The Turonian–Coniacian Boundary in Western Georgia Based on Planktonic Foraminifera

Abstract

Upper Cretaceous sediments are widespread in western Georgia (the Gagra–Java zone). Upper Turonian–lower Coniacian deposits in the western part of the Abkhazia–Racha facies type (the basins of the Chanis–tskali and Ochkhomuri rivers) are represented mainly by firm carbonate rocks, in which the Gumurishsk suite is exposed. Within the Odishi–Okriba facies type, terrigenous–volcanic rocks are significant, and were formed within the general background of carbonate sedimentation. The layers are enriched with a tuffaceous material, and are found with white and red calcareous rocks of the Mtavari Suite (K₂t₃–K₂st). This suite is composed of brick-red, brown, or yellowish-grey basalts and porphyritic effusive rocks, with interlayers of calcareous deposits and sandstones. In most previous studies of the stratigraphy of the Abkhazia–Racha and Odishi–Okriba facies types, the focus was placed on macrofauna. On the basis of the present detailed examination of planktonic foraminifers in the Abkhazia–Racha and Odishi–Okriba facies types, the Marginotruncana coronata Zone is defined at the beginning of the Coniacian. The appearance of the species Marginotruncana coronata identifies the lower boundary of the Coniacian.