Taphonomic pathway of exceptionally preserved fossils
in the Lower Ordovician of Morocco
Farid Saleh, Bernard Pittet, Pierre Sansjofre, Pierre Guériau, Stefan Lalonde,
Jean-Philippe Perrillat, Muriel Vidal, Victoire Lucas, Khadija El Hariri,
Khaoula Kouraïss, et al.
To cite this version:
Farid Saleh, Bernard Pittet, Pierre Sansjofre, Pierre Guériau, Stefan Lalonde, et al.. Taphonomic
pathway of exceptionally preserved fossils in the Lower Ordovician of Morocco. Geobios, Elsevier,
2020, 60, pp.99-115. 10.1016/j.geobios.2020.04.001. hal-03005069
HAL Id: hal-03005069
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Taphonomic pathway of exceptionally preserved fossils in the Lower
Ordovician of Morocco
Farid Saleh Bernard Pittet Pierre Sansjofre Pierre Guériau Stefan
Lalonde Jean-Philippe Perrillat Muriel Vidal Victoire Lucas Khadija
El Hariri Khaoula Kouraiss Bertrand Lefebvre
PII:
S0016-6995(20)30029-2
DOI:
https://doi.org/doi:10.1016/j.geobios.2020.04.001
Reference:
GEOBIO 904
To appear in:
Geobios
Received Date:
6 February 2020
Revised Date:
27 February 2020
Accepted Date:
3 April 2020
Please cite this article as: Saleh, F., Pittet, B., Sansjofre, P., Guériau, P., Lalonde, S., Perrillat,
J.-P., Vidal, M., Lucas, V., Hariri, K.E., Kouraiss, K., Lefebvre, B.,Taphonomic pathway of
exceptionally preserved fossils in the Lower Ordovician of Morocco, Geobios (2020),
doi: https://doi.org/10.1016/j.geobios.2020.04.001
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Taphonomic pathway of exceptionally preserved fossils in the Lower
Ordovician of Morocco
Farid Saleh a,*, Bernard Pittet a, Pierre Sansjofre b, Pierre Guériau c, Stefan Lalonde d, Jean-Philippe
Perrillat a, Muriel Vidal d, Victoire Lucas a, Khadija El Hariri e, Khaoula Kouraiss e, Bertrand Lefebvre a
a
Univ. Lyon, Université Claude Bernard Lyon 1, ENS de Lyon, CNRS, UMR 5276 Laboratoire de
b
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f
Géologie de Lyon : Terre, Planètes, Environnement, F-69622 Villeurbanne, France
MNHN, Sorbonne Université, CNRS UMR 7590, IRD, Institut de minéralogie, des Matériaux et de
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Cosmochimie, Paris, France
Institute of Earth Sciences, University of Lausanne, Géopolis, CH-1015 Lausanne, Switzerland
d
Univ. Brest, CNRS, IUEM Institut Universitaire Européen de la Mer, UMR 6538 Laboratoire
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c
Département des Sciences de la Terre, Faculté des Sciences et Techniques, Université Cadi-Ayyad,
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e
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Géosciences Océan, Place Nicolas Copernic, 29280, Plouzané, France
BP 549, 40000 Marrakesh, Morocco
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* Corresponding author. E-mail address: farid.saleh@univ-lyon1.fr (F. Saleh).
Corresponding editor: Emmanuel Fara.
Abstract
The Fezouata Shale in Morocco is the only Lower Ordovician Lagerstätte to yield a diverse
exceptionally preserved marine fauna. Sediments of this formation have yielded soft to lightly
Page 1 of 37
sclerotized taxa that were previously unknown from the Ordovician. Yet the taphonomic pathway of
fossils from this formation remains poorly understood. Here, based on drill core material, a close
association between exceptional preservation and a specific sedimentary facies is evidenced in the
Fezouata Shale. This facies corresponds to calm sea-bottoms, sporadically smothered by distal storm
deposits. The patterns of exceptional preservation in this facies indicate that most animals were dead
and decayed on the seafloor prior to their burial by distal storm deposits. Furthermore, contrasted
elemental and molecular compositions between fresh-cored and altered materials show that surface
deposits of the Fezouata Shale were substantially affected by recent weathering. This weathering
resulted in the leaching of organic materials from fossils originally preserved as carbonaceous
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compressions and the transformation of pyrite into iron oxides. Understanding the processes behind
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the current patterns of soft tissue preservation in the Fezouata Shale is essential prior to any
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paleontological description, especially of taxa with no current representatives.
Keywords:
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Depositional environment
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Sedimentary facies
Lagerstätten
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Mineralization
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Fezouata Shale
1. Introduction
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Konservat-Lagerstätten have revolutionized our understanding of metazoan evolution and
diversification, owing to the preservation in these deposits of soft-bodied and lightly sclerotized
organisms that normally are not preserved (Caron et al., 2006, 2010; Smith and Caron, 2010;
Gutiérrez-Marco and García-Bellido, 2015; Lerosey-Aubril et al., 2017; Knaust and Desrochers,
2019). These deposits are particularly abundant in Cambrian Series 2 and 3, providing critical insights
into the Cambrian Explosion, one major pulse in animal evolution (Butterfield, 1995; Liu et al., 2008;
Zhang et al., 2008; Duan et al., 2014; Lei et al., 2014). The younger Fezouata Biota (late
Tremadocian) was discovered in the early 2000s in the Central Anti-Atlas of Morocco, and is the only
Lower Ordovician Lagerstätte to yield a diverse exceptionally preserved fauna (Van Roy et al., 2010,
2015a), providing key information on the transition between the Cambrian and Ordovician (Lefebvre
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et al., 2016). Anatomical information found in fossils from this deposit is critical for deciphering the
evolution of major animal phyla (Vinther et al., 2008, 2017; Van Roy et al., 2015b; Lefebvre et al.,
2019).
The general depositional environment of the Fezouata Shale is constrained, and is stormdominated with an indirect influence of tides (Martin et al., 2016; Vaucher et al., 2017). The processes
behind the formation of sedimentary structures related to this environment were explained in recent
works (Vaucher et al., 2016, 2017). Two types of exceptional preservation have been documented in
the Fezouata Shale: the first one occurs in concretions (Gaines et al., 2012a). This type of preservation
requires vigorous sulfate reduction around carcasses, resulting in the establishment of prominent
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chemical gradients around dead animals and leading to the early precipitation of minerals around non-
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biomineralized tissues (Gaines et al., 2012a). The other type of preservation is associated with shale
(Martin et al., 2016). In these levels, fossils occur exclusively at bed junctions and not within beds
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(Vaucher et al., 2016, 2017), strongly supporting the view that organisms were smothered on the
seafloor under a new blanket of distal storm deposits, rather than having been carried in sediment
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flows (Saleh et al., 2018).
However, the step-by-step mechanism behind this type of preservation remains largely
unexplored. Most fossils collected in shales are preserved as molds or imprints on the sediments
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(Martin et al., 2016), but it is unclear whether these organisms were originally preserved as
carbonaceous compressions. Other non-biomineralized tissues, such as trilobite digestive tracts and
echinoderm water-vascular systems, are preserved in 3D red to orange iron oxides (Gutiérrez-Marco et
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al., 2017; Lefebvre et al., 2019). Considering that numerous diagenetic mechanisms may alter the
original anatomy of fossil organisms over geological time, deciphering the taphonomic processes at
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play in the Fezouata Shale is essential for palaeontological interpretations, especially for taxa without
extant representatives. Consequently, the aim of this study is to provide insights into soft tissue
taphonomy in the Fezouata Shale based on a detailed sedimentological investigation constraining the
facies in which exceptional preservation occurred, in addition to a careful geochemical analysis
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deciphering the mechanism leading to the current patterns of preservation in this facies.
2. Geological context
During the Ediacaran (600 Ma), the Panafrican Orogeny led to the formation of the Gondwana
supercontinent. Gondwana extended from the South Pole to intermediate latitudes in the Northern
Hemisphere. A rifting phase took place in its western part at the end of the Cambrian. At the
beginning of the Ordovician, a long-term transgression resulted in the flooding of Gondwanan margins
by epicontinental seas (Destombes et al., 1985). The entire Lower Ordovician succession in the Zagora
region in Morocco was deposited in a generally shallow environment at high latitude, close to the
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palaeo-South pole (Torsvik and Cocks, 2011, 2013; Fig. 1(A)). These deposits unconformably overlie
middle to upper Cambrian strata and are separated by an unconformity from the overlying lower to
middle Darriwilian (Middle Ordovician) deposits of the Tachilla Fm. (Choubert, 1952; Destombes et
al., 1985). The Fezouata Shale (Tremadocian–Floian) consists of blue-green to yellow-green sandy
mudstones and siltstones that coarsen upwards. They are up to 900 m thick in the Zagora region
(Vaucher et al., 2016). The long-term transgression at the beginning of the Ordovician was followed
by a regression leading to the deposition of massive dark brown sandstones characteristic of the Zini
Fm. (late Floian) above the Fezouata Shale (Martin et al., 2016). The Lower Ordovician succession
was interpreted to have been deposited in a storm-wave dominated sedimentary environment (Martin
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et al., 2016) indirectly influenced by tides (Vaucher et al., 2017). The corresponding
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palaeoenvironments range from the foreshore (S-SE) to the upper offshore (N-NW) (Vaucher et al.,
2017). In intermediate settings of the Ternata plain, the Bou Izargane locality (Fig. 1(B)), studied in
the present work, has yielded abundant exceptionally preserved fossils including lightly cuticularized
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arthropods, sponges, and soft parts of echinoderms (Van Roy et al., 2015a; Botting, 2016; Lefebvre et
al., 2019). This locality (Fig. 2) exposes part of the lower interval with exceptional preservation, dated
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as late Tremadocian (Araneograptus murrayi graptolite Zone; Lefebvre et al., 2018).
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3. Material and methods
3.1. Field work
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In February 2018, a 6.5 m core was drilled at the top of the Bou Izargane section
(30°30'00.5" N; 5°50'56.7" W; Fig. 2: core 1), in the Ternata plain, ca.18.5 km N of Zagora
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(Morocco), and a second 6.7 m core (Fig. 2: core 2) was made 6.8 m below the first one to cover most
of the sedimentary succession in this locality. Both cores are temporally stored at the University of
Brest, France. The outcrop in this locality was logged and highly excavated in 2014 and yielded
hundreds of exceptionally preserved fossils, registered in the collections of the Cadi-Ayyad
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University, Marrakesh.
3.2. Sediment preparation and analyses
The cores were cut and scanned, using a core XRF-scanner, for major elements (Si, Al, K)
expressed as oxides (wt% SiO2, Al2O3 and K2O) at the University of Brest, France. Then, they were
described for their lithology, grain size, depositional sedimentary structures, and bioturbation intensity
and size, and drawn on a 1:1 scale at the Laboratoire de Géologie de Lyon, France. The uppermost
2 m of each core are extremely weathered and show the same greenish color as on the outcrop. The
lower, fresher portions of the cores range from dark grey to black in color. Twelve thin sections were
Page 4 of 37
made from the cores. Transect analyses combined into elemental maps (Table S1, Appendix A) from
both green and black core sediments were made on nine samples using a Bruker M4 Tornado microXRF instrument operating at 50 kV, 600 A. This mapping of the major elements was done to better
visualize discrete lithological changes in the facies and to determine the composition of silty to very
fine sand grains (Fig. 3). In addition, around 100 Raman spectra were collected from nine core
specimens (Table S1, Appendix A) using a Labram HR800 - Jobin Yvon Horiba spectrometer
equipped with semi-confocal optics at the University of Lyon, France. A microscope with a 100
objective was used to focus the excitation laser beam, 532 nm exciting line, on a 1-3 µm size spot and
to collect the Raman signal in the backscattered direction. Acquisitions were performed using two
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accumulations of 30 s and a laser power of about 5 mW on the sample surface.
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3.3. Statistical approach
Cores give precise information in terms of sedimentary facies and their evolution, but only
minimal information on the vertical occurrences of exceptionally preserved fossils. Conversely, field
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and hand sample observations made at Bou Izargane provide important information on the occurrence
of exceptional preservation, but with unprecise information on the facies in which exceptional
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preservation occurred, due to surface weathering. Thus, the stratigraphic sequence from the core was
compared to the field-based sequence logged along the same section and described by Vaucher et al.
(2016; Fig. 4(A, B)). Cores were made starting at the upper surface of the outcrop from which the
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original log was made. For more precision, correlations between cores and outcrop were made based
on comparisons of facies defined in Vaucher et al. (2017) and in the present study (Table 1). Using
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this correlation, a statistical approach was developed to link these two distinct, though complementary
sets of data gathered from outcrops (i.e., occurrences of exceptional preservation) and from drill cores
(i.e., detailed sedimentary facies).
The obtained 13.2 m-thick core succession was divided into 22 60 cm-thick successive
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intervals. Then, the proportion of each sedimentary facies identified was calculated in these intervals.
A Principal Component Analysis (PCA) was performed to identify the facies accounting for the
largest variance between the 22 intervals (Hammer et al., 2001). Facies that are homogeneously
distributed are less likely to explain discrepancies in occurrences of exceptionally preserved fossils
and therefore were removed from further statistical analysis. The facies exhibiting the highest
dissimilarity (i.e., with the largest variance) were selected for a Classical (hierarchical) Cluster
Analysis (CCA). CCA allows investigating the heterogeneities in terms of sedimentary facies between
the 22 intervals by separating them into clusters (Hammer et al., 2001). Vertical alternation of
intervals between the clusters was plotted against the pattern of soft tissue preservation in the field to
check any direct link between the sedimentary facies and exceptional fossil preservation. Then, a
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Similarity Percentage analysis (SIMPER; Hammer et al., 2001) was made to identify which facies
caused the highest dissimilarity between these clusters and thus, to decipher the correlation of different
facies with the absence/presence of exceptional preservation. Finally, a student t-test was applied to
investigate whether the difference in the proportions of facies causing the dissimilarity between
clusters was significant.
3.4. Fossil analyses
Twenty fossil specimens (Table S1, Appendix A) collected from late Tremadocian localities
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in the Zagora area, Morocco, and registered in the paleontological collections of the Cadi-Ayyad
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University, Marrakesh, Morocco (acronym: AA), Lyon 1 University, Villeurbanne, France (UCBLFSL) and the Musée des Confluences, Lyon (ML), were included in this study. Some of these fossils
were analyzed using a FEI Quanta 250 scanning electron microscope (SEM) equipped with
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backscattered and secondary electron detectors in addition to an energy-dispersive X-ray analyzer
(EDX) operating at accelerating voltages ranging from 5 to 15 kV. At low energies, light elements
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such as C can be detected, while at higher energies, detection of heavier elements is optimized. Some
samples were analyzed using a synchrotron beam X-ray fluorescence at the DIFABBS beamline at the
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Soleil synchrotron, Paris, France, in order to determine the minor-to-trace elemental composition of
4. Results
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4.1. Core description
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the fossils, as well as of the surrounding matrix.
Both cores are dominated by Si-rich (Fig. 3(A)), quartz dominated (Fig. 4(A)), normally
graded beds having an erosive base (Figs. 3(A), 4(A)). The thickness of these beds varies from 0.2 to
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2.5 cm. Intervals with finer grains exist between these beds (Figs. 3(A), 4(A)); these levels are Al- and
K-rich (Fig. 3(A)) and are likely more argillaceous (Fig. 4(A)). Mn and Co are present around and
within the layers with the coarsest grains especially in greenish sediments (Fig. 5). The coarsest layers
bear wavy laminations. These wavy layers are hummocky cross stratifications (HCS) with a
centimeter- to decimeter-scale estimated wavelength (Figs. 3, 5), as also observed on outcrops
(Vaucher et al., 2017). Occasionally, these HCS are associated with Ca-rich deposits (Figs. 3, 5). The
distribution of Fe in the cores positively correlates with the general distribution of both Al and K (Fig.
3(A)). In fresh and lightly altered sediments, Fe correlates with S as well (Fig. 3), when pyrite is
present (Fig. 4(B)). This pyrite is generally surrounded by a halo of C-rich organic material (Figs.
4(B), 6). An absence of both pyrite and C is evidenced in surface sediments that are extensively
altered (Fig. 6). Fe-rich minerals in these recently weathered sediments are iron oxides.
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Evidence for bioturbation is abundant in the cores (Figs. 3, 5). Bioturbation is mainly
horizontal (i.e., less than 1-2 cm in depth). Some escape burrows have been observed in coarsegrained layers (Fig. 3(A)). Only one 5-cm vertical bioturbation occurs in the uppermost part of the
sedimentary succession. A detailed mm-scale description of the two combined drill cores is given in
Fig. 7.
4.2. Facies identification
Five sedimentological facies are defined from the core and are designated herein as Fc1 to
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Fc5. Fc1 is the finest grained facies (Fig. 8(A)); it is homogeneous and mostly composed of
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argillaceous material (Fig. 9(A)). Fc2 contains coarser siliciclastic layers (Fig. 9(B)) showing a normal
grading with a considerable amount of fine sediments in between (Fig. 9(B)). Sediments in Fc3 are
coarser than in Fc2 (Fig. 8(A)). Fc3 consists of stacked, normally graded layers with little to no fine-
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grained sediments in between, and in rare occasions some small HCS are present (see Vaucher et al.,
2016, 2017 for direct evidence for HCS; Fig. 9(C)). Fc4 is made of coarser sediments than in Fc3 (Fig.
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8(A)) and contains abundant wavy laminations (HCS; Fig. 9(D, E)). Fc5 consists of coarse siltstones
(Fig. 8(A)) containing sometimes Ca-rich deposits (Fig. 9(F)).
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The increase of quartz (SiO2) and decrease of clays (K2O and Al2O3) from Fc1 to Fc5 is
shown in Fig. 8(B). Bioturbation is mostly present in Fc2, Fc3, Fc4, and Fc5 (Fig. 9) and may vary in
intensity within the same facies (Fig. 9(D, E)). Pyrite occurs mainly in Fc2, Fc3, Fc4, and Fc5, with
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the largest pyrite crystals being observed in Fc2 (Fig. 9(B)).
4.3. Statistical analyzes
Principal component analysis shows that most of the variance between the 22 defined intervals
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is related to Fc2, Fc3, and Fc4 (Fig. 10(A)). Fc1 and Fc5 can be excluded from further statistical tests
because they contribute to less than 5% of heterogeneities between intervals. Based on variations in
the proportion of Fc2, Fc3, and Fc4 in the 22 intervals, two clusters were extracted (Fig. 10(B)). The
alternation of intervals between Cluster 1 and Cluster 2 fits with 95% fidelity the presence/absence of
exceptional preservation in these deposits, validating that this type of preservation is directly linked to
the sedimentary facies in the Fezouata Shale (Fig. 10(D)).
Fc2 and Fc4 are responsible for 81% of the difference between the two clusters (Fig. 10(C)).
Fc2 is abundant in Cluster 1 (Fig. 10(C)) which is correlated with intervals bearing exceptional
preservation (Fig. 10(D)). Fc4 is abundant in Cluster 2 (Fig. 10(C)) that is correlated with intervals
where exceptional preservation is absent (Fig. 10(D)). The differences in the distribution of Fc2 and
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Fc4 in Cluster 1 and Cluster 2 are significant (p = 0.003 and p = 2.8.10 -7, respectively). The difference
in the proportion of Fc3 between the two clusters is not significant (p = 0.1), indicating that this facies
did not significantly contribute to the differences observed between intervals with and without
exceptional preservation.
4.4. Fossil preservation
Red/orange 3D fossils from the Bou Izargane locality appear to be preserved in iron oxides
(Fig. 11(A)). In these samples, iron is present in two different morphologies: abundant in small
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euhedral crystals (Fig. 12(B, C)), and dispersed as framboid-shaped minerals (Fig. 12(D)). The
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obtained SEM spectra, at low voltage, show a low concentration of C in these fossils (Fig. 11(A)) in
comparison with the abundance of C around pyrite in fresh deposits (Figs. 4(B), 6). C is abundantly
present in fresh sediments (Fig. 6) but absent in cuticularized to lightly sclerotized fossils preserved as
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2D imprints as well (Fig. 11(B)). In both 3D and 2D modes of preservation, thin star-like (Fig. 12(E,
G)) iron-rich minerals (Fig. 12(H)) may cover parts of the fossils. The majority of this Fe in star-
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shaped minerals is found in fossils that are covered by Co and Mn-rich deposits (Fig. 13) in rose-like
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minerals (Fig. 12(F, G)).
5. Discussion
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5.1. General depositional environment
The increase in SiO2 and decrease of Al2O3 and K2O from Fc1 to Fc5 (Fig. 8; Table S2,
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Appendix A) is indicative of the energy at which sediments were deposited. In open marine
environments, Si-rich sandstones (i.e., quartz) are found in high-energy proximal settings, while Aland K-rich clays are generally found in lower energy, more distal environments. In the Fezouata
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Shale, Si is associated with the coarsest grained sediments (Figs. 3, 5) originating from the shallowest
settings (i.e., beach to the SE of Zagora; Vaucher et al., 2017). In this sense, the finest grained
sediments (Figs. 3, 5) belong to more distal settings and are K- and Al-rich. The dominant clay
mineral in this formation is illite (Saleh et al., 2019). The presence of oscillatory structures in Fc4 and
Fc5 (Fig. 9) and the absence of these structures in other facies support this interpretation. HCS are
sedimentary structures first described as characteristic of storm deposits (Harms et al., 1975). Wave
oscillation induces wave orbitals in the water column that decrease in size with depth. In a shallow
environment, wave orbitals form large HCS on the seafloor (Vaucher et al., 2016, 2017). Conversely,
in deep environments, these orbitals dissipate before attaining the sediment and thus leave no trace on
the seafloor. Furthermore, the abundance of normally graded beds in the core (Figs. 3(A), 4(A))
indicates that sediment was deposited by successive events of decreasing energy. These event beds can
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be formed either during storms or turbiditic events. In the Fezouata Shale, the monotonous alternation
of event beds with the background sedimentation and the occurrence of HCS favor the interpretation
of event sediments as storm deposits. In this sense, the high frequency of storm events is another
indication of less distal sites, more affected by storm wave oscillations (Vaucher et al., 2016). The
absence of event layers in Fc1 (Fig. 9(A)) indicates that this facies is characteristic of settings below
the Storm Wave Base (SWB). Fc2 shows some event beds isolated in the background sedimentation
and an absence of HCS (Fig. 9(B)). This facies is characteristic of settings below the SWB, but more
proximal than Fc1. Fc3, showing stacked storm events, and rarely HCS (Fig. 9(C)), is more proximal
than Fc2. Fc3 was deposited around the SWB. In Fc4, HCS are abundantly present (Fig. 9(D, E)),
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revealing a more proximal environment above the SWB with higher energy than what is observed in
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Fc3. The coarse grains constituting Fc5 (Fig. 9(F)) and the presence of HCS with a wavelength
estimated to be around 10 cm were deposited closer to the Fair Weather Base (FWB). Due to the
coarse-grained and high porosity of Fc5, elemental enrichment (e.g., Mn; Fig. 5) may occur and alter
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the original elemental distribution of this facies (Fan et al., 1992). The Ca-rich deposits in some
laminae (Figs. 3(A, C), 5) may resemble carbonate cement deposited in deep settings (i.e., basin) of
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some Cambrian Lagerstätten (Gaines et al., 2012b). Carbonate cements are used to explain the
presence of exceptional preservation in some deposits due to their ability to block exchange between
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sediments and the water column thus depriving oxidants of attaining dead carcasses (Gaines et al.,
2012b). However, critical differences exist between these cements and the observed carbonate laminae
in the Fezouata Shale. Carbonate cements from the Cambrian are deposited at the top of turbiditic
events (Gaines et al., 2012b), while in the Fezouata Shale, Ca-rich deposits occur only at the base of
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oscillatory structures with a coarse lithology and a high porosity (Figs. 3(A, C), 5). If carbonate
precipitation occurred in the Fezouata Shale, its original Ca source must be the bioclasts observed in
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thin sections cutting through the bottom of storm deposits (Fig. 4(C)) especially because carbonates
are not evidenced elsewhere in this formation (Vaucher et al., 2016). Thus, the most distal facies is
Fc1 and the most proximal facies is Fc5 with Fc2, Fc3, and Fc4 in between, respectively. This model
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of facies is in accordance with outcrop-based sedimentological models for the Fezouata Shale from
which an outcrop to cores correlation was made (Table 1).
5.2. Facies for exceptional preservation
Exceptional preservation requires burial by event deposits (Vaucher et al., 2016). In the
Fezouata Shale, this condition was present in the entire core (Fig. 10(D)) except in Fc1 that constitutes
only 6% of the studied deposits (Fig. 7). Another requirement for exceptional fossil pyritization is iron
availability. Iron was a limited element in the Fezouata Shale environment (Saleh et al., 2019). In
these deposits, Fe supply was likely associated to periods with high seasonality leading to high iron-
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rich continental fluxes to the sea (Saleh et al., 2019) (Fig. 10(D)). Thus, during intervals with
enhanced Fe availability exceptional preservation could occur in intervals I-4, I-5, I-6, I-7, I-12, I-13,
I-14, I-20, I-21, and I-22 (Saleh et al., 2019; Fig. 10(D)).
Although the general conditions for exceptional preservation were occurring in many
intervals, the presence of exceptionally preserved fossils at Bou Izargane is more restricted (i.e.,
exceptional preservation occurred in I-4, I-5, I-6, I-7, I-13 and I-20; Fig. 10(D)). Thus, the absence of
soft parts, e.g., in I-14, is possibly related to the original absence of living organisms on the sea floor.
This hypothesis is confirmed by the absence of benthic fauna (both hard and soft parts) in this interval,
showing that environmental conditions on surface sediments were probably not favorable for the
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colonization of this environment (Saleh et al., 2018). Statistical analyses show that an alternation of
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clusters, which are reconstructed based on the proportion of different sedimentary facies, can predict
with a 95% fidelity the presence and location of intervals with exceptional preservation discovered in
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the field (Fig. 10(D)). Levels with higher proportions of Fc2 and lower proportions of Fc4 have a
higher potential to yield exceptional preservation (Fig. 10(C)). This is because Fc2 combines rather
calm environmental conditions with lower energy events compared to other facies, allowing living
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organisms to colonize the sea floor (Saleh et al., 2018), in addition to burial during event deposition, a
prerequisite condition for exceptional preservation (Vaucher et al., 2017).
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The unique negative correlation between the alternation of clusters and the patterns of
exceptional preservation is exemplified in I-10 (Fig. 10(D)). I-10 yielded a considerable number of
mineralized skeletons (Saleh et al., 2019). The absence of exceptional preservation in this facies does
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not result from the original absence of living organisms, but may be due to the lack of berthierine in
this level (Saleh et al., 2019). Berthierine is an iron-rich clay mineral that can be deposited in the
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sediments from a primary clay precursor under anoxic conditions (Tang et al., 2017). It is documented
in most intervals with exceptional preservation in the Cambrian (Anderson et al., 2018), and only in
specific levels of the Fezouata Shale in which exceptional preservation occurred (Saleh et al., 2019).
In experimental studies, it was shown that berthierine slows down bacterial decay due to the damage
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of bacterial cells (McMahon et al., 2016). However, some authors interpreted its presence as a
symptom of the same conditions that led to exceptional fossilization, rather than a cause for soft tissue
preservation (Anderson et al., 2018). In order to further investigate this discrepancy, future work
should study the timing of berthierine formation and its exact geographical distribution in consecutive
sediment laminae.
5.3. Taphonomic pathway of fossils in Fc2
5.3.1. Burial and decay
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In the Fezouata Shale, fossils were preserved in situ (Vaucher et al., 2017; Saleh et al., 2018)
in Fc2 (Fig. 14(E)). Fc2 combines a fine grain size and the occurrence of event deposits favoring
burial. Both conditions are necessary for exceptional preservation (Gaines et al., 2012b). However, in
distal settings comparable to Fc2, burial occurred only during strong storms (Saleh et al., 2018),
causing a delay in the start of the fossilization process. For instance, in one interval with exceptional
preservation 600 fossils were discovered, but only a limited number of them show soft tissue
preservation (about 30 stylophorans, 10 trilobites, and 5 marrellomorphs; Lefebvre et al., 2019).
Furthermore, the single preserved hyolithid specimen with soft parts from the Fezouata Shale shows
totally decayed tentacles (Martí Mus, 2016). These two examples of soft tissue preservation suggest
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that organisms were most probably dead and decaying on the sea floor prior to their burial (Fig. 14(A-
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C)). Pre-burial decay was also used to explain the absence of completely cellular animals (i.e., without
cuticle, sclerites, or minerals) from the Fezouata Shale (Saleh et al., 2020) in contrast to most
Cambrian Lagerstätten. In the Cambrian, soft-bodied and lightly sclerotized organisms were killed
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during obrution events and transported by the same event to another facies for their preservation
leading to a smaller exposure to pre-burial decay (Gaines, 2014). This taphonomic process can explain
5.3.2. Authigenic mineralization
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(Moysiuk et al., 2017; Saleh et al., 2020).
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the abundance of soft cellular animals and hyolithid tentacles in sites such as the Burgess Shale
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Experimental approaches have shown that pyrite can form under different circumstances
(Rickard and Luther, 1997; Grimes et al., 2002). Pyrite can precipitate in the water column, surface
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sediments and even under deep burial under anoxic conditions. However, selected soft anatomies
replicated by pyrite minerals are often associated with active, localized sulfate reduction in iron-rich
pore waters during early diagenesis resulting in a strong concentration gradient, and confining pyrite
precipitation to dead carcasses (Farrell, 2014). Under sulfate-reducing conditions, bacteria transform
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organic matter and sulfates into HS- and then to hydrogen sulfides H2S, which react with Fe in a series
of reactions to form pyrite (Raiswell et al., 1993; Schiffbauer et al., 2014). In the Fezouata Shale,
anoxic conditions leading to sulfate reduction were established at the time of burial at the bottom of
storm deposits (Vaucher et al., 2016, 2017) leading to pyritization of some tissues deposited under
event beds. The chemical stress generated by oxygen depletion in the sediment is also evidenced by
horizontal biological traces that are shallow with some escape burrows crossing event deposits. As
bioturbation depth is minimal, it is most probable that the sediment was anoxic a few centimeters
below storm deposits. Further ichnological work should investigate biological traces in detail in order
to test this scenario and constrain oxygenation between the bottom of the water column and the
sediments.
Page 11 of 37
Because in open marine environments sulfates are not limited, pyritized tissues are those
providing sufficient organic material to form H2S (Jørgensen, 1982; Jørgensen et al., 2019; Fig.
14(D)). Laboratory experiments made under surface-sediment conditions have shown that the most
labile decaying soft parts produce considerable amounts of H2S, which reacts with iron to form nuclei
for pyrite framboids (Butler and Rickard, 2000). However, less labile soft parts produce less H 2S, and
thus fewer nuclei leading to the precipitation of mainly cubic, and sometimes octahedral minerals
(Gabbott et al., 2004). In the Fezouata Shale, framboid minerals in 3D fossils are scarce (Fig. 12(D))
and much less abundant than euhedral pyrite (Fig. 12(C)); this emphasizes the removal of a
considerable quantity of organic material, due to burial delay and oxic decay, prior to the permissive
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chemical conditions for pyrite precipitation. If a tissue did not provide sufficient H2S to form nuclei
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when anoxic conditions occur (e.g., non-cellular cuticles made of polysaccharides) for neither
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euhedral nor framboidal minerals, it remains preserved as a carbonaceous compression.
5.3.3. Late diagenesis, metamorphism, and modern weathering
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Although mineralogical and chemical evidences favor the idea of an early authigenic
pyritization of some soft tissues in the Fezouata Shale, the geochemical signal of these minerals in
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fossils from outcrops is clearly altered. The absence of S-rich minerals in surface fossils (Fig. 11)
indicates that pyrite was oxidized and S was partly leached after early diagenesis (Ahm et al., 2017).
This can be due to either metamorphism or modern weathering. According to mineral distributions in
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shales from the Fezouata Lagerstätte, sediments did not experience high burial temperatures as only
3 km of sediments were deposited above the Fezouata Shale (Ruiz et al., 2008; Saleh et al., 2019; Fig.
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14(F)). This is confirmed by a Raman signature representative of fresh organic matter (i.e., low burial
temperatures around 200°C) characterized by the presence of the D4 band and the absence of the D2
band, as well as by the higher intensity of the D1 band compared to the G band (Rahl et al., 2005;
Kouketsu et al., 2014; Fig. 6). These temperatures are lower than in other deposits with soft tissue
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preservation in which the D4 band is less pronounced and the G band has a higher intensity than the
D1 band (Topper et al., 2018). Thus, it is more likely that the removal of S from pyrite in green
surface sedimentary rocks of the Fezouata Shale results from modern weathering rather than from
metamorphism. In the Draa Valley, this formation is exposed to abundant water circulations, as
revealed by the numerous abandoned terraces near the outcrops and by the abundance of water wells
in the area (Warner et al., 2013). Fast pyrite oxidation may be induced by Mn-oxides that are abundant
in circulating waters in arid environments with occasional rain similar to the Draa Valley (Potter and
Rossman, 1979; Warner et al., 2013). Depending on Mn-oxide quantities in circulating waters, the
outcome of pyrite oxidation may differ. When the quantity of Mn-oxide is high enough to fully
oxidize pyrite, the resulting products of this reaction are Fe-oxides and Mn-sulfates (Larsen and
Page 12 of 37
Postma, 1997). Additionally, since manganese oxides are much stronger adsorbents of elements such
as Co and Ni than iron oxides (McKenzie, 1980), the reduction of manganese oxide may cause a major
release of these elements in the surrounding environment (Postma, 1985). If the quantities of Mnoxides are not sufficient to fully oxidize pyrite, pyrite oxidation by H2O molecules and atmospheric O2
will take place and unleash considerable amounts of sulfates, thus reducing the pH of the environment
and contributing to the dissolution of nearby carbonates (Larsen and Postma, 1997). In the Fezouata
Shale, it seems that both pyrite oxidation pathways were operational. Mn-oxides altered pyrite,
contributing to the initial precipitation of Co- and Mn-rich deposits. The latter reaction may lead to a
diffusion of Mn in the sediments, which would explain the distribution of Mn around the appendages
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of the analyzed marrellomorph (Richard et al., 2013; Fig. 13). Subsequently, H 2O transformed the
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remaining pyrite into Fe-oxides and sulfuric acid, and was also responsible for the dissolution of Ca
(Lucas, 2019) and the poor preservation of skeletal elements of different groups with preserved soft
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parts, such as echinoderms (Lefebvre et al., 2019).
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5.3.4. Original mode of preservation and comparison with the Chengjiang Biota
The contrast between the presence of C in fresh sediments (Figs. 4(B), 6) and its absence from
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surface deposits (Fig. 6) may be also the result of modern weathering. The association of C to pyrite
crystals in fresh sediments (Fig. 4(B)) suggests that the original mode of preservation in the Fezouata
Shale includes both carbonaceous compressions and pyrite replicates. In this sense, flattened fossils
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(Fig. 11(B)) were most probably originally preserved as 2D carbonaceous films. However, due to
recent weathering, C was leached from originally non-pyritized structures and pyrite was transformed
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to iron oxides in 3D mineralized tissues (Fig. 14(G)). Similarities in terms of taphonomic pathway of
soft tissues in the Fezouata Shale are particularly high with the Chengjiang Biota. Early studies of the
Chengjiang Biota have emphasized the role of pyrite in replicating some tissues within individual
fossils (Gabbott et al., 2004; Zhu et al., 2005). Later works focusing on less weathered material
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demonstrated that the role of pyrite in the preservation of the Chengjiang Biota fossils may have been
overestimated (Forchielli et al., 2014). Instead, carbonaceous films comprise the major original
component of preservation in the Chengjiang Biota, and only some soft tissues were selectively
replaced by pyrite (Edgecombe et al., 2015). However, C was lost in outcrop fossils probably due to
the extensive activity of recent weathering (Gabbott et al., 2004; Gaines et al., 2008), as it is likely the
case for the Fezouata Shale.
6. Conclusions
Page 13 of 37
In this study, detailed sedimentological facies identified in the Fezouata Shale based on fresh
core material offer unique insights into the mechanisms at play in the exceptional preservation. Distal
environments of the Fezouata Shale below the SWB were inhabitable by living individuals. Dead
organisms were exposed to pre-burial decay. At time of burial, based on observations of minimal
bioturbation in the core, permissive anoxic conditions were established few cm below surface
sediments. Under these conditions, pyrite replicated selectively some soft tissues, while the rest
remained carbonaceous. Carbonaceous parts were then flattened due to compaction while pyrite
replicates kept their 3D morphology. Afterward, carbon was leached from 2D compressions and
surface sediments due to recent weathering. This weathering altered the original chemical signal of
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pyrite transforming it to iron oxides. When extensive weathering occurred, Mn and Co-rich deposits
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precipitated in addition to some star-shaped iron oxides that have nothing to do with the original
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anatomy of the fossils.
Acknowledgments
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This paper is a contribution to the TelluS-Syster project ‘Vers de nouvelles découvertes de
gisements à préservation exceptionnelle dans l’Ordovicien du Maroc’ (2017) and the TelluS-
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INTERRVIE projects ‘Mécanismes de préservation exceptionnelle dans la Formation des Fezouata’
(2018) and ‘Géochimie d'un Lagerstätte de l'Ordovicien inférieur du Maroc’ (2019), all funded by the
INSU (Institut National des Sciences de l'Univers, France), CNRS. This paper is also a contribution to
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the International Geoscience Programme (IGCP) Project 653 – The onset of the Great Ordovician
Biodiversification Event. The Raman facility in Lyon (France) is supported by the INSU. The authors
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thank Yves Candela, Lukáš Laibl, Eric Monceret, Martina Nohejlová, Stephen Pates, and Daniel
Vizcaïno for assistance during field work in Montagne Noire or Morocco. The authors also thank
Lukáš Laibl, Lorenzo Lustri, Fransesco Perez Peris, Claude Colombié and Gilles Montagnac for
assistance during XRF, SEM and Raman spectroscopy analyses. Allison Daley is also thanked for
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facilitating access to the Fezouata Shale collections in Lausanne. Brian Pratt and all anonymous
reviewers are also thanked for their constructive reviews on earlier versions of the manuscript.
Appendix A. Supplementary information
Supplementary information (including Table S1 and S2) associated with this article can be
found, in the online version, at:
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Table and Figure captions
Table 1. Definition and associated depositional environment of outcrop facies (defined as F1, F2, and
F4 in Vaucher et al., 2017) and core facies (defined as Fc1 to Fc5 in this study).
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Current location in the Ternata plain (30°30'00.5" N; 5°50'56.7" W).
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Fig. 1. Bou Izargane in the Ternata plain, Zagora area, Morocco. A. Paleographical location. B.
Fig. 2. Sedimentary succession in the Fezouata Shale with a focus on the succession in Bou Izargane
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showing the alternation of thin background siltstones with coarse siltstone- to thin sandstonedominated event levels. The positions of the drilled cores are indicated next to the sedimentary
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succession. The question marks in the stratigraphic column indicate intervals where characteristic
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graptolite assemblages could not be identified (Lefebvre et al., 2018).
Fig. 3. X-Ray Fluorescence maps of slightly altered core sediments of Fc3 at 405 cm (A), and nonaltered deposits showing elemental distributions in Fc2 at 15 cm and Fc4 at 65 cm (B, C). Si is
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abundant in event beds that are normally graded (i.e., NGB). Al and K are positively correlated in
background sediments. Fe and S are partially correlated in sediments highlighting the presence of
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pyrite minerals. S most probably indicates organic matter when it is not correlated to Fe. In slightly
altered deposits, Mn and Co coexist in coarse sediments bearing sometimes Ca-rich bioclasts. Scale
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bars: 10 mm (A, B), 5 mm (C).
Fig. 4. A. Quartz-rich normally graded bedding (NGB) in the core alternating with clay-rich
background sediments seen in thin section from sediments at 240 cm. B. Pyrite crystals (Py) in the
fresh sediments surrounded by a halo of organic material (Om) at 250 cm. Pyrite and organic matter
were identified based on Raman Spectra in Fig. 6. C. Bioclasts (Bioc) possibly of trilobite fragments
from sediments at 262 cm. Scale bars: 5 mm (A), 1 mm (B, C).
Fig. 5. X-Ray Florescence maps of altered green core sediments of an intermediate Fc3-Fc4 facies at
1320 cm. These sediments show the absence of S-rich materials, except in the bottom part of the slab.
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Mn and Co are enriched in these sediments in comparison with fresh material in Fig. 3(B, C). Scale
bar: 5 mm.
Fig. 6. Raman spectra on thin sections (for analyzed material see Table S1, Appendix A) showing the
presence of both pyrite and C in fresh core sediments, as well as the replacement of pyrite by iron
oxides and the absence of organic C in weathered surface core slabs. The Raman spectra on organic
material show the characteristic peaks of C.
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Fig. 7. Facies evolution over the Bou Izargane succession. Most of normally graded beds are
discontinuous due to bioturbation. Hummocky cross stratifications occur in the coarsest event
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deposits. Bioturbation is randomly distributed in the cores.
Fig. 8. Lithology in the Fezouata Shale. A. Quartz grain size evolution in Fc1to Fc5 seen in thin
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section with crossed-polarized light and /4 gypsum plate. B. Quantifying the increase of quartz
content (SiO2) and decrease of clay content (Al2O3, K2O) (likely illite) between Fc1 and Fc5 (shown in
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A). Scale bars: 150 μm (Fc1), 200 μm (Fc2), 300 μm (Fc3), 400 μm (Fc4), and 400 μm (Fc5).
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Fig. 9. Sedimentary structures in Fc1 (A), Fc2 (B), Fc3 (C), Fc4 (D, E) and Fc5 (F). Normally graded
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beds are frequently discontinuous due to bioturbation in B-F. Scale bars: 5 mm.
Fig. 10. Statistical analyses performed on facies evolution along the core from Bou Izargane. A.
Principal Component Analysis of the 22, 60 cm-thick intervals defined along the core. B. Classical
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(hierarchical) Cluster Analysis of the 22 intervals according to Fc2, Fc3, and Fc4 distributions in these
intervals. C. Similarity Percentage (Simper) analysis showing which facies are responsible for the
differences between Cluster 1 and Cluster 2. D. Facies evolution (Fc1 to Fc5 in this study) along the
core, correlated to outcrop succession (F1, F2, F4 from Vaucher et al., 2017) and clusters 1 and 2
obtained from the cluster analysis. Intervals bearing thin layers with exceptional preservation of fossils
on the field are shown in dark grey, whereas intervals that did not yield any exceptional preservation
are in light grey. Intervals with or without iron availability are shown according to Saleh et al. (2019).
A 95% fit is observed between the alternation of clusters and levels with and without exceptional
preservation, indicating that this type of preservation is strongly correlated with the sedimentary
facies.
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Fig. 11. Preservation mode of fossils in the Fezouata Shale, evidenced by SEM-EDX point
spectroscopy of surface samples using accelerating voltages from 5 to 15 kV in order to enhance
signal from light elements (C, O) and to promote fluorescence of heavier elements such as transition
metals (Fe) in 3D fossils (A) and matrix and fossil imprints (B). Analyzed regions are marked as pink
circles in the sample photographs. Scale bars: 5 mm (A), 10 mm (B).
Fig. 12. A-G. SEM images of minerals in samples from the Fezouata Shale. Iron rich minerals in
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white (A), euhedral iron-oxides (B, C), framboidal iron-rich minerals (D), star-like iron oxides (E) in
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addition to rose-shaped mineral (isolated, F) and next to smaller star-like minerals (G). H. SEM
spectra showing that the star-like minerals are iron oxides and the rose-shaped ones are rich in
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manganese. Scale bars: 1 mm (A), 5 μm (B, E-G), 10 μm (C, D).
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Fig. 13. Elemental maps of an extensively altered marrellomorph arthropod, AA-BIZ31-OI-39.
Red/orange zones of the analyzed fossil are iron rich. Iron is preserved as star-like iron oxides. Scale
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bar: 10 mm.
Fig. 14. Mechanism for soft part preservation in the Fezouata Shale (A-F), and recent weathering
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explaining the preservation state of fossils (G). A: Living organism on the sea floor; B: Dead organism
starts to decay prior to burial; C: Anoxic conditions are established due to burial, at the base of storm
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events, H2S forms in decaying carcasses; D: Pyrite precipitation in specific tissues while the rest
remains as carbonaceous material; E: With time, a facies similar to Fc2 is observed, combining
conditions for both the colonization of the environment by a benthic fauna and for the preservation of
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this fauna; F: With compaction, fossils were preserved as 2D C-rich films with occasionally 3D
pyritization; G: Recent weathering effect removes C from the fossils and alters the chemical signal of
pyrite.
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Table 1.
Depositional setting
Below storm weather base
Above storm weather base
Below Fair weather base
Below storm weather base
Below storm weather base,
more proximal than Fc1
Around the storm weather
base
Above the storm weather base
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Description
Argillaceous siltstones with sparse intercalations of siltstones
(mm-thick)
F2 Coarse siltstones with hummocky cross-stratification (HCS) of
cm-scale wavelength
F4 Fine sandstones with centimetric to decametric HCS, the
laminations are underlined by thin layers of coarser quartz
grains
Fc1 Homogenous, composed of argillaceous material
Fc2 Siltstones with normally graded beds separated by argillaceous
material
Fc3 Siltstones with stacked normally graded beds
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Fc4 Siltstones with stacked normally graded beds and abundant
HCS
Fc5 Coarse siltstones/fine sandstones with abundant bioclasts
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Below fair weather base
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