Marcelo Cordeiro Thales

Palaeodrainage on Marajó Island, northern Brazil, in relation to Holocene relative sea-level dynamics

A large area in northeastern Marajó Island, northern Brazil, has been characterized geomorphologically, applying information acquired from Landsat imagery. This study was combined with detailed sedimentologic analysis of continuous cores, which provided a record of depositional settings developed in this area through the Holocene. The results revealed well-preserved, meandering to anastomosed drainage networks of wide palaeochannels that were superimposed by a narrower palaeochannel system. In both cases, the sedimentary record consists of sands, heterolithic deposits and muds, locally rich in plant debris. The strata are organized into fining upward successions that reach approximately 18 m thick in the wide channels and 4 m thick in the narrow channels. Sedimentary features suggestive of a coastal location for the wider palaeochannels and reworking of sediments by tidal currents include the prevalence of well to moderately sorted, rounded to sub-rounded, fine- to medium-grained sands displaying foreset packages separated by mud couplets, suggestive of tidal cycles. The data presented herein point to a rise in relative sea level reaching the Lake Arari area during the early to late/mid Holocene. This event was followed by a relative sea level drop. Tectonics seem to have contributed to an overall lowering in relative sea level in the study area since the mid-Holocene, which does not follow the same pattern recorded in other areas along the northern Brazilian coast.

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.

Processos de ocupação nas novas fronteiras da Amazônia: o interflúvio do Xingu/ Iriri

Este trabalho apresenta os primeiros resultados do esforco conjunto de varias instituicoes, organizadas em torno da rede Geoma (Rede Tematica de Pesquisa em Modelagem Ambiental da Amazonia) para avancar a compreensao dos novos padroes e processos de estruturacao do territorio nas novas frentes no sul do Para, analisando padroes de desmatamento e os processos que dao origem a esses padroes. Busca-se, aqui, produzir os subsidios necessarios para o desenho de politicas publicas responsaveis, que nao privilegiem um unico aspecto do problema, como a abertura de estradas, por exemplo. Aponta-se, entao, a partir desses primeiros resultados, que apenas uma solucao integrada que procure estruturar os principais agentes e processos na cadeia produtiva seria possivel para minorar os efeitos do desmatamento e nortear o desenvolvimento integrado para a regiao, com beneficios para a floresta e para as populacoes que ali vivem.