Ancient and modern anastomosing rivers: in sights from sedimentological and geomorphological case stud ies of the Tri assic, Neogene and Holocene of Poland

Abstract

We re view the three re gional anastomosing flu vial sys tems, both an cient and mod ern. The di no saur-bear ing up per Tri as sic suc ces sion in Krasiejów (S Po land) is com posed of siltstones and claystones that are di vided into three fa cies as so ci a tions. One of the flu vial as so ci a tions is characterized by fea tures typi cal of a low-en ergy anastomosing river sys tem in a trop i cal semiarid cli mate, in ter preted as the re sult of ac cu mu la tion in deep, wide and low-sin u os ity palaeochannels with pro nounced ver ti cal ac cre tion. De po si tion from sus pen sion pre dom i nated in flows of very low stream power. The upper Neo gene muddy suc ces sion in a tec toni cally ac tive area (Kleczew Graben, cen tral Po land) in cludes a great num ber of flu vial palaeochannels filled with sand and/or mud. These rib bon-shaped flu vial bod ies are deep and wide, and rep re sent chan nels show ing very lim ited lat eral mi gra tion. They were filled mostly under low-energy con di tions, and their mapped course shows an “anabranching” pat tern in plan view. The palaeochannels are tran si tional from sand-to mud-dom i nated. The Ho lo cene up per Narew River (NE Po land) rep re sents a mod ern anastomosing flu vial sys tem. The in ter con nected chan nels form an anabranching pat tern. The chan nels are straight to slightly sin u ous, rel a tively deep and wide. Interchannel, low-lying “is-lands” are cov ered by peat-form ing plants. De spite the low stream power, in-chan nel de po si tion is dom i nated by sand trans-ported as bedload. The chan nel banks are sta bi lised by veg e ta tion, which ef fec tively pre vents their lat eral migration.