Colour origin of Tortonian red mudstones within the Mersin area, southern Turkey

• This paper investigates reddening in mudstones.
• The reddening is defined by using XRD, ICP-AES, and SEM–EDX analyses.
• The reddening is caused by finely disseminated hematite crystals.
• The reddening is an early diagenetic origin process.
• The reddening in the floodplain deposits indicates a semi-arid climate.

Fluvial red mudstones of Tortonian age (overbank deposits) are widespread in the Mersin area in southern Turkey. The XRD analysis reveals that the mudstones consist predominantly of smectite, containing 3.0 to 6.6 wt.% Fe2O3, of which ≤ 1% is present as hematite. However this is evidently sufficient hematite to yield a red colour to the whole rock. SEM images show that very fine hematite crystals are disseminated in the mudstones as pore-filling cement between smectite flakes. After reddening, some of the clay and hematite were most likely leached and accumulated with smectite in the shrink–swell fractures as infill. Reddening in the mudstone took place in a terrestrial environment and the hematite pigment formed from intrastratal water by inorganic precipitation at the initial stage of diagenesis. The free Fe2 + was most likely released from the Fe-bearing minerals in an aqueous reducing environment, with hematite being precipitated as cement during the dry periods.