Levisoni Zone Research Articles

Toarcian ammonites of the Polymorphum Zone, Amellago area (eastern central High Atlas), Morocco, with a discussion on ammonite turnover events

In the Amellago area (central High Atlas), the widely distributed Toarcian succession consists of marls and marly limestones of the Tagoudite and Agoudim formations. We here describe, for the first time, the ammonites of the Polymorphum Zone from the Tagoudite Formation, including Canavaria cf. rosenbergi Fucini, C. aff. zancleana (Fucini), Dactylioceras (Eodactylites) mirabile (Fucini), D. (E.) aff. mirabile (Fucini), D. (E.) pseudocommune Fucini, D. (E.) simplex (Fucini), D. (Orthodactylites) aff. crosbeyi (Simpson), Lytoceras gr. villae Meneghini, Neolioceratoides cf. hoffmanni (Gemmellaro) and Praepolyplectus sp. This ammonite assemblage is closely correlated with those reported from the Polymorphum Zone of several basins of the Tethyan margins and the Northwest European Subboreal Realm, indicating marine communications between the different Moroccan Jurassic basins and the central Atlantic trough. The overlying Agoudim Formation yields ammonites of the Levisoni Zone in its base and rare ammonites of the Bifrons Zone in the middle part. Four significant changes in the ammonite assemblages during the early-middle Toarcian and their probable causes are also discussed.

Brachiopodes toarciens de la coupe d’Aït Athmane, Haut Atlas centro-oriental (Maroc)

The Aït Athmane section is located in the eastern part of the central High Atlas, about 20km north of Errachidia. Its Toarcian series consists of silty marl and silty marly limestone. Its lower part, of early (Levisoni Zone?) to middle Toarcian (Bifrons and Gradata? Zones) age, is rich in brachiopods (rhynchonellides and terebratulides). Its upper part, of late Toarcian age, is rich in terebratulids. Among eighteen reported brachiopod taxa, six are new for the central High Atlas. Their specific assemblages and stratigraphic distributions are similar to those of the Toarcian brachiopods reported from several basins of northern and southern margins of the Tethys and the Subboreal domain, particularly of western France.

Ammonites du Toarcien du Haut Atlas central (Maroc)

In the central High Atlas, the Toarcian Stage is represented by a marl and marly limestone series of variable thickness. Two sections have been studied: Amellago (500m) and Aït Athmane (100m). Ammonites from the two sections allowed to characterize the Polymorphum, Levisoni, Bifrons, Gradata, and Speciosum zones. The Polymorphum Zone provided, among others: Dactylioceras (Eodactylites) mirabile, D. (E.) pseudocommune, Dactylioceras (Orthodactylites) aff. crosbeyi and Neolioceratoides aff. hoffmanni; the Levisoni Zone provided a rich ammonite fauna composed of, Calliphylloceras nilssoni, Dactylioceras (Orthodactylites) cf. semiannulatum, Eleganticeras exaratum, Harpoceras falciferum, H. pseudoserpentinum, H. serpentinum, H. subplanatum, Hildaites cf. forte, H. levisoni, H. cf. serpentiniformis, H. cf. subserpentinus, H. striatus, H. wrighti, Lytoceras siemensi, Lytoceras sp., Maconiceras soloniacense, Phylloceras sp., Polyplectus pluricostatum et Polyplectus sp.; the Bifrons Zone provided Eleganticeras sp., Harpoceras subplanatum?, Hildoceras bifrons, H. lusitanicum, H. semipolitum, H. sublevisoni, Hildoceras sp., Porpoceras gr. vortex–verticosum; the Gradata Zone was characterized by Pseudocrassiceras bayani, P. frantzi, Pseudocrassiceras sp., Pseudocrassiceras sp. indet.; and the Speciosum Zone provided an Hammatoceras aff. insigne. These species have been described and illustrated for the first time for the central High Atlas. Most of them are common to several basins of the north Tethyan margin and the Subboreal Domain of NW Europe.

The onset of the Early Toarcian flooding of the Pliensbachian carbonate platform of central Tunisia (north–south axis) as inferred from trace fossils and geochemistry

Abstract The flooding of the Lower Jurassic shelf in the North Gondwana Palaeomargin during the early Toarcian occurred on a fragmented and irregular topography affected by differential subsidence – owing to the activity of listric faults along the North–South Axis of Tunisia – that favoured lateral changes in facies and thickness at a kilometric scale. The onset of Toarcian sedimentation (Polymorphum ammonite Zone, NJT5c nannofossil Subzone) in two adjacent sections was characterized by the deposition of limestones under high-energy conditions. The Châabet El Attaris section was located in a depressed sub-basin, and recorded restricted environmental conditions owing to water stagnation and an oxygen-depleted sea bottom. Therefore, dark mudstones developed, with increased total organic carbon contents and enhanced accumulation of redox-sensitive elements. The sedimentation of limestones bearing gutter cast structures is related to gravity flows probably linked to storm activities. These processes favoured the remobilization of sediments at the seafloor, as well as oxygen input to bottom waters, as shown by the record of trace fossils including Zoophycos , Ophiomorpha , and secondarily, Chondrites and Diplocraterion . The thinly interbedded dark mudstones are locally rich in thin-shelled bivalves that re-colonized the sea bottom after the sedimentation of these high-energy deposits, and subsequently underwent mass mortality related to the return of oxygen-depleted conditions. The Kef El Hassine section is located in the upper part of a tilted, less subsident block, as indicated by its reduced thickness compared with the Châabet El Attaris section; the absence of dark mudstones implies oxic conditions. The Polymorphum Zone consists of limestones showing evidence of sedimentation under high-energy conditions, along with hardgrounds. The occurrence of Zoophycos (deep-tiers) in the upper part of some limestone beds of the Polymorphum Zone is linked to minor erosive processes. The top of the high-energy sequence – below the deposits of a marly interval corresponding to the Levisoni Zone – is interpreted as a hardground given the high content of belemnites and Arenicolites , some of them boring on the eroded Zoophycos and Thalassinoides . This study shows that the sedimentary expression of the Jenkyns Event is not uniform across Tunisia, supporting the importance of local conditions in determining the development of anoxic conditions.

Sedimentary organic matter and early Toarcian environmental changes in the Lusitanian Basin (Portugal)

The objective of this study is to investigate the relationship between early Toarcian climatic events and the composition of kerogen assemblages in the Lusitanian Basin (Portugal). In particular, we aim to understand how the Pliensbachian–Toarcian Event (Pl–Toa Event) and Toarcian Oceanic Anoxic Event (TOAE) affected the continental areas of the western Iberian margin and how possible variations in continental sources of organic matter (OM) were expressed in marginal-marine and hemipelagic depositional environments during the early Toarcian.We present here a characterisation [total organic carbon (TOC), total sulphur (TS), δ13C in kerogen concentrate (δ13CKerogen), and palynofacies] of kerogen assemblages from several uppermost Pliensbachian (emaciatum ammonite Zone)–lower Toarcian sections in the Lusitanian Basin (western Iberian margin), including the Peniche section, which contains the Toarcian GSSP. In all sections, TOC concentration is low (average 0.4 wt.%), with the highest values reaching up to 2.1 wt.% in a discrete level located approximately 10.5 m above the base of the levisoni Zone at Peniche. The TOAE negative carbon isotope excursion is observed in kerogen concentrate at the base of the levisoni Zone throughout the basin. Palynofacies analysis demonstrates that the kerogen assemblages are mostly of terrestrial affinity, with the dominance of the Phytoclast Group and terrestrial palynomorphs, and with punctual increases in amorphous organic matter, freshwater (e.g. Botryococcus sp.) and marine microplankton (dinoflagellate cysts, acritarchs, and prasinophyte algae) in specific stratigraphic intervals. A change in palynofacies assemblages associated with the TOAE is observed around the base of levisoni Zone. Although with slight differences between sections, the TOAE interval records an increase in non-opaque, translucent, phytoclasts (NOP) and cuticle fragments, and is also associated with an increased contribution of terrestrial palynomorphs (increase in sporomorphs and Classopollis in tetrads and agglomerates) and decrease in marine palynomorphs. The increases in NOP and terrestrial palynomorphs support the postulated enhancement of the hydrological cycle and increased export of terrestrial OM into marine environments during the early Toarcian, especially during the TOAE, in the western Iberian margin. Understanding the impact of the early Toarcian climatic events on land-based ecosystems may provide important insights into current climate change.

Changes in foraminiferal assemblages and environmental conditions during the T-OAE (Early Jurassic) in the northern Lusitanian Basin, Portugal

The foraminiferal assemblages and other microfossils from the Rabaçal area (Maria Pares section) in the Lusitanian Basin (Portugal) provide new insights regarding the Toarcian Oceanic Anoxic Event (T-OAE). Complementary analysis of the δ13C, total organic carbon (TOC) and geochemical redox and palaeoproductivity proxies reflect environmental changes that affected the microfossil assemblages. Detailed analyses of foraminiferal assemblages (their diversity, abundance, morphogroups and lifestyle) and geochemistry led us to discern phases of the biotic crisis and subsequent stages of recovery. Environmental instability, which began in the lattermost part of the Polymorphum Zone, is marked by a lesser abundance of foraminifera and ostracods, increased opportunist foraminifera, fluctuations in TOC values and the enrichment factor of phosphorous (PEF), as well as the onset of a negative carbon isotopic excursion (CIE). The peak phase of the biotic crisis corresponds with the lowest values for foraminiferal diversity and abundance (including a barren sample for benthic microfossils). The abundance and diversity of echinoderms, brachiopods, dinoflagellate cysts and calcareous nannofossils dropped sharply during the peak phase. This biotic crisis also entails extinction of the Order Metacopina and a local disappearance of echinoderms and brachiopods. Anoxic conditions at sea bottom did not develop, given the almost continuous record of foraminifera and trace fossils, yet oxygen-depleted conditions occurred. The lower part of the Levisoni Zone contains thin nodular limestones corresponding to a tempestitic–turbiditic facies. In this context, the re-oxygenation of bottom waters was favoured, as indicated by the presence of Thalassinoides and the low values of TOC and redox-sensitive elements (Mo, Ni and U). The survival phase in the wake of the biotic crisis is characterized by greater diversity and abundance of foraminifera and echinoderms (holothuroids and ophiuroids). The lowest δ13C values are recorded during this phase, with a dominance of opportunist (Eoguttulina) and facultative opportunist (Paralingulina tenera) foraminifera. Finally, recovery after the biotic crisis is evidenced by high values for specialists and potentially deep infaunal foraminifera, indicating oxic conditions in the sediment.

Ichnological analysis at the Fonte Coberta section (Lusitanian Basin, Portugal): Approaching depositional environment during the Toarcian oceanic anoxic event (T-OAE)

The Toarcian Oceanic Anoxic Event (T-OAE) is commonly related with an extended marine mass extinction and global oceanic anoxic conditions. However, recently it has been demonstrated that local palaeotopography/palaeogeography can determinate the absence of anoxic conditions, being other limiting environmental parameters of higher signifi cance. The detailed study of the Fonte Coberta section (Lusitanian Basin, Portugal) shows significant changes in the macrobenthic community at the lower Toarcian (Levisoni Zone) sediments during the event. In the study section, this interval presents a high degree of bioturbation with a relatively abundant and moderately diverse trace fossil assemblage dominated by Thalassinoides, with punctual Spongeliomorpha. The ichnotaxonomical study conducted on this trace fossil assemblage has allowed the recognition of a single species of Thalassinoides, T. suevicus Rieth, with two different morphotypes (I and II), as well the differentiation of Spongeliomorpha aff . iberica Saporta. The distribution of these ichnospecies, including the differentiated morphogroups, along the studied interval, corresponding to the T-OAE, allows refining depositional conditions as changes in the substrate consistency related to variations in relative sea level. Ichnological analysis reveals as a proxy to improve sequence stratigraphy characterization for the Lusitanian Basin during the T-OAE.

Uppermost Pliensbachian-Lowermost Toarcian ostracod assemblages from the Western Tethys: Comparison between Traras Mountains (Algeria), Subbetic (Spain), and Algarve (Portugal)

Major changes in ostracod assemblages have been recorded in the Lower Jurassic of Western Tethys related to biotic crises of the Pliensbachian-Toarcian boundary and the Polymorphum-Levisoni boundary (Toarcian Oceanic Anoxic Event). The study is focused on the ostracods assemblages from Benzerka section (Traras Mountains, Algeria) and their comparison with sections from External Subbetic and Algarve Basin (South Iberian Palaeomargin). Three stratigraphic intervals coincident with the ammonite zones have been differentiated with important fluctuations in the composition of ostracod assemblages. In the ostracod assemblages from the Traras Mountains the turnover related to this benthic biotic crisis is shown by assemblages previous to the biotic crisis (uppermost Pliensbachian) mainly composed by families Healdiidae and Cytherellidae to assemblages just after the T-OAE (upper part of the Levisoni Zone) with assemblages dominated by families Protocytheridae and Polycopidae and the extinction of family Healdiidae (suborder Metacopina). The Toarcian Oceanic Anoxic Event (T-OAE) probably resulted in restricted conditions but a barren interval due to oxygen-depleted conditions in this setting is not recorded. In this context, the change of components of ostracod assemblages looks gradual. In the Southiberian Palaeomargin, represented by the External Subbetic the turnover of ostracods assemblages is also recorded but the families involved are different than in Traras Mountains. Therefore, different phases have been differentiated in the ostracod turnover occurring from Upper Pliensbachian (assemblages dominated by families Pontocyprididae, Healdiidae and Bairdiidae) to end of Lower Toarcian (assemblages dominated by family Cytherellidae and the extinction of family Healdiidae). In the Algarve Basin the families Healdiidae and Bairdiidae were dominant previously to the Toarcian Oceanic Anoxic Event. Differences in the composition of ostracod assemblages between Traras Mountains and External Subbetic may be related to the fragmentation of these palaeomargins related to early stages of rifting in the Western Tethys. Moreover, the incidence of oxygen-depleted conditions was higher in External Subbetic than in Traras Mountains where some ostracod genera survive during the biotic crisis.

Ostracod assemblages from the uppermost Pliensbachian and Lower Toarcian of the Traras Mountains (Tlemcen Domain, north Algeria)

This work presents a taxonomic study of the Upper Pliensbachian-Lower Toarcian ostracods from the Traras Mountains situated in northwestern of Algeria. The ostracod fauna comprises 8 families, 11 genera and 20 species. Important changes in the composition of the ostracod assemblages have been recorded related to the Pliensbachian-Toarcian boundary and the Polymorphum-Levisoni zonal boundary (the biotic crisis related to the Toarcian Oceanic Anoxic Event). This turnover is denoted by the uppermost Pliensbachian assemblages dominated by the families Healdiidae and Cytherellidae to the upper part of Levisoni Zone with the assemblages dominated by the families Protocytheridae and Polycopidae. Ten species of ostracods are recognised for the first time from the Lower Toarcian of the North Gondwana palaeomargin (Bairdia kempfi, Bairdiacypris dorisae, B. triangularis, Ektyphocythere bizoni, Ogmoconcha convexa, Paracypris sp. and Polycope cincinnata). The comparison with other regions (intrafamilial comparison and qualitative comparison) reveals close relationships between the location of the basins and their taxonomic compositions.

Evidence of terrestrial organic matter deposition across the early Toarcian recorded in the northern Lusitanian Basin, Portugal

The Pliensbachian–Toarcian (Lower Jurassic) in the Lusitanian Basin (Portugal) is particularly well exposed through marl-limestone alternations very rich in nektonic and benthonic macrofauna. In the general context of Toarcian Oceanic Anoxic Event (T-OAE), the Lower Toarcian (Polymorphum and Levisoni ammonite zones) marks important sedimentological changes in the marine paleoenvironment. To better understand the paleoenvironmental changes that occurred during the latest Pliensbachian–early Toarcian, a high-resolution study of organic matter was undertaken in the Alcabideque section, in the northern sector of the basin. Based on the analysis of 14 samples from a section 20m thick, this study involved organic geochemistry [total organic carbon (TOC), sulfur and biomarkers] and palynofacies. The results confirm that sediments are particularly poor in organic matter, with the highest TOC values reaching 0.41wt.% around the top of Polymorphum Zone.The palynofacies analysis provides evidence that the organic matter largely consists of particles belonging to phytoclasts and palynomorphs groups. The phytoclasts present were generally degraded and oxidized, with significant representation in the Emaciatum and Polymorphum zones. The palynomorphs were mainly composed by sporomorphs, with a sporadic increase in marine palynomorphs (mainly dinocysts) at the top of the Emaciatum and Polymorphum zones. Continental organic matter was predominant at the base of Levisoni Zone (T-OAE), through the deposition of brownish marls (Chocolate Marls), which was confirmed by the occurrence of tetrads and polyads (pollen grain agglomerates), and by biomarkers, with the major presence of long-chain n-alkanes, n-alkane C17, C20 tricyclic terpane, C24 tetracyclic terpane and C29 regular sterane. The gammacerane index shows a peak at the top of Polymorphum Zone, clearly associated with the increase of marine palynomorphs and the maximum marine influence in the series.

Latest Pliensbachian –Early Toarcian brachiopod assemblages from the Peniche section (Portugal) and their correlation

In the present paper we describe the distribution of brachiopods in the proposed Toarcian GSSP (Global Boundary Stratotype Section and Point) at Peniche. We differentiated four assemblages in the stratigraphic interval from the upper Emaciatum Zone of the Pliensbachian to the Levisoni Zone of the Toarcian. Assemblage 1 clearly presents Northwestern European affinities, with many species that are also present in Southern England. In the last levels of the Pliensbachian and in the Mirabile Subzone (the first level of the Toarcian), Assemblage 2 contains taxa that still present Northwestern European affinities, but with a more restricted, even endemic, distribution. The majority of the species in these levels are known from the Iberian Range in Spain and other neighboring basins. An important faunal change takes place in the Semicelatum Subzone, coinciding with the base of the Cabo Carvoeiro Formation, giving rise to Assemblage 3. In this assemblage we observed a clear decrease in the size of the specimens, coinciding with the establishment of the Koninckella Fauna; this fauna is found in several localities in both Northwestern European and Mediterranean areas where the paleoenvironment is relatively deep or poorly oxygenated. Brachiopods disappear in Peniche just above the Polymorphum - Levisoni zonal boundary, as has been observed in several other localities in Western Tethys. Their renewal is marked by the presence of Soaresirhychia bouchardi several meters above the extinction level, constituting Assemblage 4.

Foraminiferal assemblages and geochemistry for interpreting the incidence of Early Toarcian environmental changes in North Gondwana palaeomargin (Traras Mountains, Algeria)

The Early Toarcian was characterised by important environmental changes and a mass extinction event usually related to a global oceanic anoxic event. The analysis of ecostratigraphic fluctuations of foraminiferal morphogroups, elemental geochemical proxies and C and O stable isotopes of the Mellala section (Tlemcen Domain, North Algeria) makes it possible to determine the incidence of the anoxic event in this sector of the north Gondwana palaeomargin. The end of the Pliensbachian is characterised by a diverse foraminiferal assemblage with equilibrium species suggesting good oxygen and nutrient availability. The beginning of the Toarcian (Polymorphum Zone) evidences major changes in foraminifera with the disappearance of species, decreasing proportions of epifauna and shallow infauna, and fluctuations in diversity and dominance of Lenticulina toarcense and Lingulina tenera confirming a perturbation in the palaeoecological conditions in the sea-bottom. Redox proxies (Co/Al, Cr/Al and V/Al) with local maximum values suggest a decrease in oxygenation degree. A negative excursion of δ13C is recorded right at the Polymorphum/Levisoni Zone boundary, and the subsequent disappearance of epifauna, decreasing diversity and abundance of foraminifera (foram/100g) would be related to the accentuation of stressing conditions. Also at the Polymorphum/Levisoni Zone boundary, suboxic waters at the sea-bottom indicate the maximum values of redox proxies (Co/Al, Cu/Al, Cr/Al and V/Al). The upper part of the Levisoni Zone is more calcareous, with increasing diversity of shallow infauna and a decrease in potentially deep infauna related to more favourable conditions. The incidence of the Toarcian Oceanic Anoxic Event in this context was very low in comparison with the Saharan Basin (Raknet El Kahla section, Saharan Atlas), where a benthic barren interval, higher total organic carbon and redox proxies are recorded. The low incidence of the biotic crisis and the rapid recovery of assemblages in the Tlemcen Domain is compared with the high incidence and delayed recovery in the Saharan Basin, where the palaeogeography determined restricted water circulation between the Saharan Craton and the Oran Massif.

Pacing of the Toarcian Oceanic Anoxic Event (Early Jurassic) from astronomical correlation of marine sections

The Early Toarcian Oceanic Anoxic Event (OAE) in the Early Jurassic Period is associated with a major negative carbon isotope excursion (CIE), mass extinction, marine transgression and global warming. The Toarcian OAE is thought to have been caused by flood basalt magmatism, and may have been a trigger for mass extinction. However, these proposed causes of the Toarcian OAE and associated biotic crisis are not adequately resolved by a precise chronology. The duration of the Toarcian OAE has been estimated to be anywhere from ~0.12 to ~0.9Myr, most recently 0.74 to 3.26Myr from U–Pb dating. The CIE associated with the Toarcian OAE has a similar pattern at numerous localities, and there is evidence that the marine carbon isotope variations recorded astronomical forcing signals. Here we estimate a duration of ~620kyr for the main negative CIE, ~860kyr for the polymorphum zone and >1.58Myr for the levisoni zone based on 405-kyr astronomical eccentricity tuning of the marine section at Peniche (Portugal). This 405-kyr tuned series provides a ~2.5Myr continuous high-resolution chronology through the Early Toarcian. There are 6, or possibly 7 short eccentricity cycles in the main CIE interval at Peniche. To confirm this astronomically based estimate, we analyzed three other sections at Yorkshire (UK), Dotternhausen (Germany), and Valdorbia (Italy) from marine carbon isotopic series. These four stratigraphic sections from Early Jurassic western Tethys record the Toarcian OAE with ~6 prominent carbon isotope cycles in the CIE that span a 600±100kyr duration. The Peniche 405-kyr tuned series indicates that the pre- and post-CIE intervals experienced strong precession–eccentricity-forced climate change, whereas the CIE interval is marked by dominant obliquity forcing. These dramatic and abrupt changes in astronomical response in the carbon isotopes point to fundamental shifting in the Early Toarcian paleoclimate system that was directly linked to the global carbon cycle.

The Toarcian oceanic anoxic event in the Western Saharan Atlas, Algeria (North African paleomargin): Role of anoxia and productivity

We used an integrated analysis of benthic foraminiferal assemblages, geochemical proxies (paleoproductivity, redox, and detrital proxies), and C and O stable isotopes to characterize the Toarcian oceanic anoxic event along the North African paleomargin, more precisely in the Saharan Atlas (Algeria). Special emphasis was placed on time intervals previous to the anoxic event and the recovery of normal conditions after this event. The Fisher9s alpha diversity of the foraminiferal assemblages decreased with respect to the end of the Pliensbachian in the polymorphum Zone, with lower values of shallow infaunal forms and an increase in opportunists ( Reinholdella , Lenticulina , and Eoguttulina ). This indicates fluctuations in the oxygenation degree of the infaunal microhabitat in the polymorphum Zone despite the fact that redox proxies do not show any fluctuations, indicating that dysoxic conditions were not reached. The lower part of the levisoni Zone reflects an abrupt paleoenvironmental change, evidenced by the extinction of foraminifera, enrichment in redox-sensitive elements, increase in detrital and paleoproductivity proxies, and fluctuations of δ 13 C and δ 18 O. An anoxic or strongly dysoxic event happened by the polymorphum - levisoni zone boundary in the Western Saharan Atlas, coincident with increasing eolian detrital input and paleoproductivity. The upper part of the levisoni Zone represents a return to normal oxic conditions, and the slow recovery of benthic foraminiferal assemblages dominated by opportunistic forms ( Lenticulina , Eoguttulina , and Reinholdella ) of low diversity. The Toarcian oceanic anoxic event occurred in a generalized transgression that may have favored water stratification and confinement of bottom waters in the subbasins developed in the Atlasic domain. This research demonstrates the potential of the integration of data from foraminiferal assemblages and a wide range of geochemical proxies for understanding anoxic events during the Mesozoic.

Foraminiferal morphogroups as a tool to approach the Toarcian Anoxic Event in the Western Saharan Atlas (Algeria)

The present contribution constitutes the first morphogroup analysis of foraminiferal assemblages applied to the Toarcian Oceanic Anoxic Event (T-OAE) in the Maghreb region, which represented the northern palaeomargin of Gondwana. The ecostratigraphic and palaeoecologic analysis of the foraminiferal assemblages has allowed interpretation of the evolution of the biotic crisis associated with the T-OAE in the Saharan Atlas deposits and the recovery of foraminiferal assemblages in the sea-bottom after the event. The foraminiferal assemblages of the Uppermost Pliensbachian (Emaciatum Zone) represent optimum ecological conditions (good oxygen and nutrient availability in the infaunal microhabitats) as evidenced by the high diversity of K-strategists and the low diversity of r-strategists (representing a minor proportion in the assemblages). The beginning of the Toarcian (Polymorphum Zone) shows an initial phase of unstabilization of the benthic environment, affecting some particular species while opportunistic forms proliferate (Epistomina and Lenticulina). Variations in the oxygenation degree affecting the infaunal microhabitat favoured stress-tolerant species with extensive niche such as Lenticulina and Eoguttulina. Later, a benthic azoic interval includes the lower part of the Levisoni Zone. In this interval, a slight increase of total organic carbon (TOC) values recorded can be related to dysoxic or anoxic conditions in low-oxygen bottom-water, beyond the ecological tolerance of opportunist forms. After the benthic mass extinction interval, a slow colonisation is registered (upper part of the Levisoni Zone) with a dominance of r-type strategists (Lenticulina, Reinholdella and Eoguttulina).

Faune corallienne des récifs toarciens du Moyen Atlas marocain, première approche

Abstract Des récifs ont été décrits dans le Toarcien supérieur du Moyen Atlas Marocain [Elmi et al., 2002]. Ils sont localisés dans le synclinal de Awragh-Afennourir (Moyen Atlas occidental) immédiatement au sud du Moyen Atlas tabulaire. Ce sont des biohermes placés au sommet de blocs basculés. Leur épaisseur varie de 1 à 12 m. Ils reposent sur une formation datée de la zone à Levisoni, sous zone à Gemma et sont couverts par des sédiments appartenant à la partie supérieure de la zone à Meneghini. Leur paléolatitude était de 18°N. Un ensemble de 18 échantillons de coraux collectés par S. Elmi sont ici analysés. Ils constituent un témoignage exceptionnel de faune corallienne récifale pour cet étage. Les taxons appartenant aux genres Cladophyllia, Diplocoenia ?, Montlivaltia (deux espèces), Thecosmilia (deux espèces), Isastrea ?, Latomeandra, Periseris, Microphyllia (trois espèces), et Proleptophyllia (deux espèces), ont été identifiés. La faune inclut 12 formes coloniales et 7 solitaires. Parmi les spécimens coloniaux, les formes méandroïdes avec un haut niveau d’intégration colonial sont bien représentées et fournissent un argument fort pour la présence de zooxanthelles symbiotiques. L’examen des textures micritiques et des croûtes microbialitiques, associé avec le développement important des microarchitectures pennulaires suggèrent un récif sous le niveau d’action des vagues de beau temps et des eaux plutôt mésotrophiques. Les coraux, leur forme et leur diversité, permettent de diagnostiquer des récifs de zone photique, à une profondeur et une latitude optimale. Les extensions stratigraphiques de plusieurs genres sont agrandies : nouvelle première occurrence pour Cladophyllia, Latomeandra et Periseris (peut être aussi Diplocoenia) et nouvelle dernière occurrence pour Proleptophyllia classiquement considéré comme Pliensbachien. La seule monographie disponible à caractère général sur les coraux du Lias du Maroc [Beauvais, 1986] identifie 15 taxons génériques pour le Toarcien. Tous proviennent du Haut Atlas et seulement 3 taxons sont communs avec les résultats présentés ici. La vue générale est celle d’une faune qui ressemble davantage à celle des communautés récifales du Dogger qu’à celle du Pliensbachien.

Toarcian carbon isotope shifts and nutrient changes from the Northern margin of Gondwana (High Atlas, Morocco, Jurassic): Palaeoenvironmental implications

The Early Toarcian is marked by a global perturbation of the carbon cycle and major marine biological changes. These coincide with a general decrease in calcium carbonate production and an increase in organic carbon burial, and culminate in the so-called Toarcian Oceanic Anoxic Event. It is believed that the environmental crisis was triggered by the activity of the Karoo–Ferrar large igneous province. In order to further document the Early Toarcian palaeoenvironmental perturbations, carbon isotope, total organic matter, calcareous nannofossils and phosphorus content of the Amellago section in the High Atlas rift basin of Morocco were investigated. This section is extremely expanded compared to the well-studied European sections. Its position along the northern margin of the Gondwana continent is of critical importance because it enables an assessment of changes of river nutrient input into the western Tethyan realm. The carbon isotope curve shows two negative excursions of equal thickness and amplitude, at the Pliensbachian–Toarcian boundary and at the transition from the Polymorphum to the Levisoni Zone. This confirms the supra-regional nature of these shifts and highlights the possible condensation of the first “boundary” shift in European sections. Phosphorus content is used to trace palaeo-nutrient changes and shows that the two negative carbon isotope shifts are associated with increased nutrient levels, confirming that these episodes are related to enhanced continental weathering, probably due to elevated greenhouse gases in the atmosphere. In the High Atlas Basin, the increase in nutrient levels at the Pliensbachian–Toarcian boundary is moreover likely to be the main factor responsible for the coeval demise of the Saharan carbonate platform. A middle Toarcian event, centered on the boundary between the Bifrons and Gradata Zones, characterized by a positive carbon isotope excursion and nutrient level rise, is documented in the Amellago section.

TOARCIAN NANNOBIOHORIZONS FROM LUSITANIAN BASIN (PORTUGAL)AND THEIR CALIBRATION AGAINST AMMONITE ZONES

The calcareous nannofossil content of the Toarcian succession, widespread in the Lusitanian Basin (Central Portugal), has been studied on the basis of a closely spaced sampling and a refined lithostratigraphic frame dated by ammonites. The almost continuous calcareous nannofossil record allows to calibrate the biohorizons and zone boundaries (NJ5/NJ6, NJ6/NJ7 NJ7/NJ8) with respect to the ammonite zones, and to characterize Toarcian nannofossil zones. The easily recognizable events include the LCO of Calcivascularis jansae (Levisoni Zone), the FOs of Carinolithus cantaluppii and Carinolithus superbus (Levisoni Zone), the FO of Discorhabdus striatus (Bifrons Zone), the AE of Lotharingius hauffii (Gradata Zone) and the FO of Watznaueria contracta (Aalensis Zone). The other events are helpful to increase the resolution, albeit they are defined by taxa with a low abundance and discontinuous occurrence. They comprise the LOs of Biscutum grande and Biscutum finchii (Levisoni Zone) and the FOs of Discorhabdus criotus (Gradata Zone), Retecapsa incompta (Bonarelli Zone) and Triscutum sullivanii (Speciosum Zone). Both biohorizons and zone boundaries are compared with previous findings from the Basque-Cantabrian area and Iberian Range (Spain) as well as from NW Europe and Italy/S France.

Évolution des peuplements de foraminifères pendant la crise toarcienne à l’exemple des données des monts des Ksour (Atlas saharien occidental, Algérie)

The micropalaeontologic analysis of the Lower Toarcian from the Ksour Mountains (western Saharan Atlas, Algeria) allows us to recognize several assemblages of benthic foraminifera and to obtain information about the deposit environment. These assemblages dominated by Nodosariids are organized in four successive stages of settlement (normal, survival, extinction, and repopulation). This distribution gives new information on the environmental conditions and on their effect on the change in the vertical distribution of benthic foraminifera. The quantitative evolution documents a progressive deepening of the environment that is controlled by local tectonic and sedimentary dynamics. The result is an ecosequence depending both on the general sea-level rise and on a progressive confinement of the bottom water. The normal stage occurred at the beginning of the Polymorphum zone; uncoiled and small sized Nodosariids species indicate the opening of the Atlasic furrow. After, during the middle of this zone, these species were associated with Polymorphinids and Ceratobuliminids, which indicate a deep but isolated and confined environment (survival stage). The ecosequence ended with an episode without foraminifera; it is the extinction stage dated in the Polymorphum and Early Levisoni zones. This stage corresponds to an umbilicus setting. Such palaeophysiographic conditions exaggerated the general coeval hypoxy, responsible of the main Toarcian crisis. Then, from the late Levisoni zone to the end of the Middle Toarcian, the ecosequence evolution is inverted, as a consequence of the filling of the deep parts of the umbilicus, which were less partitioned and more oxygenated (repopulation stage).

Estratigrafia Química (COT, δ13C, δ18O) e Nanofósseis Calcários na Passagem Pliensbaquiano–Toarciano no Perfil de Peniche (Portugal): Resultados Preliminares

The marl-limestone succession that encompasses the Pliensbachian–Toarcian Stage Boundary (Lower Jurassic) and crops out at Peniche (Lusitanian Basin, Portugal), was chosen as one of the candidates for the establishment of Toarcian GSSP. Chemostratigraphy analyses, of the Upper Pliensbachian (spinatum Zone)-Lower Toarcian (levisoni p.p. Zone) portion, were based on total organic carbon (TOC) (68 samples), the isotope carbon (δ13C) and oxygen (δ18O) (38 samples) on the whole rock sample. Presenting an absolute variation of around -2.0‰, the δ13C and δ18O values decrease from the middle part up to the uppermost part of spinatum Zone, with smaller values in the lowermost part of the polymorphum Zone. In the Lower Toarcian the δ13C data shows a positive trend (spread of + 2.0‰) with a maximum value in the middle-upper portion of the polymorphum Zone and a minimum in the lowermost part of the levisoni Zone. The δ18O values show a general tendency to decrease within the polymorphum Zone, with the smallest values observed in the lower portion of the levisoni Zone. In general the TOC values are low, around 0.2%, in the spinatum Zone, upwards they increase to 0.5% in the polymorphum Zone, whilst they decrease again to 0.2% in the lowermost levisoni Zone. Calcareous nannofossils assemblages were investigated in 12 slides of marly samples collected around the Pliensbachian – Toarcian Stage Boundary that, according to the adopted zonation, proposed for NW European, lies in the NJ5b biozone. Abundant and well preserved nannofossils assemblages comprise 12 genera and 18 species. The genera Schizosphaerella and Lotharingius are dominant. Calcivascularis jansae, a characteristic taxon of the Lower Jurassic tethyan nannofossils assemblages, is abundant in the whole investigated interval. Biscutum grande is the other tethyan taxon present in studied succession. The occurrences of C. jansae and B. grande support the tethyan affinity of the calcareous nannofossils assemblages recovered from the Pliensbachian – Toarcian transition sampled at Peniche section.