Paleoenvironment controls on organic matter accumulation in transitional shales from the eastern Ordos Basin,China

To investigate the paleoenvironmental controls on organic matter accumulation of Upper Paleozoic shales in the eastern Ordos Basin,China,26 shale samples were collected from two wells drilled into the Shanxi and Taiyuan Formations.The total organic carbon(TOC)content,mineral compositions and elemental geochemistry of the samples were analyzed.Quartz(35.42%)and clay minerals(48.34%)are the dominant minerals and trace elements(Li,Cs,Cu,V,Co,and Cr)are commonly enriched in the shale samples compared to the Upper Continental Crust.C-values(ranging from 0.2 to 4.5),chemical indices of weathering(CIW)values(48.82 to 99.11),and Sr/Cu ratios(1.00 to 11.05)suggest that the paleoclimate was humid in the study area during the Late Paleozoic.Elemental redox indices(e.g.,Al_(2)O_(3)/(Al_(2)O_(3)+Fe_(2)O_(3)),V/Cr,Ni/Co,V/(V+Ni)and U/Th)indicate a dysoxic to oxic paleoenvironment characterized by transi-tional sedimentary deposits in a continental margin setting.In addition,chemical index of alteration(CIA,ranging from 77.92%to 98.36%)and CIW(89.19%to 99.11%)values suggest that there was intense chemical weathering in the study area,while the Al_(2)O_(3)-CaO^(*)+Na_(2)O-K_(2)O(A-CN-K)ternary diagram demonstrates that the shales were not subjected to potassium metasomatism during diagen-esis.Al_(2)O_(3)/TiO_(2)and TiO_(2)/Zr ratios,as well as REE characteristics suggest a felsic source rocks and discount seawater as an REE source.Ce anomalies indicate an oxic environment with terrigenous input during black shale deposition,and LREE enrichment with negative Eu anomalies suggests that both shale formations were affected by detrital input rather than hydrothermal fluids.The correlation of TOC(ranging from 1.10%to 6.39%,with an average of 2.77%)with trace elemental redox indices(Sr/Cu,Sr/Ba,V/Cr,and U/Th)indicates that a warm-humid,dysoxic to oxic environment preserved much of the organic matter.