Orbital cyclostratigraphy of the Devonian Frasnian–Famennian transition in South China
Abstract Superbundlesets, bundlesets, bundles and laminae were formed in the carbonate successions of the Devonian Frasnian–Famennian transition in Guangxi, South China. Their hierarchical structures, ratio relationships and arrangement order within conodont zones are lasting and stable in the stratigraphical sequences, and can be traced across the different facies belts and sedimentary basins. The hierarchically organized superbundlesets, bundlesets, bundles and laminae correspond to a long eccentricity, eccentricity, obliquity and precession and sub-Milankovitch cyclothem, respectively. The ratio of long eccentricity to eccentricity is 1:4 below the boundary between Frasnian–Famennian and 1:3 above the boundary between Frasnian–Famennian, eccentricity to obliquity is 1:3, and eccentricity to precession is 1:6 in the Devonian. Since the eccentricity periodicity of 100,000 years remains essentially steady through geological time, the periodicity of the long eccentricity, obliquity, precession and sub-Milankovitch cycle was 400,000, 33,333, 16,667 and 8000 or 17,000 years. These ratios are different from those of the Permian, Cretaceous and Quaternary, and coincide well with those calculated theoretically by Berger et al. Science, 255 (1992) 560. This reinforces the hypothesis of the slowing of the earth's rotation and the lengthening of precession and obliquity periods since at least the Devonian time. On the basis of the hierarchical Milankovitch cyclothems, the chronostratigraphical subdivision and correlation of the Frasnian–Famennian transition can be made at the resolution of 100,000s and 10,000s of years. The duration of the upper rhenana Zone, linguiformis Zone, and lower, middle and upper triangularis Zone is 0.6, 0.8, 0.3, 0.3 and 0.3 Ma, respectively.