Transformation of garnet epidote amphibolite to eclogite, western Dabie Mountains, China

Omphacite and garnet coronas around amphibole occur in amphibolites in the Hong'an area, western Dabie Mountains, China. These amphibolites consist of an epidote–amphibolite facies assemblage of amphibole, garnet, albite, clinozoisite, paragonite, ilmenite and quartz, which is incompletely overprinted by an eclogite facies assemblage of garnet, omphacite and rutile. Coronas around amphibole can be divided into three types: an omphacite corona; a garnet–omphacite–rutile corona; and, a garnet–omphacite corona with less rutile. Chemographic analysis for local reaction domains in combination with petrographical observations show that reactions Amp + Ab + Pg = Omp +Czo + Qtz + H₂O, and Amp + Ab = Omp ± Czo + Qtz + H₂O may lead to the development of omphacite coronas. The garnet–omphacite–rutile corona was formed from the reaction Amp + Ab + Czo + Ilm ± Qtz = Omp + Grt + Rt + H₂O. In garnet–omphacite coronas, the garnet corona grew during an early stage of epidote amphibolite facies metamorphism, whereas omphacite probably formed by the reactions forming the omphacite corona during the eclogite facies stage. It is estimated that these reactions occurred at 0.8–1.4 GPa and 480–610 °C using the garnet–clinopyroxene thermometer and omphacite barometer in the presence of albite.     

Late Pleistocene-Holocene Paleoclimatic History of the Southern Kerala Basin, Southwest India

The South Kerala Sedimentary Basin (SKSB) constitutes one of the most significant landward extensions of the southwest offshore sedimentary basins of India, and is situated between 8^o45^' and 10^o15^' N latitudes. With a maximum width of about 20 km and incorporating a 700 m thick sedimentary succession ranging in age from Early Miocene to Holocene, this belt lies almost entirely under water or alluvium-covered coastal plains. In this study, we use two continuously cored bore holes at Eruva (7.25m deep) and Muthukulam (3m deep) separated by a distance of about 7km to investigate the depositional environment as well as paleo ecology of the depocentre and climatic aspects during the Late Pleistocene and Holocene. Results from C¹⁴ dating of shell fragments from Eruva (depth zone: 2.10–6.64m) yielded ages in the range of 36.2 to 42.5 kyBP corresponding to the late Pleistocene. Wood fragments in the Muthukulam core sample (depth zone: 1.27–3.00m) gave C¹⁴ ages in the range of 3.7 to 7.2 kyBP indicating a Holocene history. The lower half of the Eruva bore hole indicates a marginal marine environment with an abundant supply of terrestrial carbonaceous debris probably corresponding to a period of abnormally high rainfall recorded in many parts of the globe covered by the Asian summer monsoon. The sediments in the upper part of this bore hole indicate a continuation of this environment but with much less input of terrestrial organic carbon. The lower part of the bore hole from this locality, corresponding to the Holocene transgression, is similar to the lower part of Eruva bore hole in the case of TOC. Deposition took place in water bodies with considerable marine influence but receiving high amounts of terrestrial plant debris-mostly in the form of finely divided particles mixed with mud. This transgressive sequence was also deposited during a time when the Asian summer monsoon was abnormally high in intensity as indicated by many examples in India, Africa, Madagascar and elsewhere. It is significant that during the deposition of this part, the sea level was probably the same as present or higher suggesting possible lowering of the stream velocities and resultant deposition of only muddy sediments laden with terrestrial organic material along the coast. The upper part of the section shows a progressively reduced rainfall pattern culminating in a period of very low precipitation with the development of a paleosol, which is traceable all over the SKSB where Late Holocene sediments are available. This period also witnessed aeolian activity modifying the sand ridges in the ridge-runnel systems formed by the Holocene regression.     

Le Silurien de la région d'Oulad Abbou (Meseta occidentale,Maroc) : une sédimentation péritidale sous contrôle tectonique

In the Oulad Abbou syncline, western coastal Meseta, the Silurian deposits exhibit siliciclastic or mixed siliciclastic/carbonate tidal facies that recorded alkaline basalt flows and syn-sedimentary deformations. These facies are staked into peritidal shallowing upward sequences reflecting the evolution from an infratidal to a supratidal environment. These sequences recorded low-amplitude and high-frequency sea-level variations. The built-up of these rhythmic sequences is related to distensive tectonic that allowed the development of isolated platform from extensive siliciclastic influx. This tectonic event is well recorded in the palaeogeographic evolution of the northern Gondwana platform during the Lower Palaeozoic time. To cite this article: A. Attou, N. Hamoumi, C. R. Geoscience 336 (2004).     

顺义地裂缝成因与顺义-良乡断裂北段第四纪活动性讨论

本文通过钻孔地层对比方法研究了顺义一良乡断裂北段的第四纪活动性，资料显示该断裂在第四纪期间呈现强弱交替的分期活动特征，距今315万年以来有3个较强活动期和3个较弱活动期，前三者分别距今266～315、171～228、73～147万年，后三者距今分别为228．266、147～171、0～73万年。中更新世晚期以来断裂活动不明显。超量开采地下水导致地面不均匀沉降是造成顺义地裂缝现今活动的主要原因。     

A detailed receiver function image of the sedimentary structure in the Bohai Bay Basin

If the site at which receiver functions are constructed is filled by sediments, then the waveforms from these receiver functions are dominantly controlled by the sedimentary structures within the first few seconds after the direct P arrival. Based on this observation, waveform data collected at 44 temporary seismic stations have been used to image the sedimentary structure of the Bohai Bay Basin, a major continental petroliferous basin in Eastern China. An adapted hybrid global waveform inversion method was applied to the receiver functions to extract structural information beneath each of the stations. The derived S-velocity structure provides for the first time, a basin-scale seismic image of detailed sedimentary stratification. The sedimentary cover of the basin is about 2-12 km thick, consisting of Cenozoic, Mesozoic, and Paleozoic strata from top to bottom. The structural features presented in the S-velocity image coincide quite well with the depression-uplift type of tectonic system in the Bohai Bay Basin. The reconstructed morphology of the sedimentary layers provides seismological evidence for the two-stage evolution of the intracontinental basin that were caused by an intensive tectonic regime transition in late Mesozoic immediately following the lithospheric reforming of the Eastern China continent.     

Western Alpine back-thrusting as subsidence mechanism in the Tertiary Piedmont Basin (Western Po Plain, NW Italy)

Basin formation dynamics of the Tertiary Piedmont Basin (TPB) are here investigated by means of cross-section numerical modelling. Previous works hypothesised that basin subsidence occurred due first to extension (Oligocene) and then to subsequent loading due to back-thrusting (Miocene). However, structural evidence shows that the TPB was mainly under contraction from Oligocene until post Pliocene time while extension played a minor role. Furthermore, thermal indicators strongly call for a cold (flexure-induced) mechanism but are strictly inconsistent with a hot (thermally induced) mechanism. Our new modelling shows that the TPB stratigraphic features can be reproduced by flexure of a visco-elastic plate loaded by back-thrusts active in the Western Alps in Oligo-Miocene times. Far-field compression contributed to the TPB subsidence and controlled the basin infill geometry by enhancing basin tilting, forebulge uplift and erosion of the southern margin of the basin. These results suggest that the TPB subsidence is the result of a combination of mechanisms including thrust loading and far-field compressional stresses.     

Sedimentary aspects and paleoenvironmental evolution of a rift basin: Salta Group (Cretaceous–Paleogene), northwestern Argentina

The rift history of the Salta basin is related to the evolution of the Central Andes and to the activity of the Pacific margin, owing to its geographic location. Sedimentation occurred from the Neocomian to the Paleogene, with deposits reaching up to 5,000 m in thickness. Paleoenvironmental analysis reveals an evolutionary history controlled by tectonic and climatic changes. Isolated grabens characterized the early synrift stage; differential subsidence provoked distinct environments in the southern and northern subbasins. In the southern subbasins, alluvial-fan, fluvial-fan and lacustrine deposits prevail, whilst in the northern subbasins eolian and fluvial environments dominate. During the Maastrichtian, two major factors controlled the basin fill: the decrease in tectonic subsidence and a relative sea-level rise as recorded in South America. An extensive and shallow Atlantic marine ingression installed a carbonate system coincident with mainly humid conditions until the Danian. Until the Middle Eocene, the fluvial and lacustrine environmental evolution of the sag basin was controlled especially by the alternation of temperate with dry and humid periods. Paleontological records reflect these climatic changes and show their relationship to the sedimentation regime.     

利用地震属性研究准噶尔盆地西北缘拐19井区下侏罗统三工河组沉积环境

在准噶尔盆地西北缘，寻找岩性圈闭油气藏已是至关重要的问题。但人们几乎还是用常规的手段来寻找岩性油气藏，没有实质性进展。本文是以利用宽方位角采集的地震数据为基础，对数据进行高保真资料处理和参考标准层的层拉平解释。利用前人的区域地质研究成果和钻井解释成果对侏罗系的沉积环境进行了精细的描述，确定了拐19井区在下侏罗统三工河组的出油层段附近的沉积环境由湖泊相－三角洲前缘相－河流相－湖泊相的演化过程。利用地震属性解释的结果也能较好地反映目标区的沉积环境变迁，并且与区域地质、测井解释结果相吻合，为寻找油气提供了很好的依据。由此得出利用地震属性可以进行沉积环境变迁分析，为油田寻找岩性圈闭及油气藏做出贡献。     

珠江三角洲江村ZK2钻孔沉积物粒度的环境意义

运用MALVERN公司2000型粒度仪对珠江三角洲地区的江村ZK2钻孔作粒度分析，江村钻孔按粒度的偏态值可划分为两个河相与海相或湖泊沼泽相的沉积交替过程，同时与粒度参数、年代数据以及前人所做的孢粉等证据相结合，大致看出研究区气候变化的4个千年尺度的气候波动：第一阶段为较长时间的冷干期，该段时间约为20-10．7kaB．P．；第二阶段为回暖期，时间大致在10．7-7．5kaB．P.,总体比较湿润；第三阶段为升温期，该时期约在7．5-5kaB．P．之间，在此期间各有一次干湿交替；第四阶段为降温期，时间大约出现在5kaB．P．至今，这是一个波动性较大的时期，也各有一次干湿交替。     

Lithostratigraphic position and petrographic characteristics of R.A.T. (“Roches Argilo-Talqueuses”) Subgroup, Neoproterozoic Katangan Belt (Congo)

The Neoproterozoic Katangan R.A.T. (“Roches Argilo-Talqueuses”) Subgroup is a sedimentary sequence composed of red massive to irregularly bedded terrigenous-dolomitic rocks occurring at the base of the Katangan succession in Congo. Red R.A.T. is rarely exposed in a continuous section because it was affected by a major layer-parallel décollement during the Lufilian thrusting. However, in a number of thrust sheets, Red R.A.T. is in conformable sedimentary contact with Grey R.A.T which forms the base of the Mines Subgroup. Apart from the colour difference reflecting distinct depositional redox conditions, lithological, petrographical and geochemical features of Red and Grey R.A.T. are similar. A continuous sedimentary transition between these two lithological units is shown by the occurrence of variegated to yellowish R.A.T. The D. Strat. “Dolomies Stratifiées” formation of the Mines Subgroup conformably overlies the Grey R.A.T. In addition, a transitional gradation between Grey R.A.T. and D. Strat. occurs in most Cu–Co mines in Katanga and is marked by interbedding of Grey R.A.T.-type and D. Strat.-type layers or by a progressive petrographic and lithologic transition from R.A.T. to D. Strat. Thus, there is an unquestionable sedimentary transition between Grey R.A.T. and D. Strat. and between Grey R.A.T. and Red R.A.T.The R.A.T. Subgroup stratigraphically underlies the Mines Subgroup and therefore R.A.T. cannot be comprised of syn-orogenic sediments deposited upon the Kundelungu (formerly “Upper Kundelungu”) Group as suggested by Wendorff (2000). As a consequence, the Grey R.A.T. Cu–Co mineralisation definitely is part of the Mines Subgroup Lower Orebody, and does not represent a distinct generation of stratiform Cu–Co sulphide mineralisation younger than the Roan orebodies.