Cyclic sedimentary record of "red-greyish green" beds in the first Member of Eocene Kongdian Formation(Ek1), Dongying Sag: An example from the Well Wang 46 in Wangjiagang area

Abstract

Vertically superimposed "red-greyish green" assemblages are developed in the first member of the Eocene Kongdian Formation (Kong1 member) in the Dongying sag of the Bohai Bay Basin, showing certain cyclic sedimentary characteristics.Inorder to better understand its depositional process and response to paleoclimate variation, the study took the Wang 46 well, which is located in the Wangjiagang area, as an example.Calibrated by the stratigraphic ages, cyclostratigraphy analysis has been conducted in the formation using cores, logging cuttings and gammaray(GR) curve to reveal its cyclic depositional process.The red beds correspond to flooded lacustrinemud and sandy beach deposited during flood events, while the greyish greenrhythm beds belong to the shallow lacustrine sand beach sediments in the intermittent wet period, and the two types of deposits are superposed with abrupt contacts.The continuous wavelet and spectral analysis of the GR curve indicate that the depositional process of the Ek1 member is controlled by the Milankovitch eccentricity and precession cycles, resulting in the thicknesses of the cyclically deposited formations are 39.4 m and 6.9 m, respectively.The "floating" astronomical time scale of the Ek1 member has been established by the band filtering of the 125 ka eccentricity cycle, and the Ek1 member is found to have 33 eccentricity cycles.The apparent average deposition rate changes between 0.236 m/ka and 0.408 m/ka.The eccentricity and precession cycles have controlled the depositional process: the maximum eccentricities mainly correspond to frequent occurrences of temporary lacustrine, on the contrary, it mainly developed flooded lacustrine.And, the temporary and perennial flooded lacustrine occurred during the periodsof minimum and maximum precessions, respectively.

Keywords

• dongying sag
• eocene
• cyclic sedimentary
• orbital parameter
• red-bed