Depositional environment of a Kimmeridgian carbonate 'black band' (Akkuyu Formation, south-western Turkey)

The Upper Jurassic Akkuyu Formation of south-western Turkey consists of pale-coloured carbonate facies, but includes a peculiar 'black band' at its base. In order to characterize the depositional environment of this level, a sedimentological, organic and inorganic geochemical study was performed on 30 samples. Three units are distinguished. Unit 1 consists of black marlstones, very rich in organic carbon (22-30%) with HI (hydrogen index) values around 600. The organic matter is an amorphous homogeneous material even at nanoscopic scale. This unit is also characterized by a very high concentration of V ( >1000 p.p.m.) and a delta(13)C(org) value around -26.5 parts per thousand. Unit 2 shows thin-bedded black limestones with chert layers. Its organic content varies from 2% to 6%. As in unit 1, the HI values are uniformly high (600), and gas chromatography shows a high quantity of hopanes. The V concentration is also high (approximate to 400 p.p.m.). The unit has a high Sr/Ca ratio, an organic carbon isotopic ratio close to -28.7 parts per thousand, and diffuse, nanoscopically amorphous organic material that is closely associated with pyrite. Unit 3 corresponds to the white limestones overlying the 'black band'. This calcitic facies is almost devoid of organic carbon (<0.1%). The delta(13)C(org) values are similar to those of unit 1 (-25.5 parts per thousand to -26 parts per thousand). Deposition occurred in a basinal environment surrounded by a carbonate platform. Primary productivity was high but not exceptional; terrestrial organic inputs were scarce. Bottom waters were probably euxinic during the deposition of unit 1. The presence of bath hopane-rich and sulphur-rich nanoscopically amorphous organic matter may be the result of strong sulphate-reducing bacterial activity. Coupled with low amounts of iron, this led to intense vulcanization of the organic matter and thus good preservation. With the onset of unit 2 deposition, environmental conditions became less extreme, perhaps because of cooler water temperatures. Finally, the depositional conditions became oxic, leading to the accumulation of pale-coloured organic-poor limestones of unit 3.