The genesis of flint nodule layers

Abstract

The upper 40 m of the Maastrichtian Gulpen Formation (Maastricht, The Netherlands) is characterised by a regular succession of approximately 75, thickening upwards, laterally continuous flint nodule layers. Flint nodule layers formed when detrital skeletal opal dissolved during late diagenesis and concentrated at sites of relatively high early diagenetic authigenic silica polymorph concentration. Early diagenetic authigenic silica polymorphs precipitated in the anoxic redox zones of bacterial sulphate and/or carbondioxide reduction, that was situated below and parallel to the sediment surface. Highest concentrations of authigenic silica were reached during periods of slowest deposition when sediment resided for a relatively long period in the anoxic redox zone. The rhythmic vertical variation of the flint nodule concentration has been measured with automatic image analysis and reflects the influence of the periodic variation of the Earth's orbital parameters (precession index) on climate, oceanography and periodically varying deposition rates. The relation between the periodic variations of the depositional and early diagenetic conditions and the rhythmically bedded flint nodule layers is simulated with a numerical model. It is shown how the rhythmic variation of authigenic mineral concentrations in Chalk can be used to reconstruct the variation of the dynamics of the shallow marine subtropical Chalk sea.