Glacial Geology Research Papers - Academia.edu (original) (raw)

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.
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.
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.
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.