Análise de proveniência sedimentar e evolução holocênica do Delta do Rio Parnaíba através de testemunho por vibração

Abstract

The present work comprises the results of the description, analysis and interpretation of vibracores collected in recent sediments of the Parnaíba river delta, located on the border between the states of Piauí and Maranhão, Barreirinhas basin. Because of the low number of inhabitants and the low-level socioeconomic development of this region of Northeastern Brazil, the Parnaíba River delta is a rare example of a delta depositional system that still presents a large part of its natural features with few human interventions. In this context, 9 cores were collected using the vibration core method, resulting in a recover of 14,86 meters of sediments. Grain size, compositional, morphoscopic and mineralogical analysis were carried out in a total of 295 samples throughout all the cores in order to carry out a study of sedimentary provenance in the delta and identify if the changes in the depositional environment are related to a probable, delta lobe swicthing process, responsible by the isolation of the western portion of the delta during the Holocene. Ascending granodecrescence was observed in the five western cores. At the top, they present very fine sands or mud with high contents of organic matter and calcium carbonate (CaCO3). In the intermediate section occurs a variation between very fine sand and medium sand with present a decrease in the contents of organic matter and CaCO3. While at the base there is fine sand with low levels of organic matter and CaCO3. The transition between the muddy and sandy environments occurs abruptly, but in agreement, between depths of 42 and 72 cm. The mineralogy at the base of the cores located in the western portion of the delta, close to the city of Tutóia is very similar to the mineralogy observed in the cores located in the main channel of the Parnaíba river. Thus, this provenance study provides evidence that supports the hypothesis of the existence of a relatively recent connection between the Parnaíba River and the western portion of the delta plain, indicated by historical information. Possibly, the main channel ran through the western portion and the great tidal channels of the delta, in reality, would be old distributional channels, which were abandoned due to the lobe switching process.