Estudo hidrodinâmico e sedimentológico do Estuário Guajará - Belém (PA)

Abstract

This work presents the hydrodynamic and sedimentological characteristics of the Guajará Estuary, in the vicinity of the city of Belém. Its approach covers geological and geomorphological aspects, the behavior of waves, winds, tides, tidal currents and the investigation of the physical-chemical parameters of the waters, with the objective of sketching a quantitative model for this environment in addition to presenting a proposal for the evolution region physics. From the oceanographic point of view, the Guajará Estuary is characterized by the absence of marked thermal stratification, being classified as partially mixed, type B in the Pritchard classification (1955), with appreciable lateral variations in salinity. It is subject to a circulation mechanism controlled by strong tidal currents, which define easily individualized flood and ebb channels. Two main wind systems are responsible for the formation of several sets of waves with characteristics that will depend on their speed, intensity, duration time and direction in relation to the geography of the estuary. The tides and tidal currents interfere decisively in the formation of waves. The dynamic tide has an average amplitude in syzygy and quadrature, in the Port of Belém, around 3.0 m and 2.5 m respectively. The tidal currents on the surface, during the moments of change of direction, show a complex mechanism of circulation, always obeying the characteristics of individualization of the flood and ebb channels. The salt tide inflow mechanism also follows this pattern. Conductivity proved to be the most important parameter for defining the salt tide intrusion mechanism. Important variations in the average values ​​of pH, O2 concentration, concentration of suspended material and conductivity in estuarine waters follow, approximately, the seasonal variations during the year marked by very rainy periods followed by the dry season. Hourly variations during tidal cycles, as well as variations arising from the fortnightly movements of the Moon (syzygy and quadrature) are similarly important. The water temperature follows the air temperature variations in the region, with expressive daily amplitudes. The Holocene sediments that fill the estuarine trough are predominantly sandy, ranging from selected to well selected, sometimes silty. The sedimentation is marked by the hydrodynamic conditions of the environment, being recognized two important textural facies: Muddy Facies and Sandy Facies. The distribution of these textural facies apparently oscillates as a function of tidal periodicity. The pelitic fraction of the estuarine sediments is composed of kaolinite, illite, smectite and stratified illite-smectite, with traces of chlorite still occurring. In the sandy fraction, quartz is the main mineral in association with iron oxide grains. Among the transparent heavy minerals, staurolite, tourmaline, kyanite, zircon, epidote and amphiboles predominate, in addition to garnet, rutile and andalusite. As main source areas, the Tocantins and Guamá rivers are pointed out. In addition to these, the Barreiras Group and the Holocene Lands must supply the estuarine bed with sediments. A geological-geomorphological evolutionary model for the Guajará Estuary is proposed, assuming the age of its implantation from the Lower to Middle Holocene, associated with the manifestations of the Flandrian Transgression.