Geochemical and palynological characteristics of the Maykop sections in the north-west of the Shamakha-Gobustan region (Azerbaijan)

Abstract

The article provides a comparative analysis of the results of complex palynological and geochemical studies of rocks from natural outcrops of Lagich, Angekharan and Agsu in the north-west of the Shamakhi-Gobustan region. A comparison of the distribution features of palynomorph species with the lithofacies variability of sediments made it possible to reveal the conditions of sedimentation during the formation of the described sections. In addition, the age of the units identified in the Maikop deposits in Lagich and Angekharan was clarified by analogy with the formations and horizons of similar ages in the Western and Central Ciscaucasia. The boundary of Eocene and Oligocene deposits in the Lagich section is assumed to be in the interval of 80–115 m. Late Eocene deposits occurring at the base of the section are apparently an analogue of the boundary deposits of the Beloglinskaya and Pshekhskaya formations of the Ciscaucasia. The presence of cold-water dinocysts of the species I.recurvus and calcareous nannofossils Coccolithus pelagicus in the boundary interval corresponds to the existing ideas about climate cooling at the end of the Eocene - beginning of the Oligocene. The formation of sediments in the upper intervals of the section probably occurred in more open marine conditions, relatively warm water, but with low levels of nutrients (nitrates). This interval of sediments with a characteristic phytoplankton complex can be correlated with the middle part of the Khadum horizon, an analogue of the Nizhny Morozkinsky subformation and, possibly, with the upper clayey part of the Polbinsky formation of Western and Central Ciscaucasia. The accumulation of the lower units of the studied part in the Angekharan, dated to the Early Oligocene, most likely occurred in a marine, shallower environment with possible short-term transgressive impulses. Overall, it is possible to suggest a proximal environment of high nutrient content in well-mixed waters within the inner neritic zone. An analogue of the deposits of this interval can be the Lower Morozkinsky Formation and the lower parts of the Upper Morozkinsky Formation of the Ciscaucasia. The significant increase in deflandroids in the upper part of the Angekharan section is similar to the changes noted during the transition to the zone of Deflandrea spinulosa f. Majkopica in the upper part of the Batalpashinskaya Formation of the late Oligocene in Ciscaucasia. The accumulation of sediments probably occurred in a marine basin with signs of stagnation in brackish, eutrophic environment, with a short-term supply of material from the photic zone. The conclusions about the peculiarities of changes in the depositional conditions in the study area are also confirmed by the results of geochemical studies. The deposits of the studied sections are represented by both terrestrial and aquatic types of organic matter (OM). Terrigenous OM was more typical in rocks from the Lagich and Agsu outcrops. The OM of rocks from the Lagich section varies from marginally mature to mature. In the Angekharan section the OM is thermally immature, as evidenced by the low Tmax values. Despite the small thickness of the Agsu outcrop, rock samples from this section contain kerogen of various stages of transformation from immature to mature. The carbon isotope composition (CIC) of Corg in sediments within the studied sections is in the δ13C range, characteristic of Oligocene sediments in the Paratethys. There is a higher enrichment of the 13C isotope in OM in sediments from the Agsu section.