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Sand- and mud-filled fluvial palaeochannels in the Wielkopolska Member of Neogene Poznań Formation, central Poland

Widera M.

Annales Societatis Geologorum Poloniae

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2013

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Vol. 83, No. 1

19--28

EN

This study focuses on the single- and multi-storey fluvial palaeochannel lithosomes encased in mud- rich floodplain deposits in the alluvial succession of the late Neogene Wielkopolska Member of the Poznań For- mation, central Poland, well-exposed in the lignite mining pits of the region. The fluvial lithosomes include both sand-filled and mud-filled channel varieties. The channel-fill facies are not diagnostic for any particular type of fluvial system, as the fine- to very fine-grained sandy deposits are massive to trough cross-stratified and also the muddy deposits are massive to weakly flat-laminated. The scarcity of lateral accretion bedding precludes the possibility of meandering rivers, whereas the low width/thickness ratios of the palaeochannels preclude braided rivers. The width/thickness ratio is in the range of 4.5–14 (averaging 7.5) for sand-filled channels and in the range of 6–10 (averaging 9) for mud-filled ones, which indicates narrow ribbons in general classification of fluvial channel belts. The origin of the alluvial succession is attributed to a W-/NW-directed anastomosing river system characterized by laterally inactive cut-and-fill channels with cohesive and vegetated banks. The sand-filled chan- nels conveyed water and sediment discharges on a perennial basis, whereas the mud-filled conduits are thought to have been the cut-and-abandoned branches of the system, filled by overbank flooding from adjacent active channels. Minor lateral migration of channels occurred probably during periods of minimum subsidence rate, when the fluvial system was forced to develop lateral accommodation for its discharges.

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Braided-river and hyperconcentrated-flow deposits from the Carboniferous of the Lublin Basin (SE Poland) - a sedimentological study of core data

Waksmundzka Maria I.

Geologos

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2012

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Vol. 18, No. 3

135--161

EN

Fining-upwards cyclothems found in five boreholes in the Carboniferous (Lower Bashkirian) of the Lublin Basin were analysed sedimentologically. It was established that the cyclothems represent fluvial deposits, and the lithofacies were grouped into lithofacies associations. Most lithofacies associations represent three types of sand-bed braided rivers: (1) high-energy, (2) deep and (3) distal sheetflood-affected. Other associations represent hyperconcentrated flows. Both coarse-grained (type I) and fine-grained (types IIa and IIb) occur among the fining-upward cyclothems. The formation of most thick cyclothems was related mainly to allocyclic factors, i.e. a decrease in the river’s gradient. The thickest fining-upward cyclothems are characteristic of hyperconcentrated flows and braided-river channels. The aggradation ratios were commonly high. During the early Namurian C and early Westphalian A (Early Bashkirian), the eastern part of the Lublin Basin was located close to the source area. The sedimentary succession developed due to a transition from high-energy braided-rivers and hyperconcentrated flows to lower-energy braided rivers, controlled by a rise of the regional base level.

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Glacifluwialne facje strumieni przeciążonych zawiesiną na przykładzie plejstocenskich osadów wschodniej Jutlandii i Pomorza Zachodniego

Pisarska-Jamroży M.

Przegląd Geologiczny

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2007

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Vol. 55, nr 6

503-510

EN

Beverage & Culbertson (1964) defined hyperconcentrated flow as a flow of water-sediment mixture with a behaviour intermediate between that of a debris flow (mudflow) and that of a stream flow. However, the essence of this definition has largely been misunderstood and lost in the subsequent literature. It would appear that almost any deposit can possibly be attributed to a hyperconcentrated flow, because this genetic label has been attached to: non-stratified deposits with normal or inverse-to-normal grading, as well as deposits that were stratified; some of these deposits had polymodal and only other bimodal grain-size distribution, occasionally bearing outsized cobble gravels and boulders. Arguably, the Beverage and Culbertson original definition implies a turbulent, non-Newtonian fluidal flow with pseudoplastic rheological behaviour, intermediate between that of a mudflow (plastic) and a stream flow (Newtonian fluid), which may suggest sediment deposition by rapid dumping from suspension (Lowe, 1988; Vrolijk & Southard, 1997), rather than tractional emplacement. The study areas in Western Pomerania and east Jutland are located in transition fan and glaciofluvial fans (Weichselian glaciation). Three assemblages of deposits derived from hyperconcentrated flow are exposed: massive cobble gravel (monofacial association GCm), massive sand (monofacial association Sm) and massive sand and planar-cross bedded sand (lithofacies association Sm, Sp). The reason for significant grain-size distribution diversity of sediments derived from the same depositional process was difference in competence flow which came out from discharge flow changes. Despite different grain framework grain-size distribution, grains within hyperconcentrated flows were mostly moved by turbulences and dispersive grain pressure.