Laminated–bioturbated cycles in Maastrichtian chalk of the North Sea: oxygenation fluctuations within the Milankovitch frequency band

Abstract

The Maastrichtian chalk of the southern Central Graben, Danish North Sea, is a homogeneous pure white coccolithic chalk mudstone deposited in a deep epeiric shelf sea, which covered large parts of northern Europe. The sediment displays a pronounced cyclicity marked by decimetre‐thick bioturbated beds alternating with slightly thinner non‐bioturbated, mainly laminated beds. The laminated half‐cycles consist of alternating millimetre‐thick, graded, high‐porosity laminae and non‐graded, low‐porosity laminae. The cyclicity has been interpreted previously as caused by periods of slow background sedimentation and bioturbation interrupted by periods of rapid deposition of laminated beds, with the latter reflecting random and local resedimentation processes. Based on textural and structural analysis, the millimetre‐scale, non‐graded laminae are interpreted as having been deposited directly from pelagic rain of pelleted coccoliths representing the primary production. The graded laminae were deposited from small‐volume, low‐density turbidity currents and suspension clouds. The sedimentation rates of the cyclical chalk are similar to those known elsewhere, and the lamination is interpreted as having been preserved from destruction through bioturbation by anoxic conditions at the seafloor. Bioturbated–laminated cycles are thus formed by slow sedimentation during alternating seafloor redox conditions probably on a Milankovitch scale. A direct implication of this interpretation is that the cycles are areally widespread, probably extending throughout the southern Central Graben area and may be useful for correlation and high‐resolution cyclostratigraphy in the chalk fields of the Danish North sea. If the laminated half‐cycles represent a few rapid resedimentation events, with a high sedimentation rate as suggested by most workers, then the sediment would not be truly cyclic, but would represent event sedimentation within a pelagic background represented by the bioturbated beds. In this case, the cycles would have very limited potential for correlation.

Environmental change prior to the K–T boundary inferred from temporal variation in the morphology of cheilostome bryozoans

“…Anoxic events have been inferred to occur on a global scale prior to the boundary as a result of environmental perturbations from Deccan volcanism (see Glasby and Kunzendorf, 1996), and indeed the boundary clay in the Danish Basin has been suggested to represent an anoxic event (Kajiwara and Kaiho, 1992;Hart et al, 2005). What's more, alternating bioturbated and laminated chalks in other parts of the Danish basin imply the regular occurrence of dysoxic events during the Maastrichtian (Damholt and Surlyk, 2004), although no laminations were observed at Nye Kløv.…”

High-frequency sea-level changes recorded in deep-water carbonates of the Upper Cretaceous Dol Formation (island of Brač, Croatia)

“…However, in other deep-water sediments, the presence of high-frequency sea-level changes responsible for a cyclic sedimentary signature with periodicities consistent with orbital (so-called Milankovitch) forcing has been demonstrated in many studies (e.g. Savrda & Bottjer 1986, 1989Masetti et al 1991;Erba & Silva 1994;Damholt & Surlyk 2004;Heard et al 2008). In many shallow-water carbonate platform settings, the cyclically stacked building blocks of 3rd-order sequences can equally be attributed to orbital forcing of sea-level changes.…”

How depositional texture and diagenesis control petrophysical and elastic properties of samples from five North Sea chalk fields

“…Scholle et al (1998) They interpreted winnowing processes to have caused the lamination together with periods of rapid deposition as evidenced by the presence of escape burrows. Damholt & Surlyk (2004) interpreted the biogenic structures as central parts of Zoophycos burrows rather than escape burrows. They interpreted the apparent lamination as resulting from low-density turbidity currents, or pelagic rain of pelleted coccoliths under anoxic conditions so as to preserve lamination.…”

“…The depth scale is in km below mean sea-level (msl). Andersen (1995), Kristensen et al (1995), Scholle et al (1998), Damholt & Surlyk (2004). b Doyle & Conlin (1990).…”