bluuebunny/crossref_metadata_2025_split huggingface dataset:
Miocene goethitic and chamositic oolites, northeastern Colombia
Authors: JAMES HAROLD E. , VAN HOUTEN FRANKLYN B. 
Published: June 2006

ABSTRACTThe 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

ABSTRACTThe 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

ABSTRACTPetrographical 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

ABSTRACTThree 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

ABSTRACTWeekly 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

ABSTRACTObservations 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

ABSTRACTThe 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

ABSTRACTField 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

ABSTRACTAn 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

ABSTRACTSkeletal 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

ABSTRACTTurbidite 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

ABSTRACTPliocene 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

ABSTRACTThe 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

ABSTRACTAbundant 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

ABSTRACTThe 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

ABSTRACTCalculations 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.