PROCESY EROZJI I AKUMULACJI GLACJALNEJ W POŁUDNIOWEJ CZĘŚCI BASENU GOTLANDZKIEGO W ZAPISIE RZEŹBY PODŁOŻA CZWARTORZĘDU ORAZ ROZPRZESTRZENIENIA OSADÓW PLEJSTOCEŃSKICH

Abstract

PROCESSES OF GLACIAL EROSION AND ACCUMULATION RECORDED IN THE RELIEF OF THE SUB-QUATERNARY SURFACE AND DISTRIBUTION OF PLEISTOCENE DEPOSITS IN SOUTHERN PART OF THE GOTLAND BASIN

Abstract. The sub-Quaternary topography is divided into a number of blocks by several meridionally oriented faults. Western part of this area is composed of Pridoli claystones and marly shales. Central and north-eastern parts consist of different types of Upper Devonian limestones. Upper Cretaceous silty sandstones occur in the east and south-east. A thin layer of Eocene sediments unconformably overlies Pridoli deposits in the south-western area. The main features of the sub-Quaternary topography are a result of glacial erosion. This relief reflects two directions of glacial transgression. One of them is oriented SW' and W'wards, towards the Słupsk Trough. The other is directed S' and SSE'wards, towards the Gulf of Gdańsk. This relief, shaped by glacial erosion, is densely cut by subglacial valleys of different depths. It is especially seen on the eastern slope of the Southern Middle Bank. Pleistocene deposits are rather thin in most of the study area. They are slightly thicker, up to 35 m, in the Southern Middle Bank. Seismoacoustic data shows here the presence of two or three morainic horizons whose continuity is disturbed by subglacial valleys. The greater thickness of Pleistocene deposits reflects a higher accumulation rate of morainic material transported in the lower portion of the ice sheet, along the distal part of a regional elevation extending from Gotland Island to the Southern Middle Bank. Over most of the area the deposits are 3-9 m thick. In this region, the Pleistocene is generally composed of a single horizon of subaqueous till deposited during gradual recession of the last ice sheet. In local depressions, this till horizon is covered by glaciolacustrine and glaciomarine sediments of the Baltic Ice Lake where the Pleistocene sequence reaches up to 14 m in thickness. Small thickness of glacial deposits in most of the study area is a result of dominant erosional processes during the Pleistocene in the central Baltic Sea and in the northern part of the Southern Baltic Sea. Petrographic composition of tills composing the Southern Middle Bank region reveals significant affmities. The constant and high content of carbonate rocks on one hand, and the low content of crystalline rocks on the other hand indicate a short transport of morainic material. Petrographic composition of subaqueous tills varies due to a depth of accumulation, pointing at various distribution of glacial debris in basal part of the ice sheet. The last palaoegeographical event of the Pleistocene was the formation of a water reservoir called the Baltic Ice Lake which, during the Holocene, has transformed into the recent Baltic Sea.