Orbitally forced glacio-eustatic origin of third-order sequences and parasequences in the Middle Permian Maokou Formation, South China

Abstract

Abstract Astronomical forcing on the development of depositional sequences has been of particular interest to researchers in recent years. To investigate the forming mechanism of depositional sequences in the Middle Permian Maokou Formation, outcrop sequence stratigraphic analysis and cyclostratigraphic analysis were conducted on one outcrop section and one drilling well in South China. Up to ninety-two meter-scale 6th-order parasequences, thirty-one 5th-order parasequences, eight 4th-order parasequences and two 3rd-order sequences were recognized based on vertical facies changes and stratal stacking patterns. Astronomical cycles (i.e., eccentricity, obliquity and precession) were detected by spectral analysis. An ∼3.1 Myr temporal framework and a high-resolution sedimentation rate curve were constructed by cyclostratigraphic analysis. The similar ratios of average thickness for the high-frequency sequences and the spectral periodicities suggest an astronomical forcing origin for the parasequences. The 4th-, 5th-, and 6th-order high-frequency parasequences correlate well with long eccentricity, short eccentricity and obliquity cycles, respectively. The durations of local and global 3rd-order sea-level sequences match the s4-s3 obliquity modulation cycles. Our results suggest that tectonic and autocyclic processes did not exert a major influence on the development of depositional sequences and that in the Middle Permian Maokou Formation the development of 3rd-order sequences and 4th-, 5th- and 6th-order parasequences was controlled by orbitally-driven glacio-eustatic sea-level changes. The coevolution trends in obliquity power, sea-level change and sedimentation rate hold important implications for stratigraphic correlation and sequence stratigraphic subdivision on a regional scale.