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

Year 2018, , 59 - 74, 15.12.2018

Kamal Haji Karım Sherzad Tofeeq Al-barzınjy Polla Azad Khanaqa

https://doi.org/10.19111/bulletinofmre.428342

Abstract

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 fields. 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 defined it as
well bedded and fine 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
confirmed and justified by field 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.

Keywords

chalky limestone, Late Cretaceous, Zagros geology, pseudo-bedding, depositional bedding, Kometan Formation, Thalassinoides

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