Magnetic fabric of a Jurassic clasic sequence from Oaxaca-Puebla, southern Mexico and inferred paleocurrent flow

Abstract

The magnetic fabric from two sections of a Mesozoic continental sequence from northern Oaxaca-Southern Puebla is interpreted as primary in some sites and secondary in others according to criteria based on (i) the anisotropy degree and its range of variation in each site, (ii) the magnetic fabric geometry and (iii) agreement with other geological indicators. The magnetic fabric was obtained from the low-field anisotropy of magnetic susceptibility (AMS) of 122 specimens from 10 stratigraphical levels. The origin interpretation is related with magnetic mineralogy and with the AMS effects of imparting isothermal remanent magnetization (IRM) and laboratory heating at 130°C and 400°C steps (in pilot specimens). Primary magnetic fabrics suggest a palaeocurrent system oriented NW (270° to 320°) for the lower section (Piedra Hueca Formation) and NNE to NW (24° to 310°) for the upper section (Otlatepec Formation). Secondary fabrics are interpreted near the contact between both units and are related to weathering and to deformation and strain direction. The AMS interpreted as of primary origin show high-coercivity magnetic phases, compare well with other palaeocurrent indicators, have low anisotropy degrees and show similar or equivalent AMS after IRM and heating steps. The AMS interpreted as of secondary origin have some low or low-intermediate coercivities show the higher and more variable anisotropy degrees, and usually loose or show a very different AMS after IRM and heatings teps . Initial AMS, after IRM AMS and after heating steps AMS was measured 3 to 6 times in each specimen in order to determine measurement precision. Statistical analyses of the repetitive measurements were performed in each specimen obtaining specimen-means of the principal AMS directions and identifying some magnetically isotropic specimens.