Refining the Jurassic-Cretaceous boundary: Re-Os geochronology and depositional environment of Upper Jurassic shales from the Norwegian Sea

Abstract

Limited chronological information for Upper Jurassic rocks hampers precise age assignment of the Jurassic-Cretaceous (J/K) boundary. This study presents Re-Os geochronology, Rock-Eval pyrolysis and geochemical data for Upper Jurassic rocks in the Nordland VII area, offshore northern Norway. Four intervals of organic-rich shale were investigated, two from the Krill and one from the Alge Member of the Hekkingen Formation and one from the underlying Måsnykan Formation (drilled cores 6814/04/U-01 and 6814/04/U-02). Rock-Eval pyrolysis data indicate immature organic matter with mixed (Type II/III) oil-gas potential in these units. Among the four intervals, Re-Os isochroneity prevails only for the Hekkingen Formation samples; data from the shale interval in the otherwise sand-rich Måsnykan Formation, deposited in a shoreface-to-foreshore marine environment, are not isochronous. Non-isochroneity of the samples from relatively proximal setting is attributable to non-conservative elemental (desorption of Os from sediments to dissolved phase) and isotopic exchange of Os at this depositional site. The Re-Os isochron age for the Alge Member (153.2 ± 7.3 Ma (Model 3); MSWD = 29; n = 9) is consistent with its biostratigraphic Kimmeridgian age (152.1–157.3 Ma). A Re-Os isochron for the lower part of the Krill Member yields an age of 144.5 ± 1.4 Ma (Model 3; MSWD = 3.7; n = 10) with an initial 187Os/188Os ratio of 0.552 ± 0.007. Given the interval's lower Tithonian biostratigraphic age, the Re-Os age suggests that the J/K boundary must be a few Myr younger than its currently assigned age of ~145.0 Ma (2017/02 time scale; www.stratigraphy.org). A Re-Os model age for shales from the upper part of the Krill Member places an upper age limit for the J/K boundary at 142 (±2) Ma. Hence, the J/K boundary is between 142 Ma and 144.5 Ma. Fe and S concentrations for the Hekkingen Formation show strong correlation and the S/Fe slope (~1.19) mimics the stoichiometric S/Fe ratio for pyrite (~1.15). A Fe-S-TOC ternary diagram for these samples points to an iron-limiting, sulphidic depositional environment. Anoxic to sulphidic depositional conditions for the Hekkingen shales are also evident from relatively higher authigenic enrichment of Mo compared to U. Unlike the Hekkingen Formation, shales from the more sandy Måsnykan Formation have lower S/Fe ratios and lower enrichment factors for redox-sensitive elements, indicating relatively oxygenated marine conditions during deposition.