Ediacaran Fluviolacustrine Depositional Systems of the Amane-n’Tourhart and Tifernine Basins (Anti-Atlas, Morocco): Facies Analysis, Petrography, Paleoenvironments, and Climatic–Volcanic Controls

Abstract

This study provides sedimentological and stratigraphic insights into the Ediacaran fluviolacustrine successions of the Amane-n’Tourhart and Tifernine basins. The Amane-n’Tourhart Basin developed in a post-caldera volcanic setting along the margin of the Oued Dar’a Caldera, whereas the Tifernine Basin formed in a pre-caldera tectono-volcanic context associated with caldera development. The successions provide valuable information about the sedimentary processes operating in late Ediacaran continental environments. Field observations, facies analysis, and petrography reveal a variety of siliciclastic, carbonate, mixed siliciclastic–carbonate, and volcaniclastic facies. These facies form associations indicative of alluvial fan, floodplain, and shallow-water lacustrine settings. Alluvial fan deposits are dominated by conglomerates and sandstones forming braided systems. Fluviolacustrine successions show a transition from clay-rich siltstones with calcareous nodules to nodular and massive limestones, marking a gradual shift from fluvial to lacustrine conditions. Laminated limestones and stromatolites indicate intermittent microbial activity that contributed to carbonate precipitation. Sedimentation was strongly influenced by volcanic inputs and climatic fluctuations, alternating between humid and arid conditions. These factors drove cycles of channel incision, sediment infill, and lake expansion–contraction, illustrating the dynamic interplay of volcanism and climate that modulated deposition in these Ediacaran continental basins, with broad relevance to our understanding of this critical window in the Earth’s history.

1. Introduction

The Ediacaran period (ca. 635–538 Ma), the final interval of the Neoproterozoic Era, represents an important period in Earth history, marked by profound environmental, tectonic, geochemical, and biological transformations that set the stage for the Phanerozoic biosphere. It began with the termination of the Marinoan Snowball Earth glaciation, recorded globally by glacial diamictites and cap carbonates that reflect extreme climatic and carbon-cycle perturbations and major changes in ocean chemistry and redox conditions [1,2,3,4,5]. These environmental shifts were followed by the emergence and diversification of complex macroscopic life, notably the Ediacara biota, a diverse assemblage of soft-bodied organisms, that provides crucial insights into early multicellular eukaryotes and stem-group animals [6,7,8].

Simultaneously, the final breakup of Rodinia and the assembly of Gondwana exerted a major control on rift, passive-margin, and intracratonic sedimentary basin development, as well as sedimentation patterns and ocean circulation, thereby promoting the widespread deposition of mixed carbonate–siliciclastic successions in shallow marine and intracratonic Neoproterozoic basins worldwide [9,10]. The period is also characterized by large-amplitude carbon isotope excursions, which serve as key chemostratigraphic markers and reflect dynamic interaction among tectonics, climate, ocean redox state, and biological productivity [11,12]. Despite significant advances, the Ediacaran remains incompletely understood, and integrated stratigraphic, sedimentological, paleontological, and geochemical studies are essential to constrain the timing, drivers, and consequences of early animal evolution and Earth System reorganization.

The Ediacaran succession of the Anti-Atlas Belt of Morocco is predominantly continental and mainly represented by the Ouarzazate Group (ca. 580–543 Ma) along the northern margin of the West African Craton. It hosts key paleontological and sedimentological archives, including microbialites, microbially induced sedimentary structures (MISS), and soft-bodied Ediacaran fossils, offering valuable insights into the environmental context of early multicellular life along the peri-Gondwanan margin [13,14,15,16]. Owing to its stratigraphic continuity and well-preserved records, the Anti-Atlas Belt constitutes a reference region for studying the interactions among volcanism, climate, ocean chemistry, and biological innovation during the Ediacaran.

While Ediacaran life is primarily recorded in marine carbonate and siliciclastic successions, relatively little is known about the terrestrial counterparts of these environments. Only rare locations worldwide preserve Ediacaran sections deposited in fluvial and lacustrine settings, including the sedimentary successions of the Amane-n’Tourhart and Tifernine localities (Ouarzazate Group) in the Anti-Atlas Mountains of southern Morocco. Despite some previous work [17,18,19,20,21,22,23,24,25,26,27], the sedimentology and facies architecture of these sections have received limited attention.

A comprehensive investigation of these sections is critical to place them within the temporal and spatial framework of the Ediacaran. The present study aims to develop a temporally calibrated record of depositional settings, paleogeography, and life habitats in the mid-to-late Ediacaran terrestrial sedimentary successions of the Ouarzazate Group. Sedimentology and facies analyses were applied to characterize depositional environments and diagenetic processes.

Previous depositional models proposed for the Amane-n’Tourhart and Tifernine basins (e.g., [20,22,23,24,25,26,27]) have provided valuable insights into the regional geological framework, basin evolution, and general depositional environments. These studies significantly advanced our understanding of the stratigraphic architecture and paleogeographic evolution of the area. However, they did not explicitly address the vertical and lateral facies’ variations, paleocurrent trends, and the role of microbial mats in sedimentary dynamics. These aspects are specifically investigated in the present study, allowing a refined reconstruction of depositional processes and basin-scale sedimentary architecture. In this context, the present study integrates sedimentological, stratigraphic, and paleocurrent analysis of five measured sections to refine depositional models and assess fluviolacustrine dynamics along the northern margin of the West African Craton during the mid-to-late Ediacaran. Fluviolacustrine dynamics have been controlled by a complex and evolving interaction between rivers and lakes, where river inflow, lake-level fluctuations, and climatic and volcanic controls produced diverse deposits, with sediments of varying grain size, often modulated by wet/dry cycles. These Ediacaran terrestrial systems are crucial for reconstructing continental paleoenvironments and understanding the life habitat in such settings. More generally, these rocks and this study provide a novel, if not unique, window into continental processes, the complex network of drivers, and the related products during a critical time in the Earth’s history.

2. Geological Setting

The Anti-Atlas Belt, located in southern Morocco along the northern margin of the West African Craton (WAC) (Figure 1a,b), represents a major segment of the Pan-African (883–541 Ma) orogenic system. It is composed of Paleoproterozoic to Neoproterozoic basement rocks, locally uplifted to altitudes exceeding 1 km due to Cenozoic tectonic events [28,29]. These basement rocks are unconformably overlain by a thick, relatively undeformed succession of Ediacaran to early Paleozoic volcanic, volcaniclastic, and sedimentary successions, most notably the Ouarzazate Group [21,30,31]. The Anti-Atlas inliers expose numerous, mainly WSW–ENE-trending lithostratigraphic structures (Figure 1b), within which the Ouarzazate Group forms an extensive, volcanics-dominated succession documenting subaerial volcanism and sedimentary processes [21,31,32]. A detailed overview of the regional geology of the Anti-Atlas Belt is provided through past studies [31,33,34,35], and the lithostratigraphic framework of the Ediacaran succession was recently described [15].

The Ouarzazate Group (“PIII” or XIII of Choubert [36]) is a significant Proterozoic lithostratigraphic unit of the Anti-Atlas Belt that was formed during the late Ediacaran period (590–543 Ma). It includes extensive volcanic as well as intrusive rocks associated with the early stage in the Iapetus Ocean opening. It belongs to the Central Iapetus Magmatic Province (CIMP), and covers a vast area of ~2 × 10⁶ km², with thickness locally exceeding 2.5 km and an estimated magmatic volume of ~1 × 10⁶ km³ [33,37,38,39,40,41]. Lithologically, it is dominated by densely welded ignimbrites, tuffs, and lava flows, ranging from andesitic basalts to rhyolites in composition, with high-K, calc-alkaline to tholeiitic affinities [21,34,39,40,42]. The Ouarzazate Group was not affected by the main Pan-African deformation at ca. 647 Ma and was deposited on a highly differentiated basement topography, which, coupled with the large and rapid variations in thickness of the Ouarzazate Group itself, strongly suggests that this group was deposited in an extensional/transtensional setting [38,43,44]. Ouarzazate Group magmatism has been attributed to either continental arc volcanism and eventual slab break-off that resulted in asthenospheric upwelling or mantle plume activity [21,31,33,39,41,44,45,46,47,48]. Geochronological data constrain the earliest Ouarzazate Group volcanism to 590–588 Ma, whereas the termination of rift-related volcanism is nearly synchronous across the Anti-Atlas Belt at 561–543 Ma [21,31,34,39,40,46,48,49]. The Ouarzazate Group calderas with large volumes of pyroclastic flow deposits (ignimbrites) and ash falls developed extensively over a large part of the Anti-Atlas Belt [21,42,44,50,51].

The Oued Dar’a Caldera, located southwest of the Saghro Massif, is a large, rectangular volcanic depression (~11 × 18 km) infilled with densely welded rhyolitic to dacitic ash-flow tuffs [21,40]. The caldera formed along a northeast-trending left-lateral strike-slip fault system and is bounded by major strike-slip faults [21]. Preserved intra-caldera fill locally exceeds 500 m in thickness, with an estimated eruptive volume of 100–200 km³, which is consistent with caldera-scale ignimbrite accumulation [52,53]. The southwestern margin is characterized by coarse-grained volcaniclastic deposits interbedded with lacustrine beds containing microbialites [17,18,19,20,21,22,24,25,26,27,42]. The Tizgui area (Figure 1c) forms part of this caldera and exposes well-preserved volcanic and volcano-sedimentary deposits within the Oued Dar’a Volcanic Complex [21,40].

Two study areas were investigated: the Amane-n’Tourhart and Tifernine sections (Figure 1c–e). They are located along the western margin of the Saghro Inlier, the type locality of the Ouarzazate Group [19]. The Amane-n’Tourhart section is located between the villages of Amane–Issougri and Ait Saoun along the Ouarzazate–Agdz Road at the 464 km sign and has been a classical stop for multiple geological field excursions [19,54]. The Tifernine section lies ~8 km southwest of Ait Saoun, and it is separated from the Amane-n’Tourhart section by the younger Adoudou Formation [19,31,55].

The Amane-n’Tourhart succession (Figure 1d) forms a lenticular unit approximately 1 km long and ~30 m thick, resting unconformably on a succession of andesitic lava flows and associated pyroclastic rocks. The unit is slightly folded and faulted and consists of fluviolacustrine and volcano-sedimentary deposits associated with post-caldera volcanic units of the Oued Dar’a Caldera, including peperites and andesitic breccias at the top [40]. Choubert [17] first proposed a lacustrine origin for these meter-thick carbonate deposits, formed contemporaneously with volcanic activity, which was an interpretation later confirmed and refined [30,54]. Structural observations indicate that tectonics did not exert a primary control on sedimentation. However, several faults affected the unit after or during deposition. An approximately W–E-trending fault exposed at the level of the Oued is interpreted as syn-depositional, whereas a subvertical NE–SW-oriented fault with a steep SE dip cuts the central part of the succession (Figure 1d), affecting both stromatolitic (Ls2, Ls3) and associated carbonate facies (Fm1, Ln, and Lm). An additional NW–SE-trending fault, inferred near the roadway (Figure 1d), is considered to be post-depositional. The succession also displays soft-sediment deformation structures, including large-amplitude folds affecting sandstone beds (Sm) overlying the Lm carbonates. These beds locally exhibit a dish-shaped geometry in the central part of the unit, truncated by the NE–SW fault (Figure 1d).

The Tifernine succession of the Ouarzazate Group (Figure 1e) similarly consists of lacustrine deposits concordant with andesitic lava flows at the base and overlain by porphyritic andesitic lavas [30,31]. Although Álvaro [20] initially placed the Tifernine section stratigraphically higher than Amane-n’Tourhart, U–Pb zircon geochronology indicates that the Amane-n’Tourhart succession is slightly younger (564–561 Ma), whereas the Tifernine deposits are older, constrained between ~590 Ma and ~566 Ma [40,47,48]. This age relationship is consistent with regional 1:50,000 geological mapping of the Bou Azzer area [56,57]. In the Tifernine area, the carbonate succession (Ll/Fm2) forms a monoclinal structure dipping toward the SE and is affected by post-depositional dextral strike-slip faults observed along the Oued (valley) of Tifernine (Figure 1e).

These studied successions recording a period between 590 Ma and 561 Ma provide critical insights into sedimentary deposition in close association with Ediacaran volcanic activity, allowing detailed reconstruction of volcano-sedimentary dynamics at the western border of the Saghro Inlier and within the context of large-scale, caldera-related volcanism in the Ouarzazate Group. These successions also represent a largely unexplored archive of continental Ediacaran environments.

Figure 1. (a) The Anti-Atlas Belt at the northern margin of the West African Craton (WAC); (b) geological map of the Anti-Atlas Belt in southern Morocco modified after Gasquet and Ait Lahna [33,35]; (c) geological map of the western Saghro with location of Amane-n’Tourhart (d) and Tifernine (e) study areas (modified from Álvaro [20]); (d) geological map of the Amane-n’Tourhart area (this study); (e) geological map of the Tifernine area modified from the Bou Azzer 1:50,000 geological map [56,57] (positions of sections AT01, AT02; AT03 and AT04).

Figure 1. (a) The Anti-Atlas Belt at the northern margin of the West African Craton (WAC); (b) geological map of the Anti-Atlas Belt in southern Morocco modified after Gasquet and Ait Lahna [33,35]; (c) geological map of the western Saghro with location of Amane-n’Tourhart (d) and Tifernine (e) study areas (modified from Álvaro [20]); (d) geological map of the Amane-n’Tourhart area (this study); (e) geological map of the Tifernine area modified from the Bou Azzer 1:50,000 geological map [56,57] (positions of sections AT01, AT02; AT03 and AT04).

3. Materials and Methods

Given depositional facies with fluctuating accommodation space in sedimentary basins (e.g., lakes), a detailed lithostratigraphic, sedimentological, and petrographic analysis was carried out. This effort involved reconstruction of depositional environments and hydrodynamic conditions, along with possible controls on sedimentation linked to paleoclimate and volcanism. Both fieldwork and laboratory analysis were performed in this study.

3.1. Fieldwork

Two study areas, Amane-n’Tourhart and Tifernine, were selected in the southern Saghro Inlier based on field data (Figure 1b). The study sections were logged in detail for their lithostratigraphy and sedimentology at both large and small scales. Conventional lithofacies analysis was performed to document facies based on their geometry, thickness, lithology, sedimentary structures, grain size, and composition of the matrix. The fieldwork led to logging with a Jacob’s staff of four lithostratigraphic sections in the Amane-n’Tourhart area (AT01, AT02, AT03, and AT04) (Figure 2) and one in the Tifernine area (TF) (Figure 3). The necessity for four logged sections in the Amane-n’Tourhart area is due to its lithological complexity. Tifernine, on the other hand, is lithologically homogeneous (Figure 1e and Figure 4). These spatially separated and more or less contemporaneous sections have enabled us to establish lithostratigraphy of these Ediacaran fluviolacustrine deposits. As a result, fifteen facies were identified in the study areas. These facies were further subdivided on the basis of their genetic typology. Ninety samples were studied and analyzed in the laboratory for texture, structure, and mineralogy.

The results of the facies analysis of the Amane-n’Tourhart and Tifernine successions are illustrated in Table 1. Lithofacies’ types were grouped into lithofacies associations, taking into consideration the stratal stacking patterns and depositional environments. The facies associations include alluvial fan, floodplain, and shallow-water lacustrine deposits.

Table 1. Fluviolacustrine facies of the of Amane-n’Tourhart and Tifernine area.

Facies	Lithology	Sediment Characteristics	Geometry/Thickness	Interpretation	Figures
Gmm1	Matrix-supported conglomerate. Volcaniclastic conglomerate (peperite).	Sub-rounded andesitic clasts in reddish sandstone matrix; poorly sorted; mud cracks on bedding planes; gradational contacts	Flat to lenticular beds, ~25 cm thick	Volcano-sedimentary deposit (peperite) formed by lava–sediment interaction	Figure 4c–e,h
Gmm2	Matrix-supported conglomerate.	Sandstone and siltstone clasts, sub-angular to sub-rounded, oriented NE–SW, poorly sorted	Lenticular, channel-shaped base, ~30 cm thick	Deposition by migrating 3D dunes or longitudinal bars within fluvial channels or debris flows	Figure 4b
Gcm	Clast-supported conglomerate.	Poorly sorted, sub-rounded clasts (mm–cm in size) with volcanic or carbonate material depending on site	Tabular/lenticular bodies, 0.2–0.25 m thick	Hyperconcentrated flood flow, debris flow, or channel bar (including small-scale bar) deposits	Figure 4c,d,f
Gt	Cross-bedded conglomerate.	Trough cross-bedding, scoured base, upward fining, mixed detrital grains	Lenticular 0.6 m thick	Channel-fill deposits formed by high-velocity river flows	Figure 4a,g
Sm	Massive sandstone.	Massive, pink to purple, locally silicified, normal grading, no sedimentary structures	Lenticular; 0.2–2 m thick	Rapid deposition from high-energy flows in channels or mid-channel bars, which are simple and, straight	Figure 5a,c,e–g
Sh	Planar bedded sandstone.	Planar bedding, iron-rich with detrital alternations of detrital material, ripple marks	Tabular/lenticular, decametric–metric	Tractional deposits under upper flow regime in channels or on floodplain, flood-related	Figure 5b,d,h–j
Fm1	Massive silty mudstone with limestone nod-ules.	Red–purple siltstone with white carbonate nodules, microsparitic to sparitic calcite	Tabular/lenticular, 0.3–3.5 m thick	Flood-plain environment with fluvial channels, transitional to carbonates	Figure 6a–c,e–g
Fm2	Massive, silty mudstone.	Highly friable, non-erosive base, fine-grained	Tabular/lenticular, 0.15–4 m thick	Suspension fallout during waning floods	Figure 6e,f
Lm	Massive limestone.	Sparitic to microsparitic calcite, spherulitic, partially silicified	Tabular/lenticular, up to 1 m thick	Lacustrine to deep-water, fluviolacustrine carbonate deposits	Figure 6a–c,h,i
Ln	Nodular limestone.	Alternating micritic and detrital lamina, spherulitic and stromatolitic textures	Lenticular/tabular, 0.25–1.25 m thick	Transitional facies between fluvial (Fm1) and lacustrine (Lm) facies	Figure 6a–c,h,i
Ll	Laminar limestone.	Horizontal or inclined microbial lamina, mud cracks, ripple marks, MISS	Tabular/lenticular, 0.5–4 m thick	Shallow-water, lacustrine environment with periodic emersion	Figure 7 and Figure 8
Lmc	Massive limestone with scattered clasts.	Limestone with detrital quartz, plagioclase, and iron oxides	Lenticular, 0.2–0.35 m thick	Deposition in relatively deep lacustrine setting with increasing energy up section	Figure 6e,f,l
Ls1	Inclined, columnar stromatolitic limestone.	Alternating red, detrital and white, carbonate lamina; inclined columns	Biohermal; up to 5 m thick	Formed in high-energy, submerged environment in the lake	Figure 9a–d and Figure 10
Ls2	Domal stromatolitic limestone.	Linked vertical domes up to 80 cm in diameter, red–white alternations	2 m thick	Formed in aquatic settings with laminar flow (palustrine or low-energy riverbeds)	Figure 9e–g
Ls3	Planar laminated stromatolitic limestone.	Alternating red–white lamina; micro-domal to planar structures, silicified	0.3 m thick	Shallow-water lacustrine stromatolites	Figure 9e–i
Volcanic facies	Andesitic breccia, Andesites, Rhyolites.	Porphyritic texture, altered plagioclase, pyroxene, quartz in microlitic matrix	—	Volcanic unit (pre- and post-caldera stage), underlying sedimentary succession	Figure 11 and Figure 12

Table 1. Fluviolacustrine facies of the of Amane-n’Tourhart and Tifernine area.

Facies	Lithology	Sediment Characteristics	Geometry/Thickness	Interpretation	Figures
Gmm1	Matrix-supported conglomerate. Volcaniclastic conglomerate (peperite).	Sub-rounded andesitic clasts in reddish sandstone matrix; poorly sorted; mud cracks on bedding planes; gradational contacts	Flat to lenticular beds, ~25 cm thick	Volcano-sedimentary deposit (peperite) formed by lava–sediment interaction	Figure 4c–e,h
Gmm2	Matrix-supported conglomerate.	Sandstone and siltstone clasts, sub-angular to sub-rounded, oriented NE–SW, poorly sorted	Lenticular, channel-shaped base, ~30 cm thick	Deposition by migrating 3D dunes or longitudinal bars within fluvial channels or debris flows	Figure 4b
Gcm	Clast-supported conglomerate.	Poorly sorted, sub-rounded clasts (mm–cm in size) with volcanic or carbonate material depending on site	Tabular/lenticular bodies, 0.2–0.25 m thick	Hyperconcentrated flood flow, debris flow, or channel bar (including small-scale bar) deposits	Figure 4c,d,f
Gt	Cross-bedded conglomerate.	Trough cross-bedding, scoured base, upward fining, mixed detrital grains	Lenticular 0.6 m thick	Channel-fill deposits formed by high-velocity river flows	Figure 4a,g
Sm	Massive sandstone.	Massive, pink to purple, locally silicified, normal grading, no sedimentary structures	Lenticular; 0.2–2 m thick	Rapid deposition from high-energy flows in channels or mid-channel bars, which are simple and, straight	Figure 5a,c,e–g
Sh	Planar bedded sandstone.	Planar bedding, iron-rich with detrital alternations of detrital material, ripple marks	Tabular/lenticular, decametric–metric	Tractional deposits under upper flow regime in channels or on floodplain, flood-related	Figure 5b,d,h–j
Fm1	Massive silty mudstone with limestone nod-ules.	Red–purple siltstone with white carbonate nodules, microsparitic to sparitic calcite	Tabular/lenticular, 0.3–3.5 m thick	Flood-plain environment with fluvial channels, transitional to carbonates	Figure 6a–c,e–g
Fm2	Massive, silty mudstone.	Highly friable, non-erosive base, fine-grained	Tabular/lenticular, 0.15–4 m thick	Suspension fallout during waning floods	Figure 6e,f
Lm	Massive limestone.	Sparitic to microsparitic calcite, spherulitic, partially silicified	Tabular/lenticular, up to 1 m thick	Lacustrine to deep-water, fluviolacustrine carbonate deposits	Figure 6a–c,h,i
Ln	Nodular limestone.	Alternating micritic and detrital lamina, spherulitic and stromatolitic textures	Lenticular/tabular, 0.25–1.25 m thick	Transitional facies between fluvial (Fm1) and lacustrine (Lm) facies	Figure 6a–c,h,i
Ll	Laminar limestone.	Horizontal or inclined microbial lamina, mud cracks, ripple marks, MISS	Tabular/lenticular, 0.5–4 m thick	Shallow-water, lacustrine environment with periodic emersion	Figure 7 and Figure 8
Lmc	Massive limestone with scattered clasts.	Limestone with detrital quartz, plagioclase, and iron oxides	Lenticular, 0.2–0.35 m thick	Deposition in relatively deep lacustrine setting with increasing energy up section	Figure 6e,f,l
Ls1	Inclined, columnar stromatolitic limestone.	Alternating red, detrital and white, carbonate lamina; inclined columns	Biohermal; up to 5 m thick	Formed in high-energy, submerged environment in the lake	Figure 9a–d and Figure 10
Ls2	Domal stromatolitic limestone.	Linked vertical domes up to 80 cm in diameter, red–white alternations	2 m thick	Formed in aquatic settings with laminar flow (palustrine or low-energy riverbeds)	Figure 9e–g
Ls3	Planar laminated stromatolitic limestone.	Alternating red–white lamina; micro-domal to planar structures, silicified	0.3 m thick	Shallow-water lacustrine stromatolites	Figure 9e–i
Volcanic facies	Andesitic breccia, Andesites, Rhyolites.	Porphyritic texture, altered plagioclase, pyroxene, quartz in microlitic matrix	—	Volcanic unit (pre- and post-caldera stage), underlying sedimentary succession	Figure 11 and Figure 12

Figure 4. Field photos and photomicrographs of conglomeratic facies; (a) cross-stratified conglomerate (Gt); (b) matrix-supported conglomerate (Gmm2) characterized by oriented sandstone pebbles; (c,d) matrix-supported conglomerate (Gmm1) containing andesite blocks (volcano-sedimentary facies interpreted as peperites) with clast-supported conglomerate (Gcm); (e) mud cracks developed at the top of the Gmm1 bed; (f) clast-supported conglomerate (Gcm) in the Tifernine area; (g) photomicrograph in cross-polarized light of facies Gt; (h) photomicrograph in cross-polarized light of facies Gmm1 (Peperite) (Qz: Quartz, Plg: Plagioclase, And: Andesite, and IO: Iron oxide).

Figure 4. Field photos and photomicrographs of conglomeratic facies; (a) cross-stratified conglomerate (Gt); (b) matrix-supported conglomerate (Gmm2) characterized by oriented sandstone pebbles; (c,d) matrix-supported conglomerate (Gmm1) containing andesite blocks (volcano-sedimentary facies interpreted as peperites) with clast-supported conglomerate (Gcm); (e) mud cracks developed at the top of the Gmm1 bed; (f) clast-supported conglomerate (Gcm) in the Tifernine area; (g) photomicrograph in cross-polarized light of facies Gt; (h) photomicrograph in cross-polarized light of facies Gmm1 (Peperite) (Qz: Quartz, Plg: Plagioclase, And: Andesite, and IO: Iron oxide).

Figure 5. Field photos and photomicrographs of sandstone facies; (a) massive sandstone (Sm) showing poorly defined laminations; (b) horizontally bedded sandstone (Sh); (c) massive sandstone (Sm) with stromatolitic lamina; (d) ripple marks observed at the top of sandstone beds; (e,f) photomicrographs in crossed polarized light of facies Sm from the Amane-n’Tourhart area; (g) photomicrograph in crossed polarized light showing facies Sm of arkosic sandstone from the Tifernine area; (h–j) photomicrographs in crossed polarized light of facies Sh from the Amane-n’Tourhart area (And: Andesite; Qz: Quartz; Plg: Plagioclase; Cl: Calcite; C: Clay; IO: Iron oxide).

Figure 5. Field photos and photomicrographs of sandstone facies; (a) massive sandstone (Sm) showing poorly defined laminations; (b) horizontally bedded sandstone (Sh); (c) massive sandstone (Sm) with stromatolitic lamina; (d) ripple marks observed at the top of sandstone beds; (e,f) photomicrographs in crossed polarized light of facies Sm from the Amane-n’Tourhart area; (g) photomicrograph in crossed polarized light showing facies Sm of arkosic sandstone from the Tifernine area; (h–j) photomicrographs in crossed polarized light of facies Sh from the Amane-n’Tourhart area (And: Andesite; Qz: Quartz; Plg: Plagioclase; Cl: Calcite; C: Clay; IO: Iron oxide).

Figure 6. Field photos and photomicrographs of different facies (Fm1, Ln, Lm, and Lmc) from the Amane-n’Tourhart area; (a) erosional base of massive siltstone–mudstone facies with carbonate nodules (Fm1), nodular limestone (Ln), and massive limestone (Lm); (b) erosional base of facies Fm1, Ln, and Lm showing a tabular morphology; (c) the cross-sectional view highlights the distinction between facies Fm1, Ln, and Lm; (d) stromatolitic buildup at the top of facies Lm; (e,f) massive limestone with scattered clasts (Lmc) displaying tabular and lenticular shapes; (g) photomicrograph of facies Fm1, showing spheroidal nodules with spherulites; (h,i) photomicrographs of facies Ln; (j,k) photomicrographs of facies Lm; (l) photomicrograph of facies Lmc (Stm: Stromatolite; Qz: Quartz; Plg: Plagioclase; Dl: Dolomite; Cl: Calcite; Si: Silica; Sph: Spherulite; MM: Microbial mat; N: Nodule).

Figure 6. Field photos and photomicrographs of different facies (Fm1, Ln, Lm, and Lmc) from the Amane-n’Tourhart area; (a) erosional base of massive siltstone–mudstone facies with carbonate nodules (Fm1), nodular limestone (Ln), and massive limestone (Lm); (b) erosional base of facies Fm1, Ln, and Lm showing a tabular morphology; (c) the cross-sectional view highlights the distinction between facies Fm1, Ln, and Lm; (d) stromatolitic buildup at the top of facies Lm; (e,f) massive limestone with scattered clasts (Lmc) displaying tabular and lenticular shapes; (g) photomicrograph of facies Fm1, showing spheroidal nodules with spherulites; (h,i) photomicrographs of facies Ln; (j,k) photomicrographs of facies Lm; (l) photomicrograph of facies Lmc (Stm: Stromatolite; Qz: Quartz; Plg: Plagioclase; Dl: Dolomite; Cl: Calcite; Si: Silica; Sph: Spherulite; MM: Microbial mat; N: Nodule).

Figure 7. Field photographs of carbonate limestone facies (Ll) from the Tifernine area. (a) Irregular lamination in carbonates with pelitic seams; (b) laminated limestone facies (Ll) showing large undulations at the base of the bed; (c) laminated limestone (Ll) with asymmetric ripple marks; (d) micro-ripples organized as an “elephant-skin”, representing microbial structures (MISS: Microbially induced sedimentary structures); (e) intraclastic, laminated limestone facies (Lli); (f) flat-pebble conglomerate at the top of the laminated limestone facies; (g) tufeted microbial structure developed at the top of the laminated limestone bed; (h) symmetrical ripples observed within the laminated limestone (Ll).

Figure 7. Field photographs of carbonate limestone facies (Ll) from the Tifernine area. (a) Irregular lamination in carbonates with pelitic seams; (b) laminated limestone facies (Ll) showing large undulations at the base of the bed; (c) laminated limestone (Ll) with asymmetric ripple marks; (d) micro-ripples organized as an “elephant-skin”, representing microbial structures (MISS: Microbially induced sedimentary structures); (e) intraclastic, laminated limestone facies (Lli); (f) flat-pebble conglomerate at the top of the laminated limestone facies; (g) tufeted microbial structure developed at the top of the laminated limestone bed; (h) symmetrical ripples observed within the laminated limestone (Ll).

Figure 8. Photomicrographs showing the carbonate limestone facies (Ll) from the Tifernine area; (a) photomicrograph in cross-polarized light from the lower part of the unit, rich in detrital clasts and microbial mats; (b,c) photomicrographs in cross-polarized light showing pelitic laminae (b) and microsparitic calcite (c); (d) photomicrograph in cross-polarized light from the middle part of the unit showing carbonate crystals associated with iron oxides; (e,f) photomicrographs in polarized light under high magnification of facies (Ll) showing extracellular polymeric substances (EPSs); (g,h) photomicrographs in cross-polarized light from the upper part of the unit showing microsparitic calcite (g) and a sandstone lamina (h) (Qz: Quartz; Cl: Calcite; Dl: Dolomite, Plg: Plagioclase; EPS: Extracellular polymeric substance; MM: Microbial mats; IO: Iron oxide).

Figure 8. Photomicrographs showing the carbonate limestone facies (Ll) from the Tifernine area; (a) photomicrograph in cross-polarized light from the lower part of the unit, rich in detrital clasts and microbial mats; (b,c) photomicrographs in cross-polarized light showing pelitic laminae (b) and microsparitic calcite (c); (d) photomicrograph in cross-polarized light from the middle part of the unit showing carbonate crystals associated with iron oxides; (e,f) photomicrographs in polarized light under high magnification of facies (Ll) showing extracellular polymeric substances (EPSs); (g,h) photomicrographs in cross-polarized light from the upper part of the unit showing microsparitic calcite (g) and a sandstone lamina (h) (Qz: Quartz; Cl: Calcite; Dl: Dolomite, Plg: Plagioclase; EPS: Extracellular polymeric substance; MM: Microbial mats; IO: Iron oxide).

Figure 9. Field photographs of stromatolitic limestone facies (Ls) from the Amane-n’Tourhart area. (a) A bedding surface of a stromatolite column showing an isolated stromatolite; (b) bedding-plane view of inclined Ls1 facies with small-sized structures; (c) inclined, columnar Ls1 facies; (d) a block of Ls1 showing the evolution in columnar stromatolites: at the base stratiform 1”, in the middle inclined 2, and at the top subvertical 3; (e–g) dome-shaped Ls2 facies; (h) small, isolated dome-shaped Ls3 facies; (i) Ls3 facies with soft-sediment deformation structures showing disorganized lamination.

Figure 9. Field photographs of stromatolitic limestone facies (Ls) from the Amane-n’Tourhart area. (a) A bedding surface of a stromatolite column showing an isolated stromatolite; (b) bedding-plane view of inclined Ls1 facies with small-sized structures; (c) inclined, columnar Ls1 facies; (d) a block of Ls1 showing the evolution in columnar stromatolites: at the base stratiform 1”, in the middle inclined 2, and at the top subvertical 3; (e–g) dome-shaped Ls2 facies; (h) small, isolated dome-shaped Ls3 facies; (i) Ls3 facies with soft-sediment deformation structures showing disorganized lamination.

Figure 10. Photomicrographs showing microstructural features of stromatolitic limestone facies (Ls1 and Ls2) from the Amane-n’Tourhart area. (a) Photomicrographs under crossed polarized light of the Ls2 facies showing alternating dark, siliciclastic, and light carbonate laminae, with synsedimentary micro-deformation textures and a small, normal micro-fault within a siliciclastic lamina; (b) columnar stromatolites in the Ls1 facies displaying spherulitic textures within the siliciclastic laminae, partial dissolution of the Ls facies, and silica replacement; (c,d). photomicrographs under crossed polarized light showing extracellular polymeric substances (EPS) and microbial mats (MMs) (Qz: Quartz; Cl: Calcite; Si: Silica; Sph: Spherulite; IO: Iron oxide, MM: Microbial mat).

Figure 10. Photomicrographs showing microstructural features of stromatolitic limestone facies (Ls1 and Ls2) from the Amane-n’Tourhart area. (a) Photomicrographs under crossed polarized light of the Ls2 facies showing alternating dark, siliciclastic, and light carbonate laminae, with synsedimentary micro-deformation textures and a small, normal micro-fault within a siliciclastic lamina; (b) columnar stromatolites in the Ls1 facies displaying spherulitic textures within the siliciclastic laminae, partial dissolution of the Ls facies, and silica replacement; (c,d). photomicrographs under crossed polarized light showing extracellular polymeric substances (EPS) and microbial mats (MMs) (Qz: Quartz; Cl: Calcite; Si: Silica; Sph: Spherulite; IO: Iron oxide, MM: Microbial mat).

3.2. Laboratory Work

The work began with the preparation of more than 30 thin sections at the Department of Geology, the Faculty of Sciences Semlalia (Marrakesh, Morocco), and the Department of Earth and Planetary Sciences, University of California Riverside (USA). We performed a petrographic study and digital microphotography on these thin sections using an Olympus polarizing microscope (Olympus Corporation, Tokyo, Japan) at the Department of Geology, Faculty of Sciences Semlalia, Marrakesh (FSSM). Other laboratory analyses were conducted at “The City of Innovation” (a research center attached to the University Sidi Mohamed Ben Abdellah of Fez). These analyses included the study of microtextures and microstructures using a Scanning Electron Microscope (SEM). Selected carbon-coated polished thin sections were examined using secondary electron (SE) and back-scattered electron (BSE) imaging with a JEOL JSM-IT500HR SEM (JEOL Ltd., Tokyo, Japon). The instrument is equipped with a Schottky field-emission gun (FEG) (JEOL Ltd., Tokyo, Japon) that provides high beam brightness and long-term stability. The microscope was operated at beam currents of 10–20 nA and accelerating voltages ranging from 0.5 to 30 kV, allowing optimization of imaging conditions for both surface topography and morphology (SE mode) and compositional contrast (BSE mode). The high brightness and probe current stability of the Schottky FEG enabled high-resolution imaging and reliable analytical performance, particularly for mineralogical and microtextural investigations.

We also used Stereonet v.11 software, which is available through open access at https://www.rickallmendinger.net/ (7 September 2025), to analyze the elongation of columnar stromatolites and conglomeratic clasts, as well as the orientation of ripple marks. We measured the preferred orientation of the longest (a-) axis in elongated stromatolites and pebbles. For ripple marks, measurements correspond to the orientation of ripple crests, which are perpendicular to the paleoflow direction. This methodology ensures reliable, reproducible interpretations of paleocurrent indicators.

Imbrication in conglomerates is a primary sedimentary fabric defined by the shingled arrangement of gravel-sized clasts (pebbles and cobbles) stacked like toppled dominoes. In this configuration, flat a–b planes of tabular or discoidal clasts dip systematically in a preferred direction, commonly upstream. Since the clasts dip against the flow direction, imbrication constitutes a reliable paleocurrent indicator. Measurement of clast orientation, therefore, provides a quantitative constraint on paleoflow trends and depositional hydraulic conditions. Field measurements were conducted on well-exposed, clast-supported conglomerates displaying clear imbrication. For each clast, the dip direction and dip angle of the a–b (flat) plane were recorded. The paleoflow direction was inferred as opposite to the dip direction of the imbricated clasts. Fifty measurements were collected to ensure statistical robustness.

4. Results and Interpretations

In the Amane-n’Tourhart area, four detailed stratigraphic logs were measured along sections AT01, AT02, AT03, and AT04 (Figure 2) to document the vertical and lateral organization of lithofacies, which exhibited a high degree of heterogeneity. Sections AT01 and AT04 record a complete succession from the underlying andesites at the base to the brecciated andesites at the top, whereas AT02 and AT03 are incomplete due to faulting, folding, and minor shear zones. The succession can be divided into a lower part, approximately 11 m thick, composed of clay-rich siltstone with calcite nodules (Fm1), nodular limestones (Ln), massive limestones (Lm), and massive limestone with scattered carbonate clasts (Lmc), with occasional intercalations of massive siltstone–mudstone throughout (Fm2), forming an interval with minor variations in thickness. The upper part is about 15 m thick, consisting of massive sandstone (Sm), horizontally bedded sandstone (Sh), matrix-supported conglomerates (Gmm), clast-supported conglomerates (Gcm), and stromatolitic limestones (Ls1, Ls2, and Ls3). This interval exhibits more complex internal architecture and minor lenticular intercalations. The total succession thickness is thus roughly 26 m. Section AT02 corresponds primarily to the lower part, whereas section AT03 mainly represents the upper part, while AT01 and AT04 preserve both parts in their entirety, allowing a detailed analysis of the vertical arrangement and lateral variations in lithofacies. In the Tifernine area, a single, laterally continuous section was measured (Figure 3) with a total thickness of 40 m, composed of laminar limestone (Ll) and massive siltstone–mudstone (Fm2), showing regularly arranged alternations and consistent thickness along the outcrop. Together, these lithostratigraphic observations provide a comprehensive framework for the detailed facies analysis presented below.

4.1. Facies Analysis

Facies analysis was performed based on textural characteristics, sedimentary structures, and allochemical and orthochemical compositions observed in the field and examined in the laboratory. Following the frameworks of Miall and Walker [58,59,60,61], thirteen distinct facies were identified and grouped into three main categories. The clastic (detrital) facies comprises conglomerates, sandstones, and muddy siltstones, representing the detrital components of the succession. The carbonate facies include massive, laminar, nodular, and stromatolitic limestones, distinguished by sedimentary structures and forming intervals with variable resistance to erosion. The facies were classified according to the approach of Arena and Larena [62,63]. Mixed facies consist of nodular carbonates containing silty grains and limestones with quartz grains and pebbles, representing transitional facies between detrital and carbonate groups. In addition, a volcanic facies group was recognized, which varies spatially. In the Amane-n’Tourhart area, dacitic andesite underlies fluviolacustrine deposits capped by andesitic breccia, whereas in the Tifernine area, microlitic andesite underlies the carbonate succession, and rhyolite occurs at its top [64,65,66].

4.1.1. Siliciclastic Facies

Siliciclastic facies have been described and interpreted from the coarsest (conglomerates) to the finest (silty mudstones). In the following description, the letters are used as follows based on the classification scheme of Miall [58] and Walker and Cant [61]: G (gravel) for conglomeratic facies, S (sand) for sandstones, and F (fine) for siltstones and mudstones.

• F1: Matrix-supported conglomerates (Gmm)

Description: This facies is absent in the Tifernine area but occurs at Amane-n’Tourhart (Figure 1d and Figure 2), where it forms continuous-to-lenticular layers and is represented by two distinct types. The first type (Gmm1; Figure 2 and Figure 4c,d), about 25 cm thick, is interpreted as a volcano-sedimentary deposit (peperite). It is composed of sub-rounded to poorly sorted volcanic (andesitic) fragments dispersed within a reddish-to-violet sandstone matrix, with mud cracks visible at the top of the bed (Figure 4e). Microscopically, it consists of large pyroclastic andesitic clasts embedded in a sandy matrix composed of quartz and plagioclase grains, showing no sharp contrast between the clasts and the matrix (Figure 4g). The second type (Gmm2; Figure 2 and Figure 4b), approximately 30 cm thick, displays a channel-shaped erosive base and contains a sandstone–limestone matrix with sandstone and siltstone clasts. These clasts are oriented and imbricated towards the southwest (NE–SW orientation), ranging from sub-rounded to angular and varying in size from millimeters to centimeters.

Interpretation: Facies Gmm1 is interpreted as peperitic deposits formed by the interaction between hot, andesitic magma and unconsolidated, water-saturated sediments. Mingling of fresh volcanic fragments with the sedimentary matrix, together with the absence of sharp boundaries, indicates a syn-eruptive magma–sediment interaction typical of shallow subaqueous or marginal lacustrine environments [67,68,69]. The development of mud cracks at the top of the bed suggests subaerial exposure subsequent to emplacement. In contrast, facies Gmm2 represents a channelized deposit generated by tractional currents within a confined fluvial setting [59,65,66]. The erosive base, internal cross-bedding, and upward decrease in grain size indicate deposition during the migration of three-dimensional dunes or longitudinal bars under waning flow conditions.

• F2: Clast-supported conglomerate (Gcm)

Description: This facies is characterized by a high proportion of clasts relative to the matrix, with the clasts embedded within the matrix. The matrix composition varies: at Amane-n’Tourhart, it consists of sandstone (Figure 4c,d), while in Tifernine, it is limestone (Figure 4f). The facies form tabular or lenticular bodies with bases that are either gullied or flat, with thicknesses ranging from 0.25 m (Amane-n’Tourhart) (Figure 2) to 0.2 m (Tifernine) (Figure 3), and lateral extension on a meter scale. The clasts are sub-rounded, with sizes ranging from millimeters to centimeters. The clast compositions differ depending on the matrix. In the Amane-n’Tourhart area, they are volcanic (andesitic) and fresh, interpreted as peperite (volcano-sedimentary rock) (Figure 4c,d). In Tifernine, the clasts are carbonate, primarily limestone (Figure 4f).

Interpretation: Facies Gcm reflects contrasting depositional processes between the two areas. At Amane-n’Tourhart, it is interpreted as a peperitic facies formed through direct interaction between andesitic lava and unconsolidated, water-saturated sediments, producing in situ fragmentation and the mingling of magma and sediment. This interpretation is supported by the presence of fresh volcanic clasts embedded with a detrital matrix, which is typical of magma–sediment mingling in shallow, subaqueous environments [67,68,69]. In contrast, the Gcm facies in the Tifernine area, characterized by carbonate clasts in a calcareous matrix, is interpreted as a product of short-lived, high-energy sedimentary reworking along a shallow lacustrine margin, resulting in local erosion and redeposition of carbonate beds [65,70,71,72]. Overall, this facies records syn-eruptive peperitic processes at Amane-n’Tourhart and reworking of carbonate material at Tifernine, illustrating the interplay between volcanic activity and sedimentation within the basin.

• F3: Cross-stratified conglomerate (Gt)

Description: Positioned above Gmm (the second type) (Figure 2 and Figure 4a), this facies is 0.6 m thick and features coarse-grained clasts at the base, which become more abundant towards the top but decrease in size. The clasts range from centimeters to millimeters in size. This facies is distinguished by a scoured base and trough cross-bedding, and it has a carbonate matrix. Microscopically, this facies is composed of sub-angular grains of quartz, plagioclase, calcite, sandstone, and andesite set in a micritic to microsparitic matrix (Figure 4h).

Interpretation: Facies Gt [66] is interpreted as a minor channel-fill deposit within a fluvial system, formed under high-energy conditions in the deepest section of the channel. The development of scoured base and trough cross-bedding indicates deposition from concentrated, high-velocity flows capable of transporting coarse-grained clasts, with the upward fining reflecting waning flow energy during deposition [58,73,74]. The mixed composition of clasts suggests derivation from multiple upstream sources, including both volcanic and detrital materials, while the micritic to microsparitic matrix points to rapid deposition with limited reworking.

• F4: Massive sandstone (Sm)

Description: This facies forms lenticular beds 0.2–2 m thick, with a non-erosive base and lateral extent ranging from decimeters to meters (Figure 2 and Figure 5). It comprises massive sandstones exhibiting normal grading from coarse-grained sandstone at the base to fine-grained sandstone and massive siltstone and mudstone at the top, sometimes silicified, particularly at AT03. Coarse-grained sandstones are dominated by quartz (~65%) and plagioclase (~25%) associated with altered andesite fragments and opaque minerals such as hematite and magnetite, cemented by micritic calcite or silica. Fine-grained sandstones display a similar mineralogical composition, but with higher quartz contents (~70%) and comparable plagioclase proportions (~25%). However, they contain a higher proportion of matrix, resulting in poorer sorting. Plagioclase grains commonly show partial alteration to secondary minerals. In the Tifernine area, above laminated limestone (Ll), there is a friable, massive arkosic sandstone (Sm) composed predominantly of plagioclase (~98%), with minor quartz and andesitic lithic fragments set in a sparse clay and iron-oxide matrix (Figure 4 and Figure 7c).

Interpretation: The massive sandstone facies (Sm) is interpreted as deposits from channels and/or lateral accretion, formed by rapid accumulation from bedload and/or suspended load transported by high-energy flows during flood events [64,65,75,76,77]. Normal grading from coarse- to fine-grained material and the absence of well-developed sedimentary structures suggest an episodic high sedimentation rate under a shallow, high-energy flow regime, where suspended solids settled rapidly [78,79]. This facies reflects episodic, high-energy fluvial processes capable of transporting both coarse- and fine-grained detrital material, including volcanic fragments and altered plagioclase grains, within confined channels or along lateral bedforms. Its occurrence above carbonate and fine-grained deposits indicates a shift toward higher-energy flow conditions and enhanced sediment supply within the depositional system.

• F5: Horizontally bedded sandstone (Sh)

Description: This facies is absent in the Tifernine area, but occurs in Amane-n’Tourhart (Figure 2) as horizontally stratified, lenticular to tabular beds of medium- to coarse-grained reddish sandstones. Laminae are at the millimeter scale, flat to slightly wavy, parallel, and generally horizontal, with flow directions varying locally (e.g., N10 in AT01, N160–N35 in AT03). The facies commonly grades to stromatolitic (Ls1) or conglomeratic (Gmm, first-type peperite) facies. Microscopically, lighter laminae are rich in detrital grains dominated by quartz (~65%) and plagioclase (~20%), with subordinate altered andesite fragments, and they display minimal cement or matrix. In contrast, darker laminae are cement- and matrix-rich, composed mainly of ferruginous carbonate micrite or clay. Opaque minerals, mainly iron oxides, are ubiquitous, and microbial mat textures are well- preserved (Figure 5d,f).

Interpretation: The horizontally laminated sandstones (Sh) [66] are interpreted as traction-dominated deposits formed under upper-flow regime conditions in shallow channels or on flood plains during episodic high-energy flows. The planar laminae reflect laminar to minimally turbulent transport of medium- to coarse-grained sediment, while alternating grain-rich and matrix-rich laminae indicate fluctuations in flow competence and sediment supply [80].

• F6: Massive siltstone–mudstone (Fm2)

Description: This facies has a non-erosive, flat base and consists of friable, mud-rich siltstones that are highly susceptible to erosion. It ranges in thickness from 0.15 to 4 m (Figure 2 and Figure 5e). It underlies and is associated with the massive sandstone facies, but always occurs below the muddy siltstone facies with limestone nodules (Fm1) (Figure 2 and Figure 5e).

Interpretation: Deposition of this facies, characterized by a non-erosive base and representing the upper part of normally graded fluvial deposits, indicates the accumulation of fine-grained sedimentary material transported in suspension by muddy flows during flood events [64,65,81,82].

4.1.2. Carbonate Facies

• F7: Massive limestone (Lm)

Description: In the Amane-n’Tourhart area (Figure 2), this facies is lenticular to tabular with an erosive base, up to 1 m thick, and is whitish to yellowish in color. It is always above the Ln facies (nodular limestone) or facies Fm1 (muddy siltstone with limestone nodules). At the base, carbonate material becomes more abundant than detrital material, nodules progressively evolve from solitary to coalescing, and the detrital material disappears in the case of muddy siltstone. In places, especially toward the top, stromatolitic structures are apparent (Figure 6d). Limestone consists of either grains, forming grainstone, or stromatolites, forming bindstone. Thin-section analysis of this facies reveals that it is composed predominantly of sparite and, locally, microsparite, and lacks micrite (Figure 10d). Large spherulitic structures are notably observed, ranging from 5 mm to 1 cm in diameter. Calcite is partially replaced by silica (Figure 10d). This facies is absent in the Tifernine area.

Interpretation: The upward transition from mud-rich siltstone with carbonate nodules (Fm1) through nodular limestones (Ln) to massive limestones (Lm) reflects a gradual increase in carbonate deposition and a progressive reduction in detrital input. This stratigraphic trend records a shift from a shallow-water, fluvially influenced depositional setting to a more stable, low-energy lacustrine environment dominated by chemical sedimentation. Across these facies (Fm1–Ln–Lm), carbonate spheroids are commonly observed. Spherulite growth is favored by high levels of Mg and silica in highly alkaline solutions, resulting in rapid carbonate crystal growth, with or without any microbial influence [83,84,85]. These spheroidal or spherulitic features are interpreted as chemical precipitates formed in supersaturated, quiescent lacustrine waters, reflecting episodes of limited clastic supply and increased carbonate saturation [86,87,88]. Their development throughout the transition indicates progressive stabilization of the depositional system and enhanced chemical precipitation in the basin under semi-closed hydrological conditions [89].

• F8: Nodular limestone (Ln)

Description: This facies occurs only in the Amane-n’Tourhart area. It is lenticular to tabular in shape, has an erosional base, and ranges from 0.25 to 1.25 m in thickness (Figure 2). It forms the transition zone between the Fm1 facies (muddy siltstone with carbonate nodules) and the massive limestone facies (Lm); its color is variegated with white nodules and red siltstone (Figure 6a–c). Further, it is calcareous when carbonate nodules are abundant (as is the case for AT02) (Figure 2 and Figure 6c). This facies includes rudstone, formed during periods with high-energy water conditions. The nodules are commonly irregular to spheroidal in shape and display concentric and radial internal microfabrics. Under the microscope, this facies displays two distinct components: red, detrital material-rich and white, micritic laminae. White-to-yellow nodular parts consist of micritic-to-microsparitic calcite, with some contribution of detrital components (Figure 6b), showing spherulitic (Figure 6c) or microstromatolitic textures. Red, siltstone laminae comprise quartz and altered plagioclase grains within a micritic matrix.

Interpretation: Presence of both stromatolitic and micritic-to-microsparitic nodules suggests that carbonate precipitation occurred through a combination of physicochemical and microbial processes [89,90]. Such stromatolitic–nodular associations correspond to the “mobile buildups” described previously [91], which develop in very shallow lacustrine-to-palustrine settings where carbonate forms within soft, unlithified sediments. These buildups remain unstable and may be displaced, deformed, or partially reworked by low-energy currents or water-level fluctuations, resulting in irregular stromatolitic lamination and nodular carbonate structures. In these settings, carbonate precipitation was mainly driven by CO₂ degassing, evaporation, and increased alkalinity of pore waters, while microbial films acted as nucleation centers for localized carbonate precipitation [92]. Presence of detrital grains indicates siliciclastic input from an adjacent floodplain, which is consistent with deposition along the margin of a shallow, low-energy lake [87]. Overall, the Ln facies represents the onset of lacustrine carbonate sedimentation, marking the transition from fluvial sedimentation (Fm1) to chemical, carbonate precipitation (Lm) [20]. It reflects a shallow lacustrine–palustrine environment characterized by fluctuating hydrological conditions, periodic exposure, and early diagenetic cementation leading to the development of nodular textures [93,94]. This facies thus records the progressive establishment of lacustrine conditions in a continental depositional system.

• F9: Laminar Limestone (Ll)

Description: This facies is typical for the Tifernine area, occurring in tabular-to-lenticular beds, is 0.5–4 m thick (Figure 3), composed of horizontal to locally inclined pelitic laminae at centimeter- to millimeter-scale, and has occasional sandstone beds. The facies displays mud cracks, micro-ripples, cross-bedding, and E-W-oriented ripple marks (N110) (Figure 7). Some beds show soft-sediment deformation and are brecciated, forming the Lli facies. Microscopically, basal beds show micritic and microsparitic laminae of calcite, sandy laminae with quartz, plagioclase, and andesitic fragments in a ferruginous matrix, and pelitic seams with replaced extracellular polymeric substances (EPSs), microbially induced sedimentary structures (MISSs), and iron oxides (Figure 8). In the middle part, sandy laminae and pelitic seams are rare, while micritic and microsparitic calcite and micrometer-scale replaced EPS structures are more pronounced. The upper part resembles the basal beds, with microsparite and the reappearance of thin, sandy laminae or sandstone beds containing altered quartz and plagioclase.

Interpretation: Grainstones of this facies, along with mud cracks, ripple marks, laminae, and pelitic seams, indicate deposition in a shallow, brackish lacustrine environment characterized by fluctuating hydrological conditions, episodic high-energy events, and periodic emersion [63,92,95]. The laminae contain microbial structures, such as tufted microbial mats and MISS, along with iron oxides and small, replaced, rounded-to-elongated EPS [96,97,98,99,100,101,102]. These features suggest that microbial activity played a significant role in sediment stabilization, promoting localized carbonate precipitation and contributing to the formation of laminated, organic-rich beds with associated EPS. The coexistence of detrital grains with micritic carbonate reflects a mixed origin, where carbonate accumulation could have been controlled by both physicochemical processes, such as CO₂ degassing, weathering, organic matter remineralization, evaporation, and increased alkalinity of interstitial waters, and by microbial mediation at low-energy, intermittently exposed lacustrine margin [89,90,92]. Overall, this facies records a dynamic, shallow-lacustrine system characterized by episodic flooding, periodic exposure, microbial colonization, and early diagenetic cementation, reflecting progressive establishment of lacustrine conditions within a continental depositional framework.

4.1.3. Mixed Facies

• F10: Massive siltstone–mudstone with carbonate nodules (Fm1)

Description: This facies is characterized by a tabular and sometimes laterally discontinuous (lenticular) shape, with an erosive base, and varies in thickness from 30 cm to 3.5 m (Figure 2 and Figure 6a–f). This facies always underlies the Ln facies (nodular limestone), and it underlies/is associated with the Lmc facies (massive limestone with scattered clasts). It is composed of red and purple, clay-rich siltstone with white carbonate nodules (Figure 6c). These nodules are solitary and frequently spherulitic. Microscopically, this facies resembles the nodular limestone facies, displaying two distinct components. The reddish, clay-rich siltstone component contains fine-grained quartz and plagioclase within micritic cement (Figure 6a). The nodular component is less abundant in this facies (Fm1) compared to the Ln facies. The nodules consist of microsparitic-to-sparitic carbonate and contain spherulitic structures that are smaller in size than those observed in Ln and Lm facies (Figure 6a).

Interpretation: Fm1 represents deposition in shallow, low-to-moderate-energy fluvial channels, where episodic, ephemeral flows transported mud and silt and allowed localized carbonate precipitation along intermittently exposed margins [58,62]. Evaporation and elevated alkalinity during early diagenesis promoted the formation of small spherulitic carbonate nodules, while input of fine-grained quartz and plagioclase reflects proximal siliciclastic sources [89,90,92]. This facies marks the initial phase of carbonate precipitation in a fluvial–lacustrine transitional setting, preceding more extensive nodular and massive carbonate deposition.

• F11: Massive limestone with scattered clasts (Lmc)

Description: This red-to-purple, pebbly massive limestone facies occurs as lenticular beds 0.2 to 0.35 m thick and extends laterally for meters (Figure 2 and Figure 6e,f). Generally, it is massive with poorly defined laminae (sometimes planar), and it contains clasts. Microscopic observations reveal that the facies consists of grainstone, with carbonate crystals set within a micritic and/or microsparitic matrix, containing abundant quartz and altered plagioclase grains. Iron oxide minerals and microbial mats are abundant throughout this facies (Figure 6f).

Interpretation: This clastic limestone facies, characterized by basal laminations and a notable presence of detrital silt-sized quartz, was deposited in a relatively deep and tranquil lacustrine environment. The grainstone textures and the abundance of detrital quartz indicate transport and deposition under non-negligible hydrodynamic energy, as such grains require traction or suspension processes capable of overcoming settling in low-energy conditions. Gradual rise in the lithoclast content towards the top of the facies suggests an increase in depositional energy, likely due to either a decrease in water level transporting clastic material to the deeper parts of the lake [63,71,87].

• F12: Stromatolitic limestone (Ls)

Description: Stromatolitic beds are abundant in the Amane-n’Tourhart area, but absent in Tifernine, occurring mainly at the top of the lacustrine succession (Figure 2). Following Freytet and Verrecchia [91], three types can be distinguished based on morphology. Ls1, the inclined columnar stromatolites (Figure 9a–d), form thick bioherms up to 5 m, with small-to-medium-sized domal columns, oriented NW–SE to E–W parallel to paleocurrents (Figure 14a). Laminae alternate between red, detrital, and white, carbonate components. This facies is sometimes interlayered with Lm or Fm1 facies and could be partially silicified. Ls2, domal stromatolites, develop on the Ls1 facies, forming vertical, attached, or isolated domes up to 2 m in size (Figure 9e–g), with laminae alternating between detrital and carbonate components, frequently replaced by silica. Ls3, stratiform stromatolites, represent the uppermost stromatolitic deposits before stromatolite disappearance, forming planar laminae or small isolated domes up to 30 cm in size (Figure 9h), with alternations of red, detrital, and pale, calcareous or siliceous components. Laminations can be either continuous or discontinuous, locally displaying soft-sediment deformation (Figure 9i). Microscopically, red laminae are composed of quartz grains embedded in a ferruginous cement, whereas white laminae consist of micritic-to-sparitic carbonate occasionally containing spherulites and synsedimentary micro-deformation structures within beds (Figure 10a,b). Partial preservation of EPS and ferruginous, irregularly laminated microbial mats (Figure 10c,d) suggests microbial mediation during early diagenesis, while locally developed silicification (Figure 10b) suggests early diagenetic replacement under silica-rich pore-water conditions.

Interpretation: The stromatolite facies Ls1–Ls3 represent a progressive evolution in microbial carbonate accumulation in shallow-water, lacustrine environments with variable hydrodynamics. The Ls1 facies (inclined columnar stromatolites) developed under fast-flowing, turbulent conditions, associated with river banks, weirs, small rapids, or minor waterfalls, where microbial mats, including EPS, grew on sloped substrates and were impacted by paleocurrents [89]. Alternating red, detrital, and white, carbonate laminae reflect episodic siliciclastic input and carbonate precipitation. The Ls2 facies domal stromatolites formed in a slower-flowing, laminar zone, such as a protected riverbed area or a palustrine setting, often on a microtopographic high. These facies laminae are frequently affected by silicification with preserved EPS, indicating microbial mediation [62,87]. The Ls3 facies (stratiform stromatolites), the stratigraphically highest microbial carbonate deposits, occur in thin beds (≤30 cm thick) as planar laminae or small domes, composed of red, detrital, and white, marly or siliceous components with rare, carbonate-poor components. Soft-sediment deformation records episodic lake-bottom disturbance. Taken together, these facies indicate the onset, development, and eventual decline of stromatolitic growth under fluctuating hydrodynamic, sedimentary, and geochemical conditions, marking the transition from active microbial carbonate precipitation to non-carbonate, lacustrine deposition [90,92].

4.1.4. Volcanic Facies

In the Amane-n’Tourhart area, fluviolacustrine deposits lie unconformably on an andesitic–dacitic substratum (Figure 11a). Brecciated andesites (Figure 11b) occur at the top and contain lithic clasts within an andesitic lava. These andesites also exhibit Neptunian dikes infilled with sediments younger than the lava, occupying open fractures. A dike emplaced after the basin development cuts across both the sedimentary and volcanic units (Figure 11c). Both types of lava (andesites and breccia andesites) are associated with the post-caldera stage and represent the youngest volcanic units in the Oued Dar’a Caldera (ODC) succession [40]. Porphyritic andesites at the base of the formation contain crystals of plagioclase and pyroxene, both of which are altered and embedded within a microlitic groundmass (Figure 11a). Brecciated andesites at the top of the formation are characterized by phenocrysts of pyroxene (clinopyroxene and orthopyroxene) and altered plagioclase, together with plagioclase microlites and quartz clasts (Figure 12b).

In the Tifernine area, carbonate deposits are underlain by microlitic andesites and overlain by brecciated rhyolites (Figure 11d). Microscopically, andesites at the base of the carbonate formation are altered and display a microlitic texture with a few rare plagioclase phenocrysts smaller than 1 mm (Figure 12c). In contrast, the rhyolite at the top of the formation shows a porphyritic texture characterized by strongly altered plagioclase phenocrysts and quartz crystals, with plagioclase microlites embedded in a glassy matrix (Figure 12d).

Figure 11. Field photographs showing volcanic facies from the Amane-n’Tourhart and Tifernine areas. (a) Andesitic lava from the Amane-n’Tourhart area; (b) andesitic breccia overlying the fluviolacustrine deposits of the Amane-n’Tourhart area, crosscut by a Neptunian dike; (c) dike cutting the fluviolacustrine unit in the Amane-n’Tourhart area; (d) panoramic view of the Tifernine area showing the Tifernine lacustrine unit underlain by andesites and overlain by rhyolites, with the carbonate unit offset by a dextral fault (And: Andesite; And B: Andesitic breccia; ND: Neptunian dike; D: Dike; C: Carbonate; Rh: Rhyolite; F: Fault).

Figure 11. Field photographs showing volcanic facies from the Amane-n’Tourhart and Tifernine areas. (a) Andesitic lava from the Amane-n’Tourhart area; (b) andesitic breccia overlying the fluviolacustrine deposits of the Amane-n’Tourhart area, crosscut by a Neptunian dike; (c) dike cutting the fluviolacustrine unit in the Amane-n’Tourhart area; (d) panoramic view of the Tifernine area showing the Tifernine lacustrine unit underlain by andesites and overlain by rhyolites, with the carbonate unit offset by a dextral fault (And: Andesite; And B: Andesitic breccia; ND: Neptunian dike; D: Dike; C: Carbonate; Rh: Rhyolite; F: Fault).

Figure 12. Photomicrographs of volcanic facies. (a) Porphyritic andesite in the Amane-n’Tourhart area; (b) brecciated andesite in the Amane-n’Tourhart area; (c) microlitic andesite in the Tifernine area; (d) rhyolite in the Tifernine area (Qz: Quartz; Plg: Plagioclase; CPx: Clinopyroxene; OPx: Orthopyroxene; IO: Iron oxide).

Figure 12. Photomicrographs of volcanic facies. (a) Porphyritic andesite in the Amane-n’Tourhart area; (b) brecciated andesite in the Amane-n’Tourhart area; (c) microlitic andesite in the Tifernine area; (d) rhyolite in the Tifernine area (Qz: Quartz; Plg: Plagioclase; CPx: Clinopyroxene; OPx: Orthopyroxene; IO: Iron oxide).

4.1.5. Biogenic Features of the Stromatolitic Limestone (Ls) Facies in Amane-n’Tourhart and the Laminar Limestone (Ll) Facies in Tifernine

Biological processes played a fundamental role in the development of the laminated structures. Scanning Electron Microscope (SEM) observations reveal three populations of micrometric microscopic structures distinguished by recurring morphologies, size ranges, spatial distribution, and consistent association with stromatolitic lamination (Figure 13). Their repeated occurrence, organization parallel to laminae, and close relationship with micritic carbonate precipitates support a biogenic origin according to widely accepted criteria for Precambrian biosignatures [89,90,101]. On this basis, these structures are interpreted as fossilized microbial remains rather than abiotic precipitates.

The observed components include calcified micritic aggregates, calcified filamentous organic structures, and micrometric spheroidal-to-tubular bodies interpreted as bacterial-like microbial remains. These occur exclusively within laminated facies, notably, stromatolites (Ls) and laminated limestones (Ll) (Figure 13). Calcified micritic aggregates occur as irregular-to-elongated or locally subcircular forms, approximately 300 μm in length and 10–15 μm in width. Calcified filamentous structures are elongated elements displaying tubular, irregular morphologies with rough terminations (Figure 13c–h). These filaments are locally curved and occasionally tapered. Their walls, measuring a few micrometers in thickness and up to 5–10 μm in diameter, are composed of calcite (Figure 13f), consistent with early mineralization of organic templates [103]. These filamentous structures are preferentially preserved within intergranular and fenestral spaces of the lithified sediments, where early cementation favored their preservation. Their morphology, size range, and spatial organization are comparable to filamentous microbial remains reported from ancient lacustrine and peritidal carbonates, and they are therefore interpreted as fossilized microbial filaments, without implying precise taxonomic affinity [20,104].

Micrometric microscopic plate-like to tubular substances, interpreted as fossilized extracellular polymeric substance (EPS)-coated organic filaments, occur either as isolated structures (Figure 13e,f–i) or in clusters of filamentous structures (Figure 13c,d) [65,66]. These features range from ~20 to 60 μm and are consistently attached to detrital grains or calcite crystals, reflecting their role in trapping, binding, and mineral nucleation during early diagenesis. In ancient systems, such EPS-like fabrics are widely regarded as robust indicators of microbial activity, even in the absence of molecular biomarkers [90,101,105].

Although direct identification of specific microbial taxa is not possible, the convergence of morphological, sedimentological, and petrographic evidence strongly supports the presence of an active microbial consortium influencing carbonate precipitation and resulting in laminations developed during deposition. This interpretation is further supported by lipid biomarker evidence from the Amane-n’Tourhart stromatolites reported by Carrizo et al. [27], which documents the presence of bacteria-derived compounds, confirming active microbial communities during carbonate deposition. In addition, the microbial textures and carbonate–bacteria interactions identified here closely resemble those described by Chraiki et al. [24] in coeval Ediacaran carbonate systems. Together, these independent petrographic and geochemical datasets provide convergent evidence for a significant bacterial contribution to carbonate precipitation and lamination development in these Ediacaran lacustrine environments.

Figure 13. Scanning Electron Microscope images showing the biogenic features in stromatolitic limestones (Ls1) and (Ls2) and laminar limestone (Ll). (a,b) Elongated calcified structures in Ls1 and Ls2; (c,d) calcified organic filaments and EPS in Ls1; (e,f) EPS attached to detrital grains in Ls1 and Ls2; (g) elongated calcified structure in Ll; (h) calcified organic filaments in Ll; (i) EPS attached to calcite crystals in Ll (Qz: Quartz; Cl: Calcite; EPS: Extracellular polymeric substances).

Figure 13. Scanning Electron Microscope images showing the biogenic features in stromatolitic limestones (Ls1) and (Ls2) and laminar limestone (Ll). (a,b) Elongated calcified structures in Ls1 and Ls2; (c,d) calcified organic filaments and EPS in Ls1; (e,f) EPS attached to detrital grains in Ls1 and Ls2; (g) elongated calcified structure in Ll; (h) calcified organic filaments in Ll; (i) EPS attached to calcite crystals in Ll (Qz: Quartz; Cl: Calcite; EPS: Extracellular polymeric substances).

4.2. Facies Associations

In the study areas, the sedimentary facies are within successions stratigraphically organized into facies associations, which record sedimentary processes that occurred within their depositional environments. These facies associations reflect the vertical and lateral arrangement of deposits formed in depositional environments. The stratigraphic arrangement was controlled by cyclicity driven by water-level fluctuations, channel migration, lake filling, progradation and retrogradation of alluvial and fluvial systems, and the expansion or shrinkage of the lacustrine system.

• Amane-n’Tourhart area

  (a)

  Alluvial fan facies association

This facies association is made up of a succession of conglomerates (Gcm, Gmm, and Gt), sandstones (Sm and Sh), and clay-bearing siltstones (Fm1 and Fm2). It reflects periods of incision of river channels, followed by phases of filling during floods with high sediment and water flow, and eventually periods of decreased water energy in the aftermath of floods [80,106,107]. This facies association is characteristic of alluvial fan deposits. Dominated by coarse, reddish-to-purple clastic material, it represents a braided-type fluvial system likely located in the proximal and medial parts of the alluvial fans. The conglomerates and sandstones correspond to debris-dominated flows deposited in channels and bars [58,108]. Abandonment or migration of the channels allowed deposition of massive siltstone–mudstone beds with calcareous nodules, corresponding to a low-energy floodplain environment.

• (b)

  Lake facies association

This facies association records a progressive transition from fluvial to lacustrine environments within the studied basin. It begins with clay-bearing siltstones containing calcite nodules (Fm1), which indicate deposition under low-energy fluvial conditions dominated by fine-grained sediment settling in overbank or shallow floodplain settings associated with ephemeral channels [106]. The carbonate nodules are interpreted as early diagenetic features, formed within the sediment through localized carbonate precipitation during periods of reduced sedimentation and enhanced pore-water saturation, rather than as primary lacustrine carbonate deposits. Their development reflects intermittent water ponding and fluctuating hydrological conditions, which promoted carbonate supersaturation in pore waters and nodule growth during early burial. Although Chraiki et al. [24] originally interpreted Fm1, Ln, and Lm as thrombolitic facies, carbonates of these facies do not represent thrombolites, but are instead interpreted here as early diagenetic nodules.

The Fm1 facies is succeeded by nodular limestones (Ln), marking a transitional zone between fluvial and shallow-water, lacustrine conditions. The nodules, including micritic and microsparitic components, reflect early diagenetic carbonate precipitation in shallow-water, low-energy lake environments, where microbial mats locally contributed to carbonate nucleation. Detrital grains, including quartz and plagioclase, indicate a siliciclastic input from nearby floodplains by wind or current, while the presence of small stromatolites suggests microbial activity under fluctuating hydrological conditions [89,90]. The succession culminates with massive limestones (Lm), representing deposition in a more persistent, deeper-water, lacustrine setting, where carbonate precipitation was largely chemical, under relatively stable water-column conditions [63,109]. The transition from Fm1 through Ln to Lm thus captures a gradual shift from predominantly siliciclastic, fluvial deposition to carbonate-dominated, lacustrine sedimentation, highlighting variations in water depth, hydrology, and sediment supply across the lake basin.

• Tifernine area
  • Lacustrine facies association

The siltstone–mudstone facies (Fm2) with laminar limestone (Ll) represents deposition in the calm, low-energy offshore zone of the lake [87]. Fine-grained sediments settled from suspension under quiet conditions, forming thin, laterally continuous laminae of microsparitic-to-sparitic limestone interbedded with claystone and sandstone laminae in the lower and upper parts of the Tifernine Formation. Within the framework of Carroll and Bohacs [72], such facies indicate a relatively stable, semi-closed lacustrine basin, where sedimentation is controlled by the interplay between accommodation space, water-level stability, and sediment–water flux. The persistence of fine lamination and minimal siliciclastic input suggests a stratified, low-energy water column with limited episodic disturbance, allowing for the preservation of primary sedimentary structures and subtle microbial or early diagenetic features. These characteristics emphasize the influence of basin type and hydrologic balance in shaping offshore carbonate sedimentation and provide insight into the quiet, offshore dynamics of this stage of the lacustrine system.

5. Discussion

5.1. Paleo-Environmental Reconstruction of the Amane-n’Tourhart and Tifernine Sedimentary Successions

The sedimentary evolution of the Amane-n’Tourhart area reflects several distinct phases associated with different sedimentary systems.

(a) 

Phase 1: Alluvial fan system

This phase is marked by a system of alluvial fans developed close to the major W–E fault (Figure 1d). It is characterized by clast-supported conglomerates at the base, and by massive clay-bearing siltstones containing carbonate nodules stratigraphically higher (Figure 15a). Siliciclastic deposits are organized into channels and truncated unstratified and stratified successions with decreasing grain size, reflecting emplacement by active alluvial systems. The imbrication of pebbles and cross-bedding observed in the conglomerates indicate a paleoflow direction oriented NNE–SSW (Figure 14b). The conglomerates were deposited in a channel environment, while the massive clay-rich siltstones with carbonate nodules were formed in calmer, more extensive environments, interpreted as floodplains.

Figure 14. (a) Inclined, columnar stromatolites with NW-SE orientation in station 1 of the AT01 section, WNW-ESE orientation in station 2 of the AT01 section, and W–E orientation in station 3 of the AT04 section; (b) imbrication of pebbles in the conglomerate facies shows a general NNE–SSW orientation in the AT02 section; (c) ripple mark crest directions measured within the Sh facies in the section AT03.

Figure 14. (a) Inclined, columnar stromatolites with NW-SE orientation in station 1 of the AT01 section, WNW-ESE orientation in station 2 of the AT01 section, and W–E orientation in station 3 of the AT04 section; (b) imbrication of pebbles in the conglomerate facies shows a general NNE–SSW orientation in the AT02 section; (c) ripple mark crest directions measured within the Sh facies in the section AT03.

(b) 

Phase 2: Fluviolacustrine system

This phase is marked by a gradual transition from fluvial siliciclastic deposition, represented by clay-rich siltstones with calcite nodules, to carbonate deposition, such as nodular limestones and limestones with scattered clasts (Figure 15b). This stratigraphic trend reflects a gradual decrease in fluvial influence and the emergence of a lacustrine water body, evolving from a relatively agitated to a calmer environment. All these deposits correspond to a fluviolacustrine transitional environment that gradually evolved into a shallow-water, lacustrine system.

(c) 

Phase 3: Shallow-water lacustrine system

This phase corresponds to an expansion of the lacustrine system, marked by deposition of nodular to massive limestones, corresponding to depositional conditions in a relatively calm, moderately deep water column (Figure 15c). The abundant presence of stromatolites suggests local, microbially mediated carbonate deposition. The paucity of siliciclastic material suggests the protected and stable nature of this lacustrine environment.

(d) 

Phase 4: Fluviolacustrine system

This phase is characterized by significant detrital input, combined with absent or very limited carbonate precipitation (Figure 15d). The fluviolacustrine system ripples are predominantly symmetrical to weakly asymmetrical and show a preferred NW–SE crest orientation (Figure 14c) perpendicular to wave oscillation directions. Their morphology indicates low-energy traction currents or wave-influenced flow in shallow water, rather than strongly unidirectional fluvial currents.

Silicified stromatolites, formed when high-temperature, silica-charged geothermal fluids mixed with basin waters, were also developed during this phase. They contain red, detrital laminae, which are far more abundant than the white, carbonate laminae.

These stromatolites reflect a dynamic evolution of the environment:

• In association with strong fluvial currents, small, inclined columnar stromatolites were developed with orientations NW–SE, WNW–ESE, and W–E, suggesting a paleoflow shifting from NW to W (Figure 14a). This alignment is interpreted to be hydrodynamically controlled, reflecting persistent shallow-water flow within the fluviolacustrine system.
• Subsequently, a decrease in hydrodynamic energy led to a calmer environment, conducive to the formation of fully developed, vertical domal stromatolites (Figure 9e–g).

(e) 

Phase 5: Alluvial fan system

This final phase of sedimentary deposition is characterized by the development of an alluvial fan system, represented mainly by conglomerates and, more rarely, sandstones (Figure 15e). It reflects a resumption of coarse-grained, detrital input, marking the terminal phase of the sedimentary system before the return to volcanic activity.

In contrast, in the Tifernine area, sedimentation is dominated by a single depositional system: the lacustrine system, like the phase 3 of the Amane-n’Tourhart area (Figure 15c), is associated with a very shallow-water column. Shallow-water depth is indicated by the presence of clay laminae accompanied by microconglomerates, and mud cracks, along with evidence for repeated emersion and associated water-escape structures. The laminar limestones locally contain thin (millimeter to centimeter) sandstone beds. These laminar limestone beds are interbedded with clay-rich siltstones containing carbonate concretions. Imbrication in microconglomerates indicates a weak W–E paleoflow.

Figure 15. Schematic 3D diagrams illustrating the depositional evolution of the Amane-n’Tourhart sedimentary system: (a) First alluvial fan stage; (b) first fluviolacustrine stage; (c) lacustrine carbonate deposition; (d) second fluviolacustrine stage with stromatolite growth; (e) second alluvial fan stage marked by volcano-sedimentary (peperitic) facies and renewed volcanic activity.

Figure 15. Schematic 3D diagrams illustrating the depositional evolution of the Amane-n’Tourhart sedimentary system: (a) First alluvial fan stage; (b) first fluviolacustrine stage; (c) lacustrine carbonate deposition; (d) second fluviolacustrine stage with stromatolite growth; (e) second alluvial fan stage marked by volcano-sedimentary (peperitic) facies and renewed volcanic activity.

Factors that controlled fluviolacustrine deposition include changes in supplied sediment composition, reflecting changes in the depositional environment or sediment inputs, which can be linked to external controlling factors. Sedimentation of the fluviolacustrine deposits at the Amane-n’Tourhart area and lacustrine deposits at the Tifernine area was further influenced by several key factors, such as climate and volcanic activity.

5.2. Sedimentary Deposits in the Amane-n’Tourhart and Tifernine Areas

The geodynamic setting of the two study areas is linked to a volcanic caldera in which the Ouarzazate Group was deposited. In the Amane-n’Tourhart area, sedimentary deposits vary according to the type of sedimentary system. Alluvial fan deposits are dominated by conglomerates, including locally derived andesitic boulders and imbricated sandstone pebbles with a more distal source. Fluviolacustrine deposits consist mainly of quartz, plagioclase, and occasional andesite fragments derived from underlying volcanic units [17,19,54]. Carbonate intervals formed in relatively low-relief, lacustrine settings, where gentle topography allowed water to pond and remain stagnant long enough for chemical precipitation to occur. In these flat areas, detrital influx was limited, preventing dilution of carbonate minerals, while warm climatic conditions enhanced chemical weathering of the surrounding volcanic rocks, supplying the necessary ions for carbonate formation and bicarbonate. Carbonate sediments subsequently underwent early-to-late-stage silicification, with silica precipitating from geothermal fluids both at the sediment–water interface and in fractures and pores [22]. Subsurface, high-temperature weathering of volcanics likely supplied silica.

In contrast, the Tifernine area is largely characterized by carbonates chemically deposited in a lacustrine setting, with Ca²⁺ derived from the surface weathering of underlying volcanic rocks [110,111]. Detrital input was minimal, reflecting an environment dominated by chemical weathering rather than siliciclastic influx. Álvaro et al. [20] inferred that the Tifernine succession is older than the Amane-n’Tourhart succession, based on lithostratigraphic correlation; however, U–Pb geochronology by Oukhro et al. [40] indicates that the Amane-n’Tourhart succession is slightly younger, ca. 561–564 Ma, with the Tifernine succession being older than ca. 566 Ma, which is consistent with the Bou Azzer regional mapping [57].

5.3. Climatic Control on Fluviolacustrine Deposition

Sedimentary systems in the Amane-n’Tourhart and Tifernine areas have been strongly influenced by paleoclimatic conditions, which imposed a strong control on the nature of deposits and sedimentary dynamics, as well as weathering and precipitation. For example, the alluvial fan deposits suggest arid-to-semi-arid climatic conditions, favorable for the development of ephemeral water courses. This climatic framework, characterized by episodic torrential rainfall, favored physical weathering resulting in abundant production of coarse debris and torrential floods [106]. The predominance of siltstone–mudstone facies reflects mudflows.

Fluviolacustrine systems suggest alternating humidity and aridity, especially suggesting alternation between terrigenous and chemical or biochemical sedimentation. Carbonate intervals (nodular and massive limestones) were formed under a hot and wet climate, conducive to alteration of volcanic rocks and the enrichment of runoff water in ions such as calcium and bicarbonate, promoting precipitation of calcium carbonate in the basin, particularly in the Amane-n’Tourhart area [110,111]. Conversely, periods marked by a cessation or reduction in carbonate precipitation coincided with more arid climatic conditions. These periods are reflected sedimentologically in an increase in detrital input, as well as by the establishment of alluvial fans. These deposits suggest an episodic intensification of erosion in watersheds, linked to torrential rainstorms [64,66,106,112].

The Tifernine area points to the effects of a hot, wet, and then dry climate. Laminar limestone deposits indicate an initial period of a hot and wet climate that favored alteration of caldera and post-caldera volcanic rocks and release of ions to runoff water. These waters, bearing high bicarbonate content, evolved through the reaction of CO₂ with volcanics, leading to the precipitation of carbonates in the basin. The overlying sedimentary record indicates a progressive lowering of the water table, which is consistent with sedimentary structures such as mud cracks, pellets, and MISS. These structures reflect significant evaporation and insufficient runoff to compensate for water loss, indicating a transition to a drier climate.

5.4. Volcanic Control on Fluviolacustrine Deposition

In the context of the Amane-n’Tourhart and Tifernine sedimentary successions, volcanic activity elsewhere in the caldera provided volcaniclastic material that fed alluvial fans and channels, changing topography and the way sediment was delivered to lakes. Between eruptions, the depositional system switched to fluviolacustrine processes, allowing fine carbonate and microbial facies to develop. This interplay can explain the transition from conglomerates and sandstones deposited in braided alluvial fans to carbonate muds and stromatolitic limestones deposited in a lacustrine setting. Volcanic pulses drove the supply of coarse-grained material, resulting in the development of alluvial fans, while quiescent periods enabled lake expansion and deposition of carbonates and microbial activity. Therefore, volcanism in these settings was not a second-order factor, but an integral forcing mechanism intertwined with tectonics and climate. It dictated sediment supply, basin morphodynamics, hydrology, chemistry, and facies evolution. Recognizing its role enables a more nuanced interpretation of stratigraphic architecture. Volcanic eruptions in the caldera were contemporary with deposition of fluviolacustrine deposits in the two areas studied [21,40]. This volcanism could have enriched surface waters with dissolved inorganic carbon of magmatic origin. In addition, alteration of volcanic minerals released Ca²⁺, and other cations like Mg²⁺ and Na⁺, contributing to carbonate precipitation in lacustrine environments [110,111]. Volcanism also stopped stromatolite growth. Following deposition of the stromatolitic facies Ls1, Ls2, and Ls3, magma (Gmm1 and Gmc), containing still-hot, fresh andesitic fragments, was injected into and flowed over unlithified sediments, curtailing microbial ecosystem in the Amane-n’Tourhart area. This event marks a renewed volcanism, followed by an emplacement of andesitic breccia flows.

6. Conclusions

This study presents a detailed lithostratigraphic, sedimentological, and petrographic analysis of Ediacaran fluviolacustrine successions from the Amane-n’Tourhart and Tifernine basins in the Anti-Atlas of Morocco. Thirteen sedimentary facies grouped into four depositional assemblages (siliciclastic, carbonate, mixed siliciclastic–carbonate, and volcanic) define three main facies associations: alluvial, fluviolacustrine, and lacustrine, recording a wide range of continental depositional environments from proximal fluvial domains to fully lacustrine settings.

Facies stacking patterns reveal an overall upward transition from coarse-grained, high-energy alluvial deposits to fine-grained lacustrine sediments, reflecting increasing accommodation space, decreasing depositional energy, and progressive basin infilling. In the Amane-n’Tourhart Basin, this evolution is marked by a vertical succession from coarse siliciclastic deposits to fluviolacustrine mixed siliciclastic–carbonate facies, and finally to laminated and stromatolitic lacustrine carbonates, documenting the establishment of persistent lacustrine conditions with reduced clastic input and enhanced chemically and microbially mediated carbonate precipitation. In contrast, the Tifernine Basin preserves a shallow lacustrine system, characterized by fine-grained laminated carbonates, episodic siliciclastic influx, and frequent subaerial exposure, indicating strong hydrological variability and climate-driven lake-level fluctuations.

Sedimentation was strongly influenced by syn-depositional volcanism associated with the Ouarzazate Group, which controlled basin morphology, subsidence patterns, sediment supply, and the spatial distribution of alluvial, fluviolacustrine, and lacustrine environments. Volcanic activity also contributed to favorable lacustrine geochemical conditions, promoting sustained carbonate production. Superimposed on this volcanic control, climatic fluctuations between humid and arid conditions modulated runoff intensity, sediment flux, and lake-level variations, resulting in alternating siliciclastics- and carbonate-dominated depositional phases.

The relative influence of volcanism and climate differed between the two basins. The Amane-n’Tourhart Basin records a dominant volcanic control related to its caldera setting, whereas the Tifernine Basin preserves a fluviolacustrine record more sensitive to climatic forcing. Overall, these successions provide a well-preserved archive of late Ediacaran continental environments and offer new insights into volcano–sedimentary interactions, lacustrine dynamics, and carbonate factory development coincident with the emergence of early animals.

While basin shape and architecture were likely defined by fault-controlled subsidence before deposition in association with a caldera formation, and the preservational extent of sedimentary basins was determined by movement along post-depositional faults, there is no clear evidence for tectonic activity during deposition, such as a gradual increase in stratigraphic thickness of sedimentary units toward bounding faults.