Cyclostratigraphy and astronomical tuning during the Oligocene in the Jizhong Depression, Bohai Bay Basin, northeastern China
Abstract The Dongying Formation (Ed) and the first member of the Shahejie Formation (Es1) in the central Raoyang Sag cover about 1705 m-long sedimentary series of terrestrial lacustrine infill. We employed the gamma-ray (GR) log data as a paleoclimatic and paleoenvironmental proxy to conduct the high-resolution cyclostratigraphy research in well W33 and W62. Prominent sedimentary cycles in decameters and meters were revealed by multi-taper (MTM) power spectra, evolutionary fast Flourier transformation (FFT) and correlation coefficient analyses in the Es1 and Ed intervals. The 405 kyr long eccentricity, 125 kyr and 100 kyr short eccentricity, 40 kyr obliquity, and 23 kyr precession can be recognized with a wavelength ratio of approximately 20:5:2:1, respectively. By tuning the 405 kyr filter output in well W33 to the orbital target curve of La2010d astronomical solution, we established an astronomical time scale (ATS) that provided absolute age controls during the Oligocene and shed light on the event correlation among isolated depressions in the Bohai Bay Basin. Sediment accumulation rates (SARs) were derived from the extracted 405 kyr long eccentricity and were tested with an average of 17 cm/kyr. Age boundaries of Ng/Ed1, Ed1/Ed2, Ed2/Ed3, Ed3/Es1 and Es1/Es2 are 23.03 Ma, 24.34 Ma, 25.53 Ma, 29.05 Ma and 33.10 Ma, respectively. Based on the ATS and SARs, we discussed the depositional evolution within age boundaries, cause of high SARs and origin of formation overpressure. After the largest lake transgression, shallow-water deltas and fluvial plains transitionally dominate the central Raoyang Sag with shallowing water depth. Thin interlayers of mudstones and sandstones originate from shallow-water deltaic systems with low sand-to-mud ratios. Under high SARs enhanced by mountain uplift and low sand-to-mud ratios, formation overpressure results mainly from high porosities in under-compacted mudstone intervals.