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Astronomical time scale of middle Miocene Hanjiang formation in Pearl River Mouth Basin,South China Sea
TIAN Shi-feng1,2,CHEN Zhong-qiang2,ZHA Ming1
(1. School of Geosciences in China University of Petroleum, Qingdao 266555,China;2. School of Earth and Environment in the University of Western Australia, 35 Stirling Highway y Crawley, Western Australia 6009, Australia)
Abstract:
Cyclostratigraphic analysis was undertaken with several different spectral analysis methods on the Hanjiang formation of Langhian age (middle Miocene) in the Pearl River Mouth Basin of South China Sea, which consists of deltaic silici-clastic and neritic shelf carbonate rhythmic alternations. Within biochronological constraint, a high-resolution astronomical time scale was constructed through astronomical tuning of the natural gamma-ray well-logging record to the 65°N insolation curve and obliquity curve, which are from the most recent astronomically calculated variation of the earth's orbit. The results show that the Hanjiang formation cycles are controlled by variation of the earth's orbital period. The astronomically tuned time scale can be used to calculate astronomical ages for geological events and bioevents recognized throughout the period,and the results are only slightly different from earlier estimates in South China Sea. The astronomical time scale resolves the sedimentation history for the Langhian stage (middle Miocene) with a variation that strongly resembles that of the earth's orbital eccentricity. The main factor controlling the variability of the sedimentation rate in the Hanjiang formation may be related to the 405 ka long period eccentricity.
Key words:  cyclostratigraphy  Pearl River Mouth Basin  middle Miocene  Langhian stage  Hanjiang formation  Milanko-vitch cycles  sedimentation rate