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\DOI{10.5802/crgeos.301}
\datereceived{2025-04-23}
\daterevised{2025-06-27}
\dateaccepted{2025-07-07}
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\dateposted{2025-07-23}
\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{research-article}

\TopicFR{S\'edimentologie, stratigraphie, g\'eologie des bassins}
\TopicEN{Sedimentology, stratigraphy, basin geology}

\title{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}

\alttitle{Le devenir de min\'{e}raux ferrif\`{e}res authig\'{e}niques
au cours de la diagen\`{e}se pr\'{e}coce des d\'{e}p\^{o}ts marins
sableux : glauconite, pyrite et magn\'{e}tite comme autant de souvenirs
fugaces des premiers moments de d\'{e}p\^{o}t}

\author{\firstname{Nicolas} \lastname{Tribovillard}\CDRorcid{0000-0003-3493-5579}\IsCorresp}
\address{Universit\'{e} de Lille, UMR 8187 LOG -- Laboratoire
d'Oc\'{e}anologie et de G\'{e}osciences, Univ. Lille, CNRS, Univ.
Littoral C\^{o}te d'Opale, IRD, 59000 Lille, France}
\email[N. Tribovillard]{nicolas.tribovillard@univ-lille.fr}

\author{\firstname{Olivier} \lastname{Averbuch}\CDRorcid{0000-0002-8480-1176}}
\addressSameAs{1}{Universit\'{e} de Lille, UMR 8187 LOG -- Laboratoire
d'Oc\'{e}anologie et de G\'{e}osciences, Univ. Lille, CNRS, Univ.
Littoral C\^{o}te d'Opale, IRD, 59000 Lille, France}

\author{\firstname{Fran\c{c}ois} \lastname{Guillot}\CDRorcid{0000-0003-0334-0943}}
\addressSameAs{1}{Universit\'{e} de Lille, UMR 8187 LOG -- Laboratoire
d'Oc\'{e}anologie et de G\'{e}osciences, Univ. Lille, CNRS, Univ.
Littoral C\^{o}te d'Opale, IRD, 59000 Lille, France}

\author{\firstname{Viviane}\nobreakauthor\lastname{Bout-Roumazeilles}\CDRorcid{0000-0001-6917-818X}}
\addressSameAs{1}{Universit\'{e} de Lille, UMR 8187 LOG -- Laboratoire
d'Oc\'{e}anologie et de G\'{e}osciences, Univ. Lille, CNRS, Univ.
Littoral C\^{o}te d'Opale, IRD, 59000 Lille, France}

\author{\firstname{Sandra} \lastname{Ventalon}}
\addressSameAs{1}{Universit\'{e} de Lille, UMR 8187 LOG -- Laboratoire
d'Oc\'{e}anologie et de G\'{e}osciences, Univ. Lille, CNRS, Univ.
Littoral C\^{o}te d'Opale, IRD, 59000 Lille, France}

\author{\firstname{Armelle} \lastname{Riboulleau}\CDRorcid{0000-0002-2717-8330}}
\addressSameAs{1}{Universit\'{e} de Lille, UMR 8187 LOG -- Laboratoire
d'Oc\'{e}anologie et de G\'{e}osciences, Univ. Lille, CNRS, Univ.
Littoral C\^{o}te d'Opale, IRD, 59000 Lille, France}

\author{\firstname{Marion}\nobreakauthor\lastname{Delattre}}
\addressSameAs{1}{Universit\'{e} de Lille, UMR 8187 LOG -- Laboratoire
d'Oc\'{e}anologie et de G\'{e}osciences, Univ. Lille, CNRS, Univ.
Littoral C\^{o}te d'Opale, IRD, 59000 Lille, France}

\shortrunauthors

\keywords{\kwd{Late Jurassic}\kwd{Tithonian}\kwd{Boulonnais}\kwd{Glauconitized bryozoans}}

\altkeywords{\kwd{Jurassique sup\'{e}rieur}\kwd{Tithonien}\kwd{Boulonnais}\kwd{Bryozoaires glauconitis\'{e}s}}

\thanks{Programme Tellus Syster of the French Institut des Sciences de
l'Univers (INSU), Monique Gentric}

\begin{abstract}
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.\looseness=-1
\vspace*{-.3pc}
\end{abstract}

\begin{altabstract}
La zone littorale (incluant les milieux estuariens) est ici
consid\'{e}r\'{e}e comme un r\'{e}gulateur des flux de fer entre le
milieu terrestre et le milieu marin, observ\'{e}s \`{a} travers les
s\'{e}diments jurassiques du Boulonnais (extr\^{e}me nord de la
France). Certaines \'{e}tapes de d\'{e}p\^{o}t au cours du
Kimm\'{e}ridgien ont \'{e}t\'{e} accompagn\'{e}es de mouvements de
failles syns\'{e}dimentaires qui ont provoqu\'{e} des injections de
sable, soit dans des sables d\'{e}j\`{a} en cours de lithification
(transformation en gr\`{e}s), soit dans des niveaux argilo-calcaires.
Les mat\'{e}riaux sableux ciment\'{e}s pr\'{e}cocement et ceux
ench\^{a}ss\'{e}s dans un milieu peu perm\'{e}able (argiles)
\'{e}taient comme fig\'{e}s dans leur \'{e}volution
diag\'{e}n\'{e}tique et prot\'{e}g\'{e}s de la plupart des
transformations ult\'{e}rieures. On y observe des min\'{e}raux
authig\'{e}niques ferrif\`{e}res pr\'{e}cocement pi\'{e}g\'{e}s et
pr\'{e}serv\'{e}s : pyrite, glauconite et magn\'{e}tite. La glauconite
peut prendre des formes inhabituelles : colonies de bryozoaires
\'{e}pig\'{e}nis\'{e}es en glauconite, filaments endolithiques mimant
des structures microbiennes, film enveloppant un polyframboide de
pyrite. Toutes ces formes inhabituelles, ainsi que la majorit\'{e} des
grains de glauconite, n'ont pas \'{e}t\'{e} pr\'{e}serv\'{e}s dans les
sables non ciment\'{e}s ou non prot\'{e}g\'{e}s pr\'{e}cocement, et le
fer contenu dans ces min\'{e}raux a \'{e}t\'{e} lib\'{e}r\'{e} dans le
milieu environnant lors des stades ult\'{e}rieurs de la diagen\`{e}se.
En g\'{e}n\'{e}ralisant \`{a} partir de cet exemple, le milieu littoral
peut constituer un pi\`{e}ge \`{a} fer jouant pr\'{e}cocement,
favorisant la formation de min\'{e}raux ferrif\`{e}res qui auraient
ensuite tendance \`{a} dispara\^{i}tre lors de la diagen\`{e}se,
lib\'{e}rant du fer dans le milieu marin. De plus, les objets
ciment\'{e}s pr\'{e}cocement, dont la porosit\'{e}/perm\'{e}abilit\'{e}
a donc \'{e}t\'{e} rapidement obstru\'{e}e, sont pr\'{e}cieux dans la
mesure o\`{u} ils peuvent enregistrer des stades diag\'{e}n\'{e}tiques
qui ne sont plus retrouv\'{e}s par la suite. C'est le cas ici pour les
min\'{e}raux ferrif\`{e}res, et cela offre l'occasion de s'interroger
sur la mise en \'{e}vidence du r\^{o}le de l'activit\'{e} microbienne
dans la formation de la glauconite.
\end{altabstract}

\maketitle

\twocolumngrid

\end{noXML}

\section{Introduction}

\vspace*{-.3pc}

\looseness=-1
Estuaries and coastal settings are environments at the interface
between the emerged lands and the marine domain; they are known to be
very reactive milieus. In fact, they collect everything that rivers
transport in particulate, colloidal or dissolved form, they accumulate
this material temporarily and redistribute it, in a more or less
sequential way, towards the marine environment
\citep[e.g.,][and reference therein]{Burdige2011}. In particular, these
phenomena are crucial for iron, which is an essential element for
marine biodiversity, starting with microbial populations
\citep{Canfield2006, Lauferetal2016, Adhikarietal2017}. Such proximal
deposits are often rich in organic matter, both of autochthonous marine
origin and of allochthonous terrestrial origin, and this organic matter
can support intense microbial activity, which in turn impacts a
significant number of early diagenetic reactions and growth of
authigenic minerals \citep{Jorgensen2006, Bianchietal2016,
Arellanoetal2019, Khanetal2024}. Woody debris, known to be refractory,
that is, difficult to degrade, are most often the only organic remains
observed in ancient coastal sediments, typically sandy deposits,
indurated or not \citep[e.g.,][]{Tyson1995}. In these sands and
sandstones, the more labile organic matter has most often been entirely
remineralized {via} bacterial reactions capable of generating or
inducing the formation of authigenic minerals, typically pyrite
\citep[][and references therein]{Jorgensenetal2019}.

We are interested here in the reactions occurring during the early
diagenesis of sandy deposits, and in the authigenic minerals that
result from them. Sands and sandstones are often porous and permeable
media (at least during part of their diagenetic curse) and fluid
circulations often induce the dissolution of chemically fragile
minerals, so much so that the sandstones of shoreface environments only
contain very poorly soluble minerals (usually, quartz and heavy
minerals). The memory of these fleeting authigenic minerals can
nevertheless be preserved if early cementation can seal the porosity,
thus preventing the fragile minerals from dissolution. From this
perspective, we study here sandy deposits that underwent selectively
located, early cementing, and we compare the mineralogical composition
of uncemented sands and that of their contemporary equivalents,
cemented early. The sediments studied belong to the geological
formation of Gr\`{e}s de Ch\^{a}tillon (Kimmeridgian) cropping out
along the cliffs of Boulonnais \citep[northernmost France, Strait of
Pas-de-Calais; Figure~\ref{fig1};][]{Geyssantetal1993,
Deconincketal1996, DeconinckBaudin2008}.

\begin{figure*}
\includegraphics{fig01}
\caption{\label{fig1}Location of the area studied here in the
Boulonnais area. Maps and photograph from the \textit{Geoportail}
website of the French \textit{Institut G\'{e}ographique National}.}
\end{figure*}

This study aims to broaden our understanding of the transitional,
screening, way-through that the coastal zones are. As reminded above,
efforts have mainly focused on the fate of organic matter in estuarine
and coastal sediments, with the aim of better understanding this
compartment or stage of the carbon cycle. Progress can still be made
regarding the fate of iron in these specific environments. Iron and
manganese have been extensively studied in coastal environments,
particularly with regard to their involvement in the remineralization
of organic matter, through phenomena such as the reduction of Fe and Mn
oxyhydroxides, the reduction of sulfate ions and the formation of
pyrite \citep[e.g.,][]{Burdige2006, Burdige2011, Jorgensenetal2019}.
However, processes are examined through {in vitro} experiments
and numerical modeling, but they are not observed {in vivo}: the
situations are too complex because intertwined biotic and abiotic
phenomena occur simultaneously, being almost impossible to decipher.
Furthermore, the literature emphasizes the coupled reduction reactions
of sulfate ions and metal species, which logically leads to the study
of the formation of iron sulfide minerals such as mackinawite or pyrite
\citep[e.g.,][]{Rickard2012, Rickard2024}. This type of work has not
been so much interested in the genesis of glauconite or magnetite, and
this is what this paper aims to address. The present work is justified
insofar as it highlights the complexity of the relationships between
iron-bearing minerals such as pyrite, glauconite, magnetite and
hematite. Indeed, these relationships are again difficult to observe
{in vivo} since they occur at intermediate time scales which
most often escape direct observation: on the one hand, {in situ}
observations require instrumentation that is difficult to set up, and
little data exists to our knowledge; on the other hand, when ancient
deposits can be observed in outcrop, the traces of the existence of
transitory and fragile mineral phases have long since disappeared. The
results of the present study show in particular that the
transformations between pyrite and glauconite can occur in both ways,
sometimes successively. This work also shows that diagenesis can lead
to a selective disappearance of certain forms of glauconite grains, in
particular those presenting the lowest maturity, which questions our
understanding of the formation and evolution of this mineral.
\looseness=-1

\section{Material and methods} \label{sec2}

The Kimmeridgian and Tithonian geological formations visible along the
Boulonnais coastline were deposited on the shallow continental platform
(shoreface to lower offshore) terminating the eastern part of the
London-Brabant Basin \citep{Mansyetal2003}. These deposits correspond
to a succession of sandstone formations (shoreface) and marl formations
(offshore). See detailed descriptions in \citet{Mansyetal2007} and
\citet{DeconinckBaudin2008}. For the present study, the focus is set on
the transition between the underlying Gr\`{e}s de Ch\^{a}tillon Fm.
(\textit{Eudoxus} ammonite zone, \textit{Contejeani} sub-zone) and the
overlying Argiles de Ch\^{a}tillon Fm.\ (\textit{Autissiodorensis}
sub-zone, i.e., the lowermost sub-zone of the \textit{Autissiodorensis}
ammonite zone). The uppermost part of the Gr\`{e}s de Ch\^{a}tillon
Formation is made up with sandstones and sands. The sandstones are in
the form of meter-size concretions, passing laterally to unconsolidated
sands (Figure~\ref{fig2}). The sedimentary structures observed are
cross-stratifications (hearing bone style) and bioturbations. The
facies are interpreted to represent shallow shoreface-type
environments. The topmost part of the Gr\`{e}s de Ch\^{a}tillon is an
uncemented level of rust-colored sand, rich in small shell fragments
and woody remnants. The boundary between the Gr\`{e}s de Ch\^{a}tillon
et Argiles de Ch\^{a}tillon formations is a whitish carbonate bed of
authigenic origin \citep{Hatemetal2014, Hatemetal2016}. The Argiles de
Ch\^{a}tillon starts with a meter-thick claystone bed
(Figure~\ref{fig2}), corresponding to offshore conditions
\citep{DeconinckBaudin2008, Hatemetal2016}. 

\begin{figure*}
\includegraphics{fig02}
\caption{\label{fig2}(A)~Picture of the basal claystone level of the
Argiles de Ch\^{a}tillon Fm. (1), overlying the top part of the
Gr\`{e}s de Ch\^{a}tillon. This latter is almost not cemented except
for rounded concretions (2~\&~3). The whitish level separating the two
formations is illustrated with the lower part of~(B),
together with the orange-color topmost level mentioned in the text, as
well as herring bone-like structures (1) and synsedimentary fault~(2).}
\end{figure*}

The upper part of the Gr\`{e}s de Ch\^{a}tillon Fm. underwent
syndepositional tectonic movements \citep[Supplementary Material
Figure~S2][]{Hatemetal2014, Hatemetal2016}. The synsedimentary faults
were of small dimensions (Figure~\ref{fig2}) or medium dimension
(Figure~\ref{fig3}), and they affected mainly the transition zone
between the two formations at stake here (namely, the Gr\`{e}s de
Ch\^{a}tillon and overlying Argiles de Ch\^{a}tillon). Well visible at
the place called Cran du Noirda, north of Audresselles, the
synsedimentary fault clearly cuts the sandstone concretions. It means
that, when the displacement occurred, the sand was locally already
indurated enough to get clearcut and remain so. In {addition}, the
fault movements induced some injections of soft sand into indurated
sand blocks \citep[{Figure}~\ref{fig3}; Supplementary Material
Figure~S2;][]{Hatemetal2014, AverbuchGuillot2015}. It means that
the sand blocks were already indurated but not lithified yet, because
water-impregnated soft sands could be injected into the concretions
being formed. Complete cementation must have occurred short after, to
account for the preservation of these structures (clearcut fault planes
and sand injectites) over geological times. Such sand injectites are
observed close to each synsedimentary fault affecting the stratigraphic
episode discussed here, between Audresselles and the place called
Pointe du Ridden, along the coastline (Figure~\ref{fig1}). The
synsedimentary movements also induced another type of sand injection.
Meter-scale lobes of sand were injected from the underlying uncemented
sand levels of the Gr\`{e}s de Ch\^{a}tillon Fm., through the
calcareous transition bed, and into the basal claystone level of the
Argiles de Ch\^{a}tillon Fm. (Figure~\ref{fig4}). These injected lobes
of sands are still uncemented \mbox{today}.\looseness=-1

\begin{figure*}
\includegraphics{fig03}
\caption{\label{fig3}Structures pictured at the Cran du Noirda (see
Figure~\ref{fig1}) accompanying the synsedimentary fault movement.
(A)~Indurated injectites visible on the cemented nodules. (B)~The fault
plane itself, also visible in Figure~\ref{fig1}), affecting the
transition zone between the Gr\`{e}s de Ch\^{a}tillon and Argiles de
Ch\^{a}tillon formations. The fault cut some sandstone nodules (lower
arrow in B,~C). (D)~A soft sand injective penetrating the Argile de
Ch\^{a}tillon Fm.}
{\vspace*{-.15pc}}
\end{figure*}

\begin{figure*}
\includegraphics{fig04}
{\vspace*{.3pc}}
\caption{\label{fig4}Sand injectites in the lower part of the Argiles
de Ch\^{a}tillon Formation fueled by sand from the underlying Gr\`{e}s
de Ch\^{a}tillon Formation. (A)~The arrows point to unconsolidated sand
lobes injected into the base of the Argiles de Ch\^{a}tillon Fm.
Circle: a wooden folding meter for scale, on top of the whitish level
separating the two formations. (B)~Close up view of the sand injected
into the basal marly bed of the Argiles de Ch\^{a}tillon Fm., with
relicts of the marly beds embedded within the sand.}
\end{figure*}

The sampling for the present study was carried out at the Cran du
Noirda and Cran Mademoiselle (north to Audresselles town;
Figure~\ref{fig1}). Several facies were sampled: (1)~uncemented, soft
sand (4~samples) and cemented sandstones (7~samples of concretions and
sand injectites) of the Gr\`{e}s de Ch\^{a}tillon, and (2)~soft sand
injectites (4~samples) and their hosting clay stones (4~samples) in the
basal part of the Argiles de Ch\^{a}tillon.

Thin sections of the sandstones have been made and observed through a
photonic microscope. The procedure for the extraction and analysis of
the glauconite grains is detailed in \citet{Tribovillardetal2023a,
Tribovillardetal2023b}. In a few words, the samples were treated with
an acid solution of HCl to digest the carbonate cement and release all
the non-HCl soluble particles. The clay faction was removed through
repeated rinses. The particles thus extracted were then oven dried
before magnetic separation using a Frantz Isodynamic apparatus to
collect the particles sensitive to magnetic separation (namely, here,
glauconite {a priori}). The particles were imaged using a
scanning electron microscope (SEM) equipped with a EDS-type analytical
probe (FEI Quanta~200 SEM equipped with a Quantax QX2 Roentec
energy-dispersive X-ray spectroscopy system). The grains were also
analyzed by X-ray diffraction (XRD) to determine their mineralogy
\mbox{according} to the standard protocol described in
\citet{BoutRoumazeillesetal1999} and \citet{Tribovillardetal2021}. XRD
was carried out on both oriented mounts and non-oriented mounts to
fully \mbox{discriminate} glauconite from illite. Raman
microspectroscopy was performed with a JY Horiba, Labram HR800UV,
equipped with an electronically cooled charge-coupled device (CCD)
detector, using 532~nm laser excitation. Spectra were obtained using
100${\times}$ magnification objective.

\section{Results} \label{sec3}

The observation of the thin sections of sandstones yields the presence
of dominant quartz grains, accessory glauconite grains, and some
green-stained quartz grains, all of them being cemented with calcite
(Supplementary Material Figure~S1). The cement is made with equant spar
drusy mosaics and poikilotopic spar. Some tiny, brown color, rhombs of
dolomite are observed. 

\begin{table*}[t!]%tab1
\caption{\label{tab1}Specific content of the three facies considered in
this study}
\fontsize{9.7}{12}\selectfont\tabcolsep4pt
\begin{tabular}{ccc}
\thead
Uncemented sands &
Soft injectites (uncemented) &
Cemented injectites and sandstones\\
\endthead
\parbox[t]{5cm}{\raggedright Magnet-sensitive quartz grains frequently stained with glauconite} &
\parbox[t]{5cm}{\raggedright Magnet-sensitive quartz grains frequently stained with glauconite} &
\parbox[t]{5.1cm}{\raggedright Magnet-sensitive quartz grains frequently stained with glauconite}\vspace*{7pt}\\

\parbox[t]{5cm}{\raggedright Magnet-sensitive wood fragments, sometimes associated with altered pyrite} &
\parbox[t]{5cm}{\raggedright Magnet-sensitive wood fragments, frequently associated with altered pyrite} &
\parbox[t]{5.1cm}{\raggedright Magnet-sensitive wood fragments, almost always associated with fresh pyrite and/or glauconite}\vspace*{7pt}\\

\parbox[t]{5cm}{\raggedright Rare non magnet-sensitive wood fragments} &
\parbox[t]{5cm}{\raggedright Common non magnet-sensitive wood fragments}\vspace*{7pt} &
\parbox[t]{5.1cm}{\raggedright Frequent non magnet-sensitive wood fragments}\\

\parbox[t]{5cm}{\raggedright Rare dark green (almost black) glauconite grains with relatively narrow size distribution} &
\parbox[t]{5cm}{\raggedright Frequent dark green (almost black) glauconite grains with relatively narrow size distribution}\vspace*{8pt} &
\parbox[t]{5.1cm}{\raggedright Abundant glauconite grains with a color array from light green to black, and a wide grain-size distribution}\\

Not observed & Not observed &
\parbox[t]{5.1cm}{\raggedright Abundant glauconitized bryozoans (light green, large sized)}\vspace*{8pt}\\

Not observed & Not observed &
\parbox[t]{5.1cm}{\raggedright Common small-dimensioned automorphic magnetite grains}\vspace*{2pt}
\botline
\end{tabular}
{\vspace*{.35pc}}
\end{table*} 

After HCl leaching and clay size-mineral elimination, the X-ray
diffraction analysis showed that all the green clay minerals examined
here are glauconite {stricto sensu (Supplementary Material
Figure~S3)}, as was the case in all our previous studies
\citep{Tribovillardetal2021, Tribovillardetal2023a,
Tribovillardetal2023b}. Raman spectrometry showed that magnetic
minerals (reactive magnets), other than glauconite, are made up of
magnetite, which is associated with apatite. The populations of grains
may be described according to their response to magnetic separation.
First, the \mbox{fraction} of the sediments or rocks that are
\textit{not sensitive} to magnetic fields is made of quartz grains,
pyrite framboids and small-dimensioned wood fragments, plus rare,
unidentified accessory minerals, also termed ``heavy'' minerals.
Second, the magnetic-field \textit{sensitive} fraction consists of
glauconite and magnetite grains, both minerals being sensitive to
magnetic fields, plus green-stained quartz, pyrite framboids and small
dimension, pyritized, wood fragments. Quartz, pyrite and wood fragments
are not sensitive to magnetic fields, therefore, their presence in this
fraction of rocks implies that they are closely associated with a
mineral sensitive to magnetic fields. Visibly, the green-stained quartz
grains must be associated with glauconite and this may also be true for
the pyrite and woody particles. The associated magnet-sensitive phase
may also be magnetite.

The magnet-sensitive fraction may be distinguished according to the
facies into three categories: cemented sandstone blocks and injectites
of the Gr\`{e}s de Ch\^{a}tillon, soft injectites pushed into the basal
claystones of the Argiles de Ch\^{a}tillon Fm., and the sandy material
still in place, which fueled the various injectites and that has not
been cemented (Table~\ref{tab1}). Hereinafter, these three facies will
be termed sandstones, soft injectites and sand, respectively.
Glauconite grains are commonly observed in the sandstones and in the
soft injectites, but only rarely in the sands. Some differences are
detected between the two facies bearing abundant glauconite grains. In
the sandstones, the glauconite grains show a gradation of colors, from
pale green to dark green, almost black, whereas the glauconite grains
are all dark green in the soft injectites. A grain size gradation is
also observed using SEM images (we were unable to use a laser-equipped
grain sizer because of a too-small number of glauconite grains). The
range of the glauconite grains is wider toward small dimension
particles in the sandstones than in the soft injectites where only
large grains are presents (Figure~\ref{fig5}).\looseness=-1

\begin{figure*}[p!]
\includegraphics{fig05}
\caption{\label{fig5}(A)~A population of magnet-sensitive particles
after magnetic separation, showing a relative wide array of green
colors. Noteworthy, the presence of quartz grains and a pyrite
polyframboid. (B,~C)~SEM images of the particles, with notably a
pyritized wood fragment and a glauconitized bryozoan~(C). (D)~The
peculiar grain with sort of endolithic filaments, looking like a
microbial structure. (E,~F)~Glauconitized bryozoans. Sometimes, the
grains are almost coated with glauconite~(F). If complete, the coating
would give the visual impression of a solid grain.}
\end{figure*}

\looseness=-1
In addition, peculiar grains display specific structures of evident
biological origin (Figures~\ref{fig5}B, C, E, F and~\ref{fig6}B). 
The peculiar grains are most probably
glauconitized fossils of bryozoans (cyclostomes~?); see for instance
\citet{HaddadiHamdane1996} and \citet{HaraTaylor2009}. Most often,
glauconitized \mbox{bioclasts} are echinoderm fragments, and
glauconitized bryozoans are seldom mentioned
\citep[e.g.,][]{Giresseetal1980} and when it is the case, glauconite
\mbox{occurs} as chamber infilling rather than the results of
microstructure epigeny \citep{Banerjeeetal2012, Banerjeeetal2016a,
Banerjeeetal2016b, Bansaletal2020, Mohammedetal2025}.
The SEM image shown Figure~\ref{fig5}F illustrates one type of grain
commonly observed in the sandstones, that is, worn glauconite
disclosing their inner structure that seems to be a fossil bryozoan
colony. An intriguing particle was observed (Figure~\ref{fig5}D;
Table~\ref{tab2} for EDS analyses), yielding an inner structure made of
glauconite-made filaments, which evokes microbial structures. The
microbial origin cannot be ascertained and only one such grain was
identified. Accordingly, another sample exhibited a peculiar structure,
as if a framboid of pyrite had been wrapped with a glauconitized film;
this biofilm looks like a microbial biofilm (Figure~\ref{fig6}E and
Table~\ref{tab2} for EDS analyses). Thus, in addition, to glauconitized
bryozoan fossils, our observations allowed us to identify rare and odd
structures, resembling microbial filaments and biofilms. More
frequently, many grains are made with pyrite and glauconite tied
together, as well as intricate wood fragments and glauconite or pyrite
(Figures~\ref{fig6}A--C and~F). Pyrite
is in the form of framboids and polyframboids, and/or euhedral
crystals. Pyrite may have grown onto glauconite grains
(Figures~\ref{fig6}A, B) but the converse is also true, with
glauconite having in some way coated pyrite framboids
(Figure~\ref{fig6}D). Many wood fragments are pyritized, as illustrated
in Figure~\ref{fig6}F. The fact that such pyritized objects are
magnet-sensitive implies that a sensitive mineral phase is in
association because pyrite does not react to magnetic fiels (at room
temperature). Some ferrimagnetic grains were extracted using a simple
magnet. As shown with Figures~\ref{fig7}A and~B, the grains
are of small dimensions and dominantly euhedral. SEM analysis using EDS
indicates that these grains are made with magnetite and euhedral
apatite crystals are associated with magnetite (Figure~\ref{fig7}B). In
our samples, magnetite was not associated with glauconite{\break} nor
pyrite. 

\begin{figure*}[p!]
\includegraphics{fig06}
\caption{\label{fig6}SEM images of the chronology of the growth of
authigenic minerals. (A--C)~Pyrite (whitish or light grey)
tetrahedrons or framboids over glauconite grains or bryozoans (dark
grey). (D)~The dark grey glauconite developed over a polyframboid of
pyrite, and tetrahedrons or octahedrons of pyrite grew in turn over the
glauconite. (E)~A sort of glauconitized sheet wraps a pyrite
polyframboid. (F)~Pyritized wood fragment.}
\end{figure*}

\begin{figure*}
\includegraphics{fig07}
{\vspace*{-.1pc}}
\caption{\label{fig7}(A)~Magnetic minerals surrounding the tip of a
steel needle. (B)~SEM image of the magnetite grains bearing authigenic
apatite (elongated, dark grey crystals indicated with arrows).}
{\vspace*{-.45pc}}
\end{figure*}

\begin{table}[t!]%tab2
\caption{\label{tab2}Chemical analyses (through EDS-equipped SEM) of
glauconite particles, expressed in weight~\%\vspace*{-.3pc}}
\begin{tabular}{c}
\tbody
\inlinefig{fx01}
\end{tabular}
\tabnote{Grey numbers: pellet-like grains and epigenized bryozoans;
blue numbers: the grain with an inner structure made up with glauconite
filament, pictured in Figure~\ref{fig5}D; pink numbers: the film
wrapping a polyframboid, pictured in Figure~\ref{fig6}E.}
\vspace*{-.5pc}
\end{table}

\looseness=-1
In the soft injectites, the mineral assemblage sensitive to magnetic
fields is less diversified (Table~\ref{tab1}). No magnetite and no
glauconitized microfossils or microstructures were observed. Only
smoothsurface, dark-green, glauconite grains are present and their size
range is narrow compared to the grains of the sandstones. In addition,
magnet-sensitive quartz grains and wood fragments are common. The
magnet-sensitive wood fragments are frequently associated with altered
pyrite. The remaining components of the soft injectites, that is, the
fraction that is not sensitive to magnetic fields, are quartz grains
and more or less abundant wood fragments
(Figures~\ref{fig5}B,~C).

\looseness=-1
The sands of the topmost part of the Gr\`{e}s de Ch\^{a}tillon Fm. are
hosting lithified sandstone blocks and they fed the soft injectites
(Figure~\ref{fig2}). Their mineral composition is even poorer, relative
to the first two facies: quartz grains are dominant, accompanied by
accessory wood fragments. Glauconite grains, magnet-sensitive,
green-coated quartz grains and wood fragments associated with altered
pyrite are rare. At the boundary between the Gr\`{e}s de Ch\^{a}tillon
and Argiles de Ch\^{a}tillon formations, the topmost sand bed is
orange-colored and rich in bioclasts. Through photonic or electronic
microscope observation, the orange particles look like small flakes or
altered framboids, made of iron oxides as evidenced by EDS analysis. 

\section{Interpretations} \label{sec4}
\subsection{Fast forming authigenic minerals} \label{sec4.1}

The mineral composition of the three facies discussed here is
contrasted in terms of authigenic-grain preservation through early
diagenesis. As reminded above, the sandstones and lithified injectites
were cemented during the earliest diagenesis. The hosting sands have
not been cemented and display a poorer mineral composition compared to
the sandstones. However, being lateral equivalents, both facies were
identical from a compositional point of view, at the time of sediment
deposition. Therefore, the early cementation of some parts of the sand
deposits sealed the pore space, thus preventing further fluid
circulation and thence, fragile grains were protected against
dissolution. Thus, magnetite and rounded, pellet-like, glauconite
grains were preserved from oxidation, together with the peculiar
glauconite grains showing an inner structure: bryozoan fossils and the
grains looking like microbial structures (filaments and wrap).
Accordingly, small glauconite grains, with a relatively large specific
surface, were also preserved. All these objects cannot have formed
during later diagenesis, because the sealed porosity must have
prevented any subsequent mineral precipitation by suppressing any ion
supply {via} pore fluids. Consequently, the fact that these
objects were already present prior to early cementation evidences that
glauconite and magnetite formed \mbox{during} the earliest diagenesis
steps. It thus confirms that under certain circumstances, glauconite
may be a fast-forming mineral. On the contrary, in the uncemented
sands, where fluid circulation could take place, all these fragile
objects disappeared (including the smallest glauconite grains), and
together with them, the record of some benthic organisms, i.e., in the
present case, bryozoans. The soft injectites seem to be an intermediate
situation. The sandy material that was injected into the claystones
must have been the same as that present in the sands and sandstones of
the gr\`{e}s de Ch\^{a}tillon Fm. This material has been caged within a
level of sediments with low permeability but it was not sealed by a
cementing phase. Thus, fluid circulation was possible and the injected
material was only partly sheltered from\break oxidation.

The fact that fluid circulation must have been a little bit easier
within the basal level of the Argiles de Ch\^{a}tillon compared to the
sealed sandstones and lithified injectites is important to determine
the time when glauconite and magnetite were formed: before injection or
after it? Did the material of the \textit{soft injectites} already
contain grains of magnetite or glauconite, or did these grains appear
later in the injected material? The authigenic formation of such grains
requires dissolved ions being available in pore fluids. As magnetite
and glauconitized bryozoans are absent from the soft injectites, it is
suggested that the authigenesis of these two minerals was not possible
in this medium, to be put in relation to the low permeability of the
claystones at the base of the Argiles de Ch\^{a}tillon. A fortiori, as
said above for early lithified materials, such authigenesis would have
been even more hampered in a medium being sealed precociously through
early cementation. Consequently, it is inferred that glauconite and
magnetite appeared during the earliest diagenesis, prior to any
synsedimentary fault movement. Later, while some localized places
within the sands were undergoing incipient cementation, some syndeposit
movements triggered the injection of sands either at the uppermost
level of the Gr\`{e}s de Ch\^{a}tillon or in the lowermost level of the
Argiles de Ch\^{a}tillon. The sands thus injected into sandstones being
cemented were also cemented almost contemporaneously, and therefore,
protected from further fluid circulation. Their authigenic content was
``locked'' and this memory kept intact. Besides, the sands injected
into the Argiles de Ch\^{a}tillon were not so well protected from
further exchanges with fluids, and only part of their authigenic
content was preserved (partial loss of memory). Magnetite and bryozoan
fragments disappeared, as well as small glauconite grains. Lastly, the
uncemented sands of the uppermost part of the Gr\`{e}s de Ch\^{a}tillon
were washed drastically through time, losing almost any evidence of the
authigenic phases but rare, dark-green colored glauconite grains (an
almost complete loss of memory). The fact that only dark-green coloured
grains are observed suggests that the light-green and pale-grey
glauconite grains, which are the immature ones, had more porosity in
them to alter and get removed from the sands
\citep{Baldermannetal2013}.\looseness=-1

A first information is that within such shoreface sands, authigenic
phases can develop during the earliest steps of diagenesis. This point
must be stressed on, with respect to glauconite. This work reinforces
the growing view considering that glauconite grains can form in a wide
range of marine environments and of durations (from the earliest
diagenesis to millions of years; \citet{Banerjeeetal2015,
Banerjeeetal2016a, Banerjeeetal2016b};
\citet{Tribovillardetal2023b, Tribovillard2024} and references therein).
Nevertheless, the earliest \mbox{diagenesis} is not a quantified
duration {per se}. To refine a bit the datation, an on-going
study of the age of the carbonate cements of the Jurassic sandstones of
the Gr\`{e}s de Connincthun Fm. must be mentioned
\citep{Blaiseetal2025}. This formation is older than the Gr\`{e}s de
Ch\^{a}tillon: \textit{Aspidoceras caletanum} ammonite zone of the
middle Kimmeridgian \citep{Mansyetal2007}. This formation was also
affected by synsedimentary movements accompanied by fluid seepages that
induced an early cementation \citep{Hatemetal2014}. The paper by
\citet{Blaiseetal2025} reports U--Pb datations yielding an age of est
148.9 ${\pm}$ 5.7~Ma, that is, very{\break} close to the
Kimmeridgian--Tithonien boundary, according to the 2024/12 version of
the International Chronostratigraphic Chart
(\url{https://stratigraphy.org/ICSchart/ChronostratChart2024-12.pdf}).\ 
Such results cannot be over interpreted.\ It may just be stated that
the cements probably induced by fluid circulations have about the same
age as that of the early lithified objects at stake in this paper.\ 
These results give support to the interpretations of
\citet{Hatemetal2014} about fluid circulation but the precision of the
ages is not adapted to the questions at stake\break here.

\subsection{Pyrite and glauconite co-occurrence} \label{sec4.2}

Our SEM observations show that pyrite may have grown onto glauconite as
well as glauconite may have coated, or even wrapped pyrite framboids.
Accordingly, because pyrite must not react to magnetic fields, if
pyrite particles and pyritized wood fragments are sensitive magnets, it
may be inferred that a part of their pyrite load has been converted to
glauconite (or magnetite). Whereas the pyritization of wood fragments
is rather common \citep[e.g.,][]{Rickard2012}, the close association of
glauconite and pyrite is seldom reported \citep{Fanningetal1989,
RabenhorstFanning1989}. Pyrite is a mineral where iron is present in
its reduced form. Glauconite and magnetite contain simultaneously the
reduced and the oxidized forms of iron (Fe$^{2+}$ and Fe$^{3+}$).
Therefore, the formation or growth of these authigenic \mbox{minerals}
results from contrasted redox conditions. As a consequence, the
observed co-occurrence of pyrite and glauconite suggests that these
authigenic minerals must have fed on a common pool of iron. However,
following slight redox variations within the pore space, the presence
of dominant Fe(II) would have led to pyrite precipitation, whereas an
iron pool with both Fe(II) and Fe(III) coexisting would have induced
the formation of glauconite (and magnetite). In addition, the presence
of pyrite growing on glauconite as well as glauconite growing on pyrite
implies that the redox conditions were fluctuating in both ways within
the pore space during the earliest diagenesis, prior to cementation
\citep{CanfieldBerner1987, Suketal1990, Brothersetal1996, Roberts2015,
Rungeetal2023}. 

\subsection{A bio-induced origin?} \label{sec4.3}

Some rare glauconite grains allow a microbial structure to be
hypothesized: bacterial filaments, fungi, or an abiotic phenomenon that
mimics a biological structure (Figure~\ref{fig5}D). Besides, pyrite
framboids wrapped by biological biofilms, such as the structure visible
in Figure~\ref{fig6}E, has already been reported \citep{Rickard2021}
but it is the first time, to the best of our knowledge, that such a
biofilm made of glauconite is reported. The biofilm looking so thin
must have been delicately mineralized through the ion by ion
replacement called epigenization. It is also the first time that such
glauconite-made filamentous structures are reported. It may be wondered
whether glauconite fossilized biological (bacterial?, fungal?)
structures or whether microbial activity induced glauconite
precipitation. In addition, SEM images show that magnetite and apatite
are intricately associated, showing euhedral morphologies typical of
authigenic formation. Such a mineral association is not uncommon and
may be interpreted as the result of bacterially mediated reactions,
e.g., dissimilatory iron reduction \citep{Huetal2025}. As a
consequence, a bundle of concordant clues suggests that the authigenic
processes highlighted in this study are directly linked to microbial
activity.

The role of bacteria in the formation of sedimentary pyrite is beyond
doubt \citep{Rickard2012, Rickard2024} but this role is not considered
in the genesis of glauconite. Nevertheless, \citet{Geptneretal2017}
reported the formation of finely dispersed glauconite within algal
borer trails and holes, in Oligo-Miocene sediments of Kamchatka. In
addition, bacterial activity is involved in the redox reactions
affecting the iron pool of the pore space  \citep[iron(II)-oxidizing
bacteria or IOB, as well as iron(III)-reducing bacteria or IRB;
e.g.,][]{Jorgensen2006, MiotEtique2016,Rickard2024}. Consequently, the
interrelationships of IOB and IRB within the pore space must have made
iron(II) and/or iron(III) available for the authigenic formation of
various iron-bearing minerals (glauconite, magnetite, pyrite) following
subtle variations in the redox status at so little scale
\citep{Mathonetal2024, Vosteenetal2024}. This \mbox{indirect} role of
bacteria, {via} iron chemistry, together with the presence of some
structures strongly resembling bacterial features mentioned above,
allow us to question the possible role of bacteria in the formation of
\mbox{glauconite}.

\subsection{Organic matter remineralization and oxygen consumption}
\label{sec4.4}

Small dimension woody fragments are common to frequent in the facies
studied here. They are less abundant and less well preserved in the
sands than in the soft injectites and in the sandstones. The presence
of such recalcitrant organic products is expected, because fragile
(also termed labile) organic compounds are rapidly remineralized in
such coarse sediments \citep[e.g.,][]{Tyson1995}. Sands being a porous
and permeable milieu, oxygen-deprived conditions, inducing reducing
conditions, are not expected to develop. Nevertheless, the presence of
reduced iron-bearing minerals in the sandstones testifies to the
development of reducing conditions in the pore space during very early
diagenesis, most probably resulting from organic matter respiration or
remineralization. The presence of scattered pyrite means that
sulfide-reducing bacteria were locally thriving, probably accompanied
by other bacterial consortia \citep{Jorgensen2006}, all of these being
heterotrophic and therefore, consuming organic matter. Such bacterial
populations usually do not use woody fragments, favoring labile organic
products. Nevertheless, the woody fragments are less frequent in the
sands, compared to the other two facies, namely, soft injectites and
sandstones. This observation could be accounted for by the priming
effect \citep{Bianchi2011, Yangetal2023}. Whether the priming effect
was the cause or not, our observations suggest that the unconsolidated
sands were an environment that made possible the loss of part of the
pool of recalcitrant woody organic matter, in addition to all of the
fragile organic material. Incidentally, such reactions to degradation
of recalcitrant or labile organic matter may account for the fact that
the shoreface sandstones are so frequently lean for organic matter,
although biological production and activity are intense there. On the
other hand, when the sediments have been ``frozen'', one might say, by
early cementation, the provisional richness in organic debris of these
sands can be observed.

\subsection{The shoreface environment, a trap for iron?} \label{sec4.5}

This study evidences how reactive the shoreface environment may be,
with respect to early diagenetic, microbially mediated or -related,
reactions involving iron. The uncemented sands yield an expected
mineralogical composition with dominant quartz grains, wood fragments
and some accessory minerals, e.g.\ glauconite grains. However, early
diagenesis, when it locked the pore space and turned the sands into
early-cemented sandstones and injectites, has somehow ``frozen'' the
situation as it was shortly after the sediment was deposited. Thanks to
this ``snapshot'', all the consequences of the biogeochemical activity
of this sedimentary environment can be observed and highlighted.
Notably, iron-bearing minerals bear witness to this intense activity.
It is thus visible that, although the shoreface sediments are porous
and permeable, that is, favorable to aerobic conditions and oxidizing
reactions, redox conditions can turn rapidly to suboxic or anoxic,
allowing pyrite, glauconite and magnetite to develop simultaneously or
successively. Such diagenetic reactions trapped reactive iron. As they
occurred shortly after deposition, iron was trapped early, at the depth
below the seafloor where grains are no longer mobile and where the
dissolved oxygen consumed by organic decay is not replenished fast
enough, all this ending with suboxic/anoxic conditions. If such sands
are not cemented fast enough, they will release the trapped iron,
leaving sandstones showing the typical assemblage of quartz, wood
fragments and calcitic bioclasts (plus possible accessory minerals). 

The following two questions may arise: (1)~why did (do) these
authigenic minerals disappear \mbox{during} subsequent steps of
diagenesis?, (2)~why did the sands get cemented early? As regards to
the first question~(1), it is expected that, within the sands not
lithified yet, once all the reactive, labile organic matter is
consumed, the porous and permeable milieu prevents suboxic/anoxic
conditions to maintain. In~addition, under oxygen-deprived conditions,
denitrifying bacteria are able to oxidize pyrite, while other microbial
population reduce Fe$^{3+}$ and Mn$^{4+}$ ions through pyrite oxidation
\citep{Jorgensenetal2009}. Magnetite and glauconite may thus turn into
iron oxides (e.g., hematite). Such rust-colored oxides are abundant in
the topmost 15 cm of the Gr\`{e}s de Ch\^{a}tillon
(Figure~\ref{fig2}B). This peculiar rusty level does not result from
supergene weathering by either meteoric water or seawater: as a matter
of fact, first, it is sheltered from {per descensum} meteoric
circulations by the thick, overlying, claystone and shale accumulations
of Kimmeridgian and Tithonian ages. Second, the erosional retreat of
the coastline maintains the freshness of the outcrop and this level can
be observed with the same visual characteristics all along the
coastline between the Pointe du Ridden and the Cap de la Cr\`{e}che,
that is, between the Cap Gris Nez and the city of Boulogne-sur-Mer
(Figure~\ref{fig1}). As regards to the second question~(2), a common
way for sands to get lithified early is the lithification process known
as beachrock \citep{TuckerWright1990, Neumeier1999, Hibneretal2025} but
the Gr\`{e}s de Ch\^{a}tillon does not match this case: rock thin
sections do not show early cements but equant spar drusy mosaics and
poikilotopic spar cements. In the Boulonnais area, all the fine-grained
carbonate beds (therefore excluding the numerous coquina beds) and
carbonate nodules of the Jurassic formations studied so far resulted
from bacterially mediated, early diagenetic precipitation of CaCO$_3$.
The bacterial activity was fueled by cold, methane-bearing, fluid
seepages. The seepages were linked to synsedimentary tectonic movements
\citep{Hatemetal2014, Hatemetal2016}. It can be hypothesized that the
same mechanism applies to the sandstone at stake here \citep{Blaiseetal2025}. 
Whatever the cause of the early cementing,
the present works highlights the cornerstone importance of such
early-lithified objects to visualize and identify the fauna (here
bryozoans) and authigenic minerals (glauconite, magnetite) initially
present and since disappeared. Authigenic carbonate beds, nodules,
concretions, septaria, early cemented sandstones (plus, here, lithified
injectites) are thus useful (if not essential) tools for reconstructing
paleo-ecological or -environmental conditions during deposition or
shortly after it.

\subsection{Estuarine environments} \label{sec4.6}

Here it is insisted on the fact that the redox degree may condition the
variety of authigenesis: pyrite is forming when iron is dominantly in
the reduced form, glauconite and/or magnetite when iron is present both
in the reduced form and the oxidized one. In sedimentary environments,
pyrite results from sulfate reducing reactions, induced by microbial
activity (see \citet{Burdige2006} or \citet{Rickard2012} for reviews).
The formation of glauconite and/or magnetite implies that sulfate
reduction was not complete (otherwise, iron would have been entirely
turned into Fe(II)). Such a limitation of sulfate reduction can result
from the redox status or from the sulfate pool itself. Sulfate
abundance could be the limiting factor and sulfate limitation could be
due to dilution by freshwater. The Boulonnais area is interpreted to
have been close to a (large?) estuary \cite{Hatemetal2017};
therefore, a dilution of the seawater (and its sulfate content) cannot
be ruled out. This statement is based on assumptions and might look
like a bit speculative; nevertheless, our hypothesis is worth
exploring. Modern occurrences of authigenic glauconite grains encompass
many estuarine
and littoral environments \citep[e.g.,][to mention a
few]{ChafetzReid2000, ElAlbanietal2005, Banerjeeetal2015,
Banerjeeetal2016a, Banerjeeetal2016b, Bansaletal2018,
RubioLopezPerez2024}.
To give some support to the hypothesis, it may be reminded that
dolomite can be observed in thin sections. Dolomite may have several
origins and one of them is linked to sulfate limitation
\citep[e.g.,][]{TuckerWright1990}. Consequently, estuarine
environments, with their freshwater inflow, may have been specific
places over geological times, favorable to the rapid authigenic
formation of glauconite grains, provided reactive iron was present. Of
course, this scheme does not preclude the existence of other mechanisms
and environments prone to {authigenic} glauconite \citep[e.g.,
see][for a \mbox{review}]{Giresse2022}.

\subsection{Life on Mars?} \label{sec4.7}

The title is a bit provocative but NASA reported the presence of
sandstone nodules beside the so-called ``Nankluft Plateau'' on Mount
Sharp on the planet Mars (image from Curiosity Mast Camera;
\url{https://assets.science.nasa.gov/dynamicimage/assets/science/psd/mars/downloadable\_items/3/9/39117\_mars-rover-curiosity-sandstone-nodule-PIA20324.jpg}).
Such nodules could have recorded and ``kept in memory'' fleeting,
short-lived, minerals such as those studied here, that may be related
to biotic processes \citep[e.g.,][and references
therein]{Siljestrometal2024}. 
\vspace*{-.25pc}

\section{Conclusions} \label{sec5}

Examining sediments where the pore space had been sealed during the
earliest steps of diagenesis allowed us to highlight the following key
points.
\begin{itemize}
\item Glauconite can form very precociously.
\item If the role of micro-organisms in the formation of pyrite (and
magnetite) is beyond doubt, the question for the formation of
glauconite here appears to be entirely legitimate.
\item The shoreface sands can be a milieu with unsuspected oxygen-poor
conditions triggering the early precipitation of iron-bearing minerals.
The iron trapped by these authigenic minerals is gradually released
into the marine environment as they disappear due to the permeability
of the hosting sands. The land-sea transition zone is therefore an
important regulator of the iron flux to the ocean.
\item Early cemented sediments are in such circumstances the memory of
early diagenesis.
\item Because they may limit the abundance of sulfate and, therefore,
the intensity of sulfate reduction reactions, estuarine environments
with a suitable reactive-iron load could be places prone to the
formation of glauconite (and magnetite) grains.
\end{itemize}

In addition, it may be wondered whether the priming effect accounts for
the degradation of recalcitrant organic matter in shoreface
environment.

This work shows that early cemented sediments can record diagenetic
steps that uncemented or lately cemented sediments are unable to ``keep
in memory''. For geochemists, carbonates are not good recorders of the
diagenetic sequence because they make a system that is generally not
well closed or sealed: for instance, upon recrystallization, they can
release or incorporate elements or specific isotopes. In spite of such
pitfalls, this study shows that early carbonate cementation is a
pre-requisite to be able to infer paleoenvironment reconstructions of
coastal, clastic dominated deposits and milieus. With a little
audacity, one could even propose that the memorial quality of early
cemented deposits makes them ideal targets for searching for traces of
biological (microbial) activity on the planet Mars.

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\section*{Acknowledgments}

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This work has been performed using the Plateform CARMIN -- Ulille
infrastructure and technical support. Our thanks go to the Programme
Tellus Syster of the French Institut des Sciences de l'Univers (INSU)
for funding our work. We thank Monique Gentric for the
financial/administrative management, and Noe Frisch for the technical
support of this project, as well as the LOG laboratory and the
Department of Earth Sciences of the University of Lille for their
support. We thank our referees for their useful and appreciated
comments, as well as the CR Geoscience's staff.

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\section*{Declaration of interests}

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The authors do not work for, advise, own shares in, or receive funds
from any organization that could benefit from this article, and have
declared no affiliations other than their research organizations.




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\section*{Supplementary data}

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Supporting information for this article is available on the journal's
website under \printDOI\ or from the author.

\CDRsupplementaryTwotypes{supplementary-material}{\cdrattach{crgeos-301-suppl.pdf}}

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