Reconstruction of continental glaciation in the Oldřichov Highland: co-operation of geomorphology and other research methods.

Transkript

1 Geomorfologický Geomorfologické sborník mapování 2 a inventarizace tvarů ČAG, ZČU v Plzni, 2003 Reconstruction of continental glaciation in the Oldřichov Highland: co-operation of geomorphology and other research methods Daniel Nývlt nyvlt@cgu.cz Česká geologická služba, Oddělení kvartéru, Klárov 3/131, Praha 1 Northern Bohemia area was repeatedly affected by the continental glaciation during the Middle Pleistocene (MACOUN AND KRÁLÍK 1995, NÝVLT 1998). This part of the Czech territory represented the marginal region and the ice sheet reached this area only during short periods in the course of glacial maxima. The extent of the glaciation has been greatly influenced by the local geomorphology. The mostly rolling upland topography within the wider area is separated by the mountain ridges. Ice crossed over the ridge of the Jizera Mts. only via the lowest transfluence pass the Oldřichov Col (478 m a.s.l.), which represents the highest known place bearing traces of continental glaciation in Northern Bohemia (NÝVLT 2002), and penetrated to the southern valley during its maximum advance (NÝVLT AND HOARE in press). The Oldřichov Highland is situated in the western part of the Jizera Mts. with the highest hills over 700 m a.s.l. (Špičák 724 m, Ostrý hřeben 714 m and Stržový vrch 711 m), the lowest parts of the northern foothills lie at about 350 m a.s.l. The Highland is characterised by strongly broken denudation relief controlled mainly by the properties of the underlying rocks (NÝVLT 2002). The surface is covered by the mesoscale landforms produced by weathering and stripping the regolith so just hard cores of rocks in form of tors and castle-koppies remained. Other periglacial phenomena like cryoplanation terraces, frost cliffs, talus and block fields are also present. In the rocky bottoms of streams potholes occur (NÝVLT 2002). The Oldřichov Col represents the lowest mountain pass on the main ridge of the Jizera Mts. building the watershed between the Lusatian Nisa and the Smědá rivers. The valley southward from the Oldřichov Col, is an old preglacial valley modelled by the consequent stream of the Jeřice River and its subsequent, mainly left, tributaries. The northern valley of the Holubí Brook is trough-shaped with a well-developed backwall. The axes of these tectonically originated valleys set the Špičák-Stržový vrch Ridge off the Ostrý hřeben Ridge which both go approximately at right angles to the main ridge of the Jizera Mts. (Nývlt 2002). The studied area lies within the Lusatian (West Sudetes) Region on the contact of Krkonoše-Jizera Massif in the E and Krkonoše-Jizera Unit in the W. The northeastern part of the valley is created by porphyritic medium- 171

2 grained biotite granite Jizera granite (KLOMÍNSKÝ 1969). On the other hand, Krkonoše-Jizera Unit, which built the western part of the area of concern, is represented by metagranites and gneisses (DOMEČKA 1970, CHALOUPSKÝ ET AL. 1989), these rocks show many similarities in the chemical composition of the protholith with eastern Lusatian granitoids (like Rumburg granite, or Zawidow and Václavice granodiorite) NÝVLT AND HOARE (in press). The ice sheet crossing over the Oldřichov Col has already been mentioned by GRAHMANN (1957) and KRÁLÍK (1989), the later suggested two or three ice sheet advances over the col into the valley south of the col. The first of these (first Elsterian advance) was the strongest and the glacier could progressed about 5 km deep to the valley to the vicinity of Mníšek near Liberec (KRÁLÍK 1989). As the glacier penetrated so far the valley, it must have filled a considerable part of the valley and produced erosional and/or depositional forms on the bedrock and changed the local geomorphology (NÝVLT 2002). By contrast, no erosional or depositional evidences of the glacier s presence were identified on the micro- or mesoscale in the valley southward from the Oldřichov Col (NÝVLT 2002). The only evidence is a body of proglacial valley sandur Mníšek glaciofluvial sand and gravel, which was accumulated by the glacial outwash. These sediments have subsequently been eroded by the Jeřice River forming the glaciofluvial terrace and now survive only as relics on the left side of the valley (NÝVLT 2002). The palaeocurrent measurements provided evidence for the material input from the Oldřichov Col direction (NÝVLT 2002). The reduction of local material up-section and the enrichment of near rocks in the same direction are well shown in the section. This means that after the ice front had reached the col, the meltwaters flowed to the southern valley, erosion and subsequently deposition of local rocks occurred. Furthermore, the frontal layers of the glacier were prevalently loaded by the local rocks (NÝVLT 2002). The more distal material, transported mainly subglacially or englacially (the nea r material could also been transported supraglacially), was deposited later during the continuing glacier pushing through the Old řichov Col. The relatively small content of Nordic rocks was particularly influenced by the position of the accumulation beyond the Oldřichov Col, as nordic rocks are usually transported in subglacial or englacial zones significantly further from the ice sheet front (NÝVLT AND HOARE 2000, NÝVLT 2002). The release of near and nordic rocks from the middle and upper glacier layers an d its enrichment, documented in the upper part of the section, occurred during the ice sheet retreat (NÝVLT AND HOARE 2000). Further details on the sedimentology and sedimentary petrology are available in the papers by NÝVLT (2002) and NÝVLT AND HOARE (2000, in press). 172

3 The body of sediments was accumulated during the second Elsterian glaciation according to the results of Nordic indicator analyses (NÝVLT in press). However, other correlation with the stratigraphic scheme of the North European glaciations has also been proposed by MACOUN AND KRÁLÍK (1995). More details on the stratigraphic position of the accumulation could be found in NÝVLT (in press). Longitudinal and cross profiles of both valleys were constructed based on the 1 : scale topographic map. The reconstructed glacier surface, its maximum advance, preglacial and immediate postglacial surface and morphological trimlines have also been reconstructed based on available geomorphological data. These results led to the general reconstruction of the glaciation in the studied area (NÝVLT 2002, NÝVLT AND HOARE in press). The re-modelling of slopes caused by intensive weathering of bedrock on the contact of the glacier body with the rock has been used to reconstruct the maximum extent of the glaciation by PROSOVÁ (1981) in Northern Moravian mountains. The ordinary trimline describes the upper limit of the glacier erosion in weathered bedrock on valley sides (BENN AND EVANS 1998). The morphological trimline, indicated by the change in the slope shape, shows on the other hand only stagnation of the glacier, not the maximum reach of the glacier in the valley (NÝVLT 2002). The accuracy of the use of the trimlines (ordinary and morphological) to reconstruct the ice sheet is therefore limited, especially as the process of abrasion and plucking at the glacier bed requires a minimum ice-thickness that could have amounted to first tens of metres. However, the combination with available data about the distribution and architecture of sedimentary bodies helped to reduce the error in the accuracy of the reconstruction. The limit of this method is ~ ±10 20 m (NÝVLT 2002). Other specific limits of use of the trimline method arise in the study area. The glacial impact on the Oldřichov Highland is quite old (Middle Pleistocene), so overprinting by younger geomorphological processes (mainly periglacial and fluvial) is quite extensive (NÝVLT 2002). As a next limit the lithological properties of the local bedrock must be mentioned, which is susceptible to both types of weathering, especially to the frost action documented by periglacial phenomena such as cryoplanation terraces, frost cliffs, and block fields in the upper part of valleys and on hill summits (NÝVLT 2002). The asymmetry of longitudinal profiles of the valleys northward and southward from the Oldřichov Col is clearly visible. These valleys originated on a fault zone at right angles to the main Jizera Fault Zone (NÝVLT 2002). Both were preglacially modelled by fluvial action since the Pliocene until the early Middle Pleistocene. The analysis of aerial photographs together with the cross-profiles shows a flat and wide bottom with steep slopes on the sides of the northern valley in comparison with the valley south of Old řichov Col 173

4 (NÝVLT 2002). This flattening and widening of the northern valley, together with its steep slope (16 in comparison to less than 10 in the southern valley) was most probably created by the ice sheet erosion (NÝVLT 2002). The absence of glacial and other types of Quaternary sediments in the upper part of the valley northward from the Oldřichov Col, was affected by the glacier excavating its soft bed and/or proglacial melt -water erosion during the retreat phase and also by the subsequent fluvial and periglacial erosion of unconsolidated material (NÝVLT 2002). The fluvial and periglacial activity was also responsible, together with the lithological properties of local bedrock, for the absence of micro- and meso-scale glacial landforms within the area (NÝVLT 2002). The palaeogeographical and chronological reconstruction of the glaciation in the Oldřichov Highland could be summarised as follows: The continental glacier was checked in the course of its southern advance through the Frýdland Hilly Land by the barrier of the Jizera Mts. The ice sheet reached up to ~ m a.s.l. in the northern foothills of the mountains. The only possibility for the continuing penetration to the south was the upward movement through narrow valleys against the flow of recent rivers. Indeed, only the Oldřichov Col was the transfluence pass low enough to enable the crossing over of the ice sheet lobe (NÝVLT 2002). The northern valley was first filled to the altitude m a.s.l. (depending on the distance from the col) by the ice, but due to the subsequent pushing of the advancing ice mass stood the ice sheet front up to the Oldřichov Col (478 m). The glacier lobe penetrated ~ m beyond the col to the southern valley during its maximum advance, this corresponds approximately to the beginning of the accumulation of the sedimentary body of the Mníšek glaciofluvial sand and gravel (NÝVLT AND HOARE in press), because the proglacial sedimentation was strongly limited during the glacier advance. The extent of the outlet glacier in the valley south of the col is therefore considerably smaller than was previously supposed by KRÁLÍK (1989). The maximum thickness of the ice in the northern valley did not exceed ~50 60 m, near the Oldřichov Col the ice thickness remained <10 m (NÝVLT 2002). References BENN, D. I., EVANS, D. J. A. (1998): Glaciers & Glaciation. 734 pp., London. DOMEČKA, K. (1970): Předvariské granitoidy Západních Sudet. Sborník geologických věd, Geologie, 18, , Praha. GRAHMANN, R. (1957): Ausdehnung und Bewegungsrichtung des Inlandeises in Sachsen. Berichte der geologischen Gesellschaft, 2, 4, , Berlin. CHALOUPSKÝ, J. et al. (1989): Geologie Krkonoš a Jizerských hor. 288 pp., Praha. 174

5 KLOMÍNSKÝ, J. (1969): Krkonošsko-jizerský granitoidní masív. Sborník geologických věd, Geologie, 15, 7 133, Praha. KRÁLÍK, F. (1989): Nové poznatky o kontinentálních zaledněních severních Čech. Sborník geologických věd, Antropozoikum, 19, 9 74, Praha. MACOUN, J., KRÁLÍK, F. (1995): Glacial history of the Czech Republic. In: Ehlers, J., Kozarski, S., Gibbard, P. L. (eds.): Glacial deposits in North East Europe , Rotterdam. NÝVLT, D. (1998): Kontinentální zalednění severních Čech. Geografie-Sborník ČGS, 103, 4, , Praha. NÝVLT, D. (2002): Geomorphological aspects of glaciation in the Oldřichov Highland, Northern Bohemia, Czechia. Acta Universitatis Carolinae-Geographica, Praha. NÝVLT, D. (in press): Leitgeschiebestatistik der Mníšek glaziofluviale Sande und Kiese. Archiv für Geschiebekunde, Hamburg. NÝVLT, D., HOARE, P. G. (2000): Valounové analýzy glacifluviálních sedimentů severních Čech. Věstník ČGÚ, 75, 2, , Praha. NÝVLT, D., HOARE, P. G. (in press): Petrology, provenance and clast shape of the Mníšek glaciofluvial sand and gravel. Journal of Sedimentary Research, Tulsa. PROSOVÁ, M. (1981): Oscilační zóna kontinentálního ledovce. Jesenická oblast. Acta Universitatis Carolinae Geologica, 25, 3, , Praha. 175

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