© Yuefeng Shen, 2017
Les systèmes biosédimentaires et la diagénèse d'une
rampe carbonatée Ordovicienne, Bassin de Tarim, Chine
Thèse
Yuefeng Shen
Doctorat interuniversitaire en sciences de la Terre
Philosophiæ doctor (Ph. D.)
Les systèmes biosédimentaires et la diagénèse d’une
rampe carbonaté e Ordovicienne, Bassin de Tarim, Chine
Thè se
Yuefeng Shen
Sous la direction de :
Fritz Neuweiler, directeur de recherche
Bingsong Yu, codirecteur de recherche
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Résumé
L'enregistrement biosédimentaire Ordovicien du bassin de Tarim offre la possibilité de s'attaquer à quatre grands enjeux scientifiques liés au « grand événement de biodiversité ordovicienne » (Great Ordovician Biodiversification Event ou GOBE en anglais): i) la phylogenèse des organismes incertae sedis, ii) la paléodiversité des producteurs primaires benthiques (algues calcaires, calcimicrobes), iii) la nature des précipités authigénique des fonds marins (automicrite), et iv) la diagénèse en termes d'évolution de la porosité et de l'enregistrement géochimique des perturbations environnementales provoquant des changements biosédimentaires majeures (éponges, crinoïdes contre algues benthiques). L'analyse typologique, morphométrique et microstructurale du microfossile problématique
Halysis Høeg, 1932, conclut pour une algue verte siphonnée avec une affinité à Bryopsidales,
Udoteaceae, morphotype Flabellia petiolata (Turra) Nizamuddin 1987. Les monticules d’Halysis (Katian) font partie d'une rampe carbonatée peu profonde et subtidale dominée par des tapis de sable de granules d'algues. L’accrétion de monticules était contrôlée par des phénomènes autocycliques, produisant des épisodes sédimentaires et d’enfouissement suivis de lacunes et de périodes de croissance d’algue.
Dans le bassin de Tarim, la diversité des producteurs primaires benthiques augmente considérablement au cours de la zone de Belodina confluens d’Ordovicien supérieur (Katian). Par rapport aux courbes de diversité dérivées d'autres régions (Laurentia, Baltoscandia), dans le bassin de Tarim, il y a une propagation de la diversification d'environ 4 Ma. La courbe de diversité des producteurs primaires benthiques sont semblables à celles enregistrées par certains groupes de fossiles herbivores et suspensivores (Échinodermes eleuthérozoaires, gastéropodes).
Cinq types de précipitations authigénique des fonds marins (automicrite) sont présents dans les monticules carbonatés de calathid-demosponge (Darriwilian), tout d'abord interprété comme « carbonate microbien ». Une bonne corrélation de la fluorescence et de la cathodoluminescence des automicrites indique que l'organominéralisation induite et soutenue a produit de l'automicrite, probablement par la perminéralisation de substrats organiques non vivants adsorbant des complexes métal-humâtes dissous. À l'aide de six
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paramètres et de dix-sept caractères, quatre automicrites se révèlent non microbiennes au lieu de représenter probablement des reliques de métazoaires calcifiés (éponges, structures d'attachement des invertébrés tendus). Un automicrite est d'origine microbienne, mais l'âge de l'après-monticule réussit une disconformité.
En utilisant un ensemble de séquences paragénétiques, un échantillonnage géochimique des composants spécifiques a été effectué pour déterminer la variation de la composition isotopique stable au carbone et à l'oxygène. Il existe deux niveaux stratigraphiques distincts séparés par Δδ13C ≈ +2.5 ‰ (PDB). Les deux niveaux suivent la même tendance partielle de
diminution de δ18O typique pour l’augmentation de la température pendant l'enfouissement.
Les valeurs de δ18O les moins modifiées sont également séparées selon les mêmes deux
niveaux stratigraphiques (Δδ18O ≈ +2.0 ‰). Cette excursion positive couplée de δ13C-δ18O
est considérée comme le résultat d'une augmentation du taux d'enfouissement du carbone organique (formation de roches mères d'hydrocarbures) et d'un refroidissement climatique subséquent provoquant un changement biosédimentaire majeure (éponges, crinoïdes contre algues benthiques) le long de l'intervalle limite Sandbian-Katian.
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Abstract
The Ordovician biosedimentary record of the Tarim Basin offers the opportunity to tackle four major scientific issues related to the Great Ordovician Biodiversification Event: i) the phylogenesis of organisms incertae sedis, ii) the paleodiversity of benthic primary producers (calcareous algae, calcimicrobes), iii) the nature of authigenic sea-floor precipitates (automicrite), and iv) diagenesis in terms of porosity evolution and the geochemical record of environmental perturbations causing major biosedimentary turnovers (sponges, crinoids
versus benthic algae).
The typological, morphometric and microstructural analysis of the mound-forming microproblematicum Halysis Høeg, 1932 concludes for a siphonous green alga with an affinity to Bryopsidales, Udoteaceae, morphotype Flabellia petiolata (Turra) Nizamuddin 1987. Early Katian Halysis mounds form part of a shallow-subtidal carbonate ramp dominated by algal-pellet sand sheets. Their accretion was controlled by autocyclic drivers such as increments of sediment flux and burial followed by episodes of omission and algal growth.
In the Tarim Basin, the diversity of benthic primary producers increases substantially during the Upper Ordovician (Katian) Belodina confluens Zone. Compared to diversity curves derived from other regions (Laurentia, Baltoscandia), in the Tarim Basin there is a protraction of diversification by about 4 Ma. The global diversity curve of benthic primary producers is similar to those derived from some herbivorous and suspension-feeding fossil groups (eleutherozoan echinoderms, gastropods).
Five kinds of authigenic sea-floor precipitates (automicrite) are present in Darriwilian calathid-demosponge carbonate mounds, altogether formerly interpreted as ‘microbial carbonate’. A good correlation of fluorescence and cathodoluminescence of automicrites indicates that induced and supported organomineralization produced automicrite, probably
via the permineralization of non-living organic substrates adsorbing dissolved metal-humate
complexes. Using six parameters and seventeen characters, four automicrites turn out to be non-microbial instead likely represent relics of calcified metazoan tissue (sponges,
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attachment structures of stalked invertebrates). One automicrite is microbial in origin but is post-mound in age succeeding a disconformity.
Using a set of paragenetic sequences, component-specific geochemical sampling was performed to determine the variation of carbon and oxygen stable isotopic composition. There are two distinct stratigraphic levels separated by Δδ13C ≈ +2.5‰ (PDB). Both levels display a subparallel trend of decreasing δ18O typical for increasing temperature during burial. Least altered δ18O values are equally separated along the two stratigraphic levels (Δδ18O ≈
+2.0‰). This coupled positive δ13C-δ18O excursion is considered the result of an increasing
burial rate of organic carbon (formation of hydrocarbon source rocks) and subsequent climatic cooling causing a biosedimentary turnover (sponges, crinoids versus benthic algae) along the Sandbian-Katian boundary interval.
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Content
Résumé ... iii Abstract ... v Content ... vii Table list ... x Figure list ... xi Acknowledgement ... xvi Foreword ... xviii Introduction ... 11.1 The Ordovician biosedimentary systems ... 1
1.1.1 The Great Ordovician Biodiversification Event (GOBE) ... 2
1.1.2 The Ordovician paleoenvironmental system ... 4
1.1.3 The Ordovician carbonate reefs and mounds ... 12
1.2 The Ordovician of the Tarim Basin ... 14
1.2.1 Tecto-sedimentary evolution of the Tarim Basin ... 14
1.2.2 Ordovician petroleum system of the Tarim Basin ... 21
1.2.3 Diagenesis of Ordovician carbonate reservoirs of the Tarim Basin ... 22
References ... 24
Chapter 2 Halysis Høeg, 1932 in Ordovician carbonate mounds, Tarim Basin, NW China 35 Abstract ... 35
2.1 Introduction ... 36
2.2 Material and methods ... 39
2.2.1 Area of study and lithostratigraphy ... 39
2.2.2 Sampling and methods ... 39
2.3 Result ... 40
2.3.1 Halysis morphometry ... 40
2.3.2 Halysis wall microstructure ... 43
2.3.3 Halysis carbonate mounds ... 46
2.3.4 Halysis mineralogy ... 49
2.4 Discussion ... 54
2.4.1 The significance of Halysis morphospecies ... 54
2.4.2 Taxonomic affinity of Halysis ... 56
2.4.3 Halysis carbonate mounds ... 57
2.5 Conclusions ... 60
Acknowledgements ... 61
References ... 62
Chapter 3 Taphocoenoses and diversification patterns of calcimicrobes and calcareous algae, Ordovician, Tarim Basin, China ... 66
Abstract ... 66
3.1 Introduction ... 67
3.2 Study area, material and methods ... 68
3.2.1 Lithostratigraphy of the Bachu Uplift ... 68
3.2.2 Sampling and methodology ... 69
3.3 Taphocoenoses of calcimicrobes and calcareous algae ... 72
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3.3.2 Algal calcimicrobial mounds ... 75
3.3.3 Algal mounds ... 75
3.3.4 Algal reefs ... 76
3.3.5 Calcimicrobial mounds ... 76
3.4 Diversity curves of calcimicrobes and calcareous algae – a discussion ... 77
3.4.1 Regional diversity curves ... 77
3.4.2 Global diversity curves ... 77
3.5 Conclusions ... 80
Acknowledgements ... 81
References ... 82
Chapter 4 Questioning the microbial origin of automicrite in Ordovician calathid-demosponge carbonate mounds ... 85
Abstract ... 85
4.1 Introduction ... 86
4.2 Area of study, stratigraphy and methodology ... 87
4.2.1 Area of study and stratigraphy ... 87
4.2.2 Methodology ... 88
4.3 Calathid-demosponge carbonate mounds ... 90
4.3.1 Geometry and lithology ... 90
4.3.2 Depositional cycles and microfacies ... 91
4.3.3 Skeletal community ... 96
4.3.4 Automicritic fabrics ... 98
4.3.5 Infiltrated sediments ... 104
4.3.6 Fluorescence and cathodoluminescence ... 107
4.4 Discussion ... 109
4.4.1 Organomineralisation ... 109
4.4.2 Questioning the microbial origin of automicrite ... 110
4.4.3 Reassessment of the origin of automicritic fabrics ... 113
4.4.4 Analogs and perspective ... 116
4.5 Conclusions ... 118
Acknowledgement ... 119
References ... 120
Chapter 5 Biosedimentary, diagenetic and geochemical compartmentalization and its implications on reservoir potential, Ordovician, northwestern Tarim Basin, China ... 130
5.1 Introduction ... 131
5.2 Regional geology ... 133
5.2.1 Study area, stratigraphy ... 133
5.2.2 Regional petroleum system ... 137
5.3 Material and methods ... 138
5.4 Results ... 139
5.4.1 Biosedimentary compartments ... 143
5.4.2 Diagenetic compartments ... 149
5.4.3 Geochemical compartments ... 167
5.5 Discussion ... 171
5.5.1 Biosedimentary compartments - environmental change and biotic evolution .. 171
5.5.2 Diagenetic and geochemical compartments - differential diagenesis and the Ordovician carbon cycle ... 174
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5.5.3 Implications for reservoir potential ... 176
Conclusions ... 177
Acknowledgements ... 178
References ... 179
Chapter 6 Summary and perspective ... 185
6.1 Problematic organisms ... 185
6.2 Paleodiversity ... 186
6.3 Automicrite ... 186
6.4 Diagenesis, chemostratigraphy and reservoir potential ... 187
References ... 189
Appendix ... 190
Appendix 1 Element concentration of Halysis skeleton and in zoned calcite cements .. 190
Appendix 2 Coordinates of studied sections and distribution of rock samples with respect to stratigraphic members ... 194
Appendix 3 Distribution of thin sections with respect to stratigraphic members (Numbers of thin sections are equivalent to numbers of rock samples) ... 198
Appendix 4 Occurrence and distribution of calcimicrobes and calcareous algae in thin sections ... 202
Appendix 5 Systematic description of calcimicrobes and calcareous algae ... 208
Appendix 6 Stratigraphic age and paleogeographic domain of calathid sponge carbonate mounds ... 218
Appendix 7 Occurrence of all five automicrites in reported Ordovician calathid sponge carbonate mounds ... 231
Appendix 8 Carbon and oxygen stable isotopes ... 235
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Table list
Table 2-1 Case studies reporting abundant Halysis Høeg, 1932 represent tropical to subtropical paleolatitudes of the Iapetus and Paleo-Tethys Ocean (Early Ordovician to Middle Devonian, ordered by year of publication) ... 38 Table 5-1 Twenty-four facies units (FU) present in four Mid-Late Ordovician carbonate
biosedimentary systems (BS), Leyayilitag ridge, NW Tarim Basin, China ... 140
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Figure list
Figure 1-1 The Great Ordovician Biodiversification Event (GOBE)... 2
Figure 1-2 Ordovician paleogeography with distributions of major lands and marine carbonate deposit ... 6
Figure 1-3 Ordovician sea-level, geochemical composition, atmospheric oxygen and carbon dioxide curves ... 8
Figure 1-4 Reconstruction of an Ordovician food web ... 12
Figure 1-5 Evolution of the taxonomic, trophic and space structure of marine ecosystems through the Early Paleozoic ... 13
Figure 1-6 Dominant contributors to the formation of Ordovician reef/mound... 14
Figure 1-7 Location, surrounded tectonic setting and main tectonic units of the Tarim Basin ... 15
Figure 1-8 Seismic interpretation profile across the Tarim Basin, showing the distribution of major unconformities (a) and variation of the tecto-sedimentary framework (b) ... 18
Figure 1-9 Generalized Phanerozoic tectonostratigraphy of the Tarim Basin ... 19
Figure 1-10 Ordovician paleogeography and facies maps of the Tarim Basin ... 20
Figure 2-1 Compilation of the various morphologies and life habits proposed for the calcarous microproblematicum Halysis Høeg, 1932 ... 37
Figure 2-2 Area of study, structural and geological context ... 41
Figure 2-3 Lithostratigraphy, sampling and facies of the Middle to Upper Ordovician boundary interval, Leyayilitag ridge, Bachu Uplift ... 42
Figure 2-4 Representative examples of Halysis with large, medium and low tube numbers (thin-sections) ... 44
Figure 2- 5 Frequency of Halysis tube numbers observed in thin-section ... 45
Figure 2-6 Branching of tubes in Halysis ... 45
Figure 2-7 Cross-plots of morphometric attributes of Halysis ... 46
Figure 2-8 Wall microstructure of Halysis ... 47
Figure 2-9 Synthetic stratigraphic section across a sequence of Ordovician (Katian) Halysis carbonate mounds, Bachu Uplift, Tarim Basin, NW China ... 48
Figure 2-10 Spar-cemented cavities in Ordovician Halysis carbonate mounds ... 50
Figure 2-11 Benthic community associated with Halysis carbonate mounds (thin-sections) ... 51
Figure 2-12 Spar-cemented cavities in Ordovician Halysis carbonate mounds ... 52
Figure 2-13 Cementation history of Ordovician Halysis carbonate mounds ... 53
Figure 2-14 Compilation of morphometric attributes of Halysis produced from Early Ordovician to Middle Devonian carbonate sedimentary rocks ... 55
Figure 2-15 The basic sedimentary and biostratonomic processes involved in the formation of Halysis carbonate mounds match the deposition-omission model of Kidwell (1991) ... 59
Figure 3-1 Study area, structural and geological context ... 70
Figure 3-2 Synthetic litho- and biostratigraphic section of the Middle to Late Ordovician boundary interval, Leyayilitag ridge, Tarim Basin, NW China ... 71
Figure 3-3 Calcimicrobes of the Middle to Late Ordovician boundary interval, Leyayilitag ridge, Tarim Basin, NW China (thin-sections) ... 73
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Figure 3-4 Calcareous algae of the Middle to Late Ordovician boundary interval, Leyayilitag ridge, NW Tarim Basin, China ... 74 Figure 3-5 Regional and global diversity curves of calcimicrobes and calcareous algae along
the Middle to Late Ordovician boundary interval ... 79 Figure 4-1 Study area, location of stratigraphic sections and stratigraphic subdivision of the
respective Lower to Middle Ordovician sequence ... 89 Figure 4-2 Size, geometry and stacking pattern of Ordovician calathid-demosponge
carbonate mounds ... 91 Figure 4-3 Synthetic stratigraphic section (not to scale), depositional cycles and microfacies
associated with Ordovician calathid-demosponge carbonate mounds ... 92 Figure 4-4 Stratigraphic architecture and microfacies associated with Ordovician calathid-demosponge carbonate mounds, lower part of depositional cycle, debris and infiltrated sediment, MF-1 and MF-2 ... 93 Figure 4-5 Stratigraphic architecture and microfacies associated with Ordovician calathid-demosponge carbonate mounds, middle part of depositional cycle, well-bedded encrinite, MF-3 and MF-4 ... 94 Figure 4-6 Stratigraphic architecture and microfacies associated with Ordovician calathid-demosponge carbonate mounds, upper part of depositional cycle, carbonate mound, MF-5 and MF-6 ... 95 Figure 4-7 Skeletal community of calathid-demosponge carbonate mounds ... 97 Figure 4-8 Introducing the various automicritic fabrics present in calathid-demosponge-automicrite boundstone (MF-6), polished slab; sample BY22-1-5 ... 98 Figure 4-9 The in-situ peloidal-spiculiferous fabric (AM-1) of calathid-demosponge
carbonate mounds (photomicrographs). ... 100 Figure 4-10 The in-situ peloidal fabric (AM-2) of calathid-demosponge carbonate mounds
(photomicrographs) ... 101 Figure 4-11 The aphanitic-microtubular fabric (AM-3) of calathid-demosponge carbonate
mounds (photomicrographs) ... 101 Figure 4-12 The minipeloidal fabric (AM-4) of calathid-demosponge carbonate mounds
(photomicrographs) ... 102 Figure 4-13 The laminoid-cerebroid fabric (AM-5) of calathid-demosponge carbonate
mounds (photomicrographs) ... 103 Figure 4-14 Infiltrated sediments (IS) of calathid-demosponge carbonate mounds
(photomicrographs) ... 105 Figure 4-15 Compilation of organisation and succession of automicritic fabrics present in
calathid-demosponge carbonate mounds (thin-sections, schematic drawings) ... 106 Figure 4-16 Comparative illustration of normal light/fluorescence (A–M) and normal
light/cathodoluminescence (N–Y) micrographs of automicritic fabrics of calathid-demosponge carbonate mounds ... 108 Figure 4-17 Multi-parameter and multi-character analysis of automicritic fabrics of calathid-demosponge carbonate mounds ... 114 Figure 4-18 Conceptual illustration of the formation of calathid-demosponge carbonate
mounds from a primary, level-bottom community (A) to an ecological bioherm draped by crinoid-rich sediment (E). ... 115 Figure 5-1 Study area, location of stratigraphic sections of the respective Lower to Middle
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Figure 5-2 Litho-biostratigraphy and outcrop photographs through the Middle to Late Ordovician sequence of the Leyayilitag Ridge, NW Tarim Basin, China ... 135 Figure 5-3 Burial history of the Ordovician sediments of the study area... 138 Figure 5-4 Depositional geometries, stacking patterns, facies units and primary to early
secondary porosities present in sponge mounds, encrinites and sponge biostromes (BC-1, = Yijianfang Formation) ... 144 Figure 5-5 Depositional geometries, stacking patterns, facies units and primary to early
secondary porosities present in marine red bed (BC-2, = Tumuxiuke Formation) .... 146 Figure 5-6 Depositional geometries, stacking patterns, facies units and primary to early
secondary porosities present in algal mounds and pellet limestones (BC-3, = lower part of the Lianglitag Formation)... 148 Figure 5-7 Depositional geometries, stacking patterns, facies units and primary to early
secondary porosities present in ooid-rich peritidalites (BC-4, = second member of the Lianglitag Formation) ... 149 Figure 5-8 Petrogenetogram illustrating the paragenetic sequence of the diagenetic
compartment 1 (DC-1)... 152 Figure 5-9 Thin-section micrographs of normal light, stained, cathodoluminescence and
fluorescence showing early and shallow burial diagenetic phenomena of the Ordovician diagenetic compartment 1 (DC-1) of the northwestern Tarim Basin, China. ... 152 Figure 5-10 Thin-section micrographs of normal light, stained, cathodoluminescence and
fluorescene showing deep burial and telodiagenetic phenomena of the Ordovician diagenetic compartment 1 (DC-1) of the northwestern Tarim Basin, China ... 155 Figure 5-11 Petrogenetogram illustrating the paragenetic sequence of the diagenetic
compartment 2 (DC-2)... 156 Figure 5-12 Thin-section micrographs of normal light, stained, and cathodoluminescence
showing diagenetic phenomena of the Ordovician diagenetic compartment 2 (DC-2) of the northwestern Tarim Basin, China ... 157 Figure 5-13 Petrogenetogram illustrating the paragenetic sequence of the diagenetic
compartment 3 (DC-3)... 158 Figure 5-14 Thin-section micrographs of normal light, stained, cathodoluminescence and
fluorescence showing early diagenetic phenomena of the Ordovician diagenetic compartment 3 (DC-3) of the northwestern Tarim Basin, China ... 161 Figure 5-15 Thin-section microphotographs of normal light, stained and
cathodoluminescence and SEM photomicrograph showing various deep-burial diagenetic phenomena of the Ordovician diagenetic compartment 3 (DC-3) of the northwestern Tarim Basin, China ... 161 Figure 5-16 Thin-section micrographs of normal light and stained showing different
generations of fractures and their associated cements and inclined stylolites of the Ordovician diagenetic compartment 3 (DC-3) of the northwestern Tarim Basin, China ... 163 Figure 5-17 Petrogenetogram illustrating the paragenetic sequence of the diagenetic
compartment 4 (DC-4)... 163 Figure 5-18 Thin-section micrographs of normal light, stained, cathodoluminescence and
fluorescence showing different diagenetic phenomena the Ordovician diagenetic compartment 4 (DC-4) of the northwestern Tarim Basin, China ... 165
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Figure 5-19 Component-specific geochemical sampling for carbon and oxygen stable isotopic composition based on paragenetic sequences, Mid-Late Ordovician, Tarim Basin ... 167 Figure 5-20 Carbon and oxygen stable isotope data of specific carbonate components from
the Middle to Late Ordovician of the northwestern part of the Tarim Basin, China.. 170 Figure 5-21 Biosedimentary, diagenetic, geochemical compartmentalization and porosity
evolution of Mid-Late Ordovician interval of northwestern Tarim Basin, China ... 173 Figure 5-22 Comparison of carbon and oxygen stable isotope data from the least-altered
carbonate components of the Middle to Late Ordovician of the northwestern Tarim Basin and data reported from Ordovician outcrops within the Tarim Basin (Liu et al., 2016; Zhang and Munnecke, 2016) as well as from Middle to Late Ordovician marine cement of Tobin and Bergstrom (2002) and Tobin et al. (1996). ... 175 Figure 6-1 Comparison of the global and regional (Tarim Basin) Ordovician reef/mound
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Acknowledgement
This thesis is the result of four-and-a-half years’ work. I would never have finished this thesis without the help and support of many other people who I want to express my gratitude. First, I would like to thank my supervisor, Prof. Dr. Fritz Neuweiler who gave me the opportunity to do my PhD research at Université Laval. Throughout my entire doctorate study, he provided me with an ardent, patient, systematic and thorough supervision, step by step. I hope I interhabited his constantly renewed interest in carbonate sedimentology and his dynamic, rigorous and intellectual cogitation on doing research.
Second, I would like to thank my former master supervisor and doctorate co-supervisor, Prof. Bingsong Yu (China University of Geosciences, Beijing) who gave me generous encouragement and support during my PhD study. His professional skill and rich experience of the regional geology of the Tarim Basin gave me confidence and new perspective during my PhD research.
I would like to thank Dr. André Desrochers (Ottawa University) for agreeing to become one of my doctoral jury members, for allowing me to discover the fascinating geology of Anticosti Island in summer, 2015, and for his critical and constructive comments on one of my submitted manuscripts.
I would like to thank Dr. Denis Lavoie (Geological Survey of Canada, Quebec) for agreeing to become one of my doctoral jury members and for allowing me to discover the Ordovician outcrops around Quebec City in summer, 2015.
I would like to thank Prof. Dr. Axel Munnecke (Friedrich-Alexander Universität Erlangen-Nürnberg) for agreeing to become one of my doctoral jury members and for his constructive comments on my submitted manuscripts.
I would like to thank Prof. Dr. Adrien Immenhauser (Ruhr-Universität Bochum) and his staff for the geochemical analysis and cathodoluminscence observation during my stay in Bochum, Germany. I am also grateful to Prof. Jisuo, Jin (Western University) for his constructive comments on one of my submitted manuscripts.
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Last but not least, I would like to acknowledge the Chinese Scholarship Council (CSC) for financial support. I appreciate travel grants from the International Association of Sedimentologists (IAS), from the 5th IGCP 591 Annual Meeting committee, from the 1st International Carbonate Mound Conference (ICMC) committee, from Bourse Quebec 2008 and from Faculty of Science and Engineering (Université Laval).
This thesis benefited from the discussion with Prof. Dr. Judith McKenzie (ETH Zürich) on microbial and biological origin of carbonate rocks, from the discussion with Prof. Dr. Helmut Weissert (ETH Zürich) on ocean anoxic events (OAEs), from the discussion with Robert Riding on the evolution of Paleozoic calcimicrobes and calcareous algae, and from the discussion with Prof. Dr. Adrien Immenhauser on carbonate rock diagenesis and geochemistry.
I appreciate the professional help and friendship of my colleagues Stéphanie Larmagnat, Amira Abassi and Antoine Hlavaty. I also would like to thank my dear friends Jiafan Tian, Pengfei Hou, Shuo Sun and Fanyu Qi for their academic and daily help and support.
I received technical support from the department professionals Edmond Rousseau, Martin Plante, André Ferland, Marc Choquette, Guylaine Gaumond, Caroline Bédard, Marcel Langlois, Pierret Therrien and Olivier Rabeau. I thank Marjorie Lapointe-Aubert and Jiafan Tian for correcting the French version of the abstract.
Finally, I would like to thank my parents Shanfan and Songqin and my beloved Mengzhou. Thank you for your selfless and meticulous care and support.
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Foreword
This thesis includes two published articles and one accepted article of respective journals.
The paper entitled of “Halysis Høeg, 1932 in Ordovician carbonate mounds, Tarim Basin, NW China” (Chapter 2) co-authored by Yuefeng Shen & Fritz Neuweiler was published on
Palaios on September 18, 2015.
The paper entitled of “Taphocoenoses and diversification patterns of calcimicrobes and calcareous algae, Ordovician, Tarim Basin, China” (Chapter 3) co-authored by Yuefeng Shen & Fritz Neuweiler was published on Canadian Journal of Earth Sciences on July 13, 2016.
The paper entitled of “Questioning the microbial origin of automicrite in Ordovician calathid-demosponge carbonate mounds” (Chapter 4) co-authored by Yuefeng Shen & Fritz Neuweiler was accepted by Sedimentology on May 10th, 2017.
1
Introduction
A biosedimentary system refers to the influence of organisms on the geometry, fabric, texture, mineralogical-chemical composition (calcite, aragonite, silica, phosphate, organic carbon) and the diagenetic potential of sedimentary deposits. Biosedimentary systems are the result of biological evolution, autecological constraints and environmental change. Well known examples of biosedimentary systems encompass the Archean-Proterozoic marine microbial system, encrinites, tropical coralgal reefs and modern cold-water coral mounds. The geological endproduct of a biosedimentary system corresponds to its biosedimentary record. The biosedimentary record contains skeletons (biomineralization), authigenic sea-floor precipitates or automicrites (organomineralization), trace fossils or ichnotaxa, and biomarkers. To date, our understanding of the community structure, trophic structure, diversity and early diagenesis of a biosedimentary system is limited due to a number of phylogentically unresolved fossil groups and the polygenic nature of automicrite.
Marine carbonate biosedimentary systems not only reveal the interaction and co-evolution of living forces through Earth’s history but also have a huge potential for oil and gas exploration and production. It is estimated that more than 60% of the world’s oil and 40% of the world’s gas are hosted in carbonate rocks. Famous examples of such petroleum provinces include the Tertiary carbonate platform reservoirs of the Middle East and the Devonian reef systems of western Canada.
The Ordovician carbonate sequences exposed in the northwestern Tarim Basin, Xinjiang Uyghur Autonomous Region, NW China, offer an outstanding example for the study of biosedimentary systems. The exposed stratigraphic range covers an episode of global biodiversification, contains a number of problematic fossils, displays several carbonate mounds with automicrite in rock-forming abundance and is located in one of China’s most important oil and gas provinces.
1.1 The Ordovician biosedimentary systems
The Ordovician marine carbonate biosedimentary record, represents a great revolution of biosedimentary systems in respect to community structure of carbonate buildups, trophic
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structure and paleodiversity. It attracts more and more attention in the recent decades after the introduction and definition of the “Great Ordovician Biodiversification Event” (GOBE, Webby et al., 2004).
1.1.1 The Great Ordovician Biodiversification Event (GOBE)
The term “Great Ordovician Biodiversification Event (GOBE)” was introduced by Webby and others to label the IGCP 410 project (Webby et al., 2004). This term, derived from the “Ordovician Radiation” of Droser et al. (1996), defines a series of Ordovician diversification events establishing the “Paleozoic Evolutionary Fauna” progressively replacing the “Cambrian Evolutionary Fauna” (Sepkoski and Sheehan, 1983; Sheehan, 1996; Fig. 1-1), stretching from the beginning to the end of the Ordovician Period (nearly 46 m.y. of earth history). The punctuation part of the “Ordovician Radiation” was during Middle to Late Ordovician, an interval of around 25 m.y. (Dapingian to Katian, prior to the end Ordovician extinction event), when a cascade of diversification resulted in increased biodiversity at species, genus, and family levels (Sepkoski, 1981, 1995; Droser and Sheehan, 1995; Sheehan, 2001).
Figure 1-1 The Great Ordovician Biodiversification Event (GOBE), modified after Sepkoski (1995,
3
The beginning and the duration of the GOBE is still under debate. Along the Ordovician radiation, individual fossil groups show different curves with diversity peaks at different stratigraphic levels. Some Paleozoic fossil groups, such as brachiopods, show a dramatic increase of diversity during a relatively short time span in the Floian-Darriwilian (Early and Middle Ordovician, Harper et al., 2004; Rasmussen et al., 2016). Other fossil groups such as corals, diversify much later, but also very rapidly, during the Sandbian (Late Ordovician, Webby et al., 2004). The diversification trend of some fossil groups appears resolved while others including some subphyla of echinoderms and phytoplanktons are still unsettled (Nardin and Lefebvre, 2010; Servais et al., 2008, 2016). Thus, the GOBE could not be regarded as a single event but as a series of different diversity increases of marine planktonic and benthic organisms at different time scales and age initiated in the Cambrian and was only interrupted by the end-Ordovician extinction. And in a large scale, the GOBE, together with the Cambrian Explosion, could be part of a long-term radiation that commenced in the Late Precambrian and peaked during the early Devonian radiation (Servais et al., 2016). In the following chapter 4 (manuscript by Shen and Neuweiler accepted by Sedimentology) the onset of the GOBE will be discussed.
During the calculation of biodiversity, many problematic and phylogenetically unresolved fossil groups were wrongly categorized, especially some calcimicrobes, calcareous algae and some sponges. For example, the macrofossil calathid sponge by most authors is considered a coralline sponge which in gross morphology resembles archaeocyathids (Billings, 1865; Guo, 1983; Church, 1991; Zhang, 1995; Liu et al., 1996, 2005; Wang et al., 2011; Li et al., 2015, 2017a, b). However, in biodiversity curves calathid sponges were classified as green algae together with other receptaculids (Nitecki et al., 2004). This problem is also commonly corresponding to some problematic calcareous microfossils. An example of such microfossils will be discussed in the following chapter 2 (manuscript by Shen and Neuweiler, published in Palaios).
There is a lack of biodiversity curves for some important fossil groups. For example, there are only qualitative assessments on the diversity patterns of Ordovician calcimicrobes and calcareous algae (Chuvashov and Riding 1984; Roux 1991; Nitecki et al. 2004; Trotter et al. 2008). Servais et al. (2008) compiled diversity curves for marine phyto- and zooplankton in
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combination with diversity curves of several marine invertebrates, but calcimicrobes and calcareous algae were not even mentioned. In the following chapter 3 (manuscript by Shen and Neuweiler, published on CJES), both global and regional biodiversity curves for calcimicrobes and calcareous algae will be discussed.
The so-called “hidden diversity” in function of taphonomy, preservation and interpretation is not clear (Sepkoski 1988; Jackson and Johnson, 2001; Stewart and Owen, 2008). For example, there is an overestimation of microbial microcrystalline Ca-carbonate in Ordovician carbonate reefs/mounds. Some of these microcrystalline Ca-carbonate fabrics could represent metazoan tissues (Neuweiler et al., 2009). In the following chapter 4 (manuscript by Shen and Neuweiler accepted by Sedimentology), some parts of this issue will be discussed in detail.
The triggers of the GOBE in terms of biosedimentary turnovers are far from being resolved. By comparing the fossil diversity curves with the timing of major paleoenvironment changes, several authors suggested that these biological radiation events are directly correlated with changes in the physical environment controlled by tectonic evolution (Miller and Mao, 1995), volcanic eruptions (Botting, 2002; Bergström et al., 2004; Barnes, 2004), opening and closing of oceanic basins (Walker et al., 2002), sea-level oscillations (Hallam, 1992; Barnes, 2004), climate changes (Saltzman and Young, 2005; Trotter et al. 2008), ocean oxygenation (Marenco et al., 2016), and/or an asteroid breakup event (Schmitz et al., 2008). However, as Sepkoski and Sheehan (1983) have already pointed out, there seems to be “no immediately obvious physical trigger for such a great burst of evolutionary activity” that could have caused the Ordovician biodiversification. In the following chapter 5 (manuscript in preparation), some parts of this issue will be discussed.
1.1.2 The Ordovician paleoenvironmental system
The great Ordovician marine radiation occurs in association with the change of the Ordovician paleoenvironmental systems. This system comprises regional and global paleogeography, ocean circulation, sea-level oscillation, climate change, seawater chemistry fluctuation and nutrient level revolution.
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1.1.2.1 Paleogeography and ocean circulation
During the Ordovician period, there were four major continents and some microcontinents (Wilde, 1991; Scotese and Mckerrow, 1991; Fig. 1-2). The craton of Laurentia was straddling the Equator represents most of present-day North America, Greenland, and part of Scotland. The craton of Siberia-Kazakhstan was lying east of Laurentia, along and slightly north of the Equator. The craton of Baltica was in the subtropical to temperate paleolatitudes corresponding to the present-day Scandinavia and north-central Europe. The supercontinent Gondwana was composed by present-day Southern Europe, Africa, South America, Antarctica and Australia. Throughout the Ordovician, this immense supercontinent ranged from the South Pole to the Equator. These continents were separated by three major oceans (Scotese and McKerrow, 1991; Fig. 1-2). The Iapetus Ocean separated the Laurentia craton and the Baltica craton as well as the microcontinent of Avalonia represents present-day England, New England and maritime Canada. The Prototethys Ocean separated Baltica, Siberian-Kazakhstan from Gondwana and a series of small blocks. The single body of Panthalassic Ocean, like the modern Pacific Ocean, covered almost the entire Northern Hemisphere.
The Ordovician oceanic mode is generally considered consisting of warm seas, gentle latitudinal and vertical gradients, sluggish circulation, low-nutrient ("oligotrophic") conditions, expanded oxygen minimum zones below the thermocline, and high sea levels (Fischer and Arthur, 1977; Martin, 1996). There are a few reports on the Ordovician ocean circulation (Christansen and Stouge, 1999; Pohl et al., 2016; Rasmussen et al., 2016). Recently, Pohl et al. (2016) reconstructed the Ordovician ocean-surface circulation patterns based on simulations using FOAM, a coupled ocean–atmosphere general circulation model, and state-of-the-art paleogeographic reconstructions (Torsvik and Cocks, 2009, 2013). This study shows that ocean-surface circulation patterns were highly sensitive to atmospheric CO2
levels. In fact, cooler climatic conditions appear to have increased the pole-to-equator temperature gradient, invigorating global-ocean circulation and thus leading to improved oxygenation of the deep ocean (Kidder and Tomescu, 2016; Pohl et al., 2016).
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Figure 1-2 Ordovician paleogeography with distributions of major lands and marine carbonate
deposit, modified from Golonka and Kiessling (2002) and Scotese and McKerrow (1991). Tarim Basin encircled in yellow.
1.1.2.2 Sea level
Following the breakup of the supercontinent Rodinia near the end of the Proterozoic, the seafloor spreading rate peaked in association with greatest on-average decreases in depths of oceani basins during the Ordovician (Vail et al., 1991). Thus, the Ordovician is characterized
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as a time of maximum eustatic high within the Paleozoic first-order cycle (Vail et al., 1991). It was depicted that shallow epeiric seas were covering at least 40%, or even 60% of the land areas during the Ordovician supporting the formation of voluminous carbonate deposits (Vail et al., 1977; Hallam, 1992). Sea level fluctuated continuously throughout the Ordovician. A second-order pattern is subdividing the Ordovician into three highstand and three lowstand “intervals” (Fig. 1-3). According to traditional British stages (revised subdivision in parenthesis), these are the Early to Mid-Tremadoc Highstand Interval (Tr1, Fig. 1-3), the Late Tremadoc-Early Arenig Lowstand Interval (Tr2-Fl1, Fig. 1-3), the Mid Arenig Highstand Interval (Fl1-Dp1, Fig. 1-3), the Late Arenig-Early Llanvirn Lowstand Interval (Dp2-Dw3, Fig. 1-3), the Late Llanvirn-Caradoc Highstand Interval (Dw3-Ka4, Fig. 1-3) and the Ashgill Lowstand Inerval (Ka4-Hi2, Fig. 1-3). The most important transgression occurred in the Caradoc, perhaps the largest of the entire Phanerozoic (Hallam, 1992). The end of the Ordovician Period is marked by a pronounced fall of sea level of about 160 meters which was triggered by the rapid expansion of continental ice sheets on Gondwana (Fig. 1-3).
1.1.2.3 Climate
The solar luminosity of the Ordovician period was set 3.5% below present-day values (Gough, 1981). The level of atmospheric carbon dioxide (pCO2) was high, some 14 to 18
times of modern preindustrial levels (Yapp and Poths, 1992; Berner,1994; Gibbs et al.,1997). The sea surface temperature was elevated through Early to Middle Ordovician, then decreased dramatically during the Middle Ordovician and reached its minimum during the end of Ordovician leading to the end-Ordovician glaciation and mass extinction (Trotter et al., 2008; Fig. 1-3). Crowell (1999) underlined that the Ordovician greenhouse-icehouse transition under persistently high pCO2 (Crowley and Baum, 1995) is anomalous in
comparison with all other Phanerozoic ice ages. It was Kump et al. (1995) and Saltzman and Young (2005) who stressed the need for a substantial pCO2 drawdown in the run-up to the
end-Ordovician Hirnantian glaciation. Kump et al. (1999) proposed increased rates of weathering (pH, exposed mountain chains, evolution of land plants; Rubinstein et al., 2010) as a potential mechanism. Saltzman and Young (2005) pushed forward increased rates of burial of organic carbon in marine sedimentary basins (radiation of marine plankton, stagnant basins, anoxia, initiation of hydrocarbon source rocks, positive δ13Ccarbonate excursions). This
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latter concept is in good accordance with our current understanding of the global mid-Cretaceous oceanic anoxic events (Schlanger and Jenkyns, 1976; Jenkyns, 1980; sluggish oceans, important source rock generation). Since the work of Kump et al. (1999) and Saltzman and Young (2005), the challenge was to provide convincing geological evidence for climatic perturbations (pCO2 drawdown: regional scale, global scale, frequency,
net-effect) at the dawn of the end-Ordovician glaciation.
Figure 1-3 Ordovician sea-level, geochemical composition, atmospheric oxygen and carbon dioxide
curves. Sea-level curve based on Nielsen et al. (2004); δ13C
carb curve complied by Bergström
et al. (2009); Oxygen isotope compositions of bioapatite of conodonts from Canada and Australia with 2-sigma error bars complied by Trotter et al. (2008); Ca concentration curve and Mg/Ca ratio curve based on Hardie (1996), Stanley and Haride (1998) and Stanley and Hardie (1999); 87Sr/86Sr curve from Shields and Veizer (2004); Atmospheric O2 curves from
Algeo and Ingall (2007), Berner (2006) and Bergman et al. (2004); Atmospheric CO2 curve
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1.1.2.4 Sea water chemistry
Th Ordovician sea-water was in the stage of ‘Calcite Sea’ (Porter, 2010), which means extensive and pervasive low-magnesium calcite (LMC) precipitated on shallow marine sea floors (Sandberg, 1983; Hardie, 1996). The LMC-dominated mineralogy probably resulted from a high-level of calcium concentration (>50 meq/L) and low Mg/Ca ratios (~1) in the sea water caused by high sea-floor spreading, which is in accordance to the overall high sea level during this time (Stanley and Hardie, 1999; Fig. 1-3).
The Ordovician is also characterized by a large drop in 87Sr/86Sr values, from ca. 0.7090 to 0.7079 (Yang and Wang, 1994; Qing et al., 1998; Shields et al., 2003; Fig. 1-3), and this general trend is explained in terms of the reduction in rates of tectonic uplift generated by the waning of Pan-African mountain-building (Qing et al., 1998; Shields et al., 2003). A major drop in seawater 87Sr/86Sr is observed at the Middle-Late Ordovician boundary (Darriwilian– Sandbian transition; Qing et al., 1998; Veizer et al., 1999; Shields and Veizer, 2004). This drop is one of the most rapid changes in 87Sr/86Sr recorded for the entire Phanerozoic.
A generalized δ13Ccarb curve was published by Bergström et al. (2009) based on data from
Argentina (Tremadocian to Dapingian), Estonia (Darriwilian to Sandbian), and North America (Katian to Hirnantian) (Fig. 1-3). Multiple positive δ13C
carbonate excursions stretching
from the Darriwilian to the uppermost Katian via Sandbian were identified both locally and globally (Bergström et al., 2001, 2006, 2007, 2010; Kaljo et al., 2007; Ainsaar et al., 2010; Fig. 1-3). These excursions include the Mid-Darriwilian (MDICE, Ainsaar et al., 2007, 2010; Schmitz et al., 2010; Albanesi et al., 2013; Edwards and Saltzman, 2016; Young et al., 2016), the Sandbian (SAICE, Leslie et al., 2011), the Guttenberg (GICE, Bergström et al., 2010; many other authors), the Rakvere (KOPE, Bergström et al., 2010), the Fairview (Bergström et al., 2010), the Waynesville (Bergström et al., 2010), the Whitewater (Bergström et al., 2010), the Elkhorn (Bergström et al., 2010), and the Paroveja (Bergström et al., 2010) excursions. The two most prominent of those anomalies are the Guttenberg positive δ13C excursion (GICE, Bergström et al., 2001) of lowermost Katian age (△13C
carb ~ +2.5) and the
Mid-Darriwilian positive δ13C excursion (MDICE, △13C
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Ordovician sea-water stable oxygen isotope data were measured either from well-preserved brachiopod shells (Brenchley et al., 1994, 2003; Marshall et al., 1997; Veizer et al., 1999; Shields et al, 2003; Hints et al., 2010) or conodont bioapatites (Lehnert et al., 2007; Trotter et al., 2008; Buggisch et al., 2010). Both data from brachiopod shells (Shields et al., 2003; from Laurentia, Baltica, South China and Australia) and conodonts (Trotter et al., 2008; from Australia and Canada) show an overall trend towards heavier values during the Ordovician, with maximum values observed in the Hirnantian (Fig. 1-3).
The sulfur isotopic composition of Ordovician seawater was not well established and only presented by a few authors (Thode and Monster, 1965; Claypool et al., 1980; Fox and Videtich, 1997; Thompson and Kah, 2012; Present et al., 2015; Kah et al., 2016; Young et al., 2016). Carbonate-associated sulfate (CAS) was regarded as a reliable proxy for marine sulfate δ34S (Burdett et al., 1989; Strauss, 1999; Kampschulte et al., 2001; Lyons et al., 2004).
However, by showing wide variation in the isotope composition of CAS within single geologic samples, Present et al. (2015) suggested the potential of CAS of reflecting more complicated processes of deposition and diagenesis. Ordovician marine S-isotope records from CAS, in association with marine sulfide and sedimentary pyrites from the Argentine Precordillera, western Newfoundland and South China show the presence of persistent marine euxinia along continental margins from the Early Ordovician through at least the Middle Ordovician (Thompson and Kah, 2012; Kah et al., 2016). A dramatic change of the sulfur isotope record in the early Darriwilian reflects an oxidation event during which a substantial portion of euxinic waters were oxidized over a short interval (Kah et al., 2016). Young et al. (2016) documented a co-variation of C- and S-isotopes for Middle–Late Ordovician successions from the Appalachian Basin and Arbuckle Mountains. New findings indicate that two major events in which large (12 ‰) negative shifts in δ34S
CAS are linked to
positive shifts in δ34Spyrite (+10 ‰) and δ13Ccarb (+2 ‰).
1.1.2.5 Trophic chain
The Ordovician marine trophic regime differs fundamentally from that of the Cambrian what concerns dominant autotrophs, dominant heterotrophs, trophic structure, oxygenic tolerance and evolutionary strategy (Servais et al., 2008; Kanygin, 2008; Prothero et al., 2013; Figs. 1-4, 1-5). In terms of dominant autotrophs, there is a transition from a Cambrian
calcimicrobe-11
dominated benthic community via an Early Ordovician calcimicrobe-dominated benthic community with some algae to a Middle-Late Ordovician algae-dominated benthic community and a few calcimicrobes. Pelagic autotrophs proceed from a Cambrian-Early Ordovician acritarch-dominated community to a Middle-Late Ordovician chitinozoan-acritarch dominated community. In terms of dominant heterotrophs, benthic grazing and deposit-feeding groups (trilobites, soft-bodied worm-type organisms) decrease along the Cambrian-Ordovician boundary interval. Benthic filter-feeders proceed from an Early Cambrian archeocyathid-siliceous sponge dominated community via a Middle Cambrian to Early Ordovician eocrinoid, sponge and calathid dominated community to a Middle to Late Ordovician cnidarian, echinoderm, brachiopod, bryozoan, pelecypod and gastropod dominated community. Pelagic zooplankton and nekton proceed from a Cambrian meroplankton-dominated community to an Ordovician graptolite, radiolarian, nautiloid, agnostid, and conodont-form dominated community. The Ordovician trophic structure is characterized by detritivore-based food whereas the Cambrian is characterized by grazing-based food. In terms of oxygenic tolerance, there is a transition from a Cambrian-Early Ordovician oxyphobe heterotroph-dominated community to a Middle to Late Ordovician oxyphile heterotroph-dominated community. The Ordovician evolutionary strategy (tendency) is a coherent evolution characterized by adaption for biotic settings, whereas the Cambrian evolutionary strategy is a noncoherent evolution characterized by adaptation for abiotic settings. Yet, the relation between marine epibenthic primary producers (calcimicrobes and calcareous algae) and herbivores and suspension-feeders is unclear. Chapter 3 (manuscript by Shen and Neuweiler, published on CJES) will discuss this issue.
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Figure 1-4 Reconstruction of an Ordovician food web (modified from Dott and Prothero, 1994).
1.1.3 The Ordovician carbonate reefs and mounds
During the Ordovician period, along with the diversification and spread of planktonic and suspension feeding organisms, benthic reef- and mound-forming communities receive special attention to explore the Great Ordovician Biodiversification Event (Sepkoski, 1997; Webby et al., 2004; Harper, 2006; Trotter et al., 2008; Servais et al., 2008, 2009, 2010; Prothero, 2013; Harper et al., 2015). Early and Middle Ordovician carbonate buildups recorded the transition from the Cambrian benthic evolutionary community with calcimicrobes such as Girvanella, Renalcis and Epiphyton, toward the Paleozoic benthic evolutionary community characterized by clonal, massive to sheet-forming eumetazoan skeletons (coralline sponges, corals, bryozoans). This transition is associated with a rising of Ordovician flora (e.g. Dasycladales, Bryosipidales; Chuvashov and Riding, 1984; Alberstadt and Repetski, 1989; Brunton and Dixon, 1994; Zhuravlev, 1996; Webby, 2002; Kiessling, 2009; Adachi et al., 2009, 2011, 2013; Hong et al., 2014, 2015; Fig. 1-8).
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Figure 1-5 Evolution of the taxonomic, trophic and space structure of marine ecosystems through the
Early Paleozoic (modified from Kanygin, 2008).
Webby (2002) made a detailed summary for the Ordovician reef patterns and their possible controls. Kiessling et al. (2002) and their “Paleoreef” database offered very comprehensive information on Ordovician carbonate buildups. However, as Webby (2002) pointed out, these Ordovician reefs/mounds, with few exceptions, remain inadequately documented and further study is certainly required. The constructional elements of these carbonate reefs/mounds are not well identified and/or defined taxonomically. Again, taking the Calathiums for example, they were wrongly classified as green algae rather than sponges thus misleading their contribution in the reef construction and paleoenvironmental interpretation. Except the benthic metazoan community, the contribution of a benthic flora (particularly calcareous algae and new taxa of calcimicrobes) to reef construction is biased. Few examples of Ordovician algal reefs/mounds were well documented. For example, Kröger et al (2016a) emphasized the contribution of Palaeoporella skeletons in the formation of Boda mud-mounds. The following chapters 2 and 3 will present some examples of algal reefs/mud-mounds. Except a few examples (Larmagnat and Neuweiler, 2015), little is currently known about the biological, sedimentary, taphonomic and diagenetic controls and their complicated interplays
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on the formation of Ordovician reefs/mounds in terms of biomineralization, organomineralization and cementation (diagenesis). Finally, though several authors (Adachi et al., 2011; Hong et al., 2014; Li et al., 2015; Kröger et al, 2016b) have tried to establish detailed phases of Ordovician reef/mound evolution, a higher-resolution trend for both regional and global reef/mound evolution is still under process. A case of regional reef/mound evolution will be presented in the discussion chapter (Chapter 6).
Figure 1-6 Dominant contributors to the formation of Ordovician reefs/mound (modified after
Rowland and Shapiro, 2002; Webby, 2002; Adachi et al., 2011; Hong et al., 2014, 2015; Li et al., 2015, own data).
1.2 The Ordovician of the Tarim Basin
The Ordovician of the Tarim Basin represents one case to track the evolution of Ordovician marine carbonate biosedimentary systems (Cai et al., 2008; Zhang et al., 2014; Liu et al., 2012, 2016; Shen and Neuweiler, 2015, 2016; Zhang and Munnecke, 2015; Li et al., 2016).
1.2.1 Tecto-sedimentary evolution of the Tarim Basin
The Tarim Basin is the largest inland sedimentary basin of China, located in the Xinjiang Uyghur Autonomous Region, NW China (Fig. 1-7). The basin is surrounded by the Tianshan Fold Belt to the north, the Kunlun Fold Belt to the southwest and the Altyn Uplift to the southeast (Fig. 1-8; Yu et al., 2016). The Tarim Basin is a cratonic basin developed on an
15
Archean to Mesoproterozoic crystalline basement and covered by thousands of meters of Neoproterozoic to Cenozoic deposits.
Figure 1-7 Location, surrounded tectonic setting and main tectonic units of the Tarim Basin
(modified from Shen and Neuweiler, 2015, 2016; Yu et al., 2016). D = Depressions; U = Uplifts. The Tarim Basin experienced a complex tecto-sedimentary evolution from the Neoproterozoic to the Cenozoic. There are eight large-scale tecto-sedimentary sequences identified (Dai et al., 2009; Lin et al., 2012; Figs. 1-8, 1-9). The basin developed on a crystalline basement, which belongs to a part of the so-called “Xinjiang” Craton, unconformably (seismic line Tg10) covered by lower Sinian (Early Neoproterozoic) morainic conglomerates and terrigenous clastic deposits as well as upper Sinian (Late Neoproterozoic) dolomite and mudrock in a rift or aulacogen setting (tecto-sedimentary sequence I; Figs. 1-8, 1-9). The basin evolved from a Cambrian to Ordovician passive continental margin and intracratonic depression setting to a Late Ordovician to early Silurian foreland or retroarc foreland and depression setting (tecto-sedimentary sequence II; Sloss’s sedimentary sequence Sauk and Tippcanoe, more regional scale; Figs. 1-8, 1-9). Huge amounts of limestone and dolomite accumulated within restricted and open carbonate platform environments during this phase. Intense tectonic movement occurred at the end of the Middle to Late Ordovician and resulted in the change of the basin background from a
16
passive to an active tectonic setting. This led to the development of the Tazhong uplift, the Tangguzibasi, and the northern depressions at the end of the Late Ordovician. There is continuous shallowing and uplifting along the northern basin margin with large-scale of terrigenous depositional wedges progradated from the north to south in the southeastern slope of the basin in association with deltaic and shallow marine deposits as well as some fluvial deposits in a cratonic inland depression setting from the Silurian to Middle Devonian (tecto-sedimentary sequence III; regional scale; Figs. 1-8, 1-9). The basin experienced weak extension during Late Devonian to Permian with clastic shoreline, fluvial and deltaic deposits in a cratonic inland depression setting (tecto-sedimentary sequence IV; regional scale; Figs. 1-8, 1-9). Widely distributed Carboniferous to Permian mafic volcanic rocks suggest an extensive setting, which was likely in relation to backarc extension. During Late Permian to Triassic, the tectonic setting of the basin changed to compression, indicated by the formation of an unconformity that mainly developed along the northern uplift belt (tecto-sedimentary sequence V; part of Sloss’s Absaroka sedimentary sequence; regional scale; Figs. 1-8, 1-9). Alluvial, fluvial deltaic and lacustrine deposits with thick lacustrine dark mudrock were accumulated in an inland depression and marginal foreland basin setting during this stage. Rapid subsidence to strong uplift and deformation occurred from the Late Triassic to Cretaceous, results in superimposition and reformation of differently orientated protobasins filled with a series of regional depositional cycles bounded by major unconformities and consisting of extremely thick of alluvial and lacustrine deposits (tecto-sedimentary sequence VI; Figs. 1-8, 1-9). This tecto-sedimentary sequence corresponds to Sloss’s Zuni sedimentary sequence. The Jurassic Tarim basin was a downwarping depression and is inferred to be formed under relatively quiet tectonic setting. Terrigenous deposits of inland depression dominated by alluvial, fluvial and shallow to deep lacustrine deposits were accumulated during this stage. The Cretaceous marginal depressions along the northern and southwestern margins of the basin are regarded to form in a foreland tectonic setting. This northern southwestern foreland depression contains completely Cretaceous system with near-shore and shallow marine deposits. Paleogene and Neogene alluvial, fluvial and shallow lacustrine deposits in a foreland basin setting corresponds to the seventh scenario of the development of the Tarim Basin (tecto-sedimentary sequence VII; Figs. 1-8, 1-9). It corresponds to Sloss’s Tejas sedimentary sequence. The marine transgression occurred during the Early Eogene and
17
the marginal depressions accumulated thick beds of gypseous mudstones and salt deposits, which serve as an important seal bed in the basin. During the Neogene, the marginal depressions became subjet of a compressional regime and were intensively faulted and thrusted, resulting in series of faulted and folded structural belts along the foreland margins. The Quaternary deposits (tecto-sedimentary sequence VIII; Figs. 1-8, 1-9) of the Tarim Basin are characterized by alluvial and fluvial deposits being covered by eolian deposits of the Takla Makan Desert.
During the Ordovician, the Tarim Block together with other small continental blocks of China (North China-Korean, South China and Qaidam) probably was part or close to the Australian-Antarctic realm of east Gondwana (Metcalfe, 1996). It was located in subtropical to tropical paleolatitudes (Wang et al., 2013; Fig. 1-2). The Tarim Block was facing the Kazakhstan plate to the south and the Amuria plate to the north across the Prototethys Ocean (Fig. 1-2). The Ordovician Tarim Basin was composed of three platforms in the southwestern, northern and central parts, namely the Southern Platform, the Northern Platform and the Bachu-Central Platform as well as three depressions (minibasins) in the southwestern, northwestern and northeastern parts (namely the Southwestern Basin, the Awati Basin and the Manjiaer Basin) (Gao and Fan, 2015; Yu et al., 2016; Figs. 1-10). The Lower Ordovician succession essentially is dolomite and dolomitic limestone. The Mid- to Upper Ordovician is made up of reef and shoal carbonate deposits (Li et al., 2012). Siliciclastic deposits of the upper part of the Upper Ordovician onlap these deposits, and the carbonate platform became entirely drowned (Gao et al., 2006). Toward the end of the Ordovician, the deep-water basin was shallowing up rapidly and subsequently eroded during uplift (unconformity between the Ordovician and the Silurian, Lin et al. (2012)).
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Figure 1-8Seismic interpretation profile across the Tarim Basin, showing the distribution of major unconformities (a) and variation of the tecto-sedimentary framework (b) (from Lin et al., 2012).
19
20
Figure 1-10Ordovician paleogeography and facies maps of the Tarim Basin (adapted from Gao and Fan, 2015). (A) Early-Middle Ordovician; (B) Late Ordovician. The study area is located on the northwestern part of the Central-Bachu platform.
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1.2.2 Ordovician petroleum system of the Tarim Basin
A series of oil and gas fields have been discovered in the Lower Ordovician of the northern and southwestern Tarim Basin and in the Middle to Late Ordovician of the central Tarim Basin (Lv et al., 2004; Chang et al., 2013; Pang et al., 2013; Wang et al., 2014), attracting more and more commercial investment. The Ordovician petroleum system of the Tarim Basin comprises three groups of source rocks, three main types of reservoirs and two seal units. There are three successions of source rocks present in the Ordovician of the Tarim Basin, the Lower to Middle Ordovician Heitu’ao Formation (Tremadocian to Dapingian), the Middle Ordovician Saergan Formation (Darriwilian to Sandbian) and the Upper Ordovician Yingan Formation (Katian) (Chen et al., 2012). The black shale of the Heitu’ao Formation is most widespread and is distributed in the eastern Tarim Basin with a thickness between 40 to 80 meters and an average relic total carbon content (TOC) of 1.0 to 3.0% (Lu et al., 2012; Chen et al., 2014). The graptolitic black shale of the Saergan Formation is restricted to the western part of the Tarim Basin with a maximum thickness of 50 meters and an average TOC of 1.0 to 5.0% (Gao et al., 2012; Ju et al., 2014). The calcareous black shale of the Yingan Formation is restricted to the western part of the Tarim Basin. Its maximum thickness is 100 meters and its TOC content is around 1% (Gao et al., 2012).
There are three main types of carbonate reservoirs present in the Ordovician of the Tarim Basin. In the Lower Ordovician there are dolomite-type reservoirs distributed in the central and northern part of the Tarim Basin (Jiang et al., 2014). The Mid- to Upper Ordovician reservoirs are characterized by reef/shoal-type systems distributed in the central, northern and western part of the Tarim Basin (Gao and Fan, 2013). Karst-type reservoirs are present through the entire Ordovician located in the central and northwestern part of the Tarim Basin (Zhang et al., 2016).
There are two cap rocks present in the Ordovician of the Tarim Basin. The Upper Ordovician Sangtamu Formation is a basin-wide mudrock with a minimum thickness of 50 meters and an average thickness of 570 meters. This formation comprises the best and most important Ordovician cap rock of the Tarim Basin (Zhang et al., 2014). The Middle Ordovician Tumuxiuke (Qiaerbake) Formation is another sealing unit. It is a condensed section
22
composed of argillaceous nodular limestone with a thickness in the range of tens of meters (Zhu et al., 2013). Due to its reduced thickness, this succession is considered a less important sealing unit.
1.2.3 Diagenesis of Ordovician carbonate reservoirs of the Tarim Basin
Diagenetic analysis of Ordovician carbonate reservoirs of the Tarim Basin generally focuses on the construction of paragenetic sequence, the hydrothermal alteration, the igneous intrusion, the fracturation and the hydrocarbon migration. Regional and basin-wide paragenetic sequence of Ordovician carbonate reservoirs were primarily constructed by Ni et al. (2010), Zhang et al. (2013a) and Jia et al. (2016). Ni et al. (2010) and Zhang et al. (2013a) discussed the diagenetic processes and porosity evolution from Ordovician oilfields of the Yingmaili-Halahatang area and the Tahe area. Jia et al. (2016) illustrated petrographic and geochemical constraints on diagenesis and deep burial dissolution of Ordovician carbonate reservoirs in the subsurface of the central Tarim Basin (Tazhong area). A group of studies emphasized the hydrothermal alteration on the Ordovician carbonate reservoirs of the Tarim Basin (Chang et al., 2003; Wu et al., 2007; Zhu et al., 2010a, b, 2011; Zhu and Meng, 2010; Li et al., 2011a, b; Cui et al., 2012; Dong et al., 2013; Jiang et al., 2014; Xu et al., 2014, 2015; Zhang et al., 2014). Based on a combination of petrographic analysis and geochemical constrains, Zhu and Meng (2010) and Cui et al. (2012) confirmed a hydrothermal origin of a series generations of silicification in the Ordovician carbonate reservoirs. Multiple constrains from petrology, isotope geochemistry and fluid inclusion microthermometry indicate a hydrothermal origin and alteration of multiple dolomitizations within the Ordovician carbonate reservoirs (Zhu et al., 2010a, b, 2011; Dong et al., 2013). Petrologic alteration, mineralogical paragenesis, fluid-inclusion homogenization temperature and salinity, trace elements, carbon, oxygen sulfur stable isotopes, strontium isotopic composition, rare earth elements analysis further provided confidence on the hydrothermal activity within the Ordovician carbonate reservoirs (Wu et al., 2007; Li et al., 2011b; Jiang et al., 2014; Jia et al., 2016). Zhu et al. (2011) and Xu et al. (2014, 2015) discussed the reservoir potential and mechanisms of direct alteration (marmorisation) of carbonate succession with the contact of magma (diabase) intrusion. Most of these studies on the hydrothermal alteration were interpreted as a result of the magma intrusions of the Early23
Permian Tarim Large Igneous Province (TLIP; Li et al., 2011c; De Booder, 2013; Yang et al., 2013; Chen et al., 2014). Hydrothermal models for fluid mixing and reservoir formation of Ordovician carbonate succession were proposed by Wu et al. (2007), Zhu et al. (2010), Jiang et al. (2014) and Xu et al. (2015). The effect of fractures on the carbonate reservoir potential of the Ordovician succession of the Tarim Basin was discussed by several authors (Han et al., 2010; Zhang et al., 2010; Qu et al., 2011; Zhang et al., 2011). These studies revealed multiple generations of tectonic fracture networks and their effects on hydrocarbon and fluid migration. Organic geochemistry and molecular biomarkers of bitumens preserved in reservoir pores indicate there were three generations of hydrocarbon migrations in the Ordovician carbonate reservoirs (Zhang et al., 2013b). However, all of these studies were dealing with the entire Ordovician carbonate succession in general. There was no high-resolution facies-dependent paragenetic sequence (petrogenetogram, Schroeder, 1984, 1988) established. The following chapter 5 (manuscript in preparation) will try to establish such kind of high-resolution petrogenetogram for each biosedimentary system in association with their geochemical proxies.
