松辽盆地松科2井早白垩世沙河子组天文年代标尺

An astronomical time scale for the Early Cretaceous continental strata in the Songliao Basin,Northeastern China

对松辽盆地白垩纪国际大陆科学钻探工程——松科2井的沙河子组自然伽马测井数据(GR)开展了详细的旋回地层学研究。频谱分析结果揭示,沙河子组上段(3336~4727 m)具有142~188 m、44~57 m、14 m和7~9 m的沉积旋回;下段(4727~5960 m)具有170~205 m、44~61 m、16~18 m和7.5~11.5 m的沉积旋回。这些沉积旋回的周期之比近似早白垩世天文轨道周期之比20:5:2:1,将这些沉积旋回依次解释为405 ka长偏心率、约100 ka短偏心率、约37 ka斜率和约20 ka岁差旋回。相关系数法(COCO)通过统计检验获得上、下段地层沉积速率约为40 cm/ka和56 cm/ka,支持了该天文周期的解释。对405 ka长偏心率沉积旋回信号进行带通滤波,并以松科2井3961 m处凝灰岩样品的锆石U-Pb二次离子探针年龄(U-Pb SIMS)113.9±0.9 Ma为锚点,建立了松科2井沙河子组(112.3~118.2)±0.4 Ma的"浮动"天文年代标尺,其持续时间约为5.92±0.4 Ma。本研究为确定早白垩世升温过程中松辽盆地重大环境和地质事件的年代提供了新的年代学约束,为该时期全球海、陆相地质事件对比提供了年代学基础。

The Songliao Basin(42°25'~49°23'N and 119°40'~128°24'E),the longest-lived and largest terrestrial basin of the Cretaceous Period,is located in Northeast China.The Mesozoic-Cenozoic terrestrial strata are uniformly distributed across the basin and consistent of volcanic,volcaniclastic,alluvial-fan,fluvial and lacustrine sediments,overlying the Precambrian to Paleozoic metamorphic and igneous rocks and Paleozoic to Mesozoic granites basement.The Songke-2(SK-2)borehole drilled 7018 m depth and recovered a continuous 4134.81 m Early Cretaceous cores in the Songliao Basin.It provides a unique opportunity to improve our understanding of correlation of strata and events between marine and terrestrial systems.In this study,the gamma-ray(GR)logging data were used as the paleoclimate proxy to conduct a detailed cyclostratigraphic analysis on the Lower Cretaceous Shahezi Formation of the SK-2 borehole,covering a depth range from 3336 m to 5960 m with a thickness of 2624 m.Power spectral analysis revealed significant sedimentary cycles of 142~188 m,44~57 m,14 m and 7~9 m in the upper part(3336~4727 m)of Shahezi Formation,and sedimentary cycles of 170~205 m,44~61 m,16~18 m and 7.5~11.5 m in the lower part(4727~5960 m).The ratio of the wavelengths of these cycles is approximately 20:5:2:1,corresponding to long orbital eccentricity(405 ka),short orbital eccentricity(ca.100 ka),obliquity(ca.37 ka),and precession cycles(ca.20 ka).Correlation coefficient(COCO)method obtained optimal sedimentary accumulation rates(SAR)of ca.40 cm/ka and ca.56 cm/ka in the upper and lower parts respectively,supporting the astronomical forcing interpretation.About fifteen 405 ka long eccentricity cycles in total was extracted through bandpass filters.A"floating"astronomical time scale of the Shahezi Formation of SK-2 borehole was established from 112.3±0.4 Ma to 118.2±0.4 Ma with a duration of ca.5.92±0.4 Ma by tuning to the U-Pb SIMS age 113.9±0.9 Ma[11]at depth 3961 m of a tuffaceous bed.This new"floating"astronomical time scale provides a new geochronological constraint on the ages of major environmental and geological events in the Songliao Basin during the Early Cretaceous,and provides a geochronological basis for the correlation of global marine and continental geological events during this period.