Magnetic gradient map of the mine tailings in Portman Bay (Murcia, Spain) and its contribution to the understanding of the bay infilling process

Abstract

Abstract The present study is a pilot magnetic gradient survey inserted in a set of geological and geophysical works that are planned to understand the infilling process of the Portman Bay (Murcia, Spain). In a period of 33 years (from 1957 until 1990) the Portman Bay has been silted up with mine tailings. This mining waste, after being discharged into the sea, was transported by littoral currents which sorted it and concentrated the densest mineral fractions (mainly magnetite and other iron oxides). In this sedimentary context, the magnetic gradient map obtained did not detect any massive accumulation of magnetic minerals, but rather a regular distribution following parallel-banded structures in the successive contours of the bay. It has been observed that one of these magnetic bands perfectly superimposes on the ancient coastline visible in aerial photo of 1972, interpreting that this coincidence is explained by the fact that the magnetite was concentrated along the sandy ridges, oriented according to the refraction of the littoral currents. In this document we analyse the magnetic gradient map obtained in a restricted sector of the Portman Bay. This sector covers an area of 2 ha and it was chosen as a first test. The significant results suggest the suitability of this method to be conducted to the whole bay. The main objective of this study is to show the usefulness of magnetic gradient method to obtain a paleogeographic reconstruction of the infilling process of this bay.