Forced regressive wedge in the Mesoproterozoic Koldaha Shale,Vindhyan basin,Son valley,central India

The present paper highlights the sequence development within the Mesoproterozoic Koldaha Shale Member of the Kheinjua Formation, Vindhyan Supergroup which records the occurrence of a forced regressive wedge and associated discontinuity surfaces at the base of the wedge. Nine lithofacies have been identified within the study area that are grouped into three lithofacies associations varying in depositional setting from outer shelf, through shoreface-foreshore-beach to continental braidplain. The outer shelf sediments are aggradational to slightly progradational representing highstand systems tract. The rapidly progradational, wedge-shaped shoreface to foreshore-beach succession occurs sharply or erosively above the outer shelf sediments and is bounded by a regressive surface of marine erosion (RSME) at the base and by a subaerial unconformity at the top. This, along with its downstepping trajectory, supports deposition of this sedimentary wedge during falling sea level. A laterally extensive soft sediment deformation zone occurs at the base of the wedge.The forced regressive wedge is incised by fluvial braidplain deposits that rest on an erosive surface representing a sequence boundary. The thin braidplain deposits are the product of aggradation during a subsequent early rise in relative sea level, and thus, they are inferred to represent a lowstand systems tract. The constituent architectural elements that characterize the braidplain deposits are downstream accretion elements and small channel elements. Further landward, the base and top of the shoreface wedge merge to form an unconformity across deposits that rest directly on the outer shelf sediments. The identification of forced regressive wedges has significant economic importance in view of the potential occurrence of hydrocarbons within the Proterozoic formations.     

Loss with Time of Dredged Sea Sand in Tidal Embankment Subject to Sea Level Variation

In this study, loss with time of dredged sea sand in a tidal embankment subject to sea level variation was examined through the centrifugal model test. The experimental results demonstrate that a differential subsidence occurs on the surface of the dredged-sea-sand fill and that the largest subsidence was observed just above the damaged portion of the geosynthetic mat. In addition, image analysis provided the largest displacement vector at the damaged portion of the geosynthetic mat, the shear-strain localization from the damaged portion of the geosynthetic mat through the center of the slope surface, and the crest of the dredge-sea sand fill. These displacement vectors and shear strains occurred early in the experiment and increased over time. Therefore, the loss of dredged sea sand can occur rapidly with damage to the geosynthetic mat and can possibly induce differential subsidence and cracks at the surface of the dredged-sea sand fill.     

Microbial mat-induced sedimentary structures in siliciclastic sediments: Examples from the 1.6 Ga Chorhat Sandstone, Vindhyan Supergroup, M.P., India

This paper addresses macroscopic signatures of microbial mat-related structures within the 1.6Ga-old Chorhat Sandstone of the Semri Group — the basal stratigraphic unit of the Vindhyan succession in Son valley. The Chorhat Sandstone broadly represents a prograding succession of three depositional facies ranging from shallow shelf to coastal margin with aeolian sandsheet. The mat-mediated structures were generated because of plastic or brittle deformation of sand, turned cohesive and even thixotropic because of microbial mat growth. Mat growth also favoured abundant preservation of structures that usually have low preservation potential. Prolific growth of microbial mat in the subtidal to intertidal zone of the Chorhat sea was facilitated due to lack of grazing and burrowing activities of organisms in the Precambrian. It further indicates low rate of sedimentation between the storms, as also attested by frequent superposition of storm-beds, even near the storm wave base. It also reduces erosion and that, in turn, would imply low sediment concentration in flows leading to development of bedforms that are likely to be smaller in size and isolated from each other in a single train in contrast to those that form in mat-free sands.     

Gully Initiation and Implications for Management of Scour Holes in the Vicinity of the Jabiluka Mine, Northern Territory, Australia

A track across a burnt grass swale was used intensively on the Jabiluka Mineral Lease (located adjacent to Kakadu National Park in the seasonally wet tropics of the Northern Territory, Australia) for a short time period during the 1998 dry season. Repeated vehicle passes over the burnt grass increased soil bulk density and locally disrupted the root and algal mat, lowering the critical shear stress for sediment transport. Overland flow during the next wet season was above average and eroded eleven discontinuous, flow‐aligned scour holes in the wheel ruts where the track crossed grassed sandy swales. Although the site was burnt again during the next dry season, the scour holes did not coalesce during the second wet season, which was wetter than the previous one, because infrequent traffic bypassed the eroded section allowing grass to re‐establish. Scour holes on vehicle tracks in the Kakadu region are an intermediate but reversible stage in the development of gullies in grassed swales. Treatment of scour holes by soil conservation works may prevent gully formation.     

提高深基坑喷锚网支护效果的技术措施

结合工程设计和施工经验及有关资料,讨论了应用超前锚管、边坡补强、加强劲梁等措施来保证喷锚网支护的开挖深度和提高支护效果的技术方法     

海洋黏土超固结性和结构性对沉垫基础离底吸附力的影响

我国近海区域海床表面广泛存在一层深厚软黏土,具有一定的超固结性与结构性。自升式海洋平台沉垫基础的离底吸附力对海上平台的起浮回收影响显著。通过数值方法模拟了沉垫基础的作业工况,将可以描述土体超固结性和结构性的弹塑性本构模型通过UMAT子程序嵌入有限元软件ABAQUS中,采用可以描述材料强度破坏和刚度衰减的黏结接触模型模拟基础底与海床的相互作用,并通过现场模型试验对数值模型进行了验证。在此基础深入探讨了黏土海床超固结性和结构性对基础离底吸附力的影响规律。研究结果表明：沉垫基础上拔时,底部会产生负孔隙水压力,随上拔时间先减后增,且沉垫基础边缘位置率先发生离底;黏土超固结性越强,离底吸附力越大;黏土结构性越强,离底吸附力越小。     

An Experimental Investigation on Wave Loading on a Platform Mat

Overall wave loading and local hydrodynamic pressure distributions on a platform mat in reg-ular waves for shallow waters are experimentally investigated in order to examine the severity of thenonlinear effects and shallow water effects.Wave focusing phenomenon is observed in the tests.The meas-ured results may also provide a comparison basis for the theoretical development to consider the nonlinearinteraction between waves and viscous flow by introducing viscosity into wave theories.     

微生物席沉积学:一个年轻的沉积学分支

现代实例和岩石记录的研究表明,微生物席是一个特别的微生物群落,这个特殊的微生物群落就像一个复杂的食物网一样,群落中的每一个组成成员紧密相互依赖,从而构成了地球上形成最早、延续时间最长的生态系.微生物席在沉积岩中留下了丰富而且复杂的记录,在碳酸盐岩中最为典型的产物就是叠层石,在碎屑岩中最具有代表性的产物就是"微生物诱发的...