Les calcretes et dolocretes de la serie permo-triasique du Haut-Atlas central (Maroc)

International audienceDans le Haut Atlas central (Maroc), le Permo Trias est constitué d'une série rouge détritique. L'ensemble est subdivisé en six formations. La formation F3 correspond à des épandages de dépôts alluviaux de nature gréso conglomératique armés de plusieurs niveaux pédologiques, foyers d'une importante carbonatation diagénétique. Cette épigénie carbonatée a provoqué une forte réduction de la porosité. La morphologie du faciès, la géométrie des corps sédimentaires et l'analyse de la minéralogie du micro-faciès de la formation F3, ont révélé la présence de deux types d'encroûtements carbonatés: l'un calcaire ou calcrète et l'autre dolomitique ou dolocrète. Ces encroûtements calcrètes et dolocrètes ont été attribués à un environnement pédogénétique d'une part et/ou aux circulations de nappes phréatiques sous couverture d'autre part. Ce système de pénéplanation témoigne d'une période de pédiplanation associée localement à une accalmie tectonique à l'échelle régionale du Haut - Atlas central. In the Central High Atlas (Morocco), the PermoTrias comprises a redbed detrital sequence, which is subdivided into six formations. Formation F3 comprises extensive sandy conglomeratic alluvial deposits with several soil horizons affected by an important carbonate diagenesis. This carbonate epigenesis has strongly reduced the porosity. Facies morphology, geometry of sedimentary bodies and the analysis of the mineralogy of microfacies of formation F3 have revealed the presence of two types of carbonate cement : one is limestone or calcrete and the other is dolomitic or dolocrete. These calcrete and dolocrete cements have been attributed to a pedogenetic environment and/or to circulating of groundwater. This peneplanation system is evidence of a pediplanation period which is locally associated with a regional tectonic lull in the Central High Atlas

Early Stage Biomineralization in the Periostracum of the ‘Living Fossil’ Bivalve Neotrigonia

A detailed investigation of the shell formation of the palaeoheterodont ‘living fossil’ Neotrigonia concentrated on the timing and manufacture of the calcified ‘bosses’ which stud the outside of all trigonioid bivalves (extant and fossil) has been conducted. Electron microscopy and optical microscopy revealed that Neotrigonia spp. have a spiral-shaped periostracal groove. The periostracum itself is secreted by the basal cell, as a thin dark pellicle, becoming progressively transformed into a thin dark layer by additions of secretions from the internal outer mantle fold. Later, intense secretion of the internal surface of the outer mantle fold forms a translucent layer, which becomes transformed by tanning into a dark layer. The initiation of calcified bosses occurred at a very early stage of periostracum formation, deep within the periostracal groove immediately below the initialmost dark layer. At this stage, they consist of a series of polycyclically twinned crystals. The bosses grow as the periostracum traverse through the periostracal groove, in coordination with the thickening of the dark periostracal layer and until, upon reaching the mantle edge, they impinge upon each other and become transformed into large prisms separated by dark periostracal walls. In conclusion, the initial bosses and the external part of the prismatic layer are fully intraperiostracal. With later growth, the prisms transform into fibrous aggregates, although the details of the process are unknown. This reinforces the relationships with other groups that have the ability to form intraperiostracal calcifications, for example the unionoids with which the trigonioids form the clade Paleoheterodonta. The presence of similar structures in anomalodesmatans and other euheterodonts raises the question of whether this indicates a relationship or represents a convergence. The identification of very early calcification within an organic sheet has interesting implications for our understanding of how shells may have evolved.Coordinated Research Projects CGL2010-20748-C02-01 (AGC, EMH) and 02 (CS) (DGI, Spanish MICINN); the Research Group RNM363 (Consejería de Economía, Investigación, Ciencia y Empleo, Junta de Andalucía); and the FP7 COST Action TD0903 of the European Community

Etude de la matiere organique particulaire marine par analyses des acides gras. Etude de la nature du materiel organique en Mer Celtique et Mer d'Irlande; elaboration d'un modele de reconnaissance des classes planctoniques; evaluation de la qualite nutritionnelle des particules marines

SIGLEINIST T 77644 / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Fatty acid composition in deep hydrothermal vent symbiotic bivalves.

Fatty acids in deep hydrothermal vent bivalves have been analyzed. Their composition is completely different from that of a littoral mussel collected in the Mediterranean sea. The distribution of fatty acids in the littoral mussel is characterized by a predominance of polyunsaturated fatty acids (20:5n-3, 22:6n-3) reflecting the planktonic origin of the food. Vent bivalve fatty acid distribution is dominated by an abundance of the monounsaturated acids (double bond in the n-7 position) 16:1n-7, 18:1n-7, and 20:1n-7 which are clearly of bacterial origin and give an indication of the symbiotic bacterial activity in the bivalves. Differences between the fatty acid composition of the bivalves from two hydrothermal sites (13 degrees N and Galapagos) and differences between the mantle and the gill were observed and are discussed with respect to vent activities at the two sites and species metabolic capacities as a function of ecological conditions

Le remplissage synrift au permien et au trias du bassin de Tahanaout (Haut Atlas de Marrakech, Maroc) géodynamique et organisation sédimentaire

The perno-triasic basin of Tahanaout reflects a tectono-sedimentary organization typical of synrift basins. The genetic modes of its replenishment, reviewed in light of the sedimentary heterogeneity and the position of the sediments reveal an evolution in 3 episodes: individualization, differentiation and homogenization. The tectonics expresses itself every time, by a reorganization of the deposit areas, a dissymmetry of the syntectonic gutters and structures in the tilled blocks, within a NS extensional regime (episode 1) and NW-SE even (episode 11 and 111). The intermediate sequences follow each other in sequences of deposit that reflect and evolve in fluvio-deltaic environments, since the cone toward the alluvial or deltaic plain according to a topographic gradient. The sedimentary model suggests the existance basins in vertical relays in severa1 lateral progradation phases interrupted by phases of silt-clay sedimentary aggradations. The generalized semi-arid climate and the closed country environment plays an important role in the configuration of this type of basins, whose borders are essentially clastics, of an extended alluvial plain.<br><br>Le bassin permo-triasique de Tahanaout refl&egrave;te une &eacute;volution typique des bassins synrifts. Les modalit&eacute;s g&eacute;n&eacute;tiques de son remplissage, r&eacute;v&egrave;lent une organisation en 3 &eacute;pisodes tectonos&eacute;dimentaires: l&rsquo;individualisation, la diff&eacute;renciation et l&rsquo;homog&eacute;n&eacute;isation du bassin. La tectonique s&rsquo;exprime par la cr&eacute;ation de goutti&egrave;res syntectoniques et de structures en blocs bascul&eacute;s et s&rsquo;inscrit dans un r&eacute;gime extensif N-S (&eacute;pisode I) puis NW-SE (&eacute;pisodes II et III). La s&eacute;dimentation continentale s&rsquo;organise en plusieurs cycles de progradation &ndash; r&eacute;trogradation; dans laquelle, les s&eacute;quences du milieu retracent toute la dynamique des environnements fluvio-delta&iuml;ques, depuis le cone vers la plaine alluviale ou delta&iuml;que selon un gradient topographique d&eacute;croissant. Le mod&egrave;le s&eacute;dimentaire propose un remplissage en plusieurs phases de progradation lat&eacute;rales interrompues par des phases d&rsquo;aggradations s&eacute;dimentaires silto-argileuses. Le climat semiaride g&eacute;n&eacute;ralis&eacute; joue un r&ocirc;le important dans la nature et l&rsquo;hydrodynamisme des apports ainsi que l&rsquo;&eacute;laboration des cycles s&eacute;dimentaires