Abstract

The paper presents magnetobiostratigraphical data on the Cretaceous sediments stripped by well 8 in the southern side of the Om’ basin within the Om’–Lar’yak facies zone (southern West Siberia). The biostratigraphical data show that the sediments under study formed in the Albian–Maastrichtian. The componential analysis of natural remanent magnetization, based on thermal demagnetization and demagnetization with an alternating magnetic field, revealed the characteristic magnetization component. This confirms the paleomagnetic data used to compile the Cretaceous paleomagnetic section of the well. A paleomagnetic description was obtained, and a magnetobiostratigraphic key section of the Cretaceous sediments of the well was compiled on the basis of comprehensive data. It comprises five Upper Cretaceous regional horizons and same-named formations (Pokur, Kuznetsovo, Ipatovo, Slavgorod, Gan’kino), which have not been studied paleomagnetically at all in West Siberia. The magnetobiostratigraphic section comprises Albian–Maastrichtian stratigraphic units (43.5 Myr) and consists of three magnetozones. For example, the Pokur, Kuznetsovo, and Ipatovo Formations (total thickness 210 m), which show normal polarity with small reversed-magnetization horizons, form one long normal-polarity zone, N(al-st). The Slavgorod and Gan’kino Formations (total thickness 75 m), separated by a sedimentation gap, form two reversed-polarity magnetozones, R1(km) and R2(mt).

Reference datums (paleontologically well-constrained magnetozones) were used to correlate the magnetobiostratigraphic section with the common magnetostratigraphic and magnetochronologic scales. The long normal-polarity magnetozone N(al-st), spanning the Albian, Cenomanian, Turonian, Coniacian, and Santonian, matches the Dzhalal hyperzone and chron C34 (∼ 112–83.6 Ma). The reversed-polarity zones, spanning most of the Campanian (R1(km) (Slavgorod Formation)) and Maastrichtian (R2(mt) (Gan’kino Formation)), match chrons C33(r) and C31(r) in the absolute chronology (∼ 83.6–80 and 71–68.5 Ma, respectively) with a gap two chrons long (C33(n), C32).

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