新疆柯坪大湾沟中、上奥陶统旋回地层学研究及其地质意义

CYCLOSTRATIGRAPHIC STUDIES OF THE MIDDLE-UPPER ORDOVICIAN AT DAWANGOU SECTION, KALPIN, XINJIANG AND THEIR GEOLOGICAL IMPLICATIONS

通过对新疆柯坪地区中、上奥陶统萨尔干组和坎岭组高分辨率磁化率数据进行旋回地层学研究,识别出405 kyr长偏心率,111 kyr短偏心率,38.6 kyr、34.1 kyr和31.0 kyr斜率,以及21.8 kyr、20.2 kyr、18.5 kyr和18.2 kyr岁差旋回,其中斜率信号突出。滤波结果显示萨尔干组和坎岭组记录了13个405 kyr长偏心率沉积旋回。通过天文校准得出萨尔干组的沉积时限是~2.63Myr,沉积速率为4.94m/Myr;坎岭组的沉积时限是~2.64Myr,沉积速率为5.30 m/Myr。萨尔干组滞流盆地相页岩比坎岭组陆棚灰岩沉积速率低。研究层段的0.16—0.17 m沉积旋回信号明显,表明地轴斜率是研究区中、晚奥陶世重要的气候驱动因子。旋回地层学研究结果还赋予生物化石带地质时代意义,大湾沟剖面所产牙形石带Eoplacognathussuecicus、Pygodusserra、Pygodusanserinus、Baltoniodus variabilis和Baltoniodus alobatus的综合延续时限大于5.27 Myr;所产笔石带Pterograptus elegans、Glossograptus hincksii、Nemagraptus gracilis和Corynoides americanus的综合延续时限为~5.27 Myr。

High-resolution cyclostratigraphic studies on the magnetic susceptibility series of the Middle-Upper Ordovician Saergan and Kanling formations in Kalpin, Xinjiang are conducted. A hierarchy of Milankovitch cycles, including 405 kyr long eccentricity, 111 kyr short eccentricity, 38.6 kyr, 34.1 kyr and 31.0 kyr obliquity, and 21.8 kyr, 20.2 kyr, 18.5 kyr and 18.2 kyr precession cycles are identified. Among these cycles, the obliquity shows the strongest signals. We also establish floating astronomical time scales of 13 long eccentricity cycles by tuning the 405 kyr cycle and accordingly estimate the duration of the Saergan Formation to be 2.63 Myr and calculate the sedimentary rate as 4.94 m/Myr, while the duration of the Kanling Formation is 2.64 Myr and the sedimentary rate is 5.30 m/Myr. The results show the sedimentary rate of the Saergan Formation is lower than that of the Kanling Formation. The strongest obliquity signal(0.16-0.17 m) demonstrates that the obliquity orbital forcing likely drove the cyclic changes of paleoclimate and paleoenvironment. The cyclostratigraphic results are also used to estimate the durations of the conodont zones at the Dawangou section, including Eoplacognathus suecicus, Pygodus serra, Pygodus anserinus, Baltoniodusvariabilis and Baltoniodus alobatus zones, to be more than 5.27 Myr, and the duration of the graptolite zones, including Pterograptus elegans, Glossograptus hincksii, Nemagraptus gracilis and Corynoides americanus zones, as 5.27 Myr.