河南禹州大风口剖面二叠纪地层古地磁研究

A STUDY ON PALAEOMAGNETISM OF PERMIAN STRATA IN THE DAFENGKOU SECTION, YUZHOU, HENAN PROVINCE

本区晚古生代含煤地层发育完整,出露良好。前人已作了大量的生物地层工作。我们对其系统地进行了古地磁研究工作后,确认本区二叠纪基亚曼(Kia-man)反极性时的存在。其古纬度为11.2°N(P_1)-15.6°N(P_2),为本区在此阶段处于低纬度气候带,由热带雨林气候逐渐北移过渡为热带干湿季气候的论点提供了重要依据。

Through systematic measurements and analyses of 187 specimens from Permian strata at the Dafengkou section of western Henan Province by gradual thermal demagnetization and alternating demagnetization, stable characteristic remanent components were obtained from 142 specimens. In the geographic coordinates these components differ evidently from those of the recent geomagnetic field. At one sampling site, there are often normal and reversed polarities which are nearly parallel in the opposite direction, so this also satisfies the requirements for the reversed test. Therefore, it may be confirmed that these characteristic remanent components were primary. On that basis the authors has also calculated the mean magnetism direction and the various palaeomagnetic parameters with the sampling site, the rank of the geological formation and the e-poch as a unit. The proportion of reversed polarity within the 58 Early Permian specimens may attain 69%; the 102 Late Permian specimens, 55%. So it is confirmed that the entire Permian mainly witnessed reversed polarity. This character of polarity sequence may correspond with that of the universally acknowledged Permian Kiaman reversed polarity epoch. Therefore, the exis-tense of the Permian Kiaman reversed polarity epoch is confirmed. Working on the basis of the variation of the palaeolatitute values (P11 -11. 4°N, P12 - 11°N ,P21 -15. 4°00N and P22 -15. 7°N) , the authors suggest that, during the period of 35 Ma in the Early Permian, the block was long situated in the low latitudes, i. e. in the tropic rain forest climate area. In the Late Permian, the block migrated gradually northwards to higher latitudes, i. e. in the arid -humid seasonal climate area. Important coal-bearing strata of this are were developed concentrately in the Early Permian, a large amount of hygrophilous and thermophilous arborescent plants, such as Lepidoden-dron, Calamites and Cordaites, and many species and genera of Filices and pteridosperms such as Gigantopterids, Pecopterids and Taeniopterids were very flourishing. In the end of the early Late Permian, gigantopteris plants declined suddenly and extinguished gradually. In the late Late Permian, the amount of feldspathic material increased. In sporepollen, the hygrophilous and ther-malphilous pteridophytic spores declined suddenly, while gymnosperous pollen with saccata were dominant. This indicates that the changes of the climate zone were the direct result of the variation of palaeolatitutes of this area.