珠江口盆地早中新世碳酸盐岩生长发育、消亡的历程与受控因素

The process of growth,development and extinction of the Early Miocene carbonate rocks and controlling factors in the Pearl River Mouth Basin

碳酸盐岩(台地)的消亡是沉积学研究的前沿和热点科学问题。南海东北部珠江口盆地东沙隆起所发育的下中新统珠江组碳酸盐岩是我国海相碳酸盐岩的最高层位,东沙台地珠江组碳酸盐岩的生长、繁盛和消亡过程提供了中国南海乃至世界范围内一个典型的受构造、海平面变化和物源共同控制的研究实例。依据采自钻井岩心钙质超微化石和浮游有孔虫化石带将珠江组的上界置于钙质超微化石带NN4与NN5的界线,即中、下中新统界线,下界置于浮游有孔虫N4的底部附近,即位于中新统与渐新统界线。有孔虫属为N4-N8带,钙质超微化石为NN2-NN4带,底界年龄为23.03Ma,顶界年龄为15.97Ma,地震反射上位于T40-T60之间。而其中碳酸盐岩地层最早于21Ma左右开始生长,最晚于16.5Ma被泥岩淹没。早期的东沙隆起北低南高,21Ma以后,东沙隆起沉降而丧失了向珠一凹陷提供物源的功能,沉降较快的东沙隆起北北部地区发育碳酸盐岩;初期为一套碳酸岩缓坡沉积,20Ma以后隆起整体被淹没,开始了大规模的碳酸盐岩建造,实现了由碳酸盐岩缓坡向台地的转变,随沉降向南推移,隆起逐渐转变为北高南低,整个碳酸盐岩台地持续向东南方向退缩,台地沉积逐渐萎缩为局部礁滩复合体,北部古地貌的高部位残存零星点礁;16.5Ma以后,沉降中心向珠二凹陷迁移,北部、北西部碎屑物质持续向东沙隆起前积导致台地消亡。结合珠江组沉积时期的地质事件的分析,本文认为早期碳酸盐岩的消亡是由于这一时期的全球海平面下降到最低位,引起区域上的物源供给加快,灰岩直接被北部沉积物退覆淹没所致;20±0.5Ma^18.3±0.5Ma的碳酸盐岩的消亡时间受制于沉降造成的相对海平面的变化,基底的火山作用及沉降中心的迁移等事件,最晚一期碳酸盐岩(流花地区碳酸盐岩)的消亡应该是构造反转后,北部物源对凹陷的持续填平补齐作用引起碳酸盐岩的生长环境变化所致。由此看,陆源碎屑的注入、沉降中心的迁移、相对海平面的升降及原始古地貌形态是碳酸盐岩生长发育及消亡的主要控制因素。

The disappearance of the carbonate platform is the frontier and hot field of sedimentary research. The Lower Miocene Zhujiang Formation carbonate deposited in the Dongsha uplift of Pearl River Mouth Basin is the latest marine carbonate in China. The process of origin, development and perished of the Zhujiang Formation carbonate is controlled by the tectonic movement, sea-level change and provenance, the research of this pattern can be served as a classical case study in the South China Sea and even in the world. According to the calcareous nannofossils and planktonic foraminifer zones collected from the drilling core, the upper bond of the Zhujiang Formation and the Middle-Lower Miocene boundary are classified in the boundary between calcareous nannofossil NN4 zone and NN5 zone, the lower bond of the Zhujiang Formation and the Miocene-Neocene boundary are divided in the bottom of foraminifer N4 zone. The ages of the foraminiferal N4-N8 zones and the calcareous nannofossils NN2-NN4 zones are determined, the lowest and the topmost boundary are 23.03Ma and 15.97Ma respectively, and the seismic reflection is located between T40 and T60. The carbonate platform firstly formed in 21Ma, and then was submerged by mudstone in 16.5Ma. Earlier Dongsha uplift was higher in the south and lower in the north. After 21Ma, because of subsidence of the Dongsha uplift, the function of the provenance to Zhu 1 depression was lost. Carbonate platform was developed in the north of Dongsha uplift which subsided quickly. At the early stage, there was a carbonate ramp. After 20Ma, the whole Dongsha uplift was submerged, and then extensive development of carbonate formations was start, including the transition from carbonate ramp to platform. As the settle expanded to the southern, the Dongsha uplift gradually changed to higher in the north, the whole platform expanded continuously to the southeast, the platform sedimentary is gradually shrinking to local reel flat complexes, the top of northern paleogeomorphic remanet fragmentary patch reefs. After 16.5Ma, the center of subsidence had been migrated to Zhu 2 sag, then the fragments from northern and northwest prograde continuously to Dongsha uplift, ultimately led to the termination of the platform. Combined with the analysis of geological events in the Zhujiang Formation period, this paper argues that the demise of early carbonate rocks is due to the global sea level down to the lowest at this time, which accelerate the supply of sediment in the region. The limestone is directly submerged by the sediment from the north. The disappearance time of carbonate rocks （formed from 200.5Ma to 18.30.5Ma） is controlled by the change of relative sea level, volcanic action of basement and the migration of subsidence center. The dead time of the late phase carbonate may be after the tectonic inversion and the growth environment of carbonate rocks has changed which are caused by clastic sediment from the north. Therefore, the terrigenous clastic injection, subsidence center migration, relative sea level changes and ancient landform control the growth and death of carbonate rocks.