New age constraints for Neogene sediments of the Sahabi area, Libya (Sirt Basin) using strontium isotope (87Sr/86Sr) geochronology and calcareous nannofossils

For almost half a century researchers have attempted to determine the absolute ages of Neogene sedimentary units in the Sahabi area of Libya. The age of these sediments is of particular interest to paleontologists who have worked on important vertebrate remains in the area since the late 1970s. To date, several geochronological methods have been attempted; however, no precise ages have been obtained for the various units. In this paper we report data for calcareous nannofossils and strontium isotope (87Sr/86Sr) analysis of macrofossils, which can be used to infer age of deposition of part of the Neogene section (Formation M). Because most Sahabi fossils are extensively altered by various diagenetic processes, including gypsification and dolomitization, we carefully screened tens of samples to select unaltered material for analysis. Among the many fossils collected from Formation M and analyzed by XRD, only two, Balanus sp. and a Cubitostrea digitalina show no evidence of diagenetic alteration of their shells, and thus retain their original low-Mg calcite (LMC) mineralogy. The strontium isotopic values from these fossils were plotted against the marine strontium isotopic curve, in order to infer the absolute age. The Balanus sp. has a 87Sr/86Sr value of 0.708917, whereas C. digitalina has a 87Sr/86Sr value of 0.708908. Based on the current estimated error (0.000008), the value of 0.708917 indicates an ages between 9.33 Ma and 8.59 Ma, centered on 8.99 Ma, whereas the value of 0.708908 indicates ages between 9.61 and 9.03 Ma, centered on 9.36 Ma. The calcareous nannofossils found in this formation belong to biozone NN10b-NN11a and provide an age of 8.23 Ma. These ages fall in the Late Miocene period (Tortonian Epoch). Therefore, Formation M should be considered Late Miocene (Tortonian) rather than Middle Miocene (Serravallian) as proposed by many previous workers. Age dating of Formation M will help in revising the stratigraphic nomenclature as well as in re-evaluating the stratigraphic position of this formation according to the Libyan Stratigraphic Code through the Earth Science Society of Libya (ESSL).