Recognition and Division of High-resolution Sequences based on the Milankovitch Theory: A case study from the Middle Jurassic of Well Ary301 in the South Turgay Basin

Key words: 
• South Turgay Basin /
• Milankovitch cycle /
• spectral analysis /
• astronomical time scale /
• high-frequency sequence

Abstract: It aims to achieve the division and correlation of high-resolution sequences by integrating the Milankovitch cycles with cycles of different scales so as to introduce such a high precision time scale into the high-resolution sequence stratigraphic classification. This work takes the Well Ary301 in Aryskum graben of the South Turgay Basin in Kazakhstan as an example. Based on different sedimentary characteristics, spectral analysis and continuous wavelet transform are used to Intervals I-IV of natural gamma ray data, respectively, which demonstrate that well-preserved Milankovitch cycles are contained in the sedimentary strata. Aryskum graben deposition process is obviously effected by astronomical orbital period, the formation thickness of 31.9~39.5 m is affected by the long eccentricity of 400 kyr, the formations thickness of 11.9~14.2 m and 6.7~8.8 m are affected by the short eccentricity of 125 kyr and 95 kyr, respectively. Cycle curves analyzed by filtering are compared with the astronomical model theory, which helps establish the Karagansky Formation of floating astronomical time scale. Finally, 11.5 middle term cycles and 47 short term cycles are identified, the filtering curves with eccentricity cycles of 400 kyr and 125 kyr are identified as the reference curves to divide middle term and short term cycles to ensure the scientific and unified division scheme in the study area, and a natural standard unaffected by man-made factors is proposed for high-frequency sequence division.