The eastern Sakarya Zone is consensually known as a back-arc setting during the Late Cretaceous period. The region has records, which are in accordance with the environments unstable and carbonate sedimentation formed into different stages of subduction-related conditions. The Tonya Formation that outcropped in the area represents the uppermost part of the Mesozoic sequence in the eastern Sakarya Zone. The Tonya Formation is mainly composed of calciclastic turbidites, including thin grey-red pelagic limestone. The Tonya Formation was subdivided into the Sahinkaya Member basing on its sedimentological properties and fauna contents. The Sahinkaya Member is interpreted as a thick carbonate succession of the Maastrichtian in the study area. To decipher the facies and depositional environment of the Sahinkaya Member, two measured sections were studied in the Cayrba-Calkoy area in terms of microfacies analyses. Here, six microfacies types were distinguished based on their depositional texture, petrographic characteristics, and fauna content. Bio-lithoclastic rudstones, grainstones, and packstones are common texture in the carbonates. For the first time, the conglomerate levels have been defined within the member and correspond to an unconformity surface or hiatus together with angular differences in the layers, which have developed due to regional tectonic events such as erosion/uplift and magmatic intrusions. The facies characteristics of the carbonates and the fossil fauna findings included in the Sahinkaya Member of the Tonya Formation point to the development of a slope/toe of slope environment. All the sedimentological properties, combined with the regional data, suggest that the member was deposited at the shore of the back-arc Black Sea basin during the northward subduction of Neotethyan Oceanic Lithosphere and affected by the outcrop of the subduction-related magmatic products.