Reply to comment on 'Orbital cyclostratigraphy of the Devonian Frasnian–Famennian transition in South China'

摘要

Although we cannot entirely exclude possible subjective factors under the condition of less-then-ideal outcrop exposure in the course of grouping laminae or beds, classification of hierarchical boundaries is objectively based mainly on the lateral clarity, continuity and stability of boundaries and cyclothems. A highly ranked sedimentary boundary is usually the surface above and below which apparent variations of the stratigraphical architecture pattern take place. The thickness and lithology of grouped beds/laminae are also important criteria in the classification of hierarchical cyclothems and boundaries in similar sedimentary facies and environments. We suggest that hierarchical changes of cyclothems, boundaries and lithology in stratigraphical sequences are common indicators of episodic sedimentation, which correspond ultimately to cyclic or periodic climate and sea level fluctuations related, in some degree, to Milankovitch cycles in tectonically relatively stable regions and periods. Therefore, hierarchical cyclothems, boundaries and lithology can reasonably be selected as proxies of tracing Milankovitch cycles. In contrast to proxies of chemical elements and isotopes recorded in strata, hierarchical cyclothems, boundaries and lithology are not easily influenced by diagenesis and epidiagenesis, even lower-graded metamorphism, which is especially truthful for the pre-Cenozoic sedimentary records. It is difficult to conclusively reply whether a possible bolide impact could have modified orbital parameters and climatic oscillation periods. But, if our deduction that the bundle, bundleset and superbundleset cycles must have been controlled by orbital forcing is considered to be supported by data, the assumption that impact event(s) could have altered long eccentricity periodicity of the early Famennian may be a more suitable, useful and inspirational working hypothesis than the synsedimentary tectonism. Our recent works indicate that of the 15 Frasnian superbundlesets studied in the Du'an and Yangdi sections, 14 show a 1:4 ratio with bundlesets, that is, the ratio of the Frasnian (from the conodont falsiovalis Zone to linguiformis Zone) and early Famennian (from the conodont Lower triangularis Zone to Upper triangularis Zone) long eccentricity to eccentricity is 1:4 and 1:3, respectively.