Kamal Haji KARIM, Sherzad Tofeeq AL-BARZINJY, Polla Azad KHANAQA

Üst Kretase Zagros tebeşirli kireçtaşının (Kometan Formasyonu) jeolojisi ve eleştirel değerlendirmesi (Süleymaniye, KD Irak)

Tebeşirli kireçtaşları (Kometan Formasyonu) Geç Türoniyen – Orta Kampaniyen yaşlı olup,Kuzeydoğu Irak’ta, Süleymaniye bölgesindeki Yüksek, İmbrike ve Bindirme Zonlarındayüzeylemektedir. Bu kireçtaşları yanal olarak, Kuzeybatı ve Batı Irak’a doğru sırasıyla Bekhme veMushurah formasyonlarına geçmekte olup, resifal eşdeğerleri ise Orta Irak’taki petrol sahalarınınaltında yer almaktadır. Stratigrafi k olarak üstteki Shiranish Formasyonu (Orta-Geç Kampaniyen) ilealttaki Gulneri Formasyonu (Geç Senomaniyen-Erken Türoniyen) arasında yer almaktadır. 40-120metre arası bir kalınlığa sahip olan birim pelajik ortamda çökelmiştir. Önceki bütün çalışmalarda,tabaka yüzeyleri üzerinde veya yakınında yoğun çört nodülleri ve stilolitlerin olduğu, tabakadüzlemleri belirgin ve ince taneli derin deniz fasiyesindeki kireçtaşları olarak tanımlanmıştır.Fosil kayıtlarında yalnızca nannofosil ve planktik formlar ile az miktardaki bentik foraminiferlererastlanır. Bütün bunların aksine, son zamanlarda yapılan bir çalışmada yukarıdaki tüm özelliklerreddedilmiş ve litoloji, bol miktarda Thalassinoides iz fosili içeren masif, homojen bir kireçtaşıolarak değiştirilmiştir. Buna ilaveten tabakalanma yüzeylerinin aslında gerçek tabakalanma olmadığı(yalancı tabakalanma); depolanma ile değil, stilolitleşme ile meydana geldiği sonucuna varılmıştır.Bu çalışma Kretase yaşlı tebeşirli kireçtaşlarının jeolojisine ilişkin bilgileri güncelleme ve eleştirel birdeğerlendirme üzerine odaklanmakta; yukarıda belirtilen tüm fi kirlerin karşılaştırılmasına ek olarakyeni fi kirleri de tanıştırmayı amaçlamaktadır. Önceki yazarların belirttiği Thalassinoides, masifl ikve yalancı tabakalanma özellikleri tartışılmış ve bulunmayışları arazi ve laboratuvar çalışmalarıile teyit edilmiş ve doğrulanmıştır. Ayrıca masifl ik durumu reddedilmiş ve tabaka düzlemlerininbelirginliği (nispeten ince tabakalı) bütün kesitlerde formasyonun temel bir özelliği olarak ortayakonmuştur. Bu çalışma ayrıca çört nodülleri ve çört tabakalarının; biyotürbasyon, kırıntılı malzeme,derecelenme, laminasyon ve renk değişiminin yokluğuna bağlı olarak iz fosiller ile bir ilişkisininolmadığını göstermektedir. Öte yandan Kometan Formasyonunun Zagros önülke (foreland)havzasında depolanması fi kri reddedilmekte; ofi yolit üzerlemesi ve önülke oluşumu öncesindeNeotetis Denizi’nin (ya da okyanusu) kıtasal kenarında depolandığı fikri kabul edilmektedir.

Geology and critical review of the Upper Cretaceous Zagros chalky limestone (Kometan Formation) from Sulaimani Governorate, Northeastern Iraq

The chalky limestone (Kometan Formation) is aged Late Turonian–Middle Campanian and cropping out in the High, Imbricate and Thrust Zones of northeastern Iraq, Sulaimani area. It laterally changes to Bekhme and Mushurah formations toward northwest and west Iraq respectively and its reefal equivalents occurs too in subsurface of Central Iraq in the oil fi elds. Stratigraphically, it is located between Shiranish Formation (Middle-Late Campanian), at the top, and Gulneri Formation (Late Cenomanian-Early Turonian) at its base. It has the thickness of about 40-120 meter and deposited in pelagic realm. All previous studies have defi ned it as well bedded and fi ne grain limestone of deep marine facies with concentration of chert nodules and stylolites on or near bedding surfaces. The only recorded fossils are nannofossils, planktonic forms and rare benthonic forams. On the contrary, a most recent study has declined all the above features and changed lithology to massive, homogenous limestone with abundant Thalassinoides trace fossils. It additionally concluded that the bedding surfaces are not true bedding (pseudo-bedding) and formed by stylolization not by deposition. The present study is focusing on update the geology of the Cretaceous chalky limestone and critical review in addition to comparison of all the above ideas and introducing new ones. The features of the Thalassinoides, massiveness and pseudo-bedding of previous authors discussed and their absence confi rmed and justifi ed by fi eld and laboratory works. Moreover, the massiveness is refused and well bedding (relatively has thin beds) is proved as an intrinsic characteristic of the formation in all section. The study also shows absence of relation of the chert nodules and chert bedding with traces fossils due to absence of bioturbation, detrital clasts, bioclasts, sorting, lamination and color change. Moreover, the idea of deposition of Kometan Formation in Zagros Foreland basin refused and assumed as deposition of continental margin of Neo-Tethys Sea (or ocean) before ophiolitic obduction and foreland generation.

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