Ministromatolite Modular Reef Framework, ~1.5 Ga, Yesanpo, North China

ABSTRACT

Earth's earliest known framework reef, ~1500‐million‐year‐old Yesanpo Mound in the Wumishan Formation, North China, consists of columns and cones built by innumerable conjoined originally aragonitic ministromatolites, each ≤ 2 cm wide. The Mound contains numerous irregular, decimetric, cavities. These are now filled with carbonate mud, giant ooids, and originally aragonitic marine cement. The role of ministromatolites in constructing this modular framework very broadly resembles that of polyps in present‐day scleractinian coral colonies. Yesanpo Mound preserves geochemical signatures and traces of organic filaments consistent with suboxic conditions, yet it predates the earliest reported reef‐dwelling animals by ~600 Myr. Similar spaces in reef framework may have provided ideal habitats for cryptic biota during the late Proterozoic. These are prime targets in the continued search for early animal life.

1. Introduction

Marine reefs—in place calcareous deposits created by sessile organisms (Riding 2002)—have a history that can be traced from the appearance of stromatolites in the Archean, and their increase in the Proterozoic, to the rise—and ultimate dominance—of algae and invertebrates in the Phanerozoic (Fagerstrom 1987; Riding 2002; Kiessling et al. 2003; Stanley 2003; Webb 1996; Wood 1999, 2011). Many microbial reefs have relatively dense interiors, whereas algal‐invertebrate reefs can be self‐supporting cavernous frameworks. For example, large present‐day microbial columns at Shark Bay, Australia, characteristically have illuminated external surfaces and dense dark interiors (B. W. Logan 1961; Logan et al. 1970; Playford et al. 1976). In contrast, coralline algal‐scleractinian coral reefs with branching skeletons that grow upward and outward, can create irregular framework with dark cavernous interior voids and illuminated outer surfaces (Ginsburg and Schroeder 1973). These cavities provide protective habitat for mobile and sessile organisms that tolerate, or prefer, less‐illuminated—and often less‐oxygenated—conditions. Growth cavities such as these often constitute 30%–75% or more of total reef volume in present‐day reefs (Ginsburg 1983). As a result, cryptic substrates may equal or exceed illuminated reef surfaces in area (Buss and Jackson 1979; Garrett et al. 1971; Ginsburg 1983; Gischler and Ginsburg 1996; Jackson et al. 1971; Jackson and Winston 1982; Kobluk 1988; Kobluk and van Soest 1989; A. Logan 1984; Pearman et al. 2018; Richter et al. 2001; Steyaert et al. 2022). Cryptic habitats help to make present‐day coral reefs one of Earth's most biodiverse environments (Scheffers et al. 2010).

The connection between skeletal reef cavities and cryptic invertebrate diversity can currently be confidently traced back to near the Proterozoic‐Cambrian transition (Kobluk and James 1979; Penny et al. 2014; Riding and Zhuravlev 1995; Saylor et al. 1995; Zhuravlev and Wood 1996); as, for example, in very small Cloudina framework within ~0.55 Ga Ediacaran thrombolite reefs (Penny et al. 2014). Even earlier examples may include sponge‐like fabrics in millimetric cavities within microbial reefs ~0.89 Ga (Turner 2021). Prior to this, throughout much of the Precambrian, cryptic biota has rarely been reported, and reefs appear to have been almost entirely microbial (Cantine et al. 2020). Currently, the oldest known cryptobionts are microscopic bacterium‐like fossils within millimetric cavities ~1.89 Ga (Hofmann and Grotzinger 1985). Much larger—centimetric—cavities within ‘tented’ microbial fabrics are locally common in the Late Archean, Paleoproterozoic (Sumner 1997, 2000), and Mesoproterozoic (Bartley et al. 2015; Tang et al. 2017).

Here we describe an example of cavernous microbial reef framework in the ~1.5 Ga Wumishan Formation, at Yesanpo, North China (Figure 1). In this relatively small lensoid deposit, ~3 m thick and ~15 m wide, the principal components are very small, ≤ 2 cm, ministromatolites. Despite their small size, these constructed a relatively large, and rigid, carbonate framework of decimetric columns and cones, traversed by irregular centimetric spaces open to the reef surface. Ministromatolites are well‐known components of extensive biostromes and large domes in the Paleoproterozoic and Mesoproterozoic (e.g., Donaldson 1963; Grotzinger and Kasting 1993). However, so far as we are aware, Yesanpo Mound is the first example to be described in which ministromatolites formed interconnected cavernous reef framework. We propose that this prolonged incremental growth, by replication of small sub‐units (ministromatolites), reflects modular increase, and is very broadly comparable with that of present‐day scleractinian coral polyps. We infer that modular growth may have allowed ministromatolite accretion to respond to local variations in conditions, such as light, water flow, and internal sedimentation, although direct evidence for these responses is currently lacking. We suggest that—later in the Proterozoic and under oxygenated conditions—similar complex frameworks with relatively large protected crevices could have provided ideal pre‐formed cryptic habitat for early metazoans.

FIGURE 1.

Geological setting, outcrop sketch, and field photograph of Yesanpo reef at the base of Member 3 of the Mesoproterozoic Wumishan Formation, Yanliao Basin, northern China. (a) Geological setting of the Mesoproterozoic Wumishan Formation. Left: Extent of Wumishan Formation in the Yanliao basin, northern China (modified after Wang et al. 1985). Right: Generalized stratigraphic column showing the horizon of Yesanpo reef at the base of Wumishan Member 3. (b) Yesanpo Mound erosively overlies wavy laminite at the top of Member 2 and is conformably overlain by parasequence Unit B (thin bedded cherty dolostone). Parasequence Unit C—typically micritic dolostone—appears to be absent at this section. The scale only applies to the reef and the adjacent underlying units. (c) Field photograph of the same view as panel (b) showing Yesanpo reef directly overlying bedded carbonates at the top of Wumishan Member 2.

1.1. Digitate Stromatolite Bioconstructions

Narrow, vertically elongate, finger‐like (digitate) laminated columnar carbonates in north‐west Russia were named Gymnosolen by Steinmann (1911), who thought they were coelenterates. Pia (1927) recognized Gymnosolen as a distinct stromatolite morphotype, and Cloud (1942) discussed numerous examples of Gymnosolen ‘digitate stromatolites’ of various ages. A variety of terms has since been used to describe branching in stromatolites (e.g., branched/branching, dendritic, digitate), together with qualifiers for size (mini‐, micro‐, macro‐) (e.g., Hofmann 1969; Walter 1972; Grey and Awramik 2020; Shapiro and Wilmeth 2020). Here we use ministromatolite as defined by Hofmann and Jackson (1987): a small digitate stromatolite ~200 μm to 20 mm in width. Hofmann (1969) designated even smaller columns as ‘microstromatolites’, ‘because their characteristics can only be properly resolved under the microscope’. Similar morphologies, but without specific size attributes, have also been termed microdigitate (Grotzinger and Hoffman 1983; Grotzinger and Kasting 1993). Ministromatolites have variously been regarded as: (1) organic (e.g., Donaldson 1963), (2) essentially abiogenic products of seafloor precipitation (Grotzinger and Kasting 1993), and (3) combinations of inorganic and organic processes (Hoffman 1975; Hofmann and Jackson 1987). It has long been recognized that ministromatolites are widely distributed and locally common in shallow‐water Paleo‐Mesoproterozoic carbonates. Ministromatolite column arrangement can range from isolated and laterally linked to laterally conjoined (pseudocolumns) (Donaldson 1963; Hoffman 1975; Medvedev et al. 2005; Mei 2008; Tang et al. 2013c; Ramsay et al. 2025, fig. 2). They form biostromal sheets (e.g., Donaldson 1963) on tidal flats (Hoffman 1975; Grotzinger and Hoffman 1983; Grotzinger and Read 1983), and also participate—with larger stromatolites—in constructing meter‐scale steep‐sided domes (e.g., Twenhofel 1919; Knight and Keefer 1966; Hofmann and Snyder 1985; Hofmann and Davidson 1998).

Ministromatolites in the Wumishan Formation have often been given Linnean names (Cao and Liang 1974; Liang et al. 1984; Qiu and Liang 1993; Cao and Yuan 2006, 344–345), and they form irregular cones and branching columns north‐east of Beijing (Tang et al. 2013a). In Yesanpo Mound—in addition to locally forming conjoined columns and tent‐like morphotypes—ministromatolites created irregular subvertical to locally overhanging framework. In these examples, the degree of ministromatolite lateral linkage (isolated, conjoined) largely determined overall geometry of the resulting macrofabric. Evidence of biogenicity in ministromatolites has been reported in silicified Wumishan examples based on cell‐like coccoid structure (Tang et al. 2013b; Yuan et al. 2022; Yang et al. 2022). Silicified cellular microfabric and organic filaments are also locally preserved in Wumishan giant ooids (Tang et al. 2015; Shi et al. 2024). We have not observed cell‐like structure in our samples, but organic filaments occur locally in fabrics that were originally aragonite (see Section 4.5). Broadly, we interpret Yesanpo ministromatolites as likely products of both organic and inorganic CaCO₃ precipitation (i.e., hybrid carbonates; Riding 2008; Riding and Virgone 2020). Lee and Riding (2026) described Cambrian ministromatolites forming thrombolite clots—as in Aitken's (1967) classic thrombolite—through gradation to ‘ghost’ forms in which macroscopic lamination is indistinct or absent. Locally, lamination in Yesanpo ministromatolites is also indistinct, and similar examples in the area have been described as thrombolite (Tang et al. 2013a).

2. Wumishan Formation

The Wumishan Formation (~1.52–1.47 Ga; Li et al. 2014; Sun et al. 2025) is up to ~3.3 km thick (Chen et al. 1980) and extends laterally for hundreds of kilometers in the mountainous region (Taihangshan, Yanshan) west‐southwest and north‐northeast of Beijing (Cao 1991; Chen et al. 1980; Mei and Tucker 2013; Zhu 1982) (Figure 1a). Wumishan is likely one of Earth's largest well‐preserved carbonate platforms. During this interval, North China is interpreted to have been part of the Nuna supercontinent (Zhang et al. 2012). The Wumishan Formation, named by Kao et al. (1934), is conventionally divided into four members (Chen et al. 1980), each containing innumerable laterally persistent disconformity‐bound upward‐shallowing peritidal parasequences (Mei and Tucker 2013). At Yesanpo, 85 km WSW of Beijing, these parasequences—individually typically at least several meters thick—generally consist of three units: (A) lower thrombolite, (B) middle chert‐rich laminite, (C) upper argillaceous dolostone (Tang et al. 2013a, 2013b, 2013c).

3. Samples and Methods

Yesanpo Mound is exposed on the north side of Yesanpo Main Road (coordinates 39° 40′ 4.02″ N, 115° 27′ 29.51″ E), approximately 800 m east‐southeast of Sanpozhen Railway Station, and 85 km west‐southwest of central Beijing. Thirteen samples collected from the mound were cut into slabs. From these, only fresh, central parts were selected for thin‐sectioning and powdering. Thin sections were prepared from separate fresh chips for petrographic examination to identify primary and secondary components. Microfabrics were observed using a Stereo Discovery V20 microscope for large‐area observation, and a Zeiss Axio Scope A1 microscope for high‐magnification imaging. Fluorescence microscopy was performed using a Zeiss Axio Scope A1 with a mercury lamp (UV excitation 365 nm). Images were acquired with a color charge‐coupled device (CCD) camera; the blue fluorescence shown is true color. For geochemical analysis, in order to sample the least altered fabrics that best preserve a seawater signal, powder was micro‐drilled from selected fresh chips using a dental drill equipped with a diamond‐impregnated bit. The drilled material was ground to ~200 mesh in an agate mortar, washed with Milli‐Q water, and dried, to remove soluble inorganic salts as much as possible.

For each sample, a separate aliquot of powder was used for iodine/major element and rare earth element and Yttrium (REE + Y) analyses, because the two protocols employ different digestion acids (3% HNO₃ vs. 0.3 M acetic acid). Analyses of iodine (I) and major elements (Ca, Mg, Mn, Sr) were conducted following the methods described by Xu et al. (2025), using an Inductively Coupled Plasma Tandem Mass Spectrometer (ICP‐MS/MS; Thermo iCAP TQ) at the National Research Center for Geoanalysis in Beijing. Approximately 24 mg of dried powder was dissolved in 4 mL of 3% HNO₃, and centrifuged. For iodine analysis, a 1 mL aliquot of the supernatant was mixed with 1 mL of 3% tertiary amine solution (for iodine stabilization; Lu et al. 2010) and 4 mL of Milli‐Q water, prior to measurement. Analyses were conducted within 24 h of sample preparation, and the sensitivity was tuned to ~50 kcps for a 1 ppb iodine standard. For major element analysis, 1 mL of supernatant was diluted with 9 mL of 3% HNO₃. Analytical precision for iodine, based on replicate analyses of the dolostone standard JDo‐1, was better than 5.4% (Table S1), and the detection limit of I/(Ca + Mg) was on the order of 0.1 μmol/mol.

Rare earth elements and yttrium (REE + Y) were analyzed at the National Research Center of Geoanalysis, Beijing. Samples were digested following Zhang et al. (2015) and measured using ICP‐MS/MS. Approximately 100 mg of rinsed powder was digested with 0.3 M acetic acid. Digestion was performed stepwise, dissolving ~30% of the carbonate at each step, based on Ca and Mg contents. The first fraction was discarded, and the second fraction was collected for analysis to minimize potential terrigenous contamination. After centrifugation, the supernatant was transferred to a polytetrafluoroethylene (PTFE) vessel, mixed with 0.5 mL of concentrated HNO₃, evaporated to dryness, re‐dissolved in 0.5 mL of sub‐boiling HNO₃, and diluted with Milli‐Q water prior to measurement. The accuracy, based on replicate analyses of reference material JDo‐1 (Imai et al. 1996), was generally better than 80% (Table S2). REE + Y data were normalized to Post‐Archean Australian Shale (PAAS; Pourmand et al. 2012) and denoted by the subscript SN. To avoid a spurious negative Ce anomaly, due to La enrichment, Ce anomalies were calculated as Ce_SN/Ce_SN* = Ce_SN/(Pr² _SN/Nd_SN) (Lawrence et al. 2006).

4. Yesanpo Mound

4.1. Location and Stratigraphy

Yesanpo Mound occurs at the base of Wumishan Member 3. The mound is essentially flat‐based, lensoid, up to ~3 m thick and ~15 m wide in outcrop (Figures 1b,c and 2a), and erosively overlies wavy laminite at the top of Wumishan Member 2. It represents parasequence Unit A at the base of Wumishan Member 3, and is conformably overlain by parasequence Unit B, composed of ~1.5 m of interbedded thinly bedded dolostone and lenticular chert layers (Yuan et al. 2022). Unit C—which in this area is commonly micritic dolostone—appears to be absent in this section.

FIGURE 2.

Field photographs of Yesanpo Mound at the base of Wumishan Member 3, showing mound morphology, internal structures, and cavity fills. (a) View north‐east, shows overall lensoid mound shape and complex internal structure. The flat mound base erosionally overlies bedded lithotypes, at the top of Member 2, which consist of wavy laminite (dark) and argillaceous dolostone (light). (b) Complex columns composed of numerous ministromatolites, internally patterned by narrow irregular sub‐radial vugs. (c) Detail of panel b, narrow cavity filled by pale secondary dolomite and buff‐colored chert. (d) Columns, left and right, with intervening interspace filled by pale secondary dolomite and crossed by bridges (lower part), and with partially replaced giant ooids (upper part). The high‐relief, buff‐colored, chert is secondary. (e) Outcrop photograph of columns in the lower part of Yesanpo Mound, showing primary cavities (shaded in black) between sub‐erect, closely spaced microbial columns. These framework cavities range from a few millimeters to approximately 6 cm in width. (f) Same photograph as panel (e) showing that the cavities are now occupied by both replacive pale gray fine‐grained dolomite (low relief) and coarse replacive chert (high relief).

4.2. Components, Structure, and Fabrics

Prominent primary macroscopic components of Yesanpo Mound include microbial cones and domes, together with distinctive steep sided asymmetric lobate columns, 10–30 cm high and up to 15 cm wide, as well as tent‐like structures, all locally separated by narrow irregular cavities filled by fine‐grained carbonate, as well as depressions containing giant ooids (Figures 2, 3, 4, 5). The cones, columns and domes are often irregularly conjoined. Columns, well seen in the lower part of the mound, are steep‐sided to irregular, and vertically elongate, as well as locally asymmetrically coniform, with smooth to lobate sides and conical to rounded terminations. They are commonly laterally patterned by sinuous elongate vugs, 2–3 mm wide and 5–10 cm long, emphasized by pale dolomite (Figure 2b–d). Column interspaces, ranging from narrow to broad, and locally very irregular and sinuous, are mainly occupied by fine‐grained dolomitic or silicified matrix (Figures 2 and 3). These columns, cones and domes create complex, largely irregularly juxtaposed or conjoined associations, locally framework, largely infilled by fine‐grained matrix and, more locally, pockets of giant ooids (Figures 4 and 5), together with extensive botryoidal—originally aragonite—crusts (Figure 6). A conservative estimate based on field observation of mound mesofabrics, such as those illustrated in Figures 2, 3, 4, 5, suggests that cryptic space—locally with good connectivity—could account for up to ~10% of the exposed mound volume.

FIGURE 3.

Base of Yesanpo Mound showing columns: (a) Complex, irregular columns with irregular interspaces emphasized, and locally enlarged, by pale‐colored replacive silica. (b) Closely spaced columns with laterally amalgamated bases. These disconformably overlie thinly laminated carbonates at the top of Wumishan Member 2.

FIGURE 4.

Internal mound macrofabrics: (a) Tall, smooth‐sided cones, locally separated by fine‐grained layered carbonate, and patchily silicified; upper mound. (b) Giant ooids in dolomitized (pale gray)—partially replacive—matrix, and with patchy silicification (pale buff, high relief); mid‐mound. (c) Ministromatolites (dark grey), patchily replaced by dolomite (pale gray); mid‐mound. (d) Asymmetric ministromatolite lobate cone/dome (center) with small columns. Above and below (lower left) this lobe, complex inter‐column fill is patterned by irregular dolomite (pale gray, low relief) and silica (buff, high relief); mid‐mound. (e) Conjoined steep‐sided ministromatolite columns: Conical (left), irregular (right), with adjacent—locally bridged—cavities, largely occupied by light‐colored replacive dolomite (left) and silica (right). Giant ooids in the upper part appear to be partly amalgamated with the columns. Lower‐mid mound.

FIGURE 5.

Aligned ministromatolites (dark) forming a dome, below a cavity (top left) with giant ooids. Individual ministromatolites, left, amalgamate to the right, forming denser and more interconnected masses. Low relief replacive dolomite is light‐colored, top left. Replacive silica, lower center and right, has high relief. Mid‐mound.

FIGURE 6.

Thin‐section photomicrographs. (a) Distinctly laminated conjoined ministromatolites, originally aragonite, now dolomite (dark), with irregularly replacive dolomite (light color). (b) Detail of well‐laminated ministromatolites with fibrous fabric—now dolomite, interpreted to be after aragonite, within replacive dolomite (light color). Overall isopachous, locally hemispheroidal, laminae suggest a mainly precipitative origin; remains of organic filaments (not visible) suggest biogenic influence, likely bacterial (cf. Tang et al. 2014) (and see Figure 7). (c) Botryoids (originally aragonite, now dolomite) with overlapping laminar to concentric growth bands, encrusting a marine dissolution surface (possibly due to organic matter degradation) on dense dark ministromatolitic reef fabric. At the top, the botryoidal layers are, in turn, truncated by dolomite. (d) Coated giant ooids (dolomite) with silicified interspaces. (e) Small botryoids, with laminar concentric growth bands and radial crystal fabric emphasized by organic matter. The botryoids are interpreted as originally aragonite marine precipitates, likely mainly inorganic but incorporating filamentous (possibly bacterial) organic matter (see Figure 7).

4.3. Thrombolitic‐Stromatolitic Ministromatolite Reef Fabric and Framework Cavities

The cone‐, dome‐, column‐ and tent‐shaped morphotypes are often asymmetric and irregular. They all appear to consist essentially of myriads of ministromatolites, typically 2–4 mm wide and 2–10 mm high, ranging from isolated and juxtaposed to laterally linked (Figures 4d and 5). The ministromatolites themselves are typically composed of fibrous isopachous, flat to convex, dark–light banded fibrous crystalline layers (Figures 6, 7, 8) with blunt crystal terminations (arrow in Figure 7a), consistent with an original aragonite mineralogy. Locally, organic filaments and relics with autofluorescence are preserved within dolomite and silica in both the ministromatolites and in the fibrous cortices of giant ooids (Figure 7). These features are consistent with organic filaments, although it cannot be determined whether they are syndepositional or younger. Over short distances, shrubby radial‐dendritic thrombolite mesofabrics range from isolated lobate millimetric columns to amalgamated framework (Figures 4d and 5). Although these fabrics are commonly locally altered by both dolomitization and silicification (Figure 4), column shape and arrangement, and inter‐column sediment distribution, locally remain well‐defined. Simpler stromatolitic, as opposed to ministromatolite, fabrics are mainly apparent in steep‐sided juxtaposed lobate columns (Figure 4e) in the lower part of the mound, and especially in coniform structures (Figure 4a) in the upper part of the mound. Overall, therefore, key macroscopic features of Yesanpo Mound are (i) irregular, locally juxtaposed, lobate to coniform, decimetric columns—some with laminated coniform caps, and (ii) interconnected sinuous matrix‐filled growth cavities up to 1 cm wide and 4 cm long (Figure 2b–f). Original aragonite ministromatolite and giant ooid fabrics, now dolomitized, are often locally destroyed by pale‐colored, partially fabric‐selective, dolomite and cross‐cutting silicification (Figures 4c, 5, and 8), contrasting with the generally dark gray to black carbonate of the giant ooids and ministromatolites.

FIGURE 7.

Evidence of organic relics in originally aragonite crystal fibers within ministromatolite and giant ooids. (a) Low‐magnification photomicrograph of a ministromatolite showing fibrous crystals (now dolomite) with organic relics. The crystal fibers are oriented perpendicular to the laminae. The arrow indicates a blunt crystal termination. (b) High‐magnification photomicrograph of the area in panel (a), showing fibrous aragonite (now dolomite) with organic relics. (c) Fluorescence photomicrograph, of the same area as in (b), showing autofluorescence of organic relics preserved in fibrous aragonite. (d) Low‐magnification photomicrograph showing fibrous aragonite (now dolomite) with organic relics in the cortex of a giant ooid. The fibers are oriented perpendicular to the cortex. (e) High‐magnification photomicrograph of the area in panel (d), showing fibrous aragonite (now dolomite) with organic relics. Arrows indicate organic relics. (f) Fluorescence photomicrograph of the same area as panel (e), showing autofluorescence of organic relics preserved in the fibrous aragonite (now dolomite). The blue fluorescence shown in (c, f) is true color.

FIGURE 8.

Very small ministromatolite columns—now dolomite—with corroded margins within replacive pale dolomite. Vertically and laterally, the columns show complex, locally amalgamated arrangements and generally distinct lamination in thin‐section.

Vertical changes in mound structure are evident in the field. The mound base is formed by nearly erect and often very closely juxtaposed columns that abruptly, locally erosively, overlie wavy laminated dolostone of Wumishan Member 2 (Figure 3b). Upwards, columns locally become sinuous and lobate, and also often markedly asymmetric, with thrombolitic interiors that can amalgamate into both domes and narrow steep‐sided coniform caps, 2–5 cm wide and 10–30 cm high, patterned by small elongate laminated anastomose columns (Figure 2b–d). In the mid‐ to upper mound, delicately layered steep‐sided columns are often separated by fine pale‐colored dolomitized, channel‐ and pocket‐filling, giant ooids—individually up to ~5 mm across—within pale fine‐grained dolomitic matrix (Figures 2d and 4b,e). Toppled or broken cones and columns have not been observed. The original fine‐grained matrix is commonly silicified. Cavity systems, ranging from narrow interspaces between columns to larger areas beneath coniform masses, are typically sinuous, highly irregular, up to several centimeters in width, and often more in height (Figures 2b–f and 3). Cavity fills, predominantly fine‐grained carbonate sediment, either occupy entire cavities or are geopetal, with sparry aragonite/dolomite cement, now preserved as dolomite.

4.4. Depositional Environment

Near the mound base, steep‐sided columns with flange‐like, locally asymmetric, lateral branches (Figure 2b,e), suggest irregular accretion as well as lateral sediment accumulation and/or current influence. Fine‐grained carbonate, presumably water‐column precipitate and/or eroded sediment, accumulated in pockets and between columns. Mainly steep‐sided initial columns at the base of the mound (Figure 3a,b) can quickly become irregular upward, and are eventually locally overgrown and overlain by more coniform ministromatolite accumulations, together with pockets filled by giant ooids (Figures 2d and 4b). These fabrics suggest rapid mound accretion and expansion, in relatively shallow water, likely influenced by illumination as well as locally strong and variable current activity. Giant ooid fabrics also occur 10.5 km to the southeast of Yesanpo, at Shierdu (Tang et al. 2015), in the upper part of underlying Member 2. Water depth appears to have remained shallow, and strongly current‐influenced, throughout mound accumulation. Yesanpo Mound is overlain by fine‐grained laminite with chert bands. Mound termination, associated with accumulation of fine‐grained carbonate interpreted as water‐column precipitate, may reflect shallowing (Tang et al. 2013b).

4.5. Synsedimentary and Burial Diagenesis

Major element compositions of the various Yesanpo Mound fabrics are listed in Table S3. Mg/Ca molar ratios of the samples are all close to 1.0, reflecting now essentially dolomitic mound composition. Mn/Sr ratios, consistently below 10 (ppm/ppm), suggest early‐stage dolomitization and relatively weak late diagenetic alteration (cf. Kaufman and Knoll 1995). The carbonate content in our mound samples is approximately 80 wt%, despite local silicification. Overall, these data suggest that Yesanpo Mound fabrics, especially matrix, were strongly affected diagenetically—by both early and late dolomitization and late silicification.

Based on field and laboratory observations, we infer the following genetic/diagenetic sequence of events for Yesanpo Mound carbonates:

1. Synsedimentarily calcified aragonite ministromatolites colonized eroded seafloor, forming irregular columns and cones that, near the mound base, are closely juxtaposed. As the mound gained relief, columns quickly tended to become asymmetric; interspaces developed and often enlarged upward, accumulating mainly fine‐grained aragonite sediment. Cones formed locally, and the larger irregular interspaces locally accumulated aragonite giant ooids. Interpretation of primary aragonite is based on fibrous crystal morphology. All fabrics experienced pervasive synsedimentary dolomitization and subsequent patchy silicification. Very low Sr values may reflect Sr‐loss during aragonite transformation to dolomite (Jacobson and Usdowski 1977). Ministromatolite fabrics locally preserve organic matter within both dolomite and silica (Figures 6b and 7). We interpret the organic matter as likely primary and in situ (Tang et al. 2013b, 2014). The giant ooids can also preserve organic matter (Figures 6e and 7). Overall, we interpret the depositional environment as illuminated (based on ministromatolite abundance), and current swept (based on localized giant ooid formation) shallow water. The mound matrix (now dolomite, probably originally aragonite) preserves a slight negative Ce anomaly, suggesting very low O₂ concentrations (i.e., suboxic conditions) (see Section 5.6).

2. Etching of mound cavity walls (Figure 6c) suggests localized acidic solution, possibly due to organic matter decomposition (cf. Walter and Burton 1990). Cavity fills, locally giant ooids but mainly fine‐grained carbonate sediment, either occupied entire cavities, or were geopetal with sparry aragonite/dolomite cement.

3. Botryoidal aragonite cementation, locally with organic matter preservation, suggests sustained seawater saturation with respect to aragonite, and is associated with synsedimentary dolomitization, indicating elevated Mg²⁺ (see Section 5.5).

4. Localized early silicification (Figures 4d and 6d) is consistent with elevated seawater Si‐saturation (Shi et al. 2024; Tang et al. 2013b; Yuan et al. 2022).

5. Primary aragonite fabrics underwent burial recrystallization, dolomitization, and silicification, evidenced by common pressure‐solution stylolites.

Overall, this diagenetic evolution reflects widely inferred prevalent Mesoproterozoic seawater chemistry: elevated Mg²⁺ concentrations favoring aragonite precipitation and synsedimentary dolomitization (Grotzinger and Kasting 1993), and high dissolved silica levels leading to widespread silicification (Siever 1992) (see Section 5.4). Microbial activity likely enhanced silica precipitation in porewaters during early diagenesis (Shi et al. 2024; Tang et al. 2014), with most chert forming in peritidal settings, or near the sediment–water interface.

4.6. I/(Ca + Mg) and REE + Y

Analytical results for I/(Ca + Mg) and REE + Y in the 13 samples are presented in Table S3, and corresponding REE + Y patterns are shown in Figure 9. All measured I/(Ca + Mg) values are below the detection limit, with Mg/Ca molar ratios close to unity, and Mn/Sr weight ratios substantially below 10. The REE + Y data from the primary fabric of Yesanpo reef yield Ce anomaly (Ce_SN/Ce_SN*) values ranging from 0.83 to 1.09, with an average of 0.97 ± 0.14. The REE + Y patterns are generally flat to heavy rare earth element (HREE) depleted, as reflected by Pr_SN/Yb_SN ratios ranging from 0.96 to 4.00. Y/Ho ratios are consistently above 24 and slightly below 36. Thorium concentrations are very low (< 0.04 ppm), and no correlation is observed between Ce_SN/Ce_SN* and Th (see Section 5.6).

FIGURE 9.

REE + Y data of Yesanpo Mound carbonates. (a) PAAS‐normalized REE + Y patterns of all samples (n = 13). Most patterns are non‐marine in appearance; only a few resemble seawater. (b) Ce_SN/Ce_SN* versus Y/Ho cross‐plot. The negative correlation is interpreted as an alteration trend: the lower‐right field reflects stronger diagenetic overprinting and/or detrital contamination; the upper‐left field identifies the best‐preserved samples approaching primary seawater composition. (c) REE + Y patterns of the best‐preserved subset (three samples). These display HREE enrichment, super‐chondritic Y/Ho, and negative Ce anomalies in the two highest‐ranked samples (Ce_SN/Ce_SN* ≤ 0.86, as low as ~0.83), with the anomaly decaying in the third, consistent with a primary suboxic seawater signal progressively overprinted by diagenesis.

5. Discussion

5.1. Introduction

Reef framework increases habitat, as shown by cavities that host coelobionts (e.g., Riding and Zhuravlev 1995; Wood 1993), thereby enhancing biodiversity (Close et al. 2020; Ginsburg 1983). Sponge‐like fabric (Turner 2021) in thrombolitic reefs ~890 Ma (Aitken and Narbonne 1989; Turner et al. 1993, 2000) is currently the earliest reported example of possible invertebrate cavity dwellers that we are aware of. Cryptic habitats, locally with fauna, certainly occur in Late Ediacaran thrombolite reefs in Namibia (Grotzinger 2000; Grotzinger et al. 1995; Penny et al. 2014; Saylor et al. 1995; Uahengo et al. 2020). The interconnected decimetric cavities, described here in ~1.5 Ga Yesanpo ministromatolite framework, predate these Tonian and Ediacaran examples by ~600 and ~1000 Ma respectively. Yesanpo Mound is therefore an unusually early example of framework structure, and it contained cavities that were open to the reef surface. Although it long predates the earliest currently suggested occurrence of metazoans (≥ ~890 Ma), Yesanpo reef demonstrates that complex framework with protected crevices existed ~1.5 Ga. In addition, coeval ministromatolite fabrics, similar to Yesanpo, contain silicified putative cyanobacteria, consistent with localized oxygenation in this environment (Tang et al. 2013b). It therefore seems conceivable that partially oxygenated pre‐formed cryptic habitats—similar to some present‐day examples (e.g., Richter and Wunsch 1999)—may have been available to evolving metazoans during the late Mesoproterozoic and Neoproterozoic.

5.2. Environmental Setting

Distinctive fabrics such as wavy laminite and crystal fans (Tang et al. 2014), that are common in Wumishan platform carbonates, suggest rapid synsedimentary carbonate precipitation. The location of Yesanpo Mound immediately overlying an erosional disconformity at the base of Wumishan Member 3, together with the pockets of giant ooids (Figures 2d and 4b,e)—typically ~5 mm and locally 10 mm in size—that it contains, is consistent with agitated shallow water conditions above fair‐weather wave‐base. We infer that Yesanpo Mound developed in shallow‐water conditions on an erosional surface transgressed by rising sea‐level. Ministromatolite growth and calcification may have been facilitated by influx of upwelling alkalinized, nutrient‐rich seawater. Although we have not observed direct evidence of biogenicity at Yesanpo, ministromatolites in Huailai county, Hebei Province (Tang et al. 2013a; Wang et al. 2021; Yuan et al. 2022), and in the Jixian section (Yang et al. 2022), preserve signs of small cells suggestive of photosynthetic organisms such as cyanobacteria. It is therefore likely that cavernous thrombolite and ministromatolite mounds, similar to Yesanpo, also formed elsewhere on flooding surfaces of the Wumishan platform, and possibly more widely globally.

5.3. Yesanpo Modular Reef Framework

Yesanpo Mound is a lensoid, locally cavernous, framework of regular to irregular, partially interconnected, lobate columns and cones, composed of synsedimentarily lithified ministromatolites and traversed by systems of synsedimentarily filled cavities. In Phanerozoic reefs, synsedimentary cavity occlusion by sediment and marine cement is well‐known (e.g., Walther 1888; Scoffin 1971; Wood 1999). So far as we are aware, this style of organization and construction is relatively unusual in Precambrian stromatolite accumulations. Whereas well‐documented ministromatolite aggregations are typically domes and biostromes (see Section 1.1), in Yesanpo Mound they form cones and interconnected steep‐sided and asymmetric lobate columns. This complex Lego‐like construction contrasts with the much more common layered arrangement of ministromatolites in domes and biostromes. It strongly suggests a degree of modularity on the part of Yesanpo ministromatolites. We suggest that—very broadly—this may be compared with the role of polyps in coral colonies. In this growth strategy, repetition of sub‐units (modules), e.g., polyps and ministromatolites, may facilitate responses to variations in conditions such as light, currents and sedimentation, thereby enhancing overall accretion of a mass of irregular columns and cones (Figures 2b–d, 3a, and 4e). Modular growth in scleractinian corals is achieved through repetitive budding of polyps, enabling colonies to respond to local conditions (illumination, water flow, sedimentation); this directed accretion enhances growth and survival (Rosen 1986; Paz‐García et al. 2015; Medellín‐Maldonado et al. 2022). We suggest that similarly, albeit in a much less controlled manner, small‐scale replication of ministromatolites, may have facilitated modular growth (Figures 4, 5, 6) that enabled relatively large columns to respond to water flow and illumination as they accreted (Figure 2b–f). Yesanpo ministromatolites, typically a few to 10 mm in width, are broadly comparable in size with present‐day scleractinian polyps (El‐Bawab 2020). In this view, the varied shapes of Yesanpo columns could be a ‘functionally relevant structural response’ (Sanna et al. 2023) to the interplay of currents and light, as can occur in both shallow and deep‐water coral frameworks at the present‐day (Todd 2008). In contrast, it seems that much larger stromatolites typically accrete by replicative lamina superimposition, without forming a framework (Petrov and Semikhatov 2001; Semikhatov and Raaben 2000). Ministromatolite accretion created structural and compositional heterogeneity in Yesanpo Mound that localized significant synsedimentary and diagenetic alteration, as discussed next.

5.4. Yesanpo Mound: Environmental Factors Influencing Genesis and Diagenesis

Distinctive features of Yesanpo reef's complex structure and composition include: (i) ministromatolite framework, (ii) a variety of additional primary carbonate components (fine‐grained carbonate, aragonite crusts, localized giant ooids), (iii) synsedimentary dolomitization and silicification, (iv) subsequent localized silicification. The factors that determined this heterogeneity in structure and composition can be linked to regional and global, as well as to local, conditions. The inception, and apparent rapid accretion, of Yesanpo Mound may be directly associated with its location, at the base of Wumishan Member 3, on a relatively planar transgressive marine‐flooding surface, directly and erosively overlying Member 2. Key factors could include: (i) CaCO₃ precipitation promoted by warming and degassing of transgressive upwelling water (Burton and Walter 1987), (ii) microbial growth, enhanced by increased nutrient availability from the combined effects of terrestrial runoff, upwelling, and organic matter degradation (Radice et al. 2019). The aragonite‐dolomite‐silica composition of Yesanpo Mound likely reflects broad, probably global, environmental conditions, particularly seawater chemistry, during the early Mesoproterozoic, such as: (i) Levels of dissolved Mg²⁺ sufficient to inhibit calcite relative to aragonite precipitation (Hardie 2003; Pan et al. 2021) (see Section 5.5, below). (ii) Relatively elevated Si‐concentrations in seawater, long inferred for Precambrian seawater in general (Maliva et al. 2005; Siever 1992) and consistent with synsedimentary silicification of organic walled microfossils in Wumishan carbonates (Shi et al. 2024; Tang et al. 2013c). (iii) High Fe²⁺ concentrations—in generally low‐oxygen seawater—could have inhibited calcite precipitation (Meyer 1984), whilst permitting seafloor aragonite precipitation under high carbonate supersaturation (Katz et al. 1993; Sumner and Grotzinger 1996), consistent with interpretations of primary mineralogies of adjacent bedded Wumishan carbonates (Tang et al. 2013a; Yuan et al. 2022). Yesanpo ministromatolites, the primary in situ frame‐building components of the mound, were originally composed of aragonite (fibrous in morphology with blunt rather than sharp terminations), and synsedimentarily coated by aragonite marine cement (Figure 6c). These primary aragonite fabrics underwent early dolomitization, followed by both synsedimentary and later burial silicification (Figure 4a–e). Yesanpo ministromatolites may have been more biogenic than has, in some cases, been inferred for ‘microdigitate stromatolites’ in general (e.g., Grotzinger and Kasting 1993). This is supported by the presence of cell‐like fabrics at other Wumishan locations (Tang et al. 2013b; Yuan et al. 2022), and the presence of organic filaments at Yesanpo (Figures 6b,e and 7). It is also consistent with the relative irregularity of Yesanpo ministromatolite constructions (Figures 2b–d, 3a, and 4d), especially when compared with forms interpreted as probably largely inorganic (Hofmann and Jackson 1987), and with their ability for modular construction (Section 5.3).

Overall, the physical heterogeneity of Yesanpo Mound, together with its varied mineralogical composition, facilitated significant early and burial diagenetic alteration. Irregular syndepositional surfaces between ministromatolite fabrics and adjacent botryoidal aragonite cements (Figure 6c) could be due to corrosion linked to bacterial production of CO₂. Dissolution of aragonitic mound components during synsedimentary and burial diagenesis likely provided routes for diagenetic fluid flow, and these components are now replaced by dolomite and silica. This profound and sustained alteration of mound components is in marked contrast to contemporaneous adjacent flat‐lying, off‐mound, fine‐grained carbonates and laminar crusts which, compositionally, are more homogeneous and relatively unaltered.

5.5. Aragonite Sea

Marine Ca‐carbonate precipitation reflects ambient seawater chemistry, and especially ionic concentrations that depend on redox conditions and alkalinity (Stumm and Morgan 1996). Common ions that can significantly influence the precipitation and mineralogy of Ca‐carbonate in seawater include iron, phosphate, and magnesium (Morse et al. 2007). Calcite can be inhibited relative to aragonite by the presence of Fe²⁺ and/or Mg²⁺ (Meyer 1984). In Phanerozoic carbonates, alternation of Aragonite Sea and Calcite Sea conditions has been attributed to variations in abundance of sulfate and, particularly, Mg²⁺, relative to Ca²⁺, as well as to temperature (Balthasar and Cusack 2015; Bots et al. 2011; Lowenstein et al. 2001; Sandberg 1983). In the mid‐Archean, the influence of dissolved iron abundance on Ca‐mineralogy may have locally promoted laterally contiguous marine conditions respectively favoring precipitation of Fe‐carbonate, aragonite and calcite (Riding et al. 2022, fig. 14). Long‐term fluctuations between Aragonite Seas and Calcite Seas have been suggested during the Proterozoic (Hardie 2003), with Aragonite Sea intervals—characterized by aragonite and high‐Mg calcite precipitation—becoming dominant in the Neoproterozoic (Hood et al. 2011; Hood and Wallace 2018; Sumner 2002). We interpret Yesanpo reef framework as being originally aragonite (fibrous crystals with blunt rather than sharp terminations), in common with the mineralogy of associated lithotypes such as wavy laminite (Tang et al. 2014). Based on Wumishan carbonate fabrics overall, we infer likely Aragonite Sea conditions in this region ~1.5 Ga.

5.6. Redox Conditions

The mid‐Proterozoic has been considered a period of persistently low atmospheric oxygen, with estimates of ≤ 0.1%–1% of present atmospheric level (PAL) (Liu et al. 2021; Planavsky et al. 2014). However, this view is being revised by new, direct evidence. Analyses of primary fluid inclusions in ~1.44 Ga surficial halite suggest that atmospheric O₂ may, intermittently, have reached ~3.7% PAL (Park et al. 2025), a concentration potentially sufficient to sustain early animal respiration (Mills et al. 2014). This finding aligns with regional geological evidence from the North China Craton that indicates dynamic redox conditions in shallow seawater (Xu et al. 2025; Yan et al. 2025; Yang et al. 2024; Zhang et al. 2016). Specifically, the Wumishan Formation preserves a long‐term positive trend in carbonate carbon isotopes, interpreted to reflect sustained organic carbon burial and a gradual tendency toward oxygenation (Guo et al. 2013). High‐resolution carbonate I/(Ca + Mg) data indicate overall low background levels of oxygenation, punctuated by positive fluctuations (Sun et al. 2025). Consistent with this, Wumishan pyrite sulfur isotope records suggest that widespread seafloor microbial mats may locally have sustained “oxygen oases” (Jia et al. 2022).

We employed two complementary geochemical proxies to reconstruct local redox conditions in Yesanpo samples: carbonate I/(Ca + Mg) ratios and Ce anomalies. In the modern ocean, dissolved iodine occurs as iodide (I⁻) and iodate (${\mathrm{IO}}_3^{-}$), of which only iodate is incorporated into the carbonate mineral lattice (Lu et al. 2010). Carbonate I/(Ca + Mg) ratios can reflect iodate concentrations in the depositional environment. However, due to the slow oxidation kinetics of I⁻, surface waters may retain low ${\mathrm{IO}}_3^{-}$ concentrations even under high atmospheric O₂ levels (Lu et al. 2016). In contrast, carbonate Ce anomalies provide an independent constraint, since Ce³⁺ is oxidized to Ce⁴⁺ in oxygenated seawater and preferentially removed by scavenging, generating negative Ce anomalies (Elderfield et al. 1981; Webb and Kamber 2000). These proxies also differ in their susceptibility to post‐depositional alteration: I/(Ca + Mg) is relatively insensitive to detrital contamination, but can be overprinted by dolomitization and diagenesis (Hardisty et al. 2017; Hashim et al. 2022), whereas the Ce anomaly is generally more resistant to dolomitization (Tostevin et al. 2016; Yang et al. 2024), but may be weakened by siliciclastic contamination (Ling et al. 2013; Planavsky et al. 2010). Consequently, combined application of these proxies strengthens local redox reconstructions.

Yesanpo reef carbonates exhibit I/(Ca + Mg) ratios below the detection limit (Table S3). Such low values could indicate anoxic shallow seawater (Hardisty et al. 2014; Lu et al. 2010). However, they could also reflect iodate loss during dolomitization (Hashim et al. 2022; Wörndle et al. 2019), or other types of diagenetic alteration (Hardisty et al. 2017). Low Mn/Sr ratios (1.24 ± 0.90 ppm/ppm) in our samples argue against strong fluid‐mediated diagenetic alteration (Kaufman and Knoll 1995). Rather, extensive dolomitization in our samples (Mg/Ca = 1.09 ± 0.02 mol/mol) suggests that dolomitization‐driven iodine removal could be responsible for low iodine concentrations, by overprinting the primary I/(Ca + Mg) signal. Although the low Sr concentrations (tens of ppm) indicate substantial loss of Sr during dolomitization, shale‐normalized REE + Y patterns and Y/Ho ratios are commonly preserved through such recrystallization and are considered relatively robust redox proxies in Proterozoic carbonates (e.g., Tostevin et al. 2016; Yang et al. 2024).

Overall, the slightly negative—to absent—Ce anomalies that we observe are consistent with locally suboxic to anoxic shallow water depositional conditions. Lack of correlation between Ce_SN/Ce_SN* values and Th content suggests minimal siliciclastic contamination (Table S3). The shale‐normalized REE + Y patterns are characterized by depletion in heavy REEs (HREEs) and low Y/Ho ratios (< 36) (Table S3, Figure 9), which differs from the HREE‐enriched signature typical of open seawater. HREE‐depletion might be explained by seawater–freshwater mixing (Tostevin et al. 2016). This is consistent with HREE depletion commonly observed in modern river waters (Goldstein and Jacobsen 1988; Sholkovitz 1992, 1995). We consider a hydrothermal origin less likely, since our samples lack the positive Eu anomaly typical of high‐temperature fluids (cf. Tostevin et al. 2016). Slightly negative Ce anomalies in our samples further support suboxic conditions during Yesanpo reef formation.

To further evaluate the fidelity of the REE + Y signals, we examined the relationship between Ce_SN/Ce_SN* and Y/Ho ratios (Figure 9b). The samples exhibit broadly negative correlation, which we interpret as a mixing or alteration trend: samples with higher Y/Ho and lower Ce_SN/Ce_SN* represent the best‐preserved fabrics that most closely approach primary seawater compositions, whereas those with low Y/Ho and higher Ce_SN/Ce_SN* indicate increased diagenetic overprinting and/or siliciclastic contamination. When only the best‐preserved subset is plotted (Figure 9c), the REE + Y patterns are HREE‐enriched, display super‐chondritic Y/Ho ratios, and exhibit slightly negative Ce anomalies (Ce_SN/Ce_SN* as low as ~0.83), closely resembling modern seawater and indicating suboxic depositional conditions. When the best‐preserved subset is expanded to three samples, the seawater‐like signature persists in the top two (Ce_SN/Ce_SN* ≤ 0.86) but decays in the third, consistent with progressive diagenetic overprinting. The Ce anomalies preserved in this subset are therefore interpreted as primary, whereas the absence of negative Ce anomalies in the remaining samples is attributed to diagenetic masking and minor detrital contamination.

Yesanpo synsedimentary fibrous aragonite cements locally preserve organic filaments, possible remains of photosynthetic microbes (Tang et al. 2013b). This is consistent with Yesanpo Mound being an ‘oxygen oasis’ that could have supported eukaryote evolution. In contrast to present‐day reef cavities, which are often characterized by low oxygen levels, Proterozoic microbial reefs, such as Yesanpo, may have been relatively more oxygenated than adjacent seawater, if abundant cyanobacteria were associated with the ministromatolites. This is consistent with the view that, although algae diversified during the Mesoproterozoic, their overall contribution to shallow marine oxygenation may have remained much less than that of cyanobacteria (Luo et al. 2016). Zhao et al. (2020) reported sedimentary structures related to photosynthesis and oxygen release in well‐preserved microbial mats and microfossils 1400–1600 Ma in age from northern China, including the Wumishan Formation.

Nonetheless, despite possible transient increases in oxygenation reported from the surrounding sedimentary sequence (Sun et al. 2025), and tented cavities in the Gaoyuzhuang Formation that preserve distinct negative Ce anomalies (Tang et al. 2016; Zhang et al. 2018), definitive evidence for oxic conditions in Yesanpo Mound is lacking. Rather, the Ce anomaly detected in our Yesanpo samples provides qualitative, rather than quantitative, constraint on local oxygen levels, and—most plausibly—indicates deposition under suboxic conditions (dissolved O₂ < 10 μM; Liu et al. 2021). Thus, although Yesanpo Mound generated cryptic habitats physically suitable for early animals, current Ce data do not indicate that contemporaneous dissolved oxygen levels were persistently sufficient to maintain metazoan respiration. The metabolic nature of the ministromatolite‐building microbes remains uncertain, and the possibility that they generated local oxygenation is conjectural. Studies suggesting that atmospheric oxygen may have reached levels capable of supporting early animals, such as sponges (~4% PAL), near or after ~1.4 Ga (Mills et al. 2014), have been supported by subsequent work (Canfield et al. 2021; Xu et al. 2025; Yang et al. 2024; Zhang et al. 2016). However, such oxygenation is likely to have been relatively episodic over geologic time‐scales, and it may not be characteristic of the Mesoproterozoic as a whole. Subsequent global decline in oxygen suggested during the Tonian (Guilbaud et al. 2015; Hou et al. 2025) further underscores the likelihood that widespread, stable and prolonged oxic marine conditions may not have been fully established until quite late in the Proterozoic.

5.7. Giant Ooids

Giant ooids, ≥ 2 mm in size, have been related to conditions of elevated CaCO₃ precipitation and water movement (Dai et al. 2022; Li et al. 2013; Sumner and Grotzinger 1993; Trower 2020; Trower et al. 2022; Trower and Grotzinger 2010), and can be closely associated with stromatolites in the Proterozoic (Swett and Knoll 1989). Their local abundance in Yesanpo Mound cavities (Figures 2d and 4b,e) is consistent with rapid CaCO₃ precipitation associated with agitation of supersaturated seawater in spatially limited wave‐ and current‐influenced reef pockets. Elsewhere, within the same succession, giant ooids locally form beds in Wumishan Members 2 and 3, and can contain bacterium‐like filaments (Tang et al. 2015; Yuan et al. 2022).

5.8. Reef Cavities

Yesanpo Mound raises questions regarding reef cavity formation and its apparent scarcity in the Precambrian. Stromatolite reefs commonly develop interspaces, and these are typically sub‐vertical and upward‐opening (Awramik and Grey 2005; Bertrand‐Sarfati and Eriksson 1977; Grotzinger 2000; Playford and Wallace 2001). Enclosed cavities appear to be less common. Until now, the largest such cavities—widely reported in Late Archean and Proterozoic benthic microbial carbonates—may be centimetric spaces in cuspate and ‘tented’ microbial fabrics (Bartley et al. 2015; Hofmann and Masson 1994; Riding and Virgone 2020; Sumner 1997, 2000). These appear to have had few interconnections and may generally have lacked external access, thereby reducing their accessibility to vagile organisms. In contrast, although they are very irregular, the spaces between Yesanpo ministromatolite columns provide an early example of replicative and interconnected biogenic cavity formation. In addition to enhancing reef surface area, these protected spaces largely appear to have had external access, anticipating cryptic reef niches of the early Phanerozoic (Kobluk 1988; Kobluk and James 1979; Riding and Zhuravlev 1995; Zatoń et al. 2018; Wood et al. 1993; Zhuravlev and Wood 1996), as well as the Neoproterozoic (Turner 2021).

5.9. Preformed Cryptic Reef Habitat

A wide variety of cavities can form within and between microbial carbonate columns and domes, as in late Proterozoic Nama Group thrombolites described by Grotzinger et al. (2000, figs 5 and 6). Since cavities that are open to the surface may be relatively quickly synsedimentarily infilled, sessile cryptic organisms occupy more protected spaces. Nama Group examples include Namacalathus (Grotzinger et al. 2000) and Cloudina (Penny et al. 2014). Yesanpo Mound, especially in its lower part, is a complex and cavernous framework of irregular lobate columns and cones. If these spaces had remained unfilled for a sufficient length of time, they could have provided diverse habitats for cryptic biota: spaces within firm substrates, open to water circulation, but protected from strong currents and fluctuations in temperature. At this age, so far as we are aware, Yesanpo cavernous spaces formed by ministromatolites are currently unique. Certainly, these cavities, reaching up to 6 cm in width, are significantly older, and often larger, than millimeter‐ to centimeter‐scale spaces described in putative framework reefs in the Neoproterozoic (Aitken and Narbonne 1989; Bartley et al. 2015; Grotzinger 2000; Grotzinger et al. 1995; Penny et al. 2014; Saylor et al. 1995; Sumner 1997; Turner et al. 1993, 2000; Webb 1996). Cryptic sponge‐like fossils have been reported in incipient microbial framework in 0.89 Ga Little Dal reef, Canada (Turner 2021). Although oxygenation has long been widely thought to have remained relatively low until the late Neoproterozoic (Lyons et al. 2014; Och and Shields‐Zhou 2012), there are increasing indications of transitory and pulsed marine oxygenation in the Mesoproterozoic, e.g., 1.57 Ga (Shang et al. 2019; Xie et al. 2023; Xu et al. 2023; Zhang et al. 2018), 1.48 Ga (the Wumishan Formation; Sun et al. 2025), 1.4 Ga (Xu et al. 2025; Yan et al. 2025; Yang et al. 2024; Zhang et al. 2016), 1.25 Ga (Meng et al. 2025), and 1.0 Ga (Tang et al. 2025). It is therefore possible that, as marine oxygenation increased, reef frameworks similar to Yesanpo provided cryptic niches suitable for early animals. These certainly existed within microbial reefs in the Late Ediacaran (Penny et al. 2014) and Early Cambrian (Kobluk and James 1979). As animals evolved, it therefore seems likely that suitable cryptic habitats could already have long been available. From this perspective, Mesoproterozoic and Neoproterozoic microbial reef cavities present inviting targets in the search for early animal life.

6. Conclusions

1. ~1500 Ma Yesanpo Mound (~3 m thick, ~15 m wide), at the base of Member 3 of the Wumishan Formation, North China, is currently Earth's oldest known cavernous reef framework. It consists of myriads of conjoined originally aragonitic ministromatolites, each ≤ 2 cm wide, strengthened by synsedimentary aragonite marine cement. The ministromatolites form irregularly interconnected decimetric columns and cones. Interspaces were synsedimentarily filled by fine‐grained sediment, pockets of giant ooids (reflecting locally elevated current energy), and marine aragonite botryoidal cement.

2. Ministromatolites are the key components of this modular reef. They functioned as very small discrete units, a role very broadly comparable to that of biomimetic/modular polyp organization in present‐day scleractinian corals. This allowed Yesanpo framework accretion to respond to variations in light, currents and sedimentation, creating irregular columns, cones and tent‐like structures. These complex fabrics contrast with generally much better layered, relatively cavity‐poor, arrangements that have hitherto been widely reported in Paleo‐Mesoproterozoic ministromatolite domes and biostromes.

3. Yesanpo Mound accumulated in shallow, illuminated, current‐swept conditions. Its growth and calcification may reflect warming and degassing of transgressive upwelling seawater, together with increase in nutrient availability, during inception of Wumishan Member 3 accretion. The ministromatolites were originally micritic and composed of fibrous aragonite. They were synsedimentarily dolomitized and silicified, and then further silicified during burial diagenesis, preserving organic matter that locally resembles filaments. Slightly negative Ce anomalies (indicated by REE analyses) suggest mainly suboxic marine conditions during Yesanpo Mound formation.

4. Yesanpo Mound reef framework, containing interconnected centimetric cavities well suited to cryptic biota and forming in potentially oxygenated shallow‐water, existed ~1.5 Ga. Currently, this predates the earliest reported reef‐dwelling animals by ~600 million years. Yesanpo Mound suggests that similar habitats could have been available to early marine animals from the mid‐Proterozoic onward.

Funding

This study was supported by the National Natural Science Foundation of China (grant 42372233), “Deep‐time Digital Earth” Science and Technology Leading Talents Team Funds for the Central Universities for the Frontiers Science Center for Deep‐time Digital Earth, China University of Geosciences (Beijing; grant 2652023001), Chinese “111” Project (B20011).

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Table S1: Analytical results of iodine and major elements (ppm) in dolostone standard JDo‐1.

Table S2: Analytical results of trace elements (ppm) in dolostone standard JDo‐1.

Table S3: Geochemical data of Yesanpo Mound.

Data Availability Statement

Our data are archived at Tang et al. (2026).