Jacques Rey

Sequence stratigraphy and biological signals: statistical studies of benthic foraminifera from Liassic series

Abstract A study of benthic Foraminifera from the Toarcian in Quercy (Aquitaine Basin, France) shows that micropaleontological assemblages are influenced by sea level changes. The percentages of coiled, and straight Nodosarridae (Lenticulina, Citharina) differ during periods of low sea level to high sea level. Statistical analysis of all the taxa in the successive levels of a section (Multiple factorial Analysis of Correspondences, methods of automatic clustering) show that there is a perfect distribution of the samples into groups corresponding to the type of systems tract: differences within the foraminiferal benthic biocoenosis allow the successive systems tracts to be separated. The first axis in the Multiple Analysis of Correspondences coincides with a gradient of biotic capacity and seems to be connected with the sea level changes. The second axis could be linked with depth. The renewal rate of taxa and the factorial diversity of the sampling levels show fluctuations associated with 4th, 3rd and 2nd order variations of the sea level. These connections are caused by intricate processes with ecological, biogeographical and bioevolutionary influence. However, the micropaleontological assemblages can be good indicators of systems tracts and unconformities. This is particularly important within deposits that have homogeneous lithology.

Le Bassin lusitanien (Portugal) à l'Aptien supérieur–Albien : organisation séquentielle, proposition de corrélations, évolution

The Lusitanian Basin (Portugal) during the late Aptian-Albian: sequential arrangement, proposal of correlations, evolution. Three transgressive-regressive 2nd-order cycles were identified in the Upper Aptian-Albian fluvial and marine deposits of the Lusitanian Basin. Its widespread nature, probably including eustatic origin, allows correlation between the southern package, with precise stratigraphic positioning, and the northern series with a poorly constrained age. The main unconformities can be related to the onset of an oceanic crust, in the western margin of Galicia during the Late Aptian, in the bay of Biscay during the Early Albian, and, to the northwest of the Galicia triple point, during the Middle to Late Albian transition, and, at the Albian-Cenomanian boundary, to a probable compressive event with Africa due to the

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.

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.

Micropaleontological assemblages, paleoenvironments and sedimentary evolution of Cretaceous deposits in the Algarve (Southern Portugal)

Abstract The Cretaceous of the Algarve comprises some lithostratigraphic units (carbonatic in shallow-water environments, siliciclastic in estuarine or fluviatile areas) which are essentially dated by their micropaleontological assemblages which contain calpionellids, lituolids, orbitolinids, involutinids, dasycladacean algae, charophytes. The formations are interpreted in the following terms: one regressive-transgressive cycle (late Oxfordian-basal Valanginian), two regressive sequences (Hauterivian-Barremian, Bedoulian) and one transgressive sequence (Gargasian-middle Cretaceous), separated by three major discontinuities at the base of the Valanginian, lower Bedoulian and upper Bedoulian. The sedimentary evolution and the geometry of deposits are controlled by various geodynamic factors the influence of which is analyzed: the relative changes of sea level are the main factor which determines the repartition of the formations in the different isotopic zones; the rate of subsidence on this passive continental margin (well marked during the Aptian and Albian) and sedimentary influxes increasing during the Hauterivian-Barremian contribute to the vertical regressive or transgressive evolutions. Orogenic movements occur only in the Neocomian, near a diapiric area. These factors record, on the south portuguese continental shelf, different stages of the history of the Northern Atlantic: the Neocimmerian and the Bedoulian riftings induces major discontinuities and sedimentary gaps. The beginning of the expansion of the Northern Atlantic determines the increasing of subsidence during the Aptian and Albian.

Fluctuations of environmental factors as seen by means of statistical analyses in micropaleontological assemblages from a Liassic series

A comparative statistical study of foraminiferal and ostracode assemblages in Domerian marls from the Aquitaine basin (France) shows that the taxonomic structure of the populations fluctuates through time. Multiple Correspondences Analyses (M.C.A.) processing presence-absence data (table of taxa in the successive sample levels) were conducted. The first axis (F1) of this M.C.A. is related to faunal diversity and expresses a biotic capacity gradient. The F2 axis coincides with accomodation space and can be used to identify systems tracts and sequence boundaries. The F3 axis is interpreted as representing an oxygenation gradient. Short-term fluctuations at the systems tract or half systems tract level appear in the residuals of regressions and the distance between samples.

Opportunistic development of Ogmoconcha amalthei (ostracod) in the lower Liassic of Quercy (SW France): an indicator of sedimentary disturbance

Abstract Ostracod assemblages have been studied in the sedimentary units of the lower Liassic of Quercy (SW France), at the Carixian–Domerian boundary (Pliensbachian). Ogmoconchella transversa was found to decrease in frequency towards the base of the Domerian while at the same time the O. pseudospina population increased. It is also noteworthy that the occurrence of small-sized species coincides with dominance of species of the Ogmoconcha amalthei group. This large-sized species makes up half or more of the ostracod assemblages in the periods of environmental disturbance indicated by sequence stratigraphy. The biocoenotic indices (number of taxa, faunal turnover and equitability) seem to confirm environmental disturbance at these levels. Correspondence analysis (CA) distinguishes such samples from those in which Ogmoconchella and Polycope species are more common. The appearance or development in the Domerian of species that were rare or absent throughout the Carixian indicates a change of environmental conditions. The opportunism of species of the Ogmoconcha amalthei group can be used to determine periods of disturbance in sedimentary units.

Le Valanginien du bassin de Safi (Maroc atlantique) et sa faune d'ammonites. Implications paléobiogéographiques

Abstract The study of ammonite assemblages collected from the Calcaire inferieur and the Argiles brunes in the region of Safi (Morocco) has allowed us to pinpoint the age of these formations and to attribute them, respectively, to the Boissieri Zone ower part of the Campylotoxus Zone and to the upper part of the Campylotoxus Zone pper Valanginian. These units are also reinterpreted in terms of sequence stratigraphy. In addition, the analysis of the faunal spectrum shows that, although differing in some respects, these ammonite associations are similar to those of other peri-Tethyan platforms, while differing considerably from the basin assemblages.

Apport des ostracodes à l’interprétation séquentielle du Crétacé inférieur (Berriasien–Valanginien) du bassin d’Essaouira–Agadir (Maroc)

Resume L’analyse de la faune d’ostracodes du Berriasien–Valanginien du bassin d’Essaouira–Agadir (Maroc) a permis de mettre en evidence une relation etroite entre l’evolution sequentielle de la serie (etablie sur la base de donnees sedimentologiques, biostratigraphiques et geochimiques) et l’evolution de parametres statistiques appliques aux ostracodes. Ainsi, le nombre d’individus et le nombre d’especes presentent des fluctuations comparables et suivent parfaitement l’evolution sequentielle de la serie. Les classes de frequence dominante et fortuite sont les plus significatives, le taux de renouvellement specifique montrant des valeurs extremes, des « pics » en limites de sequences et a la base des prismes transgressifs. Pour citer cet article : A. Rossi et al., C. R. Palevol 2 (2003) 133–141.

Les événements du passage Lias–Dogger dans le Quercy (France)

In the Quercy basin, the passage from the Liasic marls to the Dogger carbonates depends on three interdependent allocyclic factors. The long wavelength allocyclic factor (eustatism) shows a certain uniformity of the variations of the accommodation to the basin scale. The regional allocyclic factor (uplift) gives way to a partial isolation of the platform. The weak wavelength allocyclic factor (local tectonics) leads to a parcelling with various blocks of different scale.