bluuebunny/crossref_metadata_2025 huggingface dataset:
Lithologic transition and bed thickness periodicities in turbidite successions of the Antola Formation, Northern Apennines, Italy
Authors: MARTINI I. PETER , SAGRI MARIO , DOVETON JOHN H.
Published: June 2006

The Antola Formation of Upper Cretaceous age crops out extensively in the Northern Apennines and consists of graded units of calcareous sandstones, sandstones, marlstones, and shales. It can be subdivided into the Cerreto, Antola Marlstone, Bruggi, and S. Donato Members on the basis of bed thicknesses and percentage of shales. Although the whole formation is interpreted as a deep‐sea basin plain deposit, the members constitute lateral facies subdivisions which range from proximal, thick‐bedded turbidities that show a prevalence of thinning upward cycles in bed thicknesses to distal turbidites that show predominantly thickening upward cycles and have a high percentage of shale.

Repetitive patterns in the lithological sequence of the turbidite association are generally distinctive and are satisfactorily described as first order Markov chains. Only the Antola Marlstone Member has an additional second order Markov property. Imaginary eigenvalues of the transition probability matrices of all but the Bruggi Member demonstrate a strong cyclic character in the lithologic ordering within the formation. The behaviour of the Antola Marlstone and of the Bruggi may reflect the influence of a secondary ophiolitic intra‐basinal source of clastics that contributed sandy turbidites and olistostromes.

Systematic long‐term variations in the sequence of bed thickness development in some sections of the Antola Formation are often subtle and equivocal, and pose special problems in interpretation. Fourier analysis was applied to the task of partitioning fundamental wavelengths from “background noise” introduced by essentially random depositional processes. In all members there is (1) strong short‐term wavelength of two to three beds indicative of alternating thin and thick beds and judged to be typical of turbidite sequences; (2) an intermediate wavelength ranging from about five beds (proximal facies), eight beds (distal) to nine beds (very distal), which have both thinning and thickening upward trends, interpreted respectively as valley fill due to shifting talwegs of low density turbidity currents, and to progradational, flat turbidite lobes; (3) a poorly defined long‐term wave‐length of from thirty to greater than sixty beds that may be related to an unspecified trend in the evolution of the sedimentary basin. Phase angles associated with the coniputed power spectra give indications as to the asymmetry (thickening or thinning upward) or symmetry of the representative units.
Quaternary iron‐manganese deposits and associated pelagic sediments (radiolarian clay and chert, gypsiferous mud) from the Tyrrhenian Sea
Authors: CASTELLARIN ALBERTO , SARTORI RENZO
Published: June 2006

Various types of pelagic sediments occur on a dolomitic basement located at some 1000 m depth on the eastern slope of the Tyrrhenian Sea, i.e. in the most internal, collapsed and presently submerged portion of southern Apennines. The deposits include laminated limonites, muds with manganosiderite nodules, radiolarian clays, opal chert, gypsiferous muds, lutites with calcareous plankton, and all are of Quaternary age. They are interpreted as products of the interactions between submarine hydrothermal activity and deep‐sea sediments, and represent the first documented case of submarine hydrothermal sediments not directly connected to active ridges or volcanic buildings and deposits. The iron‐rich sediments are very similar to many other reported examples of submarine thermal activity. The radiolarian clay displays very abundant and almost exclusively radiolarian tests, and no calcareous fossils, but shows numerous dissolution traces of carbonate skeletons. It has been interpreted as deriving from a primary bloom of siliceous plankton followed by an extensive leaching of the calcareous tests on the seafloor. The radiolarian chert shows a very early and yet unknown Opal‐A cement. The immediate source of the cement is biogenous silica, whereas the ultimate source is the bloom of siliceous plankton triggered by the hydrothermal and volcanic activity. The gypsum muds are the result of a hydrothermal chemical remobilization of the Messinian sulphate beds which overlie the dolomitic basement.
Fair weather and storm sand transport on the Long Island, New York, inner shelf
Authors: LAVELLE J. W. , SWIFT D. J. P. , GADD P. E. , STUBBLEFIELD W. L. , CASE F. N. , BRASHEAR H. R. , HAFF K. W.
Published: June 2006

Both spring‐summer and fall‐winter sand transport have been observed on the Long Island, New York, inner shelf at water depths of 20‐22 m using a radio‐isotope sand tracer system. The extent of dispersal of the tagged, fine sand was measured at 3 week intervals in two 70 day experiments. In the late spring and early summer, movement was primarily diffusive in nature, extending 100 m around the line of tracer injection, while late fall‐winter patterns had strong advective features, including an ellipsoidal outline extending approximately 1500 m westward of the injection points after the passage of several storms with strong northeasterly winds. Near‐bottom current observations made with Savonius rotor sensors identify the event responsible for the bulk of the transport over the 135 day observation period as a storm flow of 2 days duration. Tracer and current observations together suggest that westward winter storm flow along the Long Island shelf is the major mechanism of sand transport at these depths on a yearly time scale. A least‐squares fit of several of the observed winter patterns with a plume model yields average sediment mass flux lower bounds of 3.2 × 10−3 gm/cm/sec and 1.7 × 10−1 gm/cm/sec for ‘typical’ and extreme winter storm activity.
Estuarine sedimentation in the Eocene of southern England
Authors: GOLDRING R. , BOSENCE D. W. J. , BLAKE T.
Published: June 2006

A temporary section in the Cuisian Bagshot Beds, which has been mapped in detail, displayed estuarine sediments with interlayered sands and muds, fine sands, channel‐fill sands and intraformational (mainly mud clast) conglomerates. The facies show rapid lateral and vertical changes in grain size and bed form and a restricted suite of trace fossils including Ophiomorpha nodosa and Arenicolites sp.

The sequence is shown to have been deposited in a subtidal channel where tidal, wave and fluvial processes were dominant at different times. The following points are considered to be characteristic of sedimentation in this environment: (i) correlation is difficult and facies predictability is low; (ii) there are frequent lateral facies changes from the channel to the subtidal bank environment; (iii) pene‐contemporaneous erosion removes considerable amounts of sediment; (iv) load structures may be exposed, eroded, buried and reactivated; and (v) muddy layers and bioturbated horizons offer similar resistance to penecontemporaneous erosion.
Analysis of log‐probability plots of recent Atlantic sediments and its analogy with simulated mixtures
Authors: BEIN AMOS , SASS EYTAN
Published: June 2006

The log‐probability plots of some shallow to deep Atlantic sediments are analysed and compared with simulated mixtures of log‐normal populations. The plots consist characteristically of two smoothly connected segments which display different slopes. The point of maximum curvature (PMC) between the two segments is shown to reflect the granulometric parameters (mean and sorting) and quantitative proportions of two mixed overlapping populations. This understanding is contrary to the concept which relies heavily on the break points of cumulative plots as indicating truncation of grain size distributions and therefore reflecting hydrodynamic processes.

The particular behaviour of the cumulative plots from the Atlantic indicates fining of the two component populations with depth and distance from the shore, accompanied by an increasing dominance of the finer population. This behaviour is compatible with sorting processes which operate in sediments transported through individual grain movements, rather than through mass transportation or turbidity currents.
Modern sediments and sedimentary processes in Lake Rudolf (Lake Turkana) eastern Rift Valley, Kenya
Authors: YURETICH RICHARD F.
Published: June 2006

Tertiary sediments around Lake Rudolf (now Lake Turkana) in the East African Rift Valley have yielded abundant palaeontological and palaeoanthropological remains. The present study provides a basis for interpreting the ancient lake environment and furthering our knowledge of rift valley lacustrine deposits.

Bottom sediments in Lake Rudolf are fine‐grained (average 71% clay) well laminated and have montmorillonite, kaolinite and illite as the principal clay minerals. The sediments are relatively poor in silica (40–45%) but rich in Fe2O3 (10%). Both mineral proportions and chemical composition change systematically over the area of the lake and delineate four sedimentological provinces: (1) iron‐rich, silty kaolinitic muds (Omo Delta); (2) iron‐rich, fine‐grained montmorillonite muds (North Basin); (3) silty montmorillonite muds rich in Na2O and K2O (Central Delta); and (4) argillaceous calcite silts (South Basin). Omo Delta and North Basin sediments are derived from the volcanics of the Ethiopian plateau; the source of Central Delta sediments is the Precambrian metamorphic terrain of the rift valley margin; the South Basin has a restricted detrital input.

The water in the lake is alkaline (pH 9.2) and moderately saline (TDS = 2500 p.p.m.). Comparisons with influent water from the Omo River indicate a 200‐fold concentration for the lake water. Models based on equilibrium between sediments and water column account for most of the non‐conservative chemical components in the lake water.

Sedimentation rates are high (about 1 m per 1000 years) and the dominance of detrital sediments makes Lake Rudolf unusual in comparison with other closed‐basin lakes in the African Rift Valley although some similarities with ancient rift valley deposits are suggested.
Description and hydraulic interpretation of grain size cumulative curves from the Platte River system
Authors: VIARD J. P. , BREYER JOHN A.
Published: June 2006

Log‐probability plots of grain‐size distribution from the Platte, North Platte, and South Platte rivers are composed of four or five straight line segments. The line segments are grouped, dividing each curve into three regions. These regions are interpreted as subpopulations moved by different transport mechanisms. Consideration of the criterion for suspension and calculation of shear velocities associated with dominant discharges support this interpretation. The grain size cumulative curves are similar to each other but distinct from curves of fluvial systems transporting only fine‐grained material, the difference being the presence of a subpopulation of grains moved in traction transport. One of two possible relationships seems to exist between the grain‐size distributions and flow conditions within the Platte River system. Estimated shear velocities derived by varying flow conditions within reasonable limits predict a range of grain sizes within which the break between the intermittent suspension and traction loads should occur. This break appears to be associated with intermediate shear velocities if truncated normal distributions are assumed; but if overlapping distributions are assumed, the ‘break'is associated with estimated maximum shear velocities.
Development of relief on a Middle Jurassic cemented sea floor: origin of submarine pseudo‐anticlines in the Bathonian of Normandy
Authors: FÜRSICH F. T. , PALMER T. J.
Published: June 2006

On the foreshore east of Luc‐sur‐Mer, Normandy, an Upper Bathonian hard‐ground which caps a sponge biostrome has been thrown into a series of troughs and ridges, the latter sometimes with very steep flanks. Faunal orientation and encrustation of extensive undercuts demonstrate that these structures are of a syn‐sedimentary origin. They are interpreted as submarine pseudo‐anticlines which have a composite origin. The following three factors are thought to have contributed to their formation: (a) a primary morphological differentiation of the underlying biostrome; (b) sediment expansion caused by force of crystallization during hardground formation; and (c) differential compaction, being greater where the hardground overlies mainly inter‐reef mud as compared with the rigid sponge framework. The main episode of this compaction occurred at an early stage in the life of the hardground. Subsequent erosion, colonization by encrusters and borers, and burial beneath a fossiliferous clay were the final stages in the history of these pseudo‐anticlines.
Sedimentation processes in a short residence‐time intermontane lake, Kamloops Lake, British Columbia
Authors: PHARO C. H. , CARMACK EDDY C.
Published: June 2006

Kamloops Lake in central British Columbia is a deep, intermontane lake fed by the strong and seasonally variable flows of the Thompson River. Considerations of lake‐river interaction, supported by physical and geological evidence, suggest that sediment transport and deposition within the lake is controlled by three interdependent but distinct processes: delta progradation at the lake‐river confluence which results in delta topset and foresee bedding; sediment density surges originating along the delta face which result in turbidite sequences lakeward from the base of the delta; and dispersal by the interflowing river plume which, due to Coriolis effects, results in a higher sedimentation rate and greater fraction of coarser material along the right‐hand side (Northern Hemisphere) of the lake in the direction of flow.
Downward development of overgrowths from echinoderm fragments in a submarine environment
Authors: GÖRÜR NACI
Published: June 2006

Fragments of echinoderms in the Karaisali Limestone (Miocene) of the Adana Basin, Turkey, show large syntaxial overgrowths which are restricted mostly to their undersides, whilst the upper surfaces are covered by micrite. Although this pendant style of the overgrowths is reminiscent of vadose zone ‘gravitational cement', they appear to have formed in the open voids of sediments in the marine environment, simultaneously with, as well as after, the internal deposition of micrite.
Origin and diagenesis of lacustrine algal bioherms at the margin of the Ries crater, Upper Miocene, southern Germany
Authors: RIDING ROBERT
Published: June 2006

The marginal carbonate facies of the Miocene Ries meteorite crater lake in southern Germany contain bioherms up to 7 m high and 15 m across built by the green alga Cladophorites. The algae were externally encrusted during life by micrite, probably precipitated in response to photosynthetic uptake of CO2, which produced tufts of fine (100 μm diameter), calcareous tubes. Coalescence of tufts, together with incorporation of peloidal and skeletal sand, created nodules and cones of algal tufa which in turn formed larger masses some of which are in the form of compound cones up to 2 m high. The bioherms are constructed by beds and groups of these cones and masses, and are surrounded by poorly cemented peloid, ostracod and gastropod sands. Five depositional and diagenetic stages of development can be distinguished: (1) growth and calcification (probably calcitic) of Cladophorites in shallow fresh‐ or slightly brackish water; (2) emergence due to a temporary fall in lake level and veneering of the algal tufa and adjacent sediments by laminated sinter; (3) resubmergence and deposition of peloidal and skeletal sands; (4) burial and partial phreatic dolomitization, together with dissolution of aragonite and penecontemporaneous deposition of thin isopachous rims of dolomite rhombs; (5) local vadose cementation by rhombs, spar and spikes of low‐magnesian calcite. Stages 1–3 probably occurred several times. Most of the biohermal and surrounding sediments were produced by biological processes in the lake; subaerial sinter deposition and meteoric cementation have contributed relatively minor amounts of material and the majority of the sediments retain porosities of 10–30%.
Upslope flow of turbidity currents on the northwest flank of the Cearà Rise: western Equatorial Atlantic*
Authors: DAMUTH JOHN E. , EMBLEY† ROBERT W.
Published: June 2006

A piston core (RC16‐57) raised from the northwestern flank of the Ceará Rise contained several turbidites up to 62 cm thick with grain sizes ranging from clay to coarse sand. These turbidites were similar in composition to terrigenous turbidites found throughout the Amazon Cone, continental rise and abyssal plains of the western Equatorial Atlantic. The core site (RC16‐57) on the Ceará Rise, however, was 156 m above the level of the adjacent Amazon Cone (the source of the turbidites). Thus the turbidity currents which deposited these beds apparently had to flow upslope for 17 km to reach the core site. Sub‐bottom reflectors observed on a 3.5 kHz echogram that extended from the Amazon Cone upslope past the core site suggested that these and deeper turbidites extended from the cone up the rise flank to distances of up to 40 km from the cone/rise boundary and to elevations up to 400 m above the level of the cone at the base of the rise. An equally plausible explanation could be that the turbidity currents that deposited these sediments were in excess of 400 m in thickness and thus would not require uphill flow to reach their observed location on the rise flank. The absence of terrigenous turbidites from the bases of topographic knolls on the continental rise and abyssal plains throughout the western Equatorial Atlantic indicated, however, that turbidity currents were normally less than 100 m thick and hence would seem to rule out this explanation. The average gradient of the rise flank in this region was about 1 : 1000 (\sim 0.5°).
Wave‐induced liquefaction: a modern example from the Bay of Fundy
Authors: DALRYMPLE ROBERT W.
Published: June 2006

Soft‐sediment deformation features occur commonly on parts of intertidal sand bodies in Cobequid Bay, Bay of Fundy. These features are small‐ to intermediate‐sized, slump‐like bodies, 1‐3 m2 in area and located on the crest and upper stoss side of ebb megaripples. External modification of these slumps indicates that they formed before complete emergence. The deformed cross‐bedding within these bodies extends to a depth of 0.15‐0.35 m and shows that deformation occurred during slumping and flowage of liquefied sand down the megaripple stoss side. Field evidence and calculations strongly indicate that this liquefaction results from the impact of 0.1‐0.3 m high waves breaking against the megaripple lee faces. Neither rapid drawdown of the water level nor earthquake shocks are reasonable alternative explanations.

Indigenous wave activity provides an attractive substitute to tectonism as an explanation of soft‐sediment deformation in ancient shallow‐water sediments. Slow wave‐induced compaction may also account for the relative scarcity of deformation structures in shallow marine sandstones.
Point bar and floodplain formation of the meandering Beatton River, northeastern British Columbia, Canada
Authors: NANSON GERALD C.
Published: June 2006

This study examines the morphology, sedimentology and genesis of the point bars and floodplain of the Beatton River. The formation of point bars occurs in distinct stages. An initial point bar platform composed mainly of coarse sediment is formed adjacent to the convex bank of a migrating meander bend, and is the base on which develops a single scroll bar of fine traction and suspended load. With continued sedimentation, the scroll bar grows, eventually supporting vegetation and becoming a floodplain ridge. Scroll bars form with greatest size and frequency in rapidly migrating bends, and the shape of the meander bend appears to determine both the location of the initial bar deposit, and its direction of growth up or downstream. Approximately one‐half of the floodplain sediment is derived from suspended load, and the initiation of a scroll bar appears to be due to excessive deposition of suspended load in a zone of flow separation over a point bar platform. The critical flow condition for the initiation of a scroll bar does not occur with the same recurrence interval on different shaped meander bends, however, the average recurrence interval within the study reach is approximately every 30 years. Sedimentation rates on point bars and on the floodplain indicate two relatively distinct stages of floodplain alluviation. The most rapid is for surfaces less than 50 years old, although sediment accumulation still persists on surfaces up to 250 years in age. Although frequently flooded, surfaces older than this accumulate very little sediment. Despite 2–3 m of overbank deposition, the amplitude of floodplain ridges is maintained by secondary currents which sweep sediment from the swales towards the ridge crests.
Surge marks: are they water‐escape structures?
Authors: DALRYMPLE ROBERT W.
Published: June 2006

In his recent paper on surge mark formation and morphology, Bull (1978) has provided considerable new and interesting information on these peculiar features. Of particular note is his observation that the laminae within surge marks are continuous and not dissected as would be expected if the surge marks were erosional rills: the previously proposed mechanism of formation (High & Picard, 1968; Picard & High, 1973). Consequently, it becomes necessary to formulate an alternative explanation for surge mark origin, and Bull (1978, p. 885) offers the suggestion that: ‘Surge marks may be the result of selective depositional/erosional processes together with external deformation pressures.’
The relation between the crystallography of quartz, and upturned aeolian cleavage plates
Authors: WELLENDORF WILLIAM , KRINSLEY DAVID
Published: June 2006

The relation between the crystallography of artificially abraded quartz sand grains and aeolian surface textures has been studied using an X‐ray precession camera and the scanning electron microscope (SEM). Crushed Brazilian quartz was sieved to between 250 and 350 μm, eroded in a paddle wheel device which simulated aeolian action at 8 m s−1 for 3 h, and photographed with the SEM. A typical grain was selected and over 1100 photographs were combined to create a 3 × 3 m photomosaic; because of the large size, it was possible to observe and measure the angular and linear relations between the various features. After the c axis direction in the grain was determined by precession X‐ray analysis, the most important aeolian features were related to the crystallography of quartz. The upturned plates or cleavage plates probably represent traces parallel to r(10Ī1) and z(01Ī1) cleavages in quartz. Blocky areas appear to be an expression of a cleavage parallel to m(10Ī0). In addition, plate spacing on thirty‐five experimental, modern, and ancient quartz sand grains is fairly constant. Assuming that abrasion in most sedimentary environments acts in the same manner with respect to quartz crystallography, much of the fine silt and clay in sediments and sedimentary rocks may be the result of cleavage following abrasion parallel to the r and z cleavage planes, while the less common blocky particles are probably the result of cleavage parallel to the m planes.
Early diagenetic carbonate precipitation and pore fluid migration in the Kimmeridge Clay of Dorset, England
Authors: IRWIN HILARY
Published: June 2006

In the argillaceous sequence of Kimmeridge Clay a carbonate rich bed is composed of ferroan dolomite cement with varying amounts of excess CaCO3, and Fe2+ substitution in the Mg2+ sites. The isotopic and chemical compositions change symmetrically about the centre of the band proving that it grew by vertical accretion during diagenesis. Textural and isotopic evidence shows that growth centred on a horizon rich in primary carbonate which became dolomitized and assimilated during production of diagenetic carbonate. This accounts for the lateral extent of the concretion. Early central diagenetic carbonate was produced from organic matter by bacterial fermentation (δ13C =+0.59‰) and later marginal carbonate by abiotic breakdown, (δ13C tending towards — 2.73‰). δ18O values range from — 1.56 to — 4.46‰ because the dolomite precipitated during progressive burial. As burial increased, magnesium, whose dominant source was trapped seawater, became depleted while the relative availability of Fe2+, whose source was dominantly reduced detrital oxides, increased. Dolomitization and the source of diagenetic components for dolomite formation are discussed. Diffusion and pore fluid migration transported ions to the site of precipitation. Early cementation of the band served to influence pore fluid migration, but thereafter pore fluid migration controlled carbonate precipitation.
Grain size characteristics of Namib Desert linear dunes
Authors: LANCASTER N.
Published: June 2006

The results of a systematic sampling of linear dunes in the northern Namib sand sea show that they are composed of two distinct groups of sands: a fine, well‐sorted, near‐symmetrical crest and slip face population, and a coarse, moderately sorted, positively skewed plinth population. These may be explained in terms of the aeolian sorting processes operative on the dunes, which involve differential rates of movement of creep and saltation load on windward flanks and avalanching on leeward slopes, leading to the concentration of coarse sands towards the dune base.
Mechanical effects of micro‐organisms on intertidal bedform migration*
Authors: de BOER P. L.
Published: June 2006

Attention is called to the ability of bottom‐dwelling micro‐organisms such as algae to stabilize sandy sediment surfaces in non‐carbonate environments and to influence sand transport.
Depositional models for fine‐grained sediment in the western Hellenic Trench, Eastern Mediterranean
Authors: STANLEY DANIEL JEAN , MALDONADO ANDRÉS
Published: June 2006

Sediment in tectonically active, topographically restricted settings of the western Hellenic Arc, eastern Mediterranean, consists primarily of clayey silt and silty clay. Failure of metastable sediment temporarily stored on relatively steep slopes is triggered by earthquake tremors and eustatic oscillations. Redeposition of these materials by gravitative transport has resulted in markedly different lithofacies from site to site. Most piston cores include three Late Quaternary stratigraphic units that can be correlated with sections in other parts of the eastern Mediterranean; numerous radiocarbon‐age determinations enhance the correlation.

Seven fine‐grained sediment types are identified in cores from eight distinct depositional environments. Some muds are closely related to specific environments (slump and debris flow deposits on slope and high‐relief environments), or to time (well laminated mud during the latest Pleistocene‐mid‐Holocene), or to both (uniform and faintly laminated muds restricted to trench basins). Turbiditic and hemipelagic muds are common throughout the study area. Mud distribution patterns correlate closely with calculated sedimentation rates.

We propose two depositional models for these sediments. The first emphasizes downslope transformations resulting in progressively reduced flow concentration during transport: from slump and debris flow–>turbidity current–>low density turbidity current or turbid layer mechanisms. The distal end‐member deposits settling from low concentration flows are thick, rapidly emplaced, fine‐grained uniform muds closely associated with faintly laminated muds. These were ponded in flat trench basin‐plains. Planktonic and terrigenous fractions in the turbiditic, finely laminated and uniform muds record mixing of materials of gravitative and suspension origin during redeposition. This sequence prevails under conditions of minimal stratification of water masses, as characterized by the present Mediterranean.

In the second model developed for conditions of well‐developed water mass stratification, well laminated rather than uniform mud prevails as the end product of low concentration flows. These very finely laminated and graded muds record particle‐by‐particle settling from detached turbid layers concentrated along density interfaces; they include material from turbid layers complemented by the normal ‘rain’ of pelagic material. Stratification barriers resulted in region‐wide distribution of such deposits, in both slope and trench environments.
Differentiation of environments of dolomite formation, Lower Cretaceous of Central Tunisia
Authors: M'RABET ALI
Published: June 2006

Combined field, sedimentological, mineralogical, isotopic and geochemical study of the Lower Cretaceous dolomites of Central Tunisia has demonstrated considerable diversity in origin. Environments of dolomite formation include deep phreatic, karst, lacustrine and evaporitic sabkha. All four groups of dolomite are composed of non‐stoichiometric and/or disordered crystals which are more or less rich in calcium and in iron. Petrographic fabrics are of three types: replacement, recrystallization and cementation. These three fabrics are proposed among the various criteria for the different environments of dolomitization. Average isotope (δ18 O and δ13 C) contents for these four dolomite groups range from −10·5 to +0·4%o (PDB) and −3·9 to + 3·7%o respectively. The distribution of strontium is related both to the degree of recrystallization and to the palaeosalinity. Ferrous iron, also very common, is regarded as an indicator of relatively deep reducing conditions, mainly in meteoric groundwaters. Sodium distribution is related to inclusions within the dolomite, its distribution being relatively constant in all four groups; it cannot be regarded as a reliable criterion for palaeosalinity of dolomitizing fluids. This study confirms that dolomitization may occur under widely different palaeoenvironments, either at the surface or during burial. With the exception of the sabkha environment, dolomitizing fluids seem to have been essentially meteoric.
Calcrete conglomerate, case‐hardened conglomerate and cornstone ‐ a comparative account of pedogenic and non‐pedogenic carbonates from the continental Siwalik Group, Punjab, India
Authors: TANDON S. K. , NARAYAN DEVENDRA
Published: June 2006

The occurrence of authigenic carbonates formed in three different environmental situations, within the continental Siwalik Group, has been used to compare the lithological and petrographic characters of the contrasted lithofacies. The three lithofacies are: (1) calcrete conglomerate, (2) case‐hardened conglomerate, (3) cornstone (pedogenic, nodular calcrete).

The calcrete conglomerate facies laterally intertongues with the channel conglomerates. It consists of pisolites which are interpreted to have formed from carbonate‐rich spring waters emerging on to the gravelly substrate of dry, abandoned channels. The laminae characteristics of these pisolites are distinctly different from those of marine origin and also from comparable biogenic materials.

Case‐hardened conglomerate occurs in the youngest part of the Siwalik stratigraphic column, in boulder conglomerates having limestone as the principal clast component. This lithofacies has resulted from cementation of the conglomerate through continued dissolution and re‐precipitation of calcite, by meteoric water, downwards from the surface. It displays a coarsely crystalline, sparry calcite cement with no evidence for displacive growth or replacement by calcite.

Cornstones (nodular calcrete) occur in several sedimentary cycles of the Middle Siwalik Sub‐Group. These are immature and commonly associated with thinly‐bedded sandstones (levée) and red shales (overbank). This lithofacies is a result of concentration of carbonate through capillary action associated with pedogenic activity. Ooids developed in cornstone are essentially micritic in nature and usually composed of less than five indistinct laminae.
Aeolian oriented clasts in beach sediments
Authors: JONES B. G.
Published: June 2006

Aeolian processes may cause shoreward tilting of discoidal pebbles in a beach sequence. Subsequent covering by washover, back‐berm, plane‐laminated sand may give a spurious indication of palaeo‐sea position unless a regional sedimentological analysis is undertaken.
Diffusion of sediment on the lee of dune‐like bedforms: theoretical and numerical analysis
Authors: CHAKRABARTI CHINMOY , LOWE DONALD R.
Published: June 2006

Theoretical and empirical analyses of flow structure, sediment transport, and sediment size characteristics at the crest of dune‐like bedforms indicate that it is possible to describe, at least semi‐quantitatively, the diffusion and deposition of sediment on the leeside of such structures. A numerical program based on this analysis simulates the grain‐size distribution and deposition rate on the leeside of dunes for specified flow conditions and bed material. Evaluation of flow and sediment variables through the numerical simulation program shows that flow velocity, flow depth and sediment size have a strong influence on the deposition rate and texture of leeside sediment before avalanching. Sorting of the bed material, in particular, appears to exert a strong control on both the grain‐size and the deposition‐rate gradients.
Diagenetic trends in the siliceous facies of the Monterey Shale in the Santa Maria region, California
Authors: PISCIOTTO KENNETH A.
Published: June 2006

X‐ray diffraction and oxygen isotopic analyses of outcrop and subsurface samples of siliceous rocks were used to reconstruct thermal and diagenetic histories of the Miocene Monterey Shale near Santa Maria, California. Within many stratigraphic sections soft, porous diatomaceous rocks change gradationally to underlying hard and brittle chert, porcellanite, and siliceous shale; the accompanying silica mineral zones are, in descending stratigraphic order: (1) biogenic silica (opal‐A), (2) cristobalitic silica (opal‐CT), and (3) microcrystalline quartz.

Boundaries between silica mineral zones and stratigraphic horizons are often discordant. Within the opal‐CT zone, the d(101)‐spacing of opal‐CT decreases in a smooth non‐linear fashion from about 4 10 Å to 4‐04 Å. In the Santa Maria Valley and Bradley oil field areas the thicknesses of the opal‐CT zones are greater and the present thermal gradients less than in the adjacent Orcutt oil field. Thin opal‐CT zones at shallow maximum burial depths apparently correlate with higher thermal gradients.

Using present thermal gradients and reconstructed maximum burial depths from well data in the Santa Maria region, the ranges in temperatures for the top and base of the opal‐CT zone are 38–54 °C and 55–110 °C, respectively. The temperature difference between these two boundaries ranges from 17 to 60 °C. In comparison, temperature ranges for these two boundaries computed from oxygen isotopic compositions of opal‐CT and quartz, extrapolated experimental quartz‐water fractionations, and assuming δO18= 0%o for the isotopic composition of the equilibrating fluid are 18–56 °C and 31–80 °C for the top and base of the opal‐CT zone, respectively. The temperature difference between these boundaries is 11–36 °C using this method.

Thermal gradients and sedimentation rates strongly influence rates of silica transformations. Reconstructed thermal and diagenetic histories of siliceous rocks of the Monterey Shale at four well sites in the Santa Maria region demonstrate that most silica conversions probably occurred during the last 3–4 Myr in response to accelerated rates of sedimentation (and therefore burial heating) during the Pliocene.
Determination of recent deposition rates in Lake Constance with radioisotopic methods
Authors: DOMINIK JANUSZ , MANGINI AUGUSTO , MÜLLER GERMAN
Published: June 2006

Sedimentation rates were determined with the 210Pb method in eight sediment cores from Lake Constance. The rate of deposition in the main basin (Obersee) varies from about 0.06 g cm−2 y−1 in the central part to 0.13 g cm−2 y−1 in the eastern part of the lake and then increases rapidly towards the Rhine delta.

In the central lake area the rate of deposition has been approximately constant since 1900, and dating with the 210Pb method is in good agreement with sedimentological observations. In the Konstanzer Trichter area, the deposition rate has been increasing since about 1955 as a result of eutrophication and subsequent high carbonate production.

Dating with 137Cs is fairly accurate for sediments deposited at a high rate, but is questionable for slowly accumulating ones.

A positive correlation of 210Pb fluxes and sedimentation rates indicates that 210Pb flux into sediments follows the distribution pattern of solids. 210Pb profiles in four sediment cores interpreted in terms of a constant flux model display synchronous fluctuations of the sedimentation rate; however, their relation to long‐range particulate input variations remains to be proved.

Sedimentation rates determined with the 210Pb method were used to calculate recent nutrient and heavy metal fluxes. Anthropogenic fluxes of Zn and Pb are in the same range of magnitude as in other polluted areas in Europe and America.
Flume experiments on the origin of flaser bedding
Authors: HAWLEY NATHAN
Published: June 2006

A series of flume runs using a 12·5 h tidal cycle was conducted in order to determine whether or not flaser beds could be formed in nearshore subtidal areas as a result of purely tidal activity. The effects of clay composition, sand size, bed thickness, and maximum current velocity were investigated. The resistance to erosion of mud beds 10 mm thick or less is low; shear stresses approximately equal to those needed to initiate sand movement resulted in rapid erosion of the mud beds. In the light of the low resistance to erosion of the beds, and the necessity for increased thicknesses and very long time periods for their resistance to be increased substantially, it is concluded that the formation of these beds is unlikely to result from purely tidal action unless very large amounts of mud are available. A mechanism which more readily permits the deposition of thicker beds and longer consolidation times, such as storm action, is preferred.
The shape of rock particles, a critical review
Authors: JANKE N. C.
Published: June 2006

Barrett's (1980) critical review of shape measures omitted any reference to a compound shape measure developed in 1963 (Janke, 1963, 1966). This measure formed the basis for the only published equations known to this author predicting settling velocities of a wide size‐ and shape‐range of non‐spherical particles over an appreciable range of settling velocities, through the Stokes regime to Reynolds numbers of about 100,000. The form factor of the measure was also shown to be useful in quantitatively representing shape aspects of sieving data (Janke, 1970),
REPLY
Authors: KRINSLEY DAVID
Published: June 2006

Janke has corroborated the results of Wellendorf & Krinsley (1980) which indicated that small quartz particles are platy in nature. This is not the first time that fine, platy quartz has been described in the literature; Krinsley & Smalley (1973) believed that quartz particles became increasingly flatter with decreasing size and that a cleavage mechanism could operate below about 100μm. If Janke's work were extended to smaller sizes, perhaps this idea of increasing flatness with decreasing size would be confirmed or denied.
Maceration of calcareous skeletons
Authors: LEWY Z.
Published: June 2006

In the December issue of Sedimentology (1979, 26) two papers on the disintegration of calcareous skeletons appeared. In both articles the authors (Alexandersson, 1979; Fitzgerald, Parmenter & Milliman, 1979) describe skeletal degradation, producing fragments and even single calcium carbonate crystallites in the present marine environment. At least part of this maceration (Alexandersson, 1978) is attributed to the breakdown of the skeletal organic matrix on the sea bottom. Both studies were carried out on samples from the sediment surface of marine cold‐water regions. There, another destructive process is active: etching and dissolution of the calcareous particles by the ambient carbonate‐under‐saturated water (e.g. Alexandersson, 1975, 1978). The possible coexistence of these biological and physicochemical destructive processes and the lack of information from cores have left open questions in the above‐mentioned two studies. Some of these questions were discussed by me (Lewy, 1975). My observations and conclusions may now have a wider significance.
Pyrite in ammonite‐bearing shales from the Jurassic of England and Germany
Authors: HUDSON J. D.
Published: June 2006

Pyrite occurs both in normal clays and shales with a benthic fauna (Oxford Clay, England, and Lias ε, Germany) and in highly bituminous shales (Lias ε, Germany). In normal shales it is present in small quantities as early framboids, but more conspicuously as internal moulds of fossils, especially ammonites. The pyrite in these is petrographically varied; several types of internal sediments and chamber linings are described and illustrated by reflected‐light and scanning electron microscopy. Most striking are pyrite stalactites, suspended from the roofs of ammonite chambers, which were later filled by calcite or baryte. Pyrite formed in reducing micro‐environments, while the sediment generally was not wholly anoxic. Most pyrite pre‐dates compaction of sediment, breakage of fossils and solution of shell aragonite. Variable rates and conditions of reduction of sea water sulphate are reflected in δ34S values ranging from −55 to +44. Stalactites probably started to form when the ammonite chambers were partially gas‐filled.

In the bituminous Lias ε shales pyrite occurs abundantly as early framboids and micro‐nodules. Larger nodules show a variety of forms, some of which post‐date compaction of the sediment. Pyrite is not associated with the abundant flattened ammonites. δ34S values in shales are grouped about a mode near −20. Pyrite formed over a long time‐span, and throughout the sediment, not just in protected cavities.

Contrasts in pyrite types can be related to differing depositional environments and organic contents of the shales. Pyrite is an important mineral in diagenetic mineral parageneses which can be deduced by studying fossil void‐fillings and concretions, and which help define the diagenetic history of a shale.
Surface textures of turbidite sand grains, Laurentian Fan and Sohm Abyssal Plain
Authors: WANG YING , PIPER DAVID J. W. , VILKS GUSTAVS
Published: June 2006

Surface textures of quartz grains have been examined from five samples from the Laurentian Fan and Sohm Abyssal Plain, representing varied transport distances and power of the depositing turbidity current. The grains retain their primary irregular shape derived from glacial erosion, and glacial surface textures are preserved in dish‐shaped depressions. These features have been superimposed by a slight rounding of edges and an abundance of collision‐induced markings, particularly mechanical V‐forms. The most intense current modification of this sort occurs in mid‐Wisconsinan or earlier sands that have been transported over 1000 km to the distal Sohm Abyssal Plain by turbidity currents. Collision textures probably develop during grain flow on the steep continental slope: delicate resedimented shelf foraminifera are preserved in the same turbidites and most have been transported exclusively in suspension.
The superficial sediments of the English Channel and its Western Approaches
Authors: LARSONNEUR CLAUDE , BOUYSSE PHILIPPE , AUFFRET JEAN‐PAUL
Published: June 2006

The English Channel and its Western Approaches constitute a 700 km long epicontinental sea located in a temperate environment and in a tectonic setting where subsidence is minimal.

A sedimentological survey of this region reveals quite distinct provinces: a central sector (Western Channel) where predominantly bioclastic sediments are widely represented, bounded by two mainly terrigenous‐rich zones, the outer terrace of the Celtic Sea and the Eastern Channel. Lateral sedimentary variations of decreasing grain size are interpreted in terms of current velocity patterns. The various types of such sequences may relate to the degree of mixing of older lithoclastic sediments with the Holocene bioclastic supply.

A scenario for the evolution of the recent sedimentation of this epicontinental sea is presented. Starting from a permanent marine zone—at least since the Würm period—that bounds the Bay of Biscay, the Holocene transgression progressed over the English Channel. In the Western Channel earlier terrigenous deposits were gradually overlapped by bioclastic sediments that originated on the pre‐Mesozoic rocky substratum and which were particularly extensive off the Armorican Massif. In the subsequently submerged Eastern Channel the pre‐Holocene clastic source has undergone comparatively less modification and still crops out in most of the area. With the opening of the Straits of Dover, at about 9000 yr BP, submersion was complete and new hydrodynamic conditions developed as the eustatic level stabilized. The present sediment distribution is in harmony with the hydrodynamic setting except on the southern rise of the Celtic Sea where both morphology and sedimentary patterns still largely reflect pre‐Holocene conditions.
Size–density sorting of sand‐size spheres during deposition from bedload transport and implications concerning hydraulic equivalence
Authors: STEIDTMANN JAMES R.
Published: June 2006

It is well known that sediment sorting according to size, shape and density occurs, but the exact mechanisms involved are poorly understood. To assess the effects of size and density, sand‐size spheres of two densities were transported and deposited under controlled flume conditions. Observations on the motion of discrete particles show that grains smaller than bed‐roughness grains move continuously and have the same transport velocities regardless of density. For grains near and slightly larger than the roughness, movement is intermittent and, for a given size, heavy particles move more slowly than lights. For grains much larger than bed roughness grains, movement is continuous over the rough surface and light and heavy grains have nearly the same transport velocities.

Analyses of bulk sediment deposited from plane‐bed transport, show that the size and proportion of heavies decreases and that of lights increases with distance transported. For ripple bed transport, however, the size relations between associated light and heavy grains remains essentially unchanged with transport distance and the proportion of light and heavy grains is extremely variable.

These results suggest that size‐density sorting in plane‐bed transport is a function of the transportabilities identified in the discrete grain studies but that sorting in ripple‐bed transport is related to deposition on, and recycling through, the bed forms. Application of these findings to the concept of hydraulic equivalence implies that some indication of bed configuration may be necessary for the concept to be useful.
Burrows and trails produced by Quinqueloculina impressa Reuss, a benthic foraminifer, in fine‐grained sediment
Authors: SEVERIN KENNETH P. , CULVER STEPHEN J. , BLANPIED CHRISTIAN
Published: June 2006

The benthic foraminifer Quinqueloculina impressa Reuss, was buried in various types of sediment in order to assess its capability for producing sediment disturbances and thus, ichnofossils. Silts and silty‐clays showed distinct burrows; fine sands did not. Two types of burrows were produced: fairly straight, vertical burrows from 4 cm below the water‐sediment interface to within 1 cm of the sediment surface, and a horizontal and vertical maze‐like burrow system in the top centimetre of the sediment. Individuals moving on the sediment surface also produced visible trails. When the sediments were dried the burrows were always destroyed; in some cases the surface trails were preserved. We propose that the vertical burrows are escape structures and that the horizontal and vertical maze‐like structures are living burrows.

Ichnofossils similar to the escape structures and surface structures have been described. Presence of these ichnofossils indicates a low energy sedimentary environment and a lack of macrofaunal bioturbation. The presence of escape structures may indicate a pulsatory pattern of sedimentation.