EPR stratigraphy applied to the study of two marine sedimentary sections in southwestern Venezuela

This work explores the application of the electron paramagnetic resonance (EPR) technique, a high sensitivity spectral method, as a means of characterizing stratigraphic facies and identifying depositional environments. We have studied two marine sedimentary sections, of mid-Cretaceous and mid-Eocene ages, that are located in southwestern Venezuela. The different paramagnetic species (e.g. manganese, free radicals and different forms of Fe) have been identified and their relative concentrations measured from the EPR spectra. These results were then integrated to those from petrographic and magnetic susceptibility analyses (room and high temperature) as well as to some other rock magnetic data such as Qn and S-ratios. Because Mn2+ content seems to be finely tuned to variations of redox conditions, it could serve as a sensitive index of paleoenvironmental changes in stratigraphic columns with large lithological contrasts. Conversely, the presence and concentration of distinct Fe species proves to be quite useful characterizing different depositional environments at those stratigraphic sections that exhibit similar lithologies throughout. The integration of EPR results with rock magnetic data allows the identification of diagenetic events that could have taken place after sediment deposition.