Upper Albian and Cenomanian (Cretaceous) ammonites from the Debarsu Formation (Yazd Block, Central Iran)

Abstract

New ammonite faunas consisting of 13 taxa provide the first reliable biostratigraphic dating of the Debarsu Formation of the Yazd Block, west-central Iran, indicating several levels in the Upper Albian and Lower Cenomanian, while a foraminiferal assemblage places the top of the Formation in the Middle Turonian. Among the identified ammonite taxa, Acompsoceras renevieri (Sharpe, 1857) is recorded from Iran for the first time. The upper part of the lower Upper Albian is proved by the occurrences of mortoniceratines of the Mortoniceras (M.) inflatum Zone in the lowermost part of the Debarsu Formation. For the upper Upper Albian (traditional Stoliczkaia dispar Zone), the M. (Subschloenbachia ) rostratum and M. (S.) perinflatum zones are proved by their index taxa. However, there is no evidence of the terminal Arrhaphoceras (Praeschloenbachia) briacensis Zone. The upper part of the lower Lower Cenomanian Mantelliceras mantelli Zone (M. saxbii Subzone) is proved by M. saxbii and M. cf. mantelli. Below, there is an ammonite-barren interval of ca. 100 m in thickness between M. (S.) perinflatum zonal strata and the M. saxbii Subzone. The upper Lower Cenomanian is documented by the presence of typically M. dixoni zonal ammonites such as Acompsoceras renevieri. Upper Cenomanian and Turonian ammonites have not been found in the upper part of the Debarsu Formation, but micro-biostratigraphic evidence (planktonic foraminifers) from the uppermost part of the formation indicate that the formation ranges into the Turonian. For the development of the major tectonic unconformity at the base of the overlying Haftoman Formation (which yielded Lower Coniacian inoceramids near its base), only 2–3 myr remain, stressing the geodynamic activity of Central Iran during mid-Cretaceous times.