Abstract The Khor Al Adaid embayment of southern Qatar represents a unique shallow-water mixed siliciclastic-carbonate coastal depositional system that developed in a hyper-arid climatic setting over the past 6000 years. The embayment, which was formed during the Flandrian transgression as a result of flooding across a partially fault-controlled incised fluvial drainage, is supplied by quartz-rich sands delivered by wind-blown dunes migrating southward across the surface of Qatar. These offshore-migrating eolian-derived sediments are being redistributed by tidal currents in an otherwise low-energy coastal zone, where in situ formation of carbonate mud and a low-diversity skeletal assemblage is ongoing within salinity-restricted environments. Three depositional sectors are delineated: 1) an energetic, linear, fault-controlled Entrance Channel into which the eolian dunes spill directly; 2) a relatively deep (up to 20 m) Outer Lagoon, interpreted to represent a flooded karst-collapse structure; and 3) a sprawling, low-energy, shallow ( Physical oceanographic modeling, integrating multi-seasonal current meter and tidal gage measurements, demonstrates tidal current flow velocities are relatively high in both the Entrance Channel and at the constricted entrance to the Inner Lagoon. Associated flow expansion into less confined areas results in deposition of the eolian-derived sands as flood-tidal deltas, one in the Outer, and two in the Inner Lagoon. A weakly-developed ebb-tidal delta occurs where the Entrance Channel debouches into the Arabian Gulf. Flood-tide dominance is also apparent in Entrance Channel deposits, where sand accumulates in seaward-terminating “ebb barbs” along the margins of the flood-dominant thalweg. Such flood-tide dominance of the thalweg is unusual and likely reflects the absence of river discharge. Evidence of significant inverse estuarine circulation (seaward flow of a brine along the embayment floor) is mostly absent in spite of the landward increase of salinity, where concentrations reach more than double normal seawater salinity in the Inner Lagoon. Modeling results show that seaward-flowing brines formed in the Inner Lagoon are trapped in the relatively deep Outer Lagoon, and that mixing by tidal currents in the energetic Entrance Channel precludes the formation of vertical density gradients there. Because siliciclastic mud is essentially absent, and most of the sediment forming the deltas consists of sand, upper intertidal deltaic deposits that would normally consist of mud are absent, resulting in the flood-tide deltas having a pronounced lobate geometry similar to that of fluvial deltas formed predominantly of sand. Away from tidal deltas, low-energy lagoons are floored by carbonate mud of local production, with minor gypsum precipitating in the Inner Lagoon. Wind-generated waves only locally influence sedimentation, forming complex nearshore bars along some lagoonal shorelines. The overall landward increase in salinity is accompanied by a decrease in the diversity of benthic fauna and their skeletal remains. A fairly diverse faunal assemblage is observed in the Entrance Channel near the Arabian Gulf, including colonial corals, whereas the inner lagoon assemblage is dominated gastropods belonging to Pirenella cingulata. The remnants of large eolian dunes are preserved in the low-energy setting of the Inner Lagoon, segmenting the waterbody, and locally increasing restriction. The sedimentology of the arid-zone coastal Khor Al Adaid embayment may serve as an analogue for environmental settings that were perhaps more commonplace in arid zones of flooded continents during greenhouse times.

中文翻译：

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干旱风成地貌中的混合硅质碎屑-碳酸盐-蒸发岩沉积：卡塔尔 Khor Al Adaid 潮汐主导的沿海海湾

摘要 卡塔尔南部的 Khor Al Adaid 海湾代表了过去 6000 年来在超干旱气候环境中发展起来的独特的浅水混合硅质碎屑-碳酸盐海岸沉积系统。海湾是在弗兰德海侵期间形成的，这是由于洪水穿过部分断层控制的下切河流排水系统而形成的，由富含石英的沙子提供，这些沙子由风吹沙丘向南迁移穿过卡塔尔表面。这些离岸迁移的风成沉积物正被潮汐流重新分布在一个低能量的沿海地区，在那里，在盐度受限的环境中，碳酸盐泥和低多样性骨骼组合正在原位形成。描绘了三个沉积扇区：1) 一个充满活力的、线性的、风成沙丘直接溢出的断层控制入口通道；2) 一个相对较深（高达 20 m）的外泻湖，解释为代表一个淹没的岩溶塌陷结构；3) 一个庞大的、低能量的、浅的（物理海洋学模型，结合了多季节电流计和潮汐计测量，表明在入口通道和内泻湖的狭窄入口处的潮汐流流速相对较高。相关的流量扩展到较不受限的区域导致风成沙沉积为潮汐三角洲，一个在外湖，两个在内泻湖。一个弱发育的退潮三角洲发生在入口海峡进入阿拉伯湾。在入口河道沉积物中也有明显的洪水优势，沙子沿着洪水主导的 thalweg 边缘以向海终止的“退潮倒钩”堆积。thalweg 的这种洪水主导地位是不寻常的，可能反映了河流流量的缺乏。尽管盐度向陆地增加，在内泻湖中浓度达到正常海水盐度的两倍以上，但几乎没有明显的河口逆循环（盐水沿海湾底部向海流动）的证据。建模结果表明，在内泻湖中形成的向海流动的盐水被困在相对较深的外泻湖中，并且在能量入口通道中被潮汐流混合阻止了在那里形成垂直密度梯度。因为基本上没有硅质碎屑泥，形成三角洲的大部分沉积物由沙子组成，没有通常由泥组成的上潮间带三角洲沉积物，导致潮汐三角洲具有明显的叶状几何形状，类似于主要由沙子形成的河流三角洲。远离潮汐三角洲，低能量泻湖被当地生产的碳酸盐泥所覆盖，内泻湖有少量石膏沉淀。风力产生的波浪仅在局部影响沉积，沿着一些泻湖海岸线形成复杂的近岸坝。盐度向陆地整体增加伴随着底栖动物群及其骨骼遗骸多样性的减少。在阿拉伯湾附近的入口通道中观察到相当多样化的动物群，包括殖民地珊瑚，而内泻湖群则以属于 Pirenella cingulata 的腹​​足动物为主。大型风成沙丘的遗迹保存在内泻湖的低能量环境中，分割水体，局部增加限制。干旱区沿海 Khor Al Adaid 海湾的沉积学可以作为环境设置的类似物，这些环境可能在温室时期被淹没大陆的干旱区更为常见。