STRATIGRAPHY, SEDIMENTOLOGY AND GEOCHEMISTRY OF THE OLIGOCENE – LOWER MIOCENE MAIKOP GROUP IN DAGESTAN, NE CAUCASUS

Abstract

A relatively complete section of the Maikop Group (Oligocene – lower Miocene) is exposed along the Sulak River valley in Dagestan (NE Caucasus) and contains a depositional record for this part of the Eastern Paratethys. At the Sulak River outcrop, the Maikop Group is ca. 1200 m thick and can be divided into six lithologically‐defined formations: these are from the base up the Khadum Formation (Rupelian), the Miatly Formation, the Lower Clayey Formation, the Mutsidakal Formation (Chattian), the Riki Formation and the Zuramakent Formation (lower Miocene). The Khadum Formation rests on the upper Eocene Belaya Glina Formation and the boundary is marked by a sharp lithological transition from pale‐coloured, bioturbated limestones below to black organic‐rich shales above.Biostratigraphic studies of calcareous nannoplankton in samples from the Sulak River section allowed the position of the Eocene – Oligocene boundary at the base of the Maikop Group to be defined. The boundary occurs within the CP16 Zone near the division between the CP16a and CP16b subzones. This is consistent with the age of the boundary at a reference outcrop along the Kheu River in Kabardino‐Balkaria in the Central North Caucasus, some 200 km west of Dagestan. A positive oxygen stable isotope anomaly occurs at the top of the Belaya Glina Formation.Samples of the Maikop Group are characterized by variations in TOC content ranging between 0.14 and 11.06 wt. %. The highest values were measured in both carbonate‐ and clay‐rich samples from the Khadum Formation, and the lowest (less than 0.5 wt.%) in sandstones from the overlying Oligocene Miatly, Lower Clayey and Mutsidacal Formations. Samples of the lower Miocene Riki and Zuramakent Formations have moderate TOC values (on average more than 1.5 wt.%). Results of Rock‐Eval pyrolysis show that Maikop samples contain kerogen Types II and III which is distributed unevenly throughout the formations. Clay‐rich rocks in the upper part of the Khadum Formation (Solenovian Member) with Type II kerogen have the greatest oil‐generating potential, with initial hydrogen index values estimated at 400‐600 mg HC/g TOC. The Miatly Formation sandstones and siltstones contain migrated bitumen which is recognized from increased values of Rock‐Eval S1 and the high Production Index (S1/(S1+S2). Overlying Oligocene – lower Miocene rocks contain mainly Type III kerogen, although increased TOC values obtained from samples of the Riki Formation indicate that it may have minor gas source potential.Samples of Maikop Group sediments from the Sulak section were analysed for their contents of Mo, S, Fe, Mn, V, Ni and other elements. A stagnation coefficient (Mo/Mn x 100) was calculated and was interpreted as a measure of the intensity of anoxia in the Maikop palaeobasin. Anoxic conditions are interpreted to have reached a maximum in the Rupelian and Aquitanian during deposition of the Khadum and Riki Formations respectively. However, geochemical conditions were unstable and the oxygen concentration in the bottom waters varied widely over time. Thus, the Zuramakent Formation at the top of the Maikop Group was largely deposited in normally oxygenated conditions.