Petrographic Characteristics Research Articles

Relationship between coal composition, coal facies, and desorbed methane gas content: Thermal histories and exhumation processes in the middle Magdalena Valley basin, Colombia

We analyzed a total of 25 coal samples from high volatile bituminous to medium volatile bituminous range from 2 wells drilled in the Landázuri area of the Middle Magdalena Valley Basin at the Eastern Colombia Cordillera. This study reveals the relationship between the measured gas concentration, petrographic characteristics, and coal facies. We identify that gas desorption in coal seams is related to maceral composition.Three coal seam groups were defined in Landázuri 1 based on petrographic and statistical evaluation. Two groups were identified in Landázuri 2 based on gas content.The evaluation of maceral content and preservation of reflectance vitrinite found that the highest desorbed gas values correlate with the abundance of inertinite and structured inertinite macerals, followed by the percentage of vitrinite. Structured inertinite lumens may serve as macro/mesopores which allow for higher gas desorption because they are free of mineral matter or other macerals.Using existing coal facies diagrams, two types of coal facies were differentiated: lower delta plain (fen, marsh) and back-barrier low moor developed in a humid environment, where reotrophic conditions prevail and herbaceous and aquatic/marginal vegetation predominate.The relationship of these transitional facies with gas content shows are not directly related to the higher gas values measured at Landázuri. Instead, the relationship between the high degree of destruction of organic matter in these facies (marine incursions and other flows) and the measured gas content shows the importance of quantifying the structured macerals as potential gas retainers.Using low-temperature thermochronometers apatite fission-track, and (U–Th)/He in zircon, combined with vitrinite reflectance data, we reconstructed time-Temperature histories for areas near the wells. Differences in gas content can be explained by an asynchronous behaviour of the burial-cooling processes in this part of the Colombian Eastern Cordillera. Our work shows it is possible to distinguish gas desorption in coal seams based on the maceral composition.

Organic geochemical and petrographic characteristics of the Cambrian-Ordovician organic-rich marine shales in Scandinavia

This work used organic geochemistry and organic petrology to study the depositional environment, organic matter characteristics, and thermal maturity of the Cambrian-Ordovician organic-rich marine shales in the Baltic Basin. The main macerals in Cambrian samples include alginite, bituminite and solid bitumen, while zooclastic macerals become the major proportion of organic matter in the Ordovician samples. As the maturity increase, solid bitumen becomes abundant and dispersed. Semifusinite-like maceral was observed only in Furongian of well DBH15/73, which probably indicates the local intrusion of Permo-Carboniferous dolerite dikes. The programmed pyrolysis results show that immature, early mature, and over-mature samples are developed. However, the data of high-uranium Furongian samples were greatly altered by igneous intrusives from local dikes. HIo calculation model is simulated based on pyrolysis data and fitted according to the least-square fitting method. The simulated fitting HIo: 400 mg HC/g TOC (369.5 mg HC/g TOC, 430.5 mg HC/g TOC as 95% confidence bounds) is within the worldwide marine shale HIo and indicates a marine anoxia and transgressive sea with shallow water column (organofacies B).

Metal source and ore precipitation mechanism of the Ashawayi orogenic gold deposit, southwestern Tianshan Orogen, western China: Constraints from textures and trace elements in pyrite

The metal source and ore precipitation mechanism of orogenic gold mineralization are not yet well understood, partly because ore metals may be derived from different sources. Pyrite is a dominant Au-hosting mineral in the Ashawayi orogenic gold deposit in the southwestern Tianshan orogen, western China. Petrographic features of pyrite in host rocks and orebodies define four generations: diagenetic preore (Py1), hydrothermal early-ore (Py2), main-ore (Py3), and late-ore (Py4) pyrites.Trace element abundances were analyzed in situ by femtosecond laser ablation inductively coupled plasma mass spectrometry (fs-LA-ICP-MS) to unravel the pyrite formation history. Preore Py1 contains the lowest Cu, Mo, Se, Au and As contents, consistent with a diagenetic origin. Py2 has higher Au and As contents than Py1 and may have formed by the reaction between hydrothermal fluid and preexisting Py1, as indicated by diagenetic pyrite-like As/Ni and Bi/Au ratios but lower hydrothermal pyrite-like Sb/Au ratios in Py2. Hydrothermal pyrite (Py3) contains more abundant As (1723–65182 ppm) and Au (0.32–107 ppm) but lower Co and Ni contents than Py2, suggesting a greater hydrothermal fluid contribution. Oscillatory zoning and abundant mineral inclusions (e.g., arsenopyrite and chalcopyrite) in porous Py3 indicate that fluid boiling was responsible for gold deposition during the main-ore stage. Py4 is a relict of hydrothermal pyrite (Py3) but not diagenetic pyrite, as supported by Py4 and Py3 clustering into a class based on hierarchical cluster analysis. The application of a machine learning method (i.e., artificial neural network) to the syn-ore pyrite indicates that the Ashawayi gold deposit has affinity to those from orogenic-type gold deposits worldwide. Our study, therefore, highlights that ore metals in orogenic gold deposits may originate from different sources, such as Au and As, which are largely sourced from metamorphic fluids, while Co and Ni are mainly released from preore sedimentary pyrite, fluid boiling and fluid-rock interaction triggered precipitation of Au and other metals.

Magma Source and Petrogenesis of the Early Cretaceous Granites in The Liaodong Peninsula: Evidence from In Situ Apatite Sr-Nd and Zircon Hf-O Isotopes

Apatite Sr-Nd and zircon Hf-O isotopes are broadly used to trace magma sources and constrain magma evolution processes, further improving our understanding of the origin of granitoids. We present zircon U-Pb ages, whole-rock major and trace elements, and whole-rock Sr-Nd-Hf, zircon Hf-O, and apatite Sr-Nd isotopic data for the coarse-grained quartz monzonite, biotite monzogranite, and granite porphyry in the Yushulinzi pluton in the Liaodong Peninsula, the eastern North China Craton, to establish their magma sources and petrogenesis. The coarse-grained quartz monzonite, biotite monzogranite, and granite porphyry were formed contemporaneously, with zircon U-Pb ages of 123–119 Ma. They share enriched whole-rock Sr-Nd-Hf and zircon Hf isotopic compositions, and the coarse-grained quartz monzonite has crust-like δ18O values (5.7–6.7‰). The coarse-grained quartz monzonite and biotite monzogranite have variable apatite (87Sr/86Sr)i ratios and negative apatite εNd(t) values. These isotopic characteristics indicate that the different rock types in the Yushulinzi pluton were derived from the partial melting of ancient crustal material in the North China Craton. Their geochemical and petrographic characteristics indicate that the crystal-melt segregation model can be employed to elucidate the genetic links among different rock types, with the coarse-grained quartz monzonite representing crystal accumulation and the biotite monzogranite and granite porphyry representing interstitial melts extracted from a crystal-rich magma chamber. Furthermore, the variable apatite Sr isotopic compositions and subtle differences in the peak zircon εHf(t) values of the studied rock samples confirm the possibility of a contribution from shallow crustal components and materials with high εHf(t) values during magma evolution, which is not readily revealed by their whole-rock Sr-Nd-Hf isotopic compositions. These results demonstrate that in situ apatite Sr-Nd and zircon Hf-O isotopic analyses have the potential to provide distinctive insights into the magma sources and evolution of magmatic systems.

Methane release effects on foraminiferal tests in northern South China Sea

Upward diffusing methane in gas hydrate geological systems is consumed in the sediment and water column by a series of biogeochemical reactions, which not only affect living foraminifera but also lead to early diagenetic alteration of buried foraminiferal tests. Previous studies of the impact of methane release events on fossil foraminifera focused mainly on carbon and oxygen isotopes, with little attention given to other geochemical proxies to date. Here, we examine the test wall microstructure and analyze the elemental and stable carbon-oxygen isotopic compositions of buried foraminifera from gas hydrate-bearing sediments at Site GMGS2-16 in the northern South China Sea. Our results show that diagenetic alteration of foraminiferal tests at Site GMGS2-16 is mainly linked to high-Mg calcite overgrowths. Test δ13C covaries negatively with the degree of diagenetic alteration (based on petrographic characteristics) but positively with Mg/Ca ratios. With increasing diagenetic alteration, Ba/Ca, Mn/Ca, Fe/Ca, Mo/Ca, U/Ca, V/Ca, Ni/Ca and Co/Ca also generally increase, but Sr/Ca and Cu/Ca exhibit variable changes. We infer that foraminiferal tests are highly susceptible to alteration by methane-bearing fluids, and that the elemental ratios of diagenetically altered tests are potentially reliable proxies for paleo-methane release events. At Site GMGS2-16, the overall pattern of diagenetic alteration of foraminiferal tests records frequent vertical fluctuations of the sulfate-methane transition zone (SMTZ) caused by variable fluid flux and methane release rates, with two discrete large-scale methane release events having maximum ages of ~0.47 ka and ~170 ka.

Performance of Statistical and Intelligent Methods in Estimating Rock Compressive Strength

This research was conducted to forecast the uniaxial compressive strength (UCS) of rocks via the random forest, artificial neural network, Gaussian process regression, support vector machine, K-nearest neighbor, adaptive neuro-fuzzy inference system, simple regression, and multiple linear regression approaches. For this purpose, geo-mechanical and petrographic characteristics of sedimentary rocks in southern Iran were measured. The effect of petrography on geo-mechanical characteristics was assessed. The carbonate and sandstone samples were classified as mudstone to grainstone and calc-litharenite, respectively. Due to the shallow depth of the studied mines and the low amount of quartz minerals in the samples, the rock bursting phenomenon does not occur in these mines. To develop UCS predictor models, porosity, point load index, water absorption, P-wave velocity, and density were considered as inputs. Using variance accounted for, mean absolute percentage error, root-mean-square-error, determination coefficient (R2), and performance index (PI), the efficiency of the methods was evaluated. Analysis of model criteria using multiple linear regression allowed for the development of a user-friendly equation, which proved to have adequate accuracy. All intelligent methods (with R2 > 90%) had excellent accuracy for estimating UCS. The percentage difference of the average of all six intelligent methods with the measured value was equal to +0.28%. By comparing the methods, the accuracy of the support vector machine with radial basis function in predicting UCS was (R2 = 0.99 and PI = 1.92) and outperformed all the other methods investigated.

Cyclicity related to solar activity in lacustrine organic-rich shales and their significance to shale-oil reservoir formation

The formation mechanism of micron- to centimeter-scale sedimentary cycles in lacustrine shales is a hot topic of research, because these small-scale sedimentary cycles significantly influence shale-oil distribution heterogeneity. High-frequency paleoenvironmental evolution is an important controlling factor for the formation of small-scale sedimentary cycles. However, the driving factors of high-frequency paleoenvironmental evolution and the formation process of sedimentary cycles under its constraint remain speculative. In this study, which focuses on lacustrine shales, we find that the alternating deposition of variable thickness of organic-rich lamina (ORL) and silty-grained felsic lamina (SSFL) form sedimentary cycles on the micron to centimeter scale in the Chang 73 sub-member of the Yanchang Formation in the Ordos Basin. Based on detailed petrographic characterization, in-situ geochemical parameter testing, and high-resolution cycle analysis, the formation process of cyclical sedimentary records and related paleoenvironmental evolution are investigated. Three solar activity cycles were identified from the shales, namely the 360–500 yr, 81–110 yr, and 30–57 yr cycles (cycles I, II, and III, respectively). High-frequency paleoenvironmental evolution caused by solar activity induced lake-level fluctuation, which further controlled silty-grained sediment deposition and organic matter preservation in deep lake areas. Cycle I controlled relatively long-term lake-level fluctuation, driving several pairs of SSFL and ORL deposition at the centimeter scale. Cycles II and III were short-term cycles and acted on the millimeter to micrometer scale, further complicating the sedimentary strata forming during the period of lake-level fall induced by cycle I. The cyclic deposition of SSFL and ORL correspond to cycle III. Lake-level fluctuation influenced by cycle II mainly caused SSFL thickness variation in each lamina couplet. During the period of lake-level rise induced by cycle I, periods of lake level rise during cycles II, and III show cyclic variation in reducibility, and are not thought to control the supply of coarse-grained sediments in to the deep lake areas. Frequent lake-level fluctuation promotes lamina couplet formation in thickly-bedded shales, which creates favorable conditions for shale-oil accumulation. Oil produced from ORL can migrate-locally into dissolved feldspar porosity in SSFL and therefore is able to accumulate in shales, which creates high potential for future oil exploration in thickly-bedded lacustrine shales.

Sedimentary environment controls on the lacustrine shale lithofacies: A case study from the Nanpu depression, Bohai Bay Basin

Organic-rich shale of the Paleogene Shahejie Formation in the Nanpu Sag Bohai Bay Basin was analyzed using petrographic and elements analysis. These analyses were used to identify the lithofacies, reconstruct the sedimentary paleoenvironment and their relation. This paper adopted the division scheme and naming method of “Total organic carbon (TOC) + sedimentary structure + mineral composition” to carry out lithofacies identification and division. The petrographical characteristics indicated that there are 15 types of lithofacies. According to the lithologic changes, we divided the study interval from the fourth sub-member of the third member of the Shahejie formation into three parts from the bottom up. The lower part has an evident fluctuation in paleoclimate, salinity, productivity, and redox condition. The middle part has cold and arid climate conditions, the weakest weathering, the highest paleo-productivity, and anoxic conditions. The upper part was deposited under humid and warm climate conditions, strongest weathering, brackish-fresh water, lowest paleo-productivity, and dominant oxic condition. Paleo-productivity and redox conditions are the basic conditions for the enrichment and preservation of organic matter, the paleo-climate and paleo-salinity were the critical factors controlling the mineral composition and laminae development. Under warm and humid climate conditions, the shallow lake mainly develops organic-poor massive felsic shale and siltstone; the semi-deep lake is mainly developed with organic-medium laminated/layered felsic shale; the deep lake mainly develops organic-rich laminated/layered clayey shale and mixed shale. Under dry and cold climate conditions, the shallow lake area mainly develops organic-poor felsic shale, and the semi-deep lake mainly develops organic-rich layered carbonate shale and organic-rich massive mixed shale; the deep lake mainly develops organic-rich laminated/layered clayey shale.

To be or not to be Alpine: New petrological constraints on the metamorphism of the Chenaillet Ophiolite (Western Alps)

AbstractThe Chenaillet Ophiolite represents a very well‐preserved portion of Ligurian‐Piedmont ocean in the Western Alps. It is formed from an oceanic lithospheric succession comprising exhumed mantle, various mafic intrusives (i.e., gabbro sensu lato), and a world‐renowned sequence of pillow basalts. Apart from scarce breccias closely related to oceanic lithosphere, no sedimentary cover is exposed. Historically, the Chenaillet Ophiolite has been known for its very low temperature–low pressure Alpine metamorphism, ascribed to obduction processes. However, studies aimed at constraining the peak pressure–temperature (P–T) conditions of Alpine metamorphism are virtually lacking, the general focus having been so far on its high temperature metamorphism and geochemical features. In this paper, we investigate two kinds of rocks: gabbro and albitite/alkali syenite, whose petrographic features shed light on the complex metamorphic history of the Chenaillet Ophiolite. Detailed analyses of mineral assemblages, blastesis/deformation relationships, and mineral chemical data allow two metamorphic events to be distinguished: an earlier, high temperature event (already reported in the literature) and a second, later low temperature, high pressure event, recognized here for the first time. The low temperature, high pressure event is strikingly testified by the occurrence of lawsonite relicts in the gabbro and of interstitial omphacite in the albitite. Thermodynamic modelling (i.e., via isochemical phase diagrams) performed on a gabbro sample suggests for this unit a minimum of 9 kbar and 300°C and a maximum of 15 kbar and 450°C. Overlapping these P–T conditions with those inferred for the albitite based on the observed mineral assemblage allows the Alpine peak metamorphism to be constrained to 10–11 kbar and 340–360°C. These P–T conditions suggest a thickness of the overlying nappe stack of about 35–40 km, which is incompatible with obduction or burial processes, and instead consistent with subduction processes related to the Alpine orogeny. We argue that, opposite to the common belief that the Chenaillet Ophiolite escaped Alpine metamorphism, our new data strongly support the idea that it experienced low temperature‐blueschist‐facies metamorphism, whose evidence can still be tracked in those (few) rocks that better recorded and preserved it. This finding generates new challenging questions regarding both subduction and exhumation processes in complex orogens such as the Western Alps.

Shedding new light on Late Antique pottery from Histria, Romania

Ninety seven Late Antique pottery shards excavated at Histria, Romania (4th −7th c. AD) were investigated using PIXE (Particle Induced X-ray Emission) and Optical Microscopy (OM). This study aimed to offer answers on raw materials and technological choices and to provide additional insights on the provenance of these artefacts, broadening our knowledge on fine red slip ware from this period.The pottery fragments subjected to archaeometric analyses originate from tableware with typology well described in the archaeological literature, mostly manufactured in Oriental or North-African workshops, and imported to Histria alongside other commodities.The statistical analysis of the PIXE data was generally coherent with the suggested typological attribution and provenance of the original vessels.The petrographic characterization classified the shards into six groups according to the matrix, carbonatic concretions or ceramoclasts. Clues about the firing temperatures were suggested by particular clasts or by the untransformed carbonatic concretions.

Pulsed turbidite and methane seep records in the north western South China Sea since the last glacial maximum

The small-scale event layers in the continental margin contain abundant dynamic environment information, and pose a challenge to the interpretation of continuous sedimentary records, giving geological significance to their accurate identify and possible genesis. Here, pulsed turbidite layers since the last glacial maximum (LGM) in a gravity core in the northwestern South China Sea (SCS) was analyzed to investigate the precisely identification, possible causes and the role of marine environmental change during the late Quaternary in formation of these small-scale event layers in the SCS. Eight potential pulsed turbidite layers, according to the petrographic characteristics, grain size parameters and element geochemistry, were identified. Meanwhile, indicators including total sulfur (TS)/total organic carbon (TOC) ratio, CaCO3 content, and chromium-reducible sulfur (CRS) revealed these horizons were mostly related to methane seep events. Constrained by foraminifera shells AMS14C results, these events were determined to have occurred from the LGM to early Holocene, Similar records in the northern and southern slopes suggests the universal occurrence of these small-scale layers in the SCS. The comprehensive analysis showed that the development of these event layers over the past 25 ka can be divided into three stages, 25-15.5 ka, 15.5-7 ka and 7 ka to present. Late Quaternary Ocean environment changes, especially sea level and bottom water temperature, controlled the occurrence of regional small-scale event layers in the SCS. The regional scale mechanism is that the pressure and temperature change affect the stability of hydrate and the methane seepage, and thus the strata stability. Corresponding to the lowest, the rapid increase and the highest levels of the sea level and bottom water temperature, the temporal evolution pattern of small-scale event layers in the SCS showed a highest, decreased and lowest frequency, respectively. The linkage between the late Quaternary marine environmental change and turbidite deposition through gas activities in this study can act as a useful reference for further understanding the continental margin sedimentary process.

Geochemistry and Petrology of Metasediments Associated with Gold Mineralization in Imonga Area, Eastern DR Congo

The Imonga sector is located in the Eastern of DR Congo, precisely within the Maniema province located in the Central African Karagwe Ankole belt, which forms together with the Kibara belt a Mesoproterozoic geological structure. This study gives fresh information on the origin of the metasediments in this zone, their geochemical and petrographic characteristics, and the genetic model of the gold mineralization that they contain. Field observations, as well as petrographic and geochemical studies reveal that the metasediments consist of sericitoschists and quartzites. Petrographic work revealed varying proportions of sericite and quartz with subordinate iron-oxide minerals. Geochemically, most of the analyzed metasediments displayed higher SiO 2 contents and enriched in Fe 2 O 3 . They belong to the groups of shales and sand (sometimes rich in iron), litharenite and grauwakes derived from quartzose sedimentary and mafic igneous provenance. Their protholites are intermediate altered (70 < CIA < 90) andesitic, granite and granodioritic rocks emplaced in a geotectonic active continental margin. Gold mineralization in the Imonga sector occur in veins. Gold mineralization is controlled by veins which occur as micro veins with small thickness. The mineralization consists of specific grains and nanoparticles pyrite associated with gold, hematite and goethite. The hydrothermal alteration accompanying this mineralization consists of silica, goethite and sericite-carbonate. Keywords: Metasediment, sericitoschist, quartzite, sericite, goethite, vein, Gold, Imonga, Eastern DR Congo DOI: 10.7176/JEES/13-2-04 Publication date: March 31 st 2023

Reference section of the Uk Formation of the Upper Riphean (Karatavian) in the Alatau anticlinorium (Southern Urals)

Research subject. The deposits of the Uk Formation of the Upper Riphean (Karatavian) in the Alatau anticlinorium (Southern Urals) were studied in the section of the right bank of Basu river and its tribute Manaysu located to the east of the Kulmas settlement.Results. The structure of the sections of Uk Formation along Basu River (Kulmas) in the Alatau anticlinorium and its stratotype at the Yuryzan river near town of Ust-Katav in the Suleimanovo anticline is identical: the Lower Uk Subformation is represented by irregular alternation of sandstones, siltstones and shales with rare layers of limestones and dolomites with Patomella kelleri Raaben and Tungussia bassa Kryl. stromatolites and microphytolites of the IV complex, while the Upper Uk – by limestones with Linella ukka Kryl. and Linella simica Kryl. stromatolites and microphytolites of the IV complex. However, the sandstones and siltstones of the Lower Uk Subformation in the stratotype at the Yuryuzan river (Ust-Katav) are polymictic, while in the reference section along Basu river (Kulmas) they have mostly quartz composition. These petrographic features may be a result of different provenance. In the rocks of Uk Formation in the Basu section (Kulmas), the epigenetic processes (dolomitization and silicification) are strongly manifested. In the limestones of the Upper Uk Formation the caverns and fractures along with layers enriched by a bitumen (?) are observed.Conclusions. The reference section of the Uk Formation along the river Basu (Kulmas) is proposed as a hypostratotype, in which the deposits of the lower subformation of the Uk suite are most fully represented in comparison with the stratotype along the river Yuryuzan (Ust-Katav). Cavernous-fractured limestones of the Uk Formation with layers enriched by a bitumen (?), may be regarded as an interesting search object for hydrocarbons, taking into account that a large hydrocarbon deposit has been discovered in the silicified cavernous-fractured dolomites of Kamovskaya Formation of the Upper Riphean of the Siberian platform.

Deciphering the diagenetic evolution of the lower Carnian succession in the Gorno and Pian Resinelli sulphide mining districts (Southalpine Domain, N Italy)

The stratabound Mississippi Valley-type (MVT) sulphide deposits of the Gorno and Piani Resinelli mining districts are hosted in the lower Carnian carbonate succession, mainly peritidal limestones, of the Lombardian Basin (Southalpine Domain, Northern Italy). In addition to the formation of the well-known sulphide deposits, these carbonate sediments experienced a complex diagenetic evolution, including different stages of dolomitization, silicification, dissolution, brecciation, calcite cement precipitation. Stratigraphic and petrographic features suggest that, apart from an early calcite cementation and a first penecontemporaneous dolomitization affecting the supratidal beds, diagenetic processes and sulphide precipitation, are intimately and genetically associated. They document the occurrence of a Carnian hydrothermal system developed shortly after deposition at shallow burial depth, confirmed by fluid inclusion microthermometry and U-Pb radiometric datings.

Internal isotopic variability of Neogene carbonate concretions: Constraining formational growth mechanisms and isotopic disequilibrium

ABSTRACTCarbonate concretions collected from the Dominican Republic present a valuable opportunity to evaluate the internal isotopic variations within concretions that have never been exposed to deep burial or structural deformation. Here, three concretions from the Neogene (Late Miocene–Early Pliocene) Cibao Basin are investigated, utilizing a multi‐isotope (δ13C, δ18O, δ34SCAS and ∆47 values) high‐resolution approach, to constrain the microenvironmental conditions associated with multiple stages of concretion growth. Isotopic variability and potential disequilibrium effects, which can influence geological interpretations utilizing concretions, are also considered. The petrographic characteristics and geochemical profiles indicate internal differences relating to concretion growth mechanisms and environmental changes, driven by sea‐level fluctuations. The δ34S values of carbonate‐associated sulphate indicate a closed system environment; however, the overall values are influenced by sulphide oxidation within the sediments, resulting in a complex signal. The ∆47‐derived temperatures of the concretions range between 29 to 55°C, indicating significantly warmer temperatures than are measured from the host sediments, which average 24°C. This indicates that carbonate concretion ∆47 values are in disequilibrium with their environments of formation, likely related to ion diffusion in the pore fluids or isotopic fractionation associated with microbial processes. Here geochemical variations within concretions are utilized to assess the environmental conditions and microbial interactions after sediment deposition. However, for future studies, caution should be taken when using concretions for making environmental assessments as the signals can be influenced by a multitude of processes, even prior to diagenetic alteration.

Relación entre vulcanismo dacítico monogenético y poligenético. Caso de estudio del domo Tapias (<95 ka) y volcán Cerro Machín (<50 ka), Colombia

The Tapias lava dome is located ~5 km SE of the explosive, Cerro Machín polygenetic volcano, both located on the Colombian Central Cordillera. The edifices have been interpreted as generated by the same magmatic system; however, the genetic and temporal relations between both edifices have not been established. In this work, the Tapias dome is studied using petrographic characterizations, mineral-, whole-rock- and isotopic chemistry, geothermobarometric calculations, and a K/Ar radiogenic age, and subsequently it is explored the relationship with the Cerro Machín. The rocks from Tapias dome are porphyritic with phenocrysts of plagioclase (An28-48), amphibole (tschermakite and magnesiohastingsite), biotite, quartz, and Fe-Ti oxides, embedded in a glassy groundmass. Petrographically, two types of amphibole crystals were recognized: type I, which is homogeneous, and type II, which displays different rims. The composition of Tapias is dacitic (SiO2: 66.59 wt.%) with calc-alkaline affinity. Our geothermobarometric calculations indicate that type I amphibole crystallized at 871-874°C, between 10.2 and 10.4 km depth. The core of the type II amphibole crystallized at 852-920°C, between 8.5 and 12.3 km depth, while its rim did it at higher temperatures (880-973°C), but at similar depths (9.1-11.8 km). The K/Ar radiogenic age obtained for the groundmass shows an emplacement age not older than 95 ± 0.05 ka. When comparing the results to the Cerro Machín, it is noted that the mineral and compositional characteristics are nearly identical. These results suggest that indeed both volcanoes share the same magmatic system and are fed by magma hosted between ~9 and ~12 km depth. Moreover, it is proposed that the vent location of both edifices, as well as the time gap between the eruption at Tapias and the beginning of the activity at the Cerro Machín (~50 ka), are the result of crustal deformation caused by the interaction between the Otú-Pericos and Ibagué faults.

Sedimentological and Petrographical Characterization of the Cambrian Abbottabad Formation in Kamsar Section, Muzaffarabad Area: Implications for Proto-Tethys Ocean Evolution

The current sedimentological and petrographical research of the Abbottabad Formation has been carried out in order to understand the formation and evolution of the Proto-Tethys Ocean during the Cambrian on the northern margin of the Indian Plate. The Muzaffarabad region is located east of the Upper Indus Basin and the southern part of the Hazara Kashmir Syntaxis. The geological history of the region varies from the Precambrian to the recent period. The Cambrian Abbottabad Formation is well exposed along the Hazara Kashmir Syntaxis at the core of the 500-m-thick Muzaffarabad anticline. The Abbottabad Formation is an unconformity-bounded allo-stratigraphic unit. It has an unconformable lower contact with the Late Precambrian Dogra Formation and an unconformable upper contact with the Paleocene Hangu Formation. The Abbottabad Formation has been divided into four lithofacies, from bottom to top, namely, thinly interbedded dolomite and shale, cherty-stromatolitic dolomite, oxidized limonitic-brecciated zone, and quartzite, with significant lithological changes. Petrographic studies revealed four types of dolomites: fine crystalline dolomite (Dol. I), dolomitic cryptocrystalline chert (Dol. II), algal mat-stromatolitic dolomite (Dol. III), and intraclastic-dolo-grain stone (Dol. IV). The mineral composition of dolostone was analyzed using X-ray diffraction (XRD) and found to be consistent with previous petrographic studies. The dolomite mineral content decreased from base to top, while chert increased towards the top. Elemental weight percentages through energy dispersive X-ray (EDX) analysis show different elements constitute the minerals found in the dolostone, as confirmed by petrographic and XRD analysis. Using outcrop data, facies information, and geochemical data, a modified depositional model of the Abbottabad Formation was developed. During the Early Cambrian period, the formation was deposited in a shallow subtidal to supratidal setting of the Proto-Tethys Ocean. The top of this deposit marks the Cambrian–Paleocene boundary. Because of the progressively coarsening outcrop sequences, this formation seems to be at the very top of the Proto-Tethys Ocean’s shallow marine system.

In‐Situ and Triple Oxygen Isotope Characterization of Seafloor Drilled Cherts: Marine Diagenesis and Its Bearing on Seawater Reconstructions

AbstractCherts are used to reconstruct the evolution of seawater δ18O and temperature over geological time. However, given the influence of marine diagenesis, reconstructing seawater from the isotope composition of cherts is not straightforward, resulting in ambiguity of interpretation. Here, we present a detailed isotope and petrographic investigation of deep‐sea drilled 135–40 Ma cherts with focus on the effects of marine diagenesis. We combined triple O‐isotope data with in‐situ δ′18O‐16OH/16O measurements using secondary ion mass spectrometry (SIMS). We also provide electron microprobe maps, traditional δ′18O measurements from petrographically diverse domains, and δD and H2O wt.% values. The bulk δ′18O values range between 29‰ and 38‰ in our collection, while SIMS δ′18O data reveal significant intra‐sample heterogeneities up to 6‰ related to distinct petrographic features (e.g., filled radiolarian tests) and to micrometer‐scale variations in silica forms. Further, the δ′18O—Δ′17O values of these seafloor‐drilled cherts plot near and under equilibrium curve. Both triple‐O and SIMS δ′18O results reflect diagenesis in presence of marine pore waters at temperatures higher than ambient seawater, which is especially appreciable in cherts deposited on young oceanic crust. Despite the relatively constant δ18O seawater values over last 135 Ma, the marine silica spanning between 0 and 135 Ma occupies a wide compositional space in the δ′18O—Δ′17O rather than an equilibrium curve. The δ′18O values of cherts from modern‐seafloor positively correlate with the oceanic crustal age at the time of deposition, hinting at the importance of the heat flux in the diagenetic recrystallization of marine silica.

Geochemistry and zircon geochronology of the Late Triassic volcanic–sedimentary successions in northern Tibet: Implications for the provenance and tectonic evolution of the Mesozoic Qiangtang Basin

The Late Triassic volcanic–sedimentary successions deposited in the Qiangtang Basin are linked with the initiation of the Mesozoic Qiangtang Basin. However, the nature of this basin is hotly debated. Here, we synthesize petrographic, mineralogical, geochemical, and detrital zircon U–Pb age data of these volcanic–sedimentary successions to constrain their chemical weathering history, provenance, and tectonic setting and to reconstruct the tectonic evolution of the Mesozoic Qiangtang Basin. The chemical weathering indices, especially the chemical index of alteration (CIA), are controlled by grain size and hydraulic sorting rather than sediment recycling. Low compositional and textural maturities, high index of compositional variation (ICV) values, and low CIA and plagioclase index of alteration (PIA) values in most samples suggest that the sediments from the Fanghu and north Woruoshan sections are predominantly from low-maturity sources that may have experienced a weak chemical weathering history. Based on provenance-sensitive geochemical indices (La/Th versus Hf, La/Sc versus Co/Th, and Sc/Th versus Cr/Th) and petrographic features, the detrital contributions to the Nadi Kangri Formation siliciclastic rocks are predominantly proximal felsic rocks. Detrital zircon age spectra suggest that the Nadi Kangri Formation siliciclastic rocks exhibit a provenance change from the Fanghu to the well QK-1 area, although all of the rocks were derived from the Qiangtang terrane itself. The Fanghu section was primarily sourced from synchronous magmatic rocks and pre-Triassic strata from the North Qiangtang and Central uplift belt, while well QK-1 was primarily sourced from recycled sediments from the South Qiangtang and Central uplift belt. Therefore, we propose that the Nadi Kangri Formation volcanic–sedimentary successions were deposited in a rift basin and that its evolution was likely related to the interaction of the rapid expansion of the Meso-Tethys Oceanic crust, collapse of the collisional orogen, and rollback of the Jinsha Paleo-Tethys subducting slab during the Late Triassic.

Integrated geophysical, petrophysical and petrographical characterization of the carbonate and clastic reservoirs of the Waihapa Field, Taranaki Basin, New Zealand

Despite the economic importance of the Waihapa Field in Taranaki Basin in New Zealand, the structural setting, petrographic and petrophysical properties of its Cenozoic reservoirs are still poorly understood. This research integrates a 3D seismic survey with core and well logs data, as well as petrographic description in order to discuss the structural setting and the reservoir characterization of the Cenozoic reservoirs along the Waihapa Field, Taranaki Basin, New Zealand. A major thrust fault was interpreted dissecting both the Kapuni (Paleocene- Eocene) and Ngatoro (Oligocene- Early Miocene) groups and another opposite thrust fault affecting the Ngatoro Group and the Waihapa anticline. These compressional structures were resulted from the regional compression which affected the Taranaki Basin during the Miocene. Wai-iti Group (Miocene) is dissected by a set of NE-SW normal faults. These normal faults remark the end of the compressional phase and the beginning of a new extensional phase along the Taranaki Basin.The petrographic data of the studied reservoirs indicate that the Late Miocene Mount Messenger Formation is composed of feldspathic wacke, micaceous feldspathic quartz arenite and mudstone microfacies, whereas the Oligocene Tikorangi Formation is composed of five microfacies (bioclastic grainstone, bioclastic packstone, sandy wackestone, highly fractured silty mudstone and sandy glauconitic packstone microfacies). Downward, the Upper Eocene Mangahewa Formation is composed of ferruginated quartz arenite, micaceous quartz arenite, and ferruginated calcareous quartz arenite microfacies. Cementation and compaction are mostly affected the Mangahewa Formation while fracturing and dissolution are primarily affected the Tikorangi and the Mount Messenger formations respectively. The studied core data was discriminated into four petrophysical static rock types (PSRTs). The PSRT1, representing the Mount Messenger reservoir, has the best reservoir quality (av. porosity = 23.3%, av. permeability = 45.0 md, and meso pores, 2.60 μm) while the PSRT4 of Tikorangi Formation has the lowest reservoir quality (av. porosity = 5.5%, av. permeability = 0.01 md, and nano pores, 0.07 μm). The PSRT2 (av. porosity = 4.17%, av. permeability = 0.26 md, and micro pores, 0.59 μm) and PSRT3 (av. porosity = 5.93%, av. permeability = 0.08 md, and micro pores, 0.22 μm) are petrophysically considered transitional rock types between the PSRT1 and the PSRT4.However, Tikorangi reservoir is already producing oil from the borehole scale fractures. This work represents the workflow applied during integrating seismic, petrographical and petrophysical data in order to characterize carbonate and clastic reservoirs for any further hydrocarbon development plans along the field.