Limits to the age of the Lapstone monocline,N.S.W.—a palaeomagnetic study

The age of formation of the Lapstone Monocline has been long considered to be late Pliocene/early Pleistocene (the Kosciusko Uplift) but it is now generally thought to be much older. Palaeomagnetic data from haematite‐rich beds within the Hawkesbury Sandstone on and about the monocline indicate that it formed before the oldest haematite was introduced to these beds. The age of this oldest haematite is 15 ± 7 Ma. On the basis of these data, the age of the monocline is unlikely to be less than 8 Ma, probably exceeds 15 Ma, and could be older than 22 Ma.     

Sedimentary record of Late Paleozoic rift and break-up in Northern Gondwana: a case history from the Thini Chu Group and Tamba-Kurkur Formation (Dolpo Tethys Himalaya,Nepal)

The Gottero unit of the Northern Apennines, Italy, is representative of the sedimentary cover of the Ligure-Piemontese oceanic lithosphere. This unit consists of a thick sedimentary sequence that includes Valanginian-Santonian pelagic deposits and Campanian-early Paleocene turbiditic deposits. The latter are overlain by early Paleocene trench deposits related to frontal tectonic erosion of the accretionary wedge slope. This sequence is interpreted as recording trenchward motion of the oceanic lithosphere.

The Gottero unit records a pre-Late Oligocene, complex deformation history related to subduction and accretion events. This deformation history has developed through underthrusting (D1a), underplating (D1b and D1c) and later exhumation (D2a and D2b) episodes. The folding phase related to the main underplating sub-phase (D1b) is predated by a sub-phase (D1a) connected to rapid fluid escape and followed by a sub-phase dominated by the development of shear zones (D1c). The D1b sub-phase is characterized by similar folds and a slaty cleavage developed under P/T conditions of 0.4GPa/210°-270 °C. The D1c sub-phase, characterized by west-verging thrusts, is particularly signficative in understanding the dynamics of the Ligure-Piemontese accretionary wedge because it testifies active shortening of the Gottero unit also after its transfer to the prism. In addition, sub-phase D1c represents the transition from the sub-phases connected to accretion and the tectonics dominated by extension, characterized by parallel folds and low-to high-angle normal faults. The gravity driven extension is represented by the D2a and D2b sub-phases and can be interpreted as the result of the thicknening of the Ligure-Piemontese accretionary wedge, produced by continuous underplating at its base but also by shortening of the previously underplated units. These final tectonic events resulted in the exhumation of the Gottero unit to the surface during the Early Oligocene, when this Unit became one of the source areas of the conglomerates deposited in the Tertiary Piedmont basin.

This deformation history suggests the occurrence of a complex sequence of deformations during the transition from accretion to exhumation, even in the intermediate levels of the accretionary wedge.     

非饱和岩石中的纵波频散与衰减: 双重孔隙介质波传播方程

本文采用Rayleigh理论描述纵波激励下非饱和岩石中气泡的局域流体流动,从经典力学的哈密顿原理导出了双重孔隙介质中的波传播方程,即Biot-Rayleigh方程.方程的格式简洁,参数少,所有相关参数物理可测,因此,方程具有较好的物理可实现性.基于相同的岩石与前人理论对比,初步验证了本理论的有效性.对三个地区的砂岩储层进行了分析,结果显示:地震频段内纵波对储层是否含气非常敏感,但对含气饱和度指示性不佳,且随着孔隙度降低,纵波频散与衰减在中低频段更为显著;含甲烷与含二氧化碳的砂岩储层均呈第三类AVO响应特征,很难从叠前分析技术中鉴别;理论预测的纵波频散随饱和度与频率变化的趋势与特征,与多频段实验观测结果一致.     

基于微CT技术的砂岩数字岩石物理实验

数字岩石物理技术可弥补传统岩石物理实验的诸多不足,为岩石物理学研究提供一个新平台.本文以常规砂岩为研究对象,利用微CT扫描结合先进的图像处理技术建立了具有真实孔隙结构特征的三维数字岩芯模型;应用Avizo软件内含的多种形态学算法进行数字岩芯孔隙结构量化及表征研究,统计获取了孔隙度、孔隙体积分布及孔径分布特征,建立了等价孔隙网络模型;将Avizo与多场耦合有限元软件Comsol完美对接,实现了孔隙尺度的渗流模拟并计算获得绝对渗透率,对于考虑固相充填孔隙的情况,模拟计算了岩石有效弹性参数,并与近似Gassmann方程良好验证.本文所提出的将Avizo与Comsol结合使用的方法丰富了现有的数字岩石物理研究手段,为其大规模发展提供了一条新途径.     

X-ray Computed Tomography study of the influence of consolidants on the hydric properties of sandstones for stone conservation studies

X-ray Computed Tomography (X-ray CT) has been used to study the petrophysical characteristics of a Jurassic sandstone from Asturias (Northern Spain) used as building stone in several monuments in the region. CT monitoring of water capillarity tests has made it possible to visualize the movement of water inside the samples, to relate this movement with texture characteristics, and to measure the height reached in successive images, thereby determining the capillary penetration coefficient; based on this coefficient, the effective capillary pore radius has also been estimated. An advantage of the use of CT is that, as the information comes from the sample interior, border effects that can be generated in the external faces can be avoided.The CT was also used to visualize how a commercial organosilicon consolidant penetrates inside the rock by means of capillarity, a usual way of consolidant application in stone restoration processes. The coefficient of capillary penetration of the product, determined on the basis of the measurements made on the tomographic images, is 53% less than that of water. The influence of rock texture characteristics on the capillarity both of the consolidant and the water was also observed.The samples underwent CT water capillarity studies before and after the consolidant application, proving that the treatment improves the rock's hydric properties. The coefficient of capillary penetration decreased by 24% following the application of the consolidant, whereas the effective pore radius decreased by more than 40%.     

Stable and clumped isotopes in desert carbonate spring and lake deposits reveal palaeohydrology: A case study of the Lower Jurassic Navajo Sandstone,south‐western USA

Carbon, oxygen and clumped isotope (Δ₄₇) values were measured from lacustrine and tufa (spring)‐mound carbonate deposits in the Lower Jurassic Navajo Sandstone of southern Utah and northern Arizona in order to understand the palaeohydrology. These carbonate deposits are enriched in both ¹⁸O and ¹³C across the basin from east to west; neither isotope is strongly sensitive to the carbonate facies. However, ¹⁸O is enriched in lake carbonate deposits compared to the associated spring mounds. This is consistent with evaporation of the spring waters as they exited the mounds and were retained in interdune lakes. Clumped isotopes (Δ₄₇) exhibit minor systematic differences between lake and tufa‐mound temperatures, suggesting that the rate of carbonate formation under ambient conditions was moderate. These clumped isotope values imply palaeotemperature elevated beyond reasonable surface temperatures (54 to 86°C), which indicates limited bond reordering at estimated burial depths of ca 4 to 5 km, consistent with independent estimates of sediment thickness and burial depth gradients across the basin. Although clumped isotopes do not provide surface temperature information in this case, they still provide useful burial information and support interpretations of the evolution of groundwater locally. The findings of this study significantly extend the utility of combining stable isotope and clumped isotope methods into aeolian environments.     

The provenance,petrology and sedimentology of building stone in Bromyard,Herefordshire, UK

A multi-disciplinary study was undertaken of the Lower Old Red Sandstone building stones of Bromyard, Bromyard Downs and Bringsty Common, Herefordshire, UK, to trace their original quarry sources. 196 stone structures were recorded in detail and 3 broad lithological groups recognised. Combining old maps, archival sources, Light Detection and Ranging (LiDAR) data and testimony of local residents, the historic quarries in the area have been mapped. Field studies of the geology of quarries and outcrops have established the area of origin of two distinctive coarse grained lithologies. Intraformational conglomerates have been demonstrated, with some confidence, to be derived from Bromyard itself. Coarse, pebbly, quartzose sandstones have been traced to Bringsty Common and shown to be used only in the church in Bromyard before the 20th century. The origins of fine grained sandstone are found to be more complex. Through a combination of fieldwork and archival research, Clater Park Quarry is indicated as the origin of high quality green sandstones in Bromyard. 15 samples of sandstone from the Bromyard Downs, have been petrographically analysed by point counting to determine their clastic and mineralogical proportions. Early results of this analysis are presented which show modal variation in lithics, feldspars and cements that may serve to link quarries to buildings. Finally, it is suggested that at least some of the diversity of stone present in Bromyard is the result of slight variation in depositional processes in the immediately underlying bedrock.     

Seismic inversion as a predictive tool for porosity and facies delineation in Paleocene fluvial/lacustrine reservoirs,Melut Basin,Sudan

The Melut Basin is a rift basin in the interior Sudan linked to the Mesozoic-Cenozoic Central and Western African Rift System. The Paleocene Yabus Formation is the main reservoir deposited in heterogeneous fluvial/lacustrine environment. Delineation of channel sandstone from shale is a challenge in reservoir exploration and development. We demonstrate a detailed 3D quantitative seismic interpretation approach that integrates petrophysical properties derived from well logs analysis. A porosity transform of acoustic impedance inversion provided a link between elastic and rock properties. Thus, we used seismic porosity to discriminate between different facies with appropriate validation by well logs. At the basin scale, the results revealed lateral and vertical facies heterogeneity in the Melut Basin. Good reservoir quality is observed in the Paleocene Yabus Formation. The sand facies indicated high porosity (20%) corresponding to low acoustic impedance (20000–24000 g ft/(cm3.s)). However, lower quality reservoir is observed in the Cretaceous Melut Formation. The porosity of sand/shale facies is low (5%), corresponding to high acoustic impedance (29000–34000 g ft/(cm3.s)). This suggests that the Yabus Sandstone is potentially forming a better reservoir quality than Melut Formation. At the reservoir scale, we evaluated the facies quality of Yabus Formation subsequences using petrophysical analysis. The subsequences YB1 to YB3, YB4 to YB7 and YB8 to YB10 showed relatively similar linear regressions, respectively. The subsequence of YB4 to YB7 is considered the best reservoir with higher porosity (25%). However, subsequence YB1 to YB3 showed lower reservoir quality with higher shale volume (30%). This attributed to floodplain shale deposits in this subsequence. Similarly, the high porosity (20%) recognized in deeper subsequences YB6 to YB9 is due to clean sand facies. We learnt a lesson that appropriate seismic preconditioning, exhaustive petrophysical analysis and well log validation are important keys for improved reservoir quality prediction results in fluvial/lacustrine basins.     

Permeability Upscaling Measured on a Block of Berea Sandstone: Results and Interpretation

To physically investigate permeability upscaling, over 13,000 permeability values were measured with four different sample supports (i.e., sample volumes) on a block of Berea Sandstone. At each sample support, spatially exhaustive permeability datasets were measured, subject to consistent flow geometry and boundary conditions, with a specially adapted minipermeameter test system. Here, we present and analyze a subset of the data consisting of 2304 permeability values collected from a single block face oriented normal to stratification. Results reveal a number of distinct and consistent trends (i.e., upscaling) relating changes in key summary statistics to an increasing sample support. Examples include the sample mean and semivariogram range that increase with increasing sample support and the sample variance that decreases. To help interpret the measured mean upscaling, we compared it to theoretical models that are only available for somewhat different flow geometries. The comparison suggests that the nonuniform flow imposed by the minipermeameter coupled with permeability anisotropy at the scale of the local support (i.e., smallest sample support for which data is available) are the primary controls on the measured upscaling. This work demonstrates, experimentally, that it is not always appropriate to treat the local-support permeability as an intrinsic feature of the porous medium, that is, independent of its conditions of measurement.     

云冈石窟砂岩中碳酸盐胶结物化学风化及相关文物病害研究

云冈石窟地区云冈组上段砂岩中碳酸盐胶结物的含量普遍在10%以上,其主要矿物成分为含铁方解石和含铁白云石。在碳酸盐胶结物化学风化过程中发生的化学反应包括碳酸盐矿物溶解、Fe2+ 离子氧化及Fe3+ 离子水解,以及石膏、泻利盐、水碳镁石、球碳镁石等矿物结晶。水、砂岩中裂隙和层理发育、大气污染等是影响碳酸盐胶结物化学风化的主要外因。碳酸盐胶结物化学风化引发了多种文物病害,对云冈石窟的文物价值造成严重破坏。