Nature and origin of magnetic minerals within the Middle Jurassic shallow-water carbonate rocks of the Paris Basin, France: implications for magnetostratigraphic dating

The Middle and Upper Jurassic Bathonian-Oxfordian shallow-water carbonate rocks from the Paris Basin, France, consist mainly of oolitic and bioclastic limestones that are hydrocarbon reservoirs in the subsurface. Despite a preliminary positive study, these deposits have been considered to be largely remagnetized (Rochette, private communication), and hence not amenable to palaeomagnetic dating. To establish their magnetic mineralogy and test this remagnetization hypothesis, we have used an integrated investigation combining petrographic, geochemical, rock-magnetic and palaeomagnetic measurements on samples extracted from five cores from the Paris Basin and from outcrops in Burgundy. Magnetic minerals in the Bathonian-Oxfordian carbonates include: (1) primary biogenic single-domain magnetite and detrital multi-domain Ti-magnetite and their oxidized form, maghemite; (2) authigenic spheres of magnetite probably related to hydrocarbons; and (3) goethite, either restricted to ferruginous ooid layers or resulting from surficial alteration, notably replacement of pyrite framboids. Rock-magnetic experiments carried out on 68 samples reveal H _cr/ H _c and M _rs/ M _s ratios ranging from 1.88 to 5.58 and 0.017 to 0.314, respectively. These values are clearly distinct from diagnostic values for a chemical remagnetization. Pyrrhotite was not identified within these sediments. Moreover, the average H _cr/ H _c ratio of 3.14 is significantly different from the value of 1.333 for natural pyrrhotite (Dekkers 1988). These results have a direct implication for the preservation of the primary magnetization; consequently, these deposits are selectively amenable for magnetostratigraphic dating and possible regional correlations.     

Determining Wettability of Fractured Carbonate Reservoirs

Wettability plays a pivotal role in oil recovery processes in reservoirs owing to the fact that it has a direct impact on the proportion of oil recovery. Reservoir rocks are a complex of a variety of mineral types, and each mineral may have a different wettability. Typically most aquifer materials such as quartz, carbonate, and dolomite are water-wet prior to oil migration. Some oil recovery processes are designed to alter the wetting preference of the oil formation to be more oil-wet. There are three techniques used in the laboratory for the characterization of the capillary pressure properties of core samples: mercury injection, centrifuge, and porous plate. This article provides an in-depth review of wettability. It also entails a case study from one of the Iranian carbonate reservoirs and measurements, which have been carried out for selected samples to clarify the nature of the wettability of reservoir rock. Samples were chosen from three wells of oil field which is composed of limestones and dolomites, rarely inter-bedded by shales, and their wettability was measured by Amott–Harvey method, which is one of the main methods for measuring wettability. After cleaning and drying the samples, measurements were carried out. According to the results, it can be said that the range of Amott–Harvey index for these samples is between ?0.48 and ?0.8. In Amott–Harvey method, if IAH varies from ?1 to ?0.3, the rock is oil-wet. Therefore, it can be concluded that the reservoir is oil-wet.     

Visible and near-infrared spectral survey of Martian meteorites stored at the National Institute of Polar Research

Martian meteorite chip samples stored at the National Institute of Polar Research (NIPR) have been studied by a visible and near-infrared (VNIR) spectrometer. Measured spots are about 3 × 2 mm in size, which are clearly marked on photographs of the meteorite chips. Rock types and approximate mineral compositions of studied meteorites have been identified or obtained through this spectral survey with no sample preparation required. This study demonstrates that this kind of spectral survey is effective in classifying and describing Marian meteorites, and that such a VNIR spectrometer on a Mars rover would be useful for identifying these kinds of unaltered Mars rocks. Further studies which utilize a smaller spot size are desired for improving the accuracy of identifying the clasts and mineral phases in the rocks.     

Provenance of siliciclastic and hybrid turbiditic arenites of the Eocene Hecho Group,Spanish Pyrenees: implications for the tectonic evolution of a foreland basin

The Eocene Hecho Group turbidite system of the Aínsa‐Jaca foreland Basin (southcentral Pyrenees) provides an excellent opportunity to constrain compositional variations within the context of spatial and temporal distribution of source rocks during tectonostratigraphic evolution of foreland basins. The complex tectonic setting necessitated the use of petrographic, geochemical and multivariate statistical techniques to achieve this goal. The turbidite deposits comprise four unconformity‐bounded tectonostratigraphic units (TSU), consisting of quartz‐rich and feldspar‐poor sandstones, calclithites rich in extrabasinal carbonates and hybrid arenites dominated by intrabasinal carbonates. The sandstones occur exclusively in TSU‐2, whereas calclithites and hybrid arenites occur in the overlying TSU‐3, TSU‐4 and TSU‐5. The calclithites were deposited at the base of each TSU and hybrid arenites in the uppermost parts. Extrabasinal carbonate sources were derived from the fold‐and‐thrust belt (mainly Cretaceous and Palaeocene limestones). Conversely, intrabasinal carbonate grains were sourced from foramol shelf carbonate factories. This compositional trend is attributed to alternating episodes of uplift and thrust propagation (siliciclastic and extrabasinal carbonates supplies) and subsequent episodes of development of carbonate platforms supplying intrabasinal detrital grains. The quartz‐rich and feldspar‐poor composition of the sandstones suggests derivation from intensely weathered cratonic basement rocks during the initial fill of the foreland basin. Successive sediments (calclithites and hybrid arenites) were derived from older uplifted basement rocks (feldspar‐rich and, to some extent, rock fragments‐rich sandstones), thrust‐and‐fold belt deposits and from coeval carbonate platforms developed at the basin margins. This study demonstrates that the integration of tectono‐stratigraphy, petrology and geochemistry of arenites provides a powerful tool to constrain the spatial and temporal variation in provenance during the tectonic evolution of foreland basins.     

The electrical conductivity of sandstone, limestone and granite

summary . The electrical conductivity of cylindrical cores of Westerly granite, Indiana limestone and Nugget, St Peter and Kayenta sandstones was measured at about 25°C in vacuo , in air, and after saturation in distilled water, tap water, and 0.1 MNaCl solution. The three-electrode technique with a guard ring and the twoelectrode technique without a guard ring were used. Core aspect ratio over the range of 2.00 to 0.25, as well as frequency over the range of 50 Hz to 10 kHz, influences the conductivity of all rocks, especially those measured in vacuo . Measurements from water-saturated samples using a guard ring are not appreciably different from those obtained without a guard ring. The conductivity of rocks saturated in 0.1 MNaCl solution changes least with a change in aspect ratio; for these rocks a linear relationship, known as Archie's Law, exists between log porosity and log conductivity. No simple correlation was found between those factors in rocks saturated with tap or distilled water. Thus, it appears Archie's Law is of questionable value for correlating laboratory data from rocks saturated with low-conductivity fluids.     

Constraining recycled detritus in quartz‐rich sandstones: Insights from a multi‐proxy provenance study of the Mid‐Carboniferous,Clare Basin,western Ireland

Quartz‐rich sandstones can be produced through multiple sedimentary processes, potentially acting in combination, such as extensive sedimentary recycling or intense chemical weathering. Determining the provenance of such sedimentary rocks can be challenging due to low amounts of accessory minerals, the fact that the primary mineralogy may have been altered during transport, storage or burial and difficulties in the recognition of polycyclic components. This study uses zircon and apatite U‐Pb geochronology, apatite trace elements, zircon‐tourmaline‐rutile indices and petrographic observations to investigate the sedimentary history of mineralogically mature mid‐Carboniferous sandstones of the Tullig Cyclothem, Clare Basin, western Ireland. The provenance data show that the sandstones have been dominantly and ultimately sourced from three basement terranes: older Laurentian‐ associated rocks (ca. 900–2500 Ma) which lay to the north of the basin, peri‐Gondwanan terranes (ca. 500–700 Ma) to the south and igneous intrusive rocks associated with the Caledonian Orogenic Cycle (ca. 380–500 Ma). However, the multi‐proxy approach also helps constrain the sedimentary history and suggests that not all grain populations were derived directly from their original source. Grains with a Laurentian or a Caledonian affinity have likely been recycled through Devonian basins to the south. Grains with a peri‐Gondwanan affinity appear to be first cycle and are potentially derived from south/southwest of the basin. Taken as a whole, these data are consistent with input into the basin from the south and southwest, with the reworking of older sedimentary rocks, rather than intensive first‐cycle chemical weathering, likely explaining the compositional maturity of the sandstones. This study highlights the need for a multi‐proxy provenance approach to constrain sedimentary recycling, particularly in compositionally mature sandstones, as the use of zircon geochronology alone would have led to erroneous provenance interpretations. Zircon, together with U‐Pb geochronology from more labile phases such as apatite, can help distinguish first‐cycle versus polycyclic detritus.     

The detrital record of late‐Miocene to Pliocene surface uplift and exhumation of the Venezuelan Andes in the Maracaibo and Barinas foreland basins

A multidisciplinary approach, combining sediment petrographic, palynological and thermochronological techniques, has been used to study the Miocene‐Pliocene sedimentary record of the evolution of the Venezuelan Andes. Samples from the Maracaibo (pro‐wedge) and Barinas (retro‐wedge) foreland basins, proximal to this doubly vergent mountain belt, indicate that fluvial and alluvial‐fan sediments of similar composition were shed to both sides of the Venezuelan Andes. Granitic and gneissic detritus was derived from the core of the mountain belt, whereas sedimentary cover rocks and uplifted foreland basin sediments were recycled from its flanks. Palynological evidence from the Maracaibo and Barinas basins constrains depositional ages of the studied sections from late Miocene to Pliocene. The pollen assemblages from the Maracaibo Basin are indicative of mountain vegetation, implying surface elevations of up to 3500–4000 m in the Venezuelan Andes at this time. Detrital apatite fission‐track (AFT) data were obtained from both stratigraphic sections. In samples from the Maracaibo basin, the youngest AFT grain‐age population has relatively static minimum ages of 5 ± 2 Ma, whereas for the Barinas basin samples AFT minimum ages are 7 ± 2 Ma. With exception of two samples collected from the Eocene Pagüey Formation and from the very base of the Miocene Parángula Formation, no evidence for resetting and track annealing in apatite due to burial heating in the basins was found. This is supported by rock‐eval analyses on organic matter and thermal modelling results. Therefore, for all other samples the detrital AFT ages reflect source area cooling and impose minimum age constraints on sediment deposition. The main phase of surface uplift, topography and relief generation, and erosional exhumation in the Venezuelan Andes occurred during the late Miocene to Pliocene. The Neogene evolution of the Venezuelan Andes bears certain similarities with the evolution of the Eastern Cordillera in Colombia, although they are not driven by exactly the same underlying geodynamic processes. The progressive development of the two mountain belts is seen in the context of collision of the Panama arc with northwestern South America and the closure of the Panama seaway in Miocene times, as well as contemporaneous movement of the Caribbean plate to the east and clock‐wise rotation of the Maracaibo block.     

基于允许流失量和正负地形源汇理论的喀斯特关键带土壤侵蚀研究

喀斯特地区下垫面的特殊复杂性使得地表发生显著坡面径流需要达到更大的降水累积阈值,碳酸盐岩岩石成土速率的缓慢和空间异质性导致喀斯特地区的土壤存量被严重高估和均一化等,因此,传统经典的土壤侵蚀模型在喀斯特地区难以适用。本文依据喀斯特关键带岩性的差异,确定其有效降水侵蚀阈值,并对降雨侵蚀力进行重新测算;根据碳酸盐岩化学成分的差异,计算其成土速率并作为土壤允许流失量;通过地貌—水文分析法提取喀斯特洼地空间分布信息,对喀斯特关键带土壤侵蚀算法进行多次改进和创新。结果显示：① 传统算法忽视了喀斯特关键带下垫面的特殊性,致使其平均降雨侵蚀力被高估47.35%,且喀斯特区域的平均降雨侵蚀力仅相当于非喀斯特区域的59.91%;② 传统算法可能将一些无土或少土可流区计算为土壤的高侵蚀量区,而连续性碳酸盐岩、碳酸盐岩夹碎屑岩、碳酸盐岩与碎屑岩互层的土壤允许流失量仅分别为0.21 t ha^-1 yr^-1、1.2 t ha^-1 yr^-1、2.89 t ha^-1 yr^-1;③ 传统算法通常将有坡度和耕作的洼地视为土壤侵蚀的高发区,但其实际应是地表侵蚀的沉积区,喀斯特洼地在空间上的连续性与碳酸盐岩的分布区基本重合;④ 传统算法高估土壤侵蚀面积27.79%,土壤侵蚀量47.72%。总之,传统经典模型会大大高估喀斯特地区的土壤侵蚀量,因此,应该建立一种精确适用的模型。另外,由于喀斯特地区的成土速率慢而土层薄、总量少,土壤允许流失量远低于非喀斯特区域侵蚀标准,应制定适用于喀斯特地区的土壤侵蚀分类分级标准和风险评价方法。     

Reliability of palaeointensity methods using alternating field demagnetization and anhysteretic remanence

Summary. Using natural volcanic rocks which acquired thermoremanence (TRM) in known fields, reliability of various palaeointensity methods using alternating field (AF) demagnetization were evaluated. Natural remanence (NRM), TRM and anhysteretic remanences (ARM's) before and after heating were stepwisely AF demagnetized following Shaw's method.
It was found that the coercivity spectra of TRM and ARM in these samples are very similar, and that even when changes occurred during heating, the changes for two remanences are quite similar in many samples. Therefore, Shaw's method of palaeointensity determination, which incorporates ARM checks to the conventional comparison of NRM and TRM coercivity spectra, gives results as reliable as those obtained by the Thellier method. Many examples were demonstrated in which TRM and ARM intensities changed substantially by heating, but without changes in the shape of their coercivity spectra. Such changes cannot normally be detected and erroneous palaeointensities with apparent internal consistency would be obtained by usual AF demagnetization methods.
Although ARM is quite similar to TRM, the rate of acquisition of ARM and TRM in weak fields varies by a factor of five among the present samples. To determine palaeointensities from the linear relation between ARM and TRM, it is necessary to determine experimentally the relative acquisition rate of these remanences. Therefore, methods based only on NRM-ARM relations would not give palaeointensities with acceptable errors.     

湖泊碳酸盐在过去环境变化研究中的应用

国内外的研究成果表明,湖泊沉积碳酸盐矿物及其微量元素、同位素分布特征记录了全球或区域古气候环境演变历史,可以从中提取大量定量化的古气候参数。因此,可以根据湖泊碳酸盐矿物的含量和碳、氧同位素变化等信息推知其形成时期的气候环境,重建古环境,揭示气候环境演变的规律。湖泊碳酸盐是一种研究古环境演化重要的代用指标,在过去气候环境变化研究中有广阔的应用前景。     

Rates Of Postglacial rock weathering on glacially scoured outcrops (Abisko–Riksgränsen area, 68°N)

Ice–polished quartz veins, feldspar phenocrysts and quartzite layers were used as reference surfaces to assess the impact of Postglacial rock weathering in Lapland (68°N). Over 3200 measurements were carried out on roches moutonées and glaciofluvially scoured outcrops distributed within three study areas covering 8 km². Inferred weathering rates demonstrate that 10,000 years of Holocene weathering did not significantly modify the geometry of Weichselian rock surfaces. However, rates of general surface lowering range from 1 to 25, depending on the rock type, with average values at 0.2 mm ka⁻¹ for homogeneous crystalline rocks (irrespective of their acidity and grain size), 1 mm ka⁻¹ for biotite–rich crystalline rocks, and 5 mm ka⁻¹ for carbonate sedimentary rocks. Accelerated rates were recorded in weathering pits and along joints with values up to ten times higher than on the rest of the rock surface. Comparisons with cold and temperate areas suggest that solution rates of carbonate rocks are highly dependent on climate conditions, whilst granular disintegration of crystalline rocks operates at the same rate whatever the environment. It probably means that microgelivation is not efficient on ice–polished crystalline outcrops even under harsh climate conditions, and that granular disintegration proceeds under various climates from the same ubiquitous combination of biochemical processes. Last, the weathering state of Late–Weichselian roches moutonées can be usefully compared to that of Preglacial tors of the nearby Kiruna area.     

Rates Of Postglacial rock weathering on glacially scoured outcrops (Abisko–Riksgränsen area, 68°N)

Ice–polished quartz veins, feldspar phenocrysts and quartzite layers were used as reference surfaces to assess the impact of Postglacial rock weathering in Lapland (68°N). Over 3200 measurements were carried out on roches moutonées and glaciofluvially scoured outcrops distributed within three study areas covering 8 km². Inferred weathering rates demonstrate that 10,000 years of Holocene weathering did not significantly modify the geometry of Weichselian rock surfaces. However, rates of general surface lowering range from 1 to 25, depending on the rock type, with average values at 0.2 mm ka⁻¹ for homogeneous crystalline rocks (irrespective of their acidity and grain size), 1 mm ka⁻¹ for biotite–rich crystalline rocks, and 5 mm ka⁻¹ for carbonate sedimentary rocks. Accelerated rates were recorded in weathering pits and along joints with values up to ten times higher than on the rest of the rock surface. Comparisons with cold and temperate areas suggest that solution rates of carbonate rocks are highly dependent on climate conditions, whilst granular disintegration of crystalline rocks operates at the same rate whatever the environment. It probably means that microgelivation is not efficient on ice–polished crystalline outcrops even under harsh climate conditions, and that granular disintegration proceeds under various climates from the same ubiquitous combination of biochemical processes. Last, the weathering state of Late–Weichselian roches moutonées can be usefully compared to that of Preglacial tors of the nearby Kiruna area.     

Experimental Study on the Effect of Moisture Content on Bituminous Coal Porosity Based on 3D Reconstruction of Computerized Tomography

Bituminous coal in the Xutuan Coal Mine of the Huaibei Mining Bureau (China) is the research object of this study. The influence of moisture content on the porosity of the bituminous coal was investigated from a microscopic perspective by using a high-solution 3D X-ray micro-analyzer. The threshold segmentation method was used to segment the scanning slices of the coal samples. The threshold values of the various media were in the following order (from large to small): minerals, water, matrices, and fractures. The scanning volume and actual volume proportions of the different media in the coal samples with different moisture contents were calculated. The accuracy of the computerized tomography (CT) scanning method in measuring the coal moisture content was verified by comparison with the results obtained using the weighing method. 3D reconstructed coal samples, with different moisture contents, were analyzed, as well as separately reconstructed fractures and water in the coal samples with different moisture contents. The heterogeneity and anisotropy of the coal mass were explained quantitatively by the CT scanning intensity. A commonly used fracture classification method indicated that the primary fracture in the coal sample was a type A fracture. The results of the analysis of water in the coal fracture indicated that the porosity of bituminous coal decreased with the increase in moisture content in conditions of atmospheric pressure and a short immersion period. However, a certain level of porosity remained evident, and the degree of fracture development of the coal samples remained unchanged. This is attributed to the minor volumetric change in the minerals in the coal samples, as the water does not completely occupy the fractures in the coal samples, and the dissolution of the minerals by water is therefore not significant. The reasons for the moisture content affecting gas adsorption, seepage, and strength of a coal body were analyzed from a microscopic perspective. In addition, the types of fractures and water in the coal samples were classified by employing statistics and analyses of volume, surface area, specific surface area, and aspect ratio of the fractures and the water in the coal samples with different moisture contents.

     

Ordovician tectonic shift in the western North China Craton constrained by stratigraphic and geochronological analyses

The western North China Craton (W-NCC) comprises the Alxa Terrane in the west and the Ordos Block in the east; they are separated by the Helanshan Tectonic Belt (HTB). There is an extensive debate regarding the significant Ordovician tectonic setting of the W-NCC. Most paleogeographic reconstructions emphasized the formation and rapid subsidence of an aulacogen along the HTB during the Middle–Late Ordovician, whereas paleomagnetic and geochronologic results suggested that the Alxa Terrane and the Ordos Block were independent blocks separated by the HTB. In this study, stratigraphic and geochronologic methods were used to constrain the Ordovician tectonic processes of the W-NCC. Stratigraphic correlations show that the Early Ordovician strata comprise ~500-m-thick tidal flat and lagoon carbonate successions with a progressive eastward onlap, featuring a west-deepening shallow-water carbonate shelf. In contrast, the Late Ordovician strata are composed of ~3,000-m-thick abyssal turbidites in the west and ~400-m-thick shallow-water carbonates in the east, defining an eastward-tapering basin architecture. Early Ordovician detrital zircons with ages of ~2,800–1,700 Ma were derived from the Ordos Block; the Late Ordovician turbidites were sourced from the western Alxa Terrane, based on zircon ages clustered at ~1,000–900 Ma. The petrographic modal composition and zircon age distribution imply a provenance shift from a stable craton to a recycled orogen in the Middle Ordovician. These shifts define a tectonic conversion from a passive continental margin to a foreland basin at ~467 Ma, resulting in the eastward progradation of the turbidite wedge around the HTB, the eastward backstepping of the carbonate platform in the east and the eastward expansion of orogenic thrusting in the western Alxa Terrane. This tectono-sedimentary shift coincided with the advancing subduction of the southern Paleo-Asian Ocean beneath the Alxa Terrane, generating the western Alxa continental arc and the paired retro-arc foredeep in the east under a compressional tectonic regime.     

Thermal and exhumation histories of the northern subalpine chains (Bauges and Bornes—France): Evidence from forward thermal modeling coupling clay mineral diagenesis,organic maturity and carbonate clumped isotope (Δ47) data

Assessing the thermal evolution of sedimentary basins over time is a major aspect of modern integrated basin analysis. While the behavior of clay minerals and organic matter with increasing burial is well documented in different geological and thermal settings, these methods are often limited by the temperature ranges over which they can be precisely applied and by the available material. Here, we explore the emergent Δ₄₇ clumped isotope geospeedometry (based on the diffusional redistribution of carbon and oxygen isotopes in the carbonate lattice at elevated temperatures) to refine time‐temperature paths of carbonate rocks during their burial evolution. This study provides a reconstruction of the thermal and exhumation history of the Upper Cretaceous thrust belt series in the western subalpine massifs (Bauges and Bornes, French Alps) by a new approach combining for the first time available data from three independent geothermometers. The investigated area presents two zones affected by contrasting thermal histories. The most external zone has undergone a relatively mild thermal history (T < 70°C) and does not record any significant clay mineral diagenetic transformation. By contrast, the internal zone has experienced tectonic burial (prealpine nappes) in response to thrusting, resulting in overheating (T > 160–180°C) that induced widespread clay mineral diagenetic transformations (progressive illitization from R0 to R1 and R3 illite‐smectite mixed‐layers), organic matter maturation (oil window) and Δ₄₇ thermal resetting with apparent equilibrium temperatures above 160°C. The three employed geothermal indicators conjointly reveal that the investigated Upper Cretaceous rocks have suffered a wide range of burial temperatures since their deposition, with a thermal maximum locally up to 160–180°C. High temperatures are associated with the tectonic emplacement of up to 4 km of prealpine nappes in the northern part of the studied area. Finally, a forward thermal modeling using Δ₄₇, vitrinite reflectance and clay mineral data, is attempted to precisely refine the burial and exhumation histories of this area.     

Effects of Fe3+ on Dissolution Dynamics of Carbonate Rocks in a Shallow Burial Reservoir

Dissolution of carbonates in acidic fluids, which has attracted much research attention in recent years, is of great significance for the formation of high-quality reservoirs. The dissolution stage under low temperature and low pressure in shallow burial is one of the most important processes of reservoir dissolution and transformation. However, the dissolution dynamics of carbonate rocks in shallow burial and their formation have been controversial for a long time, and there are still disputes in the dissolution processes about how associated minerals and accessory minerals (e.g., pyrite) in carbonate reservoirs influence the formation of secondary pores. Additional metal ions in acidic fluids can change fluid properties and dissolution processes, and consequently affect reservoir quality. However, there are few laboratory studies done on the effect of associated minerals on the dissolution dynamics of carbonates. To clarify the specific impact of Fe-bearing associated minerals and Fe³⁺ on the dissolution of carbonates in shallow burial reservoirs, six samples of typical carbonate rocks in the Zigui area of Hubei Province, China were studied. The dissolution kinetics of carbonates in dilute hydrochloric acid and sulfuric acid solutions containing metal ions (Fe³⁺/Ca²⁺/Mn²⁺) at ambient temperature and pressure (T?=?25 °C, P?=?1 atm) were studied, by laboratory dissolution experiments combined with numerical simulations using PHREEQC. The results show that the Fe³⁺ is of great significance on the dissolution of carbonate rocks, while the influences of Ca²⁺ and Mn²⁺ are relatively weak. The dissolutions degrees of micritic limestone (ZG-L25), dolomitic limestone (ZG-L7) and dolomite (ZG-D9) were better than the other carbonates under the influence of metal ions (Fe³⁺/Ca²⁺/Mn²⁺) in acid solutions. Therefore, the dolomite reservoir of the Cambrian Qinjiamiao Formation, the dolomitic limestone reservoir of the Tianheban Formation and the limestone reservoir of the Triassic Daye Formation in the Zigui area are potential high-quality reservoirs. The carbonate reservoirs associated with Fe-bearing minerals were easier to dissolve and formed secondary pores under shallow burial. This process is beneficial to the formation of high-quality reservoirs. Moreover, the addition of Fe³⁺ into hydrochloric acid solution may be conducive to improving the reservoirs acidizing effect. Furthermore, the results gave innovative results from multiple perspectives of geo-material science and computational geosciences, which may provide new avenues for in-depth study of carbonate dissolution in shallow burial based on water–rock reaction, chemical dissolution, computational simulation, and geological background.

     

Rock control on microweathering of bedrock surfaces in a periglacial environment

Microweathering of ice-smoothed bedrock surfaces was investigated in the Røldal area of Hardanger Plateau (60°), southern Norway. Postglacial rates of weathering were determined from surface lowering using quartz veins as reference surfaces. Weathering processes are inferred from assessment of weathering rind formation, surface hardness, and the preservation of small-scale glacial erosional features.Surface lowering rates for a range of metamorphic rocks vary from 0.05 to 2.20 mm ka^− 1 and are broadly comparable with those obtained from crystalline rocks in other periglacial environments. The mean rate of surface lowering at 0.55 mm ka^− 1 is low and demonstrates the relatively small impact of microweathering on postglacial landscape evolution. Variations in bedrock microweathering can be explained by lithological variation. Amphibolite and mica-rich bedrock surfaces experience greater denudation and weakening, least weathering rind formation, and abundant preservation of glacial striae, despite greater surface lowering. Conversely, quartz-rich bedrock surfaces are most resistant to denudation and weakening, but have greater weathering rind formation and fewer preserved striae. Postglacial microweathering is achieved primarily through granular disintegration involving detachment and removal of mineral grains and weakening from increased porosity. Granular decomposition is manifest in the formation of weathering rinds. Analysis of interactions between weathering indices indicates that rind accumulation is limited by microerosion.A conceptual model is proposed that illustrates the temporal interrelationships between in situ and erosional facets of microweathering in two contrasting mineral assemblages. The model proposes that cyclic processes of in situ disintegration, decomposition, and erosion are at work. The relative balance between these processes varies with lithology so that in more resistant quartz-rich rocks the net effect is minimal surface lowering and accumulation of weathering rind. In weaker, amphibolitic and micaceous rocks, the net effect is greater surface lowering and minimal accumulation of weathering rind. The results of the research demonstrate the important influence of rock properties, notably mineral composition, in postglacial microweathering of crystalline bedrock in a periglacial environment.     

Coastal tropical morphogenesis of the Islands in the Andaman Sea (Thailand)

The characteristics of topography of the coastal zones composed of carbonate and crystalline rocks are considered. In the coastal zones there occur outcropping tropical karst, rock ledges shaped like greywethers or coastal balls having a subaerial origin. An outer resemblance of these convergent formations with abrasion landforms is highlighted.     

Comparison of radiocarbon and OSL dating methods for a Late Quaternary sediment core from Lake Ulaan,Mongolia

Both radiocarbon and optically stimulated luminescence (OSL) dating methods were applied to test their suitability for establishing a chronology of arid-zone lacustrine sediments using a 5.88-m-long core drilled from Lake Ulaan, southern Mongolia. Although the radiocarbon and OSL ages agree in some samples, the radiocarbon ages are older than the corresponding OSL ages at the 550-cm depth horizon (late Pleistocene) and in the 100–300-cm interval (early to late Holocene). In the early to late Holocene, radiocarbon ages are consistently older than OSL ages by 4,100–5,800 years, and in the late Pleistocene by 2,700–3,000 years. Grain-size analysis of early to late Holocene sediments and one late Pleistocene sediment sample (550-cm depth) indicates that eolian processes were the dominant sediment-transport mechanism. Also, two late Pleistocene sediments samples (from 400- to 500-cm depths) are interpreted to have been deposited by both eolian and glaciofluvial processes. Accordingly, the radiocarbon ages that were older than the corresponding OSL ages during the Holocene seem to have been a consequence of the influx of ¹⁴C-deficient carbon delivered from adjacent soils and Paleozoic carbonate rocks by the westerly winds, a process that is also active today. In addition to the input of old reworked carbon by eolian processes, the late Pleistocene sediments were also influenced by old carbon delivered by deglacial meltwater. The results of this study suggest that when eolian sediment transport is suspected, especially in lakes of arid environments, the OSL dating method is superior to the radiocarbon dating method, as it eliminates a common ‘old-carbon’ error problem.     

Single-grain zircon dating of the metamorphic and granitic rocks from the Biscayarhalvøya-Holtedahlfonna zone, north-west Spitsbergen

The Biscayarhalvøya-Holtedahlfonna zone (BHZ) in north-western Spitsbergen is a north-south trending, narrow horst, with crystalline basement rocks exposed under a Devonian unconformity. Previous K-Ar, ⁴⁰Ar/³⁹Ar and Rb-Sr analyses have confirmed the occurrence of Caledonian thermal events, and Grenvillian ages have been obtained by conventional zircon U-Pb and single-zircon Pb evaporation methods. A total of 55 zircon grains from three samples (an augen metagranite, a micaceous schist and a granitic neosome of migmatite) have been analysed by the single-zircon Pb evaporation method. The grains with the age range of ca. 950-1100 My (million years) are the major component in all three samples, suggesting tectono-thermal activity in that period. The detrital versus resorption orgin of the rounded shapes of these grains from the granitic neosome is not clear yet. Therefore, the ages of the migmatization and of the sedimentary protoliths are not concluded. The youngest presumed detrital grain from the granitic neosome is 1060 My old. The metagranite, cutting the Richarddalen unit, yielded grains with an age of ca. 950 Mya. A granite dyke with an age range of 955-968 My cuts the Biscayarhuken unit in the northern Liefdefjorden area. These indicate the sedimentary protoliths of the Richarddalen and Biscayarhuken units are pre-Neoproterozoic. The youngest detrital zircon ages of ca. 940 My indicate Neoproterozoic sedimentary protoliths of the Solanderfjellet micaceous schists. A significant population of zircon grains with an age range of 1600-1900 My in all three samples suggests a wide exposure of these rocks in the source areas during Meso- and Neoproterozoic times. Several Archean ages have also been obtained. The results are generally conformable with those obtained from north-western Spitsbergen.