六盘山地区中生代煤系地层时代归属重新厘定

Redefinition of stratigraphic age of Mesozoic coal measure strata in Liupanshan area

为正确划分六盘山地区具"上三叠统或下-中侏罗统"之争的中生代煤系地层,通过野外沉积现象观察、镜下矿物学研究并结合砂岩碎屑锆石LA-ICP-MS U-Pb同位素定年,重新厘定了该套煤系地层的时代。六盘山地区争议地层普遍发育铁质结核,砂岩差异风化严重,这与邻区下-中侏罗统延安组地层相似,而与邻区上三叠统延长组差异较大;争议地层砂岩样品中含13颗中生代年轻碎屑锆石((188±3)^(254±8)Ma)(占总数17%),其中3颗碎屑锆石U-Pb年龄小于200 Ma,最年轻的碎屑锆石年龄限制了争议地层最大沉积年龄小于(188±3)Ma。争议地层中发育的晚三叠世孢粉组合,是三叠系孢粉化石再旋回的结果,鄂尔多斯盆地西南缘强烈的印支运动导致三叠系很快抬升至地表遭受剥蚀,为近区早—中侏罗世地层提供物源,古生物化石亦随之混入近区沉积地层中。根据上述证据和讨论,将六盘山地区中生代煤系地层时代重新归为早—中侏罗世,这一结论具有重要科学研究和油气勘探价值。

The stratigraphic age of Mesozoic coal measures in Liupanshan area has been an unsolved problem for a long time. The controversy is about whether it is of Upper Triassic or Lower-Middle Jurassic. The former is coeval with Yanchang Formation （the main oil-bearing series in Ordos Basin） while the latter is coeval with Yan＇an Formation （the main coal-bearing series in Ordos Basin）. This article tries to redefine its age by observing field sedimentary phenomena, studying its mineralogy under microscope, and combining with LA- ICP-MS U-Pb isotope dating of detrital zircons from sandstones. The controversial strata （Mesozoic coal measures） in Liupanshan area generally develop iron nodules and suffer from serious differential weathering, which is similar to Lower^Middle Jurassic Yan＇an Formation but quite distinct from the Upper Triassic Yanchang Formation in the neighboring area, indicating different provenance. A sandstone sample from the controversial strata contains 13 grains of Mesozoic young detrital zircons （188±3-254±8 Ma） and U-Ph ages of the three detrital zircons are smaller than 200 Ma, indicating that its maximum depositional time must be younger than 188± 3 Ma. Besides, the phenomenon that the controversial strata contain Triassic sporopollenin assemblages, once as one of the most valid evidence for Triassic viewpoint, is probably the result of recycled fossils, because Indosinian Movement in Western Ordos Basin was so strong that it uplifted the Triassic strata which was eroded soon after its deposition and provided provenance for Jurassic deposits and caused Triassic fossils mix into Jurassic strata. The above evidences and discussion prove that the stratigraphic age of Mesozoic coal measures should be redefined as Early-Middle Jurassic, and this conclusion is significant both in scientific research and oil-gas exploration.