Showing papers in "Sedimentology in 1974"

Interdistributary bay sequences and their genesis

Abstract: Interdistributary bay sedimentation is dominated by flood-generated incursions from the distributaries. There are three processes by which sediment-laden flood waters can be transferred to the bay: overbank flooding, crevassing and avulsion. These processes combine to produce a family of sedimentary sequences, the majority of which are small-scale coarsening upwards sequences representing infilling of the bay. Subordinate related sequences include those of small-scale (crevasse) channels and larger scale (distributary) channels. Spits or beaches at the mouth of open interdistributary bays produce larger scale, wave-dominated coarsening upwards sequences. A model of bay sedimentation concerning a recently initiated (hypothetical) distributary is presented. Four phases of sedimentation (overbank flooding, crevasse splay, minor mouth bar—crevasse channel couplets and avulsion) characterize the history of the distributary, and it is postulated that the phases are genetically related to alluvial ridge development.

Inlet sequence: a vertical succession of sedimentary structures and textures created by the lateral migration of tidal inlets

Abstract: Tidal inlets, narrow, comparatively deep gaps between barrier islands, are channels through which tidal currents flow vigorously as the water enters and leaves the backbarrier lagoons, bays, and intertidal flats and marshes. Many geologists have compared tidal inlets to river channels and have speculated that if an inlet shifts laterally, it should deposit a distinctive sequence of sediments, analogous to the point-bar sequence left by a meandering stream channel. Fire Island Inlet, located 56 km east of New York City, has migrated WSW at a mean rate of 64 m/year during the period 1825–1940. Waves approaching the coast, predominantly from the southeast, have shifted sediment along the shore toward the WSW. Deposition of sediment on the east side of the inlet has forced the tidal currents to erode the west side of the inlet, thus causing lateral migration. Because hydraulic conditions vary from the channel floor to the subaerial part of the spit which is present on the ENE side of the channel, sedimentary structures and textures vary systematically with depth. We have determined the various sedimentary environments associated with the modern Fire Island Inlet, sampled and described the sediments from these environments, and have collected samples from corings made on that part of Fire Island through which the inlet has migrated. On the basis of our studies, we propose an inlet sequence which is formed by the lateral migration of a tidal inlet. The sequence includes five major units, as follows on p. 492. The sediments belonging to various units in this sequence have been identified in four borings made on those parts of Fire Island through which the Fire Island Inlet has migrated since 1825. This sequence should be applicable to other inlets also. We think that the boundary between deep channel and shallow channel units remains relatively fixed at −4.5 m, whereas the thickness of the deep channel unit is determined by the depth range between −4.5 m and the total depth of the inlet. Hence, the main source of variation in the inlet sequence will be the thickness of the deep-channel unit. Most of the sediments of the inlet sequence are incised below mean low water; hence they will almost certainly be preserved in the geologic record, even if all other associated sediments from barrier environments located above mean low water are not preserved. Because of the great variability possible in rates of lateral migration of inlets along the shore compared with the rates of barrier displacement perpendicular to the shore, inlet sediments may be preserved as elongate lenses, or as widespread blankets. The shape of inlet deposits reveals much about the behaviour of barriers during a submergence. Hence, inlet sediments should shed new light on sediments of the continental shelves and on basal transgressive sands in the geologic record.

Measurements of sand transport by wind on a natural beach

Abstract: Bagnold's (1954) and Kawamura's (1951) formulae may be used for the calculation of the sand movement on a natural beach, provided the shear stress velocity U* > 0·D4 m/s. Great discrepancies have been found between calculated and measured sand transport rates for U* < 0·D4 m/s, mainly because of the capillary forces acting on a wet beach. The measured critical shear velocity U*c at the beginning of sand movement on a clean dry beach agrees very well with that predicted by Bagnold's formula. On a dry beach where the sand grains are stuck together, U*c was found to be about 10% higher. On a wet beach U*c appeared to depend on the moisture content of the surface layer. Grain size is a determining parameter in the U*c-moisture content relation. When the angle a between the wind direction at sea and the dune face is between 15° and 85° the streamlines of the wind will bend in the vicinity of the dune face. In consequence this may influence the direction of sediment movement.

Vesiculated tuffs and associated features

Abstract: Vesiculated tuffs are tuffs that contain vesicles between the ash particles Formation of the vesicles is the result of trapping of steam, the transporting agent of volcanic base surges, in wet, muddy or sticky ash deposited by the base surges Vesiculated tuffs are described from various maars and tuff-rings in Europe (Iceland, France, Germany) and USA together with associated surface features such as: gravity flowage ripples, mud flow channels, current ripples, and current ridges Other features described are: plastering of ash against obstacles and vesiculated accretionary lapilli, the latter containing vesicles in the outer layer Vesiculated base surge deposits probably contained as much as 20–30% of interstitial water and fell out of the base surge clouds en masse owing to non-free flow and consequent accretion

Lithofacies relations in the Late Quaternary Niger Delta complex

Abstract: A study of cores from thirty-three coreholes drilled in various parts of the Niger delta has shown tidal channel sand to be the dominant lithofacies type in the uppermost 30 m of the deltaic complex. Below 30 m fluviatile sand becomes predominant. Coastal barrier sand is present in the uppermost 5 m of the present coastal belt, but chances for preservation of this lithofacies appear to be small. The Post-Glacial deltaic sediments can be divided into three units. 1 Alluvial valley-fill sands and conglomerates deposited during the strong Post-Glacial sea level rise. 2 An onlapping complex of lower coastal plain deposits which contains a lower member of fine grained lagoonal and mangrove swamp deposits and an upper member of tidal channel and coastal barrier sands. This complex is thought to have been deposited during the strong Post-Glacial rise in sea level and is locally as much as 25 m thick. 3 An offlapping complex of fluviomarine and coastal deposits which contains a lower member of marine clay and silt and an upper member of tidal channel and coastal barrier sand. The presence of this late Holocene complex indicates that deltaic progradation was resumed as soon as the rapid rise in sea level slowed down. The offlapping complex is locally as much as 35 m thick.

A scanning electron microscope study of surface textures of quartz grains from glacial environments

Abstract: Surface textures of quartz sand grains from several glacial environments at the Feegletscher, Switzerland examined by means of scanning electron microscopy are described. The difference between supraglacial and subglacial material is very slight. At the moment the interpretation of these textures must be done with care until the full statistical relation to environments can be ascertained. This does not necessarily invalidate earlier investigations although it does mean that glacial and extraglacial environments in the vicinity of glaciers are apparently not distinguishable. Examination of surface precipitation features suggests a sequence of events which can be used to help discriminate between different ages of deposits in an area. A similarity between some of the surface debris from moraine samples and those seen in loess and quickclay deposits is also suggested.

Structure and directional properties of some valley sandur deposits in southern Iceland

Abstract: Sediments in the valley sandurs of Markarfljot and Landmannalaugar accrete in complex lateral (transverse) bars. Directional structures within the bars are more variable in orientation than the channels which contain them. This greater variability is mainly due to the diversification of the flow, at the low stage, as it moves through the accretion topography of the sandur. Different directional structures preferentially record different stages of the flow: imbrication and lineation form mainly in response to the high flow stage, and have a low degree of dispersion; cross-stratification records the low stage of flow, and has a wide dispersion. Cross-stratification is produced in four main ways: by small deltas, by megaripples, by ripple topped bars, and by migrating accretionary bar banks. Small deltas form at low flow stage; accretionary banks at high.

Facies relationships in Pleistocene outwash gravels, southern Ontario: a model for bar growth in braided rivers

Abstract: Facies relationships in Pleistocene braided outwash deposits in southern Ontario demonstrate the presence of a large braid bar with adjacent side channel. The core of the bar is up to 6 m high, and consists of crudely horizontally stratified gravels. Downstream from the core is the bar front facies, consisting of large gravelly foresets up to 4 m high, rounded off in many places by reactivation surfaces. Upstream from the core is the bar stoss side facies consisting of several sets (individually up to 35 cm thick) of tabular cross-bedding, arranged in coarsening-upward sequences. The stoss side—core—bar front relationships are continuously exposed in one 400 m long quarry face which is cut almost parallel to the palaeoflow direction. A transverse quarry face shows the side channel facies, which consists of trough cross-bedded sands. Gravel layers can be seen to finger from the main gravelly bar into the sandy side channel, but they do not reach the base of the channel. This surprising relationship indicates that gravel moved only in the topographically higher parts of the system. After deposition in the side channel, and growth upstream and downstream from the bar core, the entire system aggraded. Crudely horizontally stratified, and imbricated gravel sheets were laid down as a bar top facies. Grain size analyses indicate strongly bimodal distributions, implying that much of the sand in the spaces between pebbles and boulders filtered in after the gravel had been deposited. This interpretation is strengthened by velocity calculations—mean velocities in excess of 300 cm/s would be needed to roll the gravel as bed load, but at such a velocity, a large amount of sand would be transported entirely in suspension. In a final section of the paper, our results are combined with other work on braided systems in an attempt to formulate a more general facies model.

Geochemical and mineralogical influence of Ordovician Island Arcs on epicontinental clastic sedimentation. A study of Lower Palaeozoic sedimentation in the Oslo Region, Norway

Abstract: The mineralogy and geochemistry of clastic sediments in the 1000 m thick Lower Palaeozoic marine sequence of the Oslo Region is discussed on the basis of 200 chemical (XRF) and mineralogical (X-ray diffractometer) analyses. The composition of these sediments is considered to largely be a function of the relative supply from land of the continental shield (Baltic Shield) and from island are systems to the NW in the Trondheim Region. The relative abundance of chlorite and illite is discussed in light of recent knowledge about their distribution in modern oceans. Middle Cambrian to Lowermost Ordovician black shales contain only illite as the dominant clay mineral and have a high potassium content. These sediments are thought to be derived from land on the continental shield, which during this period probably had a low relief and a warm climate where chlorite is not stable. Chlorite is introduced first in the Arenigan (L. Ordovician) and this is interpreted as evidence for transport of clastic chlorite from a developing island arc system in the Trondheim Region to the west. The highest chlorite/illite ratio in the Middle and Upper Ordovician sediments of the Oslo Region corresponds to a Taconic orogenic phase. The content of clastic chlorite in the Lower Palaeozoic of the Oslo Region is thus shown to be a sensitive indicator of palaeoenvironment and tectonic evolution of the Caledonian geosyncline. This conclusion is also supported by a systematic increase in Mg, Fe, Ni, and Cr from Lower to Middle and Upper Ordovician beds. High Cr values exceeding 0–1 % Cr in the Middle/Upper Ordovician shales are due to small grains of clastic chromite resembling those in the serpentinites of the Trondheim Region. This also suggests transport from exposed island arcs onto the epicontinental shelf.

Carbonate banks and slump beds in the Upper Cretaceous (Upper Turonian-Santonian) of Haute Normandie, France

Abstract: Large scale sedimentary structures present in the Upper Turonian to Santonian chalks of Haute Normandie (northern France) represent the remains of a carbonate bank complex which formerly extended over an area of at least 1500 km2. Cliff exposures along the Channel coast from St Valery-en-Caux to Cauville and along the Seine from Sandouville to Lillebonne show sections of banks up to 50 m high and 1500 m across, their internal structures picked out by hardgrounds, nodular chalks and horizons of burrow flint. Associated with banks are slump sheets up to 20 m thick, slump scars, sedimentary breccias, injection phenomena and faults contemporaneous with sedimentation. Later diagenetic features include extensive dolomitization and silicification. These structures compare closely with the Waulsortian banks of the Palaeozoic, and bryozoan bioherms known from the Upper Cretaceous and Palaeocene of Denmark. Frame-building, sediment trapping and stabilizing organisms are absent, and bank development and stabilization was probably due to a plant covering, either algal or of marine angiosperms. Banks generated much of their own sediment, whilst a pelagic constituent (calcareous nannofossils and Foraminiferida) is also present. The distribution of the bank complex is related to a basement controlled swell area, whilst the life of the complex was limited to a relatively shallow water, regressive episode in the predominantly transgressive Upper Cretaceous history of the region. Les falaises littorales du Pays de Caux comprises entre Antifer et St Valery-enCaux, et les affleurements de la basse vallee de la Seine permettent d'observer des formations du Turonien superieur-Senonien inferieur qui presentent des stratifications irregulieres soulignees par de nombreux hardgrounds, des horizons de craie noduleuse et des cordons de silex. Ces structures sont identifiees a des accumulations de calcilutite et calcarenite sous forme de bancs sous-marins dont la hauteur peut atteindre 50 m et qui couvrent une surface superieure a 1500 km2; ils apparaissent au-dessus de hardgrounds subhorizontaux qui indiquent un haut-fond regional stable. Des glissements sous-marins sont associes a ces bancs et engendrent des niveaux avec des deformations souples atteignant 20 m d'epaisseur. Des breches apparaissent localement et contiennent des blocs bascules de hardgrounds fragmentes lors du glissement; on y observe aussi de petites failles intrasedimentaires et des phenomenes d'injection. Aucun organisme constructeur ou capable de pieger et retenir le sediment n'a ete observe. La stabilisation de ces bancs serait due a une couverture vegetale (algues ou angiospermes marines) dont on sait qu'elle peut disparâitre sans laisser de trace lors de la fossilisation. La croissance de ces bancs serait realisee par un apport de sediment comprenant une part de nourrissage autochtone comme cela existe pour les bancs recents en eau peu profonde, associee au depot d'une fraction pelagique.

Mass physical properties, sliding and erodibility of experimentally deposited and differently consolidated clayey muds (Approach, equipment, and first results)

Abstract: On experimentally deposited kaolinite, illite, and Ca-bentonite consolidated under their own load or by additional vertical pressure, the progress of compaction in relation to excess pore water pressure, mass physical properties, gravitational mass movements in a tilted tank, and erodibility under running water in a flume were studied. The very low consolidated sediments near the mud/water interface do not obey the generally used theory in soil mechanics. They show a different, non-linear relationship between void ratio or water content and depth below the sedimentary surface on the one hand, or effective overburden pressure and shear strength on the other. The same is true of other physical properties such as permeability, which changes considerably with depth and time of consolidation. High sedimentation rates on slopes induce shallow sediment flow, whereas at low rates and critical slope angles different types of slope failures including the breaking up of water-rich sediment into sharp boundered blocks are observed. Flume studies on soft clay muds show three different types of erosion: continuously suspending, discontinuous erosion of crumbs or shreds, and wavy deformation of the clay surface with disintegration of particles from the crests. The critical tractive stress depends not only on clay type, void ratio, and shear strength, but significantly also on the ‘geologic history’of the clay (i.e. deposition from thin suspension or dense slurry, fabric, consolidation and swelling generating minute inhomogeneities etc.). The experiments may lead to a better understanding of all mechanical processe's including pore-water flow taking place near or not far below the sediment/ water interface.

Regularities of rare-earth element distribution in the sedimentary shell and in the crust of the earth

Abstract: After summarizing the vast analytical material it was possible to establish the differences in rare-earth element (REE) distribution in geosynclines and on platforms. It is shown that the heavier REE composition in sediments of geosynclines and the lighter one in sediments of platforms was initially created by processes of endogenic differentiation and then inherited by sedimentary series of these zones. On the basis of the calculation of the REE content in rocks of various shells of the Earth crust the increase of the role of heavy lanthanides into the depths of the crust is shown. The obtained estimate of REE abundance in the lithosphere led to the conclusion that the crust of our planet, despite the repeated reworking of its matter by sedimentary processes, metamorphism and granitization, has inherited features peculiar to products of tholeiitic magmatism—the most widely spread type of mantle fusion.

Tidal flat associations from a Lower Proterozoic carbonate sequence in South Africa

Abstract: An association of structures is documented from a Lower Proterozoic carbonate sequence in South Africa: this is dominated by finely laminated material which may display flat (smooth), small domical (blister), crinkled (tufted) and pustular (mamillate) surface. The lamination may display overfolded structures and contain flat encapsulated features. Lens-like flat-pebble breccias are common, while edgewise breccias were seen less frequently. Small-scale features suggestive of vertical algal moulds were also recorded. This facies is considered to be analogous to contemporary tidal flat sedimentation developed in specific settings at Shark Bay in Western Australia, and at Abu Dhabi where it is the dominant growth form. This analogy is most notably clear in relation to the protected embayments at Shark Bay, and as a result, an inner intertidal to marginal supratidal environment is suggested for the specific horizons under consideration from the Transvaal Dolomite. Other structures are associated with this facies. These include larger domes, coarser bedding, oolites, ripple-marks and, in one case, columnar stromatolites. Using the basic facies as an environmental datum, a model is developed in which these latter structures extend into the outer intertidal zone. They are also considered to be associated with varying states of turbulence in the intertidal regime.

Holocene sediments and magnetic stratigraphy from Lakes Zug and Zurich, Switzerland

Abstract: Piston cores 7 m and 6.5 m long were collected from Lakes Zug and Zurich respectively. Radiocarbon age determinations, pollen assemblage zones and varve counts indicate dates of 5000–6000 years B.P. (Zug), and 12,000–13,000 years B.P. (Zurich) for the base of the cores. Declination of the magnetic remanence varies through 70° and inclination varies through 30°. Although the correlation with the geomagnetic secular variation curve covering the last 15,000 years as determined from Lake Windermere, England (Creer et al., 1972) is not precise, the results suggest that Lake Zug and Lake Zurich sediments have been recording the broad outline of past secular variations of the earth's magnetic field. Since intensity of magnetization and susceptibility correlate markedly with lithology, a detailed sediment stratigraphy is presented. The amount and texture of the detrital input appears to be a controlling factor for the natural magnetic remanence. Intensity varies from 90 μG in zones of organic, sulphide pigmentation and those with ultra-fine laminations to 0.8 μG in impure lacustrine chalk. Susceptibility ranges from 9 μG/Oe in laminated, fine-grained glacially derived muds to 0.5 μG/Oe in impure lacustrine chalk. Intensity of magnetization also varies systematically within individual turbidites with lowest values in the coarse-grained, basal fractions. Slumped beds were identified on the basis of erratic anomalies in magnetic declination and inclination measurements.

Ironstones in the Upper Oxfordian of southern England

Abstract: The Upper Calcareous Grit, the last of the four upward shallowing cycles that comprise the Corallian Beds of southern England, is relatively enriched in iron minerals, having local developments of chamosite oolite mudstone and much more widespread deposits of sand and mud containing variable amounts of siderite and disseminated chamosite. The chamosite oolite mudstones have a restricted fauna dominated by oysters and probably accumulated in slightly hyposaline lagoons where the ooids formed from mixed iron-, alumina- and silica-bearing gels. Siderite was produced during diagenesis from iron carried on the surface of clay minerals. This intimate association with the terrigenous clay fraction means that siderite occurs in sediments deposited in a variety of environments ranging from offshore shelf to lagoonal. The most important factor responsible for ironstone development was a very low rate of clastic supply throughout Upper Calcareous Grit times. The iron was probably derived by normal processes of weathering and erosion of sedimentary rocks exposed around the basin margin, but this cannot be conclusively proved and quite different iron sources may have been involved.

Geochemistry and mineralogy of a recent sabkha along the coast of Sinai, Gulf of Suez

Abstract: Most efforts in the study of sea-marginal sabkhas have concentrated on the Persian Gulf, but little is known about the sediments and mineralogy of sabkhas marginal to other seas. The purpose of this paper was to present some geochemical and mineralogical observations in a recent sabkha on the coast of Sinai along the Gulf of Suez. The sabkha is composed of coarse clastic sediments with marine-derived groundwater at depth of about 1 m. The general morphology, climate and water salinity of the Gulf of Suez resemble those of the Persian Gulf, despite the fact that the content of authigenic evaporites in this sabkha is more sparse. The evaporite minerals accumulated only in the upper 30–40 cm of the sabkha, below that and down to the groundwater table, there is no accumulation of evaporites. Laterally, the salinity of the groundwater in the sabkha and the concentration of evaporites in the sediments above it increase constantly with distance from the shore. In contrast to the Persian Gulf where anhydrite is a major evaporite mineral, in Belayim gypsum is the only calcium sulphate mineral in the recent sabkha. Anhydrite is found only in an old elevated sabkha where it recrystallized from gypsum. The gypsum occurs as interstitial crystal concentrations or lithified horizons almost exclusively at the depth of 20–40 cm below the sabkha surface. Above that, in the uppermost horizons, there is in situ accumulation of interstitial halite crystals. The total concentrations of gypsum and halite are almost equal in this sabkha. The sea water recharge in El Belayim is almost exclusively by seepage through the sabkha sediments and not by flooding. The groundwater under this sabkha is only slightly more saline than the Gulf water, thus, not heavy enough for extensive downward refluxing. The major hydrodynamic process must be upward migration of the brines from the groundwater, precipitating on the way gypsum and later halite with some magnesite. Since the sediments of the sabkha are too coarse to support extensive capillary movement, the brines must, therefore, migrate upwards due to ‘evaporative pumping’.

Shale fissility: relation to bioturbation

Abstract: Fissility in shales appears directly related to the parallel orientation of mineral grains in the rock fabric. In two shale sequences examined, fissility increases along a gradient of decreasing bioturbation. Normal marine mudrocks should be characterized by lack of fissility due to the randomized fabric produced by bioturbation, while azoic marine mudrocks should exhibit good fissility due to the lack of biogenic reworking and the preservation of an originally horizontal particle arrangement.

Halite oolites and ripples in the Dead Sea, Israel

Abstract: Clastic features in recent halite deposits are observed along the beaches of an artificially dammed part of the Dead Sea. These features include halite oolites (termed halolites in this paper) and ripples. Halite precipitates initially either at the brine surface or on the floor. It is suggested that moderate increase of wave agitation shifts the balance towards brine-surface crystallization, and keeps the growing halite grains in constant motion. In this way rippled structures are formed. A further increase of wave energy leads to the growth of coated halite grains. The accumulation of the various halite grains along the beach, to form soft rippled floor and oolitic beach ridge is brought about during shoreward winds. During calm periods the bulk of the halite crystallizes directly on the floor. It develops into a hard crust which assumes the morphology of the substrate, including the ripple forms.

Diagenetic alteration of smectite in argillaceous sediments of the Rhinegraben (SW Germany)

Abstract: Clay minerals of about 600 samples from drill cores in the Tertiary shales of the Rhinegraben, were analysed by X-ray diffraction methods. It was found that the abundance of smectite decreases with increasing amounts of mixed layer clays and illites, suggesting a diagenetic alteration of smectite with increasing temperatures of the sediments as was also observed in other sedimentary basins. As for the relation between smectite alteration and temperatures as measured in sample depths, in the marine Graue Schichtenfolge (Middle Oligocene) when temperatures reach 70°C and over, smectite no longer is to be found. In the limnic Bunte Niederroderner Schichten (Upper Oligocene) the maximum temperature of smectite occurrence is about 80°C. These temperatures also fit the results of former field studies of the thermal stability of smectite. In the brackish to limnic Obere and Untere Hydrobienschichten (Lower Miocene) however, smectite seems to have disappeared already at a temperature of over 30°C. Although the smectite distribution in the latter formations may depend partly on its inhomogenous deposition there is also evidence for the diagenetic alteration of smectite in these formations. The rapid disappearance of smectite in these formations was possibly caused by a greater availability of potassium ions since high permeability of these strata provide extraordinarily good mobility of the pore solutions. In the older Lymnaenmergel formation (Upper Eocene) a more advanced stage of smectite alteration is found as compared to the other formations at corresponding temperatures. This is considered to be the consequence of the longer duration of diagenesis and the rock salt inclusions of this saline formation which might have enriched the pore solutions with potassium ions.

The relationship of porosity and angle of repose to mixture proportions in assemblages of different sized materials

Abstract: Rotating drum experiments on the repose angles of mixtures of glass spheres have shown that φr (angle of shear) is strongly influenced by the proportions of the mixture. It was found that φr reached a peak value for the minimum porosity mixture; where the pore spaces between the large particles were just filled with small material; which was attributed to increased dilatation on the shear plane during avalanching. The geomorphic significance of these observations, in terms of slope development, is discussed. Secondly, the results of the experiments showed that, although more constant than φi (limiting angle of repose), φr was subject to some variation. Thus φr, as measured in a rotating drum, is not a true constant and can not be exactly analogous to φ'cv (angle of internal sliding friction at constant volume) as measured in a shearbox test—as has been previously suggested. It is tentatively suggested that at least some of the variability in φr is attributable to the magnitude of the immediately preceding value of φi, in that an unusually high value of φi, favours a lower value of φr due to the greater amount of kinetic energy released on failure.

Early marine lithification of the nodular limestones in the Silurian of New Brunswick

Abstract: A sequence of irregular nodular limestones in the Silurian of northern New Brunswick appear to have undergone early marine lithification. Nodules and crusts formed soon after deposition in the argillaceous lime sediments by sporadic diagenetic precipitation of calcium carbonate and were subsequently modified by differential compaction and minor flowage. While lack of both borings and encrusting organisms on the nodules indicates they were not ‘hard grounds’, computed porosities at the time of lithification based on the ratio of carbonate to insoluble residues suggest lithification depths within a few tens of feet of the sediment-water interface. Consideration of the availability of calcium carbonate suggests lithification very close to the sediment surface.

Non‐combustible suspended matter in surface waters off eastern Asia*

Abstract: A total of about 1100 well-distributed samples of suspended matter in surface waters off the length of eastern Asia are available. From these samples, 180 were selected for detailed examination of the non-combustible fraction using optical and electron microscopy along with computer methods of particle measurement and counting. The results showed that, generally, all major components of the suspended matter are most abundant in the nearshore belt (combustible fraction, mineral grains of silt size, skeletal debris, and clay minerals), the result of mechanical transport of detrital sediment and chemical transport of nutrients from the land. Mineral grains of silt size average about 2%, skeletal debris plus clay minerals—23%, and combustible organic matter—75% of total sample weights, but the last two categories vary over a wide range depending upon geographical positions of the samples. Most evident is an oceanward decrease in percentage and concentration of the total noncombustible fraction and an oceanward increase in median diameter of the mineral grains.

The isotopic composition of Tertiary carbonates from the Mainz Basin: an example of isotopic fractionations in ‘closed basins’

Abstract: The carbon and oxygen isotopic composition of seventy-nine samples of biogenic carbonates from the Mainz Basin Tertiary (Oligocene and Lower Miocene) was analysed. Most samples were mollusc shells still consisting of aragonite. Assuming only small temperature effects, salinity trends derived from isotope data are consistent with palaeontological results from the region: a salinity cycle ranging from fresh water-brackish (Lower Oligocene) towards marine (Middle Oligocene) and brackish-fresh water (Upper Oligocene) was found. Within the Lower Miocene, a trend of decreasing salinities is suggested. Though the isotopic salinity trends coincide rather well with palaeontological salinities, the absolute oxygen isotope ratios indicate an unusual isotopic environment enriched in 18O. Isotope fractionation is explained by evaporation of a closed basin (Rupelton excluded) with fresh water influx from surrounding land areas in a subtropical climate. Enrichment in 18O by repeated evaporation processes is paralleled by increasing concentration of Sr. Increasing fresh water influx during the Oligocene is due to climatic changes with a trend of more humid conditions towards the younger rock strata.

Les minerais de fer de l'Ordovicien inferieur du bassin de Bretagne-Anjou, France*

Abstract: RESUME Les ressources en mineral de fer du bassin de Bretagne-Anjou proviennent essentiellement de quatre couches de mineral sedimentaire situees dans le gres armoricain (Arenig). Les parageneses les plus frequentes reunissent le quartz, la magnetite, la siderite et des silicates (bavalite, stipnomelane). La paragenese a hematite et la paragenese a pyrite sont plus rares. L'etude petrographique permet en outre de definir deux types de minerais: un type a allochems et un type sans allochems. L'examen des variations de parageneses et de puissance des couches de minerai permet de reconstituer en partie l'histoire de la sedimentation ferrifere: le fer est mis en solution sur les reliefs cadomiens emerges. Lors de la transgression paleozoique le fer parvient dans le bassin de sedimentation qui est peu profond et encombre d'iiles et de hauts-fonds. Suivant les conditions locales le fer precipite alors soit sous ses formes oxydees (magnetite, hematite), soit sous ses formes reduites (bavalite, siderite, pyrite). Les facteurs essentiels de la differenciation des parageneses sont l'epaisseur de la tranche d'eau, l'agitation de l'eau et la teneur du sediment en matiere organique. Dans certains cas un phenomene de concretionnenment au sein du sediment donne naissance aux allochems qui peuvent alors etre repris, mis en suspension et deposes. ABSTRACT The iron resources of the ‘Bretagne-Anjou’ basin proceed essentially from four layers of sedimentary ore in the lower gres armoricain (Arenig). The more frequent paragenesis combine quartz, magnetite, siderite and silicates (bavalite, stilpnomelane). The hematite and pyrite parageneses are rarer. Two ore types were distinguished petrographically: one type with allochems and one without allochems. The study of both paragenesis and thickness variations of the ore layers enables one to reconstitute the sedimentation history: iron is dissolved on the emerged cadomian reliefs and transported to the sedimentary basin, which was shallow and strewn with islands and shoals. Then, according to local conditions, iron precipitated either under its oxydized forms (hematite, magnetite) or under its reduced forms (silicates, pyrite, siderite). The essential factors of parageneses differentiation were the depth of the basin, the water agitation and the organic matter content of the sediment. In some cases little concretions were formed in the sediment and gave rise to allochems which can be moved away later and deposited elsewhere.

Size and shape sorting in a Dutch tidal inlet

Abstract: A tidal inlet system with an outer tidal, delta, situated between two barrier islands along the north coast of Holland was studied for size and shape sorting. With size data different sand types can be distinguished and in individual samples distinct grain populations can be recognized in some cases. Graphs of shape values, plotted against the size intervals of samples also reveal the presence of different grain populations, together with their genetical significance. The following conclusions could be drawn. There is no sand transport directly from island to island. Sand up to 400 μm enters the tidal inlet, is sorted out in the tidal flat area and partly re-enters the sea via the outer tidal delta. On the delta, the sediment is split up again in different populations. A lag deposit is left behind on the frontal part of the delta. The rest of the sand either re-enters the tidal inlet cycle or contributes to the beach building of the next island. In the offshore environment, sand movement by wave-induced currents is restricted to the shallow zone. In deeper water, part of the sediment is relatively immobile and has preserved inherited characteristics from the early Holocene transgressive phase. In front of Ameland, fossil barrier-face deposits-are present, off Schiermonnikoog the sea floor contains old tidal channel deposits.

Effect of man's activities on distribution of trace elements in sub‐bottom sediments of Lake George, New York*

Abstract: A core from the southern mesotrophic basin of Lake George, New York, shows enrichment in the concentration of trace elements near its top. By contrast, a core from the northern oligotrophic basin shows, with the exception of manganese, an almost constant trace-element concentration throughout the core. Man's activities in the watershed surrounding the southern basin are responsible for enrichment in the trace-element concentration of newly deposited bottom sediments. Increase in trace-element concentration is not paralleled by increase in organic-carbon concentration.

Determination par activation neutronique des rapports Cl/Br des inclusions fluides de divers gypses. Correlation avec les donnees de la microcryoscopie et interpretations genetiques

Abstract: RESUME Le gypse, comme la plupart des mineraux, contient en inclusions des liquides contemporains de la cristallisation dont l'analyse permet de connaitre la composition du milieu de genese—le role des circulations secondaires devant etre discute. Nous avons choisi de determiner les rapports Cl/Br des inclusions car ce rapport est constant dans l'eau marine actuelle et bien connu dans de nombreux types d'eaux naturelles qu'il caracterise. L'activation neutronique permet de doser les tres faibles quantites de chlore et de brome qui se trouvent dans les cavites mesurant en moyenne 10−6mm3. Cette technique n'a encore ete que tres rarement utilisee dans l'etude des inclusions et la methode mise au point ici ecarte tout risque de pollution et donne directement le rapport Cl/Br (a 10% pres) apres une seule irradiation par echantillon. Nous rapportons les resultats de 58 manipulations faites sur une douzaine de gisements de types differents; leur etude microcryoscopique avait ete effectuee au prealable. La connaissance des salinites (en equivalents NaCl) et des rapports Cl/Br des inclusions liquides nous permet de proposer des hypotheses genetiques pour les gisements anciens d'origine discutee, apres comparaison avec les gisements actuels ou recents. Ont eteetudies des cristaux de gypse primaires de marais salants de France, de mangrove de Nouvelle-Caledonie, de la Sebkha el Melah en Tunisie, ainsi que des cristaux eocenes du bassin de Paris, oligocenes du Sud de la France et miocenes de Sicile. ABSTRACT Gypsum contains liquid inclusions which fill mainly primary cavities. Analysis of inclusions may give some information about the mineral formation process. In places, diagenetic solutions have filled secondary cavities along cleavage planes or replaced initial liquid inclusions in primary cavities; the presence of such secondary inclusions must, therefore, be taken into consideration. This study presents our results on the Cl/Br ratio of liquid inclusions in gypsum. The Cl/Br ratio is practically constant in present sea water, and has been measured in many different types of waters. The Br content of halite has also been extensively studied to trace the compositional changes of a salt-depositing brine. In gypsum Br is present only in microscopic fluid inclusions. For this reason we used the neutron-activation method. We adopted the technique in order to prevent any contamination and obtain directly Cl/Br ratio within 10% accuracy. In order to ascertain that liquid inclusions reside mainly in primary cavities of gypsum, we selected our samples only after microscopic observation. Fifty-eight samples from different types of gypsum were analysed. The salinities of the inclusions, obtained through the data on freezing, and the Cl/Br ratio, obtained through activation-analysis, give us clues as to the nature of the fluids depositing the gypsum. Recent gypsum deposits from French salt-pans of Mediterranean and Atlantic coasts were sampled. The latter has apparently been diagenetically altered by meteoric water; the salinity of many inclusions having been changed while the Cl/Br ratio (315 and 350) in both remains close to the value of present sea water (#300). Liquid inclusions of Recent crystals from New Caledonia, deposited in mangrove swamps are enriched in Br (Cl/Br = 150 and 200). The enrichment might be related to the presence of abundant organic materials. Inclusions in crystals from Sebkha el Melah (Tunisia) also show high Br content (Cl/Br = 194). In this case, the brine was highly concentrated and was saturated with NaCl. The Upper Miocene gypsum from Sicily was studied to test the various models for the genesis of this important evaporite formation of the Mediterranean. Freezing data show some decrease of salinities, probably by groundwater diagenesis, but the Br content is high. The Cl/Br ratio is 175, and this value is similar to that of Sebkha el Melah. Samples from three thick Eocene and Oligocene sequences near Paris and in the South of France were studied. Freezing data and Cl/Br ratio of their inclusions indicate that those gypsum deposits have crystallized in marine environments receiving considerable influx of river-waters.

Packing and resistance to compaction of shells

Abstract: The separated valves of bivalve molluscs, certain gastropods, and many brachiopods closely resemble the regular geometrical form known as the shell. The packing concentration of natural accumulations of such organic shells may be placed within bounds with the aid of packing models making use of ordered arrangements of equal geometrical shells, either conical, cylindrical or spherical. The low packing concentrations, comparable with 0·1–0·2, indicated by these models are confirmed by experiments using the shells of four common British species of bivalve or gastropod mollusc. Packings of these shells have a substantial intrinsic strength, and experimentally appear able to support without failing sedimentary overburdens equivalent to loads comparable with 1000 kg mass/m2. The observed and predicted low packing concentrations suggests that natural shell beds can hold relatively very large amounts of pore fluids or mineral cements. The resistance to compaction of the packings means that the high original porosities of natural shell beds have a good chance of being permanently preserved.