宁波盆地白垩系古地磁特征兼论馆头组朝川组和方岩组的层位归属

浙东宁波盆地白垩纪陆相火山岩系,经古地磁研究表明,以往认为时代属于早白垩世早期的馆头组,晚期朝川组和晚白垩世早期的方岩组均呈正向磁化,平均磁化方向基本一致。据绝对年龄和古生物旁证,磁化时间属白垩纪中期正向极性超时。因此,馆头组的层位应作相应的提高,从白垩系下部上升到白垩系中部;朝川组随之位于其上。方岩组纳入白垩系中部,在馆头组之下还应有一套以正负极性交迭为特征的白垩纪早期沉积物。从而对以往所谓的上侏罗统磨石山群的时代提出疑问。从古地磁方向特征的规律,支持了白垩系三分说。

PALAEOMAGNETIC CHARACTERISTICS OF CRETACEOUS IN NINGBO BASIN OF EASTERN ZHEJIANG AND DISCUSSION ON STRATIGRAPHIC SEQUENCE OF GUANTOU FORMATION, CHAOCHUAN FORMATION AND FANGYAN FORMATION

Based on the study of palaeomagnetism of Cretaceous terrestrial volcanics of Ningbo basin, eastern Zhejiang, authors indicate that Guantou formation, Chaochuan formation and Fangyan formation, the ages of which were considered as the early stage of early Cretaceous, late stage of early Cretaceous and early stage of late Cretaceous respectively, show normal polarities with their average magnetization in the same direction. According to the assistant evidences of isotope ages and palaeontological data, the magnetization occurred during the normal polarity super-chron in the middle stage of Cretaceous. Therefore the layer of Guantou formation should be correspondingly heightened from the lower part of Cretaceous to the middle part of the Cretaceous. Chaoshuan formation should then overlie it. Fanhyan formation should belong to the middle part of the Cretaceous, but the sequence is still above Chanchuan fornation, apparently beneath Guantou formation. There should still be a suite of the early Cretaceous sediments charcterized by alternation of . normal-reverse polarities. Thus authors doubt the age of Moshishan group which used to be considered as the upper Jurassic. The three-divided hypothesis of the Cretaceous is sup- ported by the regularity of characteristic magnetizations.