Lithic Clasts Research Articles

The Response of the Rivers of NW Greece to Late Quaternary Neotectonics, as Interpreted from Detrital Petrology

The modern drainage systems of the fold and thrust belt of the external Hellenide orogen of NW Greece are principally orogen-parallel. Late Quaternary changes in river courses have resulted from neotectonic deformation associated with the Katouna–Stamna fault and with footwall uplift in developing transverse grabens. This study assesses the impact of neotectonic deformation on river patterns and basin deposition. River sands show differences in modal abundance and varietal geochemistry of heavy minerals and fine sand lithic clasts, determined by scanning electron microscope, that allow identification of past river supply to raised fluvial terrace and beach deposits. In the past 200 ka, footwall uplift south of developing grabens at Lake Trichonis and the Amvrakikos Gulf promoted orogen-transverse flow, diverting the Arachthos-Louros rivers to the west, causing reversal of drainage to the north in the lower reach of the Acheloos River. A raised terrace gravel south of Preveza records the southwestward flow of a large paleo-Arachthos river, confirmed by sand petrology in beaches farther south on the Echinadon Sea coast. The use of varietal heavy minerals and lithic clasts is a rapid and powerful tool for tracking tectonically-induced changes in river patterns.

Composition and Petrology of a Mush-Bearing Magma Reservoir beneath Tenerife

AbstractDeciphering the dynamics of sub-volcanic magmatic processes requires a detailed understanding of the compositional and textural relationships between melt and crystals. To examine these relationships, we investigated material from one of the largest caldera-forming explosive eruptions on the ocean island of Tenerife, the 312-ka Fasnia event. This eruption ejected juvenile pyroclasts of melt-bearing, partially crystalline cumulate nodules alongside phonolitic pumice and accidental lithic clasts. Nodules contain an average of 26% melt that is preserved as vesiculated and microcrystalline basanite in segregations, pathways and interstitial domains. Both the microcrystalline groundmass and crystal framework are generally unaltered as this crystal ‘mush’ remained supra-solidus until the eruption. We find no surficial or intrinsic evidence that the nodules were transported from their reservoir in a ‘carrier’ magma, and it is most likely that the mush was in situ when it was explosively fragmented and ejected during eruption. As such, the nodules preserve a record of the proportions and relationships between the crystal framework and pre-eruptive melt in an active magma mush reservoir, importantly, capturing a snapshot of the sub-volcanic system at a single point in time. We have analysed >100 of the mush nodules from the massive lithic breccia facies within the Fasnia Member of the Diego Hernández Formation. These cumulates span a diverse range of alkaline plutonic lithologies, from wehrlite and pyroxenite, through hornblende gabbros, to monzodiorite and syenite. Their textures record a range of crystallization environments, including both crystal- and melt-rich groundmass domains, and invasion of near-solidus domains by ascending reactive melts. In addition, the cumulus phases record complex interactions between felsic and mafic magmas throughout their development, providing evidence for mush remobilization and disequilibrium. Relative homogeneity of melt compositions through the mafic and felsic lithologies testifies to melt mobility through the cumulates. Nevertheless, all melts are of different basanite-intermediate composition to the juvenile phonolitic pumice ejected during the same eruption. This observation implies that the mafic–felsic cumulate mush and the phonolite did not experience significant two-way mixing and existed as separate crustal reservoirs. However, the Fasnia eruption simultaneously fragmented and removed material from both reservoirs, implying the mafic system was subjacent to the felsic, but they did not form a contiguous body.

The Evolution of Permian Source-to-Sink Systems and Tectonics Implications in the NW Junggar Basin, China: Evidence from Detrital Zircon Geochronology

The basin type of the Junggar Basin changed during the Permian, but the time constraint of the tectonic evolution remains unclear. Besides, the fan deltas developed in the Permian in the Mahu Sag in the northwestern of the oil-rich basin. However, the provenances of the sedimentary systems remain unclear. Based on petrology and detrital zircon U-Pb ages, this study investigates the source-to-sink systems evolution and tectonics implications. Abundant lithic clasts in sandstones with low compositional and textural maturity imply proximal sources. The dating results showed a dominant peak (310–330 Ma) and a secondary peak (400–440 Ma) in the northern Mahu Sag, only one peak at 295–325 Ma in the central Mahu Sag, several peaks at 270–350 Ma in the southern Mahu Sag, and multiple peaks at 370–450 Ma in the Zhongguai Uplift. Thus, the north-western Junggar Basin was divided into four major source-to-sink systems, with adjacent central West Junggar as the main provenance and northern and southern West Junggar as the secondary provenance. The proportion of sediment supply from the southern and northern West Junggar is higher during the Middle-Late Permian. It suggests that the source-to-sink systems show inheritance and evolve from a single provenance into a complex provenance, indicating the uplift of West Junggar. The tectonic inversion may occur early in the Middle Permian and the response to tectonic activity is stronger in the southern West Junggar than in the northern West Junggar.

Machine learning for stone artifact identification: Distinguishing worked stone artifacts from natural clasts using deep neural networks.

Stone artifacts are often the most abundant class of objects found in archaeological sites but their consistent identification is limited by the number of experienced analysts available. We report a machine learning based technology for stone artifact identification as part of a solution to the lack of such experts directed at distinguishing worked stone objects from naturally occurring lithic clasts. Three case study locations from Egypt, Australia, and New Zealand provide a data set of 6769 2D images, 3868 flaked artifact and 2901 rock images used to train and test a machine learning model based on an openly available PyTorch implementation of Faster R-CNN ResNet 50. Results indicate 100% agreement between the model and original human derived classifications, a better performance than the results achieved independently by two human analysts who reassessed the 2D images available to the machine learning model. Machine learning neural networks provide the potential to consistently assess the composition of large archaeological assemblages composed of objects modified in a variety of ways.

Geochemistry of lunar meteorite Northwest Africa 11962 and its potential source region/crater in the Procellarum KREEP terrane

AbstractLunar meteorite Northwest Africa (NWA) 11962 is a regolith breccia with a diverse range of mineral and lithic clasts. For the present study, major and trace element contents and selected isotopic compositions were determined on a homogenized bulk powder of NWA 11962 by instrumental neutron activation analysis, thermal ionization mass spectrometry, and inductively coupled plasma mass spectrometry. The chemical composition of the sample points toward an origin of the meteorite from within the Procellarum KREEP terrane (PKT). Samarium‐Nd and Rb‐Sr isotopic compositions and concentrations show a similarity to those of Apollo 15 soils and KREEP basalt. Highly siderophile element (HSE) abundances and ratios, as well as the Re/Os isotopic system, are often used as tracers of different impactor types. The 187Os/188Os and 187Re/188Os isotopic ratios are within the range of ordinary chondrites, yet some of the siderophile element and highly siderophile element ratios are comparable to those of iron meteorites. Using Fe, Th, and Ti abundances of lunar surface regolith, measured by the Lunar Prospector gamma‐ray spectrometer, we attempt to constrain potential lunar source regions for NWA 11962. By matching these possible source regions with coordinates of recently (<1 Ma) formed lunar impact craters (so‐called lunar cold spots), we localized a potential source crater of NWA 11962 at the western rim of the PKT close to Sinus Medii.

Complex arrangement of early pyroclastic and intrusive rock records onset of intraplate Dunedin Volcano

Little is understood about the birth of composite volcanoes because products of early eruptive activity are buried by products of later eruptions. The oldest rocks of the Dunedin Volcano (active 16.0–11.0 Ma) are exposed at Otapahi (Allans Beach) and allow us to study the inception of a long-lived composite volcano. Seven rock units are visible at Otapahi, investigated with fieldwork and detailed componentry. A bedded deposit of phonolitic pumice clasts is vertically cross-cut by at least three diatreme deposits, as well as a compositionally diverse array of dikes emplaced penecontemporanously with the diatremes. The complicated cross cutting relationships among diatremes and different generations of dikes allow inference of the relative timing of events. Componentry revealed lithic clasts from the underlying Cenozoic sedimentary sequence and basement schist in the oldest diatreme, thus indicating it quarried to basement depths. A lack of coherent cognate igneous clasts in the oldest diatreme is also consistent with the earliest eruption in the area having formed the ~60 m thick bedded pumice deposit. Younger diatremes lack any identified lithic fragments from underlying strata but contain abundant basaltic clasts with compositions matching those of dikes injected penecontemporaneously. The increase of igneous clasts in the younger diatreme deposits, together with the decreasing number of cross-cutting dikes records a chaotic succession of diatreme formation and repeated dike intrusion. This interplay of exogeneous with endogenous processes built an early submarine eruptive centre, and initiated development of a long-lived intraplate composite volcano with activity that persisted across the next 5 million years.

Alkalic pyroclast morphology informs on fragmentation mechanisms, Trindade Island, Brazil

Quantitative morphological analysis of juvenile pyroclasts is commonly used to understand fragmentation mechanisms and eruptive styles. Pyroclast morphology has been chiefly studied on conventional tholeiitic and calc-alkaline suites, while the relationship is more poorly constrained for alkaline melt compositions. Here, we analyze the shape of high-Na alkalic juvenile pyroclasts from the Trindade Complex, Desejado, Morro Vermelho, and Paredão Formations on Trindade Island, Brazil, and classify particle populations using multivariate cluster analysis. We expand the field of particle morphology to low-viscosity alkaline suite and interpret fragmentation mechanisms and eruptive styles on products of dry magmatic and phreatomagmatic explosive eruptions. In contrast to tholeiitic and calc-alkaline series magmas, low-viscosity alkaline olivine nephelinitic compositions generate a wide variety of fragments. It includes achneliths, Pele's tears, spheres, sideromelane shards, and tachylyte, formed either by lava fountaining, Strombolian eruptions, or ash explosions, through ductile or ruptile magmatic and secondary fragmentation mechanisms. Phonolitic coarse ash and lapilli fall deposits are dominated by angular juvenile glassy pyroclasts with irregular contours and slightly rounded shapes resulting from dry magmatic fragmentation. The fine ash fall and surge phreatomagmatic deposits chiefly contain lithic clasts, crystal fragments, and blocky and angular juvenile glassy fragments.

Emplacement and eruptive style of high-grade ignimbrites from fissure vents: The Las Mellizas Ignimbrite, Caviahue-Copahue volcanic complex, southern Andes

The Pliocene to Recent Caviahue – Copahue volcanic complex is located in the Southern Volcanic Zone of the Andes. The Las Mellizas Ignimbrite is widely distributed in the intra-caldera region of the previously formed Caviahue caldera. Detailed field work led us to identify four facies associations for Las Mellizas Ignimbrite and to characterize the pyroclastic facies as dominantly vitrophyric, eutaxitic and rheomorphic, with limited occurrences of welded agglomerates and lithic breccias. Pyroclastic dykes with subvertical eutaxitic fabric were found and interpreted as fissure vents of this ignimbrite. Their orientations follow either the southern Caviahue caldera rim, or approximately the current extension direction as determined by previous neotectonic studies. Thermobarometry and hygrometry estimations were carried out based on compositional analyses of minerals (plagioclase, ortho- and clinopyroxenes, FeTi oxides) and glass. We interpret that Las Mellizas Ignimbrite was deposited from high-temperature and concentrated pyroclastic density currents, formed by almost fully collapsed low eruptive columns (pyroclastic fountaining). Low pre-eruptive pressures (0.04–0.06 GPa) and moderate water content (2.0–3.4 wt%) plus the low crystallinity of the magma (<10 vol% of phenocrysts) points to a shallow magma chamber and shallow fragmentation level, which is also supported by lithic clast composition. The eruption trigger is interpreted as mainly due to magmatic processes, and aided by tectonic activity. The presence of topographic scarps, outcrops distribution and stratigraphic discontinuities north of the Copahue volcano, in addition to co-ignimbritic lithic breccias, suggest that a small collapse may have occurred, nested on the western Caviahue caldera rim. In this context, the fissure vents represent peripheral extra-caldera pyroclastic conduits.

Temperature and basinal fluid controls on feldspar diagenesis, Lower Cretaceous sandstones, Scotian Basin, Canada

The passive-margin Scotian Basin accumulated thick Upper Jurassic–Lower Cretaceous deltaic sandstones and shales, which were influenced by high heat flow in the mid-Cretaceous and deformed by salt tectonics. This study takes advantage of the complex thermal history and record of saline fluid flow in the basin to determine the relative importance of burial depth, temperature, salinity of basinal brines, overpressure, and sandstone permeability in controlling the dissolution of detrital feldspar and growth of authigenic K-feldspar and albite. Feldspar grains and volcanic lithic clasts from two transects, with differing thermal and salt tectonic histories, were studied by scanning electron microscope (SEM) backscattered electron images, energy-dispersive spectroscopy, SEM cathodoluminescence (CL) imaging, and hot-cathode CL microscopy. Published fluid inclusion measurements are integrated with burial history curves and timing of deformation on salt detachment surfaces to provides a new synthesis of the post-Jurassic thermal and saline fluid flow history of the basin. Surface-controlled dissolution of K-feldspar and growth of authigenic K-feldspar rims occur from 1.5 to 3.1 km. More destructive transport-controlled dissolution, down to 3.9 km, is more effective in thick-bedded permeable sandstones that acted as fluid pathways. Albitization of detrital K-feldspar and sodic plagioclase, and of feldspars in volcanic lithic clasts was facilitated by high Na+ content of formation waters migrating preferentially through such pathways at the time of silica and carbonate cementation, with strong albitization coinciding with transport-controlled dissolution of K-feldspar. Authigenic albite partially fills pores created by K-feldspar dissolution. Overpressured conditions enhance albite authigenesis, as a result of slower fluid advection. Episodic release of hot, saline, overpressured fluids in the zone of surface-controlled dissolution of K-feldspar favoured albitization but had no detectable effect on K-feldspar dissolution. Basin hydrology plays a definitive role in feldspar diagenesis, producing patterns that cannot be interpreted as a simple consequence of temperature and activity of Na+.

Late Quaternary Drainage Rearrangement Prevents the Vegetation Development in the La Tatacoa Intermontane Basin of the Colombian Andes

The Tatacoa Desert (TD) is a Cenozoic basin between the Central and Eastern Cordilleras in the southern Colombian Andes. Its sparse vegetation and Badlands morphology resemble a semi-arid environment, despite receiving ∼1,300 mm of mean annual precipitation. Based on optically stimulated luminescence dating in two fluvial terraces and drainage morphometric analysis, we propose that such “aridification” was imposed by a drainage reorganization during the Late Pleistocene. No river coming from the neighboring mountains flows into the TD. This makes the Tatacoa rivers unable to balance the evapotranspiration effect, causing a surficial water deficit. A fluvial terrace (TAT-2) works as a divide between the Tatacoa catchments and the Cabrera River, the latter coming from the Eastern Cordillera and bounding the TD towards the north. Paleocurrent measurements on its sedimentary lithic clasts point to a W-NW-directed flow into the TD. Thus, OSL ages around 74.1 ± 7.0 ka in this terrace suggest the TD was drained by an Eastern Cordillera catchment during the Late Pleistocene. At the western end of the TD, a dissected fluvial terrace (TAT-1) lying 80 m above the Magdalena River ages between 6.3 ± 1.5 and 7.7 ± 1.5 ka, representing the Holocene incision rates of 10.8 ± 0.3 mm/y. Coupled OSL data and river longitudinal profile analysis show a high susceptibility to erosion of the Tatacoa rocks and estimate incision rates ranging from 2–5 mm/y to 10–20 mm/y within the TD. The higher rates are concentrated along knickpoints related to anticlines and thrust faults. Thus, a northward shift of the Cabrera River and a westward migration of the Magdalena Valley have drastically reduced the surficial water availability in the Tatacoa and the development of vegetation, increasing erodibility. Potential climatic and tectonic drivers include 1) terrace accumulation during the wet periods of Marine Isotope Stage 5A and 1; 2) fault-induced incision of the Magdalena River in the NW, which would have favored northward migration of tributaries such as the Cabrera River; and 3) activity on the Baraya thrust fault, which would have prevented rivers from the Eastern Cordillera to enter the desert.

Stratigraphy and palaeogeography of the Cretaceous Blue Spur Conglomerate, Otago, New Zealand

ABSTRACT The non-marine Blue Spur Conglomerate in southeast Otago hosts the largest gold paleoplacer in New Zealand, but its formal stratigraphic relationships have thus far not been clear. The name reflects the extensive formation of ferrous iron-bearing diagenetic clay. We present a reference section for the unit, which is in part late Haumurian in age (c. 81–66 Ma). The unit is defined as a formal Member of the nearby, but not contiguous, Late Cretaceous Taratu Formation, Onekakara Group, Haerenga Supergroup. The Blue Spur Conglomerate contains three principal clast types: proximal schist debris, quartz pebbles and recycled distal greywacke cobbles, whereas the wider Taratu Formation is dominated by quartz pebble conglomerate with subordinate lithic clasts. Rare clasts of silicified quartz pebble conglomerate (silcrete) in both units attest to the recycling of older mature quartz sediments. The Blue Spur Conglomerate formed locally at the base of active normal fault scarps that controlled a broad valley in which the regionally extensive Taratu Formation accumulated. Detrital gold in the Blue Spur Conglomerate had both proximal and distal sources, whereas the Taratu Formation elsewhere received only distal gold.

Initial uplift of the Qilian Shan, northern Tibet since ca. 25 Ma: Implications for regional tectonics and origin of eolian deposition in Asia

Abstract Sedimentary rocks in northern Tibet record uplift of the Tibetan Plateau and its potential connection with the evolution of the central Asian aridity, therefore offering a typical example of tectonic-climate linkage. The coarse-grained conglomeratic and sandy red beds of the Lulehe Formation (Fm.) in the northern Qaidam Basin (QB), northern Tibet, have long been held as synorogenic sediment accumulation. There is, however, a heated debate on its source area (the Qilian Shan, the east Kunlun Shan, or Qimen Tagh) and initiation age (ca. 52, 25.5, or ca. 21 Ma, respectively). These proposals lead to distinctly different mountain building processes of the giant Qilian Shan during the Cenozoic. One view argues that the Qilian Shan began to uplift substantially as a simultaneous far-field response to the India-Asia collision at 55–50 Ma. In contrast, others claim that significant rise of the Qilian Shan and thus northeastward expansion of the Tibetan Plateau did not occur until 19 or 12 Ma. Based on an updated magnetostratigraphic framework for the Cenozoic sediments in the northern QB, here we conducted structural, paleocurrent, pebble composition, zircon grain shape and surface texture, and detrital geochronological analyses of the Lulehe Fm., in the northern QB. The results indicate that the Lulehe Fm. was produced essentially by an initial rush of lithic clasts derived from the deformed Mesozoic sedimentary cover, which can be attributed to initial rise of the Qilian Shan since ca. 25 Ma. This finding leads additional credence to the argument that the onset of significant uplift of mountain ranges along the periphery of the plateau occurred nearly synchronously from the latest Oligocene through early Miocene. These prolonged elevated Mesozoic sediments covering the fold-thrust belts of the northern QB, on the other hand, may have acted as a sustained source of material for the Miocene eolian deposits in the western Chinese Loess Plateau.

Jurassic and Cretaceous clastic petroleum reservoirs of the West Siberian sedimentary basin: Mineralogy of clays and influence on poro-perm properties

• Kaolinite is a predominant clay for Jurassic while chlorite for Cretaceous reservoirs. • Secondary chloritization might not have affected the coarse-grained lithic clasts . • A lowland relief predominated in Jurassic and volcanic upland relief in Cretaceous. The West Siberian Sedimentary Basin (WSSB) is the main commercial and the biggest sedimentary basin of Russia and plays a huge role in the Russian oil and gas industry. Three deep wells intersecting full sections of the basin main Jurassic and Cretaceous reservoirs have been studied. This basin consists of two major petroliferous sedimentary sequences: uppermost is of Lower Cretaceous (the Neocomian Sedimentary Sequence) and the oldest petroliferous sandstones of Jurassic age (the Urman-Togur-Tyumen sequence). All the main oilfields of the WSSB are found in sandstone reservoirs, and this paper describes their petrography and mineralogy, emphasizing on the nature and origin of the clay fraction (<2 µm). Each section was characterized by granulometry and mineral composition using optical microscopy, X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM) techniques. The uppermost Lower Cretaceous sequence, that appears to be related to volcanogenic debris of high iron content, is characterized by abundant chlorite of similarly high iron content. The clay mineralogy of the petroliferous sandstones in the lowermost Jurassic sequence is not so chloritic but is more kaolinitic, a feature believed to be inherited from weathering of lateritic type soil profiles in the source area, as well as post-sedimentary authigenic conversion of feldspars.

A compilation and characterisation of lithics in kimberlite and common maar-diatremes and tephra ring deposits

Maar-diatreme volcanoes are the second-most common type on land, occurring in volcanic fields within all major tectonic environments. Their deposits typically contain an abundance of lithic fragments quarried from the substrate, and many contain large, deep-sourced lithic fragments that were erupted to the surface. Primary volcaniclastic deposits fill the diatreme structure formed during eruption. There is negligible inelastic deformation of diatreme-adjacent country rock, indicating that country rock is removed to create the diatreme structures, either by being shifting downward below observable levels, ejected upward to contribute to surficial deposits, or dissolved and hidden in magma erupted or intruded at depth. No previous study has systematically reviewed and analysed the reported lithic fragments of maar-diatreme systems. We present a comprehensive compilation from published work of lithic characteristics in maar ejecta rings and in diatreme deposits of both common and kimberlite maar-diatremes. For maar-diatremes and their tephra ring deposits, we find no correlations among lithic clast sizes, shapes, depositional sites, and excavation depths. This is difficult to reconcile with models involving systematic diatreme deepening coupled with tephra-ring growth, but consistent with those involving chaotic explosions and mixing. Larger amounts of data are needed to further examine how these types of volcanoes operate.

Sedimentological analysis of ash-rich pyroclastic density currents, with special emphasis on sin-depositional erosion and clast incorporation: The Brown Tuff eruptions (Vulcano, Italy)

The sedimentological, lithological and textural characteristics of the Brown Tuffs (BT) pyroclastic deposits, combined with their grain-size, componentry and geochemical glass compositions, are here investigated to obtain information on the transport and depositional mechanisms of the corresponding pyroclastic density currents (PDCs). The BT are widespread reddish-brown to grey, ash-rich pyroclastic deposits generated by pulsating hydromagmatic explosive activity from the La Fossa Caldera on Vulcano island during the c. 80–6 ka time-stratigraphic interval, and then distributed on most of the Aeolian Islands and Capo Milazzo peninsula (Sicily) and in the Tyrrhenian and Adriatic Sea regions. Near the source area on Vulcano, the BT are characterised by alternating massive and planar to cross stratified lithofacies that result from the stepwise, repeating aggradation of discrete PDC pulses. This alternance is regulated by either fluid escape or granular flow depositional regimes at high clast concentration or grain by grain traction deposition in the waning diluted stages of the PDCs. Most of the BT on Vulcano show intermittently stratified and massive ash deposits resulting from a pervasive post-depositional disruption of the primary structures. This is induced by upward fluid expulsion associated with dissipation of pore pressure between layers at different grain size (fine to coarse ash) and porosity, as outlined by distinctive upwards bends and pillar-type escape structures through the fluid-filled cracks and rupture points. Massive BT deposits with a faint colour and grain-size banding are widely recognised on Lipari, the nearby island of Vulcano. Based on the presence, at the base of BT depositional units, of cm-thick amalgamation bands containing pumice lapilli, scoria and lithic clasts ripped-up and embedded from the loose underlying pyroclastic units, they are interpreted as deposited by ash-rich PDCs laterally spreading from La Fossa Caldera and moving to Lipari. During their motion to Lipari these currents (likely) crossed a narrow and shallow sea-water inlet which did not stop their advancement but influenced the grain size distribution of those spreading on the Lipari mainland. In this paper, the mechanism of clast erosion and incorporation is outlined across the whole island of Lipari by means of field study, grain-size, and geochemical glass analyses on the different components of the mixed basal bands of the BT. This suggests that the BT PDCs maintained enough flow power as to erode the substratum, hence likely impacting the territory, over a distance up to at least 16–17 km from the volcanic source. Evidence that the BT PDCs exerted a high shear-stress over the loose substratum is also provided by undulated, recumbent flame and rip-up structures at the base of some depositional units in southern and central Lipari. In order to form such bed granular instabilities between the BT and the underlying deposits we calculate that the currents had at least a shear velocity of ca. 2 m s−1 and a shear stress in the range of 1‐4.5 kPa. These results add new insights on the large-scale hazard at the Aeolian Islands and shed new lights on the widespread transport and depositional dynamics of ash flows spreading over the sea and reaching nearby islands, and their interactions with the substratum and the pre-depositional topography.

Carbonate cementation in the Tithonian Jeanne d’Arc sandstone, Terra Nova Field, Newfoundland: Implications for reservoir quality evolution

AbstractPetrographic and in situ geochemical analyses were carried out on cores of the fluvial sandstone (&lt;50 to &gt;4 mm; sublithic quartz arenite) dominated Tithonian Jeanne d’Arc Formation interval from wells C‐09 and E‐79 of the Terra Nova oilfield to understand diagenetic control on reservoir quality of the formation. Lithic clasts are dominantly carbonates and shales. Successively, early near‐surface dolomite cementation, eogenetic dolomite dissolution, burial calcite cementation and its subsequent dissolution are the major diagenetic events that controlled the reservoir quality of the sandstone. Dolomite cement grades from mostly Fe‐rich (FeCO3 ca 12 ± 2 normalized mol.%) in the upper part of the formation to Fe‐poor (&lt;0.07 normalized mol.%) in the lower part of the formation in both wells. The dolomite and calcite cements generally have low Sr and Na; enriched Y and low δ18O and δ13C values relative to typical marine carbonates. Strontium is enriched in the Fe‐poor (higher Mg) dolomite relative to the Fe‐rich phase. Mineral liberation analyses of shale samples reveal common Fe‐chlorite and minor glauconite. Furthermore, considerable amounts of solutes in the early diagenetic pore fluid were likely derived from water—rock interaction with underlying Oxfordian Rankin Formation marine carbonate that was also exposed in the watershed of the study area at the time. Together, these suggest that the dolomites were formed from an early diagenetic bicarbonate‐rich pore fluid of mixed meteoric and seawater origin. The origin and distribution of early dolomite cementation and other successive diagenetic events have a depositional cycle control. Episodic post‐deposition transgression of seawater and/or compactional fluid expulsion from shales into overlying sandstones led to formation of dolomite cement. Subsequent infiltration of organic acid‐charged meteoric‐water into underlying dolomite‐cemented sandstones resulted in early dissolution. This porosity was mostly occluded by re‐precipitation of calcite. Increase in the abundance of relic corroded dolomite crystals with depth in each cycle indicates that early dissolution was most effective at the top of the cycles. Iron depletion in the calcite cement with depth through the formation points to a local derivation of reactants from in situ dolomite species; implying a stratigraphic control. Calcite cementation was succeeded by continuous mesogenetic dissolution concomitant mechanical compaction. The responsible low‐pH pore fluid attained its composition via enhanced H+ contribution from silicate burial diagenesis. The control of depositional settings on the major diagenetic processes makes it a key contribution to region‐wide reservoir quality prediction for the formation.

Petrology and geochemistry of lunar feldspathic meteorite Northwest Africa 11111: Insights into the lithology of the lunar farside highlands

AbstractWe performed a petrological, mineralogical, and geochemical study of the lunar feldspathic meteorite Northwest Africa (NWA) 11111. This meteorite contains several types of lithic clasts, including feldspathic clasts, mafic‐rich clasts, granulites, impact melt breccias, minor basaltic clasts, and highly evolved clasts cemented in a recrystallized fine grain matrix. Both mineral chemistry and geochemical characteristics indicate a lunar origin for NWA 11111. The bulk analysis suggests that NWA 11111 is a typical feldspathic lunar meteorite, which is consistent with its large population of anorthositic clasts and plagioclase fragments. A comparison of geochemical data made by lunar orbiter missions indicates that this meteorite was likely launched from the Feldspathic Highland Terrane on the lunar farside. The chemical zoning, coupled with extensive exsolution lamellae (up to 20 μm in width) occurring in pyroxene across three sections of NWA 11111, demonstrates that this meteorite contains components derived from the surface to about 10 km of lunar crust. Magnesian anorthosite clasts are commonly present in the meteorite, indicating that magnesian anorthosite probably represents an important lithology in the lunar farside crust. Basaltic clasts in NWA 11111 range from a very low‐Ti to a low‐Ti mare basalt, possibly representing cryptomare on the lunar farside. Although a KREEPy signature for NWA 11111 is not evident, highly evolved clasts containing various silica polymorphs and/or K‐feldspar are present. They may originate from late‐stage residual liquids. Lithic clasts and mineral fragments within NWA 11111 provide new insights into the diversity of lunar crust lithology and magmatic processes on the lunar farside. This meteorite also offers rocky materials from a wide vertical section of lunar crust.

Extremely high-grade, lava-like rhyolitic ignimbrites at Osham Hill, Saurashtra, northwestern Deccan Traps: Stratigraphy, structures, textures, and physical volcanology

In many continental flood basalt (CFB) provinces of the world, widespread silicic lavas and ignimbrites overlie the mafic lavas. Our knowledge of this post-flood basalt silicic volcanism in the Deccan Traps CFB province of India remains limited. Here we describe the stratigraphy, structures, and textures of the rhyolites of Osham Hill in Saurashtra, in the northwestern Deccan Traps. Basaltic lavas forming the lower parts of the hill are overlain by the “Lower Rhyolite” (LR), which is tens of meters thick and shows intense rheomorphic deformation (flow banding and folding) at microscopic, outcrop, and tens of meters scales. The LR is overlain by a “Green Tuff and Pitchstone” (GTP) band 20–40 m thick, containing massive green tuff to strongly rheomorphic, commonly spherulitic pitchstone, both types showing sharp but irregular contacts. The GTP is overlain by the “Upper Rhyolite” (UR), with an ~100 m preserved thickness. This resembles the LR in many structural features, and also contains a thin (~1 m), laterally discontinuous basal layer of unbedded, nonwelded lapilli tuff. The LR and UR locally contain basal autobreccias which are unlike crumble breccias of rhyolitic lava flows. In thin section the LR and UR are microcrystalline and lava-like, though they contain many relict vitroclasts (glass shards and pumice fragments). The LR also shows rare millimeter-size basaltic lithic clasts. The GTP contains abundant vitroclasts. We interpret the LR and UR as extremely high-grade, intensely welded, lava-like ignimbrites, and the GTP as a low-grade ignimbrite with varying degrees of welding and rheomorphic deformation. The absence of sedimentary interbeds and soil horizons suggests that the whole Osham ignimbrite sequence was deposited subaerially and rapidly. The eruptive vent locations are unknown. However, a rhyolite dyke intruding basalts at the southeastern end of Osham Hill is pyroclastic, showing a eutaxitic to microcrystalline interior and margins of welded and rheomorphic tuff-pitchstone. Basaltic lithic fragments found in the dyke margins suggest that it may be the feeder of the LR. The LR and UR show many close similarities to “Snake River-type” ignimbrites (large-volume, metaluminous, high-temperature, lithic- and crystal-poor, intensely welded, rheomorphic and lava-like ignimbrites). These render Osham Hill a newly recognised and important locality of such ignimbrites.

Revealing High‐Manganese Material on Mars at Microscale

AbstractHigh‐manganese materials would provide valuable information about the history of a water‐rich and oxidizing environment on early Mars. However, the detailed texture and components of these high‐manganese materials are unknown. In martian breccia meteorite NWA 11220, one unique lithic clast enriched in high‐manganese material (~5 vol%) has been recognized. Here, we report the morphological, micro‐textural, mineralogical, and geochemical features of the martian high‐manganese material at the microscale for the first time. Our results show that (a) high‐manganese materials are characterized by micron‐sized grain (&lt;50 μm), complex associated‐phase (i.e., apatite, enstatite, magnetite, ilmenite, and amorphous matrix), and chemical heterogeneity; (b) nano‐sized (&lt;500 nm) amorphous manganese‐oxide veins or grains are the only manganese‐bearing phases in the high‐manganese materials. These new observations provide new evidence for revealing the geological processes related to the high‐manganese materials on Mars and provide important information for future in‐situ exploration and data interpretation of high‐manganese materials on Mars.

Petrography and Shock Metamorphism of the Lunar Breccia Meteorite NWA 13120

Lunar meteorites are the fragments of rocks that fell on Earth because of the impacts of asteroids on the Moon. Such rocks preserve information about the composition, evolutionary process, and shock history of the lunar surface. NWA 13120 is a recently discovered lunar breccia meteorite having features of strong shock, which is composed of lithic and mineral clasts in a matrix of very fine-grained (&lt;10 μm) and recrystallized olivine-plagioclase with a poikilitic-like texture. As the most abundant lithic clasts, the crystalline impact melt (CIM) clasts can be divided into four types according to their texture and mineral composition: (1) anorthosites or troctolitic anorthosite with a poikilitic-like texture, but the mineral content is different from that of the matrix; (2) anorthosites containing basaltic fragments and rich in vesicles; (3) troctolitic anorthosite containing metamorphic olivine mineral fragments; (4) troctolitic anorthosite containing troctolite fragments. Based on the petrology and mineralogy, NWA 13120 is a lunar meteorite that was derived from the ferrous anorthosite suite (FANs) of the lunar highlands, while its texture suggests it is a crystalline impact melt breccia. In addition, we infer that the parent rock of NWA 13120 is a lunar regolith breccia enriched in glass fragments. During the shock process, at pressures of more than 20 GPa, all plagioclase fragments were transformed into maskelynites, and olivine fragments occurred metamorphism. The post-shock temperature led to the partial melting of the basaltic fragments. Subsequently, all glass with diverse components in the parent rock were devitrified and recrystallized, forming the common olivine-plagioclase poikilitic-like texture and different CIM clasts. Meanwhile, the devitrification of maskelynite formed the accumulation of a large number of plagioclase microcrystals. Therefore, NWA 13120 is a meteorite of great significance for understanding the local shock metamorphism of lunar rocks on the lunar surface.