Abstract Composite islands that originate from central bars are one of the most frequent types of islands in the Parana 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 Parana 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.

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复合岛的建筑、沉积相和年代学：巴西巴拉那河上游模型

摘要 起源于中央坝的复合岛屿是巴拉那河和世界上许多大河中最常见的岛屿类型之一。本研究的目的是突出岛屿形成过程以及在 101 至 104 年的时间里，一条大型亚支热带河流上的岛屿的演变。选择巴西帕拉纳河上游的 Mutum 岛（平均流量 = 9000 m3 s-1）是因为它的面积很大（长度 > 15 公里）以及在过去 8 ka 中一直活跃的形成过程。我们的分析使用了 60 年系列的卫星图像和航拍照片，以及来自 11 个振动岩心和露头的 16 个绝对日期（14C 和 OSL）。穆图姆岛是由一个中央酒吧的稳定、垂直加积和植物发育形成的。岛屿扩张是通过增加横向和正面杆来实现的。岛状沉积物表现出三种主要的岩体：与河道过程相关的岛面以下 3 m 以上的粗至细基底砂沉积物；由吞并作用形成的2-4 m厚的中砂泥层；以及岛上洪水流速减慢形成的上部块状泥浆。存款的年龄从岛上的中心（8200 BP）到银行（现在）逐渐减少。岛屿沉积物的主要横向扩展导致了与其他分支河流所描述的不同的沉积结构。本研究表明，从全新世开始，穆图姆岛的形成和演化相对稳定。