Deformed sedimentary sequences of Late Cretaceous-middle Eocene age located between the Sakarya continent and the Anatolian Complex have been interpreted as the fill of a forearc basin. The history and structure of this basin are critical for understanding the evolution of the Neo-Tethyan subduction in the Middle East. I test and elaborate upon this interpretation on the basis of three basic outcrop areas of Upper Cretaceous-lower Tertiary sedimentary sequences confined to a northeast-southwest-trending belt in north-west Ankara. These are the Alci, Kinik and Orhaniye areas. They expose two different sedimentary sequences called Alci and Orhaniye. In the Alci area, the sequence starts at the base with the continental red clastic rocks on the erosional surface of an underlying ophiolitic mélange and continues upward into the shallow-marine reefal buildup of the late Maastrichtian age. The upper half of the Alci sequence is separated from its lower half by a local disconformity and dominated by the continental debris-flow deposits composed mostly of volcanic material. By contrast, both the Kinik and Orhaniye areas are characterized by a thick, continuous and coarsening-upward sequence overlying conformably the underlying ophiolitic mélange. The lower half of the Orhaniye sequence is dominated by a deep-marine flysch, its middle part by continental clastic material, and the topmost part by the shallow-marine reefal buildup.
Lower contact relationships, lithologic composition, facies distribution, internal structures, and the regional tectonic settings of both sequences strongly suggest that the above-mentioned three outcrops preserve the remnants of a peripheral accretionary forearc basin forming the north-northwestern margin of the Haymana-Polath accretionary forearc basin during middle Campanian-middle Eocene time. This peripheral basin is here named "Orhaniye accretionary forearc basin."