High-resolution cyclostratigraphic analysis from magnetic susceptibility in a Lower Kimmeridgian (Upper Jurassic) marl–limestone succession (La Méouge, Vocontian Basin, France)

Abstract

High-resolution magnetic susceptibility (MS) analysis was carried out on a Lower Kimmeridgian alternating marl–limestone succession of pelagic origin that crops out at La Méouge (Vocontian Basin, southeastern France). The aim of the study was to characterize the strong, dm-scale sedimentary cyclicity of the succession at a very high resolution, and to analyze the cycles for evidence of astronomical forcing. From marl to limestone, MS varies progressively and closely tracks the highest frequency cyclicity corresponding to the basic marl–limestone couplets. Long-term wavelength cycling modulates the high-frequency cyclicity (couplets), and appears to be controlled by clay content. Spectral analysis of the MS record reveals the presence of the complete suite of orbital frequencies in the precession, obliquity, and eccentricity (95–128 ka and 405 ka) bands with very high amplitude of the precession index cycles originating from dm-scale couplets. 405 ka-eccentricity cycles are very pronounced in the MS maxima of the marl members of the couplets, suggesting eccentricity-driven detrital input to the basin. 405 ka-orbital tuning of the MS maxima further sharpens all of the orbital frequencies present in the succession. These results are similar to those of previous studies at La Méouge that used carbonate content observed in field. Our results are also in accordance with cyclostratigraphic studies in Spain and Canada that report dominant precession index forcing. By contrast, in the Kimmeridge Clay (Dorset, UK), obliquity forcing dominates cyclic sedimentation, with weaker influence from the precession index. Ammonite zone duration estimates are made by counting the interpreted precession cycles, and provide an ultra-high resolution assessment of geologic time. In sum, this study demonstrates the power of the MS as a proxy in characterizing the high-resolution cyclostratigraphy of Mesozoic sections, particularly in alternating marl–limestone successions, and for high-resolution correlation and astronomical calibration of the geologic time scale.