2
Diversity of Types of Mineralization

2.1. Silicification

There are many different processes involved in silicification, which, despite different geological histories, may lead to very similar exceptional fossils. Silicified fossils may result from what is known as primary silicification, roughly contemporaneous with the sediment deposit and the death of the organisms in question, or from secondary silicification, which occurs millions of years after the sedimentation of the organisms. In certain cases, fossilization may be the result of processes that combine prompt and delayed silicification.

Early silicification can occur by the involvement of endogenous silica, which is present in a highly localized or highly diffuse form in the architecture of an organism. For example, the spicules of a sponge - once they are dissolved - can supply a portion of the amorphous silica which then precipitates, silicifying the mass of the sponge's body (other possible examples can be found in diatoms and radiolaria). Relatively early and exogenous silicification may occur by the reuse of amorphous silica from siliceous organisms, such as sponges, but precipitating in the remains of other organisms associated with the sponges, such as mollusks and echinoderms.

Delayed silicification tends to use exogenous silica, involving the amorphous silica carried by the circulation of fluids (percolation, alteration, leaching, hydrothermalism) from one sedimentary medium to another. Thus, fossiliferous chalk beds covered by (stratigraphic superposition) or adjoining (lenticular systems) sandy formations subject to leaching may see the silicification of fossils contained therein.

2.1.1. Primary silicification

In the case of primary silicification, we can distinguish between cases resulting from the hydrothermal impregnation of organisms near to silica-rich fluid sources, and cases resulting from diagenetic or synsedimentary recrystallization of the remains of non-siliceous organisms.

2.1.1.1. Hydrothermal or volcanic silicification of land-dwelling organisms

Example of the Rhynie chert

The region of Rhynie in Scotland was the site of a great deal of volcanic activity in the late Devonian (around -396 Ma), which led to the formation of hot springs, generating silica-rich fluids (Channing and Edwards 2009). Vents and natural events (such as geysers) brought, or ejected, these "siliceous gels" to the surface, where they covered various organisms and impregnated them with silica, even silicifying their cellular ultrastructures (Kerp 2018). The most remarkable silicification was of plant matter - notably, the earliest forms of subvascularized terrestrial plants (tracheophytes), such as Rhynia, which were originally named in reference to the name of the fossiliferous deposit site (see Plates 2.1A and 2.1B). The paleontological assemblage also includes bryophytes, lycophytes such as Asteroxylon, fungi such as ascomycetes and freshwater algae such as charophytes. In addition to these plant remains, the site contains small arthropods and nematodes, remarkably preserved in 3D (Dunlop and Garwood 2018). The paleoenvironment, which was the backdrop to this exceptional fossilization, aside from its volcanic context, was a marsh. The Rhynie cherts reflect the fertile soils for varied plants that were part of the process of plant colonization of the dry land.

Plate 2.1. Fossils silicified in a volcanic context

NOTE ON PLATE 2.1.- A and B: polished sections of the Rhynie cherts, from the Devonian of Scotland. A: general view of a chert with a multitude of sections of axes of Aglaophyton; B: wide view of a piece of chert with sections (basically transversal) of axes of Aglaophyton; C: general view of carbonized wood impregnated with infiltration of hyalite opal in a volcanic context, Le Fau, Cantal; D and E: terrestrial gastropods of the genus Helix, mineralized with orbicular lussatite, coming from Tertiary alterites from Mine des Rois, Dallet, Puy-de-Dôme (collections: A and B: Nigel Trawin, University of Aberdeen; C: Maison de la minéralogie du Massif central, Antignac; D: Néraudeau-IGR; E: J.-C. Le Sager; photos: A and B: C. Strullu; C and D: D. Néraudeau; E: M. Guillo). Scale bars: 1 cm.

Example of Neogene wood with hyalite opal

In Cantal, and more broadly in Auvergne, volcanic activity contributed to the fossilization and mineralization of terrestrial organisms and, more specifically, plant remains.

Thus, in addition to silicification with lussatite (see section 2.1.2.2), wood was preserved in the late Neogene (around -11.5 to -2.5 Ma) in carbonaceous form (carbonized to a greater or lesser degree) by volcanic ash or lava flows, and sometimes originally mineralized by the percolation of silica-rich fluids, which crystallized in the form of hyalite opal (the vitreous form with the formula SiO2nH2O).

This exceptional process is notably seen in the Fau sector, in Pays de Salers, and in other sites in Puy-de-Dôme (Flörke et al. 1973; Lynne and Campbell 2004; Barailler 2023) (see Plate 2.1C). The impregnation of the wood with opal makes for a striking contrast between the black color of the carbonized wood and the luminous transparency of the hyalite opal.

2.1.1.2. Early diagenetic silicification of marine organisms

The Anglo-Paris Basin contains a thick Cretaceous series of chalk deposits, mainly laid down between the end of the Cenomanian (around 95 Ma) and the start of the Maastrichtian (around 70 Ma). This chalk corresponds to a relatively deep marine deposit (50 to 200 m depending on the sites and periods), in the context of a substrate favorable to the development of diverse benthic organisms such as echinoderms (for example the sea urchins Echinocorys and Micraster), bivalve mollusks (for example Inoceramus) and sponges (for example Siphonia).

These organisms, as the result of the degradation of their siliceous "gridlike" skeleton and, above all, their siliceous spines - known as "spicules" - produced huge quantities of microscopic silica debris which accumulated in the chalk as the strata were laid down.

The proliferation of burrowing organisms during certain periods of the deposition remobilized this silica with the effect of concentrating it. Limnivorous burrowers, as they passed through, ingested the sediment, consumed and thus removed the organic matter, and naturally excreted mud with an altered pH and chemical composition, poorer in organic carbonate particles and richer in siliceous particles, which the burrowing organism was unable to assimilate. This results in precipitations of silica where the mineral was present in excess concentrations - i.e. in the burrows created by the organisms.

Silicified burrows within the chalk therefore form remarkable beds (see Figure 2.1A): black in color, winding and labyrinthine, networks of flint, as can be seen in the cliffs of Étretat (Normandy, north-western France) (see Plate 2.2A).

In addition to silicified burrows, the flint-bearing chalk beds contain the remains of invertebrates and vertebrates that have been partially or completely transformed into silica, particularly near the burrows, such as sponges, echinoids and the teeth of selachians (see Plate 2.2B to 2.2E).

These primary flints, and the fossils they contain, can be found in alterites, after the dislocation and dissolution of the limestone gangue (see Figure 2.1C).

Plate 2.2. Fossiliferous primary flint

NOTE ON PLATE 2.2.- A: chalk beds rich in winding black flints, interspersed between white chalk layers with lower flint content, in the cliffs of Étretat, Seine-Maritime, dating from the Turonian; the winding form of the flints reflects the bioturbation of the levels at which they were formed; B: twisted nodule of flint which has partially silicified and encapsulated a sea urchin (Echinocorys) in the process of nodulization; C: flint nodule which has totally silicified and encapsulated a sea urchin (Echinocorys), whose calcareous test was dissolved by decalcification; D to F: Turonian flint fossils from the Cher region (central France): D: large bivalve (Inoceramus) totally silicified in a lump of flint; E: shark tooth (Squalicorax) encapsulated in a flint nodule, the large cuspid of its crown having remained apatite; F: lignitic wood caught within a flint nodule, without having totally lost its carbonaceous nature (collections: B and C: IGR; D to F: K. Chartier; photos: A: taken in situ by E. Lasseur; B to E: D. Néraudeau; F: K. Chartier). Scale bars: 1 cm.

Figure 2.1. Flint formation

NOTE ON FIGURE 2.1.- Flint formation can occur in various geological contexts. Synsedimentary flints, known as "primary flints", are formed at the time of the initial deposition of the sediment containing the flints - generally chalky limestone - and are encased in those chalky beds (A). The alteration of the surface of these flint-containing beds dissolves the surrounding carbonate rock, but...