Onset of Maikop sedimentation in the Talysh (Azerbaijan): climate or tectonics?

Abstract

The Maikop series are composed of a thick succession of black shales, and are considered the most important source rock for hydrocarbons in the Caspian basin. The type section of this formation is located near the city of Maykop in Russia, north of the Greater Caucasus. Corresponding anoxic sediments can be traced in a vast area from Austria to China. Deposition of these sediments is linked to anoxic conditions that prevailed since the birth of the Paratethys and are related to decreased basin circulation. Due to the lack of (micro) organisms or ash layers, the onset of Maikop deposition has not been dated accurately so far. The Maikop is presently assumed to be of Oligocene to Miocene age. Precise dating of the start of Maikop sedimentation could aid in understanding the role of tectonics and climate change on the onset of anoxia in the Paratethys basin, as both have affected the region during the Paleogene. The Arabia-Eurasia collision has caused uplift, which could have caused a restriction in interaction with the Tethys Sea further south. Alternatively, basin restriction might have been caused by a major glacio-eustatic regression, associated with the transition from greenhouse to icehouse climate around the Eocene-Oligocene transition (EOT). In the Talysh, the mudstone-dominated sedimentary succession of the Pirembel formation is over 1.5 kilometers thick. This formation is generally interpreted as the lower part of the Maikop series. A multidisciplinary stratigraphic approach is used in this study, integrating magneto- and biostratigraphy of the transition to the Pirembel formation, combined with radiometric dating and geochemical analyses of the volcanic formation that underlies the Pirembel formation. Three sections (AZ14, AZ15 and AZ16) were sampled. Biostratigraphic analyses have aimed at dinoflagellates, large benthic foraminifera and nannoplankton. Additionally, samples have been analyzed using magnetostratigraphy, thin sections and x-ray fluorescence. Ten samples were prepared for Ar-Ar dating. All samples were barren for dinoflagellates. Large benthic foraminifera were found in the base of sections AZ14 and AZ15 and interpreted to be of Priabonian age. Nannoplankton analyses place section AZ15 within Mediterranean nannoplankton zones MNP18 and MNP19, which are also within the Priabonian. Magnetostratigraphic analyses of section AZ14 have not yielded any high quality results due to (at least partly) remagnetization. Magnetostratigraphy places the base of section AZ15 in chron C17n.2n. The youngest chron that seems evident is C16n. A sedimentation rate is estimated for section AZ15 of 18 cm/kyr. Using this rate, the base of the section is estimated at 38 Ma, the base of the Pirembel formation at 37.3 Ma and the top of the section at 35.1 Ma. Deposition of the Pirembel formation, taken by some researchers as the onset of the Maikop series consequently started before the EOT. If the Pirembel formation truly represents anoxic sediments of the Maikop series, the anoxia is most likely related to a restriction in basin circulation caused by tectonics. Slumps and olistostromes are indicative for an active tectonic regime, as well as the presence of around 2 km of volcanics of the Peshtasar formation, just below the Pirembel formation. However, around 37.3 Ma, the formation of the first Antarctic ice sheets is reported, which might result in a drop in sea level, leading to basin restriction. A climatic cause can thus not entirely be ruled out. Volcanics are trachybasalts and basaltic trachyandesites with trace element signatures characteristic for arc-lavas, and are interpreted to have formed in an arc setting. This is in contrast with previous interpretations that placed these volcanics within a back-arc setting. Basalts were not affected by assimilation and fractional crystallization processes, and are all primary melts. Three groups are evident within section AZ16, both paleomagnetically and compositionally. These groups probably represent three short, intense episodes of magmatism. The basalts of section AZ14 can be linked to the middle and upper group of section AZ16. All basalts have shoshonitic compositions and are possibly linked to slab-breakoff during the Paleocene-Eocene.