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Mineralogy and grain-size distribution of clay-rich rock units of the Algarve Basin (South Portugal)

Published online by Cambridge University Press:  09 July 2018

M. J. Trindade ,
F. Rocha ,
M. I. Dias  and
M. I. Prudêncio
M. J. Trindade*
Affiliation:
Campus Tecnológico e Nuclear, Instituto Superior Ténico, Universidade Técnica de Lisboa, EN 10, 2686-953 Sacavém, Portugal GeoBioTec - GeoBiociências, GeoTecnologias e GeoEngenharias, Universidade de Aveiro, Portugal
F. Rocha
Affiliation:
Departamento de Geociências, Universidade de Aveiro, Campus Universitário de Santiago, 3810-193 Aveiro, Portugal GeoBioTec - GeoBiociências, GeoTecnologias e GeoEngenharias, Universidade de Aveiro, Portugal
M. I. Dias
Affiliation:
Campus Tecnológico e Nuclear, Instituto Superior Ténico, Universidade Técnica de Lisboa, EN 10, 2686-953 Sacavém, Portugal GeoBioTec - GeoBiociências, GeoTecnologias e GeoEngenharias, Universidade de Aveiro, Portugal
M. I. Prudêncio
Affiliation:
Campus Tecnológico e Nuclear, Instituto Superior Ténico, Universidade Técnica de Lisboa, EN 10, 2686-953 Sacavém, Portugal GeoBioTec - GeoBiociências, GeoTecnologias e GeoEngenharias, Universidade de Aveiro, Portugal
*
*E-mail: mjtrindade@itn.pt
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Abstract

A detailed survey of the most clay-rich rock units of the Meso-Cenozoic geological section exposed in the Algarve Basin (South Portugal) was performed and data were analysed for the grain-size distribution and mineralogy (whole rock and clay fraction), aimed at a compositional study of the clay-rich sediments and their depositional environment. Granulometry was obtained using wet sieving and laser diffraction by attenuation of X-rays, and the mineralogical study was carried out by X-ray diffraction.

Most clay-rich rock units of the Algarve are classified as silty clays and clayey silts, and only a minority is coarser. The mineralogical study enabled us to define two main types of clays: (1) noncalcareous clays, consisting largely of quartz and clay minerals, with goethite as the typical Fe-rich phase (sediments of Carboniferous, Neogene and Quaternary age and Cretaceous siliciclastic clays); and (2) calcareous clays, which can be calcite-rich clays (Middle and Upper Jurassic) or dolomiterich clays (Triassic and Lower Jurassic), the latter typically containing hematite as an accessory phase. Plagioclase, K-feldspar, and Ti-oxides are often accessory phases, whereas ankerite, anhydrite, gypsum and opal are rare.

In the clay fraction illite generally predominates, resulting probably from weathering of preexisting rocks, as well as the less frequent Fe-chlorite, pointing to incipient chemical alteration under semi-arid climatic conditions. Kaolinite occurs in diverse proportions, being especially abundant in Cretaceous and Cenozoic units; it is mainly related to chemical weathering in continental environments under humid conditions. As the Atlantic Ocean opened during Triassic and the continental environment evolved to a shallow-marine environment with evaporitic conditions, smectite became more expressive, being sometimes accompanied by other Mg-rich phases (chlorite, sepiolite, corrensite and palygorskite). Especially during the Cenozoic the proportion of different phases in the clay mineral association of the sediments reflects the control of tectonic movements and fluctuations in sea level during their deposition.

Keywords

Algarve BasinPortugalgranulometryclays-rich rock unitsclay mineralsmineralogypalaeoenvironments
Type
Research Article
Information
Clay Minerals , Volume 48 , Issue 1 , March 2013 , pp. 59 - 83
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 2013

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