Radiolarian and Planktonic Foraminiferal Biochronology of the Soğukçam Limestone Group, Elmadağ Olistostrome, and Unaz Formation (ankara Region, Central Türkiye): Insights Into the Cretaceous Evolution of the Sakarya Continent and Overlying Units

Abstract

The Ankara region (central T & uuml;rkiye) comprises a part of the Sakarya Terrane and the accretionary remnants inherited from the Izmir-Ankara-Erzincan (IAE) branch of the Northern Neotethys. The Sakarya Terrane is characterized by a pre-Jurassic basement overlain by Jurassic-Cretaceous sedimentary assemblages, collectively known as the Sakarya Continent Cover. In this study, we aim to elucidate the Jurassic-Cretaceous evolution of the Sakarya Terrane through detailed geological mapping in the regions of Haymana, Yakac & imath;k, north of Alag & ouml;z and west of Memlik, located to the west and southwest of Ankara city, central T & uuml;rkiye. By analyzing radiolarian and planktonic foraminiferal assemblages, we provide precise dating for three key lithological units: the So & gbreve;uk & ccedil;am Limestone Group (part of the Sakarya Continent Cover), the Elmada & gbreve; Olistostrome, and the Unaz Formation (from the overlying units). The oldest rock unit exposed in the Haymana region, south of Ankara city, is the Bilecik Limestone Group, consisting of Tithonian to lower Berriasian platform carbonates. A drowning unconformity separates the Bilecik Limestone Group from the overlying middle Berriasian-uppermost Albian So & gbreve;uk & ccedil;am Limestone Group, which has been elevated to "group" status in this study. The So & gbreve;uk & ccedil;am Limestone Group is subdivided into two formations-the Seyran Formation and the Akkaya Formation- separated by a disconformity surface. The Seyran Formation, at the base, consists of middle Berriasian limestone breccia in a micritic pelagic matrix, and upper Berriasian-lower Aptian micritic clayey, cherty pelagic limestones with abundant and diverse radiolarians in its upper part. The disconformity between the Seyran and Akkaya formations represents a small depositional gap corresponding to the middle to late Aptian. The lower part of the Akkaya Formation contains lower to middle Albian debris flow deposits while the upper part is composed of clayey pelagic limestones rich in glauconite and planktonic foraminifers, dating to the latest Albian. Overall, the Akkaya Formation is roughly assigned to the Albian stage.