Calcareous Nannofossils Biostratigraphy of the Lower Part of Balambo Formation (Lower Cretaceous) in Azmir Anticline - Northeastern Iraq

Abstract

The present research includes a biostratigraphic study of calcareous nannofossils of the lower part of Balambo Formation (Lower Cretaceous) in Azmir anticline - northeastern Iraq. Lithologically, the lower part of the studied section (Balambo Formation) consists of limestone containing nodules of chert, marly limestone, with intercalations of green and gray-blue shale. In this work, it has been relied on calcareous nannofossils assamblages in biostratigaphy and age determination of this part because it does not contain planktonic foraminifera. First occurrences (FOs) and last occurrences (LOs) of marker species were used to identify biozones. The section was divided into four main biozones and two subzones and its correlation with collection from global, regional, and local biozones. Thus, the age of this part of the formation extends from the Late Early Hauterivian - Late Aptian. These are from older to younger: 1- Speetonia colligata Interval Zone (part) 2- Lithraphidites bollii Interval Zone 3- Micrantholithus hoschulzii Partial range Zone 4- Chiastozygus litterarius Interval Zone Hayesites irregularis Interval Subzone Eprolithus floralis Interval Subzone (part)