Anoxic sinkholes are unique geomorphological features of modern reef systems and in terms of “lake-like” sediment successions very special marine depositional environments. This study presents results of a sedimentological survey, which focuses on the “Great Blue Hole”, a circular Pleistocene karst cavity, situated in the shallow eastern lagoon of Lighthouse Reef Atoll (Belize, Central America). The investigated Blue Hole is located within the tropical cyclone belt and acts as consequence of its unique geomorphology as a giant sediment trap for both fair weather and storm sedimentation. As an important innovation step, we applied a multi-proxy approach based on textural, compositional and geochemical data to ensure a more reliable differentiation between sediments with fair weather and storm condition affiliation. Sediments having formed under both conditions were texturally categorized by determination of classical sedimentary parameters such as mean grain size and sorting. Sediment composition has been quantified in thin sections based on 40 samples discretely taken along a core from the bottom of the 125 m deep and 320 m wide cavern. Classification of sedimentary facies follows a modified scheme for unconsolidated carbonate sediments based on the Dunham nomenclature. Fair-weather sediments are generally finer than 20 μm and consist on average of 81% carbonate fine material, 12% Halimeda platelets, 3% coral fragments, 2% organic material, 1% mollusc shells, 1% foraminifer tests, and minor portions of coralline red algae and echinoderm plates (0–1%). Intercalated event bed sediments are coarse-silt to sand-sized (> 20 μm) and differ in terms of generally lower fine carbonate material amounts (50%) and increased abundances of over-washed Halimeda platelets (28%) and coral fragments (6%). Combined analyses of this study have revealed four primary sedimentary facies with affiliation either to the background (mudstone) or cyclonic event sedimentation ( Halimeda wackestone, algae-rich packstone, coralgal rudstone) as well as a transitional sub-facies indicating storm-induced background overprinting (background wackestone). The striking compositional and textural differences of the Halimeda wackestone, algae-rich packstone and coralgal rudstone facies are considered to be another helpful (semi)-quantitative proxy to identify event beds of tropical cyclones in unconsolidated sinkhole sediment successions.

中文翻译：

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淹没落水洞的全新世沉积物：“大蓝洞”的相分析，灯塔礁，伯利兹

缺氧天坑是现代珊瑚礁系统的独特地貌特征，在“湖状”沉积层序方面是非常特殊的海洋沉积环境。本研究展示了沉积学调查的结果，重点是“大蓝洞”，这是一个圆形更新世岩溶洞穴，位于灯塔礁环礁（伯利兹，中美洲）东部浅水泻湖中。被调查的蓝洞位于热带气旋带内，由于其独特的地貌，它是一个巨大的沉积物圈闭，适合晴天和风暴沉积。作为重要的创新步骤，我们应用了基于纹理、成分和地球化学数据的多代理方法，以确保更可靠地区分具有晴朗天气和风暴条件的沉积物。在两种条件下形成的沉积物通过确定经典沉积参数（例如平均粒度和分选）来进行质地分类。基于从 125 m 深和 320 m 宽的洞穴底部沿岩心离散采集的 40 个样本，已对薄片中的沉积物成分进行了量化。沉积相的分类遵循基于 Dunham 命名法的未固结碳酸盐沉积物的修改方案。晴天沉积物通常小于 20 μm，平均由 81% 的碳酸盐细粒物质、12% 的 Halimeda 片晶、3% 的珊瑚碎片、2% 的有机物质、1% 的软体动物壳、1% 的有孔虫试验和少量的珊瑚红藻和棘皮动物板块 (0-1%)。夹层事件床沉积物为粗粉砂至砂粒大小 (> 20 μm），不同之处在于通常较低的细碳酸盐材料量 (50%) 和增加的过度洗涤的 Halimeda 血小板 (28%) 和珊瑚碎片 (6%)。对这项研究的综合分析揭示了四种主要沉积相与背景（泥岩）或气旋事件沉积（Halimeda wackestone、富含藻类的灰岩、珊瑚砂岩）以及表明风暴引起的背景叠印的过渡亚相（背景石板）。Halimeda 泥岩、富含藻类的泥灰岩和珊瑚粉岩相的显着成分和结构差异被认为是另一种有用的（半）定量代表，用于识别未固结的落水洞沉积序列中热带气旋的事件层。