Exceptional preservation of Cretaceous biota (plants and invertebrates) in flint-bearing alterites from Moragne (western France)

Cretaceous fossil inclusions were recently discovered in silica-rich nodules from the diachronic alteritic deposits of the Moragne hill, Charente-Maritime, western France. Nodules contain marine organisms such as sponge spicules, putative algae, echinoids (Periaster undulatus Agassiz and Desor) and rudists (Sphaerulites foliaceus Lamarck; hippuritids and radiolitids). These inclusions co-occur with fragmented pinnae of ferns as well as diverse structures of conifers including leafy axes (Geinitzia sp.; Glenrosa carentonensis Moreau, Neraudeau, Tafforeau and Depre) and isolated ovulate scales or subtending bracts of cones. The palaeontological assemblage shows that the alterites from Moragne are condensed deposits resulting from the alteration of the Cenomanian to Turonian-Senonian regional series. Conifer leafy axes display diverse kinds of preservation: permineralization in silica, empty external casts of the gross morphology, and casts filled by ferruginous siltstone. Propagation phase-contrast X-ray synchrotron microtomography revealed exceptional preservation of some leafy axes up to the cell level. We support a multi-phase silicification including at least: (1) a precocious phase occurring during the sedimentation of the Upper Cretaceous deposits or during the earliest diagenesis, and that could be linked with the abundance of sponge spicules; then (2) a late phase occurring during Eocene-Oligocene period that corresponds to a hard climate event with strong and long paedogenitic alteration associated with an important phase of silicification ( nodulation). The alterites from Moragne are an uncommon example of long-term time-averaged deposits caused by compaction and selective dissolution of matrix in the context of a late continental weathering.