Paleoenvironmental Implications of Authigenic Magnesian Clay Formation Sequences in the Barra Velha Formation (Santos Basin, Brazil)

Maurício Dias Da Silva,Márcia Elisa Boscato Gomes,Yaro Parizek-Silva,Leonardo Borghi,Aristóteles De Moraes Rios-Netto,André Mexias,Manuel Pozo,Paulo R A Netto,Camila Wense Ramnani

doi:10.3390/min12020200

Abstract

The characterization of Mg-clays in rock samples (well P1) from the Barra Velha Formation (Early Cretaceous) allowed the establishment of mineral assemblages on the basis of their kerolite and Mg-smectite (stevensite and saponite) content. Kerolite-rich assemblages (A and B) rarely contain saponite. Assemblage B is composed of kerolite-stevensite mixed layers, while assemblage A consists of more than 95% kerolite. Mg-smectite-rich assemblages (C and CB) are made up of both Mg-smectites. The predominance of stevensite in the lower interval of the stratigraphic succession suggests evaporative conditions, higher salinity and pH, which would favor its authigenesis by neoformation. In the upper portion, the occurrence of thick kerolite-rich intervals suggests regular water inputs, contributing with a decreasing in salinity and pH, favoring the neoformation of kerolite and later kerolite-stevensite mixed layering. The saponite would be the result of the transformation from Al-smectite into Mg-smectite in a Mg2+ rich medium. The results indicate that lake hydrochemical processes would have allowed the establishment of a basic depositional sequence, from base to top, as follows: (i) initial lake expansion stage marked by the occurrence of saponite, (ii) later kerolite neoformation, (iii) formation of kerolite-stevensite mixed layer with increasing salinity, and (iv) neoformation of stevensite, marking a final stage of maximum salinity (evaporation) and alkalinity of the lake.

Highlights

In recent years there has been a resurgence on the topic of magnesian clay minerals, both in their genetic aspects and applications
Intervals composed mainly of kerolite (>95%, assemblage A) presenting thicknesses of up to 10 m occur, generally associated with low gamma ray values (Figure 4c). These intervals are intercalated with submetric levels which contain more than 95% Mg-smectite in the clay fraction, and levels with variable contents of Mg-smectite and kerolite (>50% kerolite – assemblage B, >50% Mg-smectite – assemblage CB), the
The distance between the Minerals 2022, 12, 200 gamma ray values (Figure 4c). These intervals are intercalated with submetric levels which contain more than 95% Mg-smectite in the clay fraction, and levels with variable contents of Mg-smectite and kerolite (>50% kerolite—assemblage B, >50% Mg-smectite—assemblage CB), the cited assemblages were proposed by [2] see Table 1

Summary

Introduction

In recent years there has been a resurgence on the topic of magnesian clay minerals, both in their genetic aspects and applications These minerals give rise to deposits of economic interest in various parts of the world with a wide variety of industrial and technological applications [1,2,3]. The Santos Basin is located southeast of the Brazilian continental margin, between parallels 23o and 28o south, covering 350,000 km, up to a bathymetric depth of 3000 m. It includes the coastline of Rio de Janeiro, São Paulo, Paraná and Florianópolis states. The Santos Basin is classified with other Brazilian and West African basins as passive margin basins [9,10]

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