Early diagenetic silica precipitation, in relation to redox boundaries and bacterial metabolism, in late cretaceous chalk of the Maastrichtian type locality

Abstract

Silica concretions, known as chert, flint or silex, are common in Late Cretaceous chalks of the Maastrichtian type locality (Maastricht, the Netherlands). They show differences in shape, size and distribution that can be related to depositional structure and texture of the carbonates. It is suggested that the source of the silica was biogenic opal (diatoms), dissolving in the sediment after deposition. Precipitation of dissolved opal started during early diagenesis, at the boundary between oxidizing and reducing sediment. Anaerobic decomposition of organic matter by archaebacteria lowered the hydrogen ion concentration in pore fluids. The alkaline conditions at the boundary between oxidizing and reducing sediment, caused dissolved biogenic silica to polymerize and precipit ate. Later , during further burial, a high concentration of primary silica precipitates enhanced further precipitation. The concentration gradient that was generated in this way, forced dissolved silica to diffuse from the surrounding sediment towards the sites of primary high silica precipitation. After all biogenic opal had been dissolved to nourish the growing 'protonodules', further precipitation resulted in the lowering of dissolved silica in pore fluids below the saturation level of the new polymorph. Then this polymorph started to dissolve and precipitated as a lesser soluble and more ordered polymorph at the most dense parts of the 'protonodule'. This process, resulting in the generation of dense, sharp rimmed nodules, ended when all silica was precipitated as stable quartz. Silica concretions in chalks can be used to determine palaeoredox zones and the depositional and early diagenetic conditions of the chalks.