Late Pennsylvanian carbonate platform facies and coral reef: new insights from southern China (Guizhou Province)

The Pennsylvanian is characterized by intense paleoenvironmental changes related to glacio-eustatic sea-level fluctuations and major tectonic events, which affected the evolution of biocommunities. Most known Pennsylvanian tropical reefs and mounds are predominantly composed of calcareous algae (e.g. phylloid algae, Archaeolithophyllum), calcareous sponges, fenestrate bryozoans, Tubiphytes, and microbialites. However, in Houchang (southern China), the Late Pennsylvanian carbonate platform records a large coral reef lacking any analogs in age (Gzhelian), size (80–100 m thick) and composition (high biodiversity). The large coral reef developed at the border of the Luodian intraplatform basin. The intraplatform basin is characterized by the deposition of green algal grainstone, coated grain grainstone and bioclastic packstone, grainstone, floatstone and rudstone in shallow-waters. In the deep-water shelf, lithofacies are composed of burrowed bioclastic wackestone, microbioclastic peloidal packstone, grainstone, and fine-grained burrowed wackestone and packstone. In this context, the coral reef developed on a deep-shelf margin, in a moderate to low energy depositional environment, below the FWWB. The scarcity of Pennsylvanian coral reefs suggests global unfavorable conditions, which can be attributed to a complex pattern of several environmental factors, including seawater chemistry (aragonite seas), paleoclimatic cooling related to continental glaciation, and the biological competition with the more opportunistic and adaptive phylloid algal community that occupied similar platform margin paleoenvironments. The existence of the large Bianping coral reef in southern China, as well as a few additional examples of Pennsylvanian coralliferous bioconstructions, provides evidence that coral communities were able to endure the Late Paleozoic fluctuating paleoenvironmental conditions in specific settings. One of such settings appears to have been the deep shelf margin, where low light levels decreased competition with the phylloid algal community.


Introduction
The Carboniferous is a critical interval in Earth History, characterized by major cyclic paleoenvironmental changes such as climate, seawater chemistry and high frequency glacio-eustatic sea-level fluctuations, which directly influenced the evolution and development of biological communities (Brand 1989;Heckel et al. 2007;Heckel 2008;Davydov et al. 2012;Wang et al. 2013). The Mississippian is considered as a period of metazoan reef-building collapse subsequent to the Late Devonian extinction events (Sandberg et al. 1988; Thompson and Newton 1988;Buggisch 1991;Webb 2002;Kaiser et al. 2015;Barash 2017). During the Pennsylvanian and Permian, new types of organic carbonate buildup communities evolved that were composed of various combinations of calcareous algae, bryozoans, calcareous sponges and microbialites (Copper 1986(Copper , 2002West 1988;Wahlman 2002;Webb 2002). Sparse Carboniferous shallowwater buildups containing corals have been described, but corals generally played only minor roles in their construction (Wilson 1963;Wahlman 2002;Ogar 2012).
Sparse large Pennsylvanian coral-rich reefs have been described, such as the Bashkirian-Moscovian reefs of the Akiyoshi Group in Japan, which contain abundant tabulate and rugose corals (Ota 1968;Nagai 1985;Fagerstrom 1987;Nagai 1990, 1994;Nakazawa 1997;Wahlman 2002), and the Kasimovian-Gzhelian coral reef (80-100 m thick and 700 m wide) reported in Guizhou, southern China Gong et al. 2012). The well-developed coral reef reported in Bianping (Ziyun County, Guizhou) is considered to be the largest Pennsylvanian organic carbonate buildup with a coralliferous reefal framework in the world . To understand the overall context leading to the development of this exceptional reef growth (platform architecture and environmental conditions), two sections located in Zhongxinzhai and Brickyard villages, respectively (Houchang, Ziyun, Guizhou), have been investigated, and selected samples have been analyzed for petrographic characteristics, biostratigraphic age (fusulines), and chemostratigraphy (Sr isotopes). The existence of the large Pennsylvanian coral reef, which persisted only in a specific setting as shown in the present study, holds the potential to improve our current understanding of the collapse and recovery of coral reefs.

Southern China paleogeography
During the Pennsylvanian and Early Permian (Asselian-Sakmarian), in the Guangxi Autonomous Region and Guizhou Province (southern China), sedimentation occurred in the Dian-Qian-Gui Basin at the passive continental margin of the Yangtze Craton. During the Carboniferous, the Dian-Qian-Gui Basin records several elongate depressions (SW-NE axis), separated by more broad structural highs, resulting from pre-Carboniferous syndepositional faulting (Tsien et al. 1988;Shen 2002). During the Late Pennsylvanian-Early Permian (Asselian), the area consisted of a tropical epicontinental sea, dominated by carbonate platform facies associated with intraplatform depression facies (Shen and Qing 2010;Wang et al. 2013;Fig. 1). During the latest Sakmarian, a major sea-level fall occurred, which produced a series of coal-bearing strata and an extensive sedimentary hiatus throughout South China (Wang et al. 2013).  Feng et al. 1998;Yao and Wang 2016). Ziyun (Guizhou) is the study area. LD Luodian Basin

Study area
The study area around the town of Houchang (Ziyun County, Guizhou Province) exhibits folded Devonian to Triassic carbonate rock units (Fig. 2). During the Carboniferous, the Houchang area was located at the margin of the Luodian intraplatform basin (Fig. 1), where numerous bioconstructions have been reported (Yao and Wang 2016).
The Gzhelian Tubiphytes patch reefs (Bianping village, Houchang) are 0.7-3 m wide and 0.3-0.5 m thick and are   , which might represent calcimicrobes during this time (Flügel 2010;Yao and Wang 2016). These reefs were interpreted to have grown at a shallow platform margin (Yao and Wang 2016). The Gzhelian carbonate mud mound (Bianping village, Houchang) is built mainly by calcimicrobes (Yao and Wang 2016) associated with crinoids. It measures 2.5 m width and 1 m thickness. The mud mound was interpreted as growing at a deeper platform margin (Yao and Wang 2016).

Global paleoclimate and glacio-eustatic fluctuations
The Carboniferous is commonly regarded as a time of global greenhouse-icehouse climate transition. Recent studies suggest multiple pulses of glaciation and deglaciation (Fielding et al. 2008a), with resulting high-frequency sea-level fluctuations Ross 1987, 1988;Haq and Schutter 2008).

Late Paleozoic Ice Age
The Late Paleozoic Ice Age (LPIA) began with short-lived Mississippian (Tournaisian and Late Viséan) localized glacial events (e.g. South America, Isbell et al. 2003;Caputo et al. 2008;Fielding et al. 2008a;Henry et al. 2008). The onset of widespread glaciation occurred at the Viséan-Serpukhovian boundary in Gondwanaland (present-day South America and eastern Australia, Fielding et al. 2008a). During the earliest Bashkirian, Icehouse conditions expanded further, becoming widespread across South America (Holz et al. 2008;Rocha-Campos et al. 2008;Henry et al. 2008) and Australia (Fielding et al. 2008b;Mory et al. 2008). Further expansion of continental glacial ice centers in Southern Africa (Stollhofen et al. 2008;Isbell et al. 2008), Oman and Saudi Arabia ) occurred at the Bashkirian-Moscovian boundary. During the early late Pennsylvanian, isotopic and paleobotanical records suggest a period of relative climatic warming Pfefferkorn et al. 2008). Then, the latest Pennsylvanian-earliest Permian interval corresponds to a widely expanded glacial phase, with the growth of large ice sheets across Gondwana and accumulation of ice in the northern hemisphere . Ice sheets are inferred to have been at their maximum extent around the Pennsylvanian-Permian boundary and persisted until the Late Sakmarian , focused on Antarctica, Australia, southern Africa and South America (Fielding et al. 2008a).

Petrography
The dataset consists of three stratigraphic sections. Two sections were measured in the Zhongxinzhai (section 1) and Brickyard villages section 2; Fig. 4), respectively, which are five km apart. The third section was measured along the large Bianping coral reef Zhang et al. 2010).
Thin sections from 123 samples were analyzed using a petrographic microscope. Lithofacies were classified following Dunham (1962) and Embry and Klovan (1971) and grouped according to grain assemblages and texture to interpret the depositional environments.

Sr/ 86 Sr dating
Twenty-eight carbonate samples were selected for Sr isotope analyses. Powders of bulk samples were collected, avoiding veins during drilling. The ratios were measured using a Thermo Neptune PLUS Multi-Collector inductively coupled plasma mass spectrometer, in static mode (University of Geneva). The method follows that described in Samankassou et al. (2018). Due to a systematic difference between measured and nominal standard ratios of the SRM987 of 87 Sr/ 86 Sr (0.710248), the internally corrected 87 Sr/ 86 Sr values were additionally corrected by a value of − 0.025‰ per amu for external fractionation (McArthur et al. 2001). Measured sections 1 (Zhongxinzhai) and 2 (Brickyard) showing the lithofacies successions, the stratigraphic distribution of agediagnostic key fusuline foraminifera, strontium absolute age dates (section 2 only), and chronostratigraphy. Section 1 is through relatively shallow-water ramp facies, and section 2 is through a relatively deeper water slope to basin succession 1 3 3 Page 8 of 37

Fusuline biochronology
A total of 46 fusulinid samples from section 1 and seven samples from section 2 were investigated to establish fusulinid-based chronostratigraphic framework (Fig. 4). Fusulines are the most important biostratigraphic index fossils used in this analysis, and were recently calibrated with high-precision U-Pb zircon absolute age-dates in the Carboniferous successions of the Donets Basin and Urals (Davydov et al. 2010;Schmitz and Davydov 2012).

Section 2 (Brickyard)
Samples collected at Brickyard are poor in fossil content compared to the Zhongxinzhai section, with fusulines occurring at only a few horizons (Figs. 4,6). Especially in the lower part of the section, carbonates are entirely dolomitized, which hampered biostratigraphic age-dating. Therefore, dating for this section relied on both Sr isotope chemostratigraphy and fusuline biochronology. The 87 Sr/ 86 Sr ratios range from 0.70798 to 0.70791. 87 Sr/ 86 Sr values from some key samples are summarized in Table 2. These results indicate ages ranging from the Serpukhovian (330 Ma) to Asselian (earliest Permian) (296 Ma; Howarth and McArthur 1997;McArthur et al. 2001McArthur et al. , 2012. In the lower part of section 2, where fossils occurrences are poor, Sr isotope analysis was more important than biostratigraphy, but age dates based on isotopes and biostratigraphy consistently agreed in the middle and upper parts of the section (Fig. 4).
In the lower part of section 2, sample MM477B gives an age of ca. 330 Ma based on Sr isotope geochronology. The data show that this stratigraphic interval probably corresponds to the Serpukhovian. Two samples (MM470 and MM483; the latter was collected in the vicinity of section 2) from the middle of the section contain Triticites shikhanensis, Schwageriniformis cf. postparallelos, and Quasifusulina sp. (Fig. 6a-c). This assemblage is similar to that seen in the Kasimovian part of section 1. Moreover, a Gzhelian age (300.5 Ma) is obtained from sample MM469 by the Sr isotope dating. Thus, the middle part of section 2 is correlated to the Kasimovian and possibly to the Gzhelian. Higher up the section, two samples (MM467 and MM468) collected from thin limestone intervals contain Pseudoschwagerina sp., Rugosofusulina brevis, Schwagerina cf. nux, and others ( Fig. 6f-i). An Asselian age is demonstrated for the limestones by these fusulines (Leven and Scherbovich 1978;Leven 2009). In the uppermost ~ 100 m-thick part of section 2, represented by bioclastic limestone, four samples were examined for age determination by fusulines. Sample MM464 yields schwagerinids and Ozawainella sp. (Fig. 6d). Although the youngest stratigraphic occurrence of the genus Ozawainella has not been much discussed among fusuline workers until now, available data suggest that its last occurrence is in the Asselian (Leven and Scherbovich 1978;Leven et al. 1992). This implies that the level of sample MM464 is pre-Sakmarian. Above this level, a large schwagerinid having a peculiar style of septal fluting occurred in sample MM461 (Fig. 6k). Although the thin-section of the specimen is very poor, it is probably referable to an advanced large Dutkevitchia species, such as D. complicata, D. splendida, or D. devexa, all of which have large shells with characteristic irregular-shaped bubble-like septal flutings in the axial ends of their tests. Just above, sample MM460 contains a large spherical fusulinid with loose coiling, a thick spirotheca, and almost unfluted septa that is probably referable to Robustoschwagerina (Fig. 6l). Based on these lines of evidence, a Sakmarian age is concluded for the uppermost part of section 2 (e.g. Leven and Scherbovich 1978;Kobayashi and Altiner 2008;Leven 2009). The overall age assessment based on fusulines in section 2 is in good agreement with the Sr chemostratigraphic data. The latter also suggests an earliest Permian age (Asselian) for the upper part of the section (Table 2). Therefore, section 2 can be correlated from the Serpukhovian to the lowermost Permian although a lower-middle Pennsylvanian interval is not clearly recognized (Fig. 4).
Interpretation The predominance of the grainstone texture suggests high-energy environments. The abundance of green algae points to a depositional setting within the photic zone, from a lagoonal environment to shallow shoals (Mamet 1991). However, the association of green algae, Tubiphytes sp. and phylloid algae strongly suggest open-marine conditions (e.g. Mamet 1991; Dawson and Racey 1993). This interpretation is supported by the occurrence of suspension feeders (bryozoans, brachiopods and echinoderms) which require well-oxygenated waters and normal salinity (Wilson 1975).
Interpretation Cortoids resulted from destructive micritization process linked to the activity of microboring organisms. These microborers, light dependent, are commonly interpreted as an indicator of shallow-marine conditions (< 20 m; Swinchatt 1969; Lees and Miller 1985;Aretz and Herbig 2008). Additionally, the occurrence of green algae points to a setting within the photic zone. In Spain, Epimastopora, associated with other dasycladaceans and oncoids, is abundant in outer-shelf facies and occurs commonly in shallow and high-energy environments (Della Porta et al. 2002). Therefore, shallow and agitated waters, located above the FWWB in the photic zone, such as those prevailing in shoals (e.g. Kumpan et al. 2014;Udchachon et al. 2014;Erfani et al. 2016), are inferred.
Interpretation Autochthonous bioclastic packstone, grainstone, floatstone and rudstone are characterized by poorly sorted whole fossils and fragments, which suggest in situ components. The occurrence of burrows suggests shallow lagoon with open marine circulation, deep shelf or mid and outer ramp settings (Henderson et al. 1999;Flügel 2010). However, the association of dasyclad green algae, phylloid algae and calcimicrobes points to a depositional setting within the photic zone (Mamet 1991). In addition, the occurrence of suspension feeders (e.g. bryozoans, brachiopods and corals) which require well-oxygenated waters and normal salinity (Wilson 1975) suggest open marine depositional environment. All these elements allow to constrain the depositional environment to a reef flank setting (Flügel 2010), in a shallow subtidal environment, located in the photic zone. Allochthonous bioclastic packstone, grainstone, floatstone and rudstone is characterized by abraded fossil fragments, associated with mud clasts. Grains occur in chaotic order or present grading texture. Skeletal fragments consist of reef-derived organisms, likely transported down by turbidites, deposited in forereef position, reef slope, toe-ofslope, and basinal settings (Flügel 2010).

Microbial boundstone
Description Microbial boundstone is characterized by stromatolitic internal structures and wrinkle structures on the weathered surface, which is composed of abundant marine cement and encrusting microbial micrite. Three microscopic structures of the microbial boundstone have been identified, including encrusting, laminar, and grid-shaped microbial fabrics (Fig. 10, Huang et al. 2019). Abundant microbial carbonate (e.g. thrombolitic textures, microstromatolites, and microbial ooids) and calcimicrobes (e.g. Girvanella, Ortonella, and Wetheredella-like) occur in the deposits between microbial boundstone and the coral Ivanovia (Huang et al. 2019). Subordinate contributors include corals, crinoids, bryozoans, algae, and sponges.
Interpretation Microbial boundstone, in association with the coral Ivanovia, form the Zhongxinzhai mound (Houchang town, Guizhou). The association of microbial boundstone with numerous calcimicrobes, algae, corals and bryozoans, may indicate that the microbial boundstone was formed in the euphotic zone, with normal salinity condition (Gong et al. 2007a;Zhang et al. 2009;Huang et al. 2019). Ivanovia encrusts hard substrates and favors shallow, clear, highenergy environments (Zhang et al. 2009), which display an alternating encrusting growth pattern with the microbial boundstone, suggesting a shallow and clear water.
Interpretation Phylloid algae are commonly reported as growing in shallow waters, under moderate energy conditions, within the photic zone (e.g. Beauchamp et al. 1989;Wahlman 2002;Gong et al. 2007a, b). This interpretation is supported by the occurrence of dasyclad green algae and Tubiphytes sp., commonly growing in shallow-waters (Mamet 1991). The high diversity biotic association including brachiopods, bryozoans, and foraminifera, attest for open marine conditions and well-oxygenated waters (Oertli 1964;Wilson 1975). Phylloid algal mounds are extensively documented in the literature as growing in relatively shallow subtidal settings (e.g. Toomey et al. 1977;Soreghan and Giles 1999;Samankassou and West 2002).
Interpretation The grainstone texture suggests agitated waters. In addition, Ivanovia are common in intertidal and shallow subtidal environments, in warm and clear marine environment within the photic zone (Taylor and Wilson 2003;Zhang et al. 2009). Therefore, the Ivanovia cf manchurica boundstone formed likely in shallow-water and high energy depositional environment, above the FWWB.
Interpretation Chappell (1980) suggested that branching corals appear as moderately light-dependent, living in calm waters under high sediment influx, in subtidal settings. Scrutton (1999) suggested that fasciculate growth forms were favored by rapid sedimentation rates and less frequent scouring event. The enclosing sediments composed of micrite with scarce skeletal debris (algae, crinoids, brachiopods) confirm low-energy conditions (Scrutton 1999). Therefore, the branching coral boundstone is interpreted as growing in a low energy depositional environment, below the FWWB.
Interpretation The burrowed bioclastic wackestone could have originated as either lagoonal deposits or as basinal background sedimentation, deposited below the FWWB. The occurrence of burrows suggests shallow lagoon with open marine circulation, deep shelf or mid and outer ramp settings (Henderson et al. 1999;Flügel 2010). The lack of light-dependent dasyclad algae allows to restrain the depositional environment to the dysphotic zone (Mamet 1991). Therefore, the origin of the burrowed skeletal wackestone is interpreted as in situ sedimentation, deposited in a lowenergy environment below the FWWB and certainly below the SWB (e.g. Casier et al. 2004).

Fine-grained burrowed wackestone and packstone
Description Dark-colored fine-grained burrowed wackestone and packstone are composed of microfossils, including sponge spicules, shell debris and echinoderm fragments (Fig. 12c, d). Bioclasts are scattered within a dense and strongly burrowed mud matrix. Locally, skeletal fragments are densely packed or sparsely distributed due to the intense burrowing.
Interpretation The skeletal assemblage, composed of finegrained benthic biodetritus including spicule sponges, are common in deep-marine environments, both in slope and basinal settings (Wiedenmayer 1980;Jach 2001). The abundance of burrows supports this statement, occurring commonly in the deep shelf, mid and outer ramp settings (Henderson et al. 1999;Flügel 2010). Therefore, the fine-grained burrowed wackestone and packstone were deposited in outer ramp, deep shelf, and basinal settings with aerobic to possibly dysaerobic bottom conditions (Flügel 2010).

Shallow-water section (section 1)
The lithofacies succession of section 1 suggests shallowwater depositional environments (Fig. 13). The rock succession is dominated by thick limestone beds (> 2 m), dated as Kasimovian to Gzhelian in age. Lithofacies include coated grain grainstone, green algal grainstone, bioclastic packstone, grainstone and rudstone, bioclastic wackestone and floatstone, phylloid algal boundstone, microbial boundstone and burrowed wackestone. From 44 to 59 m, a microbial mound (15 m high, 200 m wide, Fig. 14) was identified, with the secondary contribution of phylloid algae and the coral Ivanovia. The substrate consists of bioclastic wackestone and packstone composed of various organisms including microbialite debris, crinoids, foraminifera, bryozoans, brachiopods, gastropods and algae, along with peloids and coated grains. The base of the mound is characterized by phylloid algal bafflestone (2 m thick and 8 m wide) and the core is built by microbial boundstone and the coral Ivanovia. The grain assemblage suggests a subtidal paleoenvironment, deposited in shallow-waters, within the photic zone (Huang et al. 2019).

Bianping coral reef section
The Bianping coral reef section (Fig. 16) was described by Gong et al. (2004). The reef substrate is composed of bioclastic grainstone, including foraminifera (Triticites, Textularia), brachiopods (Martinia) and crinoids. The lower part of the reef (10 m thick) consists of three patch-reefs, built by phylloid algae, calcimicrobes and the coral Ivanovia cf manchurica, respectively. These patch-reefs are overlain by bioclastic packstone and grainstone, prior to be capped by a brachiopod-rich packstone layer (8 m thick), dominated by Choristites brachiopods, foraminifera (Triticites, Textularia) and corals (Fomitchevella). Above, the coral reef core (37 m thick) is dominated by the Fomitchevella branching colonial corals. Between the coral framework occur bioclast packstone deposits, composed of fusulines (Triticites, Schwagerina), brachiopods (Choristites, Martinia) and crinoids. The basal part of the reef (5 m) is composed of isolated coral patch-reef frameworks with interstitial packstones of densely packed bioclasts of crinoids, fusulines and algal debris. Above, branching corals built a large and massive framework (15 m thick), capped by a bioclastic packstone layer (13 m thick) composed of fusulines and brachiopods. The upper 4 m of the reef is composed of massive coral framestone. Between the coral framework occur bioclastic packstone deposits. Finally, the reef cover consists of bioclastic packstone and grainstone, including fusulines (Triticites, Schwagerina), brachiopods (Choristites, Squamularia) and crinoids.

Depositional environment of the Bianping coral reef
The depositional environment of the Bianping coral reef is currently not well constrained. However, its lithofacies succession provides precious information about the depositional environment (Fig. 16). The reef substrate, composed of bioclastic grainstone, including foraminifera brachiopods and crinoids, is indicative of a high-energy depositional environment, located above the FWWB. Then, the biotic assemblage of the lower part of the reef (calcimicrobes, Ivanovia cf. manchurica and phylloid algae) suggests shallow subtidal settings, deposited around the FWWB within the photic zone. Conversely, the reef core composed of branching coral framestone (Fomitchevella, Fig. 11c) and bioclastic packstone indicates a slightly lower-energy and deeper environment, deposited below the FWWB. The reef cover, composed of bioclastic packstone and grainstone suggestive of a shallower environment, deposited in more agitated waters, around the FWWB. Fig. 9 Bioclastic grainstone, packstone, rudstone and floatstone lithofacies type. The lithofacies is composed of diverse bioclasts, including a bryozoans, brachiopods, molluscs, b, f Tubiphytes, c mud-filled burrows in foraminifer-rich packstone matrix, d Gyroporella sp., e phylloid algae fragments, foraminifera, Pseudogyroporella sp., g fistuliporid bryozoans, h Garwoodia sp. and gastropods. B bryozoan, Br brachiopod, F foraminifera, G gastropod, Gar Garwoodia sp., P Pseudogyroporella, Tu Tubiphytes sp
All of these Pennsylvanian coralliferous bioconstructions were dated from Bashkirian-Kasimovian, with a Gzhelian "reefless lag time", and grew in environments ranging from shallow-waters (above the FWWB) to the low-energy environment (below the FWWB). The reefsize remains small, measuring 10's centimeter to meter thick (< 6 m), except in Japan (large atoll). The reefal assemblages were commonly composed of corals, chaetetids, calcareous algae, foraminifera, crinoids, molluscs, Fig. 11 Phylloid algal and coral boundstone lithofacies types. a Phylloid algal boundstone. Pores are filled by fine-grained peloidal packstone and marine radiaxial calcite cement. Algal thalli are draped by conspicuous marine cement or encrusted by microbialite and Tubiphytes prior to the precipitation of marine cement in interstitial space. Matrix contains common fusuline foraminifera. b Ivanovia coral. c Branching rugose corals (Fomitchevella) formed large colonies (> 50 cm), preserved in their original up-right position. F foraminifera, Ph phylloid algae, RFC radiaxial fibrous cement ◂ bryozoans, associated locally to worm tubes, microbialites, stromatolites and Tubiphytes.

Platform model
The Houchang area is located at the border of a shelfal depression, inherited by pre-Carboniferous tectonic deformations (Tsien et al. 1988;Shen 2002). The Kasimovian-Gzhelian shallow-water limestones are composed of coated-grain grainstone and green algal grainstone lithofacies, deposited in shoals and/or above the FWWB. Bioclastic packstone, grainstone and rudstone deposited around the FWWB. The lack of siliciclastic deposits points to a low sediment influx, possibly due to a far distance from the Yangtze Craton. In shallow-waters, during the Kasimovian-Gzhelian, the persistence of stenohaline organisms such as bryozoans, brachiopods and foraminifera, tends to reflect normal salinity conditions (Oertli 1964;Flügel 2010). However, during the Kasimovian, the abundance of staffellids (Fig. 13), commonly considered to have wider environmental tolerance (Vachard et al. 2010), associated to sporadic schwagerinids documents possibly a time interval with slightly higher salinity, related to a restricted environment and/or semi-restricted conditions, likely controlled by sea-level fluctuations. Conversely, the Late Kasimovian-Gzhelian deep-water rocks are composed of microbioclastic peloidal packstone and grainstone, in situ fine-grained burrowed wackestone and packstone deposited in a low energy depositional environment, marls and thin allochthonous bioclastic packstone, grainstone, floatstone and rudstone layers (calciturbidites). The scarce and thin calciturbidite layers (< 1 m thick), indicate a starved basin, with a low sediment export from platform to adjacent basins. The transition from shallow-water (section 1) to basinal environments (section 2) points to a ramp, possibly distally steepened, or a slope (Fig. 17). From the Gzhelian to Early Permian (Asselian-Sakmarian), the deepwater rock succession presents a shallowing-upward sequence, dominated by the deposition of bioclastic packstone, grainstone and rudstone layers, which seems to indicate that the shelfal depression gradually became infilled, certainly due to the global sea-level fall (Early Permian glaciation; Fielding et al. 2008a;Rygel et al. 2008).
Within the Timan-Pechora Basin (Northern Russia), elongate seafloor structures are reported during the Pennsylvanian-Early Permian, consisting of narrow structural highs separated by broad depressions (Wahlman and Konovalova 2002). On the top and flanks of structural highs, light-colored and commonly fusuline-rich bioclastic grainstone-packstone shoals and Palaeoaplysina bioherms were deposited. Conversely, in the deeper-water shelfal depression, dark-colored argillaceous fenestrate bryozoan-crinoidal wackestone and packstone were accumulated. In the distal part of the shelf, near the western margin of the Ural Through, a carbonate bank grew on a slope or deep-ramp paleogeographic highs. The surrounding deep-water seafloor facies consist of dark-grey cephalopod-bearing marls, shales and siltstones (Wahlman and Konovalova 2002).
The sedimentary depositional systems of the Dian-Qian-Gui Basin (southern China) and Timan-Pechora Basin (Northern Russia) appear to be similar in terms of lithofacies and structures. However, the reported respective reefs differ in composition. In southern China, shallow-water reefs are composed of calcimicrobes (Huang et al. 2019), red algae (Tan 1991;Yao and Wang 2016), phylloid algae (Gong et al. 2007a, b), Tubiphytes (Chang et al. 2008;Guan et al. 2010;Yao and Wang 2016) and corals Gong et al. 2012), whereas in the Timan-Pechora Basin bioherms are composed of Palaeoaplysina and bryozoan-Tubiphytes (Wahlman and Konovalova 2002). The bioherm compositions can be explained by the paleolatitudes: southern China was located in subequatorial position (15°S), whereas Northern Russia was located in subtropical to temperate paleolatitudes (20-30°N; Webb 2002). Therefore, the Dian-Qian-Gui Basin (southern China) can be considered as a warm-water counterpart of the temperate Timan-Pechora Basin (Northern Russia).

Discussion
The scarcity of Pennsylvanian coral reefs documents unfavorable conditions globally for their development. However, the exceptional occurrence of the Bianping coral reef, reported in southern China, reveals specific settings favoring the development of coral communities. This occurrence raises several questions: How do environmental conditions Fig. 12 Burrowed bioclastic wackestone, microbioclastic peloidal packstone-grainstone and fine-grained burrowed wackestone-packstone lithofacies types, which represent a deep-water, low energy paleoenvironment. a Burrowed bioclastic wackestone. Biogenic components consist of foraminifera, debris of echinoderms and phylloid algae. Locally, scarce peloids occur. b Laminated microbioclastic peloidal packstone and grainstone with fine peloids and exhibiting mm-scale layering. c Fine-grained burrowed wackestone and packstone composed of small-sized fossil fragments, scattered within a dark and strongly burrowed matrix. Burrows are infilled with microbioclastic peloidal packstone and grainstone sediments. d Finegrained burrowed wackestone. Matrix consist of dark grey micrite. Bioclasts include small echinoderm fragments and spicule sponges. Paleoclimate: which impact on coral reef communities?

Bashkirian-Moscovian
The Early Pennsylvanian (Bashkirian-Early Moscovian) coincides with a major glacial phase, with the expansion of ice-sheets in South America, southern Africa and Australia (Fielding et al. 2008a). Crowley et al. (1996) and Bruckschen et al. (1999) have estimated a tropical sea-surface temperature (SST) of about 20 ± 5 °C, 5 °C lower than that prevailing during the Viséan.
The tropical sea-surface temperature is one of the primary controlling factors in the distribution of modern tropical coral reef ecosystems (Kleypas et al. 1999;Lough 2012). Currently, reef-building corals grow optimally between 23° and 29 °C (Spalding and Brown 2015), and spend 70% of the time within a 3 °C SST range (27-30 °C; Lough 2012). Consequently, during the last decades, coral reefs occurred in the warmer part of the tropical oceans, living within 30° latitude of the equator. Most warm-water corals exhibit a symbiotic relationship with zooxanthellae algae, from which they derive much of their nutrition. However, this relationship breaks down under seawater warming, inducing episode of coral reef bleaching and mortality today (e.g. Baker et al. 2008;Hoegh-Guldberg 2011;Spalding and Brown 2015). Therefore, modern coral reefs, sensitive to seawater temperature, represent good indicators of climate changes.
Currently, even if a symbiosis between Paleozoic corals and algae is debated (Scrutton 1998;Copper and Scotese 2003), the Paleozoic coral reefs appear as adapted to soft substrates, warm waters and shelf seas (Scrutton 1998). The unequivocal evidence for this hypothesis is the radiation of the Paleozoic coral-stromatoporid reefs during the Devonian (Emsian-Givetian), the warmest period of the Phanerozoic (Copper and Scotese 2003). During this time-interval, the warm sea-surface temperature favored the formation of coral-stromatoporid mega-reefs and stretched them to high latitudes of at least 40°-50°. Based on these findings, Paleozoic and modern coral reefs have in common an affinity for warm waters. Consequently, such as modern corals, it can be assumed that Paleozoic coral reefs were also affected by climate changes, especially cooling.
The investigation of the Carboniferous coral reef distribution tends to confirm this hypothesis. Indeed, during the Viséan (Mississippian) prior to the glaciation, numerous coralliferous reefs have been reported (e.g. Australia, Pickett 1967;Pickett and Wu 1990;Webb 1987Webb , 1998Shen and Webb 2005;UK and Ireland, Wolfenden 1958;Adams 1984;Aretz and Herbig 2003;Aretz et al. 2010;Canada, Schenk et al. 1984;Ukraine, Ogar 2012;China, Gong et al. 2012;Yao and Wang 2016;Maillet et al. 2020;Morocco, Rodriguez et al. 2012;Kazakhstan, Cook et al. 1994), distributed from Kazakhstan to Australia, in equatorial to the subequatorial position (Fig. 18a). During the Serpukhovian and the initiation of the mid-Carboniferous glaciation, a clear decline of coralliferous reefs is observed (Fig. 18b) and the rare surviving coral reefs (e.g. Kazakhstan and USA) disappear during the Early Bashkirian, except in Japan. The mid-Carboniferous coral reef decline fairly correlated with the mid-Carboniferous climate transition, from the greenhouse to icehouse conditions. This correlation suggests that Paleozoic corals were most likely affected by cooling, and this environmental parameter played certainly a key role in the scarcity of Early Pennsylvanian coral reefs.
During the Bashkirian-Moscovian, new coral reefs developed in the Donets Basin (Ukraine), Cantabrian Mountains (Spain) and Moscow Basin (Russia) (Fig. 18c). All of these Early Pennsylvanian coralliferous reefs grew in lower latitudes, in the warmer part of the ocean, at the borders of the Paleo-Tethys Ocean and at the eastern border of the Panthalassa Ocean. The range of latitudes is significantly narrower than that of the Viséan, and climate cooling appears the most likely cause.

Kasimovian-Gzhelian
During the Late Pennsylvanian (Late Moscovian-Early Gzhelian), a relative climate warming occurred (Fielding et al. 2008a). While glacial retreat is observed in some regions (e.g. eastern Australia), other areas record continuing or intermittent glacial conditions (e.g. Oman, India, western Australia; Fielding et al. 2008a). However, despite the favorable climate warming, this period records a subordinate decline of coralliferous reefs, with only a few examples reported (e.g. Austria-Italy, southern China, Fig. 18d; Table 3). Therefore, paleoclimate can hardly explain the distribution of the Late Pennsylvanian coralliferous reefs, which leads to explore additional inhibiting factors for this period.

Late Pennsylvanian: could biological competition explain the scarcity of coral reefs?
The Late Pennsylvanian (Kasimovian-Gzhelian) is characterized by sea-level fluctuations of high amplitude, varying from 10 to ~ 120 m Ross 1987, 1988;Maynard and Leeder 1991;Haq and Schutter 2008;Rygel et al. 2008;Davydov et al. 2012), with a periodicity within the Milankovitch ranges (Harrison et al. 1979;Ross and Ross 1987;Heckel et al. 2007). These environmental conditions promoted the proliferation of phylloid algae, becoming the most abundant shallow-waters reef builders until the Permian. This radiation is attributed to the rapid reproduction and growth rates of phylloid algae, which enabled them to colonize shallow-water areas rapidly and outcompete other organisms for optimum living space. This opportunistic growth strategy was ideally suited to the cyclic glacioeustasy of the Pennsylvanian-Early Permian, which caused shallow-shelf areas to be repeatedly exposed and drowned at relatively frequent intervals (Wahlman 2002). Therefore, shallow-water shelf and shelf-margin ecological niches were newly occupied by phylloid algae (Wilson 1975), to the detriment of less adaptative and less opportunistic corals, making their recovery difficult to succeed.
In southern China, corals and phylloid algae do not occur in the same horizons (Fig. 13). Within the Bianping coral reef, phylloid algae contributed to the initiation of the reefbuilding under shallow and agitated waters (above or around the FWWB). However, with the increase of water depth, phylloid algae disappeared and gave way to brachiopods and branching colonial corals. Thus, the present study demonstrates that coral communities were able to colonize a deeper water setting, where they were not in competition with the contemporaneous opportunistic phylloid algal community that dominated shallower water environments. This coral growth strategy could explain the discovery of the Bianping coral reef in southern China and possibly the auloporid coral mounds reported in the Carnic Alps (Austria-Italy), growing in low-energy environments. However, the scarcity of coral reefs worldwide, even in deeper environments, leads to explore additional inhibiting factors.

Pennsylvanian aragonite seas
Throughout the Phanerozoic, seawater had oscillated between aragonite and calcite seas, favoring the precipitation of either high-Mg calcite and aragonite or low-Mg calcite (Sandberg 1983 ; Fig. 19). The intervals of calcite and aragonite production are thought to be caused primarily by secular variations in the Mg/Ca ration of seawater (e.g. Stanley and Hardie 1998). These oscillations tend to Fig. 15 Lithofacies successions and paleoenvironments, from section 2 (Brickyard). During the latest Mississippian-earliest Pennsylvanian, lithofacies are commonly dolomitized, inhibiting the original texture to be identified. During this time-interval there is a probable unconformity, with the lack of Bashkirian deposits. During the Kasimovian-Early Permian (Asselian), lithofacies suggest toe-of-slope and basinal settings, accumulated in the Luodian intraplatform Basin. During the late Early Permian (Sakmarian), lithofacies indicate likely reef slope settings, which suggests a regression. a Field photograph of alternating of well-bedded fine-grained burrowed wackestone and packstone, thin marl and scarce allochthonous packstone and grainstone layers (< 80 cm thick). b Field photograph of chert belts. c Field photograph of microbioclastic peloidal packstone and grainstone turbidite bed unit displaying small-scale ripple cross-lamination. The underlying layer is composed of fine-grained burrowed wackestone and packstone ◂ correspond to changes in the mineralogy of abiotic as well as biotic carbonates Hardie 1998, 1999;Ries et al. 2006;Kiessling et al. 2008). For example, in the Early and Mid-Paleozoic calcite seas (Ordovician-Mississippian), reefs were dominated by calcitic tabulate, heliolitid, rugose corals and calcitic stromatoporoid sponges (Wood 1987). Conversely, in the Late Paleozoic-Early Mesozoic aragonite seas, reef-builders were dominated by phylloid algae (Pennsylvanian-Permian), scleractinian corals (Early Mesozoic) and high-Mg calcite ancestral coralline algae (Stanley and Hardie 1998). Therefore, the fluctuations of seawater composition impacted biocommunities, constraining them to evolve (e.g. Hautmann 2006) or disappear.
This theory would suggest that all Pennsylvanian coral reef genera, including the rugose corals Yuanophylloides, Axolithophyllum, Monophyllum, Caninia, Amandophyllum, Caniniostrotion, Tschussovskenia, Petalaxis, Fomitchevella and Cladochonus, Syringopora, and tabulate coral Multithecopora (Table 3), precipitated intermediate-Mg calcite skeletons. However, this hypothesis is currently not demonstrated and it cannot be excluded that some of these corals precipitated low-Mg calcite skeletons.

How could the coexistence of aragonite and high-Mg calcite components with low-Mg calcite corals be explained?
From the Early to Late Pennsylvanian, a brief drop in Mg/Ca ratio of seawater is recorded, from about 4.2-2 mol (Stanley and Hardie 1998; Fig. 19). In theory, this decline could promote the coexistence of aragonite, high-Mg calcite and low-Mg calcite skeletons during the Kasimovian-Gzhelian. However, several bioconstructions, including calcite corals, have been also reported from the Early Pennsylvanian (Bashkirian-Moscovian, Table 3), when the Mg/Ca ratio of seawater was high (about 4.2 mol), which seems to invalidate this first hypothesis. Fig. 17 Kasimovian-Gzhelian platform model, records from Houchang (southern China). Coated-grain grainstone and green algal grainstone lithofacies types deposited in shallow waters and shoals. In situ bioclastic pack-, grain-, rud-and float-stone lithofacies type deposited around the FWWB, within the photic zone, whereas allochthonous bioclastic pack-, grain-, rud-and float-stone were reworked and deposited from peri-reefal to basinal settings. The burrowed bioclastic wackestone, microbioclastic peloidal packstone and grainstone, fine-grained burrowed wackestone and packstone and marls deposited in low energy environment, below the SWB, into the slope and basinal environment. In Houchang, phylloid algal reefs, microbial mounds, sponge mounds and Tubiphytes reefs grew in shallow-waters (Yao and Wang 2016). Conversely, the large Bianping coral reef was initiated in shallow-waters, mainly built by Ivanovia, calcimicrobes and phylloid algae, but grew with the increase of water-depth allowing the development of Fomitchevella branching corals, below the FWWB Other studies (e.g. Adabi 2004) reveal that, even if biotic and abiotic carbonates follow the general trend of calcite seas and aragonite seas (Sandberg 1983), a coexistence of organisms with calcite and aragonite mineralogy is possible, depending on environmental conditions. In the "aragonite seas", the precipitation of high-Mg calcite and aragonite would be favored in low latitudes and shallow-waters.
Conversely, the precipitation of low-Mg calcite biota and cements would be possible thanks to cooling, decreasing of light, high paleolatitudes or increasing paleobathymetry (Wahlman 2002;Adabi 2004). This assumption is currently used to explain the occurrence of the large Viséan-Moscovian reef complex in the tropical seas of Japan. The abundance of corals in the Akiyoshi reef complex is currently attributed to the location: it formed atoll-like reef on a seamount, where they would have been exposed to the upwelling of cold open-ocean seawater (Wahlman 2002).
However, worldwide, Early Pennsylvanian coralliferous bioconstructions were mainly present in low latitudes (Fig. 18c) and shallow-waters (Table 3). Therefore, the precipitation of low-Mg calcite corals cannot be attributed to the decreasing of light, high paleolatitudes or increasing paleobathymetry. The remaining hypothesis would be climate cooling. However, this statement appears incompatible with the development of tropical corals, which raises new questions about the factors controlling this coexistence.

How can the existence of the Late Pennsylvanian Bianping coral reefs be explained?
During the Late Pennsylvanian, the main factor inhibiting the development of coral buildups was probably biological competition with calcareous algal communities, and seawater chemistry might also have played a role. During this period, only two coral reefs have been reported in Austria-Italy and southern China. It is interesting to note that both reefs occur in a low-energy depositional environment, below the FWWB. This depositional environment differs from that of the Early Pennsylvanian, where coral reefs grew commonly in shallow waters, above the FWWB. Therefore, the deeper depositional environment where the competition of corals with phylloid algae is reduced most likely favored the development of coral communities building the large reef of the present study.

Conclusions
1. In Houchang (southern China), the Kasimovian-Gzhelian carbonate platform records a high diversity of bioconstructions, including Tubiphytes reefs, sponge reefs, phylloid algal reefs, microbial mounds and a large coral reef, growing at the margin of a shelfal depression. 2. The existence of the large Bianping coral reef is uncommon for the Pennsylvanian, lacking any analogs in age (Gzhelian), size (80-100 m thick) and composition (Fomitchevella, phylloid algae, the corals Ivanovia and Antheria, microbialites, brachiopods, fusulines, crinoids, algae, bryozoans, Tubiphytes). 3. The large coral reef (southern China) was initiated in shallow waters, with the deposition of phylloid algal, the coral Ivanovia and microbial patch reefs. But the reef core composed of branching corals formed when water-depth increased, with the reef growth occurring below the FWWB. 4. The carbonate platform records the deposition of green algal grainstone, coated grain grainstone and bioclas-tic packstone, grainstone, floatstone and rudstone in shallow-waters, and burrowed bioclastic wackestone, microbioclastic peloidal packstone and grainstone, and fine-grained burrowed wackestone and packstone in the deep-water shelfal depression. In this context, the coral reef developed in the deep shelf margin, in a moderate to low energy depositional environment. 5. The investigation of the Pennsylvanian environmental conditions reveals global inhibiting factors leading to the scarcity of Pennsylvanian coral reefs: -The Early Pennsylvanian cooling fairly correlates with a decline of coralliferous bioconstructions. However, during the Late Pennsylvanian, despite the short-lived climate warming, this period records a subordinate decline of coral buildups. Therefore, paleoclimate could explain the collapse of the Early Pennsylvanian coral reefs but cannot explain the distribution of the Late Pennsylvanian reefs. -The Late Pennsylvanian coincides with the radiation of phylloid algae, more opportunistic and adaptative than corals, that occupied most of the shallow and warm waters. Consequently, the biological competition could explain the scarcity of coral reefs during the Late Pennsylvanian. -The Carboniferous is characterized by the transition from calcite (Mississippian) to aragonite seas (Pennsylvanian). Paleozoic corals, precipitating low to intermediate Mg calcite skeletons, were likely affected by the seawater chemistry evolution, which could explain the coral reef collapse. However, additional investigations on coral skeletons are needed to confirm this hypothesis. 6. During the Late Pennsylvanian, the main factor inhibiting the development of coral buildups was probably biological competition with calcareous algal communities, and seawater chemistry might also have played a role. However, the deeper depositional environment where the competition of corals with phylloid algae is reduced most likely favored the development of coral communities building the large reef of the present study.
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