Sedimentology of Lower Triassic (Induan Stage) Mixed Carbonate-Siliciclastic Succession of an Epeiric Carbonate Ramp at the Northeastern Passive Margin of the Arabian Plate, Northern Iraq-Kurdistan Region

IF 2.2 4区 地球科学 Q3 GEOSCIENCES, MULTIDISCIPLINARY
Geological Journal Pub Date : 2025-01-07 DOI:10.1002/gj.5124
Sardar M. Balaky, Irfan Sh. Asaad, Mazin Y. Tamar-Agha, Ahmed E. Radwan
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Abstract

The Mirga Mir Formation (Lower Triassic) was studied for its sedimentology and lithostratigraphy from two outcrops (Nazdur and Beduhe areas) c. 20 km apart, in the Northern Iraq-Kurdistan Region. The collected samples were investigated by standard polarised microscope, X-ray diffraction and scanning electron microscopy to infer their mineralogical characteristics, provenance and paleodepositional environments. The Mirga Mir Formation is a mixed carbonate-siliciclastic succession consisting of thin-, medium-, occasionally thick-bedded, and yellowish-grey limestones, sandy dolomitic and argillaceous limestones, alternating with thin- to thick-bedded grey shale/marl. Thin-laminated siltstones and fine- to coarse-grained sandstones are seen in the lower and middle parts of the formation. The petrographic study of carbonates and clastics (sandstones) showed that most limestones are carbonate mud (micrite). The skeletal grains consist principally of thin-shelled pelagic bivalves (Posidonia), serpulid worm tubes (Spirorbis), microgastropods, ammonoids, calcispheres, brachiopods, dasyclad green algae, miliolid foraminifers and ostracods. Non-skeletal grains include ooids, peloids, intraclasts and extraclasts. Depending on field observations, five different lithologic units were recognised. These are in ascending order: oolitic limestone unit, sandy limestone-shale unit, shale-limestone unit, argillaceous limestone-shale unit and brecciated-slumped limestone unit. Sandstones occur as thin beds within the lower and middle parts of the studied formation. In addition, thick coarse-grained sandstone beds occur in the middle part of the Nazdur section only. Petrographically, thin sandstone beds consist of fine-grained quartz. The thick sandstone bed is an immature, litharenite and consists of rock fragments (mostly sedimentary), quartz and feldspars. The X-ray diffraction analysis and scanning electron microscope micrographs of shales revealed that the abundant clay minerals in the Mirga Mir Formation are illite, followed by kaolinite and chlorite. In addition to illite-chlorite mixed layers, non-clay minerals include calcite and quartz, with low feldspars. Based on detailed microfacies analysis of the limestones, 12 microfacies types from the carbonates and mixed detrital carbonates are distinguished. These facies were subdivided, according to their environmental interpretation, into five facies associations: Offshoal/basinal, Foreshoal/slope, Shoal, Back Shoal/Lagoon. The study introduces a good example of a gently sloping epeiric carbonate ramp that shows a gradual shallowing of the basin from the offshoal/basinal sediments to the foreshoal/slope and backshoal/lagoonal environments.

Abstract Image

伊拉克-库尔德斯坦北部阿拉伯板块东北被动缘表相碳酸盐岩斜坡下三叠统(Induan期)碳酸盐-硅屑混合沉积学
在伊拉克北部库尔德斯坦地区,从两个相距约20 km的露头(Nazdur和Beduhe地区)对Mirga Mir组(下三叠统)进行了沉积学和岩石地层学研究。利用标准偏光显微镜、x射线衍射和扫描电镜对样品进行了矿物学特征、物源和古沉积环境分析。米尔加米尔组是一个混合碳酸盐-硅质碎屑序列,由薄层、中层、偶尔厚层、黄灰色灰岩、砂质白云岩和泥质灰岩组成,与薄层至厚层的灰色页岩/泥灰岩交替存在。地层下部和中部可见薄层粉砂岩和细粒至粗粒砂岩。对碳酸盐岩和碎屑(砂岩)的岩石学研究表明,大多数灰岩为碳酸泥(泥晶)。骨骼颗粒主要由薄壳的远洋双壳类动物(Posidonia)、蛇形虫管类动物(Spirorbis)、小腹足类动物、菊石类动物、石壳类动物、腕足类动物、壳绿藻、千层有孔虫和介形虫组成。非骨骼颗粒包括卵状、球质体、内裂粒和外裂粒。根据现场观察,识别出了5种不同的岩性单元。由大到小依次为鲕粒灰岩单元、砂质灰岩-页岩单元、页岩-灰岩单元、泥质灰岩-页岩单元和角砾-塌陷灰岩单元。砂岩以薄层的形式出现在被研究地层的下部和中部。此外,厚的粗粒砂岩层只出现在纳兹杜尔剖面的中部。岩石学上,薄砂岩层由细粒石英组成。厚砂岩层是一种未成熟的岩屑岩,由岩石碎片(主要是沉积岩)、石英和长石组成。页岩x射线衍射分析和扫描电镜结果显示,米尔加米尔组黏土矿物以伊利石为主,高岭石次之,绿泥石次之。除了伊利石-绿泥石混合层外,非粘土矿物包括方解石和石英,长石含量低。通过对灰岩微相的详细分析,区分出碳酸盐岩和混合碎屑碳酸盐岩的12种微相类型。这些相根据其环境解释被细分为五个相组合:近海/盆地、前滩/斜坡、浅滩、后滩/泻湖。该研究介绍了一个缓坡的表面层碳酸盐斜坡的好例子,该斜坡显示了从浅海/盆地沉积物到前滩/斜坡和后滩/泻湖环境的盆地逐渐变浅。
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来源期刊
Geological Journal
Geological Journal 地学-地球科学综合
CiteScore
4.20
自引率
11.10%
发文量
269
审稿时长
3 months
期刊介绍: In recent years there has been a growth of specialist journals within geological sciences. Nevertheless, there is an important role for a journal of an interdisciplinary kind. Traditionally, GEOLOGICAL JOURNAL has been such a journal and continues in its aim of promoting interest in all branches of the Geological Sciences, through publication of original research papers and review articles. The journal publishes Special Issues with a common theme or regional coverage e.g. Chinese Dinosaurs; Tectonics of the Eastern Mediterranean, Triassic basins of the Central and North Atlantic Borderlands). These are extensively cited. The Journal has a particular interest in publishing papers on regional case studies from any global locality which have conclusions of general interest. Such papers may emphasize aspects across the full spectrum of geological sciences.
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