Mineral Diagenesis Research Articles

Mudstone diagenesis in the Cenomanian–Turonian Eagle Ford Group in the San Marcos Arch area. PART I: Chemostratigraphy, early diagenesis, bitumen expulsion and migration pathways

The Cenomanian–Turonian Eagle Ford Group (EFG) is composed of massive, laminated, and/or burrowed limestones, marls, and lime mudstones deposited below storm-wave base on the drowned south Comanche Shelf. Also, eolian deposited volcanic ash layers are common being preserved as individual ash beds or reworked into the sediment. An investigation of diagenesis relative to burial delineated a complex diagenetic history as a result of changes in burial history and associated changes of water chemistry and redox conditions. The first part of this comprehensive study sheds light on initial depositional redox conditions, early mineral diagenesis, and expulsion and migration pathways of generated bitumen and oil from kerogen of the EFG in the study area (maximum burial depth <6000 ft [1828.8 m]). The EFG followed a diagenetic pathway from the deeper marine directly into the burial environment (not affected by meteoric diagenesis). Original depositional porosity of the EFG marl could be as high as 70–80%, whereas present-day porosity values have been decreased to around 10% because of burial related diagenesis. At shallow burial depths, compaction was the mechanism responsible for most porosity loss followed by cementation. The texture and fabric of these cements increase mechanical strength of the EFG mudrocks significantly. The shallow, immature EFG limestones and marls have abundant interparticle micropores, high porosity, permeability, and water saturation with relatively low resistivity. With thermal maturation, bitumen and hydrocarbon generation increases oil saturation and alters wetness of the rock. The migration and filling of the bitumen and possibly oil would limit further cementation of calcite and quartz. The interaction between diagenesis of mineral matrix and organic matter (OM) evolution is significant and notably impacts cementation in mudstones at relatively shallow burial depth. The impact of OM is also more remarkable in mudrocks compared to that in sandstone and carbonate counterparts.

Mudstone diagenesis with depth and thermal maturity in the Cenomanian–Turonian Eagle Ford group. PART II: Diagenetic processes and paragenetic sequence

The Cenomanian–Turonian (C–T) Eagle Ford Group (EFG) is composed of massive, laminated, and/or burrowed limestones, marls, and lime mudstones deposited on the drowned south Texas shelf below storm-wave base. A burial diagenesis investigation using seven cored wells from shallow to deep burial (depth ranges from 7684 to 13,670 ft [2342 to 4166.6 m]; BHT from 183 to 285 °F [83.9–140.6 °C]) and from east (San Marcos Arch) to west (Maverick Basin) delineated a complex diagenetic history as a result of changes in water chemistry, redox condition, and burial history. The second part of this comprehensive study shows the variety of diagenesis and paragenesis. This study focuses especially in the organic-rich Lower Eagle Ford (LEF) member deposited largely under anoxic/euxinic conditions. Processes such as near seafloor sulfate-reduction, Fe-reduction, and methanogenesis drive various chemical reactions during lithification. Feldspar diagenesis included albitization and transformation of K-feldspar to kaolinite and albite, and from kaolinite to Mg-chlorite with depth. Clay mineral diagenesis is complex with authigenic kaolinite, mixed kaolinite/Mg-chlorite, and Mg-chlorite form in biota tests and matrix. Additionally, calcite dissolution, burial dolomitization, late Fe-chlorite replacement, and oxidation of pyrite to marcasite are also observed. Detritus-derived smectite starts to transform to mixed layered clay, illite-smectite (I/S) in the oil window; however, illitization trends were not observed in depth. This is because most smectite forms from alteration of volcanic ashes and the volcanic origin of the disorientated smectite did not follow regular illitization kinetics. The increase in Mg content from illitization transforms not only the kaolinite to chlorite, but precipitates authigenic dolomites. The clay minerals precipitated in the original interparticle pores, further reducing porosity, and impacted permeability, fluid saturation, wettability, and capillary pressure. With thermal maturation, bitumen and oil conversion increases oil-wetness of the rock and the resistivity measurement from wireline logs.

Unraveling mudstone compaction at microscale: A combined approach of nanoindentation mapping and machine learning data analysis

Compaction processes within mudstone samples across varying depths (723.5–3213.5 m) in the Vienna Basin were investigated, focusing on porosity changes and the resulting micromechanical response in the clay-rich, fine-grained fraction (“clay matrix”). A novel approach combining nanoindentation mapping with machine learning data analysis was developed to efficiently extract representative micromechanical parameters of the clay matrix. Emphasis was put on capturing the properties of the fine-grained, clay-dominated composite rather than individual mineral phases, enabling the analysis of compaction-induced strength changes at microscale. A significant enhancement in the mechanical strength of the clay matrix with increasing burial depth was observed. Reduced elastic modulus (Er) and hardness (H) increased from 6.8 ± 3.4 to 22.6 ± 7.5 GPa and from 0.2 ± 0.2 to 0.9 ± 0.2 GPa, respectively, over the depth interval of 2490 m. Moreover, a strong correlation between depth and porosity and consequently micromechanical properties of the clay matrix exists. This highlights the substantial influence of intra-clay porosity on mechanical properties, which follow a general compaction trend with depth. Broad ion beam-scanning electron microscopy (BIB-SEM) analysis was used for textural investigations at micro-to nanoscale and indicated that the reduction in porosity predominantly resulted from mechanical compaction rather than mineral diagenesis, which would have been noticeable by signs of significant dissolution or cementation. The correlation coefficient matrix between multiscale porosity measurements and nanoindentation affirmed that the porosity loss was closely linked to the enhanced mechanical properties of the fine-grained composite clay matrix, while other compositional variables like total clay mineral content showed weak correlations. Empirical mathematical equations were derived to describe the mechanical properties as generalized functions of depth and porosity. These may be used as geomechanical input for future basin analysis and geoenergy applications (e.g., seal rock studies in a geological storage context). This study introduces a new approach to unravel compaction processes in fine-grained rocks at microscale, emphasizing the interplay between intra-clay porosity and micromechanical changes during proceeding burial.

Deciphering Mineralogical Composition and Clay Diagenesis in Exposed Shales of Jaintiapur, Surma Basin: Unveiling Insights into Burial Diagenesis

Mineralogical analysis of exposed shale samples in the Jaintiapur region was conducted utilizing X-ray diffraction (XRD) techniques. The studied shales are characterized by illite, chlorite, illite/ smectite mixed layer clay, and kaolinite as the predominant clay minerals, with a relatively lower proportion of smectite. Additionally, the non-clay fraction consists mainly of quartz, feldspar, dolomite, and trace amounts of pyrite. Notably, illite and chlorite exhibit prominent abundance within the clay portion, while quartz is the prevailing non-clay mineral. Furthermore, the investigation reveals a notable enrichment of illite and chlorite with the advancement from younger to older stratigraphic formations in the studied shale samples, indicating greater intensity in clay mineral diagenesis in deeper stratigraphic formations. The transition from smectite to illite minerals can release Si, Ca, Fe, and Mg ions, potentially entering nearby sandstones and leading to cementation with quartz, chlorite, and calcite. This process aids in retaining pore water in shales. In this context, overpressure development influenced by clay diagenesis is not primarily due to cementation while undergoing smectite to illite conversion but due to illite packet formation within smectite, reducing shale permeability. During this conversion, interlayer water shifts to free water, causing volume reduction compounded by the low permeability of shale, trapping fluids in the pores. This expansion of pore water, reduced velocity, and increased pore pressure suggest that mineral transformation with burial depth might contribute to overpressure development. However, the observed decline in smectite clays in sediments transitioning from younger to older, culminating in their absence in older shale-dominated formations, explains the presence of diagenetic illite and chlorite. This study underscores the progressive diagenetic transformation in clay mineralogical composition with progressive burial across the sedimentary succession. The Dhaka University Journal of Earth and Environmental Sciences, Vol. 12(2), 2023, P 69-82

Mineral diagenesis in lacustrine organic-rich shales: Evolution pathways and implications for reservoir characteristics

Porosity in shale reservoirs is profoundly influenced by diagenetic processes that shape the evolution of pore space. In the Songliao Basin, the first member of the Qingshankou Formation (K2qn1) shale serves as a typical lacustrine clay-rich shale oil reservoir. It exhibits variable mineralogy and thermal maturity, with its reservoir quality strongly influenced by these variations. This study integrates microscopic observations, including field emission scanning electron microscopy (FE-SEM), cathodoluminescence (CL), and organic petrology, with bulk rock investigations employing X-ray diffraction (XRD), Rock-Eval pyrolysis, and elemental analysis, to reconstruct the diagenetic pathways leading to changes in porosity. The findings reveal that K2qn1 shale is primarily composed of extrabasinal grain assemblages, such as detrital quartz and clay minerals, with sedimentary organic matter mainly as lamalginite. As maturity exceeds 0.8 %Ro, the presence of authigenic clay minerals, primarily characterized by chlorite, becomes widespread, often associated with bitumen impregnation. Additionally, authigenic dolomite and pyrite form during the synsedimentary to early diagenetic stages. In immature to marginally mature stages (<0.6 % Ro), mechanical compaction is the primary factor leading to porosity reduction. Pores associated with organic matter become visible when maturity reaches the oil window (>0.7 % Ro). At the peak of hydrocarbon expulsion (approximately 1.2 % Ro), a high frequency of organic matter shrinkage pores at interfaces to clay minerals is present, likely playing a significant role in enhancing permeability. Although localized occurrences of dissolution and cementation can be observed, the development of these diagenetic processes is limited and exerts minimal influence on the physical properties of the bulk rock. This study significantly contributes to our understanding of the mechanisms governing diagenetic processes and reservoir evolution in markedly heterogeneous lacustrine shale, considering the diverse origins of grain assemblages.

Investigating the potential of African land snail shells (Gastropoda: Achatininae) for amino acid geochronology

Aragonitic calcium carbonate (CaCO3) terrestrial mollusc shells with complex shell microstructures, such as those in the African subfamily Achatininae, have the potential to be used to build amino acid geochronologies across the African continent. However, as different microstructural shell layers are likely to have different protein compositions, sampling strategies need to be developed to identify the most appropriate shell portion to target. To test possible variability in protein degradation rates between microstructural layers, sampling of a single microstructural shell layer (the ‘nacreous’ layer) was compared to sampling all three aragonitic layers ('3AL') in modern and fossil shells. Reliable isolation of the nacreous layer in all samples proved impractical, and additional complications arose due to mineral diagenesis induced by sampling with a drill. Pleistocene fossils of Lissachatina sp. and modern specimens of Achatina tavaresiana Morelet, 1866 were shown to have an intra-crystalline protein fraction. The 3AL shell portion adhered more closely to closed-system behaviour in heated modern, and fossil, samples. The intra-crystalline protein degradation (IcPD) patterns of Achatininae fossil samples were not consistent with IcPD under forced degradation experiments at high temperatures in the laboratory. However, reliable degradation trends were observed in the 3AL shell portion, demonstrating the potential of fossil achatinids for building relative amino acid geochronologies across Africa.

Porosity in mudstones and its effectiveness for sealing carbon capture and storage sites

Abstract Mudstones represent top-seals for many carbon capture sites as they tend to have the correct petrophysical properties, including suitable porosity and pore-size distribution. The pore network of mudstones is thus pivotal for many carbon capture and storage (CCS) projects. The key to understanding the effectiveness of top-seals is an appreciation of the controls on the pore network. For this reason, schemes to classify pore body size, pore type and pore throat size are presented. Pore types include primary and secondary interparticle and intraparticle pores and pores associated with organic matter and fractures. The most relevant mudstone pore body sizes for CCS top seals are likely to be between &lt;62 µm and 1 nm. Pore throat sizes are classified as nano- (&lt;10 nm), transition- (10 nm–0.1 µm), meso- (0.1–0.625 µm), and macro-pore throats (&gt;0.625 µm). Petrophysical, geochemical, and geomechanical properties control porosity and the CO 2 sealing integrity of mudstones; these properties are, in turn, controlled by the rate and extent of compaction, mineral diagenesis and overpressure. The success of a CCS top-seal relies on pore throats in intact top-seal being sufficiently small, and fracture pressure (typically minimum horizontal stress, σ hmin ) not being exceeded by CO 2 pressure. CO 2 sorption, especially by smectite in top-seals, may improve the nanoscale sealing efficiency of clay minerals. The systematic workflow presented here will help facilitate the new drive to understand mudstone properties, as they are essential for establishing safe and durable CO 2 containment.

Clay mineral diagenesis in alternating mudstone‐sandstone beds from Late Palaeozoic strata of the Anadarko Basin, United States

Closely interbedded mudstone and sandstone are rarely the focus of clay mineral diagenetic studies; as such, it is not understood if these various rock types follow the same or different diagenetic pathway(s) during burial. Over a depth span of ~2,300 to 4,200 m, 95 core samples of alternating Pennsylvanian mudstone/sandstone from the hydrocarbon‐rich Anadarko Basin in Oklahoma have been characterized petrographically, mineralogically, and chemically. The investigated fine‐grained sediment is argillaceous to siliceous mudstone, dominated by Illite/mica and mixed‐layer illite–smectite. The sandstone is variably classified as quartz and mudclast‐rich litharenite to sub‐litharenite, which is commonly carbonate‐cemented. This contribution provides evidence for a very similar diagenetic evolution of the clay mineral assemblages across all rock types despite the discrepancies in porosity and permeability of the host rock. We posit that the incorporation of micrometre‐scale mudclasts through bioturbation of the sandstone may explain the relatively uniform compositional characteristics of illite–smectite, chlorite, and kaolinite. Inferences are drawn from a detailed clay mineral quantification of these interbedded lithologies aid to our understanding of the clay mineral distribution and speciation in Pennsylvanian strata of the Anadarko Basin, which further helps to constrain the reservoirs' physical, chemical, and structural properties.

Clay mineral transformations of mesozoic mudstones in the central Junggar Basin, northwestern China: Implications for compaction properties and pore pressure responses

The clay mineral transformation of deeply buried mudstones is closely related to the mudstone microstructure, petrophysical properties, and pore fluid pressure (pore pressure). The influence of the burial history on the microstructure and compaction behavior of mudstones has been the focus of recent research. In this study, the mineral composition, mudstone microfabric and petrophysical properties of Mesozoic overpressured mudstones in the central Junggar Basin were analyzed to explore the influence of clay mineral diagenesis on compaction processes and overpressure responses in diagenetically altered mudstones. The clay minerals of these Mesozoic mudstones are mainly composed of mixed smectite/illite layers (I/S) and illite with small amounts of kaolinite and chlorite, but they do not contain discrete smectite. The Mesozoic mudstone underwent clay mineral diagenesis, the smectite in I/S transformed to illite at depths of 4000–4800 m (temperatures of 90–105 °C), and the I/S changed from the R0 type to the R3 type. The recrystallization and growth of clay minerals continued with increasing stress with depth, leading to the substantial realignment of clay particles observed at the microscale. The combination of the preferential alignment of clay particles and precipitation of authigenic cements caused significant decreases in the porosity of the mudstone. The results of this study support the viewpoint that the effective stress continues to work in diagenetically altered mudstones, although porosity loss is enhanced by clay fabric destabilization caused by clay mineral transformation. The top depth of smectite-to-illite transformations is consistent with the top of the overpressure, indicating that overpressure occurs simultaneously with clay diagenesis in diagenetically altered mudstones. The decrease in porosity and increase in density of mudstone caused by clay diagenesis changes the overpressure logging response to a certain extent and affects the accuracy of the pressure prediction results. The sonic transit time-density cross plot can be used to effectively distinguish the compaction stage and the cause of overpressure in diagenetically altered mudstones. Clay mineral diagenesis cannot be ignored in pore pressure analysis and pressure prediction in deeply buried mudstones. This research is of great importance for deepening the theory of mudstone compaction and has practical significance for pore pressure prediction in deeply buried layers.

Holocene Paleosecular Variations Recorded by Relict Magnetic Minerals in the Anoxic Black Sea Sediments

Continuous paleosecular variations reconstructed from sedimentary archives have remarkably deepened our insights into the dynamics of the Earth's magnetic field as well as the chronological purpose. Nevertheless, to construct reliable sedimentary paleomagnetic records in diagenetic reducing sediments is challenging generally due to the pervasive magnetic mineral diagenesis. The relict magnetic minerals are residuals after diagenesis and probably able to record a depositional remanent magnetization, thus it is worthy to explore their paleomagnetic potentials. For this study, two Black Sea sediment cores covering the past 8 ka were subjected to mineralogical and paleo- and rock magnetic analyses. Paramagnetic pyrite framboids are pervasive in the studied sediments deposited under anoxic bottom water conditions in the Black Sea. In addition, relict magnetic minerals of ferrous hemoilmenite, Fe-Mn and Fe-Cr spinels, and magnetite inclusions are also present in the studied cores. Compared to the previous published paleomagnetic results from the same cores over the last glacial period (20-30 ka) which are dominated by detrital (titano-)magnetite particles, the studied relict magnetic mineral samples exhibit a similar behavior in recording the depositional remanent magnetization. Furthermore, the obtained paleosecular variations spanning the past 8 ka reproduce the high intensity patterns observed in the regional archeomagnetic and volcanic datasets. Thus, the successful reconstruction of paleomagnetic secular variations from the anoxic Black Sea sediments greatly extend the application of paleomagnetism in sediments deposited in water with a reducing sub-surface environments from where paleomagnetic data are generally sparse.

Effect of Hydrocarbon Presence and Properties on the Magnetic Signature of the Reservoir Sediments of the Catcher Area Development Region, UK North Sea

This paper presents a detailed study investigating the effect of hydrocarbon presence on magnetic mineral diagenesis in sediments from the Catcher Area Development (CAD) region, UK North Sea, between 1,000 and 1,500 m (True Vertical Depth Sub-Sea). Magnetic analysis of core samples from hydrocarbon fields of the region and nearby dry-well sandstones (background) was carried out to determine if their signatures can serve as a proxy for understanding petroleum reservoir systems. From the background samples, nanometric and micron-sized magnetite, hematite and titano-iron oxides, were identified. Hydrocarbon presence in the reservoir sediments was found to diminish the iron-oxide signature and favour the precipitation of hexagonal pyrrhotite, siderite and potentially vivianite, lepidocrocite, greigite and paramagnetic iron sulphides. Hexagonal pyrrhotite was found at the oil-water transition zones. This relationship is possibly related to biodegradation at this interface. Siderite was found in increased abundance at shallower depths within the reservoir, which we attribute to hydrocarbon vertical migration and biodegradation. The interbedded shales also experienced significant magnetic mineral diagenesis that depended on its proximity to the hydrocarbon plume. These findings suggest that mineral magnetism can be applied to the identification of oil-water transition zones, reserve estimation, production planning and the determination of hydrocarbon migration pathways. It also suggests that mineral magnetic methods can be used to estimate the timing of hydrocarbon migration.

New understanding of overpressure responses and pore pressure prediction: Insights from the effect of clay mineral transformations on mudstone compaction

Chemical compaction dominated by clay mineral transformations, commonly develops in deeply buried mudstones, and prominently affects the petrophysical properties of mudstones and fluid flow. This paper takes the Neogene mudstones in the Ledong Slope of the Yinggehai Basin as research object, analyzes the relationship between chemical compaction and overpressure, establishes a new multisegment normal compaction trends (NCTs) and reevaluates the overpressure mechanisms. The results show obvious clay mineral transformation with increasing depth, and the top of the clay mineral transformations is approximately equal to the depth of the overpressure. The logging responses of overpressured mudstones are affected by clay mineral diagenesis, showing a 45% increase in the sonic transit time at 3500-5000 m and a 10% decrease in the density at the same interval, and the density response exhibits a relative lag. A method comprising an inversion algorithm calibrated by measured pressure data is proposed to establish multisegment NCTs. Furthermore, new loading-unloading curves comprising a mechanical loading curve and chemical loading curve are established to reevaluate the overpressure mechanisms. The loading-unloading curves show that the pressure gradient generated by mudstone compaction maybe less than 1.8. While the pressure data with pressure gradient larger than 1.8 fall on the unloading curve, indicating elevated sandstone pressures rather than in situ mudstone pressures. The multisegment NCTs were proven to provide a satisfactory overpressure estimate in the Ledong Slope as well as the relative contribution of different overpressure mechanisms. This paper provides new insight into mudstone compaction and overpressure responses, as well as a meaningful reference for the identification of multiple-overpressure generation mechanisms and the prediction of pore pressure in deeply buried formations.

A New Quantitative Approach for Element-Mineral Determination Based on “EDS (Energy Dispersive Spectroscopy) Method”

With the continuous development of hydrocarbon exploration, how to efficiently, economically, accurately, and comprehensively obtain mineral species, composition, and structure and diagenesis information has become one of the hot topics in both the academia and industry. By scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS), a new method of qualitative mineral identification and quantitative measurement is established. Typical tight sandstone reservoir rock samples in the Ordos Basin are selected; through the element surface scanning image of “mineral element composition” and “pixel element combination”, mineral types are distinguished, and mineral parameters such as types, characteristics, and content are rapidly and accurately determined. Meanwhile, such results achieved via the new method are compared with conventional XRD and TIMA methods. The results show that the new method exhibits several advantages: cost advantages compared to XRD experiment analysis technology and TIMA system and ability to analyze low content minerals which XRD techniques are hard to identify; it allows quantitative characterization on the phenomenon of mineral miscibility, which is of great significance to explore the mineral diagenetic evolution.

Discussion of “Quantitative evaluation of thixotropy-governed microfabric evolution in soft clays”

The diagenesis of clay minerals and thermal evolution of organic matter (OM) are consistent in space and time in most petroliferous basins. Within these processes, the synchronism of smectite illitization with peak hydrocarbon generation is one of the most important phenomena and the illitization process is believed to play an important role in catalytic effects on the conversion of OM to oil and gas. The variation in the layer charge of montmorillonite (Mt) is a key indicator of illitization. However, the effects of layer charge on OM pyrolysis have not been investigated. In this study, a series of homologous reduced-charge Mt (LiMt) samples were prepared by changing the temperature of heat treatment based on the Hofmann–Klemen effect. The 12-aminolauric acid and Mt were mechanically mixed to prepare clay–OM complexes. Pyrolysis experiments were conducted in a confined gold capsule system, and the gas components were evaluated by gas chromatography. The experimental results show charge reducing changed the solid acidity of Mt and further influence the catalytic activity of Mt on the OM pyrolysis. The solid acidity of Mt first increased and then decreased as the layer charge decreased, while the water content decreased. These sites, especially Brønsted (B) acid sites, catalyzed OM pyrolysis via the carbocation mechanism. The content of iso‐alkanes was controlled by the content of water in Mt because excess water molecules hindered the isomerization reactions occurring at the B acid sites. The CO2 content was affected by both the amount of Lewis (L) acid sites and the water content. These fundamental results suggest that changes of the layer charge in Mt which occurs during the processes of smectite illitization may play an important role in hydrocarbon generation. These findings are important for understanding the organic-inorganic interactions between clays and OM during sedimentation and diagenesis.

Effects of montmorillonite charge reduction on the high-temperature/high-pressure pyrolysis of organic matter

The diagenesis of clay minerals and thermal evolution of organic matter (OM) are consistent in space and time in most petroliferous basins. Within these processes, the synchronism of smectite illitization with peak hydrocarbon generation is one of the most important phenomena and the illitization process is believed to play an important role in catalytic effects on the conversion of OM to oil and gas. The variation in the layer charge of montmorillonite (Mt) is a key indicator of illitization. However, the effects of layer charge on OM pyrolysis have not been investigated. In this study, a series of homologous reduced-charge Mt (LiMt) samples were prepared by changing the temperature of heat treatment based on the Hofmann–Klemen effect. The 12-aminolauric acid and Mt were mechanically mixed to prepare clay–OM complexes. Pyrolysis experiments were conducted in a confined gold capsule system, and the gas components were evaluated by gas chromatography. The experimental results show charge reducing changed the solid acidity of Mt and further influence the catalytic activity of Mt on the OM pyrolysis. The solid acidity of Mt first increased and then decreased as the layer charge decreased, while the water content decreased. These sites, especially Brønsted (B) acid sites, catalyzed OM pyrolysis via the carbocation mechanism. The content of iso‐alkanes was controlled by the content of water in Mt because excess water molecules hindered the isomerization reactions occurring at the B acid sites. The CO2 content was affected by both the amount of Lewis (L) acid sites and the water content. These fundamental results suggest that changes of the layer charge in Mt which occurs during the processes of smectite illitization may play an important role in hydrocarbon generation. These findings are important for understanding the organic-inorganic interactions between clays and OM during sedimentation and diagenesis.

The acid test: An experimental microarchaeological study of guano-driven diagenesis in tropical cave sediments

Caves in tropical regions form a key resource for those reconstructing human evolution and dispersals. However, the complex sedimentary records they contain remain under-interpreted because of the poorly constrained effects of humid tropical climates upon archaeological site formation processes. Guano-driven phosphatic diagenesis impacts archaeological preservation in caves, but atypical features observed in guano layers in prominent Southeast Asian archaeological sites suggested that hot and humid conditions had promoted the formation of distinctive sedimentary environments with unknown effects on assemblage taphonomy. Few reference data exist that are relevant to geomorphic processes in tropical caves and this laboratory-based modern analogue study was designed to conclusively relate analytical observations to sedimentary palaeoenvironments and explore their effects on assemblage taphonomy. Stratigraphic models were kept under simulated tropical conditions, each containing an identical range of faunal remains, organic materials and minerals, which was buried in wet bat guano. These were excavated, one per month over 24 months, while nine further stratigraphic models were used to control for environmental variables. Sedimentary environments were investigated using sedimentological characterisations of excavated sediments, while post-depositional alterations to buried materials were investigated through optical and scanning electron microscopic analysis of thin-section samples combined with chemical characterisations of excavated samples through Fourier-transform infrared spectroscopy (FTIR). Experimental conditions prevented the sediment acidification that is typically associated with guano-driven diagenesis, but buried materials were severely, and distinctively, diagenetically altered. Organic decomposition in guano, bamboo and charcoal was associated with the activities of sulphate-reducing bacteria, but diagenetic alterations affecting charcoal remain poorly resolved due to variation in the experimental assemblage. Bone was discoloured and affected by dissolution, recrystallisation and alteration to secondary minerals, including gypsum, while carbonates reprecipitated as calcium phosphates and non-stochiometric mineral species. Diagenesis of clay minerals was observed, but related diagenetic trajectories remain poorly resolved. These alterations provide micromorphological and mineralogical indicators of sedimentary palaeoenvironments in the earliest stages of burial and this experiment provides a framework within which to understand the taphonomy of archaeological materials in tropical caves.

Diagenesis and genesis of clay minerals in the Triassic sandstones of the Panchet and Parsora formations, Damodar–Son Basin, India

The Triassic sandstones of the Panchet and Parsora formations are deposited in the Damodar and Son basins of the Peninsular India. Though both the formations have similarity in depositional and lithification processes, their diagenetic history and authigenic clay minerals formation have little differences. To improve understanding of diagenesis and associated clay minerals, the present study examined in Triassic sandstones. The Lower Triassic Panchet sandstones are poorly sorted arkose and originate from nearby granitoid terrain. The early diagenetic assemblages of the sandstones are kaolinite–iron oxide, montmorillonite, quartz and cement in the sandstones formed in oxy-acidic environment under shallow burial condition. Authigenic chlorite and recrystallised interstitial components such as orthomatrix are the main late diagenetic products formed, respectively, at oxy-acidic and reducing alkaline conditions where carbonate and allogenic iron cements were emplaced as the latest events. The Panchet and the Parsora formations are the riverine Triassic deposits and have almost identical early diagenetic assemblages. The kaolinisation in sandstone is loss of feldspar and is more pronounced in the Parsora Formation. The late diagenetic products in both the formations essentially similar and differ only in their relative chlorite content, which was formed under elevated temperature as an effect of tectonic activity. Allogenic iron and carbonate cements are post-tectonic.

Diagenesis of Magnetic Minerals in Active/Relict Methane Seep: Constraints From Rock Magnetism and Mineralogical Records From Bay of Bengal

In this study, we conducted a comprehensive investigation of rock magnetic, mineralogical, and sedimentological records of sediment cores supplemented by a high resolution seismic data to elucidate the controls of structural and diagenetic (early vs. late) processes on the sediment magnetism in active and relict cold seep sites in the Bay of Bengal. Two distinct sediment magnetic zones (Z-I and Z-II) are defined based on the down-core variations in rock magnetic properties. The sediment magnetism is carried by complex magnetic mineral assemblages of detrital (titanomagnetite, titanohematite) and authigenic (fine-grained greigite) minerals. Overall, the magnetic susceptibility varies over one order of magnitude with highest values found in relict core. Uppermost sediment magnetic zone (Z-I) is characterized by higher concentration of magnetite as seen through elevated values of magnetic susceptibility (χlf) and saturation isothermal remanent magnetization (SIRM). A systematic gradual decrease of χlf and IRM1T in Z-I is attributed to the progressive diagenetic dissolution of iron oxides and subsequent precipitation of iron sulfides. Magnetic grain size diagnostic (ARM/IRM1T) parameter decreases initially due to the preferential dissolution of fine-grained magnetite in the sulfidic zone (Z-I), and increases later in response to the authigenic formation of magnetite and greigite in methanic zone (Z-II). Distinct low S-ratio and χlf values in methanic zone of relict core is due to increased relative contribution from highly preserved coercive magnetic (titanohematite) grains of detrital origin which survived in the diagenetic processes. A strong linkage between occurrence of authigenic carbonates and greigite formation is observed. Two plausible mechanisms are proposed to explain the formation and preservation of greigite in Z-I and Z-II: 1) decline in methane flux due to massive hydrate accumulation within the active fault system and formation of authigenic carbonate crust in the sub-surface sediments hindered the supply of upward migrating fluid/gas; thereby limiting the sulfide production which preferentially enhanced greigite formation in Z-I and 2) restricted supply of downward diffusing sulfide by the carbonate layers in the uppermost sediments created a sulfide deficient zone which inhibited the pyritization and favoured the formation of greigite in the methanic zone (Z-II).

Controls of mass transport deposit and magnetic mineral diagenesis on the sediment magnetic record from the Bay of Bengal

We conducted rock magnetic, mineralogical, sedimentological and geochemical analyses on a sediment core (MD161/Stn-11) retrieved from a complex marine sedimentary system of Krishna-Godavari (K-G) basin to delineate the control of mass transport deposits (MTD's) and methane-induced diagenesis on the sediment magnetic record. Four sediment magnetic zones (Z-I, Z-II, Z-III, Z-IV) were defined based on rock magnetic signatures. The sediment magnetic signal is mainly carried by complex magnetic mineral assemblages of detrital (titanomagnetite, titanohematite) and diagenetic (pyrite) minerals. Changes in rock magnetic properties are mainly controlled by fluctuations in supply of detrital magnetic particles, onset of MTD's and differential rate of methane-influenced magnetic minerals diagenesis in the studied sediment core. Downcore reduction in magnetic susceptibility followed by subsequent precipitation of iron sulfides within sediment magnetic zone (Z-I) representing the period of normal sedimentation can be attributed to diagenetic dissolution caused by anaerobic oxidation of methane coupled to sulfate reduction. Decline in magnetic susceptibility and increase in sediment grain size within MTD-rich sediment intervals (Z-II, Z-III, Z-IV) is linked to loss of finer magnetic grains due to diagenetic dissolution and dilution caused by increase in concentration of diamagnetic minerals. Lower values of magnetic grain size diagnostic (ARM/SIRM) parameter indicate loss of finer and selective retention of coarser magnetic particles due to diagenetic dissolution beyond 12 mbsf. Elevated content of total organic carbon (TOC) content in Z-III and Z-IV can be attributed to the efficient preservation of labile organic matter due to rapid sediment deposition. A conceptual model is presented to explain the control of mass transport deposit and magnetic mineral diagenesis on the sediment magnetic record.

Fluid Accumulation, Migration and Anaerobic Oxidation of Methane Along a Major Splay Fault at the Hikurangi Subduction Margin (New Zealand): A Magnetic Approach

AbstractUnderstanding the locus of fluid flow along thrust and splay faults is important to understand the hydraulic properties of accretionary systems and fault mechanics. Here, we use rock magnetic techniques in combination with backscattered electron imaging to depict the locus of enhanced magnetic mineral alteration within the Pāpaku fault, an active splay fault of the subduction interface at the northern Hikurangi Margin. The Pāpaku fault was cored at Site U1518 during Expedition 375 of the International Ocean Discovery Program and we report room temperature magnetic parameters, complemented by first‐order reversal and thermomagnetic curves in the depth interval 250–400 m below seafloor (mbsf). The ∼60‐m wide Pāpaku fault zone comprises two main slip zones, referred to as the upper main brittle (304–321 mbsf) and lower subsidiary (351–361 mbsf) fault zones, and an intervening zoned, termed the lower ductile deformation zone. Two narrow zones, at the top of the main brittle fault zone, and one in a sand‐rich interval above the subsidiary fault zone, experienced enhanced magnetic mineral diagenesis, which resulted in the recrystallization of ferrimagnetic greigite to paramagnetic pyrite. We propose that secondary magnetic mineral diagenesis was driven by anaerobic methane oxidation within these intervals, which occurs in the presence of methane and sulfate. We relate the observed changes to the fault parallel transport of fluids which is restricted to two damage zones. Overlying compacted and clay‐rich sediments likely act as a barrier to upward advective flow through the fault zone and into the hanging wall.