bluuebunny/crossref_metadata_2025_split huggingface dataset:
Modern sediments and sedimentary processes in Lake Rudolf (Lake Turkana) eastern Rift Valley, Kenya
Authors: YURETICH RICHARD F. 
Published: June 2006

ABSTRACTTertiary 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

ABSTRACTLog‐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

ABSTRACTOn 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

ABSTRACTKamloops 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

ABSTRACTFragments 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

ABSTRACTThe 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

ABSTRACTA 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

ABSTRACTSoft‐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

ABSTRACTThis 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

SUMMARYIn 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

ABSTRACTThe 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

ABSTRACTIn 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

ABSTRACTThe 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.
Depositional models for fine‐grained sediment in the western Hellenic Trench, Eastern Mediterranean
Authors: STANLEY DANIEL JEAN , MALDONADO ANDRÉS 
Published: June 2006

ABSTRACTSediment 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

ABSTRACTCombined 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

ABSTRACTThe 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

ABSTRACTAeolian 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.