Kopalne osady aluwialne rzeki roztokowej z okresu interglacjału lubelskiego między Włocławkiem a Ciechocinkiem

Janusz W. Jeziorski



Alluvial deposits of a braided river of the Lublin Interglacial age occur between Włocławek and Ciechocinek and are known from 11 exposures and 46 boreholes. From a stratigraphic point of view they are located beneath a glacial complex of the Warta Glaciation and above tills of the Odra Glaciation.mMost commonly there are medium-grained (l-2φ), coarse-grained (0-1 φ) sands and locally there are fine-grained and very fine-grained sands (2-4 φ). Gravels are the usual admixture. Deposition of sands and gravels occurred in cycles. In the section Mikorzyn 2 (Fig. 7) there are two sequences (6,5-7 m thick each) with upwards fining grains. In the middle part of the sections Nieszawa 1 and 2 the stratified sets indicate upward coarsening (from 2 to 0,5 φ). Among cross beddings the trough one is the most common. It constitutes 48-91% of all bedding types. Thicknesses of multi sets that contain such bedding varies from 1-2 dm to 3,8 m. Tabular cross-bedding constitutes 9-50% of all beddings. Its multisets are from a few dm to 7,2 m. Sandy-gravel deposits contain single armored clay balls, to 4 cm in diameter, and deformed clayey inserts. In the exposures Nieszawa 1-3 there are single shells of the snail Viviparus politus Makowska. Comparison of single deviation and mean grain diameter enabled to distinguish 3 groups of deposits (Fig. 8), different due to dynamics of the environment. The group A represents channel bars composed of trough- bedded medium-grained sands, the group B – horizontal lamination of the upper flat bottom and trough-bedded deposits from medium-grained sands to fine gravels of channel bars. Coarser grain size is accompanied with decreasing sorting. The group C indicates very bad sorting and is typical for the environment with considerably varying dynamics and changing transporting force. Such deposits fill proglacial erosive channels. Mineral composition indicates transformation cycle of amphiboles, garnets and staurolite (Fig. 5). Such changes reflect longer transport and longer mechanical treatment of material. The alluvial zone width is estimated for at least 12 km i.e. 2 - 5 times more than in the case of the valley of the Eemian or Mazovian interglacials age in this area. Similar width was also ascribed to the Vistula ice marginal streamway at the end of Vistulian. A constant and generally northern direction of flow of the braided river indicates that to the north or northwest from the studied area there must have been an ice-free zone during the deposition of alluvia. Thus, a retreat of the Odra Glaciation ice sheet from its maximum extent occurred at distance of at least 250-400 km to the north.

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