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Previous studies have shown that within fine-grained dolomites, recrystallisation that occurs at shallow burial depths (<1 km) and low temperatures (<40 °C) can alter the carbonate clumped isotope signature of the minerals. Dolomites are, therefore, no longer representative of the environment of deposition, but rather capture temperatures present during burial. Currently, we do not understand isotopic re-setting of carbonate clumped isotopes during calcite recrystallisation during burial within micritic formations and whether the temperatures measured are representative of maximum burial temperatures reached. This study thus aims to determine if recrystallisation in fine-grained calcitic micrites show similar degrees of isotopic alteration to early dolomites; this is important as temperatures measured could be representative of temperatures reached during burial and serve as a proxy to reconstruct the thermal evolution of a formation and burial diagenetic processes. We have combined carbonate clumped isotope palaeothermometry with SEM/EDS imaging to study calcite recrystallisation under moderate burial (<1.8 km).

We analysed 17 samples collected from outcrops located on the West of the Eagle Ford Shale outcrop belt, Texas. The Eagle Ford is an ideal study location as it contains abundant fine-grained carbonate mixed with clastic material. The burial depth and temperature reached are understood through previous studies using organic proxies, giving this study a reasonable calibration for temperatures obtained through clumped isotope palaeothermometry. Results confirm that the Eagle Ford Shale is mixed formation, with nearly all samples showing 50:50 carbonate to clastic material. Clumped isotope analysis show variability in both measured clumped isotope temperatures (T(Δ_{47 calcite}), 35 to 105 °C) and calculated water oxygen isotope composition (δ¹⁸O_vsmow, -1.93 to 6.96 ‰). The results show higher temperatures than reconstruction based on organic matter matutuation index. This indicates that thermal evolution of calcite recrystallisation can differ from that of organic matter transformation, probably because different kinetics exist for the two reactions. Organic matter transformation depends on both temperature and time, whereas carbonate recrystallisation can occur instantaneously and depends on temperature and the fluid present. A correlation also exists between T(Δ_{47 calcite}) and δ¹⁸O_vsmow; the higher T(Δ_{47 calcite}), the higher δ¹⁸O_vsmow. This correlation is interpreted to be evidence of burial recrystallisation via dissolution/re-precipitation within a closed system, therefore with minimal to no change in the δ¹⁸O_calcite. We suggest that even with the large range of measured T(Δ_{47 calcite}), these variations could be a result of recrystallisation via dissolution/re-precipitation during burial and therefore representative of maximum burial temperatures, which are not recorded by the organic temperature proxies.

How to cite: Robinson, A. S., John, D. C. M., Dale, D. A., and Osborne, D. M.: Application of Clumped Isotope Palaeothermometry to reconstruct thermal evolution of recrystallised calcite in fine-grained micrites, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2721, https://doi.org/10.5194/egusphere-egu2020-2721, 2020.

The objects of the current study are glendonite pseudomorphs forming the central part of cannon-ball carbonate concretions found within Miocene terrigeneous sediments of Sakhalin island (easternmost part of Russia). Twelve samples of glendonites and host carbonate concretions were examined using optical and cathodoluminescence microscopy, EDX analysis, powder X-ray diffraction and isotopic analysis. The aim of the study is to determine the origin of the concretions and the relationships between the concretion and glendonite occurrence.

Glendonites and host cannon-ball concretions were found within terrigeneous sediments of Bora (Lower Miocene) and Vengeri (Upper Miocene) formations. These formations are composed of laminated sandstones, siltstones, argillites and siliceous rocks. Dropstones are often found within these sediments as well as cannon-ball carbonate concretions, some of them with glendonites in central part. 60-90% of the cannon-ball concretion is occupied by sandy limestone (with high-magnesium calcite) and occasionally contains dolomite and pyrite. Central part of the cannon-ball concretion is occupied by glendonite (single crystal-like or star-like cluster of crystals). Glendonites are composed of several calcite generations. Rosette-like calcite crystals (“ikaite-derived calcite”) are composed of low-magnesium calcite, they are non-luminescent. Needle-like calcite cement is composed of high-magnesium calcite or dolomite and show bright-yellow cathodoluminescence. The rest of the glendonite is occupied with low-magnesium radiaxial fibrous or sparry calcite with dark-red cathodoluminescence.

Isotopic ratios of glendonites are close to those of host concretions. For host concretions δ¹³С varies from -20.3 to -14.9 ‰PDB, δ¹⁸О varies from +1.7 to +2.7 ‰PDB; for glendonites δ¹³С varies from -18.1 to -1.9 ‰PDB, while δ¹⁸О varies from +0.7 to +3.4 ‰PDB.

Close mineralogical and isotopic composition of the studied glendonites and host cannon-ball concretions suggest they were formed in similar geochemical environment. Association of glendonite occurrence along with dropstones is an indicator of cold conditions, which is well-corresponding with view on glendonites as a proxy for cooling events. Cementation of surrounding sediment (formation of the cannon-ball concretions) and glendonite formation was simultaneous and occurred during early diagenesis in the sulfate-reduction zone. The source of calcium and magnesium ions was seawater (δ¹⁸О values are characteristic for seawater). Ikaite was replaced with low-magnesium calcite; the replacement was favored by organic matter decay (δ¹³C values are characteristic for organic matter). Cementation of the cannon-ball concretion with high-magnesium calcite occurred together with needle-like high-magnesium calcite growth in the glendonite with increasing concentration of magnesium due to calcite extraction from the pore water. The remaining pore space was subsequently filled with radiaxial fibrous or blocky sparry calcite during burial diagenesis.

The study is supported by RFBR, project number 20-35-70012.

How to cite: Vasileva, K., Ershova, V., Vereshchagin, O., Rogov, M., Tuchkova, M., Kostrov, Y., Khmarin, E., and Pokrovsky, B.: Multi-proxy study of cannon-ball concretions with glendonites from Paleogene-Neogene sediments of Sakhalin Island: implication for concretion growth and ikaite-calcite trans-formation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4041, https://doi.org/10.5194/egusphere-egu2020-4041, 2020.

The "black chimney" type of hydrothermal vents in the modern deep sea have become a popular research topic in many disciplines. Due to the actual conditions, the research on palaeo-thermal vents in geological history is relatively low. Fortunately, the discovery of hydrothermal vents and bio-fossils from the Chang 7 source rocks of the Yanchang Formation of the Triassic in the Ordos Basin, China, provides the best evidence for deciphering hydrothermal activity during geological history. Here, we report a case study. Through ordinary sheet observation, scanning electron microscopy and electron probe observation, layered grained siliceous rocks, dolomites, and hydrothermal mineral combinations, such as pyrite + dolomite + gypsum and calcite + barite, are found. Their unique petrological characteristics, mineral composition, and structure confirm the existence of palaeo-thermal fluid vents. We further analysed the geochemical characteristics and in situ isotope characteristics. The study found that Cs, U, Th, Pb, Ba and other trace elements of the sample showed positive abnormalities, in which values of U/Th were high; in addition, the enrichment of major elements such as Sr, Mn, and the in situ sulphur isotopes of pyrite reached 7.89%-10.88%. This study of hydrothermal vents over geological history is expected to provide new insights on the life forms of various extreme microorganisms in hydrothermal environments and on their formation of high-quality source rocks.

How to cite: You, J., Liu, Y., and Zhou, D.: Discovery and geological significance of the Magma-hydrothermal micro-jets at the bottom of a lake: A case from the Chang 7 section of the Yanchang Formation of the Triassic in the Ordos Basin, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6331, https://doi.org/10.5194/egusphere-egu2020-6331, 2020.

Chemostratigraphy has become a key tool to study shallow-water carbonate systems and propose insightful time correlations where biostratigraphic markers are limited. However, the bulk geochemical signal of shallow-water carbonate deposits commonly results from the superposition of local and global trends. Additionally, the shallow-water carbonate deposits frequently undergo intense diagenetic alteration obliterating the original seawater signature.

Based on three well-constrained Upper Jurassic sections of the French Jura, this study aims at discussing the control of the original depositional environment on the bulk geochemical signature of ancient shallow-water carbonates. Using isotope ratios (δ¹³C, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr), elemental concentrations (Ca, Mg, Fe, Mn, Sr, Al, U) and statistical methods, this paper shows that two main processes, closely linked to the depositional environments and associated conditions, control the overall signature of bulk samples of the studied deposits: the detrital input and the diagenetic effects. The detrital input, identified by increase in Fe, Al, Mn and U concentrations, is the highest in very proximal areas (supra- and intertidal domains) affected by terrestrial organic matter and pedogenetic material influx, and in distal realms (open sea) characterized by fine terrigenous fraction deriving from continental landmass erosion. The diagenetic effects can be subdivided into two processes: the dolomitization and the diagenetic imprint. The dolomitization, associated to increase in Mg and δ¹⁸O, mostly concerns supra- and intertidal deposits affected by refluxing evaporitic-derived brines. The diagenetic imprint, mainly associated to decrease of δ¹³C, δ¹⁸O and increase in ⁸⁷Sr/⁸⁶Sr, is the most important in platform margin deposits associated to high primary porosities enhancing fluid-rock interactions during burial and/or meteoric diagenesis. Because of these processes, time correlations are overall very difficult to establish between the studied sections.

The combined analyses of depositional environments and geochemical signal finally led to the conclusion that the concept of “geochemical facies” might represent an interesting tool to discuss depositional conditions and diagenetic effects along specific depositional models. This integrated study provides (1) relevant results to step back on challenging chemostratigraphic interpretations in shallow-water carbonate settings and (2) new insights into the complex sedimentological, diagenetic and geochemical interactions in shallow-water carbonate depositional systems.

How to cite: Courgeon, S., Makhloufi, Y., Vimpere, L., Meyer, M., and Samankassou, E.: Depositional environment control on the geochemical signal of ancient shallow-water carbonates (Upper Jurassic, French Jura), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6963, https://doi.org/10.5194/egusphere-egu2020-6963, 2020.

Dissolution of foraminiferal calcite above the lysocline can occur within the water column, at the sediment-water interface, and/or within the sediment column as a result of low in-situ carbonate ion concentrations. The dissolution of foraminiferal shells, which are widely used for paleoceanographic studies, has the potential to influence the weight and the chemical composition of the whole test. Their partial dissolution has been suggested to significantly bias their δ¹⁸O and δ¹³C signals, while the trace element ratios decrease as dissolution progresses. Despite the significant Atlantic importance on the climate system, and that of the carbonate system on atmospheric CO₂ concentration, there have been only a few studies examining carbonate preservation along depth transects in the North and South Atlantic, and mostly by indirect means.

In order to assess the preservation potential of the central Atlantic basins, a set of 16 Atlantic surface sediment (core-top) samples along the mid-Atlantic Ridge was deployed. The samples span from approximately 30°N to 30°S and are situated along the mid-Atlantic Ridge from an average water depth of 3700 m, well above the 4200 m modern lysocline, with roughly equal bottom water ΔCO₃²⁻ values (23 ±4 μmol/kg). Typically 15 pre-weighed shells of three different planktonic foraminifera species, widely used in paleoceanographic research, were picked from each sample (300-355 μm) and scanned using a GE vtomex s high-resolution micro-CT scanner. The species under consideration, namely Globigerinoides ruber s.s. (white), Trilobatus trilobus and Globorotalia truncatulinoides, inhabit different water depths and are known to have different geochemistries and thus preservation potentials. The preliminary analysis of the tomographs suggests that although carbonate sediments from the eastern basins below from the south equatorial upwelling zone are more corroded their initial geochemistry is not greatly altered by dissolution.

How to cite: Zarkogiannis, S., Kontakiotis, G., and Antonarakou, A.: Preservation potential assessment of central Atlantic biogenic carbonate deposits using X-ray micro-computed tomography (XMCT), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11397, https://doi.org/10.5194/egusphere-egu2020-11397, 2020.

The Miocene Dam Formation in the Al-Lidam area of Eastern Saudi Arabia consists of a succession 

of mixed siliciclastic-carbonate sequences that were deposited during Miocene (Burdigalian) 

times. Stratigraphic equivalents of the Dam Formation occur as hydrocarbon reservoir intervals in 

the Arabian Plate. Reservoir quality of carbonate rocks is controlled by a combination of 

depositional setting and post-depositional diagenetic factors. 

In this study, fifteen lithofacies were identified as they were deposited on a low angle dipping 

carbonate ramp, under supratidal, beach, intertidal and shallow subtidal conditions. Carbonate 

diagenesis has been examined using: thin-section petrography, SEM, XRD and 

cathodoluminescence. These analytical tools have shown that the intertidal lithofacies are 

influenced by extensive meteoric dissolution and minor cementation. Marine diagenesis was 

restricted to beach grainstone and subtidal lithofacies, in the form of aragonite and high magnesium 

calcite cement. Shallow burial conditions were inferred by grain contacts represented by point, 

suture and concavo-convex contacts. Mimetic dolomitization for the whole succession was also 

observed. Three fourth - order, shallowing upward sequences were identified in the study area, and 

they are separated by two sequence boundaries. A clear relation between sequence surfaces and 

diagenetic processes was observed; meteoric diagenesis and dolomitization increases upwards in 

each sequence. Porosity and permeability measurements have shown that the highest values are 

associated with the HST of each sequence, followed by the TST and the LST. The results of this 

study can help in understanding of diagenetic processes, and consequently in developing better 

and more accurate predictions of the porosity and permeability distribution within hydrocarbon 

reservoirs.

How to cite: Salih, M., Abdullatif, O., Al-Ramadan, K., and Bashri, M.: Carbonate Diagenesis in a Sequence Stratigraphic Framework; Case study from Miocene Dam Formation, Eastern Saudi Arabia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21365, https://doi.org/10.5194/egusphere-egu2020-21365, 2020.

The upper Miocene to Pliocene interval of the Browse Basin on the Australian North West Shelf (NWS) records a significant paleo-environmental change in its sedimentary record concerning the decay of middle to late Miocene tropical reefs. Seismic observations towards the Pliocene show a clear landward migration of carbonate build-ups in the eastern part of the basin, and very high subsidence rates seem to have outpaced most reef growth in distal shelf-edge positions. Nevertheless, the Scott Reef and the Seringapatam Reef were able to withstand shelf-edge drowning, which indicates a significant contribution of inversion-related uplift for reef survival. The contribution of basin subsidence as a driving factor for this reef decay and survival is still discussed and has not been studied in detail. This study provides an estimate for the laterally and through time changing late Miocene/Pliocene subsidence pattern. A 3D paleo-environmental reconstruction was generated by 3D quantitative backstripping, integrating 3D paleo-waterdepth information derived from seismic-based depositional system interpretation. The base of this analysis is a giant 2D and 3D seismic-reflection data set (>130.000 km²) integrated with borehole data (logs, cores, cuttings), new Sr-isotope dating, X-ray diffractometry (XRD) and microfacies analyses, supporting paleo-bathymetric correction and ties to global sea-level data. The seismic-reflection data is covering a study area extending over 130.000 km² and is supported by industry borehole data (logs, cores, and cuttings), SR-Isotope dating, X-Ray diffractometry (XRD) and microfacies analysis.

How to cite: Thronberens, S., Back, S., Reuning, L., and Bourget, J.: Basin subsidence and Miocene/Pliocene sedimentary change in the Browse Basin, NW Australia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21627, https://doi.org/10.5194/egusphere-egu2020-21627, 2020.

Marine biogenic carbonate deposits are important climate archives as environmental conditions during mineral genesis are recorded chemically in the crystal lattice. When exposed to diagenetic alteration metastable carbonate phases as aragonite are prone to transform into more stable calcite, Mg-calcite or dolomite resulting in the loss of the original geochemical information. As diagenetic alteration is often heterogeneous, numerous carbonate archives are characterized by multi-phase compositions, including potentially unaltered remains of primary formation. Consequently, high-spatial-resolution identification of adjacent carbonate mineral phases is of key interest to constrain diagenetic fronts, as well as the degree and pattern of a carbonate archive alteration, and the deciphering of relevant geochemical proxy information.

The aim of the present study is to contribute to the localization of potentially unaltered localities within heterogeneous carbonate samples, enhancing the retrieval of original geochronological and proxy information. Using natural tropical fossil coral samples as an example, we present an approach for the 3D-identification of multiple carbonate phases using µ-computed tomography scans with a resolution on a µm scale. The straightforward discrimination of aragonite, calcite, and dolomite from CT-imaging is principally hampered by the similar X-ray densities of these carbonate phases. To overcome this problem, reference material blocks for each of the three carbonate minerals were scanned together with the carbonate sample. Using the AVIZO 2019 software package in combination with a self-developed Tool command language (Tcl) script each of the carbonate reference blocks within the scanned volume was systematically subsampled for its voxel grey values. The obtained data set was statistically analysed and a robust mean voxel grey-value was calculated for each reference mineral. Subsequently, these mean grey values were used for the automated selection of seed points for subsequent image segmentation throughout the entire scanned volume based on a self-developed Tcl script. After seed point definition mineral identification was carried out throughout the volume using the watershed algorithm as a region-based image segmentation method. The final result yielded an approximation of the 3D-distribution of identified carbonate phases throughout the sample on a µm scale, which represents an excellent starting point for subsampling strategy development. 

How to cite: Krause, S., Liebetrau, V., Engelkes, K., Büsse, S., Jurikova, H., Nehrke, G., Gorb, S., and Eisenhauer, A.: 3D identification and quantification of multi-phase diagenetic carbonate mineralogy using µ-computed tomography: roadmap to original isotope geochemistry of altered archives , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21797, https://doi.org/10.5194/egusphere-egu2020-21797, 2020.

In terms of reservoir properties distribution carbonate rocks are very heterogeneous. Moreover, the types of porosity in carbonate rocks is very diverse. In our study of the Upper Marrat Formation near Khasm-adh-Dhibi (central Saudi Arabia) we have documented the pore system complexity and are deconvolving the impact of various post-depositional processes on porosity and permeability evolution of the formation. The Upper Marrat Formation is exposed in the central part of the Arabian plate in a north-south elongated mountain belt. It forms the lower part of the thick Jurassic petroleum-rich succession. The sediments forming the Upper Marrat Formation were deposited during the Early Jurassic time, the Toarcian. The Upper Marrat Formation shows fossiliferous biomicrite to sparse biomicrite carbonates with an evaporite deposit at the top. It is bounded by clayey units at both the top and the base. In general, because of the muddy matrix of the Upper Marrat, sediments are very tight and show low permeability. During the last 175 My, the Upper Marrat has been subjected to a series of diagenetic and tectonic processes. The initial micro- and intergranular porosity was reduced due to early compaction and cementation, however, during later diagenesis and tectonism, porosity and permeability were enhanced. The dominant diagenetic porosity in the Upper Marrat sediments is vuggy porosity, followed by fabric selective intragranular porosity. Many of the horizons in the Upper Marrat are heavily burrowed and mostly filled with sand-sized grains showing a higher porosity than the matrix. Dolomite is limited to evaporite strata and contain extensive inter-crystalline porosity produced during dolomite formation. Tectonism has enhanced porosity through the development of micro- and macro-fractures.  The different sized and orientated micro-fractures are important while they enhance permeability by connecting different pore types. Then extensive macro-fracture network has a major impact on the reservoir qualities, both porosity and permeability. The heavily fractured formation shows numerous fractures sets with NNE to SSW and ENE to WNW orientations. Fractures are mostly vertical to near-vertical; they are nearly all open, and often crosscut beds, or end at bedding planes. These fractures are the most abundant porosity type and their connectivity results in a very high permeability. In conclusion, initial porosity and permeability, and subsequent diagenetic and tectonic processes reduced and enhanced the porosity and permeability development of the sediments of the Early Jurassic Upper Marrat Formation.

How to cite: Ahmed, S., González, L., Reijmer, J. J. G., and ElHusseiny, A.: The complexity of carbonate porosity distribution in the Upper Marrat Formation, Central Saudi Arabia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22663, https://doi.org/10.5194/egusphere-egu2020-22663, 2020.