渤海湾西岸晚更新世沉积的差异性特征*

位于渤海湾西岸黄骅坳陷的次级构造单元歧口凹陷的BQ1孔,深95.6m;位于相邻次级凸起单元北大港潜山构造带的BQ2孔,深32.8m。两孔相距约18km。对岩性、古生物(有孔虫、介形虫和软体动物)、测年(OSL和AMS ¹⁴ C)和古地磁等基础数据的研究表明,两孔中可资对比的晚更新世海相层和陆相层具明显的差异性空间分布特征。其中,晚更新世60~35kaB.P.的第2海相层(Ⅱ海)在凹陷区BQ1孔厚约10.7m、在凸起区BQ2孔厚5.7m;BQ1孔该海相层顶、底板埋深分别比BQ2孔低约11.5m和16.5m。其他相同层位,同样表现出类似的空间位置(厚度、埋深)的差异。进入全新世,两孔第1海相层(Ⅰ海)的底板位于相同埋深,暗示至此差异性沉积过程趋于平缓,两孔所在地区在相对平稳的构造与沉积背景下开始了全新世滨海平原的近现代造陆过程。

DIFFERENT DEPOSITIONAL PROCESSES OF BOREHOLES BQ1 AND BQ2 IN THE LATE PLEISTOCENE ON THE WEST COAST OF BOHAI BAY

Borehole BQ1, ca.95.6m in depth, is located in the Qikou Hollow Zone, a secondary subsiding unit of the Huanghua Depression Basement Tectonics, west Bohai Bay. Borehole BQ2, ca.32.8m in depth and 18km away from the BQ1, is drilled in the Dagang Palaeo Buried Hill Belt, a secondary salient unit, in the same Depression. Multidisciplinary studies including lithology (grain size, nodules, colour and facies analysis), palaeontology (molluscs, foraminifera and ostrocoda), chronology (OSL and AMS ¹⁴ C) and palaeomagnetism (intensity, inclination, declination and susceptibility), etc. have been carried out. A number of terrestrial layers and marine layers, including transitional layers, are recognized, stratigraphically as the Marine Bed Ⅰ, Terrestrial Bed Ⅰ, Marine bed Ⅱ, Terrestrial Bed Ⅱ, and Marine Ⅲ,… from top downwards for the two cores. A major result reveals that there existed obvious differences on composition, thickness and burial depth of the same layers of the two cores in the Late Pleistocene Period. (1) 80~60kaB.P. , the Terrestrial Bed Ⅱ. In BQ1 it consists of fine sand and silt deposited in alluvial-floodplain-shallow lake environment, with frequent hiatuses; in BQ2 it is composed of clay and silt in floodplain environment. The top of the bed in BQ1 is 16.6m lower than in BQ2 and the thickness of the bed in BQ2 is only one fourth of that in BQ1. (2) 60~35kaB.P. , the Marine Bed Ⅱ. It is intertidal laminated silt and clay or massive mud in bay environment in BQ1 while it is composed of brackish sediments of transitional facies in BQ2. The top and bottom of the bed in BQ1 are 11.5m and 16.5m lower, respectively than in BQ2, while the thickness of the bed in the latter is only a half of that in BQ1. (3) 35~20kaB.P. , the Terrestrial Bed I. During that period, the subsiding unit was a water catchment area and thus the bed in BQ1 consists of well sorted channel sand. That in BQ2 is composed of flooding clay and silt with calcium carbonate nodules, palaeosols and hiatuses. The top and bottom of the bed are 3.9m and 11.5m lower in BQ1 than those of BQ2 and the thickness in BQ2 is only one third of BQ1. (4) 20~10kaB.P. , the Hiatus. It was the Last Glacial Maximum and almost nothing had been deposited in either cores. (5) After 10kaB.P. , the Marine Bed Ⅰ. The thickness of the bed is 13.77m and 13.07m respectively in BQ1 and BQ2 with the similar depositional rates. During 8.6~7.9kaB.P. (uncalibrated AMS ¹⁴ C dates), the bottoms of the same layers in two cores are almost at the same level which suggests that the different patterns of depositional process were faded out, and the development of the Holocene coastal plain started with identical tectonic and depositional background.