The Ba Moussa West Coral fauna, a new Early Carbiniferous Coral assemblage from central Morocco.

Mississippian rocks are common in the Moroccan Meseta. They have been studied and described by French geologists since the beginning of the twentieth century. The Mississippian stratigraphic successions are clearly different in the western and in the eastern parts of the Meseta. The succession was considered quite continuous from the Devonian to the Serpukhovian. However, sedimentation in the eastern part of the central Meseta (Azrou-Khenifra Basin) is more complicated. It took place in both a shallow-water carbonate platform and a deeper water flysch basin, within a tectonically active setting, involving movements of blocks, and transgressions and regressions that produced some gaps and unconformities. Sedimentation during the Tournaisian, early and mid Visean in the basin is regarded as being absent by some authors, whereas continuous or sporadic sedimentation during that time interval is suggested by others.

In a paper published in the Journal of Palaeogeography on 11 February 2020, Sergio Rodríguez of the Universidad Complutense de Madrid and the Instituto de Geociencias at the Consejo Superior de Investigaciones Científicas, Ian Somerville of the School of Earth Sciences at University College Dublin, Pedro Cózar, also of the Instituto de Geociencias at the Consejo Superior de Investigaciones Científicas, Javier Sanz-López of the Departmento de Geología at the Universidad de Oviedo, Ismael Coronado of the Institute of Paleobiology, Felipe González of the Departmento de Ciencias de la Tierra at the Universidad de Huelva, Ismail Said, also of the Universidad Complutense de Madrid, and Mohamed El Houicha of the Laboratoire de Géodynamique et Géomatique at the Université Chouaïb Doukkali, report the recent discovery of a relatively rich Mississippian (early Visean) Coral fauna in the southern part of the Azrou-Khenifra Basin, describe the Corals in detail and their host limestone rocks, and comment on their comparison and affinity with other coeval Coral assemblages in North Africa, Europe and southwest Asia. The microfossil content was also studied to enhance the biostratigraphic discussion and significance of the Coral fauna.

The beginning of Carboniferous sedimentation in the Khenifra region, which lies in the southern part of the Azrou-Khenifra Basin and contains the largest Mississippian outcrops in the eastern central Meseta, is usually considered to occur within the widely known late Visean transgression. However, two early Visean transgressions have been cited. The first one is imprecisely located as “to the north of Ba Moussa (point 1)”. The second one was equated with the base of V2b of mid Visean age.

In the southwestern margin of the Azrou-Khenifra Basin at Sidi Lamine and Tabainout, a thick shallow-water carbonate succession with basal Mississippian conglomerate and sandy limestone can be seen to rest unconformably on older (Ordovician) tilted siltstones and sandstones. A similar relationship is seen at the southeastern margin of the basin at Tiouinine where shallow water sandy limestones rest unconformably on red Ordovician sandstones.

(a) Location of Khenifra in central Morocco; (b) Geological map of Azrou-Khenifra Basin with Ba Moussa West coral fauna locality and other Coral localities mentioned in the text; (c) Simplified geological sketch map of Ba Moussa West area and the location of the studied limestone horizons BMW1 and BMW2. hV-Fm1, Lower Visean; hV-Fm2, Upper Visean. Rodríguez et al. (2020).

The eastern part of the Azrou-Khenifra Basin, northwest of Khenifra, is a region of mostly deep-water rythmic mudstones. However, recent field investigations at Ba Moussa West, northwest of a nappe folded as a north-south trending syncline, and approximately 3 km northwest of Khenifra city margins, have discovered two pale gray weathering limestone units within a thick dark gray siltstone and shale rhythmic succession. These limestones contain abundant corals that form the focus of this paper. The limestone units form two distinct parallel ridges, some 50m apart, and traceable laterally for over 200 m. They form prominent features on the landscape, compared to the subdued topography of the more easily eroded mudstones which encase the limestones. The beds dip steeply to the east (70°) and in places can be vertical. The two ridges expose respectively, 4.90m and 4.10m thicknesses of well-bedded limestones (with beds ranging typically from 10 to 40 cm thick) with thin dark gray shale interbeds.

(a) View looking south of limestone ridge (BMW1) about 5 m thick showing steeply dipping beds overlain and underlain by softer shales; (b) Limestone bed with large angular quartzite and sandstone lithoclasts (beside coin) succeeded by thin laminated sandy limestone and black shales, in turn overlain by bioclastic limestone rich in Corals; solitary Rugose Coral Siphonophyllia (black arrows) and Cerioid Tabulate Coral Turnacipora (white arrow), coin diameter is 2.5 cm; (c) Close-up view of richly bioclastic limestone bed with sharp base, showing abundant transverse sections of Siphonophyllia and Sychnoelasma (black arrows), hammer length is 40 cm; (d) Coarse-grained crinoidal limestone with longitudinal and transverse sections of Siphonophyllia khenifrense; (e) Thin section of rudstone at BMW1 showing bioclasts and lithoclasts. Abbreviations: br, brachiopod; bz, bryozoan; co, coral; cr, crinoid; gr, gastropod; st, sandstone; (f) Thin section of rudstone at BMW2 showing bioclasts and lithoclasts. Abbreviations: br, brachiopod; co, coral; cr, crinoid; sl, siltstone; st, sandstone. Rodríguez et al. (2020).

The limestones are variable in composition and texture, comprising coarse-grained, bioclastic and lithoclastic calcirudites, rich in crinoids, thick-shelled Brachiopods and relatively abundant Corals. The limestone beds consist of numerous sedimentary events. Some have sharp, erosive bases and show grading with laminated tops. Large angular lithoclasts of sandstone and siltstone (up to 20 cm in diameter) occur in some beds. Other limestones are buff weathered, fine-grained, laminated calcarenites. Under the petrological microscope two microfacies are differentiated. The first microfacies, which is less common, is a laminated Crinoidal wackestone-packstone containing small fragments of Crinoidal plates, Corals and Bryozoans. The second one, which is dominant, is a polymictic rudstone with fragmented Corals, Crinoids, Bryozoans, Brachiopods, Trilobites, Gastropods, Bivalves, Foraminifers and angular to subangular grains of quartzite sandstone and siltstone. The disposition of siliciclastic clasts and bioclasts is random in some beds, suggesting rapid sedimentation, but in some beds, most clasts are disposed mainly parallel to the stratification. The fragmentation of bioclasts is also variable.

The limestones can be regarded as proximal debris flow and multistorey high-density turbidite bodies, with numerous event beds, deposited in a prevailing succession of distal turbidite beds. Thus, the coral assemblage is allochthonous and may have been transported far from its original depositional shelf setting.

The two limestone horizons (BMW1 and BMW2) were sampled and corals were collected. Samples from BMW1 contain almost entire Brachiopods and Corals, whereas in BMW2 most bioclasts are completely broken and very few Coral specimens are identifiable at generic or specific level. The coral assemblage is relatively rich, but their diversity is quite low (5 genera and 7 species). The assemblage comprises solitary Rugose Corals and Tabulate colonies. Many corals are well preserved and nearly complete, missing only the apexes and showing sometimes compressed calices when they show few skeletal elements and are filled with muddy sediment. However, others are completely fragmented or crushed or have lost much of their dissepimentaria. Fifty specimens were collected, of which 38 have been definitively identified.

Thin sections of samples were studied to describe the microfossil content. Owing to the brecciated character of many beds, including boulders of large size, only the fine-grained limestones yield Foraminifers. Assemblages are relatively abundant in those fine-grained limestones, although specimens are commonly crushed, and diversity is limited to a few genera. Assemblages from BMW1 are slightly richer than BMW2, although this may be the result of more intense sampling and sectioning.

A large sample from limestone BMW1 (3.8 kg weight) was etched with 8%–10% buffered formic acid solution, following the standard technique to avoid damaging. The low abundance of Conodont elements includes one complete P1 element and six broken elements with upper surface damaged and a few with surface dissolution, which could be in relation to significant transport and resedimentation of elements. The colour of Conodonts shows values of 4.5 to 5 for the alteration index. Reworking of Conodonts may be causing a higher colour alteration index value, but small recrystallised apatite surface is observed in Conodonts. Some specimens preserve a smooth surface, but etched surfaces with pits are often discerned. It suggests short heating on proximity to an igneous intrusion.

Conodonts from samples of BMW1. (a)–(b) Fragment of element of Kladognathus sp., DGO 15624, and detail of the face where breakage shows a lamellar inner structure and small apatite crystal 2–3 μm in size interpreted as recystallized and, later, slight dissolution; (c)–(e) Aboral and oral views of Mestognathus cf. beckmanni, DGO 15625, and detail of the margin of the platform with a strong dissolution located on the ornamentation of ridges and carina causing the inversion of surface relief; (f) Oral view of Polygnathus lobatus with pits due to dissolution of the Conodont surface, DGO 15622; (g) Gnathodus pseudosemiglaber, DGO 15623; (h)–(i) Oral and aboral views of Polygnathus inornatus, DGO 15621. Conodonts are stored in the Museum of Geology of the University of Oviedo. Rodríguez et al. (2020).

The allochthonous shales embedding the limestone horizons were sampled for palynomorphs. A total of 12 shale samples were crushed and dissolved following the classical extraction techniques. After complete removal of carbonate and silicate minerals, the organic remains were oxidized with Fuming Schulze solution and mounted in slides for microscope analysis. Palynomorphs recovered from shales are dominated by phytoclasts and, in minor proportions, by spores, whereas organic-walled marine microphytoplankton and amorphous organic matter are virtually absent. The reduced number of spores and their irregular state of preservation precluded further taxonomic identification. The large proportion of equidimensional to lath-shaped phytoclasts and the absence of marine components may be explained by the intense reworking and effective dilution associated to low-density turbidity currents. The brownish-black to black colour of spores and phytoclasts points to a thermal alteration index which essentially agrees with the colour alteration index values observed for conodonts from the limestone sample.

The Coral assemblage from Ba Moussa West contains a new species of Siphonophyllia, with other solitary Rugose Corals, such as Sychnoelasma urbanowitschi, Cravenia lamellata, Cravenia tela, and Cravenia rhytoides. Colonial Tabulate Corals recorded include Turnacipora megastoma, and Pleurosiphonella crustosa. Themost abundant specimens collected belong to the genus Siphonophyllia (20) and Turnacipora (7). Most other species are represented only by three specimens or less.

The assemblage is similar to that described from lower Visean (Arundian) Moel Hiraddug Formation in North Wales, UK. In both regions the large Siphonophylliid Corals represent the dominant component in dark gray bioclastic limestone and shale lithofacies, in which colonial Rugose Corals are absent. However, the Ba Moussa succession has a lower diversity Coral assemblage and the specimens are not as well preserved. This may be explained by the sedimentological setting at Ba Moussa, with the Corals occurring in graded limestone beds containing large exotic clasts, interpreted as debris flow and proximal turbidite deposits.

The stratigraphic range of Sychnoelasma urbanowitschi, and the three species of Cravenia (Cravenia lamellata, Cravenia tela, and Cravenia rhytoides) is very restricted, typically diagnostic of the early Visean throughout Western Europe. Turnacipora megastoma occurs also, typically in the early Visean.

The Ba Moussa West assemblage has similarities with Tafilalt in Eastern Morocco, where a richer early Visean solitary Rugose assemblage is recorded including Cravenia, Siphonophyllia and Sychnoelasma, but where colonial Rugose genera are also absent. Similar assemblages containing dominant Cyathopsids plus Sychnoelasma, Pleurosiphonella and Micheliniids have been reported in Canada and United States, and in Mid-Asia.

The Ba Moussa limestone beds are clearly older than other Mississippian sections in the Khenifra area, as confirmed by the associated Foraminifers and Conodonts. Coral assemblages from Tabainout and Sidi Lamine, 20 km and 30 km respectively, further west of Ba Moussa West, at the western margin of the Azrou-Khenifra Basin, contain fasciculate and massive colonial Rugose Coral genera (Siphonodendron and Lithostrotion) of late Visean (Asbian) age. Both sections have basal transgressive deposits with in situ shallow-water limestones containing ooids and Calcareous Algae. At Tiouinine, 8 km southeast of Khenifra on the eastern margin of the basin, very rich and diverse late Visean (Brigantian) Coral assemblages form a reefal tract. The early Visean age of the Ba Moussa West limestone correlates with the early Visean age of the transgressive point 1, located to the north of Ba Moussa.

The assemblage in samples from BMW1 contains the Foraminifers Earlandia vulgaris, Earlandia elegans, Endothyra spp., Endothyra similis, Endolaxina sp., Endothyranopsis (Eosinopsis) sp., Eosparastaffella sp., Eosparastaffella concinna, Eosparastaffella evoluta, Eosparastaffella interiecta, Eosparastaffella macdermoti, Eosparastaffella aff. macdermoti, Eosparastaffella ovalis, Eosparastaffella simplex, Eosparastaffella tumida subsp. 1, Eosparastaffella vdovenkoae, Eotextularia diversa, Granuliferella sp., Globoendothyra sp., Lapparentidiscus sp.,? Lituotubella sp., Mediocris mediocris, Mediocris ovalis, Mediocris aff. ovalis, Omphalotis sp., Pseudoplanoendothyra sp., Septabrunsiina sp., Septaglomospiranella sp., Spinobrunsiina sp., Spinolaxina sp., Tetrataxis sp. and Urbanella (Brenckleites) fragilis. The Algospongia recorded are very common Kamaena delicata and Palaeoberesella lahoseni, as well as Stacheoides spissa and Exvotarisella sp.

(a) Eotextularia diversa, BMW1, Pc4367; (b) Latiendothyranopsis sp., BMW2; (c) Omphalotis sp., BMW1, Pc4364; (d) Eoparastaffella tumida, BMW1, Pc4364. (e) Granuliferella sp., BMW1, Pc4364; (f) Mediocris aff. ovalis, BMW1, Pc4366; (g) Eoparastaffella simplex, BMW1, Pc4366; (h) Eoparastaffella ex gr. simplex (Eoparastaffella tumida subsp. 1), BMW1, Pc4367; (i) Eoparastaffella aff. concinna, BMW1, Pc4365; (j) Eoparastaffella evoluta, BMW2; (k) Eoparastaffella vdovenkoae, BMW1, Pc4366; (l) Eoparastaffella macdermoti, BMW1, Pc4364; (m) Eoparastaffella ovalis, BMW1, Pc4367; (n) Endolaxina sp., BMW1, Pc4367; (o) Pseudoplanoendothyra sp., BMW1, Pc4364; (p) Endothyranopsis (Eosynopsis) sp., BMW1, Pc4364. Scale bar same for all figures. Rodríguez et al. (2020).

The assemblage is characterized by a high diversity in Eoparastaffella species, and in particular, the first species with pointed periphery in the last whorl, Eoparastaffella tumida subsp. 1 and Eoparastaffella ex gr. simplex. Although the marker for the base of the MFZ9, as well as the marker for the base of the Visean, Eoparastaffella tumida subsp. 1 is derived from Eoparastaffella simplex from the basal levels of the MFZ9, and thus, the assemblages can be attributed to the base of the Visean. It is noteworthy for the occurrence of Eoparastaffella concinna and Eoparastaffella evoluta, also derived from Eoparastaffella simplex in more advances stages of the MFZ9.

The foraminiferal assemblage recorded in BMW2 is composed of Earlandia minor, Earlandia vulgaris, Endothyra spp., Endothyra ex gr. bowmani, Endothyra prisca, Endothyra similis, Eotextularia diversa, 'Glomospira' sp., Eoparastaffella sp., Eoparastaffella concinna, Eoparastaffella interiecta, Eoparastaffella macdermoti, Eoparastaffella simplex, Eoparastaffella tumida subsp. 1, Eoparastaffella vdovenkoae, Mediocris mediocris, Latiendothyranopsis sp., Omphalotis sp., Plectogyranopsis sp., and Pseudoplanoendothyra sp. This assemblage also contains the pointed and slender Eoparastaffella, including Eoparastaffella. concinna, which is a more evolved form than the ancestral stock of pointed Eoparastaffella. In consequence, the assemblage is also assigned to an advanced stage in the MFZ9. The Algospongia recorded in those levels contain Palaeoberesella lahoseni, Kamaena delicata, Issinella sp., and Exvotarisella sp.

The Conodont fauna studied in samples from BMW1 includes Polygnathus inornatus, Polygnathus lobatus (that is usually related with the first species), and a fragment of Polygnathus sp. These taxa were usually described in the early to mid Tournaisian Siphonodella–Polygnathus inornatus Assemblage Zone in the British Isles. However, it has been indicated that Polygnathus inornatus ranged up to the upper Tournaisian Gnathodus typicus Conodont Zone in Cornwall (UK). Polygnathus inornatus have been reported in the upper Tournaisian Scaliognathus anchoralis Zone of the Moravia-Silesia and the Dinant-Namur basins, and in the earliest Visean, just at the first occurrence of Pseudognathodus homopunctatus in the Belgian area. A late Tournaisian to early Visean age is supported by the occurrences of one P1 element of Gnathodus pseudosemiglaber, one P1 fragment of Mestognathus sp. and one P2 element probably corresponding to Kladognathus sp. The fragment of Mestognathus sp. shows dissolution of the carina and ornamentation of the platform, and the blade and the dorsal part of the platform are broken. The parapet area is close to that described in Mestognathus praebeckmanni. The secondary keel seems to be formed with a basal groove, as in Mestognathus beckmanni, but the specimen is broken. The first occurrence of Mestognathus beckmanni was indicated just below the lower boundary of the Visean Stage at the Global Boundary Stratotype Section in the Pengchong section, South China and in a few localities of Western Europe, although it is often recorded in Visean beds. The early Visean Pseudognathodus homopunctatus species is lacking in Rodríguez et al.'s sample.

The new species of Siphonophyllia is named Siphonophyllia khenifrense, which refers to the town of Khenifra within the Azrou-Khenifra Basin in Morocco. Seventeen whole specimens were recovered, all from Ba Moussa West, as well as 29 transverse sections and 15 longitudinal sections.

The whole specimens are cylindrical Corallites between 20 mm and 40 mm in alar diameter and recorded fragments are up to 20 cm long, often without calice. The dissepimentarium is often abraded. The outer wall is thin.

Siphonophyllia khenifrense. (a)–(c) Holotype DPM BMW1-1: (a) DPM BMW1-1A, transverse section., (b) DPM BMW1-1B, transverse section., (c) longitudinal sections; (d)–(e) DPM BMW1-6: (d) transverse section, (e) longitudinal sections; (f)–(g) DPM BMW1-20: (f) longitudinal sections, (g) transverse section.; (h) DPM BMW2-16, transverse section; (i) DPM BMW2-4, transverse section; (j) Wall microstructure in Siphonophyllia khenifrense, DPM BMW1-1, L, Lamellae; (k) Septal microstructure in Siphonophyllia khenifrense, DPM BMW2-16, Gr, Granular axial septum; F, Fibronormal middle zone; L, Lamellar external zone. Black arrows indicate the position of the cardinal septum. Corals are housed in the Geodinamica, Estratigrafía y Paleontología Department of the Universidad Complutense de Madrid. Rodríguez et al. (2020).

The tabularium diameter varies from 17 mm in immature stage to 31 mm in adult stage. The tabularium is wide, 3/5 to more than 4/5 Corallite diameter; the variation in tabularium width is a function of the age of the specimen (immature vs mature Corallite) and variation in the width of the dissepimentarium, which although generally narrow, can also be variably preserved. The number of major septa ranges commonly between 42 and 61, but up to 68 may be present. The septa are long, almost reaching the axis in immature stage but withdrawn from the centre in mature adult stage. They are straight to slightly flexuous in the tabularium, thinning axially and straight to sinuous in the dissepimentarium. Major septa are strongly thickened in the tabularium but are thin in the dissepimentarium; septa can be slightly thicker in cardinal quadrants and thinner in counter quadrants. The minor septa are also thickened where they penetrate slightly into the tabularium, but not as thick as majors; in the dissepimentarium they are thin. They are variable in length, from 1/4 to 1/3 length of majors. The cardinal septum is slightly shorter in most mature Corallites and located in a closed small cardinal fossula. It is often flanked by two major septa which are shorter than the others. Counter septum is inconspicuous, but shorter in late adult stages.

The dissepimentarium is narrow (typically 1/10 to 1/5 Corallite diameter) and mainly composed of interseptal regular dissepiments. The dissepiments are more irregular in the external part of the dissepimentarium, with occasional lonsdaleoid dissepiments. Typically 3 to 6 rows of slightly angular concentric dissepiments are present in the dissepimentarium. In longitudinal section, the dissepiments are small and elongate. They are declined to the tabularium from 60° to 70°.

The tabulae are mostly complete flat domes with some splitting; horizontal, medially sagging and convex tabulae can be present, sloping down peripherally to prominent gutters. They are relatively widely spaced numbering between 6 and 12 each centimetre.

The wall microstructure is microlamellar, as well as the septal stereoplasm and thickenings of tabulae and dissepiments. The septal mesoplasm is granulofibrous with incipient development of microtrabeculae. The tabulae and dissepiments are microgranular.

At least four transgressive phases have been differentiated in the Azrou-Kenifra Basin which were related with fault activity and resedimentation on the margins of tectonic blocks. The early Visean Corals at Ba Moussa West are the oldest occurrence in this basin, and are an important fauna differentiated from the commonly described faunas in late Visean beds of the western margin of the basin at Sidi Lamine and Tabainout, as well as in the northern part of the basin at Adarouch.

The early Visean age in the MFZ9 is older than the previously considered age for North Ba Moussa point 1 (Zone 11 or equivalent MFZ10), in spite of Foraminifer species that was based on their zonal correlation, Earlandia vulgaris and Eotextularia diversa, are also occurring in samples from BMW1 and BMW2 (assigned here to the MFZ9).

The Ba Moussa West succession is a resedimented body of shale, siltstone and limestone with early Visean microfossils and Corals, indicating that the probable age of sedimentation was very close to that of skeletal growth of the components. The corals and microfossils correspond to shallow-water taxa dwelling on a neighbouring sedimentary relief. The coralline assemblage shows a distinctive dominance of solitary rugosans, the absence of colonial Rugosans and occurrence of colonial Tabulate Corals. Moreover, the solitary forms are dominated by Siphonophyllia khenifrense and Sychnoelasma urbanowitschi, and the Tabulate Coral Turnacipora megastoma. A similar association of Siphonophyllia aff. garwoodi and Sychnoelasma urbanowitschi is known from the early Visean of the Laval syncline in Normandy (north France), although with colonial Rugosans there (Solenodendron spp.). This colonial genus is not recorded in the Azrou-Khenifra Basin first until the late Visean.

This colonial genus is not recorded in the Azrou-Khenifra Basin first until the late Visean. However, none of the seven listed key taxa of this subzone are recorded in Morocco, although the genera Siphonophyllia, Cravenia and Sychnoelasma are present. Perhaps of greater significance though, is that whereas Siphonophyllia hawbankense is only recorded in the underlying upper Tournaisian RC4ß1 subzone, a new taxon Siphonophyllia hawbankense subsp. A which starts in this subzone, extends into RC4ß2 subzone. The strong possibility exists though, that this corresponds to the small Siphonophyllia urbanowitschi of Ba Moussa, which represents the transition to larger typical forms in RC5 Zone.

The Ba Moussa West Coral fauna, although quite restricted in its diversity, nevertheless, contains typical elements of the Western European Coral province (which includes North Africa and Nova Scotia). In particular, the dominance of solitary Rugosa and Tabulate Corals is a feature of the early Visean assemblages which are recognised in northwest Europe: Normandy (north France), southern Belgium, southwest Province, North Wales, Craven Lowlands and South Cumbria (Great Britain), and Dublin Basin (Ireland). Similar early Visean faunas with solitary rugosans are known in the eastern part of the Anti-Atlas region at Tafilalt in eastern Morocco and in the Béchar Basin in Algeria. The late Tournaisian to early Visean Rugose Coral fauna from Tafilalt is richer than that from Ba Moussa. It is dominated by solitary genera, both undissepimented (Sychnoelasma, Cravenia) and dissepimented (Bifossularia, Cyathoclisia, Clisiophyllum, Siphonophyllia, Palaeosmilia, Amygdalophyllum), and is lacking colonial Rugosans.

Palaeogeographic distribution of the Coral taxa recorded in Ba Moussa West in the Palaeotethys region and around Laurentia and Baltica. (s) Siphonophyllia, (u) Sychnoelasma urbanowitschi, (c) Cravenia, (t) Turnacipora, (p) Pleurosiphonella. (1) Ba Moussa West, (2) Tafilalt, (3) Midcontinent, (4) Western Interior, (5) Canadian Rockies, (6) Carnic Alps, (7) Western Europe, (8) Eastern Europe, (9) Moscow Basin, (10) Ural Mountains, (11) Tian-Shan (Northwest China), (12) Turkey, (13) Transcaucasia, (14) Iran, (15) Himalaya, (16) South China. Rodríguez et al. (2020).

It was previously considered that since the Azrou-Khenifra Basin only had late Visean and younger Coral assemblages, so too the Jerada Basin in northeast Morocco, they were isolated from other marine basins in the early Visean. Connections among the Azrou-Khenifra Basin, northwest Europe, Tafilalt, and other Saharian basins in Algeria (Béchar Basin) were open from the Asbian and Brigantian (late Visean). The Ba Moussa West Corals, Foraminifers and Conodonts suggest that marine seaways were available for migrations between the Azrou-Khenifra Basin and other regions from the early Visean. Similar early Visean faunas with solitary Rugosans are known in the eastern part of the Anti-Atlas region at Tafilalt, in eastern Morocco and in the Béchar Basin in Algeria. The marine connections between northwest Europe and the southern part of the Azrou-Khenifra Basin is supported by similar early Visean assemblages recognized in northwest Europe with abundant solitary Rugose and Tabulate Corals, but with colonial Rugosans: Normandy (north France), southern Belgium, southwest Province, North Wales, Craven Lowlands and South Cumbria (Great Britain), and Dublin Basin in Ireland.

In relation to the tabulate corals, the Tabulate Turnacipora megastoma in the Ba Moussa West assemblage was also known from Central Saharian basins, but also from the Chadian-Arundian (early Visean) locations in northwest Europe (UK, Ireland, France, Germany?). The occurrence of Pleurosiphonella crustosa is the first report in North Africa and suggests marine connection with the Urals. It was first described from the upper Tournaisian of Transcaucasia and its age range extends here slightly into the early Visean. The dispersion between southwest Asia (Armenia, Taurides and Alborz) and the Azrou-Khenifra Basin, via Tafilalt, Béchar and Sinai, is poorly established. Some solitary Rugosans (Siphonophyllia) are common to all areas, but others, such as Kueichouphyllum and the colonial form Eokoninkocarinia, indicative of Asiatic affinity are clearly absent in Morocco.

A new early Visean Coral assemblage has been discovered transported in the rhythmic facies deposits of the southern part of the Azrou-Khenifra Basin, northwest of Khenifra, Morroco. The Ba Moussa West coral fauna includes the new species Siphonophyllia khenifrense, as well as Sychnoelasma urbanowitschi, Cravenia lamellata, Cravenia tela, Cravenia rhytoides, Turnacipora megastoma and Pleurosiphonella crustosa. The early Visean age of the Coral assemblage is supported by microfossil data, which confirms a previous hypothesis that indicated a first transgression during the early Visean in the Carboniferous of the Meseta. The allochthonous coral assemblage was recovered from coarse-grained proximal limestone debris flow and turbidite beds within a fault-bounded rhythmic unit in the eastern part of the basin. No evidence remains of the former early Visean shallow-water platform from which the Corals were derived. All other in situ platform carbonate rocks around the southern margin of the Azrou-Khenifra Basin are of late Visean (Asbian–Brigantian) age. The early Visean Ba Moussa West Coral fauna can be compared with that from the Saharian basins of southeast Morocco and Algeria. Most of the genera and species in the Ba Moussa West assemblage are identical to those in Western Europe, indicating possible marine connections. The new Rugose species described, Siphonophyllia khenifrense, is probably endemic to North Africa. Its ecological niche in northwest Europe was occupied by Siphonophyllia cylindrica or Siphonophyllia aff. garwoodi.

The microfossil determinations provide greater precision in the age dating of the Ba Moussa West limestones. The foraminiferal assemblages from BMW1 can be attributed to the lowermost Visean (MFZ9). Similarly, the Conodont fauna recovered from the same beds, although sparse, suggests a late Tournaisian to early Visean age.

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Coralline Red Algae from Middle Eocene tropical and mid-latitude regions.

Rhodolith (Coralline Red Algae) beds are ubiquitous sea-floor ecosystems distributed all over the world. Equivalent dense to loose concentrations of Rhodoliths, as major components of carbonate, are also common in the rock record in tropical and cold-temperate settings. They can be the result of the accumulation of Rhodoliths where they originally grew (autochthonous concentrations), or nearby (parautochthonous Rhodolith beds), or can be accumulated in a different site due to long-distance transport (allochthonous concentrations). Reworked deposits can accumulate either offshore or onshore of the original place of Rhodolith development. The oldest records of calcified Coralline Algae in the Early Cretaceous are fragments dispersed in the sediment or thin crusts attached to Corals. Coralline Algae became important carbonate components during the latest EarlyCretaceous, and Rhodoliths were increasingly common during the Late Cretaceous. There are no quantitative studies on the volumetric contribution of Rhodolith beds to the global carbonate production through time. Qualitative data have only been compiled for Oligocene–Neogene Coralline Algal-dominated deposits. Although most of this information is based on the Mediterranean record, the data suggest an important increase in Rhodolith bed occurrences during the Oligocene and early Miocene. Interestingly, this substantial development coincidewith the highest Coralline diversity in the Chattian–Burdigalian (Late Oligocene-Early Miocene, 28.1-18.97 million years ago) interval. Global temperature, palaeoceanographic circulation patterns, and major palaeogeographic changes in the Tethys were important factors controlling the diversification of Coralline Algae. Before this diversity peak, during the Eocene, Rhodoliths were widespread worldwide, but information on their relevance in marine ecosystems is scarce. The Eocene was a transient period of time between greenhouse and icehouse conditions. Global temperature rose in the early Eocene up to the highest values ever reached during the Cainozoic, during the Early Eocene Climatic Optimum. Then, temperature progressively decreased during the rest of the Eocene, the Long-term Eocene Cooling, up to a drastic cooling at the Eocene–Oligocene transition due to the onset of the Southern Hemisphere glaciation.

In a paper published in the journal Diversity on 23 March 2020, Julio Aguirre and Juan Braga of the Departamento de Estratigrafía y Paleontología at the Universidad de Granada, Victoriano Pujalte and Xabier Orue-Etxebarria at the Departamento de Estratigrafía y Paleontología at the Universidad del País Vasco, Edward Salazar-Ortiz of the Servicio Geológico Colombiano, Daniel Rincón-Martínez of the Ecopetrol Centro de Innovación y Tecnología, Manuel Abad of the Departamento de Biología y Geología, Física y Química Inorgánica at the Universidad Rey Juan Carlos, and Fernando Pérez-Valera of the Departamento de Ciencias de la Tierra y del Medio Ambiente at the Universidad de Alicante, present the results of a study of Middle Eocene Rhodoliths from four areas, two in Southern Spain, one in Hispaniola (Dominican Republic), and one in Colombia, made in order to analyze the palaeonvironmental conditions in which Rhodoliths developed in different palaeolatitudinal contexts. This will also contribute to understanding the long-term role of coralline algae in marine ecosystems during a time interval when substantial climate changes were taking place at global scale.

Parallel to high global temperatures, sea level several tens of meters higher than in the modern oceans prevailed during the early and middle Eocene, promoting the expansion of large carbonate platforms. In this context, oceans were devoid of large coral reef structures, particularly affecting low latitude areas; the so-called Early–Middle Eocene 'reef gap'. In contrast, large carbonate ramps largely dominated by larger benthic Foraminifers and Algae developed.

As far as is known global diversity of Coralline Algae was moderate during the Eocene, predating the great diversification event during the Oligocene–Early Miocene. During the Middle Eocene, Coralline Algal diversification shows a slight decline coinciding with a major macroevolutionary turnover in Algal assemblages, characterised by a substantial increase of members of the Order Hapalidiales and a decrease of members of the Family Sporolithales.

Palaeogeographic reconstructions during the middle Eocene show that Southern Spain was in mid latitudes while Hispaniola and Colombia were in lower latitudes closer to the equator.

Samples with rhodoliths were collected in the different study areas. Except in one of the study areas in Southern Spain (Salinas Menores), sediments were well lithified, precluding the extraction of isolated rhodoliths to make measurements. Therefore, Rhodolith morphology is based on sections in the outcrops. Samples were cut to produce a total of 56 thin sections.

Aguirre et al. studied two sites in the Betic Cordillera: the Salinas Menores Ravine and Sierra del Zacatín

(A) Geographic location of the study areas in Southern Spain (red rectangles). (B) Location of the Salinas Menores Ravine section (blue star). (C) Location of the Sierra del Zacatín area (blue star). Red lines indicate the main roads; blue lines and patch mark rivers and water reservoir, respectively; green lines mark tracks. Aguirre et al. (2020).

The Salinas Menores outcrop is close to Dehesas de Guadix and Villanueva de las Torres villages (Northeast Granada province, Southern Spain). The study deposits belong to the Cañada Formation, which includes redeposited materials due to gravitational flows unconformably overlying different Paleogene–Mesozoic rocks. The section consists of grey-greenish marls intercalating turbidite beds of packstone–rudstone. The marls also incorporate large, up to hundred metre olistoliths derived from the Paleogene–Mesozoic basement. The top of the section is a channelized body, up to 3 m thick, of rudstones. Rhodoliths are dispersed in the marls. Planktic Foraminifera assemblages are characterized by Subbotina jacksonensis, Subbotina senni, Catapsydrax unicavus, Acarinina bullbrooki, Acarinina punctocarinata, Acarinina praetopilensis, Morozovelloides bandyi, Morozovelloides crassatus, and Globigerinatheka spp. This assemblage indicates a Lutetian age (Middle Eocene, 47.8-41.2-million-years-old).

Stratigraphic column from Salinas Menores ravine. Aguirre et al. (2020).

The Sierra del Zacatín is a southwest–northeast trending mountain range northeast of Nerpio (Albacete) in Southern Spain. The study section is located at the southwest of the sierra. The Sierra del Zacatín is mostly made up of Paleocene to Eocene sediments. The middle Eocene succession consists of about 40 m thick limestones dominated by larger benthic Foraminifers, followed by fragments of Echinoids, Bryozoans, and Bivalves. In the middle of the section, Corals become common together with Coralline Algae, which occur as Rhodoliths but are mostly intergrown with Corals. The presence of Alveolina boscii, Alveolina stercusmuris, and Alveolina praespira at the base of the limestones, as well as Alveolina fusiformis, Alveolina elongata, and Nummulites aturicus at the top indicates a Lutetian–earliest Bartonian age (Middle Eocene).

Stratigraphic column from Sierra de Zacatín. Aguirre et al. (2020).

The Dominican Republic shares with Haiti the Hispaniola Island, one of the Greater Antilles separating the Caribbean Sea from the Atlantic Ocean. The Bahoruco Peninsula is the southern extreme of the island and, therefore, the farthest area from the convergence zone of the Caribbean and North America plates, which runs east–west between Hispaniola and the Bahamas. The Cainozoic succession in the Bahoruco Peninsula includes a series of Eocene to Pliocene carbonate units overlying Upper Cretaceous volcanic rocks. The lowest carbonate unit, Polo Unit, consists of whitish limestones, 100 to 250 m thick. The lower part of the unit comprises roughly bedded bioclastic wackestones and Rhodolith rudstones. Coralline Algae are the main components in the latter lithofacies with secondary Corals, Molluscs, and benthic Foraminifera. The larger benthic Foraminifer assemblages include Discocyclina, Lepidocyclina, Nephrolepidina, Operculina, Eorupertia?, Rotalia, Sphaerogypsina, and Cushmania. Upwards in the succession, bedding definition increases as the grain size decreases. The finer-grained limestones in the upper part of the Polo Unit change gradually to wackestones and mudstones included in the Neiba Formation. Larger benthic Foraminifers together with planktic Foraminifers (Acarinina, Globigerina, and Globigerapsis) suggest a Middle (Late) Eocene age.

The inset shows a map of Hispaniola with indication of the study area (red rectangle). Location of the Polo section (blue star). Red lines indicate main roads. Aguirre et al. (2020).

The studied rhodoliths were collected at 18°5’13”N–71°17’33”W, 1 km to the northwest of the hamlet Polo.

Stratigraphic column from Polo. Aguirre et al (2020).

The study Rhodoliths occur in middle Eocene deposits in the San Jacinto Fold Belt, in northwestern Colombia. The San Jacinto Fold Belt is an elongated antiform structure with a northeast–southwest direction. Upper Paleocene-Eocene sedimentary rocks in this belt formed on an intraoceanic Cretaceous Caribbean arc. Two middle Eocene units crop out in the San Jacinto Fold Belt: the Arroyo de Piedra and Chengue formations. Their age assignment is based on planktic Foraminifers and calcareous nannoplankton. The Chengue Formation consists of mudstones, siltstones, and redeposited bioclastic carbonates, up to 300 m in thickness. The Arroyo de Piedra Formation mainly includes bioclastic limestones, sandstones, siltstones, and mudstones.

The inset shows a map of Colombia with indication of the study area (red square). Location of sections (blue stars). Red lines indicate main roads; blue lines and patches mark rivers and lakes, respectively. Aguirre et al. (2020).

Stratigraphic columns from Colombia. Aguirre et al. (2020).

At Salinas Menores Ravine Rhodoliths occur scattered in the planktic foraminifer-rich marls. They are spheroidal to ellipsoidal and range from 2 to 6 cm in largest diameter. Rhodoliths are made up of Coralline Algae intergrown with Bryozoans and encrusting Foraminifers. The Peyssonneliacean Alga Polystrata alba is another common component. The nuclei of Rhodoliths are fragments of Corals, Coralline Algae or wackestone–packstone dominated by Miliolid Foraminifers. On the Rhodolith surface, Algae are predominantly encrusting, with a few warty growth forms. Internally, Rhodoliths are concentric with encrusting Algae followed by warty and fruticose Algal thalli. In terms of Algal assemblages, Rhodoliths are composed of members of the orders Hapalidiales, Sporolithales, and Corallinales. Corallinales is the most diversified group represented by Neogoniolithon spp., Spongites spp., Lithoporella sp., Hydrolithon cf. lemoinei, Subterraniphyllum thomasii, laminar thalli of Karpathia sphaerocellulosa, as well as unidentifiable fragments of geniculate forms showing cell fusions.

(A) Rhodoliths dispersed in planktic Foraminifer-rich marl from Salinas Menores area. (B) Coralline Algal crusts (white veneers) intergrown with Coral from Sierra del Zacatín. (C) Spheroidal Rhodolith made up by the superposition of thin crusts of Coralline algae (Sierra del Zacatín). (D) Autochthonous Rhodolith rudstone from Colombia (CECG Quarry). (E) Redeposited Rhodolith rudstones from Colombia (Punta Brava Quarry). (F) Larger benthic Foraminifer-dominated limestones from Sierra del Zacatín. Aguirre et al. (2020).

At Sierra del Zacatín Coralline Algae mostly occur as thick crusts attached to and intergrowing with Corals. Rhodoliths, although present, occur scattered in the sediment in dispersed and loosely packed Rhodolith beds. Rhodoliths are mainly spheroidal and ellipsoidal, up to 4-5 cm in largest diameter. They are made of Coralline Algae intergrown with encrusting benthic Foraminifers, Bryozoans, Corals, Serpulids, and Vermetids. Other Rhodophytes, such as Parachaetetes asvapatii included in the family Elianellaceae and the Peyssonneliacean Polystrata alba, can be important Rhodolith builders. Algal thalli both on the surface and in the Rhodolith interior are largely encrusting, with very few warty Algal growth forms. A few fruticose thalli are observed in the inner parts of some Rhodoliths.

Coralline Algal assemblages are dominated by Sporolithales, while Hapalidiales and Corallinales are rare.

In the Dominican Republic Rhodoliths occur either densely concentrated in rudstones together with Coralline Algal debris or dispersed in a fine-grained matrix in bioclastic wackestones. They are spheroidal to ellipsoidal and a few centimeters in size. Their internal structure is generally concentric, made up of encrusting to warty thalli although fruticose thalli are also common. Crustose Coralline Algae are intergrown with Peyssonneliaceans, Bryozoans, and encrusting Foraminifers. Rhodoliths are bioeroded by Sponges (Entobia) in varying degrees.

The most abundant components of the Algal assemblages are members of the order Sporolithales followed by Hapalidiales. Members of the order Corallinales are not present in the Dominican material.

At the Colombian outcrops Rhodoliths are the main components in roughly bedded, whitish to light beige rudstones. Coralline Algal fragments, larger and small benthic Foraminifers, and minor fragments of Bivalves and Echinoderms also occur as bioclasts. Rhodoliths also occur in grainstones–packstones to rudstones intercalated in mudstones and siltstones. These lilthofacies appear in decimeter-scale plane-parallel, plane-convex, and channelized beds with rip-up intraclasts of siltstone and mudstone. Red algal fragments are the main components in packstones and in the rudstone matrix (packstone). Geniculate Coralline Algae and larger benthic Foraminifers are also common.

In both types of lithofacies, Rhodoliths are ellipsoidal to spheroidal, and are millimeters to several centimeters in size; they can be fruticose and branching or have concentric to box-work internal structure. The nuclei made up of bioclasts are relatively small compared with the Algal cover (one-fifth to one-sixth in size). Peyssonneliaceans are commonly intergrown with crustose Corallines and occasionally are the only components in some spheroidal nodules. Both kinds of Red Algae are intergrown with encrusting Foraminifers in 30% of nodules.

Coralline Algal assemblages are mainly characterized by members of the orders Hapalidiales and Sporolithales, with anecdotal occurrences of Lithoporella sp.

In the Salinas Menores, Rhodoliths are scattered in the marls, not concentrated in particular beds. The sediment trapped in the inner voids of the Rhodoliths and that surrounding them show different micropalaeontological content. Miliolid Foraminifers are very abundant in the sediment filling up the internal cavities of Rhodoliths. These benthic Foraminifers are typical inhabitants of protected lagoons as well as shallow inner-platform settings. However, the sediment engulfing the Rhodoliths is rich in planktic Foraminifers characteristic of deep environments. The micropalaeontological content suggests that these Rhodoliths are displaced from their original place of growth. The Miliolids trapped within the Rhodoliths when they were growing suggest that the Algal nodules formed originally in a shallow platform. The abundance of members of the order Corallinales is typical of shallow water settings. Rhodoliths were afterwards transported to deeper basinal areas accumulating as allochthonous components. The presence of turbidites, channeled bodies, and olistoliths of varying sizes intercalated in the marls attests that downslope transport was common during the deposition of these materials.

The major biotic components of the middle Eocene carbonates in the Sierra del Zacatín are larger benthic Foraminifers. Coralline Algae are present but as secondary representatives of the fossil assemblages. They occur mainly as crusts, attached to and intergrown with Corals, and as Rhodoliths, which are dispersed in the larger benthic Foraminifer-dominated carbonates. Algal crusts occur on top of Coral colonies of varying sizes, indicating that colonies were preserved in their original growth position. In addition, geopetal structures infilling internal voids of Rhodoliths indicate normal polarity. This is consistent with absence of substantial reworking or lateral displacements of the Algal nodules.

In the lower Polo Unit in the southern Dominican Republic, spheroidal to ellipsoidal Rhodoliths mainly composed of Hapalidiales and Sporolithales indicate relatively deep (several tens of meters) shelf environments. Abundant Peyssonneliacean Red Algae are also characteristic of relatively deep shelf settings. The larger benthic Foraminifers associated to rhodoliths also suggests this kind of shelf environment. Lithofacies changes indicate deepening upwards in the succession.

The Rhodolith rudstones in the San Jacinto Fold Belt in northwestern Colombia probably formed in similar relatively deep-water, mesotrophic shelf environments. Foralgaliths of Hapalidiales and encrusting Foraminifers are also characteristic of calm-water conditions. An increase of Hapalidiales and Peyssonneliaceans in Red Algal assemblages with depth was reported in the Priabonian of Austria, Late Eocene–Late Oligocene in Northeast Italy, and Miocene of Southern Spain. Rhodolith beds dominated by Hapalidiales and Peyssonneliaceans are generally recorded in middle-ramp settings in both modern and ancient depositional systems.

The bed geometries, internal structures, and rip-up clasts clearly indicate that the packstones and rudstones intercalated in mudstones and siltstones in the San Jacinto Fold Belt are sediment gravity flow deposits. The Rhodoliths in this lithofacies were removed from shallower shelf settings and redeposited in deeper marine environments in which the autochthonous sediments were mudstones and siltstones with planktic Foraminifers. Bed geometry and dimensions indicate that these redeposited carbonates accumulated in small channel and lobe systems, downslope of the shelf in which Rhodolith rudstones formed. The Coralline Algal composition does not differ significantly among Rhodolith rudstones from mid-platform and those transported into deeper settings. This suggests that redeposited Rhodoliths originally grew in the same middle platform palaeoenvironments.

The Rhodoliths studied by Aguirre et al. are dominated by Hapalidiales and Sporolithales. Representatives of Corallinales are mostly limited to the pervasive presence of laminar thalli of Lithoporella sp. and calcified segments of geniculates (branching fronds), except in Salinas Menores, where this order is  relatively abundant and diverse. Extant species of the order Sporolithales are most diversified in relatively deep tropical waters, although they also occur in shallow waters. Along the evolutionary history of the Coralline Algae, Sporolithales expanded worldwide and reached its highest diversification during the Late Cretaceous, when greenhouse conditions prevailed. Afterwards, the species richness progressively decreased as the planet underwent a general decline in temperature. 

Selected Coralline Algal species identified in the different study areas. (A) Sporolithon nummuliticum from the El Salvador Creek section (Colombia). (B) Sporolithon sp. from the Sierra del Zacatín section. (C) Lithothamnion camarasae from the Salinas Menores section. (D. ‘Palaeothamnium’ kossovense from the Polo section. (E) Neogoniolithon sp. 1 from the Salinas Menores section. (F) Lithoporella sp. from the Salinas Menores section. (G) Subterraniphyllum thomasii from the Salinas Menores section. (H) Distichoplax biserialis from the Sierra del Zacatín section. Aguirre et al. (2020).

Hapalidiales, which outnumber other orders in Coralline Algal assemblages in modern cold, high-latitude waters and deeper low-latitude settings, diversified during the Eocene, becoming more abundant than Sporolithales. The Cainozoic decline in temperature started by the end of the Early Eocene and accelerated at the end of this epoch with the onset of glaciation in Antarctica.

In terms of relative abundance, the Coralline Algal assemblages in mid-latitude Southern Spain and in the tropical Dominican Republic and Colombia show varying proportions of Sporolithales and Hapalidiales. The number of species belonging to these two groups varies in the different study areas. In Salinas Menores, Hapalidiales and Corallinales encompass higher species richness than Sporolithales. Complied data from the literature show that Hapalidiales started to diversify in the Ypresian (early Eocene) while Sporolithales slightly declined during the Eocene. Aguirre et al.'s data confirm the increasing replacement of Sporolithales by Hapalidiales during the greenhouse Middle Eocene.

The occurrence of Subterraniphyllum thomasii in the Salinas Menores section is remarkable. This species was particularly abundant during Oligocene times, and some authors have considered it as a biostratigraphic indicator of this epoch. Nonetheless, in the original description of the species, it is indicated that Subterraniphyllum thomasii also rarely occurs in Late Eocene and Aquitanian (Early Miocene) sediments. The presence of Subterraniphyllum thomasii in Salinas Menores extends back its occurrence to the Lutetian (early Middle Eocene).

Distichoplax biserialis is an extinct Coralline Alga characterized by laminar thalli with an isobilateral organization. Distichoplax biserialis is particularly abundant in Palaeocene and Early Eocene carbonates and became gradually extinct during the Eocene. In Aguirre et al.'s study areas, this species is virtually absent except for a few small fragments of thalli found in the Sierra del Zacatín, confirming its rarity in the middle Eocene.

Ecological factors required for the healthy development of Rhodolith beds in recent oceans are well-oxygenated bottom conditions, low sedimentation rates, low content of suspended particles, and moderate water energy. Except where Rhodoliths were transported from shallower settings, Rhodoliths in the rest of the study areas formed in oxygenated conditions, as shown by the prolific abundance of accompanying faunas, such as Sea Urchins, Corals, larger benthic Foraminifers, Bryozoans, and Molluscs. In addition, carbonate sedimentation devoid of terrigenous particles indicates low sediment supply and, consequently, clear waters. Finally, absence of sedimentary structures suggests that turbulence was low to moderate.

Although local palaeoenvironmental conditions were a priori favorable for Rhodolith bed development, Rhodoliths are major constituents in the study tropical Middle Eocene shallow platform deposits, whereas larger benthic Foraminifers with varying proportions of Calcareous Red Algae dominate in carbonate deposits at mid latitude. Similarly, Eocene deposits worldwide are mostly characterised by Rhodoliths and Coralline Algal fragments dispersed in larger benthic Foraminifer-dominated carbonates, and the few examples of Rhodolith beds were found so far in the Early or Late Eocene. The low proportion of densely packed Rhodolith beds during the Eocene, and particularly during the Middle Eocene, coincides with a relative decline in Algal diversity and with a significant decline in reef ecosystems.

Distribution of Rhodolith beds (red circles) and larger benthic Foraminifer-dominated carbonates with variable proportions of Rhodoliths (blue circles) during the Eocene. Diamonds indicate Rhodolith beds (red diamonds) and larger benthic Foraminifer-dominated carbonates with dispersed Rhodoliths (yellow diamonds) but with no chronological precision. Aguirre et al. (2020).

An earlier study performed a detailed facies analysis of Middle Eocene to lower Oligocene ramp carbonates in different localities from Central and Southern Alps. Interestingly, Coralline-Algal dominated facies (marl, Rhodolith, Algal debris, and crustose Algal facies) are frequent in Late Eocene and Early Oligocene deposits, while Middle Eocene carbonate facies were largely dominated by larger benthic Foraminifers with subordinate Algal debris and local Rhodolith concentrations in middle ramp settings. 

Likewise, at several seamounts southeast of Japan, Middle–Late Eocene shallow water carbonates mainly dominated by larger benthic Foraminifers have been described. Coralline Algae, forming Rhodoliths or as fragments in rudstones to packstones, occur in lesser abundance. They became dominant afterwards, in Oligocene-to-Pleistocene carbonate deposits in the same western Pacific areas.

Profuse development of larger benthic Foraminifers mostly takes place in oligotrophic conditions, although they can be also important in nutrient-rich tropical sediments in upwelling areas. Regarding Coralline Algae, it is not clear whether nutrient contents do actually promote the development of Rhodolith beds. Present-day Coralline Algae withstand strong annual variations in nutrient conditions, from nearly depleted settings to high levels of nutrients. However, it seems that profuse Rhodolith beds mainly occur in mesotrophic conditions. The largest Rhodolith beds in tropical latitudes occur nowadays on the eastern Brazilian shelf, in areas with relatively reduced development of Coral reefs. Here, Rhodolith beds extend from shallow subtidal settings to the shelf margin and thrive under mesotrophic conditions, with mean seawater temperatures higher than 20°C on the sea floor, and low terrigenous sedimentation, which is generally limited to near-shore areas. Similarly, extensive Rhodolith beds are found in the Amazon River mouth in the northwestern Brazilian platform, associated to the so-called Great Amazon Reef System. This is a complex of carbonate buildups including Scleractinian Corals, encrusting Coralline Algae, Sponges, and Rhodolith beds developed in the marginal areas of siliciclastic influx from the Amazon River under mesotrophic conditions. 

In subtropical latitudes in the Gulf of California, Rhodoliths spread throughout the gulf. Large and dense Rhodolith beds extend from shallow subtidal zone to about 40 m depth and occur in a wide spectrum of environmental conditions, with extreme variations of temperature (8–32°C), and in mesotrophic waters in the middle part of the Gulf of California. Fine sediment input and related anoxia seem to be strong limiting factors for Rhodolith development.

In a similar way, the Rhodolith beds in the Mediterranean occur in mesotrophic areas with reduced sedimentation and far from high nutrient influx. In the subtropical Western Pacific, on the shelves around the Ryukyu Islands, Rhodolith beds develop in nutrient poor waters lacking significant upwelling. 

The greenhouse conditions prevailing during great part of the Eocene favored the establishment of productive equatorial ocean waters and oligotrophic conditions widespread in middle and high latitudes. Palaeoceanographic models as well as type of sediments show that productive upwelling zones were located in low latitudes, particularly in the Pacific, during the Eocene. There is evidence of low nutrient conditions at midlatitude in the southeastern Atlantic Ocean during the Middle Eocene Climatic Optimum. In Aguirre et al.'s study cases, the Middle Eocene deposits of the Chengue Formation in Colombia were formed in mesotrophic waters according to their micropalaeontological content. Aguirre et al. hypothesise that latitudinal gradient in oceanic productivity might account for the formation of Rhodolith beds and Rhodolith rudstone lithofacies in tropical areas, whereas larger benthic Foraminifer-enriched lithofacies prevailed in mid latitudes.

A precise reconstruction of environmental variables in middle Eocene carbonate records is difficult, which is generally true for Palaeogene larger benthic Foraminifer- and Coralline Algal-dominated sediments. The impact of high temperatures due to high levels of atmospheric carbon dioxide during the Eocene, and particularly during the hyperthermal events, on Rhodolith bed development needs to be assessed. In this regard, sustained anomalously high summer temperatures led to high mortality rates of Coralline Algae in rhodolith communities along the western coast of Australia. In addition, and taking into consideration the discussion made above, the prevailing oligotrophic conditions at global scale accounting for the general prevalence of larger benthic Foraminifers during the Middle Eocene and the relative decline of Rhodolith beds worldwide requires further analyses. More calibration studies, essentially geochemical, for reconstructing water temperature and palaeoproductivity, and knowledge of the depth habitats of benthic and planktic organisms are needed to define the multifactor settings that drove the carbonate grain associations found in low- and mid-latitude regions during the Middle Eocene.

In two tropical settings, the Dominican Republic and Colombia, Middle Eocene Coralline Algae occur as dense concentrations of Rhodoliths in Rhodolith rudstone lithofacies. Rhodoliths are ellipsoidal to spheroidal in shape and are composed by encrusting to warty Coralline Algal thalli in association with benthic Foraminifers, Bryozoans, Corals, and other calcareous Red Algae such as Parachaetetes asvapatii and Polystrata alba.

In mid-latitude areas in Southern Spain, Coralline Algae occur in two different contexts. In Salinas Menores ravine, Rhodoliths are spheroidal to ellipsoidal and occur dispersed in planktic Foraminifer-rich marls. In the Sierra del Zacatín, larger benthic Foraminifers dominate the Middle Eocene carbonate deposits and Rhodoliths are scarce. Coralline Algae mostly occur attached to and intergrown with Corals. Rhodoliths consist of encrusting and warty (occasionally fruticose) Algal thalli intergrown with Bryozoans, Corals, and benthic Foraminifers.

In all the study areas, Coralline Algal assemblages are dominated by Hapalidiales and Sporolithales. The order Corallinales is scarcely represented, except in Salinas Menores, where its members are relatively abundant and diverse. Within this group, Subterraniphyllum thomasii is found in the Salinas Menores section. The oldest previously known record of this species is from the Late Eocene; therefore, Aguirre et al.'s finding extends the occurrence of Subterraniphyllum thomasii back to the Middle Eocene.

Rhodoliths in the two tropical areas developed in relatively deep platform settings (tens of meters of water depth) as shown by the larger benthic Foraminifers and Coralline Algal assemblages. In the Salinas Menores section, Miliolids in the internal voids indicate that Rhodoliths grew in a shallow-water inner platform or lagoon and were re-deposited in deep outer-shelf settings.

During the greenhouse conditions in the Early–Middle Eocene, shallow-water carbonate platforms from the tropics to intermediate latitudes were depauperate in Rhodolith beds. The key palaeoenvironmental factors accounting for this decline remain elusive up to now. Extremely high global temperatures due to high atmospheric carbon dioxide concentrations could negatively affect Coralline Algae. In addition, distribution of oceanic productivity might account for the main carbonate producers in marine shelves: mesotrophic conditions associated with upwelling areas in tropical regions could have favored the development of Rhodolith beds, such as those in the Dominican Republic and Colombia, while oligotrophic conditions in mid–high latitudes catalysed the widespread dominance by larger benthic Foraminifer assemblages, as observed in Southern Spain.

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Ancient forest on Welsh coast exposed by storm.

A forest of petrified trees on the coast of Ceredigion County, Wales, has been exposed by a storm this week. The forest, which comprises the stumps of hundreds of Pine, Pinus, Alder, Alnus, Oak, Quercus, and Birch, Betula, stumps stretches from Ynyslas to Borth, and is thought to be between 4500 and 6000 years old. While the forest is usually covered by sediment, it is periodically exposed following storms, having last been seen in 2014, and is even found in local folklore, as Cantre'r Gwaelod, a mythical kingdom on the west coast of Wales that sunk into the sea, sometimes known as the 'Welsh Atlantis'.

Exposed tree stumps on the Ceredigion Coast in May 2019. Matthew Horwood/Getty Images.

Most of the trees show a distinctive growth pattern of growth, with most of the roots spreading along the surface, with only a few roots extending downwards as anchors. The exception to this rule is the Alder stumps, which have consistently deeper root systems. This style of growth is typical of trees growing in wetland environments with a high water table, where most trees struggle to get oxygen to deep roots submerged in water, something which Alder trees are adapted for such environments. Alder trees typically lower the water table where they live, and often form the first stage of the colonisation of wetland environments by terrestrial woodlands, but in this case the reverse seems to have happened, with the waters rising and eventually drowning the trees.

 Exposed tree stumps on the Ceredigion Coast in May 2019. Matthew Horwood/Getty Images.

A study of the Ceredigion Submerged Forest published in the journal New Phytologist in 1938 by Harry Godwin and Lily Newton, based largely on pollen and Foraminifera extracted from boreholes by Florence Campbell James, suggested that an ancient Reed-bed trapped a raised bogland behind it, which was then colonised by first the peat-forming Moss Sphagnum sp., then an Alder woodland, which was in turn overwhelmed by marine waters.

The Ceredigion Submerged Forest exposed in May 1923. Challinor in Godwin & Newton (1938).

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