辽东湾北部钻孔磁性地层年代框架及地质意义

辽东湾地属渤海,其沉积历史和特征,是揭示渤海海域晚新生代以来构造演化与地理格局形成过程的重要窗口。目前该区缺少精确年代学控制的长序列研究钻孔。文章基于磁性地层学方法,结合粒度分析、AMS ¹⁴C测年和光释光测年数据,建立了该区北部JXC-1孔和YKC-2孔的年代地层框架。JXC-1孔地理坐标为40°24'43.38″N,121°03'23.37″E,水深约22 m,进尺70.3 m,取芯率89.2%;YKC-2孔地理坐标为40°25'53.95″N,121°36'30.20″E,水深约14 m,进尺70.1 m,取芯率85.3%。研究结果显示：JXC-1孔底界年龄为1.2 Ma,YKC-2孔底界年龄为0.7 Ma。根据沉积速率特征,JXC-1孔可分为2个阶段：阶段Ⅰ年龄为0~0.78 Ma,埋深0~45.18 m,沉积物以粉砂、细砂为主,位于布容正极性时（Brunhes）,记录了Mono Lake、Levantine-Biwa Ⅲ和Calabrian Ridge Ⅱ等极性事件;阶段Ⅱ年龄为0.78~1.20 Ma,埋深45.18~70.30 m,沉积物以粉砂为主,位于松山负极性时（Matuyama）,贾拉米洛正极性亚时（Jaramillo）埋深为61.6~66.2 m。YKC-2孔主要位于布容正极性时（Brunhes）上部,包括Gothenburg、Blake等极性事件。JXC-1孔的布容松山倒转边界（MBB）位于45.18 m,与渤海及周边其他钻孔存在较大差别,YKC-2孔则未打穿MBB。通过不同构造单元沉积记录和新构造运动响应对比,发现辽东湾北部晚更新世以来沉积厚度存在"东厚西薄"的模式。整个渤海及周边地区沉积相对连续,沉积厚度呈现以渤中坳陷为中心向四周减薄的特征。这表明,在新近纪构造基底控制的基础上,受郯庐断裂晚更新世以来的活动性影响,叠加了区域新构造运动,二者共同控制了区域内沉积厚度差异。这一研究深化了该区构造及沉积演化模式。

Magnetic stratigraphy of boreholes in the north of Liaodong Bay and its significance

The sedimentary history and characteristics of the Liaodong Bay area is an important window to determine the tectonic evolution and geographical pattern formation process of the Bohai Sea since Late Cenozoic.The Liaodong Bay is also the only area in Bohai Sea that lacks of accurate chronology framework for the long sequence research.Based on grain size analysis,magnetostraitigraphy,AMS 14C and OSL dating data,this study constructed a chronological framework for holes JXC-1 and YKC-2 in the north of Liaodong Bay.The JXC-1 hole is located at 40°24'43.38″N,121°03'23.37″E,where the water depth is about 22 m.The drilling footage of the JXC-1 hole is 70.3 m with the coring rate of 89.2%.The YKC-2 hole is located at 40°25'53.95″N,121°36'30.20″E,where the water depth is about 14 m.The drilling footage of the YKC-2 hole is 70.1 m with the coring rate of 85.3%.The paleomagnetic results showed that the bottom boundary age of the hole JXC-1 and YKC-2 are 1.2 Ma and 0.7 Ma,respectively.According to the age-depth model,the hole JXC-1 can be divided into two stages:the buried depth of stageⅠ(0~0.78 Ma)is 0~45.18 m,and sediments are mainly composed of slit and fine sand.The buried depth of stageⅡ(0.78~1.20 Ma)is 45.18~70.30 m,sediment is dominated by silt.Results show that the core from the hole JXC-1 consists of two normal magnetozones,which can be correlated to Brunhes and Jaramillo.The age of hole YKC-2 is within the Brunhes normal Chron.Combined with referenced magnetostraitigraphy of cores in this area,in the northern part of Liaodong Bay,the sedimentary thickness is thicker at the eastern part than the western part since Late Pleistocene.The deposition is relatively continuous in the whole Bohai Sea and its surrounding area in the Late Quaternary.The sediment thickness gradually thins from the Bozhong depression to surrounding areas.On the basis of the control of the neotectonic basement in the neotectonics in the superimposed regions,sedimentary thickness differences can be attributed to the neotectonic movement in the superimposed regions.The neotectonic movement in Liaodong Bay is influenced mainly by the activity of the Tanlu Fault since Late Pleistocene.