Origin of flints in the Upper Cretaceous and Danian Chalk of England, Denmark and the North Sea: Evidence from their geological setting, mineralogy, geochemistry, paramagnetic properties and ultrastructure

Abstract

Two types of flints are recognized in the Upper Cretaceous Chalk of England, the North Sea and Denmark. Alpha-quartz/moganite flints are associated with the high quartz smectite-mica clay assemblage that probably derived its silica from mainly oceanic sources related possibly to the development of the Atlantic Ocean, and opal-CT flints that are associated with the low-quartz smectite-mica clay assemblage that derived its silica ultimately from the deep weathering of continental rocks on the Mid-European Island. Eight different groups of alpha-quartz/moganite flints are described based on mineralogy, colour, core/cortex definition, size, relationship to bedding, and location. The presence of authigenic paramagnetic and non-paramagnetic minerals demonstrate that certain of these groups of flints developed in the oxic and suboxic diagenetic zones although the majority formed in the anoxic zone. Their major, minor and trace element chemistry is discussed in relation to the timing of their development. A predictive schematic model is put forward linking flint development with diagenesis, burrow-type and the plumbing system of the Chalk. The origin of flint veins and sheets is discussed within the context of the known relationship between pore size and the degree of super saturation needed for cement precipitation.