中国科学院南京地质古生物研究所机构知识库

珠江口盆地白云凹陷L2-X井中新世地层沉积连续,含丰富的有孔虫化石,是研究区较为典型的中新统钻井剖面之一。在L2-X井1 490～2 640 m井段内分析有孔虫岩屑样品127个,共鉴定有孔虫71属110种,其中浮游有孔虫12属37种,底栖有孔虫59属73种。根据浮游有孔虫标志种,在中新世地层中共识别出10个有孔虫化石带或联合化石带。依据有孔虫分异度、丰度及组合特征,对早中新世—中中新世的沉积环境演化特征进行了分析。井段2 565～2 640 m(下中新统下部),受白云运动影响,为浅水大陆架向深水大陆坡过渡的沉积环境,之后开始稳定地接受深水大陆坡沉积;井段2 050～2 040 m(N8b亚带,中中新统下部),浮游有孔虫的含量急剧降低,这表明L2-X井在该时期可能发生了一次快速沉降,水体进一步加深;井深1 680 m之上(N13带,中中新统上部),浮游有孔虫的百分含量升高、玻璃质底栖有孔虫逐渐占据优势,底栖深水种逐渐消失,指示了在该界面之上,L2-X井的水体逐渐变浅,由深水大陆坡逐渐向浅海外陆架沉积环境转变。L2-X井识别出的胶结质壳有孔虫,形态类型以M4a和M4b占主导地位,可用于指示海底有机质含量变化。基于胶结质壳有孔虫的丰度和占底栖类型百分比,可以初步识别出海底有机质含量变化的3个阶段:阶段Ⅰ(2 640～2 290 m, N4带-N6带),相对较低的有机质含量;阶段Ⅱ(2 280～1 710 m, N6带-N13带),相对较高的有机质含量;阶段Ⅲ(1 700～1 490 m, N13带-N14带),相对较低的有机质含量。海底有机质含量的变化,可能是由于表层生产力水平的波动所引起。
Well L2-X contains the typical Miocene sections in the Baiyun Sag, Pearl River Mouth Basin. A diverse foraminiferal fauna of 110 species from 71 genera has been recovered from the well, including 37 planktic species in 12 genera and 73 benthic species in 59 genera. The detailed foraminiferal biostratigraphic work allowed recognition of 10 early to middle Miocene planktic biozones. Three major turnovers of the sedimentary environment from early to middle Miocene in L2-X were discussed based on the foraminiferal assemblages, abundance, diversity, and percentage of planktic foraminifers. The first turnover occurred at 2 560 m(lower Lower Miocene) with a transitional environment of the shelf to the slope changed to the slope upward. The interval of 2 050～2 040 m(N8 b subzone, lower Middle Miocene) recorded a sharply decreasing percentage of planktic foraminifers, indicating rapid subsidence and deepened sedimentary environment in the study area. The foraminiferal assemblages above 1 680 m(N13 zone, upper Middle Miocene) changed, representing a shallowing upward, from the slope to the outer shelf. The agglutinated benthic foraminifers in L2-X were mainly attributed to the morphotypes M4 a and M4 b, which could be used as indicators for organic matter flux. Three stages of paleoproductivity were discussed preliminarily based on the abundance and percentage of the agglutinated benthic foraminifers, including Stage Ⅰ(2 640～2 290 m, N4-N6) and Stage Ⅲ(1 700～1 490 m, N13-N14), representing relatively lower organic matter flux and possibly lower surface productivity, and Stage Ⅱ(2 280～1 710 m, N6-N13), relatively higher organic matter flux and possibly higher surface productivity.