南京地区新近纪砂砾层的沉积环境演变

EVOLUTION OF SEDIMENTARY ENVIRONMENT OF NEOGENE GRAVEL BEDS NEAR NANJING

南京地区分布着一套新近纪的松散砂砾沉积,曾被统称为雨花台砾石层。依据岩性、层序、沉积构造等特征,我们解译了4个地点砂砾层的沉积环境。研究发现,洞玄观组的沉积环境类似于曲流河沉积,六合组类似于砂质辫状河沉积,雨花台组则属于砾质辫状河沉积。沉积环境的不同印证了前人的观点,即这些砂砾层是不同时期形成。古流向显示,从中新世到更新世早期本区的流域格局与目前的大体一致。而河流沉积环境的演化,却指示河道的坡降具有不断增大的趋势。我们认为这可能是全球海平面不断下降导致侵蚀基准面降低所引起的,而差异性的构造运动似乎不是主要的驱动因素。

A suite of unconsolidated gravel sediments, once generally known as the Yuhuatai Gravel Bed, distributes in Nanjing region. Recent studies revealed that the Yuhuatai Gravel Bed can be divided into four formations,i, e. the Dongxuanguan Formation （the Puzhen Formation）, the Liuhe Formation, the Huanggang Formation, and the Yuhuatai Formation. Except for the Huanggang Formation which is mainly composed of basalt, three formations are investigated in order to understand their sedimentary environments. The Fangshan section （the Dongxuanguan Formation） （31°53′30.2″N, 118°52′26.4″E） in Jiangning District is located in the south of the Changjiang （Yangtze） River. Other sections are located in the north of the Changjiang River, they are the Lanhuatang section （the Yuhuatai Formation）（31°58′42.3″N, 118°33′31.3″E） and the Jiangpu Sandpit section （the Puzhen Formation） （ 32 °09′37.0″N, 118 °39′55.5″E ） in Pukou District and the Lingyanshan section （ the Liuhe Formation） in Liuhe District （ 32 °26′49.9″N, 118°55 ′50.0″E ）. Paleontological evidence indicates that the ages of the Dongxuanguan Formation and the Liuhe Formation are 17 N llMa and 10 -8Ma respectively; while the Yuhuatai Formation was deposited during Pliocene. The chronostratigraphic correlation suggests that the Yuhuatai Formation continued to deposit in the Early Pleistocene and was terminated during the period of 1232 -869ka. We reconstructed the sedimentary environment of the gravel beds according to their depositional facies, such as lithology,sedimentary structure, sequence etc. We found that the sedimentary environment of the Dongxuanguan Formation is similar to that of the Puzhen Formation; both of them were comprised of 6 cycles of the depositional sequences similar to that of a meandering river. Each cycle mainly began with a channel bottom deposit and ended with a floodplain deposit. This finding further confirmed that the Dongxuanguan Formation is contemporaneous with the Puzhen Formation. The Liuhe Formation shows 2 aggradation cycles similar to that of a sandy braided river. Each began with a channel bottom deposit and ended with a channel bar deposit or a floodplain deposit. The Yuhuatai Formation exhibited 4 depositional cycles of a gravelly braided river. It is comprised of alternating channel deposits and gravelly bar deposits ,frequently interrupted by sand drapes. The Dongxuanguan Formation was deposited in a meandering river,indicating a small river gradient. The Liuhe Formation was deposited in a sandy braided river, indicating a moderate river gradient. The Yuhuatai Formation was deposited in a gravelly braided river, indicating a large river gradient. Increasing river gradient suggests that the base level of erosion of Nanjing was continually lowered. We analyzed the change of the global eustatic sea level and found that the Dongxuanguan Formation was deposited in a period of high global eustatic sea level. The Liuhe Formation was deposited in a period of lower global eustatic sea level. The Yuhuatai Formation was deposited in a period of the lowest global eustatie sea level. As a result,the lowering of the erosion base level in Nanjing can be briefly ascribed to the lowering of the global eustatic sea level. Assuming that the lowering of the base level of erosion is entirely induced by the lowering of the global eustatie sea level,we can estimate the eustatic sea level change from the base level of erosion. At present,the main rivers near these sites belong to meandering rivers with the riverbed altitude at about 5m. Since the top deposit of the Dongxuanguan Formation is located at an altitude of about ll0m, we find that the base level of erosion dropped 105m. Compared with the period of about 15Ma, the present global eustatic sea level dropped about 100m. It is clear that the lowering of the base level of erosion approximately equals to the fall of the global eustatie sea level. Therefore,we conclude that the change of the sedimentary environment in Nanjing since about 15Ma is essentially controlled by the fluctuation of the global eustatic sea level. Differential tectonic movement seems not to be the primary forcing factor.