Evolution Of Paleoenvironment In Qiong-dongnan Basin During Cenozoic

This paper is based on major national technology special project " The researchon The evaluation about the potential rich hydrocarbon SAG in deepwater area ofNorthern South China Sea" child topics " The research of paleoenvironment andpaleoclimate in deepwater area of Northern South China Sea "(2011ZX05025-002-03), basing on the paleontology information of the oil and gasdrill and LS33-1-1.212samples from drill LS33-1-1,30genera and185species ofplanktonic foraminifera were identified. In drill LS33-1-119planktonic foraminiferabiozones were recognized (Zone P19-Zone N22) mainly in terms of last occurrenceevents of planktonic foraminifera. A detailed planktonic foraminifera biostratigraphyand chronostratigraphy of LS33-1-1were set up. Based on the statistics of planktonicforaminiferal abundance, diversity, assemblage characters, this paper is to try torecover paleoenvironment since Oligocene from paleontologic aspects.In the drilling, sedimentary geologic age for Yacheng group is dating to earlyOligocene, mainly belong to Rupelian Stage, and amount to palnktonic foraminiferalzones P18a to P21a. Depositional environment change is coast to continental shelf,mainly a transgressive process. In this period, Water temperature is colder andfluctuations frequently.The geologic age of Lingshui group is from Late Oligocene to early Miocene,which belongs to the bottom of Chattian Stage and Aquitanian Stage, as the bottom ofpalnktonic foraminiferal zones P21a to N4a. Early sedimentary environment of thestage is coast-continental shelf, Water temperature by low to high. In late of the stagelarge-scale transgression was happened and the depositional environment by theoffshore environment transition to bathyal environment, water temperature slightlydecreased and remained stable.The geologic age of Sanya group is from Early Miocene to Middle Miocene,which is belongs to the upper part of Aquitanian Stage and Burdigalian Stage, thelower part of Langhian Stage, as palnktonic foraminiferal zone supper part of N4, N5,N6, union zone N7and N8, and the bottom of N9.In this stage the large scale ofregressive event early, sedimentary environment and gradually evolved into thelittoral environment, to maintain the water temperature stability.In the drill, The geologic age of Meishan group is middle Miocene, belongs toupper part of the Langhian Stage, Serravalle stage, lower part of the Tortonian Stage,and amount to palnktonic foraminiferal zones upper part of N9, N10, N11, N12, N13,N14and N15.It is a transgression time on the whole, the depositional environmentfrom shore shallow water environment mutation bathyal environment. In this stage,early water temperature remained stable, in the middle water temperature increases,the late water temperature drops rapidly and frequently during fluctuations. The geologic age of Huangliu group is Late Miocene, belongs to upper part ofTortonian Stage and middle-upper part of Messinian Stage, which as N16-N18unionpalnktonic foraminiferal zones. In the early of the stage there was a regression,gradually increased after the water temperature decrease. Large-scale transgressionwas happened and sedimentary environment as bathyal environment, watertemperature lower before they rise in the late of this stage.The geologic age of Yinggehai group is Pliocene, belongs to upper of MessinianStage, Piacenzian stage and lower part of Gelasian stage, which as the top ofN16-N18union palnktonic foraminiferal zones, N19palnktonic foraminiferal zonesand N20-N21union palnktonic foraminiferal zones. In Early of this stage, there was aregressive process, in the middle a transgressive took place, late regressive changeagain, the depth of the water first and then deep shallow. The water temperatureelevated entire period.The geologic age of Ledong group is Pleistocene, belongs to middle-upper partof Gelasian stage, Calabrian stage, amount to top of the N20-N21union palnktonicforaminiferal zones and N22. During this stage, a transgressive the events andregressive events happened, the late depositional environment into the continentalslope environment, similar to the depositional environment today. The period of theseawater temperature is decreased.Over all, Studies suggest that the general trend of the study area since paleowater depth changes since the Oligocene to increase gradually, to the late EarlyMiocene to Pliocene appear bathyal environment, as the maximum period of paleowater depth, then the water depth is gradually decreased. The study area since theOligocene to the early Miocene, planktonic foraminifera water content remaineddominant position. Since the middle Miocene to Pleistocene planktonic foraminiferacontent, warm water to maintain the dominant position, the overall trend oftemperature is increasing.