Serge Elmi

The middle jurassic Bathonian-Callovian ammonites zones of the Argentine-Chilean andes

Abstract The biostratigraphically important Ammonitina of the Chacay Melehue key section situated in Neuquen Province (Argentina) are taxonomically revised and illustrated. The classical section of Caracoles (Antofagasta Province, Chile) is also reviewed. New taxa are: Stebnocephalites n. gen., (type species: Indocepbalites gerthi Spath ), Xenocephalites stipanicici n. sp. (=“X.berrerodouclouxi” Stipanicic , nomen nudum) and Oxycerites (Alcidellus) obsoletoides n. sp. A succession of seven biostratigraphic units is identified,spanning the interval from the early Upper Bajocian (#1) up to the Middle Callovian (#7), capped by the gypsum of the Tabanos Formation. Four of these assemblage zones (#3–6) can be very widely recognized, from Mexico to the southern Andes. They are therefore used to define a provincial standard chronozonation for the Andean Upper Bathonian Lower Callovian, although the primary Bathonian Callovian boundary cannot be precisely located. The zonation is as follow: 1:(?) Megasphaeroceras rolundum Assemblage Zone, Lower Upper Bajocian; 2: Cadomites-Tulitidae mixed assemblage, (? Lower-) Middle and Upper Bathonian; 3: steinmanni Zone, index Lilloettia steinmanni ( Spath ). Uppermost Bathonian, with two local horizons: Stebnocepbalites gertbi horizon (Argentina) and Cboffatia jupiter horizon (North Chile); 4: Vergarensis Zone, index: Eurycephalites vergrensis ( Burckh. ), near Bathonian Callovian boundary; 5: Bodenbenderi Zone, index: Neuqueniceras (Frickites) bondenbenderi ( Tornq. ), Lower Callovian; 6: Proximum Zone, index: Hecticoceras proximum Elmi , uppermost Lower Callovian; 7: Reineckeia assemblage, Middle Callovian.

Physiography, palaeotectonics and palaeoenvironments as controls of changes in ammonite and brachiopod communities (an example from the early and middle jurassic of western Algeria)

Abstract Comparative studies of the palaeogeographical distributions of brachiopods and ammonites in western Algeria and its constituent basins give evidence of the control played by physiological differentiation. This factor, dependent upon tectonics, conditioned the bathymetry as well as the sedimentary context and the supply of nutrients. After a marked provincialism, caused by the large extent of carbonate platforms which prevailed during the Early and Middle Liassic, exchanges became easier. But from the Domerian to the Bathonian, the splitting of the margins into a mosaic of small subbasins bounded by steep seamounts and/or islands precented a wide distribution by currents. This led to diversification of the faunal assemblages inside the North African basins. The brachiopod communities consisted of abundant specimens of varied species on the shelves but were rare or absent in (more or less wide) epioceanic areas. Among ammonites the dominant morphological type varied from one palaeogeographical setting to another. Platycones were dominant on the shallow outer shelves, oxycones were abundant on the slopes, and the proportion of sphaerocones and serpenticones was higher in largely open basins and on outer shelves offering good connections with more open seas.

Le Toarcien des carrières Lafarge (Bas-Beaujolais, France): Cadre biostratigraphique de référence pour la région lyonnaise

Resume Les carrieres Lafarge du Val d’Azergues (Bas-Beaujolais, Rhone) permettent une observation precise des depots toarciens et de leurs variations. Grâce a des fouilles etalees sur plusieurs annees, la stratigraphie detaillee a pu etre etablie pour la premiere fois. Elle montre la succession de six ensembles lithologiques bien reperes par des faunes generalement riches, ce qui autorise un parallelisme precis avec la succession de reference etablie dans le Centre-Ouest de la France. De bas en haut: “Calcaires jaunes a Ammonitella ”, “Marnes inferieures” (avec un grand Ichthyosaure), “Bancs a Porpoceras ”, “Marnes medianes”, “Bancs a Haugia et a Grammoceras ”, “Marnes superieures”. Le niveau exact de quelques especes a pu etre precise, en particulier pour celles du genre Pseudolillia . Des lacunes sedimentaires affectent la base du Toarcien, le sommet du Toarcien moyen et le Toarcien terminal. Elles correspondent a l’exageration de perturbations dues a des courants amenant materiaux terrigenes et oolithes ferrugineuses depuis des zones hautes tres voisines.

Palaeogeography, physiography, palaeoenvironments and brachiopod communities. Example of the Liassic brachiopods in the Western Tethys

Abstract Within the Western different and characteristic brachiopod communities, as well as other benthic and necto-benthic faunas, have evolved in various palaeoenvironments resulting from the differentiation of a highly controlled submarine topography. This physiography depends on palaeogeographical conditions which were induced by eustatic changes, exaggerated or lessened by local tectonic movements. The characteristics of brachiopod communities, their location in the tectono-sedimentary context, the reconstruction of biotooe conditions from the study of sediments, allow us to propose a pattern in which the relationships between the various characteristic brachiopod communities and the different palaeoenvironments are established. These environmental settings include: (1) inner carbonate platforms; (2) protected carbonate ramps and mud flats; (3) hinge zones between mud flats and basins; (4) low-energy outer platforms and gentle slopes; (5) medium- to high-energy outer platforms; (6) episodically waved and winnowed outer platforms and shoal borders; (7) slopes (transitional to basin); (8) umbilicus and narrow troughs; (9) epioceanic basins. Then, according to this model, the recognition and the distribution through time and space of these different brachiopod communities, makes it possible to reconstruct the palaeogeographical evolution, and to give a more accurate of the tectonic framework (buried antithetic faults) when the palaeogeographical evolution is mainly due to tilted-blocks tectonic (example of the Portuguese Lias).

Biostratigraphy of the upper Bajocian-middle Callovian (Middle Jurassic), South America

Abstract The biostratigraphic division of the upper Bajocian-middle Callovian of South America is based on ammonites from different sections of the following provinces and regions: Neuquen, Mendoza, and San Juan in Argentina; Malleco, Linares, Talca, Atacama, Antofagasta, and Tarapaca in Chile. The complete upper Bajocian-middle Callovian succession includes the following biostratigraphic units: the Megasphaeroceras magnum assemblage zone, lowermost upper Bajocian; the Cadomites-Tulitidae mixed assemblages, (?lower) middle and upper Bathonian; the Steinmanni zone, index Lilloettia steinmanni (Spath), uppermost Bathonian, with two local horizons—Stenocephalites gerthi horizon (Argentina) and Choffatia jupiter horizon (northern Chile); the Vergarensis zone, index Eurycephalites vergarensis (Burck.), near the Bathonian-Callovian boundary; the Bodenbenderi zone, index Neuqueniceras (Frickites) bodenbenderi (Tornq.), lower Callovian; the Proximum zone, index Hecticoceras proximum Elmi, uppermost lower Callovian; and the Rehmannia (Loczyceras) patagoniensis horizon, middle Callovian.

Âge et contexte dynamique des calcaires à brachiopodes téthysiens (Zeilleriidés multiplissés) du Grand pic de l'Ouarsenis(Tell algérien)

Abstract The vertical range of the brachiopods in the Ouarsenis depends closely on the sedimentary evolution and on the local geodynamic controls. During the Late Sinemurian and the Early Carixian, several Zeilleriid coquinas have accumulated on a shallow carbonate platform. These brachiopods are well levelled in the biochronologic scale and their age is confirmed by the discovery of an ammonite (Gemmellaroceras sp.). Aborted reefs were built laterally following tectonic movements. The sedimentary evolution has happened during a relative sea-level rise. The brachiopods appeared at the beginning and disappeared when the filling progressed. A forced regression occurred at the end of the Early Carixian. It is marked by incisions, emersions and karstifications. Large bivalve limestones (Cochlearites, Opisoma) have been deposited later. Their local chronological position ranges from the Middle Carixian to the Late Carixian (Demonense and Dilectum Zones).

Evolution de l'architecture des monts d'Oujda (Maroc oriental) pendant le Trias et au debut du Jurassique

Isopach maps of the Triassic-Lower Liassic volcano-sedimentary successions in the Oujda Mountains (eastern Morocco) have been drawn and complemented by the study of the directions of the doleritic dykes. New interpretations of the dynamics of this part of the northern Gondwana margin during the beginning of the Mesozoic rifting are proposed. The tectonic pattern of the Oujda Mountains was controlled by the conjugate motion of N070 degrees E-N090 degrees E (Atlasic) and N150 degrees E-N160 degrees E faults. This extensional regime led to the differentiation of several strike-slip or pull-apart basins delimited by conjugate faults: Oujda, Ahajfoune, Oued El Himer, Tiouli, North Jerada and Metroh depocentres. Some of these are distinct from the others and have sustained a differential subsidence. They have undergone a differential subsidence but this is almost always balanced by the rate of sedimentation and by the basaltic flows which underline the palaeostructures.

Le passage toarcien-aalenien et l'age des calcaires a Cancellophycus dans la region de Macon (Saone-et-Loire)

Lithologic studies and paleontologic studies of ammonites from a section between Vergisson and Davaye have provided more detailed information on the stratigraphy of the upper Toarcian and lower Aalenian in Macon (central France). The upper boundary of the Toarcian is marked by the Pleydellia buckmani bed recognized for the first time at the eastern border of the French Central Massif. The Tmetoceras scissum zone is recognized in the Aalenian. The Cancellophycus limestones represent the basal Ludwigia murchisonae zone (Costileioceras sinon bed) and the T. scissum zone. Two tables show proposed correlations between Macon, Mont d9Or, and the southern Jura mountains.