The contrasting origins of glauconite in the shallow marine environment highlight this mineral as a marker of paleoenvironmental conditions

Glauconite is an authigenic mineral reputed to form during long-lasting contact between a nucleus (a pre-existing phyllosilicate) and seawater. This protracted contact makes it possible to subtract the ions necessary for the construction of the neoformed phyllosilicate, here, glauconite (a mineral very close to an illite, rich in K and Fe). As a result, glauconite is often associated with sediments deposited in a transgressive context with a strong slowdown in the rate of sedimentation and a relatively large water layer thickness. This is the case of the Cenomanian chalk of Boulonnais (north of France). Being chemically and physically resistant, glauconite is a mineral that is often reworked, like quartz grains. This is frequently the case of the Jurassic deposits of the Boulonnais, where glauconite, almost ubiquitous, either in traces or in significant proportions of the sediments, presents a grain size sorting attesting to its transport and reworking. However, these Jurassic deposits are shallow (shoreface, upper offshore), which supports the idea that the “glauconite factory” was itself in the shallow areas of the Boulonnais. The only identified Jurassic facies of the Boulonnais where glauconite is both relatively abundant, large in size and unsorted (non reworked) are oyster reefs that formed at the outlet of cold seeps linked to a late-Jurassic synsedimentary tectonic (Kimmeridgian, Tithonian). Our work makes it possible to hypothesize that isolated oyster reefs were environments combining the redox conditions and in contact with seawater favoring the authigenic formation of glauconite. The weakly reducing conditions necessary for the formation of glauconite here are attested by the contents of metallic trace elements sensitive to redox conditions (vanadium, germanium, arsenic, in this case). Our work thus adds a new element to the understanding of the mechanisms of formation of glauconite in shallow environments

A novel approach to volcano surveillance using gas geochemistry

The aim of this paper is to test a simple damage model of a cohesive granular medium to study the relationship between the damage and velocity of elastic waves. Our numerical experiments of edometric compression show that the mi- croscopic deformation quickly becomes very heterogeneous, while our simulations of elastic waves propagation show that a small amount of damage induces a dra- matic decrease in the elastic velocity. This shows that cohesive discrete media are very sensitive to strain field heterogeneity, and that the wave velocities in these media can measure subtle transient deformation processes, such as earthquake initiation phases

Poly-phased fluid flow in the giant fossil pockmark of Beauvoisin, SE basin of France

The giant Jurassic-aged pockmark field of Beauvoisin developed in a 800 m wide depression for over 3.4 Ma during the Oxfordian; it formed below about 600 m water depth. It is composed of sub-sites organized in clusters and forming vertically stacked carbonate lenses encased in marls . This fine-scale study is focused on a detailed analysis of petrographical organization and geochemical signatures of crystals that grew up in early to late fractures of carbonate lenses, surrounding nodules, and tubes that fed them. The isotopic signature (C, O and Sr) shows that at least three different episodes of fluid migration participated to the mineralization processes. Most of the carbonates precipitated when biogenic seepage was active in the shallow subsurface during the Oxfordian. The second phase occurred relatively soon after burial during early Cretaceous and thermogenic fluids came probably from underlying Pliensbachian, Late Toarcian or Bajocian levels. The third phase is a bitumen-rich fluid probably related to these levels reaching the oil window during Mio-Pliocene. The fluids migrated through faults induced by the emplacement of Triassic-salt diapir of Propiac during the Late Jurassic and that remained polyphased drain structures over time

La production de sel à partir des eaux naturelles : approche minéralogique, géologique et physico-chimique

Résumé : Les sels se forment par évaporation des eaux naturelles de diverses natures (océaniques, continentales, mixtes). Le principal minéral formé par évaporation est la halite (NaCl), accompagné de tout un cortège d’autres minéraux évaporitiques, qui cristallisent selon une séquence parfaitement définie. Certaines périodes de l’évolution de la Terre ont connu la formation de grands gisements, certains ayant été exploités depuis l’Âge du fer (Hallstatt). Loin des gisements ou des circuits commerciaux, le sel a été extrait à partir de saumures dans des marais salants ou des ateliers utilisant des fourneaux. L’approche pluridisciplinaire de l’étude de la production du sel (archéologie expérimentale, minéralogie, physico-chimie) est un élément essentiel de compréhension des contraintes de la production de sel dans l’Antiquité.Abstract : Salt deposits form by evaporation of natural waters of different origins (oceanic, continental, mixed). The most important mineral produced by evaporation is halite (NaCl), accompanied by a large variety of evaporitic minerals, crystallizing according to a well-defined sequence. During particular periods of Earth history, large salt deposits formed, some being exploited since Iron Age (Hallstatt). Far from mines and trade routes, salt was extracted in salt marshes or by boiling of brines. The multidisciplinary study of salt production and origin (experimental archaeology, mineralogy, physico-chemistry) is an essential component for understanding constraints on salt production in Antiquity

Evidence for fluid overpressuring along an Oligocene out-of-sequence thrust in the Shimanto accretionary prism, SW Japan

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