Contourite-drift archive links Late Devonian bioevents with periodic anoxic shelf water cascading Open Access

Contourite deposits and morphosedimentary features are powerful tools for deciphering thermohaline circulation patterns (Knutz, 2008). Of particular significance are overflow-related contourites, as they provide detailed insights into the timing and nature of paleoceanographic changes (e.g., Toucanne et al., 2007; de Weger et al., 2020).

We use a contourite archive to unravel erosional and depositional processes associated with periodic anoxic bottom currents that traversed the Tafilalt Platform (in the eastern Anti-Atlas of Morocco) during Late Devonian evolutionary events (Fig. 1E), culminating in the Kellwasser crisis at the Frasnian–Famennian boundary. These bioevents are closely associated with coeval hypoxia, specifically in the closing Paleotethys Ocean, and are characterized by either widespread sudden extinctions (Frasnes event, Givetian–Frasnian boundary) or rapid radiations (Timan event, early Frasnian and Rhinestreet event, Middle Frasnian; e.g., House, 2002; Racki, 2005; Becker et al., 2020). Our study is the first to use contourite deposits for the reconstruction of the circulation patterns and paleoceanographic mechanisms associated with these Devonian global evolutionary events.

The Tafilalt area is part of the eastern Anti-Atlas, an ENE-trending foreland fold belt of the Variscan Orogen intersecting the southeast-trending intra-continental Variscan Ougarta Belt (Fig. 1A). During the Middle-Late Devonian, a basin and ridge topography characterized the passive, northwestern continental margin of Gondwana (Wendt et al., 1984; Soulaimani and Burkhard, 2008; Baidder et al., 2016; El Hassani, 2023). The Tafilalt Platform formed a terrace on the northern rim of Gondwana’s broad epicontinental sea, which included intra-shelf basins such as Reggane, Ahnet, and Ghadames farther southeast. The Tafilalt Basin southeast of the Tafilalt Platform connected to the Reggane Basin, while the Rheris Basin in the north was part of the deeper slope, facing the rifted Meseta domain and the Paleotethys Ocean (Figs. 1B and 1D; e.g., Wendt, 2021). The Tafilalt Ridge in the south linked to the euphotic shelf of the Maider Platform (Lubeseder et al., 2010). Persistent weak-to-intermediate bottom currents at a water-mass interface controlled the area, forming an extensive carbonate contourite terrace (Tafilalt contourite terrace [TCT]), at ~200–300 m depth (Hüneke et al., 2023). From south to north, the TCT features a contour-parallel channel, bounded by a small, low-mounded drift, followed by a sheeted drift with abraded surfaces and condensation layers. The varying, mostly oxic, bottom currents captured most of the pelagic rain, creating a complex interplay of sediment accumulation, winnowing, reworking, and bypassing, resulting in low net accumulation rates of condensed cephalopod limestones (Hüneke et al., 2023).

Located on the inner TCT, our 20 key study sites with Givetian to Frasnian records are distributed over an area of 4.5 km (north-south) by 5.5 km (west-east) in the hill ranges of Jebel Ihrs, Jebel Amelane, and Mdoura (Fig. 1C). We investigated the sites bed-by-bed, documenting lateral thickness and facies variations and paleocurrent indicators, and sampled for thin sections and conodont biostratigraphy.

We identified five facies associations (FA1–FA5; see item S2 in the Supplemental Material¹), composed of nine facies (F1–F9; S3), based on the sedimentological criteria of Hüneke et al. (2023). Our work focuses on the organic-rich coquinas (ORCs; FA3), calcareous sandy contourites predominantly composed of millimeter-sized planktic styliolinids (Fig. 2I), with elevated total organic carbon (TOC) content and pyrite framboid size distribution indicating deposition by energetic dysoxic–anoxic bottom currents. They are interbedded with nodular and bedded cephalopod limestones (FA1 and FA2) and bedded crinoid limestones (FA4), representing pelagites and muddy-to-sandy contourites. Hiatal intraclast concentrations (FA5) are gravel-lag contourites. We traced the ORCs across the terrace at specific stratigraphic levels, e.g., the Lower and Upper Styliolinite (LSty, USty; Fig. 1E) of the early Frasnian (MN1–MN4), and the Lower Rhinestreet equivalents (LRh) of the middle Frasnian (MN6–MN7; Figs. 1E and 2A). Although highly variable, thicknesses of up to 2.6 m for FA3 bedsets were documented. Traction structures, such as ripple cross-lamination, show NNW-directed current-induced bedload transport with a wide spread to the southwest and northeast (Fig. 2F). The degree of bioturbation, evaluated with the bioturbation index (BI) of Taylor and Goldring (1993), is very low (BI 0–1; Figs. 2A and 3A). Biostratigraphic gaps and minor erosional surfaces, identified as erosional contourite features (Fig. 1E), typically occur at the base of ORCs.

A significant erosional feature is a northwest-trending, along-slope channel on the inner, upslope part of the TCT, up to 8.5 m deep and ~5 km wide (aspect ratio of 0.0017), traceable from Jebel Ihrs to El Kachla (Figs. 1B, 2B, and 4B). In this area, the terrace-wide hiatuses identified at the base of ORCs converge into a wide-ranging biostratigraphic gap from the middle Givetian (semialternans conodont zone) to the middle Frasnian (up to 7 m.y.; Fig. 1E). The channel is largely filled with thick (up to 6 m), organic-rich Kellwasser limestone facies (Frasnian–Famennian boundary) traceable toward the terrace with decreasing thickness. Locally, in the channel (Fig. 1B, Mech Irdane), up to 3-m-thick, middle Frasnian bedded crinoidal limestones (coarse-grained contourites of FA4) are interstratified.

The most pronounced depositional feature is a small, mounded drift, 1.5–3 km wide, bounding the downslope margin of the channel (Fig. 1B), exposed at Mdoura and Jebel Ihrs. This drift (Fig. 2B) differs from the central terrace by an increase in (1) overall sediment thickness, despite more intermittent accumulation, indicated by larger biostratigraphic gaps (Fig. 1E); (2) thickness and abundance of coarse-grained bioclastic contourites (FA3 and FA4) at the expense of muddy intervals; (3) grain-supported textures at the expense of mud-supported textures; (4) traction structures; and (5) hiatal intraclast concentrations (FA5).

The geometry of early-middle Frasnian deposits indicates a steeper, channel-ward drift flank, as bed thicknesses rapidly increase within ~400 m, from <0.5 m in the most condensed sections close to the channel (IH1 and IHS in Fig. 2B) to 5.3 m near the reconstructed drift crest (JI and IH5 in Fig. 2B). The fence diagram (Fig. 2B) presents the drift-internal stacking pattern of mid-Givetian to mid-Frasnian strata. It shows a successive lateral, channel-ward shift of coarse-grained contourites (FA3 and FA4) and upslope-directed accretion, both also known from modern drifts (Rebesco et al., 2014). The decrease in thickness toward the external margin of the mounded drift is moderate from 5.3 m to 3.4 m (AM2 in Fig. 3B). The number of coarse-grained contourites (FA3 and FA4) decreases with distance from the drift crest, in favor of interstratified fine-grained contourites (FA2).

Conodont assemblages from stratigraphic key levels (sections IH1, IH2, IH5, JI, MDW-E, MDE in Fig. 1C), specifically from the base of thick ORCs, covering the major basal Frasnian hiatus (BFH), indicate that the evolution of the channel-related mounded drift began during the early Frasnian (MN3–MN4) at ca. 377–378 Ma.

For most of the Middle-Late Devonian, the TCT was controlled by weak-to-intermediate, clear-water bottom currents under mostly oxic conditions associated with a water-mass boundary (Hüneke et al., 2023). However, our study of drift-scale features and ORCs (FA3) provides evidence for additional periodic overflows of dense, anoxic shelf water, which passed the TCT before cascading into the deep western and northwestern marine realm. The overflows caused wide-ranging hiatuses by non-deposition and local erosion, followed by the deposition of ORCs as migrating bedforms. These two-phase circulation events are best exemplified by (1) the Basal Frasnian Hiatus (MN1), overlain by the Lower Styliolinite (MN2); (2) the hiatus of MN3, covered by the Upper Styliolinite (MN4); and (3) the hiatus of the late MN6, draped by the ORC of MN7 (Fig. 1E). The large-scale erosional features, locally associated with hiatal concentrations of intra-clasts (FA5; Figs. 2E and 3E), indicate initial current speeds between 0.3 m/s and 0.7 m/s (Wynn and Masson, 2008). The bioclastic sand sheets of FA3, with an effective grain size of very fine- to fine-grained quartz sand and current-ripple cross-lamination, formed at decreasing current speeds between 0.4 m/s and 0.2 m/s (Boguchwal and Southard, 1990; Baas, 1999; Hüneke, 2013). Because in the Southern Hemisphere, ripple migration within the frictional bottom boundary layer is slightly oblique to the right of the main current core (Rebesco et al., 2014; Hernández-Molina et al., 2018), the paleocurrent data indicate a west-northwest–directed flow.

The overflows channeled through Ougarta Trough and Tafilalt Basin were steered leftward across the TCT by the Coriolis force, placing the current core at the inner part of the terrace, and erosion at the southern (left), upslope flank. With the reduced current velocity at the northern (right) flank of the channel, a small, mounded drift was formed, with thicker bedsets of coarse-grained contourites (FA3 and FA4). Bottom-current filaments, generated by friction in the bottom Ekman layer (WÅhlin and Walin, 2001) and deviating at an angle of 35–40° from the main flow, produced north-trending small erosional furrows during peak flow and distributed thicker ORC sheets on the northern terrace (MDW in Fig. 2B).

We conclude from the morphosedimentary features and paleocurrent data that the anoxic overflows originated southeast of the TCT within the epicontinental sea of northern Gondwana. High-saline, dense water generated by high evaporation in the shallow shelf seas of the Devonian (greenhouse) circum-equatorial ocean (Wilde and Berry, 1986) accumulated within intra-shelf basins before cascading down-slope. The Reggane, Ahnet, Berkine, and Ghadames basins were locations of widespread anoxic (hot-shale) deposition during the Frasnian (Lüning et al., 2003). Their restriction from the open ocean and a high terrestrial nutrient input resulted in eutrophication and recurring episodes of photic-zone euxinia (Algeo and Scheckler, 1998). These conditions have been suggested to result in hot-shale deposition and in enhanced preservation of organic matter (Riboulleau et al., 2018; Carmichael et al., 2019; Kabanov and Jiang, 2020).

Because the deposition of overflow-induced ORCs coincided with sea-level rises (Fig. 1E; Haq and Schutter, 2008; Becker et al., 2020) and short-lived (typically ≤1 m.y.) hothouse conditions associated with intense global warming (Kidder and Worsley, 2010; Kabanov and Jiang, 2020), anoxic shelf-water cascading was probably caused by an increase in dense anoxic water masses in the shelf basins, yielding spill over.

The northern margin of Gondwana being entirely in the Southern Hemisphere, the dense anoxic water initially flowing northward was diverted to the west over the TCT by the Coriolis force, independent of the surface-water flow direction. Similar to the present Mediterranean outflow water (MOW), a modern analogue in the Northern Hemisphere forming similar morphosedimentary features (Hernández-Molina et al., 2014; Fig. 4C), the warm but highly saline overflows most likely cascaded farther downslope to a level of similar density at an intermediate depth. While decelerating, the Coriolis force caused them to spread toward the ocean interior as an oxygen-deficient layer above well-ventilated deep water, as evidenced by ostracod data reported by Crasquin and Horne (2018).

The stratigraphic record of the TCT challenges the model of an oxygen-minimum zone spreading from deep slopes onto the shelf and into intra-shelf basins (e.g., Lüning et al., 2003; Crasquin and Horne, 2018), and thus the idea of a bottom-up process behind the Devonian bioevents. Instead, the co-occurrence of Devonian bioevents and anoxic overflows, originating from intra-shelf basins experiencing runoff-related eutrophication, indicates a causal link between the two, supporting the model of photic-zone eutrophication and a top-down control (Algeo and Scheckler, 1998; Carmichael et al., 2019). Our model (Fig. 4A) further agrees with the ostracod fauna pattern reviewed by Crasquin and Horne (2018). We conclude that the overflows of dense, anoxic shelf water greatly contributed to, if not drove, the Devonian bioevents. This global impact is plausible if penecontemporaneous overflows on other continental margins discharged similar anoxic water masses oceanward, likely controlled by sea-level fluctuations (Carmichael et al., 2019).

We thank science editor Urs Schaltegger, one anonymous reviewer, and reviewer Adriano Viana. Their feedback improved our manuscript’s focus and quality significantly. Invaluable for this contribution were also Sylvia Weinert’s exceptional thin section preparation and Marie-Elaine van Egmond’s English correction. This work is part of the “Devonian contourites in oceanic passageways between Gondwana and Laurussia” (HU 804/8-1) project, funded by the Deutsche Forschungsgemeinschaft.