Abstract
Nineteen subsurface sections and a large number of thin sections of the Mauddud limestone (age Albian–Early Cenomanian) were studied to unravel the depositional facies and environments. The allochems in the Mauddud Formation are dominated by bioclasts (reach 23%) and peloids (reach 60%), whereas intraclasts are less abundant (reach 2.3%). The sedimentary microfacies of the Mauddud Formation includes lime mudstone, wackestone, wackestone–packstone, packstone, packstone–grainstone, in addition to dolostone lithofacies and green shale lithofacies. These microfacies have been deposited in shallow warm marine environment of varying salinities and energy levels. Cementation, neomorphism, dolomitization, compaction, and dissolution are observed affecting variably both ground mass and particles. The formation displays various extents of dolomitization and is cemented by calcite and dolomite. Dolomitization increases toward the north of the study area and exhibits different textures. Similarly cementation shows a variety of textures. In addition, authigenic minerals, such as glauconite and pyrite, are scattered within the groundmass and along solution surfaces. The formation has gradational contact with the underlying Nahr Umr Formation but is unconformably overlain by the Ahmadi Formation, despite local conformity. Thus, the vertical bioclast analysis indicates that the Mauddud Formation is characterized by four major depositional cycles, which control the distribution of reservoir quality as well as the patterns of calcite and dolomite, cement distribution. Petrographical study shows that the fossil represents the main Allchem, Peloids, whereas intraclasts come second in abundance. Calcite (more than 50%) and dolomite (as diagenetic products, range between 20% and 50%) are the predominant mineral components of Mauddud Formation. Fossils were studied as an environmental, age, and facial boundary indicators. Five major depositional microfacies are recognized. These depositional microfacies have been subdivided according to their primary and diagenetic constituents into ten submicrofacies. The determined four major depositional cycles were representing normal sequential regression from base upward. The lateral analysis shows the same regressive cycle and by using the lithofacies association concepts to build the depositional model of the Mauddud Formation environment.
المستخلص
تم دراسة تسعة عشر مقطعا تحت سطحيا وعدد كبير من الشرائح الصخرية لتكوين المودود الجيري ( عمر الالبي – السينوماني المبكر) لاعطاء صورة جيدة عن بيئته الترسيبيه وسحناته الصخريه .
تتكون الحبيبات الهيكلية لصخور تكوين المودود من المتحجرات (تصل الى 23%) والدمالق (تصل الى 60%)
بينما تكون الحبيبات المفتته داخليا اقل شيوعا لتصل الى 2،3%. شملت السحنات الدقيقة لتكوين المودود سحنات الوحل الجيري والواكي والمتراص والحبيبي اضافة الى سحنات صخارية الدولومايت والسجيل الاخضر. ان جميع السحنات الدقيقة كانت قد ترسبت في بيئة بحرية ضحله ودافئه ذات ملوحة ومستويات طاقة متنوعة . شملت العمليات التحويرية (السمنته والتشكل الجديد والدلمته والتراص والاذابه) كل من الحبيبات الهيكلية والحشوه القاعديه للصخور الجيريه لتكوين المودود . وكانت عملية الدلمته مؤثرة بدرجات وباشكال وانسجة مختلفة وتزداد باتجاه الشمال من منطقة الدراسة لتشابه عملية السمنته ذات الانسجة المتنوعه. اضافة الى ذلك فقد كانت معادن الكلوكونايت والبايرايت متناثره في الحشوه الداخليه وعلى طول اسطح تاثيرات عملية الاذابة. ان تكوين المودود يمتد متوافقا مع تكوين نهر عمر الذي يحده من الاسفل بينما يكون غير متوافق مع تكوين الاحمدي الذي يقع في اعلى تكوين المودود. لقد اظهرت التحاليل الستراتيكرافية بان المودود يتكون من اربع دورات ترسيبيه تمثل فترات التراجع البحري من الاسفل الى الاعلى وتكون مؤثره في في توزيع معادن الكالسايت والدولومايت والماده السمنتيه .
بينت الدراسات المجهرية بان المتحجرات شكلت غالبية الحبيبات الهيكلية ثم الدمالق والحبيبات المفتتة داخليا .
وصلت نسبة الكالسايت الى 50% والدولومايت تراوحت بين 20 – 50% وهما المعدنين الرئيسيين في صخور تكوين المودود. اظهرت الدراسة خمسة سحنات دقيقة رئيسيه قسمت الى عشرة سحنات ثانوية اعتمادا الى مكوناتها المعدنيه والسحنية وتاثيرات العمليات التحويريه عليها.ومن خلال معرفة التغيرات العمودية والجانبية السحنية امكن بناء الموديل الترسيبي لبيئة ترسيب تكوين المودود في الابار المدروسة في وسط وجنوب العراق.
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Al-Dabbas, M.A., Jassim, J.A. & Qaradaghi, A.I. Sedimentological and depositional environment studies of the Mauddud Formation, central and southern Iraq. Arab J Geosci 5, 297–312 (2012). https://doi.org/10.1007/s12517-010-0256-5
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DOI: https://doi.org/10.1007/s12517-010-0256-5