Characterization of Upper Jurassic organic-matter-rich carbonates in the Gissar Mountains (Tubiegatan section, Uzbekistan)

International audienceNew investigations are currently underway on the Middle-Upper Jurassic carbonates of the Gissar Mountains in the Amu-Darya Basin. Located in the southwest of the Gissar Mountains (SE Uzbekistan), the Tubiegatan section corresponds to 30-meter-thick limestone, overlain by 10-meter-thick dark limestone and marl interval. These dark, “black shale-type deposits” were also recognized in cores from the subsurface of the Amu-Darya Basin. The organic-rich deposits interval is precisely dated to the Luciaeformis and Schilli Subzones (Transversarium Zone, Middle Oxfordian) by ammonites. A transdisciplinary approach including sedimentology, organic matter characterization (Rock-Eval pyrolysis, palynofacies), bulk and clay mineralogy (XRD), carbon and oxygen isotopes of carbonate and organic fractions and whole rock trace-element geochemistry were used to elucidate the origin of these organic-rich deposits. Carbon-isotope curves were also established to help to correlate this interval with the partly contemporaneous carbonate platform. Both Tmax from Rock Eval analysis and clay mineral assemblages indicate a pronounced burial diagenesis (oil window).Despite this burial, paleoenvironmental and paleoclimatic can still be processed. Rock-Eval pyrolysis analyses performed on 65 samples show that TOC values range from 0 (peloidal grainstone) to 6 % on average for the thinly laminated black limestone. Interestingly, in this dark organic-rich deposits interval, higher TOC were measured in limestone (2% on average), while lower TOC values (0.5% on average) characterized marl and claystone, which is relatively unusual. Rock-Eval parameters (IH, IO) coupled to palynofacies observations (predominant amorphous organic matter) point to a probable marine, algal/microbial origin for the organic matter (Type II), with rare continental inputs mostly located in the upper part. Trace elements, as well as palynofacies and sedimentological observations demonstrate local O2-depleted conditions promoting the preservation of organic matter. Such paleoenvironmental conditions may have supported bacterial mineralization processes related to anaerobic oxidation of methane, which would explain the very low δ13C values (-5‰ to -12‰) measured in laminated facies. From the base of the organic-rich interval, a progressive decrease then disappearance of kaolinite, balanced by illite and illite/smectite mixed-layers (type R1), is observed. This coincides with the development of terrestrial phytoclasts inputs, suggesting a probable aridification in the Amu Darya Basin during the Middle/Late Oxfordian. Overall, the organic-rich deposits of Tubiegatan could record the onset of the disconnection of the Amu-Darya Basin from the open sea in the south, induced by compression and uplifts in the Afghan and Central Iranian blocks.This disconnection, combined with a Tethys-wide aridification could have led to the progressive increase of evaporation and subsequent deposits on the carbonate platform, followed by the massive uppermost Jurassic evaporites (salt, anhydrite) of the Gaurdak Fm