Composition and Formation Conditions of Lower Eocene Shallow-Marine Carbonates in Southern Armenia

Abstract

Nummulite facies are known to be a good indicator of Eocene shallow-water paleoenvironments. Nummulitic limestones are widespread in the Lower Eocene of Armenia—the Tethys/northern Peri-Tethys transition zone, but they are studied insufficiently so far. The objective of this work was an integrated study of Lower Eocene deposits in the typical Urtsadzor section (Ararat region, Armenia): detailization of the stratigraphic subdivision based on larger benthic foraminifers (LBF); restoration of sedimentation settings based on the facies analysis and tectonic conditions of the basin origin, and evolution in the Early–Middle Eocene by studying the rock fracture system and formational analysis. For the first time, the Ypresian shallow benthic zones SBZ 9-10 and SBZ 10-11 of the Tethyan Chronostratigraphic Scale were identified in Kotutz and Sevan formations—an important contribution to the development of a modern stratigraphic chart of Armenia. Restoration of depositional settings based on the microfacies analysis was accomplished for Lower Eocene carbonate rocks of Armenia for the first time. Nine microfacies were determined and interpreted in the Ypresian part of the Urtsadzor section. Evolution of paleobiocoenoses represented mainly by nummulites, orthophragmines, and red algae was traced through the section. A transgressive succession of microfacies types was revealed from the littoral to lower–middle parts of the middle ramp. Comparison of the section with the adjacent Shagap and Landzhar sections allowed us to estimate the facies variability of rocks and the main direction of transgressional expansion. Ypresian LBF in paleobiocoenoses of southern Armenia are similar to the Peri-Tethyan varieties at the generic and specific level, and distinguished from the Tethyan biocoenoses by the absence of porcelaneous foraminifers (alveolinids and soritids). The South Armenian Ypresian paleobiota is distinguished from the assemblages in northern Armenia and northern Peri-Tethys by the abundance of red algae remnants. Wide Late Ypresian transgression resulted in the proliferation of LBF in the shallow oligotrophic basins, and their hydrological differences provoked the diversification of biocoenoses. The study of jointing systems in nummulitic limestones healed with aluminous products of karstification demonstrated that the jointing was formed at the Early/Middle Eocene transition due to the northeastern to sublatitudinal tension during uplift of the southern rift shoulder of the emerging Middle Eocene Shirak–Sevan–Megri rift. Rifting was preceded by the Early Eocene attenuation of tectonic activity and expansion of a wide carbonate sedimentation basin with development features typical of pre-rift basins.