Architecture, sedimentary facies and chronology of a composite island: A model from the Upper Paraná River, Brazil

Abstract

Composite islands that originate from central bars are one of the most frequent types of islands in the Paraná River and in many large rivers in the world. The aim of this study is to highlight island-forming processes and the evolution of an island on a large anabranching tropical river over a period of 101–104 yr. Mutum Island in the Upper Paraná River, Brazil (mean discharge = 9000 m3 s−1), was chosen because of its large size (>15 km in length) and by the formative processes that have been active over the last 8 ka. Our analysis used a 60-yr series of satellite images and aerial photos, and 16 absolute dates (14C and OSL) from 11 vibro-cores and outcrops. Mutum Island was formed by the stabilization, vertical aggradation, and vegetal development of a central bar. Island expansion occurred through the addition of lateral and frontal bars. Island deposits exhibit three main lithosomes: coarse to fine basal sand deposits more than 3 m below the island surface related to channel processes; middle sandy mud layers 2–4 m thick formed by annexation processes; and upper massive mud formed by flood flow deceleration over the island. The age of deposits decreased from the center (8200 BP) to the banks (present day) of the island. The predominantly lateral expansion of island deposits resulted in a different depositional architecture to that described for other anabranching rivers. This study demonstrates that the formation and evolution of Mutum Island has been relatively stable since the beginning of the Holocene. The model presented for the Mutum Island represents a useful contribution on the understanding of large anabranching river systems.