Coarse-tail Grading Research Articles

Erosion and deposition by supercritical density flows during channel avulsion and backfilling: Field examples from coarse-grained deepwater channel-levée complexes (Sandino Forearc Basin, southern Central America)

Erosion and deposition by supercritical density flows can strongly impact the facies distribution and architecture of submarine fans. Field examples from coarse-grained channel-levée complexes from the Sandino Forearc Basin (southern Central America) show that cyclic-step and antidune deposits represent common sedimentary facies of these depositional systems and relate to the different stages of avulsion, bypass, levée construction and channel backfilling. During channel avulsion, large-scale scour-fill complexes (18 to 29 m deep, 18 to 25 m wide, 60 to > 120 m long) were incised by supercritical density flows. The multi-storey infill of the large-scale scour-fill complexes comprises amalgamated massive, normally coarse-tail graded or widely spaced subhorizontally stratified conglomerates and pebbly sandstones, interpreted as deposits of the hydraulic-jump zone of cyclic steps. The large-scale scour-fill complexes can be distinguished from small-scale channel fills based on the preservation of a steep upper margin and a coarse-grained infill comprising mainly amalgamated hydraulic-jump zone deposits. Channel fills include repeated successions deposited by cyclic steps with superimposed antidunes. The deposits of the hydraulic-jump zone of cyclic steps comprise regularly spaced scours (0.2 to 2.6 m deep, 0.8 to 23 m long) infilled by intraclast-rich conglomerates or pebbly sandstones, displaying normal coarse-tail grading or backsets. These deposits are laterally and vertically associated with subhorizontally stratified, low-angle cross-stratified or sinusoidally stratified sandstones and pebbly sandstones, which were deposited by antidunes on the stoss side of the cyclic steps during flow re-acceleration. The field examples indicate that so-called spaced stratified deposits may commonly represent antidune deposits with varying stratification styles controlled by the aggradation rate, grain-size distribution and amalgamation. The deposits of small-scale cyclic steps with superimposed antidunes form fining-upwards successions with decreasing antidune wavelengths, indicating waning flows. Such cyclic step-antidune successions form the characteristic basal infill of mid-fan channels, and are inferred to be related to successive supercritical high-density turbidity flows triggered by retrogressive slope failures.

Quartz arenites of the Cambro-Ordovician Kamouraska Formation, Quebec Appalachians, Canada: I. Deep-water depositional processes in a continental-slope environment

The Kamouraska Formation is an uppermost Cambrian – lowermost Ordovician quartz-arenite-dominated unit of controversial origin deposited on the southeastern slope of Laurentia bordering the Iapetus Ocean. It is exposed in the Quebec Appalachians on the south shore of the St. Lawrence Estuary. The formation consists of basal polymictic conglomerate and overlying massive sheet-like quartz arenite. The conglomerate beds are reversely and reversely to normally graded. The quartz arenite beds are generally massive, although they may show coarse-tail grading. Beds containing full or partial Bouma sequences are rare. Paleoflow directions from ripple-cross lamination, ripple marks on bed surfaces, and sole marks point towards southeast, south, and southwest. The clastic sediments of the Kamouraska were transported into the deep sea by sediment gravity flows that evolved from hyperconcentrated to concentrated density flows, and then to turbidity currents. The depositional environment is interpreted to have been a southwest-trending meandering submarine canyon. The exposed part of the canyon deposits is slightly oblique to the strike of slope. If correct, our interpretation establishes the preservation of continental-slope deposits in more distal deep-water siliciclastic sedimentary rocks of the Taconian orogen in Quebec, which traditionally have been interpreted as submarine-fan and (or) basin-plain deposits. The orientation of a canyon near parallel-to strike of the slope may have been controlled by syn-depositional growth faults. The coarsest hyperconcentrated flows, which deposited the conglomerate, were restricted to the deepest parts of the canyon during its early stages of development, whereas the concentrated density flows that deposited the massive quartz-arenite beds covered a wider area.

Multi-component particle-size segregation in shallow granular avalanches

A general continuum theory for particle-size segregation and diffusive remixing in polydisperse granular avalanches is formulated using mixture theory. Comparisons are drawn to existing segregation theories for bi-disperse mixtures and the case of a ternary mixture of large, medium and small particles is investigated. In this case, the general theory reduces to a system of two coupled parabolic segregation–remixing equations, which have a single diffusion coefficient and three parameters which control the segregation rates between each pair of constituents. Considerable insight into many problems where the effect of diffusive remixing is small is provided by the non-diffusive case. Here the equations reduce to a system of two first-order conservation laws, whose wave speeds are real for a very wide class of segregation parameters. In this regime, the system is guaranteed to be non-strictly hyperbolic for all admissible concentrations. If the segregation rates do not increase monotonically with the grain-size ratio, it is possible to enter another region of parameter space, where the equations may either be hyperbolic or elliptic, depending on the segregation rates and the local particle concentrations. Even if the solution is initially hyperbolic everywhere, regions of ellipticity may develop during the evolution of the problem. Such regions in a time-dependent problem necessarily lead to short wavelength Hadamard instability and ill-posedness. A linear stability analysis is used to show that the diffusive remixing terms are sufficient to regularize the theory and prevent unbounded growth rates at high wave numbers. Numerical solutions for the time-dependent segregation of an initially almost homogeneously mixed state are performed using a standard Galerkin finite element method. The diffuse solutions may be linearly stable or unstable, depending on the initial concentrations. In the linearly unstable region, ‘sawtooth’ concentration stripes form that trap and focus the medium-sized grains. The large and small particles still percolate through the avalanche and separate out at the surface and base of the flow due to the no-flux boundary conditions. As these regions grow, the unstable striped region is annihilated. The theory is used to investigate inverse distribution grading and reverse coarse-tail grading in multi-component mixtures. These terms are commonly used by geologists to describe particle-size distributions in which either the whole grain-size population coarsens upwards, or just the coarsest clasts are inversely graded and a fine-grained matrix is found everywhere. An exact solution is constructed for the steady segregation of a ternary mixture as it flows down an inclined slope from an initially homogeneously mixed inflow. It shows that for distribution grading, the particles segregate out into three inversely graded sharply segregated layers sufficiently far downstream, with the largest particles on top, the fines at the bottom and the medium-sized grains sandwiched in between. The heights of the layers are strongly influenced by the downstream velocity profile, with layers becoming thinner in the faster moving near-surface regions of the avalanche, and thicker in the slowly moving basal layers, for the same mass flux. Conditions for the existence of the solution are discussed and a simple and useful upper bound is derived for the distance at which all the particles completely segregate. When the effects of diffusive remixing are included, the sharp concentration discontinuities are smoothed out, but the simple shock solutions capture many features of the evolving size distribution for typical diffusive remixing rates. The theory is also used to construct a simple model for reverse coarse-tail grading, in which the fine-grained material does not segregate. The numerical method is used to calculate diffuse solutions for a ternary mixture and a sharply segregated shock solution is derived that looks similar to the segregation of a bi-disperse mixture of large and medium grains. The presence of the fine-grained material, however, prevents high concentrations of large or medium particles being achieved and there is a significant lengthening of the segregation distance.

Spatio-temporal evolution of velocity structure, concentration and grain-size stratification within experimental particulate gravity currents

We describe a flume study of the spatio-temporal evolution of particulate gravity currents. Time series of the vertical structure of flow in terms of the forward component of velocity, flow concentration and grain-size of suspended sediment were co-measured at a different position along the flume for each of a series of nominally identical flows. The data are combined to show the spatial evolution of a single idealised flow. Such a flow, fed into the flume from an overhead reservoir, first propagates as a quasi-steady jet. The subsequent evolution of coherent spatial and temporal trends in velocity, grain-size and concentration are developed by the internal action of the flow itself, rather than being inherited from the flow generation mechanism. Quasi-steady input currents evolved down flume into surge-type flows that wax very rapidly upon arrival then progressively wane. Thus the velocity history of currents at source may differ from that experienced downstream, indicating that flow steadiness measured (or interpreted) in downstream positions may not necessarily be indicative of the flow generation mechanism. The fore-most parts of the flow travelled more rapidly than the hind-most parts and thus gradually drew further away; as a consequence, the duration of the experimental currents systematically increased along the length of the flume (the flows ‘stretched out’). Natural scale flows may therefore wane more rapidly in proximal regions than distal ones. This may impact upon sedimentation style, with higher sedimentation rates potentially more prevalent in proximal than in distal regions. Grain-size maxima are recorded in the head of the current. Convincing evidence of coarse tail lag behind the head is only patchily developed. Within the flow body a consistent pattern of upward fining then coarsening is observed. This can be related to an upward flux of coarse particles from the head, which subsequently settle downwards into the body. At the natural scale such a phenomenon may have implications for the development of coarse tail grading. q 2003 Elsevier Ltd. All rights reserved.

Transport and Emplacement of the 15.4 Ma Rhyolitic Ignimbrites from Gutai Mts., Eastern Carpathians

The 15.4 Ma rhyolitic ignimbrites are the first volcanic products from Gutâi Mts. The study of their sedimentary structures reflects the mechanisms involved in the transport and emplacement of the parental flows, essential to inffering the style of eruption and the source evolution and location. Primary sedimentary structures show massive deposits emplaced from mass flows, sequence of units showing the normal coarse-tail grading of the dense clasts and the reverse coarse-tail grading of the pumice clasts. They reflect concentrated laminar flows or dilute, subcritical flows emplaced by progressive aggradation. Secondary sedimentary structures are represented by the eutaxitic texture or welding, cooling textures (columnar jointings, spherulitic textures) and gas-escape pipes. They assess the hot state deposition with low degree of fluidization, low cooling rates and gas-retention regime. Secondary sedimentary structures are represented by the eutaxitic texture or welding, cooling textures (columnar jointings, spherulitic textures) and gas-escape pipes. They assess the hot state deposition with low degree of fluidization, low cooling rates and gas-retention regime.

Field characteristics of deposits from spatter-rich pyroclastic density currents at Summer Coon volcano, Colorado

Abstract The Oligocene, deeply eroded Summer Coon composite volcano contains mafic andesite deposits that are massive to poorly bedded, have abundant flattened and deformed spatter clasts, have varying proportions of dense lithic clasts, and are supported mostly by a coarse-ash matrix. Although superficially these deposits resemble typical facies from Strombolian eruptions (emplaced ballistically, by fallout, and by rolling and local grain-avalanches down steep cone slopes), there are several lines of evidence that lead to an interpretation that the deposits were emplaced by pyroclastic density currents. These include local coarse-tail grading, deformation of spatter clasts in a down-flow direction, incorporation of matrix ash and lapilli into flattened spatter clasts, imbrication of large clasts, plastering of spatter on stoss sides of large lithic blocks and lenses of lithic-rich material on lee sides, deposition on angles less than the angle of repose, and a paucity of clast shapes associated with Strombolian mechanisms. The deposit characteristics are consistent with rapid sedimentation from a low-particle-concentration, turbulent flow onto an aggrading bed. We infer two potential mechanisms for generating these density currents: (1) explosive magma–water interaction involving lithic debris and relatively unfragmented melt; and (2) collapse of oversteepened upper cone slopes due to rapid accumulation of spatter from voluminous Strombolian eruptions.

Sedimentary features of the seismo-turbidites, Lake Biwa, Japan

Abstract Seismo-turbidite layers are deposited at the foot of the steep slope of the half graben in which Lake Biwa has accumulated. Sedimentary studies, especially of grain-size distribution of selected turbidites provide excellent information for clarifying the distinctive features of general seismo-turbidites. Development of the two constituent sublayers (lower sandy sublayer and upper silt sublayer), lack or limited appearances of traction-indicating structures and characteristic grain-size distribution in the two constituent sublayers are most remarkable and significant. The characteristic grain-size distribution features are coarse tail grading in a relatively thick sandy sublayer, exceptionally rapid distribution grading through the thin boundary interval between the two sublayers, height increase and fluctuation of the silt mode, and high clay content. Brief discussion was made on the causes of these sedimentary features. The need and importance of further comparative studies of various turbidites known triggering forces are emphasized.

Emplacement mechanism and lateral correlation of pyroclastic flow and surge deposits in northen St Kitts, Lesser Antilles

Thick accumulations of pyroclastic deposits have emanated from the Mt Misery volcano on St Kitts, Lesser Antilles, during two major eruptive episodes (2-3000 and 41000 a BP). Systematic logging of coastal sections in the northern part of the island has revealed a variety of deposits, including pyroclastic flows, surges and intermittent air fall. Palaeosols and one laterally persistant surge deposit provide important marker horizons, permitting correlation of the intervening and less persistent flow deposits. Block and ash flows (traditionally termed nuée ardentes) can be subdivided into andesitic Pelean-type and basaltic andesite St Vincent-type deposits, closely corresponding to the products of historic pyroclastic eruptions in the Lesser Antilles. Finer grained ash flows are equally voluminous and comprise scoria, semi-vesicular andesite and mixed magma and ash deposits. A significant pyroclastic surge component has been recognized with characteristic bedforms. Field evidence suggests that there is a continuous gradation between flow and surge deposits. Collectively the pyroclastic flow deposits are all of the small volume type and are commonly stratified, with poor to moderate sorting and coarse tail grading. Where thickly developed they show an internal depositional sequence compatible with the standard ignimbrite flow unit. Well developed bedforms include steeply incised U-shaped gullies eroded by the infilling flows. The depositional characteristics of these pyroclastic flows appears to be a function of the physical properties of the magma on eruption, initial flow magnitude, potential energy and degree of topographic channel formation. In general grain-size and the erosive potential of the flow increases with increasing channel formation, whilst sorting and the degree of stratification decrease. The development of graded bedding is, however, apparently unaffected.

Hemipelagic Rocks at Bissex Hill, Barbados: Sedimentology, Geochemistry, and Depositional Environment

ABSTRACT The island of Barbados exposes the crest of the Barbados accretionary prism of the Lesser Antilles forearc. The sedimentology of Lower Eocene hemipelagite was studied in three fault-bounded packets near Bissex Hill. Three facies were identified based on coherent stratigraphic successions. Facies 1 hemipelagite strata are well layered to massive and consist of radiolarite layers (T layers), matrix-supported radiolarian mudstone (F layers), layers with an intermediate abundance of radiolarian tests (N layers), mud-rich (M) layers containing less than 20% radiolarian tests, rare quartzose sandstone intervals, mud-chip breccia beds, and wispy, laminated, bioturbated units. Many T and N layers have parallel laminations and sharp basal contacts. Few T and N layers have coarse-tail grading, cour structures, and floating mud clasts. Most T and N layers were deposited by turbidity or bottom-water currents. F layers probably represent waning-flow current or suspended-load deposits. Wispy, laminated units are most likely bioturbated T, N, and F layers. Facies 2 strata are wispy, laminated terrigenous mudstones greater than 10 cm thick. Thorough bioturbation indicates slower rates of deposition relative to facies 1 strata. Facies 3 is mudstone composed of sparse radiolarian tests floating in a terrigenous-mud matrix. Facies 3 deposits are interpreted as extensively bioturbated sediment which, prior to reworking, contained fewer intercalated T layers. Bulk-rock chemistry indicates that the hemipelagites were derived from terrigenous and biogenic sources. Weight-percent Al2 UB>O3 correlates with the percentages of clay-sized material in T, N, and F layers. Low total organic carbon (bitumen-free; 0.06-0.83%) and Type III kerogens indicate an oxic depositional environment. The hemipelagic rocks were deposited in abyssal-plain or outer-trench-slope environments.

Waulsortian-Type Buildups and Resedimented (Carbonate-Turbidite) Facies, Early Mississippian Burlington Shelf, Central Missouri

ABSTRACT The crinoidal carbonates of the Lower Mississippian (Osagean) Burlington Limestone in central Missouri consist of individual and multiple interconnected Waulsortian-type buildups and associated resedimented debris (carbonate-turbidite) facies. This association of Waulsortian-type buildups and turbidity-current sedimentation developed on the foreslope, toe-of-slope, and basinal near-toe-of-slope areas of the prograding Burlington Shelf. The Waulsortian-type buildups contain a core facies of dolomitized, bryozoan-crinoid, lime mudstone and a flank facies of symmetrically disposed, thin-bedded crinoidal packstone and grainstone. The core developed where bryozoan-crinoid thickets trapped carbonate fines of upslope (turbidity-current) origin. Lime-mud hardgrounds in the cores developed during prolonged breaks in the influx of lime mud. The flank beds developed by in situ disarticulation of indigenous crinoids and minor gravity-slide processes. The resedimented-debris facies are laterally continuous and normally graded grainstone and packstone. The beds show 1) a subdivision of Bouma intervals A and B; 2) coarse-tail grading; 3) scours, flutes, and load casts; 4) a generally random fabric except for crinoid-ossicle alignment in Bouma interval B; and 5) no articulation of crinoid stems or calyxes. These features, combined with the absence of infiltration textures, escape burrows, shell imbrication, and hummocky cross-stratification, are useful in distinguishing these resedimented beds from lag-suspension deposits of storm origin. The lower part of the Burlington (5-15 m thick) consists exclusively of Waulsortian-type buildup core and flank facies. These buildups rest on a thin, laminated dolomitic shale bed (starved-basin sediment) and represent basinal near-toe-of-slope deposition. The upper and middle parts of the Burlington Limestone, corresponding to foreslope and toe-of-slope depositional settings, consist of Waulsortian-type buildups and resedimented-debris facies. Considering approximate modern analogues, the resedimented debris probably came from various linear, near-shelf margin sources. The resedimented debris contains algal-bored grains from the shelf margin and foreslope biogenic grains and clasts.

A transgressive storm- and fair-weather wave dominated shelf sequence: Cretaceous Nimar Formation, Chakrud, Madhya Pradesh, India

Abstract The lithologic assemblage of the Cretaceous Nimar Formation in the area around Chakrud, Madhya Pradesh, India, has been classified in five marine facies: (A) polymictic conglomerate resting on granite basement, largely representing a lag deposit; (B) coarse-grained pebbly sandstone with normal and reverse coarse-tail grading and also frequency grading; (C) sandstone, with alternate hummocky cross-stratified and small-scale wave-rippled units; (D) mudstone; and (E) limestone. Facies C bears the definite stamp of alternations between storm- and fair-weather waves. Probable parameters of the fair-weather waves have been determined from the grain size and morphometric parameters of the wave ripples. The depth of the generation of the wave corresponds to a modern mid-shelf region. Facies B shows gradational contact with facies C and was deposited shoreward above fair-weather wave base. Bed shear stresses calculated from variable grain-size parameters within individual cross-laminae indicate that the nature of the current that deposited the pebbly sands was similar to that of storm currents. It has been postulated that storm ebb surges initially moved as fluid gravity flows pushing ahead the bed loads, and eventually turned into density currents farther offshore carrying most of the sediment in suspension. The other two facies, D and E, were deposited largely below storm wave base. The overall fining-upward sequence reflects gradual deepening of the basin. Local aberrations in the facies sequence as represented by short-phase coarsening-upward sequences suggest temporary reversals of this trend.

A proximal ignimbrite breccia facies on santorini , Greece

Prominent among the pyroclastic deposits of Santorini are several thick, widespread lithic breccia deposits, which are found in intimate association with ignimbrite. At least three of these breccias are interpreted, on the basis of field and grain-size criteria, as having originated by the segregation of lithic clasts from active pyroclastic flows. They therefore record the occurrence of three large, previously unrecognized ignimbrite-forming eruptions of the volcano. The breccias of the 18,500 yr. B.P. Cape Riva eruption include two types. The first type is a thin, basal ground breccia, which overlies a strong erosion surface. This breccia shows pinch and swell structures and is strongly enriched in lithic and crystal components. It is considered to have formed by strong fluidization due to incorporation of air into the head of an active pyroclastic flow. The second, and predominant, type consists of thick co-ignimbrite lag breccias (up to 25 m), which overlie the ground breccia. These deposits are generally clast-supported, poorly sorted breccias which in places grade both vertically and laterally into non-welded pumiceous ignimbrite. They consist of well-defined, normally graded units which show coarse tail grading of lithic and pumice clasts. Each breccia unit is underlain by a thin, inversely graded ignimbrite basal layer, and correlates laterally with a flow unit of the associated ignimbrite. The lag breccias are therefore thick equivalents of the 2b lithic concentration zones of Sparks et al. The lag breccias and ignimbrite contain abundant lithic segregation structures that are characteristic of strong gas fluidization. These structures, the presence of basal layers, and the gradation into normal ignimbrite, suggest that the lag breccias originated by the segregation of lithic clasts within the bodies of dense, but strongly fluidized pyroclastic flows. The Cape Riva breccias occur within a few kilometers of their source vent and are interpreted as proximal facies of their associated ignimbrite. The presence of the ground breccia indicates that within this distance, the pyroclastic flows had developed the head and body regions characteristic of gravity currents. The deflation of the pyroclastic flow bodies, within a few kilometers from source, to particle concentrations sufficient to permit the generation of basal layers and coarse tail grading, is incompatible with present theories of column collapse. It is postulated that high pressures at the base of the collapsing Cape Riva eruption column were sufficient to significantly compress the dilute particle-gas mixture of the column close to the source vent. Subsequent sedimentation, as the pyroclastic flows moved laterally, increased the density further to the point where the observed sedimentary features could form. Simultaneous decompression of the gas phase resulted in strong fluidization, and the segregation of the lag breccias.

The pyroclastic deposits of the 1875 eruption of Askja, Iceland

The 1875 explosive eruption of Askja, Iceland was part of a series of regional volcanic and tectonic events which took place in the northern rift zone in 1874 and 1875. These events were marked by regional seismicity, graben formation and a basaltic fissure eruption at Sveinagja, and the plinian eruption of Askja on 28-29 March. Crustal rifting caused basaltic magma to be mixed with rhyolitic magma, triggering the plinian eruption. A caldera, Oskjuvatn, was formed in Askja measuring 3 x 4 km and 267 m deep. Six distinguishable pyroclastic layers can be recognized. The main eruption began with a small sub-plinian pumice eruption forming layer B. The next phase produced a fine-grained, poorly sorted pumice and ash deposit with well developed stratification (layer C), which contains base surge beds near source and is interpreted as phreatomagmatic in origin. The main plinian phase of the eruption lasted 6 h and formed a coarse-grained, poorly bedded pumice-fall deposit (layer D) which contains 75% of the total ejecta. Late-stage explosions formed a layer of lithic clasts (layer E). Isopach and grain-size isopleth maps show that the vents migrated from south to north along a line 1.5 km long in the area now occupied by Oskjuvatn. The intensity and column height of the eruption increased with time as shown by reverse grading and an increasing dispersal index in successive layers. Most of the ejecta is composed of white rhyolitic pumice and ash. Lithics consist of rhyolitic obsidian, partially fused trondhjeimite, and basalt fragments: layer D contains 2.1 mass % lithics. All layers contain abundant grey pumice clasts consisting of intimate mixtures of dark brown basaltic and brown rhyolitic glasses. The mass percentage of mixed pumice in layer D is 4.7, of which 40 % is basaltic glass. These mixed pumice clasts are concentrated at distances of 30-80 km in layer D by aeolian sorting. A grey, crystal-rich, andesitic pumice occurs as inclusions in the white pumice. Layer D shows a systematic decrease in median grain diameter, but no change in cr^ with distance from source. Layer C shows no change in median grain diameter, but a decrease in with distance from source. Phreatomagmatic deposits such as layer C can be readily distinguished from plinian deposits on a Md$ against cr^ diagram, on a against a* (skewness) diagram and on the F against D plot of Walker (1973). The downwind, coarse-tail grading in layer C is attributed to fall-out of fine ash as clumps and aggregates. The total grain-size distributions of both layers D and C show bimodality. In layer D a minor mode in the ash size classes reflects secondary processes of fragmentation by collisions in the vent and column, whereas the major mode is due to disruption of magma by expanding gases. In layer C the fine mode is dominant and represents extensive fragmentation by explosive interaction with water. Field and grain-size studies of layer D show that impact breakage is of major importance near source.

The role of fluidization in the emplacement of pyroclastic claws: An experimental approach

Abstract A series of experiments was carried out to review the process of fluidization for a number of particulate materials having various sorting and grain shape characteristics. The up (increasing gas velocity) curve on a gas velocity/bed-pressure drop plot for a poorly sorted mixture of irregularly shaped particles is divided into three sections; non-expanded, expanded, and segregating. These sections are used to define a threefold genetic classification of pyroclastic flows which can be directly linked to conditions within a semifluidized parent flow. Type 1 flows are ungraded and mostly result from hot-avalanche flows and pyroclastic flows formed of relatively dense material. Type 2 and type 3 flows are mainly pumiceous ignimbrites and are distinguished by expansion induced coarse-tail grading, coupled with segregation structures in the latter. The implications of this classification are discussed with reference to the flow regimes, deposition slope angles, crystal concentration and “fossil fumaroles” variously developed in the flows. The relevance of the source of fluidizing gas is discussed in relation to the zoning of flow types within a single flow unit and it is shown how this, and its attendant structures, can be used to estimate the relative importance of each gas source during and after flow emplacement.

Mass flow deposits on a mid-Tertiary carbonate shelf, southern New Zealand

SummarySheet-like and channel-filling conglomerates and pebbly biomicrudites occur within a sequence of cross-stratified shallow-water bioclastic limestones, the Forest Hill Formation of Late Oligocene to Early Miocene age. The largest channel, 50 m wide, 9 m deep, and at least 1.2 km long, is infilled with four separate conglomerate units of pebble- to boulder-sized limestone lithoclasts in a sparse muddy matrix (biomicrudite). Both the conglomerates and associated pebbly biomicrudites possess features characteristic of high concentration mass flows - both of laminar flow (preferred clast orientation, inverse grading or massive bedding) and of turbulent flow (normal coarse-tail or distribution grading and stratification). Clast imbrication (a- or b-axes) is suggestive of gradual deposition. Some flows travelled far enough to allow both vertical and horizontal differentiation of the coarse sediment. Facies models which have been proposed for conglomerates of deep water (turbidite) association can also be applied to the small-scale neritic conglomerates. Formation and remobilisation of such coarse grained facies in a shelf setting is attributed to eustatic sea-level changes and/or to local tectonism.

Graded Beds as Paleogravimetric Cameras: ABSTRACT

Physicists over the past century have postulated that the universal constant of gravitation, G, is decreasing with time. Assuming an annual rate of decrease of 10-10 parts per year the gravitational acceleration of the earth, g, would have been 22% greater two billion years ago. Distal turbidites occur in geologic deposits as old as 2.5 billion years. Utilizing the relations developed by Scheidegger and Potter, a functional relation between g and measurable characteristics of a graded bed may be constructed. This relation, a paleogravimeter, was tested in terms of sensitivity to changes in g as well as possible confounding effects such as sediment concentration, density, and grain packing. Only the sediment concentration in the turbidity current had a measura le effect. By utilizing Middleton's result that the transition between distribution grading and coarsetail grading occurs at particle concentrations of 30%, a selection criteria can be established for the camera. In this respect it is meaningful to measure only beds deposited from currents of the same particle concentration. When this is done the paleogravimetric change can be measured. The paleogravimetric camera can be improved by substituting more realistic relations for Stokes' Law and allowing viscosity to vary as suggested by Roscoe. Preferably however, a purely physical model for sedimentation from a turbidite should be developed in place of the Scheidegger-Potter relation. End_of_Article - Last_Page 674------------

Characteristics of widespread pyroclastic deposits formed by the interaction of silicic magma and water

We have recognized a type of pyroclastic deposit formed by the interaction of water and silicic magma during explosive eruptions. These deposits have a widespread dispersal, similar to plinian tephra, but the overall grain size is much tiner. Several deposits studied can be associated with caldera lakes or sea water and water/magma interaction is proposed to account for the fine grain size. Several examples have been studied, including the Oruanui Formation, N.Z., and the Askja 1875 deposit. Both show little downwind decrease in median diameter, a downwind decrease in sorting (σφ) (more evident in the Askja deposit) and coarse tail grading. The Askja example has base surge deposits near source and some Oruanui members show multiple thin beds near source; both are common features of phreatomagmatic deposits. Isopachs of the Askja deposit indicate a source under Lake Oskjuvatn in Askja Caldera and those of the Oruanui indicate a source under the NW part of Lake Taupo. In terms of dispersal area, volume and calculated eruption column heights, these deposits are similar to plinian. However, their extreme fragmentation due to magma/water interaction, superimposed on fragmentation imparted by carlier vesiculation, gives a much finer and more complex grain size distribution than plinian counterparts. The field of phreatomagmatic equivalents to plinian pumice deposits was unoccupied onWalker’s (1973) classification of explosive volcanic eruptions. Such deposits are the phreatomagmatic analogue of plinian deposits and the name « phreatoplinian » is proposed.

EXPERIMENTS ON DENSITY AND TURBIDITY CURRENTS: III. DEPOSITION OF SEDIMENT

Turbidity currents were formed by releasing suspensions of plastic beads (density 1.52, median diameter 0.18 mm) from a lock into a horizontal water-filled flume. Graded beds were formed; the mechanism of deposition was studied by motion photography and the size grading by 150 size analyses.Deposition of sediment took place behind the head even at a time when there was no deceleration of the head: the greater part of the thickness of the bed was deposited during a period of rapid decline in velocity of flow within the body of the current. The mechanism of deposition and the type of grading differed for beds deposited from suspensions with concentrations less than and greater than about 30% by volume. Low concentration suspensions formed 'distribution grading' in which all percentiles showed vertical grading and at least the coarser half of the distribution showed lateral size decrease away from the gate. High concentration suspensions formed 'coarse-tail grading' in which there was almost no lateral size variation and the vertical grading was shown only by the coarsest few percentiles (except at the top of the bed).In high concentration flows the bed did not accumulate layer by layer, as it did in low concentration flows, but was deposited first as an expanded 'quick' layer, which was deformed by shearing and waves produced by the entrained water flowing over the still plastic bed.In both types of graded beds the sorting coefficient (standard deviation of the logarithm of the settling velocity) decreased upward within the bed, and to a lesser extent also laterally away from the gate. The skewness reached a maximum near the center of the bed and became negative at the top.