The Asmari Formation Revisited: Changed Stratigraphic Allocation and New Biozonation

The essential biostratigraphy of the Asmari Formation was outlined in the early 1950’s and thereby the only published record of the biostratigraphy before the Asmari Formation was formally described in the mid 1960’s. The traditional Iranian biostratigraphy is based on unpublished reports. Unfortunately, these reports were written in a period when the Aquitanian stage was under debate. Recent studies have shown that sediments ascribed to the Miocene “Aquitanian” are in fact Late Oligocene, Chattian in age. This was proved by the application of Sr-isotope stratigraphy to cored sections from 10 Iranian oil fields and 14 outcrop sections, within the framework of a high resolution sequence stratigraphic study down to fourth order cycles (van Buchem et al., in press). For each section, the strontium dates were plotted against the fossil ranges. The strontium ages necessitate a revision of the interpretation of the Chattian-Aquitanian boundary. The Chattian/Aquitanian boundary is marked by a major faunal turnover, with the general extinction of Archaias species and Miogypsinoides complanatus. The previous age interpretation of the early, unpublished zonations necessitates a revision, and the establishment of an updated biozonation. This new zonation and the stratigraphic ranges of selected key-species are presented here.