The importance of local mean time in remote sensing for mapping megatidal zones

The local mean time is rarely debated in applied remote sensing, although it is a key parameter, especially for the geological mapping of coastal areas with high tidal ranges. French test sites are used to illustrate the role of this parameter. Depending from location and for an average time of acquisition of 11:00 UTC, the exposed tidal flat ranges from 55% (Baie des Veys), 80% (Mont Saint-Michel) to 100% (Cap-Ferret). Eight multisource satellite images were used in the Mont Saint-Michel Bay to draw the limit of the sea and evaluate the corresponding tidal flat's surfaces for various times of acquisition. The relation emphasizes some geometrical properties of the lower part of the intertidal zone. A NW–SE profile allows one to identify a distal part with a slope of 0.2% and a proximal part with a slope of only 0.06%. JERS 1-OPS and ALOS-AVNIR-2 data have been also compared to evaluate the progression of the halophytic vegetation, covering more than 6 km² between 1992 and 2007. From a methodological point of view, the ALOS-AVNIR-2 acquired on October 2007 is the first satellite data covering the Mont Saint-Michel Bay with a water elevation of only 2.56 m, i.e. a tidal flat free of water over 80% of its surface.     

Paleo-fluids characterisation and fluid flow modelling along a regional transect in Northern United Arab Emirates (UAE)

In the Northern Emirates, Jurassic and Lower Cretaceous platform carbonates of the Musandam parautochthonous units are tectonically overlain by siliciclastic units of the Hawasina–Sumeini allochthon, which derive from the former paleo-slope domain and a more distal basinal portion of the Arabian margin of the Tethys, respectively. All these tectonic units display numerous evidences of paleo-fluid circulations, accounting for dolomitisation and recrystallisation of the rock matrix (Musandam Platform units), as well as cementation of fractures. Polymict breccias of Upper Cretaceous Ausaq Formation which underlay the sole thrust of the Hawasina–Sumeini allochthon also record episodes of hydraulic fracturing, whereas fluid inclusion data indicate precipitation at high temperature in relation to paleo-fluid flow. Petrography of thin-sections (conventional and cathodoluminescence microscopic techniques) as well as fluid inclusion and stable isotopes analyses, were combined with micro-tectonic studies. These analytical data document (1) the paragenetic sequence of diagenetic products for the Musandam Platform (which constitutes a carbonate reservoir analogue) and Sumeini units of the Dibba Zone, as well as (2) the nature of the paleo-fluids circulating along fractures and the sole thrust of the Hawasina–Sumeini allochthon. The main results of this petrographic approach are qualitative, evidencing (1) the rapid and vertical transfer of hot fluids in the vicinity of the former slope to platform transition, accounting for episodes of hydrothermal dolomitisation, as well as (2) early (i.e. pre-orogenic) and late (i.e. post-orogenic) episodes of emersion of the carbonate units, accounting for additional interactions with meteoric fluids and karstification. In order to better link these diagenetic events with the overall burial, thermal and kinematic evolution of the Arabian margin, basin modelling with Ceres2D, including fluid flow and pore-fluid pressure modelling, was subsequently performed along a regional transect (D4) located in the vicinity of the samples localities and cross-cutting the Northern Oman Mountains from Dibba in the east up to the Arabian Gulf in the west. New subsurface constraints provided by deep seismic profiles were used to constrain the architecture of the cross-section, and to test various hypotheses on the lateral and vertical connection, timing and hydrodynamic behaviour of the faults. This Ceres basin modelling also provides new quantitative estimates of the paleo-fluid pathways, of the timing and velocities of the fluid transfers and of the evolution of pore-fluid pressures. Ultimately, this integration of petrographic studies on surface samples and coupled kinematic and fluid flow basin modelling provides an updated scenario for the succession of tectonically controlled episodes of fluid rock interactions, namely dolomitisation and karstification recorded in the Mesozoic platform carbonates of the Northern Emirates.     

A new amber deposit from the Cretaceous (uppermost Albian-lowermost Cenomanian) of southwestern France

A Cretaceous amber deposit has recently been discovered in a quarry of Charente-Maritime (southwestern France), at Cadeuil. This paper presents the sedimentary and palaeoenvironmental settings of the uppermost Albian-lowermost Cenomanian series including the amber deposit. A preliminary analysis of the amber samples reveals diverse fossil arthropods (a few mites and at least 20 insect families within 9 orders), as well as numerous micro-organisms, mainly algae and mycelia. A myceloid colony of bacteria, a flagellate algae and four especially well preserved insects are illustrated (Diptera Dolichopodidae, Diptera Chironomidae, Hymenoptera Parasitica, and Heteroptera Tingidae). The abundance of the limnic micro-organisms is discussed in terms of bloom events. Their relative scarcity in almost all the amber pieces containing fossil arthropods is attributed to differences in the origin of resin: production along trunk and branches for amber with arthropods; production by aquatic roots for amber rich in algae. The absence of pollen and spores in amber is attributed to differences in the respective periods of resin and palynomorph production, which may be related to a seasonal climate during the Albian-Cenomanian transition in Western Europe.     

Shoshonitic liquid line of descent from diorite to granite: the Late Precambrian post-collisional Tismana pluton (South Carpathians, Romania)

The post-collision late-kinematic Tismana pluton belongs to the shoshonitic series. It is part of a Late Precambrian basement within the Alpine Danubian nappes of the South Carpathians (Romania). This pluton displays an exceptionally complete range of compositions from ultramafic to felsic rocks (granites). Widespread mingling/mixing relationships at all scales give rise to a variety of facies. A liquid line of descent from the diorites to the granites is reconstructed by considering the variation in major and trace elements (REE, Sr, Rb, Ba, Nb, Zr, Hf, Zn, V, Co, Cr, U, Th, Ga, Pb) from 33 selected samples as well as mineral/melt equilibrium relationships. The first step of fractional crystallization is the separation from a monzodioritic parent magma of a peridotitic cumulate similar to the ultramafic rock found in the massif. A possible contamination by lower crustal mafic component takes place at this stage. The second step marks the appearance of apatite and Fe–Ti oxide minerals as liquidus phases, and the third step, saturation of zircon. Mixing by hybridisation of magmas produced at different stages of the evolution along the liquid line of descent is also operating (endo-hybridisation). As depicted by Nd and Sr isotopes, fractional crystallization was combined to an important early contamination by a mafic lower crust in a deep-seated magma chamber and to a later and mild contamination by a felsic medium crust in an intermediate chamber. The mingling essentially occurred during the final emplacement in the high-level magma chamber. The monzodioritic parent magma, identified by major and trace element modelling, is shown by Sr and Nd isotopes to have its source in the lithospheric mantle or in a juvenile mafic lower crust derived from it. The necessarily recent enrichment in K₂O and associated elements of the lithospheric mantle is likely to be related to the preceding Pan-African subduction period. The partial melting of this newly formed deep source has to be linked to a major change in the thermal state of the plate.     

Characterization of Mediterranean magnetotactic bacteria

Magnetotactic bacteria are a diverse group of motile prokaryotes that are ubiquitous in aquatic habitats and cosmopolitan in distribution. In this study, we collected magnetotactic bacteria from the Mediterranean Sea. A remarkable diversity of morphotypes was observed, including multicellular types that seemed to differ from those previously found in North and South America. Another interesting organism was one with magnetosomes arranged in a six-stranded bundle which occupied one third of the cell width. The magnetosome bundle was evident even under optic microscopy. These cells were connected together and swam as a linear entire unit. Magnetosomes did not always align up to form a straight linear chain. A chain composed of rectangle magnetosomes bent at a posi- tion with an oval crystal. High resolution transmission electron microscopy analysis of the crystal at the pivotal position suggested uncompleted formation of the crystal. This is the first report of Mediterranean magnetotactic bacteria, which should be useful for studies of biogeochemical cycling and geohistory of the Mediterranean Sea.     

Post-collisional melting of crustal sources: constraints from geochronology,petrology and Sr,Nd isotope geochemistry of the Variscan Sichevita and Poniasca granitoid plutons (South Carpathians,Romania)

The Sichevita and Poniasca plutons belong to an alignment of granites cutting across the metamorphic basement of the Getic Nappe in the South Carpathians. The present work provides SHRIMP age data for the zircon population from a Poniasca biotite diorite and geochemical analyses (major and trace elements, Sr–Nd isotopes) of representative rock types from the two intrusions grading from biotite diorite to biotite K-feldspar porphyritic monzogranite. U–Pb zircon data yielded 311 ± 2 Ma for the intrusion of the biotite diorite. Granites are mostly high-K leucogranites, and biotite diorites are magnesian, and calcic to calc-alkaline. Sr, and Nd isotope and trace element data (REE, Th, Ta, Cr, Ba and Rb) permit distinguishing five different groups of rocks corresponding to several magma batches: the Poniasca biotite diorite (P₁) shows a clear crustal character while the Poniasca granite (P₂) is more juvenile. Conversely, Sichevita biotite diorite (S₁), and a granite (S₂*) are more juvenile than the other Sichevita granites (S₂). Geochemical modelling of major elements and REE suggests that fractional crystallization can account for variations within P₁ and S₁ groups. Dehydration melting of a number of protoliths may be the source of these magma batches. The Variscan basement, a subduction accretion wedge, could correspond to such a heterogeneous source. The intrusion of the Sichevita–Poniasca plutons took place in the final stages of the Variscan orogeny, as is the case for a series of European granites around 310 Ma ago, especially in Bulgaria and in Iberia, no Alleghenian granitoids (late Carboniferous—early Permian times) being known in the Getic nappe. The geodynamical environment of Sichevita–Poniasca was typically post-collisional of the Variscan orogenic phase.     

Monitoring of playa evaporites as seen with optical remote sensing sensors: case of Chott El Jerid,Tunisia, from 2003 to present

Chott El Jerid in the Zone of Chotts of Tunisia is one of the largest endorheic basins in the world. During the dry period, from May to August, it is generally covered by continental evaporites which result from the desiccation of the lake formed after a flooding event. This lake comprises runoff water from the surrounding relief and also water resurgence. In order to map and monitor these evaporitic surfaces (mineral composition and evolution in space and time), optical multisource, multispectral, and multidate satellite data have been used. Landsat 4–5 TM, Landsat 8 OLI, SPOT 6, and Landsat Surface Reflectance (LSR) constitute the main data set. The central part of the Chott, north and south of the road crossing the Chott over 70 km from Tozeur to Kebili, has been particularly studied, because it corresponds to the major evaporite accumulation zone. These evaporites precipitated as concentric layers (a relatively rare pattern), mainly north of the road, after several recent flooding events during the last 15 years. Winds can play a significant role in the development of the evaporite layers. Image interpretation associated with field data shows that after the final desiccation of the playa lake, the mineralogy of the salt crust comprised an assemblage dominated by halite south of the road and by gypsum north of the road. Halite and gypsum are the only minerals to be identified using satellite remote sensing data. Sulfates such as gypsum can be identified thanks to a drop in reflectance in the short-wave infrared (SWIR) range caused by vibrations of the SO₄ group. Gypsum crusts are more widely distributed than halite crusts. LSR data are particularly suitable for multitemporal comparison because they are calibrated and atmospherically corrected. The classical bull’s eye pattern characterizing evaporitic deposits (from carbonates along the rims to halite, gypsum, and finally potassium-magnesium minerals in the center of the basin) is deeply disturbed by the road crossing Chott El Jerid.