bluuebunny/crossref_metadata_2025 huggingface dataset:
Size and shape sorting in a Dutch tidal inlet
Authors: WINKELMOLEN A. M. , VEENSTRA H. J.
Published: June 2006

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.
Les minerais de fer de l'Ordovicien inférieur du bassin de Bretagne‐Anjou, France*
Authors: CHAUVEL JEAN‐JACQUES
Published: June 2006

Les ressources en mineral de fer du bassin de Bretagne‐Anjou proviennent essentiellement de quatre couches de mineral sédimentaire situées dans le grès armoricain (Arénig). Les paragenèses les plus fréquentes réunissent le quartz, la magnétite, la sidérite et des silicates (bavalite, stipnomélane). La paragenèse à hématite et la paragenése à pyrite sont plus rares. L'étude pétrographique permet en outre de définir deux types de minerais: un type à allochems et un type sans allochems.

L'examen des variations de paragenèses et de puissance des couches de minerai permet de reconstituer en partie l'histoire de la sédimentation ferrifère: le fer est mis en solution sur les reliefs cadomiens émergés. Lors de la transgression paléozoïque le fer parvient dans le bassin de sédimentation qui est peu profond et encombré d'iîles et de hauts‐fonds. Suivant les conditions locales le fer précipite alors soit sous ses formes oxydées (magnétite, hématite), soit sous ses formes réduites (bavalite, sidérite, pyrite). Les facteurs essentiels de la différenciation des paragenèses sont l'épaisseur de la tranche d'eau, l'agitation de l'eau et la teneur du sédiment en matiére organique. Dans certains cas un phénomène de concrétionnenment au sein du sédiment donne naissance aux allochems qui peuvent alors être repris, mis en suspension et déposés.
Non‐combustible suspended matter in surface waters off eastern Asia*
Authors: HONJO SUSUMU , EMERY K. O. , YAMAMOTO SATOSHI
Published: June 2006

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.
Early marine lithification of the nodular limestones in the Silurian of New Brunswick
Authors: NOBLE J. P. A. , HOWELLS K. D. M.
Published: June 2006

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.
Interdistributary bay sequences and their genesis
Authors: ELLIOTT T.
Published: June 2006

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.
Vesiculated tuffs and associated features
Authors: LORENZ V.
Published: June 2006

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.
Vanished evaporites: revisited
Authors: SCHREIBER B. CHARLOTTE
Published: June 2006

Detrital particles were found as a sand‐silt layer in a deep‐sea Messinian Stage (Upper Miocene) salt deposit in the Mediterranean Basin (Balearic Sea). These particles include worn and limonite‐coated quartz, feldspar, glauconite, volcanic glass, reworked Messinian foraminifera and some lutecites. The lutecites are not original to the salt and taken in their association with the other detritus, especially the foraminifera, suggest syndepositional reworking of exposed Messinian evaporites.
Quartz‐sandy, grazing‐contoured stromatolites from coastal embayments of Mauritania, West Africa
Authors: SCHWARZ HANS‐ULRICH , EINSELE GERHARD , HERM DIETRICH
Published: June 2006

A new occurrence of Recent stromatolites different from those known up to now has been discovered on tidal flats of the Bay of Saint‐Jean (near Cape Timiris). Their most remarkable features are predominance of quartz sand instead of carbonate, characteristic surface‐contouring by grazing fish, absence of cementation, intensive reworking by crabs, and connection with saline sabkha deposits.

Entrapment of sand grains and great resistance of the algal sheaths are most important for stromatolitic growth, although today destruction appears to prevail over accretion. Quartz sand of aeolian dunes and carbonatic pellets of aragonitic ooze feeders are the main constituents of the stromatolitic sediments.

According to the tidal range the following zonation of the algal mat exists: (A) knoll and cuspate zone, (B) flat zone, (C) gas‐domed zone, and (D) crinkle zone.
Deposition of fine‐grained sediments in the abyssal environment of the Algéro‐Balearic Basin, Western Mediterranean Sea
Authors: RUPKE N. A.
Published: June 2006

Two depositional processes control the mud accumulation on the southern Balearic Abyssal Plain: pelagic settling at a rate of 10 cm/1000 years, and turbidity currents at an average frequency of > 3 per 2000 years. Thermo‐haline bottom flow has little effect on the abyssal sediment distribution. Just over half of the Late Quaternary section is made up of turbidite mud. Distinctive properties of turbidite mud are: structural, textural, and compositional continuity from the underlying turbidite sand‐silt layer into the overlying mud, grading within the mud layer, a ratio of carbonate percent with the underlying turbidite sand‐silt layer of about 0.5, and a proportion of sand of > 1%. Those of (hemi)pelagic mud are: bioturbation, an average of 8% of sand consisting largely of remains of foraminifera and pteropods, a grain size distribution which is virtually normal with a median around 9 φ, and very poor sorting; in general, the properties of (hemi)pelagic muds are the same in widely separated localities and depths in cores. In some instances the clay mineral ratios of the turbidite mud layer are markedly different from those of the overlying (hemi)pelagic mud layer.
Relict magnesian calcite oolite and subsidence of the Amazon shelf
Authors: MILLIMAN JOHN D. , BARRETTO HENYO T.
Published: June 2006

Oolitic sediments occur over large portions of the outer continental shelf off northernmost Brazil and southern French Guiana. Carbon‐14 dates, stable isotope values and analogies with similar deposits from other areas in the world indicate that the ooids were formed in shallow water (possibly protected hypersaline environments) during low stands of sea level. In contrast to both modern ooids and other relict ooids, however, the Amazon shelf ooids are composed entirely of magnesian calcite (12 mol % MgCO3) and possess radial crystal microstructures.

A comparison of water depths and ages of the Amazon ooids with the generally accepted eustatic sea level curve indicates that the Amazon shelf subsided more than 100 m 16‐21 thousand years ago. Prior to and subsequent to the subsidence, however, the outer shelf remained relatively stable.
Mass‐emplaced sand‐fingers at Mararoa construction site, southern New Zealand
Authors: CARTER R. M.
Published: June 2006

During work on a dam construction site in South Island, New Zealand, outflowing ground water resulted in extensive local mass‐transport of the sands being excavated. A series of small scale sand fans was built up, probably mainly by laminar mass‐flow processes. The surfaces of the fans were made up of a series of complexly interdigitating sand‐fingers that are inferred to have been emplaced by viscous plug‐flow. Other sedimentary processes associated with the building and synsedimentary destruction of the fans included rapid grain‐flow, liquefaction and progressive slumping. Although small in scale, the Mararoa mass‐flow fans may be a close analogue for some of the many‘fluxoturbidite’ or‘proximal flysch’ facies described in the sedimentological literature, and their geologic implications are therefore briefly discussed.
Possibilities of palaeopedology
Authors: BUURMAN P.
Published: June 2006

Although fossil hydromorphic soils are very abundant in both Tertiary and older deposits in Western Europe, they are either not recognized or their study is not undertaken. A short account is given on the pedogenetical processes in hydromorphic soils and some of the features that can be traced and others that cannot are discussed. Two examples illustrate conclusions that can be drawn from the study of fossil hydromorphic soils.
Calcareous crust (caliche) profiles and early subaerial exposure of Carboniferous carbonates, northeastern Kentucky
Authors: WALLS RICHARD A. , HARRIS W. BURLEIGH , NUNAN W. E.
Published: June 2006

Sequences of laminated limestones found within thin Carboniferous carbonate strata of northeastern Kentucky were studied to determine their origin and palaeo‐environmental significance. These laminated zones are strikingly similar to Holocene and Pleistocene surficial calcareous crusts (caliche) profiles that occur in various parts of the world.

Carboniferous laminated carbonates are associated with shallow marine carbonate units, palaeokarst, and overlying palaeosol zones. A typical laminated profile ranges in thickness from 1 to 2 m and contains brecciated, light olive‐grey to brown micrite that lacks distinctive bedding. Structures and textures common in most profiles include: (1) calcareous and silicious laminae (laminae form diffuse, alternating light and dark bands that generally parallel bedding but often fill fractures and vugs within the rocks); (2) particles (allochems, and micrite and microspar fragments) coated by brown microcrystalline calcite; (3) brecciated texture; (4) circular to elliptical fossil moulds (occur in sinuous patterns and fill fractures within the rocks); (5) large and small scale fracture patterns.

Subaerial weathering and vadose diagenesis of carbonate mud banks or islands is suggested as a mechanism for the formation of these Carboniferous calcareous crust profiles. These ‘crusts’ formed by a combination of solution (karsting), brecciation, and soil development that transformed an exposed marine biomicrite (‘host’ rock) into a porous subsoil rubble. Laminated ‘crusts’ and coated particles developed as the result of dissolution and reprecipitation of CaCO3 and SiO3 from the soil and carbonate bedrock.

Carboniferous laminated carbonates in northeastern Kentucky are often referred to as ‘algal limestones’ because of their superficial similarity to some modern and ancient algal structures. This study, however, reveals numerous characteristics that can only be explained by diagenesis in a subaerial environment.
Computer program for presentation of grain‐size data by the graphic method
Authors: SLATT ROGER M. , PRESS DAVID E.
Published: June 2006

A computer program is described for a Hewlett‐Packard desk‐top Calculator (Model 9820A)‐Plotter (Model 9862A) which plots a histogram, frequency polygon and/or cumulative curve, as well as standard statistical parameters and percentages of gravel, sand, silt, and clay from raw weight data from a sediment grain‐size analysis. The program utilizes the graphic method for calculating statistical parameters in contrast to other commonly used computer programs derived for the method of moments.

This program is advantageous because (1) it is rapid, (2) it allows direct comparison with grain‐size data in the literature which have been computed by the graphic method, (3) either a complete presentation of a distribution can be obtained on one sheet of paper or a series of cumulative curves or frequency polygons can be prepared on a single graph which is ready for publication photocopying, (4) automatic extrapolation of data points from a cumulative curve eliminates inconsistencies arising from manual extrapolation, and (5) the desk‐top calculator is more convenient to use, eliminates punching of data cards, and can be operated by someone without a knowledge of computers and programming. Disadvantages of the program are: (1) some authors feel critical percentiles are more accurately read off cumulative curves plotted on a probability ordinate than on an arithmetic ordinate, (2) the cumulative curve is a series of straight‐line segments, (3) the frequency polygon is not as accurate a representation of grain‐size distribution as a frequency curve and (4) size class intervals for the histogram and frequency polygon are standardized at 0.5φ, even though gravel and mud fractions are analysed at whole φ intervals. For most studies, the advantages of the program outweigh the disadvantages.
87Sr/86Sr composition of evaporitic carbonates and sulphates from Miocene sediment cores in the Mediterranean Sea (D.S.D.P., Leg 13)
Authors: CLAUER NORBERT
Published: June 2006

The 87Sr/86Sr ratios of evaporitic carbonates and sulphates from Miocene sediment cored in the Mediterranean Sea show a depletion of 87Sr when compared to the isotopic composition of the Miocene contemporaneous marine strontium: 0.70803 versus 0.70936. The arrival into the evaporitic environment of strontium brought by continental waters can explain this difference.

The variation of the 87Sr/86Sr ratios is, nevertheless, noticeable only when the influence of the continental waters is already well marked. This is proved when one compares the results obtained with strontium, to the results of isotopic analysis made on oxygen, carbon, sulphur and hydrogen taken from the same samples.
Depositional environments of Upper Miocene (Messinian) evaporites of Sicily as determined from analysis of intercalated carbonates
Authors: SCHREIBER B. CHARLOTTE , FRIEDMAN GERALD M.
Published: June 2006

Various workers have suggested that the Upper Miocene (Messinian) evaporites of the Sicilian Basin formed in a topographic basin of considerable relief, filled with hypersaline water. Our studies indicate that this basin contained shallow water, at least during the deposition of the carbonate rocks intercalated between the gypsum beds. We recognize four basic kinds of limestone: (1) pelletal and pisolitic limestone; (2) skeletal limestone; (3) oöitic limestone, and (4) laminated lime‐stone‐dolostone. Modern analogs suggest that three of these four kinds of carbonate must have formed close to or above sea level. The evidence supporting this contention includes pellets with algal coatings, pisolites, quiet‐water oöids, and algal laminates. Therefore we suggest that the evaporites associated with these carbonates may likewise have formed in relatively shallow water. An alternative conclusion would be that the level of the sea, and the salinity, underwent irregular patterns of profound change.
Changes in intertidal dunes during two spring‐neap cycles, Lifeboat Station Bank, Wells‐next‐the‐Sea, Norfolk (England)
Authors: ALLEN J. R. L. , FRIEND P. F.
Published: June 2006

Twice‐daily observations over a fixed range were made covering three consecutive sets of spring tides. Some dune population dimensions (e.g. mean wavelength) show a long‐lasting (at least weeks) alteration during the most vigorous springs, but a short‐lived (tides or days only) change over the weaker ones. Other properties (e.g. mean height) experience merely short‐lived alterations. The changes seem to be linked to the creation during spring tides of new dunes of an initially small size (minor dunes), which introduces new matured dunes into the populations if there is sufficient subsequent sediment transport, and to the limited ability of individual dunes to respond to changing hydraulic conditions during their life‐spans. The dunes are active during only a very small proportion of each spring‐neap cycle, but the bank is permanently covered by them, and cross‐stratification caused by dune movement is the only internal structure. Without a knowledge of the history and dynamics of the dune populations, the quality of the environment at Lifeboat Station Bank cannot be satisfactorily assessed from the sedimentary products.
Lower Silurian transgressive barrier islands, southwest Wales
Authors: BRIDGES PAUL H.
Published: June 2006

Barrier islands developed on the southeastern flanks of a volcanic terrain during the Lower Silurian transgression of southwest Wales. The barriers are preserved in transgressive sequences overlying basalts and comprising from base upwards: lagoon→barrier island→offshore marine sediments. The thickness of the barrier island sediments varies from 5 m to 28 m. Comparison with modern barriers suggests that the thin sequences result from narrow (<2 km), steadily transgressing barrier islands, whereas the thicker sequences represent broad (2–4 km), slowly transgressing forms. In one case the barrier became narrower as the rate of migration accelerated in response to decreased fluviatile sediment supply caused by rising sea‐level. Despite the high preservation potential of inlet fill deposits, the latter are generally absent in these Silurian barriers because inlet migration was slow compared with the rate of barrier retreat. Possibly much shell material was dissolved during early diagenesis.
Bed topography and grain size in open channel bends
Authors: BRIDGE JOHN S.
Published: June 2006

Engelund's (1974a, b) theoretical bed topography model for curved channels is modified and shown to fit natural channels when using values for the dynamic bed load friction coefficient of about 0·4–0·5. The dependence of this coefficient on grain size for flow in the lower flow regime is discussed. Using the bed topography model simultaneously, Allen's (1970a, b) theory for grain size variation over point bar profiles is shown to be broadly applicable. The interaction between sediment, flow and bed topography in bends is therefore adequately described.
Glacial lake sedimentation, Austerdalsisen, Norway
Authors: THEAKSTONE WILFRED H.
Published: June 2006

Sediments deposited in a lake at the front of a glacier in the Svartisen area, Norway, have been studied between 1957 and 1974. Until 1959, they were almost completely covered by an outwash plain (sandur), but subsequent erosion has exposed glacial lake sediments more than 70 m deep within a rock basin about 2·5 km long and 1 km wide. The basin was filled by sand and silt carried from beneath the glacier Austerdalsisen by two rivers, each of which deposited a delta in the lake. As the deltas advanced, laminated pro‐delta silt was covered by crossbeds of fine sand and silt, and by near‐horizontal sheets of fine sediments laid down between the delta‐fronts and the distal end of the rock basin. Although both slumping and loading caused minor disturbance of sediments at the lake floor, deformation was of local significance only. Movement of a mass of sediment across the floor, probably triggered by a ‘seismic event’ related to movement of the glacier or to calving at the floating tongue, created a recumbent fold in laminated sand and silt, but transfer of sediment over the lake bed was rare once it had been deposited. Varves are not common at Austerdalsisen, indicating that water temperature, lake chemistry or variations of water and sediment discharge from the glacier were unfavourable for their formation; rhythmic deposition from density flows of sediments carried from beneath the glacier rarely occurred within the Austerdalsisen basin.
Quantitative evaluation of the depth of the western Mediterranean before, during and after the Late Miocene salinity crisis
Authors: RYAN WILLIAM B.F.
Published: June 2006

Quantitative geophysical calculations which take into consideration the isostatic loading of sediment overburden, the overlying water cover, and the thermal cooling history of the continental edge, and its adjacent oceanic lithosphere, demonstrate the foundering of the margins of the western Mediterranean had already commenced in the Aquitanian stage of the early Miocene. The calculations are based on magnitudes and rates of sediment accumulation observed along a profile of three commercial boreholes into the subsurface of the continental shelf of southern France. By the time of the late Miocene (Messinian) salinity crisis, the depth of the seafloor within the Balearic basin exceeded 2.5 km.

Sea‐level fluctuations induced by evaporitic draw‐down permitted the exposure of large tracts of the former submerged continental margins to subaerial processes. The measured magnitude of sediment removal by erosion and channel incision near the outer shelf of the modern Gulf of Lion surpasses 1 km.

The subsidence history of this shelf platform south of France provides new evidence that the continental lithosphere behaves as if it is rigidly coupled to its oceanic counterpart commencing with the initial phase of the pull‐apart. No major vertical fault displacements have subsequently offset their overlying crustal layers.

The sedimentary shaping and construction of the margin seaward of the Rhône delta resulted in a pronounced shelf edge migration and slope progradation during the pre‐salinity crisis Miocene. It has taken 5 million years of predominant upbuilding to establish a new equilibrium profile similar in cross‐section to the precrisis depositional surfaces created by outbuilding.
Size distribution of quartz in mudrocks
Authors: BLATT HARVEY , SCHULTZ DOUGLAS J.
Published: June 2006

Sixteen penecontemporaneously deposited mudrock‐sandstone pairs ranging in age from Devonian to Cretaceous were treated chemically to isolate the quartz and chert fraction from the other constituents of the rocks. The amount of crystalline silica was determined from the chemical treatments; its size distribution was determined using a combination of normal and Micro Mesh sieving and settling tube analysis; the crystalline silica coarser than 10 μu was examined petrographically to determine the amount of chert.

Percentages of crystalline silica in the mudrocks range from 6·7 to 46·7 % and average 27·6 (σ= 10·7). Mean grain size ranges from 4·4 φ to 7·3 φ and averages 6·1 φ. The crystalline silica fraction is a poorly sorted medium to fine silt consisting of one‐eighth sand, six‐eighths silt, and one‐eighth clay size sediment. Percentage of crystalline silica and mean size of crystalline silica in the thirty‐two mudrocks and sandstones are positively correlated; r= 0·685, which is significant at the 99 % level. The best fit linear regression line is: Y= 102·7–11·3X.

Extrapolation of the regression line indicates that, on the average, crystalline silica is lacking in mudrocks in grain sizes finer than 9·1 φ (1·μm), a result consistent with observations by clay mineralogists.

The crystalline silica fraction of both the mudrocks and associated sandstones averages 4% chert in the >10 μm portion. There is no correlation between the mean grain size of the crystalline silica and the percentage of chert in it.
Controls of variation in suspended sediment concentration in the River Rother, West Sussex, England
Authors: WOOD P. A.
Published: June 2006

Suspended sediment concentrations were determined for samples collected from the River Rother, West Sussex, and rating loops constructed for several hydrographs. The rating loops often exhibit hysteresis with a greater suspended sediment concentration for a given discharge occurring on the rising limb than on the falling limb. A comparison of these loops indicates that the hysteresis, and the suspended sediment concentration (for a given discharge) become progressively reduced when storm events occur in rapid succession.

Various types of rating curves can be identified, and a model of suspended sediment concentration and discharge constructed. Controlling factors of suspended sediment concentration are river discharge, duration and frequency of the storm event, the length of time between successive events, and the time of year.
The sliding bed facies in esker sands and gravels: a criterion for full‐pipe (tunnel) flow?
Authors: SAUNDERSON HOUSTON C.
Published: June 2006

The Guelph esker (Ontario, Canada) consists of a sinuous, steep‐sided and segmented ridge which comprises poorly sorted, matrix‐supported sands and gravels. These sands and gravels were probably deposited during the sliding bed stage which has been observed by others in closed‐conduit hydraulic experiments. The poor sorting probably resulted from a high concentration of bed‐material load in the lower part of a subglacial tunnel, sorting being restricted to that produced by particle collisions. Inclusive graphic standard deviation is characteristically large for the sands and gravels, indicating that virtually all sizes available were in transport. The overall grain size distribution shows a characteristic undulatory shape on arithmetic probability paper, mostly because of selective removal of pebble gravel and granule sizes. This poorly sorted fades is believed to be diagnostic of transport in a subglacial tunnel flowing full of water, and may be used to identify subglacial conditions in other eskers. Deltaic sands and gravels occur downcurrent of the esker and contain a greater diversity of structures; climbing‐ripple cross‐laminae, parallel laminae and massive structure, deposited in large‐scale foresees at the end of a subglacial tunnel. These deltaic sands and gravels grade distally into outwash sands and gravels.
Caliche profile formation, Saldanha Bay (South Africa)
Authors: KNOX GORDON J.
Published: June 2006

A sequence of gradational lithification events can be observed in caliche profiles, in the Saldanha Bay area (South Africa), from friable lightly cemented aeolian calcarenites or littoral shelly deposits through an intermediate semi‐indurated zone to an upper strongly indurated zone (calcrete).

Lightly cemented sediment fabrics exhibit bridge and meniscus cements, micritic druses and vadose compaction phenomena. The middle semi‐indurated zones exhibit coated grains in which irregular borings and/or tubules with tangential acicular fibres contribute to coated grains. Random networks of acicular fibres also occur in void spaces. In fully indurated upper layers of the caliche profiles, fabrics of micriteand microspar (in voids) occur in complex brecciated macro‐fabrics.

The features represent changes in a sequence from the friable primary sediments to the calcretes. Fresh‐water vadose flushing leaches grains and causes formation of meniscus and bridge cements and uneven druses. In the middle zone, inorganic processes are aided by the action of micro‐organisms; fungi, bacteria or algae which produce tubules and irregular borings; the overall effect of which is to break down original detrital carbonate particles and enclose them in a crypto‐crystalline micrite. The acicular fibres probably result from evaporation of supersaturated solution. Mechanical processes cause fracturing, which repeated many times gives complicated brecciated fabrics within the upper indurated zone.
The development of sediment threshold curves for unusual environments (Mars) and for inadequately studied materials (foram sands)
Authors: MILLER M. C. , KOMAR P. D.
Published: June 2006

Generalized curves for grain threshold under liquid flow and under gas flow (air), based on laboratory data summarized in a previous paper, are utilized to generate a series of threshold curves for unusual sedimentary environments and little‐studied sedimentary materials of interest to geologists. These are aeolian and water flow threshold on the planet Mars, and the threshold of foraminiferal sands in the deep‐sea. Such extrapolations from the generalized curves are necessary because of the difficulty or impossibility of obtaining satisfactory data for these sediments or environments, and the procedures demonstrate the importance of the general threshold curves.
Lake circulation induced by density currents: an experimental approach*
Authors: LAMBERT ANDRÉ M. , LÜTHI STEFAN M.
Published: June 2006

This short note reports a series of density current experiments designed to model turbidity underflows caused by flood‐stage discharge of lake‐tributaries. In a 5.8 m long tank filled with freshwater, saltwater was fed in continuously, flowing down a 15°‘delta’ slope onto a horizontal floor. These density currents maintained steady state characteristics. The main objectives of this investigation were to determine (1) the flow regime of the density currents and (2) the underflow‐induced movements in the freshwater. Reynolds numbers for thirty‐five runs ranged from 70 to 4100. Experiments with laminar flow reproduced kinematic (Froudian) models of underflows measured in the Walensee (Switzerland). Flow was rapid on the slope (Froude number, Fr > 1) and tranquil (Fr<1) on the floor. Turbulent flow experiments yielded velocity profiles (with a maximum at the flow interface) which approximate natural conditions. Movements in formerly stagnant water body are induced by interfacial shear stress: a layer of freshwater is dragged along by the density current and replaced by the backward flow of an equal amount of overlying water (mass conservation). Extrapolated to a natural setting, circulation induced by underflows is probably an important mechanism for oxygenating deep lacustrine basins.
Last‐glacial alluvial fan sedimentation in the Lower Derwent Valley, Tasmania
Authors: WASSON R. J.
Published: June 2006

Alluvial fans of the last glacial age in the lower Derwent Valley in southeastern Tasmania were built by debris flows and stream flows. The deposits were derived from periglacial and nivational slope mantles at the highest altitudes in the fan catchments, and from regolith of uncertain affinities at the lowest elevations. The apical and middle parts of the fans commonly consist of coarse‐grained debris flow and water‐laid deposits, while the distal deposits are predominantly water‐laid sheetflood silts and clays. Channels are important in the proximal parts of the fans and are mostly filled with water‐laid sediments, less commonly by debris flow deposits. A first approximation to the mean velocity of the last stage of debris flow in a channel is between 4 m/sec and 6 m/sec. A group of ‘water‐laid’ sediments shows very poor separation of particle sizes, and two explanations have been offered for these sediments. Firstly, the large quantities of unsorted debris available from the catchments produced moderately well‐sorted slurries, or, secondly, openwork stream gravels were infiltrated by later water flows charged with fines. The streams crossing the fans also produced water flows with relatively low suspended sediment concentrations resulting in reasonably good sorting. The debris flows and ‘water‐laid’ muddy sediments cannot be distinguished using CM plots. The debris flows display considerable variation in thickness and grain‐size characteristics, which is attributable to properties inherent in debris flow behaviour.
Recognition of ancient tidal inlet sequences: an example from the Upper Silurian Keyser Limestone in Virginia
Authors: BARWIS J. H. , MAKURATH J. H.
Published: June 2006

The Upper Silurian Keyser Limestone is a relatively thin (< 85 m) unit of lagoonal, barrier, and shallow offshore sediments that crops out in the central Appalachians. Lithologies include massive micritic limestones to calcarenites, calcisiltites, and calcareous quartz arenites. The barrier lithofacies is preserved predominantly as tidal inlet channel‐fill. Its presence is supported by two lines of evidence: (1) the sequence of sedimentary textures and structures resembles that observed in modern inlets, and (2) the sequence occupies a position immediately above a disconformity, and is accompanied by an abrupt vertical change in faunal diversity, which is interpreted as representing the transgression of open marine over back‐barrier environments

The inlet channel sequence comprises fine‐ to medium‐grained, well‐sorted quartz arenites that disconformably overlie sediments deposited on carbonate tidal flats (laminated, mudcracked pelmicrites). The sandstone displays a fining‐upward texture, and contains a broken and abraded mixed fauna. Cross‐bedding is bipolar, with major modes oriented obliquely to depositional strike. Decimetre‐scale sets of planar and trough cross‐beds grade upward to centimetre‐scale sets of ripple cross‐lamination, washed‐out ripples, and plane beds. This sequence represents the change from deep to shallow channel environments, and is attributed to lateral inlet migration. The inlet sequence was preferentially preserved during marine transgression because of its relative thickness and lower stratigraphic position with respect to overlying and adjacent barrier‐beach sediments.

The vertical relationships of this inlet‐lagoon complex emphasize that care must be taken in interpreting shallow‐water transgressive sequences. Vertical ‘jumps’ in faunal diversity accompanied by scour surfaces could be misconstrued as major unconformities. Instead, such sequences may represent the shoreface erosion normally associated with the transgressive migration of barrier islands. Whether or not the faunal jump is accompanied by a barrier lithosome is greatly dependent on the geometry, frequency, and migration rate of tidal inlets.
Bedforms produced by fine, cohesionless, granular and flakey sediments under subcritical water flows
Authors: MANTZ PETER A.
Published: June 2006

Experiments have been conducted in a 10 m long laboratory flume to investigate the bedforms which develop from fine, cohesionless sediment beds. Two grades of near uniformly sized silica grains (of median nominal diameters 15 and 66 μm) and six grades of micaceous flakes (ranging in median nominal diameter from 15.5 to 76 μm) were used. A steady subcritical water discharge, which was increased in steps after several hours, was applied to a flat bed of each grade. The developing bedform sequence for fine granular beds was identified as many small‐sized primary ripples, isolated primary, transverse primary, secondary ripples and then possibly dunes; this development was almost the same as that observed for coarser grains. The sequence for fine flake beds differed from grains. Only the single bedform type of parting lineations was observed; with increased discharge, the lineations began to oscillate and eventually enter into fluid suspension. The low discharge parallel lineations were thought to be generated by ‘streaks’ or lanes of transversely alternate high and low velocity fluid which have been reported to exist in the viscous sub‐layer of a turbulent‐smooth boundary, whilst the higher discharge wandering lineations were attributed to low velocity streak ‘bursts’.
Variations in the size of coarse clastic particles over the surface of a small sandur, Ellesmere Island, N.W.T., Canada
Authors: BALLANTYNE C. K.
Published: June 2006

For a small sandur in the Canadian Arctic, trend surface analysis of fifty‐three samples shows that less than 40% of clast size variation is explained by downsandur fining. Further variations are explained by considering different levels of the sandur surface separately. In the main flow area, which carries water even during periods of low flow, channel deposits are invariably coarser than bar deposits although the difference lessens and overall sorting improves with distance downsandur. The flood surface area, which carries water only during high discharges, is less affected by downsandur fining than the rest of the sandur, and the bar‐channel relationship is weaker. The high flood surface which occupies most of the proximal zone of the sandur is probably an abandoned main flow surface, as indicated by clast size characteristics similar to those on the present main flow surface.
Petrographic variations within thick turbidite sequences: an example from the late Palaeozoic of eastern Australia
Authors: KORSCH R. J.
Published: June 2006

The subdivision of thick sequences of turbidite sediments has been problematical because of the monotonous nature of the units. One method, of using detailed detrital petrography for a large number of specimens, has delineated variations with a sequence of Late Palaeozoic age in eastern Australia. The rocks occur within a single structural block and are all members of one sedimentary petrographic province. They have been subdivided into three stratigraphic units (Moombil Beds, Brooklana Beds and Coramba Beds) and greywackes from these units are quartz‐poor to quartz‐intermediate, feldspathic or volcanolithic types. Dacitic volcanism has provided most of the detritus and the contribution from non‐volcanic sources is small. The Coramba Beds are further subdivided into four petrographic units which are parallel to the stratigraphic boundaries. These lithostratigraphic units are based on the presence or absence of detrital hornblende, and the relative ratio of volcanic lithic fragments to feldspar. Vertical petrographic variations within the entire sequence indicate that although the acid volcanic source was predominant throughout the time of deposition, there is a noticeable increase in the contribution from intermediate‐volcanic, acid‐plutonic, low‐grade metamorphic and sedimentary sources towards the top of the sequence. Detrital hornblende is also present in the upper parts of the sequence.
Carbonate cements: their regional distribution and interpretation in Mississippian limestones of southwestern New Mexico
Authors: MEYERS W. J.
Published: June 2006

Calcite cements in Mississippian skeletal packstones and grainstones of southwestern New Mexico are dominated by echinoderm‐syntaxial, inclusion‐free calcites that can be divided into four major compositional zones (from oldest to youngest: zone 1, 2, 3, 5) based mainly on varying Fe11 and MnII contents. These compositional zones are interpreted as‘time stratigraphic’ units as indicated by petrographic evidence for age gaps between zones, and by consistency of their ages on a regional scale. As such, these cement zones can be correlated over most or all of the approximately 30,000 km2 of study area, based on similarity of age, number and sequence of major zones. The inclusion‐free calcite cements comprise approximately 95% of the total cements, of which the pre‐Pennsylvanian zones (zones 1, 2, 3) make up about 60%, and the post‐Mississippian zone 5 makes up about 40% of the total cements. These cements are interpreted as meteoric phreatic on the basis of MnII and FeII content, crystal clarity, cement morphology, substrate selectivity, low Mg content, and absence of marine and vadose characteristics.

In the southern part of the study area zone 2 contains significant amounts of meteoric‐marine mixing‐zone phreatic cements. These mixing‐zone cements are identified by their similarity in morphology, luminescence and substrate selectivity to the inclusion‐free meteoric phreatic cements, but contain microdolomite inclusions indicative of former high‐Mg calcites. Their restriction to the south is interpreted to have resulted from relatively long residence time of the mixing zone in the south during zone 2 precipitation.

Strictly marine subtidal and beachrock cements make up less than 1% of the total cements, and meteoric vadose cements are virtually absent.

Regional distribution of the pre‐Pennsylvanian cement zones suggests a model of cementation during a world‐wide late Mississippian eustatic regression identified by Vail & Mitchum (1976). Specifically, pre‐zone 1 and zone 1 meteoric phreatic cements formed during regression within a shallow oxygenated (?) groundwater system; zone 2 formed during the later part of the regression and during stillstand in a deep‐seated, more extensive flow system; zone 3 formed during subsequent transgression in a shallow groundwater system. The post‐Chester, pre‐Pennsylvanian unconformity resulted mainly in microkarsting and weathering. The main difference, other. than scale, between this model and those derived from diagenetic studies of Quaternary limestones is that it implies that major cementation occurred during sea‐level changes in cpeiric settings, rather than only during stillstands.
Bed forms and stratification types of modern gravel meander lobes, Nueces River, Texas
Authors: GUSTAVSON THOMAS C.
Published: June 2006

All major streams draining the southwestern flank of the Edwards Plateau in south‐central Texas transport large volumes of gravel and sandy muddy gravel and are developing meander lobe sequences consisting predominantly of coarse gravel. The largest of these streams, the Nueces River, has a sinuosity index of 1.3 and an average stream surface slope of 1.8 m/km in the study area. Stream discharge is variable and has ranged from no flow to more than 17,000 m3/s.

Mean clast b‐axis length for the ten largest clasts at thirteen sample sites ranged from 2.5 to 10.8 cm. Velocities of 2.7‐4.4 m/s 1 m above the stream bed are required to transport these clasts. Stream velocities of these magnitudes occur about once in 8 years when discharge of the Nueces River exceeds 3300 m3/s. Mean grain size of Nueces River alluvium ranges from 1.2 to 3.4 cm. At a flow depth of 1 m, sediment of this size has a critical erosion velocity of 1.8‐3 m/s. Velocities of this magnitude occur about once in two years when discharge exceeds 340 m3/s. Under these conditions flow is subcritical, with critical shear stresses on depositional surfaces ranging from 6.4 to 12.7 kg/m2.

Gravel clasts are imbricated and channel bed forms are predominantly transverse gravel bars with slip faces ranging up to 2 m high and wavelengths in excess of 100 m. Stratification includes graded planar crossbeds and horizontal beds. Lower lateral accretion face sediments are also predominantly transverse bars; upper lateral accretion face deposits occur as longitudinal gravel ridges deposited in the lee of vegetation and, less commonly, as chute bars. Near the upper limit of meander lobes where vegetation is heavy, mud and muddy sand occur as overbank deposits; in these deposits sedimentary structures other than desiccation cracks are rare.

Sedimentary sequences in gravel meander lobe systems deposited by low sinuosity streams are graded or non‐graded horizontal beds and planar cross‐beds overlain by mud and muddy sand interbedded with horizontally bedded gravels. Sequences may be several metres thick, but probably do not exceed 8‐10 m in thickness. These deposits in turn are overlain by overbank deposits of mud and muddy sand. Similar sedimentary sequences occur in the extensive Quaternary terraces that parallel the Nueces River.