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

Abstract

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 limited lateral migration. They were filled mostly under low-energy conditions, 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.