Cyclicity in the Middle Eocene Çayraz Carbonate Formation, Haymana Basin, Central Anatolia, Turkey

Abstract

The Haymana Basin in central Anatolia, Turkey, developed on a fore-arc accretionary wedge during the Late Cretaceous and the Middle Eocene. Turbidite deposits fill the center of the basin and grade northwestward into shallow marine clastics and carbonates, which in turn pass into lacustrine and fluvial deposits toward the margin. The Middle Eocene Çayraz Formation represents the youngest paleotectonic unit of the Haymana Basin. It is composed of two shelf systems (SS), each formed by a package of nummulitic banks and intervening calcareous mudstones, 150–250 m thick, overlying 70–100 m thick monotonous mudstones. Within each of the two shelf systems, two orders of coarsening-upward and shallowing-upward sequences are recognized. The smaller order sequences, 3–10 m thick, are termed basic units (BU). Four types of basic units have been distinguished. These are made up of either calcareous mudstone overlain by a nummulitic bank, or silty mudstone overlain by sandy to conglomeratic beds. Each basic unit represents a specific depositional environment ranging from quiet lagoon or open marine to more energetic bank or sandy bar. Basic units are vertically associated to form three types of 15—35 m thick basic sequences (BS), which represent the time and space evolution of the shelf systems. Owing to their small scale, their lateral association, and the condensed nature of their bounding surfaces, basic units are considered to be allocyclic transgressive-regressive cycles tuned to Milankovitch bands. At the scale of basic sequences (BS) and shelf systems (SS), the lateral and vertical distribution of the sequences, and the presence of onlap structures strongly suggest sea-level changes. The sequential organization, thicknesses of basic units and basic sequences, and an estimated sedimentation rate favour sea-level changes controlled by Milankovitch climatic cycles. Amalgamation or condensation of basic units in basic sequences, and the existence of locally accentuated onlaps suggest a coupled effect of discontinuous subsidence.