The Cretaceous Marine Onlap on Palaeozoic Deposits (Smara–Lâayoune Basin, South Morocco)

The Cretaceous marine onlap on Palaeozoic deposits (Smara–Lâayoune Basin, South Morocco). Comparison with neighbouring regions Mohamed Abioui, Serge Ferry, Danièle Grosheny, Nourrissaid Içame, Emmanuel Robert, Mohammed Benssaou

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Mohamed Abioui, Serge Ferry, Danièle Grosheny, Nourrissaid Içame, Emmanuel Robert, et al.. The Cretaceous marine onlap on Palaeozoic deposits (Smara–Lâayoune Basin, South Morocco). Com- parison with neighbouring regions. Comptes Rendus Géoscience, Elsevier Masson, 2019, 351 (7), pp.498-507. 10.1016/j.crte.2019.09.003. hal-03155821

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Distributed under a Creative Commons Attribution - NoDerivatives| 4.0 International License Comptes rendus - Geoscience 351 (2019) 498e507

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Stratigraphy, Sedimentology The Cretaceous marine onlap on Palaeozoic deposits (SmaraeLaayoune^ Basin, South Morocco). Comparison with neighbouring regions

* Mohamed Abioui a, Serge Ferry b, , Daniele Grosheny c, Nourrissaid Içame a, Emmanuel Robert d, Mohammed Benssaou a a Universite Ibn Zohr, Laboratoire de geologie appliquee et geo-environnement (LAGAGE), Departement de geologie, Faculte des sciences, BP 81106, Cite Dakhla, 8000 Agadir, Morocco b 6D, avenue du General-de-Gaulle, 05100 Briançon, France c Universite de Lorraine, UMR CNRS 7359 GeoRessources, BP 70239, 54506 Vandœuvre-les-Nancy, France d Universite de Lyon, Laboratoire de geologie de Lyon: terre, planetes et environnement (LGLTPE), campus de la Doua, 2, rue Rapha€el- Dubois, 69622 Villeurbanne cedex, France article info abstract

Article history: The Cretaceous marine transgression proceeded through successive steps from the Albian Received 24 March 2019 to the Turonian (dated with ammonites). The onlapping wedge begins with coastal Received in revised form 25 September 2019 transgressiveeregressive short-term sequences on massive, probably ﬂuvial sandstones to Accepted 26 September 2019 be correlated with the very thick continental Lower Cretaceous succession found in the Available online 14 November 2019 Puerto Cansado well in the Tarfaya sub-basin to the north. A second step, of probable Handled by Sylvie Bourquin Cenomanian age, reached the Palaeozoic basement. A third, more pronounced step Keywords: occurred during the earliest Turonian with platy laminated limestone overlain by marl- Albian stone bearing pyritized ammonites. At early Turonian peak transgression, a marine Cenomanian connection was possibly established between the Atlantic and the Tethyan margins, be- Turonian tween the Anti-Atlas and the Reguibat Shield. From large-scale correlation integrating OAE2 what occurred along the southwestern shoulder of the Atlas rift, the South Moroccan CTBE Atlantic margin may have undergone a short-lived tectonic uplift around the Cenomanian eTuronian boundary. © 2019 Académie des sciences. Published by Elsevier Masson SAS. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/ ).

1. Introduction remain poorly studied since the pioneer work of Choubert et al. (1966). Most recent studies have focused on the In southern Morocco, the Cretaceous deposits of the western, coastal part of the basin and the connection with easternmost part of the TarfayaeLaayoune^ eBoudjour Basin offshore deposits, mostly for oil exploration or water sup- (TLBB) or TarfayaeDakhla Basin as also known in the plies. Two subbasins, the Tarfaya and the SmaraeLaayoune literature (Fig. 1A), onlapping the Palaeozoic basement, subbasins, were studied as part of a Cooperative Pro- gramme between the Universities of Agadir (Morocco) and Lorraine (France), in order to reﬁne the stratigraphy and depositional environments of Cretaceous deposits pinching * Corresponding author. E-mail addresses: [email protected] (M. Abioui), serge.ferry@ out on basement rocks of the southern termination of the yahoo.fr (S. Ferry), [email protected] (D. Grosheny), n. Anti-Atlas and the Reguibat Shield. Here are presented the [email protected] (N. Içame), [email protected] (E. ﬁeld data acquired in the Smara area. Those of the Tarfaya Robert), [email protected] (M. Benssaou). https://doi.org/10.1016/j.crte.2019.09.003 1631-0713/© 2019 Académie des sciences. Published by Elsevier Masson SAS. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). M. Abioui et al. / Comptes rendus - Geoscience 351 (2019) 498e507 499

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.

2. Description of the sections

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

2.1. 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 fine- grained 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. Fig. 1. Location of the study area. A, Simplified geological map showing the The yellowish claystoneedolomite succession under B1 area of study (Laayoune^ sub-basin), including that of a companion one under can be interpreted as an overall prograding sequence over the completion (Tarfaya sub-basin), and whose results are partially cited in the basal transgressive sandstone bed. This sequence is probably text. The dotted heavy green line representing the early Turonian shoreline e e at maximum transgression, as discussed in the text. Localities cited in to be subdivided into three transgressive regressive (T R) Choubert et al. (1966) on the northern border of the Tindouf syncline, and subesequences (a, b and c, Fig. 2). The first white limestone discussed in the text, are also indicated (B, El Bouirat; GK, Gour Kneifida; M, bed B1 is a laminated restricted deposit, probably repre- Merkala; T, Touiela (Garet el Houissane); TZ, Tarf Zaïda, Z, Zaak, or Zag). senting a maximum flooding, as well as the second, wavy- White star, location of Puerto Cansado well; black star, location of Wadi bedded B2 limestone. Chebeika. B, detailed map of the Smara area with the location of the logged sections (blue dots). Age assignments (Fig. 2) will be discussed further. subbasin are under completion. A special focus is also given 2.2. Smara-II section on CenomanianeTuronian boundary deposits (Cen- e omanian Turonian Boundary Event or CTBE). This event, This section (Fig. 2) is located in a more distal position also known as Oceanic Anoxic Event 2 (OAE2), has attracted than section I. It begins with bioturbated sandstone with much attention over the past decades on a worldwide basis thin clayey interbeds, overlain by a sandstone bed bearing (see Kuhnt et al., 2009, 2017, and references therein for the Tarfaya subbasin, which hosts the CTBE black shale). But few is known on the corresponding proximal shallow- Table 1 water deposits, devoid of organic-rich shales. In this Coordinates of the sections. respect, still unpublished results regarding the southern Gara El Berd 263905200N, 115403400W shoulder of the Atlas belt have been integrated for com- Saoula El Hamra 265100600N, 121500300W 0 00 0 00 parison in the present study, as part of a continued effort Smara-I 26 37 39 N, 11 58 28 W Smara-II 264002600N, 115905200W (Grosheny et al., 2017a, 2017b) to track a possible tectonic Smara-IIIa 264000900N, 115903900W imprint on the CTBE environmental event on a large scale. SMA-IIIb 263901700N, 115904200W Six sections have been logged west of Smara, in order to Smara-IV start 263602000N, 120201500W 0 00 0 00 reconstruct a transect (Fig. 1B). The Cretaceous coastal Smara-IV finish 26 32 20 N, 12 09 44 W 500 .Aiu ta./Cmtsrnu esine31(09 498 (2019) 351 Geoscience - rendus Comptes / al. et Abioui M. e 507

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. M. Abioui et al. / Comptes rendus - Geoscience 351 (2019) 498e507 501

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

A. observation hiatus, a thick sequence of yellowish, bioturbated, very sandy claystone ﬁnishes up with a cap of Fig. 1 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, wavy-

ll. Location of the area in bedded white limestone bed, that we interpret as being fi 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., Nanno- vascoceras constrictum Renz and Alvarez (1979), 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 (Kuhnt et al., 2009). Other age assignments will be discussed further.

2.3. 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, laterally- continuous sandstone overlain by a light-grey, cross- bedded, 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

Lower Albian transgressive marine deposits in the Abteh area (onshore Tarfaya sub-basin). A to B, unitsﬁ discussed inne-grained the text. IVF, incised valley sandstone cut by root casts, and overlain by a bioturbated, coquina-rich, gravelly sandstone bed, which

Fig. 3. 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 502 M. Abioui et al. / Comptes rendus - Geoscience 351 (2019) 498e507 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 sandstone- prone 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 com- mon (Ouaja et al., 2002).

2.4. Smara-IV section

The section (Fig. 2) is a composite one, logged from different outcrops (Fig. 1B) along the road linking Laayoune^ 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 Fig. 4. Geological map of Cretaceous deposits along the road from Smara to also present between the platy limestone bed B1 and the TaneTan showing a discomformity between lower Albian deposits (unit b) described and dated farther north (see Fig. 3) and (? middle) Albian marine wavy bed B2. The remaining of the section is a complex sequences dominated by thick, greyish, cross-bedded sandstone beds (unit alternation of claystone to marlstone (most often covered) c). Unit a, full continental deposits (mostly red claystones); unit d, Miocene and shallow-water facies like massive dolomite, laminated deposits. dolomite bearing wave-ripples, bioturbated fine-grained limestone, some of these beds being rich in bivalves or gastropods (Nerinea), sandy limestone, and coarse-grained spectacular vertical root casts. A covered recessive interval fi to gravelly, coquina-rich sandstone. The coarse-grained whose upper part shows a ne-grained sandstone bearing sandstone bed, by the mark 40 m, has been traced over small sinusoidal (? wave) ripples is found below the sum- several hundred metres. It shows a laterally-continuous, mital ledge made of the same laterally-continuous, evenly- fi even or low-angle stratification characteristic of a beach strati ed, coarse-grained sandstone as below, but without facies. In addition, stratal evidence of forced regression has the root casts. Its base is also sharp. fi been found in the nesting of successive beach prisms to- On a sequence stratigraphic point of view, the rst two ward a western (Atlantic) lowstand palaeoshore. The last sandstone units belong to two successive sequences, given fi beds of the section were logged on an isolated outlier, north their fully different features. The next unit, under the rst fl of the road. Its summit is a coquina-rich dolomite capped root casts surface, is probably uvial. The next one is by a karstified stromatolite bed, indicating emergence. The considered tidal due to occurrence of clayey rip-up clasts fi upper part of the section, above the lower Turonian within the megaripples, therefore representing the rst ammonite-bearing marlstone, probably covers the upper evidence of a marine transgression. The two upper, ledge- Turonian, extending possibly into the Coniacian. forming sandstone beds, are probably beach deposits, representing either a full TeR sequence for the lower one, e 2.5. Gara El Berd section or the shallow-water top of a T R sequence for the second one. This is quite clear for the first one, with its sharp base The section (Fig. 2) is logged on the flank of an isolated and the root casts at its top. The upper ledge could cap, in a outlier south of the main road to Smara, in a saddle be- forced regressive regime (owing to its sharp base), the fi tween the Palaeozoic horst of the SMA-I section and the underlying ne-grained sandstone of the recessive interval. Palaeozoic cover of the Reguibat Shield to the east (Fig. 1B). At the base, the same massive sandstone and the light-grey, 2.6. Wadi Saquia El Hamra section cross-bedded coarse-grained sandstone found in the SMA- III section are outcropping. The remaining of the section is This section (Fig. 2) is located close to the confluence different. A first package of yellowish, megarippled, poorly- between the main Wadi Saquia El Hamra (SAO) and a little cemented sandstone beds capped by an indurated bed of wadi coming from the north. No intermediate section could the same facies overlies the light-grey sandstone. Its be logged along the banks of the Wadi Saoula El Hamra indurated cap is cut by vertical root casts indicating expo- (Fig. 1B), except for some isolated outcrops. The section, sure. This emergence or abandonment surface is overlain which is in the most distal position, begins with the same by medium-grained, megarippled sandstone bearing two lower units than in the Gara el Berd section. Especially, numerous clayey rip-up clasts within the bottomsets, pre- the light-grey gravelly sandstone unit, which is easily sumably indicating a tidal environment. This sandstone recognizable, can be almost continuously spotted along the unit is sharply overlain by a 4-m-thick coarse-grained left bank of the Wadi Saquia El Hamra. The overlying de- sandstone bed bearing laterally-continuous even stratifi- posits are an alternation of marine claystone, sandy clay- cation or low-angle cross-bedding, and also cut by stone, bioturbated sandstone, dolomitic coquina sandstone, M. Abioui et al. / Comptes rendus - Geoscience 351 (2019) 498e507 503 and evenly-stratified coarse-grained sandstone (inter- Given the pervasive dolomitization found in Cretaceous preted as a beach facies). In the middle of the section, about deposits of the Smara area, such lateral facies changes five metres of large-scale, high-angle cross-bedded aeolian could explain that either the bioturbated limestone or sandstone are overlain by about five metres of cross- sandstone caps of the overlying sequences (SMA-IV) could bedded, fluvial gravelly sandstone. These continental de- pass to full dolomite sequences (SMA-II). posits are overlain by a marine succession roughly similar Farther east of the Smara area (Fig. 2, left part), along the to the previous one. northern border of the Tindouf syncline (Fig. 1A), the de- The section therefore shows two marine transgressive scriptions of Choubert et al. (1966) point to a large episodes made of several short-term shallow-water TeR CenomanianeTuronian transgression within what looks sequences, separated by an overall regressive interval of like a corridor between the Anti-Atlas and the Reguibat continental deposits. Shield. Although their biostratigraphic data are poor, they are enough to support the hypothesis of a large Cen- 3. Correlation of the sections omanian onlap, as discussed above. But the detail of what could have occurred around the CenomanianeTuronian The correlation diagram (Fig. 2) includes both our data boundary farther east cannot be deduced from the avail- and the sections of Choubert et al. (1966), redrawn from able data. 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 4. Discussion basal, massive, structureless sandstone and its cover of light-grey gravelly sandstone make everywhere the sole of 4.1. Age assignments marine deposits. The first marine TeR sequences of the distal SAO section (first marine flooding) are correlated Regarding the age of the deposits, neither the thin- with those at the base of the Gara el Berd section owing to sections cut in limestone or dolomite beds nor the their characteristics, especially because of the look-alike, washing residues of claystone have given microfauna of any coarse-grained sandstone beach facies that caps them in stratigraphic interest, probably due to the overall shal- both sections. The overlying continental interval of the SAO lowness of the depositional facies. The logged sections section is not recorded updip, as well as the sandstone therefore remain poorly constrained regarding the age of marine sequences (second marine flooding) under the the deposits. The only biostratigraphic data of importance thick yellowish laterally-continuous claystone sequence in the Smara area are the lower Turonian pyritized am- (underlined in yellow, Fig. 2) representing a third marine monites found above the B1 limestone bed. flooding. This flooding was more pronounced than the two In the Tarfaya sub-basin, a 600-m-thick Aptian to Albian former ones, as it transgresses the Palaeozoic basement of section (Grosheny et al., 2012), similar in thickness to that the horst. The transgressive ravinement surface of this third of the Puerto Cansado exploration well (Viotti, in Choubert flooding may have truncated the sequences below. The et al., 1966) was logged along the present-day shore, from overlying yellowish claystone sequence can be subdivided the mouth of the Wadi Draa, west of Tan Tan city, to the into three TeR subsequences (a, b and c, Fig. 2) which can village of Akhfennir (location in Fig. 1). Onshore sections be easily traced in the most proximal sections (SMA-I and show that the sedimentary wedge thins when approaching SMA-III), less in the SMA-II section because their regressive the Palaeozoic basement. summital facies (coquina-bearing dolomite) fades away North of the Smara area, along the road from Smara to within the distal claystone. Tan Tan (red star, Fig. 1A), serial sections were logged over a The fourth marine flooding is at the base of the B1 platy ten of kilometres in the first marine deposits (Fig. 3). These limestone bed. The basal transgression surface looks like begins with a incised valley fill beginning with fluvial, erosional, especially in the SMA-II section, where the b and gravelly, red deposits overlain by marine yellowish sand- c underlying subsequences are lacking. In this section, the stone, and next by a number of metre-scale, probably short- B1 bed rests on a bioturbated, coarse-grained, gravelly, term transgressiveeregressive (TeR) marine sequences coquina-bearing sandstone bed representing a trans- beginning with bioturbated, sandy-gravelly deposits fol- gressive systems tract. This bed was not found in other lowed by green claystone, and capped by yellowish sand- sections, and is thought to be truncational. stone, often evenly laminated (? beach facies), representing Above the wavy-bedded B2 limestone, the correlation the regressive half-cycle of the sequences. In some se- diagram (Fig. 2) shows an interesting feature within the quences, channelled sandstone (? “pocket-size” incised next sequence. The top of the sequence is represented in valley fills) cut into the previous ones, below the next the SMA-III section by a partly dolomitized, coarse-grained transgressive, coarse-grained lag deposit. These deposits sandstone bearing aligned, overturned bivalve shells have given a lower Albian ammonite (Mellegueiceras sp.) of within an even stratification, characteristic of a beach the Tardefurcata zone, although its range is suspected to facies. Laterally (SMA-II and SMA-IV sections), structureless begin in the latest Aptian in Tunisia. This outcrop is very dolomite beds are found instead. It is suggested that these close to the Palaeozoic basement. The underlying A (full dolomite beds could result from the full dolomitization of a continental) and B (red claystone and coarse-grained, beach sandstone, as it has been often found in look-alike greyish, gravelly sandstone units (Fig. 3), which should sequences of the so-called Continental Intercalaire (or correspond to the thick continental succession of the “Purbecko-Wealdien”) of South Tunisia (Ouaja et al., 2002). Puerto Cansado well, are here very thin. Unit B could 504 .Aiu ta./Cmtsrnu esine31(09 498 (2019) 351 Geoscience - rendus Comptes / al. et Abioui M. e 507

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 d13 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. M. Abioui et al. / Comptes rendus - Geoscience 351 (2019) 498e507 505 correspond to the upper reaches of full marine Aptian de- the Agadir transect in the western High Atlas, based on posits found in the lower Wadi Draa (Grosheny et al., 2012). planktic foraminifera and carbon isotopes, Jati et al. (2010) The lower Albian sequences can be traced to the south showed that the CenomanianeTuronian boundary black (unit b, Fig. 4, see location in Fig.1A). They extend over large shale is not late Cenomanian in age, but early Turonian distances due to a very low dip. The underlying red deposits from its base instead, as it is deposited on a transgressive (unit a) are thicker here. Lower Albian sequences are surface after an emergence that lasted part of the latest overlain by at least 30 m of cross-bedded, coarse-grained to Cenomanian. The occurrence of a coarse-grained sandstone gravelly greyish sandstone alternating with green clay- transgressive bed under the B1 limestone in the SMA-IV stone. The occurrence of claystone rip-up clasts, mud section, as well as the probable truncation of the drapes within the cross-bedding suggest a tidal deposi- yellowish Cenomanian sequence below (Fig. 2) are evi- tional environment. The sequences, around five metres dences that the B1 bed could be early Turonian in age, as in thick each, are bounded by emergence surfaces showing the Taghazoute section along the Agadir transect (Jati et al., root casts. The mapping of unit c shows that it is bounded at 2010), instead of being correlated with the nearby Tarfaya its base by a disconformity (Fig. 4). This peculiar facies is black shale. Therefore, contrarily to what is found in many traced to the north along the banks of Wadi Chebika. It is locations where the CeT boundary layers are full trans- overlain by the deeper upper Albian deposits dated by gressive, the eastern border of the Laayoune^ eSmara sub- ammonites (Collignon, 1966). basin would have undergone a short-lived uplift to These additional observations allow us to suggest an explain the emergence and the earliest Turonian trans- Albian age for the lower part of the marine deposits of the gressive truncation of the Cenomanian sequence. Smara area. The first full-marine unit of the SAO section (first marine flooding, Fig. 2) could be lower Albian, and the 4.3. CenomanianeTuronian boundary palaeogeographic second marine flooding be late Albian in age. But this feature on a regional scale would require more robust biostratigraphic data. In the Tarfaya sub-basin, Kuhnt et al. (2009) showed that The data obtained in the Smara area regarding what major deepening occurred at the very base of the Cen- happened around the CeT boundary are integrated (Fig. 5) omanian. Deposits become finer-grained and host a num- into a larger-scale picture, including what is known from ber of organic-rich layers, which culminate with the latest the western reaches of the Atlas belt (Agadir-Essaouira Cenomanian black shale (Oceanic Anoxic Event 2, or OAE2). basin) and the many data acquired along the southern It has been suggested above that the B1 platy limestone bed shoulder of the Atlas rift (Benyoucef and Meister, 2015; of Smara could be the lateral equivalent of the Benyoucef et al., 2017; Ettachfini et al., 2005; Lebedel CenomanianeTuronian boundary black shale of the Tarfaya et al., 2013, 2016; Lezin et al., 2012), the Kem-Kem sub-basin, given its stratigraphic position just under the embayment (Cavin et al., 2010), up to the Ghardaïa area marlstone bearing the pyritized lower Turonian ammon- (Nouh-Mefnoune, 2006) in the large ites. As a consequence, the transgressive yellowish clay- CenomanianeTuronian lagoon of the Algerian Sahara. stone sequence of Smara (Fig. 2) could be the lateral Although the observations of Choubert et al. (1966) suggest equivalent of the deepening Cenomanian sequence of the a pinching out of the most transgressive Turonian deposits Tarfaya sub-basin, but under a more proximal facies. In by the Tarf Zaïda and Merkala localities on the northern summary, the stepped transgression found in the border of the Tindouf syncline (Fig. 1A), a full marine Laayoune-Smara^ sub-basin would have about the same connection cannot be excluded between the Atlantic and timing than in the Tarfaya sub-basin, i.e. two moderate the Tethys along the saddle between the Anti-Atlas and the Albian transgressive pulses separated by a (? mid-Albian) Reguibat Shield during the early Turonian peak trans- regressive trend, then two major transgressive pulses, one gression, therefore connecting the Smara and the Kem- at the base of the Cenomanian (third flooding, Fig. 2), and Kem embayment. At that time, the Anti-Atlas massif could the second at the base of the Turonian (fourth flooding, have been an island (the supposed palaeoshoreline is Fig. 2). indicated in Fig. 1A). What occurred just before this peak transgression, that 4.2. Exact timing of the B1 laminated limestone vs the OAE2 is during the latest Cenomanian CTBE/OAE2 event, is more complicated, and despite what has been suggested above in The above suggestion that the B1 laminated limestone the Smara area about a large Cenomanian transgression could be at first sight the lateral equivalent of the CeT black followed by a stronger Turonian one. The correlation shale of the Tarfaya sub-basin is to be discussed more scheme (Fig. 5) is based on the tracking of short-term thoroughly. It has been shown that the CeT black shale depositional sequences with the help of the CTBE d13C present in deep-water deposits of the Central Atlantic isotope signal (work in progress) within the CeT boundary covers mostly the uppermost Cenomanian (Forster et al., deposits along the southern border of the Atlas belt. As well 2008; and other references therein), as well in onshore as in the Saharan lagoon (Busson et al., 1999; Grosheny et deposits of the Tarfaya sub-basin (Kuhnt et al., 2009). This is al., 2013), the southern shoulder of the Atlasic rift experi- based on the d13C positive shift, age-calibrated in the enced a latest Cenomanian transgression represented reference section of Pueblo in the US Western Interior (Fig. 5) by two or three short-term shallow-water carbon- (Kennedy et al., 2005), and correlated on a larger scale in ate sequences exactly spanning the CTBE d13C shift. This many local reference sections of western Europe and North transgressive pulse is recorded along the Saharan Atlas up Africa, according to a huge literature on the OAE2. Along to Tunisia (Grosheny et al., 2013; Bachari et al., 2019), and 506 M. Abioui et al. / Comptes rendus - Geoscience 351 (2019) 498e507 therefore is opposed to what is found along some segments indicative of forced regression may even be found between of the Atlantic margin. In the Smara area (Fig. 2), an the highstand marlstone of a given sequence and the exposure coupled with truncation of upper Cenomanian transgressive sandstone of the next sequence. Such unusual sequences is recorded instead, as explained above. The patterns are commonly found on very low-sloped ramp same sequence stratigraphic disturbance was also found profiles, where the slightest sea-level drop can be marked along the southwestern shoulder of the Atlasic rift (uplifted by sharp facies changes. In the Cenomanian of the western area, Fig. 5), in the Agadir basin (Jati et al., 2010), although a High-Atlas, Essafraoui et al. (2015) developed a sequence coeval deepening (subsiding area, Fig. 5) is found to the stratigraphic model for such deposits, including the north in the Essaouira basin (work in progress). The areas occurrence of regressive fluvial deposits above a conti- that underwent either a deepening or an uplift from the nental aggradation surface. The latter are not present in the sedimentary record are shown in Fig. 4. Evidences of local Laayoune^ basin, but the observed patterns of Late Creta- uplifts are along the Atlantic margin, in opposition to the ceous sequences indicate a very low-sloped depositional remaining of the African craton to the east, which subsided profile, responsible of the evidences of stepped regression during the CTBE. Other evidences are found along the during the regressive half-cycle of depositional sequences. Iberian and French Atlantic margin (as discussed in Grosheny et al., 2017b). On the Atlantic margin of France for 5. Conclusions instance, Desmares et al. (in press) showed that the CTBE beds are lacking on the eastern termination of the Armor- In the onshore part of the SmaraeLaayoune^ sub-basin, ican Massif, while they are present in the Paris Basin to the the Cretaceous marine transgression began presumably in east, and also on the coastal section of Charentes to the the early Albian. It proceeded by successive steps to onlap west (refs. in Lasseur et al., 2008). This suggests that the the Palaeozoic basement in the Cenomanian, to reach a massif underwent an uplift just before the early Turonian maximum in the early Turonian. At that time, the Anti-Atlas transgression. We think that the same might have occurred massif may have been an island, isolated from the Reguibat on the western part of the Smara area, which would explain Shield by a narrow seaway connecting the Atlantic, the the hiatus found in the SMA-IV section (Fig. 2). Allocyclic Tethys and the Saharan lagoon through the SmaraeKem processes are excluded, given the very good lateral conti- Kem embayments. The CenomanianeTuronian boundary nuity of metre-scale sequences in the area. The hiatus is is marked here by a short-lived emergence, as on the therefore explained by the truncation of the sedimentary western termination of the Atlas belt (Agadir area), succession by the wave ravinement surface of the early contrarily to what is more commonly found elsewhere Turonian transgression on an area that was uplifted during (flooding). The whole sedimentary data, including the the CTBE event. southern shoulder of the Atlas rift under study, point to a Grosheny et al. (2017a) suggested that such sequence brief tectonic uplift (? inversion) of the Atlantic side of the stratigraphic heterogeneity was the result of the short- Anti-Atlas and the Reguibat Shield during the lived tendency to tectonic inversion of the Atlantic CenomanianeTuronian Boundary Event. margin during the CTBE event, as a result of an hypothe- sized tectonic pulse recorded on a larger scale. All available Acknowledgements stratigraphic data suggesting basement deformations during the CTBE along the South Moroccan Atlantic margin are This paper is a contribution to the Cooperative Pro- in Fig. 5 (lower box). gramme between the Universities of Lorraine (France) and Although considered a passive margin, the Moroccan Agadir (Morocco) e Programme Hubert-Curien Volubilis Atlantic margin underwent some major uplifts during its (Grant MA/14/305) -, and part of the ongoing PhD work of Mesozoic to Cenozoic history (Benabdellouahed et al., the first author. 2017; Bertotti and Gouiza, 2012; Gouiza et al., 2010; The careful reading and sound comments of an anony- ^ Lepretre et al., 2017, 2018; Meghraoui et al., 1998; mous reviewer helped to improve the manuscript, Westaway et al., 2009). The short-lived uplift inferred although all the recommendations could not be met due to e here around the C T boundary could be part of this com- the short format of C. R. 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