Analysis of the ferromagnetic contribution to the susceptibility by low field and high field methods in sedimentary rocks of the Southern Pyrenees and Northern Ebro foreland basin (Spain)  Oscar Pueyo Anchuela, 1 Antonio M. Casas-Sainz, 1 Emilio L. Pueyo, 2 Andr es Pocov  ı Juan 1 and Andr es Gil Imaz 1 1 Grupo de investigaci  on Geotransfer, Departamento de Ciencias de la Tierra, Universidad de Zaragoza, C/Pedro Cerbuna, nº 12, CP. 50.009 Zaragoza, Spain; 2 Instituto Geol  ogico y Minero de Espa~ na, Unidad de Zaragoza, C/Manuel Lasala, 44 - 9º, B. 50006 Zaragoza, Spain ABSTRACT Magnetic susceptibility in rocks is the sum of the contribu- tions of different magnetic particles (paramagnetic, diamag- netic and ferromagnetic s.l.). These contributions can be measured at variable temperatures or at variable fields (hys- teresis loops). Both are time-consuming techniques that can- not be routinely used in magnetic fabrics analysis. In this study, we propose a simplified method to determine the ferro- and paramagnetic contributions to the susceptibility, based on the discrete measurement of susceptibility at two different fields (near 0 and 2.5 T). The results obtained in samples from the Southern Pyrenees and Ebro Basin indicate that the ferromagnetic contribution vary considerably within homogeneous susceptibility values and within the expected range of paramagnetic values. Standard bulk susceptibilities higher than 200 10 À6 SI contribute significantly to the ferro- magnetic fraction (>50% on average). Terra Nova, 0, 1–8, 2013 Introduction Magnetic susceptibility (v) is an intrinsic property of the matter and can be defined as the ratio between the applied field and the induced magnetization when a substance is exposed to an external magnetic field (v = M/B). Three main magnetic behaviours can be expected: para- magnetism, diamagnetism and ferro- magnetism. Diamagnetism and paramagnetism depend on the elec- tronic interaction at the orbital scale, and ferromagnetism s.l. depends on electronic interaction at the supra- orbital scale. Standard bulk magnetic susceptibility in rock samples is the addition of these behaviours. How- ever, certain ranges of values can shed some light on the origin of the susceptibility: (i) negative values between 10 À5 and 10 À9 (SI units) are usually due to diamagnetic minerals; (ii) positive high values between 10 À2 and 10 À3 are related to ferromagnetic carriers; and (iii) intermediate values, between 10 À4 and 10 À6 , lie in the potential paramagnetic domain, due either to a complete control of para- magnetic particles or to different mixtures of ferromagnetic and para- magnetic particles. Accurate identification of the dif- ferent contributions is usually carried out by: i) measurement at different fields (analysis of susceptibility under progressively increasing magnetic fields; Rochette et al., 1983; Rochette and Fillion, 1988; : Borradaile and Werner, 1994; Richter and van der Pluijm, 1994; Mart  ın-Fern andez and Ferr e, 2007); and ii) monitoring sus- ceptibility changes (according to the Curie-Weiss law) during heating/ cooling experiments (Schultz-Kru- tisch and Heller, 1985; Rochette, 1987; Jover et al., 1989; Hrouda, 1994; Richter and van der Pluijm, 1994; Hrouda et al., 1997). These analyses are commonly applied to a limited number of samples, and the results obtained are extrapolated to groups of samples with similar lithol- ogy, susceptibility or analogous rela- tionships between susceptibility and the anisotropy of the AMS ellipsoid. In this study, we propose a system- atic approach for the analysis of the origin of magnetic susceptibility in sedimentary rocks from the Southern Pyrenees and Northern Ebro (fore- land) basin. The study focused on the Upper Cretaceous (marls, sand- stones and carbonate rocks), Palaeo- cene–Eocene (limestones, sandstones, shales and marls) and Oligocene– Miocene (sandstones, mudstones and carbonates) units. Our main goal was to classify the rock samples depend- ing on the contribution of ferromag- netism s.l. to the magnetic susceptibility where to analyse: (i) the origin of the susceptibility; (ii) the correlation among AMS anisot- ropy, bulk magnetic susceptibility and susceptibility changes; and (iii) the usefulness of the bulk susceptibil- ity value as an indicator of origin. The methodology is based on the measurement and interpretation of differences between magnetic suscep- tibilities at high and low fields. Methodology Susceptibility values at low and high fields were obtained from 245 rock samples, using a PPMS device (Physical Property Measurement Sys- tem, Quantum design; http://www. qd-international.com/) at room tem- perature (RT; 25 °C). Measured specimens were subsampled from AMS standard cores with a maxi- mum size of 1 cm, representative of Correspondence: Óscar Pueyo Anchuela, Ciencias de la Tierra, Universidad de Zar- agoza, C/Pedro Cerbuna, nº 12, Zaragoza 50003, Spain. Tel.: 97 67 621 27; e-mail: opueyo@gmail.com © 2013 Blackwell Publishing Ltd 1 doi: 10.1111/ter.12037