The age of the Asmari Formation: Oligocene or Miocene?

Category Paleontology and Stratigraphy
Group GSI.IR
Location International Geological Congress,oslo 2008
Author Laursen, Gitte V.۱; Monibi, Saeed۲; Allan, Tony L.۳; Pickard, Neil A.۱; Hosseiney, Ahmad۴; Vincent, Benoit۵; Hamon, Youri۵; Moallemi, Ali۲; Drullion, Gilles۶
Holding Date 08 October 2008

Although the Asmari Formation is one of the most prolific oil producing sequences in the world, relatively little is published of its stratigraphic palaeontology. The essential biostratigraphy of the Asmari Formation was outlined in the early 1950’s. This was 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the Asmari Formation was formally described in the mid 1960’s. The traditional Iranian biostratigraphy is based on unpublished reports.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, the reports were written in a period when the Aquitanian stage was under debate. Thus, sediments ascribed to "Aquitanian" may in fact be Chattian in age.
To address the questions about the interpretation of the Chattian-Aquitanian and the fossil ranges, strontium isotope dating was applied 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). For each section, the strontium dates were plotted against fossil ranges. The strontium ages necessitate a revision of the interpretation of the Chattian-Aquitanian boundary. The study verifies a condensed nature of the Aquitanian in the oilfields (but expanded in some of the outcrops) and shows that part of the Asmari Formation, previously referred to the Miocene, belongs to the Chattian of the Oligocene. The sequence stratigraphy also shows evidence for condensation/non-deposition in the Aquitanian interval.
In summary, we present evidence that the previous assignment of much of the Asmari Formation to the Aquitanian is incorrect, and that the interval referred to the "lower Aquitanian" is in fact Chattian. Our data shows that, within the Oligocene, reticulate Nummulites are restricted to the Rupelian, in agreement with their known range elsewhere. The Chattian/Aquitanian boundary is marked by a major faunal turnover, with the general extinction of Archaias species and Miogypsinoides complanatus. Strontium and biostratigraphy date the sequence boundary/stage boundary to the very latest Chattian and it has been correlated to a basin-wide drawn-down event that results in precipitation of a basin-wide evaporite (the Basal Anhydrite). Faunal extinction is associated with this basin desiccation.
The Aquitanian is more difficult to identify on the basis of foraminiferal marker species, and requires further integrated taxonomic and stratigraphic study of the major groups such as the miogypsinids. A generality is that when Miogypsina spp. and Elphidium sp. 14 occur together, it is probably Aquitanian. The Burdigalian is easily picked by the occurrence of Borelis melo curdica. In addition to accurately constraining the time range of stratigraphically important species, the study showed the facies dependence of important markers. For example, Peneroplis evolutus ranges from Rupelian to Burdigalian in our dataset, and Eulepidina has a probable facies disappearance in the latest Oligocene in our study area.