat_H6; shown in Figure 11, and Table S1) was accreted by a single mat type, with variations in microfabric evolution.The other head collected from the Playford Transect (Play_H1; shown in Figure 11, Table S1), contains fabrics formed by a variety of mat types and microfabrics.
Collected in the subtidal zone, microbialite head Goat_ H6 (Figure 12) was colonised by colloform mat at the time of collection.The entire head displays a laminated fabric, with steeply convex laminae forming columns, indicative of colloform mat type (Figure 12A,B,C).The upper ca 2 cm of the head exhibit fine, discontinuous laminae with horizons of micropeloidal micrite forming Microfabric 0 (Figure 12D1).With increasing depth in the head, dark micropeloidal micrite infills and thickens laminae, forming Microfabric A (Figure 12D2).At approximately 10 cm depth, botryoidal cement fills interlaminar spaces causing further thickening of layers, forming Microfabric AB (Figure 12D3).The observation of a consistent microfabric produced by colloform mat types suggests that this microbialite grew entirely within the subtidal zone.
The second head, Play_H1, was collected in the upper intertidal zone with a pustular mat displaying a hard crusty surface.Subsurface fabrics were formed by three mat types displaying Microfabrics 0-AB (Figure 13A,B,C) as follows: (1) Clotted fabrics produced by pustular mats make up the upper 2 to 4 cm of the head.Thin sections in the uppermost portion of this area show miniclots to mesoclots of micrite surrounded by fine flocculent micrite, which is characteristic of Microfabric 0 (Figure 13D1.1).Below Microfabric 0 and extending around the margin of the head is a 1 to 2 cm thick band of clotted micrite, characteristic of pustular mat fabric that has undergone thickening to form mesoclots to maxiclots characteristic of Microfabric A (Figure 13D1.2).The large, dense micritic clusters incorporate both fresh and micritised sediment grains and are surrounded by dispersed micrite (Figure 13D1.2). ( 2) Transitional mat fabrics, showing discontinuous laminae of aligned clots, form the next 6 or 7 cm of the head (Figure 13A,B,C).The laminations are composed of miniclots to mesoclots in a sediment-rich matrix (Figure 13D2).Clots enlarge with depth, forming Microfabric A (Figure 13D3).(3) The bottom 20 cm of the head display continuous laminations characteristic of smooth mat fabrics (Figure 13A,B,C).Sediments with dark micritic rims form medium-scale laminae (Figure 13D4), characteristic of Microfabric A. Sediment-rich horizons were further thickened by fringing aragonite cement (Figure 13D5), forming Microfabric AB.The fabrics displayed by this head represent a shallowing upward sequence from (i) lower intertidal (smooth mat) to (ii) intertidal (transitional mat) to (iii) upper intertidal (pustular mat).
The detailed descriptions of these two heads (Figures 12 and 13), together with the fabric maps produced for the remaining 43 heads collected from all around Hamelin Pool (Figure 11), reveal that Hamelin Pool microbialites display a bidirectional evolution of fabrics, with upward evolution representing changes in surface mat type, and downward changes representing subsurface microfabric evolution.Colours on the fabric maps show predictable upward sequences from blue, to red, to orange and finally to green, representing the upward growth of mats into shallower water.Of the 45 heads, 21 were built by a single mat type, 13 were built by two mat types and 10 were built by more than two mats (Figure 11).In heads constructed by a single mat type, four were built entirely by transitional mats, seven by smooth mats, 10 by colloform mats and one by an unknown mat type, classified as 'other' (Figure 11).No single head was built entirely by pustular mats, and pustular mats were most often observed on top of transitional mat microfabrics, with smooth mats transitioning to pustular mats observed only once (Goat_H3; Figure 11).Subsurface observation of pustular mat fabric below transitional mat fabrics was also only observed once (T1_H3; Figure 11), which may have been tipped over and moved to deeper water.Transitional mats overlie smooth mats or the initial substrate on which the microbialite nucleated, never directly overlying colloform mat fabrics (Figure 11).Many heads collected in the subtidal zone were composed entirely of colloform mat microfabrics (Playford_ H4; Goat_H6 and H7; T9a_H1, H2 and H5; Figure 11), suggesting that these microbialites grew entirely in the subtidal zone.Elsewhere, colloform mat microfabrics were overlain by smooth mat microfabrics (Figure 11); subsurface observation of colloform mat microfabrics transitioning directly to pustular mat microfabrics was observed once (T2b_H2; Figure 11), suggesting a rapid change in relative sea level.
Fabrics formed by any given mat type typically show predictable downward evolution from Microfabric 0 to Microfabric A to Microfabric AB.Microfabric A is abundant in Transects 1 and 11 in the north, and T4b, Goat, and Transect 9 in the central portions of the pool (Figure 11).In contrast, Microfabric A is less pervasive in T2, T3, T4 and T7a (Figure 11).Microfabric AB is abundant in Transects 1, 2, 4, 7a and 9 (Figure 11) and was not observed in Transects 6 or 11 (Figure 11).In two of the 45 heads, both formed by smooth mat (T3c_H1; T6b_H1; Figure 11), fabrics progressed from Microfabric 0 directly to Microfabric B (Cement Infilling), bypassing Microfabric A (Micritic Thickening).Several of the sampled heads included a base rock (depicted in black), often representing allochthonous cobbles.

FABRIC EVOLUTION
The internal fabrics of Hamelin Pool microbialites show a bidirectional evolution; an upward evolution is interpreted to reflect changes in surface mat type as the head grew upward into shallowing water, while downward evolution corresponds to changes in microfabric composition as former surface mats were subsequently buried by new generations of mats.Simplified cartoons depicting this bidirectional fabric evolution are shown in Figure 14.
The upward evolution from colloform, to smooth, to transitional, to pustular mat fabrics (Figures 11 and 13), which is associated with growth into shallower water, is consistent with observations from previous studies (Collins & Jahnert, 2014;Jahnert & Collins, 2011, 2012).This shallowing upward sequence could simply reflect upward growth of the structure or could be associated with a drop in relative sea level.Two periods of stromatolite growth in Hamelin Pool have previously been recognised based on radiocarbon dating: a first phase occurring between 2000 and 1100 years ago when sea level was higher and a second phase occurring during the last 900 years associated with a relative fall in sea level of 1.5 m along Australia's western coastline (Collins et al., 2006;Collins & Jahnert, 2014;Davies, 1970;Jahnert & Collins, 2012;Playford et al., 2013).The sea-level regression caused a seaward shift of the microbialite system leaving some microbialites stranded while also providing new substrate for deeper growth of colloform mats (Collins & Jahnert, 2014;Jahnert & Collins, 2012).As falling sea level and upward growth of a microbialite could both result in similar shallowing upward sequences, the relative contribution of each process in fabric evolution is difficult to deconvolve.Some heads that were likely stranded by the fall in sea level, and which have surface descriptions of 'other' (Table S1, Figure 11), are presently topped with bronze caps (T4b_H1), red caps (T4b_H2; Goat_H1) and black films (Playford_H1; Goat_H2).Age dating is needed to further resolve growth histories.
The downward evolution of fabrics in Hamelin Pool microbialites is a result of early taphonomic processes.It has long been recognised that progressive burial of microbial mats is accompanied by bacterial decomposition of cyanobacteria, other light-dependent organisms, and their associated EPS (Black, 1933;Dupraz et al., 2009).The decomposition of these organic components is commonly linked with calcium carbonate precipitation (Arp et al., 1998(Arp et al., , 1999(Arp et al., , 2012;;Chafetz & Buczynski, 1992;Défarge, 2011;Défarge et al., 1996;Dupraz et al., 2009;Reitner et al., 1995).Thus, early taphonomy in Hamelin Pool microbialites is interpreted to correspond to degradation of organic matter by heterotrophic communities.S2.
This interpretation could be confirmed via geochemical analyses like analyses of stable carbon and sulphur isotope ratios of the carbonate, as well as measurements of total organic carbon content.In Hamelin Pool, early taphonomy of microbialites is characterised by predictable changes in microfabric, as discussed below.

| Original architecture- Microfabric 0
Primary framework consisting of variable proportions of micrite and sediment grains is established in a thin microbial mat occupying the upper few millimetres of the microbialite.Various microbial communities within this biofilm are efficiently recycling elements, producing and consuming organic matter (cf.Braissant et al., 2009;Dong et al., 2022;Dupraz et al., 2008Dupraz et al., , 2009) ) and precipitating the first micritic products associated with the calcification of Entophysalis.Micrite precipitation is believed to occur along the edges of the cell envelopes encapsulating the coccoid colonies (Golubic, 1983;Hoffman, 1976;Horodyski & Vonder Haar, 1975;Reitner et al., 1996;Vitek et al., 2022).As illustrated by Reitner et al. (1996, plates 1-13), the strongly acidic mucilaginous substances between the polysaccharide envelopes encapsulating Entophysalis contain abundant heterotrophic bacteria and calcification may be associated with metabolic activity of these bacteria.Precipitates range from amorphous nanobulbous calcium carbonate phases to tabular and bladed micrite-sized aragonite (Vitek et al., 2022).This initial micrite, which may display micropeloidal textures with subangular porosity (Vitek et al., 2022), is suspended within an organic matrix of microbial cell tissue and EPS products.Variations in the abundance and composition of EPS may be driven by environmental conditions (Blanco et al., 2019;Decho, 2011;Kim & Chong, 2017;Suosaari et al., 2022b;Zhao et al., 2016).

| Micritic Thickening-Microfabric A
As subsequent generations of microbial mats build upward, former surface mats become subsurface features within the microbialite.Here, remaining organic matrix is degraded rapidly through the joint activity of heterotrophic degradation and the release of EPS-degrading enzymes by the expired biofilm cells, which allows for efficient recycling of nutrients (cf.Arp et al., 1998Arp et al., , 1999Arp et al., , 2012;;Défarge et al., 1996;Dong et al., 2022;Dupraz et al., 2008;Laspidou & Rittmann, 2002;Reitner et al., 1995).These processes lead to further micrite precipitation that enlarges clots and thickens laminae.Formation of micrite cement infilling pores in Hamelin Pool microbialites has been documented by Diaz and Eberli (2022), who showed that organomineralisation is actively triggered by the byproducts of autotrophic or heterotrophic metabolisms, or passively induced on EPS surfaces.In addition, EPS replacement by carbonate has been documented in various hypersaline ponds in Eleuthera and San Salvador, Bahamas (Dupraz et al., 2004(Dupraz et al., , 2013;;Glunk et al., 2011;Sforna et al., 2017).In the rock record, formation of peloidal micrite in early Cretaceous reef crusts of Spain was suggested to result from calcification of bacterial aggregates driven by decay and hydrolysis; shrinkage associated with organic decay created small water-filled spaces that became filled with microspar cement (Riding & Tomás, 2006).

AB or B
Finally, as the surface of the microbialite continues to build upward, the mostly degraded former surface mats are buried even deeper down-head within the microbialite fabric.Heterotrophic microbial activity continues, modifying the microenvironment and altering porewater chemistry, driving further lithification of the structure.Through the complete degradation of biofilms and subsequent increased accommodation space, micritic surfaces provide nucleation sites for spar cement (Diaz & Eberli, 2022;Diaz et al., 2022).These spar cements appear similar to those observed in modern reefs (Ginsburg & James, 1976;Harris et al., 1985;James & Ginsburg, (1979), with aragonite precipitating as botryoids and sparry fringes in open cavities; there is no evidence that botryoids are replacing micrite, as described by Ge et al. (2021).In addition to the spar cements, occasional occurrences of irregular accumulations of micropeloids and associated microspar in cavities may represent the final decay of residual bacterial biofilms (Chafetz, 1986;Diaz & Eberli, 2022;Diaz et al., 2022;Dupraz & Strasser, 2002;Reitner, 1993;Riding, 1991Riding, , 2000) ) that survived degradation that occurred during the stage of Micritic Thickening.Studies showing the presence of fatty acids of bacterial origin within similar peloidal precipitates and stable carbon isotope values F I G U R E 9 Schematic cartoons shown in false colours and emphasising components that depict the downward evolution of fabrics.Pie charts show the percentages of each component.Percentages of sediment and micrite in the original framework are listed below the pie charts; (for pustular, smooth and colloform see Vitek et al., 2022; for transitional see Figure S2).(A) Pustular mat fabrics (A1) Original fabric composed of miniclots to mesoclots of micrite surrounded by dispersed micrite.(A2) Shows an evolution of the Original fabric to Microfabric A, composed of mesoclots to maxiclots that are enlarged as a result of additional micrite precipitation and the incorporation of dispersed micrite into the clots.In addition, new patches of dispersed micrite may precipitate in the subsurface.(B) Transitional mat fabrics (B1) Original fabric composed of aligned miniclots to mesoclots of micrite and dispersed micrite forming irregular laminae.(B2) The evolution of Original fabric to Microfabric A, composed of mesoclots to maxiclots of micrite that enlarged as a result of additional micrite precipitation and the incorporation of dispersed micrite into the clots.(B3) Shows an evolution of Microfabric A to Microfabric AB, with the addition of spar cement and minor peloidal micrite precipitating in pore spaces.interpreted to reflect carbon isotope fractionation by microbial production of organic matter are evidence for precipitation of carbonate within and around clumps of bacteria (Chafetz, 1986).Peloidal precipitation was thus interpreted to have been induced by the vital activity of bacterial colonies.Such micropeloid accumulations represent a proportionally minor fraction of the carbonate deposited during this stage of microfabric evolution and were only occasionally observed in microfabrics mapped as AB in the Hamelin Pool microbialites.More work is needed to define taphonomic processes and potential environmental controls on taphonomic alteration and accretionary mat type.For instance, geochemical analyses of original architecture and taphonomic products could help to define taphonomic processes, while examination of regional trends in environmental conditions may elucidate relationships between hydrodynamics and taphonomic evolution.While tidal zone is important in determining the distribution of accretionary mat types, detailed analyses of complex seasonal and regional trends in sea-level variation within Hamelin Pool (Burne & Johnson, 2012;Suosaari et al., 2016b), would be critical to more accurately relate accretionary mat type to tidal zone position.Similarly, comparison of taphonomic fabrics with the regional bathymetric gradients and energy profiles (Table S1) does not indicate obvious relationships between taphonomic alteration and hydrodynamics, except the observation that cement infilling to form Microfabric AB was not observed in low energy areas, such as Transects T6a and T11 (Figure 11).More detailed analysis, including correlation of fabrics with bathymetric profiles and energy gradients localised to sampled heads, could reveal other regional trends.S1. generation of accreting mat.Despite taphonomic evolution, results indicate that the original mat type can still be recognised and mapped within the heads.Thus, the subsurface fabrics serve as modified fingerprints of original surface mats.

Mesoscale
Results also update work by Reid et al. (2003), which was the first study to describe extensive micrite in Hamelin Pool microbialites.Reid et al. (2003) recognised two distinct fabrics: grainy microbialites termed calcarenite, and muddy microbialites, termed micritic; these fabrics were further classified based on the presence or absence of lamination.Based on results from the present study, fabrics described by Reid et al. (2003) have been re-evaluated, most of which were regarded as primary depositional textures, to incorporate new insights into bidirectional fabric evolution and taphonomy (Figure S10).Overall, the present study affirms that micrite is an important component of Hamelin Pool microbialites as previously concluded (Reid et al., 2003), while simultaneously evolving the original conclusions by distinguishing between original and secondary fabrics.In addition, these new results enable a reassignment of the calcarenite and micritic fabric designations to the traditional terminology of pustular, smooth and colloform mat fabrics (Jahnert & Collins, 2012;Logan et al., 1974;Playford, 1990;Playford et al., 2013;Suosaari et al., 2016a).
Extending beyond Hamelin Pool, the present study endorses the potential utility of microbialites as a general indicator of water levels.Large-scale distribution of microbialites has traditionally been interpreted as an indicator of the littoral zone (cf.Bouton et al., 2016;Eymard et al., 2019;Osborne et al., 1982;Surdam & Wray, 1976;Vincens et al., 1986).In lake systems, successions of microbialites have previously been used as a proxy for palaeolake stabilisation levels (cf.Benson et al., 1990;Casanova, 1987;Casanova & Hillaire-Marcel, 1992;Hillaire-Marcel et al., 1986).With improved correlation of energy and tidal regimes, microbialite fabrics might be useful in reconstructing short-lived or fine-scale events from a sequence stratigraphic perspective, acting as true 'living stratigraphic columns'.Future work linking microbialite microfabrics to water levels both within Shark Bay and beyond is warranted.
Taphonomic evolution of fabrics, recognised in this study in Hamelin Pool microbialites, can also be considered in a broader context.While microbialite taphonomy was observed to enhance the original architecture produced by surface mats in Hamelin Pool, taphonomic evolution was shown to result in the destruction of the original architecture in some structures from the Exuma Cays Bahamas (Planavsky & Ginsburg, 2009).Planavsky and Ginsburg documented a downward evolution from laminated structures at the tops of heads to unlaminated, or thrombolitic, fabrics at depth.These authors concluded that variations in the amount and style of 'penecontemporaneous diagenesis', rather than differences in surface communities were the primary cause of fabric evolution, suggesting that remodelling of the laminated fabric by physical and metazoan disruption, micritisation, secondary cementation and localised carbonate dissolution produced irregular, clotted fabrics at depth (Planavsky & Ginsburg, 2009).
Differences in the taphonomic evolution of microbialites in the Bahamas and Hamelin Pool reflect differences in accretionary mat types and abundance (or paucity) of metazoans.In Hamelin Pool, laminated and clotted fabrics are primary fabrics associated with specific accretionary mat types, whereas clotted fabrics in Bahamian microbialites are considered to be diagenetic alteration of fabrics that were originally laminated.In addition, in the Bahamas, metazoans such as boring sponges, bivalves and worms are abundant in slabbed sections and disrupt original laminated fabrics (Planavsky & Ginsburg, 2009).In contrast, metazoans were rarely observed in slabbed sections of Hamelin Pool microbialites.Although not completely excluded (Edgcomb & Bernhard, 2013;Skyring & Bauld, 1990), metazoans are limited by the hypersaline environment of Hamelin Pool (Playford & Cockbain, 1976).Future studies of early taphonomy in other microbialite settings should thus consider whether early taphonomy enhances or obscures original microfabrics.
Fabric evolution of Hamelin Pool microbialites has further implications for biosignature formation and preservation, in particular geochemical signatures.As microbialites are products of biology and the environment, geochemical records from microbialites may provide critical insights into the co-evolution of life and the Earth system as a whole (i.e.Dietrich et al., 2006;Dong et al., 2022;Lenton et al., 2014;Lepot, 2020).Some authors advocate that the role of microbial community metabolisms active during accretion can be interpreted as a chemical biosignature, through stable carbon, oxygen, sulphur, nitrogen, iron and molybdenum isotope ratios and elemental concentrations, like rare earth elements (Andres & Reid, 2006;Bontognali et al., 2012;Corkeron et al., 2012;Giovanna, 2015;Johannesson et al., 2014;Lepot, 2020;Londry & Des Marais, 2003;Stüeken et al., 2016;Thomazo et al., 2011;Valdivieso-Ojeda et al., 2014).In contrast, others interpret the geochemistry of microbialites to represent high-fidelity archives of sea water chemistry, redox and environmental conditions through time (Chagas et al., 2016;Hohl et al., 2015;Hohl & Viehmann, 2021;Petrash et al., 2016;Voegelin et al., 2009;Webb & Kamber, 2000).Understanding the evolutionary history of a microbialite in terms of initial architecture and early taphonomy will therefore be critical for valid interpretation of both morphological and geochemical signatures.The geochemical composition of accretionary products produced in an active surface mat at the top of a structure could be strongly modified by early taphonomic processes, and the chemistry of products formed prior to burial or subaerial exposure must be differentiated from those formed during later diagenesis.Disentangling fabric evolution is critical to understanding what is being captured in the rock record: depositional environment and initial architecture, early taphonomy or later stage diagenetic alteration.
Finally, overall insight into microbialite fabric evolution in Hamelin Pool provides a framework for interpreting microbialites of all ages.For example, spectacular modern microbialites at Lago Sarmiento, southern Chile, show subsurface textural alteration that reflects early taphonomy (Airo, 2010).Isolated carbonate granules embedded in thick (1 cm) mucous-rich mats of filamentous cyanobacteria at the surface of the structures undergo a gradual amalgamation to form a solid thrombolitic framework of micropeloidal micrite and microspar within a few centimetres depth, obliterating any original stratification (see figure 28 in Airo, 2010).A similar taphonomic textural alteration from sediment-rich filamentous mats with sparse precipitates at the surface to clotted textures within 1 to 2 cm depth, suggested to be associated with EPS degradation, is apparent in Cretaceous stromatolites of the Leza Formation illustrated by Suarez-Gonzalez et al. (2019; Figure 13).Additionally, early Archean stromatolite 'pseudocolumns' show a systematic change in fabrics from the base to top, with chert-filled laminoid fenestrae in the base (Allwood et al., 2009).Are these changes in fabric related to changing environmental conditions associated with upward growth of the structures, or is this a downward taphonomic evolution?Investigations in a well-studied modern system like Hamelin Pool can better constrain interpretations derived from microbialite carbonate archives, spanning billions of years of geological time.

CONCLUSIONS
Documentation of bidirectional fabric evolution in microbialites of Hamelin Pool holds the following new insights: 1.Each microbialite-building mat type in Hamelin Pool (pustular, transitional, smooth and colloform mats) forms distinct mesoscale and microscale fabrics, including micritic clots and laminae.2. Microfabrics formed by each mat type show a downward evolution as surface mats transition into the subsurface of a head.Micrite precipitation below the surface enhances original clots and laminae, making them larger and denser, in a process referred to as 'Micritic Thickening'.Subsurface precipitation is thought to be associated with organic decay.Microspar and spar precipitate in void spaces left by degrading organic matter in a process referred to as 'Cement Infilling'.3. Mat type combined with microfabric formed the basis for a fabric classification system that was used to map fabrics within heads around Hamelin Pool. 4. The upward evolution of fabrics corresponds to changes in surface mat type as the head grows upward into shallowing water.Unexpected sequences of microfabrics could provide critical information about sea-level changes. 5. Downward fabric evolution corresponds to changes in microfabric induced by the progressive decay of microbial organic matter in the subsurface of the head.This early taphonomic evolution represents an important stage of microbialite growth occurring between surface processes responsible for the construction of the original architecture and the onset of diagenesis.Hamelin Pool microbialites present a unique opportunity to constrain the morphological and geochemical changes associated with early taphonomy without the confounding impacts of diagenetic alteration.6.The early taphonomic evolution described in this study is relevant for the interpretation of modern microbialites across the globe.These findings indicate that a commonly held perception that modern microbialite microfabrics are fingerprints of accretionary microbial mats is an oversimplification.In many cases, microbialite microfabrics are likely fingerprints of accretionary mats modified by early taphonomy.

ACKNO WLE DGE MENTS
We thank Phil Playford for inspiration and guidance; the Geological Survey of Western Australia; Hamelin Station (during the tenure of Brian and Mary Wake, and Bush Heritage Australia) and multiple competent field assistants for logistical support; the Western Australian Department of Parks and Wildlife (formerly Department of Environment and Conservation) and the Federal Department of Sustainability, Environment and Population and Communities for field access and sampling permits; Barry Kayes at Aradon for assistance in microbialite slab preparation; Paul Hagan for initial petrographic analysis, Gregor Eberli for early thoughtprovoking discussions and Tim Gooding for assistance with SEM.We acknowledge Malgana as the traditional custodians of Gathaagudu and their connections to land, sea and community.We pay our respects to their elders past and present and extend that respect to all Aboriginal and Torres Strait Islander peoples.The manuscript benefitted from detailed reviews by Robert Madden, Pablo Suarez-Gonzalez and an anonymous reviewer.This project was funded by Chevron, BP, Repsol and Shell.#10 in the Hamelin Stromatolite Contribution Series (publications by a University of Miami-led research consortium).

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I G U R E 3 Terminology used to describe clots (A) and laminae (B).Adapted from Grey and Awramik (2020).

F
I G U R E 4 Mesoscale features as observed on polished slabs (left) and corresponding thin section scans (right).(A) Clotted mesofabric formed by pustular mat.Polished slab appears unlaminated.Thin section scan shows miniclots and mesoclots above the dashed red line and mesoclots and maxiclots below the dashed red line.Sample HP12_Goat_H3.(B) Mesofabric formed by transitional mat.Polished slab appears unlaminated.Thin section scan shows aligned clots forming discontinuous laminae; miniclots and mesoclots occur above the dashed red line and mesoclots and maxiclots occur below the dashed red line.Sample HP12_T4b_H4.(C) Laminated mesofabric formed by smooth mat.Polished slab and thin section scan appear laminated.Thin section scan shows a thickening of laminae from fine (above dashed red line) to medium (below dashed red line).Sample HP12_Goat_H1.(D) Laminated mesofabric formed by colloform mat.Polished slab and thin section scan appear laminated.This section scan shows discontinuous fine laminae above the dashed red line and continuous medium laminae below dashed red line.Sample HP12_Play_H4.F I G U R E 5 Photomicrographs illustrating Original Architecture, designated Microfabric 0, formed by each mat type.(A) Unlaminated microfabric formed by pustular mat with small micritic clots (<2 mm in diameter, red arrows in (A) and dispersed micrite (black arrows in B); (B) shows the boxed area in (A); thin section HP12_Goat_H3.1.(C) Weakly laminated microfabric formed by transitional mat with aligned micritic clots forming discontinuous horizons in the sediment-rich matrix; (D) shows the boxed area in (C); thin section HP12_Play H1.3.(E) Laminated microfabric formed by smooth mat with continuous horizon of micrite; (F) shows the boxed area in (E); thin section HP12_T2b_H3.1.(G) Laminated microfabric formed by colloform mat with convex upward micritic horizons; (H) shows boxed area in (G); thin section HP12_Goat_H7.1.

4. 2 |
Pathways of microfabric evolution by mat typeSchematic cartoons synthesising the downward evolution of microfabrics formed by the four different mat types are shown in Figure9, and Figures S5 through S8 .

F
Photomicrographs illustrating Micritic Thickening of clots and laminae to form Microfabric A. Red dots in A, B and J indicate locations for SEM imagery shown in Figure 8. (A) shows a clot, several millimetres in size, that has become enlarged and denser due to precipitation of reddish brown to dark micrite and microspar within pore spaces, creating a mottled appearance; (B) shows the boxed area in (A); thin section HP12_T1_H3.3.Compare with the small porous clots in Figure 5A,B.(C) Dispersed micrite adjacent to dense clots.In (D), which is the boxed area in (C), the translucency of the micrite at the edge of the clot (arrows) suggests that the dispersed micrite is becoming incorporated into the clot; thin section HP12_Play_H1.1.(E) Thick dense laminae with irregular patches of red-brown micrite creating a mottled texture.(F) The boxed area in (E); thin section HP12_Play H1.8.Compare with the thin, porous laminae in Figure 5E through H). (G) Thick dense laminae with mottled textures of red-brown and grey micrite.(H) The boxed area in (G); thin section HP12_ Goat H7.1.Compare with the thin, porous laminae in Figure 5E through H). (I) Thick dense laminae composed of grains rimmed with dark micrite and microspar fringe cements; dark micrite also encompasses granules of golden micrite (arrows in J). (J) Boxed area in (I); thin section HP12_Goat_H5.3.

F
Photomicrographs illustrating Cement Infilling by spar and micropeloidal micrite to form Microfabric AB.Red dots in (A), (B) and (H) indicate locations of SEM images shown in Figure 8. (A) Botryoidal aragonite infilling interlaminar pore spaces, fusing and further thickening laminae, and sometimes obliterating the layered structure.(B) Boxed area in (A); thin section HP12_T9_H1.3.(C) Acicular aragonite crystals, some in fan-shaped bundles, forming fringing cement in empty pores.(D) Boxed area in (C); thin section HP12_T2b_H1.4.(E) Dark micropeloidal cement infilling cavity.Peloids about 50 μm in diameter have fuzzy borders and are rimmed with microspar.(F) Boxed area in (E); thin section HP12_T9_H1.3.(G) Dark red micropeloidal cement infilling.Borders of the red peloids are more distinct than the fuzzy dark peloids in (E, F) but are also rimmed with microspar.(H) Box in (G); thin section HP12_T9_H2.3.

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I G U R E 8 SEM showing crystal morphology of microfabric components.(A) Red-brown micrite composed of nanobulbous, equant, bladed and acicular aragonite crystals less than 10 μm in size from sample HP12_T1_H3.3 (see Figure 6A #1 for location).(B) Grey micrite composed of nanobulbous, equant and bladed aragonite crystals less than 4 μm in size from sample HP12_T9_H1.3 (see Figure 7A #1).(C) Red-brown micrite composed of nanobulbous, equant, bladed and acicular aragonite crystals less than 10 μm in size from sample HP12_T9_H2.3 (see Figure 7H #2).(D) Dark micritic rims composed of equant and acicular aragonite crystals up to 10 μm in size from sample HP12_T1_H3.3 (see Figure 6B #2).(E) Dark micrite composed of acicular aragonite crystals less than 4 μm long from sample HP12_Goat_H5.3(see Figure 6J #3).(F) Botryoidal aragonite cement from sample HP12_T9_H1.1 (see Figure 7B #3).(G) Isopachous fringe cement composed of acicular aragonite crystals (ca 2 μm in width) from sample HP12_T9_H2.3 (see Figure 7H #4).(H) Dark micropeloidal aragonite cement composed of nanobulbous, equant or acicular crystals less than 4 μm in size from sample HP12_T9_H2.3 (see Figure 7H #5).EDS quantifications are available in Table (C) Smooth mat fabrics (C1) Original fabric showing fine, continuous laminae composed of micrite and sediment.(C2) Shows an evolution of the Original fabric to Microfabric A, highlighting the evolution of pre-existing fine laminae thickened into medium-scale laminae by the addition of micrite.(C3) Shows an evolution of Microfabric A to Microfabric AB with spar cement and minor peloidal micrite precipitating in pore spaces.(D) Colloform mat fabric (D1) Original fabric showing steeply convex, discontinuous, fine laminae.(D2) An evolution of the Original fabric to Microfabric A; pre-existing laminae are thickened to form continuous, medium-scale laminae by the addition of micrite.(D3) An evolution of Microfabric A to Microfabric AB; aragonite cement, commonly as botryoidal cement, is precipitated along the underside of laminae.Minor peloidal micrite may also be present infilling pores.Corresponding cartoons showing comparisons with cartoons drawn in the approximate colour of planepolarised light and used for Figure 10 are shown in Figures S5 through S8.T A B L E 1 Distinguishing features of accretionary mat types: Original and early taphonomic fabrics.
Recognition of bidirectional fabric evolution within the microbialite heads of Hamelin Pool adds new perspectives F I G U R E 1 0 Proposed fabric classification scheme.Colours indicate accretionary mat type, arranged in columns; patterns indicate taphonomic stage, arranged in rows.Arrows indicate dominant pathways of fabric evolution.Schematic cartoons depict fabrics as diagrammed in Figures S5 through S8.Abbreviations for the initial microfabric of each mat type are coded as MatType 0 (for example, Sm 0 refers to the original architecture produced by smooth mat), Micritic Thickening forming Microfabric A is coded as MatType A (for example, Sm A ), and Micritic Thickening with cement infilling, Microfabric AB is coded as MatType AB (for example, Sm AB ).See text for further details. to understanding microbialite growth in Hamelin Pool and beyond.In Hamelin Pool, the results extend work by Vitek et al. (2022), which documented initial fabrics in pustular, smooth and colloform surface mats.New results document the downward evolution in initial architecture as the former surface mats are covered by a new F I G U R E 1 1 Mapped fabrics in 45 microbialite heads, arranged by transect.For legend of colours and symbols, see classification scheme in Figure 10.For transect locations, see Figure 1 and Figure S9.For surface descriptions, see Table

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Detailed fabric mapping of a head formed entirely by colloform mat-HP12-Goat-H6.See text for details.(A) Polished slab.(B) Thin sections scans, from boxed areas in (A).(C) Mapped fabrics; see Figure 10 for explanation of colours and patterns.(D) petrographic images showing details of boxed areas in (B); abbreviation indicating microfabric is shown at the bottom of each image.F I G U R E 1 3 Detailed fabric mapping of a head formed by multiple mat types-HP12_Play_H1.See text for details.(A) Polished slab.(B) Thin sections scans, from boxed areas in (A).(C) Mapped fabrics; see Figure 10 for explanation of colours and patterns.(D) Petrographic images showing details of boxed areas in (B); abbreviation indicating microfabric is shown at the bottom of each image.

F
Cartoons depicting bidirectional fabric evolution in Hamelin Pool microbialites.(A) Directionality of processes: Left arrow indicates upward evolution corresponding to changes in accretionary mat type as the head grows up into shallow water; right arrow shows downward evolution corresponding to taphonomic stages accompanying organic degradation.(B) Fabrics in an idealised microbialite head showing upward evolution from laminated structures to clotted fabrics accompanying upward growth of mats into shallower water (left arrows), combined with downward evolution of microfabrics accompanying downward degradation of organic matter produced by each mat type as surface mats move into the subsurface (right arrows).