A Cenomanian-Turonian drowning unconformity on the eastern part of Kopet-Dagh basin, NE Iran

Abstract

A Cenomanian-Turonian interval in the Mozduran pass section, eastern part of Kopet-Dagh basin (NE of Iran), was measured and studied in order to interpret the biostratigraphy and paleoecology (based on distribution of planktonic foraminifera). This section consists of 52.5-m light and gray marl and coincides with the Aitamir and Abderaz formation boundary. Based on study of 47 samples, 11 planktonic foraminifera genera and 27 species were encountered, and the following zones have been defined: Rotalipora cushmani Total Range Zone and three zones of Helvetoglobotruncana helvetica, Dicarinella primitiva-Marginotruncana sigali, and Dicarinella concavata as condenced zones. This study shows that the sediment record is incomplete in the section. This unconformity is just after a drowning succession, places in the Late Cenomanian-Early Turonian Whiteinella archaeocretacea Zone, and interpreted as a drowning unconformity. A paleoenvironmental model has been formulated to explain the successive paleocommunity changes during this drowning of the platform. We attribute the drowning of some parts of the platform to the occurrence of the Cenomanian-Turonian oceanic anoxic event. The impingement of anoxic waters over the platform could produce the drastic reduction of the carbonate producing observed in the stratigraphic section and therefore a reduction in carbonate accumulation rates. Subsidence and the Late Cenomanian-Early Turonian sea level rise were then able to drown the platform.