The Göktaş fan delta complex in Manavgat Basin, South Türkiye: a model for stratigraphic development of coarse-clastic littoral wedges and spatial-facies prediction

Abstract

The Göktaş fan delta complex is a regressive-transgressive wedge deposited in the late Serravallian to late Tortonian as a wave-dominated, dip-slope coastal system associated with the antithetic hangingwall faultline of the Manavgat Basin (South Türkiye), whose footwall was submerged. The gravelly fan delta was sourced from a pre-existing valley in the basin’s mountainous northern hinterland. The composite wedge, more than 230 m thick, consists of a series of regressive-transgressive basic wedges stacked upon one another and extending up to 18 km from the basin margin. A forward-stepping set of forced-regressive wedges is overlain by an aggradational to a backstepping set of normal-regressive wedges, split by a forced-regressive one in the middle. The fan delta complex has a timespan of ca. 4 Ma. It is considered to have formed in the transitional, fall-rise turnaround phase of two successive 3rd-order eustatic cycles, punctuated by rapid tectonic subsidence and fan delta drowning (4th-order maximum-flooding surfaces separating the basic wedges). Tectonics was probably responsible also for sporadic, brief events of 5th-order marine flooding, whereas minor shoreline shifts of 6th-order were likely due to climatic fluctuations and autocyclic lateral switching of fluvial activity. Based on the fan delta complexes in the Manavgat Basin and adjoining Köprü Basin, a model is suggested for the depositional architecture and facies anatomy of wave-dominated, shoal-water fan deltaic successions, with emphasis on the sedimentation processes and response to relative sea-level changes. In the hierarchical organisation of the fan delta complex, the normal-regressive basic wedges consist of highstand (HST) and transgressive systems tract (TST). In contrast, the forced-regressive wedges comprise highstand (HST), falling-stage (FSST), lowstand (LST), and transgressive systems tract (TST). The systems tracts differ in geometry and spatial partitioning of facies. The HST has a moderately thick, short-radius alluvium comprising moderately deep palaeochannels and extending basinwards over progradational mouth-bar facies, underlain and passing into a narrow belt of wave-worked shoreface facies grading into tempestitic offshore-transition deposits. The FSST has a thin and poorly preserved alluvium, including basal deposits of overdeepened bypass channels and incised valleys, that passes basinwards into progradational delta-front deposits developed as either mouth-bar facies underlain by shoreface deposits (possibly with offshoot turbiditic channels and lobes in offshore-transition zone) or a large foreset of avalanching strandplain deposits overlying offshore-transition facies. The LST has a moderately thin, basinward-thickening alluvium with mainly shallow palaeochannels, which overlies mouth-bar facies and passes basinwards into an aggradational delta-front succession of alternating beach and shoreface facies, the latter far extended as a passage to offshore-transition deposits. The TST has the thickest alluvium, comprising shallow to moderately deep palaeochannels and thinning basinwards rapidly, truncated by a landward-rising transgressive ravinement; the ravinement surface is overlain by a blanket of alternating upper/lower shoreface facies, passing landwards into beach facies and seawards into offshore-transition deposits. The model may serve to predict facies distribution in fan deltaic littoral wedges and assess reservoir quality in petroleum exploration. © The Author(s), under exclusive licence to Springer Nature Switzerland AG 2024.