赣南晚白垩纪红层磁学研究及其地质意义

磁性地层学是界定沉积物地质年代的一种重要方法,该方法的应用建立在沉积物记录了可靠的特征剩磁基础上.红层的磁性地层年代学研究对于我国华南地区晚中生代以来古环境、古气候的演化具有极为重要的意义.但是,目前对于红层古地磁可靠性的解释存在广泛的争议,为更好地理解并尝试解决具体红层剖面的古地磁可靠性问题,迫切需要综合岩石磁学和其他非磁学手段对沉积物中的载磁矿物性质进行系统厘定.本文通过对赣南地区两个典型晚白垩纪红层剖面(信丰XF剖面和留车LC剖面)进行详细的岩石磁学分析,并结合扫描电镜和沉积相特征,判定出磁性矿物以赤铁矿为主,含有少量针铁矿、磁赤铁矿和磁铁矿.其中,赤铁矿是最主要的剩磁载体,且与钛铁矿共存,综合磁学与非磁学结果判定其记录了原生的沉积剩磁.在获得了可靠的古地磁信息基础上,结合前人对该区域生物地层学和年代学研究并与邻区地层比对,初步判断出LC剖面与XF剖面地质年代分别为晚白垩纪早期Cenomian期和晚白垩纪晚期Maastrichtian期.XF剖面依据ESR测年、南雄盆地地层比对和沉积相的判断,地层极性时段初步界定为C30n,即68.196~66.398 Ma,这为进一步探讨该地区的古环境和古气候演化提供了可参考的年龄标尺.

Magnetic research of Late Cretaceous red beds in southern Jiangxi province and its geological implications

Magnetostratigraphy is an important method for dating sedimentary sequences. Its effective application is based on the record of a reliable characteristic remanent magnetization (ChRM). Magnetostratigraphy of red beds is significant for the study of Late Mesozoic paleoenvironment and paleoclimate evolution in South China. However, enormous controversies exist in the reliability of the paleomagnetic data recorded by red beds. To address this issue, a systematic investigation should be carried out on the magnetic minerals of sediments through rock magnetic and non-magnetic methods (e.g., Scanning Electron Microscope, SEM). In this study, we carried out a detailed investigation on the rock magnetic characteristics of the two Late Cretaceous red beds sections (Xinfeng (XF) section and Liuche (LC) section). Based on rock magnetic and SEM results, we conclude that (a) hematite is the main magnetic mineral, coexisting with a small amount of goethite, maghemite and magnetite. (b) Hematite is the most important remanence carrier, which records a primary Detrital Remanent Magnetization (DRM). Combining the reliable paleomagnetic information and previous biostratigraphic and chronologic studies, the ages of LC section and XF section are Cenomanian and Maastrichtian, respectively, compared with the stratigraphy of adjacent area. Furthermore, according to ESR dating and analysis of the stratum in Nanxiong Basin, the age of XF section can be determined at the C30n (68.196~66.398 Ma). Acquirement of the ages of the two sequences provides a reliable time scale for paleoclimate and paleoenvironment evolution in this area.