晚松山时南黄海的古长江三角洲

PALAEO-YANGTZE RIVER DELTA IN THE SOUTH YELLOW SEA DURING LATE MATUYAMA CHRON

早更新世晚期（加拉米洛亚时开始到松山时结束），中国东部陆架发生海侵。随着海侵的推进，在南黄海中部出现了较厚的三角洲沉积。在沉积层的垂直序列上，发现了分流河口沙坝、分流河道和分流河口间湾等三角洲前缘沉积亚相的交替。这是一个海侵序列的三角洲层序。在海侵进程中三角洲的进积速率与海水进侵速率基本平衡。南黄海晚松山时古长江三角洲的发现，从一个侧面证明了浙闽隆起带在早期第四纪仍然是继承晚第三纪的一个构造上的隆起及地貌上的屏障，它制约着古长江流向东北方向。

The Palaeo-Yangtze River had already been as nearly large as its present scale in the end of Late Pliocene. During the early period of Early Pleistocene (after Olduvai subchron), when a low sea level remained for a longer term in the Yellow Sea Shelf area, in the central part of the sea were formed thicker fluvilly deposited sediments, which provided the oldest evidence ever revealed for the traces of Quaternary Palaeo-Yangtze River in the South Yellow Sea. In consequence of marine transgression proceeding towards Eastern China Shelves during the late period of Early Pleistocene (from the beginning of Jaramillo subchron to the end of Matuyama Chron), the delta deposition with thicker sediments occurred geographically in the centre of the sea. The early Early Pleistocene fluvial sediments are covered by the transgressive delta of late Early Pleistocene. The interval 79.82-91.33m from Borehole QC2 collected on the South Yellow Sea Shelf, located at 122°16'E and 34°18'N and under a water depth of 49.05m, records this palaeo-deltaic sedimentary sequence. Overlain by fluvial deposits, the sequence can be easily recognized to consist of 4 beds labelled with letters a,b,c and d from bottom to top: The bed d, belonging to interdistributary bay sediments, comprises silty clay interbedded with laminated silt and lag deposits in bottom, as well as a great amount of foraminiferas showing monotonic species and very low diversity. The bed c, deposited in distributary riyer mouth bars, is dominated by smallscale, low-angle cross-bedded and, locally, ripple bedded, medium, to fine sands burrowed by livings, not rich in foraminiferas which are of shallow coastal origin and mostly wore off and in low diversity; additionally, there are mollusks analogous to those in modern intertidal zone and fresh water, typical of sea water environment mixing with rivers. The bed b, resulted from distributary channels and containing tabular-cross bedded sands, shows 4 evident upwards-fining sedimentary cycles representing subaqueous fluvial multicycle aggrations from channels to point bar to flood lands, dominant foraminifera Ammonia becarii var. distributing interrupedly in the layer.The bed a, also attributable to the deposition of river mouth bars and seperated from the underlying strata by a erosive surface, is composed of low-angle crossbedded fine sands and silty clay, in which are found 3 turbidite layers generated from rapid sedimentary events. The above-mentioned beds a,b,c and d make up a deltafront sedimentary sequence, indicative of subenvironmental changes from river mouth bars through distributary channels to river mouth bars and finally to distributary bays. The bed a overlies the deposits formed in a branching channel plain, instead of the prodelta sediments as in normal progressive delta sequences. Studies in palaeomagnetism demonstrate that the delta-front deposits are between the base of Jaramillo subchron and the B/M boundary, representing a span of 0.24 Ma (from 0.97Ma B. P.-0.73 Ma B.P.). Study of the grain size characteristics is helpful to recognizing the Yangtze River Delta. The sediments of river mouth bars are similar in grain size, with mean grain size of 2.5-4.4φb, poorly or middlingly sorted,and positively skewed; the probability curve is characterized by two saltation subpopulations, both intersecting between 2.25-2.8φ, which may be interpreted as scouring/reflucence bountary point, an important mark indicating bimodel currents reworking of sediments and representing bidirectional interaction among rivers, tidal currents and wave in the river mouth area. A high content of suspended load in the river mouth bars points to the character of distal river mouth bars. The distributary channel subfacies also shows two saltation subpopulations in the probability curve, because it was near the river mouths and frequently affected by tidal currents. The interdistributary bay deposits are dominated by suspeded load,intercalated with a number of silty laminations due to disturbance of waves and shore currents. From the content and