Microfacies Analysis of the Carbonate Rocks in Ora Formation (Late Devonian early Carboniferous) in North Iraq

Abstract

The outcrop sections of Ora Formation (late Devonian-Early Carboniferous) in the Nazdor and Ora localities at the Northern Thrust Zone of Iraq were selected for this study. Lithologically the Ora Formation composed of clastic and carbonate rocks. The lower part in the Nazdor section cosists of crossbedded quartz arenite sandstone interbedded with successive laminated siltstone and shale. The lithology of the upper part in the Nazdor section and the upper part in Ora section, are composed of mixed silicalcastic (shale, siltstone and sandstone) and carbonate interbedded lithology. The petrographic study shows that the carbonate rocks consist of micrite and sparite groundmass. The skeletal grains include shallow water brachiopods, bryozoans , echinoderms , in addition to ostracods , trilobite , pelecepoda , gastropoda and calcispheres , while nonskeletal grains include peloids and lithoclasts of carbonate and noncarbonat. Ten microfacies types were identified. There are silty mudstone, laminated silty peloidal packstone, lithoclast breccia, wackstone, floatstone, bryozoa grainstone, sandy laminated grainstone, grainstone/rudstone and boundstone. Facies analysis of the upper part of the Ora Formation in Nazdor and Ora section indicate that the rocks were deposited mainly in inner margins of homoclinal carbonate ramp with minor units were deposited in middle of outer ramp margins.