Reconnaissance microfossil and magnetic stratigraphy of the Miocene Sayula-Isla sequence, Veracruz, Mexico

Abstract

A pilot biomagnetostratigraphic study has been performed on the paleontologically well constrained Early-Middle Miocene sequence at Sayula-Isla (southern Veracruz, Mexico). Rock magnetic investigations included isothermal remanence (IRM) acqui- sition and hysteresis experiments. IRM acquisition curves were found very similar for all samples. Saturation is reached in mod- erate fields of the order of 150-200 mT, which points to some spinels as remanence carriers. Judging from the ratios of hysteresis parameters, it seems that all samples fall in the pseudo-singledomain (PSD) or multidomain (MD) grain size region. In most of studied units one to two paleomagnetic components could be recognized. Small secondary components were easily removed applying 100-180°C. Some of the samples yielded unstable thermal behavior suggesting evidence for multicomponent magnetiza- tion. Characteristic remanent magnetization components have been defined after 250°C for these samples. The comparison of the Sayula-Isla polarity zonation poses some problems because of scarce microfossil content in studied sections. More data are required to better constrain the local Geomagnetic Polarity Time Scale, but reasonably good correlation was obtained for the Early-Middle Miocene biozones in the Salina del Istmo basin.