Late Neoproterozoic carbonate productivity in a rifting context: the Adoudou Formation and its associated bimodal volcanism onlapping the western Saghro inlier, Morocco

Abstract

An interval of episodic carbonate productivity, lithostratigraphically recognized as the ‘Calcaires inférieurs’ (upper member of the Adoudou Formation), took place across the Neoproterozoic–Cambrian transition onlapping the western Saghro inlier, Morocco. Sedimentation of the ‘Calcaires inférieurs’ was highly variable: in relatively stable substrates, a peritidal-dominated mixed platform is recorded where deposition was primarily controlled by autocyclic processes and accommodation space availability, whereas, in unstable substrates, the tectonic activity associated with the inherited block-faulting basement led to deposition of complex slide sheets composed of penecontemporaneous isoclinal folds and disrupted strata. The uppermost part of the ‘Calcaires inférieurs’ displays a negative δ13C shift reaching values of −6.5‰. This shift may represent the δ13C excursion to −6‰ that marks the Neoproterozoic–Cambrian boundary in the western Anti-Atlas. Two volcanic episodes bracketed the carbonate productivity. They consist of lower basaltic flows and an upper rhyolitic ignimbrite, with a SiO2 gap between 52 and 74 wt%. The basic rocks resemble those of tholeiitic magmas in continental rifts. The felsic rocks show high light to heavy rare earth element abundances and negative Nb, Ta, P and Ti anomalies, and were probably generated as a result of either fractional crystallization coupled with relative crustal contamination, or from a different magmatic source. The lower basic flows of tholeiitic affinity predated and geochemically differ from the alkaline magmatism of the Alougoum volcanic complex (Boho jbel) that surrounds the neighbouring Bou-Azzer inlier.

Sublacustrine hydrothermal seeps and silicification of microbial bioherms in the Ediacaran Oued Dar'a caldera, Anti‐Atlas, Morocco

“…At the stratotype of the Taghdout Group, the δ 13 C background values of carbonate interbeds range from −2 to +1‰, with a negative shift from background conditions of 4 to 6‰, whereas the fluctuations of δ 18 O values range from −8·2 to −18·6‰. In the Adoudou Formation, δ 13 C values vary between −4‰ and 0‰ in the Tabia Member, and between −6‰ and +7‰ in the Tifnout Member, except for some anomalous data from the Tagrara inlier (Maloof et al ., ; Álvaro et al ., ). Hydrothermally affected carbonates of the Tabia Member display δ 13 C values from −0·2 to −3·7‰, whereas their karstic infill (speleothems) range from −8·2 to −8·6‰ (Álvaro, ).…”

Interplay of phosphogenesis and hydrothermalism in the latest Ediacaran rift of the High Atlas, Morocco

“…Synsedimentary tectonism during deposition of the Tabia Member is manifest regionally by dramatic, directional variations in thickness, regional angular unconformities, and abrupt lateral and vertical changes in lithofacies that reflect differential subsidence of fault-bounded blocks. It is also indicated by gravitational slides at several localities (Soulaimani et al, 2001;Álvaro et al, 2008a), which attest to abrupt changes in local slope and/or episodic seismicity associated with synsedimentary faulting neighbouring inherited palaeohighs and pre-Pan-African inliers. Indeed, synsedimentary faulting profoundly influenced regional bathymetric changes and submarine palaeotopography during each rifting pulse.…”

“…This episode was linked to a WNW-ESE true extension, associated with a network of NW-facing NE-SW normal faults and the onset of a N30°fissure system. Fault-controlled, differential subsidence during this rifting pulse is also supported by: (i) marked variations in thickness and lateral discontinuity of the Tabia lithofacies regionally (Demange, 1980;Chbani et al, 1999;Algouti et al, 2000;Benssaou and Hamoumi, 2001); (ii) measurements of slump folds and olistostromes neighbouring active faults and inherited escarpments, which indicate development of local slopes of variable facing directions (Piqué et al, 1995(Piqué et al, , 1999Álvaro et al, 2008a); and (iii) contemporaneous deposition of submarine escarpment-derived breccias (Demange, 1980;Soulaimani et al, 2001Soulaimani et al, , 2003. As a result, the Tabia strata accumulated not on an intact, uniformly subsiding segment of the Souss basin (southern High Atlas and Anti-Atlas platforms), but instead on a basin composed of a mosaic of differently subsiding, fault-bounded crustal blocks facing a northern basinal seafloor preserved, at present, in the northern Coastal Meseta (see a recent discussion in Álvaro et al (2008b)).…”

“…During deposition of the Tabia Member, some grabens dropped down significantly relative to adjacent uplifts and inherited inliers, and slope-related and alluvial sediments accumulated to great thickness in the troughs (Piqué et al, 1999;Algouti et al, 2000;Benssaou and Hamoumi, 2001;Soulaimani et al, 2001Soulaimani et al, , 2003Pouclet et al, 2007). Meanwhile, dike intrusion and local volcanic (tholeiitic) eruptions took place within the rift valleys (Álvaro et al, 2008a;Ezzouhairi et al, 2008). In contrast, the carbonate-dominated Tifnout Member represents a more stable, peritidal platform, wholly colonized by microbial communities.…”

Late Ediacaran syn‐rift/post‐rift transition and related fault‐driven hydrothermal systems in the Anti‐Atlas Mountains, Morocco

“…). During deposition of the Tabia Member, some grabens dropped down significantly relative to adjacent uplifts and inherited Proterozoic inliers, and slope‐related, alluvial and fluvial sediments accumulated to great thickness in the troughs (Piqué et al ., , ; Algouti et al ., ; Soulaimani et al ., , ; Pouclet et al ., ); meanwhile, dike intrusion and local volcanic eruptions took place within the rift valleys (Álvaro et al ., ; Ezzouhairi et al ., ). As pointed out by Maloof et al .…”