Farid Saleh
email: farid.saleh@univ-lyon1.fr

Bernard Pittet

Pierre Sansjofre

Pierre Gu

Stefan Lalonde

Jean-Philippe Perrillat

Muriel Vidal

Victoire Lucas

Khadija El

Hariri Khaoula Kouraiss

Bertrand Lefebvre

Pierre Guériau

Khadija El Hariri

Khaoula Kouraiss

Emmanuel Fara

Taphonomic pathway of exceptionally preserved fossils in the Lower Ordovician of Morocco

Keywords:

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 J o u r n a l P r e -p r o o f 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 compressions and the transformation of pyrite into iron oxides. Understanding the processes behind the current patterns of soft tissue preservation in the Fezouata Shale is essential prior to any paleontological description, especially of taxa with no current representatives.

 et al., 2016). Anatomical information found in fossils from this deposit is critical for deciphering the evolution of major animal phyla [START_REF] Vinther | Machaeridians are Palaeozoic armoured annelids[END_REF][START_REF] Vinther | Ancestral morphology of crown-group molluscs revealed by a new Ordovician stem aculiferan[END_REF]Van Roy et al., 2015b;[START_REF] Lefebvre | Exceptionally preserved soft parts in fossils from the Lower Ordovician of Morocco clarify stylophoran affinities within basal deuterostomes[END_REF].

The general depositional environment of the Fezouata Shale is constrained, and is stormdominated with an indirect influence of tides [START_REF] Martin | The Lower Ordovician Fezouata Konservat-Lagerstätte from Morocco: age, environment and evolutionary perspectives[END_REF][START_REF] Vaucher | A wave-dominated, tidemodulated model for the Lower Ordovician of the Anti-Atlas, Morocco[END_REF]. The processes behind the formation of sedimentary structures related to this environment were explained in recent works [START_REF] Vaucher | A genetic link between Konzentrat-and Konservat-Lagerstätten in the Fezouata Shale (Lower Ordovician, Morocco)[END_REF][START_REF] Vaucher | A wave-dominated, tidemodulated model for the Lower Ordovician of the Anti-Atlas, Morocco[END_REF]. 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 chemical gradients around dead animals and leading to the early precipitation of minerals around nonbiomineralized tissues (Gaines et al., 2012a). The other type of preservation is associated with shale [START_REF] Martin | The Lower Ordovician Fezouata Konservat-Lagerstätte from Morocco: age, environment and evolutionary perspectives[END_REF]. In these levels, fossils occur exclusively at bed junctions and not within beds [START_REF] Vaucher | A genetic link between Konzentrat-and Konservat-Lagerstätten in the Fezouata Shale (Lower Ordovician, Morocco)[END_REF][START_REF] Vaucher | A wave-dominated, tidemodulated model for the Lower Ordovician of the Anti-Atlas, Morocco[END_REF], 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 flows [START_REF] Saleh | Storm-induced community dynamics in the Fezouata Biota ( Lower Ordovician, Morocco)[END_REF].

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 [START_REF] Martin | The Lower Ordovician Fezouata Konservat-Lagerstätte from Morocco: age, environment and evolutionary perspectives[END_REF], 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 al., 2017;[START_REF] Lefebvre | Exceptionally preserved soft parts in fossils from the Lower Ordovician of Morocco clarify stylophoran affinities within basal deuterostomes[END_REF]. Considering that numerous diagenetic mechanisms may alter the original anatomy of fossil organisms over geological time, deciphering the taphonomic processes at 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 deciphering the mechanism leading to the current patterns of preservation in this facies.

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 [START_REF] Destombes | Lower Palaeozoic rocks of Morocco[END_REF]. The entire Lower Ordovician succession in the Zagora region in Morocco was deposited in a generally shallow environment at high latitude, close to the J o u r n a l P r e -p r o o f palaeo-South pole (Torsvik andCocks, 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. [START_REF] Choubert | Histoire géologique du domaine de l'Anti-Atlas[END_REF][START_REF] Destombes | Lower Palaeozoic rocks of Morocco[END_REF]. 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 [START_REF] Vaucher | A genetic link between Konzentrat-and Konservat-Lagerstätten in the Fezouata Shale (Lower Ordovician, Morocco)[END_REF]. 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 [START_REF] Martin | The Lower Ordovician Fezouata Konservat-Lagerstätte from Morocco: age, environment and evolutionary perspectives[END_REF]. The Lower Ordovician succession was interpreted to have been deposited in a storm-wave dominated sedimentary environment [START_REF] Martin | The Lower Ordovician Fezouata Konservat-Lagerstätte from Morocco: age, environment and evolutionary perspectives[END_REF] indirectly influenced by tides [START_REF] Vaucher | A wave-dominated, tidemodulated model for the Lower Ordovician of the Anti-Atlas, Morocco[END_REF]. The corresponding palaeoenvironments range from the foreshore (S-SE) to the upper offshore (N-NW) [START_REF] Vaucher | A wave-dominated, tidemodulated model for the Lower Ordovician of the Anti-Atlas, Morocco[END_REF]. 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 arthropods, sponges, and soft parts of echinoderms (Van Roy et al., 2015a;[START_REF] Botting | Diversity and ecology of sponges in the Early Ordovician Fezouata Biota, Morocco[END_REF][START_REF] Lefebvre | Exceptionally preserved soft parts in fossils from the Lower Ordovician of Morocco clarify stylophoran affinities within basal deuterostomes[END_REF]. This locality (Fig. 2) exposes part of the lower interval with exceptional preservation, dated as late Tremadocian (Araneograptus murrayi graptolite Zone; [START_REF] Lefebvre | Age calibration of the Lower Ordovician Fezouata Lagerstätte, Morocco[END_REF].

Material and methods

Field work

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 (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 University, Marrakesh.

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% SiO 2 , Al 2 O 3 and K 2 O) 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 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 micro-XRF 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 accumulations of 30 s and a laser power of about 5 mW on the sample surface.

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 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 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 [START_REF] Vaucher | A genetic link between Konzentrat-and Konservat-Lagerstätten in the Fezouata Shale (Lower Ordovician, Morocco)[END_REF]Fig. 4(A,B)). Cores were made starting at the upper surface of the outcrop from which the original log was made. For more precision, correlations between cores and outcrop were made based on comparisons of facies defined in [START_REF] Vaucher | A wave-dominated, tidemodulated model for the Lower Ordovician of the Anti-Atlas, Morocco[END_REF] and in the present study (Table 1). Using 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 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 [START_REF] Hammer | Past: paleontological statistics software package for education and data analysis[END_REF]. 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 [START_REF] Hammer | Past: paleontological statistics software package for education and data analysis[END_REF]. 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 J o u r n a l P r e -p r o o f Similarity Percentage analysis (SIMPER; [START_REF] Hammer | Past: paleontological statistics software package for education and data analysis[END_REF] 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.

Fossil analyses

Twenty fossil specimens (Table S1, Appendix A) collected from late Tremadocian localities in the Zagora area, Morocco, and registered in the paleontological collections of the Cadi-Ayyad University, Marrakesh, Morocco (acronym: AA), Lyon 1 University, Villeurbanne, France (UCBL-FSL) 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 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 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 Soleil synchrotron, Paris, France, in order to determine the minor-to-trace elemental composition of the fossils, as well as of the surrounding matrix.

Results

Core description

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 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 [START_REF] Vaucher | A wave-dominated, tidemodulated model for the Lower Ordovician of the Anti-Atlas, Morocco[END_REF]. 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.

J o u r n a l P r e -p r o o f

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.

Facies identification

Five sedimentological facies are defined from the core and are designated herein as Fc1 to Fc5. Fc1 is the finest grained facies (Fig. 8(A)); it is homogeneous and mostly composed of 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 finegrained sediments in between, and in rare occasions some small HCS are present (see [START_REF] Vaucher | A genetic link between Konzentrat-and Konservat-Lagerstätten in the Fezouata Shale (Lower Ordovician, Morocco)[END_REF][START_REF] Vaucher | A wave-dominated, tidemodulated model for the Lower Ordovician of the Anti-Atlas, Morocco[END_REF] for direct evidence for HCS; Fig. 9(C)). Fc4 is made of coarser sediments than in Fc3 (Fig. 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)).

The increase of quartz (SiO 2 ) and decrease of clays (K 2 O and Al 2 O 3 ) 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 the largest pyrite crystals being observed in Fc2 (Fig. 9(B)).

Statistical analyzes

Principal component analysis shows that most of the variance between the 22 defined intervals 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 J o u r n a l P r e -p r o o f 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.

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 euhedral crystals (Fig. 12(B,C)), and dispersed as framboid-shaped minerals (Fig. 12(D)). The 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 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 starshaped minerals is found in fossils that are covered by Co and Mn-rich deposits (Fig. 13) in rose-like minerals (Fig. 12(F,G)).

Discussion

General depositional environment

The increase in SiO 2 and decrease of Al 2 O 3 and K 2 O from Fc1 to Fc5 (Fig. 8; Table S2, 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 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; [START_REF] Vaucher | A wave-dominated, tidemodulated model for the Lower Ordovician of the Anti-Atlas, Morocco[END_REF]. 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 [START_REF] Lefebvre | Exceptionally preserved soft parts in fossils from the Lower Ordovician of Morocco clarify stylophoran affinities within basal deuterostomes[END_REF]. 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 [START_REF] Harms | Depositional environments as interpreted from primary sedimentary structures and stratification sequences[END_REF]. 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 [START_REF] Vaucher | A genetic link between Konzentrat-and Konservat-Lagerstätten in the Fezouata Shale (Lower Ordovician, Morocco)[END_REF][START_REF] Vaucher | A wave-dominated, tidemodulated model for the Lower Ordovician of the Anti-Atlas, Morocco[END_REF]. 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 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 [START_REF] Vaucher | A genetic link between Konzentrat-and Konservat-Lagerstätten in the Fezouata Shale (Lower Ordovician, Morocco)[END_REF]. 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)), revealing a more proximal environment above the SWB with higher energy than what is observed in 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 the original elemental distribution of this facies [START_REF] Fan | The process of formation of manganese carbonate deposits hosted in black shale series[END_REF]. The Ca-rich deposits in some laminae (Figs. 3(A,C), 5) may resemble carbonate cement deposited in deep settings (i.e., basin) of 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 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 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 thin sections cutting through the bottom of storm deposits (Fig. 4(C)) especially because carbonates are not evidenced elsewhere in this formation [START_REF] Vaucher | A genetic link between Konzentrat-and Konservat-Lagerstätten in the Fezouata Shale (Lower Ordovician, Morocco)[END_REF]. Thus, the most distal facies is Fc1 and the most proximal facies is Fc5 with Fc2, Fc3, and Fc4 in between, respectively. This model 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).

Facies for exceptional preservation

Exceptional preservation requires burial by event deposits [START_REF] Vaucher | A genetic link between Konzentrat-and Konservat-Lagerstätten in the Fezouata Shale (Lower Ordovician, Morocco)[END_REF]. 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 [START_REF] Lefebvre | Exceptionally preserved soft parts in fossils from the Lower Ordovician of Morocco clarify stylophoran affinities within basal deuterostomes[END_REF]. In these deposits, Fe supply was likely associated to periods with high seasonality leading to high iron- 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 colonization of this environment [START_REF] Saleh | Storm-induced community dynamics in the Fezouata Biota ( Lower Ordovician, Morocco)[END_REF]. Statistical analyses show that an alternation of 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 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 organisms to colonize the sea floor [START_REF] Saleh | Storm-induced community dynamics in the Fezouata Biota ( Lower Ordovician, Morocco)[END_REF], in addition to burial during event deposition, a prerequisite condition for exceptional preservation [START_REF] Vaucher | A wave-dominated, tidemodulated model for the Lower Ordovician of the Anti-Atlas, Morocco[END_REF].

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 [START_REF] Lefebvre | Exceptionally preserved soft parts in fossils from the Lower Ordovician of Morocco clarify stylophoran affinities within basal deuterostomes[END_REF]. The absence of exceptional preservation in this facies does not result from the original absence of living organisms, but may be due to the lack of berthierine in this level [START_REF] Lefebvre | Exceptionally preserved soft parts in fossils from the Lower Ordovician of Morocco clarify stylophoran affinities within basal deuterostomes[END_REF]. Berthierine is an iron-rich clay mineral that can be deposited in the sediments from a primary clay precursor under anoxic conditions [START_REF] Tang | Ferruginous seawater facilitates the transformation of glauconite to chamosite: An example from the Mesoproterozoic Xiamaling Formation of North China[END_REF]. It is documented in most intervals with exceptional preservation in the Cambrian [START_REF] Zhang | 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[END_REF], and only in specific levels of the Fezouata Shale in which exceptional preservation occurred [START_REF] Lefebvre | Exceptionally preserved soft parts in fossils from the Lower Ordovician of Morocco clarify stylophoran affinities within basal deuterostomes[END_REF].

In experimental studies, it was shown that berthierine slows down bacterial decay due to the damage of bacterial cells [START_REF] Mcmahon | Experimental evidence that clay inhibits bacterial decomposers: Implications for preservation of organic fossils[END_REF]. 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 [START_REF] Zhang | 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[END_REF]. 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.

Taphonomic pathway of fossils in Fc2

Burial and decay

J o u r n a l P r e -p r o o f

In the Fezouata Shale, fossils were preserved in situ [START_REF] Vaucher | A wave-dominated, tidemodulated model for the Lower Ordovician of the Anti-Atlas, Morocco[END_REF][START_REF] Saleh | Storm-induced community dynamics in the Fezouata Biota ( Lower Ordovician, Morocco)[END_REF] 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 [START_REF] Saleh | Storm-induced community dynamics in the Fezouata Biota ( Lower Ordovician, Morocco)[END_REF], 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; [START_REF] Lefebvre | Exceptionally preserved soft parts in fossils from the Lower Ordovician of Morocco clarify stylophoran affinities within basal deuterostomes[END_REF].

Furthermore, the single preserved hyolithid specimen with soft parts from the Fezouata Shale shows totally decayed tentacles [START_REF] Martí Mus | A hyolithid with preserved soft parts from the Ordovician Fezouata Konservat-Lagerstätte of Morocco[END_REF]. These two examples of soft tissue preservation suggest that organisms were most probably dead and decaying on the sea floor prior to their burial (Fig. 14(A-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 [START_REF] Saleh | Taphonomic bias in exceptionally preserved biotas[END_REF] in contrast to most Cambrian Lagerstätten. In the Cambrian, soft-bodied and lightly sclerotized organisms were killed 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 the abundance of soft cellular animals and hyolithid tentacles in sites such as the Burgess Shale [START_REF] Moysiuk | Hyoliths are Palaeozoic lophophorates[END_REF][START_REF] Saleh | Taphonomic bias in exceptionally preserved biotas[END_REF].

Authigenic mineralization

Experimental approaches have shown that pyrite can form under different circumstances [START_REF] Rickard | Kinetics of pyrite formation by the H2S oxidation of iron (II) monosulfide in aqueous solutions between 25 and 125°C: The mechanism[END_REF][START_REF] Grimes | Fossil plants from the Eocene London Clay: the use of pyrite textures to determine the mechanism of pyritization[END_REF]. Pyrite can precipitate in the water column, surface 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 [START_REF] Farrell | Pyritization of soft tissues in the fossil record: an overview[END_REF]. Under sulfate-reducing conditions, bacteria transform organic matter and sulfates into HS -and then to hydrogen sulfides H 2 S, which react with Fe in a series of reactions to form pyrite [START_REF] Raiswell | A simple three-dimensional model of diffusion-with-precipitation applied to localised pyrite formation in framboids, fossils and detrital iron minerals[END_REF][START_REF] Schiffbauer | A unifying model for Neoproterozoic-Palaeozoic exceptional fossil preservation through pyritization and carbonaceous compression[END_REF]. In the Fezouata Shale, anoxic conditions leading to sulfate reduction were established at the time of burial at the bottom of storm deposits [START_REF] Vaucher | A genetic link between Konzentrat-and Konservat-Lagerstätten in the Fezouata Shale (Lower Ordovician, Morocco)[END_REF][START_REF] Vaucher | A wave-dominated, tidemodulated model for the Lower Ordovician of the Anti-Atlas, Morocco[END_REF] 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.

J o u r n a l P r e -p r o o f

Because in open marine environments sulfates are not limited, pyritized tissues are those providing sufficient organic material to form H 2 S [START_REF] Jørgensen | Mineralization of organic matter in the sea bed -the role of sulphate reduction[END_REF][START_REF] Jørgensen | The Biogeochemical Sulfur Cycle of Marine Sediments[END_REF]Fig. 14(D)). Laboratory experiments made under surface-sediment conditions have shown that the most labile decaying soft parts produce considerable amounts of H 2 S, which reacts with iron to form nuclei for pyrite framboids [START_REF] Butler | Framboidal pyrite formation via the oxidation of iron (II) monoslfide by hydrogen sulphide[END_REF]. However, less labile soft parts produce less H 2 S, and thus fewer nuclei leading to the precipitation of mainly cubic, and sometimes octahedral minerals [START_REF] Gabbott | Preservation of Early Cambrian animals of the Chengjiang biota[END_REF]. 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 chemical conditions for pyrite precipitation. If a tissue did not provide sufficient H 2 S to form nuclei when anoxic conditions occur (e.g., non-cellular cuticles made of polysaccharides) for neither euhedral nor framboidal minerals, it remains preserved as a carbonaceous compression.

Late diagenesis, metamorphism, and modern weathering

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 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 shales from the Fezouata Lagerstätte, sediments did not experience high burial temperatures as only 3 km of sediments were deposited above the Fezouata Shale [START_REF] Ruiz | Illite crystallinity patterns in the Anti-Atlas of Morocco[END_REF][START_REF] Lefebvre | Exceptionally preserved soft parts in fossils from the Lower Ordovician of Morocco clarify stylophoran affinities within basal deuterostomes[END_REF]Fig. 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 [START_REF] Rahl | Raman spectroscopic carbonaceous material thermometry of low-grade metamorphic rocks: Calibration and application to tectonic exhumation in Crete, Greece[END_REF][START_REF] Kouketsu | A new approach to develop the Raman carbonaceous material geothermometer for low-grade metamorphism using peak width[END_REF]Fig. 6). These temperatures are lower than in other deposits with soft tissue preservation in which the D4 band is less pronounced and the G band has a higher intensity than the D1 band [START_REF] Topper | Characterization of kerogenous films and taphonomic modes of the Sirius Passet Lagerstätte, Greenland[END_REF]. 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 [START_REF] Warner | Integration of geochemical and isotopic tracers for elucidating water sources and salinization of shallow aquifers in the sub-Saharan Drâa Basin, Morocco[END_REF]. 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 [START_REF] Potter | The manganese-and iron-oxide mineralogy of desert varnish[END_REF][START_REF] Warner | Integration of geochemical and isotopic tracers for elucidating water sources and salinization of shallow aquifers in the sub-Saharan Drâa Basin, Morocco[END_REF]. 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 J o u r n a l P r e -p r o o f [START_REF] Larsen | Nickel mobilization in a groundwater well field: release by pyrite oxidation and desorption from manganese oxides[END_REF]. Additionally, since manganese oxides are much stronger adsorbents of elements such as Co and Ni than iron oxides [START_REF] Mckenzie | The adsorption of lead and other heavy metals on oxides of manganese and iron[END_REF], the reduction of manganese oxide may cause a major release of these elements in the surrounding environment [START_REF] Postma | Concentration of Mn and separation from Fe in sediments -I. Kinetics and stoichiometry of the reaction between birnessite and dissolved Fe(II) at 10°C[END_REF]. If the quantities of Mnoxides are not sufficient to fully oxidize pyrite, pyrite oxidation by H 2 O molecules and atmospheric O 2 will take place and unleash considerable amounts of sulfates, thus reducing the pH of the environment and contributing to the dissolution of nearby carbonates [START_REF] Larsen | Nickel mobilization in a groundwater well field: release by pyrite oxidation and desorption from manganese oxides[END_REF]. 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 of the analyzed marrellomorph [START_REF] Richard | Kinetics of manganese adsorption, desorption, and oxidation in coastal marine sediments[END_REF]Fig. 13). Subsequently, H 2 O transformed the remaining pyrite into Fe-oxides and sulfuric acid, and was also responsible for the dissolution of Ca [START_REF] Lucas | Mécanismes de préservation et étude géochimique de la carotte 18MFBI du Lagerstatte des Fezouata (Ordovicien inférieur[END_REF] and the poor preservation of skeletal elements of different groups with preserved soft parts, such as echinoderms [START_REF] Lefebvre | Exceptionally preserved soft parts in fossils from the Lower Ordovician of Morocco clarify stylophoran affinities within basal deuterostomes[END_REF].

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 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 (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 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 [START_REF] Gabbott | Preservation of Early Cambrian animals of the Chengjiang biota[END_REF][START_REF] Zhu | Fossilization modes in the Chengjiang Lagerstätte (Cambrian of China): testing the roles of organic preservation and diagenetic alteration in exceptional preservation[END_REF]. Later works focusing on less weathered material demonstrated that the role of pyrite in the preservation of the Chengjiang Biota fossils may have been overestimated [START_REF] J O U R N A L P R E -P R O O F Forchielli | Taphonomic traits of clay-hosted early Cambrian Burgess Shale-type fossil Lagerstätten in South China[END_REF]. Instead, carbonaceous films comprise the major original component of preservation in the Chengjiang Biota, and only some soft tissues were selectively replaced by pyrite [START_REF] Edgecombe | Unlocking the early fossil record of the arthropod central nervous system[END_REF]. However, C was lost in outcrop fossils probably due to the extensive activity of recent weathering [START_REF] Gabbott | Preservation of Early Cambrian animals of the Chengjiang biota[END_REF][START_REF] Gaines | Cambrian Burgess Shale-type deposits share a common mode of fossilization[END_REF], as it is likely the case for the Fezouata Shale.

Conclusions

J o u r n a l P r e -p r o o f

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 pyrite transforming it to iron oxides. When extensive weathering occurred, Mn and Co-rich deposits precipitated in addition to some star-shaped iron oxides that have nothing to do with the original anatomy of the fossils.

Table and Figure captions

Table 1. Definition and associated depositional environment of outcrop facies (defined as F1, F2, and F4 in [START_REF] Vaucher | A wave-dominated, tidemodulated model for the Lower Ordovician of the Anti-Atlas, Morocco[END_REF] and core facies (defined as Fc1 to Fc5 in this study). Current location in the Ternata plain (30°30'00.5" N; 5°50'56.7" W). 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.

J

  o u r n a l P r e -p r o o f 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)).

Fig. 1 .

 1 Fig. 1. Bou Izargane in the Ternata plain, Zagora area, Morocco. A. Paleographical location. B.

Fig. 2 .

 2 Fig. 2. Sedimentary succession in the Fezouata Shale with a focus on the succession in Bou Izargane 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 succession. The question marks in the stratigraphic column indicate intervals where characteristicgraptolite assemblages could not be identified[START_REF] Lefebvre | Age calibration of the Lower Ordovician Fezouata Lagerstätte, Morocco[END_REF].

Fig. 3 .

 3 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 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 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 bars: 10 mm (A, B), 5 mm (C).

Fig. 4 .

 4 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 .

 5 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.

Fig. 6 .

 6 Fig.6. Raman spectra on thin sections (for analyzed material see TableS1, 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.

Fig. 7 .

 7 Fig. 7. evolution over the Bou Izargane succession. Most of normally graded beds are discontinuous due to bioturbation. Hummocky cross stratifications occur in the coarsest event deposits. Bioturbation is randomly distributed in the cores.

Fig. 8 .

 8 Fig. 8. Lithology in the Fezouata Shale. A. Quartz grain size evolution in Fc1to Fc5 seen in thin section with crossed-polarized light and /4 gypsum plate. B. Quantifying the increase of quartz content (SiO 2 ) and decrease of clay content (Al 2 O 3 , K 2 O) (likely illite) between Fc1 and Fc5 (shown in A). Scale bars: 150 μm (Fc1), 200 μm (Fc2), 300 μm (Fc3), 400 μm (Fc4), and 400 μm (Fc5).

Fig. 9 .

 9 Fig. 9. Sedimentary structures in Fc1 (A), Fc2 (B), Fc3 (C), Fc4 (D, E) and Fc5 (F). Normally graded beds are frequently discontinuous due to bioturbation in B-F. Scale bars: 5 mm.

Fig. 10 .

 10 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 (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).

Table 1 .

 1 Fc1 Homogenous, composed of argillaceous material Below storm weather base Fc2 Siltstones with normally graded beds separated by argillaceous material Below storm weather base, more proximal than Fc1 Fc3 Siltstones with stacked normally graded beds Around the storm weather base Fc4 Siltstones with stacked normally graded beds and abundant HCS Above the storm weather base Fc5 Coarse siltstones/fine sandstones with abundant bioclasts Below fair weather base

	Description

Acknowledgments

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-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 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 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 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 andS2