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

Abstract

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 sedimentological oxymoron: highly evolved glauconite of earliest diagenetic origin

“…In agreement with our previous work conducted in the Upper Jurassic rocks and Cretaceous chalk of [Espitalié et al, 1986], based on Rock-Eval analysis data, shows that the organic matter of the samples examined here is of Type III [all details about Rock-Eval in Baudin, 2023]. the Boulonnais area [Tribovillard et al, 2021[Tribovillard et al, , 2023, the green minerals in the samples studied here were identified as glauconite using XRD (Supplementary Figure S1). The glauconite shows a high crystallinity index [0.4°2 theta; Deconinck et al, 1982].…”

“…Semi-quantitative analyses have been performed on individual grains using the EDS probe of the SEM. The concentrations in FeO and K 2 O thus obtained are consistent with those obtained previously through OES techniques [Tribovillard et al, 2023]. As illustrated with Figure 8C, a correlation is drawn between the FeO and K 2 O concentrations, and the concentrations of most of the samples are above the threshold values (K 2 O > 8%, FeO > 22% or Fe 2 O 3 > 24%) of the so-called highly evolved glauconite, first determined by Odin and Matter [1981].…”

“…At the LOG lab of University of Lille, the glauconite grains were isolated through the protocol described in Tribovillard et al [2021Tribovillard et al [ , 2023. Briefly, samples were digested with HCl to dissolve the carbonate and phosphate phases before being rinsed.…”

“…Therefore, the presence of glauconite in a deposit does not automatically mean a local authigenic (and therefore syn-depositional) formation of this mineral. Recent work [Tribovillard et al, 2021[Tribovillard et al, , 2023 has shown that examining the grain size distribution curves of glauconite makes it possible to distinguish between syndepositional glauconite and reworked glauconite.…”

The Behavior of Rare Earth Elements during Green Clay Authigenesis on the Congo Continental Shelf

“…In this study, the lower degree of Rb enrichment in authigenic clays compared to K (~6 times lower) confirms previous inferences that Rb is significantly less affected by post-depositional water-sediment exchange processes [62]. Furthermore, while the behavior of redox sensitive elements during glauconitization still remains poorly documented, our results suggest that green clay authigenesis can sequester substantial amounts of V, Mo, and Cr (up to ~85%), possibly reflecting the micro-reducing conditions associated with glauconite formation in fecal pellets [63]. Additionally, recent experimental work has demonstrated a strong binding mechanism for Cr when precipitating on clays through the substitution of Al [64], which could partly explain the observed Cr enrichment in authigenic Fe-bearing clays from the Congo Shelf.…”