Astrochronology of a middle Eocene lacustrine sequence and sedimentary noise modeling of lake-level changes in Dongying Depression, Bohai Bay Basin

Abstract

It has been demonstrated that the sedimentary deposits of Eocene succession in Bohai Bay Basin in China responded to astronomically forced climate changes. Several sections have been evaluated for cyclostratigraphy and many astronomical time scales (ATSs) have been constructed in Paleogene Bohai Bay Basin. However, the calibrations of age model and the constructed ATSs failed to reach an agreement and many of them relied on totally different orbital interpretations within the same section/borehole. In this work, we generated a new ATS for the middle Eocene lacustrine strata of middle to lower Member 3 (Es3m-l) and upper Member 4 (Es4u) of Shahejie Formation in Bohai Bay Basin. In this research, high resolution natural gamma-ray (GR) data were used for the cyclostratigraphy analysis of Es3m-l and Es4u Members of Well Fanye-1. We filtered ~11.11 m cycles of t Es3m-l and ~9.26 m cycles of Es4u which were directly correlated to the short eccentricity cycles (~100-kyr) of La2004 solution. The geomagnetic polarity ages of C18r/C19n, C19n/C19r and C19r/C20n in Geological Time Scale 2020 were adopted as age markers for the tuning in this study. This astrochronology provided astronomical time scale (ATS) in the range of 39.268 ± 0.050 Ma to 43.621 ± 0.050 Ma in Es3m-l and Es4u Member. The astrochronology of Well Fanye-1 of the same interval had been studied before. However, the ATS built in this paper used correlation coefficient (COCO) analysis and average spectral misfit (ASM) approach to provide quantitative measurement for fitting the observed sedimentary cycles to astronomical periods and given a range of plausible sedimentation rate. In this research, a sedimentary noise model was applied for the detection of high-resolution lake-level variations controlled by orbital forcing. The lake-level variations generated from the sedimentary noise model were supported by previous sequence stratigraphic interpretations. Variable sedimentation could be affected by lake-level changes which have been demonstrated by the evolutionary correlation coefficient analysis of GR series. In addition, from the ATS and thin section observations, it was concluded that carbonate-clay laminae in lacustrine shale were generated annually. Seasonal carbonate-clay laminated shale formations are controlled by orbital scale variability of monsoon and are always developed in a relatively high eccentricity amplitude under deep water environment conditions. Finally, the lake-level rise at ~40.500 Ma to ~40.050 Ma was demonstrated to be the response to Middle Eocene Climate Optimal (MECO) event in lacustrine records.