bluuebunny/crossref_metadata_2025_split huggingface dataset:
Zoned calcites in Jurassic ammonite chambers: trace elements, isotopes and neomorphic origin
Authors: MARSHALL JAMES D. 
Published: June 2006

ABSTRACTZoned calcites were found in the phragmacone chambers of three Sonniniid ammonites from marine Middle Jurassic sandstones (Isle of Skye, U.K.). Each ammonite has a unique sequence of up to nine zones of calcite which fill or partially fill the chambers. Zones are defined by changes in the density of minute opaque inclusions and variation in trace‐element composition. Proximal (early) calcites have undulose extinction and some exhibit the specific fabrics of fascicular‐optic and radiaxial fibrous calcites. Microdolomite inclusions are found in one specimen. Early calcites, interpreted as replacements after a single isopachous fringe of acicular carbonate (probably high magnesium calcite), are succeeded by blocky ferroan calcite cement. In one specimen there are two distinct generations of calcite, in the others there is a continuous mosaic incorporating both early calcites and late cement.Isotopic composition of the early calcite zones demonstrates the initial importance of organic derived carbon (δ13C =— 26‰, δ18O ‰ O). Further cementation and mineralogical stabilization took place at increased temperatures and probably after modification of the pore water isotopic composition (calcites with δ13C =— O‰, δ18O∼— 10‰). The distinctive fabrics and zonal patterns probably developed during the replacement of the precursor cement and are not primary growth features. Reversals in isotopic and trace element trends are believed to be related to the rate of neomorphic crystal growth and hence to the degree of exchange with external pore waters. Further increase in temperature, probably during Tertiary igneous activity, gave rise to the extremely light δ18O values of the late cements in the ammonite which had previously had least contact with external waters (cements with δ13C ∼ O, δ18O ∼— 20‰).
The occurrence of calcified planktonic green algae in freshwater carbonates
Authors: MÜLLER GERMAN , OTI MICHAEL 
Published: June 2006

ABSTRACT Phacotus and Coccomonas, two carbonate‐secreting, non‐marine, planktonic chlorophyte genera, are important sediment contributors mainly in hard water lakes. The calcite loricae of these flagellate algae exhibit characteristic features easily recognizable under the microscope, making Phacotus and Coccomonas important facies fossils for the recognition of ancient non‐marine environments.The fossil record of Phacotus can be traced back to the Miocene and it is probable that further research could extend this record into pre‐Tertiary periods.
Palaeosols of the Lower Jurassic Mishhor and Ardon Formations (‘Laterite Derivative Facies’), Makhtesh Ramon, Israel
Authors: GOLDBERY RON 
Published: June 2006

ABSTRACTThe Lower Jurassic sequence exposed at Makhtesh Ramon, in the south‐western Negev of Israel, comprises cyclic fluviatile sediments derived almost exclusively from the reworking of an older laterite terrain. A succession of palaeosols, having the characteristics of modern, red‐brown earths, has developed during periods of sub‐aerial exposure upon the fine grained overbank sediments. A marked seasonal palaeoclimate with accompanying poor sub‐surface drainage conditions, and some stagnation of ground water, is reflected by the distribution and mottling patterns of the iron minerals (with varying degrees of hydration), precipitation of illuviated carbonate and sulphate minerals and the development of soil microrelief.A complex history of pedogenesis and ground water alteration is shown by the rapid lateral facies change within this sequence, into one dominated by high‐alumina flint clay and bauxite. Relict pedogenetic and sedimentological textural features of the combined palaeosol/fluviatile sequence, have been identified and mapped within the flint clay/bauxite lithologies. These features include irregular voids, produced by solution of pedogenic sulphate nodules, slickensided curviplanar surfaces of soil microrelief, and depositional bedding. The field relationships of these relicts indicate the overprinting of a younger, chemical weathering event giving rise to the high‐alumina flint clay and bauxites.The geometric shape of the upper bounding surface of the ‘bauxite’ alteration zone, is irregular and has an ‘intrusive’ character with respect to the younger, overlying, unaltered sediments. This, together with abrupt changes in mottling patterns near the contact zone and subtle changes in mineralogy from a bauxitic assemblage at depth to one containing mixed‐layer clay minerals and carbonates nearer the contact, leads to the conclusion that the later stages of chemical weathering occurred within a confined system, where ground water movements were induced by intake of underlying aquifers.
Isotope signatures associated with early meteoric diagenesis
Authors: ALLAN J.R. , MATTHEWS R.K. 
Published: June 2006

ABSTRACTThe environments in which carbonate diagenesis proceeds have been documented in previous studies of Holocene and late Pleistocene sediments and limestones on Barbados, West Indies. Variations in the carbon and oxygen isotopic composition of limestones, produced during early freshwater diagenesis, have been observed in this study to occur in specific patterns. Six potentially useful patterns emerge when one views stable isotope data within a stratigraphic framework: (1) the subaerial exposure surface is characterized by strongly 12C‐enriched limestones. δ13C compositions of underlying limestones grow progressively heavier with increasing depth; (2) the subaerial exposure surface may also be marked by slight 18O‐enrichment; (3) an abrupt shift in δ18O values may differentiate sediments above the exposure surface from those below; (4) sediments altered in the marine‐meteoric mixing zone may be characterized by positive covariance between their δ18O and δ13C compositions; (5) the vadose‐phreatic boundary may be marked by a sharp increase in δ13C values in the seaward portions of a fresh groundwater system; and (6) samples contemporaneously altered in a single fresh groundwater system within an areally restricted region should display a narrow range of δ18O and a wide range of δ13C compositions.Analysis of samples from five Palaeozoic and Mesozoic formations, which contained petrographic evidence of early freshwater diagenesis, showed that isotope patterns similar to those observed in Barbados limestones have been preserved in rocks as old as Mississippian.These isotope patterns could prove to be useful for identifying diagenetically induced porosity trends in carbonate rocks. They might be used to identify limestones diagenetically altered in meteoric environments, to identify mixing zone cements and dolomites, and to trace the regional and vertical distributions of early meteoric groundwater systems in ancient carbonate formations.
Palaeokarst, tidal erosion surfaces and stromatolites in the Silurian Eke Formation of Gotland, Sweden
Authors: CHERNS LESLEY 
Published: June 2006

ABSTRACTIrregular, scalloped erosion surfaces in the shelf carbonate sequences of the lower Eke Formation (Ludlow Series, upper Silurian) in the east of Gotland, Sweden, comprise series of shallow hollows separated by sharp‐crested ridges, and cavities with sculptured, undercut walls, cut into lithified sediment. These represent analogues of the solution basins formed in modern coastal and subaerial karst terrains. Discrete erosional cavities merge on enlargement by breaching of the intervening walls to leave remnant, tapering ridges. Sets of the ridges and basins seen in surface view show an average width of basins of 1–2 m, with relief of 40–50 cm and pronounced N‐S axes for the ridges; this alignment may reflect the local drainage direction. The lowermost erosion surface passes laterally into a planar, mineralized horizon at the top of the underlying Hemse Group that was resistant to and forms the base level of erosion. Because of restricted exposure of higher Eke Formation sediments the upper limit of erosion remains unknown. There is no evidence of caliche or subaerial diagenetic textures, but solution vugs are common in the eroded limestones. Marine hard‐bottom biota attached to some surfaces, and transition from scalloped to planar surfaces indicate erosion in tidal zones, but subaerial karstic erosion is also inferred. The resubmerged karst‐eroded topography is overlain by shallow marine carbonates, including small organic buildups. Finely‐layered stromatolitic mats developed over the initial infill, in subtidal environments, and grew to form domed mounds within the erosional cavities. They abut sharply against bounding side walls and overhangs. Some emergence is evident from desiccation features in the upper parts of mounds. Biostratigraphical evidence dating the events from initial uplift and karstic erosion to covering of the drowned relief topography places the whole sequence within upper Ludlow times.
Variation in particle size distribution over a small dune
Authors: BARNDORFF‐NIELSEN O. , DALSGAARD K. , HALGREEN C. , KUHLMAN H. , MØLLER J.T. , SCHOU G. 
Published: June 2006

ABSTRACTThe variation in particle size distribution for a sequence of sand samples, collected from the surface of a small barchanoid dune along a sampling line parallel to the windflow, is studied by means of the hyperbolic distribution. The modal (or typical) value of the log size increases linearly with distance from the windward foot to the crest of the dune, and falls off steeply on the lee side. The spread of the log‐size distribution, locally near the modal value, is roughly constant on the windward side but decreases significantly down the slip face. Two other parameters of the four‐parameter hyperbolic distribution can be considered largely constant. A comparison of the log size distributions for heavy and light mineral grains, based on a selected set of the samples, showed that heavy and light minerals follow different hyperbolic patterns. However, the difference in modal size of the distributions for the two types of minerals was virtually constant among the samples.
Deposition of climbing‐ripple beds: a flume simulation
Authors: ASHLEY GAIL M. , SOUTHARD JOHN B. , BOOTHROYD JON C. 
Published: June 2006

ABSTRACTThirteen runs were made in a small recirculating flume to simulate the deposition of the climbing‐ripple sequences commonly present in fine‐grained facies of fluvial and deltaic deposits. These sequences consist of intergradational climbing‐ripple cross laminae and draped laminae. The experiments were based on the assumption that stratification type depends mainly on near‐bottom flow structure and uniform sediment fallout from an overloaded flow. Various combinations of curves of velocity versus time and of sediment feed versus time in runs lasting from 45 to 840 min were used in an exploratory program; conditions for each run were selected on the basis of experience in previous runs. The runs verified that Type A (erosional‐stoss) climbing ripples are produced by aggradation rates that are small relative to ripple migration rate, and Type B (depositional‐stoss) climbing ripples are produced by aggradation rates that are large relative to ripple migration rate. Draped lamination results from continued fallout of sediment from suspension after ripple migration ceases or almost ceases. Comparison of geometric details of the ripple stratification produced in the flume runs with that in natural sequences, supplemented by considerations on maximum and minimum migration rates of ripples, suggests times of no more than a few tens of hours for the deposition of the climbing‐ripple portions of sequences 10‐20 cm thick. Runs in which deposition of a 20 cm sequence took more than 10 h produced such atypical features of ripple geometry as sharp crests, planar lee‐side laminae, and angular toeset‐foreset contacts.
Sand waves and sediment transport around the end of a tidal sand bank
Authors: McCave I. N. , LANGHORNE D. N. 
Published: June 2006

ABSTRACTTwo detailed surveys have been made of the north‐western end of the Haisborough Sand off the Norfolk coast using echosounder, 3‐5 kHz reflection profiler and side‐scan sonar. Asymmetrical sand waves indicate north‐westerly sand movement on the southern side and south‐easterly sand movement on the steeper northern side of the bank. Secondary, superimposed megaripples, which are probably better indicators of sediment movement, give evidence of a cross‐bank component. Between the north‐westerly and south‐easterly facing sand waves on the tip of the bank there is a zone of symmetrical sand waves. These are usually taken to indicate zero net transport, but in this case the oblique orientation of megaripples in their troughs indicates transport parallel to the sand wave crests. This suggests the route by which sand travels around the end of the bank to form a roughly closed circulation. Sediment textural parameters support the notion that sand is winnowed from the foot of the bank on both sides and is transported to the middle with an overall net transport from the south to the north. Analysis of charts dating back to 1886 shows that the bank is stable within the error limits of position fixing, though that could allow more than 0‐25 km shift to the north east in 100 years to pass undetected. A box model is drawn up for the estimated sediment fluxes around the end of the bank, and implications for residence times and circulation rates are drawn from it.
Sedimentology and evaporite genesis in a Holocene continental‐sabkha playa basin—Bristol Dry Lake, California
Authors: HANDFORD C. ROBERTSON 
Published: June 2006

ABSTRACTBristol Dry Lake, a 155 km2 continental‐sabkha playa basin in the Mojave Desert of south eastern California, is filled with at least 300 m of interbedded terrigenous clastics, gypsum, anhydrite, and halite. Evaporite facies conform approximately to a bull's eye pattern with gypsum and anhydrite surrounding a basin centre accumulation of halite. Transects through Bristol Dry Lake, from the alluvial fan to the centre of the playa, reveal: (1) crudely‐bedded, alluvial fan clastics interfingering with (2) playa‐margin sand flat and wadi sand and silt, followed by (3) gypsum, anhydrite, chaotic mud halite, and clay of the saline mud flat, and (4) salt‐pan halite beds.Terrigenous clastics were deposited in Bristol Dry Lake by sheetflow and by suspension settling from ponded floodwater. Some sediment has been reworked by aeolian processes to form barchan dunes around the playa margin. Thin nodular‐like beds of anhydrite and several types of gypsum occur across most of the playa. Giant hopper‐shaped halite cubes are suspended in saline mud flat facies, suggesting that they grew displacively in brine soaked sediment just below the surface. Thick beds (4 m) of halite, in the playa centre, may have formed through a complex alternating history of subaqueous and intrasedimentary precipitation under the influence of periodic floods, intense evaporation and brine‐level lowering, and capillary discharge of brines.The stratigraphy in the playa centre is cyclic. An ideal cycle consists of: (1) chaotic mud halite at the base overlain by (2) green to red clay with abundant, giant hoppers, and at the top (3) red clay, gypsum, and anhydrite with flaser‐ to wavy‐bedded sand and silt. This type of cycle probably records a gradual progradation of mud‐flat facies over salt pans.Bristol Dry Lake sediments are nearly identical to some of the Permian evaporites of the Permian Basin region, U.S.A. and they can serve as modern analogues for ancient‐sabkha facies analysis.
The Cambro‐Ordovician Cap Enragé Formation, Queébec, Canada: conglomeratic deposits of a braided submarine channel with terraces
Authors: HEIN FRANCES J. , WALKER ROGER G. 
Published: June 2006

ABSTRACTThe Cambro‐Ordovician Cap Enragé Formation is interpreted as a deep submarine channel complex of conglomerates, pebbly sandstones and massive sandstones. The formation is up to 270 m thick, and crops out in a coastal belt 50 km long. In general terms, it has previously been interpreted as a deep sea channel deposit, with the channel about 300 m deep, at least 10 km wide and trending south‐westward, parallel to the coastal outcrops. Eight facies have been defined in this study and they have been grouped into three major facies associations. In the Coarse Channelled Association, conglomerates with carbonate boulders up to about 4 m are associated with graded‐stratified finer grained conglomerates. Facies of this association make up about 25% of all the beds in the formation. The association is also characterized by abundant major channels 1–10 m deep and up to 250 m wide. Excellent outcrop allows the reconstruction of topographic highs (bars) within the channels and the association is interpreted as a braided channel and bar system.The second association, Multiple‐Scoured Coarse Sandstones, contains some graded‐stratified fine conglomerates, along with massive to structureless coarse and pebbly sandstones, and rare cross‐bedded pebbly sandstones. Deep channels are absent, but multiple channelling on the scale of 0.5–1 m is characteristic. In the absence of the very coarse conglomerates and deeper channelling, this association is interpreted as being deposited on topographically higher terrace areas adjacent to the main braid plain.The third facies association, Unchannelled Sandstones, is characterized by massive sandstones with abundant fluid‐escape structures, classical turbidites and thin shales. In the absence of any scouring deeper than a few tens of centimetres, this association is interpreted as being deposited on an even higher and smoother terrace, farther from the braid plain.Palaeoflow directions for conglomerate facies indicate fairly consistent south‐westward transport, apparently parallel to the base of the Cambro‐Ordovician continental slope. Flow directions in the finer‐grained facies are rather variable, suggesting complex bar development and overbank spills.Thinning‐and fining‐upward sequences are present on two scales. The smaller, 1–10 m sequence, is related to channel filling and abandonment. Thicker sequences (10–100 m), with facies of the Multiple Scoured, and Unchannelled Sandstone Associations, may indicate switching of a main channel away from the area and its subsequent burial by marginal terrace and higher terrace deposits.
Isochronous fluvial systems in Miocene deposits of Northern Pakistan
Authors: Behrensmeyer Anna K. , Tauxe Lisa 
Published: June 2006

A Palacomagnetic isochron dated at about 8.1 Myr BP and detailed lithostratigraph of a 40 m interval exposed along strike for 40 km establish the depositional patterns of two contemporaneous, interfingering fluvial systems in the upper part of the Meddle Siwalik sequence.The two systems, referred to as the buff and blue‐grey, differ in unit shape, lithofacies, bedding sequence, palaeocurrent direction and sand composition. Interfingering occurs along the south‐west‐north‐east strike of the outcrops, with the palaeodrainage directions of the two systems generally perpendicular to this line. The axis of the blue‐grey system, which deposited widespread sheet sands and silts, lay toward the south west end of the study area. The more complex axis of the buff system, which deposited shoe‐string sand bodies and lage volumes of silt and clay, lay toward the north‐east. The source area for both systems was the rising Himalyan belt to the north and noth‐east of the study area. At maximum extent the blue‐grey system occupied a channel belt at least 25 km wide. Channel belt widths and depths for the buff system are 1–3 km and 3–7 m, respectively. Current directions averge 94° for blue‐grey sands and 136° for buff sands. Blur‐grey sands contain 20% more rock fragments and are otherwise less mature than buff sands.The buff system shows a verticla pattern of avulsion, palaeosol formation and floodplain aggradation which we attribute to autocyclic processes of parallel rivers. The blue‐grey system shows phases of erosion accompaniced laterally by plaeosol formation, folowed by valley fill and overfowing of interfluve surfaces. Theis pattern may be caused by allocyclic presses affecting the source area. We interpret the blue‐grey system as a major drainage from the interior Himalayas (perhaps the ancestral Indus) and the buff system as a complex of smaller drinages along the mountain front which were probably ributaries to the bluegrey syste.Vertebrate fossils including hominoid primates from the area are almost exclusively associated with lithofacies of the buff system, and this probably refects both taphonomic and palaeoecological differences between the two systems.
Cambrian skeletal halite crystals and experimental analogues
Authors: SOUTHGATE PETER N. 
Published: June 2006

ABSTRACTA suite of unusual highly skeletal halite pseudomorphs is described from the lower Middle Cambrian rocks that crop out at Ardmore in the Georgina Basin, northern Australia. The pseudomorphs are preserved as both moulds and casts within a dark and light colour‐banded chert. They are orientated parallel to bedding and represent halite crystal growth within a brine pool. Laterally equivalent rocks are characterized by sedimentary textures and structures indicative of periodic emergence and desiccation. Laboratory grown halite crystals are also described and compared with the pseudomorphs from Ardmore. Experimentally produced halite crystals formed from either: (1) brine solutions obtained as residues after organic matter extraction from phosphate rocks, or (2) solutions of NaCI and distilled water. Both pyramidal rafted hoppers and floor nucleated cuboids formed in solutions containing only NaCI and distilled water; whereas in the residue solutions, that contained humic acids, rafted pyramidal forms were absent and crystal nucleation was restricted to the floor of the evaporative dish. As brine depth decreased the halite precipitation rate increased and resulted in a suite of excrescent, highly skeletal crystals that formed as a result of brine evaporation to dryness.The variable crystal morphology depended upon both the brine depth and slope of the evaporative dish floor. Horizontally orientated chevron halite crystals formed where the evaporative dish was inclined and precipitation preferentially occurred on cube faces. Pagoda, reticulate ridge and dendritic forms represent an increasingly skeletal crystal suite characterized by the preferential precipitation of NaCI on cube edges and corners rather than faces. Using the experimentally grown crystals as analogues the pseudomorphs at Ardmore are interpreted as forming in very shallow brine pools that evaporated to dryness.
Boundary effects on stream meandering and river morphology
Authors: NAKAGAWA TAKEO 
Published: June 2006

ABSTRACT An account of stream meandering and river morphology is given for five experiments, each having different conditions at the wetted perimeter of the channel.On the basis of the present experiments, it is proposed that a necessary (but insufficient) condition for the formation of stream meandering is an appropriately small ratio of total bank shear force to total bed shear force, both per unit length downstream. In mathematical form, the criterion is expressed by

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τs and τs are an average bank stress and bed stress, respectively, and ps and pbb are an average bank wetted perimeter and bed wetted perimeter of a half‐channel, respectively. The value of α is evaluated by Kinoshita's (1957) experiment and others.It is also found that the flow pattern, channel morphology and sand waves on the bed are dependent upon the boundary conditions at the wetted perimeter.