Planktonic foraminiferal biostratigraphy of the Turonian-Coniacian transition interval in the Kopeh-Dagh sedimentary basin, NE Iran

Abstract

The Abderaz Formation was analyzed from four stratigraphic sections (Qaleh-Zo, Abderaz village, Mozduran pass (type section of the formation), and Chahchahe) in awest to east transect of the Kopeh-Dagh Basin. The Abderaz Formation is composed of light gray shale and marl alternated with limestone rich in echinoids and inoceramids. Planktonic foraminifera were studied in detail in order to determine the Turonian-Coniacian boundary with more accuracy and to highlight bio-events around this boundary in the Kopeh-Dagh sedimentary basin. Nineteen planktonic foraminifera species belonging to 12 genera were identified in the Turonian-Coniacian transition interval. Four planktonic foraminifera biozones are recognized in all studied sections; they are 1) Whiteinella archaeocretacea Partial Range Zone, 2) Helvetoglobotruncana helvetica Total Range Zone, 3) Marginotruncana sigali-Dicarinella primitiva Partial Range Zone, and 4) Dicarinella concavata Interval Zone. In all studied sections, the Turonian-Coniacian boundary is placed at the base of the Dicarinella concavata Interval Zone. As in our previous work, we confirm that the base of the Coniacian stage in the Kopeh-Dagh sedimentary basin is distinguished based on the first occurrences of Dicarinella concavata, Pseudotextularia nuttalli, and Contusotruncana fornicata, and on the last occurrences of Dicarinella imbricata, Marginotruncana paraconcavata and Marginotruncana schneegansi.