渤海湾盆地南堡凹陷南部古近系沙河街组一段古水体特征

Palaeo-water characteristics of the Member 1 of Paleogene Shahejie Formation in southern Nanpu sag,Bohai Bay Basin

渤海湾盆地南堡凹陷古近系沙河街组的古水体特征研究程度较低。以南堡凹陷南部为例,采用多种方法,分析了沙河街组一段沉积时期水体的古盐度和古水深。根据Sr/Ba值、Rb/K值和特殊藻类化石,认为其古盐度为微咸—半咸。通过Sr/Cu值和孢粉化石2种手段,恢复了研究区沙河街组一段沉积时期的古气候,认为其为潮湿气候但存在干湿波动。根据Mn/Fe值变化、干酪根类型和藻类化石以及似瘤田螺的发现,恢复了该时期的相对古水深,并结合自生黏土矿物、沉积构造和沉积相等,通过加权平均值的方法定量计算古水深,得出研究区沙一段中亚段顶部沉积时期水体较浅,为4～15m,而沙一段下亚段沉积时期水体较深,为4～30m。综合分析认为,南堡凹陷南部沙河街组一段沉积时期的古盐度和古水深在一定程度上取决于古气候条件和古地形背景。

The palaeo-water characteristics of the Paleogene Shahejie Formation in Nanpu sag of Bohai Bay Basin was poorly studied. Taking the Member 1 of Shahejie Formation in southern Nanpu sag as anexample,we studied the palaeosalinity and palaeo-water depth by many means. Based on special algae fossils and chemical element ratios including Sr / Ba and Rb / K,the palaeosalinity was interpreted as brackish water to semi-saline water. The palaeoclimate was humid with fluctuation by analyzing Sr / Cu ratio and sporopollen fossils. The relative palaeo-water depth was reconstructed according to the change of Mn / Fe ratio,kerogen types,algae fossils and Tulotomoides. By synthetic analysis with the authigenic clay minerals,sedimentary structure and sedimentary facies, the palaeo-water depth interval was calculated by weighted average method. The results show that it was shallow water sedimentary environment with the quantitative palaeo-water depth ranging from 4 m to 15 m during the depositional period of the top of middle submember of Member 1 of Shahejie Formation in Nanpu sag. During the depositional period of lower submember of Member 1 of Shahejie Formation,the palaeo-water depth changed from 4 m to 30 m. It is indicated that the palaeosalinity and palaeo-water depth depended relatively on the palaeoclimate and ancient terrain background.