Glauconite Grains Research Articles

Origin of the glauconite from the Middle Eocene, Qarara Formation, Egypt

Although the Middle Eocene sediments at the Nile Valley, Egypt have been extensively studied, the origin of glauconites in these sediments is still questionable. In this paper, the glauconites that occur in the Qarara Formation were subjected to petrographical and mineralogical studies to assess their origin.Glauconite pellets or grains have been separated from the glauconitic sandy units of the studied three sections. They are present in different colors and different forms. X-ray diffraction analysis indicated that bulk samples are composed of mixed – layer clays (illite-montmorillonite), quartz, microcline, and albite. The separated glauconite grains are present in two shapes, the first is spherical and the second is elliptical, both are green in color with different grades. They are authigenic in origin as evidenced by high sorting, presence of deep or surficial fractures, and as supported to form by alteration of fecal pellets. They are also less mature as evidenced by their pale green color and by the higher expandable layer contents in their mixed-layer clay composition.

Source areas of the Grybów sub-basin: micropaleontological, mineralogical and geochemical provenance analysis (Outer Western Carpathians, Poland)

Abstract The Grybów Unit occurring in the Ropa tectonic window was the subject of micropaleontological and geochemical investigation. Studies, based on calcareous nannofossils, proved that the level of reworked microfossil is not higher than 22 % and it varies between two sections. Quantitative analyses of the reworked assemblages confirmed the domination of Cretaceous and Middle Eocene species. The Sub-Grybów Beds, Grybów Marl Formation and Krosno Beds were assigned to the Late Oligocene and represent the terminal flysch facies. Detrital material accumulated in the Oligocene sediments originated from the Marmarosh Massif, which is the eastern prolongation of the Fore-Magura Ridge. The microscopically obtained petrological features agree with the chemical composition of the samples. Mica flakes, rounded grains of glauconite, heavy mineral assemblage, including abraded grains of zircon, rutile and tourmaline as well as charred pieces of plant tissues are reworked components. Enrichment in zircon and rutile is confirmed geochemically by positive correlation between Zr and SiO2. Zr addition is illustrated on 10×Al2O3–Zr–200×TiO2 and Zr/Sc vs. Th/Sc diagrams. Interpretation of the A–CN–K diagram and variety of CIA and CPA values indicate that the source rocks were intensely weathered granite-type rocks.

Structural-chemical features and morphology of glauconites in sedimentary iron ore of Bakchar prospect (Western Siberia)

The research embraces the investigation results of glauconites in Bakchar iron ore occurrences to evaluate the potential diversified commercial application of this mineral. The following lab methods were used to analyze the morphology, chemical composition and structure of glauconites: granulometric analysis, optical microscopy, electron microscopy, X-ray fluorescence analysis, atomic arc-emission analysis and infrared spectroscopy. Glauconite was classified according to morphology and grain color and chemical composition and some specific characteristics were also determined (relative content of absorbed water, random distribution of smectite flakes within the grain structures). The research results showed that pistacho-green glauconite grains are less subjected to alteration than greenish-yellow grains due to the content of potassium, iron, absorbed water and organic impurities.

Mineralogical transformations in sandstone: a fingerprint for prehistorical heating of Palaeolithic hearth stones

At the prehistoric site of Rosalinde in Ruien (Belgium) sandstone blocks of local origin have been found in a configuration that was tentatively interpreted by the archaeologists as a hearth. These sandstones are identified as ferruginous sandstone and glauconiferous sandstone, and each block shows a discoloured rim which is not typically observed on these lithotypes. In order to confirm their use as hearth stones, the mineralogy in the discoloured rim was studied and compared to the bulk mineralogical composition by means of optical petrography, X-ray diffraction (XRD), differential scanning calorimetry (DSC) and dithionite-citrate bicarbonate extraction (DCB). In the ferruginous sandstone, XRD and DSC illustrate the transformation of amorphous iron oxides and goethite into hematite. In the glauconiferous sandstone, however, the amount of iron oxide phases is too low for detection with XRD and DSC. In this case, the dark brown discolouration of the glauconite grains, together with the higher free iron oxide content in the rim than in the bulk as indicated by DCB analysis, are most suggestive of a mineralogical transformation due to heating. The heating temperature is estimated between 300 and 600°C. Despite the absence of charcoal, the combined evidence obtained on the two lithotypes suggests the use of this specific stone arrangement as a hearth at the prehistoric site of Ruien.

Integrated Paleoenvironmental Reconstruction and Taphonomy of a Unique Upper Cretaceous Vertebrate-Bearing Locality (Velaux, Southeastern France).

The Velaux-La Bastide Neuve fossil-bearing site (Bouches-du-Rhône, France) has yielded a diverse vertebrate assemblage dominated by dinosaurs, including the titanosaur Atsinganosaurus velauciensis. We here provide a complete inventory of vertebrate fossils collected during two large-scale field campaigns. Numerous crocodilian teeth occur together with complete skulls. Pterosaur, hybodont shark and fish elements are also represented but uncommon. Magnetostratigraphic analyses associated with biostratigraphic data from dinosaur eggshell and charophytes suggest a Late Campanian age for the locality. Lithologic and taphonomic studies, associated with microfacies and palynofacies analyses, indicate a fluvial setting of moderate energy with broad floodplain. Palynomorphs are quite rare; only three taxa of pollen grains occur: a bisaccate taxon, a second form probably belonging to the Normapolles complex, and another tricolporate taxon. Despite the good state of preservation, these taxa are generally difficult to identify, since they are scarce and have a very minute size. Most of the vertebrate remains are well preserved and suggest transport of the carcasses over short distances before accumulation in channel and overbank facies, together with reworked Aptian grains of glauconite, followed by a rapid burial. The bones accumulated in three thin layers that differ by their depositional modes and their taphonomic histories. Numerous calcareous and iron oxides-rich paleosols developed on the floodplain, suggesting an alternating dry and humid climate in the region during the Late Campanian.

Berthierine formation in reservoir rocks from the Siri oilfield (Danish North Sea) as result of fluid–rock interactions: Part I. Characterization

Berthierine as the only authigenic Fe-rich clay mineral occurs in an oil-bearing, glauconite-rich sandstone (Paleocene Heimdal Member of the Lista Formation, Danish North Sea). Based on the results of thin section, transmission and scanning electron microscopy, five different morphological types of berthierine were identified: (1) mainly as grain-coatings partly together with authigenic quartz covering detrital grains, (2) as pore-filling clay in open pores, (3) small crystals in transformed glauconite grains, (4) small crystals associated with siderite in layers of mica/micaceous glauconite, and (5) as elongated crystals in siderite concretions. Although different in occurrence, all berthierine types investigated by microprobe analysis are of similar chemical composition, suggesting similar precipitation conditions. Investigations by transmission electron microscopy reveal that fine berthierine crystals display straight parallel layers with a typical lattice fringe spacing of 0.7 nm, and that their occurrence is intimately connected to oil phases in pore space. Partly, berthierine textures resemble mineralized oil–water emulsions. Berthierine is more abundant in the oil leg than in the water leg, but occurs especially abundant at the oil–water contact. The coexistence of berthierine together with quartz overgrowth, but also its appearance in glauconite indicates that berthierine formed in situ and that it is a product of glauconite dissolution. Berthierine formation is coupled to the reduction of aqueous Fe3+ into Fe2+ derived from glauconite dissolution, and is related to fluid–rock interactions triggered by oil degradation which occurred in the oil-filled zone and at the oil–water contact. The biomarker parameters pristane/n-C17 and phytane/n-C18 indicate an increased extent of oil degradation at the oil–water contact. On the one hand, soluble oil degradation products (methane, carbon dioxide, and hydrogen) changed the hydrogeochemical conditions (such as pH, pe, alkalinity, etc.), and controlled dissolution of glauconite (and feldspar). On the other hand, the changed hydrogeochemical conditions led to berthierine and quartz precipitation in water-wet reservoir intervals.

Tidal influence in redbeds: A palaeoenvironmental and biochronostratigraphic reconstruction of the Lower Tremp Formation (South-Central Pyrenees, Spain) around the Cretaceous/Paleogene boundary

The Upper Cretaceous–Paleogene deposits of the Grey Unit, the Lower Red Unit and the Suterranya Limestone of the Tremp Formation (South-Central Pyrenees) preserve sedimentological and palaeontological features that suggest a connection with marine settings and which allow the age of these deposits to be reviewed. The deduced sedimentary environments for the Grey Unit and the Suterranya Limestone – lagoonal and coastal lake environments respectively – agree with those proposed by previous authors. However, a new depositional setting is proposed for the Lower Red Unit, that of a mixed-energy zone in a tide-dominated delta. This new interpretation is based on the presence of inclined heterolithic stratification in point bar deposits, along with flaser and lenticular bedding and the presence of in situ marine fossils – including those of planktonic foraminifera – and glauconite grains. These foraminifera were almost certainly transported, as a suspended load, from the open sea into meandering channels on flood tides. Their associated biostratigraphy suggests a Maastrichtian age for the Grey Unit and Lower Red Unit, and a late Danian age for the Suterranya Limestone. There is a significant hiatus between latter and former units that covers most of the Danian, and presumably includes the Cretaceous/Paleogene boundary.

Warming, euxinia and sea level rise during the Paleocene–Eocene Thermal Maximum on the Gulf Coastal Plain: implications for ocean oxygenation and nutrient cycling

Abstract. The Paleocene–Eocene Thermal Maximum (PETM, ~ 56 Ma) was a ~ 200 kyr episode of global warming, associated with massive injections of 13C-depleted carbon into the ocean–atmosphere system. Although climate change during the PETM is relatively well constrained, effects on marine oxygen concentrations and nutrient cycling remain largely unclear. We identify the PETM in a sediment core from the US margin of the Gulf of Mexico. Biomarker-based paleotemperature proxies (methylation of branched tetraether–cyclization of branched tetraether (MBT–CBT) and TEX86) indicate that continental air and sea surface temperatures warmed from 27–29 to ~ 35 °C, although variations in the relative abundances of terrestrial and marine biomarkers may have influenced these estimates. Vegetation changes, as recorded from pollen assemblages, support this warming. The PETM is bracketed by two unconformities. It overlies Paleocene silt- and mudstones and is rich in angular (thus in situ produced; autochthonous) glauconite grains, which indicate sedimentary condensation. A drop in the relative abundance of terrestrial organic matter and changes in the dinoflagellate cyst assemblages suggest that rising sea level shifted the deposition of terrigenous material landward. This is consistent with previous findings of eustatic sea level rise during the PETM. Regionally, the attribution of the glauconite-rich unit to the PETM implicates the dating of a primate fossil, argued to represent the oldest North American specimen on record. The biomarker isorenieratene within the PETM indicates that euxinic photic zone conditions developed, likely seasonally, along the Gulf Coastal Plain. A global data compilation indicates that O2 concentrations dropped in all ocean basins in response to warming, hydrological change, and carbon cycle feedbacks. This culminated in (seasonal) anoxia along many continental margins, analogous to modern trends. Seafloor deoxygenation and widespread (seasonal) anoxia likely caused phosphorus regeneration from suboxic and anoxic sediments. We argue that this fueled shelf eutrophication, as widely recorded from microfossil studies, increasing organic carbon burial along many continental margins as a negative feedback to carbon input and global warming. If properly quantified with future work, the PETM offers the opportunity to assess the biogeochemical effects of enhanced phosphorus regeneration, as well as the timescales on which this feedback operates in view of modern and future ocean deoxygenation.

The uppermost deposits of the stratigraphic succession of the Farafra Depression (Western Desert, Egypt): Evolution to a Post-Eocene continental event

This paper gives insight into continental sedimentary deposits that occur at the uppermost part of the stratigraphic succession present in the north-eastern sector of the Farafra Depression (Western Desert, Egypt). Using space imagery to complete the field work, the geology of the area has been mapped and the presence of a N–S oriented fault system is documented. The analysis of the morphotectonic features related to this fault system allows reconstructing the structural and sedimentological evolution of the area. The study indicates that the continental deposits were accumulated in alluvial systems that unconformably overlie shale and evaporitic rocks attributable to the Paleocene–Eocene Esna Formation. The deposits of the Esna Formation show soft-sediment deformation features, which include slump associated to dish and pillar sedimentary structures and provide evidence of syndepositional tectonic activity during the sedimentation of this unit. The outcrops are preserved in two areas on separated fault-bounded blocks. Proximal alluvial fan facies crop out in a dowthrown block close to the depression boundary. The proximal facies are made up mostly by polymictic conglomerates which occasionally contain boulders. The conglomerate clasts are mainly quartz, carbonate, anhydrite satin spar vein, mudrock, ironstone and nummulite fossils. The mid-fan facies consist of trough cross-bedded, rippled and cross-laminated quartzarenites with reworked glauconite grains and carbonate rock fragments, interpreted as deposited by distributary streams. The distal alluvial fan deposits consist of sandy marls that evolve toward the top of the sections into root-bioturbated lacustrine limestone beds that are locally silicified. The limestones are biomicrites containing characea, ostracods and gastropods with fenestral porosity.A number of features, including clast provenance (mainly from marine Paleocene and Eocene rocks), the observed fractural pattern (N–S direction related to the opening of the Red Sea), and the sedimentary relationships, suggests that the continental deposits were accumulated during the Oligocene–Miocene interval.

The palaeomagnetism of glauconitic sediments

The palaeoenvironmental significance of glaucony has long been appreciated, but accurate palaeomagnetic dating of events recorded by glauconitic horizons requires an understanding of how glauconitic sediments acquire a remanent magnetization. Pure glauconitic minerals are paramagnetic, but glauconite grains are large and slow-forming (over periods that can exceed 100kyr), with complex and variable morphologies. It is, thus, possible that small magnetic grains within glaucony particles may carry a significant fraction of the remanence in weakly magnetized sediments. Any remanence carried by glauconitic grains may therefore represent the geomagnetic field at a time significantly later than the time of deposition, or a time-averaged signal over some or all of the formation period. We investigated this problem using weakly magnetic Palaeocene glauconitic siltstones from southern New Zealand. We disaggregated the rock and separated it magnetically into glauconitic and non-glauconitic fractions. Results from stepwise isothermal remanent magnetization (IRM) acquisition, alternating-field demagnetization, temperature dependence of magnetic susceptibility, and stepwise thermal demagnetization of a triaxial IRM were used to demonstrate that the remanent magnetization is carried by single-domain or pseudo-single-domain magnetite in the non-glauconitic sediment fraction, and that the glauconite grains themselves make no contribution to the remanent magnetization. However, accurate measurement of the primary remanence is complicated by a strong viscous overprint and mineral alteration during thermal demagnetization studies. Identification of magnetite as the remanence carrier in sediments within a reducing diagenetic environment gives confidence that the remanence has a depositional origin. Glauconite does not carry a remanence; therefore, its effect is to dilute and weaken the overall magnetization. Furthermore, the use of rock magnetic parameters may be problematic when glauconite concentrations are (as in the studied sediments) orders of magnitude greater than remanence carrier concentrations, because in such cases the glauconite susceptibility can dominate that of the remanence carriers.

Framboidal and idiomorphic pyrite in the upper Maastrichtian sedimentary rocks at Gabal Oweina, Nile Valley, Egypt: Formation processes, oxidation products and genetic implications to the origin of framboidal pyrite

The upper Maastrichtian organic-rich sediments studied at Gabal Oweina, Egypt, are moderately enriched in syngenetic and diagenetic pyrite. Pyrite occurs mostly as layers or bands, group of lamina, lenses, diagenetic intercalated pockets, burrow fills and disseminated individual pyrite framboids and crystals within the host sediments. The pyritic thin bands and lamina consist mostly of unconsolidated to compact-oriented pyrite (oriented along the bedding planes) in gypsiferous–clayey matrix and less common as poorly oriented pyrite crystallites. In several cases, pyrite crystals of the latter type depict zoning, fracturing and micro-concretions. Pyritic burrow fills are composed mainly of pyrite, phosphatic ooids, microfossils, glauconitic grains, poorly graphitized carbon and native sulfur. Pyrite replaces minerals other than gypsum, sulfur or carbon. It also replaces microfossils thus turning some of the phosphatic ooids and microfossils to pyritized pseudomorphs. None of the studied phosphate ooids or framboids contains any mackinawite, pyrrhotite or greigite. Based on the microscopic and SEM observations of the micro-textures of disseminated pyrite found at Gabal Oweina section, four morphological forms of primary pyrite could be identified: (1) Grouped multiple-framboids; (2) Individual framboids; (3) Pyrite idiomorphic crystal overgrowths on framboids and (4) Single and aggregates of euhedral pyrite crystals. The multiple-framboid formation may have emerged from three successive processes: nucleation and growth of individual aggregates of the microcrystals to form combined micro-framboids (the growth of framboids); and followed by grouping of the several pyrite framboids. Direct pyrite nucleation (shell formation), crystallization, and aggregation processes might complete a single framboid. The disseminated single and aggregated euhedral pyrite crystals bear evidence indicating that their formation was via nucleation and growth of pyrite crystallites and their aggregation (to individual framboids), infilling (in the interstices by additional pyritic material), compaction and homogenization (of all these materials). Furthermore, we encounter for the first time in nature idiomorphic pyrite crystals that integrated numerous framboids, using them as their nucleation and growth sites without erasing or modifying their pristine morphology. Elemental sulfur contains minor concentration of Sb, Ni, Cd and Cu strongly suggesting their presence as submicron sulfide crystallite inclusions. SEM and microprobe investigations revealed that goethite is present as a weathering product in all morphological types of pyrite however; only an iron-sulfate (presumably melanterite) was encountered as oxidation product of the multi-framboids and the euhedral aggregate crystals. The upper Maastrichtian sediments not only contain a menagerie of pyrite morphologies but probably a complete record of the formation process and the geochemical conditions of the growth of framboids, single pyrite crystals, pyrite burrows, pyritized Mn–Fe-oxide framboids and finally their weathered products. The various pyrite forms strongly suggest a multistage process that led to their formation without any evidence for mackinawite, pyrrhotite or greigite, precursors. There is also no evidence in the Oweina sediments for post pyrite formation of mackinawite, pyrrhotite or greigite. The presence of elemental sulfur containing minor concentrations of Sb, Ni, Cd and Cu with pyrite framboids indicates that the pore solutions were geochemically supersaturated in sulfur thus inhibiting the crystallization of any iron sulfide other than pyrite. This cast considerable doubt on the assumed mackinawite or greigite precursors as prerequisite for formation of pyrite framboids.

Facies and sedimentation of Coniacian deposits of the Kraków Swell in the Wielkanoc area (southern Poland)

Coniacian deposits, ca. 1.5 m thick, cropping out in the Wielkanoc Quarry, north of Krakow in southern Poland, consist of firm, nodular and, less commonly, marly limestones with horizons of in situ, slightly phosphatized hexactinellid sponges and thick-shelled inoceramid bivalves. The succession is composed of foraminiferal-inoceramid packstones with common sand-sized quartz and glauconite grains at the base, passing upwards into foraminiferal or foraminiferal-inoceramid wackestones with or without rare glauconite. A microfacies analysis shows that planktonic foraminifers are the dominant forms, while benthic forms are rare. The facies indicates that sedimentation in the Wielkanoc area on the Krakow Swell, which separated the deeper Mid-Polish Trough Zone to the north-east and the Opole Trough Zone to the south-west, was generally calm (documented by abundant wackestones) and slow (indicated by the dominant sedimentary “coccolith system” and presence of glauconite) during the Coniacian. Rare episodes of non-deposition are recorded by episodes of phosphatization and minor intra-Coniacian discontinuity surfaces. The presence of hexactinellid sponges in the section studied are consistent with a calm environment, below the storm-wave base, with low rates of sedimentation. Subhercynian (latest Turonian–Coniacian Ilsede Phase) local tectonic movements had an important influence on the evolution of the region. They presumably led to subsidence of the Wielkanoc Block during the Early Coniacian. These movements were probably associated with activity on the Krakow–Myszkow Fault Zone

Effects of Weathering on Glauconite: Evidence from The Abu Tartur Plateau, Egypt

AbstractRecognizing weathering effects is significant for any work carried out on glauconites at the surface. The mineralogy and chemistry of glauconite grains exposed to weathering in a hot arid climate for a maximum of 42 y were studied here. The objective of the study was to find the mineralogical and chemical differences between weathered glauconite from the surface and fresh glauconite from the subsurface.One specific glauconite-bearing layer at the surface (Layer 16) of the Abu Tartur phosphate mine, located in the Western Desert of Egypt, was studied in detail and compared to a fresh, subsurface glauconitic sandstone from the underground mine.Even within this single surface layer, the brownish-green glauconite grains vary in color and chemical composition. From top to bottom, the grains show an increase in Fe and K and a decrease in Al and S. In addition, the grains show an internal color zonation caused by variation of Fe and K contents between the center and rim of the grains. The differences in color and chemical composition are even more pronounced between the weathered-glauconite grains from the surface and the fresh glauconite grains from the subsurface which are dark green and enriched in Fe and K.The clay fractions consisted of mixed-layer glauconite (illite)-smectite, with the surface samples containing more expandable smectite (50%) than the subsurface samples (20%). In the charge-distribution diagram for muscovite-pyrophyllite-celadonite, the weathered glauconites at the surface showed a clear trend from smectitic glauconite at the top to illitic glauconite at the bottom of the layer, whereas the fresh subsurface sample plotted exactly in the glauconite field.The color, mineralogy, and chemistry indicate that the surface samples were strongly altered by weathering processes and that glauconite transformed progressively into Fe-rich mixed-layer illite-smectite and then into smectites.Weathering can thus completely reverse the glauconitization process. For any chemical and mineralogical characterization of glauconites at the surface, these weathering effects must be taken into consideration.

The filament event near the Cenomanian-Turonian boundary in Tunisia: filament origin and environmental signification

AbstractNear the Cenomanian-Turonian boundary, the filament event corresponds to the massive occurrence of thin elongate shells, probably bivalves which constitute, in the Bahloul Formation, about 40% of allochems. This biosedimentary event was described in several areas, such as in Tunisia, Algeria, Morocco, North America, Oman and Tibet. In Tunisia, filaments are well represented within well laminated black shales constituting the Bahloul Formation, late Cenomanian – early Turonian in age. These black shales were deposited under anoxic conditions developed in an outer ramp environment. Filaments are particularly abundant in the upper part of the Bahloul Formation, which is topped by a condensation level exhibiting encrusted ammonites associated with frequent burrows, and phosphatic and glauconitic grains. In terms of depositional environments, the filament concentrations announce an evolution towards “normal”-marine conditions, which followed the anoxic conditions favouring black-shale formation. This trend coincided with a deepening that led to maximum flooding at the condensation surface. The petrographic study of samples taken along a N-S transect shows a clear evolution of filament structures from distal to proximal ramp. Limestones deposited in the outer ramp environment mainly contain well preserved “pelagic” bivalve shells showing a thin-shelled structure composed of elongated filaments. Laterally, toward the proximal ramp, the bivalve shells are progressively disintegrated, the filaments being arranged parallel to bedding. In an even more proximal setting, filaments appear finer, highly fragmented and arranged without preferential orientation. The Cenomanian-Turonian “pelagic” bivalves thus probably lived in an outer ramp environment and their thin shells disintegrated progressively into filaments. This happened during high energy conditions generated by storms.The filament event, which is associated to other bio-events expressed by the extinction and the first occurrence of some pelagic species, could be used to establish local to global correlations.When the ammonite Watinoceras, the main early Turonian marker, is rare or absent, the filament-rich facies could indicate the proximity of the Cenomanian-Turonian boundary.At a global scale, the filament event illustrates a sea-level rise in relation with eustatic and climatic changes. The maximum flooding of this transgressive event was identified in several localities and dated about 93 Ma. The very high sea-level conditions are favourable to accumulation and preservation of organic-rich facies such as black shales, which illustrate the anoxic event OAE2.The abundance of filaments near the Cenomanian-Turonian boundary is an indicator of high organic productivity following a climatic change to warmer conditions. The latter have clearly influenced oceanic water-masses circulation, water column stratification and nutrient partitioning.

Petrophysical properties of greensand as predicted from NMR measurements

ABSTRACT Nuclear magnetic resonance (NMR) is a useful tool in reservoir evaluation. The objective of this study is to predict petrophysical properties from NMR T 2 distributions. A series of laboratory experiments including core analysis, capillary pressure measurements, NMR T 2 measurements and image analysis were carried out on sixteen greensand samples from two formations in the Nini field of the North Sea. Hermod Formation is weakly cemented, whereas Ty Formation is characterized by microcrystalline quartz cement. The surface area measured by the BET method and the NMR derived surface relaxivity are associated with the micro-porous glauconite grains. The effective specific surface area as calculated from Kozeny9s equation and as derived from petrographic image analysis of backscattered electron micrograph9s (BSE), as well as the estimated effective surface relaxivity, is associated with macro-pores. Permeability may be predicted from NMR by using Kozeny9s equation when surface relaxivity is known. Capillary pressure drainage curves may be predicted from NMR T 2 distribution when pore size distribution within a sample is homogeneous.

Reconciling strontium-isotope and K–Ar ages with biostratigraphy: the case of the Urgonian platform, Early Cretaceous of the Jura Mountains, Western Switzerland

During the late Early Cretaceous, the shallow-water domains of the western Tethys are characterized by the widespread deposition of Urgonian-type carbonates rich in rudists, corals and other oligotrophic, shallow-marine organisms. In the Helvetic Alps, the Urgonian occurrences have been dated by ammonite biostratigraphy as Late Barremian and Early Aptian. For the more proximal occurrences in the western Swiss Jura, a recent age model based on bio-, chemo- and sequence stratigraphy has been proposed, which allows for an improved correlation with the Helvetic counterparts. In order to corroborate the recently proposed age model for the Jura, a set of well-preserved rhynchonellids collected from five different lithostratigraphical formations and members (“Marnes bleues d’Hauterive”, “Marnes d’Uttins”, basal marly layers within the “Urgonien Jaune”, “Marnes de la Russille”, “Urgonien Blanc”) has been analysed for its strontium–isotope ratios (87Sr/86Sr). In addition, K–Ar dating was performed on well-preserved glauconite grains from two different levels (“Marnes d’Uttins” and a basal layer within the “Urgonien Jaune”). The correlation of the Sr–isotope data set with a belemnite-based, ammonite-calibrated reference curve provides an age model which is coherent with recently published ages based on calcareous nannofossil biostratigraphy and the correlation of trends in chemo- and sequence stratigraphy. K–Ar dating on well-preserved glauconite grains from the “Marnes d’Uttins” and lowermost part of the “Urgonien Jaune” delivered ages of 127.5 ± 2.3 and 130.7 ± 2.6 Ma, respectively. Whereas the age of the glauconitic level near the base of the “Urgonien Jaune” is chronostratigraphically meaningful, the K–Ar age of the “Marnes d’Uttins” appears too young relative to the presently used time scale. This may be related to rejuvenation of the K–Ar chronometer due to post-depositional Ar loss, most likely during hardground formation. The ages obtained here confirm the Late Barremian age for the onset of the Urgonian platform, an age which is conform with ages obtained in the Helvetic Alps and elsewhere along the northern Tethyan margin.

Mixed glauconitic-carbonate-siliciclastic surficial sediments on the north Kaipara continental margin, northwestern North Island, New Zealand

A mosaic of siliciclastic and mixed carbonate-siliciclastic sediments and authigenic minerals occurs at shelf and slope depths (30–1015 m water depth) on the open, wave-dominated north Kaipara continental margin (NKCM) off northern New Zealand. Texture and composition define five surficial sediment facies. Facies 1 (siliciclastic sand) comprises generally well-sorted fine sands that extend to outer shelf depths. Facies 2 (glauconitic sand) is composed of 30–95% authigenic glauconite grains at 150–400 m water depth in central to northern portions of NKCM. Facies 3 (mixed bryozoan-siliciclastic sand) occurs only in northernmost NKCM and involves a conspicuous (>40%) bryozoan carbonate content. Facies 4 (pelletal mud) occupies the mid shelf (100–150 m water depth) in northern NKCM and consists of muddy sediment with >30% mixed carbonate-siliciclastic pellets of probable fecal origin. Facies 5 (foraminiferal mud and sand) contains >30% foraminiferal tests at slope depths in southern NKCM and at both slope and mid-outer shelf depths towards the north. The siliciclastic mineralogy is consistent with mainly distant provenances to the south of the NKCM in central North Island and northern South Island, while much of the bryozoan material is likely reworked from the Three Kings carbonate platform to the north. No single shelf sedimentation model explains the complex facies distributions on the NKCM because the deposits record the interplay of several present and past hydrodynamic and sediment supply controls. While truly modern deposits may occur inshore, the bulk of NKCM surficial sediments are mainly palimpsest and/or relict deposits.

An extensive erosion surface of a strongly deformed limestone bed in the Gushan and Chaomidian formations (late Middle Cambrian to Furongian), Shandong Province, China: Sequence–stratigraphic implications

In order to understand sequence development and sea-level fluctuations during the late Middle Cambrian to early Furongian on the North China epeiric platform, the present study focuses on a unique, subtle erosion surface of an extensive (approx. 100 km), strongly deformed limestone bed in the uppermost part of the Gushan Formation, Shandong Province, China. The Gushan Formation and the overlying Chaomidian Formation consist mainly of shales and a variety of carbonates that were deposited in subtidal environments (e.g., deep subtidal, shallow subtidal, shoreface/shoal, subtidal microbial flat, and restricted platform interior). Three third-order depositional sequences (S1–3) are identified, each of which comprises a thin transgressive systems tract (TST) and a relatively thick highstand systems tract (HST). Each sequence is bounded by a drowning unconformity (SB1), a subaerial unconformity (SB2), or a surface of submarine erosion (SB3). The upper sequence boundary (SB2) of sequence 1 (S1) is represented by a subtle erosion surface of an extensive, deformed limestone bed with a wide variety of soft-sediment deformation structures (e.g., lime mudstone breccias, chaotic wacke-packstone laminae and fragments, homogenized oolites, and clastic dykes), and is overlain by small sporadic microbial buildups and an extensive bioclastic grainstone bed. The deformed limestone was formed during early diagenesis by differential deformation processes (brecciation, liquefaction/fluidization, and injection) which were most likely induced by pore-water overpressure during the period of rapid sea-level fall. Despite the lack of subaerial exposure features (e.g., paleokarst, paleosol, etc.), the characteristics of the erosion surface (cutting well-lithified sediment below), the missing of a significant geological record (the Prochuangia biozone), and the worldwide correlatable positive carbon isotope excursion collectively indicate that the erosion surface developed under conditions of subaerial exposure after contemporaneous marine cementation of the deformed sediment. The missing of the Prochuangia biozone is most likely due to non-deposition at a subaerial hiatal surface. The erosion surface was submerged as a result of subsequent rise in sea level, where sporadic microbial buildups formed under suitable conditions. Freshly deposited, winnowed, shell-dominated transgressive lag deposits (containing Chuangia trilobite fragments, brachiopod shells, and abundant glauconite grains) formed with continued rise in sea level, which became, in turn, overlain by shale-dominated facies. The unique combination of the subtle erosion surface ( sensu stricto a subaerial unconformity) and the underlying deformed limestone bed provides an important criterion for recognizing the subtle changes in relative sea level on shallow epeiric platforms.

Nature of the structural and crystal-chemical heterogeneity of the Mg-rich glauconite (Riphean, Anabar Uplift)

The detailed mineralogical and structural-crystal-chemical characteristics are reported for the first time for glauconite grains extracted from the fine-platy silty-sandy dolomites at the roof of the lower subformation of the Yusmastakh Formation (Riphean, Anabar Uplift, North Siberia). Based on the complex study (X-ray diffraction, classical chemical analysis, microprobe analysis, IR-spectroscopy, thermogravimetric analysis, scanning electron microscopy with microprobe analysis, and Mossbauer spectroscopy), it was demonstrated that the studied glauconite sample is characterized by unique chemical and structural heterogeneity.

Submicroscopic accessory minerals overprinting clay mineral REE patterns (celadonite–glauconite group examples)

Samples of glauconite, representing different stages of glauconitisation, as well as different formation environments, were analysed for rare earth elements (REE) and other trace elements using a combination of bulk sample and spatially-resolved in situ techniques. The results indicate that the high-sensitivity, spatially-resolved technique of laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) produces values up to two orders of magnitude lower than the bulk sample analyses. This suggests that submicroscopic rare earth element-bearing phases are distributed within the glauconite aggregates comprising the bulk samples. Analytical scanning electron microscopy (ASEM) revealed the presence of micrometre-sized grains of apatite and pore filling precipitates of an unidentified (REE, Ca)-phosphate (approximate composition Ca 0.3–0.4(Ce 0.4La 0.1–0.2Nd 0.1)PO 4) in some glauconite grains. The inherent REE concentrations of the glauconite aggregates (i.e., glauconite crystallites without accidental mechanical inclusions or authigenic, not layer silicate mineral precipitates) was found to be relatively low (e.g., typically less than 100 ppm), and this value decreased with increasing glauconitisation (smectite–mica transformation through a series of recrystallisation processes). These results suggest that the REEs substitute for Ca in the interlayer space of the layer silicate structure and, therefore, the REE content decreases as Ca is progressively removed from the interlayer (smectite–mica transition). LA-ICP-MS, when combined with electron probe microanalysis (EPMA) or ASEM, offers an opportunity to exclude submicroscopic accessory minerals from glauconite trace element analyses, and so produces reliable trace element data for the respective minerals which host those elements. These results illustrate that accessory minerals are difficult to eliminate from clay samples, and that care needs to be taken in the interpretation of clay mineral REE distributions, irrespective of the aggregation state of the studied clay (i.e., whether finely dispersed within the sedimentary rock, or forming millimetre-sized aggregates). Model calculations showed that authigenic apatite associated with the studied green marine clays tends to have higher REE content than “bioapatites”, the total REE content being above 10 000 ppm.