CONSTRAINING THE TIMING AND DEPOSITIONAL CONDITIONS OF THE MAIKOP FORMATION WITHIN THE KURA BASIN, EASTERN AZERBAIJAN, THROUGH THE APPLICATION OF RE‐OS GEOCHRONOLOGY AND CHEMOSTRATIGRAPHY

Abstract

The Oligocene – Miocene Maikop Formation is the key source rock in the South Caspian and Kura Basins. The Maikop is composed of a thick (up to 3 km) succession of clay‐rich mudstones containing up to 15% total organic carbon (TOC). Despite decades of study, the mudstones often lack precise age control – Maikop strata rarely contain diagnostic microfaunal assemblages which can be used for dating, stratigraphic correlation, or constraining the depositional setting. Using rhenium‐osmium geochronology, this study adds important numerical age data for the Maikop Formation. Of five sample suites analysed from the Kura Basin, eastern Azerbaijan, one Re‐Os data‐set produced a significant range in 187Re/188Os versus 187Os/188Os space to yield an isochron of 17.2 ± 3.2 Ma (Early Miocene). Other sample suites yielded imprecise Re‐Os age constraints as a result of variable initial 187Os/188Os values and a limited range in 187Re/188Os versus 187Os/188Os space. The initial 187Os/188Os values of these data‐sets were compared with the known 187Os/188Os values of seawater for the past 70 Ma to provide more qualitative age constraints. Pre‐Maikopian strata from the Perikeshkul locality were found to coincide in 187Os/188Os values with an isotope excursion at the Eocene – Oligocene Transition (EOT), therefore indicating that deposition of Maikopian strata began around the EOT. While values such as this match well with global values, there are several 187Os/188Os values that are not easily explained by global ratios. Intervals with initial 187Os/188Os values that deviate significantly from global 187Os/188Os values suggest periodic basin restriction and the development of anoxia at discrete times as the basin transitioned towards a closed system. High Os abundances outside of expected global values are often coupled with enrichment in detrital elements (Al, Ti, Ga, Sc and La) and changes in basin circulation, suggesting changing basinal conditions and sediment routing dynamics related to the initial uplift of the Greater Caucasus Mountains, changes in sediment provenance, or changing proximity to the sediment source. Through generation of isochron age dates and imprecise Re‐Os age constraints from the Maikop Formation, we gain a better understanding of the timing and nature of the evolution of the South Caspian Basin during this critical time period. Better age constrains will also help to better constrain the wealth of geochemical information already gathered within this petroleum‐rich basin.