植物生态学方法在沉积环境恢复中的应用: 以南海北部白云凹陷为例^*

Application of plant ecology methods in palaeoenvironment reconstruction: a case study of Baiyun sag in northern South China Sea

摘要

新生代早期是南海北部古地理发生巨变的时期,沉积环境经历了从陆到海的转变,通过植物生态学方法准确揭示这一过程发生的时间及过程,对本地区古地理的恢复具有重要意义。本研究选取南海北部深水区白云凹陷代表性钻井样品,通过孢粉藻类分析,揭示古近纪海侵过程。研究表明: 早中始新世文昌组沉积时期,白云凹陷为未受海侵影响的河湖相环境; 晚始新世恩平组沉积时期,凹陷东部和东南部为浅海相环境,海相沟鞭藻大量出现,凹陷南部隆起带早期为河湖相环境,晚期变为海陆过渡相环境,凹陷西北部为大型河流三角洲相环境; 渐新世珠海组沉积时期,凹陷东部和东南部以陆坡深水环境为主,凹陷西北部及南部为海陆过渡相,西北部发育大型河流三角洲; 早中新世,由于全球海平面上升及南海扩张轴发生转向,整个白云凹陷大部分地区处于陆坡深海环境。综上,海侵最早发生在白云凹陷的东部和东南部,呈由东向西的推进,西部海侵发生时间最晚,这一过程与南海洋壳从东向西逐步打开完全一致。

Abstract

The palaeogeography in the northern South China Sea changed drastically in the Early Cenozoic era and the sedimentary environment underwent a transition from land to sea. The plant ecology methods are powerful in accurately revealing the transition time and processes,which is of great significance for the restoration of palaeogeography in this region. In this study,representative borehole samples are collected from the deep-water area of the Baiyun sag in the northern South China Sea to investigate the Paleogene transgression based on sporopollen algae analysis. The results show that the Baiyun sag was dominated by fluvial and lacustrine facies that was not affected by marine transgression of the Wenchang Formation in the Early to Middle Eocene. In the Enping Formation of Late Eocene,eastern and southeastern parts of the Baiyun sag developed into a shallow marine environment characterized as the development of a large number of marine sporopollen algae. The fluvial and lacustrine environments developed in the southern part in the early stage,and it transforms into marine and land transition environment. The northwestern part was dominated by large river delta environment. For the southern uplift zone of the Baiyun sag,it was characterized by fluvial lacustrine facies environment in the early stage,while it transformed into the marine and land transition environment in the late stage. The northwestern part of the depression was dominated by the large river delta environment. During the depositional period of the Zhuhai Formation in the Oligocene,it revealed a continental slope-deep sea environment in eastern and southeastern parts of the Baiyun sag,a transitional depositional environment occurred in northwestern and southern parts,and the large river delta appeared in the northwest part of the sag. Due to global sea level rise and sudden shift of the South China Sea spreading axis,most parts of the Baiyun sag were located in the continental slope environment in the early Miocene. Transgression initiated from the eastern and southeastern parts of the Baiyun sag and subsequently advanced towards the western areas,which is completely consistent with the gradual opening of the South China Sea oceanic crust from east to west.

关键词

南海 / 古近纪 / 海侵过程 / 沉积环境 / 植物生态学

Key words

South China Sea / Paleogene / transgressive process / depositional environment / plant ecology

基金资助

*国家自然科学基金项目(编号: 42076066)资助