Paleogeograficzne elementy rozwoju doliny Warty w Kotlinie Kolskiej w świetle badań w stanowisku „Koźmin Las”
Palaeogeographical elements of the Warta River Valley evolution within the Koło Basin in the light of investigations at the site „Koźmin Las”

Summary/Abstract: The “Koźmin Las” site is located in central Poland, within the extent of ice sheets of the Middle Polish Complex. Geomorphologically, it lies on the low terrace of the Warta river valley. The main interest at the site was focused on the remnants of forest, which persisted as stumps in situ, collapsed trunks and individual branches, and its relation to the underlying and overlying mineral deposits. These are the uppermost portion of the Weichselian fluvial complex in the investigated area. The origin of the forest and accompanying organic series has been estimated with the radiocarbon method to be of Alleröd and Younger Dryas age. A field study has been undertaken in 2010 in the open pit of about 160 m2 using an archaeological approach. The material for sampling purposes of palaeolocogical analyses was also collected. The objective of the study is to establish the cause of the forest destruction by answering the question about secular or catastrophic nature of the event and also to provide information on palaeoclimate parameters from a local perspective and their relation to a global data. The depositional sequence at the “Koźmin Las” site has been divided into three lithological units, termed lower, middle and upper. The lower unit consists of sand deposits and represents a braided river environment under cold climatic conditions of the LGM period. The middle unit, mainly composed of organic-rich deposits, is the one with the main assemblage of tree remnants. At the bottom of the unit deformation structures, such as flat-bottomed involutions and drop structures have been observed. The upper sandy and sandysilty unit has been recognized as overbank facies of an anabranching river. The results obtained so far show that a forest, dominated by pine, covering the riverside area, occupied the sandy substratum. Climatic conditions of the Younger Dryas were first reflected in the increasing moisture of a substratum and afterwards in the appearance of shallow pools with Cladocera, among which the most abundant are the littoral species of the Chydoridae family. Pollen diagrams indicate an open pine forest with an admixture of birch. The appearance of rush plants and aquatic species reflect stand waters. Water-logging is also supported by the results of plant macrofossils analysis. Local permafrost reactivation, which may have resulted in increasing groundwater level, is also taken into account. These deteriorating conditions at the site, resulting from local factors as well as global climatic changes, are reflected in morphology of trees. The results of dendrological studies inform about the poor species composition of the forest and the short life span of the trees (on average 30-70 years). Reaction wood indicates their depart from the equilibrium before the final knockdown. The forest destruction took place during a period of time of several decades.

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