The Nullarbor Plain in southern Australia is an uplifted succession of Cenozoic marine carbonates whose surface has been exposed for ~14 m.y. This succession of limestones, particularly in the surfical middle Miocene Nullarbor Limestone, hosts a complex and prolonged record of meteoric diagenesis. Alteration took place through 3 broad phases of diagenesis encompassing 8 stages that are interpreted to have taken place over a dramatic regional climate change. Phase 1 diagenesis occurred under a humid middle Miocene climate and involved mineralogical equilibration with meteoric fluids, calcite cementation, widespread microkarst, and regional lacustrine and palustrine sedimentation producing copious amounts of ooids. These ooids are interpreted to have formed via microbial secretions and sediment aggradation over multiple seasons of changing rainfall and soil hydration states. Cortical laminations are proposed to represent microbial mucus envelopes during wet seasons alternating with dehydration during dry seasons and precipitation of fibrous clay minerals and CaCO3 that preserve the pre-existing microbial fabrics. Phase 2 alterations took place under a more temperate climate from the late Miocene to Pliocene with a later pronounced humid interlude. This phase encompassed ~8 m.y. and was dominated by karst process where deep cave dissolution occurred at depressed water tables related to globally low sea levels and later shallow caves developed during a Pliocene sea level highstand. Phase 3 has occurred since the late Pliocene and is indicative of the onset of modern semi-arid climatic conditions. This final phase involved the creation of subsoil hollows filled with blackened limestone lithoclasts, deep and shallow dolines, and indurated pedogenic calcrete that now forms much of the surface of the Nullarbor Plain. Blackened limestone clasts have been shown to form at the B-C boundary in soil profiles where roots have their cellular structures calcified and during this process incorporate trapped organics that ultimately produce the distinctive black colouration. The importance of this comprehensive diagenetic record is its direct applicability to the understanding of ancient subaerial exposure surfaces. / Thesis (Ph.D, Geological Sciences & Geological Engineering) -- Queen's University, 2012-07-16 12:09:35.842

Thesis--Lehigh University. / Bibliography: p. 133-141.

Thesis--Lehigh University. / Bibliography: p. 133-141.

Diagenetic histories of Proterozoic and Paleozoic carbonate strata on Victoria Island, in the Canadian arctic, are poorly understood, and their potential to be associated with base metals or petroleum is unknown. Using fluid inclusion and geochemical techniques, it was determined that the diagenetic fluid compositions of two major carbonate units, the Wynniatt Formation and the “Victoria Island formation”, were largely controlled by fluid-rock reactions in reservoirs and by mixing of multiple fluids. Diagenesis of the Wynniatt Formation resulted from the progression from a shale-dominant fluid mixture to a meteoric-dominant mixture. Fluid composition of “Victoria Island formation” was a shale-dominant mixture. A change in fluid:rock from low to high was recorded during diagenesis of both units. Metals and hydrocarbons transported to the study sites were ac-quired by the fluids during interaction with the respective source reservoirs. Mixing of diagenetic fluids follows the established ‘mixing model’ used to explain many other min-eralised locations. The diagenetic fluids that affected the strata in this study were compa-rable to those that produced the Polaris Zn-Pb deposit. This similarity suggests that there is potential for mineralisation on Victoria Island.

Carbonate diagenesis encapsulates a wide range of water rock interactions that can occur within many environments and act to modify rock properties such as porosity, permeability, and mineralogical composition. These rock modification processes occur by the supply of reactant-laden fluids to areas where geochemical reactions are thermodynamically and kinetically favoured. As such, understanding the development of diagenesis requires an understanding of both palaeohydrology and geochemistry, both of which have their own complexities. However, within geological systems, both the conditions that control fluid migration and the distribution of thermodynamic conditions can change through time in response to external factors. Furthermore, they are often coupled, with rock modification exercising a control on fluid flow by altering the permeability of sediments. Numerical methods allow the coupling of multiple complex processes within a single mathematical formulation. As such, they are well suited to investigations into carbonate diagenesis, where multiple component subsystems interact. This thesis details the application of four separate types of numerical forward modelling to investigations of diagenesis within two Carboniferous carbonate platforms, the Derbyshire Platform (Northern England) and the Tengiz Platform (Western Kazakhstan). Investigations of Derbyshire Platform diagenesis are primarily concerned with explaining the presence of Pb-mineralisation and dolomitisation observed within the Dinantian carbonate succession. A coupled palaeohydrology and basin-development simulation and a series of geochemical simulations was used to investigate the potential for these products to form as a result of basin-derived fluids being driven into the platform by compaction. The results of these models suggest that this mechanism is appropriate for explaining Pb-mineralisation, but dolomitisation requires Mg concentrations within the basin-derived fluids that cannot be attained. Geothermal convection of seawater was thus proposed as an alternative hypothesis to explain the development of dolomitisation. This was tested using an advanced reactive transport model, capable of considering both platform growth and dolomitisation. The results of this suggests that significant dolomitisation may have occurred earlier on in the life of the Derbyshire Platform than has previously been recognised. An updated framework for the development of diagenesis in the Derbyshire Platform is proposed to incorporate these new insights. The Tengiz platform forms an important carbonate oil reservoir at the northeastern shore of the Caspian Sea. The effective exploitation of any reservoir lies in an understanding of its internal distributions of porosity and permeability. Within carbonate systems, this is critically controlled by the distribution of diagenetic products. A model of carbonate sedimentation and meteoric diagenesis is used to produce a framework of early diagenesis within a sequence stratigraphic context. The studies mentioned above provide a broad overview of the capabilities and applicability of forward numerical models to two data-limited systems. They reveal the potential for these methods to guide the ongoing assessment and development of our understanding of diagenetic systems and also help identify key questions for the progression of our understanding in the future.

Carbonate precipitates in biofilm were investigated from hot springs near Viterbo, Italy; Salt Pond, San Salvador; and Fowl Cay Reef, Abaco, Bahamas. Features shared by hot springs and salt ponds are supersaturation with CaCO3, abundant Spirulina, and clustered acicular aragonite crystals termed “fuzzy dumbbells.” TEM and FESEM microscopy show fuzzy dumbbells contain a core of amorphous organic matter and subhedral CaCO3 microcrystals arranged in linear fabrics. Micron- to millimeter-scale microenvironments are identified by localized dissolution, the occurrence of gothic calcite inter-grown with organic filaments, and the presence of calcite in biofilm where aragonite is chemically favored. Spherical CaCO3 precipitates in reefs were anticipated, but not encountered in TEM sections of reef biofilm. In conclusion, biofilm creates the microenvironment and organic matter provides substrate for fuzzy dumbbell precipitation. TEM is a novel technique for studying the relationship between organic matter and CaCO3 precipitation, and has potential medical, industrial, and academic applications.

The Late Pennsylvanian was a time of frequent, rapid glacioeustatic sea-level changes. These changes were recorded in the Wolfcamp D Formation of the Midland Basin as a series of cyclothems similar to those studied in the Midcontinent region (e.g., Algeo and Heckel, 2008). This study focuses on identifying the mechanisms and controls on carbonate deposition and diagenesis through the Upper Pennsylvanian Wolfcamp D Formation and evaluating the potential for these layers to be stratigraphically significant. A stepwise progression of diagenetic processes was identified through the use of δ13Ccarb and δ18Ocarb, bulk geochemical and petrographic analysis, and scanning electron microscopy. Carbonate deposition and early-burial diagenesis appears to be strongly influenced by frequent changes in sea-level and benthic redox conditions. The transition to deep-burial diagenesis was controlled by the thermal gradient in the basin and the extent of diagenesis by the amount of clays and organic matter in the surrounding mudrocks. Further diagenesis was induced through interactions with a brine following clay diagenesis. The presence of multiple phases of diagenesis in this system further highlights the need for several lines of inquiry when evaluating the post-depositional evolution of carbonates in a mud-rich setting.

Les interactions entre la fracturation, les circulations fluides et la chimie des fluides au sein de marges hyper-étendues sont encore peu décrites et sont pour la plupart localisées en mer, enfouies sous des sédiments post-rift. Le bassin sud Aquitain et la partie nord des Pyrénées constituent un cas d’étude approprié pour l’investigation de ces interactions dans un modèle de marge hyper-étendue avec exhumation du manteau durant le Crétacé inférieur puis inversée. Les données de terrain ont permis de décrire trois principaux sets de fractures. Ils ont été corrélés aux principaux événements de l’évolution géodynamique du bassin correspondant au rifting triasique, à l’hyper-extension datée Aptien-Cénomanien, et à la compression pyrénéenne. Les observations pétrographiques, les analyses Raman et microthermométriques sur les inclusions fluides, les données acquises par ICP-MS, et les analyses isotopiques ont permis de déterminer les chimies, les températures, les conditions rédox, les compositions des gaz, les signatures isotopiques de l’oxygène et du carbone, et les teneurs en terres rares des fluides parents pour les ciments précipités durant chaque épisode. Ces données ont permis le calage temporel des évènements diagénétiques majeurs. En particulier, la dolomie baroque et la chlorite ont précipité dans les fractures du set 2 durant l’hyper-extension correspondant au pic thermique à des températures supérieures à 300°C. La signature isotopique, la forte teneur en CO2, l’occurrence de H2S et les fortes salinités des fluides parents suggèrent la percolation de fluides mantelliques ascendants au travers des évaporites triasiques. La phase fin et post hyper-extension est caractérisée par de la bréchification hydraulique dans les formations les plus poreuses, une baisse des températures et des salinités, une baisse de la contribution mantellique dans les fluides parents, une fermeture du système diagénétique au cours de l’enfouissement et un passage à des conditions réductrices durant la précipitation du quartz, de la pyrite et de la calcite. La phase de compression pyrénéenne associée au troisième stade de fracturation a induit une réouverture du système diagénétique et favorisé le retour à des conditions oxydantes et à des infiltrations de fluides météoriques / Interactions between fracturing, fluid circulations and fluid chemistry on hyper-extended margins is still poorly described as most of them are located offshore, buried underneath post-rift sediments. The southern Aquitaine basin and the northern Pyrenees constitute an appropriate case study to investigate these interactions since a model of hyper extended margin with mantle exhumation during the Lower Cretaceous subsequently inverted was recently proposed. From a field study, we here describe three main sets of fractures (set 1 to set 3). They are correlated with main stages of the geodynamic evolution of the basin corresponding to the Liassic rifting, the Aptian-Cenomanian hyper-extension, and the Pyrenean compression. Petrographic observations, Raman and micro-thermometry analysis on fluid inclusions, ICP-MS, and isotope analysis permitted to determine chemistries, temperatures, redox conditions, gas compositions, oxygen and carbon isotopic signatures, and REE contents of parent fluids for cements precipitated during each episode. In particular saddle dolomite and chlorite precipitated in set 2 fractures during the hyper-extension corresponding to the thermal peak at temperatures higher than 300°C. The isotopic signature, the high CO2 content, the occurrence of H2S and the high salinity of parent fluids suggest ascending mantle fluids percolating across Triassic evaporites. The late and post hyper-extensional phase is characterized by hydraulic brecciation in porous formations, a decrease in temperature and salinity, a decrease in mantle contribution in parent fluids, a closing of the diagenetic system during burial and a switch to reducing conditions during the precipitation of quartz, pyrite and calcite. The Pyrenean compressive phase associated with the third fracturing stage induced a reopening of the diagenetic system and favored a return to oxidizing conditions and infiltrations of meteoric fluids

Dans l’étude de la diagenèse des formations silico-clastiques ou carbonatées, il est souvent difficile de contraindre la température et l’âge des différents épisodes de cimentation ou de recristallisation. Le premier objectif de ce travail de thèse était de tester le potentiel de deux outils isotopiques récents (le géothermomètre des « clumpedisotopes » Δ47 et le géochronomètre U-Pb par analyse ponctuelle sur lame mince) dans le cadre de l’étude de la diagenèse des carbonates. Le deuxième objectif était de préciser l’histoire diagénétique, thermique, ethydrogéologique des réservoirs carbonatés du bassin de Paris. Pour atteindre ces deux objectifs, nous avons étudié des ciments de calcite et dolomite collectés dans les calcaires du Jurassique Moyen du dépocentre du bassin de Paris. La bonne caractérisation pétrographique et sédimentologique de ces échantillons, et le couplage entre les géothermomètres Δ47 et inclusions fluides, nous ont permis d’accéder à plusieurs informations de premier ordre. Premièrement, nous montrons que le Δ47 permet de reconstruire simultanément et précisément la température de formation des ciments diagénétiques dans la gamme 60-100°C (avec une précision de ~3-5°C) et la composition isotopique en oxygène des fluides (δ18Owater) à partir desquels ces ciments ont précipités (avec une précision de ~ 1‰). Deuxièmement, nous présentons une amélioration des connaissances de l’histoire thermique et des périodes de paléo-circulation des fluides à l’échelle régionale dans le bassin de Paris. Notamment, le croisement du géothermomètre Δ47 avec la méthode de datation U-Pb nous a permis pour la première fois de poser des contraintes thermo-chronologiques absolues sur l’ensemble des phases de circulations de fluides étudiées. Plus largement, ce couplage Δ47 /(U-Pb) inédit dévoile un champ d’application nouveau en thermo-chronologie de basse température (10-100°C). Nous anticipons que ce travail initie de nombreuses applications pour l’étude des histoires thermiques des bassins sédimentaires, via l’analyse de la diagenèse des carbonates / Diagenesis studies of both siliciclastic and carbonate rock units face the difficulty of constraining the temperature and age of the successive episodes of mineral crystallization. The first objective of this PhD work was to test the potential of two recent isotopic tools (the "clumped isotopes" Æ47 geothermometer and the laser ablation U-Pb geochronometer ) to integrate conventional carbonate diagenesis studies. The second objective was to reconstruct the diagenetic, thermal, and hydrogeological histories of the Middle Jurassic carbonate reservoirs of the Paris Basin. To achieve these goals, we studied calcite and dolomite cements mostly collected in the basin subsurface (exploration cores). The good petrographical and sedimentological characterization of the studied samples, together with the analysis of fluid inclusions and ?47 thermometry for determining the temperature and the chemical and isotopic compositions of the mineralizing fluids, allowed us to reach several first-order information. First, we show that Æ47 allows to simultaneously and precisely reconstruct the formation temperature of diagenetic cements in the range 60-100 ¡C (with an accuracy of ~ 3-5 ¡C) and the oxygen isotopic composition of the parent fluids (d18Owater) with a precision of ~ 1 ä. Secondly, this work improves our knowledge on the thermal history and past fluid-flows occurring at the basin scale. In particular, the coupling of the Æ47 thermometer with the U-Pb chronometer permitted for the first time to set absolute thermo-chronological constraints on every fluid-flow episodes investigated. More broadly, this unprecedented Æ47/(U-Pb) coupling reveals a new field of application in the low-temperature thermo-chronology field (10-100 ¡C). We anticipate that this pioneer thermo-chronological approach will induce many future applications in the study of sedimentary basins via carbonate diagenesis studies