末次盛冰期以来长江三角洲地区的沉积相和古地理  

PALAEOGEOGRAPHY AND PALAEOENVIRONMENT IN CHANGJIANG DELTA SINCE LAST GLACIATION

作　　者：李从先[1] 陈庆强[1] 范代读[1] 张家强[1] 杨守业[1] 

机构地区：[1]同济大学教育部海洋地质开放实验室,上海200092

出　　处：《古地理学报》1999年第4期12-25,共14页Journal of Palaeogeography：Chinese Edition

基　　金：国家自然科学基金资助课题(49676288;49736220)

摘　　要：未次盛冰期低海平面期间,长江三角洲地区可划分为2个古地理单元:古河谷和古河间地.下切河谷底部侵蚀面和古河间地顶面构成了冰后期海侵沉积旋回的底界面,它相当于层序地层学中的层序界面.位于河口湾—浅海相中的最大海侵面将冰后期海侵沉积旋回分为其下的海侵层序和其上的海退层序.随着δ^(18)O3期的海平面下降,长江开始下切,至δ^(18)O2期低海面时形成巨大的下切河谷.冰后期海平面上升引发的海侵造成了长江古河谷系的充填和河床、河漫滩—河口湾和部分河口湾—浅海相的形成,尔后的进积产生了部分河口湾—浅海相及三角洲相等.溯源堆积是产生下部河流沉积单元的主要过程,其中河漫滩沉积中出现的潮汐层理和少量小个体有孔虫说明了海洋因素的影响,河口湾—浅海相泥质沉积主要形成于最大海侵之时,三角洲的进积则产生了具有多期河口坝的三角洲.古河间地表面的硬粘土层经历了沉积和成壤作用交替、持续成壤作用和早期成岩作用,它们大致分别对应于δ^(18)O3期、δ^(18)O2期和δ^(18)O1期,硬粘土层中留下了这3种作用的烙印.长江三角洲古河间地的古土壤母质属河漫滩相.持续成壤阶段河流基面和地下水位均较低,年降雨量约为 500～800mm,相当于现今的温带地区,干湿周期变化明显,地下水升降频繁.所有这些表明。During the sea level low stand of the last glaciation maximum, the present Changjiang Delta area could correspondingly be subdivided into two palaeogeographical units: incised valley and palaeointer-fluve. Erosional surface at the base in theincised valleys and the surface of the stiff clay (palaeosol) at the paleointerfluves constitute the lower boundary of the post - glacial transgressive sedimentary cycle, which corresponds to sequence boundary in sequence stratigraphy. The maximum flooding surface lying in estuarine - shallow marine muddy unit demarcates the underlying transgressive sequence and overlying regressive sequence. With the falling of sea level of δ18O stage 3 the Changjiang River began to down-cut, and a huge incised valley was formed during the lowest sea level of the stage 2. The postglacial transgression caused by sea level rising led to infilling of the incised valley and formation of the river channel, floodplain -estuary, estuarine - shallow marine and deltaic stratigraphical units. The retrogressive aggradation was the main process resulting in fluvial depositional units. The presence of tidal laminae and small - sized foraminiferas within muddy deposits of the floodplain unit demonstrated the influence of marine factors. Estuarine - shallow marine muddy deposits were basically formed during maximum transgression , then delta progradation created the deltaic sequences with multi - stage mouth sand bars. The stiff clay on the palaeointerfluve, belonging to floodplain facies, underwent deposition alternating with pe-dogenesis, ongoing pedogenesis and early diagenesis, which roughly correspond to stage 3, stage 2 and stage 1 of δ18O respectively. The stiff clay, overprinted by all these processes, contains abundant information on late Quaternary palaeoenvironment changes in the Changjiang Delta region. During the period of ongoing pedogenesis, river base level and groundwater remained lower, the annual precipitation was about 500 - 800 mm, which corresponded to that of modern temperate zones

关 键 词：末次盛冰期 长江三角洲地区 沉积相 古地理 古气候 

分 类 号：P512.2[天文地球—地质学] P531