Orbital forcing in a ‘Boreal’ Cretaceous epeiric sea: high-resolution analysis of core and logging data (Upper Albian of the Kirchrode I drill core — Lower Saxony basin, NW Germany)

Abstract

A 245 m (0–245 m subsurface) core of Upper Albian marine sediments from North Germany (research well Kirchrode I) was studied to identify sequences and cycles of sedimentation and related forcing mechanisms on sedimentation in an epeiric sea of the (sub)-‘Boreal’ realm. The core consists of monotonous gray marls with an exceptionally high sedimentation rate of 8–12 cm ka −1. In total, it represents a period of approximately 2.5–4 Ma. A silty glauconite-rich condensed layer marks a change in the succession at 132 m and is paralleled by the first occurrence of E. turriseiffelii and G. bentonensis. The lower part of the core shows lenticular bedding, siderite nodules, average CaCO 3 content of ca. 40% and an abundance of inoceramids ( Inoceramus anglicus, Birostrina lissa, Actinoceramus sulcatus). The upper part is characterised by plane bedding, common siderite layers, an average CaCO 3 content of ca. 35% and an abundance of aucellinas ( Aucellina gryphaeoides). According to lithology and ichnofacies, the succession represents a transgressive–regressive cycle with distinct bimodality of lithology, fossil, and trace fossil content together with reworking and redeposition at the base and top. We studied well-log-data, sediment composition, sediment color, and carbonate content and applied various mathematical analysis techniques to these data (principal component analysis, spectral analysis (Fourier-transform), autocorrelation). Spectral analysis revealed well-pronounced periodic cyclicity in the upper part of the Upper Albian (12, 8, 4.5, 3.4, 1.7 m). The 12 m-eccentricity cycles are the most intense cycles reflecting periodic changes in the terrigenous clastic supply. Chaotic transitional intervals occur around the Middle–Upper Albian, Upper Albian and topmost Albian parts of the core. They are characterised by non-cyclic changes in litho- and biofacies. Mathematically, these changes correspond to singularity points. They are preceded by non-orbital high-frequency variations in carbonate content and in γ-ray.