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Stefano M. Bernasconi:

Geochemical and Microbial Controls on Dolomite Formation in Anoxic Environments

A Case Study from the Middle Triassic (Ticino, Switzerland)

1994. V, 109 pages, 6 tables, 7 plates, 16x24cm, 300 g
Language: English

(Contributions to Sedimentary Geology, Volume 19)

ISBN 978-3-510-57019-5, paperback, price: 38.00 €

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Keywords

sedimentologydolomite formationanoxicgeochemicalmicrobialmiddle TriassicTicinoSwitzerland

Contents

Synopsis top ↑

The Middle Trassic Grenzbitumenzone is a 16 m thick sequence of interbedded, finely laminated organic matter-rich dolomites and black shales. Organic carbon contents in the dolomites reach 10 wt%, whereas black shales have organic carbon contents of up to 40 wt%. Geochemical calculations based on trace metal concentrations in the black shales together with paleontological and sedimentological data indicate that the sediments of the GBZ were deposited at extremely low sedimentation rates (2 to 5 m/my) in a silled shallow marine basin (30-100 m deep) under permanently anoxic conditions. Sedimentary structures indicate that the dolomites are the product of periodic turbiditic transport of calcium carbonate mud into the basin diluting a more or less constant organic matter-siliciclastic background sedimentation. Carbon isotope compositions of the dolomites range from -1.4 to -5.6%o (PDB) indicating that dolomite was formed in the sulfate reduction zone of organic matter diagenesis. No organic or inorganic geochemical evidence for methanogenetic activity is found in the sediments, suggesting that dolomite may have formed before sulfate was completely depleted from the pore waters. Sedimentary structures and the small variations in carbon isotopic compositions suggest that dolomite is mostly of replacement origin.

This study shows that the combination of slow sedimentation rates and high supply of organic matter are the main factors that favored extensive early diagenetic dolomitization in the GBZ. The slow sedimentation rate allowed enough time for magnesium and sulfate to diffuse into the pore waters. The high alkalinity produced by organic matter decomposition through sulfate reducing bacteria, combined with the availability of magnesium, led to high dolomite supersaturation in the pore-waters and to the replacement of the abundant precursor calcium carbonate.

Organic geochemical data indicate that the organic matter is immature and primarily of marine origin with a high bacterial contribution and can be classified as type II. A high contribution of bacterial lipids to the kerogen is indicated by high hopane concentrations. A model is presented in which sedimentation and organic matter accumulation and preservation is dominated by two major factors: the periodic deposition of carbonate mud turbidites and the presence of a permanently stratified water column in which cyanobacteria and chemoautotrophic bacteria formed a bacterial plate at the anoxic-oxic interface.

Content Description top ↑

In this study, the organic carbon-rich sediments of the Middle Triassic Grenzbitumenzone (GZB) in the Southern Alps along the border between southern Ticino (Switzerland) and Italy are investigated. A schematic map of the area showing the outcrops of Middle Triassic rocks is presented. Often the study of extreme and end-member situations enables a better understanding of processes that take place under more normal" earth's surface conditions.

Table of Contents top ↑

1. Scope of study and methodology 2
1.0 Introduction 2
1.1 The Grenzbitumenzone, a historical overview 2
1.2 Summary of research objectives 4
1.3 Methods 5
2. Stratigraphie framework 7
2.1 Introduction 7
2.2 General evolution of the Middle Triassic 7
2.3 Description of individual Middle Triassic formations 9
2.3.1 Lower Salvatore Dolomite 9
2.3.1.1 "Plattendolomite” 10
2.3.1.2 Diplopora Dolomite 10
2.3.2 Middle Salvatore Dolomite and Grenzbitumenzone 11
2.3.2.1 Middle Salvatore Dolomite 11
2.3.2.2 The Grenzbitumenzone11
2.3.2.2.1 Dimensions of the GBZ basin 13
2.3.3 Upper Salvatore Dolomite and San Giorgio Dolomites 16
2.3.3.1 Upper Salvatore Dolomite 16
2.3.3.2 San Giorgio Dolomite 16
2.3.4 Lower Meride Limestone17
2.3.5 ”Dolomitband” 17
2.3.6 Upper Meride Limestone18
2.3.7 "Kalkschieferzone” 18
2.3.8 Raibl Beds 18
3. Sedimentology and petrography 18
3.1 Introduction 18
3.1.1 The Grenzbitumenzone: general description 19
3.2 Description of lithologies 22
3.2.1 Rhythmic sediments 22
3.2.1.1 Laminated dolomites 22
3.2.1.2 Black shales 24
3.2.2 Non-periodic events 25
3.2.2.1 White dolomite layers 25
3.2.2.2 Bentonites 25
3.2.2.3 Cherts 26
3.2.2.4 Massive dolomites 26
3.2.2.5 Coarse resediments 27
3.2.3 Fossil content and preservation 27
3.3 Discussion: depositional environment 29
3.3.1 Origin of dolomite laminations 29
3.3.2 Origin of pure organic matter laminae 30
3.3.3 Bathymetry 31
3.4 Synsedimentary deformation structures 33
3.4.1 General characteristics 33
3.4.2 Semi-ductile deformation features 33
3.4.3 Brittle deformation (Layers 140, 144u, 88)34
3.4.4 Interpretation 35
3.5 Dolomite petrography and cathodoluminescence (CL) cement
stratigraphy 36
3.5.1 Grenzbitumenzone36
3.5.2 San Giorgio Dolomites and Lower Salvatore Dolomite 37
3.5.2.1 Salvatore Dolomite 37
3.5.2.2 San Giorgio Dolomite 37
3.5.3 Fluid inclusions 38
3.5.4 Summary 39
4. Inorganic geochemistry 39
4.1 Major and minor element geochemistry 39
4.1.1 Introduction 39
4.1.2 Results 41
4.1.2.1 Major elements 41
4.1.2.2 Trace elements 42
4.1.3 Discussion 44
4.1.3.1 Source of trace metals 44
4.1.3.2 Controls on V and Ni enrichment 45
4.1.3.3 Calculations of excess trace metals 45
4.1.3.4 Calculation of sedimentation rates and water residence times 47
4.1.3.5 Calculation of primary productivity 50
4.1.4 Summary 51
4.2 Sulfur geochemistry 51
4.2.1 Introduction 51
4.2.2 Results 53
4.2.3 Discussion 54
4.2.3.1 Controls on pyrite formation 54
4.2.3.2 Timing of pyrite formation 55
4.2.3.3 Sulfur incorporation in GBZ organic matter 57
4.2.4 Summary 58
4.3 Oxygen and carbon isotope geochemistry of dolomite 59
4.3.1 Introduction 59
4.3.2 Results 61
4.3.2.1 Dolomite crystallography and crystal chemistry 61
4.3.2.2 Isotope geochemistry of carbonates 62
4.3.3 Discussion: origin of GBZ dolomites 63
4.3.3.1 Controls on the carbon isotope composition 63
4.3.3.2 Possible temperature and composition of dolomitizing fluids 65
4.4 Carbonate fluorapatite isotope geochemistry 66
4.4.1 Results and discussion 66
4.4.1.1 Carbon isotopes 66
4.4.1.2. Oxygen isotopes 67
4.4.1.3 Carbonate substitution in the apatite lattice 67
4.5 Oxygen isotope geochemistry of cherts 68
4.5.1 Introduction 68
4.5.2 Results and discussion 68
4.6 Summary 69
5. Organic geochemistry 70
5.1 Organic matter content and type 70
5.2 Biomarker studies on chloroform extracts 70
5.3 Organic carbon isotope geochemistry 71
5.4 Summary 72
6. Summary and conclusions 73
6.1 Introduction 73
6.2 The GBZ depositional environment and basin geometry 74
6.3 Sedimentation rates 75
6.4 Origin of cyclicity 76
6.5 Controls on organic matter accumulation and preservation 77
6.6 Controls on dolomite formation in the GBZ 79
Acknowledgements 80
References 81
Appendix 91
Plates 1-7 95