Bernard Peybernès

Basement blocks and tecto-sedimentary evolution in the Pyrenees during Mesozoic times

The Alpine tectonics in the Pyrenean Range is controlled by motions of basement blocks (High Primary Range, North Pyrenean blocks). These blocks belong to different tectonic units; they are amygdaloid, wedge-shaped, squeezed at their roots, upthrust and displaced with their cover (detached or not) in a convergent shear zone between the European and Iberian plates. These basement blocks are distributed within isopic (isofacial) units with diachronous terrigenous prisms. The stratigraphical analysis of their Mesozoīc cover reveals their relative position during the Mid-Cretaceous continental break-up which induced the separation of the two previously joined plates.

Triassic palaeogeography of Tunisia

Abstract A stratigraphic, palaeogeographic and palinspastic synthesis of the Triassic successions in Tunisia is herein documented from a SSE–NNN oriented profile (Saharan Platform, Gulf of Gabes offshore, Tunisian Atlas, Tellian Units) across the northern boundary of the Gondwana Plate and the future South-Tethyan Margin. It is principally based on a reinvestigation of old data and on recent results, particularly related to new lithostratigraphic subdivisions and palaeontologic datings obtained from assemblages of benthic foraminifera, algae, palynomorphs, ammonites and brachiopods. The various Triassic successions of Tunisia are compared at the scale of the formations and sequences in the most possible precise biochronologic framework. The corresponding chart is used as reference to seven maps of the dominating depositional environments, during the following time intervals: Induan/Olenekian, Anisian, Lower Ladinian, Middle–Upper Carnian, ‘Norian’, ‘lowermost part of Rhaetian’ and ‘uppermost part of Rhaetian’. In a Peri-Tethyan domain controlled both by eustasy and extensional tectonics, the proposed maps show the alternation of regressions (marked by fluvio-deltaic siliciclastics and paralic sebkha-type evaporites) and transgressions (marked by foraminifera-bearing limestones/dolomites of internal/proximal platform, sometimes open to pelagic influences). The transgressions were particularly widespread during Carnian and ‘Rhaetian’ times and probably came from a deeper trough situated towards ENE. During a first rifting phase, the motion of normal crustal faults, strongly inclined towards the north, induced well-marked differential subsidences. In conclusion, a new palinspastic reconstruction along a south–north/SSE–NNW oriented profile is proposed. It shows a succession of shoals (flanked by lateral steps), such as the Tebaga of Medenine shoal, sub-basins (Atlas) and tilted blocks (Hairech–Ichkeul) heralding the structuring of the South-Tethyan Margin during Jurassic times.

Comparative study of the Lower and Mid-Cretaceous sequences on different Maghrebian shelves and basins-their place in the evolution of the North African Atlantic and Neotethysian margins

Abstract The Lower and Mid-Cretaceous sequences known from Morocco (coastal Essaouira-Agadir-Safi Basin and Eastern Rif Foreland), in Western Algeria (Daǐa Mountains) and finally in Southern Tunisia (Chott Range and Saharian Shelf) are presented from a biostratigraphic and sedimentologic viewpoint. This work shows, for the period considered, common features in the geodynamic evolution of the different regions, such as: 1. - generalization of the carbonate facies in the Early Cretaceous (Berrisian) and at the end of the Mid-Cretaceous (Cenomanian-Turonian) times; 2. - delineation of important discontinuities in the Valanginian and Bedoulian times. This evolution gives evidence of an undeniable influence of eustatism in this Northern part of Africa. Nevertheless, detailed differences allow us to distinguish: 3. - an “Atlantic-type” evolution characterized by discontinuities with gaps (Safi) or sudden variations of thickness (Essaouira-Agadir) at the Berriasian/Valanginian boundary, related to the Neocimmerian rifting and block-faulting phase. Later on, this evolution is underlined by a progressive acceleration of subsidence at the Aptian/Albian boundary, due to the removal of the mid-oceanic doming; 4. - a “Neotethysian-type” evolution (Eastern Rif Foreland, Western Algeria and Southern Tunisia) where local tectonic movements have probably generated the rapid growth of the external marly Bedoulian basins and the spreading of the Wealdian-type (Valanginian to Barremian) and Utrillas (Albian) terrigenous depositional systems. The latest event is linked to the Austrian tectonic phase responsible of variable gaps towards the Aptian to Albian transition.

Microfaunes, microflores etpaléoenvironnements successifs dans la formation d'El Mers (Bathonien-?Callovien) du synclinal de Skoura (Moyen-Atlas, Maroc)

In the Middle-Atlas (Morocco), the lower part of the “Red beds” (El Mers Formation), Middle Jurassic in age,contains various microfossils which permit to reconstruct the palaeoenvironmental evolution of this area for this period. Populations of abundant charophytes, collected at the base of the formation, are assigned to a new species: Porochara hians Feist & Grambast-Fessard. Species of ostracods are generally different than the European synchronous ones. The first ostracods appearing at the base of the formation are monospecific and euryhaline (continental or brackish). They are more diversified above and correspond to a typical marine depositional environment. Marine incursions occur up to the top of the formation where they are represented by limestones bearing dasycladales and benthonic foraminifera, Late Bathonian to Early Callovian in age. The transgressive trend observed at the top of the “El Mers Formation” can be correlated to the deepening-upward trend of the Transgressive Systems Tract from the Depositional Sequence DS 158,5 (3rd order eustatic cycle LZA 3-1) which corresponds to a general transgression of the sea over the palaeostructures of the Jurassic Tethyan Basin.

Stratigraphie séquentielle comparée et ≪ gradedatation ≫ par les foraminifères planctoniques du Campano-Maastrichtien et du Paléocène de quelques sites d'Europe sud-occidentale et d'Afrique du Nord

From the analysis of the open marine beds enclosing the K/T boundary along several cross-sections of SW Europe and North Africa, a subdivision is proposed into nine Campanian-Maastrichtian depositional sequences followed by five Palaeocene sequences. These sequences have been correlated from one section to another by means of planktonic Foraminifera classic biozones and supported in the Cretaceous by numeric ages coming from the grade-dating method applied to Globotruncanids. In the latter case, sequences belong to two different types separated by the presence or the absence of lowstand chaotic deposits such as breccias and olistostromes. The dark clays of the K/T boundary containing the Iridium anomaly can be regarded as a 3d order sequence boundary. They also correspond to the base of a 2d order cycle which is locally (Beam) underlined by a gap of the Early Palaeocene, the underlying Maastrichtian marls, everywhere grade-dated around 65.2–65.3 Ma, never being significantly eroded.

Sequence stratigraphy and micropaleontology of the Triassic series from the southern part of Tunisia

Abstract The Triassic series in the southern part of Tunisia, only known from the oil exploration wells in the Kirchaou area (sandy series from Early Scythian to Ladinian) and in its Djeffara/Dahar-North outcrops (alternating series from Ladinian to Rhaetian) is now subdivided into 10 Depositional Sequences (DS) sensu Vail et al. (1987), T 1 to T 10 corresponding approximately to most of the 3 rd order eustatic cycle from the Haq et al. curve (1987). Within the DS succession, 3 basal discontinuities (or sequence boundaries) appear to be important in Tunisia: the base of T 2 , correlated to the “H Discordanz” from the “Germanic Triassic” (intrascythian); the base of T 6 (intracarnian), well-marked by the Sidi Stout angular discordance from Dahar North; the base of T 8 (intranoirian additional DS, previously recognized in Germany) pointed by the cartographic discordance of Low Stand conglomerates. The shallow-water facies from the carbonate Transgressive Systems Tracts of 7 Tunisian DS contain macrofauna, such as Myophoria , and particularly 6 successive benthonic Foraminifera assemblages, F 1 to F 6 giving chronostratigraphic informations as complement of Palynomorphs and Conodonts. Belonging to the genus Meandrospira, Triadodiscus, Aulotortus, Lamelliconus, Endothyranella, Pilamminella, Agathammina, Gandinella etc.…, most of them, commonly used as zone-indices in Europe, are new in Tunisia.

Lithostratigraphy, biostratigraphy and sedimentary dynamics of the Lower Cretaceous deposits on the northern side of the western High Atlas (Morocco)

Abstract The analysis of a sedimentary succession on the northern side of the western High Atlas, from Essaouira to Amizmiz, has enabled us to identify, and describe, thirteen formations ranging in age from Tithonian to Clansayesian. The collection and determination of ammonite and echinoid faunas has allowed us to attribute a precise age to most of these units. Lower Cretaceous formations are organized into six main depositional sequences: Late Jurassic-Berriasian; Valanginian; Hauterivian-Barremian; Bedoulian; Gargasian; Clansayesian-Albian. This organization, the characteristics of the major discontinuities and the geometry of sedimentary succession allows recognition of the following events: a Late Berriasian distensive phase; a period of sea-level change and basin infilling during Valanginian, Hauterivian, Barremian and Bedoulian times under essentially eustatic control; an intra-Aptian tectonic phase; and a period of relative sea level rise and topographic levelling during Gargasian-Albian. This is essentially linked to the evolution of the North Atlantic ocean during the Early Cretaceous.

Évolution paléogéographique de la Tunisie saharienne et atlasique au cours du Jurassique

Abstract By means of palaeontologic data, the differert Jurassic formations of Tunisia are correlated along a north-south transect, from the Saharan Platform to the Northern Atlas, across the successive steps of the South-Tethyan stable palaeomargin. A palaeogeographic synthesis of the country is herein proposed, based on 11 maps established for the most important stages of this preoceanic rifting phase which is characterized by particularly active transtensional tectonics.

Reconstitution, en Corse, au Jurassique moyen–supérieur, de la marge européenne de l'océan Liguro-Piémontais, grâce à des niveaux repères à Praekurnubia crusei (foraminifère)

Abstract By means of the Upper Bathonian–Callovian isochron time-line marked by some calcareous key-beds bearing Praekurnubia crusei Redmond (benthonic Foraminifer), identified both in the ophiolitic ‘Balagne Nappe’ and in the continental ‘Prepiedmontese Zone’, an attempt of palinspastic reconstruction of the European margin of the Ligurian-Piedmont Ocean, in Corsica, during Middle–Upper Jurassic times, is herein proposed. The bayonet transect reaches to the west the autochthonous cover of the Hercynian Corsica. It shows, during the syn-rift phase (Dogger), the subdivision of the margin into tilted blocks inducing strong differential subsidences (particularly between the Caporalino half-horst and the adjacent half-grabens) and, during the post-rift phase (Malm), the sealing of the previous palaeostructures by the Kimmeridgian shelf-type limestones preceeding the general flooding of the Lowermost Cretaceous times.

Le probleme de la limite Cretace-Tertiaire en Corse-du-Sud

In the autochthonous unit of Punta-di-Calcina (southern Corsica, France), Upper Cretaceous deposits are reworked (clasts of orbitoid-bearing limestones, Globotruncanidae) within marine lowstand breccias of Danian age, that are overlying Jurassic limestones or granitic basement. These Danian breccias can be dated by means of subautochthonous planktonic foraminifera (Globigerinidae and Hedbergellidae), belonging to the Plb and Plc subzones. The K/T boundary coincides with an ante-Danian submarine erosional surface (major unconformity) underlining the gap of the whole Cretaceous. It is probably impossible to find the iridium-level of the K/T boundary along this surface. This stratigraphic framework suggests the occurrence of main (compressional ?) tectonic movements just before the beginning of the Palaeocene, inducing deep erosions on a Corsican-Sardinian block partially covered at this time by Maastrichtian marine carbonate platforms, now disappeared.