Morfogênese e evolução paleogeográfica da foz do Rio Tocantins, Estado do Pará, durante o Holoceno

Abstract

The islands have a wide geographical distribution over nearly every major tropical river. In the coastal zone of the Amazon, they occur mainly between the Amazon, Pará, and Tocantins Rivers. This extensive occurrence contrasts with the attention they have received from geomorphological studies. Consequently, insular genesis is poorly understood, particularly in the system established at the mouth of the Tocantins. This study aimed to investigate the mechanisms related to the paleogeographic evolution of the mouth of the Tocantins River, State of Pará, during the Holocene, using a multi-proxy approach (oceanographic, morpho- topographic, and structural data, well drilling records, sediment cores, sedimentary features, palynology, stable isotope analysis and 14C-dating). The results suggest that the coastal adjustment of the Tocantins River is the central stage of the regional paleogeography. This river diverged from the Amazon basin and opened a new eastern route towards the Atlantic Ocean at the end of the Pleistocene. The displacement of the laterite layer overlying the sedimentary succession reflects subsidence events concurrent with the fluvial rearrangements. This succession comprises fining-upward cycles attributed to a tidal channel fill deposit, which is likely related to the transgressive configuration. The imposition of the mouth of the Tocantins towards the sea drove changes in the Holocene landscape. First, the incised valley of Maratauíra was opened, initiating the detachment of the fringe of land from the continental margin at the mouth of the Tocantins. Subsequently, the marine incursion invaded the region, attaining a peak of 2 ± 0,5 m above the present level, forming brackish environments dominated by mangroves between 8408 and 1485 cal yr BP. Mangroves suffered a spatial decline between 1485 and 700 cal yr BP, coinciding with the formation of peat bogs. In contrast, the freshwater vegetation flourished and became dominant. The paleoecological dynamics witnessed by the landscape are consistent with a fall in the relative sea level, which forced migration from brackish conditions towards the shoreline. In the last seven centuries, given the rising island physiognomy of the present day, the main changes in the landscape have involved the rise and stabilization of the sea level in the current position, drowning of the tidal river network with plains dominated by the floodplain, and complete separation of the land fringe from the continental domain. Therefore, this insular morphology is short-term, with long-term genetic and evolutionary trends. Regarding the landscape structuration mechanism, tectonic activity and sea-level change were predominant up to ~1500 cal yr BP, whereas hydrodynamics was more involved in late evolution. Currently, the role of this forcing is expressed in the morphodynamics through the tidal regime, which subordinates ~40% of the insular stretch to estuarine flood events. In addition to the region of great potential in the scope of Physical Geography, the morphological, tectonic, paleoecological, sedimentary, and biogeochemical inter-evidence gathered in this study provides data that help in the reconstruction of the paleogeographic picture of the land- sea interface of the northeastern portion of the state of Pará during the Holocene, including new signs regarding the highstand sea level on the northern Brazilian coast.