Grain size distribution of the lagoonal deposits within the South Malé Atoll, Maldives, Indian Ocean@en

Seismic and multibeam data, as well as sediment samples were acquired in the South Malé Atoll in the Maldives archipelago in 2011 to unravel the stratigraphy and facies of the lagoonal deposits. Multichannel seismic lines show that the sedimentary succession locally reaches a maximum thickness of 15-20 m above an unconformity interpreted as the emersion surface which developed during the last glacial sea-level lowstand. Such depocenters are located in current-protected areas flanking the reef rim of the atoll or in infillings of karst dolinas. Much of the 50 m deep sea floor in the lagoon interior is current swept, and has no or very minor sediment cover. Erosive current moats line drowned patch reefs, whereas other areas are characterized by nondeposition. Karst sink holes, blue holes and karst valleys occur throughout the lagoon, from its rim to its center. Lagoonal sediments are mostly carbonate rubble and coarse-grained carbonate sands with frequent large benthic foraminifers, Halimeda flakes, red algal nodules, mollusks, bioclasts, and intraclasts, some of them glauconitic, as well as very minor ooids. Finer-grained deposits locally are deposited in current-protected areas behind elongated faros, i.e., small atolls which are part of the rim of South Malé Atoll. The South Malé Atoll is a current-flushed atoll, where water and sediment export with the open sea is facilitated by the multiple passes dissecting the atoll rim. With an elevated reef rim and tower-like reefs in the atoll interior it is an example of a leaky bucket atoll which shares characteristics of incipiently drowned carbonate banks or drowning sequences as known from the geological record.2011年于马尔代夫群岛的南马累环礁采集了地震数据、多波束数据及沉积物样品，旨在揭示该环礁泻湖沉积的地层与沉积相特征。多道地震测线结果显示，在被解释为末次冰期海平面低位期形成的暴露面的不整合界面之上，局部沉积层序的最大厚度可达15~20米。此类沉积中心主要分布于环礁礁缘侧翼的水流缓滞保护区域，或是喀斯特溶斗的充填区域内。泻湖内部水深50米的海底大多受水流冲刷，仅覆盖极薄的沉积物或基本无沉积覆盖。侵蚀性水流沟谷沿淹没斑礁展布，其余区域则以无沉积作用为特征。整个泻湖从礁缘至中心广泛分布喀斯特落水洞、蓝洞及喀斯特谷地。泻湖沉积物以碳酸盐砾屑与粗粒碳酸盐砂为主，伴生大量大型底栖有孔虫、仙掌藻屑、红藻结核、软体动物介壳、生物碎屑及内碎屑，部分组分具海绿石化特征，同时含极少量鲕粒。细粒沉积物局部沉积于伸长型小环礁后方的水流缓滞区，此类小环礁属于南马累环礁礁缘的组成部分。南马累环礁属于受洋流冲刷的环礁，环礁礁缘被多个水道切割，使得水体与沉积物可顺畅向外海输出。该环礁兼具抬升的礁缘与环礁内部的塔状礁体，是典型的“漏桶型环礁”，其特征与地质记录中所见的初始淹没碳酸盐台地或淹没序列高度吻合。