bluuebunny/crossref_metadata_2025 huggingface dataset:
Miocene goethitic and chamositic oolites, northeastern Colombia
Authors: JAMES HAROLD E. , VAN HOUTEN FRANKLYN B.
Published: June 2006

The late (?) Miocene non‐marine to paralic Guayabo Group in northeastern Colombia and adjacent Venezuela contains as many as thirty‐four thin (to 15 cm) goethitic and chamositic oolites and ooid‐bearing sandstones in 1080 m of section. This eastward prograding deltaic complex consists mainly of chert‐rich litharenite in fluvial, distributary channel, and shoreline facies, and of montmorillonite mudstone in floodplain and interdistributary embayment facies. Within this framework the oolites are restricted to a paralic association that developed during episodes of waning detrital sedimentation when distributary abandonment was followed by minor transgression across mudflats.

Most of the multilayered and symmetrical ooids are composed of goethite, rarely with a small amount of chamosite. These are essentially spherical; many of the chamosite‐rich ones have been plastically deformed. Goethitic ooids resemble those in a thin layer accumulating in brackish Lake Chad, central Africa. Chamositic ooids have affinities with those accumulating in a sea loch in western Scotland. Each of these examples is associated with detrital (silicate) sedimentation, apparently developed directly from colloidal ferric oxide and silicate precursors, and affords no evidence that primary aragonitic ooids were later placed by ironbearing oxide and clay.
Deep‐water fan‐channel conglomerates of Late Cretaceous age, southern Chile
Authors: WINN R. D. , DOTT R. H.
Published: June 2006

The exceptionally well exposed Lago Sofia conglomerate and sandstone lenses in the Upper Cretaceous Cerro Toro Formation of southern Chile are interpreted as the channel and channel margin facies of a deep‐sea fan. The north‐to‐south oriented channels formed on an elongate fan in a narrow retroarc basin between a rising cordillera to the west and the South American craton to the east. The great length of some of the channels (> 120 km) seems to reflect the long duration (> 30 m.y.) and stable nature of the basin. Enclosing the lenses is the fine‐grained Cerro Toro Formation which represents overbank turbidite flows and hemipelagic sedimentation on levee and levee flank areas. Foraminiferal assemblages suggest deposition in 1000‐2000 m of water. Most of the conglomerate has features developed by tractive currents (parallel‐ and cross‐stratified conglomerate). Most is moderately well sorted, imbricated, and has parallel to inclined stratification; large‐scale dunes up to 4 m high are exposed. Typical sediment, gravity flow structures and bedding styles (e.g. pebbly mudstones, graded conglomerate, giant flutes) are not as common in the channel deposits as are tractive features. Tractive features in the gravels apparently were developed by rolling, sliding, and saltation as the bed‐load component of highly turbulent, moderate‐ to low‐density turbidity currents flowing in a confined channel. Graded‐to‐massive conglomerates appear to have been deposited rapidly from fully turbulent flows; diamictites were deposited from debris flows in which fluid viscosity, yield strength, and buoyancy of the fluid were dominant. The three major conglomerate classes recognized do not occur in a systematic manner; vertical and lateral heterogeneity is the rule.
Sedimentology of the Lower Jurassic flint clay bearing Mishhor Formation, Makhtesh Ramon, Israel
Authors: GOLDBERY RON
Published: June 2006

Petrographical and mineralogical studies of the Lower Jurassic sequence exposed at Makhtesh Ramon, have shown the domination of a lateritic suite of epiclastic sediments of pisolite conglomerate and laterite arenite composition. Their subsequent modification by epigenetic processes of chemical weathering has given rise to the formation of flint clay, high alumina flint clay, laterite and mottled and variegated claystone. Within the Nahal Ardon area, both the transported laterite accumulates and the autochthonous lithologies occur interbedded within the same section. In all, a total of ten lithofacies have been defined and mapping of the various units, grouped into allochthonous and autochthonous classes, has established the existence of lateral facies changes between Nahal Ardon, where the sequence rests on a broad undulatory Triassic basement, and the zone to the west where it occurs as infillings of Triassic solution cavities, sink holes and irregular depressions. Vertical profile studies of the lithofacies indicate that the vertical range of ground water movements increased westwards from Nahal Ardon and resulted in the transported laterite accumulates being subjected to varying degrees of chemical weathering. The observed lateral facies changes are a direct consequence of this. Within the ‘karstic’ zone, the laterite accumulates in addition to the overlying younger carbonate lithologies have been converted to flint clay and demonstrate the intraformational genesis of the flint clay, whereby vertical ground water movements are related to recharge of aquifer zones within the underlying Triassic Mohilla Formation.
Possible retrogressive flow slide deposits from the Kongsfjord Formation: a Precambrian submarine fan, Finnmark, N. Norway
Authors: PICKERING K. T.
Published: June 2006

Three thinning and fining‐upward turbidite sequences are described from the Precambrian Kongsfjord Formation, a 3.5 km thick flysch succession. Their thicknesses range between about 2 and 5 m. They show a progressive upward decrease in bed thickness, bulk mean grain size and the ratio of the higher to lower energy division of the Bouma sequence. In one case, however, there is an initial upward bed thickness and grain size increase, with an increase in the proportion of the higher energy division. The absence of structureless mud of the Bouma E division and the presence of wavy interfaces between beds, together with similar palaeo‐currents within each sequence suggest that these sequences resulted from related depositional events. These sequences are interpreted as the deposits of retrogressive flow slides, as an alternative to the classic mechanism of channel fill after abandonment.
Mechanisms of berm development and resulting beach growth along a barrier spit complex
Authors: HINE ALBERT C.
Published: June 2006

Weekly topographic profile measurements across a southward migrating recurved‐spit complex throughout a summer period have revealed three different mechanisms of berm development, each reflected by a distinctive sedimentary sequence. Each mechanism dominates berm widening along certain sections of the active spit with transition zones separating each one. Along the straight beach sections where a net longshore transport is well developed, sand accumulates at the distal high‐tide swash mark during neap tide. These sandy accumulations are neap berms which are later redistributed over the main berm by swash occurring at spring high water. The main berm grows vertically and horizontally as a result. To the south, along the middle portion of the recurved spit, swash bars or ridge‐and‐runnel systems actively develop, migrate, and weld onto the established berms. This is the second method of berm widening and results from an excess of sand carried into this portion of the spit due to the steadily decreasing transport of the longshore current system. Berm‐ridges develop along the southernmost portion of the active recurved spit and represent the third and most rapid form of beach progradation. Wide, broad swash bars build nearly up to the spring high tide level. At neap high tide, the swash cannot extend over this feature. Wave energy is expended on the seaward margin of the swash bar initially developing a low‐angle beach face. Rapidly, this beach face steepens and a new berm (beach face and berm top) is developed on top of the swash bar. This berm structure still retains much of its swash bar or ridge appearance, hence the term‘berm‐ridge'. Numerous trenches dug into the beach provide data to model the distribution of primary sedimentary structures in recurved spits. Berm‐ridges are the most important features along rapidly accreting spits, and structures associated with these features are volumetrically the most significant. Berm‐ridges also develop arcuate, vegetated ridges separated by low lying, marsh‐infilled swales. These features are commonly seen within barrier islands and designate former inlets.
Megaripples, ridges and runnels on intertidal flats of the Wash, England
Authors: MCCAVE I. N. , GEISER A. C.
Published: June 2006

Observations have been made of parts of the channels and the outer portions of the sand banks in the Wash using 1 : 10 000 air photographs and brief ground surveys at low spring tides. Two principal structures are found. In the channels megaripples 0.3–0.6 m high and of 10–15 m modal spacing are dominantly flood orientated. They do not reverse during ebb tide. On the outer parts of banks low ridges of 0.5 m height and 50‐100 m spacing are interpreted as wave‐formed ridge and runnel structure. They contain wave‐rippled mud patches in the runnels and although they have shorewards asymmetry do not appear to migrate to any great extent. It is suggested that only in sheltered areas do the Wash intertidal flats show a lower mudflat subfacies; the norm is outer bank sand flats with ridge and runnel structure eventually overlain by Arenicola sand‐flats during progradation.
Trench‐slope channels from the New Zealand Jurassic: the Otekura Formation, Sandy Bay, South Otago
Authors: CARTER ROBERT M.
Published: June 2006

The Otekura Formation (Early Jurassic, Pseudaucella zone) at Sandy Bay comprises part of a 10+ km thick, regressive, forearc shelf and slope sequence, the Hokonui facies belt of the Rangitata Geosyncline. The Otekura Formation is dominantly fine grained, being mostly mudstone, silty mudstone and siltstone. The sediments are volcanogenic throughout. The upper 150 m of the formation contains two 20 m thick, channelized bodies of medium‐thick bedded sandy flysch, each associated with thin bedded muddy flysch interpreted as overbank turbidites. Directional indicators within the channel sequence indicate emplacement from the south‐southwest. In contrast, rare turbidites that occur below the channel sequence, within the background mudstone sediment, were emplaced from the east, i.e. at right angles to the channelized flows. The immediately overlying Omaru Formation contains more abundant macrofossils, intraclastic conglomerates, and appreciable amounts of traction‐emplaced cross‐bedded sand. Bioturbated calcareous siltstones with an in situ molluscan fauna follow (Boatlanding Formation), and are of shelf origin. The Omaru Formation is therefore interpreted as a shelf‐slope break deposit, and the Otekura Formation as an upper slope facies.

Reconnaissance studies indicate that the Otekura Formation is underlain by several kilometres of dominantly fine grained, deep water slope sediments, containing occasional sand and conglomerate filled channels similar to those here described in detail from the Otekura Formation. Such channels are inferred to form when a mass‐transported sand, derived from failure higher on the slope, ploughs erosively into the sea floor. After their incision, the channels served for a short time as conduits for downslope transport of sediment, the redeposited deposits of which are found filling each channel. Both channel fills at Sandy Bay are capped by thin‐bedded turbidites inferred to have overspilled from similar channels nearby on the slope.
Undular hydraulic jumps and the formation of backlash ripples on beaches
Authors: BROOME R. , KOMAR P. D.
Published: June 2006

Field observations are made of the formation of backwash ripples on the beach face, formed by undular hydraulic jumps generated by backwash down the beach face colliding with wave bores. Measured ripple wavelengths range from set averages of 48 to 70 cm. Within a particular set of ripples the spacing tends to decrease in the offshore direction. These observations are compared with laboratory experiments where undular jumps are generated in a flume, and with a computer simulation model which calculates both the flow within an undular hydraulic jump and the resulting sediment transport which gives rise to the backwash ripples. The computer model involves a numerical solution of the Bousssinesq equations which govern the fluid flow, and sediment transport equations which relate the sand transport rate to the local mean flow velocity. The model permits a study of the detailed time‐history of the undular jump development and the formation of the backwash ripples and shows good agreement with the field observations of backwash ripples, predicting an offshore decrease in their spacings. The laboratory experiments showed a similar result so long as the Froude number of the supercritical flow before the jump occurs is small (c. 1–4). Small differences between the computer model and experiments arose principally from the neglect of internal friction and surface tension in the model.
Miocene bioherms and associated structures in the Upper Coralline limestone of the Maltese Islands: their lithification and palaeoenvironment
Authors: PEDLEY H. MARTYN
Published: June 2006

An extensive sequence of small patch‐reefs occurs within the middle member of the Upper Coralline Limestone Formation (Upper Miocene) in western Malta. In the lower horizons of the Tal Pitkal Member these structures are lensoidal in cross section and are surrounded by coarse flanking biosparites. Towards the top of the member they become more irregular in form. Extensive biostrome developments occur in association with the later structures but unlike the patch‐reefs they were killed off periodically by episodes of exposure.

The resistance of these structures to wave action is verified by the presence of extensive mollusc borings both in patch‐reefs and biostromes. The initial binders within both structures are considered to be stromatolitic algae. Early diagenetic rims were also precipitated around allochems and added further strength to the frame work. Within this framework pelleted micrites accumulated which contrast strongly with the sparite cements of the flanking sediments.

The organic framebuilders were finally killed off by a particularly strong episode of submarine erosion, with the subsequent establishment of an oolite shoal over the entire region.
Marine maceration of skeletal carbonates in the Skagerrak, North Sea
Authors: ALEXANDERSSON E. T.
Published: June 2006

Skeletal carbonates in the sediments of the eastern Skagerrak Bay in the North Sea disintegrate into their microscopic structural elements, needles, fibres, flakes, and granules. The process, called maceration, takes place while the carbonates are exposed on the sea‐floor, in contact with ambient seawater. All kinds of calcareous shells and skeletons are thus transformed into cryptic lime mud which then may be further dissolved, or redeposited elsewhere, In the area, maceration of carbonates is a major destructive agent, causing severe depletion of calcareous organisms in the fossil record. The diagenetic regime in the Skagerrak is a complete contrast to the classical warm‐sea carbonate regime so extensively studied in areas like the Bahama Banks.
A physical model for the transport and sorting of fine‐grained sediment by turbidity currents
Authors: STOW DORRIK A. V. , BOWEN ANTHONY J.
Published: June 2006

Turbidite muds in cores from the outer Scotian continental margin, off eastern Canada, contain abundant thin silt laminae. Graded laminated units are recognized in parts of this sequence. These represent single depositional events, and show a regular decrease in modal grain size and thickness of the silt laminae through the unit. A similar fining trend is shown by both silt and mud layers over hundreds of kilometres downslope. Textural analysis of individual laminae allows the construction of a dynamically consistent physical model for transport and sorting in muddy turbidity currents. Hydraulic sorting aggregates finer material to the top and tail regions of a large turbidity flow which then overspills its channel banks. Downslope lateral sorting occurs with preferential deposition of coarser silt grains and larger mud flocs. Depositional sorting by increased shear in the boundary layer separates clay flocs from silt grains and results in a regular mud/silt lamination. Estimates can be made of the physical parameters of the turbidity flows involved. They are a minimum of several hundreds of metres thick, have low concentrations (of the order of 10−3 or 2500 mg 1−1), and move downslope at velocities of 10‐20 cm s−1. A 5 mm thick, coarse silt lamina takes about 10 h to deposit, and the subsequent mud layer ‘blankets’ very rapidly over this. A complete unit is deposited in 2‐6 days which is the time it takes for the turbidity flow to pass a particular point. These thick, dilute, low‐velocity flows are significantly different from the ‘classical’ turbidity current. However, there is mounting evidence in support of the new concept from laboratory observations and direct field measurements.
Use of clay fabric to distinguish turbiditic and hemipelagic siltstones and silts
Authors: O'BRIEN NEAL R. , NAKAZAWA* KEIJI , TOKUHASHI* SHUICHI
Published: June 2006

Pliocene and Holocene siltstones and silts in outcrops from the Boso Peninsula, Japan and in cores from the East China were studied to determine distinctive characteristics of the turbiditic (Bouma E‐Division) and hemipelagic siltstones and silts. Weathering characteristics, colour, grain size, and organic carbon‐organic nitrogen ratio, plus clay fabric proved valuable in characterizing each unit. Clay and non‐clay mineral content was uniform throughout. Clay fabric differences are pronounced. Random clay flake orientation prevails in the turbiditic interval while the hemipelagic unit has more preferred orientation. The fabric reflects different conditions of sedimentation. The turbiditic clay was deposited more rapidly in the flocculated state while the interturbidite hemipelagic clay may have formed from more slowly sedimented dominantly dispersed clay. Results suggest that clay fabric may be useful in combination with other sedimentary features in the study of mud‐turbidite sedimentation.
The Kathikas mélange, SW Cyprus: late Cretaceous submarine debris flows
Authors: SWARBRICK RICHARD E. , NAYLOR MICHAEL A.
Published: June 2006

The Maastrichtian Kathikas mélange is shown to be of sedimentary origin, being a succession of undeformed, submarine, matrix‐supported debris‐flow deposits up to 270 m thick. Internal sedimentological features include beds emphasized by colour or clast size variation, pelagic chalk interbeds, planar clast fabrics and channels. A trend of upwards‐thinning beds in the mélange is interpreted as due to debris‐flow initiation on gradually increasing slopes. Debris was shed locally from the deformed and fragmented Mamonia Complex, a series of disrupted gravity‐slide sheets of Mesozoic sedimentary rocks and deformed igneous rocks. All Mamonia lithologies are represented in the mélange, and local facies variations permit identification of individual sources. The mélange probably pre‐dates emplacement of serpentinite into the Mamonia Complex. There was also local inter‐mixing of material from the adjacent and underlying Troodos sequences. The mélange rests unconformably on both Mamonia and Troodos sequences, and formed after the main deformation episode of the Mamonia Complex. The degree of resedimentation increases gradually away from the disrupted Mamonia source rocks. The thickness and volume of the Kathikas mélange are comparable with those of recent submarine debris flow deposits on unstable or seismically active continental margins.
Evidence for currents associated with waves in Late Precambrian shelf deposits from Finnmark, North Norway
Authors: LEVELL B.K.
Published: June 2006

Abundant wave ripples with wavelengths of up to 0.3 m, top‐surface granule lags and tabular sand‐stone beds suggest that the interbedded sandstones and siltstones of the Skaergårdnes Formation are shallow marine deposits. A simple orthogonal relationship between crestal trends of wave ripples and trough axes/cross‐bed dip directions indicates that bottom currents and surface waves were closely related, suggesting that wind‐drift currents were responsible for deposition. Interbedding of the tabular sandstones with siltstone is more probably due to the patchy nature of sand cover on the sea floor than to episodic sand derivation from the coast. The top‐surface granule lags indicate local, temporary excess of sand removal over supply, and were probably formed by both wave‐winnowing and non‐depositional megaripple migration. The well developed lags in this and similar ancient sediments suggest that winnowing, leading to bed armouring and sediment bypassing may be an important shallow marine process.
The pattern of natural size distributions
Authors: BAGNOLD R. A. , BARNDORFF‐NIELSEN O.
Published: June 2006

The pattern of empirical distributions, in particular size distributions, is often best brought out by drawing a log‐histogram. The Gaussian or ‘normal’ distribution furnishes a description of the empirical distribution if the log‐histogram approximates to a parabola. In many cases, however, the log‐histogram is far from parabolic but may be closely approximated by a hyperbola. It is therefore natural to consider those theoretical probability distributions for which the graph of the log‐probability (density) function is a hyperbola. The theory and applicability of such hyperbolic distributions have been the subject of a number of recent investigations and it is the purpose of the present paper to summarize these developments, with regard to the interest they may have to sedimentologists. A precise description of the hyperbolic distributions is given and their wide applicability is indicated. Methods for fitting these distributions to data are discussed and a number of sedimentological examples are presented. Furthermore, the question of finding dynamical explanations for the occurrence of the hyperbolic shape is considered from various points of view.
Criteria for the instability of upper‐stage plane beds
Authors: ALLEN J. R. L. , LEEDER M. R.
Published: June 2006

Calculations of the critical dimensionless bed stresses that obtain when upper‐stage plane beds should revert to ripple and dune bed forms are presented. Strong support is given to the Bagnold ‘universal’ plane‐bed instability criterion and to a modified criterion suggested by Allen over a wide range of solids grain size. A reinterpretation of the mechanism of plane bed instability is based upon the extent to which significant grain concentrations in plane bed flows increase apparent fluid viscosity and decrease turbulence production over potential bed defects, thereby preventing ripple or dune propagation and growth.
Isotopic and trace element evidence for submarine lithification of hardgrounds in the Jurassic of eastern England
Authors: MARSHALL JAMES D. , ASHTON MICHAEL
Published: June 2006

Geochemical and petrographic data suggest early submarine cementation of hardgrounds from the Lincolnshire Limestone Formation, Middle Jurassic, England. The three hardgrounds, from Cowthick, Castle Bytham and Leadenham quarries, developed in tidal‐inlet, on‐barrier and lagoonal sub‐environments of a carbonate barrier‐island complex. At Cowthick early composite (acicular‐bladed) radial‐fibrous cements, which pre‐date aragonite dissolution, completely fill intergranular pore‐space at the hardground surface; away from it isopachous fringing cements decrease in thickness. Microprobe analyses demonstrate zoning within the fringes with magnesium concentrations (> 2 wt % MgCO3) higher than those in allochems or later, ferroan cement (≅0.5 wt % MgCO3, 1.7 wt % FeCO3). At Castle Bytham early granular isopachous cements, which post‐date aragonite dissolution, occur within 5 cm of the surface. At Leadenham early lithification is superficial and represented by ferruginous crusts and micritic internal sediment. Late blocky cement fills residual pore‐space in all three examples. Carbon and oxygen isotopic composition of whole‐rock samples taken at intervals away from each hardground surface demonstrate the increasing proportion of late 18O depleted cements (δ18O – 8 to – 10). Early cements must have a marine isotopic composition; different δ18O values from each hardground reflect the intensity of early lithification and exclusion of late cements at the hardened surface. There is no isotopic evidence for subaerial cement precipitation during possible emergence at Castle Bytham. Oyster samples (with δ18O, – 2.9 and δ13C, 2.4) give estimated palaeotemperatures of 22–25°C. Early cements from Cowthick are enriched in 18O and 13C (δ18O = 0 δ13C ≅ 3‰) compared to the oyster values. In conjunction with trace element data this is interpreted as evidence for high‐magnesium calcite precursor cements which underwent replacement in a system with a low water: rock ratio. The intensity of early lithification is related to depositional environment: maximum circulation of sea‐water producing the most lithified hardground (Cowthick). This is directly analogous to the formation of Recent hardgrounds.
Participation of fresh water in chert diagenesis: evidence from oxygen isotopes and boron α‐track mapping
Authors: KOLODNY YEHOSHUA , TARABOULOS ALBERT , FRIESLANDER URI
Published: June 2006

Sedimentary chert breccias are the product of two stages of silicification: one which produced ‘fragments’, the other, during which the ‘matrix’ crystallized. Fragments and matrix of twenty‐five breccias from the Senonian Mishash Formation in Southern Israel were separated and analysed for δ18O in quartz. The matrix is significantly depleted in 18O in comparison with fragments (δ18O of fragments ranges between +29.4 and +34.7‰, for matrix the range is +21.1–32.6‰). Boron has been mapped in the same rocks using the 10B(n, α)7Li reaction and α‐track etching. Fragments are significantly enriched in boron in comparison to the matrix. The correspondence between boron maps and rock texture is perfect.

These results when combined with Steinitz's (1977) report of evaporites and evaporite ghosts in the same rocks are interpreted as indicating two different environments of chert diagenesis: a first, normal marine to hypersaline stage during which the microquartz in fragments crystallized, and a second stage of matrix chertification which in most cases occurred when fresh water was interacting with the crystallizing sediment. These conclusions are corroborated by isotopic analysis of carbonates which coexist with the cherts, as well as stratigraphical and palaeontological analysis.
Clay orientation in some Upper Carboniferous mudrocks, its relationship to quartz content and some inferences about fissility, porosity and compactional history
Authors: CURTIS C. D. , LIPSHIE S. R. , OERTEL G. , PEARSON M. J.
Published: June 2006

The degree of preferred orientation of clay minerals in a series of fifteen fine‐grained sediments has been precisely measured by transmitted X‐ray goniometry. This varies systematically with quartz content. By contrast it is apparently unrelated to fissility since the one fissile sample (a ‘paper shale’) occurs at the mid‐point of the sample series ranked in order of degree of preferred orientation. In this series fissility corresponds with the presence of fine‐scale lamination. Estimates of compaction strain using the theory of March (1932) and assuming random initial clay orientation are consistent with the view that these samples started life with porosities similar to those of present‐day muddy sediments. On this basis it is concluded that preferred orientation in clay‐rich sediments results almost entirely from compaction strain. The degree to which this can be attained is limited by the presence of nonplaty particles (such as quartz grains) which prevent planar fabric development in their immediate vicinity.
Petrology of recent caliche pisolites, spherulites, and speleothem deposits from central Texas
Authors: CHAFETZ HENRY S. , BUTLER JOHN C.
Published: June 2006

Recent caliche, including nodules, pisolites, crusts, internal sediment, speleothem deposits, and spherulites, has formed within the dolomitic Cretaceous Edwards Formation of central Texas. As weathering altered the host strata, rhombic crystals of calcite were precipitated concomitantly with dissolution of the dolomite, thereby forming nodules. The highly altered dolomite (i.e. pulverulite) was then removed and spar, internal sediment, and travertine accumulated in the internodular voids. Nodular masses of calcite and dolomite are the most prominent constituent of the caliche. Some of the nodules have a well developed concentric structure as well as other characteristics similar to hypersaline pisolites. Features which appear to be useful in distinguishing caliche from hypersaline pisolites are: regional geological setting, association with other caliche and palaeosoil deposits, types of fossils present, and the presence of rhombic calcite and/or bladed sparry calcite with triangular shaped cross‐section. A brick‐like calcite texture and relict aragonite rays characterize hypersaline pisolites. Incipient neomorphism of the nodules and pisolites has resulted in the development of a radial pattern of spar within these structures. Geopetal deposits of internal sediment, including terra rossa soil, inhibited spar growth in the upward direction; consequently, spar is much better developed on the undersides of pisolites. Crusts and travertine flowstone (speleothem) deposits are intimately associated with the nodular masses and internal sediment. The brecciated thin crusts and travertine flowstone are end products of the same processes. The crusts formed during times of periodic desiccation of the growing surface while the flowstone formed when water was relatively abundant. Spherulitic bodies within the caliche, commonly 1–2 mm in diameter, display a radial texture and yet are composed of single crystals of calcite. The structures are the product of neomorphosed Microcodium or Microcodium‐like globular bodies.
Texture and structure of resedimented conglomerates: examples from Książ Formation (Famennian—Tournaisian), southwestern Poland
Authors: NEMEC W. , PORĘBSKI S. J. , STEEL R. J.
Published: June 2006

The Famennian‐Tournaisian conglomerates and sandstones of the Ksiaz Formation are interpreted as marine resedimented deposits. Matrix‐ and clast‐supported conglomerate beds are equally common, and two textural sequences (motifs) have been recognized: (I) matrix‐rich conglomerate → pebbly sandstone → sandstone, and (II) clast‐supported conglomerate → sandstone. Variation in clast type partly controls motif type, and therefore, to some extent, matrix percentage in the conglomerates generally. Grading is extremely common in both clast‐ and matrix‐supported conglomerates: inverse (19%), inverse‐to‐normal (14%) and normal (26%). The studied succession, itself part of a 4 km thick, fan delta, basin‐fill sequence, is organized into large (110–150 m) and small‐scale (5–30 m) sequences, both of which show (1) upward coarsening and thickening, (2) upward trend of sandstones and pebbly sandstone → matrix‐rich conglomerates → clast‐supported conglomerates and (3) a less clear upward tendency of massive and normally graded beds → inversely graded beds. Variation in matrix percentage in beds is therefore also partly controlled by fan processes, during the progradation of fan bodies and lobes. It is predicted that individual resedimented conglomerate beds or motifs show general downfan trends in thickness, texture and structure opposite to those evident in the vertical sequences.
A late Precambrian tidal shelf deposit, the Lower Sandfjord Formation, Finnmark, North Norway
Authors: LEVELL B. K.
Published: June 2006

The Lower Sandfjord Formation is a 1.5 km thick late Precambrian sandstone. It is a remarkably homogeneous unit consisting largely (98%) of cross‐bedded, texturally and mineralogically mature, coarse or medium sandstone, and is interpreted as a shallow marine deposit. This interpretation is based on the maturity, the exclusively tabular bed geometry, occasional sets of herring‐bone cross‐bedding and most importantly, the abundance of sheet‐like pebble layers only 1–5 grain diameters thick and sometimes overlain by thin siltstone drapes. Various different types of compound cross‐bedding, all of which show evidence of reversing currents, are interpreted as sub‐tidal sandwaves. These sets range in thickness from 0.5 to 14 m, and in conjunction with the overall abundance of cross‐bedding probably indicate strong tidal currents. A tide‐dominated current regime is also considered essential to explain the derivation of such large quantities of sand from the contemporary coasts. It is suggested that sand transport offshore took place during the erosional transgression of abandoned delta lobes. However, the predominance of a single, easterly, mode in the palaeocurrent patterns suggests that the tidal currents were reinforced by some other current system. The predominantly unimodal palaeocurrent patterns and the coarse, sand‐rich nature of the succession, taken together with the thickness do not superficially seem likely characteristics for a shallow marine sequence. Nevertheless this study appears to demonstrate that such deposits were formed on tidal shelves in at least late Precambrian time.
Pellets, ooids, sepiolite and silica in three calcretes of the southwestern United States
Authors: HAY R. L. , WIGGINS BRIAN
Published: June 2006

Pellets and ooids are widespread and locally abundant in mature calcrete profiles in the Argus Range, California; near Wickieup, Arizona; and in Kyle Canyon, Nevada. Most concentrations of pellets and ooids either overlie laminar calcrete at various levels in the calcrete profile or fill subhorizontal fractures in the petrocalcic horizon. In all three profiles the petrocalcic horizon has been thickened by the pelletal, chemically deposited fracture fillings. Pellets range from 0.02 to 8.0 mm in diameter and consist principally of micritic calcite and sepiolite. Ooid coatings are chiefly calcite and opal or calcite and sepiolite. The pellets represent small concretions, some of which grew by accretion, either in void space or by displacing adjacent sediment, and the others of which were formed by cementation of pellet‐shaped bodies of porous micrite. Ooid coatings with opal or sepiolite may have been deposited as a gel with sufficient strength for surface tension to thin the coatings over angular corners of nuclei so as to increase the roundness and sphericity of the particles. Major problems in calcrete genesis are (1) the cause of subhorizontal fractures and the mechanism for widening a fracture as sediment accumulates in it and (2) what determines the deposition of calcite, sepiolite, and opal as pellets and ooid coatings or as laminar layers.
Rhizoliths in terrestrial carbonates: classification, recognition, genesis and significance
Authors: KLAPPA COLIN F.
Published: June 2006

Rhizoliths are defined as organosedimentary structures resulting in the preservation of roots of higher plants, or remains thereof, in mineral matter. They are abundant and characteristic features of Quaternary terrestrial carbonates (calcretes and aeolianites) from coastal regions of the western Mediterranean. Field and petrographic observations indicate that five basic types of rhizoliths can be recognized: (1) root moulds, which are tubular voids that outline positions of former, now decayed roots; (2) root casts, which are sediment‐ and/or cement‐filled root moulds; (3) root tubules, which are cemented cylinders around root moulds; (4) rhizocretions s.s., which are pedodiagenetic mineral accumulations (here low magnesian calcite) around living or dead plant roots; and (5) root petrifactions, which are mineral impregnations or mineral replacements of organic matter whereby anatomical features of roots have been preserved partially or totally. Apart from rhizoliths themselves, roots of higher plants are responsible for the formation of numerous and characteristic features of pedogenetically affected terrestrial carbonates. Plant roots are responsible for, or contribute to, the formation of alveolar textures, in situ brecciation (rhizobrecciation) textures, horizontal sheet cracks, vertically elongate glaebules (concretionary soil structures) and micritization (rhizomicritization) within terrestrial carbonates. Rhizoliths, together with the above features, are products of pedodiagenesis. More significantly, rhizoliths and related features are indicators of palaeosols and hence of subaerial vadose environments in ancient (post‐Silurian) successions.
Stages in a Precambrian dolomitization, Scotland: cementing versus replacement textures
Authors: FAIRCHILD IAN J.
Published: June 2006

Dolomitization in member 1 of the Dalradian Bonahaven Formation of Islay was sequential. The sediments studied are siliciclastic tidal flat deposits, which originally contained pebbles of ferroan dolomicrite and sandy dolomicrite (type 1 dolomite). Following burial, both dolomicrite and clay‐stone pebbles, and possibly other carbonates, were replaced by coarser, fluid‐inclusion‐bearing, chemically zoned (type 2) dolomite. Ferroan, inclusion‐free, intergranular dolomite (type 3) in sandstones may be synchronous with late zones of type 2 dolomite which it closely resembles. Cathodoluminescence and microprobe analyses of type 2 dolomite characterize four ubiquitous growth stages within which there was considerable fluctuation in Fe and Mn content. There was much variation between and within samples in terms of timing of nucleation, absolute speed of growth and relative growth rate of crystals. Type 2 dolomite displays two types of intracrystalline truncation surfaces explicable by dissolution followed by overgrowth, or by replacement by a moving solution film. Type A is characterized by a single later zone cutting across earlier ones along the line of an irregular truncation surface which developed from the edges of the crystals. In type B truncations it is the interior, rather than the edges of crystals which show truncation, the truncations forming smooth closed surfaces which enclose areas filled centripetally with new zones. In general, dolomite has a pronounced tendency to form crystal faces, and thus compromise crystal boundaries, during replacement as well as during cementation. Thus, if nucleation of crystals is restricted to the margins of an area to be replaced, replacive dolomite is capable of completely mimicking a vug‐filling texture.
Quaternary pedogenic calcretes from the Kalahari (southern Africa): mineralogy, genesis and diagenesis
Authors: WATTS N. L.
Published: June 2006

The calcretes of the Kalahari are amongst the thickest in the world representing pedogenic episodes in a semi‐arid climate during Pliocene to Recent times. The descriptive morphological terminology of Netterberg is used to describe the calcrete types and a differentiation into simple and composite profiles is made. A pedogenic/diagenetic scheme has been constructed using all available data. Early calcite cementation is induced by two mechanisms. Slow evaporation and/or CO2 loss causes the precipitation of low‐Mg calcite, whereas rapid evaporation and/or CO2 loss precipitates predominantly high‐Mg calcite, in thermodynamic disequilibrium with the low Mg/Ca ratio vadose water, and minor aragonite (which transforms rapidly to low‐Mg calcite). High‐Mg calcite is also precipitated from high Mg/Ca ratio vadose waters in calcretes developed on Mg‐rich host lithologies and by capillary rise from shallow groundwaters in saline depressions. Calcite precipitation may be passive (cement), displacive or replacive, in the latter released silica migrating down‐profile to precipitate length‐slow chalcedony, clinoptilolite (saline conditions), length‐fast chalcedony and megaquartz (non‐saline conditions). Displacive introduction of calcite takes place from highly supersaturated solutions due to rapid evaporation (with CO2 loss) of vadose waters. During low‐Mg calcite precipitation (in a ‘closed’ system) the Mg concentration of the resulting solution increases. This, combined with Mg released during high‐Mg to low‐Mg calcite transformation, induces precipitation of authigenic palygorskite, sepiolite and minor dolomite. Vadose dolomite is often present whilst some dolomite may be precipitated at the mixing‐zone of vadose and phreatic waters. The proposed model applies to the Kalahari calcretes, although it may perhaps be extrapolated to other areas. Further detailed studies, involving analyses of pore water chemistry, soil microclimate, and trace element and isotopic analyses of individual cements are necessary.
A meander‐belt sandstone of the Lower Cretaceous of Southern England
Authors: STEWART D. J.
Published: June 2006

The meander‐belt deposit comprises a sandstone resting on an erosion surface and bounded above and below by massive varicoloured mudstones with rootlet traces. The sandstone unit is composed of six bodies separated from one another, horizontally, by erosion surfaces; together the bodies form a single multilateral sand body. Internally each body is composed of lateral accretion units inclined at up to 6° from the horizontal. Vertical sequences of facies show significant variations but the grain size generally fines upwards. The principal lithofacies within the sandstones are, in common ascending order, intraformational conglomerate, large‐scale cross‐bedded, horizontal bedded and small‐scale cross‐laminated sandstone, and alternate sandstones and mudstones. Current directions are normal to the true slope of accretion surfaces and show insignificant scatter within individual bodies but are very diverse overall. Five of the sand bodies are believed to represent individual point bars, and one body an abandoned channel. Together they comprise the meander belt. The river was subject to very variable discharges and carried high suspended loads. Analysis of vertical profiles indicates that grain size segregation along the length of the point bars caused differentiation of the bars into coarse‐grained heads and sandy tails.
Some new aspects of two‐dimensional turbidity currents*
Authors: LÜTHI STEFAN
Published: June 2006

A theoretical consideration of two dimensional underflows and surge‐type turbidity currents results in a general momentum equation. A number of formulae in current use are special cases of this equation, among which are the modified Chézy equation and Bagnold's criterion for autosuspension. Five dimensionless parameters are included: the Richardson number Ri (defined as the inverse square of the Froude number), the friction coefficient cf, the slope β, the dimensionless settling velocity of the sediment Vs/u and the changes in flow height with distance dD/dx. The latter is mainly a measure of the dilution by entrainment of ambient water.

For chalk powder experiments on surge type turbidity currents and on the initial front of continuous underflows the momentum equation is shown to be correct. Values for Ri range from about 1.5 at 0° slope to about 0.75 at 5° and are slightly to substantially lower than values from earlier authors. The two types of turbidity currents investigated show close similarity. A surprising attribute is their strong dilution even at very low‐angle slopes. Pelitic sedimentation is possible from the upper, dilute part of the currents, graded intervals found at the base of turbidites can be explained as bedload deposits from the lowermost, concentrated layer of the current; hydraulic jumps are expected to be rare in surge‐type turbidity currents and fronts of incipient underflows.
Particulate matter grain‐size characteristics and flocculation in a partially mixed estuary
Authors: KRANCK KATE
Published: June 2006

The concentration and grain size of the natural and deflocculated inorganic suspended particulate matter were measured along the length of the Miramichi Estuary and interpreted with respect to flocculation and transport properties. Changes in particulate matter concentration are associated with regular changes in grain‐size characteristics. In the turbidity maximum region of the estuary the suspended matter occurs mostly as large flocculated particles whereas, in the waters with lower particle concentrations, a larger proportion of the material occurs as fine material. At higher concentrations natural floc modes and inorganic grain modes vary simultaneously but at low concentrations the two modes vary inversely. This modal relationship and the variation in organic matter within the estuary is proposed to result from variation in inorganic—organic composition of flocs. Increase in settling rates due to flocculation is believed to increase the trapping effect of the estuarine circulation that produces the turbidity maximum.
Comparison of methods of size analysis for sands of the Amazon‐Solimões Rivers, Brazil and Peru
Authors: SEMINAR SEDIMENTATION
Published: June 2006

Cumulative frequency curves and bivariate textural plots of sands collected and sieved from the medium‐grained (1.94), well‐sorted (0.46) Amazon‐Solimões Rivers are not environmentally sensitive. Only 13 % of the samples plot as fluvial using cumulative frequency curves, and only one of the nine bivariate plots successfully identified the fluvial environment. Thus, identification of depositional environments of sands by size distribution based only on sieving is not a reliable technique—even when the results of diagenesis are not considered.
Petrology of glacial sand: implications for the origin and mechanical durability of lithic fragments
Authors: SLATT ROGER M. , EYLES NICHOLAS
Published: June 2006

The petrology of first cycle (unmodified) and second cycle (reworked) sand at the termini of eleven valley glaciers eroding coarse‐ to fine‐grained bedrock types is determined in order to evaluate the origin and mechanical durability of lithic sands. First cycle sands are coarse‐ to medium‐grained, poorly sorted, fine‐skewed, non‐modal lithic sands with an average composition of Q21F6L73. Grain‐size distributions do not vary with composition or source rock types, although sands derived from finer grained source rocks contain more lithic fragments than sands from coarser grained sources. By contrast, second cycle sands are medium‐grained, poorly sorted, fine‐ to coarse‐skewed arkosic to lithic sands with an average composition of Q19F40L41, and contain fewer lithic fragments than do first cycle sands.

We propose that the origin, mechanical durability and survival potential of lithic fragments are related to the types and abundances of their internal planes of weakness, and the particular stress field of the transporting medium. As a result of abrasion, glacial clasts and lithic fragments are subjected to shear stress so that fractures propagate along intracrystal and intercrystal planes of weakness resulting in a continuous grain‐size spectrum of lithic fragments and monomineralic grains irrespective of the type of source rock. Upon subsequent aqueous transport in the proglacial environment lithic fragments are subjected to point‐loading during saltation in addition to shear stress. Point‐loading produces extensional fractures which preferentially propagate along intercrystal planes of weakness, so the lithic fragments break into smaller monomineralic grains and/or lithic fragments.

Lithic fragments of very coarse sand‐size are abundant in first cycle sands, thus refuting, at least for glacial sands, the widely held view that grains of this size are deficient in nature. The presence of fewer grains of this size in second cycle sands indicates that very coarse sand‐size lithic fragments can survive recycling, but in greatly reduced proportions due to breakage.