海南玉带滩障壁型海岸现代沉积特征及沉积演化

Modern Sedimentary Characteristics and Sedimentary Evolution of Barrier-type Coast in Yudai Beach of Hainan,China

海南玉带滩是我国南海沿岸典型的障壁型海岸现代沉积场所之一。根据野外考察和室内分析,从沉积物的成分、结构、沉积构造等方面揭示了玉带滩障壁型海岸现代沉积特征。结果表明:玉带滩东侧为障壁岛海滩,沉积层序总体上表现为粒度向上变粗;中部为障壁岛沙丘,以中、粗粒石英砂为主,具平行层理、斜层理;西侧为障壁坪,以中、细砂沉积为主,夹杂一些贝壳碎片,层理构造比较明显。玉带滩西侧水体为河流作用明显强于潮汐作用的潟湖;河口沙坝发育或河流岩岛阻挡产生河汊是形成潟湖中冲积岛分布的主要原因;仅在潮汐通道外侧出现拦口沙坝堆积。现代沉积特征结合Google Earth卫星图像分析认为,该障壁型海岸沉积演化大致包括3个阶段:第一阶段,河流携带泥沙入海,在河口湾地区形成多级近新月型河口沙坝。第二阶段,由于后期水道延伸方向、水动力的不断变化,西侧河口沙坝出现合并并向冲积岛转化;在海岸拐弯处,以原有河口沙坝为主体,形成初期障壁沙坝,障壁沙坝西侧水体演变成潟湖。第三阶段,障壁岛西侧受河流冲刷作用影响,河口沙坝坝头几乎完全消失;障壁沙坝形成后,潟湖水面加宽,流速减小,河口沙坝增生合并,最终形成了三大冲积岛(沙坡岛、东屿岛、鸳鸯岛)。该障壁型海岸现代沉积发育冲积岛的障壁岛-潟湖沉积体系,而不同于传统的碎屑海岸障壁岛-潟湖沉积体系,研究其沉积特征和沉积演化为进一步认识和预测特殊类型的古代碎屑海岸障壁岛-潟湖沉积体系油气储层提供了具有参考价值的沉积演化模式。

Yudai beach in Hainan is one of the typical modern barrier-type coast sedimentary sites along the South China Sea coast.According to the field investigation and laboratory research,the modern sedimentary characteristics of barrier-type coast of Yudai beach were revealed from the the composition,structure and sedimentary structure of the sediments.The results show that the east side of Yudai beach is the barrier island beach,and on the whole,the grain size of sedimentary sequence becomes coarser from bottom to top;the middle part is the barrier island sand dunes,mainly with medium and coarse grained quartz sand,parallel bedding and oblique bedding;the west side is the barrier flat,the sediment is mainly medium and fine sand with some shell fragments,and the bedding structure is obvious.The waters on the west side of Yudai beach are lagoons with stronger river action than tidal action.The main reason for the distribution of alluvial islands in lagoons is the development of estuary sandbar or river branch generated by the obstruction of river rock islands.Only the estuary sandbar accumulation occurs outside the tidal channel.According to modern sedimentary characteristics and the satellite remote sensing images of Google Earth,the barrier-type coastal sedimentary evolution includes three stages:①The river carries sediment into the sea,forming a multi-stage approximate crescent estuary sandbar in the estuary region.②Due to the continuous changes in the extension direction of the waterway and hydrodynamic force in the later period,the sandbars in the western estuary are merged and transformed into alluvial islands;at the turning area of the coast,the original estuary sandbars are taken as the main body to form the initial barrier sandbars,and the water on the west side of the barrier sandbars evolve into lagoons.③The west side of barrier island is affected by river scouring,and the head of estuary sandbar almost completely disappears;after the barrier island sandbar formed,the lagoon water surface widens,the flow velocity decreases,and the estuary sandbar proliferates and merges,and finally three major alluvial islands(Shapo island,Dongyu island and Yuanyang island)are formed.The barrier-type coast modern sedimentation is a barrier island-lagoon sedimentary system with alluvial island,which is different from the traditional clastic coastal barrier island-lagoon sedimentary system.Study on the sedimentary characteristics and evolution has provided a valuable sedimentary evolution model for further understanding and predicting the oil and gas reservoirs of the ancient clastic coastal barrier island-lagoon sedimentary system.