Refining Middle-Upper Ordovician Conodont Biostratigraphy in the Hatuke Creek Section, Western North China Craton: Evidence from Bedding Plane Assemblages and Implications for Shale Gas Exploration

The 128.6-m-thick, shale-dominated Klimoli, Wulalike and Lashizhong formations exposed at the Hatuke Creek section in the Zhuozishan area of Inner Monglia, North China, have been investigated for conodonts. Detailed stratigraphical collections of conodonts preserved on bedding planes from graptolitic shales, supplemented by additional discrete conodonts acid-leached from limestones, enable a refinement of the conodont biostratigraphic scheme at this section. Four successive conodont biozones, ranging in age from mid Darriwilian to late Sandbian (Stage slices Dw2-Sa2), are identified: the Dzikodus tablepointensis Biozone, the Eoplacognathus suecicus Biozone, the Pygodus serra Biozone, and the Pygodus anserinus Biozone, in ascending order. New sub-biozones, based on morphotypes of the biozonal index species, are proposed for the Pygodus serra and Pygodus anserinus biozones, providing alternatives when the traditional sub-biozones are unrecognizable. The biozonation is clearly correlated with the coeval Baltoscandian, South American and South China reference standard successions. The diverse preservation states of conodont bedding plane assemblages suggest that the alteration of conodonts in graptolitic shales represents a diagenetic process, challenging the prevailing hypothesis of pre-diagenetic dissolution. This study highlights the crucial role of previously overlooked conodont bedding plane assemblages in correlating Ordovician slope/basin facies shales, which holds great potential for marine shale gas exploration.