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

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

There were left out deliberately mountain of medium height stream-side sequences at which the lacustrical character is not expressed. STRATIGRAPHICAL AND CHRONOLOGICAL RESULTS On the basis of the faunal-succession of the territories described in this paper it can be laid down as a fact that the juvenile subsided zones in Hungary can bring into connection with the crustal movements of the end of the Pleistocene and the beginning of the Holocene. The basis formed after the subsidence were filled by fluvial sediments in the beginning. It is documented by partly the gravel containing deposits, like the Early-Holocene sediments of the Fert6-tó, Lesence: Nádas-tó, Sárrét, Fejér county; or the presence of the fluvial sand in Békés county, in the Oanube-Tisza Interfluve or in the Jászság. These juvenile Holocene sediments are characterised by the occurrence of Lithoglyphus naticoides as reophyl species, and Valvata piscinalis, Valvata pulchella in view of malacology. Here have to be noted that after the fluvial period in the initial deposits of the lacustrical phase the Valvata piscinalis appears as "guiee fossil", since there is no vegetation, or only the lower plant vegetation can be found here in this period of the lake's evolution. The second period of the succesion of the lakes in case of our subsided zones can be characterised with forming calcareous deposits. It can be observed both in the case of the Great Plain and Transdanubia

, but we can find this sediment In the Nyírség, too. (SZÖÖS, Gy. - SÜMEGI, P. - HERTELENOI, E. 1991. ) The third period is characterised by developing of bogs, sometimes peat-bogs. While the polythermic species ere typical mostly in the second period like Bithynia tentaculata, Gyraulus albus. In the third period, in which we can find the terrestrial species reffering to intermittent dry phases, characteristic is the presence of Gyraulus riparius generally. It is interesting because this species does not occur in Hungary nowdays. It was the member of the Pleistocene mollusc-fauna (KROLOPP, E. 1982-83) but as the climate became warmer disappeared. And this species appeared again at the end of the Holocene (Sárrét, Fejér county; Lesence: Nádas-tó; Kolon-tó; Körösladány; Jászság). This fact is refer to fell in temperature. The occurrence of Valvata pulchella refers to fall in temperature, too. (Körösladány, Jászság, Danube-Tlsza Interfluve). The third evidence of cooling down is the presence of Bithynia leachi in the sediments. This species is the vicarious species of the Bithynia tentaculeta and was forced back by the warming up in our fauna. The vicariaty of these two species can be observed in the deposits of the Sárrét (Fejér county) the most expressively (FÖKÖH, L. 1977). According the above mentioned indicator species it can be laid down as a fact that the third period of the succession of our lakes may be put to the second cool-phase of the Holocene. Finally the fourth period, which is not natural process solely, but it has anthropogen aspects, is the course of becoming boggy or sometimes the total filling up. These successional processes beside ttre reletlve character of malacostratigraphy can be bring into connection with the absolute chronology. The radio-carbon analysis of the calcareous sediment presents basis for it. According the survey data, performed from the lower part of the calcareous deposits of Sárrét (Fejér county) the age of the sediments is 8200Í 150 BP

. While this value in case of the lakes of the Great Plain is 8000-8500 BP

. (FÖKÖH, L. 1991). In accordance with these we can state that these sediments developed at the end of the Boreel period and during the Atlanticum period. This is keeping with the dominancy of the polythermic species and the presumably extensive precipitation. This was the reel "lake-age" of our subsided zones. Compered to the age of the calcareous mud we can lay down as a fact that the beginning of the siltetion of the subsided zones can be ascribed to the Preboreal - Boreal period, while the bog-formation and peat-formation is Identiable with the fall in temperature following the Atlanticum, with the Subboreal period. If we concende this logical reasoning, it can be established that the Gyraulus riparius can be used as subboreal indicator species in the territory of Hungary, nowdays (FÖKÖH, L. 1989). 104