辽西朝阳地区含鸟化石层附近侏罗—白垩系磁性地层研究

本文通过对辽西朝阳地区含鸟化石层附近侏罗-白垩系蓝旗组、土城子组、义县组地层共1252块古地磁样吕的测试与分析，建立了以上沉积地层的磁极性序列，发现蓝旗组、土城子组地层的磁极性序列具有频繁的正、反极性、而义县组则为单一正极性，结合现有古生物和同位素年龄资料，对比国际中生代地磁极性年表，表明土城子组的磁极性序列相当于提塘期、基末里期、牛津期和卡洛期，其主体的地质时代应属晚侏罗世（J3），土城子组底部的地质时代应属中侏罗世（J2)；并且根据义县组含鸟化石层以上层位的磁性地层研究结果，认为义县组含鸟化石的正极性带可与M16正极性时相对应，义县组含鸟类化石层的时代应属早白垩世早期，辽西白垩系/侏罗系界线很有可能位于义县组/土城子组之间。

Magnetostratigraphy of the Jurassic-Cretaceous Boundary near Bird-bearing Beds in Western Liaoning

A total of 1252 samples have been collected from Jurassic-Cretaceous sedimentary rocks near bird-bearing beds in the Chaoyang area, western Liaoning, for a magnetostratigraphic study with the aim of obtaining geo-magnetic polarity sequences recorded in Jurassic-Cretaceous formations. The results show that the geomagnet-ic polarity sequences of the Lanqi and Tuchengzi Formations are characterized by repeated normal and reversal polarity, while there is only one normal polarity in the Yixian Formation. Based on fossil records and isotope data, the magnetic polarity sequence recorded in the Tuchengzi Formation may correspond to the Tithonian, Kimmeridgian, Oxfordian and Callovian stages. According to the international Mesozoic magnetostratigraphic time scale, the age of the major part of the Tuchengzi Formation should be Late Jurassic and the age of the bot-tom of the Tuchengzi Formation Middle Jurassic. Based on the magnetostatigraphic study of the upper part of the Yixian Formation and new geochronologic data, the normal polarity recorded in the Yixain Formation corre-sponds to M16, which suggests that the age of bird-bearing bed should be Early Cretaceous and that the Jurassic - Cretaceous boundary in western Liaoning is most likely to be located between the Yixian and Tuchengzi For-mations.