An abandoned estuary within Marajó Island: Implications for late Quaternary paleogeography of northern Brazil

Several previous studies have recognized paleochannels as the most typical geomorphologic features imprinted in the modern landscape of Marajó Island, northern Brazil. A characterization of these paleochannels by detailed mapping has not yet been accomplished, despite their great potential for helping reconstructing the succession of events responsible for the origin and evolution of the drainage system at the mouth of the Amazon River during the Quaternary. The present study aims to provide a detailed mapping and characterization of these paleochannels using remote sensing (i.e., SRTM, Landsat 5-TM, and Landsat 7-ETM+), integrated with sedimentological and radiocarbon data obtained from outcrops and cores. The mapped paleochannels allowed for the first time correlation of the channel-like morphologies with fining-upward sedimentary successions, typical of channel deposits. The study focuses on the Lake Arari area located in the northeast of Marajó Island, where a complex fluvial network dominated by a funnel-shaped structure attributed to a Late Pleistocene paleoestuarine system was recognized. The inner estuary was connected to three fluvial channels that ran from the east-southeast throughout an area currently occupied by Marajó Bay, implying that this bay did not exist when the estuarine system was active. Separation of the eastern side of Marajó Island due to tectonic causes related to capture of Tocantins River by northeastward-orientated faults would have interrupted the fluvial inflow, ultimately causing the abandonment of the estuarine system. Today, the central part of the funnel-shaped structure related to the Late Pleistocene paleoestuary is occupied by Lake Arari, where sediments started to accumulate no earlier than the mid-Holocene. It is proposed that this lake might have developed from the Late Pleistocene estuary as the fluvial inflow was cut off during the detachment of Marajó Island. Marajó Island serves as an example of a very dynamic site of changing landscapes throughout the Quaternary as a result of tectonic activity.