Study On The Geomorphic Development And Relative Sea-level Changes In The Coastal Alluvial Lowlands Of Liaodong Peninsula In Holocene

The current research on the paleo-environment along the coast in Liaoning province has lots of defects, such as the oversize range of study, the low density of drilling, the lack of continuous analysis on the micropaleontology in the profile, unconsidering the intersite tidal range of impact on sea level change, the lack of precise measure to the hight of drilling, and the mistakes of the choice of the mark to the paleosea-level. Now we should emphasize using the diatom analysis on the coast research which is sophisticated in the foreign country, choosing the subject that is the coast alluviation or marine deposit plain which is the minor area, sampling continuously by drilling, high-density analysing the diatom in the sediment, discriminating detailedly marine or continental sediment, using the peat or the black humus clay which covers the marine sediment on the coast plain as a marker bed to reinstate ancient sea-level hight and coastline site.The result of the study showed that the peat towards the coastal plain from the valleys in the Dagu Mountain area is fully developed. In the profile of core samplings, the altitude of the valley peats descends from top of valleys, about 4m altitude, to mouth of valleys, together with the decrease of the thickness of valley peats and finally there occurs black clay lamina. The diatom analysis from the Cores LD and DK reveals the marine facies beneath the valley peats is a transition zone, the number of marine species decreases while the freshwater species increases.There is conformity contact between the valley peats and underlain marine facies sediments. The forming of peat indicates that the process of seawater gradually exits from ravine as well as the mouths of valleys and the process of plant's following .Then the bottom of the top layer of valley peats represents the maximum sea level in Holocene. The shell-samples for dating were collected from the lower part of the peat near the mouths of valleys at the cores LD and DK and from marine facies sediments at the coastal plain, and the ¹⁴C dating for those shells indicates maximum Holocene transgression is around 6000～5000calBP, with the altitude of the sea-level at around 4 m (the average level for high-tide). The maximum altitude for sea-level at that time is believed around 1.7m higher than current sea-level. The high sea level period went on to 4200calBP, then the sea level fell to 3200～2700cal BP, about 0.5m below the current level. The sea level in this period is relatively stable, shoal head developed well. Sea water began to fall back from the west of Dayang River after 2700calBP, Dayang River entered low land, the natural levee in the northeast started to form. The regression speed is 4.8m/a from 2766cal BP to 1756AD. The retreat speed of coastline is 15.26m/a from 1495AD to 1756AD and 27.78m/a from 1756AD to 1900AD. Except the cooling of climate and the relative fall of the sea level, it is estimated that the increasing sediment discharge came from the upper stream triggered by intensified human activity which caused the destruction of the upstream forest, and therefore the silt is brought into the river by flood is also the substantial reason for the rapid expansion of Dayang River Plain towards the sea.Through samples drilling survey of the ancient lagoon which is in the western part of the Changxing Island of Liaodong Peninsula, the sediment of the land and sea phase of the ancient lagoon plain is differentiated, the identification of the ancient marine layer has an obvious consistency, the sea level is about 4.0-4.2m, the altitude represents the average high tide line of the highest sea level of the ancient lagoon plain during Holocene. It is about 5800cal.aB.P when the highest level of Holocene realized. The average sea level of the maximum period of Holocene is about 3.4-3.6m.As this height opposed to the high sea level of about 2m degree of the eastern part of Dagushan of Liaodong Peninsula, there is an upward than in the eastern part of the ocean along the river plain, In addition, there are two peaks in the hole B3 conductivity which lies in 9m depth, it reflects there have been two transgression during Holocene. The largest range of later transgression is up to about 1.2km away from the Modern coastline.It can be presumed that they were formed in the stage from early Holocene to middle Holocene (9300-8000cal.BP) according to the known ¹⁴C age dating. This regression event may have relation with the cooling event in 8200cal BP after the Younger-Dryasevent at the postglacial period.