Sedimentary Dynamic and Composition of a Tidal Channel in a Tropical Hot Semi-Arid Environment, NE Brazil

Ian Cerdeira De Oliveira Souza,Narelle Maia De Almeida,George Satander Sá Freire,Guilherme Augusto Mendonça Maia,João Capistrano Abreu Neto

doi:10.11137/2020_4_144_155

Abstract

Tidal channels comprise a peculiar and dynamic environment. This paper aims to recognize the sedimentary distribution and composition of a tidal channel located in a semi-arid climate area in order to understand the sedimentary dynamics of the region. This region has economic and environmental importance considering that several activities are developed in the area such as; salt industry and aquaculture with shrimp farming. The results and discussion presented here on the Barra Grande Port tidal channel are based on 43 superficial samples distributed in the area, in which we analyzed the grain-size distribution and the calcium carbonate and organic matter contents. The data enabled the characterization and compartmentalization of the tidal channel on five sections and the interpretation of the sedimentary dynamics of the area. The sections present an important variation in the composition and selection. The section 1 is located in supratidal zone while sections 02, 03, 04 and 05 are in intertidal zone. The grain-size mean has a tendency to decrease toward the end of the channel as well as the gravel percentage, and the carbonate and organic matter contents. Differently, the mud content and the sorting increase toward the end of the channel and the skewness becomes more positive. In a general way, the carbonate content is high throughout the tidal channel ranging from 20 to 98% while the organic matter content is low ranging from 0 to 3%. This sedimentary distribution occurs due to the development of a hydraulic dam on section 3, causing a morphological growth of this sand bar, which acts as a natural hydraulic dam, hampering the access of the tide and consequently reducing the effectiveness of the transport, resulting in the deposition of fine sediments in the sheltered areas of the channel (sections 04 and 05). The high temperatures and low rainfall of the tropical hot semi-arid climate allowed the development of carbonate sedimentation as well as the development of anthropic activities such as salt extraction in artificial salt pans which may have influenced the low levels of organic matter.

Highlights

Many intertidal flats are dissected by complex system of tidal channels, and the subtidal sediments are those which are seldom, if ever, exposed to air (Shinn, 1983)
Other factors influenced in the sedimentation of Barra Grande tidal channel, such as, the morphology, the climate, the natural biological diversity, the anthropogenic activities, among others, as it is discussed as follow
The tidal channel was compartmentalized on five sections that presented important variation

Summary

Introduction

The supratidal sediments are those deposited above normal or mean high tide and exposed to subaerial conditions most of the time because they are flooded only by spring and storm tides; spring tides occur twice each month, and storm tides, the largest of all, occur sporadically during certain seasons and are less frequent. This environment includes, in arid regions, the sabkhas. Many intertidal flats are dissected by complex system of tidal channels, and the subtidal sediments are those which are seldom, if ever, exposed to air (Shinn, 1983)

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Conclusion