Tirgan formation of the Kopet-Dagh Basin (northeast Iran) represents one of the Urgonian carbonate platforms that were deposited during the Early Cretaceous time in the northern Alpine Tethys and deformed during the Alpine orogeny. In this study, six stratigraphic sections of the shallow-water platform sediments (Tirgan formation) were measured based on microfacies and fauna abundance. Detail study of petrography, fossil content, and sedimentary structures led to the identification of fifteen microfacies belonging to four facies belts including open marine, shoal, protected lagoon, and tidal flat. The sediments of the Tirgan formation exhibit calcareous green algae, abundant ooids, oysters, bryozoans, and crinoids in inner and middle platform ramp facies and planktonic bivalves and sponge spicules in outer-platform facies. Furthermore, the absence of basinal deposits and lack of evaporate evidence point to deposition under warm-temperate and humid climate conditions. Sequence stratigraphy analysis of Tirgan formation led to distinguish a single depositional sequence in all of the sections which are composed of transgressive and highstand systems tracts with sequence boundaries of type II (SB2). The lowermost lower Aptian Tirgan sequence in the study area relatively shows a similar trend in comparison with the global curve. This basin was deepened over time so that shaly and marly sediments of Sarcheshmeh formation were placed over Tirgan conformably and may suggest a drowning event that was likely related to unusual global warming. Last, this study contributes to the better understanding of the high distribution of facies assemblages in the Urgonian carbonate platforms.

中文翻译：

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伊朗东北部 Kopet-Dagh 盆地东部早白垩世（乌尔贡阶）Tirgan 地层碳酸盐岩台地演化：沉积环境和层序地层意义

Kopet-Dagh 盆地（伊朗东北部）的 Tirgan 地层代表了 Urgonian 碳酸盐平台之一，该平台在早白垩世时期沉积在高山特提斯山脉北部，并在高山造山运动期间变形。在这项研究中，根据微相和动物群丰度测量了浅水台地沉积物（Tirgan 组）的六个地层剖面。通过对岩石学、化石含量和沉积结构的详细研究，确定了属于四个相带的 15 个微相，包括开阔海相、浅滩、受保护的泻湖和潮滩。Tirgan组沉积物在台地内、中台斜坡相中发育钙质绿藻，丰富的鲕类、牡蛎、苔藓虫和海百合类，台外相中为浮游双壳类和海绵针状体。此外，没有盆地沉积物和缺乏蒸发证据表明在暖温带和潮湿气候条件下沉积。对 Tirgan 组的层序地层分析导致在所有由海侵和高水位系统域组成的剖面中区分出单一的沉积层序，层序边界为 II 型（SB2）。研究区最下方的Aptian Tirgan层序与全球曲线相比显示出相似的趋势。随着时间的推移，这个盆地不断加深，因此 Sarcheshmeh 地层的页岩和泥灰质沉积物被一致地放置在 Tirgan 上空，这可能表明可能与不寻常的全球变暖有关的溺水事件。最后，这项研究有助于更好地了解 Urgonian 碳酸盐台地中相组合的高度分布。