PROBLEMS OF DATING ALLUVIUM USING BURIED SUBFOSSIL TREE TRUNKS - LESSONS FROM THE BLACK-OAKS OF THE VISTULA VALLEY, CENTRAL-EUROPE

Dating of alluvia of large rivers based on subfossil tree trunks is discussed using examples from the Vistula Valley near Krakow and from literature referring to Central Europe. The method, which has been used hitherto for dating alluvia based exclusively on single trunks or single generations, has led usually to erroneous results because the majority of trunks in the alluvia were redeposited. Dendrochronology is helpful in solving problems of dating alluvia, which should use only in situ trunks, i.e., trunks with bark, sapwood and branches. Equally suitable materials are stumps in growth position, which were cut down with an axe and which are preserved by an aggradating river. These stumps possess well-preserved root systems with roots of a lower order. Redeposition of trunks in alluvia is associated with lateral migration of a river channel. Channel migration leads to the washing out of subfossil trunks of different ages from older alluvia. These trunks, heavier than water, are redeposited at the level of channel lag. Simultaneously, waterside trees undermined by the river accumulate at the water level in the upper parts of point bar deposits. This results in one series of channel deposits with two layers of trunks ('a sequence of simulate aggradation'). In recent centuries, after deforestation of floodplains, this mechanism lead to the formation of a single level of black oaks at the level of channel lag. Nevertheless, the spatial pattern of redeposited trunks may be helpful in reconstruction of the direction of palaeochannel migration.