Upper Cretaceous foreland flysch deposits from the Neotethyan Intra-Pontide Ocean: geological and palaeontological evidence from the Elmadağ Olistostrome of Ankara, central Türkiye

OpenAlex İngilizce

The Ankara region of Türkiye is located on the Sakarya Continent, bounded by two Neo-Tethyan oceans (Intra-Pontide to the north and Izmir-Ankara-Erzincan to the south). In this region, the oldest lithologies of the Sakarya Continent are the Middle Triassic Dikmen Greywacke. The lower part of the Dikmen Greywacke comprises low-grade metamorphics, while its upper part is primarily represented by non-metamorphic greywacke with rare carbonates/chert interlayers and a basic volcanic breccia at the top. The Dikmen Greywacke is overlain unconformably by the Coniacian Elmadağ Olistostrome, composed of blocks of varying sizes (pebble to mega-block of 2-3km width) within a calcareous clastic to clayey carbonate matrix. Based on the benthonic Foraminifera assemblages, while detrital limestone provides a Kungurian to Changhsingian (late Early to latest Permian) age, platform carbonates yield latest Capitanian-late Changhsingian (latest Middle to latest Permian) ages. Radiolarian assemblages reveal a Changhsingian (latest Permian) age from a radiolarian chert, middle Late Ladinian (Middle Triassic) from a spiculite/radiolarian chert, latest Bajocian-early Bathonian (Middle Jurassic) from a radiolarian chert, early Hauterivian (Early Cretaceous) from a pelagic limestone, and late Hauterivian (Early Cretaceous) from a radiolarian chert. Combined geological and palaeontological evidence suggest that the Elmadağ Olistostrome has been formed in a foreland basin in front of the southwardly advancing nappes originating from the Intra-Pontide domain. The determined ages imply that the Intra-Pontide basin was rifted after the Artinskian (Early Permian), possibly during late Early Permian to latest Permian. With the gradual deepening of this basin, the first pelagic rock unit occurred in late Changhsingian (latest Permian), where platform carbonates were also present at the rim of the basin in this time interval. Due to the continuous deepening in the basin, pelagic rock units were deposited from middle Late Ladinian (Middle Triassic) to late Hauterivian (Early Cretaceous). By correlation to the other olistostrome occurrence to the south of the Ankara region, it can be suggested that the Elmadağ Olistostrome formed in the Ankara region during Coniacian (Late Cretaceous) owing to the closure of the Intra-Pontide Ocean.