6878601929

GENESIS OF THE GRUDZIĄDZ BASIN IN THE LIGHT OF ITS DEPOSITS

AND GLACIAL FORMS

Summary

The Grudziądz Basin is the largest basin-type widening of the Lower Vistula valley. Its lenght is about 20 km, its width about 18 km, and the area it covens is about 240 km². Its main characterics are three islands of the moraine plateau, ■so-called „kępy” (fig. 1), rising up to 60 m above valley floor. They are seperated from the surrounding moraine plateau by broad curves of the former Vistula bed. The problem of genesis of this basin-type form has for long been drawing the interest of geographers and geologists. So far, two hypotheses have been advanced on the issue; the first, put forward by A. Jentzsch (1911) relates the formation of the Grudziądz Basin to the former existence of a large pro-glacial lakę at the mouth of the Mątawa outwash valley; the second, advanced by R. Galon (1934), regards lateral erosion of the meandering Vistula at the decline of Pleistocene and Holocene as the main factor in the formation and evolution of the basin.

This work is a new attempt at solving the problem. It lays emphasis on the stratigraphy and genetic-facial interpretation of deposits from the last (Baltic) glaciation, especially on those from the penultimate Baltic ice-sheet in the area under consideration, and on structural analysis of glacial slope forms associated with a melting of dead-ice blocks which — as it turned out during the investiga-tions carried out — occur in that area beside fluvial landforms.

STRATIGRAPHY OF THE BALTIC-GLACIATION DEPOSITS

The discussed sector of the Lower VLstula valley is carved mainly in the Baltic-glaciation deposits. Their lower boundary was determined on the basis of paleobotanic data in the newly discovered sites of the Eem Interglacial at Gru-dziądz-Mniszek and Rządz (E. Drozdowski and K. Tobolski 1972). Materiał obtained from the test boring, located by the western shore of Lakę Rudnickie Wielkie (fig. 2), has permitted to State that Eeem Interglacial is represented here by lacu-strine deposits at an altitude of 4—10 m a.s.l. At a higher altitude rests another series of organogenic deposits the age of which — due to still unfinished paleobotanic analyses — has not been established with accuracy. It is most probable that the series represents an Early-Wiirm Interstadial. The series underlies the marking for this area bed of grey varved clays (fig. 3), in turn underlain locally with a till strata. Basing on these data, it has been established that the profile of deposits from the Baltic glaciation in the area under investigation comprises three strata of till together with fluvioglacial deposits sepereting them. Their

9 — E. Drozdowski, Geneza...