福建兴化平原的形成与古地理环境

THE GENESIS AND PALEOGEOGRAPHICAL ENVIRONMENT OF THE XINGHUA PLAIN IN FUJIAN PROVINCE

本文根据大量野外调查和室内古生物化石鉴定、^(14)C年代测定及史志记载资料,对晚第四纪兴化平原古地理环境演变以及控制该平原形成的主要因素等作了讨论。

The Xinghua Plain lies in the eastern section of Fujian Province, with an area of some 464 square kilometres and about 3-7 metres abouve sea leve (Fig. 1). This land is one of Fujian's most important farming areas for rice and fruit. It is also among the most densely populated areas in Fujian. The Quaternary sedimentary strata overlying the rhyolite and granite is only 20 metres or so in thickness (Fig. 2). The upper layer is mainly grey mud deposited in the Holocene, and the lower-sand and gravel with clay occasionally, deposited in the end of Late Pleistocene, is less than 5 meters in thickness. There are abundant paleobifossils, such as forminifera, mollusc shell and spore-pollen etc. in the sediments. A sedimentary profile in Hezhu, in which the thickness of sediments overlving the bed rock is only 2.25 metres, but fossil forminifera is quite rich in species, mainly including Ammonia tepida (Cushman), A. annectens (Parker), Elphidium hispidulum Cushman, E. advenum (Cu-shman) and Cribrononion porisutaralis Zheng etc. There are more abundant fossil foramini-fera in the Holocene sediments in the borehole CK9 of Sanjiangkou (see Fig. 3). This has indicated the changes of depositional environment since the Late Pleistocene. Oyster reefs are widely distributed in the Xinghua Plain. The Huadong oyster reef in situ, consisting of ostrea rivularis and ostrea plicatula, is 2 metres in thickness and 0.5 metres above sea level. 14C dating of the oyster sheik gives the oysters the age of 1 420 ? 60-2 575 + 105 yrs. B P. There is also oyster reef in *he west of the plain, with a distance of 14 kilometers to the present coastline, and an altitude of 6 metres or so. The oyster reef is mostly composed of 0. rivularis with the thickness of 0.3 to 0.4 meters. Radiocarbon date of oyster shell from the reef is 3610 + 100 yrs. BP. Peat is a synthetical outcome of various natural factors. So, it's a good indicater for natural environment changes. The peats are relatively poor in inland of the plain, but they are found extensively along the coast. For instance, in the north of Duobian village there are three peat layers on the hill. The highest one, with about 40 metres above sea level and 0.8 metres in thickness, gives the age of 14C dating 1 980 + 80 yrs. B P The second, with 20 metres in height, is 0.3 metres thick. The third, about 10 metres in height, 0.3 to 0.5 metres in thickness, tells the age (14C dating) of 13 910 + 400 yrs. B P. In the south of Duobian village, a peat bed stands at about 8 meters above sea levelwith the thickness of 0.8 meters. It gives the radiocarbon age of 21 740 + 520 yrs. BP. In addition, the Humian peat layyer gives the age of 19 580 + 490 yrs. BP., and the Wangcuo peat bed, 1 790 + 80 yrs. BP.Based on the analysis of palynology from the peats and other deposits, the changes, the changes of climate and plant in this area since the Late Pleistocene are as fallows: At the time of 21 740 yrs. B P., the climate was warm and damp, and since then, it turned warm and dryi round, about 19 580 yrs. B P., the climate turned cold, and mainly covered with herbal plants in this place; round about 13 910 a B P., the climate was slightly cool and dry, the vegetation in the plain mainly consisted of Gramineae and Cyperaceae etc.. From the Early Holocene to the Middle Holocene, the climate varied from warm and damp to hot and wet, but in the plain grew aquatic plants and on the hill there were chiefly covered with evergreen broad-leaf trees till 1 980 yrs. B P.. According to historical records, tide water could still reach to Shihua Bridge and Deng/hu Hill, and the land couldn't be cultivated but growing cattail only till the Sui-Tong Dynasty (A. D. 581-618). Ancient people to started build dykes to reclaim land from marshes in A D. 627, and the dykes and dams had been extended the Ninghi Bridge in the centre of the plain in the Song Dynasty (A. D. 960-1127). It's quite evident that the activities of human have made the plain extend rapidly. To sum up, the genesis of the Xinghua Plain has approximately experi