利用风暴沉积类型恢复海平面变化:以北京西山下苇甸剖面寒武纪中晚期风暴沉积为例

华北地区寒武系中发育有大量的风暴沉积,对于此类沉积物的成因前人意见趋于统一,但对其沉积环境的认识仍存在较多争议。在野外细致观察、实测的基础上,结合室内沉积学描述和分析,在北京西山下苇甸剖面寒武系第三统和芙蓉统内识别出5种沉积相类型,并根据沉积环境将其归入3种相组合,分别形成于浅海碳酸盐缓坡潮下带、深潮下带和页岩盆地环境。在研究地层内,共识别出了49层风暴沉积,基于沉积结构和构造特征,这些风暴沉积层可归入4个风暴沉积大类(原地未破碎风暴沉积、原地破碎风暴沉积、近源风暴沉积、远源风暴沉积)和12个亚类,这些风暴沉积类型形成于不同的环境。根据风暴沉积亚类在地层中的分布规律及其所指示的环境类型,恢复了北京西山寒武纪中晚期相对海平面的变化历史。通过与用不同方法恢复的北京西山和鲁西地区同时期海平面变化曲线进行对比,发现利用风暴沉积类型所恢复的海平面变化曲线与使用正常沉积类型恢复的海平面变化曲线具有相同的变化趋势。因此,风暴沉积类型可以用于恢复长时间尺度的海平面变化趋势。

Reconstructing sea-level changes from types of storm deposits:An example of the Middle and Late Cambrian at Xiaweidian section of Western Hills,Beijing

Storm deposits were extensively developed in the Middle and Upper Cambrian in North China. However,the sedimentary environments of these storm deposits are a matter of debate. Based on careful observation and measurement of the Xiaweidian section in Western Hills of Beijing,North China,five lithofacies are identified, which are further grouped into three lithofacies associations that were deposited in shallow subtidal zone,deep subtidal zone,and shaly basin, respectively. According to the sedimentary textures and structures,49 layers of storm deposits are recognized in the studied section,which are grouped into 4 types(named as:In-situ unbroken,in-situ broken,proximal,and distal storm deposits)and 12 sub-types that were formed in various sedimentary environments. A new sea-level curve is inferred from the changes of storm deposit sub-types in the Middle and Upper Cambrian at western Beijing. This sea-level curve matches well with the curve reconstructed from facies analysis of fair-weather deposits in the studied section,as well as those inferred by previous researches in Beijing and in western Shandong Province,North China. Thus,types of storm deposits can be utilized to reconstruct long-term sea-level changes in the geological history.