Diagenetic evolution of lower Jurassic platform carbonates flanking the Tazoult salt wall (Central High Atlas, Morocco)

Platform carbonates diagenesis in salt basins could be complex due to potential alterations of fluids related and non‐related to diapirism. This paper presents the diagenetic history of the Hettangian to Pliensbachian platform carbonates from the Tazoult salt wall area (central High Atlas, Morocco). Low structural relief and outcrop conditions allowed to define the entire diagenetic evolution occurred in the High Atlas diapiric basins since early stages of the diapiric activity up to their tectonic inversion. Precipitation of dolomite and calcite from both warmed marine‐derived and meteoric fluids characterised diagenetic stages during Pliensbachian, when the carbonate platforms were exposed and karstified. Burial diagenesis occurred from Toarcian to Middle Jurassic, due to changes of salt‐induced dynamic related to increase in siliciclastic input, fast diapir rise and rapid burial of Pliensbachian platforms. During this stage, the diapir acted as a physical barrier for fluid circulation between the core and the flanking sediments. In the carbonates and breccias flanking the structures, dolomite and calcite precipitated from basinal brines, whereas carbonate slivers located in the core of the structure, were affected by the circulation of Mn‐rich fluids. The final diagenetic event is characterised by the income of meteoric fluids into the system during uplift caused by Alpine orogeny. These results highlight the relevant influence of diapirism on the diagenetic modifications in salt‐related basins in terms of diagenetic events and involved fluids.

| 547 EAGE MORAGAS et Al. and focused fluid flow along the diapirs flanks, among others. The understanding of the interaction between these elements is essential in the analysis of reservoir quality linked to diagenesis (Beavington-Penney et al., 2008;Ghazban & Al-Aasm, 2010;McManus & Hanor, 1988;. The referenced studies however are mostly handicapped by the lack of continuous outcrops or well data availability. In this regard, the continuous and excellent exposures of the lower Jurassic carbonates flanking the Tazoult salt wall (central High Atlas of Morocco) represent a unique opportunity to analyse the diagenetic evolution and its implications in modifying the carbonate properties through time.
The study of the geometry and evolution of diapiric structures that were active from Early to Middle Jurassic times in the central High Atlas, and their interaction with the flanking rocks, has been addressed in later years (Bouchouata, Canerot, Souhel, & Almeras, 1995;Ettaki, Ibouh, Chellaï, & Milhi, 2007;Martín-Martín et al., 2017;Michard, Ibouh, & Charrière, 2011;Moragas et al., 2018;Saura et al., 2014;Teixell, Barnolas, Rosales, & Arboleya, 2017;Vergés et al., 2017). The presented study of diagenesis in the Tazoult salt wall is part of a prolonged work of our group in the Central High Atlas, detailed below, in which extensive field work and remote sensing mapping was carried out. Integrated analysis of both structural geology (Saura et al., 2014; and sedimentology of the lower-middle Jurassic mixed carbonate-clastic system (Joussiaume, 2016;Malaval, 2016) allowed the discovery of well-preserved and unequivocal halokinetic depositional sequences and to highlight the role of salt tectonics as an additional control on the evolution of the Central High Atlas rift basin during Early and Middle Jurassic times. Concerning the Tazoult salt wall, the study of stacks of halokinetic sequences with both carbonate and mixed carbonate/clastic deposits along the structure indicated its evolution as a 20-km long, NE-SW trending salt wall forming a structural and sedimentary high for at least 20 Myr from Pliensbachian to Bajocian times . This first study was complemented through subsidence and thermal analysis evaluated using vitrinite data (Moragas et al., 2018) and analogue models . Results indicated a close relationship between high subsidence rates and salt-related areas due to two competing mechanisms: tectonic extension and salt withdrawal. Despite the above mentioned literature, there is a lack of studies focusing on the diagenetic evolution of the rocks flanking such diapirs, and thus their analysis needs proper attention. The current erosion of the Tazoult salt wall, as well as most of the central High Atlas diapirs, allows the exposure of structural levels that are buried beneath kilometre-thick Mesozoic successions in other localities of the High Atlas. The excellent exposure of old stratigraphic units and the limited deformation related to the Alpine compression, made the Tazoult salt wall an excellent field analogue to evaluate the complexity of the diagenetic evolution of platform carbonates since early stages of the diapiric activity.
This work aims to: (i) characterise the diagenetic alterations affecting the Early Jurassic platform carbonates flanking the Tazoult salt wall, (ii) constrain the type, origin and pathways of the fluids that drive such alterations, and (iii) construct a model of the Tazoult salt wall that illustrates the diagenetic evolution of the overlying platform carbonates through time, from the onset of diapirism to the Alpine compression. The results of this study will ultimately provide information about how diapiric activity could influence diagenetic overprinting and fluid circulation in stratigraphic levels that are often not accessible. The presented study represents an analogue for similar diapiric structures in other salt basins. Moreover, it contributes to a better understanding of reservoir quality in platform carbonates attached to salt diapirs, which typically are target for geological resources exploration as oil, gas or ore deposits.

Highlights
• Diapirs located in the central High Atlas controlled the diagenetic evolution of the fringing platform carbonates. • Marine-derived and meteoric fluids characterised diagenetic stages in early phases of the Tazoult salt wall activity. • The diapir was a barrier for fluid circulation between the core and the flanking sediments during deep diagenesis. • Slivers within the core of the diapir were affected by Mn-rich fluids derived from Triassic shales and magmatic fluids.

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sandstones intruded by Middle to Late Jurassic magmatic bodies (Frizon de Lamotte et al., 2008;Hailwood & Mitchell, 1971;Jossen & Couvreur, 1990;Laville & Harmand, 1982). These anticlines were recently interpreted as diapirs that started forming and were active during, at least, Early and Middle Jurassic times (Bouchouata, 1994;Bouchouata et al., 1995;Ettaki et al., 2007;Martín-Martín et al., 2017;Michard et al., 2011;Moragas et al., 2018;Saura et al., 2014;Teixell et al., 2017;Vergés et al., 2017). The Tazoult salt wall corresponds to a NE-SW-trending elongated four-way closure salt wall slightly oblique to the more common ENE-WSW structural alignments of the central High Atlas (Figure 1b). The diapir is about 20 km long and 0.6 to 3 km wide. Its core is formed by Upper Triassic red beds and basalts from the Central Atlantic Magmatic Province (CAMP), Middle Jurassic intrusions, and slivers of Hettangian-Lower Sinemurian carbonates (Aït bou Oulli Fm.) ( Figure 2). The Tazoult salt wall is bounded in both flanks by sub-vertical Pliensbachian platform carbonates that grades vertically, and away from the flanks, to Late Pliensbachian-Early Aalenian siliciclastic-carbonate mixed deposits of the Zawyat Ahançal group (Figure 2). Well-preserved halokinetic sequences on the flanks of the Tazoult salt wall show thinning, onlaps, and truncations, documenting the diapiric activity from Early to Middle Jurassic times (Bouchouata, 1994;Bouchouata et al., 1995;Martín-Martín et al., 2017). Shallow marine Late Aalenian-Bajocian platforms carbonates were deposited as a relative uniform unit across the area. These carbonates do not display any evidence of salt tectonic activity during their deposition and thus they recorded that the Tazoult diapir became inactive at that time. Despite the Sinemurian sediments do not crop out in the studied area (Figure 2), they are expected to be located beneath Pliensbachian deposits as is reported in other localities of the central High Atlas (Mehdi, Neuweiler, & Wilmsen, 2003;Poisson, Hadri, Milhi, Julien, & Andrieux, 1998;Wilmsen & Neuweiler, 2008). The evolution of the Tazoult salt wall reported by Martín-Martín et al. (2017) is considered as the structural framework for the diagenetic study presented here. These authors analysed in detail the halokinetic sequences in both flanks of the Tazoult salt wall using remote sensing mapping and field data and presented several cross-sections along the salt structure. Base on their observations, Martín-Martín et al. (2017) define six different stages of diapiric activity and determine the relationship between sedimentation rate and diapir rise rate. According to later authors, the Tazoult salt wall underwent two major stages of diapir activity in the Late Pliensbachian and Toarcian to Aalenian times. Halokinesis was followed by the partial fossilisation of the diapir during the Bajocian, and the squeezing and welding of the structure during the Alpine compression.
The top of the Jbel Choucht platform is karstified and overlaid by the Talmest-n'Tazoult Fm. The basal part of this formation is characterised by red clay matrix breccias with Triassic and Jbel Choucht limestones clasts that passes vertically to conglomerates, sandstones, clays and marls arranged in 0.5-2-mthick beds . The Talmest-n'Tazoult Fm. passes vertically and laterally to the Aganane Fm. that constitutes the upper platform carbonates deposited during Pliensbachian times on both flanks of the Tazoult salt wall. The Aganane formation consists of well-bedded oncolithic limestones, black mudstones and marls with local large bivalve-rich (Lithiotis sp.) levels deposited in inner platform and lagoon settings (Bouchouata et al., 1995;Fraser, Bottjer, & Fischer, 2004;Lee, 1983;Martín-Martín et al., 2017).

| METHODOLOGY
Fifty-five samples were collected from the Hettangian-Lower Sinemurian (Aït bou Oulli Fm.) slivers and Pliensbachian platform carbonates (Jbel Choucht and Aganane formations) and breccias (Talmest-n'Tazoult Fm.) flanking the Tazoult salt wall. These samples, including host rock, veins and karstic fillings, were selected to identify lithology, sedimentary components, diagenetic phases and their crosscutting relationships. Sixty-five thin sections were stained with Alizarine Red-S and potassium ferricyanide to distinguish calcite and dolomite from their ferroan equivalents (Dickson, 1966). The standard thin sections were prepared and analysed using transmitted light and cathodoluminescence microscopy. Cathodoluminescence petrography was carried out on a Technosyn Cold Cathodoluminescence equipment (model 8200 MkII), with operating conditions at 15-18 Kv and 150-350 µA gun current. Stable isotopy and elemental geochemistry of calcite and dolomite phases were performed in order to characterise the type and origin of the fluids involved in the diagenetic alterations of the studied intervals (see data in supplementary material I and II). Seventy-one samples of depositional and diagenetic calcite and dolomite phases were obtained for carbon-and oxygen-isotope analysis using a microdrill to extract 60 ± 10 µg of powder directly from polished slabs. The calcite and dolomite powder were reacted with 103% of phosphoric acid for 10 min at 90°C. The CO 2 was analysed using an automated Kiel Carbonate Device attached to a Thermal Ionization Mass Spectrometer Thermo Electron (Finnigan) MAT-252. The isotopic results have a precision of ± 0.02‰ for δ 13 C and ± 0.04‰ for δ 18 O. The results were corrected using the standard technique (Claypool, Holser, Kaplan, Sakai, & Zak, 1980;Moore & Wade, 2013) and are expressed in ‰ with respect to the VPDB standard. Carbon-coated and double polished thin-sections of selected samples were used to analyse minor and trace element concentrations in a CAMECA SX-50 electron microprobe. The microprobe was operated using 20 kV of excitation potential, a current intensity of 15 nA and a beam diameter of 10 µm. The detection limits are 99 ppm for Mn, 144 ppm for Fe, 386 ppm for Mg, 89 ppm for Sr ppm and 497 ppm for Ca. Precision on major element analyses averaged 6.32% standard error at 3 sigma confidence levels.

OBSERVATIONS
In this section, we describe the distribution of all the diagenetic products in the different studied units, that is, Aït bou Oulli Fm.

Sinemurian platform carbonates)
The Hettangian-Lower Sinemurian carbonate slivers are composed of dolomicrites and dolosparites with no remnants of the original limestones (Figure 3a,b). The dolomicrites (D1) form fine to very fine crystals with dull orange luminescence under cathodoluminescence (CL). D1 is partially replaced, and thus predates, replacive dolomite 1 (RD1) that form coarser crystal mosaics (dolosparites). RD1 are characterised by anhedral to euhedral crystals with red bright luminescence (Figure 3c,d).
The Aït bou Oulli slivers show brecciated fracture corridors, up to 50 cm wide, which include several generations of calcite cements ( Figure 3e). The outer part of the corridors is characterised by a crackle texture that changes to mosaic breccia in the core. The later corresponds to a more developed cataclasites (Figure 3e Oulli Fm. clasts are subangular with size ranging from less than 1 mm to 3 cm. Calcite cement CC4-S clasts consist of translucent subhedral calcite crystals with dull orange luminescence CL pattern, and fluorite clasts (Fl) consist of subhedral fluorite crystals with purple luminescence (Figure 3g,h). Calcite cement CC6 engulfs, and thus postdates, clasts of D1, RD1, CC4 and Fl. CC6 shows bright orange luminescent subhedral to euhedral crystals featuring blocky texture. The edge of the Hettangian-Lower Sinemurian carbonate slivers (i.e, sliver-diapir contact) is commonly brecciated, being the resulting rock classified as a cemented rubble floatbreccia sensu Morrow (1982). These breccias are mainly constituted by Aït bou Oulli Fm. clasts floating in a red clay matrix with a high content of quartz and hematite. Host rock clasts are frequently calcitised (CD1), forming anhedric to euhedric crystals with dull orange luminescence. The clasts of the floatbreccias are cemented, and thus post-dated by non-luminescent calcite cement CC7 with orange bright zonation (CC7, Figure 3c,d). The breccia is partially dissolved to form a highly porous honeycomb-like texture at outcrop ("carneugle").

| Jbel Choucht Fm. (Pliensbachian platform carbonates) and Talmest-n'Tazoult Fm. (Pliensbachian breccias)
The base of the Jbel Choucht Fm. carbonates (C2), from the diapir-platform contact up to ca. 10 m away, is dolomitised, and thus post-dated by replacive dolomite 1 (RD1) (Figure 4a). RD1 replaces all components (nonfabric selective) and is characterised by well-developed euhedral crystals (up to 60 µm) with a bright orange luminescence (Figure 5a,b). The degree of replacement decreases up sequence, and thus the depositional texture made of C2 and an early generation of interparticle nonluminescent calcite cement 1 (CC1) appear only partially dolomitised. Dolomitised and non-dolomitised Jbel Choucht carbonates show mm-scale vuggy pores that are completely occluded, and thus post-dated by calcite cement CC3 (Figure 5a,b). CC3 appear as bladed (CC3a) and mosaic fabric (CC3b) calcite crystals in the outer and inner parts of the vugs, respectively. CC3 shows a zoned non-luminescent to bright orange luminescence under CL. Fractures affecting the RD1 crystal mosaics and all previously described diagenetic alterations are completely filled, and thus post-dated, by subhedric calcite cement 4 (CC4-F) composed of translucent subhedral calcite crystals with dull orange luminescence CL pattern ( Figure 5).
Calcite cement CC4-F shows similar petrographic characteristics (texture and CL pattern) than calcite cement CC4-S observed as a clast component of breccias in the Aït bou Oulli slivers.
The top of the Jbel Choucht platform carbonates is karstified and characterised by cm to dm-scale dissolution vugs filled by karstic sediments (Figure 4b-d). Two types of fillings are differentiated, a karstic sediment 1 (CS1) made up of very fine grain orange sand sediment with bright orange luminescence under CL, and a karstic sediment 2 (CS2) made of fine grain red sediment with dull reddish luminescence ( Figure 5). Vug porosity in karstic sediment is partially occluded, and thus post-dated by calcite cement 3 (CC3, Figure 5a,b). The karstic fillings are replaced, and thus post-dated by replacive dolomite 3 (RD3) that show a characteristic orange bright luminescence (Figure 5g,h). The remaining porosity after CC3 precipitation is partially cemented by well-developed coarse crystalline saddle dolomite (SD1), which show orange dull to bright luminescence CL pattern. Saddle dolomite SD1 is post-dated by (i) the precipitation of calcite cement CC5 in fractures, and (ii) by its calcitisation by calcitised dolomite 1 (CD1) (Figure 5c-f).
The Talmest-n'Tazoult Fm., which overlies the karstified and eroded surface developed at the top of Jbel Choucht carbonates, consists of breccias made of subangular to angular cm-scale clasts of Jbel Choucht carbonates and Triassic siltstones and sandstones with a red fine grain matrix equivalent to the karstic sediments (CS1 and CS2) ( Figure 4d). As occurred in Jbel Choucht Fm., the vug porosity observed in the Talmest-n'Tazoult Fm. is occluded by the precipitation of calcite cement CC3. Fracture porosity affecting CS1, CS2, RD3 and SD1 in both Jbel Choucht and Talmest-n'Tazoult Fm. is filled and thus post-dated by calcite cements 4 and 5 (CC4-F and CC5). CC4 is distinguished by its characteristic bright orange luminescence (Figure 5l,m), whereas CC5 typically form drusy mosaics made of equant spar calcite crystals with zoned luminescence ranging from dull to bright orange (Figure 5j,k).

| Aganane Fm. (Pliensbachian platform carbonates)
The primary interparticle porosity within the Aganane Fm. host carbonates (C3) is filled with calcite cements 2 and 3 (CC2 and CC3). CC2 appears rimming most depositional components and is characterised by a red bright luminescence (Figure 6b,c). CC3 fills the remaining interparticle porosity after CC2, and is characterised by mosaic fabric and zoned non-luminescent to bright orange luminescent calcite crystals. The top of the Aganane platform carbonates is slightly karstified, forming small-scale cavities filled with ochre karstic sediment 3 (CS3) that show a red dull luminescence. The Aganane Fm. limestones (C3) are partially dolomitised, and thus post-dated, by non-luminescent replacive dolomite 2 (RD2). Fractures within the karstified and partially dolomitised Aganane Fm. are filled, and thus postdated, with saddle dolomite 1 (SD1) and calcite cements 4 and 5 (CC4-F and CC5). SD1 crystals are calcitised (CD1) (Figure 6d). In the units flanking the diapiric structure, stage 2 correspond to the kasrtification of the top of the Jbel Choucht platform carbonates, the deposition of the Talmest-n'Tazoult breccias and the deposition of karstic sediments (CS1 and CS2) (Figure 7). Stages 3 and 4 are characterised by the deposition of the carbonates of Aganane Fm., the precipitation of CC2, the local karstification of the uppermost part of this formation accompanied with the deposition of karstic sediment CS3, and the partial dolomitisation of this carbonates by RD2 (Figure 7). Stage 5 is characterised by the dolomitisation of karstic sediment at the top of Jbel Chouch Fm. by RD3, and the precipitation of saddle dolomite SD1 and calcite cement CC4-F in fractures affecting all the units flanking the Tazoult salt wall. Stage 6 represents the precipitation of CC5 followed the precipitation of CC4-F (Figure 7).

| Relative timing of the diagenetic features
In the Hettangian slivers, the chronology of events is less well constrained, as calcite cement CC4-S and fluorite could form at any time between the dolomitisation that results in replacive dolomite RD1 (Stage 1) and the precipitation of the calcite cement CC6 (Stage 6) ( Figure 7). All studied units, both in the diapir core and flanking the structure, include calcitised dolomite (CD1) and calcite cement CC7 formed during the last stage of diagenesis (Stage 7, Figure 7).

| GEOCHEMISTRY
The elemental and isotopic composition analyses of the Aït bou Oulli Fm. (D1) were not possible to obtained due to the overprinting by the replacive dolomite RD1. Contrarily, the Pliensbachian carbonate of Jbel Choucht and Aganane formations show similar elemental composition but different stable isotope signal. The Jbel Choucht Fm. carbonates (C2) are characterised by high Mg content ranging from 4,330 to 5,990 ppm, low Mn content from below detection limit (d.l.) to 120 ppm, low Fe content from 216 to 830 ppm, and low Sr content from 221 to 440 ppm (see complete dataset in Appendices I and II). The stable isotope composition of the Jbel Choucht Fm. yielded δ 13 C values ranging from +0.19 to +1.57‰ VPDB and δ 18 O values ranging from −3.62 to −2.76‰ VPDB (Figure 8; see complete data set in Appendices I and II). The Aganane Fm. carbonates (C3) are characterised by Mg content ranging from 4,500 to 5,275 ppm, Mn content always below the d.l., Fe content from below the d.l. to 1,750 ppm, and Sr content from 150 to 495 ppm. The stable isotope analysis yielded δ 13 C values ranging from −1.71 to +1.93‰ VPDB and δ 18 O ranging from −8.20 to −3.47‰ VPDB .
Calcite cement CC1, filling inter-particle porosity in the Jbel Choucht Fm., is characterised by high Mg content ranging from 3,600 to 5,465 ppm, Mn content from below d.l. to 242 ppm, Fe content from below d.l. to 708 ppm, and Sr content from below d.l. to 435 ppm ( Figure 9). Calcite cement 2 (CC2) from the Aganane Fm. shows δ 13 C values ranging from +2.22 to +2.33‰ VPDB and δ 18 O values ranging from −6.90 to −6.26‰ VPDB (Figure 8a). CC1 isotope analysis and CC2 elemental analysis are not available due to sampling difficulties associated with the small crystal size. CC3 calcite cement is characterised by Mg content ranging between 1,740 and 8,555 ppm, Mn content from below d.l. to 2,200, Fe content from below d.l. to 2,170, and Sr content below d.l. to 523 ppm. CC3 yielded δ 13 C values ranging from +0.73 to +1.75‰ VPDB and δ 18 O ranging from −8.61 to −7.54‰ VPDB .
Calcite cement CC4 analysed in the Aït Bou Oulli Fm. slivers (CC4-S) is characterised by Mg content ranging from below d.l to 1,500 ppm, Mn content from 426 to 1636 ppm, Fe content from below the d.l. to 880 ppm, and Sr content from below d.l. to 294 ppm. CC4-S isotope analysis yielded a δ 13 C value of +1.10‰ VPDB and a δ 18 O value of −5.57‰ VPDB .
Calcite cement CC4-F analysed in the units flanking the diapiric structure differs from CC4-S in elemental composition. Mg content ranges from 542 to 10,394 ppm, Mn content from below the d.l. to 393 ppm (much lower than CC4-S), Fe content from below d.l. to 3,707 ppm (higher than CC4-S), and Sr content from below d.l. to 2,838 ppm (higher than CC4-S, Figure  9). In contrast, the isotopic composition of CC4-F is similar to CC4-S, with values ranging from δ 13 C values ranging from −0.33 to +1.91‰ VPDB and δ 18 O values ranging from −6.50 to −4.12‰ VPDB (Figure 8b).
Calcite cement 5 (CC5) is very similar to CC4-F in elemental geochemistry, with Mg content ranging from 611 to 7,567 ppm, Mn content from below the d.l. to 1,160 ppm, Fe content from below d.l. to 4,524 ppm, and Sr content from 154 to 1,945 ppm. In contrast, CC5 clearly differs from CC4-F in the isotopy composition, with δ 13 C values ranging from −3.38 to +0.51‰ VPDB and δ 18 O values ranging from −16.80 to −8.87‰ VPDB (Figure 8b).
CC6 shows Mg content ranging from below d.l. to 2,900 ppm, Mn content from below d.l. to 2,644 ppm, Fe content from below d.l. to 3,806 ppm, and Sr content from below d.l. and 328ppm. CC6 isotopy yielded δ 13 C values ranging from −4.30 and −3.08‰ VPDB , and δ 18 O ranging from −6.75 to −5.78‰ VPDB . CC7 is characterised by Mg content ranging from 2000 to 3,500 ppm, Mn content from below the d.l. up to 17,832 ppm, Fe content from below the d.l. to 356 ppm, and Sr content from below the d.l. to F I G U R E 8 (a) δ 18 O-δ 13 C plot of host rocks, karstic sediments and early cements, and (b) δ 18 O-δ 13 C plot of late cements. Light grey polygon in a shows the δ 18 Oδ 13 C range of Hettangian to Pliensbachian marine carbonates according to Veizer et al. (1999) andDella Porta et al. (2015). The plots show the isotopic composition of depositional and diagenetic phases arranged according to the defined diagenetic stages (shaded envelopes). Arrows indicate changes of diagenetic realms 558 | EAGE MORAGAS et Al. 348 ppm. CC7 isotopic composition shows δ 13 C ranging from −8.46 to −6.22‰ VPDB and δ 18 O ranging from −8.41 to −6.33‰ VPDB (Figure 8b).
Replacive dolomite 1 (RD1) is characterised by high Mg content ranging from 11.79% to 13.17%, low Mn content from 488 to 2,155 ppm, and low Fe content from below d.l. to 9,650 ppm. RD1 isotope analysis shows δ 13 C values ranging from +1.71 to +2.70‰ VPDB , and δ 18 O values ranging from −4.74 to −1.76‰ VPDB (Figure 8a and data set in Appendices I and II). The Pliensbachian Aganane Fm. replacive dolomite 1 (RD2) is characterised by high Fe content varying from 6.5% to 10.8%, low Mn content from 760 to 1,475 ppm, low Mg content from 5.6% to 7.4%, and Sr content below 375 ppm (Figure 9). RD2 isotope analysis yielded a δ 13 C value of −0.69‰ VPDB and a δ 18 O value of −3.12‰ VPDB .
Calcitised dolomite 1 (CD1) is characterised by highly variable Mg, Fe and Sr contents, ranging from 700 to 10,400 ppm, from below the d.l. to 4,918 and from below the d.l. to 3,089 ppm, respectively, and by a very low Mn content from below d.l. to 310 ppm. CD1 yielded δ 13 C values ranging from −7.24 to −3.25‰ VPDB and δ 18 O values that range from −6.43 to −6.00‰ VPDB . 6 | DISCUSSION

Pliensbachian)
The first stage is characterised by the deposition of the Pliensbachian platform carbonates of the Jbel Choucht Fm. (C2), and thus is dominated by marine pore fluids (Figures  8-10). Such fluids facilitated the precipitation of the calcite cement CC1 that occluded the inter-particle porosity of the host limestones. After the precipitation of CC1, the Hettangian-Lower Sinemurian Aït bou Oulli Fm. slivers and the lowermost part of the Jbel Choucht Fm. were partially replaced by RD1 (Figures 3 and 4). The distribution of RD1 in Jbel Choucht Fm., which decreases upwards and away from the diapir, suggests that dolomitising fluid most likely migrated along the diapir margin. Based on field and analytical data, F I G U R E 9 Elemental composition and stable isotope of the host rocks and diagenetic phases described in the Hettangian to Pliensbachian platform carbonates. Isotopic values for Hettangian-Pliensbachian marine carbonates are according to Veizer et al. (1999)  two main hypotheses are envisaged for the origin of RD1. The dolomitising fluids could be related to the downward percolation of seawater derived fluids along fractures (e.g. Fischer et al., 2013). In this regard, fracturing and faulting of the sedimentary overburden above salt diapirs are typically associated with reactive and active stages of diapirism (Davison, Alsop, Evans, & Safaricz, 2000;Jackson, Vendeville, & Shultz-Ela, 1994;Vendeville & Jackson, 1992). Taking into account that during early Pliensbachian times the Tazoult salt wall was in a stage of reactive-active growth linked to the Early Jurassic rifting of the central High Atlas Moragas et al., 2018;Saura et al., 2014;Vergés et al., 2017), it is suggested that the fracturing of the Jbel Choucht Fm. at that time likely provided conduits for the downward circulation of marine fluids until the crest of the diapir (Figure 10). The similar δ 13 C (from +1.71 to +2.7‰ VPDB ) and slightly depleted δ 18 O values (from −4.74 to −1.76‰ VPDB ) of RD1 compared to Early Jurassic seawater points to warm marine water as dolomitising fluid (Figure 8). The increase in temperature of seawater at shallow depths is attributed to the occurrence of high geothermal gradient associated with the rifting stage (Moragas et al., 2018).
Alternatively, the origin of RD1 could be associated with the upwards flow of dolomitising fluids using the diapir margin as a major pathway (e.g. Enos & Kyle, 2002;Masoumi, Reuning, Back, Sandrin, & Kukla, 2014). In this scenario, the upwards migration of fluids would be driven by burial compaction of the Sinemurian and Pliensbachian marly deposits that underlie the Jbel Choucht Fm. out of the diapir and basinwards, subsequently the fluids focused along the diapir margin. An upwards circulation of Sinemurian or Pliensbachian marine-derived fluids warmed at depth is supported by the slightly depleted δ 18 O and positive δ 13 C values of RD1. Furthermore, the limited volume of dolomitised rock is in agreement with a compaction driven mechanism as dolomitising fluids expelled by burial compaction commonly result in limited amount of replacive dolomite (Machel, 2004;Warren, 2000).

| Stage 2. Meteoric diagenesis (Pliensbachian)
After the deposition of the Jbel Choucht Fm., the Tazoult salt wall underwent an increased diapiric growth with respect to early Pliensbachian times . According to Martín-Martín et al. (2017), the growth of the salt wall promoted the uplift and subaerial exposure of the Jbel Choucht carbonate platform, resulting in the invasion of the platform top by meteoric waters and the karstification of the host limestones ( Figure 10). Accordingly, the depleted δ 13 C values (from −6.98 to −2.79‰ VPDB ) yielded by the karstic sediments CS1 and CS2 are consistent with a meteoric alteration (Figures 7-10). Following the meteoric alteration, the continuous growth of the Tazoult salt wall caused the erosion of the karst and the deposition of downflank syn-diapiric sedimentary breccias, conglomerates and sandstones of the Talmest-n'Tazoult Fm. The presence of Triassic clasts in the breccia indicates that the diapir core rocks were cropping out at that time (Figure 4d). These clastic deposits pass laterally and towards the top to marly facies, and finally to the Aganane platform carbonates, representing a transition to marine-dominated environment that characterised stage 3.
The occurrence of topographic highs above the regional sea level associated with diapiric structures and the rapid change in facies distribution around them and basinwards have been previously reported in the Tazoult salt wall (Joussiaume, 2016;Malaval, 2016;Martín-Martín et al., 2017;, as well as in other diapiric structures of the central High Atlas (Teixell et al., 2017) and in diapiric basins elsewhere (Counts, Dalgarno, Amos, & Hasiotis, 2019;Giles et al., 2008;Poprawski, Basile, Jaillard, Gaudin, & Lopez, 2016). However, the karstification of platform carbonates forming the crest of these diapirs and their subsequent erosion and sedimentation as clastic deposists around them has rarely been reported in the literature, and thus constitutes a key diference of the Tazoult salt wall with other case studies.

| Stages 3 and 4. Marine to meteoric diagenesis (late Pliensbachian)
The stages 3 and 4 occurred in a transitional environment with interaction between marine and meteoric fluids as recorded by: (i) the deposition of the Pliensbachian Aganane Fm. limestones (C3) in a lagoon environment (Bouchouata et al., 1995;Fraser et al., 2004;Lee, 1983), and the presence of small-scale karstic cavities (pockets and fissures) at top of the Aganane carbonate platform that are filled with sediments showing a meteoric isotopic signature (CS3 in Figures  8 and 9). The Aganane limestones (C3) show lower δ 18 O values than the values expected for the Pliensbachian marine carbonates (Della Porta, Webb, & McDonald, 2015;Veizer et al., 1999) indicating that they were later overprinted by modified-marine fluids during progressive burial of the rock (Moore, 2001). In this setting, the Aganane Fm. limestones were locally replaced by RD2. Despite the limited analytical data of RD2, the slightly depleted δ 13 C and δ 18 O values (−0.69 and −3.12‰ VPDB , respectively) together with the high iron content and the dull (non-luminescent) colour under cathodoluminescence suggests a dolomitisation from marine-derived fluids in chemically reducing conditions (Boggs, 2003;Machel & Burton, 1991). Such reducing conditions are typically related to burial environments (Banner & Hanson, 1990;Machel, 2004), implying that the replacement by RD2 likely occurred at the end of the 4 or even during the early phases of stage 5 (shallow burial).

| Stage 5. Shallow burial diagenesis (Toarcian)
The increase in sedimentation rate and siliciclastic input during Toarcian times caused a rapid burial of the Pliensbachian platform carbonates together with the increase in the rate of the Tazoult salt wall rise associated with salt withdrawal from the adjacent minibasin . This change of salt-induced dynamics marks the onset of burial diagenesis (Figure 10), which is interpreted to result in the precipitation of calcite cement CC3 in the inter-particle and vug porosity of the Pliensbachian platform carbonates flanking the Tazoult salt wall (Jbel Choucht, Talmest-Tazoult and Aganane formations). Crosscutting relationships indicate that CC3 cementation was followed by the replacement of the karstic sediments CS1 and CS3 to form RD3, and the precipitation of SD1 in fractures ( Figure 10).
The similar carbon and oxygen isotopic signature of RD3 and SD1 suggests formation from a fluid with similar geochemical characteristics (Figure 8). On the one hand, the negative δ 13 C values (from −3.56 to −0.43‰ VPDB ) likely reflect an input of light carbon from thermally decarboxylised organic matter (Moore & Wade, 2013;Spötl & Pitman, 1998). On the other hand, the relatively depleted δ 18 O values (from −7.45 to −5.13‰ VPDB ) suggests formation from relatively high temperature fluids (Allan & Wiggins, 1993;Spötl & Pitman, 1998). In this regard, saddle dolomite is considered to precipitate from hot basinal, frequently hydrothermal, fluids (Davies & Smith, 2006;Mansurbeg et al., 2016;Morad, Nader, Morad, Al Darmaki, & Hellevang, 2018), with temperatures above 60°C (Spötl & Pitman, 1998). Therefore, the most probable scenario is that RD3 and SD1 formed from light carbon and high temperature Mg-rich fluids expelled from the Zawyat Ahançal Group sediments or the Pliensbachian basinal marls, which are in lateral contact with the Pliensbachian platform carbonates, using faults, fractures and the margin of the Tazoult salt wall as major conduits ( Figure 10).
According to crosscut relationships, the formation of RD3 and SD1 dolomites was followed by the precipitation of the calcite cement CC4-F. The calcite cement CC4-S, with similar isotopic signature and petrological characteristics, precipitate in the Aït bou Oulli Fm. slivers. According to the isotopic signature, we interpret both calcite cement CC4-F and CC4-S to precipitate in a similar diagenetic environment (shallow burial), but from two different fluids as CC4-S shows higher Mn content (426-1636 ppm) than the equivalent cement CC4-F (below d.l. to 393 ppm) (Figure 9). The lack of RD3 and SD1 in the Ait bou Oulli Fm. slivers compared to the flanks of the Tazoult salt wall, and the differences in origin between calcite cements CC4-F and -S, suggest that the exchange of fluids between the flanks and the core of the diapir was very limited during this stage ( Figure 10).

Toarcian)
The continuation of sediment supply and the progressive increase in burial depth during Middle Jurassic characterised the diagenetic evolution of the Pliensbachian carbonate platforms after the Toarcian . During this stage calcite cements precipitated in the remaining porosity as: (i) CC5 in Pliensbachian carbonates and breccias, and (ii) CC6 in the Hettangian-Lower Sinemurian Aït bou Oulli slivers (Figure 10). This differentiated cementation suggests that the Tazoult salt wall acted as a physical barrier for the migration of fluids between the core and the flanking sediments as previously reported from other diapir structures like those of La Popa Basin (e.g. Smith et al., 2012).
The highly depleted δ 18 O values of calcite cement CC5 (from −16.80 to −8.87‰ VPDB ) suggest that the flanking carbonates were most probably affected by hot basinal brines. Crosscutting relationships indicate that CC5 pre-dates the stage of exhumation and uplift (Stage 7), and thus is the last cement that precipitated during deep burial diagenesis. According to burial and thermal models by Moragas et al., (2018), the maximum burial of the Pliensbachian sediments in the minibasins flanking the Tazoult salt wall occurred from Middle Jurassic to Early Cretaceous times, reaching temperatures between 150 and 250°C. The units flanking the diapir, however, are not expected to reach these temperatures as (i) they were affected by less burial than the equivalent units located in the minibasin centre and (ii) the high thermal conductivity of salt causes negative thermal anomalies in the vicinity of diapiric structures (Li, Reuning, Marquart, Wang, & Zhao, 2017;Magri et al., 2008;Petersen & Lerche, 1995;1996). Thus, the formation of CC5 would be probably associated with abnormal and relatively high temperature fluids, likely hydrothermal, documented in the area during Middle to Late Jurassic. This high temperature would be related to: (i) the emplacement of gabbros in the core of the Tazoult salt wall (see Martín-Martín et al., 2017) and/or (ii) the emplacement of vein-like Mississippi Valley-type ore deposit hosted in the Pliensbachian Jbel Choucht carbonates of the Tazoult south flank (Pb-Zn ore deposits from the Aguerd n'Tazoult mine according to Mouttaqi et al., 2011). Field observations indicate that the ore deposit post-date the magmatic intrusions, and thus the MVT deposit of Tazoult most probably formed after Middle Jurassic times. Zn-Pb ore deposits hosted in Middle Jurassic carbonates flanking salt structures equally intruded by gabbros have been recently reported in other localities of the central High Atlas. In the Ikkou Ou Ali salt wall (central High Atlas of Imilchil), Zn-Pb ore deposits have been determined to form between Late Middle Jurassic and Early Cretaceous under an extensional regime (Mouttaqi et al., 2011;Rddad et al., 2018). According to these authors, the origin of the Ikkou Ou Ali ore deposit is related to the mixing of a basement-derived hot and metal-bearing fluids that migrated upwards through faults with a sulphur-rich fluid derived from the dissolution of Triassic evaporites. Maximum temperatures reported from Ikkou Ou Ali Zn-Pb deposit and related calcite cements are up to 206°C (Rddad et al., 2018).
In contrast to the diapir flanks, the Aït bou Oulli Fm. slivers were cemented by the calcite cement CC6. Similarly to CC4-S, calcite cement CC6 shows a high Mn content (up to 2,644 ppm), which is a key difference with other diagenetic phases described here (Figure 9). The origin of a Mn-rich 562 | EAGE MORAGAS et Al. fluid exclusively affecting the carbonate slivers embedded in the Tazoult core rocks could be associated with: (i) Triassic clayely sediments of the salt wall (Chukhrov, Gorshkov, Rudnitskaya, Beresovskaya, & Sivtsov, 1980), and (ii) hydrothermal fluids associated with the intrusion of gabbro in the core of the salt wall, as magma partitioning can result in fluids rich in base metals such as Mn (Schindler, Hagemann, Banks, Mernagh, & Harris, 2016;Sharma & Srivastava, 2016) (Figure 10). Similarly, the fluorite cement that exclusively appears in the Aït bou Oulli Fm. slivers would be associated with intra-core fluids. The fluorite likely precipitated from high saline fluids related to the leaching of evaporites from the core of the Tazoult salt wall (Pique, Canals, Grandia, & Banks, 2008;Sánchez et al., 2009). Calcite cement CC6 appears cementing the breccias in the Aït bou Oulli Fm. slivers, which is composed of a variety of clasts including host rock (RD1 and D1) and several generations of cements precipitated at different times during the diapiric evolution (CC4-S and Fluorite). This highlights that fracturing and brecciation are recurrent processes not only in the sediments flanking the diapiric structure but also in the slivers embedded in the core. This observation is in agreement with numerical simulations results from Li et al. (2012), demonstrating that fragmentation and brecciation of intra-salt carbonate blocks in Oman was continuous all along the evolution of the diapiric structures.

| Stage 7. Meteoric diagenesis (Cenozoic)
The uplift of the Tazoult salt wall during the Alpine inversion promoted the exposure and erosion of the diapiric core rocks and the flanking sediments, facilitating the circulation of meteoric waters (Figures 8-10). Accordingly, this stage is characterised by the late karstification of the Hettangian-lower Sinemurian Aït bou Oulli slivers and the Pliensbachian Jbel Choucht and Aganane carbonate platforms. The interaction between the latter units and meteoric fluids likely caused the calcitisation of dolomites resulting in the calcitised dolomite CD1 and the precipitation of the calcite cement CC7. The succession of nonluminescent to orange bright rimmed luminescent cement (CC7) could be interpreted as meteoric or shallow burial in origin (Carpenter & Lonhmann, 1989;Meyers, 1974;Moldovany & Lonhmann, 1984). However, the meteoric origin of this cement is further supported by the stable carbon and oxygen isotope results, ranging from −8.46 to −6.22‰ VPDB for δ 13 C and from −8.41 to −6.33‰ VPDB for δ 18 O (Lonhmann, 1988;Moore, 2001).

| CONCLUSIONS
This study presents the diagenetic evolution of Hettangian to Pliensbachian platform carbonates that fringe the Tazoult salt wall located in the central High Atlas diapiricbasin (Morocco). Specifically, the study investigates the diagenetic alterations that affect (i) Hettangian-Lower Sinemurian carbonates distributed as slivers within the core of the diapir, and (ii) Pliensbachian Jbel Choucht and Aganane formations that appear flanking the salt wall. Moreover, the latter platforms are separated by an important karstic surface and the Talmest-n'Tazoult Fm., representing a major characteristic of the studied area compared to other case studies worldwide.
Using field and analytical data, we were able todraw the entire diagenetic evolution occurred in a diapir since the early stages of the diapiric activity up to their inversion. We recognise the following seven diagenetic stages that are linked to the halokinetic evolution of the Tazoult salt wall: Stage 1 (early Pliensbachian) to stage 4 (late Pliensbachian) occurred during the early growth of the Tazoult salt wall and are dominated by an alternation of marine and meteoric diagenetic environments. The former diagenetic processes are characterised by the circulation of dolomitising fluids along the diapir margin that resulted in the interaction with the bottom part of the host carbonates. Likewise, the continuous growth of the diapir caused the exposure and karstification of the carbonate platforms through interaction with meteoric fluids.
Stage 5 (Toarcian) and stage 6 (post-Toarcian) represent the burial of the studied carbonates and breccias and are characterised by the Tazoult salt wall acting as a barrier to fluid exchangethe flanking units and the core of the structure. The Pliensbachian platform carbonates and breccias located in the flanks of the diapir were affected by the circulation of hot basinal brines, which result in the precipitation of dolomite and calcite cements in fracture-related porosity. Contrarily, the slivers of Hettangian carbonates embedded within the core of the structure interacted with Mn-rich fluids derived from the clayey Triassic rocks and/ or hydrothermal fluids associated with the intrusion of gabbros.
Stage 7 (Cenozoic) corresponds to the uplift and exhumation event related to the Alpine compression. During this stage, the Hettangian to Pliensbachian carbonates and breccias were exposed and interacted with meteoric waters, which result in the calcitisation of dolomites and the precipitation of calcite cement.
The study of the Tazoult salt wall highlights how the diapiric activity influcences the diagenetic evolution of the fringing platform carbonates by: (i) creating fluid pathways (fractures) due to the forces caused by the growth of the salt structures; (ii) local relative water depth variation due to vertical salt movement that causes alternance of marine and meteoric diagenetic processes, and the exposure and karstification of the carbonates, (iii) diapirs and welds which act as as preferencial vertical conduits but as barriers for horizontal | 563 EAGE MORAGAS et Al. migration of fluids and (iv) salt and other evaporites dissolution that influences the chemistry of the fluids.