Mineralogy and magnetic fabric of Triassic Red Beds from the Conraria Formation, Central Portugal

• Type: Article in International Scientific Journal
• Date: 2010

Authors

C. Gomes (Author) Other; Maria Armanda Dória (Author) FCUP; A. Rocha (Author) Other; L.G. Pereira (Author) Other; Helena Sant'Ovaia (Author) FCUP

Journal

Title: Geochimica et Cosmochimica Acta

Vol. 74 No. 12, Supplement

ISSN: 0016-7037

Publisher: Elsevier

Indexing

ISI Web of Science

Scientific classification

FOS: Natural sciences > Earth and related Environmental sciences

Other information

Abstract (EN): The Conraria Formation is the lower unit of the ‘Grupo de Silves’ (Silves Sandstones Group) of Upper Triassic age. It is the sum of two subunits which have mean thickness of 40 ± 5 m and 120 ± 15 m, respectively, in the Coimbra region. The main goal of this study is to quantify the magnetic fabric of the upper subunit of the Conraria Formation and to relate it to the magnetic mineralogy. Studies of Anisotropy of Magnetic Susceptibility (AMS) were carried out on 50 samples from 11 sites in order to characterise the magnetic fabric. The magnetic susceptibility of these samples is comprised between 40.6 and 329.7 x 10-6 SI (average 170.36 x 10-6 SI). Magnetic anisotropy (described by the parameter (kmax/kint-1)x100) ranges from 0.4 to 8.3 % (average 3.7%). AMS fabric shows consistent patterns in the studied sites: NW-SE-trending low dipping magnetic foliations associated with subhorizontal N197° trending magnetic lineations. The shape parameter is always oblate, exceptionally the sites where the magnetic susceptibility is higher (292.83 and 329.68 x 10-6 SI) have prolate AMS ellipsoids. Petrographic studies were performed in the samples with the lower and the higher magnetic susceptibility. In the first type, the main mineralogy is composed by quartz, feldspar, rock (lithic) fragments (quartzites and quartz-phyllites), muscovite, tourmaline and biotite. The cement is essentially sparry calcite with thin films of iron hidroxides coating the particles. The second type is composed by quartz, white mica, carbonates associated with specular hematite (needle-shaped crystals) and tourmaline. Feldspar are rare and rock fragments are absent. The cement is essentially ferruginous. Our results show that this formation retain a primary sedimentary fabric, related to a planar distribution of the paramagnetic phyllosilicates, with Kmin closely perpendicular to the bedding plane. In this plane the magnetic lineation probably materializes the paleocurrents. The specular hematite associated with calcite is responsible for the prolate AMS ellipsoid. The general parallelism of the magnetic fabric with bedding indicates a composite fabric between a primary sedimentary fabric and an early tectonic layer parallel shortening fabric as it is attested by the magnetic anisotropy.

Language: English

Type (Professor's evaluation): Scientific

Notes: Comunicação apresentada na conferência "Goldschmidt 2010: Earth, Energy, and the Environment"