Geochemical and mineralogical characterization of a Lower Cretaceous sedimentary profile from central Algarve (Portugal)

Abstract

Geochemical and mineralogical studies were carried out in Cretaceous sedimentary rocks from a profile of the central Algarve (Portugal), with the aim of contributing to the analysis of paleoenvironmental variations, during Cretaceous, in the Algarve Basin. Major and trace analysis provided information on the provenance, sedimentary history and geochemistry of the Cretaceous sediments. The clay assemblage (illite/kaolinite) and high Zr/Rb ratios point to a strong continental influence and early diagenesis. The chemical variability for most elements is concordant with variations in the grain-size distribution, with the exception of some transition trace elements. There is a strong relationship between the levels formed under more reducing conditions (richer in organic matter) and higher positive anomalies of transition elements like Zn, Co, U, As and Sb. These observations enhance the role of organic activity in modifying the chemical composition of rocks. The mineralogical content, as the presence of reduced sulfur species (pyrite), and some field observations suggest the preservation of different redox regimes in the sedimentary column, denoting paleoenvironmental variations.