Astronomical Forcing of Origins of Eocene Carbonate-bearing Fine-grained Sedimentary Rock in Dongying Sag
[Objective]This study aims to conduct an in-depth investigation into the large set of rhythmically-characteristic lacustrine fine-grained sedimentary rocks rich in carbonate materials and organic matter developed in the Dongying Sag area of the Bohai Bay Basin in eastern China,exploring their genetic mechanisms as well as characteristics of paleoclimate and paleoenvironment changes.[Methods]Based on core and thin-section observa-tions,we classified the lithofacies according to sedimentary structural features and mineral component content.Con-sidering the differences in sedimentary environment and deposition rate,the target interval was divided into four units.The Multi-Taper Method(MTM)was applied to perform segmented spectral analysis on the natural gamma(GR)logging curve from the upper Fourth member of the Shahejie Formation(Esu4)to the lower Third member(Esl3)in well Fanye 1(FY1).[Results]Through macroscopic and microscopic sedimentary observations,we identified five types of fine-grained rocks in the Dongying Sag area:laminar argillaceous limestone,laminar calcareous mudstone,weakly laminar calcareous mudstone,lenticular argillaceous limestone and massive mudstone.Spectral analysis results revealed that all four units recorded Milankovitch cycles,including periods of 125 kyr,38.7 kyr,and 18.7 kyr.Based on volcanic ash dating data,we established a"floating"astronomical timescale with a precision of 38.7 kyr and determined that the total duration of fine-grained sedimentary deposition from the upper Es4 to the lower Es3 in the Dongying Sag was 5.3 Myr.Further analysis indicated a good correspondence between the 18.7 kyr precession cycle and changes in carbonate content,suggesting a significant influence of Earth's orbital parameters on the deposition of carbonate-bearing fine-grained sedimentary rocks.[Conclusions]Utilizing the theory of cyclostratigraphy,this study revealed characteristics of paleoclimate and paleoenvironment changes in lacustrine sediments from the perspective of astronomical cycles controlling sedimentation.Our analysis showed that Earth's orbital parameters,particularly the 18.7 kyr precession cycle and the 125 kyr eccentricity cycle,jointly drove climate changes,which in turn controlled the deposition of carbonate-bearing fine-grained sedimentary rocks.Specifi-cally,at maxima of precession or minima of eccentricity,increased sunlight and warmer lake water temperatures favored the crystallization of calcite and the deposition of limestone.Conversely,at minima of precession or maxima of eccentricity,decreased sunlight and cooler lake water temperatures resulted in fine-grained material being predominantly derived from terrestrial sources,with mudstone deposition dominating.This research has important scientific and guiding implications for unconventional oil and gas exploration and development.
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