Lithic Fragments Research Articles

Material Exchange and Migration between Pore Fluids and Sandstones during Diagenetic Processes in Rift Basins: A Case Study Based on Analysis of Diagenetic Products in Dongying Sag, Bohai Bay Basin, East China

The exchange and migration of basin materials that are carried by pore fluids are the essence of diagenesis, which can alter physical properties of clastic rocks as well as control formation and distribution of favorable reservoirs of petroliferous basins. Diagenetic products and pore fluids, resulting from migration and exchange of basin materials, can be used to deduce those processes. In this study, 300 core samples from 46 wells were collected for preparation of casting thin sections, SEM, BSE, EDS, inclusion analysis, and isotope analysis in Dongying Sag, Bohai Bay Basin, East China. Combined with geochemical characteristics of pore fluids and geological background of the study area, the source and exchange mechanisms of materials in the pore fluids of rift basins were discussed. It was revealed that the material exchange of pore fluids could be divided into five stages. The first stage was the evaporation concentration stage during which mainly Ca2+, Mg2+, and CO32- precipitated as high-Mg calcites. Then came the shale compaction stage, when mainly Ca2+ and CO32- from shale compaction water precipitated as calcites. The third stage was the carboxylic acid dissolution stage featured by predominant dissolution of plagioclases, during which Ca2+ and Na+ entered pore fluids, and Si and Al also entered pore fluids and then migrated as clathrates, ultimately precipitating as kaolinites. The fourth stage was the organic CO2 stage, mainly characterized by the kaolinization of K-feldspar as well as dissolution of metamorphic lithic fragments and carbon cements. During this stage, K+, Fe2+, Mg2+, Ca2+, HCO3-, and CO32- entered pore fluids. The fifth stage was the alkaline fluid stage, during which the cementation of ferro-carbonates and ankerites as well as illitization or chloritization of kaolinites prevailed, leading to the precipitation of K+, Fe2+, Mg2+, Ca2+, and CO32- from pore fluids.

Basin classification and tectonic framework of the Nam Pat Group, Uttaradit Province, Thailand: Implications for the Nan Suture Zone

The Nam Pat Group of the Nam Pat Basin, Uttaradit Province, Thailand is situated in the Nan Suture Zone. Two contrasting tectonic framework models of the Nan Suture have previously been proposed: as a main Paleo-Tethyan suture and as a remnant of a closed back-arc basin. The goal of this study is to analyze the tectonic framework of the Nan Suture and reevaluate the existing tectonic models. This research infers the tectonic setting of the basin based on basin-filled lithofacies distributions, provenance, and paleocurrents. Three facies groups are identified. Facies A, B, and C are polymictic conglomerate, interbedded sandstone and mudstone, and mudstone with thin-bedded sandstone, respectively. Generally, the strata strike NE-SW, steeply dip NW, and fine toward the SE. Petrographic results of sandstone samples show that the majority of lithic fragments are volcanic rocks with lesser amount of metamorphic rocks and sedimentary rocks. The modal compositions of sandstones are analyzed on QFL, QmFLt, QmPK, LmLvLs, and QpLvmLsm ternary diagrams. The plots indicate that the sandstones of the Nam Pat Group have high proportions of volcanic-arc detritus. Paleocurrents, determined from the orientation of clast imbrications and cross-stratification, are toward the SE. The results suggest that large quantities of sediment traveled southeastward from a nearby volcanic arc into the basin. The Nam Pat Group stratigraphically overlies the Pak Pat Volcanics. The chemical features of the Pak Pat Volcanics, located to the west of the basin, show that they are andesitic volcanic rocks formed as a magmatic arc. Thus, the nearby Pak Pat Volcanics are the main sediment source of the Nam Pat Group, and the basin is best interpreted as back-arc basin rather than as a forearc basin.

Provenance, U-Pb detrital zircon geochronology, Hf isotopic analyses, and Cr-spinel geochemistry of the northeast Yukon-Koyukuk Basin: Implications for interior basin development and sedimentation in Alaska

The Yukon-Koyukuk Basin is a large depression that covers ∼118,000 km2 in western interior Alaska and is divided into two subbasins by a volcanic arc assemblage. Interpretations of the depositional ...

Cyanobacterial-algal crusts from Late Ediacaran paleosols of the East European Craton

The top surface of the Ediacaran flood basalts of West Ukraine (Volyn), extruded during the break-up of Rodinia supercontinent, is weathered into several meters thick paleosol, covered by late Ediacaran (ca. 550 Ma) sediments. The paleosol grades from unaltered basaltic material into saprock and finally clay-dominated saprolite with a minor content of the organic matter. The upper parts are dominated by kaolinite and hematite. SEM images and SEM-EDS analyses revealed in one of the paleosol profiles a horizon with significant amount of morphologically recognizable organic structures, mineralized by silicates and iron oxides. HF-etching of the bulk rock samples revealed lithic fragments covered irregularly by a biocrust of colonial coccoid cyanobacteria associated with irregularly distributed unicellular algae. The cyanobacteria are preserved as mineralized common mucilage sheaths (glycocalyx), which reproduce the original shape typical of sheaths of modern coccoidal cyanobacterial biofilms, whereas the heavily mineralized algae show features characteristic of modern chlorococcaleans. The preservation of morphologically distinguishable remnants of cyanobacterial biofilms and aggregates of unicellular algae in terrestrial weathering deposits is a rare preservational phenomenon requiring rapid post-mortem permineralization of the dead microbiota. One of the main factors facilitating such a unique fossilization was supposedly a copious excretion of extracellular polymeric substances (EPS) by the biocrust forming biota in which early mineral nucleation occurred. The colonization of the Late Ediacaran land by the presumably desiccation-resistant cyanobacteria-dominated crust communities associated with algae was of importance for creating fertile soils and a soil-forming microbial ecosystem that later allowed a successful land invasion by the ancestors of vascular plants.

New archaeological insights from petrographic analysis of ceramics from the Bocas del Toro Archipelago, Panama

In this study, we conducted the first petrographic analysis of pottery from several Pre-Columbian archaeological sites located in Bocas del Toro province along the Caribbean coast of Panama. The fifty-four sherds examined in this study included surface finds collected from the sites of Red Frog (RF) and Punta Vieja Arriba (PVA) on Bastimentos Island, the Cerro Brujo (CB) site on the mainland, and excavated samples from Sitio Drago (SD), Isla Colon. Petrographic examination of sherd thin sections shows that different sandy materials (seven constituent natural and/or manually-added temper groups) were used in their production. Representative sherd compositions from each group were determined by petrographic point-counts (n=200) using two matrix (silt and clay) and 14 mineral and rock fragment categories for sand grains. The sand in RF sherds is mainly fragments of mudstone, whereas sherds from the PVA and CB sites contain different proportions of sand-sized quartz, feldspar, dense minerals, and several varieties of volcanic lithic fragments. The latter components are constituents derived from local Panamanian geologic units, including one with pyroclastic temper that likely comes from a volcanically active region, such as the El Baru Volcano. Some of the archaeological sites (SD and PVA) have a greater variety of sherd compositional/temper groups than others, implying that people inhabiting these sites were actively engaged in acquiring materials and/or ceramics from multiple sources regionally.

Silicic, high- to extremely high-grade ignimbrites and associated deposits from the Paraná Magmatic Province, southern Brazil

The Cretaceous trachydacites and dacites of Chapecó type (ATC) and dacites and rhyolites of Palmas type (ATP) make up 2.5% of the ~800.000km3 of volcanic pile in the Paraná Magmatic Province (PMP), emplaced at the onset of Gondwana breakup. Together they cover extensive areas in southern Brazil, overlapping volcanic sequences of tholeiitic basalts and andesites; occasional mafic units are also found within the silicic sequence. In the central region of the PMP silicic volcanism comprises porphyritic ATC-type, trachydacite high-grade ignimbrites (strongly welded) overlying aphyric ATP-type, rhyolite high- to extremely high-grade ignimbrites (strongly welded to lava-like). In the southwestern region strongly welded to lava-like high-grade ignimbrites overlie ATP lava domes, while in the southeast lava domes are found intercalated within the ignimbrite sequence. Characteristics of these ignimbrites are: widespread sheet-like deposits (tens to hundreds of km across); absence of basal breccias and basal fallout layers; ubiquitous horizontal to sub-horizontal sheet jointing; massive, structureless to horizontally banded-laminated rock bodies locally presenting flow folding; thoroughly homogeneous vitrophyres or with flow banding-lamination; phenocryst abundance presenting upward and lateral decrease; welded glass blobs in an ‘eutaxitic’-like texture; negligible phenocryst breakage; vitroclastic texture locally preserved; scarcity of lithic fragments. These features, combined with high eruption temperatures (≥1000°C), low water content (≤2%) and low viscosities (104–7Pas) suggest that the eruptions were characterized by low fountaining, little heat loss during collapse, and high mass fluxes producing extensive deposits.

Modal composition and tectonic provenance of the sandstones of Ecca Group, Karoo Supergroup in the Eastern Cape Province, South Africa

AbstractPetrography of the sandstones of Ecca Group, Karoo Supergroup in the Eastern Cape Province of South Africa have been investigated on composition, provenance and influence of weathering conditions. Petrographic studies based on quantitative analysis of the detrital minerals revealed that the sandstones are composed mostly of quartz, feldspar and lithic fragments of metamorphic and sedimentary rocks. The sandstones have an average framework composition of 24.3% quartz, 19.3% feldspar, 26.1% rock fragments, and 81.33% of the quartz grains are monocrystalline. These sandstones are generally very fine to fine grained, moderate to well sorted, and subangular to subrounded in shape. In addition, they are compositionally immature and can be classified as feldspathic wacke and lithic wacke. The provenance characteristics suggest the influence of plutonic and metamorphic terrains (meta-magmatic arc) as the main source rock with minor debris derived from recycled sedimentary rocks. The latter revealed that the compositional immaturity of the sandstones is a result of weathering or recycling and short transport distance. The weathering diagrams and semi-quantitative weathering index indicate that the Ecca sandstones are mostly from a plutonic source area, with climatic conditions ranging from arid to humid. The detrital modal compositions of these sandstones are related to back arc to island and continental margin arc. These results, therefore, support previous studies that infer foreland basin setting for the Karoo Basin.

The 1914 Taisho eruption of Sakurajima volcano: stratigraphy and dynamics of the largest explosive event in Japan during the twentieth century

The 1914 Taisho eruption of Sakurijima volcano was Japan’s highest intensity and magnitude eruption of the twentieth century. After a 35-year period of quiescence, the volcano suddenly rewoke a few days before the eruption, when earthquakes began to be felt on Sakurajima Island. The eruption began on January 12, 1914, from two fissures located on opposite sides of the volcano, and was characterized by a complex time evolution and changes in eruptive styles. The eruption began with a subPlinian explosive phase in which two convective columns rose from the two fissures. Both plumes were sustained for at least 2 days. This resulted in deposition of a widely dispersed tephra sequence. After this phase, the eruption evolved to a final, waning phase, shifting toward effusive activity that lasted until April 1914. During the first weeks, effusive activity was also accompanied by ash emission. The complex sequence of events, characterized by contemporaneous explosive and effusive activity, is typical of several recently observed mid-intensity eruptions, such as during the 2011 eruption of Cordón Caulle, Chile. The stratigraphic sequence of the eruptive deposits from the Taisho eruption comprises alternating coarse-to-fine lapilli beds with ash beds dispersed toward the ESE and SE. These deposits can be subdivided into three lapilli-bearing units (Units T1, T2 and T3, which correspond to the subPlinian phase) and one ash-bearing unit (Unit T4, which corresponds to the final ash venting, accompanying the first day/weeks of lava flow activity). Grain size analyses from each unit reveal a marked polymodal distribution generally described by the sum of two or three Gaussian subpopulations. Both the modes and the relative amounts of the coarse subpopulations vary with distance from vent, with those of the fine subpopulation remaining nearly constant. Within the vertical sequence, component analysis shows a progressive increase in lithic fragments, suggesting that conduit enlargement continued until the final stages of the eruption. The estimated volume of the tephra deposit of the subPlinian phase of the eruption is 0.33 ± 0.11 km3 (dense rock equivalent (DRE) volume = 0.09 ± 0.03 km3). The height of the eruption column was also assessed by using four different isopleth maps compiled based on different strategies for the characterization of the largest clasts. The maximum height attained by the eruption column is estimated at 15.0 ± 1.2 km above the vent, resulting in a maximum mass discharge rate of 3.6 ± 1.2 × 107 kg s−1 (calculated taking into account the strong effect of wind advection). Finally, different classification schemes were applied to classify the eruption, which generally straddles the fields between Plinian and subPlinian.

Diagenetic model of the Bajo Barreal formation (Cretaceous) in the southwestern flank of the Golfo de San Jorge Basin (Patagonia, Argentina)

This paper analyses the diagenetic evolution of sandstones belonging to the Bajo Barreal Formation (Cretaceous) in the Golfo de San Jorge Basin (Patagonia, Argentina). The Bajo Barreal Formation includes the main reservoirs, which are located along the western area of the basin and is composed of sandstones, conglomerates, mudstones, tuffaceous mudstones and some layers of tuffs. The principal reservoirs comprise medium-to coarse-grained sandstones, which are dominated by feldspathic litharenites and contain minor amounts of litharenites and lithic arkoses. The authigenic minerals include kaolinite, smectite, chlorite, quartz overgrowths, microquartz and calcite, with minor proportions of megaquartz, siderite, analcime, laumontite, feldspar overgrowths and illite/smectite and chlorite/smectite mixed layers. Secondary porosity is much more important than primary porosity and is produced by the dissolution of feldspar, lithic clasts and clay cements. The diagenetic history of the Bajo Barreal sandstones can be divided into seven diagenetic stages, each of which is characterized by a specific assemblage of authigenic minerals and diagenetic processes. Eogenetic conditions occur in stages 1, 2, 3 and 4. Stage 1 corresponds to shallow burial characterized by the physical reduction of primary porosity by compaction; during stage 2, rim clay cements of chlorite, smectite and clinoptilolite, as well as thin quartz overgrowths, were formed. The precipitation of pore-filling cements of kaolinite, chlorite and smectite occurred during stage 3, while stage 4 records the intense dissolution of feldspar, lithic fragments and kaolinite cements. Mesogenesis occurs in diagenetic stages 5 and 6. The former corresponds to a new phase of authigenic kaolinite, while the latter records the significant dissolution of feldspar, lithic clasts and previous cements, which produced the highest values of secondary porosity. Finally, stage 7 corresponds to the highest degree of diagenesis in the Bajo Barreal Formation (mesogenesis), which resulted in the precipitation of cements of zeolites and calcite, as well as quartz and plagioclase overgrowths.

Diagenetic Features of Jurassic Fort Member Sandstone, Jaisalmer Formation, Western Rajasthan

ABSTRACT Jaisalmer Formation consists of 360m thick succession of medium to coarse grained sandstones with interbeds of shale, claystone and occasional lignite that rest over Lathi Formation, is the basal part of the Jaisalmer basin. The rocks are exposed amidst desert, low mounds and shallow stone quarries. Sandstones were deposited in shallow marine to deltaic environments. The studied sandstones consist of abundant quartz followed by feldspar, mica, chert, rock fragments and heavy minerals. The study mainly deals with identification of various diagenetic features such as compaction, cementation and porosity evolution. During mechanical compaction rearrangement of grains took place and point, long and suture contacts were formed. The sandstones are cemented by iron oxide, silica overgrowth, carbonate and clay. Porosity has developed due to dissolution of iron, carbonate cement and feldspar grains. Dissolution and alteration of feldspar, lithic fragments and pressure solution were the main source of quartz cements. The sandstones show good amount of existing optical porosity with an average of 7.19%. Porosity reduction is mainly due to early stage of mechanical compaction and subsequent pervasive calcite and iron oxide cementation. Further, porosity reduced due to deposition of clay cement.

Provenance of Middle to Late Triassic sedimentary rocks in the Zoige Depression in theNEpart of the Songpan–Ganzi Flysch Basin: Petrography, heavy minerals, and zircon U–Pb geochronology

The Zoige Depression is an important depocenter zone within the northeast of the Songpan–Ganzi Flysch Basin, which is bounded by the South China, North China, and Qiangtang blocks, and also forms the northeastern margin of Tibetan Plateau. This paper discusses the provenance of Middle to Late Triassic sedimentary rocks in the Zoige Depression using petrography, heavy mineral assemblages, and zircon U–Pb geochronology. The results demonstrate that the detritus is derived from multiple source regions. Four distinct parent rocks can be distinguished based on the heavy mineral assemblages and lithic fragments: Provenance 1 predominantly comprises intermediate–acidic volcanic rocks; Provenance 2 includes high‐grade metamorphic rocks; Provenance 3 contains a mixture of various detrital components; and Provenance 4 primarily consists of mafic volcanic rocks. The different U–Pb ages of the zircons from the Middle to Late Triassic ranging from 260 to 280, 429–480, 792–974, and 1,800–2,500 Ma represent distinct source regions, which are comparable to the 4 provenances mentioned above: Provenance 1 (260–280 Ma), Eastern Kunlun Orogen; Provenance 2 (429–480 and/or 1,800–2,500 Ma), Qinling orogeny (mainly in North Qinling); Provenance 3 (1,800–2,500 Ma), the North China Block; and Provenance 4 (792–974 Ma), the Yangtze Block. Overall, the detritus in the Middle Triassic (Ladinian, T2zg) primarily originates from the Eastern Kunlun Orogen and North Qinling. During the Late Triassic (Early Carnian, T3z), the southern margin of the North China Block was likely transported westward to the basin by a river network between the North China and South China blocks flowing through the Qinling region, because of the predominance of the detrital zircon age ranging from 1,800 to 2,500 Ma and the occurrence of quartz sandstone with visible enlargement texture. Since the Late Triassic (Middle Carnian, T3zh), great changes have occurred in the source terrains, such as the absence of sources of the Eastern Kunlun Orogen and North China Block and the predominance of Yangtze Block. This drastic change can be explained by the Triassic collision between the South China and North China blocks, and the clockwise rotation of the South China Block progressively closed the basin and uplifted the Qinling orogeny.

Provenance, tectonic setting and source of Archean metasedimentary rocks of the Browns Range Metamorphics, Tanami Region, Western Australia

ABSTRACTThis study combines U–Pb age and Lu–Hf isotope data for magmatic and detrital zircons, with whole-rock geochemistry of the Browns Range Metamorphics (BRM), Western Australia. The BRM are medium- to coarse-grained metasandstones that consist of angular to sub-rounded detrital quartz and feldspars with minor granitic lithic fragments. The sequence has undergone partial to extensive quartz–muscovite alteration and rare-earth-element mineralisation and has been intruded by mafic/ultramafic, syenitic and pegmatitic intrusive rock units. Uranium–Pb and Lu–Hf isotopic data on detrital zircons from the metasandstones and intruding granitic rocks yield a well-defined age of ca 3.2 to ca 3.0 Ga for all samples, with relatively radiogenic ϵHf values (ϵHf = –1.7 to 5.1) indicating derivation from Mesoarchean granite basement of juvenile origin. This is consistent with geochemical and petrological data that support deposition from a granitic source in a continental rift basin setting. The timing of sediment deposition is constrained between the ca 3.0 Ga age of the source rocks and ca 2.5 Ga age of the granitic intrusive bodies that cross-cut the metasedimentary rocks. The ca 2.5 Ga zircons from the intrusive rocks have ϵHf model ages of ca 3.4 to ca 3.1 Ga, which is consistent with formation via partial melting of the BRM, or the Mesoarchean granite basement. Zircons of the Gardiner Sandstone that unconformably overlies the BRM return detrital ages of ca 2.6 to ca 1.8 Ga with no trace of ca 3.1 Ga zircons, which discounts a significant contribution from the underlying BRM. The Mesoarchean age and isotopic signatures of the BRM zircons are shared by some zircon records from the Pine Creek Orogen, and the Pilbara, Yilgarn and Gawler cratons. Collectively, these records indicate that juvenile Mesoarchean crust is a more significant component of Australian cratons than is currently recognised. This work also further demonstrates that detrital minerals in Paleoproterozoic/Archean sedimentary rocks are archives to study the early crustal record of Earth.

Syneruptive sequential fragmentation of pyroclasts from fractal modeling of grain size distributions of fall deposits: the Cretaio Tephra eruption (Ischia Island, Italy)

In this work we use fractal statistics in order to decipher the mechanisms acting during explosive volcanic eruptions by studying the grain size distribution (GSD) of natural pyroclastic-fall deposits. The method was applied to lithic-rich proximal deposits from a stratigraphic section of the Cretaio Tephra eruption (Ischia Island, Italy). Analyses were performed separately on bulk material, juvenile, and lithic fraction from each pyroclastic layer. Results highlight that the bulk material is characterized by a single scaling regime whereas two scaling regimes, with contrasting power-law exponents, are observed for the juvenile and the lithic fractions. On the basis of these results, we infer that the bulk material cannot be considered as a good proxy for deducing eruption dynamics because it is the result of mixing of fragments belonging to the lithic and juvenile fraction, both of which underwent different events of fragmentation governed by different mechanisms. In addition, results from fractal analyses of the lithic fraction suggest that it likely experienced a fragmentation event in which the efficiency of fragmentation was larger for the coarser fragments relative to the finer ones. On the contrary, we interpret the different scaling regimes observed for the juvenile fraction to be due to sequential events of fragmentation in the conduit, possibly enhanced by the presence of lithic fragments in the eruptive mixture. In particular, collisional events generated increasing amounts of finer particles modifying the original juvenile GSDs and determining the development of two scaling regimes in which the finer fragments record a higher efficiency of fragmentation relative to the coarser ones. We further suggest that in lithic-rich proximal fall deposits possible indications about the original GSDs of the juvenile fraction might still reside in the coarser particles fraction.

Genesis of tuff interval and its uranium enrichment in Upper Triassic of Ordos Basin, NW China

Recently measured high gamma ray values in the Yanchang Formation of the Upper Triassic in the Ordos Basin have added an interesting and controversial twist to the study of the formation’s uranium enrichment and genesis. High uranium and thorium contents in the tuffaceous layer cause high gamma ray values in the Yanchang Formation. Petrographic studies, major elements, rare earth elements (REEs), and trace elements have been systematically analyzed to determine the composition, geochemical environment, and diagenetic processes of the layer. The observed color of the tuffaceous layer in the study area varies from yellow to yellowish brown. The tuff consists of matrix supported with sub-rounded to sub-angular lithic fragments. These lithic fragments probably derived from pre-existing rocks and incorporated into the tuffaceous layer during volcanic eruption. Quartz, plagioclase, and biotite were observed in well to poorly sorted form, in addition to framboidal pyrite and organic laminae. Measured ratios of SiO2/Al2O3 ranged from 3.277 to 6.105 with an average of 3.738. The ratio of TiO2/Al2O3 varied from 0.037 to 0.201 with an average of 0.061, indicating that the sediments of the tuffaceous layer originated from an intermediate magma. REE distribution patterns show sharp negative Eu anomalies, indicating a reducing environment, which is suitable for uranium deposition. A reducing environment was confirmed by black shale in the base of the Yanchang Formation. Such black shale has high organic matter content that can take kerogene from mudstone and provide a reducing environment for uranium enrichment in the tuffaceous layer. Moreover, negative Eu anomalies and the REE patterns indicate a subduction-related volcanic arc environment as the magma source of the tuffaceous layers. High values of Rb, Ba, and Sr might be the result of fluid phase activities; low values of Hf and Eu indicate the involvement of crustal material during diagenesis of the tuff. Discrimination diagrams (Th/Yb vs Ta/Yb, Th/Hf vs Ta/Hf) suggest an active continental margin as the tectonic setting of source volcanoes. Plots of Nb versus Y, Rb versus Y + Nb, TiO2 versus Zr, and Th/Yb versus Nb/Yb of the tuffaceous content point to calc-alkaline continental arc-related magmatism. We concluded that uranium enrichment in the tuffaceous layer was supported by oxidation–reduction.

Petrological study of the early Mesoproterozoic Glauconitic Sandstone and Olive Shale members from the Semri Group, Vindhyan Supergroup in Central India: Implications to input from intrabasinal felsic volcanic source and glauconitization

The Semri Group of the Vindhyan Supergroup contains widespread intrabasinal felsic volcanic rocks in the Porcellanite Formation formed during the early Mesoproterozoic. This widespread felsic volcanism was followed by the deposition of siliciclastic sedimentary rocks of the Kheinjua Formation containing the Olive Shale, Fawn Limestone, Glauconitic Sandstone members, the last one dominantly arkose and litharenite. The volcanic products resulting from volcanic activity during deposition of Porcellanite Formation were predominantly fine volcanic ash or dust that settled very slowly in the water column of the basin. The ranges of 28 elemental ratios for the Olive Shale and Glauconitic Sandstone members overlapped with those of the Porcellanite Shale Formation. The contemporaneous intermingling of the detrital input from volcanic products with the terrigenous detritus received by the basin from the Bundelkhand Craton in the north is reflected in the geochemical signatures. Various plots of elemental ratios and ternary plots for the siliciclastic rocks overlap those of the Porcellanite Shale Formation and Bundelkhand Tonalite‐trondhjemite‐granodiorite (TTG) suggesting that they might be the source rocks. The mixing of detritus from volcanic products was perhaps responsible for the widespread appearance of glauconite‐bearing sediments in the subsequent deposits. Glauconitization of detrital K‐feldspars, clay peloids, and volcanic lithic fragments is evident from the petrography. The shallow marine environment, slow sedimentation rates, elevated surface temperatures and mildly reducing conditions (CO2 got released from explosive volcanism) are considered as the favourable conditions for glauconitization, which prevailed within the Vindhyan Basin. Under these conditions the detrital input received as the result of previous volcanism during deposition of the porcellanite shale (glass, pumice lithic fragments, and feldspars) were transformed into clays such as illite and glauconite during the deposition of siliciclastic rocks of the Kheinjua Formation within the basin.

Sandstone memory of a Late Paleozoic continental arc in southeast China (Lower Yangtze region)

Provenance analysis was carried out on Paleozoic siliciclastic rocks exposed in the South China Block (Lower Yangtze region, Chaohu and Nanjing areas) by sandstone petrography, U-Pb detrital-zircon geochronology, and zircon Hf isotopes. Sandstones of the Silurian Fentou Formation are feldspatho-quartzose with a few igneous and low-grade metamorphic rock fragments. Sandstones of the Devonian Wutong Formation are quartzose with a few quartz-rich metamorphic rock fragments. The Fentou and Wutong formations share similar detrital-zircon age spectra, with peaks mainly at 465−420 and 838−812 Ma. Zircons of Early Paleozoic age were with all likelihood derived from magmatic rocks associated with the Kwangsian orogeny, documented by a widespread unconformity across the South China Block. Neoproterozoic zircons were probably shed from Proterozoic basement of the Wuyi terrane in the Cathaysian block, interpreted to have been uplifted south of the Lower Yangtze region during the Kwangsian orogeny. The Permian Longtan Formation in Chaohu area and Nanjing area are litho-quartzose and feldspatho-quartzose, respectively, with dominant felsic and intermediate volcanic lithic fragments, indicating that detrital source areas did not change markedly after the Kwangsian orogeny. Zircon-age spectra from the Longtan Formation in the Chaohu area display a diagnostic peak at ~ 1866 Ma, indicating provenance from the Paleoproterozoic basement of the Wuyi terrane. Instead, detrital zircons in the Longtan Formation in the Nanjing area display peaks at ~ 445 and ~ 970 Ma, similar to those in the underlying Paleozoic sandstones. The Wuyi terrane in the Cathaysian block thus remained as a major source area for long, as supported by independent sedimentological and seismic evidence. The Longtan Formation in both Chaohu and Nanjing areas also contain detrital zircons with ages distributed continuously from 341 to 254 Ma and a major peak around 270 Ma, reflecting renewed intermediate and felsic magmatism in the source area. These zircons have e Hf( t ) ranging from −19.8 to +14.9, indicating melting of old crust and mixing with crustal magmas. Such a continuously magmatic source supplied sediments all across the South China Block from east to west. These and other lines of evidence including probability accumulation-curve analysis favor deposition in a convergent plate-tectonic setting. We suggest that source rocks were part of a continental arc developed along the southeast coast of China during the Late Paleozoic, shedding sediments to as far as north as the northern South China Block. According to previous studies, such continental arc would be related to the subduction of the paleo-Pacific plate. Magmatism generated at first by melting of old crust was mixed later on with felsic and intermediate magma newly generated during mantle upwelling and associated uplift. The continental arc remained as the main detrital source to the Lower Yangtze region and other parts of the South China Block until it was eventually denudated or buried by the Mesozoic times.

The Buritizal meteorite: classification of a new Brazilian chondrite

Abstract On August 14, 1967, the reporter Saulo Gomes, working at TV Tupi, went to a small city in the State of Sao Paulo called Buritizal to investigate reports of a meteorite fall and write a newspaper report. He actually recovered three fragments of the meteorite at a small farm. In 2014, he donated one of the fragments to the Museu Nacional of the Universidade Federal do Rio de Janeiro (MN/UFRJ). We named this meteorite Buritizal and studied its petrology, geochemistry, magnetic properties and cathodoluminescence with the intent to determine the petrologic classification of the meteorite. In this manner, the Buritizal meteorite is classified as an ordinary chondrite LL 3.2 breccia (as indicated by lithic fragments). The meteorite consists of ~ 2% of metallic Fe,Ni and many well-defined chondrules with ~ 0.8 mm in average diameter. An ultramafic ferromagnesian mineralogy is predominant in the meteorite, represented by olivine, orthopyroxene, clinopyroxene, Fe-Ni alloy, troilite and glass. The total iron content was calculated as 20.88 wt%. Furthermore, the meteorite was classified as weathering grade W1 and shock stage S3. Buritizal is the 25th observed meteorite fall recovered in Brazil, of 70 meteorites known from Brazil. Thus, the study of the Buritizal meteorite is very important and relevant for the Brazilian scientific community.

Grain composition and diagenesis of organic-rich lacustrine tarls, Triassic Yanchang Formation, Ordos Basin, China

The role of the primary detrital grain assemblage as a control on diagenetic pathways is reasonably well-understood in sandstones and limestones, but less so in mudrocks. We have documented diagenesis in mudstones from the Triassic Yanchang Formation that are dominated ([Formula: see text] by volume) by grains derived from outside the basin of deposition (terrigenous-argillaceous mudstones or tarls). Major extrabasinal grains are K-rich clay, quartz, plagioclase, K-feldspar, lithic fragments, and micas. In terms of the quartz-feldspar-lithic grain compositions, the silt fraction in these samples is classified as arkose. Grains of intrabasinal derivation include particulate organic matter, phosphatic debris, and rare carbonate allochems. The principal chemical diagenetic components in these mudrocks have strongly localized spatial distributions at micrometer to centimeter scales. Chemical diagenetic components include cone-in-cone structures, replacements of detrital feldspar, pore-filling precipitates within anomalously large pores, pore-filling solid hydrocarbon, and very minor quartz overgrowths associated with local packing flaws around silt-size detrital quartz grains. Matrix-dispersed intergranular cementation, as observed in well-known organic-rich marine mudstones, such as the Barnett Shale and the Eagle Ford Formation, is not observed in Yanchang Formation lacustrine mudstones. The authigenic features present are consistent with the thermal maturity of the units ([Formula: see text]) and are broadly similar to features observed in other mudstones that contain grain assemblages dominated by particles of extrabasinal derivation. The low porosity and the absence of significant amounts of intergranular cement indicate that compactional porosity loss and in-filling by migrated solid hydrocarbon were the major causes of porosity decline during diagenesis of Yanchang Formation mudrocks. Although the mudstones of the Yanchang Formation have a relatively high content of organic carbon and serve as source rocks in the Ordos Basin, the depositional grain assemblage is not conducive to creation of porosity, permeability, and mechanical properties that would make these mudrocks effective unconventional reservoirs.

Critical evaluation of an Upper Carboniferous tight gas sandstone reservoir analog: Diagenesis and petrophysical aspects

Upper Carboniferous sandstones are one of the most important tight gas reservoirs in Central Europe. We present data from an outcrop reservoir analog (Piesberg quarry) in the Lower Saxony Basin of Northern Germany. This field-based study focuses on the diagenetic control on spatial reservoir quality distribution.The investigated outcrop consists of fluvial 4th-order cycles, which originate from a braided river dominated depositional environment. Westphalian C/D stratigraphy, sedimentary thicknesses and exposed fault orientations (NNW-SSE and W-E) reflect tight gas reservoir properties in the region further north. Diagenetic investigations revealed an early loss of primary porosity by pseudomatrix formation. Present day porosity (7% on average) and matrix permeability (0.0003 mD on average) reflect a high-temperature overprint during burial. The entire remaining pore space is occluded with authigenic minerals, predominantly quartz and illite. This reduces reservoir quality and excludes exposed rocks as tight gas targets. The correlation of petrographic and petrophysical data show that expected facies-related reservoir quality trends were overprinted by high-temperature diagenesis. The present day secondary matrix porosity reflects the telogenetic dissolution of mesogenetic ankerite cements and unstable alumosilicates.Faults are associated with both sealed and partially sealed veins near the faults, indicating localized mass transport. Around W-E striking faults, dissolution is higher in leached sandstones with matrix porosities of up to 26.3% and matrix permeabilities of up to 105 mD. The dissolution of ankerite and lithic fragments around the faults indicates focused fluid flow. However, a telogenetic origin cannot be ruled out.The results of this work demonstrate the limits of outcrop analog studies with respect to actual subsurface reservoirs of the greater area. Whereas the investigated outcrop forms a suitable analog with respect to sedimentological, stratigraphic and structural inventory, actual reservoirs at depth generally lack telogenetic influences. These alter absolute reservoir quality values at the surface. However, the temperature overprint and associated diagenetic modification, which caused the unusually low permeability in the studied outcrop, may pose a reservoir risk for tight gas exploration as a consequence of locally higher overburden or similar structural positions.

Petrography and geochemistry of upper Triassic sandstones from the Tumengela Formation in the Woruo Mountain area, North Qiangtang Basin, Tibet: Implications for provenance, source area weathering, and tectonic setting

AbstractThe petrography and major and trace element concentrations of the sandstones from the Tumengela Formation in the Woruo Mountain area, North Qiangtang Basin, are studied to determine their provenance, intensity of weathering and tectonic setting. The detrital compositions of the Tumengela sandstone samples are dominated by quartz (58.0–70.1 %, average 64.7 %) and lithic fragments (21.8–35.9 %, average 27.3 %), but low in feldspar content (4.9–12.9 %, average 8.0 %). The sandstones can be classified as litharenite and feldspathic litharenite according to their detrital compositions, which is consistent with the geochemical data. The detrital modal compositions reflect that these sandstones are probably derived from a recycled orogenic source. The index of chemical variability (ICV) and SiO2/Al2O3 ratio values suggest that the compositional maturity and recycling were moderate. The weathering indices such as the chemical index of alteration (CIA), plagioclase index of alteration (PIA), chemical index of weathering (CIW), and Al2O3–(CaO* + Na2O)–K2O (A–CN–K) diagram indicate that the intensities of weathering in the source area were moderate. The Al2O3/TiO2, Th/Co, La/Sc, La/Co, Th/Sc, Cr/Th ratio values and the discriminant function of the Tumengela sandstones indicate that the sediments were mainly derived from felsic source rocks, while also mixed with intermediate source rocks. The comparison of rare earth element patterns and its Eu anomalies to the probable source rocks infer that the sandstones were derived from the combination of granite, rhyolite, dacite, and gneisses. The proximal central uplift belt was probably the primary provenance area as evidenced by the petrographical and geochemical features of the Tumengela sandstones. The multidimensional tectonic discrimination diagram based on major elements show a collision setting (80 %) combined with a rift setting (20 %) for the Tumengela sandstones, which is consistent with the general geology of the study areas.