Cenomanian Acanthoceratoidea (Cretaceous Ammonoidea) from the Koppeh Dagh, NE Iran: taxonomy and stratigraphic implications

Abstract

Seven species of the acanthoceratoidean genera Forbesiceras Kossmat, 1897, Mantelliceras Hyatt, 1903, Acanthoceras Neumayr, 1875 and Cunningtoniceras Collignon, 1937 are described and illustrated from the upper (i.e., Cenomanian) part of the Aitamir Formation of the Koppeh Dagh, northeast Iran. The mantelliceratines were collected from lower Cenomanian silty shales while the rest of the fauna stems from lower middle Cenomanian glauconitic sandstones in the upper part of the formation. the ammonite association allows recognition of the lower lower Cenomanian Mantelliceras mantelli and the lower middle Cenomanian Acanthoceras rhotomagense zones. the upper lower Cenomanian M. dixoni Zone is not proven by its index but is most likely represented by a unit of fossil-poor shales intercalated between the two above-mentioned zones. the lowermost middle Cenomanian Cunningtoniceras inerme Zone, however, is potentially at least partly missing due to a major sea-level fall and lowstand in the latest Early to earliest middle Cenomanian. A preliminary sequence stratigraphic interpretation of the successions suggests the presence of lower Cenomanian sequence boundaries sb Ce 1–3. The Aitamir Formation is truncated along a major regional unconformity at the base of the overlying Abderaz Formation (Turonian–Coniacian). The upper Cenomanian and most likely also (parts of) the lower Turonian are missing. This major unconformity has a tectonic origin as it deviates from the eustatic sea-level trend which was very high at this time. Furthermore, contemporaneous tectonic unconformities are also known from Central Iran and may have their origins in rotational movements of the Central-East Iranian microcontinent.