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Unusual Development of Sandur Sedimentary Succession, an Example from the Pleistocene of S Poland

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Unusual Development of Sandur Sedimentary Succession, an Example from the Pleistocene of S Poland Geologos, 2010, 16 (2): 83–99 doi: 10.2478/v10118-009-0007-9 Unusual development of sandur sedimentary succession, an example from the Pleistocene of S Poland Tomasz Salamon1, Tomasz Zieliński2 1 Department of Earth Sciences, University of Silesia, Będzińska Str. 60, 41-200 Sosnowiec, Poland; e-mail: [email protected] 2 Institute of Geology, Adam Mickiewicz University, Maków Polnych Str. 16, 61-606 Poznań, Poland; e-mail: [email protected] Abstract An atypical lithological development of outwash deposits in the Carpathians Foreland (S Poland) shows lower and middle parts of the sedimentary succession that are characterized by sinuous palaeochannels. This channel facies con- sists of laterally accreted sands derived from side bars. The sedimentary environment was a proglacial system of ana- branching channels, presumably of anastomosed type. The outwash channel pattern was most probably controlled by the raising base level of the fluvial system. Both proglacial and extraglacial waters were dammed by a sandur within a small upland valley. Aggradation and progradation of the glaciofluvial deposits resulted in progressive rising of the dammed lake level. The low hydraulic gradient of the outwash streams resulted in a sinuous planform as well as a low- energy style of deposition. Afterwards, the rising lake water was drained off through a low watershed and the entire valley became filled with outwash sediments. The bedrock morphology thus became buried and a typical unconfined sandur with a braided channel network developed during the last phase of the glaciomarginal sedimentation (upper part of the sedimentary succession under study). Keywords: outwash deposits, sedimentology, Pleistocene, Poland Introduction ser, 1993; Marren, 2005) worked this out into a depositional model: the sandur is nowadays Sandurs (outwash plains) are, as a rule, rec- the best example of a braided fluvial system. ognized as sedimentary environments domi- The extensive development of the typical nated by typically braided rivers. This was al- braided channels on sandurs is controlled by ready expressed in the early sixties of the last the following factors: a highly unstable hy- century, when Krigström (1962) published the drologic regime regulated by glacier melting, first sedimentological study of Icelandic sandur a large flow and sediment discharge during plains. Later benchmark works (among others: floods, and cohesionless channel banks that are Church, 1972; Klimek, 1972; Boothroyd & Ash- highly susceptible to erosion. Sheetflows are ley, 1975; Rust, 1975; Boothroyd & Nummedal, a secondary depositional factor for sandurs. 1978; Bluck, 1979; Dawson & Bryant, 1987; Fra- The significance of sheetflows increases espe- Tomasz Salamon, Tomasz Zieliński 84 Tomasz Salamon, Tomasz Zieliński Fig. 1. Location map and detailed sketch of the studied site. Direction of ice-sheet advance is indicated by white arrow. Line A-B is the position of the geological cross-section shown in Figure 2. cially in the distal parts of outwashes (cf. Krig- systems but also in sinuous, deep channels. In ström, 1962; Olsen & Andreasen, 1995; Aitken, the first phase of outwash development, sinu- 1998; Zieliński & Van Loon, 2003). ous channels dominated the proglacial river Since the pioneer study of Leopold & Wol- system. Up to present, no similar sedimentary man (1957), meandering rivers are usually con- successions have been described from else- sidered as opposite to braided streams. These where. Therefore, the Polish succession is in- two sedimentary environments are distinctly teresting and its interpretation is novel. The different with respect to fluvial geomorphol- purpose of the present study is to reconstruct ogy, style of deposition, and hydrological the precise palaeoenvironmental situation in conditions. Therefore meandering rivers are, which the sandur developed a sedimentary as a rule, not present on sandurs, but a few character that is quite different from the cur- studies deal with such an exceptional situ- rently acknowledged models. ation: Klimek (1972) mentioned that mean- The succession under study has been de- dering channels are formed on Icelandic out- scribed earlier by Salamon (2009) as an exam- wash plains of extremely low slope (3.1 × 10–6 ple of a relationship between the style of pro- < S < 4.5 × 10–6). This was characterized more glacial sedimentation and the morphology of precisely by Boothroyd & Ashley (1975) and the upland bedrock. The present contribution Boothroyd & Nummedal (1978): in the dis- focuses on a more detailed sedimentological tal, flat, vegetated zones of Alaskan sandurs, analysis, and contains new data which are the high-sinuosity channels are present. Point bars basis for a discussion about fluvial environ- and side bars are abundant forms within these ment and sandur development under condi- channels, whereas typical floodplain sedimen- tions of a changing base level. tation takes place beside them. Unusually developed outwash deposits have been found in the northern part of the Study area Oświęcim Basin in southern Poland. Sedimen- tological analysis pointed out that these depos- The Gardawice site under study consists of its accumulated not only in braided channel a set of large sandpits in the northern part of Unusual development of sandur sedimentary succession, an example from the Pleistocene of S Poland 85 Fig. 2. Synthetic geological cross-section through the northern part of the Oświęcim Basin (based on Salamon & Wilanowski, 2003). For location, see Figure 1. the Oświęcim Basin in the Carpathians Fore- & Wilanowski, 1999). Glaciofluvial sands of land, southern Poland (Fig. 1). It is located in Drenthian age form a surficial cover of the the watershed zone of the two largest Polish study area (Fig. 2). rivers: the Odra and the Wisła. The area around Gardawice is located about 4 km from the Gardawice is slightly undulating. The surface line indicating the maximum extent of the reaches 280–285 m a.s.l. and is gently inclined Drenthian (Odranian in Polish stratigraphy) ice to the East. The Oświęcim Basin forms part sheet (Karaś-Brzozowska, 1963; Lewandowski, of the Carpathian Foredeep and is filled with 1982). Its position is marked by an extensive Neogene deposits, mainly clays. In the North, till cover west of Gardawice. The ice sheet ad- the Oświęcim Basin meets the Mikołów Horst, vanced to this part of the Oświęcim Basin from which belongs to the Silesian Upland (Kotlicka the NW (Fig. 1). A large outwash plain then & Kotlicki, 1979) built of Carboniferous rocks. developed towards the ESE. It is important to The thickness of the Pleistocene deposits in mention here that, during the Drenthe Glacia- the study area is 40–60 m. They are represent- tion, the Wisła/Odra watershed was located ed by tills intercalated by sands, gravels and more eastwards than nowadays (Lewandow- silts (the Elsterian complex) (Fig. 2) (Haisig & ski, 1996). Wilanowski, 1999). A sandy series of intergla- The ice sheet advanced into the upper part of cial palaeovalleys is present as well (Klimek & the Bierawka drainage basin, where it formed Starkel, 1972; Kotlicka & Kotlicki, 1979; Haisig a glaciomarginal lake. The 5–10 m thick succes- 86 Tomasz Salamon, Tomasz Zieliński sion of gray clays and silts found in drill-holes This lithofacies is intercalated by sand beds (Haisig & Wilanowski, 1999) (Fig. 2) represents with various types of ripple lamination Sr (in- this ice-dammed lake. Glaciolacustrine depos- cluding climbing ripples), as well as silty sands its underlie the outwash plain under study with wavy lamination SFw (Fig. 4). The most (Fig. 2). The contact between glaciolacustrine fine-grained (silty and sandy/silty) deposits and glaciofluvial facies is at an altitude of 260 form often sheet-like packages with a large m a.s.l. horizontal extent, at least several tens of me- tres. Sometimes the sand beds are arranged in palaeochannel structures of 1.5–2.0 m deep and Description of the sandur 20–30 m wide. succession and its interpretation Another characteristic association, Si, is abundant in the lower unit. It is formed by large-scale (2–4 m thick) sets of fine-grained The exposed sandur succession is 10–20 m sand laminae (Fig. 3). Parallel laminae are thick and is mainly composed of sands. Four inclined at angles of 10–20°. Lithofacies Si glaciofluvial units have been distinguished at infills asymmetric palaeochannels which are the Gardawice site. The lowermost unit con- a few metres deep and 20–60 m wide (Fig. tains several lithofacies associations. Each up- 5). Cross-beddings indicate variations in cur- per unit corresponds with a single lithofacies rent directions of up to 40o within individual association. The lithofacies associations that palaeochannels, whereas the current direc- build up the various units are denoted by code tions measured in adjacent palaeochannels symbols (Table 1) of the most frequent beds. are strongly different. Small- or medium-scale cross-stratification of intrasets within Si pack- Table 1. Lithofacies code symbols used in text and figures ages display an opposite orientation to the (partly modified after Miall, 1978). inclined beds (Figs. 6, 7). These intrasets are Textural symbols less common, but sometimes form sand beds F silt of a few decimetres thick with trough (St) or S sand planar cross-stratification (Sp). It is also char- SF silty sand acteristic that relatively thick beds of dune (St) SG gravelly sand cosets pass in the up-slope direction into rip- pled beds (Sr) (Fig. 8). Structural symbols Lithofacies association St is composed of i large-scale inclined cross-stratification medium-grained sand. The sets with trough l low-angle cross-stratification cross-stratification are up to 25 cm thick (Fig. p tabular cross-stratification 8). They form cosets 0.7–1.5 m thick and sev- t trough cross-stratification eral dozens of metres wide. The cosets are r ripple cross-lamination commonly characterized by a thinning-up ten- w wavy lamination dency. The frequency of this association is evi- dently lower than that of the two associations mentioned earlier.
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