Fluvial to estuarine facies architecture of the Late Serravallian–Early Tortonian incised valley fill in the Adana Basin, Türkiye: depositional evolution in response to sea-level change

This study describes the sedimentary facies associations of the incised valley fill deposits, which allowed the reconstruction of the Late Serravallian–Early Tortonian palaeogeographic evolution of the Adana Basin, one of the largest peri-Mediterranean basins in southern Türkiye. The incised valley fill disconformably overlies the offshore mudstones and comprises fluvial and tide-dominated estuarine facies associations from base to top. The fluvial deposits comprise intercalated channel fill and overbank deposits. The laterally accreted point bars of the meandering river channel deposits consist of conglomerates and sandstones, while the overbank deposits form sheet-like beds of sandstones, siltstones and mudstones. These are overlain by fine- to coarse-grained marine sandstones rich in oyster fossils at the boundary. The sandstones show planar and trough cross-stratification, sigmoidal beds and herringbone cross-stratifications bounded by reactivation surfaces. These bi-directional sand bars, up to 150 cm in height, indicate tidal-bar deposits of the estuarine facies association. The biostratigraphic dating of the incised valley fill deposits suggests an age of approximately 11.78–11.19 million years, indicating the MMi8-MMi9 zone, based on planktonic foraminifera from underlying and overlying marine sediments. The deposits developed in response to the Late Serravallian relative sea-level fall that partially exposed the Adana Basin, indicating a regional forced regression and unconformity. The sediment supply led to the deposition of the thick fluvial deposits at the base of the incised valley due to the low relative sea-level rise rate. Progressive sea-level rise drowned the incised valley, transforming it into a marine embayment where tidal bars were deposited in a tide-dominated estuarine environment. This study shows that even in a microtidal setting like the Neogene Mediterranean, a tide-dominated estuarine environment can develop in response to the confinement and consequent amplification of the tidal currents in a funnel-shaped, narrow and shallow incised valley.