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2003 (December): Pleistocene sandur deposits represent braidplains, not alluvial fans. Boreas, Vol. 32, pp. 590-611. Oslo. ISSN 0300-9483.Weichselian sandar in NE Poland show characteristics that are inconsistent with the commonly accepted alluvialfan-like model for outwash deposition and sandur formation. Analysis of the lithofacies and their vertical and lateral transitions indicates that the Polish sandar developed as braidplains, not as alluvial fans. Analysis of the geomorphic conditions under which modern sandar form, indicates that these conditions (which are characterized by deposition in a narrow belt between ice-covered mountain ranges and the sea) cannot be considered representative of those that prevailed in the geological past when sandar developed as braidplains in confined valleys, to end up in a lowland area where the deposits could spread out further in lateral directions. The latter conditions have been found consistently for all Polish Weichselian sandar that were investigated in much sedimentological detail. This raises the question whether sandar are alluvial fans or not. Because the development of the sandar in NE Poland seems to be much more representative for outwash deposition than the present-day sandar in Iceland and elsewhere, the current alluvial-fan-like sandur model -based on the fairly exceptional present-day situation with deposition in a narrow belt -should therefore be replaced by the braidplain-like sandur model -based on deposition in a valley and in a wider lowland area in front -that has been established for the Polish examples.

This study investigated the Grabia River valley mire in central Poland to reconstruct its palaeoenvironmental conditions from the Younger Dryas to the present. We analysed sedimentological, biological and geochemical data from the palaeo-oxbow lake and valley mire to identify the principal hydrological trends, especially episodes of high water level. During the Lateglacial and Holocene, the Grabia River had a meandering channel, and its hydraulic parameters and the channel dimensions changed in response to climatic oscillations and vegetation development. We identified phases of high flood intensity and high groundwater level that correlate with regional and supraregional climatic events. The frequency and timing of palaeohydroclimatic oscillations show strong similarities to records from other sites in Poland and the rest of Europe. We show that various analytical methods, namely, pollen, plant macrofossils, Cladocera, Chironomidae, sedimentological, geochemical and radiocarbon data, can be effective tools for reconstructing past hydroclimatic changes in palaeo-oxbow lakes and investigating the effects of past climate changes on river environments. The high sensitivity of the biota, especially Cladocera, to changes in water level permits the reconstruction of palaeoecological changes, especially flood episodes that occurred in the river valley. In particular, the increase in the proportion of sediment-associated Cladocera and pelagic taxa was closely correlated with floods. Through comparisons with the palaeobiological data, geochemical data allowed the identification of humid phases within the fen associated with a rising groundwater table, direct fluvial activity (floods) and alluvial deposition. We also discuss the limitations of palaeohydrological reconstructions based on these proxies, especially on fossil aquatic invertebrates.

Seis mi cally-in duced soft-sed i ment de for ma tion in cre vasse-splay microdelta de pos its (Mid dle Mio cene, cen tral Po land). Geo log i cal Quar terly, 63 (1): 162-177, doi: 10.7306/gq.1456 As so ci ate ed i tor: Anna Wysocka Cre vasse-splay microdelta de pos its and their soft-sed i ment de for ma tion struc tures (SSDS) are de scribed from a tec toni cally ac tive lig nite-bear ing area. These strongly de formed siliciclastic de pos its, sit u ated be tween two lig nite benches, are typ i cal of a cre vasse-splay microdelta. They ac cu mu lated in the overbank zone of a Mid dle Mio cene river sys tem (backswamp area) where shal low ponds or lakes oc ca sion ally ex isted. The de for ma tion takes the form of de formed lam i na tion and load (load casts and flame struc tures) struc tures as well as seis mic brec cias within the first Mid-Pol ish lig nite seam. Duc tile de for ma tion struc tures were gen er ated first by liq ue fac tion and then the brec cia was formed un der brit tle con di tions. The brecciation followed a sud den tec tonic col lapse re sult ing in an in crease in pore pres sure re lated to up ward wa ter move ment. The oc currence in a tec tonic graben and char ac ter is tic mor pho log i cal fea tures sug gest an or i gin of these deformational struc tures with seis mic shocks; thus, they can be called seismites. Hence, we pro vide strong ev i dence for ac cu mu la tion of cre vasse-splay sed i ments in the stand ing wa ter of a backswamp area, and for tec tonic ac tiv ity in cen tral Po land as the Mid dle Mio cene lignite ac cu mu lated.

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A general model is presented for the small type of fans (not to be confused with sandurs) that develop subaerially in the zone immediately before an ice front. These fans have in common with other fans that a proximal, a middle and a distal subenvironment -with distinctly different sedimentary fades -can be distinguished. The characteristics of these fans differ in several respects, however, from those formed under other conditions, particularly by the high proportion of mass-flow deposits in the proximal part, by the relative scarcity of fine particles in the middle fan, and by the relatively uniform character of the sediments in the distal fan.The special character of this type of fan is ascribed to the interaction of a continuously changing distance between the ice front and the fan (as a result of alternating phases of ice advance and ice retreat), its position that may be surrounded by ice for a large part, and the irregular supply of debris-laden meltwater that comes sometimes even in the form of more or less catastrophic floods.Due to the fact that terminoglacial fans have a good preservation potential only during phases of ice retreat, these fas tend to show a slight fining-upward tendency. The slope of terminoglacial fans tends to be more gentle (rarely over 2-5°) than that of fans formed under different conditions.