Climatic and tectonic influences as recorded in the mineralogy of a Miocene clay deposit at Şile, Türkiye

Abstract Miocene lacustrine clay deposits formed in the Şile region of Türkiye rest unconformably on Cretaceous andesite, basaltic andesite, basalts and rare dacites. Factors controlling the genesis of this sequence include: (1) sediment provenance, (2) tectonic uplift and climatic regimes during syn- and post-depositional times, (3) burial diagenesis and (4) changes from surface weathering alterations (i.e. oxidation and hydrolysis reactions) in the critical zone. Clay minerals are dominated by kaolinite and illite, with their relative abundances varying in relation to the proximity of coal seams, stratigraphic sequence and in an overlying sand-rich fluvial deposit. Variations in the mineral abundances reveals cyclothem-like sequences with patterns of fining upwards (i.e. increasing clay mineral abundance) capped by thin coal seams. The Clay Mineral Alteration Index values for the Şile clay sequence indicate a slight trend of decreasing chemical weathering intensity up-section, which is consistent with regional geological data for terrain uplifting and a drying climate during the Miocene. Critical zone processes have modified the mineral and chemical assemblages, as evidenced by the appearance of iron oxides putatively formed from recent oxidation of the ferrous minerals siderite and pyrite, which are not found in the upper sections. Taken together, the evidence indicates that the clay minerals are derived from a combination of prior weathering of basement rocks, diagenesis after deposition and modern critical-zone weathering. The degree of each process is dependent on depositional history, stratigraphic position and depth below the land surface, all under the changing influences of tectonic uplift and regional climate. The Şile deposits provide an economical clay resource that could be important to the ceramics industry of Türkiye.