bluuebunny/crossref_metadata_2025 huggingface dataset:
Biosedimentological zonation of Boundary Bay tidal flats, Fraser River Delta, British Columbia
Authors: SWINBANKS DAVID D. , MURRAY JAMES W.
Published: June 2006

Boundary Bay tidal flats on the inactive southern flank of the Fraser Delta have surface sediments consisting almost entirely of very fine to fine, well to very well sorted sands which show a gradual fining‐shorewards trend. Five floral/sedimentological zones form distinct biofacies. These are, from the shoreline seaward, the saltmarsh, algal mat, upper sand wave, eelgrass and lower sand wave zones. The lower limit of the saltmarsh lies at a constant level above which the maximum duration of continuous exposure rises abruptly from ∼ 12 to 40 days. Similarly, at the lower limit of the algal mat zone the maximum duration of continuous exposure jumps from 1 to ∼ 2 days, and at the upper limit of the eelgrass zone from ∼ 0·5 to ∼ 0·8 days. These correlations between exposure and zonation are suggested to be causal.

In the algal mat and eelgrass zones microtopography of biogenic origin, only a few centimetres high, creates lateral heterogeneity within the zonal biofacies. In the upper sand wave zone, very low amplitude (a∼ 0·1 m) symmetrical sand waves (λ∼ 30 m) of probable storm‐wave origin have a similar effect. In the lower sand wave zone, sand waves (a∼ 0·5 m, λ∼ 60 m) are formed by tidal currents or wave action and physical sedimentary structures dominate over biogenic ones.

The densities of the following macrofaunal organisms which produce distinctive biogenic sedimentary structures were determined on two surveyed transects: Callianassa californiensis and Upogebia pugettensis, both thalassinidean shrimps; three polychaete worms, Abarenicola sp., Spio sp. and Clymenella sp.; the bivalve Mya arenaria and the gastropods Batillaria attramentaria and Nassarius mendicus. Callianassa excavate unlined temporary feeding burrows, whereas Upogebia build mud‐lined permanent dwelling burrows. Upogebia are restricted to below mean sea‐level where continuous exposure is < 0·5 days, whereas Callianassa extend up to a level, just below mean higher high water, where maximum continuous exposure rises abruptly from 4 to 9 days. This difference in range is probably due to the latter's greater anoxia tolerance—a necessary adaptation for life in an unlined feeding burrow.
Static breakage of granitic detritus by ice and water in comparison with breakage by flowing water
Authors: MOSS A. J. , GREEN PATRICIA , HUTKA J.
Published: June 2006

Temperature cycling between −4·5±1·0°C and 13·0±2·0°C in the presence of added water caused significant breakage of granitic detritus. Because cycling with only adsorbed water present also caused breakage, fragmentation is attributed to both the combined action of ice and adsorbed water and to the latter acting alone. Breakage evidently resulted from the gradual tensile opening of pre‐existing cracks, weakening then splitting grains. Surviving unbroken grains show evidence of a fatigue effect. Quartz split dominantly along pre‐existing subplanar microfractures whereas feldspar and biotite split along crystal cleavages.

Degree of breakage and product size distribution depend on the crystalline nature of the parent material, its previous history, and the nature and duration of the breakage process. With the first two factors the same, size distributions from adsorbed water breakage alone and from that due to adsorbed water plus ice differ slightly. Both contrast strongly with those of simulated fluviatile breakage. Whereas the latter preferentially produced 2–20 μm particles (probably debris of inter‐particle collisions), static breakage split coarser grains wherever major weaknesses occurred, producing less selective product size distributions with greater proportions of loess‐sized material (about 20‐‐60 μ). Characteristic inflections in the size distribution curves of our experimentally produced debris are also shown by samples from the sola of some frost‐affected soils.

Partially healed microfractures in plutonic quartz are normally spaced at about 1–10 μ—approaching the downward asymptotic comminution limit for brittle solids (about 1 μ). Surficial physical processes are capable of reducing only a small proportion of plutonic quartz to this size before its storage in sediments.
Evolution of a classic sand ridge field: Maryland sector, North American inner shelf
Authors: SWIFT DONALD J. P. , FIELD MICHAEL E.
Published: June 2006

The ridge and swale topography of the Middle Atlantic Bight is best developed on the Delaware‐Maryland inner shelf. Here sand ridges can be seen in all stages of formation. Several aspects of the ridge field are pertinent to the problem of ridge genesis. The first is ridge morphology. There is a systematic morphologic change from shoreface ridges through nearshore ridges to offshore ridges, which reflects the changing hydraulic regime. As successively more seaward ridges are examined, maximum side slope decreases, the ratio of maximum seaward slope to maximum landward slope decreases, and the cross‐sectional area increases. These changes in ridge morphology with depth and distance from shore appear to be equivalent to the morphologic changes experienced by a single ridge during the course of the Holocene transgression.

A second aspect is the change in bottom sediment characteristics that accompanies these large‐scale morphologic changes. Megaripples, sand waves and mud lenses appear in the troughs between nearshore and offshore ridges. These changes indicate that the storm flows which maintain ridges are less frequently experienced in the deeper sector, and that the role of high‐frequency wave surge becomes less important relative to the role of the mean flow component in shaping the sea‐floor.

A third aspect is the systematic relationship of grain size to topography. Grain size is 90° out of phase with topography, so that the coarsest sand lies between the axis of the landward trough and the ridge crest, while the finest sand lies between the ridge crest and the axis of the seaward trough. This relationship is characteristic of large‐scale bedforms.

Finally, flow was measured and transport calculated on the same ridge during a one‐month period (November 1976). Threshold was exceeded only during storm events. Mean transport was southerly and a little seaward with respect to both the ridge crest and the shoreline. These flow measurements are in conformity with the pattern of smaller bedforms. A 43‐year time series of bathymetric change for this ridge reveals a systematic pattern of landward flank erosion, seaward flank deposition, and seaward crest migration.

Sand ridges are considered the consequence of constructive feedback between an initial topography and the resulting distribution of bottom shear stress. The relationship between grain size and topography supports this model, but does not account directly for the oblique angle of the ridge with respect to the coastline. This feature may be due to a more rapid alongshore migration rate of the inshore edge of the ridge than the offshore edge, and the relationship between this migration rate, and the rate of shoreface retreat.
The control of cathodoluminescence in dolomite by iron and manganese
Authors: PIERSON BERNARD J.
Published: June 2006

Variations in the cathodoluminescent properties of carbonates are usually attributed to differing proportions of manganese (Mn2+) as the most important activator, and iron (Fe2+) as the main inhibitor of luminescence. Interactions between manganese and iron concentrations and the luminescent properties of dolomite are demonstrated by petrographic and chemical analyses of 86 samples of dolomite representing a range of depositional environments and ages (Cambrian to Cretaceous) and a wide geographical distribution (North America and Europe).

Iron and manganese are positively correlated in the dolomites, with the former showing a greater range of variation. Very small amounts of manganese are sufficient to activate the luminescence and as little as 100 ppm Mn2+ is present in highly luminescing samples. The intensity of luminescence is not proportional to the manganese concentration. Iron begins to quench luminescence as its concentration reaches 10,000 ppm. Above that level, luminescence is rapidly lost and total extinction occurs among samples containing more than 15,000 ppm Fe2+, regardless of the manganese concentration.
De glauconiarum origine
Authors: ODIN GILLES S. , MATTER ALBERT
Published: June 2006

The glauconitic facies is widespread on present‐day continental shelves from 50° S to 65° N and at water depths between 50 and 500 m, and is in particularly great abundance on the upper slope and outer shelf between 200 and 300 m. It is also common in many ancient rocks of post‐late Precambrian age.

It occurs as sand‐ to pebble‐sized, essentially green particles (granular facies) but also as a surface coating on particles and hardgrounds and as a diffuse impregnation (film and diffuse facies). We suggest the replacement of the term ‘glauconite’, which has been interchangeably used to designate a morphological form and a specific mineral, by glaucony (facies) and glauconitic smectite and glauconitic mica as end members of the glauconitic mineral family.

The widely accepted model of Burst and Hower for glauconitization requires a degraded, micaceous (2: 1 layer lattice structure) parent clay mineral. However, detailed analysis of numerous samples of Recent glaucony reveals that such a parent substrate is exceptional. The model therefore requires modification. Generally the parent material is carbonate particles, argillaceous (kaolinitic) faecal pellets, infillings of foraminiferal tests, various mineral grains and rock fragments, that pass gradually into the commonly occurring green grains.

We show that the process of glauconitization is achieved by de novo authigenic growth of automorphous crystallites in the pores of the substrate, accompanied by progressive alteration and replacement of the substrate. It is this two‐fold evolution that causes the ‘verdissement’of granular substrates, macrofossils and hardgrounds. The authigenic mineral is an iron‐rich and potassium‐poor glauconitic smectite. While new smectites are growing into the remaining pore space the earlier smectites are modified by incorporation of potassium, producing decreasingly expandable minerals with a non‐expandable glauconitic mica as the end member. This mineralogical diversity of the glauconitic mineral family explains the highly variable physical and chemical properties of glaucony. Four categories, nascent, little‐evolved, evolved and highly‐evolved glaucony are distinguished.

Glauconitization appears to be controlled by a delicate balance between degree of physical confinement of a particle and the amount of ionic exchange between the micro‐environment and ambient open marine sea water. The optimum conditions for glauconitization are those of semi‐confinement. As a result the interior of a grain is more glauconitized than its less confined periphery. Similarly, for identical substrate types, large grains (500μm) provide more favourable substrates for glauconitization than lesser confined small grains.

On a larger scale the formation of glaucony is governed by the availability of iron and potassium and the balance between detrital influx and winnowing. Low accumulation rates expose grains to the open marine environment for sufficiently long times (105‐106 years for highly‐evolved glaucony).
Significance of interdune deposits and bounding surfaces in aeolian dune sands
Authors: KOCUREK GARY
Published: June 2006

Bounding surfaces and interdune deposits provide keys for detailed interpretations of the development, shape, type, wavelength and angle of climb of aeolian bedforms, as well as overall sand sea conditions. Current alternate interpretations of bounding surfaces require very different, but testable models for sand sea deposition.

Two perpendicular traverses of Jurassic Entrada Sandstone, Utah, reveal relations among cross‐strata, first‐order bounding surfaces, and horizontal strata. These field relations seem explicable only as the deposits of downwind‐migrating, climbing, enclosed interdune basins (horizontal strata) and dune bodies consisting of superimposed smaller crescentic dunes (cross‐stratified deposits). A 1.7 km traverse parallel to the palaeowind direction provides a time‐transgressive view showing continuous cosets of cross‐strata, first‐order bounding surfaces and interdune deposits climbing downwind at an angle of a few tenths of a degree. Changes occur in the angle of climb, cross‐strata structure, and interdune deposits; these reflect changes in depositional conditions through time. A 1.5 km traverse perpendicular to the palaeowind direction provides a view at an instant in geological time showing first‐order bounding surfaces and interdune deposits forming flat, laterally discontinuous lenticular bodies. The distribution of interdune sedimentary structures in this traverse is very similar to that of some modern interdune basins, such as those on Padre Island, Texas.

Hierarchies of bounding surfaces in an aeolian deposit reflect the bedform development on an erg. The presence of three orders of bounding surfaces indicates dune bodies consisting of smaller, super‐imposed dunes. The geometry of first‐order bounding surfaces is a reflection of the shape of the inter‐dune basins. Second‐order bounding surfaces originate by the migration of the superimposed dunes over the larger dune body and reflect individual dune shape and type. Third‐order bounding surfaces are reactivation surfaces showing stages in the advance of individual dunes. The presence of only two orders of bounding surfaces indicates simple dunes.

Modern and Entrada interdune deposits show a wide variety of sediment types and structures reflecting deposition under wet, damp, and dry conditions. Interdune deposits are probably the best indicators of overall erg conditions and commonly show complex vertical sequences reflecting changes in specific depositional conditions.
Methods of computation of suspended load from bed materials and flow parameters
Authors: GHOSH J. K. , MAZUMDER B. S. , SENGUPTA S.
Published: June 2006

Computation of the grain‐size distribution of the suspended load above a sand bed must take into consideration: (1) sorting processes from the bed to the bed layer and (2) sorting between the bed layer and suspension. Grain‐size distributions of the bed layers above sand beds of three different types have been computed in this work, both by the Einstein and the Gessler methods. Using these as references, suspended load distributions have been obtained in each case by the Rouse suspension equation. A new formula has also been developed in partial modification of Hunt's method for direct computation of bed load and suspended load from a bed's grain‐size distribution and flow parameters.

Comparison of the computed data with actual observations in laboratory flumes show that no one method is particularly superior to the others, but the present method is advantageous because it affords direct computation of the suspended load from a bed's grain‐size distribution, without going through an intermediate stage (bed load). The possible sources of error in each of the methods have been discussed.
La sédimentation argileuse depuis l'Eocène sur le plateau Vøring et à son voisinage, d'après le Leg 38 DSDP (Mer de Norvège)
Authors: FROGET CLAUDE
Published: June 2006

The study of clays from sites 338 to 343 enables the main lines of the palaeogeographical evolution of the Vøring plateau, off Norway, to be determined. The following stages are proposed:

(1) In early Eocene sediments, clays provide evidence of erosion of the Scandinavian shield; detrital primary minerals (illite, chlorite) derived from crystalline rocks are dominant. This erosion results from tectonic movements coeval with the opening of the Norwegian basin. The rejuvenation of marginal relief hinders pedogenesis, which is only indicated by the presence of minerals coming from the few soils which are developed.

(2) From the Middle Eocene, primary minerals decrease while smectite becomes dominant. This trend increases through the late Eocene and at the beginning of the Middle Oligocene, where only smectite is present. During this period the basin margins are stabilized after the initial movements. In essentially pelagic marine sediments, the smectite seems to be derived from volcanic rocks.

(3) During the Middle Oligocene and early to Middle Miocene, smectite is always dominant, but primary minerals reappear, with mixed‐layers indicating a moderate continental alteration. This composition suggests a modification of the climate, mainly marked by an increase in humidity.

(4) In Pliocene and Plio‐Pleistocene sediments, primary minerals are dominant, while smectite decreases. The glacial climate is responsible for this mineralogy. Detrital minerals would come from the erosion of Scandinavian moraines and from rafting by icebergs.
Wind sedimentation tunnel experiments on the origins of aeolian strata
Authors: FRYBERGER STEVEN G. , SCHENK CHRISTOPHER
Published: June 2006

The origins and sedimentary features of grainfall‐, avalanche‐, and ripple‐produced strata have been studied experimentally in a wind sedimentation tunnel. Rate of deposition, wind velocity and wind duration have been shown to control specific sedimentary features of these types of strata.

Grainfall‐produced strata were deposited on a horizontal surface, and surfaces sloping up to the angle of initial yield for dry sand (about 34°). Thickness of a grainfall‐produced stratum depended upon rate of deposition and duration of a specific wind event. Grainfall‐produced strata were both non‐graded and graded. Graded strata were produced by changes in wind velocity which controlled size of sand in transport and flying distances of individual grains. Distinctive features of grainfall‐produced strata are: (a) gradual thinning, or tapering downwind (e.g. down the simulated slipface and across the simulated interdune; (b) extreme variability of thickness from less than 1 mm (wind gusts of a few seconds) to 10 cm or more (sustained gusts).

Aeolian avalanche‐produced strata were formed when grainfall‐produced strata steepened above the angle of initial yield and sheared downslope. A rapid transition in sedimentary features from top to bottom of the slipface characterized avalanche‐produced strata of the slump degeneration type in dry sand derived from grainfall deposition. Fadeout laminae formed near the top of the simulated slipface and about 1 m farther down the slipface were flame structures and drag folds. Near the base of the slipface, the avalanche truncated and then overrode grainfall‐produced deposits. Distinctive features of avalanche‐produced strata for a 2.5 m long slipface are the deformation structures, a thickness of 1 or 2 cm, sandflow toes, and steep dip (34°). Each avalanche‐produced stratum was roughly tabular in cross‐section parallel to wind direction, with gradual pinchout upslope.

Aeolian ripple‐produced strata were deposited on horizontal surfaces, and surfaces sloping to as much as 28°. Thickness of a ripple‐produced stratum depended upon rate of deposition, morphology of the ripple, and rate of ripple migration. A maximum thickness of several centimetres was observed for a single ripple‐produced stratum. Shape and attitude of ripple foresets was controlled by ripple morphology. Distinctive features of aeolian ripple‐produced strata are: (a) presence of ripple foresets; (b) abrupt changes in thickness of a stratum or pinchout over downwind distances of a few centimetres; (c) low average foreset‐to‐diastem angle (10–15°); (d) low ripple‐climb angle (<10°).
Transmission electron microscope study of quartz microstructures produced by aeolian bombardment
Authors: VEBLEN DAVID R. , KRINSLEY DAVID H. , THOMPSON MARK
Published: June 2006

The near‐surface structure produced by laboratory aeolian bombardment of Brazilian quartz has been studied with transmission electron microscopy (TEM). Thin specimens of the abraded surface were prepared by a one‐sided ion etching technique, and control specimens of unabraded material were prepared by standard ion etching methods. The abraded sample exhibited a vastly larger number of defects than the unabraded specimen. The abraded sample contained areas with very little surface disruption, presumably where pieces of material up to several tens of microns in diameter had broken away. Other areas of the abraded surface were extremely disrupted, with open and closed microcracks spaced from a few tens of angstroms to a few microns apart. Electron diffraction indicated significant rotation among submicron blocks bounded by the fractures. These observations suggest that surface disruption occurs primarily by a mechanism involving brittle fracture, rather than plastic deformation.

This study demonstrates the feasibility of studying the effects of abrasion on near‐surface structure with TEM. The degree and scale of fracturing also suggest a mechanism for forming submicron particles in quantity during aeolian abrasion.
Zoned calcites in Jurassic ammonite chambers: trace elements, isotopes and neomorphic origin
Authors: MARSHALL JAMES D.
Published: June 2006

Zoned 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

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

The 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

The 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

Irregular, 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

The 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

Thirteen 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

Two 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

Bristol 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

The 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

A 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.
Zabyat Formation, Semail Nappe, Oman: sedimentation on to an emplacing ophiolite
Authors: ROBERTSON A. H. F. , WOODCOCK N. H.
Published: June 2006

ABSTRACT The pelagic sediments of the Semail Ophiolite are depositionally overlain by ophiolite‐derived sediments of the Zabyat Formation, dominated by matrix‐supported rudites, with subordinate arenites and lutites. The detritus was locally derived from the ophiolite, in a deep marine environment indicated by both underlying and overlying pelagic carbonates. The Zabyat rocks are restricted to fault zones in the ophiolite which were active during emplacement. They are interpreted as debris shed from the sub‐vertical fault zones during the initial disruption of the Semail ocean floor, prior to the final emplacement on to the Arabian continental margin. This interpretation places constraints on possible mechanisms of ophiolite emplacement.
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.
The hydrodynamics of particle clusters and sediment entrapment in coarse alluvial channels
Authors: BRAYSHAW ANDREW C. , FROSTICK LYNNE E. , REID IAN
Published: June 2006

ABSTRACT Particles projecting from the bed of an alluvial channel distort the fluid stream to produce a distinctive pressure field. This has considerable significance for both the entrapment and entrainment of other particles and is a primary cause of the widespread occurrence of pebble clusters and boulder shadows. Lift and drag forces are determined on clustered hemispherical particles of varying size. In the wake of an obstructing particle both forces are shown to vary directly with particle separation in a linear fashion. On the stoss side of the cluster, drag is uniform regardless of the separation of the component particles, but lift is shown to increase when particle separation is small, so affecting stability. This mutual interference of neighbouring clustered bed particles is a vital consideration of incipient motion and is shown by field evidence to cause a wide range in transport stage for particles of similar size and shape. On average, 46% of clustered particles are entrained by flood flow compared to 87% of particles in open plane‐beds. The influence of clusters is a major determinant of sedimentary sorting.
An inlet sequence produced by migration of a small microtidal inlet against longshore drift: the Keurbooms Inlet, South Africa
Authors: REDDERING J. S. V.
Published: June 2006

ABSTRACT Lateral migration of the Keurbooms Inlet along its barrier causes stratigraphic stacking of the different sedimentary units in the inlet area thus accumulating an inlet sequence under the newly formed barrier. Deposition in the inlet is controlled by wave and tidal action. Migration is caused by sediment accretion on the downdrift side of the inlet and erosion on its updrift side. The migration direction is therefore in the opposite direction to the longshore current. This is comparatively rare as most inlets with a tendency to migrate do so in the same direction as longshore drift. Tidal Current directions in the inlet are highly variable and not bipolar.

The inlet‐associated environment is divided into a seaward wave‐dominated zone and a landward tide‐dominated zone. These zones probably have laterally interfingering facies. Only the lower part of the sequence is likely to be preserved. Recognition of ancient analogues in the rock record may be obscured by the unidirectional palaeocurrent pattern of the lower part of the inlet sequence.