Folia Historico-Naturalia Musei Matraensis - A Mátra Múzeum Természetrajzi Közleményei 17. (1992)

Füköh, L.: Malacostratigraphical Investigation of the Late Quaternary Subsided Zones of Hungary

The age of the sediment is 8200 150 B.P. according to the radiocarbon analysis (FÖKÖH, L. 1991) of the samples taken from the lime mud. In the third period of the slltatlon appeared the typical palaudal species (Lymnaeidae, Planorbldae) and those species which occurrence refer to periodical desslcation (Vallonia costata and Pupilla muscorum). The faunlstlcal attraction of the sediment is the presence of Gyraulus riparius. Fert6-tó (Neusiedler See) The formation of the Fertö-tó basin is not cleared up totally until now. There are some authors who bring it into connection with the Late-Pleistocene crustal movements (SOMOGY, S. 1969). The others emphasize the eroslonal role of the river Danube (MIKE, K. 1988). Investigations have been performed partly to clear up this problem, partly to explorate the past of the recent very poor Mollusc fauna.. (RICHNOVSZKY

, A. 1981, FÛKUH, L. 1986, FÛKOH, L. 1988 ). The exploration of the Holocene sediments were taken place in the area of Fertőrákos, FertÖboz and Balf. The oldest deposits were found in 270 - 170 cm depth. It can be supposed that after the beginning of the sedimentation crustal movements might distrub this territory. It corroborates the fact that the quality and the fauna of deposits from different depths is similar. These Early-Holocene sediments are characterized by the presence of Lithoglyphus naticoides and Theodoxus sp.. The occurence of Theodoxus reinforces Mike's theory, because we can conclude from it that there was connection between the basin and the Danube. There could be a shallow-water lake here et that time, as It is shown by the frequent occurrence of the fresh-water gastropod species. (Fig. 3.) The next period has brought a significant, rapid change in the lake's evolution. The relative frequency is 60 - 70 % of the terrestrial species in the sediment. The typical species are Vallonia costata and Vallonia pulchella. In the third period of the succession we can observe the frequency of the fresh-water species again. This changement presumably can be attributed to climatlcal causes. While the rapid water level deminutlon of the second period can be brought Into connection with the warming up of the climate, the presence of Gyraulus riparius in the third period's sediments sings fall in température. The fourth period, the further slltatlon goes on until now, too. The process, dellenated here naturally could have been accomplished differently in case of other territories, the good example for it the peat, formed in the area of Fertőrákos (FÖKÖH, L. 1986). Balatonederics, Lesence: Nádas-tó To invstigate the Holocene alongshore sediments we have choosen the territory of Lesence, Nádas-tö, which is situated north from the Lake Balaton, In the Tapolca-basin. We aimed to dig up with help of borings the sediments of the former lake-basin. The sampling was performed between 1986 and 1988 (FÖKÖH, L. 1988). According 18 samples we could study the different character of the sedimentation here. This fact, presumably Is the result of the diverse geomorphology

, formed by the Lesence-stream. The next conclusions can be drawn from the simultaneous examination of the deposits and the Mollusc-fauna. The basin's sediments were characterized by quartz and limestone gravels which were deposited by the streaming water, in the initial period (18th sample). The supportional depth of water was 500 cm approximately. The vegetation could be highly insignificant (lime mud formation). The characteristic species are the Lithoglyphus naticoides occurring emong the gravels, and the Velvets pisclnalis which can be found in the initial phase of the lime mud deposits. This type of sediment contains the Marstoniopsis soltzi, a rare gastropod species of the Hungarian Holocene fauna. The occurence of this species wae described by E. KR0L0PP from Mezölakháza, In 1982 (KR0L0PP, E. - VÖRÖS, I 1982). This occurrence is older, presumably. According the succession of the fauna (Fig. 4) this part of the basin may have been filled up rapidly and uniformly. Initially the rate of the terrestrial species is insignificant (1-6«period)

. Then it accelerates, and the slltatlon In the 9th period increases to such an extent, that the of the aquatic species decreases only 3,6%. After those samples which contain gravel and lime mud, appeers in the swampy boggy deposits of this territory the Gyraulus riparius, which does not occur in the Juvenile sediments. 99