Abstract

VAN ZELE, María Andrea; VIZAN, Haroldo  and  NIETO, Miriam Gabriela. Ciclos orbitales jurásicos registrados en rocas sedimentarias aflorantes en la garganta de Breggia (Ticino-Suiza) a partir de la suceptibilidad magnética. Geoacta [online]. 2012, vol.37, n.1, pp.53-61. ISSN 1852-7744.

Some stratigraphic sedimentary sequences are developed in epicontinental stable platforms. During the Jurassic time there were formed such platforms in the open sea named Tethys, and now their sedimentary sequencies are located in Europe. Through different methods orbital cycles have been recognized in the organogenic sediments precipitated in the platforms of Tethys Sea. Some of these sequences register orbital terrestrial cycles called precession (of approximately 20 ka), obliquity (of approximately 40 ka) and eccentricity (of approximately 100 ka). In Breggia gorge on the basis of sedimentary rocks (marls, limestones and shales) that are rhythmically repeated, these orbital cycles were determined. The aim of the present work is to analyze if the low field magnetic susceptibility measured in Jurassic rocks outcropping in this gorge, is also an indicator of the mentioned cycles. For this work 153 susceptibility data that belong to Uptonia jamesoni and Tragophyloceras ibex biozones (aprox. 189 Ma to 187 Ma) were compiled from a pre-existing study. These determinations were corrected because a linear increase was observed; then Fourier's coefficients, the spectrum of power and the variation of the quadratic error were calculated. From these results it was inferred that cycles belonging to the precession and obliquity of the axis of the terrestrial rotation and the eccentricity of the orbit of the planet, can be recorded. The linear increase observed in the susceptibility might be due to a gradual increase of paramagnetic and/or ferromagnetic minerals in the rocks of the analyzed sedimentary sequence. It is noteworthy that in sedimentary sequences studied in Dorset and Yorkshire (England) a linear decrease of 87Sr/86Sr relation was recognized for the time span that belongs to Uptonia jamesoni and Tragophyloceras ibex biozones. This decrease was interpreted as a consequence of an increase in the hydrothermal activity in the sea floor. It is interpreted that the linear increase in the susceptibility observed in Breggia gorge would have the same origin.

Keywords : Jurassic; Orbital cycles; Magnetic susceptibility.

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