The seismic property of reef flat in the Changxing-Feixianguan Formation, Northwest Sichuan Province, China

The development of a carbonate reservoir is dominated by its primary sedimentary environment. Generally, a good reservoir must be the oolite of a bank or bioclastics of an organic reef in a marginal mesa, which must deposit on the slope of the mesa. Therefore, the analyses of sequence stratigraphy are the key for predicting the presence of carbonate reservoirs. A typical sequence pattern can be seen in the Puguang gas field. As an independent lithological body, the mechanical property of a reef flat is very different from the surrounding rock, causing stress to concentrate. Stress centralization will, in turn, cause a lot of fractures and faults to develop, and ultimately, result in a gas chimney, which is a seismic smear zone beneath or above the reef flat. Known gas fields, such as the puguang gas field, also possess gas chimneys. Sequence stratigraphy patterns in the marginal mesa and gas chimney near the reservoir are very obvious in all known fields in southeast and east Sichuan Province. By analyzing the seismic features of all known gas fields, a model to predict carbonate reservoirs has been set up. Using this prediction model, we have found that there is a typical reef flat of the marginal mesa in the Malubei structure of Tongnanba structural belts whose sequence stratigraphy pattern is very much like that of the Puguang gas field; and in the Cangxi-Jiulongshan structure, there is another reef flat which is symmetrical to the Malubei reef. The Cangxi reef flat possesses the largest platform edge shallow facies domestically found, and has a typical prograding sequence. Its gas chimney is very obvious too. Because the Cangxi reef flat is in the very vicinity of the hydrocarbon kitchen and the depression is much bigger than that of the Puguang gas fields, we predict that there may be a gigascopic gas field in Cangxi County. There may also be a large gas field in the Malubei structure. Translated from Oil & Gas Geology, 2006, 27(3): 332–339 [译自: 石油与天然气地质]     

Petrogenesis and Geodynamic Implications of Late Jurassic Diorite Porphyry in the Neoproterozoic Ophiolitic Mélange of NE Jiangxi (South China)

Mesozoic magmatism is widespread in the eastern South China Block and has a close genetic relationship with intensive polymetallic mineralization. However, proper tectonic driver remains elusive to reconcile the broad intracontinental magmatic province. This study presents integrated zircon U-Pb dating, Hf isotopes and whole-rock geochemistry of the Xiwan dioritic porphyry in the NE Jiangxi ophiolitic mélange. Zircon U-Pb dating by SIMS and LA-ICP-MS methods yielded an emplacement age of ～160 Ma for the Xiwan diorite, confirming its inclusion into the Mesozoic magmatic province in SE China, instead of a component of the Neoproterozoic ophiolitic mélange genetically. The dioritic rocks have low Si02(58.08 wt%-59.15 wt%), and high Na_2 O(5.00 wt%-5.21 wt%) and MgO(4.60 wt%-5.24 wt%) contents with low TFeO/MgO ratios(1.02-1.09). They show an adakitic geochemical affinity but exhibit relatively low Sr/Y ratios(24.8-31.1) and high Y contents(14.6-18.3 ppm) compared to the Dexing adakitic porphyries. In addition, the Xiwan diorites have moderately evolved zircon Hf isotopic compositions(ε_(Hf)(t)=-6.1--0.1; T_(DM2)=1597-1219 Ma). These elemental and isotopic signatures suggest that the Xiwan diorite formed through partial melting of a remnant arc lower crust(i.e., early Neoproterozoic mafic arc-related rocks) in response to the underplating of coeval mafic magmas. In conjunction with the temporal-spatial distribution and complex geochemical characteristics of the Mesozoic magmatism, our case study attests to the feasibility of a flat-slab subduction model in developing the broad intracontinental magmatic province in SE China. The flat-slab delamination tends to trigger an asthenospheric upwelling and thus results in extensive partial melting of the overlying lithospheric mantle and lower crustal materials in an extensional setting during the Mesozoic.     

三维激光扫描仪平缓地形分区扫描方法研究

三维激光扫描仪对平缓地形扫描作业时,存在点密度不均匀、冗余数据多、有效扫描半径小等限制。为减少上述限制,提高作业效率,在深入分析三维激光扫描仪工作原理的基础上,提出分区扫描方法:根据到扫描仪中心距离的不同,将扫描范围分为若干环状区域,每个环状区域分别对应不同的竖直扫描夹角,各区域基于扫描点密度均衡的原则,设置不同的扫描参数。试验结果表明:竖直扫描角度分别为88°~89°、85°~88°、70°~85°、45°~70°的分区扫描方法为较理想的方法,与整体扫描方式相比,前者在作业效率、有效扫描半径等方面均有明显优势,数据点密度分布得到很大改善,利用效率明显提高。     

Diatom‐based tidal‐level transfer functions as an aid in reconstructing Quaternary history of sea‐level movements in the UK

This research analyses the diatom asssemblages recorded from six UK coastal sites and relates these diatom assemblages to tidal levels. The relationship between diatom assemblages and tidal levels is examined statistically in order to develop a diatom‐based tidal‐level transfer function. The results suggest that there is highly significant correlation between the diatom assemblages and water levels from mean high water of neap tides to highest astronomical tide (p = 0.01, 99 random permutations). A weighted average (WA) transfer function is thus established, and the predictive ability of this transfer function is highly satisfactory. Finally, this transfer function is applied successfully to estimate palaeotidal‐levels from fossil diatom data recorded in late Holocene coastal sequences. Copyright © 1999 John Wiley & Sons, Ltd.     

Groundwater,salt and contaminant transport in a coastal aquifer system with a low-permeability layer in the intertidal zone

Low-permeability layer (LPL), formed by natural deposit or artificial reclamation and commonly found below the intertidal zone of coastal groundwater system, can retard the ingress of seawater and contaminants, and shorten the travel time of the land-sourced contaminant to the marine environment compared with a homogenous sandy coastal aquifer. However, there is limited understanding on how an intertidal LPL, a condition occurred in a coastal aquifer at Moreton Bay, Australia, influences the groundwater and contaminant transport across the shallow beach aquifer system. We characterized the aquifer hydrological parameters, monitored the in situ groundwater heads, and constructed a 2-D numerical model to analyses the cross-shore hydrological processes in this stratified system. The calibrated model suggests that in the lower aquifer, the inland-source fresh groundwater flowed horizontally towards the sea, upwelled along the freshwater–saltwater interface, and exited the aquifer at the shore below the LPL. Whereas in the upper aquifer, the tidally driven seawater circulation formed a barrier that prevented fresh groundwater from horizontal transport and discharge to the beach above the LPL, thereby directing its leakage to the lower aquifer. A contaminant represented by a conservative tracer was ‘released’ the upper aquifer in the model and results showed that the spreading extent of the contaminant plume, the maximum rate of contaminant discharge to the ocean, and its plume length decreased compared with a simulation case in a homogenous sandy aquifer. Sensitivity analysis was also conducted to investigate the characteristics of the LPL, including its continuity and hydraulic conductivity, which were found to vary along the beach at Moreton Bay. The result shows that with a lower hydraulic conductivity and continuous layer of LPL reduced the groundwater exchange and contaminant transport between upper and lower aquifer. The findings from the combined field and modelling investigations on the impact of an intertidal LPL on coastal aquifer systems highlight its significant implications to alter the groundwater and mass transport across the land–ocean interface.     

A flux-limiting wetting–drying method for finite-element shallow-water models,with application to the Scheldt Estuary

We present a flux-limiting wetting–drying approach for finite-element discretizations of the shallow-water equations using discontinuous linear elements for the elevation. The key ingredient of the method is the use of limiters for generalized nodal fluxes. This method is implemented into the Second-generation Louvain-la-Neuve Ice-ocean Model (SLIM), and is verified against standard test cases. The method is further applied to the wetting and drying of sand banks in the Scheldt Estuary, which is located in northern Belgium and the southern Netherlands. The results obtained for both the benchmarks and the realistic problem illustrate the accuracy of the method in describing the hydrodynamics in the vicinity of dry areas. In particular, the method strictly conserves mass, and there is no transport through dry areas.     

Evidence of free convection in groundwater: Field-based measurements beneath wind-tidal flats

Free convection caused by salinity differences had not been conclusively detected or measured in the field. A field experiment at wind-tidal flats on Padre Island National Seashore, Texas, documents salinity-driven free convection with both direct (head and salinity data) and indirect (time-lapse 3-D resistivity) methods. Evaporative concentration of groundwater near the water table created unstable inverted density gradients, reduced groundwater levels, and reversed hydraulic gradients. These factors allowed plumes or fingers of more saline, denser fluid to flow downward into less-dense fluid as observed in monitoring wells and 3-D surveys. The development of density inversions can overcome the dissipating forces of dispersion and diffusion to create a sufficiently large unstable gradient to induce free convection. The development of free convective flow of variable-density fluids in groundwater can be detected and monitored through field techniques.     

黄河三角洲东营港潮坪上的贝壳沉积

黄河三角洲东营港的泥质潮坪上发育大量的贝壳沉积。贝壳的特殊形态使其很容易被水流搬运,波浪的簸选、泥坪表面的侵蚀坑穴和植物根、堤坝等障碍物对贝壳的拦阻、与地形地貌或涨潮流的水流形态有关的流速降低都可以形成贝壳堆积。潮坪表面贝壳的沉积形态包括:与涨潮流有关的坑穴状、密集窝状、陡坎状、垄沟状、席状、带状以及潮汐水道中的混合沉积等类型;与波浪作用有关的堤坝状、堤坝遮挡沉积等类型。涨潮流流速降低的方式不同,贝壳沉积形态也会相应有较大的差别。另外,潮坪表面的细微起伏、潮坪底质类型和粘结性都会对贝壳的沉积形态造成较大影响,其中尤其是以密集窝状和陡坎状为特征。从东营港贝壳的沉积特征上看,潮坪上贝壳沉积的水动力条件与常见的砂砾滩、鲕粒滩、内碎屑滩有很大的不同,不能将介壳的富集一概归为滩坝相沉积。