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Ancient and modern anastomosing rivers: insights from sedimentological and geomorphological case studies of the Triassic, Neogene and Holocene of Poland

Kędzior Artur, Widera Marek, Zieliński Tomasz

Geological Quarterly

2021

Vol. 65, No. 4

art. no. 54

We review the three regional anastomosing fluvial systems, both ancient and modern. The dinosaur-bearing upper Triassic succession in Krasiejów (S Poland) is composed of siltstones and claystones that are divided into three facies associations. One of the fluvial associations is characterized by features typical of a low-energy anastomosing river system in a tropical semiarid climate, interpreted as the result of accumulation in deep, wide and low-sinuosity palaeochannels with pronounced vertical accretion. Deposition from suspension predominated in flows of very low stream power. The upper Neogene muddy succession in a tectonically active area (Kleczew Graben, central Poland) includes a great number of fluvial palaeochannels filled with sand and/or mud. These ribbon-shaped fluvial bodies are deep and wide, and represent channels showing very lim ited lateral migration. They were filled mostly under low-energy condi tions, and their mapped course shows an “anabranching” pattern in plan view. The palaeochannels are transitional from sand- to mud-dominated. The Holocene upper Narew River (NE Poland) represents a modern anastomosing fluvial system. The interconnected channels form an anabranching pattern. The channels are straight to slightly sinuous, relatively deep and wide. Interchannel, low-lying “islands” are covered by peat-forming plants. Despite the low stream power, in-channel deposition is dominated by sand transported as bedload. The channel banks are stabilised by vegetation, which effectively prevents their lateral migration.

Tectonic-climatic interactions during changes of depositional environments in the Carpathian foreland: An example from the Neogene of central Poland

Widera Marek, Zieliński Tomasz, Chomiak Lilianna, Maciaszek Piotr, Wachocki Robert, Bechtel Achim, Słodkowska Barbara, Worobiec Elżbieta, Worobiec Grzegorz

Acta Geologica Polonica

2021

Vol. 71, no. 4

519--542

Many geological problems have not been convincingly explained so far and are debatable, for instance the origin and changes of the Neogene depositional environments in central Poland. Therefore, these changes have been reconstructed in terms of global to local tectonic and climatic fluctuations. The examined Neogene deposits are divided into a sub-lignite unit (Koźmin Formation), a lignite-bearing unit (Grey Clays Member), and a supra-lignite unit (Wielkopolska Member). The two lithostratigraphic members constitute the Poznań Formation. The results of facies analysis show that the Koźmin Formation was deposited by relatively high-gradient and well-drained braided rivers. Most likely, they encompassed widespread alluvial plains. In the case of the Grey Clays Member, the type of river in close proximity to which the mid-Miocene low-lying mires existed and then were transformed into the first Mid-Miocene Lignite Seam (MPLS-1), has not been resolved. The obtained results confirm the formation of the Wielkopolska Member by low-gradient, but mostly well-drained anastomosing or anastomosing-to-meandering rivers. The depositional evolution of the examined successions depended on tectonic and climatic changes that may be closely related to the mid-Miocene great tectonic remodelling of the Alpine-Carpathian orogen. This resulted in palaeogeographic changes in its foreland in the form of limiting the flow of wet air and water masses from the south and vertical tectonic movements.