Paleoenvironmental considerations based on geochemistry and mineralogy of a Miocene lacustrine calcrete, southern Portugal

Ana Luísa Rodrigues,Dulce Russo,Fernando Rocha,María Luz González-Regalado,Maria Isabel Dias,Guilherme Cardoso,Rosa Marques,Francisco Ruiz Muñoz,Maria Isabel Prudêncio,O Chudaev,O Shouakar-Stash,R Harmon,Y Kharaka,R Millot

doi:10.1051/e3sconf/20199806012

Abstract

A lacustrine carbonate facies has been investigated in Middle Miocene successions of the western side of the Moura-Marmelar basin, Southern Portugal in order to evaluate paleoenvironments in which carbonates formed, by using geochemistry and mineralogy. The studied formation corresponds to a recent unearthed section of a calcrete with an homogeneous carbonate facies comprising fine, whitish, and powdery material, and the only detected difference is a slightly hardness decrease towards the top, as well as an increase of weathered rock fragments (schist). The calcrete is calcium rich, even some magnesium enrichment was observed at the middle level, which is differentiated by a general depletion of all chemical elements, particularly REE, in both the bulk and clay-size fraction. Regarding the mineralogy, calcite prevails, and dolomite is only present in the middle level of the calcrete, together with a smectite enrichment, suggesting changes to more alkaline and low drainage, lacustrine and arid to semi-arid conditions.

Highlights

1 Introduction In Southern Portugal, in the Moura basin, Paleogene and Neogene formations are predominantly carbonates developed in continental environments by pedogenic processes and/or under groundwater conditions, in lacustrine and palustrine settings
For the calcretes of Southern Portugal there is a lack of information related with those issues, so in this framework, this study intends to: (i) perform a novel and detailed assessment of their chemical composition, especially of trace elements distribution, and mineralogical composition, including both the bulk sample and clay-size fraction; and (ii) establish paleoenvironmental considerations
The chemical composition of the bulk sample was obtained by X-ray Fluorescence (XRF) and Instrumental Neutron Activation Analysis (INAA), and of the clay-size fraction by INAA

Summary

Introduction

In Southern Portugal, in the Moura basin, Paleogene and Neogene formations are predominantly carbonates developed in continental environments by pedogenic processes and/or under groundwater conditions, in lacustrine and palustrine settings. A detailed study of the geochemistry and mineralogy of this calcrete is performed to better clarify its composition, development and paleoenvironmental conditions. The use of the chemical and mineralogical composition of the clay fraction yield valuable palaeoenvironmental information and has proved to be a successful tool for palaeoenvironmental reconstruction in many different sedimentary basins For the calcretes of Southern Portugal there is a lack of information related with those issues, so in this framework, this study intends to: (i) perform a novel and detailed assessment of their chemical composition, especially of trace elements distribution, and mineralogical composition, including both the bulk sample and clay-size fraction; and (ii) establish paleoenvironmental considerations. The mineralogical and geochemical data reported here intend to provide insights into the development of these type of calcrete deposits, and its usefulness for paleoenvironmental reconstruction in other regions of the world

Methods

Results

Conclusion