OCORRÊNCIA E ORIGEM DE BOLAS DE LAMA NAS PRAIAS DA ENSEADA DE JURUJUBA (BAÍA DE GUANABARA) - NITERÓI-RJ

Resumo

O objetivo deste trabalho é relatar a ocorrência, bem como determinar a origem, de bolas de lama freqüentemente encontradas nas praias de São Francisco e Charitas, no fundo da Enseada de Jurujuba (setor leste da Baía de Guanabara), após ressacas ou fortes tempestades. As bolas de lama apresentam, principalmente, as cores preta, cinza-clara, ou esverdeadas. São de diversos tamanhos e cobertas por biodetritos, quartzo ou outros fragmentos. Os estudos mineralógicos e geoquímicos permitiram a correlação entre as bolas de lama e a sequência sedimentar descrita nos testemunhos coletados na Enseada de Jurujuba. As bolas de lama cinza-clara e esverdeadas tem sua origem associada à camada de lama inferior existente à cerca de 3 metros de profundidade, dentro da sequência sedimentar da Enseada. As bolas de lama pretas são provenientes da camada de lama superior, depositada nos últimos três (3) mil anos. As dragagens realizadas na Enseada, na década de setenta, chegaram a atingir a camada de lama inferior, a qual passa a ser remobilizada pela ação das ondas e correntes durante períodos de ressaca ou tempesta-des.

OCCURRENCE AND ORIGIN OF MUD BALLS ALONG THE BEACHES OF JURUJUBA SOUND (GUANABARA BAY) − NITERÓI-RJ

Abstract

This paper studies at the occurrence and origen of armoured mud balls on São Francisco and Charitas beaches - Jurujuba Sound (eastern Guanabara Bay), frequently found in the region after storms. The mud balls are mainly black, light grey and greenish, of various sizes, and covered by biodetritus, quartz or other fragments. The mud balls are similar to ones previously studied on a variety of modern environments including marine and lacustrine shorelines. Armoured mud balls can potentially become preserved in the geological record, thus the understanding of the modern environments of formation will aid the reconstruction of paleoenvironments. Mineralogical and geochemical studies allow the correlation between the mud balls and the sedimentary sequence described in cores collected in the sound. The source of mud for the light grey mud balls is with the lower mud layer, that occurs at around 3 meters within the sedimentary sequence, while the black balls originate from the upper muddy sequence that was deposited in the last 3 thousand years. The dredging carried out in the area in the 70’s penetrated the lower mud layers, which made the mud available to be remobilized by wave and current action during periods of surges or storms.