Postglacial Deposits and Land Movement

Abstract

Postglacial deposits are formed in relating to the sea level fluctuations of Postglacial epoch. Generally they can be divided into four members, that is, the lower sand and gravel bed, the middle mud bed, the upper sand bed and the uppermost terrigenous bed.
Postglacial deposits form an alluvial plain which often coinsides with a sinking area, for instance, Toyama, Osaka basin etc. They are the upper part deposits in these structual basins.
Basin buildig and land subsidence are discused as examples of the land movement in Postglacial epoch. Basin building does not finish in early Pleistocene epoch but it is continueing in Holocene as Toyama basin.
The land subsidence is divided into natural one and artificial one. It consists largely of compaction of Postglacial deposits and partly of crustal subsidence. The compaction of Postglacial deposits has an intimate relationship with the thickness of the mud bed and the water quantity pumped out from the deposits. The annual amount of the natural subsidence including of the compaction of sediments and land movement is only of millimetre order and generally less than 5mm, on the other hand the artificial one is centimetre order (table 2).
The results of recent survey show that the pumping qantity from the diluvium deposits is larger than that from Postglacial deposits, and the land subsidence takes place diluvium deposits as well as Postglacial ones. Therefore we have to investigate not only the subsidence of Postglacial deposits but also that of the Quaternary structual basin.