Problems of detailed biostratigraphic and magnetostratigraphic correlation in the Potamidha and Apostoli sections of the Cretan Neogene

Abstract

The Utrecht working group taking part in the I.G.C.P. project no. 1, "Accuracy in time", received its initial funds in the middle of 1975. The group hastened to collect a first series of samples with which it could begin its research. The marly clays of the selected sections Apostoli and Potamidha 1 cover the major part of the marine Middle-Upper Miocene of Crete (Freudenthal, 1969; Meulenkamp, 1969). The corresponding time span, estimated to be some five million years, was thought to be sufficiently long to permit a detailed study of the evolution of the benthonic foraminiferal lineages of Planorbulinella and Uvigerina and an evaluation of the entries of some planktonic marker species. Moreover, the facies of the deposits was considered suitable for magnetostratigraphic research. The biostratigraphers sampled both sections closely with equal spacing, each sample covering an interval of five to ten centimetres. The magnetostratigraphers had their own sampling method; the position of their cores was calibrated in the field with the sampling spots of the biostratigraphers. In some parallel sections layers with volcanic ash were sampled for radiometric dating. It soon appeared that the bentonite samples were unsuitable, at best they yielded untrustworthy results. Next, the entire Apostoli section was found to give normal polarity for all samples after demagnetization, evidently the sandy-silty character of the clays of this section had provided an opportunity for a subrecent destruction of the original magnetisation in the expected reversed polarity intervals. Since the Potamidha section appeared more suitable, efforts on the Apostoli section were stopped with the exception of the investigation of the Uvigerina lineage (Thomas, in preparation). The gathering of the faunal and floral composition data on the Potamidha samples was started and the usual Utrecht counting procedure was applied, i.e. counting up to 200 specimens for benthonic foraminifera, for planktonic foraminifera and for calcareous nannofossils. While this process of data collecting was nearing completion, the working group began to be dissatisfied with the method. Another partial project was inserted, in which different counting methods were tried and compared (Zachariasse et al., 1978). A variable approach to the collecting of quantitative data appears advisable. Some parts of the numerical composition of the Potamidha faunas and floras should have been tackled with techniques other than the standard 200- counting adopted in the beginning. In the meantime Raju had completed the observations of the Potamidha Planorbulinella and the magnetostratigraphic survey had yielded tangible results which called for a nearby comparison section. Section Potamidha 2 was sampled primarily for such a repetition of the polarity scale of Potamidha 1. The problems ensuing from this comparison necessitated the continuance of the investigation of the Potamidha 1section. A second major result emerged from Raju's Planorbulinella observations, and as a result a continuation of the research downward into the older samples from the Apostoli section was considered to be highly desirable. This research was carried out by P. H. Doeven in the course of 1977. The group which had made the 200-counts on the composition of the three groups of microfossils was not enthusiastic about repeating the timeconsuming counting with the more sophisticated methods that had become available. The group felt it would be more logical to include new problems and subjects into its programme. Hence, it was decided that we would present the "primitive" data on the benthonic foraminifera, the planktonic foraminifera and the calcareous nannoplankton as they were in 1976-77. These data are sufficiently detailed to serve as a backcloth for the account of the peculiar Planorbulinella evolution and the problems that originated from the magnetostratigraphic correlations.