Intertwined fates of iron-bearing, authigenic minerals during early diagenesis of sandy marine deposits:glauconite, pyrite and magnetite as a fleeting memory of the early days

The littoral zone (including estuarine environments) is considered here as a regulator of the iron flow between the land and the marine environment, observed through the Jurassic sediments of Boulonnais (Northernmost France). Certain stages of deposition during the Kimmeridgian were accompanied by synsedimentary fault movements that caused sand injections, either in sands undergoing lithification (transforming into sandstone), or in claystone levels. The early cemented sandy material and that being hosted in a low-permeability environment (claystones) were as if frozen in their diagenetic evolution and protected against most subsequent transformations. We can see there early trapped and preserved iron-bearing authigenic minerals: pyrite, glauconite and magnetite. Glauconite can take on unusual forms: bryozoan colonies epigenized into glauconite, endolithic filaments mimicking microbial structures, a film enveloping a pyrite polyframboid. All these unusual forms, as well as the majority of glauconite grains, were not preserved in sands that were not cemented or protected early on, and the iron in these minerals was released into the surrounding environment during later stages of diagenesis. Generalizing from this example, the littoral environment can be an early iron trap, favoring the formation of iron-bearing minerals that would then tend to disappear during diagenesis, releasing iron into the marine environment. Furthermore, early cemented objects, where porosity/permeability was therefore rapidly occluded, are valuable insofar as they can record diagenetic stages that are subsequently no longer traced. This is the case here for iron-bearing minerals and this is an opportunity to question the highlighting of the role of microbial activity in the formation of glauconite.