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长江上游年产泥沙5.12×105t，经宜昌输入中游，长江中游干流及各支流年产沙约0.80×108t, 这两部分中约1.24×108t沉积于中游河湖中，4.68×108t经大通输入下游，沉淤于河道及长江三角洲或入海。长江中游的泥沙，总体是淤大于冲，但是冲淤的地理分布、时间段分布和河床断面分布不均衡。长江干流的宜昌-城陵矶段、九江-大通段冲大于淤或冲淤平衡，淤积主要发生在长江干流以武汉为中心的螺山-武汉-黄石段以及洞庭湖区和鄱阳湖区。即使在主要淤积江段，也有局部时段和江段以冲刷为主。主泓所在河槽以冲刷为主，而河道两侧或河道之间的洲滩，则以淤为主，形成高而平的漫滩，往往被人为改造成圩垸。荆江以北的江汉平原因有大堤从长江隔开，只有汉水少量泥沙供应，冲淤量很小，再加上构造沉降的累计效应，其地面高程绝大部分低于干流洲滩数米，螺山-武汉-黄石段的淤积导致该段及以上江段洪水位抬高。 干流断面冲槽淤滩使长江中游河道的典型形态呈深河谷、高漫滩，靠干堤保护堤内平原。这种断面在洪水时成为在同等水量下，水位不断抬高的原因。高水位要高堤来防堵，这就增加了堤防压力、水头压力，容易导致管涌、渗漏等险情。堤外滩及洪水位与构造沉降的堤内平原间的高差不断增大，三峡工程建成后，水库拦沙及中游河道冲深，可使同流量下水位大幅度降低，但城陵矶-武汉段冲刷量很小，不能解决该段及以下江段的泥沙淤积问题，因此江汉平原因相对于洪水位地势过低而造成的洪涝灾害及相关环境问题，仍将是中游长远的重大隐患。

本文根据古地理、古生物地理、古地磙、地球化学及火山岩工作成果，提出镇安-淅川地块是早古生代自南秦岭裂离而较早缝合于北秦岭南的一小型地块、早古生代秦岭是个多岛小洋盆，具有包括镇淅地块在内的五列岛屿。由于扬子、华北板块及其间列岛的同向不等速漂移，导致了缝合过程不同于经典的板块碰撞，这是特提斯多岛洋盆缝合的一大特点，以此解释东秦岭加里东期“碰撞而不造山”的问题。

the base of bed 27c in the meishan section changxing county, zhejiang province, china, is recommended as gssp for the basal boundary of the triassic system. the meishan section is easily accessible, well exposed, and records continuous marine sedimentation from the changxingian(latest permian) into the early triassic. the global boundary stratotype point is placed at the first occurrence of the conodont species hindeodus parvus in the evolutionary lineage latidentatus-parvus-turgida-isarcica, which is transitional between hindeodus and isarcicella. integrated stratigraphic study of the meishan section includes radiometric dating, and chemo, sequence, eco, and event-stratigraphy. the latter involves the records of possible impact events as well as volcanic, anoxic, and transgressive events. the latter three studies have identified events the appear to be recorded on a pan-tethyan or even a global scale.

biostratigraphic correlation of the ten most important permian-triassic boundary sections throughout tethys enables establishment of four conodont and ammonoid subdivisions within a stratigraphic interval, one to a few metres thick, representing less than i ma. in ascending order, they are conodonts(clarkina chargxingensis-c. deflecta zone; hindeodus typicalis interval; isarcicella parva zone and i. isarcica zone) and ammonoids(pseudotirolites-pleuronodoceras zone, lower otoceras zonel, upper otoceras zone and ophiceras zone). most of them can be traced to notrh america and the arctic region. carbon isotope investigations of 24 sections along tethys and in greenland and spitzbergen confirm the consistency of 813 c negative excursions at the permian-triassic boundary, together with an ineonsistent ir anomaly. eight sections show that in most cases there is a succession of: ir anomaly (denoting the end-permian catastrophic envirorment); 813c excursion (biomass loss, extinction); p/t boundary (origination of newcomers; potentially capable of intercontinental correlation within that short time interval). there is an intercontinental sequence boundary at the top of the permian, and a transgressive surface at the p/t boundary followed shortly by a maximum flooding surface. an intercontinental anoxia event accompanied the transgression. three delineations of the mass extinction phases and three population explosions have been recognized and can be more or less correlated in south china and the southern alps. radiometric dating of the volcanogenic boundary clays of meishan, shangsi and the main-stage siberian tunguss traps give an almost identical age of 250 ma, thus implying a synchronous interregional volcanic event. there are a few palaeomagnetic transforms within the ptb strata. thus, multidisciplinary research of the p/t boundary strata allows subdivision of this short interval into more than ten intercontinentally correlatable pars. each subdivision averages less than 100ka in duration. this may be the highest resolution obtained so far in pre-cretaceous rocks. high-frequency. milankovitch-type cyclic deposits within the p/t boundary beds enable correlation of even higher resolution in the lower yangtze area. this high resolution is made possible because the p/t transition is a time of saltation and catastrophy in geological history. causality of the aggregation of events merits further investigation.