The lowermost Mississippi River: a mixed bedrock‐alluvial channel

In this study, the distribution of channel‐bed sediment facies in the lowermost Mississippi River is analysed using multibeam data, complemented by sidescan sonar and compressed high‐intensity radar pulse seismic data, as well as grab and core samples of bed material. The channel bed is composed of a discontinuous layer of alluvial sediment and a relict substratum that is exposed on the channel bed and sidewalls. The consolidated substratum is made up of latest Pleistocene and Early Holocene fluvio‐deltaic deposits and is preferentially exposed in the deepest thalweg segments and on channel sidewalls in river bends. The exposed substratum commonly displays a suite of erosional features, including flutes that are quantitatively similar in form to those produced under known laboratory conditions. A total of five bed facies are mapped, three of which include modern alluvial deposits and two facies that are associated with the relict substratum. A radius of curvature analysis applied to the Mississippi River centreline demonstrates that the reach‐scale distribution of channel‐bed facies is related to river planform. From a broader perspective, the distribution of channel‐bed facies is related to channel sinuosity — higher sinuosity promotes greater substratum exposure at the expense of alluvial sediment. For example, the ratio of alluvial cover to substratum is ca 1·5:1 for a 45 km segment of the river that has a sinuosity of 1·76 and this ratio increases to ca 3:1 for a 120 km segment of the river that has a sinuosity of 1·21. The exposed substratum is interpreted as bedrock and, given the relative coverage of alluvial sediment in the channel, the lowermost Mississippi River can be classified as a mixed bedrock‐alluvial channel. The analyses demonstrate that a mixed bedrock‐alluvial channel boundary can be associated with low‐gradient and sand‐bed rivers near their marine outlet.     

A new Lyrodiscus (Mollusca, Gastropoda) assemblage from Saint-Acheul (Somme Valley): a reappraisal of MIS 11 malacofaunas from northern France

Recent studies undertaken at Saint-Acheul in the Somme Valley (France) have provided an opportunity to recover malacological assemblages from a tufa deposit located at the top of a Middle Pleistocene fluvial sequence. Molluscan communities are rich and, although dominated by open grassland species, contain up to 23 forest taxa. This high diversity of thermophilous molluscs allows the recognition of fully temperate climatic conditions. Moreover, the occurrence of an extinct zonitid belonging to the genus Retinella (Lyrodiscus), along with several species beyond their modern range (Platyla polita, Ena montana, Ruthenica filograna Clausilia pumila, Clausilia dubia, Macrogastra ventricosa, Perforatella bidentata, Monachoides incarnatus, Belgrandia marginata, Hygromia limbata) allows correlation of the Saint-Acheul assemblage with malacofaunas recovered in other MIS 11 tufa deposits from the Somme and Seine valleys. This age attribution is reinforced at Saint-Acheul by stratigraphy and an ESR date on quartz of 403±73 kyr from the underlying fluvial deposits. In addition, old collections containing R. (Lyrodiscus) specimens have been rediscovered, allowing taxonomic reassessment of the species. This shows that Retinella (Lyrodiscus) skertchlyi Kerney, 1976 is a junior synonym of Retinella (Lyrodiscus) elephantium (Bourguignat, 1869) and that at least two extinct species of R. (Lyrodiscus) occurred in western Europe during the Quaternary. Finally, reappraisal of these French molluscan assemblages shows that they are similar to British malacofaunas of Hoxnian age. These new results strengthen the uniqueness and biostratigraphical value of the 'Lyrodiscus assemblage'.     

Chronology of the Last Climatic Cycle (Upper Pleistocene) of the Surduk loess sequence, Vojvodina, Serbia

The Surduk loess section in Serbia provides a 20 m thick pedosedimentary record of the last interglacial–glacial climatic cycle (Upper Pleistocene). Based on optical dating, a chronostratigraphy could be established for the last climatic cycle, yielding the first numerical ages of a loess record from the middle Danube basin. Infrared-stimulated luminescence (IR-OSL) dating has been applied to the polymineral silt fraction using a multiple aliquot additive-dose protocol to determine the equivalent dose ( D _E ). Within error limits, all age estimates are in stratigraphic order. Owing to the application of shine-plateau tests, the samples showed no evidence of insufficient bleaching. The Surduk loess section comprises three major periods of soil formation. Based on the IR-OSL chronostratigraphy, the lowermost pedocomplex is attributed to the Last Interglacial and to the Early Glacial (Marine Isotope Stage – MIS 5e to 5a). The middle part of the section exhibits a succession of weakly developed brown soils and a humic horizon, named 'Surduk soil', formed during MIS 3. On top of the section, recent soil formation is related to the Holocene. Thick loess deposits are preserved between these palaeosols and are attributed to the Lower and Upper Pleniglacials (MIS 4 and 2), respectively. Estimated mean sedimentation rates are 0.1–0.2 mm/yr for the last glacial cycle, with a strong increase to 0.6 mm/yr with onset of the Pleniglacial.     

Nature mineralogique et origine des glauconies du plateau continental du Gabon et du Congo

The detailed analysis of pellets deposited on the continental shelf off Congo and Gabon (Gulf of Guinea) reveals the present formation of goethite and berthierine-granules, as well as that of glauconite. The goethite is being formed in the most oxygenized zones, independently of the initial nature of the granular support and, sometimes, takes the form of pseudooolites. The berthierine is being formed in the Ogooue delta zone. To us the deltaic environment, rather than the temperature factor, seems determinant. Principally, this berthierine is found in the form of coprolites, of initial kaolinic composition. Incrustations of calcareous debris and fillings of animalculae tests are present. This genesis is recent (post-Flandrian) and is still continuing. The glauconite exists practically uniquely in the form of coprolites, from the medium-depth zones of the continental shelf (—50 m) down to the upper continental slope (—300 m). The sèdiment in which it was formed, and is probably still being formed, is older (ante-Flandrian). The actual rate of sedimention is null when glauconite is forming. The glauconite formation may be broken down into several stages, which are found at different sampling depths and, sometimes, in the same samples. The pellets are formed first. These original pellets, of kaolinic nature, gradually become smectitic. The evolution continues, becoming a component with diffractometric behaviour identical with that of an interlayered material (illite-vermiculite) with the illite dominant. Correlatively, the K₂O percentage increases from 2–3% in the smectites to 5–6% in the most evolved component. The mineralogical composition of these last pellets is identical with that of certain fossil glauconites, the evolution of which could be followed up to the best crystallized glauconite mineral. Thus, we were able to reconstitute the formation history of the glauconite from any mineralogical support up to the typical glauconite mineral (Table l). Two conditions necessary for this formation are detected here: the time factor, as has often been pointed out, is important. However, it is, above all, the environment factor which has attracted our attention. The forming of pellets favours and determines the geo-chemical evolution through the creation of a semi-confined environment. Reactions take place there in a more concentrated environment than in the open sea, this environment is characterized by a large surface of reaction, since the granule is porous. Sedimentologically, it is possible to distinguish two groups of glauconites: one, monomineral, corresponds to more or less crystallized glauconite mineral; the other, plurimineral, consists of various TOT minerals. Only the first has the significance generally attributed to all glauconites : open epicontinental marine environment, free of sedimentation for a long time. The various cases of evolution encountered here are summarized below (Table 1).     

Palaeoenvironments associated with caddisfly-dominated microbial-carbonate mounds from the Tipton Shale Member of the Green River Formation: Eocene Lake Gosiute

Caddisfly‐dominated microbial‐carbonate mounds and avian eggshell fragments are common in a nearshore, oolite facies of the Tipton Shale Member of the Eocene Green River Formation. The fossils occur in a 9 m thick carbonate sequence exposed on the south‐west flank of Essex Mountain, Sweetwater County, Wyoming. The eggshell was determined to be of avian origin by examination of the radial eggshell microstructure by scanning electron microscopy and polarized light microscopy. Common allochems in the limestone include: ooids, pisoids, oncoids, ostracods, gastropods, intraclasts, caddisfly larval/pupal cases, fish bones, avian bones and avian eggshell fragments. Carbonate mineralogy varies between 95% calcite and 95% dolomite. Oxygen stable isotopes vary between −10·1 and −1·3‰ (V‐PDB). Carbon and oxygen stable isotopes co‐vary. Co‐variance of oxygen and carbon stable isotopes indicates that Lake Gosiute was hydrologically closed during the formation of this oolite sequence. Positive excursions of oxygen stable isotope values are correlated with dolomite, caddisfly cases and avian eggshell fragments. These factors are probably associated with lake regression and more saline conditions. Negative excursions of oxygen stable isotope values are correlated with calcite, gastropods and intraclasts. These factors are probably associated with lake transgression and lake freshening. In the 9 m oolite sequence studied there are at least two inferred transgressions, three regressions and two storm events. The caddisfly cases and avian eggshell fragments occur in sediments formed only during low‐stands of the lake as determined by other lake level proxies (dolomites and more positive oxygen stable isotope values). As avian eggshell fragments were formed under subaerial conditions, the avian eggshell fragments independently confirm lake regression. In addition, the occurrence of avian eggshell fragments during saline phases of the lake may indicate that the birds which produced the eggshell preferred saline conditions (or that their food preferred saline lake water).     

Comparing X-ray densitometry and BSE-image analysis of thin section in varved sediments

A 10-cm long section from varved sequence of Lake Nautajärvi, Finland, has been analysed using both X-ray densitometry and image analysis of Backscattered Scanning Electron microscope Images (BSEI) of thin sections. Lake Nautajärvi's clastic-organi c varves appear in X-ray images as a succession of dark and pale stripes, pale ones being the spring detrital minerogenic layers and dark ones organic matter laid down during the time of summer, autumn and winter ice cover. Owing to the sharp contrast of the minerogenic spring layer, a semiautomatic tree-ring width and density measurement DendroScan system was applied to obtain seasonal/annual data of relative X-ray density. Mean density of BSEI was also acquired. Then BSEI were processed to produce black and white images, where white pixels represent the clay-rich sedimentary matrix and black pixels the particles in the matrix: silt- or sand-sized, terrigenous grains, authigenic particles and diatoms. Measurements of the size of the detrital particles forming the varves were obtained. For each varve, mean grey-level values in BSEI correlated well with the mean grey-level values of X-ray images, but grain size obtained on BSEI did not correlate with any parameters computed from X-ray radiography.     

Palaeoenvironmental changes inferred from malacofaunas in the Lateglacial and early Holocene fluvial sequence at Conty, northern France

Environmental changes are reconstructed from a Lateglacial and early Holocene sequence at Conty, northern France. The molluscan succession is put into a chronostratigraphic framework supported by numerous radiocarbon dates. Malacofaunas from the Bølling chronozone are reported for the first time in northern France and show progressive expansion of marshy communities within organic deposits. This biozone ended in a calcareous silt with the appearance of several species of arctic-alpine affinities. These sedimentological and malacological data point to colder climatic conditions after 12 220 ± 90 BP, but before 11 640 ± 80 BP, allowing allocation to the Older Dryas event. The first part of the Allerød appears to have been drier and relatively stable. After 11 400 BP, a decline in species richness and diversity in the malacofaunas suggests increasing dryness. During the Younger Dryas, two molluscan biozones are identified in a homogeneous calcareous silt, reflecting an early wet phase followed by a drier episode. At the onset of the Holocene malacofaunas show a higher diversity, suggesting climatic improvement.     

ANALYSIS OF THE COMMON MIDPOINT GATHER BY DECOMPOSITION INTO ELEMENTARY WAVEFRONTS1

The imaginary wavefront of a common midpoint gather of seismic data is analysed in terms of elementary wavefronts. The wavefronts emitted by the sources are split into a sequence of elementary circular wavefronts. Each such wavefront is independently transmitted through the subsurface and received by a group of receivers associated with the sources. The theory is first demonstrated in the case of a one-layer medium and is then extended to the case of a multilayer medium. We show that pairs of imaginary stationary points F and F' occur in the first medium through which all the rays, corresponding to the group of sources and receivers associated with the elementary wavefront pass. The location of the points is determined by 2n equations with 2n unknowns; (2n - 1) of these equations correspond to n layers of the medium, and an additional equation is given by the geometry of the CMP gather. Applying Fermat's principle between the points F and F', we obtain an algorithm for CMP ray tracing. The wavefront sampling problem is considered but not theoretically solved. Results obtained in the general case (a multilayer medium with lateral velocity variation) are presented and compared with those obtained by other modelling techniques.