Bioclastic bottom‐current deposits of a Devonian contourite terrace: Facies variability and depositional architecture (Tafilalt Platform, Morocco)

The study examines bioclastic carbonate contourites that arise from the broad spectrum of bottom‐current related sedimentary processes ranging from deposition to erosion. The result of the intermittent accumulation of sediment are thin and condensed successions with abundant hiatuses. Such bottom‐current deposits are poorly known, since the broadly accepted contourite‐facies model, the bi‐gradational sequence, characterizes environments of contourite depositional systems as a continuous accretion of fine‐grained siliciclastic sediments. To increase current understanding of the carbonate facies within hiatal contourite records, the Eifelian–Frasnian of the Tafilalt Platform in Morocco was investigated. The succession is divided into five facies associations that are interpreted to reflect pelagic sedimentation and deposition from bottom currents on a contourite terrace, a gently inclined section of the upper slope of Gondwana shaped by a water‐mass interface. Contourite deposition was mainly controlled by oxic clear‐water currents (documented by moderately to completely bioturbated limestones with abundant hydrogenetic ferromanganese nodules, and low organic‐carbon contents), at times also by an anoxic water mass (featured by organic‐rich coquinas with absent to sparse bioturbation and predominantly syngenetic framboidal pyrites). Biostratigraphic data and the overall depositional architecture display palaeoceanographic hydrodynamic processes associated with a shifting water‐mass interface. The inner terrace was characterized by an alongslope contourite channel and a small mounded drift at its downslope margin. Energetic bottom currents furthermore caused abraded surfaces, i.e. plain areas of non‐deposition and localized erosion, and sandy condensation layers. The microfacies reflects repeated alternation between suspension deposition, winnowing of fines, bedload traction, dynamic sediment bypassing and reworking, together with concomitant seafloor cementation. Coquinas of mainly planktonic and nektonic organisms are identified as integral parts of bi‐gradational contourite sequences showing inverse and normal grading. Hiatal lag concentrations of carbonate intraclasts, ferromanganese nodules and conodonts often drape hardgrounds and erosional surfaces at the midpoint of these frequently incomplete sequences. This Devonian case provides the opportunity to investigate the spatial and temporal variability of the bed‐scale contourite sequence, also with regard to the drift‐scale depositional architecture. In addition, the identified high‐resolution record is a starting point for unravelling the pattern of oceanic circulation in the Devonian greenhouse world.

The study examines bioclastic carbonate contourites that arise from the broad spectrum of bottom-current related sedimentary processes ranging from deposition to erosion. The result of the intermittent accumulation of sediment are thin and condensed successions with abundant hiatuses. Such bottom-current deposits are poorly known, since the broadly accepted contourite-facies model, the bi-gradational sequence, characterizes environments of contourite depositional systems as a continuous accretion of finegrained siliciclastic sediments. To increase current understanding of the carbonate facies within hiatal contourite records, the Eifelian-Frasnian of the Tafilalt Platform in Morocco was investigated. The succession is divided into five facies associations that are interpreted to reflect pelagic sedimentation and deposition from bottom currents on a contourite terrace, a gently inclined section of the upper slope of Gondwana shaped by a water-mass interface. Contourite deposition was mainly controlled by oxic clear-water currents (documented by moderately to completely bioturbated limestones with abundant hydrogenetic ferromanganese nodules, and low organic-carbon contents), at times also by an anoxic water mass (featured by organic-rich coquinas with absent to sparse bioturbation and predominantly syngenetic framboidal pyrites). Biostratigraphic data and the overall depositional architecture display palaeoceanographic hydrodynamic processes associated with a shifting water-mass interface. The inner terrace was characterized by an alongslope contourite channel and a small mounded drift at its downslope margin. Energetic bottom currents furthermore caused abraded surfaces, i.e. plain areas of non-deposition and localized erosion, and sandy condensation layers. The microfacies reflects repeated alternation between suspension deposition, winnowing of fines, bedload traction, dynamic sediment bypassing and reworking, together with concomitant seafloor cementation. Coquinas of mainly planktonic and nektonic organisms are identified as integral parts of bi-gradational contourite sequences showing inverse and normal grading. Hiatal lag concentrations of carbonate intraclasts, ferromanganese nodules and conodonts often drape hardgrounds and erosional surfaces at the midpoint of these frequently incomplete sequences. This Devonian case provides the
The aim of this study is to explore fossil bioclastic contourites of calcareous composition that were formed under conditions of very low net accumulation rates. To document such an environment, the Devonian Tafilalt Platform in the eastern Anti-Atlas of Morocco was analysed. The analysis of this platform aims to refine facies models of carbonate contourites and to establish a database for future work on palaeoceanographic changes in the Devonian.
This paper investigates the Eifelian-Frasnian succession of the Tafilalt Platform, based on extensive fieldwork in south-eastern Morocco and by means of a comprehensive microfacies approach. The objectives of this study are to: (i) obtain high-resolution records of bed to microfacies-scale erosional and depositional features from different parts of the platform; (ii) evaluate the lateral facies variability; (iii) determine the drift-scale features based on architecture, general stacking patterns and dimensions of the stratigraphic units, together with the distribution and range of biostratigraphic gaps; and (iv) unravel the sedimentary processes and hydrodynamic conditions that produced the contourite system. Ideal preconditions are the extensive stratigraphic database, large-scale accessible outcrops and an overall weak tectonic deformation. The late Frasnian to early Famennian Kellwasser limestones are not considered in this study, since this facies was formed under the exceptional conditions of a major mass-extinction event and rapid sea-level changes (Stigall, 2012;McGhee et al., 2013;Carmichael et al., 2019;Percival et al., 2020), which are not relevant to the longterm depositional history on the Tafilalt Platform and its overarching controls.
The results of this contourite study will have important consequences for future research. First, instead of pelagic processes, bottomcurrent-induced processes will be put forward as a main cause of condensed sediment accumulation on deep-marine platforms. Second, the interpretation of thickness variations on the Gondwanan continental margins can be broadened from exclusively considering tectonicallyinduced subsidence rates and associated gravity processes to also include the morphological perspective of a contourite depositional system and the distribution of depocentres. Third, investigating the link between oceanic anoxic events and intensified bottom-current controlled sedimentation is essential for prospective palaeoceanographic reconstructions and the analysis of Devonian evolutionary events (e.g. Bond et al., 2004;Racki, 2005;Becker et al., 2020;Percival et al., 2020).

STUDY AREA AND GEOLOGICAL SETTING
The Devonian cephalopod limestones are part of the 3 to 4 km thick, early-mid Palaeozoic succession deposited on continental terraces of the passive north-western margin of Gondwana (Soulaimani et al., 2003;Raddi et al., 2007;Baidder et al., 2008Baidder et al., , 2016Michard et al., 2008;Soulaimani & Burkhard, 2008). The present work focuses on the Tafilalt area, where the east-west trending Anti-Atlas belt intersects with the northwest/south-east-trending Ougarta belt, both formed during the Variscan (Alleghanian-Hercynian) collision of Laurussia and Gondwana during the Pennsylvanian to Cisuralian (Fig. 1).
Large-scale open anticlines and synclines, formed by thick-skinned inversion tectonics with superimposed folding events (Burkhard et al., 2006;Baidder et al., 2016), provide excellent outcrops to study regional thickness variations and the architecture of the sedimentary units.
A much thicker record (100-400 m) of hemipelagic and density-flow deposits characterizes the slope aprons into the Maider and Tafilalt basins on both sides of the platform (Lubeseder et al., 2010), consisting of: (i) well-bedded laminated marls and calcareous mudstones, rich in lithic peloids and locally quartz, forming rhythmic limestone-marl couplets; (ii) normally-graded limestone beds showing lamination and crosslamination together with complete and partial Bouma-sequences, interpreted as turbidites; (iii) mud-rich conglomerates, representing debrites; (iv) slump deposits; and (v) thin beds of brachiopod and styliolinid coquinas, i.e. textures of wellsorted packstone and grainstone locally showing low-angle cross-bedding and cross-lamination.
Mud mounds are a characteristic morphosedimentary feature of the eastern Tafilalt Platform (Fig. 2). Although most of the mud mounds at Hamar Laghdad developed during the Emsian, some also accreted during the Eifelian-Givetian, especially the famous Hollard Mound at the eastern end of the ridge (e.g. T€ oneb€ ohn, 1991; Brachert et al., 1992;Belka, 1998;Aitken et al., 2002;Hartenfels et al., 2018;. Tabulate corals (auloporids and thamnoporids) and crinoids are the most common organisms that colonized the mounds, while calcareous algae and stromatoporoids are absent. The palaeoenvironment has been interpreted as a deep-water, below fair-weather wave base but probably within the range of major storms and under control of bottom currents (Brachert et al., 1992;.

Stratigraphic framework
The Eifelian-Frasnian sections investigated here are part of the Bou Tchrafine (d4) to Achguig (d6) formations (Hollard, 1963(Hollard, , 1981Hartenfels et al., 2018). A number of marker beds have been recognized in the succession of cephalopod limestones, which are laterally discontinuous (pinching out locally) but can be traced across the whole Tafilalt Platform and locally into the basins (Fig. 2). These marker beds are characterized by a combination of specific sedimentary features (for example, organic-rich beds, particle-supported depositional texture, beds rich in ferromanganese nodules) and a typical fauna (for example, abundant styliolinids, pumilio-type brachiopods, specific goniatites). Examples are the Lower and Upper pumilio Beds (LpB and UpB) or the Lower and Upper Styliolinite Beds (LSty and USty, Fig. 2). Detailed biostratigraphic work identified these marker beds as isochronous lithostratigraphic units (Belka & Wendt, 1992;Belka et al., 1997Belka et al., , 1999Aboussalam & Becker, 2007Gouwy et al., 2007; and publications edited by Becker et al., 2013;Hartenfels et al., 2018). The latter authors detailed several biostratigraphic gaps (Fig. 2), which are frequently associated with the known marker beds. In successions sampled bed-by-bed, the gaps can be reliably identified by conodonts, because these skeletal elements consist of apatite and would be preserved even under the influence of carbonate dissolution. In case conodonts of certain zones and subzones are missing, the duration of the biostratigraphic gaps can be estimated, since the Devonian conodont zonation is chronostratigraphically scaled (Becker et al., 2020). The most widespread biostratigraphic gaps, those of the norrisi (ca 200 ka) and MN 2-3a zone (ca 800 ka), occur in the early Frasnian succession and can be traced across the whole Tafilalt Platform (Fig. 2). They occur at the base of organic-rich marker beds (Lower and Upper Styliolinite Bed), which consist of styliolinid packstone and grainstone (styliolinid coquinas). In many cases, both the hiatuses and the marker beds were formed in times of rapid global extinctions or radiations (Fig. 2), which have been described as Devonian bioevents (e.g. House, 1985House, , 2002Racki, 2005Racki, , 2020Becker & Kirchgasser, 2007;Brett et al., 2012;Gereke & Schindler, 2012;Becker et al., 2016Becker et al., , 2020.

Palaeoceanographic setting
The Tafilalt Platform formed at the outer rim of a broad epicontinental sea that extended over large parts of North Africa (L€ uning et al., 2003Craig et al., 2008;Soua, 2014). It was located at the transition to the slope aligned along the South Meseta Fault separating the Meseta domain from mainland Gondwana Michard et al., 2010).
Oceanographic models infer an eastwarddirected (warm) surface current along the northern margin of Gondwana as part of a large anticlockwise circulation cell between Gondwana and Laurussia (Heckel & Witzke, 1979;Oczlon, 1990;H€ uneke, 2006;Abram & Holz, 2020). Eastwarddirected currents of (cold) deep water masses probably affected intermediate and deeper parts of the Gondwanan continental slope (Crasquin & Horne, 2018). The gradual narrowing of the oceanic gateways between the continental plates and the intermediate terranes triggered the intensification of both shallow-marine and deep-marine current systems during the Devonian (H€ uneke, 2006). Regional circulation patterns in the Tafilalt area have been reconstructed from the orientation of orthoconic cephalopods (Wendt, 1995) and neodymium isotopic data (Dopieralska, 2009), indicating an eastward flow of the surface water mass during the Eifelian-Frasnian.

INVESTIGATED OUTCROPS, MATERIAL AND METHODS
The current study is based on extensive fieldwork conducted in the early 2010s in a project that focused on the high-resolution stratigraphy of the Devonian platform and basin successions in the Eastern Anti-Atlas and the Moroccan Meseta. Subsequent fieldwork during annual field campaigns from 2016 to 2019 concentrated on the Eifelian to Frasnian succession of cephalopod limestones and associated lithologies on the Tafilalt Platform ( Fig. 1C). Microfacies and bed-scale sedimentary features were documented in detailed sedimentary graphic logs, resulting from bed-by-bed examination and tight thin-section sampling. Four representative and well-dated key sections from Bou Tchrafine (Fig. 3), Jebel Ihrs (Fig. 4A), Jebel Amelane (Fig. 4B) and Hamar Laghdad (= Hmar Lakhdad) (Fig. 5) are exemplified in detail in this paper to unravel the facies successions and their vertical and lateral changes. From the same areas in the central, south-western and north-eastern parts of the Tafilalt Platform, stratigraphic correlation schemes show the large-scale stratigraphic architecture (Fig. 1C). These correlation schemes are based on 45 sections logged along the kilometrelong scarp slopes to explore overall thickness variations, lateral continuity of marker beds and shell concentrations, the range and regional distribution of biostratigraphic gaps, and principal stacking patterns of the stratigraphic units.
The database includes 890 thin sections, most of them prepared at a size of 7 9 10 cm in order to document sedimentary structures and microfacies variation at the bed scale. Samples for thin sections were obtained from all different lithologies and from all parts of the measured sections. Individual key beds were documented with up to ten thin sections to investigate subtle facies variations across the beds and along their lateral continuation, in particular transitions across bed boundaries into the underlying and overlying lithologies. All thinsection photomicrographs and scans shown in the figures are oriented vertically unless stated otherwise. For thin section analysis, the textural classification of Dunham (1962) and Embry & Klovan (1972) was used and considers the recommendations by Fl€ ugel (2010) and Lokier & Al Junaibi (2016). The analysis of dense shell concentrations, termed coquinas, was based on the criteria of Kidwell (1991a). The relative percentage frequency of different components was determined by means of the visual-comparison charts of Baccelle & Bosellini (1965) and Matthew et al. (1991). The extent of biogenic reworking was quantified using the bioturbation index (BI) of Taylor & Goldring (1993).
The mineralogical composition of ferromanganese nodules and hardgrounds was determined using X-ray diffraction (Bruker D8 Advance; Bruker Corporation, Billerica, MA, USA). For quantification of total carbon (TC), finely-ground samples were analysed with a CNS elemental analyser (EuroEA 3000; Eurovector, Pavia, Italy). In a second step, the amount of total inorganic carbon (TIC) was determined by measuring the released CO 2 after dissolving the carbonate content with hot 50% H 3 PO 4 (EuroEA 3000). The subtraction of TIC from TC-content revealed the amount of total organic carbon (TOC). Calibration was performed using Eltra standards (pure CaCO 3 ). The analytical precision is AE1.66% for TC and AE2.0% for TIC. For pyrite morphological analysis, polished slabs (17 samples) were studied under Zeiss-Evo MA10 scanning electron microscope (SEM; Zeiss, Oberkochen, Germany) equipped with secondary electron (SE) probes, back-scatter detector (BSD) and an X-ray spectrometer (EDX). Then, the size of the pyrite framboids and the framboid microcrystals in the SEM pictures (1067 images) were measured using the software ImageJ Fiji (version 2.9.0).

Bedded cephalopod limestones (FA2)
Bedded cephalopod limestones form bedsets with thicknesses of decimetres to several metres, especially in the Givetian and the Frasnian (Figs 3 to 5 and 6A). The spectrum of bioclasts corresponds to that of nodular limestones, with the difference that fragmented shells are more common or even prevail ( Fig. 10B to F). The lamination preserved in many beds of this lithology is irregular and locally indistinct ( Fig. 6E, 7C, 8C and 8D). It relates to variations in depositional texture and/or bioclastic composition (Figs 10A, 10F and 11A to F). Thin (millimetre to centimetre) micro-bioclast packstone (F4) and mottled styliolinid packstone and grainstone (F5.1) are interstratified with thicker intervals of bioclast wackestone with faint lamination (F2) and more rarely thin-shelled bioclast wackestone (F1) (Figs 3 to 5 and 11E,  Figs 3 and 4). The facies boundaries are often gradual and display both inverse and (more common) normal grading, i.e. transition from mud to grain-support and vice versa (Fig. 10A). The consecutive inverse to normal-graded (bi-gradational) sequences Bioturbation index (Taylor & Goldring, 1993)  display a transition from graded bioclast wackestone (F1.3) to micro-bioclast packstone (F4) and progressively to bioclast wackestone with faint lamination (F2) (Fig. 10F). Less common are bi-gradational sequences with inverse grading from F1.3 to styliolinid packstone (F5.1) and normal grading to F1.2 or F2 (Table 2, Fig. 10A). The thickness of the bi-gradational sequences ranges from a few centimetres to over a metre, with the coarse-grained central interval forming a thin part of the sequence (millimetres to centimetres) (Figs 3 to 5).
A characteristic feature of FA2 are the uneven discontinuity surfaces, some of which show truncations and hardgrounds, locally with ferromanganese encrustations. Laterally, discontinuity surfaces can be traced over tens of metres, but in many cases they transition into gradual facies boundaries as described above. In other cases, succeeding discontinuity surfaces laterally merge into a single one (Fig. 8C). These discontinuity surfaces frequently form the sharp base of microbioclast packstone (F4) and styliolinid packstone and grainstone layers (F5) (Figs 10F and 11A to F). Above the discontinuity surfaces, the bioclastic laminae are often enriched in similar-sized conodonts and quartz grains (Fig. 10E). Trace fossil assemblage is composed of Chondrites, Planolites and ?Teichichnus. Below hardgrounds, Planolites burrows locally show a sharp upper limit and a more diffuse lower limit. Large networks of Thalassinoides are common on the tops of the beds of this facies association (Fig. 8E). The preservation of the sedimentary lamination typically varies from sharp-cut to completely disturbed due to moderate to intense bioturbation (BI 3-6, Figs 3 to 5, 8C and 11A).

Organic-rich coquina beds (FA3)
Organic-rich coquinas form distinct dark beds or bedsets with thicknesses of up to 2.8 m (Figs 3 to 5 and 6B). Individual beds locally show bedform-related morphologies, like crests and troughs of subaqueous dunes ( Fig. 8A and B). More rarely, organic-rich coquinas constitute isolated or laterally-chained lenses. Bed boundaries are typically clear-cut. Previous biostratigraphic work (Fig. 2) and additional conodont data revealed that, in many cases, biostratigraphic gaps occur at the base of separate FA3 beds or bedsets (Figs 3 to 5). Less common are biostratigraphic gaps within bedsets associated with layers of intraclasts (see FA5). The organic-rich coquinas consist of styliolinid packstone and grainstone (F5) or pumilio-type brachiopod rudstone (F6) ( Table 2). Bioturbation is absent to sparse (BI 0-1, Figs 3 to 5 and 13A). Planoliteslike burrows were found in a few beds.
Pumilio-type brachiopod rudstone beds (F6) display inverse and normal grading at the base and the top ( Fig. 12C to E), respectively.  Articulate brachiopod shells occur tightly packed and at places parallel to crosslamination. Connected by a gradual transition, the lowermost and uppermost parts of the beds tend to show disarticulated and smaller shells with more abundant styliolinids of the same size.

Hiatal intraclast concentrations (FA5)
In the late Givetian and Frasnian, intraclast layers up to a few decimetres thick or intraclast-bearing surfaces occur (Fig. 7A, D and F). They are associated with biostratigraphic gaps (see Fig. 2) or hardgrounds and minor erosional surfaces (Figs 4 and 5). Above such hiatus surfaces, often specific particle concentrations are present, consisting almost exclusively of phosphatic conodonts or ferromanganese-coated particles, i.e. grains with a high specific weight. Large goniatite shells are locally enriched in larger numbers (Fig. 4A, UMB). Carbonate-intraclast floatstones (F9.1) and ferromanganese-intraclast floatstones (F9.2) document the variability of this FA (Table 1). The former is mainly associated with biostratigraphic gaps in FA3 (Fig. 7A), as evidenced by conodont data (Fig. 2), as the latter only occurs on erosional surfaces and hardground in FA2 ( Fig. 7D to F).
Thicker layers display complex internal microstratigraphic relationships with small-scale truncating unconformities (irregular) and scour marks (U-shaped) together with composite sedimentary structures ( Fig. 16A to D). Facies palimpsests feature ferromanganese-nodule floatstone (F3) and micro-bioclast packstone (F4). Quartz grains (extraclasts) are locally abundant (Fig. 16D to F). Burrows below such layers may accommodate tubular infills of otherwise unpreserved (because they are bypassed or eroded) sediments. In some cases, sediments of this facies seal cavities below hardgrounds that were undermined by tunnelling erosion.

Stratigraphic architecture
The large-scale variation in sediment thicknesses and facies of the Eifelian-Frasnian cephalopod limestones and interbedded lithologies was obtained by correlation and lateral tracking of marker beds (Fig. 17). Stratigraphic correlation schemes from the central Tafilalt Platform at Bou Tchrafine (Fig. 18), the south-western part at Jebel Amelane and Jebel Ihrs (Fig. 19) and the north-eastern part at Hamar Laghdad ( Fig. 20) exemplify the stratigraphic architecture and are described in the following sections.

Central Tafilalt Platform (Bou Tchrafine)
The Eifelian to Frasnian succession ( Fig. 2) of mainly nodular and bedded cephalopod limestones (Figs 3, 17A and 18) represents a sediment-starved stratigraphic succession covering about 21 Myr, including condensed beds in the late Givetian to middle Frasnian. Bedsets of both nodular and well-bedded limestones (Table 1) form tabular stratigraphic units with minor thickness variations (Fig. 18). The Eifelian interval is about 14 m thick. The pre-pumilio Givetian has a thickness of ca 7 m, the postpumilio Givetian ca 4 m, and the (pre-Kellwasser) Frasnian succession ca 3.5 m. These thicknesses indicate overall very low netaccumulation rates varying between 0.6 and 2.8 mm kyr À1 .
A widespread biostratigraphic gap occurs at the base of the early Frasnian (Basal Frasnian Hiatus, BFH, Fig. 17A) and spans the lower (Figs 3 and 18). A characteristic bedset with parallel and cross-laminated styliolinid coquinas (F5 , Table 2) is present in the early Frasnian. These beds cover the basal-Frasnian hiatus and enclose another biostratigraphic gap (Fig. 3, bed boundary 84e/f) comprising the main parts of MN zones 2 and 3 (between LSty and USty in Fig. 2). Across a distance of 600 m towards the south (BT8-BT10 in Fig. 18), the lowermost beds of these coquinas vary in thickness (from 8 to 40 cm). Additional thin styliolinid coquinas occur in the middle Givetian (Upper   Fig. 18). The marker beds reflect a uniform tabular architecture, also with regard to the interbedded cephalopod limestones (FA1 and FA2).

South-western Tafilalt Platform (Jebel Amelane, Jebel Ihrs)
The tabular Eifelian to middle Givetian stratigraphic units can be traced from the central Tafilalt Platform to the south-western margin and show minor thickness and facies variations (Fig. 18). The same holds for the late Givetian to middle Frasnian limestones; however, they are missing in the western and southern scarps of Jebel Ihrs (Fig. 19A, section IH1, IH2, IH2.1), and further south at Mech Irdane, Rich Gaouz and El Kachla (Fig. 1C). As a consequence, the late Frasnian Kellwasser Limestone rests disconformably on middle Givetian cephalopod limestones in this area. Hence, the various shortranging stratigraphic gaps identified on the central Tafilalt Platform converge towards the south-west into a long-ranging hiatus of about 7 Myr that cuts down into the late and middle Givetian (Fig. 2). This area of truncated late Givetian limestones was identified as a channel trending south-east/north-west (Fig. 1C). Middle Frasnian crinoid limestones (FA4) are locally present at Jebel Ihrs (Fig. 19A, section IH1) and at Mech Irdane (Fig. 1C).
Between the north-eastern channel margin and the platform interior, the late Givetian and Frasnian succession shows a distinct lateral variation in facies and sediment thickness, as documented along the upper scarps of Jebel Amelane and Jebel Ihrs (Figs 17B, 17C and 19). Over a distance of 6 km, the thickness of the late Givetian varies between 0 m and 2 m, whereas the (pre-Kellwasser) Frasnian varies between 1.2 m and 5.5 m in thickness. The thickest intervals occur in a 2 km wide zone bordering the channel margin at Jebel Ihrs (Fig. 19). Within that zone, Frasnian organic-rich styliolinid coquinas (FA3) and crinoid limestones (FA4) form thick bedsets (Table 1, Fig. 4) that are laterally discontinuous and pinch out towards the south-west (i.e. the channel) and the north-east (i.e. the platform interior) (Fig. 19A). The Frasnian organic-rich styliolinid coquinas (FA3) at Jebel Amelane occur as discrete beds, thinning over a short distance (AM5 in Fig. 19B).
The mounded bedsets of FA3, 1.5 m in thickness close to the channel, directly cover the basal Frasnian hiatus. They represent the Upper Styliolinite and the Lower Rhinestreet Styliolinite (Fig. 17B), which are separated by an intraclast-bearing hiatus (Fig. 7A). The interbedded cephalopod limestones and the Lower Styliolinite are not preserved (compare Fig. 2).
Close to the channel margin at Jebel Ihrs, crinoid limestones of FA4 constitute thick bedsets in the late middle Frasnian (Figs 17 and 19). They are stacked on top of the mounded styliolinid coquinas of FA3, and have a maximum thickness of almost 3.0 m close to the channel margin. Consequently, FA3 and FA4 deposits of packstone to rudstone make up more than 60% of the Frasnian succession in this zone, which contrasts with a maximum of 5% in other parts of the Tafilalt Platform (Fig. 18).

North-eastern Tafilalt Platform (Hamar Laghdad)
At Hamar Laghdad, distinct variations in sediment thicknesses and facies in the Givetian-Frasnian succession were documented near the Eifelian-Givetian mud mounds (Fig. 20). Similar to their occurrence in central parts of the Tafilalt Platform, the Eifelian nodular and bedded limestone bedsets (Table 1) form tabular stratigraphic units with moderate thickness variation. The thickness of the pre-pumilio Givetian, however, decreases from 18 m in the west to 5 m in the east in close proximity to the Hollard Mound (HL2 to HL4.2 in Fig. 20). The postpumilio Givetian varies between 2 m and 5 m, and the (pre-Kellwasser) Frasnian succession between 2 m and 6 m. Many of the shortranging hiatuses identified in the central platform occur in the Hamar Laghdad area as well (Figs 5 and 20).
The organic-rich coquinas (FA3) show a remarkable increase in thickness in close proximity to the Hollard Mound (HL10 in Fig. 20).
Pelagic deposits (FA1) FA1 sediments are interpreted as pelagic deposits because they lack hydrodynamic sedimentary structures and show a predominance of shells of calcareous plankton and nekton embedded in carbonate mud (Tables 1 and 2). The fragile shell preservation and vertically embedded cephalopod shells underline an overall tranquil depositional environment. The absence of light-dependent (phototrophic and photosymbiotic) benthos signals accumulation below the euphotic zone. The well-preserved cephalopod shells indicate deposition in a water depth less than 300 m, which has been calculated based on the implosion depth for such shells (Hewitt, 1996). This interpretation aligns with previous studies on cephalopod limestones of the Tafilalt Platform (Wendt et al., 1984;Wendt & Aigner, 1985;Lubeseder et al., 2010) and other areas (Tucker, 1974(Tucker, , 1990. The lowdiverse trace fossil assemblage, tentatively assigned to the Zoophycos ichnofacies (see Hubbard et al., 2012;Svarda, 2012), is compatible with a pelagic origin.
Where FA1 is part of bi-gradational sequences together with bedded cephalopod limestones, however, the facies association may also signify fine-grained contourites resulting from pirated pelagic rain and remobilized pelagic mud in response to weak bottom currents (see below). The subtle vertical variation in skeletal grains and carbonate mud, such as in F1.3 (Table 2), may agree with a variation in primary productivity in the surface waters and a variable release of particles from suspension within a tranquil but fluctuating bottom current. The syn-depositional intensive bioturbation makes it impossible to clearly distinguish pelagites from muddy carbonate contourites removed from a bottom nepheloid layer, also because both facies represent conceptual extremes in a continuum of deep-sea sedimentary processes (see Rebesco et al., 2014).
The origin of the lime mud constituting Palaeozoic deep-sea carbonates is unclear (Servais et al., 2016). Munnecke & Servais (2008) identified different types of calcareous microfossils and nannofossils in Silurian carbonates of Gotland and concluded that calcareous plankton most probably existed already during the Palaeozoic. Tucker (1990) has suggested that the lime mud of Palaeozoic deep-sea carbonates could well be derived from the breakdown of macroskeletons, which requires a fragmentation process. In general, plankton exhibit a dramatic diversification in the Devonian (Whalen & Briggs, 2018).
The coquinas with whole shells (F5.1) represent sand-sized bioclastic contourites and constitute the C3 contourite sequence intervals (Fig. 21A), as coquinas made up of heavily fragmented shells (F4) are silt-sized bioclastic contourites and represent the C2 and C4 intervals  Fig. 21B and C). The latter display textural inversions resulting from disintegration of the fragile shells in response to varying hydrodynamic energy (Folk, 1962). The occurrence of silt-sized quartz grains (Fig. 10E) agrees with an increase in hydrodynamic energy. The coquinas are identified as: (i) hiatal or condensed shell concentrations; and, more rarely, (ii) lag concentrations (Kidwell, 1991a,b). The condensed concentrations (Figs 10A and 11A) formed because finer-grained sediment was never deposited out of suspension (total bypassing) at peak-flow conditions, or resulted from alternating small-scale deposition and erosion (dynamic bypassing). The lag concentrations (Fig. 10F) result from prolonged winnowing of carbonate mud and shell fragmentation. Abundant ferromanganesecoated particles (F3) show many of the microbial structures described by Preat et al. (2008) and indicate mostly oxic (to suboxic) conditions and low overall sediment-accumulation rates (e.g. Benninger & Hein, 2000;F€ ollmi et al., 2011;Hein et al., 2015;F€ ollmi, 2016), which implies deposition from clear-water bottom currents. Such depositional conditions are confirmed by the large extent of bioturbation (BI 3-6).
Incomplete contourite sequences typically include an erosional surface that separates a coarsening-up unit from an overlying fining-up unit (Fig. 21C, Table 2), displaying a gradual transition from C1 to C2 (for example, F1.3 to F2), abruptly followed by a gradual transition from C4 to C5 (for example, F4 to F1.2). Thin C3 intervals may occur (for example, styliolinid packstone), locally represented by starved ripples. These midcut-out bi-gradational sequences may originate from long-term variation in bottom- current velocity, or a temporal variation in local sediment supply Stow et al., 1986;Stow & Faug eres, 2008). The omission of (parts of) the C3 and C2 divisions is related to a prolonged phase of non-deposition and erosion during peak-flow, as has been documented for modern contourite successions (Stow & Faug eres, 2008). The intervening erosional surface was frequently transformed into a hardground ( Fig. 11A and H). Burrows below the hardground show sharp ceilings, indicating early marine cementation and hardening of the seafloor when the current intensified and the sedimentation rate was extremely reduced (H€ uneke, 2013). The C2 and C4 intervals (F2, F3 and F4) include carbonate mud intraclasts (Table 2), which were eroded from the hardened substrate during peak-flow conditions. Reworked intraclasts are a common component in modern carbonate drifts (Chabaud et al., 2016;L€ udmann et al., 2018;. The C4 division may contain particles with a higher specific weight compared to the calcite bioclasts (ferromanganese nodules, conodonts) and hence may form lags at the base of the normally-graded interval ( Fig. 21B and C). All of these features indicate winnowing or a long break in sedimentation halfway through the formation of a bigradational sequence. Furthermore, siliciclastic particles (mainly quartz grains) commonly occur in the C4 intervals (F4), indicating enhanced current-induced supply from more distant (external) sources, as in packstone layers of carbonate contourite sequences on the modern Bahamian slopes (Mulder et al., 2019). The low-diverse trace fossil assemblage agrees with a contourite environment (see Wetzel et al., 2008;Rodr ıguez-Tovar et al., 2019;Hovikoski et al., 2020). Thalassinoides-like traces on hiatal surfaces (Figs 8C to E and 11A to E), which are frequently filled by coquinas of styliolinid packstone to grainstone (F5), developed on semiconsolidated firmground to hardground substrates due to erosion, non-deposition and chemogenic induration (see Wetzel et al., 2008).

Deposits of energetic dysoxic-anoxic bottom currents (FA3)
The coquinas constituting FA3 (F5 and F6) are interpreted as sand-sized bioclastic contourites forming stand-alone C3 intervals within contourite sequences (see Stow & Faug eres, 2008), which in many cases are bounded by hiatuses (Fig. 21D, Table 2). The predominance of particle-supported depositional textures and the omnipresent traction structures evidence deposition from a migrating bedload in response to continuous hydrodynamic traction (Mart ın-Chivelet et al., 2008;Stow et al., 2009). Widespread shell stacking (telescoping) and layer-specific orientation patterns of the conical styliolinid shells display a unidirectional flow with small direction spread and variable speed ( Fig. 13B and F). The vertical variation in styliolinid shell size reflects sorting processes (Fig. 12F). The particle compositionabundant shells of calcareous planktonindicates bottom-current induced reworking of pelagic carbonate muds occurring within the source area. The absent to sparse bioturbation (BI 0-1) and the elevated TOC values (Table 3), together with the morphology and size distribution of pyrite framboids present in most beds (Fig. 14) suggest sediment deposition from an anoxic water mass (see Berner, 1984;Ekdale & Mason, 1988;Sagemann et al., 1991;Svarda et al., 1991;Wignall, 1994Wignall, , 2005Wilkin & Barnes, 1997;Uchman & Wetzel, 2011). The prevalence of small pyrite framboids (D < 5 lm) composed of small microcrystals (d ≤ 0.4 lm) indicates syngenetic formation at the oxic-anoxic interface within the water column with subsequent settling to the seafloor (Wilkin et al., 1996;Liu et al., 2019). Interlayers with a few discrete traces and larger framboidal pyrites (D > 5 lm) likely represent periods of dysoxic conditions. The varying subhorizontal-undulating lamination (Table 2), which includes small-scale scours  and normally graded laminae (Fig. 13A), was produced by tenuous traction together with rhythmic suspension fallout, which is consistent with deposition under the influence of weak to moderate, fluctuating bottom currents (Mart ın-Chivelet et al., 2008). Ripple cross-lamination indicates bedload transport at subcritical flow conditions (Figs 12B and 13H), as planarparallel lamination from upper-stage-plane beds most likely resulted from bedload sheets (and very low-amplitude bed waves) at Froude nearcritical flow conditions (e.g. Allen, 1984;Paola et al., 1989;Ashley, 1990;Southard & Bouguchwal, 1990;Best & Bridge, 1992;Ono et al., 2021). The brachiopod coquinas (F6), with inverseto-normal grading (Fig. 21E, Table 2), agree with varying current strength, more specifically, increasing to decreasing flow speed during bed accretion (see Stow & Faug eres, 2008). The large amount of styliolinids at the base and in the centre of the Upper pumilio Bed (Fig. 12C to E), indicates depositional conditions similar to those derived for the styliolinid coquinas (F5). The prevailing brachiopods, however, point to a specific source area outside of the Tafilalt Platform.
The crinoid floatstone facies (F7) resulted from in situ benthic carbonate production, which is a specific feature of some modern carbonate-contourite systems . Carbonate-segregating benthic organisms, such as suspension-feeding crinoids and solitary rugose corals, are most prolific when laterally supplied with food particles by weak bottom currents at water-mass boundaries (Dorschel et al., 2007;Duineveld et al., 2007;Davies et al., 2009;Mienis et al., 2012a,b;Taviani et al., 2016). The Frasnian crinoid meadows were places of preferred mud deposition from the nepheloid layer and were prone to fluctuating flow strengths, resulting in indistinctly laminated and sorted deposits with abundant chains of connected crinoid ossicles (including holdfast). The bi-gradational beds and, on a larger scale, the bedsets of FA7 and FA8 display a longterm change between stimulated benthic carbonate production and intensified sediment reworking as a result of the fluctuating bottom-current flow strength.

Hiatal lag deposits (FA5)
Concentrations of carbonate intraclasts, ferromanganese nodules, large goniatite shells and conodonts in FA5 (Table 1), draping hardgrounds and erosional surfaces, are interpreted as lag deposits. The residuum of mechanically robust shells and highly fragmented less robust shells constituting the matrix clearly results from mechanical erosion. Published conodont data reveal that biostratigraphic gaps characterize many of the associated discontinuity surfaces ( Fig. 2; Fig. 4A bed 22) or indicate extremely reduced accumulation rates during the formation of these layers (see Wendt, 1988;Aboussalam & Becker, 2007. The lag deposits are indicative of energetic currents that selectively removed fine-grained particles, excavated and reworked granule to cobble-sized intraclasts from the indurated seabed, and resulted in long periods, exceeding one biozone, with reduced sediment accumulation. Long-lasting exposure of the seafloor is evidenced by the thick ferromanganese coats around many intraclasts (Fig. 16A and F), shells and on the hardground surface (e.g. Giresse, 2008;Reolid & Nieto, 2010;F€ ollmi, 2016). Most of the smallsized ferromanganese nodules were formed by hydrogenetic accretion from ambient seawater, as indicated by the regular and closely-packed concentric microstructures, with columnar patterns or rounded segregations. The formation of the hydrogenetic nodules relies on a regular relocation process allowing the nodules to rotate and to remain at the sediment-water interface despite their very low accretion rates (Halbach et al., 1981;Hein & Peterson, 2013). Low overall sediment-accumulation rates are an important precondition (<10 mm kyr À1 ; Hein & Koschinsky, 2014). Especially contour currents are capable of winnowing and eroding fine-grained sediment, delivering oxygen, and assuring the throughput of iron and manganese ions and colloids used in the accretion of the nodules (Glasby & Read, 1976;Hartmann et al., 1989;Usui et al., 1993;Hein & Peterson, 2013).
Seaward-dipping contourite terraces are typically found on the upper and middle continental slopes at the interfaces of different water masses (e.g. Viana, 2001;Viana et al., 2002a,b;Hern andez-Molina et al., 2009, 2014Brackenridge et al., 2011;Preu et al., 2013;Ercilla et al., 2016;Steinmann et al., 2020;Wilckens et al., 2021). Water-mass interfaces are characterized by energetic geostrophic currents and associated internal waves and eddies (Reid et al., 1977;Yin et al., 2019;Miramontes et al., 2020), which Detail of (C) that shows increasing shell size and shell thickness together with gradual transition from mud to particle-support, displaying inverse grading. (E) Detail of (C) that shows large bivalved brachiopod shells gradually overlain by disarticulated, fragmented and smaller shells, exhibiting normal grading. Note minor amount of styliolinids. (F) Styliolinid grainstone gradually overlain by cephalopod rudstone (F5.3) displaying inverse grading. The styliolinid cones in lowermost part are oriented subhorizontally. In upper part, the abundant goniatites (Manticoceras) are aligned obliquely due to crosslamination. Thin section IH1.5-13v2, early Frasnian, USty. Pourtal es terraces along the seaward edge of the Florida Platform and the Marion Plateau off the Great Barrier Reef (Mullins & Neumann, 1979;Heck et al., 2004;H€ ubscher et al., 2010;. These morphosedimentary features are produced by bottom currents that are driven by the surficial water mass at 300 to 400 m water depth, or at even shallower water depths. The gently sloping terraces alternately take up thin contourite deposits or become eroded to deliver sediment down-current on associated contourite drifts, which are mainly controlled by sea-level changes and changes in the thickness of the surficial water mass (Kenter et al., 2001;Isern et al., 2002Isern et al., , 2005Eberli et al., 2010;Correa et al., 2012).

Sheeted or plastered drift
The fairly uniform thicknesses (tabular sediment architecture), the predominantly aggradational stacking pattern, and the low lateral variability of the Eifelian-Givetian facies succession (Figs 18 to 20) agree with an interpretation as a sheeted drift. Alternatively, the sheeted architecture may represent the thin upslope part of a larger plastered drift located further downslope (Preu et al., 2013;Rebesco et al., 2014). Such an increase in sediment thickness down the slope has been documented west and north of the Tafilalt Platform (Wendt et al., 1984;Wendt, 1989;Wendt & Belka, 1991;D€ oring, 2002;Lubeseder et al., 2010). The distribution of contourites in these basins, however, has so far remained unexplored. Note that cephalopods are of almost equal size due to sorting and many shells are embedded with their long axis parallel to the former ripple slipface, i.e. oblique from upper right to lower left. Thin section JI-22e2.4wV2(N-S) (Fig. 4). Obstacle scours and tails The regional thickness variations present at the eastern ridge of Hamar Laghdad (Fig. 20) suggest that the Eifelian-Givetian mud mounds formed obstacles for active bottom currents on the morphologically rather plain contourite terrace (Fig. 22B). With increasing size (up to 35 m), the Hollard Mound influenced the flow dynamics and sedimentation in the surrounding area, causing a scour in the early-mid Givetian (Fig. 20, offmound units J and K) and a lee-side tail or a luv-side bulge in the early Frasnian (Fig. 20, unit O). Such morpho-sedimentary features typically form close to mounds associated with contourite drifts due to local turbulence and enhanced current speeds (e.g. De Mol et al., 2002;Grasmueck et al., 2006;Fink et al., 2013;L€ udmann et al., 2016).

Abraded surfaces and local erosion areas
The biostratigraphic gaps identified in the late Givetian to middle Frasnian succession (Fig. 2) result from non-deposition and submarine erosion, which gave way to local disconform bed contacts, associated with lag deposits (Fig. 7A) and strata of different ages below the hiatus surface (Fig. 19). These features reveal that the contourite terrace was temporarily truncated by near-flat abraded surfaces produced by energetic bottom currents (Fig. 22B). Most distinctive are the two abraded surfaces that extended over the entire Tafilalt Platform during the early Frasnian (MN zones 1 and 3) (compare Fig. 2 with Figs 17 to 20). Similar surfaces of slightly lesser extent were produced during the mid-late Givetian and middle Frasnian. Abraded surfaces are regions eroded by strong tabular currents (Hern andez- Ercilla et al., 2011;Rebesco et al., 2014), and are often found in association with sandy contourite deposits within high-velocity zones of modern contourite depositional systems (Habgood et al., 2003;Masson et al., 2004;Hern andez-Molina et al., 2014, 2016a. The basal Frasnian erosional disconformity (BFH, Figs 17 to 20) documents widespread sediment bypassing and more intense erosion on the south-western terrace over a period of several hundred thousand years, starting in the topmost Givetian (norrisi Zone), and continuing during the earliest Frasnian (lower part of MN1 zone) (Bultynck & Walliser, 2000;Aboussalam & Becker, 2007;Hartenfels et al., 2018). As a result, a smooth indurated submarine surface was produced on a former muddy substrate (Figs 3 to 5, BFH). Such hardened seafloor surfaces are typical for carbonate contourite systems under conditions of energetic bottom-currents and low sediment supply (see . Carbonates have a high diagenetic potential and cementation in deep-marine carbonates may start close to the seafloor (Milliman & M€ uller, 1973(Milliman & M€ uller, , 1977, a mere reduction in sedimentation rate in current-swept areas already may result in rapid lithification of the seafloor (Mullins et al., 1980;Kenter et al., 2001;Malone et al., 2001). Subsequent reworking of indurated mud clasts (from the hardened seafloor) by intensified currents has been reported from modern and ancient drifts (e.g. L€ udmann et al., 2018;. The Upper Marker Bed and the Petteroceras Bed, which occur immediately below the basal Frasnian erosional disconformity (Figs 3 to 5, BFH), are enriched in these reworked carbonate-mud clasts that occur together with concentrations of corraded goniatite shells. Both marker beds indicate widespread intraclast reworking pre-dating a period of prolonged bypassing and local erosion during earliest Frasnian. The laterally discontinuous beds and bedsets of organic-rich coquinas (FA3) found on many of these erosional disconformities indicate that (centimetre to decimetrethick) bioclastic sand sheets and ribbons migrated as bedload across these abraded surfaces (Fig. 22B) and were preserved as the energetic currents decreased in intensity. Numerous minor hiatuses, limited to smaller areas and not characterized by a biostratigraphic gap, occur within the depositional record of (incomplete) contourite sequences of FA2. They are likely caused by fluctuations of the interface between the two water masses in which shearing of the water masses and internal waves transported and removed the sediment, as also shown for the present-day Saya de Malha Bank (Indian Ocean; Betzler et al., 2023). Alternatively, the projecting outer shelf-slope morphology of the Tafilalt Ridge (Figs 1B and 22B) may have influenced the local geostrophic currents forming non-permanent eddies (e.g. Viana & Faug eres, 1998). Wilckens et al. (2021) documented the link between eddy formation and localized erosional features even under conditions of low mean flow speeds on the Ewing contourite terrace of the Argentine continental margin. Such eddies are transient features and cause energetic periods during which the sediment is eroded, alternating with calm periods during which sediment deposition is favoured. The local erosion results from increased shear stress in turbulent flow compared to otherwise laminar flow (Schlichting & Gersten, 2017;Yin et al., 2019).

Channel and small mounded drift
On the south-western Tafilalt Platform, the converging disconformities, the stratigraphic sediment architecture (Figs 17B and 19), together with the lateral and vertical facies variation of the early-mid Frasnian succession display the formation of a north-west-trending channel bounded by a small mounded drift (or lev ee) at its north-eastern margin (Fig. 22B). A hiatus Hardground-bounded bioclast floatstone rich in ferromanganese-coated intraclasts (i) and ferromanganese nodules, which is truncated by an erosional surface and draped by a concentration of sand-sized intraclasts, quartz grains, ferromanganese nodules and bioclasts (F9.2). Thin section AM5-18, middle Frasnian (Fig. 4). (B) Detail of (A). Stromatolitic ferromanganese crust on lithoclast. (C) Detail of (B). Note spherical and hemispherical microbial structures (m) with agglutinated quartz grains (q). (D) Detail of (A) showing the mixture of ferromanganese nodules (n), bioclasts, abundant extraclasts (quartz grains, q), and a few intraclasts. (E) Carbonate-intraclast rudstone-floatstone with a packstone matrix of quartz and bioclasts in lower part (F9.1). Some of the intraclasts are impregnated by ferromanganese oxides. Thin section AM5-18u, middle Frasnian (Fig. 4). (F) Detail of (E) showing abundant quartz grains (q) and two different types of carbonate intraclasts (i).
covering 7 Myr is present within the channel (Fig. 1C), that crops out at the western and south-eastern scarp of Jebel Ihrs, Mech Irdane, Rich Gaouz and El Kachla (Fig. 1C) (Wendt & Belka, 1991;Walliser, 2000;Aboussalam, 2003;Dopieralska, 2003;Hartenfels et al., 2018). In central parts of the channel, such as at El Kachla (Fig. 1C), the biostratigraphic gap  Fig. 19). Note occurrence of hiatal intraclast concentration (FA5) above basal Frasnian hiatus. Boundaries marker beds, FA3, FA4 and FA2 were physically tracked to measure lateral thickness variation of depositional units, as gradual boundaries of FA1 and FA2 are shown schematically. comprises the semialternans to MN zone 10 (7 Myr), as in the northern channel margin at Jebel Ihrs the hiatus includes the semialternans to MN zone 4 (4 Myr) (Fig. 19A, section IH1). Initially, the channel resulted from an erosive incision (1-3 m) of the main bottom-current core into the late and middle Givetian succession during the earliest Frasnian. This can be inferred from: (i) disconform stratal patterns; and (ii) the uniform character of the late Givetian succession outside the channel, which contrasts with the early Frasnian showing facies and thickness variations together with widespread biostratigraphic gaps across the entire Tafilalt platform (compare Figs 2,18,19 and 20). Subsequently, during early-mid Frasnian, the channel remained largely free of sediment and became confined by low-mounded bedsets of coarse-grained contourites (FA3 and FA4), together with finer-grained contourites (FA2) at its downslope margin (Figs 17B and 19A). Compared to the central Tafilalt Platform   . 18), the channel-bounding drift formed by: (i) a preferred deposition of coarse-grained contourites (FA3 and FA4 >50%); (ii) a slightly increased cumulative sediment thickness; and in spite of (iii) a more fragmentary sediment accretion due to longer breaks in accumulation (Fig. 19A). Within the channel, merely coarse-grained contourites of FA4 were deposited intermittently and locally, such as during the middle Frasnian at Mech Irdane (Fig. 1C) (Aboussalam, 2003). Thus, the channel remained open until the late Frasnian due to persistent strong bottom currents, which caused sediment bypassing and erosion.  An erosional truncation, such as the one associated with the basal-Frasnian hiatus at Jebel Ihrs and the locations further south (BFH in Figs 17B and 19A), is used as a main criterion to identify contourite channels (Hern andez- Garc ıa et al., 2009;Rebesco et al., 2014;Miramontes et al., 2021). Such channels can be oriented along-slope, or sinuous and oblique relative to the slope. They have been described as second-order drift features from contourite depositional systems of the eastern Gulf of Cadiz (Garc ıa et al., 2009;Hern andez-Molina et al., 2014;de Castro et al., 2020b) and the Mozambique margin (Thi eblemont et al., 2019;Miramontes et al., 2020). In both cases, the contourite channels are bounded downslope by a small and low-mounded drift or a lev ee.

Basin-scale features
The interpretation of the Eifelian-Frasnian succession as deposits on a contourite terrace at the upper continental slope of Gondwana is supported by the coeval occurrence of contourite depositional and erosional features in other deep-sea domains between Gondwana and Laurussia (Oczlon, 1990;H€ uneke, 2006), like the Harz, the Carnic Alps and the Moroccan Meseta (Fig. 22A), which represent parts of the Rhenish Sea shelf, the intra-Alpine terrane and the Meguma terrane (Stampfli et al., 2013). This basin-wide co-occurrence indicates an overarching palaeoceanographic process as the driving mechanism behind the sediment redistribution. In all areas, the current velocities reached a maximum during the late Givetian and early Frasnian causing widespread submarine erosion and, more locally, the accumulation of coarsegrained contourites (H€ uneke, 2007, 2013). Aboussalam & Becker (2011) demonstrated for the global Taghanic Biocrisis that hiatuses may reflect simultaneous erosional or nondepositional episodes in deep-marine environments of different continental margins, indicating hydrodynamic events driven by global circulation rather than regional processes.
Global palaeogeographic and plate-tectonic models show that the intensified circulation during the Givetian-Frasnian influenced different ocean basins, including the distal passive margin of Laurussia, isolated terranes (Galatian and Hanseatic terrane assemblage) and the disintegrated northern continental margin of Gondwana (see Golonka, 2002;Stampfli et al., 2011Stampfli et al., , 2013von Raumer et al., 2016). The main driver of this intensified palaeocirculation is the constriction of deep-marine gateways due to the continuous convergence between Gondwana and Laurussia (H€ uneke, 2006).

Hydraulic behaviour of bioclastic grains
The hydrodynamic interpretation of the Devonian bioclastic contourite sequences is mainly based on the variation in the ratio between carbonate mud and skeletal grains (Fig. 21, Table 2). Grain-size variations should be interpreted with caution because a series of studies in carbonatedominated settings have demonstrated that grain properties, such as shape and density, also control the threshold of initiation of motion and affect the terminal settling velocity (e.g. Maiklem, 1968;Braithwaite, 1973;Flemming, 2017;de Kruijf et al., 2021). The conical styliolinid shells possess different buoyancy and settling trajectories than platy brachiopods and spherical crinoids, or the contemporaneous siliciclastics. Consequently, sediment transport not only results in progressive size-sorting but also shape-sorting (Flemming, 2017). This feature is  Wendt et al. (1984), Wendt & Aigner (1985) and Belka & Wendt (1992). Eifelian-Frasnian sediment thickness, contourite morphological features and water depth are shown at different scales. The depth of the contourite terrace is estimated at 200 to 300 m water depth. Dark-grey areas are the locations of Devonian outcrops. ascribed to the higher drag effect, i.e. higher resistance to motion, of irregularly shaped particles. Differences in particle shape can thus lead to progressive separation and selective concentration of individual shape groups in different lateral and vertical positions within a deposit (Flemming, 2017;de Kruijf et al., 2021). The effective density of the ubiquitous styliolinid shells, plugged with water by capillary force, varies between 1.4 g cm À3 and 1.6 g cm À3 (Hladil et al., 1991). Lottmann (1990) estimated that the settling velocity of whole styliolinid shells is equivalent to that of quartz spheres of a diameter approximately four times less than the maximum length of the conical test, whereby large variations from this mode occur resulting from the percentage of mud-filled shells. Complete beds of styliolinid packstone to grainstone (F5), accordingly, typically show a grain size of medium to very coarse sand, and therefore are comparable to sediments dominated by very fine to fine-grained quartz sand. Current speeds between 19 cm s À1 and 23 cm s À1 were calculated for the mobilization of such styliolinidrich coquinas (H€ uneke, 2013).

Depositional setting
Contourite terraces, which are considered to result from sedimentary processes varying between deposition and erosion (Ercilla et al., 2016;Miramontes et al., 2021), are a sedimentary environment known primarily from modern continental slopes (Viana & Faug eres, 1998;Mutti et al., 2014;Llave et al., 2015;Hern andez-Molina et al., 2017;Thi eblemont et al., 2019;Wilckens et al., 2021). The Moroccan Tafilalt Platform is identified as part of such a mixed (depositional/erosional) system positioned at the uppermost slope off the North African Epicontinental Sea (Fig. 22A) (for example, Reggane, Ahnet, Ghadames basins; L€ uning et al., 2004;Soua, 2014). Bed-scale and drift-scale features show that the flow conditions varied between tranquil (for example, C1-C2-C4-C5 contourite sequences of FA2) and vigorous (for example, C3-C4-C5 sequences with a sharp erosive base and stand-alone C3 intervals of FA3, FA4 and FA5). From the Eifelian to the middle Givetian, long periods with continuous sediment accumulation at overall low accumulation rates alternated with times of winnowing and reworking (prevailing FA1 and FA2 in Fig. 3, Table 2). As of the late Givetian to middle Frasnian, sediment accumulation became increasingly interrupted by bottom-current induced winnowing and reworking over longer periods of time (see FA3, FA4 and FA5, and abundant erosional surfaces in Figs 3 to 5). During early-mid Frasnian, long-lasting periods of nondeposition and even erosion occurred (compare biostratigraphic gaps in Fig. 2 and Figs 17 to 20).
The bathymetric position of major water-mass interfaces and their vertical variations in time are known to exert primary control on the configuration of contourite terraces, including morphological changes along the slope gradient (Hern andez- Molina et al., , 2016aPreu et al., 2013;Llave et al., 2015;Ercilla et al., 2016;Steinmann et al., 2020;Miramontes et al., 2021;Wilckens et al., 2021). In addition, density contrasts are important components of these water-mass interfaces. So far, however, very few facies criteria have been developed to characterize the effects of the various palaeoceanographic processes on sedimentation and terrace formation.
In the succession studied, contourites with widespread shell concentrations, mainly planktonic and nektonic organisms (F4, F5), evidence periods of increased hydrodynamic agitation due to a position of the water-mass interface close to the water depth of the Tafilalt Platform (Fig. 23). The formation of such contourite terraces is attributed to strong along-slope currents (Miramontes et al., , 2020 and turbulent hydrodynamic processes at the density gradient (Hern andez- Steinmann et al., 2020), which may cause widespread sediment reworking even when both geostrophic currents flow in the same direction. The contourite channel identified at the southern margin of the Tafilalt terrace, agrees with the observation that vigorous currents are observed only in the inner (landward) part of the contourite terraces, while the central and external (basinward) parts are affected by weaker bottom currents . Extensive hiatuses and lag deposits (F9) were caused in periods of a downward shifting water-mass interface or accelerating geostrophic currents, while mud-rich contourites and pelagites (F1 and F2) were deposited in periods of an upward shifting interface or decelerating currents.
Internal waves and tidal waves are secondary processes that may influence sedimentation on contourite terraces (Hern andez- Molina et al., , 2016bPreu et al., 2013;Ercilla et al., 2016;Yin et al., 2019;Llave et al., 2020). Comparable processes were identified in deep-marine dunes along the Jurassic South-Iberian margin (Pomar et al., 2012(Pomar et al., , 2019 and in a contourite channel of the Miocene Rifian corridor (de Weger et al., 2021). In the Devonian succession, however, sedimentary features indicative of oscillatory flows were not observed.
The contourite interpretation outlined above contradicts previous assumptions of frequent and widespread sediment redistribution by storm waves (Wendt et al., 1984;Wendt, 1989Wendt, , 1995Wendt & Belka, 1991;Lubeseder et al., 2010). The main arguments in favour of a periodic deposition from bottom currents opposing event deposition from storm-generated oscillatory flows and combined flows are: (i) the frequent integration of coquinas into bi-gradational sequences instead of exclusively normallygraded event beds; (ii) features of syndepositional bioturbation instead of post-depositional burrowing; (iii) the bioclastic composition corresponding to the interbedded pelagic sediments; (iv) the absence of particles with a shallowwater origin; (v) the close association of shell and intraclast concentrations with hardgrounds; and (vi) the absence of hummocky crossstratification and oscillation ripples. These characteristics also allow to differentiate basecut-out (normally graded) contourite sequences from fine-grained turbidites, which usually show post-depositional burrowing, grain compositions contrasting with the host sediment, an erosional base without features of a long-lasting break in sedimentation, and turbidite-specific facies sequences (see Stow & Shanmugam, 1980;Stow & Smillie, 2020).
The palaeoenvironment of contourites is generally interpreted as deep marine (Fig. 22B), since contourite terraces are commonly found on the upper and middle slopes (Hern andez- Preu et al., 2013;Miramontes et al., 2021) and on the outer shelves . This interpretation agrees with the absence of light-dependent benthic organisms such as photosymbiotic colonial corals, stromatoporoids and calcareous algae within both the carbonate contourites and the interbedded pelagic limestones. Their absence suggests that the sediments were deposited in the aphotic zone, which comprises parts of the deep shelf and the deep-sea bottom further downslope (Fl€ ugel, 2010). The frequent preservation of complete goniatite shells, which have a calculated implosion depth of <300 m (Hewitt, 1996), provides a lower waterdepth limit and implies terrace formation on the deep shelf or on the uppermost slope (200-300 m).
The basin-ridge topography in the eastern Anti-Atlas is basically the result of tectonic processes and differential subsidence that increased as of the early Eifelian (Wendt, 1985(Wendt, , 2021aBaidder et al., 2008;Lubeseder et al., 2010). The documented morphosedimentary features indicate a distinct influence of bottom currents and associated palaeoceanographic processes. On the Tafilalt Platform, bioclastic contourites and pelagites formed a contourite terrace and gave way to a thin series of partially condensed carbonates (10-30 m). A much thicker Eifelian-Frasnian succession is known from the Maider, Tafilalt and Rheris basins, which preserve the sediments deposited further downslope in the west, east and north, respectively (Fig. 22B). As an example, the stratal record on the slope towards the Maider Basin is up to 350 m thick (Ottara section) and correlates with more than 600 m thick deposits in the central part of the basin (Bou Dib section) (Hollard, 1974;Wendt et al., 1984;D€ oring & Kazmierczak, 2001;D€ oring, 2002;Lubeseder et al., 2010).
A thicker Eifelian-Frasnian succession is also preserved on the Tafilalt Ridge in the south of the Tafilalt Platform (Wendt et al., 1984;Lubeseder et al., 2010;. The succession with nodular limestones, peloid-rich laminated limestones, marls and claystones, as well as normally-graded limestone beds, coralrich conglomerates and quartziferous deposits thickens upslope from 80 m (Bou Maiz Syncline) to locally 400 m (Amessoui Syncline) (Fig. 23). The spur-like upslope area, rich in gravity-flow deposits, connects the Tafilalt Platform with a shallow-water carbonate factory with hermatypic corals and stromatoporoids, possibly connected with the Maider Platform further to the south-west (D€ oring & Kazmierczak, 2001;D€ oring, 2002) (Fig. 22B). Palaeocurrent measurements from turbidites (cross-lamination) and the orientation of slump folds (fold planes) indicate a predominant north-west-directed sediment transport, i.e. parallel to the spur of the Tafilalt Ridge (Lubeseder et al., 2010). Latter authors also identified iron-rich hardgrounds on the Tafilalt Ridge that could be correlated with submarine hiatuses on the Tafilalt Platform. Together these sedimentary features suggest that the Tafilalt Ridge was less affected by winnowing, reworking, non-deposition and erosion when compared with the Tafilalt Platform. The preserved sedimentary record is thicker, more complete, and the particle composition of the interbedded gravity-flow deposits signals a shallowing towards a euphotic depositional environment to the south.
Aforementioned regional features contradict the hypothesis of repeated storm-induced reworking and erosion of the carbonate muds as the main mechanism for condensed sediment deposition on the Tafilalt Platform (Wendt et al., 1984;Wendt, 1989Wendt, , 1995Wendt & Belka, 1991;Lubeseder et al., 2010). In general, there is no empirical basis in assuming any particular bimodal separation in the size of fair-weather and storm waves, or in the manifestation of such differences in the sedimentary record (Peters & Loss, 2012). In addition, storm waves and associated combined flows are most severe in shallow water and less intense in deeper parts of the shelf. Incomplete successions with erosional surfaces as numerous tempestites mark proximal (shallow) settings and more complete records with less-abundant, non-erosive tempestites prevail in distal (deep) settings (Duke, 1990;Myrow & Southard, 1991, 1996Seilacher & Aigner, 1991;Einsele, 2000;Jelby et al., 2020). The distribution of shell concentrations and hiatuses on the Tafilalt Platform and the Tafilalt Ridge contradicts the aforementioned depth relationship.
A contourite terrace provides a more adequate depositional model for the Tafilalt Platform. Above and below the bathymetric level of the interrelated water-mass interface, hydrodynamic effects are less intense and, thus, sediment deposition is more continuous and occurs at a higher accumulation rate, frequently giving way to plastered contourite drifts (Fig. 23). The styliolinid coquinas identified in the successions above (Tafilalt Ridge) and below (Maider and Tafilalt basins) the bathymetric level of the Tafilalt Platform (Fig. 22B) are probably part of contourite drifts formed under mainly depositional conditions.

Energetic bottom currents and oxygen restriction
The organic-rich coquinas classified as FA3 (Table 3) formed simultaneous to Devonian biotic crises (Lottmann, 1990;Ebert, 1993;Walliser, 1996;Aboussalam & Becker, 2011;Becker et al., 2016) and their depositional conditions are of key importance for palaeoceanographic reconstructions. These coarse-grained contourites are interpreted to result from energetic bottom currents of a mostly anoxic water mass, because bioturbation in these beds is absent or sparse (BI 0-1). The morphology and size distribution of framboidal pyrites (and microcrystals) reflects syngenetic and rarely early-diagenetic formation (Sagemann et al., 1991;Wignall, 1994Wignall, , 2005Wilkin & Barnes, 1997;Uchman & Wetzel, 2011). These pyrites (Fig. 14) that occur in most investigated organic-rich coquinas (for example, LpB, UpB, LSty, USty and LRh in Fig. 2) are indicative of anoxic conditions (Wilkin et al., 1996;Liu et al., 2019). The FA3 deposits are organic-rich (Table 3) and characterize conditions of enhanced organic-carbon burial, especially because the analysed samples are from shell concentrations with very low proportions of carbonate mud (Figs 12 and 13). Organic matter is preferentially transported as suspension load and occurs concentrated in fine-grained sediments, because most organic particles are hydrodynamically equivalent to clay-grade and silt-grade particles (Tyson, 1987).
The TOC contents measured in outcrop agree with values identified in borehole samples from Frasnian petroleum source rocks in the Ghadames Basin (0.8 to 2.8%, Riboulleau et al., 2018). In this basin and other parts of the spacious epicontinental sea of North Africa (Fig. 22A), organic matter accumulation was temporarily widespread during the Frasnian and early Famennian, resulting in organicrich shales and limestones, in part >200 m thick and TOC values of up to 14%, the so-called hot shales (L€ uning et al., 2003Soua, 2014). Sections studied at the margin of the Ahnet Basin indicate the presence of a major anoxic phase during Frasnian conodont zones MN1-2 and shortly thereafter (L€ uning et al., 2004). For the same time interval, this study documents the formation of erosional disconformities and organic-rich coarse-grained contourite beds (FA3) on the Tafilalt Platform. Based on this stratigraphic correlation, L€ uning et al. (2003) inferred the formation of an oxygen minimum zone that expanded during the early Frasnian at the upper continental slope and inundated the shelf with oxygen-poor water masses.
The organic-carbon enrichment in styliolinid coquinas on the Tafilalt Platform was interpreted to result from either episodically enhanced primary productivity (regional plankton blooms, enabling blooms of planktonic dacryoconarids and plankton-feeding brachiopods), or the presence of an oxygen-minimum zone (Wendt & Belka, 1991;L€ uning et al., 2003;Aboussalam & Becker, 2011). Such increased supply and burial of organic matter, however, does not necessarily signify deposition under tranquil and stagnant anoxic conditions. Intensified currents preferentially operating at water-mass boundaries may be closely associated with conditions of pronounced oxygenminimum zones. The Namibian continental margin provides an example (Hanz et al., 2019), where   Fig. 23. Schematic north-south cross-section of the Tafilalt Platform displaying thickness variation of the Eifelian-Frasnian lithologies, palaeogeographic interpretation and inferred water-mass interface during the middle Frasnian (compare Fig. 1C). The partially condensed succession between Bou Tchrafine and El Kachla mainly shows a sheeted architecture. Frasnian carbonates display a small mounded drift at Mdoura and Jebel Ihrs, which is recognized at the downslope margin of a channel located further south. The distribution of FA1 and FA2 is shown schematically. Sediment thicknesses from Wendt & Belka (1991), Klug (2002), Aboussalam (2003), Aboussalam & Becker (2007), Lubeseder et al. (2010), publications in Becker et al. (2013), Hartenfels et al. (2018) and own data. the relatively sharp upper boundary of the oxygenminimum zone (150-200 m below sea level) corresponds with the interface between the South Atlantic Subtropical Surface Water at the surface and South Atlantic Central Water below.
A similar ocean stratification and combination of environmental conditions is assumed for the organic-rich facies of styliolinid and brachiopod coquinas (FA3), which result from deposition with increased hydrodynamic energy from an anoxic water mass rich in particulate organic matter. The main evidence includes: (i) grain-supported depositional texture; (ii) layer-specific distinct orientation patterns of styliolinid cones; (iii) monomict composition of well to very-well sorted bioclastic particles and the partial or complete lack of interstitial carbonate mud; (iv) frequent cone-in-cone stacking of the shells; (v) current-ripple cross lamination; and (vi) parallel lamination; together with (vii) aggregations of organic matter within skeletal grains and carbonate mud; (viii) syngenetic framboidal pyrites; and (ix) absent to sparse bioturbation. The identified hiatuses and gravel lags, which may occur at the base and the top of those beds, are additional criteria for a vigorous bottom current, supporting this interpretation.
An unclear aspect of the contourite interpretation of the pumilio-type brachiopod coquinas (F6) is their composition. The predominant brachiopod shells must have been supplied from a source outside the Tafilalt Platform, since these shells are not found within the host succession, and their provenance is unknown. A possible source area were hemipelagic outer-shelf sediments rich in pumilio-type brachiopod colonies, preserved in the Dra Valley west of the Tafilalt (Ebbighausen et al., 2004(Ebbighausen et al., , 2011.

CONCLUSIONS
This study identified the Moroccan Tafilalt Platform as part of a contourite depositional system at the uppermost slope that connects the Gondwana mainland with the Meseta domain further north. The bioclastic carbonate drift is one of the rare fossil analogues of modern carbonate contourites that was formed under greenhouse climate conditions. The contourite interpretation is based on independent lines of evidence at the microfacies, bed, drift and basin scale.
The microfacies reflects repeated changes between suspension deposition, sediment bypassing, winnowing of fines, bedload traction and erosion, together with concomitant seafloor cementation. Shell concentrations (coquinas) of mainly planktonic and nektonic organisms are identified as integral parts of bi-gradational contourite sequences showing inverse and normal grading as a result of gradual variation in mean grain size, depositional texture and degree of shell fragmentation. Coquinas with whole shells constitute the C3 intervals of contourite sequences, as coquinas made up of heavily fragmented shells represent the C2 and C4 intervals. Hiatal lag concentrations of carbonate intraclasts, ferromanganese nodules and conodonts drape hardgrounds and erosional surfaces. These lag deposits contain mechanically robust shells and highly fragmented less robust shells, which result from current-induced mechanical erosion. Widespread shell-in-shell structures and layer-specific orientation patterns of the conical styliolinid shells display unidirectional flow behaviour of varying direction and speed.
The bedsets of calcareous muddy to sandy contourites (wackestone to grainstone) form coarsening-up and fining-up sequences, and display long-term increasing and decreasing current velocities. Traction structures preserved in organic-rich contourites evidence currentinduced bedload transport mostly under subcritical flow conditions. Ferromanganese and calcareous gravel-lag contourites (rudstone to packstone) often drape hardgrounds and erosional surfaces that coincide with biostratigraphic gaps, suggesting very low net accumulation rates at moderate to strong current speeds.
At the drift scale, the overall sheeted architecture of the Eifelian-Frasnian succession results from an interplay between depositional and erosional processes on a contourite terrace that presumably constitutes the thin upslope part of a larger plastered drift. Abraded surfaces and sandy condensation layers are most widespread in the late Givetian to middle Frasnian part of the drift. Obstacle scours and tails occur in the vicinity of closely associated carbonate mud mounds. The upper contourite terrace is characterized by a Frasnian contourite channel and a small mounded drift at its downslope margin, which are oriented quasi-parallel to the continental slope.
The coeval occurrence of contourite depositional and erosional features in other areas of the oceanic domain between Gondwana and Laurussia indicates an overarching palaeoceanographic process driving sediment redistribution. Individual hiatuses and coarse-grained contourites can be linked to synchronous contourite deposits on the Laurussian continental margin.
Devonian global evolutionary events and crises, known to be linked to widespread deposition of organic-carbon-rich sediments in outer shelf settings, relate to the formation of coarsegrained contourites and hiatuses, both indicating intensified bottom-current dynamics. The coincidence of intensified thermohaline deep-marine circulation with hypoxic and anoxic conditions, affecting individual basins and perhaps even spreading into the ocean, point to common palaeoceanographic causes.