Depositional system of an anastomosing river with low organic content: an example from the Oława Valley, SW Poland

Abstract

Large amounts of or ganic mat ter (OM) de pos ited on a co he sive floodplain are con sid ered a typ i cal fea ture of a low-en ergy anastomosing river in a hu mid cli mate. The O³awa River (Silesian Low land) is a multi-chan nel flu vial sys tem with lat er ally sta ble and low-gra di ent chan nels as well as inter-chan nel is lands cov ered with lush veg e ta tion. Stud ies in the O³awa Val ley were de signed to de ter mine the forms of oc cur rence and the con di tions of de po si tion of OM in re la tion to the main sed i mentary subenvironments. The re sults show a low pro por tion of OM in the sed i ments of the O³awa floodplain, usu ally dis play ing val ues typ i cal of min eral soils (up to sev eral per cent). In par tic u lar, the floodbasin and nat u ral levee de pos its have a low content of OM. The rap idly de cay ing OM there mainly ac cu mu lates in epipedons and is dom i nated by leaf and wood de bris. A some what higher con tent of OM was found in the chan nel al lu via, es pe cially in the rhyth mi cally strat i fied sed i ments of the upper part of the river bars. Plant de tri tus in the form of spe cies-rich wood, bark, re mains of leaves, fruits and seeds was de posited di rectly from the tree can opy or to gether with mud in the last phase of the flood. The re mains of di a toms, porifera and bryo zoans are typ i cal of sand ier strata. Or ganic sed i ments from fre quently flooded bars lo cally con tain more rhizodermis and epi der mis of amphiphytes. The great est po ten tial for ac cu mu la tion and long-term pres er va tion of OM is found in the abandoned chan nels. The aban doned chan nel-fills mainly con tain the re mains of autochthonous wood, rhizodermis and epi dermis. The re mains of mosses, Cladocera and Chironomidae are char ac ter is tic of this sed i men tary sub-en vi ron ment. In gen eral, how ever, the area oc cu pied by peatlands within the study area is dis pro por tion ately low (less than 1%) com pared to pub lished ex am ples of anastomosing-river, or ganic-rich floodplains. In the sec tions stud ied, the O³awa River thus rep resents a type of anastomosing sys tem in tem per ate, hu mid ar eas that has been over looked by clas si cal clas si fi ca tions and is char ac ter ised by a low or ganic sed i ment con tent.