Miocene Paleoenvironmental Evolution Based On Benthic Foraminiferal Assemblages In The Lufeng Sag, Northern South China Sea

The Miocene epoch marks the most crucial period during the Cenozoic cooling trend,characterized by continental drifting,mountain building,and other tectonic processes.The South China Sea(SCS)is sensitive to changes in global climate and paleoceanography since it lies in between the Eurasia continent and the tropical western Pacific Ocean.In recent years,the paleoceanographic and paleoenvironmental researches based on shallow water shelf are mainly focus on the Quaternary,less is known about the Pliocene and the Miocene.The thick Neocene marine sequences in the continental shelf of northern SCS,characterized by abundant marine fossils(e.g.foraminifera,coccolithus and dinoflagellates),provide ideal materials for paleoceangraphic researches linked to oil exploration.Foraminifera have proved to be useful in reconstructing paleoclimate and paleoenvironmental conditions,as the abundance,test morphology,diversity,ecological preferences,and microhabitats are very sensitive to environmental factors of water masses.In this thesis,the foraminiferal assemblages and total organic carbon content(TOC)of 171 drilling cutting samples of Hole LF14 located in the Lufeng Sag,northern South China Sea,were analyzed.The planktic foraminiferal biostratigraphic zones and framework of age of Hole LF14 have been established during the Miocene.The paleowater depths and tectonic subsidence history have been estimated with a transfer function and backstripping,repectively.The paleoenvironmental evolution of the shelf and response of benthic foraminifera in shallow shelf to changes in global climate and paleoceanography have been discussed significantly.The main results in this paper are as follows:(1)Based on the extensive collection of previous research and laboratory analysis,the distribution of planktic foraminiferal events in northern SCS during the Miocene was summarized,which provide an important planktic biostratigraphic base for the accurate determination of the age of planktic foraminiferal events and the division and correlation of biostratigraphy among the basins of northern SCS.Subsequently,9 plankonic zones and combine zones were recognized at Hole LF14 based on 14 planktic foraminiferal events and foraminiferal assemblages according to the latest updated standard of the Geological Time Scale 2012(GTS2012),including M3,M4–6,M7,M8,M9,M10,M11,M12 and M13–PL1 during the early Miocene to early Pliocene.(2)The characteristics of benthic foraminiferal assemblages,percentage of planktic foraminifera,and paleowater depths at Hole LF14 revealed that the transgression in study area started since the early Miocene(~18.7 Ma),the deposited environment changed from the land or littoral to semi–closed bay.The study area was characterized by open middle to outer shelf or even upper bathyal environment as the Dongsha Uplift was submerged by seawater after ~16 Ma.(3)The tectonic subsidence history during the Miocene was reconstructed by using the backstrpping analysis method.By comparing with previous studies in the Pearl River Mouth Basin(PRMB),the study area is highlighted by a rapid subsidence during 17.5–10 Ma.The high development rate of accommodation space and high supply of terrestrial materials were the two significant factors contributing to the high sedimentation rate during 17.5–10 Ma.The subsidence rate and sedimentation rate were low during 10–4.53 Ma,which may be related to less supply of terrigenous materials and less accommodation space caused by the Dongsha Movement.(4)The global climate is warm,characterized by low global ice volume and high sea water temperature during the Early Miocene to Middle Miocene Climate Optimum(MMCO).The dominant Uvigerina spp.assemblage was characterized by low diversity,low percentage of oxic species,and high TOC,indicating a warm,low–oxygenation and eutrophic conditions in study area during the Early Miocene to MMCO(~18.7–14.24 Ma).The global climate cooled rapidly during the middle Middle Miocene.Beyond the Uvigerina spp.assemblage,the Cibicides spp.assemblage became important during the middle–late Middle Miocene(14.24–11.54 Ma).This assemblage was dominated by epifaunal species with relative high diversity and high percentage of oxic species,suggesting high–energy,high–oxygenation and oligotrophic conditions with episodic supply of organic food.An abrupt sea level drop and significant faunal changes were recorded during 14.24–13.41 Ma,suggesting development of the East Antarctic Ice Sheets,which resulted in a drop of sea level and change in benthic foraminiferal assemblages along the shallow water shelf.The cooling trend of global climate continues during the Late Miocene to Pliocene.The dominant Cibicidoides spp.assemblage with high diversity and high percentage of oxic species,indicates mesotrophic conditions with relative high–oxygen content during 11.54–4.53 Ma.On the basis of comparison between the benthic foraminiferal assemblages at Hole LF14 with benthic foraminiferal assemblages in Western Foothills of Taiwan,the major ice sheets growth on East Antarctica related to Mi–3,resulted in variation of global sea level and invigoration of surface ocean circulation systems,which influenced the composition of planktic and benthic foraminiferal assemblages and the TOC.In addition,the appearance and continuous occurrence of Ammonia spp.and Pseudorotalia spp.since 10.02 Ma,may reveal the Late Miocene western Pacific warm pool became influential since ~10 Ma.