Textural Maturity Research Articles

Mineralogy and palaeoenvironments: the Weald Basin (Early Cretaceous), Southeast England

AbstractDetermination of the mineralogy of sedimentary rocks is important in palaeoenvironmental interpretations, especially when outcrop and field data are insufficient or not accessible. The Lower Cretaceous Wealden facies, presented here as a case study, is one such example. Petrographic analysis of this suite of sediments reveals that the arenaceous facies are mainly quartz arenites, quartzose siltstones and lack significant feldspar. The clay minerals are illite, kaolinite, illite‐montmorillonite, vermiculite and illite‐smectite. The presence of quartz arenites and kaolinite suggests that the rocks were reworked from granitic and/or gneissic rocks. Directly or indirectly, the rocks may have been obtained from a stable craton. The mineral and textural maturity, together with a travel distance of ca 300 km point to matured sources. A warm and humid palaeoclimate in the source areas resulted in extensive weathering.

Provenance and tectonic setting of the Quemoco sandstones in the North Qiangtang Basin, North Tibet: Evidence from geochemistry and detrital zircon geochronology

The geochemistry and detrital zircon geochronology of the Quemoco sandstones from the Woruo Mountain area, North Qiangtang Basin, are analysed to discriminate their provenance and tectonic setting. The sandstones were classified as litharenite according to their geochemical data. The index of chemical variability and SiO2/Al2O3 ratio values suggest that the compositional maturity and recycling are weak to moderate. The weathering indices, such as chemical index of alteration, plagioclase index of alteration, and chemical index of weathering, and A–CN–K (Al2O3–(CaO* + Na2O)–K2O) ternary plot reflect that the source areas have undergone a weak degree of weathering. The TiO2 versus Zr, La/Th versus Hf, and Co/Th versus La/Sc bivariate diagrams and multimajor elements discrimination diagram of Quemoco sandstones indicate that the provenances were primarily derived from felsic igneous rocks and may also have less intermediate igneous source rocks. The paleocurrent data and low composition and textural maturity of the Quemoco sandstones suggested that the provenances are mainly from the southwest of the study area and with proximal source features. The U–Pb ages studied here could be divided into 8 groups: 206–219 Ma, 268–283 Ma, 446–630 Ma, 700–970 Ma, 1006–1346 Ma, 1457–1843 Ma, 2087–2214 Ma, and 2331–2587 Ma. The provenances are mainly from the Central Uplift Belt and surrounding of the study area according to the detrital zircon ages and rare earth element parameters, which are composed of Late Triassic tuff, Permian basalt, Early Ordovician granite, Cambrian granite gneiss, and Precambrian crystalline basement. Two multidimensional tectonic discrimination diagrams based on major elements show a complicated tectonic setting composed of a collision, rift, and minor arc setting for the Quemoco sandstones, which is consistent with the nature of a rifted margin during the late Late Triassic.

The uppermost clastic succession of Lower-Middle Buntsandstein as a record of continous sedimentation during Early Triassic trangression in the area of Grodziec syncline

The uppermost clastic succession of Lower–Middle Buntsandstein sediments of the Grodziec syncline (North Sudetic Synclinorium) shows a high macroscopic similarity to overlaying Cenomanian deposits. That might have resulted in erroneous determination of the boundary between Triassic and Cretaceous sediments in historic drill holes. Detailed macroscopic observations were performed on cores and samples were taken for microscopic analyses. A number of features characterizing sediments of the uppermost clastic succession have been identified. These are primarily: increased kaolinite content, decrease of textural maturity, decline of bedding, lack of pebbles and argillaceous intraclasts. These features might indicate transgressive character of sediments and a deposition in a transitional: terrestrial to marine environment.

The Maturity Myth In Sedimentology and Provenance Analysis

Abstract: We often resort to mythical thinking in the face of natural phenomena that we hardly understand. Sedimentary petrology has several long-standing myths. One is that sediments increase their “maturity” through time by winnowing and sorting, physical wear, chemical weathering, diagenesis, and recycling, as if their destiny were to reach a final stage of perfection represented by quartz spheres of equal size. Such an armchair representation of supermature purity is embodied in white recycled desert sand, whereas a modern analogue for its opposite, the dark dirty rock improperly called graywacke, is yet to be found. Clay matrix does not occur in sand deposited from tractive currents, which is sorted at the instant of settling. Sand grains get rounded relatively rapidly during transport in air but very slowly when transported in water. The degree of winnowing, sorting, and rounding (“textural maturity”) has little to do with the accumulation of “modifying energy” through time. “Mineralogical maturity” does not increase notably even after thousands of kilometers of transport in high-energy environments, and not even during recycling, a physical process that in a weathering-limited denudation regime tends at most to reproduce the mineralogy of parent clastic rocks. Rather, “mineralogical maturity” decreases drastically where sedimentary rock fragments are eroded in abundance from mudrocks interbedded in the source, and sand recycled entirely from orogenic flysch or molasse may contain less than 10% quartz. If natural sediment factories were quartz distilleries, then quartzose sand should become more common through geological time. Pure quartz sand, however, is relatively rare in the modern world, and largely confined to hyperhumid equatorial latitudes or recycled eolian dunefields. A relative increase in more durable minerals is achieved effectively only by chemical processes such as selective leaching in soils, or especially during burial diagenesis, when the time and temperature available for chemical reactions are much longer and higher than at the Earth9s surface. If recycled sediments are commonly quartz-rich with very poor assemblages dominated by zircon, tourmaline, and rutile, it is because their parent sandstones have undergone diagenetic dissolution prolonged for many millions of years at temperatures of many tens of degrees before final exhumation and erosion.

Reservoir quality variations within a conglomeratic fan-delta system in the Mahu sag, northwestern Junggar Basin: Characteristics and controlling factors

The conglomeratic reservoir in the Mahu depression, northwestern margin of the Junggar basin, Northwestern China has become an important exploration target. In this study, we use well and core data to investigate the characteristics and controlling factors of conglomeratic fan-delta reservoirs. The reservoir studied here developed in a retreating fan-delta complex that is dominated by conglomerate lithofacies and conglomeratic sandstone lithofacies. A petrological analysis shows that the reservoir exhibits poor compositional maturity and textural maturity. A physical property analysis shows that the reservoir exhibits extremely low porosity and extremely low permeability, as well as strong heterogeneity in the vertical and planar views. The depositional environment and compaction result in extremely low porosity and extremely low permeability in the reservoir. About 66% porosity was destroyed by the compaction. Fan-delta evolution and provenance control the macroscopic variations in reservoir quality. Muddy matrix content, grain size and dissolution control the microscopic variations in reservoir quality. About 52.5% porosity was induced by the dissolution. This reservoir study can be compared to fan-deltas that developed in marine-connected rift basins during early stages of extension and to nearshore subaqueous fans in East China. Additionally, the results of this study may provide a reference for such systems and provide a subsurface case for similar fan-delta outcrop studies and facies reservoir modelling.

Zircon U–Pb geochronology of lower crust and quartzo-feldspathic clastic sediments from the Balagne ophiolite (Corsica)

The Balagne ophiolite from central-northern Corsica represents a continent-near paleogeographic domain of the Jurassic Liguria-Piedmont ophiolitic basin. Pillow and massive basalt lavas are primarily associated with Middle–Upper Jurassic pelagic sediments (mostly radiolarites at their base), continental-derived quartzo-feldspathic clastic sediments and ophiolitic breccias containing clasts of gabbros and basalts. The basalt-sedimentary succession is tectonically associated with a slice composed of an intrusive sequence overlain by basalt lavas. A “plagiogranite” from the intrusive sequence was dated by U–Pb zircon geochronology. Although affected by some uncertainty, mainly reflecting common Pb contamination, the U–Pb zircon data suggest a crystallization age of 159 ± 3 Ma (MSWD = 6.3), which is coeval with the formation of oceanic lower crust in the Schistes Lustres units from Alpine Corsica. The predominance of quartz grains preserving typical volcanic shape, the prevalence of prismatic zircons and the arkose whole-rock composition indicate that the continental-derived quartzo-feldspathic clastic sediments have a low degree of textural maturity. U–Pb zircon geochronology carried out on two distinct levels of quartzo-feldspathic clastic sediments identified the predominance of zircons with within error U–Pb dates at ~280 Ma; minor components at ~457, ~309 and ~262 Ma were also obtained. The U–Pb date distribution is consistent with a source magmatic material mostly developed during the Variscan orogenic collapse.

Multi‐proxy approach to palaeoenvironmental modelling: the English Lower Cretaceous Weald Basin

Palaeoenvironmental reconstructions of the Wealden (Lower Cretaceous) in southeast England have been previously based mainly on field and fossil data. This study presents new interpretations of multi‐proxy data sets such as sandstone petrography, elemental geochemistry and sideritic ironstones, supplemented by outcrop and clay mineralogical data. The dominance of quartz arenites, kaolinite and thorium and the presence of Zr in the sediments suggest that they were sourced mainly from granitic and/or gneissic rocks. Materials from metamorphic sources are secondary in abundance. The mineral and textural maturity of the sediments coupled with inferred short transport distances suggests reworking from older sources. Palaeoclimatic conditions in the source areas were generally warm and humid in nature, which supported an intensive weathering regime. The nature of the sediments suggests they may have been sourced directly or indirectly from a stable craton. The lithology, facies, sedimentary structures, architecture and gamma ray data confirm the freshwater origin of the sediments. Redox‐sensitive trace elements such as Mo, U, V and Co suggest anoxic conditions in the basin. The results presented in this paper reinforce the importance of integrating multi‐proxy data sets for interpreting the palaeoenvironmental conditions of sedimentary basins: when traditional data sets such as field and fossil data are not readily available or insufficient to enable adequate interpretations, the integration of petrographic, mineralogical and geochemical data sets may prove to be useful in similar palaeoenvironmental studies. Copyright © 2017 John Wiley & Sons, Ltd.

The milling factory: Componentry-dependent fragmentation and fines production in pyroclastic flows

Abstract In order to decipher the mobility of hazardous small-volume pyroclastic flows (PFs), we investigate here the role of clast fragmentation and production of fine particles (<2 mm) in controlling PF dynamics. We report a high-resolution textural data set (i.e., grain size distribution, clast componentry, and clast morphology) of andesitic scoriaceous PF deposits from Tungurahua volcano (Ecuador) based on a unique method and data processing procedure. The strong relationship shown between clast size and componentry in PF deposits is quantified through a componentry versus size index (CSI), which varies between −1 and +1 (all components in the coarse or fine fractions, respectively). Our results show that the incorporated old lava clasts (CSI of −0.33) control early grinding of the pristine juvenile bombs (CSI of −1), which produce smaller scoriaceous andesitic fragments (CSI of 0.26). These latter interact in the granular assemblage and release fine particles through the abrasion processes, as supported by the morphological data. The textural maturity of the deposits reveals that the “milling factory” works before sedimentation begins (i.e., on steep upper slopes) and that there is some balance between fines production and loss. The process evident here tends to redistribute stress into the moving flow and produces fines that contribute to fluidization of it. The milling factory is thus likely to enhance the mobility of geophysical flows, which has strong implications for modeling and hazard assessment purposes.

Petrography and geochemistry of the Paleogene sandstones from the Ningnan Basin, NW China: implications for provenance, weathering and tectonic setting

Petrographical and geochemical characteristics of the Paleogene sandstones in the Ningnan Basin, NW China, were studied by analyzing thin sections and major, trace, and rare earth element (REE) compositions. The sandstones of the Sikouzi and Qingshuiying formations are sublitharenite to feldspathic litharenite and characterized by low-medium compositional and textural maturity. Petrographic provenance study indicates a quartzose-recycled provenance with higher concentration of Zr in the Qingshuiying Formation. Depositional environment of the Sikouzi and Qingshuiying sandstones was mainly oxic as evidenced by lower values of δEu, δCe, V/Cr, Ni/Co, U/Th, and δU. The Chemical Index of Weathering (CIW’) and the Th/U ratios (av. 3.3 and 3.54) reveal weak and moderate weathering history in the source area. The Zr, Nb, Y, Th, and U concentrations in the Qingshuiying Formation are comparable to upper continental crust (UCC) values and the negative Eu anomalies support contribution from a felsic source. However, a significantly larger range of Ce anomalies in the Sikouzi Formation also reveals mixing of felsic and mafic/ultramafic source terrains. The petrology and geochemistry analyses of the Sikouzi sandstones imply that the provenance was mainly derived from diabase in Xiang Shan and lower Cretaceous strata in the study area whereas the Qingshuiying sandstones were derived from the granites in the southern part of Liupan Shan, which means that the northeastward expansion of the Tibetan Plateau did not make the Liupan Shan a high relief during Paleogene. The present study has let to document the origin of the Ningnan Basin that was formed during the late phase of Ordos Basin reformation, along with the other synchronous peripheral fault-bounded basins of the Ordos Basin, and was not related to the northeastern extrusion of Tibetan Plateau.

Assessing automated image analysis of sand grain shape to identify sedimentary facies, Gran Dolina archaeological site (Burgos, Spain)

Gran Dolina is a cave (Sierra de Atapuerca, Spain) infilled by a 25m thick sedimentary record, divided into 12 lithostratigraphic units that have been separated into 19 sedimentary facies containing Early and Middle Pleistocene hominin remains. In this paper, an automated image analysis method has been used to study the shape of the sedimentary particles. Since particle shape is interpreted as the result of sedimentary transport and sediment source, this study can provide valuable data about the sedimentological mechanism of sequence formation. The shape of the sand fraction in 73 samples from Gran Dolina site and Sierra de Atapuerca was analyzed using the Malvern Morphologi G3, an advanced particle characterization tool. In this first complete test, we used this method to the published sequence of Gran Dolina, defined previously through field work observations and geochemical and textural analysis. The results indicate that this image analysis method allows differentiation of the sedimentary facies, providing objective tools to identify weathered layers and measure the textural maturity of the sediments. Channel facies have the highest values of circularity and convexity, showing the highest textural maturity of particles. On the other hand, terra rossa and debris flow samples show similar values, with the lowest particle maturity.

Lithofacies and organic geochemistry of the Middle Permian Lucaogou Formation in the Jimusar Sag of the Junggar Basin, NW China

Abstract The Middle Permian Lucaogou Formation in the Jimusar Sag, Eastern Junggar Basin, is one of the highlights for unconventional petroleum exploration and development in China. Approximately 580 m-long cores from 18 wells in the Sag were carefully examined, and 500 core samples were collected. These samples were analysed by combined methods including X-Ray diffraction (XRD), thin-section observation and organic geochemical analyses. Based on texture, fabric and mineralogy, the Lucaogou Formation is mainly classified into three lithofacies: (1) shale; (2) siltstone; and (3) dolostone. They are further grouped into three lithofacies associations (LFA): (1) shale LFA; (2) shale interbedded with siltstone LFA; and (3) siltstone intercalated with shale LFA. Compared with North American Barnett shale, the XRD data of the Lucaogou Formation reflects that the concentrations of the plagioclase minerals are generally high. In addition, these rocks commonly contain high contents of brittle minerals (quartz+feldspar+carbonate+pyrite) (avg. 88%) and low contents of clay minerals (avg. 10.97%). The poorly sorted and subangular mineral grains reflect low textural maturity. The organic geochemical data of the shales reflect that the shales have high contents of total organic carbon (TOC), mainly sapropel type (II1) kerogen and matured organic matter. Thus, the shales are expected to be good-quality source rock. The transgressive–regressive (T–R) sequences analysis shows that the Lucaogou Formation can be divided into three transgressive intervals and two regressive intervals. These transgressive intervals are in favour of the formation of excellent source rocks for their low-energy, deep to semi-deep lacustrine environments. In contrast, the regressive intervals contain more rocks with brittle minerals for their relatively high energy of the sedimentation hydrodynamic conditions. The regressive intervals of the Lucaogou Formation as a petroleum reservoir are favourable for fracturing and the production of tight oil.

Compositional differences between Besko and Otryt sandstones of the Krosno Beds (Oligocene) in Eastern part of the Silesian Tectonic Unit [abstract

The aim of this research was to quantitatively and qualitatively analyse under microscope sandstones representing two lithofacies called Otryt and Besko (or Lesko) facies of the Krosno Formation (Oligocene), Silesian Tectonic Unit in the E part of the Polish Outer Carpathians. Four samples were taken into consideration — two from each lithofacies. Special attention was paid to structural and textural characteristics and mineral composition that reflects mainly provenance. The results have been compared and analysed between and within facies, in terms of source and transport of the material, mineralogical and textural maturity and sedimentary processes that affect the deposition. Ślączka & Unrug (1972) and Shideler at al. (1976) concluded that mineralogical variations exhibited by sandstones in Krosno Formation were attributed to both regional (provenance) and local (sorting) effects. The samples have been taken from the northern (Otryt) and southern (Besko) limbs of the Otryt Syncline. They represent different sources of the material - Silesian Ridge in NW in case of Besko facies and Maramures Massif in the the SE in case of Otryt facies (Wendorff 1981). The result of the analysis confirm statement about two different sources. Transport and deposition mechanism results in considerable difference in grain size and its distribution. Besko facies sandstones display classic Bouma sequence at structures (T b - c ) indicative of deposition from normally diluted, waning turbidity currents, with incremental deposition layer-by-layer (Talling 2012) and this results in very fine grained sandstones and in better sorting. Deposits of that lithofacies appear much relatively uniform and well organized (Wendorff 1987). By contrast, sedimentary features of the Otryt facies suggest deposition from rapidly decelerating high density turbidity currents — “sandy debris flows” (Talling 2012), conditions and mechanisms of transportation and deposition must have been very variable (Wendorff 1987). These sandstones are medium grained, but the bottom sample is characterized by coarse tail caused by rapid deposition. The detrital material of the top sample was deposited also of high density turbidity current, however slightly less dense than that of the bottom sample. The diversity of mineral composition between Besko and Otryt lithofacies is very clear. There are probably three reasons for that — completely different source rocks, initially different maturity of the detritus involved in density flows and the depositional processes within the basin . Besko (Leslo) lithofacies is dominated by nonundulatory quartz. Other components are rather minor. The only difference between carbonate components is in the proportion of detrital calcite/limestone (or dolomite) to carbonate cement, which may have resulted from diagenetic processes. Carbonate cements are very rich in both samples. In Otryt facies mineral composition is more differentiated. Quartz predominates, but not as distinctly as in Besko facies. The most abundant and the coarsest are polycrystalline grains. Nonundulatory quartz is similarly abundant. Altered K-feldspars and metamorphic rock fragments are important however they occur less frequently. The bottom and top samples differ in quantity of coarse to very coarse metamorphic rock fragments and polycrystalline quartz — the bottom sample is richer in such grains. This is the result of grain-size sorting — coarser grains have been deposited first and this sorting process affects the mineral composition of the top-of-bed sample in which the coarse tail is absent and the proportion of metamorphic grains is less pronounced. There is very little cement in both samples. Next to high mineralogical maturity, Besko facies sandstones show also high textural and mineralogical maturity (presence of well-rounded grains and high quartz content). On the other hand, Otryt facies sandstones are immature both texturally and mineralogically, which results from both source area composition and rapid deposition. According to provenance diagrams (Dickinson 1983), source of the Besko facies is recycled orogen, but close to craton interior field. Whereas source of the Otryt facies should be recycled orogen too. Despite the fact that all samples are the same age and occur in one formation, they are totally different. In summary, differences in mineral composition and textural features, observed under the microscope, between the analysed samples of the Besko/Lesko and Otryt lithofacies turbidite sandstones reveal different mineral composition of the source areas and are comparable to the macroscopically observed sedimentary features. The latter indicate pronounced differences in the transportation and deposition mechanisms – from normally diluted turbidity currents to rapidly decelerating high sandy debris flows.

Thermal metamorphic evolution of the Pułtusk H chondrite breccia – compositional and textural properties not included in petrological classification

The thermal history that chondrites experienced on their parent body is an aspect of their petrological classification. However, in the classification scheme, metamorphic conditions are generally limited to the peak metamorphic temperature attained, while it is known that reconstruction of the genuine thermal evolution of any rock requires identification of various metamorphic factors, definition of the temperature-time path during metamorphism and characterization of the processes responsible for heating. Study of the brecciated Pultusk H chondrite shows that the meteorite comprises both low and high petrologic type material and should be classified as a H3.8–6 chondrite. Based on the textures and mineral and chemical composition, the thermal metamorphic history of the breccia is reconstructed and it is shown to be inconsistent with the petrologic classification; the textural maturation and degree of compositional equilibrium in the meteorite do not correspond to the temperatures attained. The metamorphic conditions are shown to be a function of the primary composition of the accreted minerals and of two metamorphic phases, progressive and retrogressive. First, a prograde phase led to textural maturation and equilibration of the chemical composition of silicates and oxides. The peak metamorphic temperatures were at least ~700oC for the type 3.8. and 4 material, and up to ~1000oC in H6 clasts i.e., sufficient to locally give rise to partial melting. The following retrograde metamorphism led to compositional re-equilibration of minerals and textural re-equilibration of minerals with partial melts. The cooling rate during retrograde metamorphism down to at least ~700oC was low, which allowed potassium feldspar to form patches in Na-plagioclase and pseudobrookite-armalcolite breakdown to form an association of ilmenite and rutile. The two-phase metamorphic evolution of the Pultusk breccia was the most likely the result of impact heating, which affected the parent body in its very early history.

Reservoir characterization and origin of tight gas sandstones in the Upper Triassic Xujiahe formation, Western Sichuan Basin, China

The Western Sichuan Basin, which is located at the front of Longmen Mountain in the western Sichuan Province, China, is a foreland basin formed in the Late Triassic. The Upper Triassic Xujiahe formation in the Basin includes several tight gas sandstone reservoirs, whose porosity is less than 6.0 % and permeability less than 0.1 md. According to data of cores, thin sections, FMI, SEM and experiments, we summarize the reservoir characteristics, analyze the origin of these tight gas sandstones, and discuss the factors controlling the formation of relatively high-quality reservoirs. Xujiahe Formation is a set of terrestrial clastic rocks with low compositional maturity, low cement content, and medium textural maturity. Secondary dissolution pores are abundant, but pore spaces are small with narrow pore throats and high capillary pressures. Due to the existence of abundant fractures and intense diagenesis, these tight gas reservoirs are characterized by high reservoir heterogeneity, complex seepage system, and strong stress sensitivity. The original sedimentary facies belt and later tectonism co-control the distribution of favorable reservoirs. The intense compaction and cementation are the main factors making the Xujiahe formation to be tight.

Provenance of the Ordovician–lower Silurian Tumblagooda Sandstone, Western Australia

Detrital zircon U–Pb ages and heavy mineral assemblages provide conflicting evidence of the provenance of the Ordovician–lower Silurian Tumblagooda Sandstone, a fluvial to shallow marine, red-bed succession over 2000 m thick, within the northern Perth and Southern Carnarvon basins in Western Australia. Tourmaline composition indicates a main provenance from interior continental terranes dominated by ‘Li-poor granitoids, pegmatites and aplites’ and ‘Ca-poor metapelites, metapsammites and quartz-tourmaline rocks,’ akin to the Yilgarn Craton to the east of outcrop of the Tumblagooda Sandstone. Other possible source areas include orogens mostly to the south but lack tourmaline analyses for comparison. Taking into account the lack of garnets—a conspicuous component of the adjacent Proterozoic Northampton Inlier—the limited zircon data are compatible with the Albany–Fraser and Pinjarra orogens along the southern and western margins of Australia and/or terranes in or adjacent to East Africa and/or Antarctica, as ultimate source regions with a minor contribution from the Yilgarn Craton, as with other Phanerozoic strata in Western Australia. Whereas the textural and mineralogical maturity of the sandstone could be explained by derivation from such regions, it is more likely that the source was relatively local and that the sediment passed through several phases of reworking. The main source of ilmenite and hematite, by comparison, may have been mafic–ultramafic rocks and/or banded iron formations within the Archean Yilgarn Craton to the east or the Pilbara Craton to the northeast, mobilised by acidic meteoric waters. Iron oxides forming the earliest cements may have been derived from the oxidation of detrital hematite and ilmenite grains concentrated along some bedding laminae or transported in solution from beyond the zone of deposition. Whereas the detrital iron oxides most likely come from the craton to the east of outcrop of the Tumblagooda Sandstone, the sand grains appear to have originally come from a relatively local orogenic source.

Dating of detrital zircons from the Dabure clastic rocks: the discovery of Neoproterozoic strata in southern Qiangtang, Tibet

ABSTRACTThis article reports the results of field mapping and the petrology of clastic rocks in the Dabure area, southern Qiangtang, Tibet, together with the results of U–Pb dating of detrital zircons from these rocks. The Dabure clastic rocks are characterized by low compositional and textural maturity, and they have been affected by lower greenschist facies metamorphism. The deposits exhibit the typical features of turbidites. Altogether, 279 detrital zircons were selected for U–Pb dating, and the ages fall into five groups: 550–650, ~800, 900–1100, 1600–1800, and 2300–2500 Ma. In general, the ages of the detrital zircons that are older than ~550 Ma are similar to those found elsewhere in the southern Qiangtang and Himalayan terranes. The most reliable youngest age of a detrital zircon from the Dabure clastic rocks is ~550 Ma. In the southern part of the Tibet Plateau, strata with the same ages and lithologies as the Dabure clastic rocks are widespread, especially in the Himalayan terrane. Combining our data with previous work on the basalts in the Dabure area (the Dabure basalts), we tentatively suggest that the Dabure clastic rocks represent the late Ediacaran (~550 Ma) sedimentary record for the Qiangtang terrane, and that before the late Neoproterozoic the southern Qiangtang terrane was possibly connected to the Himalayan terrane.

Mafic magma replenishment, unrest and eruption in a caldera setting: insights from the 2006 eruption of Rabaul (Papua New Guinea)

Abstract Understanding the magmatic processes that drive unrest at silicic calderas remains a major goal in Volcanology. Rabaul in Papua New Guinea is an exceptional location because after two decades of unrest and a peak in seismicity and deformation in 1983–85, eruptive activity began in 1994 and is still ongoing. A particularly large sub-Plinian eruption occurred from Tavurvur in October 2006. Whole-rock compositions are andesitic and reflect mixing/mingling between basaltic and dacitic magmas from the same system. The magmas that fed the 2006 eruption were stored at about 930°C, with 1–3 wt% H 2 O, 25–520 ppm CO 2 , and 50–2500 ppm SO 2 in the melt. Melt inclusions hosted in pyroxene, and plagioclase phenocrysts record fractional crystallization at ≤200 MPa under relatively dry and poorly oxidizing conditions. Magma mixing/mingling is expressed as heterogeneous glass compositions, strongly zoned phenocrysts, and mafic crystal aggregates. A textural maturation from fine, acicular to large, blocky crystal clots implies different relative ages of formation. Modelling the chemical zoning of plagioclase shows that mafic–silicic interactions started a couple of decades prior to the 2006 eruption and continued until days to weeks prior to eruption. Basaltic replenishments have been driving unrest and eruption at the Rabaul caldera since the 1970s. Supplementary material: Tables and figures reporting the composition of the Tavurvur 2006, Kombiu and 1.4 ka BP caldera samples and showing thermodynamic modelling with MELTS are available at http://www.geolsoc.org.uk/SUP18816

Sandstone Petrology and Geochemistry of the Kolhan Basin, Eastern India: Implications for Basin Tectonics

The 2.2-2.1 Ga pear shaped Kolhan basin show the development of a time transgressive group in a passive rift setting caused due to the fragmentation of the Columbia supercontinent. The Kolhans while showing a variability in the thickening/thinning of the quartzo-feldspathic, quartzolithic, and quartzose sandstones, do also show a similarity in compositional and dispersal characteristics to both underlying and overlying strata, phenomenon termed here congruence. A combined petrologic and geochemical analysis of sandstone suites (congruence suite) can be used to track changes in the sediment supply from adjacent areas if (1) a long-term record of the basin fill is available (2) the source signal is preserved by “proximal” depositional conditions and (3) diagenetic alteration of sediments is limited. Provenance-derived variations in sandstone compositions are therefore a key in unraveling regional tectonic histories. The basin axis controlled the progradation direction which was likely driven by climatically induced sediment influx, a eustatic fall, or both. In the case of the incongruent shift, increased sediment supply permitted the rivers to cross the basinal deep. Temporal association of the Kolhans with tectonic structures in the belt indicates that syntectonic thrust uplift, not isostatic uplift or climate, caused the influx of quartz. The Kolhans display increasing textural and mineralogical maturity from base to top of its lithological succession. Continued regression and peneplanation heralded the deposition of supermature sandstone in the uppermost horizons of the Kolhans.

Feature, origin and distribution of calcareous interlayers: A case of Carboniferous Donghe sandstone in Hade Oil Field, Tarim Basin, NW China

The petrologic characteristics, well-logging, genetic models and distribution patterns of calcareous inter-layers in Carboniferous Donghe sandstone, Hade Oil Field were investigated systematically by using data such as outcrop, cores, thin sections, stable carbon and oxygen isotopic compositions, conventional logging curves, and dip logging curves. Calcareous inter-layers in the study area are made up of calcareous fine sandstone or calcareous siltstone of high compositional and textural maturity, in which calcareous components include calcite, ferro-calcite, dolomite, ferro-dolomite and anhydrate. Although the calcareous inter-layers are usually thin, they can be identified by combining conventional logging curves with dip logging curves. Three genetic models of calcareous inter-layers are identified, including shallow burial fresh water, evaporation and deep burial. According to the development of sedimentary facies, it is inferred that calcareous interlayers are well-developed in shore face and backshore sandstones, and likely to join together and show continuity between wells; based on features of high-resolution sequence stratigraphy, it is concluded calcareous inter-layers in Donghe sandstone have five types of occurrence: “upper calcareous-lower argillaceous” type, “upper argillaceous-lower calcareous” type, “calcareous-argillaceous-calcareous” type, purely calcareous type and “calcareous-argillaceous complex” type.

Sedimentary characteristics of braided stream—braided fan of Shawan Formation in Chunfeng Oilfield, northwest margin of Junggar Basin, China

On the foundation of the geological background, textural maturity analysis, compositional maturity analysis, mudstone color index study, and other provenance research approach were fully made use of during this study to make certain the provenance of Shawan Formation in Chunfeng Oilfield. Provenance analysis of the Shawan Formation in Chunfeng Oilfield, conducted as the first stage of this study, revealed that the main provenance direction changed from northeast to northwest gradually, with a little fluctuation, while Shawan Formation was deposited. And then synthetically using seismic data, log data, and drilling data, according to petroleum geology and reservoir sedimentology, this study analyzed the sedimentary facies indicators, single-well facies, and logging facies in Shawan Formation; clarified the distribution of the sandbodies; and reached new insights on the types of the sedimentary facies of Shawan Formation in Chunfeng Oilfield. The research indicated that braided streams were developed during the deposition of the first member of Shawan formation and in the following two periods where braided fans became dominant little by little. These three microfacies, braided channel, channel bar, and braided plain, make up a typical stacked upward-fining cycle in the braided stream system. And the braided streams flowed from the northwest to the southeast in the research region, while the first member of Shawan Formation was formed. While the second member of Shawan Formation was deposited, the braided streams shrank gradually, and the braided fan coming from the northwest began to grow, with the flow direction moving from the northwest to the northeast. From the upstream to the downstream, the braided fan can be divided into three subfacies, proximal fan, middle fan, and distal fan. And when the third member of Shawan Formation was developed, the area of the braided fan became even larger than ever before, covering over almost the whole Chunfeng Oilfield.