Astronomical forcing and sedimentary noise modeling of lake-level changes in the Paleogene Dongpu Depression of North China

Abstract

Challenges in interpreting continental sequence stratigraphy and typically uncertain geochronology hinder the understanding of paleolake evolution and hydrocarbon exploration in terrestrial basins. The Dongpu Depression in North China is a lacustrine basin with abundant hydrocarbon resources. An accurate chronology for the Paleogene stratigraphy in the Dongpu Depression is lacking, and the mechanism of lake-level variation remains unclear. In this study, we utilize high-resolution gamma ray logs to conduct a cyclostratigraphic analysis of the Shahejie Formation, which is a Paleogene succession in the Dongpu Depression characterized by sandstones and dark mudstones interbedded with thin salt rocks. Time series analysis reveals evidence for 405 kyr eccentricity cycles in the gamma ray series, which is supported by statistical modeling of optimal sedimentation rates. Tuning of the gamma ray data to this 405 kyr eccentricity cyclicity enables a 14.7-Myr-long astronomical time scale to be constructed. This astrochronology is anchored to the astronomical age of the Dongying Formation/Shahejie Formation boundary (28.86 Ma) in the Bohai Bay Basin, thus providing an absolute timescale for the studied interval that extends from 28.86 Ma to 43.59 Ma. Using this anchored astrochronology, we show that a recently established sedimentary noise model for inferring sea level change in marginal marine settings can be used to similarly infer lake level fluctuations in terrestrial basins. In particular, sedimentary noise modeling of the tuned gamma ray series reveals high-resolution changes in sedimentary noise that are indicative of lake-level variations linked to million-year scale (i.e., ∼4.8 Myr, 2.4 Myr and 1.2 Myr) astronomical forcing. These inferred changes in lake level are supported by previously published sequence stratigraphic interpretations. Moreover, an evolutionary correlation coefficient (eCOCO) analysis of the gamma ray series also indicates recurrent distortions in sedimentation that may be linked to lake level changes. This study provides new methods for the assessment of paleolake level variations, as well as insights into the connection between astronomical forcing and lake evolution across long timescales in terrestrial basins.