PRELIMINARY RESULTS ON MAGNETIC PROPERTIES OF THE GIRON GROUP COLOMBIA. IMPLICATIONS IN STRATIGRAPHY AND TECTONICS IN THE NW MARGIN OF SOUTH AMERICA

The Jurassic tectonic evolution of the northwestern corner of South America, i.e. the northern Andes, has been discussed in the past two decades but still controversial aspects are unknown. One of the most challenging issues to constraint the paleogeographic configuration as both the subduction of the Pacific Farallon plate under South America and the break-up of Pangea with the formation of the Gulf of Mexico must have influence the formation of magmatic arcs and sedimentary basins.

A major difficulty arise for the absence of reliable ages for the sedimentary units, and the understanding of the transition between volcanic-influenced sedimentary rocks and associated plutonism (e.g. Norean and Jordan formations) to “pure” continental units (e.g. Girón Group and La Quinta formation).

We present a preliminary paleomagnetic analysis on a section of the Girón Group located in the western flank of the Eastern Cordillera. The Girón Group is a continental siliciclastic sequence composed of muddy sandstones, silty sandstones, red beds and conglomerates. Our study is oriented towards to better assess the magnetic properties of the Girón Group, in order to define primary and reliable components of magnetization.

The shape of the obtained AMS ellipsoid is oblate with an average T=0.270 while the corrected anisotropy degree Pj=1.050, evidencing a very low deformation degree of the sequence. These values and the fact that after tilt correction confidence angle for K3 axis are within or close to bedding pole reveals a primary or sedimentary fabric with minor deformation. Preliminary paleomagnetic analysis in the Girón Group shows that 32 samples gave reliable demagnetization curves and well-developed magnetic components and a characteristic remanent magnetization (ChRM) could be isolated between 550°-700° C, revealing the influence of magnetite and hematite as the main magnetic carriers. After tilt correction mean directions are distributed in northern and southern polarities. Considering the magnetic mineralogy results, the AMS magnetic fabric data and the ChRM obtained, the Girón Group shows a great potential for paleomagnetic studies, including Magnetostratigraphy.