Sedimentology and depositional environment of the Late Eocene marine siliciclastic to evaporite transition in the Sivas Basin (Turkey)

Abstract The E-W oriented Sivas foreland basin in Turkey recorded a salinity crisis during the Late Eocene resulting in evaporite accumulations thick enough to trigger intense halokinesis during the Oligo-Miocene. The salinity crisis is studied thanks to three sedimentological sections crossing the transition from the last marine deposits (Bozbel Formation) to the overlying evaporitic facies (Tuzhisar Formation) preserved from halokinetic deformations. The top of the Bozbel Formation presents flood-generated hyperpycnites developed in pro-delta to delta front settings. In the central part of the basin, such facies become increasingly sediment-starved with azoic calcareous facies interlayered with organic-rich shales. Such facies are ultimately capped by thick accumulations of gypsiferous turbiditic lobe deposits. Southward, the foredeep was partly isolated from the central domain due to the propagation of an anticline. There, the basal siliciclastic turbidites become increasingly gypsum-rich and are capped by a 45 m-thick mass-transport deposit enclosing olistoliths of gypsum and of ophiolitic rocks. Such gravity collapse deposits evolve upward to the same gypsiferous turbiditic lobes observed northward. Both transitional facies record the progressive confinement of the basin from the sea, likely due to the northward propagation of the fold-and-thrust belt located farther south. The evaporites started to precipitate in piggy-back evaporitic basins, along the highs of the fold-and-thrust belt, before being reworked gravitationally in the foredeep to the north, producing the high to low density gypsum turbidites. Finally, from north to south, the reworked evaporites are extensively covered by a >100 m thick, chaotic, prismatic gypsum mass likely resulting from the hydration of anhydrite grains left as a residual phase after the leaching of a significant amount of halite. The latter formed in a hypersaline marine-fed basin and have lately allowed mini-basin salt tectonics during Oligo-Miocene times.