The Sedimentary Characteristics Of Beachrocks In Weizhou Island And Their Sea Level Indication Significance

Beachrocks are an important symbol of ancient sea level elevation.Beachrocks were widely developed in Weizhou Island of Beibu Gulf,with a total length of 5 km and a width of about 15-20 m.However,most of them were covered by vegetation or mined by local residents.In this paper,three beachrocks,Beigang,gongshanbei and Hengling,which were relatively well preserved,had been selected as the research objects.By observing the deposition and material composition characteristics of beachrocks in the field,GPS measured the geographical location and relative elevation of beachrocks,and samples were collected.The samples collected in the laboratory were made into thin sections,and the formation of beachrocks cement was analyzed by optical microscope and electron probe The trace element content and mineral composition of the cement were measured by LA-ICP-MS,and 15 relativelywell preserved coral branches were selected from the beachrocks samples,and the uranium age was determined by using high-precision uranium dating technology.Based on the comprehensive analysis of the above data,this paper drawed the following conclusions:(1)The Beigang,gongshanbei and Hengling beachrocks of Weizhou Island were all distributed parallel to the modern coast,with clear layered sedimentary sequence and low angle(4-10°) inclined to the sea.The material composition of beachrocks included terrigenous clastic and bioclastic.The terrigenous clastic of Beigang beachrocks was mainly quartz sand,while the terrigenous clastic of gongshanbei and Hengling beachrocks was not only quartz sand,but also volcanic bomb.The main bioclasts were coral branches,with a diameter of0.8-1.2 cm and a length of 1-10 cm.(2)According to the observation of optical microscope and the analysis of EPMA element content,the beachrocks was cemented as low magnesium calcite,which was in the form of equiaxed granular and blade,reflecting that its diagenetic environment was fresh water environment.The cement of Beigang beachrocks was characterized by bright and clean,continuous wafer like filling pores,determined as bright crystal low magnesium calcite,indicating that the diagenetic environment was the atmospheric fresh water undercurrent environment;most of the cement of Gongshanbei and Hengling beachrocks was of granular mosaic structure,with dirty surface,determined as microcrystalline low magnesium calcite,indicating that the diagenetic environment was theatmospheric fresh water seepage environment.However,the relatively high content of strontium element indicated that the diagenetic process experienced the erosion of seawater.(3)LA-ICP-MS analysis showed that microcrystalline calcite and bright calcite are negative anomalies of Ce,indicating that the environment of beachrocks in cementation diagenesis was oxidation environment,that was,Beigang,gongshanbei and Hengling beachrocks in Weizhou Island were the products of cementation diagenesis under the action of fresh air.(4)The uranium dating results of coralline branches showed that the age of Beigang beachrocks was 1780-836 years ago(2017 A.D.),Gongshanbei beachrocks was 1834-1337 years ago and Hengling beachrocks was 1561-672 years ago.Generally speaking,the beachrocks in Beigang,Gongshanbei and Hengling of Weizhou Island were the products of late Holocene,with the ages ranging from 1834-672 years ago.(5)According to the age of uranium series,the elevation of beachrocks and the sedimentary characteristics of coral branches in beachrocks,it was concluded that the sea level of 1834-672 years ago was about 0.82-1.24 m higher than the current sea level.Among them,the sea level of Beigang beachrocks indicator from 1780-836 years ago was about 0.91-1.16 m higher than that of today,the sea level of Gongshanbei beachrocks indicator from1834-1337 years ago was about 0.95-1.24 m higher than that of today,and the sea level of Hengling beachrocks indicator from 1561-672 years ago was about0.82-1.17 m higher than that of today.(6)The process of sea level change indicated by beachrocks in Weizhou Island was similar to the trend of Paleotemperature change in the northern part of the South China Sea in the same period,so it could be inferred that the late Holocene climate played a role in regulating sea level.