珠江三角洲晚更新世以来的沉积－古地理

本文将珠江三角洲晚更新世以来的演化分为前三角洲(约４００００—３２５００ａＢ．Ｐ．)、老三角洲(３２５００—７５００ａＢ．Ｐ．)和新三角洲(７５００ａＢ．Ｐ．—现在)三个阶段，并在此基础上探讨珠江三角洲沉积－古地理的变迁。研究结果表明，在前三角洲阶段，珠江三角洲为内陆环境，五条大河在中山三角一带汇合，然后向南东入海。磨刀门西江水道形成于第二阶段的３００００—２００００。Ｂ．Ｐ．；狮子洋、珠江和银洲湖等水道则是全新世才发展起来的。三角洲发生过两次海侵。第一次大约开始于３２５００ａＢ．Ｐ．，第二次大约开始于７５００ａＢ．Ｐ．。两次海侵形成了新、老两套三角洲沉积。

PALEO-GEOGRAPHIC EVOLUTION OF THE PEARL RIVER DELTA SINCE THE LATE PLEISTOCENE

According to the analysis of sedimentary factes and carbon dating, the authors divide the evolution process of the Pearl River Delta since the late Pleistocene(about 40 000aB.P.) into three stages called as pre-deltaic (40 000-32 500aB.P.), paleo deltaic (32 500-7 500aB.P.) and neo-deltaic(7 500-0aB.P.).The sediments of the predeltaic stage are mainly composed of pebbles and sands formed in the stream environment. No marine sediment of the stage has been found.The linear distribution of sediments in space indicates that the deposition of the stage was mainly along the paleo-river channels.At about 32 500aB.P., this area was subjected to the first time of marine transgression. The paleo-shoreline of the stage probably standed at the line from Zhini of Panyu County to the south of Xinhui City. The bottom set of the paleodeltaic sequence was made up of black and grey-black mud and silt contained a veriaty of marine organic remains. The foreset bed of the sequence consisted of sandy sediments of fluvial factes. On the top of the fluvial bed, there was a clear erosion surface on which spreaded an uncontinue layer of grano phric clayer, indicating that there was a depositional break in the later period of the stage.The second time of marine transgression started at about 7 500aB.P. and formed another suit of deltaic sequence on the erosion surface mentioned above. The area covered by the sea in the stage was much larger than that in the paleodeltaic stage with a high-tide shoreline standing from Gulao to Guangzhou and a low-tide shoreline from Xinhui to Siqiao to Zhongtang. South of the low-tide shoreline, a layer of black or grey-black marine ooze with the thickness generally between 5 and 10 meters and bearing a lot of organic remains such as Ostrca sp. and hali-diatoms was deposited. On the basis of above research, this paper further discusses the evolution of the stream framework of the delta. In the predeltaic stage,the Xijiang R., Beijiang R.and the paleo-Pearl (Zhujiang)R. (Liuxi R.) were paralell each other and all of them stretched from northwest to southeast. The Dongjiang R. flowed southwestward past Machong, Shilou and north Lingshan, and the Tanjiang R. flowed eastward past Henglan and north Zhongshan. The five main rivers mentioned above joined at about Sanjiao of Zhongshan County and then ran eastward through the Lingding Yang into sea. The Modaomen Channel of the lower Xijiang R. was probably developed between 30 000-20 000 aB.P. The oldest sediments in the channel was dated as 20 300aB.P.(14C), which is in good agreement with the TL dating of the faults controlled the channel(23 400aB.P.). From that time, the Xijian R. and Tanjian R. went to the sea through the channel. The Shiziyan Channel of Dongjiang R. was formed at 12 000-10 000aB.P. The formation of the Zhujiang Channel from Guangzhou to Huangpu and the Yinzhouhu Channel of Tanjian R. also began at that time but they developed into main channels maybe a little late.