Paleoredox reconstruction of bituminous shales from the Miocene Hançili Formation, Çankırı-Çorum Basin, Turkey: Evaluating the role of anoxia in accumulation of organic-rich shales

Abstract

The Early-Middle Miocene succession in the Çankırı-Çorum Basin (ÇÇB) represents a potentially large bituminous shale resource in Turkey. In order to elucidate paleoredox conditions during deposition of the Hançili Formation and to evaluate the role of anoxia in accumulation of the organic-rich shales, a total of 70 samples were collected using channel-profile sampling strategy. Rock Eval pyrolysis data indicates that the majority of the samples have excellent generative potential. Total organic carbon (TOC) content of the Hançili ranges from 0.61 to 13.91 wt% (5.59 wt% on average). Pyrolysis data as well as microscopic and isotopic studies of the samples indicate that the organic matter in the Hançili bituminous shales is immature to early mature, predominantly oil prone (Types I and II), and algal in origin. Moreover, the isotopic (δ13Corg) and geochemical studies suggest that the studied samples were deposited in a lacustrine setting which is consistent with the previous sedimentological inferences. Based on average enrichment factors (EF) of redox-sensitive elements, Ni (∼2.18), V (∼1.58), Cu (∼1.47) and Mo (∼1.1) are only slightly enriched relative to the average shale, whereas U (∼0.76), Pb (∼0.71) and Zn (∼0.73) are depleted. Such an enrichment pattern for redox sensitive trace elements reflects an environment with an oxic-dysoxic water column. Well-known redox-sensitive geochemical ratios (e.g., V/(V+Ni), Ni/Co) also show small-scale fluctuations in redox conditions during deposition of the Hançili Formation. TOC-S-Fe relationships suggest that the Hançili Formation were deposited under oxic-dysoxic water conditions. However, high TOC and HI along with low OI values of the samples suggest that the samples have not weathered under oxic-dysoxic regime. To investigate the role of anoxia in accumulation of the Hançili organic-rich shales, TOC content of 41 samples were plotted against the paleoredox ratios. Lack of correlation between the TOC content and redox indicators suggests that the redox conditions were not a controlling factor in accumulation of the organic-rich shales of the Hançili Formation. Other factors such as high sedimentation rate and periodic eutrophication (e.g., algal bloom) may have influenced accumulation of organic matter (OM) and compensated for the lack of preservative anoxic conditions.