渤海湾西岸沧县隆起中更新世地层间断*

渤海湾西岸天津平原南部静海区400m深CGZ5孔、大港区500m深TPZ1孔岩心的岩石地层、磁性地层、¹⁴C测年、微体古生物研究表明,2个钻孔底部分别进入到定年3.6Ma的高斯(Gauss)正极性带和定年4.5Ma的吉尔伯特(Gilbert)极性带上部;位于西侧沧县隆起的CGZ5孔2.58Ma古地磁M/G(松山/高斯,Matuyama/Gauss)极性带界线位于225m,位于东部黄骅坳陷西斜坡的TPZ1孔该界线位于208m;0.78Ma的B/M(布容/松山,Brunhes/Matuyama)极性带界线分别位于55m和68.6m。结合微体化石分析、3期海侵地层的确定以及岩石地层特征,可判断CGZ5孔和TPZ1孔上更新统底板分别位于55m 和63.4m。依据末次盛冰期低水位域潴育化黏土层位,确定全新统分别厚14.8m和16.5m。上述2孔之间的黄骅坳陷板桥主凹陷中CQJ4孔,B/M和M/G界线分别位于74m和340m。西部CGZ5孔显现整个中更新统缺失,东部TPZ1孔中更新统仅厚5.3m。推测这一地层间断可能是由于渤海海盆在中更新世初期发生快速断陷,在河流到海盆的高差加大的背景下,河流输送物大部流入海盆,海盆边缘的沧县隆起和相邻的滨海平原发生了过路作用。在“源到汇”古地理状况变迁过程中形成的东部大平原,中更新世的沉积动力学背景需要进一步研究。

Middle Pleistocene stratigraphic hiatus on the Cangxian Uplift,the west coast of Bohai Bay

This study examined core materials of the Borehole CGZ5 with 400m depth in Jinghai County and Borehole TPZ1 with 500m depth in Dagang district of southern Tianjin plain on the west coastal plain of Bohai Bay. Based on analyses of lithology,magnetostratigraphy,¹⁴C dating,micropaleontology,the present paper determined that the bottom of the two boreholes arrived at the upper parts of the Gauss and Gilbert polarity zones dated 3.6Ma and 4.5Ma,respectively. The palaeomagnetotizm boundary of Matuyama/Gauss(M/G)polarity zone dated 2.58Ma occurred at 225m and 208m in the Borehole CGZ5 on the western Cangxian Uplift and in the Borehole TPZ1 on the west slope of the eastern Huanghua Depression,respectively. In the two boreholes mentioned above,the boundary of Brunhes/Matuyama(B/M)polarity zone representing 0.78Ma occurred at 55m and 68.6m,respectively. Combining with analyses of microfossils and determination of three transgression beds,as well as lithological characteristics,the bottom of the Upper Pleistocene series was estimated to be located at depths of 55m and 63.4m in the two studied boreholes. Based on the low waterlevel redoxing clay horizon of LGM(Last Glacial Maximum),the thickness of Holocene series was determined to be 14.8m and 16.5m,respectively. The boundaries of B/M and M/G of the CQJ4 borehole,located between the two boreholes(CGZ5/TPZ1)and at the main Banqiao sag of Huanghua Depression,occurred at 74m and 340m,respectively. The absence of the entire Middle Pleistocene series was observed in the western Borehole CGZ5,whereas the thickness at the eastern Borehole TPZ1 is only 5.3m. It was speculated that this stratal hiatus was caused by the fastly fault-controlled deposition in the preliminary stage of Middle Pleistocene in the Bohai Sea basin. Under the setting of large elevation difference,the significantly high amount of sediments supplied by the rivers flowing into the sea resulted in sediment bypass on the marginal Cangxian Uplift and adjacent coastal plains. For the great east plain of China formed during the palaeogeographic changes of the “Source to Sink”system,the dynamic sedimentary setting during the Middle Pleistocene epoch needs to be further studied.