Abstract: 

The notion of fine-grained pelagic carbonates as uniform, monotonous sequences of sediments settled in a quiescent environment has been challenged over the past few decades. Fine-grained pelagic carbonates can undergo substantial reworking after their first deposition, as illustrated by an abundance of sedimentary structures, such as drifts, moats, sediment waves, and channels documented in the Upper Cretaceous Chalk Group of NW Europe. Despite chalk being a major hydrocarbon reservoir rock of the North Sea, surprisingly little is known about the physical behavior of the pelagic carbonate sediment from which the chalk formed—calcareous nannofossil ooze. This poses a serious challenge to the understanding of the depositional system and the properties of facies distribution. Experimental tests, such as those performed in laboratory flumes, are necessary to provide empirical data on this subject. However, the use of modern calcareous nannofossil ooze as an analogue for Cretaceous ooze is associated with a number of disadvantages such as generally higher noncarbonate content and smaller coccolith size of modern oozes. Here, we document a preparation method for the production of calcareous nannofossil ooze for the purpose of physical experiments, based on disaggregation of Upper Cretaceous chalk through repeated freezing and thawing. We further document the textural characteristics of the ooze compared to the original chalk, based on quantitative backscatter scanning electron image analysis, laser diffraction granulometry, and smear slides. The Upper Cretaceous chalk chosen for disaggregation is highly friable due to delicate contact cement and has a low noncarbonate content (< 2 wt %), a high porosity, friability, and good nannofossil and microfossil preservation. These characteristics allowed an effective disaggregation of the chalk matrix into its basic nannofossil and microfossil components, which show good preservation through the disaggregation process. Textural analysis of chalk used for disaggregation and the produced ooze shows no significant differences between the two, thus validating the use of the freeze–thaw method for production of experimental ooze to model the basic depositional behavior of Cretaceous chalk.

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