Carboniferous carbonate platforms and coral reefs: New insights from southern China

Thesis

Reference

Carboniferous carbonate platforms and coral reefs: New insights from southern China

MAILLET, Marine

Abstract

Metazoan reef builders receded globally during the Carboniferous resulting from the Late Devonian extinction events. During this period, shallow-water bioconstructions are commonly small and scarce, and corals play a minor role in their construction. However, in southern China, several extended coral-bearing bioconstructions have recently been discovered, dated from Late Viséan-Serpukhovian and Late Kasimovian-Gzhelian respectively. To understand the coral reef distribution and to constrain the factors that control their growth, eight sections were investigated using petrography, biostratigraphy, chemostratigraphy and geochemistry.

MAILLET, Marine. Carboniferous carbonate platforms and coral reefs: New insights from southern China. Thèse de doctorat : Univ. Genève, 2020, no. Sc. 5520

URN : urn:nbn:ch:unige-1469490

DOI : 10.13097/archive-ouverte/unige:146949

Available at:

http://archive-ouverte.unige.ch/unige:146949

Disclaimer: layout of this document may differ from the published version.

Section des Sciences de la Terre et de l’Environnement Département des Sciences de la Terre

Elias Samankassou

Carboniferous Carbonate Platforms and Coral Reefs: New Insights from Southern China

THÈSE

présentée à la Faculté des sciences de l’Université de Genève pour obtenir le grade de Docteur ès sciences, mention sciences de la Terre

Par

Marine MAILLET Paris (France) Thèse N°5520

GENÈVE

Atelier de reprographie ReproMail 2020

Section des Sciences de la Terre et de l’Environnement Département des Sciences de la Terre

Elias Samankassou

Carboniferous Carbonate Platforms and Coral Reefs: New Insights from Southern China

THÈSE

présentée à la Faculté des sciences de l’Université de Genève pour obtenir le grade de Docteur ès sciences, mention sciences de la Terre

Par

Marine MAILLET Paris (France) Thèse N°5520

GENÈVE

Atelier de reprographie ReproMail 2020

Prof. Elias Samankassou, University of Geneva, Rue des Maraichers 13, 1205 Genève, Switzerland Jury members

Prof. Giovanna Della Porta, University of Milan, Via Mangiagalli 34, 20133 Milano, Italy

Prof. John Reijmer, King Fahd University of Petroleum & Minerals, Dhahran, 31261 Kingdom of Saudi Arabia Prof. Rossana Martini, University of Geneva, Rue des Maraichers 13, 1205 Genève, Switzerland

Abstract

Metazoan reef builders receded globally during the Carboniferous resulting from the Late Devo- nian extinction events. During this period, shallow-water bioconstructions are generally small and scarce, and corals play a very minor role in their construction. However, in Southern China, several coral-bearing bioconstructions have recently been discovered, including two extended reefs, dated respectively Late Viséan-Serpukhovian (50 m high, Guangxi) and Kasimovian-Gzhelian (80-100 m high, Guizhou). To understand the coral reef distribution and to constrain the factors that control their growth, eight sections were investigated using petrography, biostratigraphy, chemostratigraphy and geochemistry.

In Langping (Guangxi), one large (50 m high and 250 m wide) and three small (6–16 m high and 10-20 m wide) coral reefs, dating from the Late Viséan–Serpukhovian, have recently been identified in two localities. These exceptional Mississippian reefs grew as part of a carbonate ramp (slope an- gle: 1–2.5°). The Late Viséan-Serpukhovian time-interval coincides with high amplitude and high frequency sea-level fluctuations, most likely related to glaciation). Due to the scarcity of extended coral reef elsewhere, it suggests that the southern China region provided an ecological refuge for corals, led by the persistence of warm oceanic currents and/or by the deep depositional environment (‘deep reef refugia’ hypothesis).

In Houchang (Guizhou), 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, 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.

The scarcity of Carboniferous coral-bearing bioconstructions worldwide suggests global unfavor- able conditions, with specific settings acting as refugia. To constrain the environmental conditions and consequently understand the global reef distribution, seawater composition was investigated using carbon and oxygen isotopes. The resulting oceanic δ¹³C curves revealed several environmental changes. On the one hand, during the Late Viséan-Serpukhovian, δ¹³C values (3‰) point to short-lived glacial events associated with the expansion of ice-sheets in South America and eastern Australia.

the other hand, during the Kasimovian-Gzhelian, the δ C values correlate with a period of climate warming, which should be suitable for tropical coral communities. However, the scarcity of Pennsyl- vanian coral reef leads to consider other inhibiting factors (e.g. biological competition, aragonite seas).

In the Helv and Dujie villages (Guangxi), the evolution of stromatolite morphologies and the cyclic alternation of subtidal and peritidal facies imply high frequency sea-level fluctuations during the latest Viséan to Serpukhovian, most likely eustatic in origin. In peritidal deposits, the δ¹³C curve exhibits a positive excursion of +4.3‰, interpreted as the result of the onset of the Late Paleozoic Ice Age.

The present work adds evidence to the current knowledge that coral reefs are able to resist harsh environmental conditions, thanks to their location (low latitudes), the persistence of warm ocean cur- rents on these carbonate platforms and the deep depositional environments. The results of the study improves significantly the understanding of the Carboniferous reef demise, after the Late Devonian extinction events, and their subsequent recovery.

Résumé

Au cours du Carbonifère, la construction de récifs métazoaires s’effondre à la suite des extinc- tions de masse de la fin du Dévonien. Les rares bioconstructions décrites sont généralement de petite taille, et les coraux jouent un rôle mineur dans leur construction. Cependant, au sud de la Chine, plusieurs récifs coralliens ont récemment été découverts, dont deux grands récifs datés respective- ment fin Viséen-Serpukhovien (50 m de haut, Guangxi) et Kasimovien-Gzhélien (80-100 m de haut, Guizhou). Pour comprendre la distribution de ces récifs et pour contraindre les facteurs qui contrôlent leur croissance, huit sections ont été étudiées au sud de la Chine en utilisant la pétrographie, la bio- stratigraphie, la chimiostratigraphie et la géochimie.

À Langping (Guangxi), un large (50 m de haut et 250 m de large) et trois petits (6-16 m de haut et 10-20 m de large) récifs coralliens, datés fin Viséen-Serpukhovien, ont été récemment découverts à deux endroits. Ces récifs mississippiens exceptionnels se sont développés sur une rampe carbon- atée (angle : 1-2.5°). La fin du Viséen-Serpukhovien coïncide avec des variations du niveau marin de grande amplitude et de haute fréquence, associées à une période glaciaire. A cause de la rareté de grand récif corallien à travers le monde, il ne peut être exclu que le sud de la Chine fournit un refuge écologique pour les coraux, grâce à la persistance de courants océaniques chauds et/ou grâce à l’environnement de dépôt profond.

À Houchang (Guizhou), au sein de la plateforme carbonatée pennsylvanienne a été découvert un grand récif corallien. Ce récif ne possède aucun analogue en terme d’âge (Gzhelien), de taille (80- 100 m d’épaisseur) et de composition (forte biodiversité). Le grand récif corallien s’est développé à la bordure du bassin“Luodian”. Plus précisement, ce dernier s’est développé au niveau de la marge profonde de la plateforme, dans un environnement de dépôt d’énergie modérée à faible, en dessous de la limite d’action des vagues de beau temps. La rareté des récifs coralliens pennsylvaniens suggère des conditions globales défavorables, qui peuvent être attribuées à plusieurs facteurs environnemen- taux. Parmis ces facteurs on compte la chimie de l’eau de mer (mers d’aragonite), le refroidissement climatique et la compétition biologique avec les communautés d’algues phylloïdes, opportunistes et adaptatives, qui occupaient des paléoenvironnements similaires à ceux des coraux. L’existence du grand récif corallien de Bianping (Chine), ainsi que l’existence de quelques bioconstructions coralliennes du même âge à travers le monde, prouvent que les communautés coralliennes ont été capables de supporter les conditions environnementales fluctuantes du Paléozoïque supérieur dans des environnements spécifiques. L’un de ces environnements favorable pourrait être la marge pro- fonde de la plateforme, où la faible luminosité pourrait avoir réduit la concurrence entre les coraux et les algues phylloïdes.

La rareté des récifs coralliens à travers le monde durant le Carbonifère, suggère des conditions globales défavorables, avec des conditions spécifiques agissant comme des refuges. Pour contraindre

sition de l’eau de mer fut étudiée en utilisant les isotopes de carbone et d’oxygène. Les courbes de δ¹³C révèlent plusieurs changements environnementaux, liés au cycle du carbone. D’une part, à la fin du Viséen-Serpukhovien, les valeurs de δ¹³C (3‰) concordent avec des évènements glaciaires, avec l’expansion de calottes en Amérique du Sud et à l’est de l’Australie. Par conséquent, le déclin récifal en zone équatoriale peut être expliqué par le refroidissement des températures. D’autre part, au Kasimovian-Gzhélien, les valeurs du δ¹³C corrèlent avec une période de réchauffement climatique, qui devrait être favorable aux communautés coralliennes tropicales. Cependant, la rareté de récifs coralliens pennsylvaniens suggère d’autres facteurs inhibiteurs (ex : compétition biologique, mers aragonitiques).

Dans les villages de Helv et Dujie (Guangxi), l’évolution des morphologies de stromatolite et les alternations cycliques des faciès subtidaux à péritidaux attestent de fluctuations du niveau marin durant la toute fin du Viséen au Serpukhovien, certainement d’origine eustatique. Dans les sédiments péritidaux, la courbe de δ¹³C enregistre une excursion positive de +4.3‰, interprétée comme étant liée au début de l’âge glaciaire de la fin du Paléozoïque.

Cette étude apporte aux connaissances actuelles des preuves que les récifs coralliens sont capable de résister aux conditions environnementales difficiles du Carbonifère, grâce à leur distribution spa- tiale (faibles latitudes), la persistence de courants océaniques chauds sur ces plateformes carbonatées et à des environnements de dépôt profonds. Les résultats de l’étude améliorent significativement la compréhension de l’extinction des récifs coralliens, suite aux crises de la fin du Dévonien, et de leur reconquête des océans.

Acknowledgments

First, I would link to thank Elias Samankassou, my supervisor, for his scientific support, the en- riching scientific exchanges and for giving me the opportunity to work in a very interesting project.

Thank you for your reviews and advice that contribute to improve the content and the structure of the manuscript.

In addition, I would like to thank people involved in the project:

• Wen-Tao Huang, Zhuo-Wei Miao, En-Pu Gong, Chang-Qing Guan, Yong-Li Zhang, Xiao Li and Zhen-Yuan Yang for preparing and managing fieldworks;

• François Gischig (University of Geneva) for the thin section manufacturing;

• Katsumi Ueno (University of Fukuoka) for his scientific expertise on foraminifera;

• Massimo Chiaradia (University of Geneva) for strontium isotope analysis;

• Torsten Vennemann (University of Lausanne) and Michael Joachimski (University of Erlan- gen-Nuremberg) for the carbon and oxygen isotope analysis.

Thank you to the jury members, Dr Giovanna Della Porta (University of Milan), Dr John Reijmer (King Fahd University of Petroleum & Minerals, Dhahran, Saudi Arabia) and Dr Rosanna Martini (University of Geneva) for their scientific expertise.

Moreover, I would like to thank the teaching and research staff, PhD candidates, students and ad- ministrative team of the Department for the great times spent together and the enriching discussions.

Finally, I would like warmly to thank my family, my friends as well as my partner, for their love and unbreakable support.

This work was supported by the Swiss National Science Foundation [grant number 200021_160019].

Content

Chapter I: Introduction

THE CARBONIFEROUS 1

Late Devonian extinction events 1

Paleogeography 1

Paleoclimate and glacio-eustatic fluctuations 2

Reef-building communities 4

Coral reefs 5

SOUTHERN CHINA SETTING 5

Paleogeography 5

Carboniferous reefs 7

Study areas 7

Objectives and main findings of the thesis 8

Chapter II: Coral reefs and growth dynamics of low-angle Carboniferous platform: Records from Tianlin, southern

China

ABSTRACT 12

INTRODUCTION 12

GEOLOGICAL SETTING 13

Paleogeogeography 13

Paleoclimate and sea-level fluctuations 14

METHODS 15

Chemostratigraphy 15

Biostratigraphy 15

Petrography 16

Restored section 16

DATA 16 DATING 16

Biostratigraphy 20

MICROFACIES ANALYSES 22

Coated-grain grainstone 22

Bioclastic grainstone, packstone and rudstone 25

Lithoclastic grainstone and rudstone (breccia) 25

Coral framestone 26

Stromatactis mudstone, wackestone and floatstone 28

Skeletal grain packstone and wackestone 28

Crinoid-rich packstone 30

Lime mudstone 30

RESULTS: MFT SUCCESSIONS AND PALEOENVIRONMENT 31

Section 1 31

Section 2 31

Section 3 33

INTERPRETATIONS 35

Nutrient and salinity 35

Late Viséan-Serpukhovian platform model 37

DISCUSSION: INFLUENCE OF ENVIRONMENTAL CONDITIONS ON

CARBONIFEROUS CORAL-REEF DISTRIBUTION 39

Background: Carboniferous reefs and mounds 39

Impact of paleoclimate and sea-level fluctuations 40

Impact of platform morphology 41

Impact of water depth and potential refugia 41

CONCLUSIONS 42

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

ABSTRACT 44

INTRODUCTION 44

Southern China paleogeography 45

Study area 45

Global paleoclimate and glacio-eustatic fluctuations 50

METHODS 50

Petrography 50

87Sr/⁸⁶Sr dating 52

Fusulinid biochronology 52

AGE 52

Section 1 (Zhongxinzhai) 52

Section 2 (Brickyard) 54

LITHOFACIE TYPES 56

Green algal grainstone 56

Coated-grain grainstone 56

Bioclastic packstone, grainstone, floatstone, rudstone 59

Microbial boundstone 61

Phylloid green algal boundstone 61

Ivanovia cf. manchurica boundstone 61

Branching coral boundstone 62

Burrowed biolcastic wackestone 64

Microbioclastic peloidal packstone and grainstone 64

Fine-grained burrowed wackestone and packstone 64

LITHOFACIES SUCCESSIONS 66

Shallow-water section (section 1) 66

Deep-water section (section 2) 66

Bianping coral reef section 66

INTERPRETATIONS 71

Depositional environment of the Bianping coral reef 71

Bianping coral reef: A world exception 71

Platform model 74

Paleoclimate: Which impact on coral reef communities? 76 Late Pennsylvanian: Could biological competition explain the scarcity of coral reefs? 79

Seawater chemistry 80

How can the existence of the Late Pennsylvanian Bianping coral reefs be

explained? 82

CONCLUSIONS 83

Chapter IV: Oceanic δ

C , environmental changes and Carbon- iferous coral reef development: Records from Tianlin and Ziyun

(southern China)

ABSTRACT 86

INTRODUCTION 86

PALEOCLIMATE AND PALEOGEOGRAPHY 87

Paleoclimate 87

Carboniferous reef builders 87

GEOLOGICAL SETTINGS 88

Southern China paleogeography and reefs 88

Measured sections 90

SELECTED SAMPLES 91

METHODS 93

Cathodoluminescence 93

δ¹³C and δ¹⁸O isotopes 93

DATA 93

Oxygen isotopes 93

Carbon isotopes 94

INTERPRETATIONS AND DISCUSSION 95

Mississippian (Tournaisian – Early Serpukhovian) 98

Mid Carboniferous (Serpukhovian – Early Bashkirian) 99

Carboniferous environmental conditions and coral reef development 100

CONCLUSIONS 103

Chapter V: The onset of the major glaciation of the LPIA:

Record from South China

ABSTRACT 108

INTRODUCTION 108

GEOLOGICAL SETTING AND BIOSTRATIGRAPHY 109

MATERIALS AND METHODS 113

MICROFACIES 113

Helv Section 113

Dujie Section 118

CARBON AND OXYGEN ISOTOPE DATA 123

DISCUSSION 126

Depositional processes and sea level changes 126

Interpretation of carbon isotope 129

Implications for the LPIA 132

CONCLUSION 132

Chapter VI: Synthesis

CORAL REEFS AND GROWTH DYNAMICS OF A LOW-ANGLE CARBO-

NIFEROUS PLATFORM: RECORDS FROM TIANLIN, SOUTHERN CHINA 135 LATE PENNSYLVANIAN CARBONATE PLATFORM FACIES AND CORAL

REEF: NEW INSIGHTS FROM SOUTHERN CHINA ( GUIZHOU PROVINCE) 136 OCEANIC δ¹³C, ENVIRONMENTAL CHANGES AND CARBONIFEROUS

CORAL REEF DEVELOPMENT: RECORDS FROM TIANLIN AND ZIYUN

(SOUTHERN CHINA) 137

THE ONSET OF THE MAJOR GLACIATION OF THE LPIA: RECORD FROM

SOUTH CHINA 138

CARBONIFEROUS CORAL REEF DEMISE AND RECOVERY, AFTER THE LATE

Latest Devonian-Tournaisian 139

Early-Late Viséan 140

Latest Viséan-Early Serpukhovian 140

Late Serpukhovian-Early Moscovian 141

Late Moscovian-Gzhelian 141

MAIN CONCLUSIONS AND RESEARCH PERSPECTIVES 144

References

Appendixes

Chapter I: Introduction

THE CARBONIFEROUS

The Carboniferous is the geological period (359.2 to 299.0 million years, Gradstein et al., 2004) referring to the first episode of widespread massive coal-bearing strata (Ramsbottom, 1984; Davydov et al., 2012). This time-interval coincides with significant environmental changes subsequent to the Late Devonian extinction events. It is a time period characterized by major continental collisions, drastic climatic changes and high-frequency sea-level fluctuations. These variations directly influ- enced the evolution of biocommunities, including that of reef-building organisms.

Late Devonian extinction events

The second of the five great mass extinctions of the Phanerozoic occurred in the Late Devonian.

The decline of species can be attributed to two discrete episodes, namely the Kellwasser and Han- genberg crises, occurring at the Frasnian/Famennian (F/F) and Devonian/Carboniferous boundaries, respectively. Marine environments were particularly affected, inducing the collapse of ecosystems and subsequent evolution of many faunal groups (Buggisch, 1991; Kaiser et al., 2015).

The biostratigraphic, faunal, sedimentological and geochemical records demonstrated that the Late Devonian extinction events were caused by a complex pattern of environmental changes (Kaiser et al., 2015). However, no consensus was reached as to the cause of the extinction (e.g. extraterrestrial impact, volcanisms, cooling or warming). Sandberg et al. (1988) suggested that the F-F extinction could be directly or indirectly attributed to extraterrestrial triggering mechanisms, with secondary contribution of glaciation. Conversely, Thompson and Newton (1988) suggested that episodic climatic warming was responsible for the mass extinction via direct thermal effects and intensification and amplification of oxygen minimum zones during an extreme polytaxic oceanic mode. Barash (2017) demonstrated that critical intervals are systematically marked by layers of black shales, deposited in euxinic or anoxic environments, interpreted as related to impact event and/or extensive volcanism.

Both phenomena would lead to greenhouse warming, darkening of the atmosphere which prevented photosynthesis, stagnation of oceans, development of anoxia and consequently induced ecosystem collapse. Finally, Joachimski and Buggisch (2002) have demonstrated, based on conodont apatite δ¹⁸O signatures, that repeated climatic cooling in low latitudes during the late Frasnian would have had a severe impact on the tropical shallow water faunas, which led to the decline in diversity.

Paleogeography

The Early Carboniferous world is characterized by two main continents including Laurussia in the Northern Hemisphere, Gondwana in Southern Hemisphere and small cratonic blocks (Blakey,

2003). Then, the Mid-Carboniferous was marked by the collision of the Laurussia and Gondwana continents, inducing the uplift of major mountain chains (Hercynian orogeny in Europe, and the Al- leghenian orogeny in North America) and the initial assembly of the Pangea supercontinent (Scotese and McKerrow, 1990; Fig. 1). The closure of the gateway between the two continents (Tethys Sea) induced overturning of the water mass, oceanic cooling (Copper, 1986; Saltzman, 2003a) and the increase of siliciclastic input to the surrounding basins.

Paleoclimate and glacio-eustatic fluctuations

The Late Paleozoic is commonly regarded as a time of a global greenhouse to icehouse climate transition. The most recent data suggest multiple phases of glaciation and deglaciation during this period, that were accompanied by high frequency sea-level fluctuations (Ramsbottom, 1973; Rams- bottom et al., 1978; Veevers and Powell, 1987).

Recent studies suggest that the Late Palaeozoic ice age began with short-lived (Tournaisian and Late Viséan) and mainly 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 (Fielding et al., 2008a; Huang et al., 2020), in South America and eastern Australia. During the earliest Bashkirian, ice expanded further, becoming widespread across South America (Holz et al., 2008; Rocha-Campos et al., 2008; Henry et al., 2008) and Aus- tralia (Fielding et al., 2008b; Mory et al., 2008). Further expansion of ice centers in Southern Africa (Stollhofen et al., 2008; Isbell et al., 2008), Oman and Saudi Arabia (Martin et al., 2008) occurred at the Bashkirian-Moscovian boundary. During the Late Pennsylvanian, isotopic and paleobotanical records suggest a period of relative warming (Frank et al., 2008; Pfefferkorn et al., 2008), followed at the Pennsylvanian-Permian boundary by a wide glacial expansion. Ice sheets are inferred to have been at their maximum extent during the Asselian to Early Sakmarian, focused on Antarctica, Aus- tralia, southern Africa and South America (Fielding et al., 2008a).

From the Early Mississippian to Mid-Carboniferous, long-term sea-level records show a global lowering, followed by a sea-level rise from the Bashkirian to Late Pennsylvanian (Haq and Schutter, 2008). Throughout the Carboniferous, third-order sea-level fluctuations have amplitudes varying from 10 m to ~100 m (Ross and Ross, 1987, 1988; Haq and Schutter, 2008; Davydov et al., 2012). From the Late Tournaisian to the Mid-Viséan, third-order cycles were characterized by a relatively long periodicity (~6 My; Haq and Schutter, 2008). Conversely, from the Late Viséan to Early Permian, fluctuations were characterized by relatively short periodicity (1 to 1.5 My; Harrison et al., 1979;

Ross and Ross, 1987).

Figure 1: Carboniferous paleogeographic maps of the world, modified from Webb (2002) and Wahlman (2002).

Reef-building communities

Even if the causes of the extinction events are still debated, a massive loss of biodiversity oc- curs in the Late Devonian (e.g. Buggisch, 1991; Kaiser et al., 2015). The extinction events seem to have particularly affected marine life, including brachiopods, trilobites and reef-building organisms.

Therefore, the latest Devonian - Mid-Carboniferous time period represents an interval with important change in the style, composition and extent of carbonate buildups (Fig. 2; Webb, 2002).

During the Devonian, shallow-water bioconstructions were widely distributed. But after the F/F extinction event, the development of shallow-water reefs collapsed resulting in a minimum abun- dance and distribution during the latest Famennian and Early Tournaisian. Rare Tournaisian reefs were built by stromatolites and thrombolites, with subordinate corals, bryozoans and algae (e.g.

Webb, 2005). Reef abundance and diversity increased again to peak during the Late Viséan. Reef organisms included corals, bryozoans, sponges, and calcareous algae along with thrombolites (e.g.

Cook et al., 1994; Mundy, 1994; Shen and Webb, 2005). Through the Late Serpukhovian, Bashkirian and Early Moscovian the abundance of shallow-water buildups decreased; they were dominated by calcareous algae, sponges (e.g. Chaetetes) and microbialites (e.g. West, 1988; Connolly et al., 1989;

Bahamonde et al., 1997, 2007; Wahlman, 2002; Della Porta et al., 2003; Hamilton, 2014). During the Late Moscovian-Gzhelian, carbonate buildups predominantly showed bafflestones composed of phylloid algae (e.g. Heckel and Cocke, 1969; Toomey, 1991; Gong et al., 2007).

Figure 2: Relative abundance of dominant shallow-water reef-builders, from Famennian to Bashkirian, modified from Webb (2002).

Coral reefs

Small and scarce Carboniferous coralliferous bioconstructions have been described (Table 1), but corals played a minor role in their construction, as reported in USA (e.g. Wilson, 1963; Nelson and Langenheim, 1980), Canada (Schenk and Hatt, 1984), UK (Wolfenden, 1958; Adams, 1984; Aretz and Herbig, 2003), Spain (Rodriguez and Moreno-Eiris, 1987; Dingle et al., 1993), Ukraine (Ogar, 2012), Kazakhstan (Cook et al., 1994), Japan (Ota, 1968; Nagai, 1985; Sugiyama and Nagai, 1990, 1994; Nakazawa, 1997), Southern China (Gong et al., 2012; Yao and Wang, 2016) and Australia (Pickett, 1967; Webb, 1987, 1998; Pickett and Wu, 1990; Shen and Webb, 2005).

However, extended metazoan reefs were reported in various settings, including (1) a Late Viséan coral fringing reef (30 m high) reported from Morocco (Rodríguez et al., 2012), (2) Late Viséan Pauciradiale bioherms (8-50 m thick, Bricklieve facies) present in NW Ireland (Aretz et al., 2010), (3) a Viséan-Moscovian metazoan atoll reported from Japan (Ota, 1968; Nagai, 1985; Sugiyama and Nagai, 1990, 1994; Nakazawa, 1997), and (4) two large coral reefs, dated Late Viséan-Serpukhovian (50 m high) and Late Pennsylvanian respectively (80-100 m high), located in southern China (Zhang et al., 2010; Gong et al., 2012; Yao and Wang, 2016) – the focus of this study.

SOUTHERN CHINA SETTING

Paleogeography

During the Early Carboniferous, the South China Bloc collided with Indo China, East Malaya and SW Borneo and together they formed a microcontinent, located at the eastern border of the Pa- leo-Tethys Ocean. This microcontinent was located at a subequatorial-equatorial position, far from the megacontinent Gondwana (Jinlu, 1989; Metcalfe, 1989; Fig. 1).

During the Carboniferous, sedimentation in southern China occurred on the passive continental margin of the Yangtze craton, in the Dian-Qian-Gui Basin, extending from Guiyang in the North to Nanning in the South, and from Quijing in the West to Gullin in the East (Guangxi Region, South

Table 1: Synthesis of Carboniferous coralliferous bioconstructions reported worldwide. 1- Guizhou, China (Zhang et al., 2010; Gong et al., 2012; Maillet et al., 2021), 2- Autria-Italy (Samankassou, 2003), 3- Moscow Basin (Ogar, 2012), 4- Nevada, US (Wilson 1963; Nelson and Langenheim 1980; Gong et al. 2012), 5- Spain (Rodriguez and Moreno-Eiris, 1987; Dingle et al., 1993; Bahamonde et al., 2015, 2017), 6- Ukraine (Ogar, 2012), 7- Japan (Ota, 1968; Nagai, 1985;

Fagerstrom, 1987; Sugiyama and Nagai, 1990, 1994; Nakazawa, 1997), 8- Kazakhstan (Cook et al., 1994), 9- Oklahoma, US (Sutherland and Henry 1977), 10- Ireland (Aretz et al., 2010), 11- Morocco (Rodríguez et al., 2012), 12- Guangxi, China (Gong et al., 2012; Yao and Wang, 2016; Maillet et al., 2020), 13- Nova Scottia, Canada (Schenk and Hatt, 1984), 14- UK: a- Southern Wales (Aretz and Herbig, 2003), b- England (Wolfenden, 1958; Adams, 1984), 15- Australia: a- New South Wales (Pickett, 1967; Pickett and Wu, 1990), b- Queensland (Webb, 1987, 1998; Shen and Webb, 2005).

Age Country Localities Bioconstruction type Main building organisms Ref.

Gzhelian China Guizhou Coral reef Rugose corals (Fomitchevella? Fomichevella?) 1

Kasimovian Austria-Italy Carnic Alps Coral mounds Auloporid corals (Multithecopora syrinx), sponge spicules, worm tubes, foraminifera, ostracods, chaetetids

2

Moscovian Russia Moscow Basin Coral biostromes and bioherms Rugose corals (Ivanovia, Petalaxis), chaetetids 3

Moscovian USA Nevada Chaetetes reef Chaetetes, fusulines, algae, rugose (Caninia, Amandophyllum, Caniniostrotion, Tschuss-

ovskenia) and tabulate corals (Syringopora, Multithecopora), bryozoans

4

Bashkirian - Moscovian

Spain Cantabrian Mountains, Northern Picos de Europa

Coral bioherms and biostromes Rugose corals, auloporid corals, Chaetetes 5

Chaetetes-coral buildups Chaetetes, rugose (Caninia) and tabulate (Multithecopora) corals

Bashkirian - Moscovian

Ukraine Donets basin Coral and coral-chaetetid biostromes Corals, chaetetids, brachiopods, calcareous algae, foraminifera, crinoids, stromatolites, microbialites, worm tubes, bryozoans

6

Viséan - Moscovian

Japan SW Japan, Akiyoshi terrane

Metazoan atoll Corals, Chaetetes, and bryozoans with encrusting foraminifers and calcimicrobes 7

Viséan- Bashkirian

Kazakhstan Pricaspian basin Coral-crinoid-bryozoan-sponge-algal-Tubiphytes buildups

x 8

Viséan - Bashkirian

USA Oklahoma Coral patch reefs Petalaxis corals 9

Serpukhovian Ukraine Donets basin Coral-chaetetid bioherms/biostromes Rugose corals (Lithostrotion, Siphonodendron, Aulina), chaetetids 6

Viséan Kazakhstan Pricaspian basin Coral-algal-sponge-bryozoan-Tubiphytes bioherms

x 8

Coral algal- bryozoan-algal mound x Coral-crinoid-bryozoan-algal mound x

Viséan Ireland NW Ireland Coral biostromes Rugose corals (Siphonodendron) associated to encrusting bryozoans, gigantoproductid brachiopods, crinoids, foraminiferans, red algae, rare green algae

10

Viséan Morocco Tiouinine Coral reef Rugose corals (Siphonodendron, Lithostrotion, Diphyphyllum, Tizraia), tubulate corals

(Michelinia, Multithecopora, Syringopora), associated to algae, microbial communities 11

Viséan China Guangxi Bryozoan-coral patch reefs Rugose corals (Thysanophyllum, Lithostrotion), tabulate corals (Syringopora), bryozoans (Fistulipora)

12

Viséan Ukraine Donets basin Coral biostromes Rugose corals (Siphonodendron junceum, Dibunophyllum) associated to brachiopods, chaetetids

6

Viséan Canada Nova Scottia Algal-coral buildups Algae and tabulate corals (Cladochonus) 13

Viséan UK Southern Wales Microbe-bryozoan-coral mounds Non-skeletal microbial framework, along with bryozoans, tabulate corals (syringoporid) 14a

Viséan Coral-microbial buildups Rugose corals (Lithostrotion, Siphonodendron), microbial crusts

Viséan England Stromatolitic-algal buildups Calcareous algae, demosponges (lithistids and Chaetetida), bryozoans, rugose corals (Lithostrotion irregulare)

14b

x Algal-foraminiferal-coral buildups Tabulate coral (Syringopora), problematic micro-organisms (Aphralysia), thrombolites,

solenoporoid algae

x New South Wales Algal patch reefs and mud mounds Algae associated to corals, crinoids, brachiopods 15a

China). The sedimentary successions show near shore siliciclastic and carbonate platform facies, deposited in an epicontinental sea (Shen and Qing, 2010; Wang et al., 2013; Fig. 3), with scarce Carboniferous bioconstructions.

Carboniferous reefs

In southern China, Carboniferous reefs mainly occurred in the Guizhou Province, Guangxi Zhuang Autonomous Region and Hunan Province (Gong et al., 2012; Yao and Wang, 2016).

In shallow-waters, the Mississippian buildups consist of coral biostromes and reefs and bryozo- an-coral reefs (Fang and Hou, 1986; Chen et al., 2013; Yao et al., 2014, 2015b). Conversely, from deep platform margins and slopes, Waulsortian-type mud mounds and stromatolite mounds were reported (Zhou and Zhang, 1991; Shen and Qing, 2010).

During the Pennsylvanian, the components occurring within the shallow-water reefs slightly change and become dominated by corals (Zhang et al., 2010; Gong et al., 2012), chaetetids-corals (Yao and Wang, 2016), red algae (Tan, 1991), Tubiphytes (Guan et al., 2010), or phylloid algae (Fan and Rigby, 1994; Gong et al., 2007). Conversely, the deep-platform margins were characterized by the growth of mud mound (Yao and Wang, 2016).

Study areas

The study areas are located in Helv, Dujie, Langping (Guangxi Autonomous Region) and Houch- ang (Guizhou Province):

• In Dujie and Helv, the dataset consist of two stratigraphic sections, including a shallow-water succession and a stromatolite-bearing deep water succession.

• The Langping area is characterized by folded units which exhibit a continuous succession of carbonate rocks ranging in age from the Late Devonian to the Triassic (geological map, Gong et al., 2012). The dataset comprises three stratigraphic sections. Two of these sections were measured along the ‘new’ coral-bearing bioconstructions, located in Xiadong and Longjiang- dong, respectively. The third section, located in Gandongzi, comprises a thick sequence of limestones, dated as Late Devonian–Early Permian based on the geological map.

• The Houchang area (Ziyun county, Guizhou Province) consists of folded units exhibiting a continuous succession of carbonate rocks with Devonian to Triassic ages. The dataset is composed of three stratigraphic sections concentrating on the Pennsylvanian. The first section include a large coral reef, located in Bianping (Zhang et al., 2010; Gong et al., 2012). The second section records a shallow-water rock succession, located close to the Zhongxinzhai village. The third section records a deep-water rock succession, located close to the Brickyard village.

Objectives and main findings of the thesis

After the Late Devonian extinction events, the lack of extended shallow-water metazoan reefs suggests a “reefless lag time” lasting until the Permian (Sheehan, 1985; Wood, 1999; Alroy, 2008).

However, the recent discoveries of large coral reefs, up to 50 m high, in southern China, dated as Mississippian and Pennsylvanian respectively, called into question the duration and causes of the aforementioned reef collapse. Within the ongoing debate about the origin of the “reefless lag time”

no consensus has been reached over the cause of the extinction, e.g. bolide impact, cooling, or warming. In addition, considering the decline of present shallow-water coral reef systems driven by global warming and rising sea levels, understanding of the survival strategy of coral communities in response to harsh changes in environmental conditions could provide insight into the development of suitable refugia conditions for modern coral reefs in the future.

Figure 3: Paleogeographic maps of southern China throughout the Carboniferous (modified from Feng et al., 1998, and Yao and Wang, 2016). Tianlin (Guangxi) is the study area. QG: Qian-Gui basin, DQGX: Dian-Qian-Gui-Xiang platform, CS: Central South platform, CLY: Central Lower Yangtze platform, SW: Southwestern platform, LD: Luodian.

In Langping, three small (6–16 m thick and 10–20 m wide; Longjiangdong village) and one ex- tended (up to 50 m high and 250 m wide, Xiadong village) coral reef were described in earlier publi- cations, but they were never studied in detail (Gong et al., 2012; Yao and Wang, 2016). In Houchang, the sediment composition and vertical extent of a reef (80-100 m high) was described (Zhang et al., 2010; Gong et al., 2012), but its overall sedimentary context was not investigated.

The present study focuses on the main aspects of the Carboniferous coral reef growths reported in southern China. The aim of the project is to address to the following questions: (1) Which global parameters led to the scarcity of coral reef development? (2) Which specific settings could lead to the development of coral reefs, despite global unfavorable conditions?

The first chapter deals with the Mississippian coral reefs, located in Langping (Guangxi). The study evaluates the factors that controlled their growth: depositional environment, platform geometry, sedimentary dynamic and environmental conditions. Three sections have been investigated, and se- lected samples were analyzed using petrography, biostratigraphy (fusulinids) and chemostratigraphy (⁸⁷Sr/⁸⁶Sr). This chapter is published in Sedimentary Geology as: Maillet, M., Huang, W.T., Miao, Z.W., Gong, E.P., Guan, C.Q., Zhang, Y.L., Ueno, K., Samankassou, E., 2020. Coral reefs and growth dynamics of a low-angle Carboniferous platform: Records from Tianlin, southern China.

The second chapter focuses on the Pennsylvanian coral reef growth, located in Houchang (Guizhou). The aim of the research presented in this chapter is to understand the overall context of the exceptional Bianping coral reef growth. Three sections have been investigated, and selected samples were analyzed using petrography, biostratigraphy (fusulinids) and chemostratigraphy (⁸⁷Sr/⁸⁶Sr). This chapter is published in Facies as: Maillet, M., Huang, W.T., Li, X., Yang, Z.Y., Guan, C.Q., Zhang, Y.L., Gong, E., Ueno, K., Samankassou, E., 2021. Late Pennsylvanian carbonate platform facies and coral reef: New insights from southern China (Guizhou Province).

In the third chapter the reconstruction of the global Carboniferous environmental changes are discussed based on geochemistry. The goal is to constrain the environmental factors which impact- ed coral reef growth and constrained their distribution. Five sections were investigated (Langping, Guangxi; Houchang, Guizhou). A series of samples were analyzed using carbon and oxygen iso- topes. This chapter is in preparation for submission as: Maillet, M., Samankassou, E. Oceanic δ¹³C, environmental changes and Carboniferous coral reef development: Records from Tianlin and Ziyun (southern China).

The fourth chapter focuses on the latest Viséan-Serpukhovian environmental conditions. In this chapter, the onset of the major Mid-Carboniferous glaciation, occuring at the latest Viséan, is dis- cussed in great detail. Two sections were investigated (Helv and Dujie villages, Guangxi), and se- lected samples were analyzed using petrography and geochemistry (carbon isotopes). This chapter is published in the International Journal of Earth Sciences as: Huang, W., Maillet, M., Zhang, Y., Guan, C., Miao, Z., Samankassou, E., Gong, E., 2020. The onset of the Major glaciation of the LPIA:

Record from South China.

Finally, the fifth chapter presents a synthesis of the results of this study, and summarizes the envi- ronmental factors inhibiting the worldwide coral reef growth throughout the Carboniferous. It is also evaluates the factors leading to the development of metazoan reefal communities in southern China.

Chapter II: Coral reefs and growth dynamics of low-angle Carbonifer- ous platform: Records from Tian- lin, southern China

Chapter published:

Maillet, M., Huang, W.T., Miao, Z.W., Gong, E.P., Guan, C.Q., Zhang, Y.L., Ueno, K., Samankassou, E., 2020. Coral reefs and growth dynamics of a low-angle Carboniferous platform: Records from Tianlin, southern China. Sedimentary Geology 396. DOI: 105550.

ABSTRACT

Metazoan reef building receded globally during the Carboniferous after the Late Devonian ex- tinction events. Shallow-water bioconstructions were generally small and scarce, and corals played a minor role in their construction. However, in southern China, one large (50 m high) and three small (6–16 m high) coral reefs, dating from the Late Viséan–Serpukhovian, have recently been discov- ered in two localities in Guangxi. To understand the occurrence of these coral reefs and to constrain the factors that controlled their growth, three sections around Langping (Guangxi) were measured, and selected samples were analysed using petrography, biostratigraphy, and chemostratigraphy. The depositional environment of these exceptional Mississippian reefs was interpreted as a low angle platform (1–2.5°), recording oligotrophic conditions and normal salinity. The three small reefs have grown around the fair-weather wave-base, whereas the large coral reef has grown in a deeper envi- ronment, between the fair-weather and storm wave-base.

During Carboniferous, the global temporal reef distribution can be partly ascribed to paleoclimate changes. However, in southern China, the growth of extended Mississippian coral reefs is restricted to a short time window, the Late Viséan-Serpukhovian, which coincides with high amplitude and high frequency sea-level fluctuations most likely related to glacial pulses. Recent studies revealed that several rugose coral species survived the mid Carboniferous glaciation event thanks to the per- sistence of warm water ocean currents maintaining tropical conditions on the platform. Due to the scarcity of extended coral reefs elsewhere, it cannot be excluded that Tianlin provided an ecological refuge for corals, related to oceanic currents. Another potential favorable factor could be the mid ramp depositional setting. The deep-water platform areas could potentially provide more stable conditions, less affected by global stress events. However, the ‘deep reef refugia’ hypothesis remains debatable as only very few studies have explored its validity at the community level. Even if the causes of the occurrence of an extended coral reef in southern China cannot be fully constrained, the existence of this large coral framework in the Late Viséan-Serpukhovian provides additional evidence that coral communities did build reefs after the Late Devonian extinction events.

INTRODUCTION

The Carboniferous is characterized by major environmental changes linked to Gondwana su- per-glaciation and significant continental collisions related to the assembly of Pangaea (Scotese and McKerrow, 1990). These events induced important environmental shifts, including a transition from greenhouse to icehouse climate conditions characterized by high-frequency and high-ampli- tude sea-level fluctuations. These events influenced significantly the composition and evolution of biological communities (Brand, 1989; Davydov et al., 2012; Wang et al., 2013).

Moreover, the Carboniferous is considered as a period of global recession in metazoan reef building after the Late Devonian extinction events (Wahlman, 2002; Webb, 2002). The causes of the Frasnian/Famennian and Hangenberg crises are still debated (e.g. bolide impact, cooling, and

sea-level drop) but are known to have induced the collapse of rugose and tabulate coral reefs, which were replaced by algae, bryozoans, calcisponges, and microbial communities (Copper, 1986, 2002;

West, 1988; Ahr and Stanton, 1996; Kirkby and Hunt, 1996; Pickard, 1996; Somerville et al., 1996;

Webb, 2002; Aretz and Chevalier, 2007; Kaiser et al., 2015). Although small and scarce examples of Carboniferous shallow-water bioconstructions do exist, corals presumably played a minor role in their construction (Wilson, 1963; Pickett, 1967; Adams, 1984; Schenk and Hatt, 1984; Fagerstrom, 1987; Webb, 1987, 1998; Pickett and Wu, 1990; Rodríguez, 1996; Aretz and Herbig, 2003; Shen and Webb, 2005; Zhang et al., 2010; Gong et al., 2012). The rarity of metazoan communities has significant implications for the profile of carbonate platforms, commonly reported as carbonate ramps for the Mississippian (Ahr, 1989; Wright and Faulkner, 1990; Elrick and Read, 1991; Bourque et al., 1995; Madi et al., 1996; Bachtel and Dorobek, 1998) and un-rimmed, high-relief, steep slope margin platforms for the Pennsylvanian (Bahamonde et al., 1997, 2004, 2008; Cook et al., 2002; Della Porta et al., 2002a, 2004; Kenter et al., 2002; Verwer et al., 2004; Chesnel et al., 2015).

Currently, only scarce examples of extended coral reefs have been described from Carbonifer- ous, including the Late Viséan coral fringing reef (30 m high) reported from Morocco (Rodríguez et al., 2012), the Late Viséan Pauciradiale bioherms (8-50 m thick, Bricklieve facies) reported from NW Ireland (Aretz et al., 2010) and the large Pennsylvanian coral reef (80-100 m high), reported from southern China (Zhang et al., 2010). In addition to these discoveries, several Carboniferous coral reefs (6-50 m high), not yet studied in detail, have recently been identified in two localities in Guangxi (Gong et al., 2012; Yao and Wang, 2016). To understand these conspicuous occurrences of coral reefs, several stratigraphic sections were measured in the Langping area (Guangxi), and selected samples were analysed using petrography, biostratigraphy (dating by fusulinids), and che- mostratigraphy (87Sr/86Sr). The purpose of this study is to explore the environmental conditions and the depositional setting of these coral reefs. Understanding of coral reefs, which persisted only in specific settings, could, in turn, improve understanding of the Late Devonian extinction events and the subsequent recovery. Considering the decline of present-day shallow-water coral reef systems endangered by global warming, we discuss whether deeper-water settings may represent potential refugia for the survival of coral reefs.

GEOLOGICAL SETTING

Paleogeogeography

During the Early Carboniferous, the South China Block collided with the Indochina, East Malaya, and Southwest Blocks, forming a micro-continent at the eastern border of the Paleo-Tethys Ocean.

This micro-continent formed an independent group located at a subequatorial-equatorial position, far from the super-continent Gondwana (Jinlu, 1989; Metcalfe, 1989).

During this period, sedimentation occurred on the passive continental margin of the Yangtze Cra-

Guiyang

Nanning Chengdu

Changsha

Nanjing

Kunming

Wuhan

DQGX QG

Tournaisian

Guiyang

Nanning Chengdu

Changsha

Nanjing

Kunming

Wuhan

DQGX QG

Viséan-Serpukhovian

Guiyang

Nanning Chengdu

Changsha

Nanjing

Kunming

Wuhan

Tianlin Ziyun Yangtze Landmass

CLY

CS

SW LD

Guiyang

Nanning Chengdu

Changsha

Nanjing

Kunming

Wuhan Yangtze Landmass

Yangtze Landmass Yangtze Landmass

Cathaysia Landmass

Cathaysia Landmass

Cathaysia Landm ass

Tianlin Ziyun

Bashkirian-Moscovian Kasimovian-Gzhelian

Deep basin facies

Pre-platform ramp facies

Shallow carbonate facies

Shallow siliciclastic rock facies

Terrestrial

facies Landmass

102° 110° 118° (E)

(N) 30°

22°

106° 114°

28°

102° 110° 118° (E)

(N) 30°

22°

106° 114°

28°

102° 110° 118° (E)

(N) 30°

22°

106° 114°

28°

102° 110° 118° (E)

(N) 30°

22°

106° 114°

28°

Tianlin ^Guilin Tianlin

Qujing Qujing Guilin

Guilin

Qujing Qujing Guilin

Figure 1: Paleogeographic maps of southern China throughout the Carboniferous (modified from Feng et al., 1998, and Yao and Wang, 2015). Tianlin (Guangxi) is the study area. QG: Qian–Gui Basin; DQGX: Dian–Qian–Gui–Xiang plat- form; CS: Central South platform; CLY: Central Lower Yangtze platform; SW: Southwestern platform; LD: Luodian.

ton, in the Dian–Qian–Gui Basin extending from Guiyang in the north to Nanning in the south, and from Qujing in the west to Guilin in the east (Guangxi region, southern China). Sedimentation records near-shore siliciclastic and carbonate platform facies, deposited in an epicontinental sea (Shen and Qing, 2010; Wang et al., 2013; Fig. 1). In southern China, scarce Carboniferous reefs were reported, occurring mainly in Guizhou Province, Guangxi Zhuang Autonomous Region and Hunan Province (Gong et al., 2012). The Mississippian shallow-water reefs consist of coral biostromes (Yashui town, Guizhou), bryozoan-coral reefs (Gandongzi village, Langping, Guangxi) and coral reefs (Xiadong and Longjiangdong villages, Langping, Guangxi). Conversely, deep water mounds consist of Waul- sortian-type mud mounds (Longdianshan Hill, Guangxi) and stromatolite mounds (Mengcun and Helv villages, Guangxi; Huang et al., 2020). The present study focusses on the coral reefs, recently discovered in Langping (Guangxi), briefly reported by Gong et al. (2012) and Yao and Wang (2016).

Paleoclimate and sea-level fluctuations

The Late Paleozoic is commonly considered as a time of global greenhouse-icehouse climate transition. The most recent data suggest multiple phases of glaciation and deglaciation, accompanied by high frequency sea-level fluctuations (Ramsbottom, 1973; Ramsbottom et al., 1978; Veevers and Powell, 1987). However, the extent and age of these pulses of glaciation are still debated (e.g. Isbell

et al., 2003; Fielding et al., 2008; Montañez et al., 2007). A consensus emerges for the mid Carbon- iferous boundary. Scientists suggest that this boundary marks the beginning of a major Gondwana glaciation and climate cooling (Veevers and Powell, 1987; Davydov et al., 2012; Chen et al., 2018;

Ahern and Fielding, 2019). This episode is associated to a global sea-level drop, widely reported (e.g.

southern China, Wang et al., 2013, Tian et al., 2019; Huang et al., 2020; Arrow Canyon, Barnett and Wright, 2008; Appalachian basin, Blake and Beuthin, 2008; Moscow basin, Kabanov et al., 2016).

Throughout Carboniferous, third-order sea-level fluctuations are commonly large, with an am- plitude varying from tens meters to ~100 m (Ross and Ross, 1987, 1988; Haq and Schutter, 2008;

Davydov et al., 2012). From the upper Tournaisian to middle Viséan, third-order cycles are charac- terized by a relatively large periodicity (long-term). Conversely, from the Upper Viséan to Lower Permian, fluctuations are characterized by relatively short periodicity (1 to 1.5 million years; Harrison et al., 1979; Ross and Ross, 1987). Second-order sea-level trends are clearly superimposed on these depositional sequence cycles. From the Lower Mississippian to Mid Carboniferous, sea-level records a global lowering, followed by a sea-level rise from the Bashkirian to Late Pennsylvanian.

METHODS

Chemostratigraphy

Twenty-one samples were selected for ⁸⁷Sr/⁸⁶Sr isotope analyses. For each sample, 30 mg of pow- dered carbonate material (bulk) were dissolved in 2.2 M high-purity acetic acid for 1–2 h at room temperature (22-24 °C) in conical 2 ml vials. The solutions were centrifuged, and the supernatant was recovered and transferred to Teflon vials, where it was dried to a residue on a hot plate. The residue was re-dissolved in a few drops of 14 M HNO₃ and dried again prior to Sr separation from the matrix using Sr-Spec resin. The Sr separate was re-dissolved in 5 ml ~2% HNO₃ solution, and the ratios were measured using a Thermo Neptune PLUS Multi-Collector inductively coupled plasma mass spectrometer in static mode (University of Geneva). The ⁸⁸Sr/⁸⁶Sr (8.375209) ratio was used to monitor internal fractionation during the run. Interferences at masses 84 (⁸⁴Kr), 86 (⁸⁶Kr), and 87 (⁸⁷Rb) were also corrected in-run by the monitoring of ⁸³Kr and ⁸⁵Rb. SRM987 standard was used to check external reproducibility, which, on the long term (>100 measurements in one year), was 10 ppm. The internally corrected ⁸⁷Sr/⁸⁶Sr values were further corrected for external fractionation by a value of -0.025‰ per amu, because a systematic difference between measured and nominal standard ratios of the SRM987 of ⁸⁷Sr/⁸⁶Sr was 0.710248 (McArthur et al., 2001).

Biostratigraphy

To perform foraminiferal biostratigraphy, 101 samples were collected from the studied sections.

Fusulinids were the most important taxon in this analysis. Russian platform fusulinid biozones and stages (and substages) were used, based on a well-constrained biostratigraphic framework (e.g. Einor,

1996; Kulagina et al., 2003; Cózar and Somerville, 2004; Somerville and Cózar, 2005; Somerville, 2008), which has recently been calibrated with high-precision U–Pb zircon ages in the Carbonifer- ous depositions of the Donets Basin and Urals (Davydov et al., 2010; Schmitz and Davydov, 2012).

Petrography

Thin sections from 240 samples were analysed using a petrographic microscope (Leica DME microscope) and classified following Dunham (1962) and Embry and Klovan (1971). Facies were grouped into microfacies types according to grain assemblages and texture to interpret hydrodynamic conditions and paleoenvironments.

Restored section

A cross-section was made to reconstruct the geology in the subsurface. The topographic profile was constructed perpendicular to the strike or trend of the tectonic structures. The geology of the surface was added to this profile, including rock types, unit boundaries, and bedding dips and faults derived from field data and/or geological maps. Surface data were extrapolated in deep areas. Reconstructed deformations were subsequently retro-deformed to a restored section (manually). The restored sec- tion was realized by preserving the rock volume and maintaining a constant bed thickness and equal lengths for all units. To un-deform the cross-section, several pin lines were established, which serve as marker horizons for measuring bed length (Dahlstrom, 1969; Woodward et al., 1989).

DATA

The study area exhibits folded units, comprising continuous succession of carbonate rocks ranging in age from the Devonian to the Triassic (Fig. 2A). The dataset originated from three stratigraphic sections (Fig. 2B). Two of these sections were measured along the ‘new’ coral-bearing bioconstruc- tions, including three small reefs (6–16 m thick and 10–20 m wide, section 1; Fig. 3) and an extended reef (up to 50 m high and 250 m wide, section 2; Fig. 4), respectively located near Longjiangdong and Xiadong villages (Langping, Guangxi). The last section (section 3) comprises a long sequence of limestones (slope succession), dated as Late Devonian–Early Permian and located close to Gan- dongzi village (Langping, Guangxi).

DATING

The stratigraphy was constrained using two independent methods, namely Sr isotopes (⁸⁷Sr/⁸⁶Sr) and fusulinid assemblages (Fig. 5).

Figure 2: (A) Geological map of the Langping area (Tianlin, Guangxi, southern China) with the location of the three studied sections, modified from Gong et al. (2012). (B) Lithofacies successions record from the three measured sections.

Figure 3: Outcrop of the three small coral reefs located near Longjiangdong village (Langping). The coral reefs range from 6 to 16 m high and 10 to 20 m wide. Section 1 crosses successively the base, core and cover of each coral reef. (A) Reef substrates are dominated by coarse skeletal grain limestones. (B) Reef cores are composed of branching colonial corals.

Figure 4: Outcrop of the extended Xiadong coral reef. The coral reef measures up to 50 m high and 250 m wide. Section 2 crosses vertically the core and cover of the large coral reef.

Figure 5: Distribution of the Sr isotope (⁸⁷Sr/⁸⁶Sr) and fusulinid samples along the three measured sections. Sr values are detailed in Table 1 and foraminiferal assemblages are summarized in Tables 2 and 3.

Chemostratigraphy

The Sr isotope compositions of selected samples are summarized in Table 1. Samples were selected in order to have an optimized and consistant stratigraphic resolution. The ⁸⁷Sr/⁸⁶Sr ratios for sections 1, 2, and 3 are 0.70771–0.707785, 0.707721–0.707824, and 0.707708–0.708203, respectively. Based on the Phanerozoic strontium curve, strontium signals of selected samples correlate with several stratigraphic intervals (Howarth and McArthur, 1997; McArthur et al., 2001). Because the study area exhibits a rock succession ranging in age from the Devonian to Triassic, ⁸⁷Sr/⁸⁶Sr values lead

Biostratigraphy

Foraminiferal (especially fusulinid) zonations are reliable biostratigraphic markers and are com- monly used to identify Carboniferous chronostratigraphic divisions (Davydov et al., 2012). In sections 1 and 2 (Table 2) and the lower part of section 3 (interval 1, Table 3), the fauna includes small fusulinids

Section Interval Sample Location (m)

87Sr/⁸⁶Sr Ages

min average max min average max

Section 1 1 119 1 0,707785 343.35 342.75 342.15 329.95 330.15/20 330.45

Viséan Viséan

2 130 16 0,707756 342.15 341.50 340.75 -

Viséan

3 146 56.3 0,707716 340.05 338.75/85 337.25 331.40 331.80 332.30

Viséan Viséan

4 155A 78.4 0,707748 341.80 341.10 340.30 330.70 330.95 331.25

Viséan Viséan

5 161 94.6 0,707714 339.85 338.45/50 336.95 331.45 331.90 332.45

Viséan Viséan

Section 2 6 110 substrate 0,707824 344.65 344.20 343.65 329.10 329.35 329.60

Viséan Viséan

7 61 2.1 0,707753 342.05 341.35 340.60 330.60 330.80/85 331.10

Viséan Viséan

8 74 23.3 0,707786 343.35 342.80 342.20 329.90 330.15/20 330.40

Viséan Viséan

9 87 57.4 0,707721 340.40 339.30/35 337.75 331.25 331.60/65 332.10

Viséan Viséan

Section 3 10 189 1.1 0,708166 375.75 376.45 377.05 -

Late Devonian

11 198 184.4 0,708202 374.30 375.20 375.90 -

Late Devonian

12 207 231.6 0,707872 346.15 345.70 345.25 -

Tournaisian

13 215 337.8 0,707763 342.45 341.85 341.15 -

Viséan

14 233 418.7 0,707793 343.60 343.05/10 342.50 329.75 330.00/05 330.25

Viséan Viséan

15 254 535.8 0,707708 331.60 332.05/10 332.80 -

Viséan

16 266 659.2 0,707997 325.30 325.65 326.00 -

Serpukhovian

17 273 765.5 0,708203 317.30 318.00/05 318.75 -

Bashkirian

18 275 942.6 0,707924 -

19 295 1065.7 0,708145 -

20 307 1170.3 0,707829 -

21 318 1242.9 0,707804 -

Table 1: Sr isotope values.

to interpret sections 1 and 2 as Viséan, whereas the section 3 is estimated as Late Devonian–Early Permian. However, for the last section, Sr values could correspond to the several potential ages and additional investigations are needed to better constrain the adequate stage within the Carboniferous.