Ammonoid taphonomy, palaeoenvironments and sequence stratigraphy at the Bajocian/Bathonian boundary on the Bas Auran area (Subalpine Basin, south-eastern France)

Bas Auran, in south‐eastern France, is the candidate area for Global Boundary Stratotype Section and Point (GSSP) of the base of the Bathonian Stage (Middle Jurassic). In the Bas Auran area, upper Bajocian and lower Bathonian deposits are made up of limestone beds alternating with marls that correspond to the ‘Marno‐calcaires àCancellophycus’, below the ‘Terres Noires’ Formation. Taphonomic analyses of the successive ammonoid fossil assemblages provide new and complementary data for biostratigraphic completeness, palaeoenvironmental setting and sequence stratigraphy. Lithologic differentiation between limestone and marl intervals resulted from alternating episodes of carbonate input and starvation. Both lithologic phases may contain evidence for sedimentary and taphonomic reworking, associated with scours, that reflects low rates of sedimentation and stratigraphic condensation. Three successive types of elementary cycles resulted from increasing rates of stratigraphic condensation, sedimentary condensation and substrate stabilization during early Bathonian. The occurrence of reelaborated ammonoids (i.e. exhumed and displaced before their final burial) implies that tractive current flows or winnowing affected the burial of concretionary internal moulds. In the lower Bathonian strata, the dominance of homogeneous concretionary internal moulds of phragmocones, completely filled with sediment, is indicative of low rates of sedimentation and sediment accumulation, respectively associated with low degrees of stratigraphic and sedimentary condensation. However, at the Bajocian/Bathonian transition, hemipelagic, bed‐scale limestone–marl alternations show a maximum value of biostratigraphic completeness and there is no evidence for taphonomic condensation in the ammonoid fossil assemblages. Taphonomic analyses of the successive ammonoid fossil assemblages and taphofacies confirm the development of a deepening phase associated with sedimentary starvation, which characterizes the last episode within the deepening half‐cycle of third and second order cycles, in the Bas Auran area of French Subalpine Basin during early Bathonian.     

ALGAL LIMESTONES WITH PSEUDOMORPHS AFTER GYPSUM FROM THE MIDDLE JURASSIC OF SCOTLAND

Hudson, J. D.: Algal limestones with pseudomorphs after gypsum from the Middle Jurassic of Scotland. Lethaia , Vol. 3, pp. 11–40. Oslo, January 15th, 1970.
Algal limestones from two horizons within the Great Estuarine Series of the Hebrides are described. That from the Mytilus Shales is a typically stromatolitic limestone formed by algal trapping and binding of micrite pellets. Calcite pseudomorphs after gypsum occur along some of the laminae. The limestones from the Lower Ostrea Beds include less regularly-stromatolitic types, and nodular algal limestones that contain tubes of calcified algae, notably Cayeuxia nodosa Anderson. The algal nodules also yield uncalcified tubes composed of organic material, that are comparable to recent Schizothrix. These algae form inter-growths similar to those recorded from the recent supratidal deposits of the Bahamas. The algal nodules are often coated with microtufa. Pellet- and spheru-lite-limestones are associated with the algae, and pseudomorphs after gypsum are again found. Isotopic data on the carbonate-carbon, and the palaeoecological implications of the algae and the pseudomorphs are discussed.     

Corals,molluscs, and paleogeography of late Jurassic strata of the Cerro Pozo Serna,Sonora, Mexico

Jurassic strata of late Oxfordian to early Kimmeridgian age are reported from the Cerro Pozo Serna, northwest-central Sonora, Mexico. The presence of these marine strata greatly alters previous paleogeographic reconstructions of the Tethyan embayment in this area. Approximately 60% of the Sonorian molluscan fauna has been previously reported from Jurassic horizons in the Gulf of Mexico and west Texas regions. The remainder of the molluscs have previously been reported from either the western interior United States or Canada. The Pozo Serna faunas seem to be zoogeographically transitional between communities present in the southern North American Tethyan realm and regions within the central North American Tethyan realm. Additionally, the Pozo Serna fauna greatly resembles contemporaneous communities reported from the Jurassic of Morocco. Corals from the Sonorian Jurassic prove to be, in large part, conspecific with well-known central European and Algerian reef-building species. The scleractinian Lepidophylliopsis gen. nov. and Macgeopsis sonorensis sp. nov. are described from central Sonora.     

Ostracods of the genus Palaeocytheridea Mandelstam in the Middle and Upper Jurassic of Europe: 3. Stratigraphy and paleobiogeography

The analysis of distribution of endemic and cosmopolitan ostracods of the genus Palaeocytheridea shows that, in the Bajocian and Bathonian, the Tethyan and Boreal-Atlantic regions of Western and Eastern Europe developed in partial isolation, while beginning from the middle of the Lower Callovian, these parts of the European paleobasin were connected. For the Middle Jurassic of Western and Eastern Europe and, in particular, for Ukraine, the ostracod zones are recognized based on stratigraphic distribution of species of the genus Palaeocytheridea.     

Microaerophilic Fe‐oxidizing micro‐organisms in Middle Jurassic ferruginous stromatolites and the paleoenvironmental context of their formation (Southern Carpathians,Romania)

Ferruginous stromatolites occur associated with Middle Jurassic condensed deposits in several Tethyan and peri‐Tethyan areas. The studied ferruginous stromatolites occurring in the Middle Jurassic condensed deposits of Southern Carpathians (Romania) preserve morphological, geochemical, and mineralogical data that suggest microbial iron oxidation. Based on their macrofabrics and accretion patterns, we classified stromatolites: (1) Ferruginous microstromatolites associated with hardground surfaces and forming the cortex of the macro‐oncoids and (2) Domical ferruginous stromatolites developed within the Ammonitico Rosso‐type succession disposed above the ferruginous microstromatolites (type 1). Petrographic and scanning electron microscope (SEM) examinations reveal that different types of filamentous micro‐organisms were the significant framework builders of the ferruginous stromatolitic laminae. The studied stromatolites yield a large range of δ⁵⁶Fe values, from ?0.75‰ to +0.66‰ with predominantly positive values indicating the prevalence of partial ferrous iron oxidation. The lowest negative δ⁵⁶Fe values (up to ?0.75‰) are present only in domical ferruginous stromatolites samples and point to initial iron mobilization where the Fe(II) was produced by dissimilatory Fe(III) reduction of ferric oxides by Fe(III)‐reducing bacteria. Rare‐earth elements and yttrium (REE + Y) are used to decipher the nature of the seawater during the formation of the ferruginous stromatolites. Cerium anomalies display moderate to small negative values for the ferruginous microstromatolites, indicating weakly oxygenated conditions compatible with slowly reducing environments, in contrast to the domical ferruginous stromatolites that show moderate positive Ce anomalies suggesting that they formed in deeper, anoxic–suboxic waters. The positive Eu anomalies from the studied samples suggest a diffuse hydrothermal input on the seawater during the Middle Jurassic on the sites of ferruginous stromatolite accretion. This study presents the first interpretation of REE + Y in the Middle Jurassic ferruginous stromatolites of Southern Carpathians, Romania.     

Seafloor erosion and sea-level change: Early Jurassic Blue Lias Formation of central England

Minor bedforms within the mudstone-dominated Early Jurassic Hettangian Saltford Shale Member (Liasicus up to Angulata Chronozone) of the Blue Lias Formation in central England, indicate weak seafloor erosion in a mid to outer ramp setting. Distal storm flows below maximum storm wave base are proposed as the most likely generative mechanism for silty scour and gutter casts that enclose concentrations of well-preserved schlotheimiid ammonites and arthropod trace fossils. Within the upper part of the Saltford Shale (probably Angulata Chronozone), a discrete layer of reworked and bioencrusted limestone nodules signifies an episode of more persistent seafloor erosion. The immediately overlying strata, transitional to the Hettangian–Sinemurian Rugby Limestone Member, are relatively bioturbated and feature fossils of macrobenthos, as well as shell concentrations resembling relatively proximal storm beds. This suggests that the reworked nodule horizon marks sea-level fall, rather than stratigraphic condensation associated with sediment starvation. The biostratigraphic evidence raises the possibility that this erosional episode correlates with a mid-Angulata Chronozone hiatus documented from the Wessex Basin, southwest England. Equally however, it could be linked to contemporaneous movement on one or more nearby faults, affecting the southern part of the English East Midlands Shelf.     

Late Triassic, Early and Middle Jurassic Radiolaria from ferromanganese-chert ‘nodules’ (Angelokastron, Argolis, Greece): evidence for prolonged radiolarite sedimentation in the Maliac-Vardar Ocean

In the Argolis, the Basal Sequence, constituting the eastern Pelagonian margin which bordered the Maliac-Vardar oceanic domain, includes shallow-water carbonates of Late Triassic-Early Jurassic, condensed pelagic limestones of Early-Middle Jurassic, radiolarian cherts of late Middle-Late Jurassic age and siliceous mudstones and sandstones rich in ophiolite fragments. Up-section, coarse breccias, including clasts of boninites derived from the ophiolite obducted onto the Pelagonian margin in Late Jurassic times crop out. Near Angelokastron a small quarry exposes pervasively sheared dark reddish-brown, radiolarian-bearing cherty shales with disrupted fragments of chert and chert nodules impregnated by ferro-manganese oxides. These shales occur in the footwall of a thrust bringing them into contact with the Pantokrator Limestone of the Basal Sequence. We collected more than 30 samples of the chert fragments and the shaly matrix. Thirteen nodules and one matrix sample yielded determinable radiolarians. Low to non-detectable concentrations of trace metals such as Co, Cr, Cu, Ni, Zn, and Pb indicate a hydrothermal origin of the ferro-manganese mineralization. The radiolarian taxa found indicate four age groups for the nodules that are embedded in the siliceous shale matrix that yielded a Middle Jurassic age (middle Bathonian). The first group includes a nodule of Late Triassic age (late Norian to Rhaetian); the second group nodules of Early Jurassic age (late early to late Pliensbachian and probably middle-late Toarcian); the third group nodules of early Middle Jurassic age (Aalenian–Bajocian); the last group finally includes nodules of late Middle Jurassic age (Bajocian–Bathonian). The presence of Upper Triassic to Middle Jurassic Mn-impregnated chert nodules in a Middle Jurassic matrix indicates a deep oceanic environment of deposition outside the Pelagonian realm (easternmost Adria Plate), which at that time was a shallow-water carbonate platform with a thin pelagic limestone cover. The chert nodules are with all certainty derived from the oceanic Maliac-Vardar domain and were, together with their host formation, tectonically emplaced onto the Pelagonian margin. We speculate that these nodules, more lithified than their matrix, were exhumed on the slope of an intra-oceanic accretionary wedge and were redeposited in the Middle Jurassic siliceous mudstones on the floor of the subducting Maliac-Vardar Ocean.     

Reworked carboniferous spores: An example from the lower jurassic of northeast Scotland

An association of contemporaneous Jurassic mega- and miospores with reworked Carboniferous mega- and miospores is described from the Lower Lias rocks of northeast Scotland. The assemblage is compared with other accounts of reworked spores in Jurassic sediments. It is suggested that such reworking was particularly common in the Lower Jurassic of northern Europe and that four situations exist in the literature concerning probable cases of reworking: (1) the spores are recognised as reworked; (2) the identity of the spores as Carboniferous species is recognised and the “range” of these species is extended into the Jurassic; (3) the reworked spores are not recognised as such, and are incorrectly assigned to Jurassic genera and species; (4) the spores are recognised as Carboniferous, but their origin is attributed to contamination rather than reworking. In conclusion, the contrasting preservational features exhibited by the reworking and contemporaneous megaspores are discussed and interpreted as the result of differing depositional and postdepositional histories.     

AMMONOIDS AND THE TRIASSIC/JURASSIC BOUNDARY IN THE HIMALAYAS OF SOUTHERN TIBET

Abstract:  The sections of Germig in the Nyalam area, southern Tibet, provide a continuous exposure of ammonoid-bearing, uppermost Triassic and basal Jurassic strata. Eighteen species (three of them new) are described and illustrated: Choristoceras marshi Hauer , C. aff. marshi, C. cf. nobile Mojsisovics , C. nyalamense sp. nov., Eopsiloceras germigense sp. nov. , Pleuroacanthites aff. biformis (Sowerby) , Rhacophyllites sp., Nevadaphyllites cf. psilomorphus (Neumayr), Neophyllites sp. indet., Neophyllites cf. biptychus (Lange), Psiloceras tibeticum sp. nov., P. calliphyllum (Neumayr), Euphyllites cf. struckmanni (Neumayr), Discamphiceras pleuronotum (Canavari), Alsatites spp., Kammerkarites frigga , and K. sp. The ammonoid fauna shows a strong affinity to that of the Northern Calcareous Alps, although diversity in the Calliphyllum Zone is markedly lower. The ammonoid succession across the Triassic/Jurassic boundary is subdivided into four zones: the Rhaetian Marshi, the basal Hettangian Tibeticum, the lower Hettangian Calliphyllum, and the middle Hettangian Pleuronotum zones. It is the only known succession across the Triassic/Jurassic boundary in the Tethyan Realm that is not condensed. The Marshi and Calliphyllum zones are correlated with the same zones in the Northern Calcareous Alps. The Tibeticum Zone, a new local zone, is transitional between the Marshi and the Calliphyllum zones in that it yields both choristoceratids and psiloceratids. Its base is taken to mark the base of the Jurassic System in the eastern Tethys.     

Episodic sedimentation on a peri-Tethyan ridge through the Middle–Late Jurassic transition (Villány Mountains, southern Hungary)

The Villány area, as a central part of the Tisza microcontinent/terrane along the European margin of Tethys, was characterized by intense subsidence in the Early and Middle Triassic, followed by a long interruption of subsidence in the Late Triassic to Middle Jurassic. During the Middle–Late Jurassic transition, marine sedimentation started with three distinct sedimentary episodes dated as Late Bathonian, Early Callovian, and Middle–Late Callovian, respectively. The succession is terminated by a thick limestone of Middle Oxfordian age. The sedimentary features, microfacies, and macroinvertebrate associations of these four stratigraphic units are documented and illustrated. The Middle to Late Jurassic sedimentary episodes of the Villány succession record an interplay of local and global factors and paleogeographical changes. At the beginning, local tectonic movements governed the main features of sedimentation, though the role of eustasy was also essential. From the mid-Callovian onwards, global climatic, biotic, and paleoceanographical changes controlled the nature and formation of the local carbonate sediments. The Callovian stromatolites are attributed to the activity of sulphate-reducing bacteria in a deep sublittoral, current-swept environment. Upwelling of eutrophic Tethyan waters is recorded by the prevalence of the Bositra filament microfacies in the Callovian. The long submarine hiatus at around the Callovian–Oxfordian transition mirrors a serious restriction of the carbonate budget, due to sudden cooling and a change in the oceanic current system (opening of a circumglobal Tethyan Passage), and to a higher amount of dissolved CO₂. In the Middle Oxfordian, the carbonate production considerably increased in accordance with the sudden global warming.     

Jurassic evolution of Southern Hemisphere marine palaeobiogeographic units based on benthonic bivalves

The distribution of benthonic Jurassic bivalve genera in the Southern Hemisphere is analysed here. For this region, palaeobiogeographic units (biochoremas) are quantitatively characterized according to their biologic contents (mainly levels of endemism). Their evolution through time is followed from the latest Triassic to the earliest Cretaceous. The Tethyan Realm is undoubtedly the most mature and persistent through time, with three subordinate units: an Australian unit restricted to the Late Triassic, a North Andean unit, which appears sporadically as an endemic centre, and an East African unit which is recognisable from Bajocian times onwards. From Late Triassic times, a South Pacific Realm has been recognised, with a Maorian Province mostly based on the distribution of monotoid genera. A South Andean unit is also recognisable through most of the Jurassic, and its reference either to the South Pacific unit or to the Tethyan Realm is a matter of debate. Being a transitional biogeographic setting between Tethyan and South Pacific first-order units, it is included in the South Pacific unit due to the common presence of antitropical (didemic) genera. The East African unit is included within the Tethyan Realm during the Jurassic, but during Early Cretaceous times, it splits into two units, one of which was regarded as part of the “South Temperate Realm” by Kauffman. The rank of all these units changed with time. Throughout the Jurassic, the ecotone between South Pacific and Tethyan palaeobiogeographic units fluctuated in position with time. The approximate latitudinal location of the ecotonal boundary area and its shift through time are recognised on the basis of faunal composition along the Andean region.     

The effect of deposition of organic matter on phosphorus dynamics in experimental marine sediment systems

The effect of deposition of organic matter on phosphorus dynamics in sandy marine sediments was evaluated using an experimental system (boxcosms) and three different strategies: (1) no supply (2) one single addition (3) weekly additions of a suspension of algal cells (Phaeocystis spec.). Macrofauna (3 species, 6 individuals of each) were added to half of the boxes. Both in the case of the single and weekly additions a clear effect of increased organic matter loading on phosphorus dynamics was found. Following the organic matter addition, porewater phosphate concentrations in the upper sediment layer increased, phosphate release rates from the sediment increased by a factor 3–5 and in the boxes to which a single addition was applied NaOH-extractable phosphorus increased substantially. The increase in phosphate release rates from the sediment was attributed to mineralization of the added material and to direct release from the algal cells. No clear effect of the presence of macrofauna on sediment-water exchange of phosphate could be discovered. The macrofauna were very effective at reworking the sediment, however, as illustrated by the organic carbon profiles. It is hypothesized that the sediment-water exchange rates of phosphate were regulated by the layer of algal material which was present on the sediment surface in the fed boxes. In the boxes to which the single addition was applied porewater phosphate concentrations were lower and NaOH-extractable phosphorus was higher in the presence of macrofauna, suggesting that macrofauna can stimulate phosphate binding in the sediment.Publication no. 40 of the project Applied Scientific Research Netherlands Institute for Sea Research (BEWON)     

Late Jurassic decapod crustaceans from northeast Japan

Abstract: Astacidean and thalassinidean macrurans (Glyphea sp., ?Eryma sp. and Protaxius sp.) and a new longodromitid crab, Planoprosopon kashimaensis, are recorded from the Upper Jurassic (upper Kimmeridgian to lower Tithonian) of Fukushima Prefecture, northeast Japan. Material was collected from the Tatenosawa Sandstone Member of the Nakanosawa Formation, Somanakamura Group, from which abundant Tethyan‐type marine invertebrates are known. Planoprosopon kashimaensis sp. nov. closely resembles P. heydeni (von Meyer), a common form in the Upper Jurassic of the Tethyan realm in Europe, and represents the oldest record of a brachyuran from the circum‐Pacific region. Similarities to contemporaneous decapod assemblages in southern Germany indicate that closely comparable, parallel decapod faunas in the Tethyan realm, inclusive of brachyurans, had already been established in the western circum‐Pacific region by the Late Jurassic.     

Pliensbachian gastropods from Venetian Southern Alps (Italy) and their palaeobiogeographical significance

Abstract: The Pliensbachian gastropods described by De Toni in 1912, coming from an isolated boulder at the foot of Mt Vedana (eastern margin of Trento Platform, Venetian Southern Alps, Italy) are revised. The fauna consists of 13 species representing nine families and eight superfamilies. Despite the low number of species, the assemblage represents the most diverse Early Jurassic gastropod fauna known for the Venetian Southern Alps. The boulder yielding the material was thought to derive from the upper part of the Early Jurassic Calcari Grigi Group, a carbonate platform unit extensively cropping out in the Mt Vedana area. The sedimentological analysis indicates a prevalently bioclastic wackstone‐floatstone, reflecting a lime‐muddy deposit undergone to an early consolidation. This and the high content of ammonoids, which is unusual for the Calcari Grigi Group, are typical aspects of a condensed pelagic sediment, presumably a fissure filling at the top of the carbonate platform succession. Palaeobiogeographical comparisons show that the fauna is composed of species occurring exclusively in pelagic limestones of the western Tethys. By contrast, it shows no relationships with the coeval faunas of the adjacent Trento platform and of the other western Tethyan carbonate platforms. These lines of evidence and the facies analysis would testify to the Pliensbachian drowning of the eastern margin of the Trento platform. In the wider context of the palaeobiogeographical history of Early Jurassic western Tethyan gastropods, the species from Vedana belong to a faunal stock which is typical for pelagic, mainly postdrowning sediments. Thus, appearance and diffusion in space and time of this stock were probably regulated by the direction, rate and pattern of the Neotethyan rifting. A new subgenus, Proarcirsa (Schafbergia) subgen. nov., and three new species, namely Ataphrus (Ataphrus) cordevolensis sp. nov., Guidonia pseudorotula sp. nov. and Proarcirsa (Schafbergia) zirettoensis sp. nov. are erected.     

Late Jurassic sea-level fluctuations in NW Switzerland (Late Oxfordian to Late Kimmeridgian): closing the gap between the Boreal and Tethyan realm in Western Europe

In Late Jurassic times, the Swiss Jura carbonate platform occupied the transition between the Paris Basin and the Tethys and thus connects the Boreal and Tethyan realm. Up to now, the lack of index fossils in the Reuchenette Formation prevented a reliable correlation between both areas (its sediments are characterised by a prominent sparseness of index fossils). Now, seven recently in situ collected species of ammonites helped to establish a new sequence-stratigraphical frame for the platform sediments of the Reuchenette Formation in NW Switzerland. Based on biostratigraphical data, five third-order sedimentary sequences were assigned to the Late Oxfordian to Late Kimmeridgian. The upper three third-order sequences correspond to the Boreal sequences Kim3–5 of Hardenbol et al. (1998). The deduced large-scale sea-level fluctuations match those from other European regions (Spain, Russia). This biostratigraphically based sequence-stratigraphical frame is a prerequisite to refine correlations within a wider area covering the Swiss Jura and parts of adjacent France and Germany. Electronic Supplementary Material Supplementary material is available for this article at     

THE EARLY JURASSIC AMMONITE BADOUXIA FROM BRITISH COLUMBIA, CANADA

Abstract:  Most species of the Lower Jurassic ammonite genus Badouxia are restricted to the eastern Pacific where they are of fundamental importance to the Hettangian/Sinemurian ammonite zonation of North America. The fauna from the type area for the genus has been restudied and four species and two transitional forms are now recognized. Badouxia forticostata and Badouxia castlensis are new. Badouxia occidentalis (Frebold) and Badouxia oregonensis Taylor are placed in synonymy with Badouxia canadensis (Frebold). These major taxonomic changes and subsequent modification to the range of B . canadensis require revisions to the current zonation for the Western Cordillera of North America. The Oregonensis Zone is re-named the Mineralense Zone. The Canadensis Zone is abandoned and its previously defined Rursicostatum and Columbiae subzones are elevated to the level of full zones. Large samples indicate that most species of Badouxia display a remarkable range of continuously variable morphology. In addition, the genus shows some of the best-documented evidence of sexual dimorphism in Early Jurassic ammonites. Badouxia spans the Hettangian/Sinemurian boundary and based on the range of different species, five stratigraphic intervals are recognized. Intervals 1–3 correlate with the north-west European Angulata Zone. The majority of interval 4 and the whole of interval 5 correlate with the north-west European Bucklandi Zone. The age of the base of interval 4 is uncertain.     

The dispersal and colonisation behaviour of the marine polychaete <Emphasis Type="Italic">Nereis diversicolor</Emphasis> (O. F. Müller) in south-east England

Recent submarine caves are inhabited by endemic faunas adapted to oligotrophism, darkness and a tranquil environment. Many of their representatives are archaic types of animals resembling fossils from very early times in evolution. This article compares fossil fauna from Jurassic neptunian dykes (originally sea bed clefts) from the Western Carpathians with the Recent cave-dwelling fauna. The ostracods Pokornyopsis feifeli are particularly important. In the Western Carpathians, these were exclusively found in the Middle/Late Jurassic fissure fillings, but in the non-Tethyan Germanic Jurassic this species was found in deep-marine claystones. They are phylogenetic forerunners of the recent genus Danielopolina inhabiting both anchialine caves and deep seas. This indicates a Jurassic migration of deep-marine fauna to cryptic habitats. Other examples of cryptic communities include the Upper Jurassic cavity-dwelling fauna dominated by serpulids and scleractinian corals. Associated suspension feeders include thecideidine brachiopods, oysters, bryozoans, sponges, crinoids and sessile foraminifers. Serpulid-dominated bioconstructions have recent analogies in the Mediterranean and Carribean seas. Different type of dyke communities represent the Late Jurassic fauna of small sized ammonites which originated from both Tethyan and Boreal paleobioprovinces. It has not been established whether these amonites were juvenile, dwarfed specimens adapted to limited cave space or size-sorted adult specimens.     

Stratigraphie et sédimentologie des « Couches rouges » continentales du Jurassique-Crétacé du Haut Atlas central (Maroc) : implications paléogéographiques et géodynamiques

In several synclines of the central High Atlas, the “Redbeds” following the closure of the marine Tethyan Atlasic trough during the Middle Jurassic are constituted by three successive formations or units of continental deposits dated recently with biostratigraphical elements. Some micropaleontological markers, mainly charophytes and ostracods, allow to precise the stratigraphy in agreement with a Bathonian-?Callovian assignment for the lower unit (Guettioua Formation), and in dating the middle and upper units. The Upper Jurassic, mainly the Kimmeridgian, is developed in the lower part of the middle unit (Iouaridene Formation). The Barremian has been recognized in this middle unit and in the upper unit (Jbel Sidal Formation). The Jurassic/Cretaceous boundary is thus delimited for the first time with micropaleontological data. These new data are very significant for the Atlasic history during the Mesozoic. The basaltic flows inserted in the continental Jurassic-Cretaceous deposits of the central High Atlas result from two separate events in the Middle Jurassic and in the Barremian. The tectogenesis in the basins is characterized by a polyphase process including notably a synsedimentary tectonic activity conspicuous in the Barremian. The evidence of marine to brackish intercalations allows moreover to date the first Cretaceous transgressive event on the NW boundary of the High Atlas during the Lower Barremian and to consider an Atlantic paleogeographical interaction. SW margin of the Tethyan trough in the Lower and Middle Jurassic, the central High Atlas is merged with the margin of the central Atlantic Ocean during the Lower Cretaceous.