The Katırınemeği and Asar fan delta complexes in the Manavgat Basin (southern Türkiye): facies architecture of small shoal-water deltas recording forced and normal regressions

Abstract

The study focuses on the late Serravallian–early Tortonian Katırınemeği fan delta complex in the west-central Manavgat Basin, SW Türkiye. It also includes data from the overlying late Tortonian–early Messinian Asar fan delta complex and outcrop sections lateral to these gravelly fan deltas. The two complexes were deposited above each other within the basin-fill succession. They were probably sourced by the same mountain canyon, whose fluvial system was temporarily deactivated by Tortonian major transgression. Each complex is a composite regressive-transgressive wedge comprising a forward-stepping set of forced-regressive basic wedges. They are coarsening-upward successions of each delta deposit indicating episodes of shallowing accompanied by shoreline progradation, overlain by an aggradational to a backstepping set of normal-regressive ones. Two additional forced-regressive basic wedges were deposited at the top of the Asar complex, with the last episode culminating in the dramatic fall of the regional sea level during mid-Messinian time. The forced-regressive basic wedges consist of shoreface facies covered by mouth-bar deposits and overlain by alluvium composed of multilateral, deeply incised, single-storey palaeochannels. The normal-regressive basic wedges consist of shoreface and foreshore/beach deposits overlain directly by alluvium. Marine processes entirely dominated the fan delta front during normal regressions. In the Katırınemeği case, only one forced regression (probably the strongest) and one later normal regression (probably the longest) were recorded by the coeval lateral shoreline. During the Serravallian sea-level highstand, the shoreline was perched on the high-relief basin margin. Hence, the trajectory of its subsequent shifts was steep, resulting in relatively little facies change in the offshore-transition zone. In the Asar case, all the regressive wedges are laterally correlative with a similar record of the contemporary non-deltaic shoreline. The shoreline’s higher sensitivity to sea-level changes was due to its separation from the high-relief basin margin by a wide strandplain accreted by waves and also alongshore sediment drift during the Tortonian highstand, which together resulted in a much greater horizontal component of the shoreline-shift trajectory. The difference in the facies architecture of basic fan delta wedges may help to distinguish between forced and normal regressions in the stratigraphic record. Recognising a set of forced-regressive basic wedges overlain by a set of normal-regressive ones may help identify the turning point of a lower-order regressive-transgressive sequence, which is not always identifiable in coeval non-deltaic succession. The correspondence or lack between the basic fan delta wedges and the coeval lateral nearshore record may reflect basin-margin configuration. © The Author(s), under exclusive licence to Springer Nature Switzerland AG 2024.