The coeval development of conglomeratic, shoal–water and Gilbert-type deltas in the post-orogenic extensional Çardak Basin, SW Türkiye: implications for accommodation and sediment supply

This study focuses on the sedimentary facies and depositional architecture of the coarse-grained shoal–water delta complex and Gilbert-type delta deposits of the Çardak Basin in southwest Türkiye. The coexistence of these deltaic deposits, which developed on the opposite margins of the basin, provides an excellent opportunity to reconstruct the palaeogeographic evolution of the Çardak Basin during the Oligocene. As the sedimentary facies and palaeogeographical changes are generally controlled by the relationship between accommodation and sediment supply, the roles of syndepositional tectonics, basin dynamics, and hinterland morphology in developing different delta types are assessed. The orogen collapse of the Lycian Nappes led to the opening of the Çardak molasse basin in the Lutetian, and the shallow marine sedimentation developed in the basin from the Lutetian into the Late Chattian. A Gilbert-type delta up to 150 m thick was deposited on the western margin, and a shoal–water delta complex consisting of individual delta packages 5–20 m thick was deposited on the eastern margin. The shoal–water delta complex formed a sequence up to 300 m thick and 10 km wide within the basin. The Oligocene gravelly deltaic sediments deposited on the Eocene shoreface deposits indicate an abrupt deepening of the basin margins and an abundant supply of clastic sediments. The abrupt changes in sedimentary facies were caused by normal faults that increased the basin’s accommodation space and uplifted the surrounding hinterland, resulting in an abundant sediment supply. From the thickness of the shoal water and Gilbert-type delta deposits, we infer that the subsidence rate of the basin varied between the western and eastern margins. While the western margin subsided rapidly, the east margin was subjected to a slower but more sustained subsidence rate. The coarse-grained deltaic sediments, the multistorey fluvial distributary channel fills and the stacking of the individual deltas upon one another and with lateral offset indicate that the active channels of the stream-dominated alluvial fan fed the shoal–water delta complex.