沉积学报2025，Vol.43Issue(1)：88-98,11.DOI:10.14027/j.issn.1000-0550.2022.165
塔里木盆地奥陶系萨尔干组旋回地层学研究
Cyclostratigraphy of the Ordovician Saergan Formation in the Tarim Basin
摘要
Abstract
[Objective]The Middle-Upper Ordovician Saergan Formation in the Tarim Basin represents an important hydrocarbon source rock deposited in a marine environment.However,the current model of the deposition of the Saer-gan Formation remains nebulous.High-precision geochronologic constraints may shed new light on the deposition his-tory of the Saergan Formation.[Methods]The identification of Milankovitch cycles,in particular,a 405 ka long or-bital eccentricity cycle,can be used to construct precise time scales for the sedimentary succession of the Paleozoic Era.The magnetic susceptibility(MS)is a measure of the degree of magnetization of a material in response to an ap-plied magnetic field,which can be a powerful tool for cyclostratigraphy and paleoclimate studies.Here,a total of 701 MS measurements from the 9 m Saergan Formation in the Subashigou section of the Kalpin area,Aksu city,Xinjiang were obtained to conduct a cyclostratigraphic analysis,including a multitaper method spectral analysis,an evolutive harmonic analysis,and bandpass filtering.[Results]The spectral analyses show significant peaks at 0.74-1.28 m,0.2-0.27 m,0.057-0.071 m,and 0.035-0.039 m with ratios of 21.14∶1,7.71∶1,5.71∶1,1.86∶1,and 1.11∶1,which are close to those of the Middle-Late Ordovician astronomical cycles.Although there is no reliable astronomical solution for the Ordovician period,astronomical calibration can be achieved using a stable 405 ka eccentricity cycle.After the 405 ka long eccentricity calibration,the spectral analysis shows periods with 100 ka(short eccentricity),30.6-31.2 ka(obliquity),and 20-21.8 ka(precession).Ten eccentricity cycles are obtained using bandpass filtering and a"floating"astronomical time scale with a duration of～3.9 Ma is established for the Saergan Formation of the Subashigou section.An amplitude modulation(AM)envelope of the interpreted 30.6 ka band of the 405 ka calibrated MS time series reveals long-term cycles with a period of～1.2 Ma for the Ordovician,which is identical with that at present.The first appearance datum(FAD)of the graptolite Nemagraptus.gracilis is used as a tie point for building the correlation between the Subashigou section and Dawangou section(Global Stratotype Section and Point auxiliary Stratotype of the Sandbian).The results show that the initiation of deposition of the Saergan Formation in the Sub-ashigou section was～1.03 Ma later than that in the Dawangou section.This supports the assertion that the Saergan black shales developed during a strong sea-level transgression process,which drowned the platform,and organic-rich sediments were deposited on the carbonate platform.Based on a further comparison among the MS series,1.2 Ma obliquity cycle,and global sea-level curve during the Middle-Late Ordovician,we suggest that the 1.2 Ma long obli-quity cycle may have controlled the sea-level changes during the time of the Saergan Formation deposition.Ice accu-mulation at the million year scale can be explained by a sustained reduction of ice melting in cool high-latitude sum-mers during an interval of a series of reduced obliquity variations(minima of the long term obliquity cycle).[Conclu-sions]In summary,the orbitally forced formation of the Saergan Formation is confirmed and a possible model of the deposition process of the Saergan shale is proposed based on the high-resolution cyclostratigraphy study in this study.关键词
塔里木盆地/中—上奥陶统/旋回地层对比/1.2 Ma斜率旋回/沉积模式Key words
Tarim Basin/Middle-Upper Ordovician/cyclostratigraphic correlation/1.2 Ma obliquity cycle/deposition model分类
天文与地球科学引用本文复制引用
高源,任传真,房强,吴怀春,时美楠,张世红,杨天水,李海燕..塔里木盆地奥陶系萨尔干组旋回地层学研究[J].沉积学报,2025,43(1):88-98,11.基金项目
国家自然科学基金项目(41925010,42072039)[National Natural Science Foundation of China,No.41925010,42072039] （41925010,42072039）
