Showing papers in "Sedimentology in 1973"

Antidune and chute and pool structures in the base surge deposits of the Laacher See area, Germany

Abstract: Grey tuffs of late Pleistocene age form broad fans radiating from the Laacher See basin. They were derived from phreatomagmatic outbursts, and transported in turbulent pyroclastic flows, in contrast with the underlying white pumice tuffs of air fall origin. Flow origin of the grey tuffs is inferred from the well-bedded plane parallel to cross-bedded tephra characteristic of base surge deposits, and a variety of other sedimentary structures, as well as grain size distributions. We recognize a tentative sequence of five main kinds of dune structures or cross-bedded strata. With some reservations these may be compared with the high flow-regime alluvial bedforms produced experimentally in flumes. Most of the cross-bedded structures in the Laacher See deposits resemble antidunes, with steep stoss sides and very low-dipping lee sides. Upcurrent migration of antidune crests is dominant close to the source, but changes to downcurrent migration at greater distances, presumably because of decay in flow energy. The most spectacular cross-bedding is somewhat similar to chute and pool structures formed under experimental condition in alluvial flumes, but not recognized in ancient sedimentary rocks. We suggest that these structures of the Laacher See tuffs formed during deposition from phreatic pyroclastic flows of very high flow energy and high sediment concentration. The antidunes apparently formed at lesser flow velocity than chute and pool structures, although interpretation of velocity conditions by examination of the deposits is difficult because of other factors such as the cohesiveness of wet material erupted by explosive phreatic volcanic activity. The large wave lengths of the dune-like structures, however, suggest unusually high velocities. The Laacher See magmas were of phonolitic to tephritic composition, and may have erupted with greater explosive energy and in greater volume than comparable basaltic eruptions.

Growth and Submarine Fossilization of Algal Cup Reefs, Bermuda

Abstract: Small charges of explosives were used to section cup-shaped reefs that occur on the margins of the Bermuda Platform. Study of these artificial outcrops, up to 10 m high, and the samples collected from them show how the reef-building community is rapidly converted to well-lithified reef rock in the marine development. The reefs, known locally as boilers and breakers, occur along the wave-swept south shore of the Bermuda Islands and around the northeast and northwest margins of the Platform. They are cup-shaped, up to 30 m in long dimensions, and rise up to the sea surface as much as 12 m above the surrounding sea floor. The reefs are built by an intergrowth of encrusting organisms, principally crustose coralline algae, an encrusting hydrozoan, Millepora sp., and an attached gastropod, Dendropoma irregulare. The growth framework of these algal cup reefs has extensive voids: large and intermediate-sized growth framework and shelter pores; borings of bivalves and sponges; and both intra- and inter-particle pores. A variety of vagile and sessile organisms (coelobites) inhabit these pores: an encrusting Foraminifera, Homotrema rubrum, is the most abundant attached coelobite; the tests of a variety of benthic Foraminifera and ostracods are common: branched coralline algae, barnacles, bivalves, ahermatypic corals, bryozoans, and burrowing crustaceans occur in varying abundance. Beginning millimetres below the living surface, internal sediments accumulate in the extensive voids. Coarse-grained skeletal sand derived from the surface of the reefs is characteristic of the larger voids; lime mud with the tests of planktonic Foraminifera and planktonic algae occurs generally in the smaller voids. Most specimens from the interior of the reefs show multiple generations of internal sediment that vary in grain size, composition, and colour. The sand-sized sediments are pumped into the voids by the frequent and intense wave action; the lime mud settles out in the smaller, less agitated pores. Cementation of internal sediments and surrounding growth frame begins centimetres below the living surface; it is so pervasive that marble-hard reef rock is developed within 1/2 m or less. The cement is principally high-magnesium calcite of micrite size, and subordinately acicular aragonite, but there are locally wide variations in crystal size and morphology. The occurrence of the cement within the reefs well below sea level, the isotope ratios of the cement crystals, the mineralogy, and the age inferred from radiocarbon age determinations of the growth frame all indicate that the cement is submarine and deposited from water of oceanic composition. The algal cup reefs of Bermuda demonstrate the reef-building ability of a community of encrusting organisms that form only crusts in the intertidal zone of the Mediterranean and Northern Brazil. The cup reefs of the northern margins of the Bermuda Platform are true reefs, not merely veneers covering eroded blocks of Pleistocene limestone. In their composition, location, and early diagenesis, the cup reefs closely resemble the algal or lithothamnion ridge of Pacific atolls. Synsedimentary cementation of internal sediments and growth frame makes a major contribution to the rigidity of these ocean-facing reefs and atoll rims. The assemblage of features that characterize the submarine fossilization of the cup reefs is widespread elsewhere in the modern seas: the floors of the Persian Gulf and parts of the Mediterranean; the margins of Pacific atolls; and the reefs off the north coast of Jamaica. This fossilization is characterized by reiterated generations of coelobites, internal sediments, and synsedimentary cements that can in time replace a major part of the original growth framework. Major variations in the sequence of these generations from pore to pore is the signature of this kind of fossilization. The same features of fossilization are described from reefs in the Devonian, Permian, and Triassic.

Distribution and nature of incorporation of trace elements in modern aragonitic corals

Abstract: The objective of this study is to locate as closely as possible the sites of strontium, magnesium, sodium, and potassium in modern aragonitic corals, specifically whether these cations are adsorbed, or are substituted in the carbonate lattice or are incorporated in organic components. In addition to locating the sites of each of these four elements we wanted to find out quantitatively how much of each element occurs at each site. The experiments in this study are based on the dissolution rate of aragonite in distilled water and on the substitution of strontium and magnesium by calcium and sodium. Special attention has been given to the occurrence of strontium, magnesium, sodium, and potassium in the organic components of the corals. The main site for strontium in the corals is in the aragonite lattice. Twenty-five per cent of the total magnesium occurs in adsorbed sites and in organic compounds. The rest of the magnesium may be located in the aragonite lattice, but it is easily removed by repeated leaching or by replacement with calcium ions. Another possibility is that magnesium may occur in a dispersed mineral phase more soluble than aragonite because magnesium was released at a higher Mg to Ca ratio than is found in the solid coral; also because no local concentration of magnesium could be detected with an electron microprobe. About 12% of the total sodium is in adsorbed sites and is included in the organic compounds. The rest of the sodium might be in the lattice replacing calcium, but the low total exchange capacity is not enough to provide the needed charge balance. Another possibility is that sodium is located in a proposed mineral phase. Potassium is in adsorbed sites and incorporated in the organic compounds to an extent greater than all the other elements studied (30% of the total potassium), but again the evidence suggests that the remaining potassium is in a proposed mineral phase. Calcite is detected on the surfaces of aragonite corals after 5 months in the substitution experiment. The change of argonite to calcite took place after the inhibitor magnesium was exchanged from the surface sites and replaced by calcium. The organic compounds in corals contain small amounts of strontium, magnesium, sodium and potassium. Strontium is preferentially enriched in the organic compounds over magnesium.

Markov chain analysis applied to an ancient alluvial plain succession

Abstract: Markov chain analysis is a comparatively simple statistical technique for the detection of repetitive processes in space or time. Coal measure cyclothems or fluvial fining-upward cycles are good examples of sedimentary successions laid down under the control of Markovian processes. Analyses of stratigraphic sections commence with a transition count matrix, a two-dimensional array in which all possible vertical lithologic transitions are tabulated. Various probability matrices may be derived from this raw data, and these are then subjected to chi-square tests to determine the presence or absence of the Markov property. This technique is applied to four types of stratigraphic succession which occur in the Devonian rocks of Prince of Wales Island, Arctic Canada. (1) A conglomerate succession of alluvial fan origin. Markov analysis is of little or no assistance in the interpretation of these rocks, in which only two principal lithologies are present. (2) A conglomerate-sandstone succession. Fluvial fining-upward cycles are detectable by visual examination of the sections and are strongly indicated by Markov analysis. (3) A sandstone-carbonate succession, of marginal marine origin, and including both marine and non-marine strata. Cyclicity is weak in these rocks, but analysis suggests that regressions took place much more rapidly than transgressions during their period of deposition. (4) A succession in which the relative proportions of the various lithologies vary markedly with age. The varying nature of the cyclic tendencies is emphasized in this case by dividing the succession into two subintervals, for the purpose of analysis.

Holocene meteoric dolomitization of Pleistocene limestones, North Jamaica

Abstract: Wholesale removal of the unstable carbonate phases aragonite and Mg-calcite, and precipitation of calcite and dolomite is currently taking place where phreatic waters (the modern water table) invade 120,000-year-old Pleistocene biolithites (Falmouth Formation), North Jamaica. Pleistocene rocks presently in the vadose zone are relatively unaltered, and consist of mineralogically unstable scleractinian biolithites. At the water table, a narrow zone of solution, a ‘water table cave’ is commonly encountered. Below the water table the rocks are invariably more highly altered than those above. Mg-calcites are very rare, and considerable dissolution of aragonite has commonly occurred. Dolomite occurs as 8–25 μm, subhedral to euhedral crystals replacing micrite, or precipitated as void linings. The isotopic composition of the dolomite (δO18=-1·0 %0, δC13=-8·4 %0), and its high strontium content (3000 p.p.m.) suggest precipitation as CO2-oversaturated meteoric groundwaters invade the mineralogically unstable biolithites, dissolve Mg-calcites and Sr-rich aragonites, and de-gas. Because some dolomitized rocks are enriched in magnesium relative to unaltered biolithites, addition of magnesium to the system is necessitated, and is probably derived from sea water in the mixing zone. Phreatic meteoric diagenesis is thus demonstrated to be a rapid process, and to be capable of dolomitization.

Radiaxial fibrous calcite: a replacement after acicular carbonate

Abstract: Radiaxial fibrous calcite, a common cavity fill in ancient limestones, is characterized by curved twin lamellae, optic axes that converge away from the substrate and subcrystals which diverge in this same direction. The optic axes radiate about three or four axes located in positions between adjacent crystals and orientated parallel with the crystal elongation. The crystals are commonly turbid with inclusions, which may be concentrated along twin lamellae and subcrystal and inter-crystalline boundaries, or form zones parallel to the substrate. Some inclusion patterns reveal the position of former crystal faces. From a consideration of the occurrences, gross morphological characters, the fabrics of the crystals and the inclusion patterns, radiaxial fibrous calcite is interpreted as a replacement of an early diagenetic acicular cement, composed of interfering bundles of radiating crystals. It is suggested that replacement takes place by a solution-precipitation process and the migration of a fluid film through the acicular host, with replacement occurring most rapidly between bundles of acicular carbonate. The optic axis pattern of the fibrous calcite is considered to be inherited from the c-axis orientations of the host acicular crystals. Fibrous calcite intercrystalline boundaries form as fractures after replacement. Some inclusion patterns record the characters of the replaced acicular cement; others, formed of impurities reorganized during the replacement, reveal the form of the replacement front.

Pebble segregation and bed lenticularity in wave‐worked versus alluvial gravel*

Abstract: Two aspects of bedding may permit visual discrimination between wave-worked and alluvial gravel. Pebbles in gravel worked by waves tend to be better segregated into discrete beds than those in alluvial deposits, and bedding in wave-worked gravel tends to be more laterally regular, or less lenticular, than that in stream gravel. Pebble segregation and lenticularity were quantitatively analysed in twenty-five different marine and fluvial deposits of Quaternary or Tertiary age in southwestern Oregon in order to evaluate their use as environmental indicators. The results indicate that these two factors combined provide a visual criterion for discriminating between many gravels of wave-worked and alluvial origin.

Precipitation and lithification of magnesian calcite in the deep‐sea sediments of the eastern Mediterranean Sea*

Abstract: Magnesian calcite is an important sedimentary component in the deep-sea sediments of the eastern Mediterranean Sea, comprising an average of 20–50% of the carbonate fraction in most areas. The lack of any obvious biogenic source, plus similarities with magnesian-rich lutites from the Red Sea and deep-sea cements from other areas suggest that this magnesian calcite was precipitated inorganically. Although the exact mode of precipitation is not understood at present, it probably occurred at the water-sediment interface under elevated salinity and temperature conditions, such as those present in the modern eastern Mediterranean. Precipitation did not occur during periods of lower temperatures and/or salinities such as during the stagnant conditions caused by the influx of fresh waters from melting Pleistocene glaciers. The eastern Mediterranean magnesium-rich sediments appear to represent an intermediate stage between normal deep-sea sediments and those from the warm hpyersaline Red Sea. Normal deep-sea carbonates are composed almost entirely of biogenic calcite, whereas the Red Sea magnesian calcite alternates with layers of aragonite that were precipitated under elevated salinity and temperature conditions brought about by lowered stands of sea level.

Features of cross-stratified units due to random and other changes in bed forms

Abstract: Because cross-stratified units depend upon the movement of bed forms, any change in the shape, size and direction of travel of the forms is reflected in the geometry of the units, notably in their relative length, breadth and thickness, mode of termination upstream and downstream, and internal discontinuities. Most models of cross-stratification so far published are unsatisfactory because they ignore the fact that real bed forms are subject to change. The changes are thought to occur at two levels of detail independently. Those at the coarser level depend on the essential non-uniformity, unsteadiness and multi-directionality of natural flows, when assessed on a suitably large scale. At the finer level, change is related to the random behaviour of individual bed forms as they interact with the adjacent flow, and it proceeds even when the flow is an equilibrium one overall. Flume experiments on current ripples show that many features of cross-stratified units can be explained by the random behaviour of bed forms. The finite streamwise length of such units, and their upstream and downstream erosional termination, is governed by the life-span (finite) of individual ripples and by the extent of net deposition on the bed. Internal discontinuities, closely resembling features described as reactivation structures, were also found to depend on the relative motion of ripples, no change of flow discharge and stage being involved. The degree of relative motion in the ripple assemblages was substantial, as measured by the fluctuating component of the ripple celerity.

Phase differences between bed configuration and flow in natural environments, and their geological relevance

Abstract: In nature where unsteady flows prevail, the characters of bed configurations lag, or differ in phase from, the flow conditions This is demonstrated from quantitative data for dune beds in the Fraser River (Canada) and the Gironde Estuary (France), where the flow conditions change ahead of the bed form by as much as 1/4 period Qualitative evidence for lag abounds, for instance, substantially unmodified dunes exposed during low water The existence of lag appears to confuse attempts to establish from field data the relationships between bed-form properties and flow conditions and, at the same time, makes difficult the confident application to field situations of formulae based on steady-state experiments or theories Unfamiliar types of field study and experiments are needed to increase understanding of the degree and precise significance of lag

From limestones to distal turbidites: a facies and trace fossil analysis in the Zumaya flysch (Paleocene—Eocene), North Spain

Abstract: The flysch of Paleocene to Eocene age outcropping in an almost unbroken cliff section at the Playa de San Telmo, Zumaya, North Spain, has been the subject of a quantitative sedimentological analysis. It is inferred that sedimentation commenced with deposition of a limestone-red shale sequence below wave base and continued with proximal and then distal turbidite sandstones deposited in what may have been a gradually deepening trough. The trough was probably oriented approximately east-west, parallel to the subsequent main tectonic trend. Sediment transport within the trough appears to have been essentially axial, with calcareous and siliceous sand derived from the east, but some siliceous sand was also laterally transported mainly from a land mass to the north. The sediments contain an abundant and varied suite of trace fossils. Thus, with the depositional environment already defined sedimentologically, it was possible to critically examine the relationship between facies, trace fossil distribution and possible water depth. The facies variations were shown to be reflected in a changing ichnofauna. Spreite such as Zoophycos and Rhizocorallium were present in limestones apparently deposited not far below wave base. These traces are replaced by rosetted, winding and meandering forms in the more proximal turbidite facies. In the more distal facies spiral and patterned trace fossils appear, winding and meandering forms are common but Zoophycos, Rhizocorallium and rosetted traces are absent. It is inferred that these changes reflect faunal distribution on the sea floor rather than preservational factors.

Experimentally produced halite compared with Triassic layered halite‐rock from Cheshire, England

Abstract: The experimental evaporation of brine in shallow vessels shows that layered halite can form by the overgrowth of crystals in mats that have foundered from their initial position of growth at the brine-air interface; by the upward and lateral growth of crystals that have nucleated on the floor; and by the overgrowth on detrital halite. Each of these processes, and its resultant crystal fabric, is described in relation to the environmental factors which influence it. The fabrics of primary halite crystals in layered halite-rock and associated deposits from the Lower Keuper Saliferous Beds of the Cheshire Basin are described and compared with those of the experimentally produced halite. Some of the ancient halite-rock may be matched with floor-nucleated crystals, and some with detrital halite; none can be shown to have grown from foundered mats. The halite-rock is regarded as having formed in shallow brine bodies, perhaps only a few centimetres deep, but of unknown extent, whilst the presence of planar solution or deflation surfaces at the top of the layered units, and disruptions in bedding similar to buckled salt-crust structures, point to periodic and perhaps prolonged emergence.

Settling behaviour related to sieve analysis of skeletal sands

Abstract: One thousand grains selected from sieved samples of a bioclastic sand have been individually measured, weighed and timed in free unhindered fall at terminal velocity in a 250 cm column of sea water. Four fall regimes are represented: straight fall, spinning and spiral modes and erratic tumbling. As size increases grains pass through this series at rates dependent upon shape and effective density. Computed best-fit curves for velocities/intermediate diameter and an equivalent sphere/intermediate diameter illustrate considerable divergencies in behaviour between the eleven grain types and five shape classes examined. Current methods of analysis of carbonate sediments, by sieving, by grain-counting of components, and by sedimentation balance, provide different kinds of information which it is impractical to consider as having any simple relationship to each other.

An appraisal of the computation of statistical parameters in grain size analysis

Abstract: A computer program has been written to overcome the limitations of two published programs that compute statistical parameters in grain size analysis. This program uses all available data, obtained from standard sedimentological analyses, regardless of phi interval, and computes statistical parameters by direct integration of a linear approximation to the cumulative curve. This has the advantage that the distribution curve is known and hence the limitations of the parameters obtained can be assessed. Furthermore, it is argued, with examples, that this approximation introduces no greater degree of inaccuracy than is already present in standard sampling and analytical procedures.

Depositional theme of a marginal marine evaporite

Abstract: We have reconstructed the depositional environment of the gypsum-carbonate-shale sequence that comprises the Upper Permian Bellerophon Formation of the southeastern Alps in northern Italy. This formation, which reaches a maximum thickness of 600 m, is roughly divided into two facies: (a) a lower dolomite-gypsum facies, and (2) an upper micritic-skeletal limestone facies. It directly overlies, with transitional contact, a thick red-bed sequence (alluvial fanglomerates, fluviatile sandstones and flood-plain siltstones) and is sharply overlain by Lower Triassic calcarenites (oolites, grapestones, pellets, flat-pebble conglomerates). The lower evaporite facies rocks are found in well-defined cycles, each of which, from bottom to top, consists of (A) thin-bedded, worm-burrowed, vuggy ‘earthy’ micritic dolomite, (B) massive to poorly laminated dark grey to black sandy dolomite carrying isolated gypsum nodules, (C) layered (thin-bedded) nodular gypsum (commonly with ‘enterolithic’ folds) with fragmented partings of dolomite, and (D) massive ‘chicken-wire’ nodular gypsum. At Passo di Valles, just east of Predazzo, and 50 km from the basin margin, we measured forty-six consecutive complete cycles, with an average thickness of 3 m per cycle. We interpret the cyclic sequence as having been deposited in a prograding shallow lagoon—sabkha complex. The worm-burrowed ‘earthy’ dolomite mud accumulated in a shallow hypersaline subtidal lagoon. The black sandy dolomite was an ‘intertidal’ sand-flat devoid of algal mats and constantly churned by burrowers (likely crustaceans). As the shoreline prograded lagoonward evaporative concentration of the groundwater induced diagenetic growth of anhydrite nodules (now gypsum) within the porous sandy dolomite. The layered nodular and ‘chicken-wire’ gypsum of the cycle cap is an extreme product of such displacive intra-sediment growth of anhydrite (now gypsum) above the water table of a completely exposed sabkha, such as is found in the Persian Gulf today. We have observed the same cyclically arranged lithologies in two other evaporite sequences in Italy: the Triassic Raibl Formation of the Southern Alps and the Upper Triassic Burano Formation of the central Apennines. We suggest that this mode of deposition is likely a very common one for at least the early stages of marine evaporite accumulation.

Fragmentation of granitic quartz in water

Abstract: Studies of grain fragmentation in natural streams have the limitation that the full size range of the debris produced is virtually unobtainable. Experiments described here for grain fragmentation in a rotating drum permitted the study of all of the debris, and a fragmentation load technique was used to relate experimentally and naturally fragmented material. The present investigation has been focused on granitic quartz. Relatively gentle collective movement in water can cause significant fragmentation of coarse, nascent, granitic quartz grains. The debris produced by rotating in a drum a range of single sieve fractions, taken from gravel in the headwaters of a stream draining granite, had continuous size distributions down to (and probably beyond) 0·06 μm. Quartz was the dominant fragmentation product in all fractions down to 2 μm and present in finer fractions. When pebbles moved with sand in these experiments, breakage of the latter was greatly increased. In comparison with that of breakage, the effect of attrition on granitic quartz was negligible. At least a proportion of granitic quartz grains are subject to a fatigue effect as a result of impacts in water. Evidently they are thus progressively weakened prior to being broken. Size analysis of debris showed a significant break at 20 μ, suggesting some special production of quartz particles just below this size. Granitic quartz is criss-crossed with partially healed cracks acquired before the zone of weathering is reached. The wholesale breakage that affects it, particularly in pebbly streams, is largely due to the reopening of these cracks. Progressive fragmentation of this material must eventually reach a stage wherein grains comprising single original crack-bounded volume elements are produced. Such grains, lacking significant internal weaknesses, must strongly resist further breakage. Possibly the preferential production of quartz grains just below 20 μm in size may represent an accumulation of these single, crack-bounded volume elements.

Miocene point-bar deposits in the Ebro Basin, Northern Spain*

Abstract: Pyrenean debris dispersed in relation to the Miocene tectonic phase filled the northern part of the Ebro basin. In the studied area (50 km south of Pamplona) the Miocene sediments are represented by fluviatile sequences consisting of flood basin and point-bar deposits forming fining-upward cycles. The point-bar character of part of these sediments is not only evidenced by the internal sedimentary structures, but also by the fact that the corresponding meander bends, about 200 m in radius, are clearly visible on aerial photographs. In this way a unique opportunity is given to compare directly the vertical profile and the surface pattern of ancient small size point bars. The internal structure consists of many imbricated and concentrically arranged bundles corresponding to the accretional topography of the point bar. The separation planes between the bundles are erosional and dip toward the channel axis. The internal structure of each bundle is festoon mega-cross-stratification whose direction points toward the convex bank, away from the channel axis. The resulting structure resembles the Epsilon cross-stratification of Allen (1965a) and its origin is the same as postulated by Allen for this kind of cross bedding.

Incorporation of uranium in modern corals

Abstract: Uranium occurs in corals at three sites: 1, in organic matter; 2, adsorbed on the surfaces of skeletal aragonite; and 3, in the aragonite lattice. Organic matter incorporates from sea water by chelation 40–70 ppm uranium; skeletal aragonite incorporates only 3 ppm. However, as the organic fraction is low (0·1%), its high concentration of uranium does not significantly affect the total concentration of uranium in the coral. A negligible concentration of uranium, 40–60 ppb, is adsorbed on skeletal aragonite from which it is readily leached or exchanged. This low concentration of adsorbed uranium (<2% of the total uranium in skeletal aragonite) is related to the very small specific surface area (1·5–1·8 m2/g of the corals.

Tide‐dominated offshore sedimentation, Lower Cambrian, north Norway

Abstract: The Duolbasgaissa Formation, Lower Cambrian, of northern Norway consists of 550 m of mineralogically and texturally mature sandstones with subordinate siltstones, mudstones and conglomerates. Four facies are defined on the basis of grain size, bed thickness and sedimentary structures. Facies 1–3 consist of a variety of erosively-based, cross-stratified and parallel-stratified sandstones interbedded with siltstone and mudstone. Many of these sandstones show evidence of deposition from waning currents. Facies 4 consists of trough cross-bedded sandstones with sets up to 4 m thick. Symmetrical ripples and bioturbation are ubiquitous. Bipolar palaeocurrent distributions are common to all facies and one mode is usually strongly dominant. Lateral facies variations and sedimentary structures suggest that deposition took place in a tide-dominated, offshore, shallow marine environment in which maximum sediment transport probably occurred when storm generated waves enhanced tidal currents. The four facies are thought to represent the deposits of various parts of tidal sediment transport paths such as exist in modern seas around Great Britain. Small scale coarsening upward sequences may represent the superposition of facies independently of changing water depth. Lack of information prevents a detailed palaeogeographic reconstruction. It is suggested that sand body shape is not accurately predictable.

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

Abstract: RESUME Sur le plateau continental du Congo et du Gabon, on constate la formation de goethite, de berthierine et de glauconies (smectite et glauconite). La berthierine caracterise un milieu deltaique a sedimentation active; les glauconies se forment sur le plateau continental a plus grande profondeur (—50a—300m). Dans un milieu original granulaire (generalement des coprolites) tous les stades d'une succession mineralogique depuis la kaolinite jusqu'a un mineral voisin de la glauconite ont ete deceles. Cette formation passe necessairement par le stade smectite ferrifere. La formation prealable de granules cree un milieu semi-confine remarquable qui est, ici, la condition primordiale des evolutions minerales. ABSTRACT 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 sediment 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).

An Upper Carboniferous tonstein of volcanic origin

Abstract: Previous work on the mineralogy of the volcanic supra-Wyrley Yard tonstein is extended to include the opaque minerals. This has led to a better understanding of the major and trace element geochemistry of the tonstein. The complete mineral assemblage is kaolinite (dominant) with minor quartz, K-mica, zircon, apatite, pyrite, sphalerite, galena, rutile, dolomite, barytes, goethite and organic matter. Information on element location in the tonstein has been obtained by using autoradiography and fission track radiography, the electron probe and by analysing separated fractions with an optical spectrograph. This has enabled the diagenetic history of individual elements to be determined, and the composition of the tonstein to be compared with igneous material. The high concentration of Th, U, Pb, Sn, Bi, Y, Be and B in the tonstein, and the low concentration of Ti, Cr, V, Ni, Cu and Co is indicative of an acidic ash. A source in the Armorican orogenic belt is postulated, based on the contrast with the basaltic Carboniferous volcanism and also the high concentrations of Sn, Pb and Zn.

Diagenetically formed ooids and intraclasts in South African calcretes

Abstract: Certain South African calcrete (caliche) beds contain structures that very closely resemble normal marine ooids and intraclasts. The ooids/intraclasts are actually non-marine in origin, and were formed by an in situ process of calcretization and selective neomorphism. The term ‘diagenetic ooids/intraclasts’ is thus applied to distinguish these grains from marine ooids/intraclasts. Quaternary calcretes contain the only known ooids/intraclasts in any South African Cretaceous or Cainozoic rocks. The diagenetic grains are believed to have formed in the following manner. Rain water percolates through an unlithified calcareous sand deposit, dissolving carbonate along its downward path. This carbonate-laden water is eventually checked in its descent and drawn upward somewhat by capillary action. Evaporation and soil suction in the uppermost zones of the sediment body cause precipitation of a concentric coating of carbonate mud around individual grains (incipient ooids) or composites of grains (incipient intraclasts). As this dissolution-precipitation cycle is repeated the micritic coatings thicken and locally push grains apart. At this stage three things can happen: (a) sparry calcite may be introduced as an intergranular cement (very rare in these calcretes), (b) wet carbonate mud may fill the pore spaces and lithify to micrite (common), or (c) wet carbonate mud may fill the pore spaces and neomorphose to microspar (common). The last process causes ooids/intraclasts to stand out distinctively, since they are opaque, micrite-coated grains set in a fabric of clear microspar. This selective neomorphism of the micritic mud in pore spaces, rather than the micritic mud forming ooids/intraclasts, is probably a function of rapidity of lithification. The micrite of the coatings must have lithified quickly, before neomorphic crystal enlargement could occur, whereas lithification of the intergranular mud was delayed long enough for the growth of microspar crystals.

Clastic sediments associated with radiolarites (Tauglboden—Schichten, Upper Jurassic, Eastern Alps)

Abstract: During the early Upper Jurassic, widespread deep-sea radiolarites were deposited in most parts of the Northern Limestone Alps. In the formation described (Tauglboden-Schichten), these pelagic sediments interfinger with local-source clastic material. Depending on the topography and the kind of material, either slides and slumps, mudflows, grain flows or turbidity currents operated and formed slump-folded beds, mud-flow breccias, fluxoturbidites or turbidites. A breccia had been traced over an area of 20 km2. Its variation is described in terms of lithological columns, bed thicknesses, maximum grain sizes and grain orientations. It forms a tongue-shaped body, which was probably a part of a submarine fan. The fluxoturbidites of the proximal area grade distally on three sides into turbidites within 3–5 km. The clastic material consists of marls and limestones of Rhaetian and Jurassic age. It was probably derived from a tectonically uplifted palaeo-high by an interplay of tectonics and gravity. The clastics were deposited on submarine fans bordering this high. In its lithology the formation closely resembles certain marginal facies of flysch troughs.

Quantitative analysis of calcite and Mg‐calcite by X‐ray diffraction: effect of grinding on peak height and peak area

Abstract: Mineralogical analysis of calcite and Mg-calcite by X-ray diffraction requires that the samples be ground to a powder. Such grinding determines the particle size of the powder and the structural damage of the minerals. Both of these in turn affect the peak intensities recorded by the X-ray machine. Most carbonate sediments are inhomogeneous; they contain both calcite and Mg-calcite which are affected differently by grinding. Such differences cause quantitative analytical results to be inconsistent with the true mineralogical abundance. The two acceptable methods of analysis—(1) measurement of peak height from the base and (2) measurement of the area under the peak—were compared to determine if sample preparation affects the quantitative results. In samples with variable and relatively small amounts of calcite and Mg-calcite the measurement of peak height yields more reproducible results than does the measurement of peak areas. Different proportions of particle size of the mineralogical components in a sample powder, affect proportionally more the peak areas than the peak heights. Extensive grinding causes structural damage of the component minerals which affects much more the peak areas than the peak heights. Thus for quantitative analyses of calcite and Mg-calcite in inhomogeneous carbonate samples which require differing grinding times and have greatly variable amounts of calcite and Mg-calcite, the peak height measurement seems to be a better method than peak area measurement.

Selective transport of heavy minerals by shoaling waves

Abstract: A model to account for the occurrence of heavy mineral concentrations as discrete lamina on beaches is presented. The model is based on the shoreward variation in the distribution of bottom fluid velocities due to shoaling, progressive, gravity, water waves. This variation causes certain portions of the sediment population to be transported shoreward at a faster rate than other portions. On moderate energy shorelines composed of sand-sized sediment the selectively transported portion may include the relatively hard-to-move heavy mineral fraction. Wave tank experiments with artificially prepared sediment populations provide empirical support for the proposed model. Under certain wave conditions it was found that the heavy mineral fraction of the sediment population moved shoreward at a faster rate than did the light mineral fraction.

Use of the Model T Coulter Counter* in size analysis of fine to coarse sand

Abstract: An electrical sensing-zone particle size analyser has been calibrated for use with sands. A 10: 90 saline/glycerol electrolyte is used with a concentration of 0-1-0.5 g/1 of suspended sand. Calibration error is ± 1.6% (± 0.02 φ). Comparison of counter and sieving results shows close agreement. Advantages of the machine are rapidity—a prepared sample can be analysed in 60 sec—and the small size of the sample required.

Carbonate cementation of some Pleistocene temperate marine sediments

Abstract: Carbonate cementation of some carbonate and quartz sands in three raised beaches of temperate origins was investigated. The carbonate of the cements was found to have been derived from the dissolution of skeletal debris. The sandstones, so produced, now possess only low-magnesium calcite, but the original sediments, like adjacent modern beach and blown sands, probably contained low-magnesium calcite, aragonite and some high-magnesium calcite, all of skeletal origin. In meteoric water the dissolution has occurred of all carbonate within minute, tubulelike, volumes of sand. Concurrent deposition in adjacent volumes of sand of low-magnesium calcite formed cements that are irregularly nodular or uneven on a small scale. Aragonite within the minute nodules has been replaced paramorphically by low-magnesium calcite. Additional local carbonate cements were formed at later dates, around and within solution pipes.

Grain size studies on turbidite components from Tyrrhenian deep sea cores

Abstract: Through an analysis of their coarse-grain composition (>63 μm), pumice-rich sandy layers from deep sea cores were identified as shelf-derived turbidites rather than deposits due to ash rain or drifting pumice. A comparison of median sizes of individual grain types and total samples yielded significant vertical and horizontal sorting trends, which allowed apparently unrelated samples from different cores to be grouped into proximal and distal parts of one turbidite type. A discrimination of samples belonging to unrelated turbidite layers was also possible. In addition, whatever the grain size, planktonic molluscs and foraminifera have Md-diameters 1·1–2·75 times larger than those of pumice. Weathered shallow-water skeletals have Md-diameters approximately equal to pumice, whereas those of augite and hornblende are 1·25–2·3 times smaller. This results in a different proportion of components in proximal and distal turbidite samples. The Md-values of planktonic foraminifera reach their upper natural limit at 250–350 μm.

Effect of compacton on chemistry of solutions expelled from montmorillonite clay saturated in sea water

Abstract: The total mineralization of solutions squeezed out of montmorillonite clay saturated in sea water was determined at different overburden pressures The subsequent fractions of expelled solutions were also analysed for various anions (Cl−, SO2-4, HCO−3, F−) and cations (Na+, K+, Mg2+, Ca2+, B3+) The results indicate that the concentrations of squeezed-out solutions during the initial stages of compaction (at pressures up to 35 kg/cm2) are slightly higher than that of interstitial solution present initially The concentration of squeezed-out solution goes through a maximum, or at least remains constant, before starting to decrease with increasing overburden pressure