At the southern limits of the Devonian reef zone: Palaeoecology of the Aferdou el Mrakib reef (Givetian, eastern Anti‐Atlas, Morocco)

Abstract

Devonian reefs of north-western Gondwana represent the southernmost record of shallow-water coral reefs in the Palaeozoic. However, few studies have attempted to date palaeoecological reconstructions of these high-latitude reefal buildups. This study provides the first detailed palaeoecological analysis of Aferdou el Mrakib, an isolated, Givetian coral-stromatoporoid reef, which developed in a semirestricted basin in the south-eastern part of the Rheic Ocean. The study documents spatial facies variability and succession of faunal replacements accompanying progressive reef accretion towards the sea surface. The investigations included both autochthonous communities found at the base of the reef and, partially, within the reef core, and allochthonous deposits of reef-derived skeletal debris that accumulated in the fore-reef setting. Contrary to some previous suggestions, the study shows that the Aferdou reef shared many characteristics of classical Middle Devonian coral-stromatoporoid buildups, including the ecological succession, limited role of calcareous algae, and development within the range of the euphotic zone, but likely below the zone of regular water agitation. Critical factors in the facies development and temporal changes in the character of reef building were the palaeobathymetry, dominant sedimentary and circulation regimes, level of wave energy, and, possibly, light availability. Distinctive features of the palaeoecology of Aferdou el Mrakib are the dominance of massive colonies of heliolitid tabulates and a subordinate role of massive stromatoporoids, both explained here primarily as a result of increased water turbidity in the high-latitude sedimentary basin.The growth of the high-latitude coral-stromatoporoid reefs in the south-eastern Rheic Ocean was favoured by a combination of the exceptionally warm climate and plate tectonic configuration typifying the Devonian. Of critical importance appears the palaeogeographic position of the Rheic, which resulted in the seawater circulation in the ocean being dominated by tropical water masses, with restricted inflow of cold water from the circumpolar oceanic circulation.

A large Middle Devonian eubrachythoracid ‘placoderm’ (Arthrodira) jaw from northern Gondwana

“…An isolated bone like the PSG of Leptodontichthys implies that the element fell off and was transported post mortem. Its reasonably good preservation, the relative completeness of rare gnathostomes findings as Maideria and invertebrates, such as trilobites, found in the host layer (Lelièvre 1995 ; Campbell et al 2002 ) as well as the argillaceous-calcareous wackestone matrix surrounding the bone suggest relatively calm waters and low-energy depositional settings (Dunham 1962 ; Jakubowicz et al 2019 ). Thus, the PSG likely did not undergo prolonged transport in a high-energy environment.…”

“…These strata yield abundant remains of sessile benthic organisms such as brachiopods, corals and crinoids. Very few vertebrates were described from the same layer, including some conodonts (Kaufmann 1998 ; Lelièvre 1995 ; Campbell et al 2002 ; Jakubowicz et al 2019 ), the arthrodire Maideria falipoui (Lelièvre 1995 ) and the dipnoan Dipnotuberculus gnathodus (Campbell et al 2002 ). Now, we can add a large arthrodire to this list.…”

“…Now, we can add a large arthrodire to this list. Previous research using conodonts dated the locality to the early to middle Givetian (Lelièvre 1995 ; Kaufmann 1998 ; Campbell et al 2002 ; Jakubowicz et al 2019 ).

Fig.

…”

Epizoans immured in the heterocoral Oligophylloides maroccanus Weyer, 2017: a unique record from the Famennian (Upper Devonian) of Morocco

“…By the Middle through Late Devonian, this tectonic segmentation resulted in the formation of several basins with distinct sedimentary histories, and accompanying spatially and temporally variable ecosystems, including both diverse assemblages of relatively shallow‐water habitats, and impoverished faunas of deep, episodically oxygen‐deprived environments (e.g. Wendt & Belka ; Belka et al ; Kaufmann ; Frey et al ; Jakubowicz et al ). During the Late Devonian, this area comprised part of a relatively narrow seafloor high separating depocentres of the Mader Basin to the west and Tafilalt Basin to the east (Tafilalt Ridge sensu Lubeseder et al and see fig.…”

“…Simplified geological map of the eastern Anti‐Atlas with indicated Devonian outcrops (slightly modified after Jakubowicz et al () adapted from Dopieralska ()). The location of Jebel Bou Ifarherioun is indicated by star.…”