Carbonate–siliciclastic interactions and depositional evolution of the Carboniferous Trakya formation (NW Türkiye): implications for deep-marine facies along the Rheic Ocean margin

Abstract The Trakya Formation, a Carboniferous mixed carbonate–siliciclastic deep-marine succession in the western part of the Istanbul Zone (NW Türkiye), provides key insights into sedimentary processes along the southern margin of the Rheic Ocean. This study focuses on the Cebeci region and aims to reconstruct the depositional architecture and evolution of a mixed carbonate–siliciclastic deep-marine system by integrating geological mapping, measured stratigraphic sections, petrographic and microfacies analyses, and U–Pb zircon geochronology. Seven lithofacies (LF) have been identified: shale (LF-1), limestone (LF-2), chert-bearing shale (LF-3), thin-bedded sandstone–shale (LF-4), polygenic conglomerate (LF-5), glauconitic sandstone (LF-6), and medium- to thick-bedded sandstone–shale (LF-7). The limestone facies (LF-2; Cebeci Limestone Member) contains four distinct carbonate-dominated microfacies (MFS-1 to MFS-4), locally including minor siliciclastic components, namely bioclastic wackestone, foraminiferal packstone, bioclastic grainstone, and bioclastic–lithoclastic microbreccia. Petrographic and microfacies observations, supported by field-scale relationships, reveal episodic carbonate accumulation alternating with siliciclastic input, reflecting variable deep-marine depositional conditions. These characteristics offer new insights into the interactions between deep-water carbonate and siliciclastic sediments, as well as the mixed depositional processes along the Rheic Ocean margin during the Late Carboniferous. The glauconitic sandstone sample yielded a youngest concordant zircon age of ca. 335 ± 5 Ma, providing a Serpukhovian–Bashkirian maximum depositional age and constraining the timing of deep-marine sedimentation within the Trakya Formation. The integration of U–Pb zircon ages with facies architecture demonstrates that sediment supply, depositional processes, and spatial organization of the submarine fan evolved synchronously during the late Carboniferous, linking provenance signals with lobe and basin-plain depositional settings. Older zircon populations at 420–400 Ma, 550–500 Ma, and 700–900 Ma record sediment contributions from Paleozoic, Pan-African, and Neoproterozoic–Mesoproterozoic crustal sources. Collectively, the integrated sedimentological, petrographic, and geochronological data demonstrate that the Trakya Formation represents a mixed carbonate–siliciclastic deep-marine fan system developed along the southern margin of the Rheic Ocean under alternating low- and high-density turbidity currents.