Mohamed Abioui
email: abioui.gbs@gmail.com

Serge Ferry
email: serge.ferry@yahoo.fr

Danièle Grosheny
email: daniele.grosheny@univ-lorraine.fr

Nourrissaid Içame
email: n.icame@gmail.com

Emmanuel Robert
email: emmanuelr.obert@univ-lyon1.fr

Mohammed Benssaou
email: m.benssaou@uiz.ac.ma

Dani Ele Grosheny

Sylvie Bourquin

The Cretaceous marine onlap on Palaeozoic deposits (Smara-Lâayoune Basin, South Morocco). Comparison with neighbouring regions

Keywords:

scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

In southern Morocco, the Cretaceous deposits of the easternmost part of the TarfayaeLâayouneeBoudjour Basin (TLBB) or TarfayaeDakhla Basin as also known in the literature (Fig. 1A), onlapping the Palaeozoic basement, remain poorly studied since the pioneer work of [START_REF] Choubert | Le bassin côtier de Tarfaya (Maroc m eridional). Tome I e Stratigraphie[END_REF]. Most recent studies have focused on the western, coastal part of the basin and the connection with offshore deposits, mostly for oil exploration or water supplies. Two subbasins, the Tarfaya and the SmaraeLaayoune subbasins, were studied as part of a Cooperative Programme between the Universities of Agadir (Morocco) and Lorraine (France), in order to refine the stratigraphy and depositional environments of Cretaceous deposits pinching out on basement rocks of the southern termination of the Anti-Atlas and the Reguibat Shield. Here are presented the field data acquired in the Smara area. Those of the Tarfaya subbasin are under completion. A special focus is also given on CenomanianeTuronian boundary deposits (Cen-omanianeTuronian Boundary Event or CTBE). This event, also known as Oceanic Anoxic Event 2 (OAE2), has attracted much attention over the past decades on a worldwide basis (see [START_REF] Kuhnt | Cenomanian sequence stratigraphy and sea-level fluctuations in the Tarfaya Basin (SW Morocco)[END_REF][START_REF] Kuhnt | Unraveling the onset of Cretaceous oceanic anoxic event 2 in an extended sediment archive from the TarfayaeLaayoune Basin, Morocco[END_REF], and references therein for the Tarfaya subbasin, which hosts the CTBE black shale). But few is known on the corresponding proximal shallowwater deposits, devoid of organic-rich shales. In this respect, still unpublished results regarding the southern shoulder of the Atlas belt have been integrated for comparison in the present study, as part of a continued effort (Grosheny et al., 2017a(Grosheny et al., , 2017b) ) to track a possible tectonic imprint on the CTBE environmental event on a large scale.

Six sections have been logged west of Smara, in order to reconstruct a transect (Fig. 1B). The Cretaceous coastal onlap is complicated here by the presence of a horst of Palaeozoic deposits. These data are complemented with sections logged farther north, along the road from Smara to Tan Tan towns, where it is possible to trace the thin coastal sequences in detail over tens of kilometres. Mapping of sequences in this area also give evidence of a disconformity between fully marine sequences and underlying, mixed, continentalemarine sequences, whose ages are discussed.

In studying sections, attention has been mostly paid to depositional facies and to identify stratigraphic surfaces having a sequence stratigraphic interest.

Description of the sections

The precise location of the sections is given in Table 1.

Smara-I section

The section (Fig. 2) is located on the northern termination of a horst of Palaeozoic deposits (Fig. 1B), here represented by Ordovician bedded quartzite bearing Tigilites burrows. The section begins with a bioturbated, very coarse-grained transgressive sandstone bed resting on brecciated Palaeozoic quartzite. It is overlain by a yellowish succession of sandy claystone passing up to mostly dolomite, either fine-grained and bioturbated, or platy, or bearing bivalves casts. A yellowish marlstone makes the passage to a whitish, platy (laminated) limestone bed (B1) about 3 m in thickness, showing aligned, large, spherical black chert nodules in its uppermost part. Yellowish finegrained dolomite overlain by a massive dolomite bed bearing bivalves casts then follows. It is overlain by a second white limestone bed (B2), about 3 m in thickness, but not laminated as B1, showing a wavy bedding instead.

The yellowish claystoneedolomite succession under B1 can be interpreted as an overall prograding sequence over the basal transgressive sandstone bed. This sequence is probably to be subdivided into three transgressiveeregressive (TeR) subesequences (a, b and c, Fig. 2). The first white limestone bed B1 is a laminated restricted deposit, probably representing a maximum flooding, as well as the second, wavybedded B2 limestone.

Age assignments (Fig. 2) will be discussed further.

Smara-II section

This section (Fig. 2) is located in a more distal position than section I. It begins with bioturbated sandstone with thin clayey interbeds, overlain by a sandstone bed bearing branched cherts. After an observation hiatus, a fine-to medium-grained bioturbated sandstone passes up to a coarser-grained sandstone with gravel lenses and indurated beds with low-angle cross-bedding. After a new observation hiatus, a thick sequence of yellowish, bioturbated, very sandy claystone finishes up with a cap of indurated bioturbated sandstone bed, bearing bivalves casts and forming a prominent ledge in the landscape. Above the overlying yellowish marlstone bed, the same platy limestone bed B1 of section SMA-I is found, with about the same thickness. This B1 bed is overlain by about 8 m of yellowish marlstone bearing numerous pyritized ammonites. These marls are overlain by a thin, wavybedded white limestone bed, that we interpret as being coeval to the bed of the SMA-I section, given its similar facies. The remaining of the section is an alternation of yellowish fine-grained dolomite and very hard crystalline dolomite forming several ledges in the landscape. No macrofossils have been found in these dolomites.

On a sequence stratigraphic point of view (Fig. 2), the thick claystoneedolomite sequence in the lower half of the section can be easily correlated with the first sequence of the SMA-I section. Its upper shallow facies is thinner than in SMA-I, which can be explained by its more distal position. At the base of the section, at least two additional TeR sandstone sequences are found vs. the SMA-I section. The uppermost dolomites are thought to represent several TeR marl-limestone sequences, but fully dolomitized.

A major discovery in this section are the pyritized ammonites found in the marlstone above the B1 platy limestone. The fauna comprises the following species: Hoplitoides (? Erichsenites) sp., Choffaticeras sp., Nannovascoceras constrictum [START_REF] Renz | Two new ammonite genera from the Lower Turonian of Venezuela[END_REF], indicative of an early Turonian age. As a consequence, the underlying laminated limestone B1 could be a lateral facies of the latest Cenomanian black shale (Oceanic Anoxic Event 2) of the nearby Tarfaya sub-basin [START_REF] Kuhnt | Cenomanian sequence stratigraphy and sea-level fluctuations in the Tarfaya Basin (SW Morocco)[END_REF]. Other age assignments will be discussed further.

Smara-III section

This section (Fig. 2) is in intermediate position between SMA-I and SMA-II (Fig. 1B). It was logged along the eastern cuesta supporting the antennas of the Moroccan phone network close to the main road (Fig. 1B). The section is similar to the SMA-II section, except for its base and its summit. At the base, under the thick yellowish claystone sequence of the former two sections, outcrops a yellowish, medium-to coarse-grained, structureless, laterallycontinuous sandstone overlain by a light-grey, crossbedded, very coarse-grained to gravelly sandstone bearing scattered, poorly-rounded pebbles (mostly translucent quartz). This sandstone is overall fining-up. The transition to the overlying yellowish claystone sequence comprises a fine-grained sandstone cut by root casts, and overlain by a bioturbated, coquina-rich, gravelly sandstone bed, which is probably the transgressive facies of the overlying claystone sequence. At the top of the section, above the probable lateral equivalent of the B2 limestone bed, an evenly-stratified coquina sandstone, about 2 m in thickness, is found instead of the crystalline dolomite bed in the nearly SMA-II section. It is probable that such dolomite is the result of the dolomitization of the sandstone cap of the sequence, as it has often been found in similar sandstoneprone coastal deposits of the Upper Jurassic to Lower Cretaceous Continental Intercalaire of southern Tunisia where strata-bound, full transformation of coarse-grained sandstone into massive, isotropic dolomite is very common [START_REF] Ouaja | Faci es de d epôt du Jurassique et du Cr etac e du bassin de Tataouine (Sud de la Tunisie). Livretguide excursion Association des S edimentologistes Français[END_REF].

Smara-IV section

The section (Fig. 2) is a composite one, logged from different outcrops (Fig. 1B) along the road linking Laâyoune to Smara, and stretching over 20 km due to the very low dip. Key beds have been continuously traced in the field to link partial sections. The base of the section is close to the SMA-I section, and shows the same lithologies, up to the second white limestone bed (B2), which is here a little thicker (5 m) and partly dolomitized. The marlstone bearing pyritized ammonites found in the SMA-II section, is also present between the platy limestone bed B1 and the wavy bed B2. The remaining of the section is a complex alternation of claystone to marlstone (most often covered) and shallow-water facies like massive dolomite, laminated dolomite bearing wave-ripples, bioturbated fine-grained limestone, some of these beds being rich in bivalves or gastropods (Nerinea), sandy limestone, and coarse-grained to gravelly, coquina-rich sandstone. The coarse-grained sandstone bed, by the mark 40 m, has been traced over several hundred metres. It shows a laterally-continuous, even or low-angle stratification characteristic of a beach facies. In addition, stratal evidence of forced regression has been found in the nesting of successive beach prisms toward a western (Atlantic) lowstand palaeoshore. The last beds of the section were logged on an isolated outlier, north of the road. Its summit is a coquina-rich dolomite capped by a karstified stromatolite bed, indicating emergence. The upper part of the section, above the lower Turonian ammonite-bearing marlstone, probably covers the upper Turonian, extending possibly into the Coniacian.

Gara El Berd section

The section (Fig. 2) is logged on the flank of an isolated outlier south of the main road to Smara, in a saddle between the Palaeozoic horst of the SMA-I section and the Palaeozoic cover of the Reguibat Shield to the east (Fig. 1B). At the base, the same massive sandstone and the light-grey, cross-bedded coarse-grained sandstone found in the SMA-III section are outcropping. The remaining of the section is different. A first package of yellowish, megarippled, poorlycemented sandstone beds capped by an indurated bed of the same facies overlies the light-grey sandstone. Its indurated cap is cut by vertical root casts indicating exposure. This emergence or abandonment surface is overlain by medium-grained, megarippled sandstone bearing numerous clayey rip-up clasts within the bottomsets, presumably indicating a tidal environment. This sandstone unit is sharply overlain by a 4-m-thick coarse-grained sandstone bed bearing laterally-continuous even stratification or low-angle cross-bedding, and also cut by spectacular vertical root casts. A covered recessive interval whose upper part shows a fine-grained sandstone bearing small sinusoidal (? wave) ripples is found below the summital ledge made of the same laterally-continuous, evenlystratified, coarse-grained sandstone as below, but without the root casts. Its base is also sharp.

On a sequence stratigraphic point of view, the first two sandstone units belong to two successive sequences, given their fully different features. The next unit, under the first root casts surface, is probably fluvial. The next one is considered tidal due to occurrence of clayey rip-up clasts within the megaripples, therefore representing the first evidence of a marine transgression. The two upper, ledgeforming sandstone beds, are probably beach deposits, representing either a full TeR sequence for the lower one, or the shallow-water top of a TeR sequence for the second one. This is quite clear for the first one, with its sharp base and the root casts at its top. The upper ledge could cap, in a forced regressive regime (owing to its sharp base), the underlying fine-grained sandstone of the recessive interval.

Wadi Saquia El Hamra section

This section (Fig. 2) is located close to the confluence between the main Wadi Saquia El Hamra (SAO) and a little wadi coming from the north. No intermediate section could be logged along the banks of the Wadi Saoula El Hamra (Fig. 1B), except for some isolated outcrops. The section, which is in the most distal position, begins with the same two lower units than in the Gara el Berd section. Especially, the light-grey gravelly sandstone unit, which is easily recognizable, can be almost continuously spotted along the left bank of the Wadi Saquia El Hamra. The overlying deposits are an alternation of marine claystone, sandy claystone, bioturbated sandstone, dolomitic coquina sandstone, and evenly-stratified coarse-grained sandstone (interpreted as a beach facies). In the middle of the section, about five metres of large-scale, high-angle cross-bedded aeolian sandstone are overlain by about five metres of crossbedded, fluvial gravelly sandstone. These continental deposits are overlain by a marine succession roughly similar to the previous one.

The section therefore shows two marine transgressive episodes made of several short-term shallow-water TeR sequences, separated by an overall regressive interval of continental deposits.

Correlation of the sections

The correlation diagram (Fig. 2) includes both our data and the sections of [START_REF] Choubert | Le bassin côtier de Tarfaya (Maroc m eridional). Tome I e Stratigraphie[END_REF], redrawn from their descriptions, in order to have a more complete picture of the marine coastal onlap. In the right part of the diagram (our data), downdip of the Palaeozoic horst (SMA-I), the basal, massive, structureless sandstone and its cover of light-grey gravelly sandstone make everywhere the sole of marine deposits. The first marine TeR sequences of the distal SAO section (first marine flooding) are correlated with those at the base of the Gara el Berd section owing to their characteristics, especially because of the look-alike, coarse-grained sandstone beach facies that caps them in both sections. The overlying continental interval of the SAO section is not recorded updip, as well as the sandstone marine sequences (second marine flooding) under the thick yellowish laterally-continuous claystone sequence (underlined in yellow, Fig. 2) representing a third marine flooding. This flooding was more pronounced than the two former ones, as it transgresses the Palaeozoic basement of the horst. The transgressive ravinement surface of this third flooding may have truncated the sequences below. The overlying yellowish claystone sequence can be subdivided into three TeR subsequences (a, b and c, Fig. 2) which can be easily traced in the most proximal sections (SMA-I and SMA-III), less in the SMA-II section because their regressive summital facies (coquina-bearing dolomite) fades away within the distal claystone.

The fourth marine flooding is at the base of the B1 platy limestone bed. The basal transgression surface looks like erosional, especially in the SMA-II section, where the b and c underlying subsequences are lacking. In this section, the B1 bed rests on a bioturbated, coarse-grained, gravelly, coquina-bearing sandstone bed representing a transgressive systems tract. This bed was not found in other sections, and is thought to be truncational.

Above the wavy-bedded B2 limestone, the correlation diagram (Fig. 2) shows an interesting feature within the next sequence. The top of the sequence is represented in the SMA-III section by a partly dolomitized, coarse-grained sandstone bearing aligned, overturned bivalve shells within an even stratification, characteristic of a beach facies. Laterally (SMA-II and SMA-IV sections), structureless dolomite beds are found instead. It is suggested that these dolomite beds could result from the full dolomitization of a beach sandstone, as it has been often found in look-alike sequences of the so-called Continental Intercalaire (or "Purbecko-Wealdien") of South Tunisia [START_REF] Ouaja | Faci es de d epôt du Jurassique et du Cr etac e du bassin de Tataouine (Sud de la Tunisie). Livretguide excursion Association des S edimentologistes Français[END_REF].

Given the pervasive dolomitization found in Cretaceous deposits of the Smara area, such lateral facies changes could explain that either the bioturbated limestone or sandstone caps of the overlying sequences (SMA-IV) could pass to full dolomite sequences (SMA-II).

Farther east of the Smara area (Fig. 2, left part), along the northern border of the Tindouf syncline (Fig. 1A), the descriptions of [START_REF] Choubert | Le bassin côtier de Tarfaya (Maroc m eridional). Tome I e Stratigraphie[END_REF] point to a large CenomanianeTuronian transgression within what looks like a corridor between the Anti-Atlas and the Reguibat Shield. Although their biostratigraphic data are poor, they are enough to support the hypothesis of a large Cenomanian onlap, as discussed above. But the detail of what could have occurred around the CenomanianeTuronian boundary farther east cannot be deduced from the available data.

Discussion

Age assignments

Regarding the age of the deposits, neither the thinsections cut in limestone or dolomite beds nor the washing residues of claystone have given microfauna of any stratigraphic interest, probably due to the overall shallowness of the depositional facies. The logged sections therefore remain poorly constrained regarding the age of the deposits. The only biostratigraphic data of importance in the Smara area are the lower Turonian pyritized ammonites found above the B1 limestone bed.

In the Tarfaya sub-basin, a 600-m-thick Aptian to Albian section [START_REF] Grosheny | La s erie apto-albienne du bassin marginal de Tarfaya (Maroc m eridional). R eunion th ematique du groupe français du Cr etac e[END_REF], similar in thickness to that of the Puerto Cansado exploration well (Viotti, in [START_REF] Choubert | Le bassin côtier de Tarfaya (Maroc m eridional). Tome I e Stratigraphie[END_REF] was logged along the present-day shore, from the mouth of the Wadi Draa, west of Tan Tan city, to the village of Akhfennir (location in Fig. 1). Onshore sections show that the sedimentary wedge thins when approaching the Palaeozoic basement.

North of the Smara area, along the road from Smara to Tan Tan (red star, Fig. 1A), serial sections were logged over a ten of kilometres in the first marine deposits (Fig. 3). These begins with a incised valley fill beginning with fluvial, gravelly, red deposits overlain by marine yellowish sandstone, and next by a number of metre-scale, probably shortterm transgressiveeregressive (TeR) marine sequences beginning with bioturbated, sandy-gravelly deposits followed by green claystone, and capped by yellowish sandstone, often evenly laminated (? beach facies), representing the regressive half-cycle of the sequences. In some sequences, channelled sandstone (? "pocket-size" incised valley fills) cut into the previous ones, below the next transgressive, coarse-grained lag deposit. These deposits have given a lower Albian ammonite (Mellegueiceras sp.) of the Tardefurcata zone, although its range is suspected to begin in the latest Aptian in Tunisia. This outcrop is very close to the Palaeozoic basement. The underlying A (full continental) and B (red claystone and coarse-grained, greyish, gravelly sandstone units (Fig. 3), which should correspond to the thick continental succession of the Puerto Cansado well, are here very thin. Unit B could correspond to the upper reaches of full marine Aptian deposits found in the lower Wadi Draa [START_REF] Grosheny | La s erie apto-albienne du bassin marginal de Tarfaya (Maroc m eridional). R eunion th ematique du groupe français du Cr etac e[END_REF].

The lower Albian sequences can be traced to the south (unit b, Fig. 4, see location in Fig. 1A). They extend over large distances due to a very low dip. The underlying red deposits (unit a) are thicker here. Lower Albian sequences are overlain by at least 30 m of cross-bedded, coarse-grained to gravelly greyish sandstone alternating with green claystone. The occurrence of claystone rip-up clasts, mud drapes within the cross-bedding suggest a tidal depositional environment. The sequences, around five metres thick each, are bounded by emergence surfaces showing root casts. The mapping of unit c shows that it is bounded at its base by a disconformity (Fig. 4). This peculiar facies is traced to the north along the banks of Wadi Chebika. It is overlain by the deeper upper Albian deposits dated by ammonites [START_REF] Collignon | Les c ephalopodes cr etac es du bassin côtier de Tarfaya. Relations stratigraphiques et pal eontologiques[END_REF].

These additional observations allow us to suggest an Albian age for the lower part of the marine deposits of the Smara area. The first full-marine unit of the SAO section (first marine flooding, Fig. 2) could be lower Albian, and the second marine flooding be late Albian in age. But this would require more robust biostratigraphic data.

In the Tarfaya sub-basin, [START_REF] Kuhnt | Cenomanian sequence stratigraphy and sea-level fluctuations in the Tarfaya Basin (SW Morocco)[END_REF] showed that major deepening occurred at the very base of the Cenomanian. Deposits become finer-grained and host a number of organic-rich layers, which culminate with the latest Cenomanian black shale (Oceanic Anoxic Event 2, or OAE2). It has been suggested above that the B1 platy limestone bed of Smara could be the lateral equivalent of the CenomanianeTuronian boundary black shale of the Tarfaya sub-basin, given its stratigraphic position just under the marlstone bearing the pyritized lower Turonian ammonites. As a consequence, the transgressive yellowish claystone sequence of Smara (Fig. 2) could be the lateral equivalent of the deepening Cenomanian sequence of the Tarfaya sub-basin, but under a more proximal facies. In summary, the stepped transgression found in the Laâyoune-Smara sub-basin would have about the same timing than in the Tarfaya sub-basin, i.e. two moderate Albian transgressive pulses separated by a (? mid-Albian) regressive trend, then two major transgressive pulses, one at the base of the Cenomanian (third flooding, Fig. 2), and the second at the base of the Turonian (fourth flooding, Fig. 2).

Exact timing of the B1 laminated limestone vs the OAE2

The above suggestion that the B1 laminated limestone could be at first sight the lateral equivalent of the CeT black shale of the Tarfaya sub-basin is to be discussed more thoroughly. It has been shown that the CeT black shale present in deep-water deposits of the Central Atlantic covers mostly the uppermost Cenomanian [START_REF] Forster | The Cenomanian/Turonian oceanic anoxic event in the South Atlantic: new insights from a geochemical study of DSDP site 530A. Palaeogeogr[END_REF] and other references therein), as well in onshore deposits of the Tarfaya sub-basin [START_REF] Kuhnt | Cenomanian sequence stratigraphy and sea-level fluctuations in the Tarfaya Basin (SW Morocco)[END_REF]. This is based on the d 13 C positive shift, age-calibrated in the reference section of Pueblo in the US Western Interior [START_REF] Kennedy | The global boundary stratotype section and point for the base of the Turonian stage of the Cretaceous: Pueblo, Colorado[END_REF], and correlated on a larger scale in many local reference sections of western Europe and North Africa, according to a huge literature on the OAE2. Along the Agadir transect in the western High Atlas, based on planktic foraminifera and carbon isotopes, [START_REF] Jati | The CenomanianeTuronian boundary event on the Moroccan Atlantic margin (Agadir basin): stable isotope and sequence stratigraphy[END_REF] showed that the CenomanianeTuronian boundary black shale is not late Cenomanian in age, but early Turonian from its base instead, as it is deposited on a transgressive surface after an emergence that lasted part of the latest Cenomanian. The occurrence of a coarse-grained sandstone transgressive bed under the B1 limestone in the SMA-IV section, as well as the probable truncation of the yellowish Cenomanian sequence below (Fig. 2) are evidences that the B1 bed could be early Turonian in age, as in the Taghazoute section along the Agadir transect [START_REF] Jati | The CenomanianeTuronian boundary event on the Moroccan Atlantic margin (Agadir basin): stable isotope and sequence stratigraphy[END_REF], instead of being correlated with the nearby Tarfaya black shale. Therefore, contrarily to what is found in many locations where the CeT boundary layers are full transgressive, the eastern border of the LaâyouneeSmara subbasin would have undergone a short-lived uplift to explain the emergence and the earliest Turonian transgressive truncation of the Cenomanian sequence.

CenomanianeTuronian boundary palaeogeographic feature on a regional scale

The data obtained in the Smara area regarding what happened around the CeT boundary are integrated (Fig. 5) into a larger-scale picture, including what is known from the western reaches of the Atlas belt (Agadir-Essaouira basin) and the many data acquired along the southern shoulder of the Atlas rift [START_REF] Benyoucef | Lithostratigraphic evolution, facies analysis and depositional environment of the Cenomanian-lower Turonian in the Guir area, Southwestern Algeria[END_REF][START_REF] Benyoucef | Litho-and biostratigraphy, facies patterns and depositional sequences of the CenomanianeTuronian deposits in the Ksour Mountains (Saharan Atlas, Algeria)[END_REF][START_REF] Ettachfini | La limite C enomanien-Turonien dans le Haut Atlas central[END_REF][START_REF] Lebedel | Geochemical and palaeoecological record of the CenomanianeTuronian anoxic event in the carbonate platform of the Preafrican trough, Morocco[END_REF][START_REF] Lebedel | The upper Cenomanian-lower Turonian of the Preafrican trough (Morocco): platform configuration and palaeoenvironmental conditions[END_REF][START_REF] Lezin | The upper Cenomanian-lower Turonian of the Preafrican trough, Morocco[END_REF], the Kem-Kem embayment [START_REF] Cavin | Vertebrate assemblages from the early Late Cretaceous of southeastern Morocco: an overview[END_REF], up to the Ghardaïa area [START_REF] Nouh-Mefnoune | Contribution a la stratigraphie de la barre carbonat ee c enomano-turonienne de la plate-forme saharienne : etude des affleurements de Ghardaïa (dorsale du M'Zab)[END_REF] in the large CenomanianeTuronian lagoon of the Algerian Sahara. Although the observations of [START_REF] Choubert | Le bassin côtier de Tarfaya (Maroc m eridional). Tome I e Stratigraphie[END_REF] suggest a pinching out of the most transgressive Turonian deposits by the Tarf Zaïda and Merkala localities on the northern border of the Tindouf syncline (Fig. 1A), a full marine connection cannot be excluded between the Atlantic and the Tethys along the saddle between the Anti-Atlas and the Reguibat Shield during the early Turonian peak transgression, therefore connecting the Smara and the Kem-Kem embayment. At that time, the Anti-Atlas massif could have been an island (the supposed palaeoshoreline is indicated in Fig. 1A).

What occurred just before this peak transgression, that is during the latest Cenomanian CTBE/OAE2 event, is more complicated, and despite what has been suggested above in the Smara area about a large Cenomanian transgression followed by a stronger Turonian one. The correlation scheme (Fig. 5) is based on the tracking of short-term depositional sequences with the help of the CTBE d 13 C isotope signal (work in progress) within the CeT boundary deposits along the southern border of the Atlas belt. As well as in the Saharan lagoon [START_REF] Busson | La grande transgression du C enomanien sup erieur-Turonien inf erieur sur la Hamada de Tinrhert (Sahara alg erien): datations biostratigraphiques, environnement de d epôt et comparaison d'un t emoin epicratonique avec les s eries contemporaines a mati ere organique du Maghreb[END_REF][START_REF] Grosheny | The Cenomanian-Turonian boundary on the Saharan Platform (Tunisia and Algeria)[END_REF], the southern shoulder of the Atlasic rift experienced a latest Cenomanian transgression represented (Fig. 5) by two or three short-term shallow-water carbonate sequences exactly spanning the CTBE d 13 C shift. This transgressive pulse is recorded along the Saharan Atlas up to Tunisia [START_REF] Grosheny | The Cenomanian-Turonian boundary on the Saharan Platform (Tunisia and Algeria)[END_REF][START_REF] Bachari | The CenomanianeTuronian Boundary Event (CTBE) in North-Central Tunisia (Jebel Serj and Bargou) integrated into regional data (Algeria to Tunisia)[END_REF], and therefore is opposed to what is found along some segments of the Atlantic margin. In the Smara area (Fig. 2), an exposure coupled with truncation of upper Cenomanian sequences is recorded instead, as explained above. The same sequence stratigraphic disturbance was also found along the southwestern shoulder of the Atlasic rift (uplifted area, Fig. 5), in the Agadir basin [START_REF] Jati | The CenomanianeTuronian boundary event on the Moroccan Atlantic margin (Agadir basin): stable isotope and sequence stratigraphy[END_REF], although a coeval deepening (subsiding area, Fig. 5) is found to the north in the Essaouira basin (work in progress). The areas that underwent either a deepening or an uplift from the sedimentary record are shown in Fig. 4. Evidences of local uplifts are along the Atlantic margin, in opposition to the remaining of the African craton to the east, which subsided during the CTBE. Other evidences are found along the Iberian and French Atlantic margin (as discussed in Grosheny et al., 2017b). On the Atlantic margin of France for instance, Desmares et al. (in press) showed that the CTBE beds are lacking on the eastern termination of the Armorican Massif, while they are present in the Paris Basin to the east, and also on the coastal section of Charentes to the west (refs. in Lasseur et al., 2008). This suggests that the massif underwent an uplift just before the early Turonian transgression. We think that the same might have occurred on the western part of the Smara area, which would explain the hiatus found in the SMA-IV section (Fig. 2). Allocyclic processes are excluded, given the very good lateral continuity of metre-scale sequences in the area. The hiatus is therefore explained by the truncation of the sedimentary succession by the wave ravinement surface of the early Turonian transgression on an area that was uplifted during the CTBE event. Grosheny et al. (2017a) suggested that such sequence stratigraphic heterogeneity was the result of the shortlived tendency to tectonic inversion of the Atlantic margin during the CTBE event, as a result of an hypothesized tectonic pulse recorded on a larger scale. All available stratigraphic data suggesting basement deformations during the CTBE along the South Moroccan Atlantic margin are in Fig. 5

(lower box).

Although considered a passive margin, the Moroccan Atlantic margin underwent some major uplifts during its Mesozoic to Cenozoic history [START_REF] Benabdellouahed | Recent uplift of the Atlantic Atlas (offshore West Morocco): tectonic arch and submarine terraces[END_REF][START_REF] Bertotti | Post-rift vertical movements and horizontal deformations in the eastern margin of the Central Atlantic: Middle Jurassic to Early Cretaceous evolution of Morocco[END_REF][START_REF] Gouiza | Kinematic and thermal evolution of the Moroccan rifted continental margin: Doukkala-High Atlas transect[END_REF][START_REF] Leprêtre | Mesozoic evolution of NW Africa: implications for the central Atlantic Ocean dynamics[END_REF][START_REF] Leprêtre | Polyphased inversions of an intracontinental rift: case study of the Marrakech high Atlas, Morocco[END_REF][START_REF] Meghraoui | Coastal tectonics across the South Atlas thrust front and the Agadir active zone, Morocco[END_REF][START_REF] Westaway | Field reconnaissance of the Anti-Atlas coastline, Morocco: fluvial and marine evidence for Late Cenozoic uplift[END_REF]. The short-lived uplift inferred here around the CeT boundary could be part of this complex history involving subsidence interrupted by uplifts. What the present study suggests is that such uplifts may have been very short in duration, based on the stratigraphic record.

Turonian to Coniacian sequences

The upper Turonian to (?) Coniacian TeR sequences (Fig. 2) have a special pattern. In some cases, the flooding marlstone of a sequence is sharply truncated by the beach facies of the regressive half-cycle or by the bioturbated transgressive sandstone bed of the overlying sequence. In some case, the nesting of successive beach prisms (Fig. 2) is indicative of stepped forced regression. In lower Albian short-term sequences (Fig. 3), small incised valley fills indicative of forced regression may even be found between the highstand marlstone of a given sequence and the transgressive sandstone of the next sequence. Such unusual patterns are commonly found on very low-sloped ramp profiles, where the slightest sea-level drop can be marked by sharp facies changes. In the Cenomanian of the western High-Atlas, [START_REF] Essafraoui | Sequence stratigraphic architecture of marine to fluvial deposits across a passive margin (Cenomanian, Atlantic margin, Morocco, Agadir transect)[END_REF] developed a sequence stratigraphic model for such deposits, including the occurrence of regressive fluvial deposits above a continental aggradation surface. The latter are not present in the Laâyoune basin, but the observed patterns of Late Cretaceous sequences indicate a very low-sloped depositional profile, responsible of the evidences of stepped regression during the regressive half-cycle of depositional sequences.

Conclusions

In the onshore part of the SmaraeLâayoune sub-basin, the Cretaceous marine transgression began presumably in the early Albian. It proceeded by successive steps to onlap the Palaeozoic basement in the Cenomanian, to reach a maximum in the early Turonian. At that time, the Anti-Atlas massif may have been an island, isolated from the Reguibat Shield by a narrow seaway connecting the Atlantic, the Tethys and the Saharan lagoon through the SmaraeKem Kem embayments. The CenomanianeTuronian boundary is marked here by a short-lived emergence, as on the western termination of the Atlas belt (Agadir area), contrarily to what is more commonly found elsewhere (flooding). The whole sedimentary data, including the southern shoulder of the Atlas rift under study, point to a brief tectonic uplift (? inversion) of the Atlantic side of the Anti-Atlas and the Reguibat Shield during the CenomanianeTuronian Boundary Event.

Fig. 1 .

 1 Fig. 1. Location of the study area. A, Simplified geological map showing the area of study (Laâyoune sub-basin), including that of a companion one under completion (Tarfaya sub-basin), and whose results are partially cited in the text. The dotted heavy green line representing the early Turonian shoreline at maximum transgression, as discussed in the text. Localities cited in Choubert et al. (1966) on the northern border of the Tindouf syncline, and discussed in the text, are also indicated (B, El Bouirat; GK, Gour Kneifida; M, Merkala; T, Touiela (Garet el Houissane); TZ, Tarf Zaïda, Z, Zaak, or Zag). White star, location of Puerto Cansado well; black star, location of Wadi Chebeika. B, detailed map of the Smara area with the location of the logged sections (blue dots).

Fig. 2 .

 2 Fig. 2. Correlation of the sections in the Smara area. Sequence stratigraphic interpretation in terms of transgressiveeregressive (TeR) sequences are indicated along the sedimentological sections. On the left are redrawn the sections published by Choubert et al. (1966) on the northern border of the Tindouf syncline. For location of sections, see Fig. 1A.

Fig. 3 .

 3 Fig. 3. Lower Albian transgressive marine deposits in the Abteh area (onshore Tarfaya sub-basin). A to B, units discussed in the text. IVF, incised valley fill. Location of the area in Fig. 1A.

Fig. 4 .

 4 Fig. 4. Geological map of Cretaceous deposits along the road from Smara to TaneTan showing a discomformity between lower Albian deposits (unit b) described and dated farther north (see Fig.3) and (? middle) Albian marine sequences dominated by thick, greyish, cross-bedded sandstone beds (unit c). Unit a, full continental deposits (mostly red claystones); unit d, Miocene deposits.

Fig. 5 .

 5 Fig. 5. Correlation of sections covering the CeT transition along the edge of the Saharan Platform from South Morocco to western Algeria, and interpretation of relative sea-level changes in terms of subsidence/uplift (see map). Sections (a) to (d) are from unpublished data, with orange background indicating shallow-water, coquina-bearing (including rudists) limestones. Ammonites mostly after Meister et al., 2017. Section (e) is redrawn from Lezin et al. (2012), (f) from Cavin et al., 2010, (g) from Benyoucef and Meister, 2015, (h) from Nouh-Mefnoune (2006). The red-lined areas on the map are areas where an emergence fully covers the CeT transition or part of it. Blue-lined are areas, where the CeT transition is within a full transgressive trend. Vertical violin bars, stratigraphic extent of the d 13 C shift of the CenomanianeTuronian Boundary Event (work in progress). Stratigraphic surfaces (upper case major, lower case minor): SB, sequence boundary; TS, transgression surface; FS, flooding surface.

Table 1

 1 Coordinates of the sections.

	Gara El Berd

39 0 52 00 N, 11 54 0 34 00 W Saoula El Hamra 26 51 0 06 00 N, 12 15 0 03 00 W Smara-I 26 37 0 39 00 N, 11 58 0 28 00 W Smara-II 26 40 0 26 00 N, 11 59 0 52 00 W Smara-IIIa 26 40 0 09 00 N, 11 59 0 39 00 W SMA-IIIb 26 39 0 17 00 N, 11 59 0 42 00 W Smara-IV start 26 36 0 20 00 N, 12 02 0 15 00 W Smara-IV finish 26 32 0 20 00 N, 12 09 0 44 00 W

Acknowledgements

This paper is a contribution to the Cooperative Programme between the Universities of Lorraine (France) and Agadir (Morocco) e Programme Hubert-Curien Volubilis (Grant MA/14/305) -, and part of the ongoing PhD work of the first author.

The careful reading and sound comments of an anonymous reviewer helped to improve the manuscript, although all the recommendations could not be met due to the short format of C. R. Geoscience articles.


Jonathan Pradelles

Loïc Perraud

Aurélien Fay

Jean-Baptiste Henry

Jessy Bustos

Estelle Guyez

Sébastien Berard-Bergery

Aurélie Le Pennec

Mohamed Abaidi

Jordan Belissard

Roughness measurement of 2D curvilinear patterns: challenges and advanced methodology

à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Roughness measurement of 2D curvilinear patterns: challenges and advanced methodology

Jonathan Pradelles a , Loïc Perraud a , Aurélien Fay a , Jean-Baptiste Henry a , Jessy Bustos a , Estelle Guyez a , Sébastien Berard-Bergery a , Aurélie Le Pennec a , Mohamed Abaidi b , Jordan Belissard b , Nivea Schuch b , Thiago Figueiro b , Matthieu Millequant b , Patrick Schiavone b,c a Univ. Grenoble Alpes, CEA, LETI, DPFT, Lithography Laboratory, F-38000 Grenoble, France b ASELTA Nanographics, 4 pl. Robert Schuman, F-38000 Grenoble, France c Univ. Grenoble Alpes, CNRS, CEA/LETI-Minatec, Grenoble INP, LTM, F-38054 Grenoble-France

The importance of measurement and analysis of pattern roughness in SEM (Scanning Electron Microscope) images has continuously grown in the past few decades since it affects performances of devices [START_REF] Oldiges | Modeling line edge roughness effects in sub 100 nanometer gate length devices[END_REF]. In parallel, non-Manhattan patternssuch as curvilinear and shapes with multiple anglesare more and more present in recent lithography landscape [START_REF] Dalton | Photonic integration improves on current technologies[END_REF], [START_REF] Koh | Manhattan or non-Manhattan?: a study of alternative VLSI routing architectures[END_REF]. Key examples are photonic devices, ILT (Inverse Lithography Technology) patterns in photomask fabrication, and DSA (Directed Self-Assembly) fingerprint patterns for early material development. In this article the challenges associated with roughness measurement on such curvilinear patterns will be presented and algorithms and methodology to tackle these challenges will be proposed.

The proposed methodology relies on robust-to-noise contour extraction algorithms [START_REF] Le-Gratiet | Contour based metrology: "make measurable what is not so[END_REF]. In this paper, an original method to evaluate and validate contour extraction algorithms, in the context of roughness measurement, is proposed. The schematic workflow is illustrated in Figure 1. Both a SEM model and a noise model are calibrated using experimental SEM images from state-of-the-art equipment and processes. This allows the generation of synthetic SEM images with arbitrary layout, programmed roughness and controlled noise (equivalent to 'frame by frame' SEM image acquisition). Edge detection algorithms and advanced 'PSD analysis' are applied to the images for roughness measurement. Same strategy may then be applied to 'real' SEM images.

The method is first carried out in 1D patterns (straight lines). For that, synthetic SEM images are generated, following the protocol of ref [START_REF] Francesco Lorusso | Need for LWR metrology standardization: the imec roughness protocol[END_REF]. From this step, an excellent correlation with the results obtained in [START_REF] Mack | Unbiased roughness measurements: subtracting out SEM effects, part 3[END_REF] is found. Additionally, to that, a very low noise sensitivity of the edge contour extraction algorithm is noticed (Figure 2).

The methodology is then extended to 2D patterns. As a first step, we make use of synthetic SEM images to discuss the potential problems raised by curvilinear patterns, such as reference determination, data interpolation and closing contours. In the same way as for 1D patterns, a programmed roughness, entered as input in the workflow of Figure 1, is found back in the analysis after contour extraction and PSD analysis. Using synthetic images for different number of frames, the contour extraction sensitivity to noise can also be explored.

Finally, the methodology is successfully applied to experimental SEM images: unbiased PSD of 2D curved patterns are determined for two classes of applications: photonic devices (as shown in Figure 3) and DSA fingerprint (as shown in Figure 4). Keywords: 2D roughness metrology, curvilinear patterns, contour metrology, PSD analysis, SEM model, synthetic images

Figure 1 :

 1 Figure 1: Schematic workflow used to measure roughness on curvilinear 2D patterns.

Figure 2 :

 2 Figure 2: 1D-pattern roughness measurement following protocol of [5] with excellent correlation with results of [6].

Figure 3 :

 3 Figure 3: (a), a ring oscillator design from a photonic device. (b) a detailed zoom of the associated SEM image, showing the variability of angles that have to be considered for the roughness measurement. (c), the unbiased PSD for the ring oscillator in (a).

Figure 4 :

 4 Figure 4: (a) a SEM image from a DSA fingerprint and zoom in detailed (highlighted in red). (b), the result of the contour extraction [4], and (c), the associated unbiased PSD.