Rekonstrukcja przebiegu zlodowacenia warty w regionie łódzkim
This study presents a reconstruction of the course of events during the Warta Glaciation in the Łódź Region. It includes an analysis of the Warta stratigraphic unit, a description of development and decay features of the Warta ice-sheet (Late Saalian ice sheet) as well as features of the environment in which it functioned. The study puts major emphasis on the reconstruction of the process of shaping glaciogenic landscape, prepared on the basis of lithofacial features of sediments they were composed of, relations to older and younger sediments, and traces of tectonic activity. The reconstruction of the course of Warta ice-sheet transgression and retreat in the Łódź Region against the background of the entire Eurasian ice-sheet during MIS 6, indicates the presence of individual features and large-space character of this glaciation in relation to the situation during the preceding warming, when glaciation extent was limited to a fragment of Fennoskandia. The distinctiveness of the Warta ice-sheet is supported by a number of conditions, one of them being the presence of complex structures which point to a wide array of glacial events, including several short stages of ice-sheet readvance in the post-maximum period. These sub-phases or phases of the cataglacial period of the Warta Glaciation were noticeable in different regions, which suggests activation of individual elements of the stream-based distribution of ice-sheet masses. It is more characteristic of complete glaciation than of a smaller unit. Independently of local dissimilarities, in areas where glacial accumulation prevailed, one basic level of glacial till occurs, relatively easy to identify with the use of various methods. The Warta Glaciation took place between the pre-Warta warming, whose climatostratigraphic rank has not been fully established, and the Eemian interglacial, which is the best documented interglacial apart from the Holocene in both marine and land areas. In the Łódź region there are several uncertain locations of pre-Warta warming and at the same time several dozen documented stands of the Eemian interglacial period, mainly in kettle holes, developed on the surface of the Warta Glaciation sediments. These stands also provide evidence for the existence of the Eemian lakeland, characterised by an abundance of kettle holes. Owing to the above asymmetry of the stratigraphic framework of the Warta unit, the former practice of including the Warta stage in the Odra Glaciation is not fully justified. If the pre-Warta warming is not considered as an interglacial, it seems correct to include the Odra stage in the wider Warta unit, which has a well-defined border with the Eemian interglacial period. The debatability of the climatostratigraphic rank of the pre-Warta warming is the reason for difficulties with determining the rank of the Warta stage. The dispute has been in progress ever since the first findings of sediments from this warming period, both in the Łódź region and in other parts of Europe. To put it simply, it often comes down to determine whether it was a fairly warm interstadial or rather a cold interglacial. Because of the presence of organic stands from pre-Warta warming which have not received clear stratigraphic definition and absolute dating, this dispute has not yielded any unequivocal conclusions yet. Irrespective of it, attention should be paid to other evidence which concerns only the cold stage of the Warta Glaciation, and not the limiting periods. The evidence includes: morphological individuality of Wartian forms and sediments on the surface, well-developed lithostratigraphic features of sediments from this stage – characteristic of complete glaciations, appropriate periglacial structures present beyond the ice-sheet extent, and global-scale records of this cool period in deep-see profiles, cave deposits etc. Duration of the Warta Glaciation (sensu stricto), i.e. as a period of advance and retreat of the ice-sheet, in the Łódź Region might have been relatively short, perhaps only several thousand years. A similar time interval is also suggested in other parts of the Wartian relief zone, including Germany among other places. It may have been a small section of the cold stage of Warta. It is possible to think so on the basis of the following: lithofacial profiles characteristic of high accumulation rate, poor development of marginal forms along the maximum extent line, underlining of glaciogenic profiles with periglacial deposits, as well as numerous analogies to the main stadial of the Vistulian (Weichselian) Glaciation. However, it must be kept in mind that the thesis about the relatively short duration of glacial events is not confirmed only by the TL and OSL absolute dating results to date. For this problem to be solved, a major advance in the dating methodology is necessary. The dynamics of ice-sheet growth and the way of clearing more diverse relief of the foreland during transgression were analysed. Considerable differences in ice-sheet dynamics were found between the two main lobes: the South Greater Poland lobe and the South Mazovian lobe, as well as within the range of individual lobes. It was dependent on the way of alimenting them in the firn fields, diversity of climatic conditions in the ablation zone, thermal properties of the ice-sheet base (dependent largely on the geothermal stream density) and other base properties, in particular the distribution of proglacial lakes and high pore-pressure underground water. The course of deglaciation was also reconstructed in detail, in both temporal and spatial aspects. During deglaciation, cases of activated ice streams with probable low climatostratigraphic rank but important for the terrain’s morphology were distinguished and denominated provisionally as sub-phases of Dobrzynka, Ner and Bzura. Deglaciation zones were marked off in individual lobes on the basis of spatial and dynamic diversification of the course of glaciofluvial accumulation processes recorded in deposits and formations. Transitional thermal regime presumably dominated in the marginal zone of the ice-sheet dome, while warm thermal regime prevailed in glacial depressions. The regime may have changed in time, form transitional to warm and vice versa. It is also possible that cold regime occurred locally in glacial elevations, mostly within the extent of the South Mazovian lobe. It was particularly important for the dynamics of glacial streams and till accumulation, with regard to both its final thickness and structure. Individual ice streams, with different bottom regime, may have moved in confluence, but at greatly varying speeds. The fact that ice-sheet adapted its shape to the existing relief during transgression had its consequences during deglaciation, which often led to permanence of major relief forms. Valleys deepened by glacial and glaciofluvial erosion were partially refilled during deglaciation. That was also where initial river flow originated later. Small glacial plateaus were usually covered with a thin layer of glaciofluvial deposits and glacial till of corresponding shape. The more efficient glaciofluvial accumulation in the watershed zones is locally significant. Sometimes, both glacial currents and subglacial trough-forming waters directly eroded the Mesozoic bedrock. There were examples of “typical” exaration and detersion as well as efficient erosion of the solid bedrock by subglacial waters. Taking place in limited areas, these processes had more linear than superficial effects. Formations along the line of maximum extent are poorly developed in the entire region, which indicates lack of longer stabilisation of ice-sheet front along this borderline. This results in the long-known difficulties with determining the exact location of this line. Metasynchronous development of individual glacial streams is also very likely. As in other segments of the Wartian zone in Poland, one can question the existence of sensu stricto proglacial valleys in the analysed region. Sandar with proximal features as well as hills and other glaciomarginal forms are rare and small. However, counterparts of these forms developed in the internal lobe zones in the form of a system of small marginal valleys, internal sandar and kames of various types. The main reasons for this kind of spatial distribution of forms and lithosomes of water accumulation include subglacial relief and ice-sheet’s vulnerability to surface decay. Analysis of relief and deposits of the Warta Glaciation suggests the occurrence of short stages of glacial stream activation during ice-sheet decay, with possible low climatostratigraphic rank. These were provisionally denoted as subphases of Dobrzynka, Ner and Bzura and their presumable extents were indicated. They did not cover a fragment of the ice-sheet dome in the South-Eastern part of the region, i.e. in the South Mazovian lobe. The same glacial stream probably underwent further advances, but subsequent kinematic waves shifted its axis in an eastward direction. Distinguishing subphases and various lithostratigraphic traces of deglaciation processes provided the basis for further division into deglaciation zones in individual lobes. A complex course of ice-sheet decay processes was found. In the scale of the Łódź region, areal deglaciation predominated, which is indicated by a matching inventory of glacial landforms: kames and kettle holes, which constituted vast concentrations. Some of the landforms, sometimes in the form of chain structures, owe their origination to active pressure from ice-sheet front on older formations as a result of rapid, short-term transgression (surge) and subsequent rapid glaciofluvial and ablative accumulation. Larger forms often indicate more complex genesis. They may have originated from glaciofluvial and ablative accumulation being superimposed on kame accumulation, after a small-scale advance of ice-sheet snout. In the analysed region, many complex forms were found. For example, part of the structure indicates kame accumulation conditions in stagnant and dead ice, another part of the form may be a result of a dynamic bulldozing, while still another part – a result of subsequent glaciomarginal accumulation. Some landforms, so far regarded as kames, should be moved to other categories. This concerns in particular hills, often asymmetric, built from transitional segments of glaciofluvial cones and glaciolacustrine deltas, with traces of ice contact, which document active glacial ice sedimentation. Stability of tunnels in ice, indicated by the presence of eskers in various deglaciation zones, proves the relationship with a single main ice-sheet advance, short duration of ice-sheet coverage, simultaneous accumulation over a considerable distance and rapid accumulation of esker deposits. Identification of diversified esker accumulation ranging from the stage of N-type tunnels to accumulation in intraglacial canyons of kame sedimentation type indicates short-term functioning of live ice and rapid progress of deglaciation. Slow glacioisostatic readjustment, related to the functioning of the Warta ice-sheet, resulted in considerable stress in the lithosphere, which could be discharged in the form of tectonic shocks. Many deformation structures, including clastic dykes of significant size, indicate the possibility of tectonic shocks of up to 6°. Perhaps, as was the case during final periods of other glaciations, tectonic activity increased during deglaciation of the Warta ice-sheet. In many places Wartian deposits are deformed to the form of diapirs. These structures could originate as a result of many different processes: unstable density stratification in hydrated deposits, deposit fluidisation resulting from high stresses in the transgressing ice-sheet, tectonic shocks, as well as differences in pressure from deposits and neighbouring ice on the underlying or adjacent plastic formations. The mechanism of their origination was largely influenced by lithological features of the deposits, and in particular their susceptibility to plastic deformations and varying pore-water pressure. The Warta ice-sheet advanced to areas influenced by short-term or mild periglacial processes, which is suggested by traces of aeolian activity of surface material and local occurrence of periglacial structures. Poorly developed permafrost was probably of insular type. During the glaciation, there was a tendency of increasing involvement of meltwaters, which warmed up and degraded the manifestations of permafrost processes. Traces of material blocks, preserving the structure, numerous in the analysed deposits, may prove their short-time freezing and carry no real paleogeographic significance. It is necessary to stress the importance of permafrost structures from Vistulian, epigenetic, e.g. sandy wedges. These structures, usually completely preserved, prove that no further (e.g. in late Vistulian) significant slope profile modifications occurred at their locations. Sometimes regarded as indicators of effective periglacial denudation, these structures may be interpreted as evidence for periglacial stabilisation of the relief. Among the Vistulian periglacial processes, some relief-forming role was undoubtedly played by aeolian processes, which is indicated by deflation-corrasion pavements and accumulation of aeolian sands in closed depressions and in deposits of Vistulian rivers.
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