Characteristics of Milankovitch Cycle in Eocene Formation, Eastern Depression of the North Yellow Sea Basin

Milankovitch cycle is the manifestation of sedimentary stratigraphy recorded, its time implication is an effective means of high resolution stratigraphic division and correlation, from the reservation information of Milankovitch cycles, which enrich stratigraphic theory, especially cyclostratigraphic theory. Characteristics of Milankovitch cycle in the eastern depression of the North Yellow Sea basin are explored in this study. Major parameters of Milankovitch cycle are worked out by J. Laskar solution in the Eocene formation: the eccentricity cycle is 125 ka and 99 ka; the obliquity cycle is 51 ka and 39 ka; and the precession cycle is 23 ka and 19 ka. It is found by frequency spectrum analysis of the GR and SP well logging of three wells that the spectrum peak cycle corresponds well to its astronomy cycle, indicating well-preserved Milankoitch cycle in the formation. Stratigraphic cycle thickness is of 13.03 to 15.89 m long-cycle, 3.70 to 5.21 m mid-cycle and 2.17 to 2.94 m short-cycle, by which the formation sedimentation rate is calculated as between 121.20 to 127.12 m/Ma. The sedimentary duration is increasingly longer and the deposition thickness is increasingly thicker from uplift along the slope toward the center of the lake basin. However, the sedimentation rate is relatively stable.As to sublayer division of Encene with the continuous wavelet transform, it can be divided into six sedimentary bodies, with each body as a separate window for spectrum analysis to calculate the duration and rate of sedimentation. From the perspective of the impact of climate change on the sedimentary environment, it is concluded that, E₆ period was low water system tract, E₅ and E₄ were lake expand system tract, E₃ was highstand systerm tract, E₂ and E₁ were lakes contraction system tract from the bottom to top sequence stratigraphy. It is confirmed that Milankovitch cycle is an effective approach for depositional cycle analysis.