bluuebunny/crossref_metadata_2025 huggingface dataset:
Depositional processes of the Suwolbong tuff ring, Cheju Island (Korea)
Authors: SOHN YOUNG KWAN , CHOUGH SUNG KWUN
Published: June 2006

The Suwolbong pyroclastic sequence in the western part of Cheju Island, Korea, comprises partly preserved rim beds of a Quaternary basaltic tuff ring whose vent lies about 1 km seaward of the present shoreline. The sequence consists of breccia, lapillistone, lapilli tuff and tuff. Eighteen sedimentary facies are established and organized into six lateral facies sequences (LFS) and seven vertical facies sequences (VFS).

The LFS 1, 4 and 5 begin with massive lapilli tuff which transforms downcurrent into either planar‐bedded (LFS 1), undulatory‐bedded (LFS 4) or climbing dune‐bedded (LFS 5) (lapilli) tuff units. They are representative of relatively ‘dry’ base surge whose particle concentration decreases downcurrent with a progressive increase in both tractional processes and sorting. The LFS 2 begins with disorganized and massive lapilli tuff and transforms into crudely stratified units downcurrent. It results from relatively ‘wet’ base surge in which sorting is poor due to the cohesion of damp ash. The LFS 3 comprises well‐sorted lapilli tuff and stratified tuff further downcurrent, suggestive of deposition from combined fall and surge of relatively ‘dry’ hydroclastic eruption. All seven vertical facies sequences generally comprise two facies units of coarse‐grained fines‐depleted lapilli tuff and an overlying fine‐grained tuff. These sequences are suggestive of deposition from base surge that consists of a turbulent head and a low‐concentration tail.

Depositional processes in the Suwolbong tuff ring were dominated by a relatively ‘dry’ base surge. The base surge comprises turbulent and high‐concentration suspension near the vent whose deposits are generally unstratified due to the lack of tractional transport. As the base surge becomes diluted downcurrent through fallout of clasts and mixing of ambient air, it develops large‐scale turbulent eddies and is segregated into coarse‐grained bedload and overlying fine‐grained suspension forming thinly stratified units. Further downcurrent, the base surge may be either cooled and deflated or pushed up into the air, depending on its temperature. The Suwolbong tuff ring comprises an overall wet‐to‐dry cycle with several dry‐to‐wet cycles in it, suggestive of overall decrease in abundance of external water and fluctuation in the rate of magma rise.
The Scots Bay formation, Nova Scotia, Canada, a Jurassic carbonate lake with Silica‐rich hydrothermal springs
Authors: De WET CAROL C. BIRNEY , HUBERT JOHN F.
Published: June 2006

During Early Jurassic time, the Scots Bay Formation accumulated in an aerobic lake on the floor of the subtropical Fundy rift valley. A diverse biota flourished in the lake as evidenced by algal stromatolites, oncolites, charophytes, ostracods, gastropods, conchostracans, fish bones, calcispheres and logs. Carbonate wackestone and packstone were deposited in the littoral zone and lime mudstone accumulated offshore on a proximal slope. Silica‐rich hydrothermal springs and seeps around and on the floor of the lake precipitated siliceous tufa and silicified adjacent carbonate strata. The tufa is characterized by the progressive sequence: (1) open‐framework, porous substrate of goethite spheres, (2) encrusting chalcedony spherulites where goethite inclusions decrease in size and number outwards towards the surfaces of the spherulites, (3) a fringe of quartz on the chalcedony spherulites and (4) coarsening inwards mosaics of quartz that filled the remaining voids. Carbonate diagenesis included conversion of low‐Mg calcite ooze to micrite and microspar‐pseudospar, and local dissolution and reprecipitation of calcite cement. At times the lake shrank in size, concentrating silica in increasingly alkaline lake water which then silicified some of the carbonate strata and logs. The latest cements were calcite, zeolites and celadonite.
Properties and origin of a very fine‐grained kaolinitic lacustrine deposit
Authors: WALKER P. H. , TAYLOR GRAHAM , EGGLETON RICHARD A.
Published: June 2006

The Rosebrook Member of the Miocene Bunyan Formation is a highly kaolinitic clay‐rock of lacustrine origin. It is ultra‐fine grained, plastic and of high cation exchange capacity (CEC) of 20–33 c mol(+)kg−1 of clay. Fabrics observed optically and by transmission electron microscopy (TEM) indicate significant post‐depositional consolidation and localized shear failure. Sedimentation and TEM analyses confirm its very fine grain size which is consistent with the relatively high CEC.

Tertiary soils and weathering profiles in the catchment of Lake Bunyan developed under relatively moist and cool conditions. Some were gibbsitic with kaolinite and smectite clay minerals. Highly weathered soils (oxisols) on basalt under modern high rainfall environments in the region are dominated by gibbsite and kaolinite. These and other highly weathered soils in the region, formed on a range of parent rocks, have high proportions of very fine grained kaolinite in their subsoils. Such materials are likely to have been abundant in the catchment of Lake Bunyan during the Miocene and would have contributed very fine grained kaolinite directly to the lake by sedimentation. However, the unusually high proportion of kaolinite in the Rosebrook Member and its euhedral crystal form suggest that some of its features developed diagenetically.
The origin of faceted garnets in sandstones: dissolution or overgrowth?
Authors: MORTON A.C. , BORG G. , HANSLEY P. L. , HAUGHTON P. D. W. , KRINSLEY D. H. , TRUSTY P.
Published: June 2006

Faceted garnets from a wide range of geological ages, environments and locations have been studied in polished grain mounts by a combination of backscattered electron microscopy and elemental mapping using energy‐dispersive X‐ray analysis. In all cases, the areas apparently showing positive relief on the faceted garnet surfaces are compositionally identical to the adjacent grain cores despite a wide variation in detrital garnet compositions. In one case, zoning within the grain core can be traced into the faceted areas on the grain surface. Thus, faceted areas must be considered to form part of the original detrital grains. Together with previously published studies on experimental garnet etching, thermodynamic conditions for garnet growth, textural relationships between faceted garnets and authigenic and detrital phases, and distribution of faceted garnets in the subsurface, this paper provides conclusive proof that faceted garnet surfaces form as a result of dissolution, not overgrowth.
Basal Waitemata Group lithofacies: rapid subsidence in an early Miocene interarc basin, New Zealand
Authors: RICKETTS BRIAN D. , BALLANCE PETER F. , HAYWARD BRUCE W. , MAYER WOLFGANG
Published: June 2006

The abrupt transition from coastal and shallow shelf sediments to bathyal sediments provides a record of rapid subsidence and deepening of the early Miocene Waitemata basin. Basal shallow marine strata (Kawau Subgroup) accumulated upon a highly dissected surface that overlies deformed Mesozoic metagreywacke. The early Miocene coast was characterized by an embayed and cliffed shoreline with numerous sea stacks and islands. Kawau Subgroup lithofacies, which include pocket beach, shallow shelf and base‐of‐cliff talus deposits, reflect rapidly changing coastline configuration and water depths as the rugged bedrock surface was buried.

The response to continued rapid subsidence and transgression in Waitemata basin was a decrease in the supply of coarse clastic sediment. Beach gravels were locally displaced to greater water depths by avalanching down steep bedrock slopes. The first bathyal turbidite facies, which abruptly overlie the shallow‐water Kawau Subgroup, include locally derived sediment gravity flows commonly ponded by remnant bedrock submarine highs. When this local supply of sediment had been exhausted, coarse sediment starvation ensued and bathyal muds accumulated. With the resumption of sediment supply and gradual burial of submarine bedrock relief, submarine fans coalesced and increased in lateral extent.

Subsidence of the Waitemata basin to bathyal depths is thought to have occurred in less than a million years. From the above hypothesis, a general model of sedimentation is proposed.
Sedimentology and origin of deposits from a small ice‐dammed lake, Leirbreen, Norway
Authors: DONNELLY ROBERT , HARRIS CHARLES
Published: June 2006

This paper describes recent proglacial lacustrine sediments exposed by the drainage of a small (probably never more than 0·03 km2) ice‐dammed lake basin at Leirbreen, central Jotunheimen, Norway. The dominant facies include ripple‐laminated, massive and horizontally‐stratified sands, massive and horizontally‐laminated silts, and irregularly‐laminated fine sands and silts. The major control on lake circulation and the nature and distribution of these facies was an underflow driven by a subglacial meltwater stream which formed the major lake input. Although much of the sedimentary sequence indicates a pulsatory input, the proximal character of this small lake prevented the development of classic varved silts. Compressional deformation of shoreline sediments was due to winter lake ice push. Other deformational processes included the grounding of icebergs, water escape and syn‐sedimentary downslope collapse. Observations from an adjacent small ice‐marginal lake at Leirbreen provide support for several of the inferences drawn from the sediments of the former ice‐dammed lake.
Diagenetic K‐feldspar pseudomorphs in the Triassic Buntsandstein sandstones of the Iberian Range, Spain
Authors: MORAD SADOON , MARFIL RAFAELA , DE LA PEŇA JOSÉ ANDRÉS
Published: June 2006

Early diagenetic K‐feldspar in the Triassic Buntsandstein of the Iberian Range (Spain) occurs as pseudomorphs after detrital K‐feldspar (Or<93) and plagioclase (Ab<96). These pseudomorphs are chemically pure (Or>99), untwinned, commonly heavily clouded by vacuoles and tiny inclusions, dark‐luminescing and are composed of numerous fine euhedral crystals of K‐feldspar. The latter property suggests that the pseudomorphs form via dissolution of detrital K‐feldspar and plagioclase and precipitation of authigenic K‐feldspar. X‐ray diffraction analysis shows that the authigenic K‐feldspar is intermediate microcline.
The role of the late Cimmerian unconformity for the distribution of kaolinite in the Gullfaks Field, northern North Sea
Authors: BJØRKUM PER ARNE , MJØS RUNE , WALDERHAUG OLAV , HURST ANDREW
Published: June 2006

A study of the Middle Jurassic Rannoch Formation in the Gullfaks Field shows that there is no relationship between the content and distribution of kaolinite and location relative to the late Cimmerian unconformity. From petrographic data most of the kaolinite is interpreted to be detrital, and only traces of authigenic kaolinite are observed. Mass‐balance calculations are used to provide guidelines as to the likelihood of kaolinite being formed and preserved in sandstones under the unconformity. The result of the calculations shows that the propagation rate of the dissolution front in the sandstones was probably on average slower than the erosion rates during the formation of the late Cimmerian unconformity. Hence, transformation of significant amounts of feldspar and mica to kaolinite probably did not take place within the sandstones which at present underlie the unconformity in the Gullfaks Field. Periods of exposure during formation of unconformities may thus be less effective causes of kaolinitization in sandstones than has been assumed.
Diagenesis of radiaxial fibrous calcites in a subunconformity, shallow‐burial setting: Upper Triassic and Liassic, Northern Calcareous Alps, Austria
Authors: MAZZULLO S. J. , BISCHOFF W. D. , LOBITZER H.
Published: June 2006

Multiple generations (I‐IV) of radiaxial fibrous calcite (RFC) cement crusts are intercalated with meteoric internal sediments and microsparitic calcite cements within karst dissolution cavities in Upper Triassic Steinplatte and basal Liassic Hirlatz carbonates in the Northern Calcareous Alps of Austria. Stratigraphic, petrographic and geochemical evidence suggest that RFC precipitated as high‐Mg calcite from marine fluids in the shallow‐burial (<180m) environment. Estimated original δ18O and δ13C isotopic compositions of Late Triassic and Early Jurassic marine waters are −1.88 to −2.21%‰ and +2.37 to +2.89%‰ (PDB), respectively. The RFC subsequently was altered to low‐Mg calcite by meteoric or mixed meteoric‐marine fluids introduced during periods of subaerial exposure.

RFC cement generations I‐IV in Steinplatte reef‐slope beds, and I‐II in Steinplatte reef and lagoon and Hirlatz samples retained near‐original marine oxygen and carbon isotopic compositions (‐1.88 and +2.89%‰ PDB, −2.11 and +2.37%‰ PDB, respectively) and petrographic identity during the conversion to low‐Mg calcite. In contrast, RFC generations III and IV in the Hirlatz and upper Steinplatte were intensely leached and partly replaced by coarse, blocky calcites of meteoric origin, with attendant modification of original crystal fabrics and isotopic compositions (−4.65 to −5.22%‰ PDB δ18O, +2.54 to +3.14%‰ PDB δ13C). These differences in diagenetic behaviour are related to contrasting processes of fluid‐rock systems and original microfabric variations in the RFC cements. The karst‐affected, phreatic‐marine burial environment of RFC formation and subsequent diagenetic alteration inferred for these rocks have not been previously documented in other ancient rocks.
Recent evolution of the Outardes Estuary, Quebec, Canada: consequences of dam construction on the river
Authors: HART B. S. , LONG B. F.
Published: June 2006

The Outardes Estuary (Quebec) is a regressive sandy system developed in a sub‐boreal climatic setting. The Outardes River drains an area of 18 780 km2 and was dammed for hydroelectric purposes in the 1960s and‘70s. By reconstructing the hydrographs and mapping the morphology of the estuary from aerial photographs over a period of 21 years, we have documented the changes in the estuary resulting from the modification of the hydrologic regime.

Until the mid‐1960s, a catastrophic spring freshet (1800–2800 m33s‐1) controlled the morphology of the estuary. The mouth was dominated by a disorganized, braided channel pattern, and large fluvial discharge‐generated transverse bars. Peripheral channel areas in the upper estuary were zones of transport at flood stages. For 18 months in 1968 and‘69 the river discharge was drastically reduced (50 m3s‐1) to accomplish the main reservoir filling. Tidal currents and waves filled and remodelled the morphology of the mouth of the estuary, while the peripheral zones in the upper estuary experienced low‐energy sedimentation. Since the generating plants were brought on‐line in 1969, the fluvial discharge has been unable to modify substantially the sand body geometry in the mouth of the estuary. In the upper estuary the peripheral regions continue to be filled in, but the thalweg is being eroded.

Based on the results of our coring programme, we suggest that major environmental changes may leave a sedimentary signature (addition or subtraction of size modes) which can be identified by the detailed analysis of grain size distributions.
Ripple formation in combined transdirectional steady and oscillatory flow
Authors: YOUNG J. S. LEE , SLEATH J. F. A.
Published: June 2006

Measurements are described of the geometry of ripples formed on beds of sand exposed to a steady current at right angles to an oscillatory flow. Four different sands were studied. The oscillation was produced by an oscillating tray set into the bed of a steady‐flow flume.

It was observed that straight‐crested ripples formed by oscillatory flow would usually develop a ‘serpentine’ form when the superimposed steady current exceeded a certain limit. For amplitudes of the tray velocity U∞ less than about 0.38 m s‐1 this limit corresponded to U∞/Ū*c>31, where Ū*c is the shear velocity measured just upstream of the oscillating tray. It is suggested that the serpentine form is caused by the interaction of vortices carried back and forth between adjacent ripples. On this assumption, the wavelength of the serpentine form would be proportional to the product of period of oscillation and near‐bed steady current velocity. The present measurements appear to support this hypothesis although there is also evidence that the wavelength is influenced by preferred spacing patterns between vortices. The measurements also show the ratio of the amplitude of the serpentine form to its wavelength to be approximately constant.

Empirical relationships are derived relating ripple geometry to flow and sediment properties. It is observed that the influence of Reynolds number and sediment properties on the geometry is very weak. It is suggested that this is typical of ripples formed with relatively low sediment transport rates. It is also found that, under the present experimental conditions, the ripple spacing in the direction of oscillation is almost independent of the magnitude of the steady current and in close agreement with the wavelengths previously measured in an oscillating water tunnel. This suggests that the additional inertia effects associated with oscillating tray rigs were not sufficient to affect bed geometry under the present test conditions.
Dynamics and facies model of a macrotidal sand‐bar complex, Cobequid Bay—Salmon River Estuary (Bay of Fundy)
Authors: DALRYMPLE ROBERT W. , KNIGHT R. JOHN , ZAITLIN BRIAN A. , MIDDLETON GERARD V.
Published: June 2006

The 40‐km‐long, Cobequid Bay—Salmon River estuary has a maximum tidal range of 16·3 m and experiences limited wave action. Sediment, which is derived primarily from areas seaward of the estuary, is accumulating faster than the high‐tide elevation is rising, and the system is progradational. The deposits consist of an axial belt of sands, which is flanked by mudflats and salt marshes in the inner half of the estuary where a funnel‐shaped geometry is developed, and by erosional or non‐depositional foreshores in the outer half where the system is confined by the valley walls. The axial sands are divisible into three facies zones: zone 1—elongate, tidal sand bars at the seaward end; zone 2—sand flats with a braided channel pattern; zone 3—the inner, single‐channel, tidal—fluvial transition. Tidal current speeds reach a maximum in zone 2, but grain sizes decrease headward (from medium and coarse sand in zone 1, to fine and very fine sand in zones 2 and 3) because the headward termination of the major flood channels prevents the coarse, traction population from entering the inner part of the estuary.

Longitudinal progradation will produce a 20‐m‐thick, upward‐fining succession, the lower 1/2–2/3 of which will consist of cross‐bedded, medium to coarse sand deposited on the zone 1 sand bars. The ebb‐dominated portion of this unit will be finer grained than the flood‐dominated part, and will contain trough crossbedding produced by 3‐D megaripples; the flood‐dominated areas, by contrast, will consist mainly of compound cross‐bedding created by sandwaves with superimposed megaripples. Headward migration of swatchways (oblique channels that link the ebb‐ and flood‐dominated areas) will create packages of ebb cross‐bedding that is orientated at a high angle to the long axis of the estuary and that contains headwardinclined, lateral‐accretion surfaces. The overlying fine and very fine sands of zones 2 and 3 will be composed mainly of upper‐flow‐regime parallel lamination. The succession will be capped by a 4‐m‐thick unit of mixed flat, mudflat and salt marsh sediments. A review of other macrotidal estuaries with tidal ranges greater than 10 m suggests that the major elements of the model have general applicability.
Carbonate mineral cycles generated by foraminiferal and pteropod response to Pleistocene climate: west Florida ramp slope
Authors: GARDULSKI ANNE F. , MULLINS HENRY T. , WEITERMAN SANDRA
Published: June 2006

Carbonate ramps are gently sloping depositional surfaces where shallow‐water, coarse‐grained facies pass basinward into fine‐grained, deep‐water sediments, with no abrupt change in slope. The objectives of this study are: (i) to integrate the depositional processes recorded in the Pleistocene stratigraphy of the west Florida outer ramp into a palaeoclimatic and palaeoceanographic framework for the eastern Gulf of Mexico; and (ii) to examine the origin of mineralogical and sedimentary cycles in the light of pteropod and planktonic foraminiferal populations corresponding to climatic oscillations.

Aragonitic, pteropod‐rich sediments with large amounts of insoluble residue occur in sediments deposited during glacial intervals; sandy calcitic sediments with abundant planktonic foraminifera accumulate during interglacials. These cycles reflect variations in biological productivity of pelagic pteropods and planktonic foraminifera, rather than preferential dissolution of either aragonitic or calcitic fractions. Species assemblages suggest that the productivity cycles are linked to changes in upwelling intensity at the margins of the Loop Current and variations in water mass salinities, as well as terrigenous dilution from the Mississippi Delta. These cycles are the response to Pleistocene glacial‐interglacial oscillations, controlled by Milankovitch orbital parameters.

Although the organisms contributing to deep‐water carbonate environments have changed through geological time, facies patterns, as well as sedimentary textures and structures, identified in the west Florida sediments provide criteria for recognition of ancient ramps. An understanding of the processes on a modern ramp slope, such as west Florida, may prove valuable in palaeoclimatic and palaeoenvironmental analysis of epicontinental carbonate sequences and ramps in the rock record.
Hydraulic controls of grain‐size distributions of bedload gravels in Oak Creek, Oregon, USA
Authors: SHIH SHYUER‐MING , KOMAR PAUL D.
Published: June 2006

Grain‐size distributions of gravels transported as bedload in Oak Creek, Oregon, show systematic variations with changing flow discharges. At low discharges the gravel distributions are nearly symmetrical and Gaussian. As discharges increase, the distributions become more skewed and follow the ideal Rosin distribution. The patterns of variations are established by goodness‐of‐fit comparisons between the measured and theoretical distributions, and by Q‐mode factor analysis. Two end members are obtained in the factor analysis, having (respectively) almost perfect Gaussian and Rosin distributions, and the percentages of the two end members within individual samples vary systematically with discharge.

Transformation from Gaussian to Rosin distribution with increasing discharge may be explained by processes of selective entrainment of grains from a bed of mixed sizes. Samples of bed material in Oak Creek follow the Rosin distribution. At high discharges, the transported bedload approaches the grain sizes of that bed‐material source and mimics its Rosin distribution. Random‐selection processes must be more important to grain entrainment at lower discharges, so that the resulting Gaussian distributions of transported bedload reflect similar distributions of bed stresses exerted by the stream flow.

The results from Oak Creek demonstrate that the competence of the flow is reflected in the entire distribution of transported gravel sizes. A sequence of layers of fluvial gravels, modern or ancient, might show systematic variations between coarse Rosin and finer‐grained Gaussian distributions, and these could be used to infer frequencies of various discharges and to establish a relationship to the source sediment. With further study, analyses of changing bedload grain‐size distributions and their transport rates will lead to a better understanding of downstream variations in grain sizes of bed sediments and how their distributions reflect the progressive development of textural maturity.
Cyclic sedimentation in the Southern Alpine Rhaetic: the importance of climate and eustasy in controlling platform‐basin interactions
Authors: BURCHELL MARK T. , STEFANI MARCO , MASETTI DANIELE
Published: June 2006

Uppermost Triassic (Rhaetic) facies, as developed in the Southern Alpine region of Northern Italy, were deposited in a rapidly subsiding, fault‐dissected trough (the Lombardy Basin) bounded by carbonate platforms. The main part of the Rhaetic succession consists of 10‐m‐scale asymmetric cycles, each divided into three parts: a lower shale portion; a central rhythmic portion consisting of repeated marl‐limestone couplets, the limestone parts of which thicken upward; and an upper, wholly carbonate unit. A study of the diagenetic history of the series demonstrates that both the major asymmetric cyclicity and the limestone‐marl couplets of the central rhythmic member (together constituting a ‘compound’ cyclic form) are fundamentally depositional in nature. It is suggested that this compound cyclicity resulted from the superposition of a low‐frequency (approximately 100 000‐year periodicity) asymmetric carbonate mud signal with a higher‐frequency terrigenous mud signal.

Field, petrographic, and geochemical investigations suggest that the basinal carbonate is predominantly allochthonous in origin, having been derived as relatively pure aragonitic mud from adjacent carbonate platforms. It is postulated that the asymmetric carbonate signal was linked to the ecological effects of eustatic fluctuation on platform carbonate systems. Repeated subaerial exposure of subtidal muds in shallow areas indicates that such sea‐level variations occurred. A model is presented in which the basinward export of carbonate was negligible in the deepening phase, increased to a maximum during shallowing and was finally halted by the emergence of large platform flats. In contrast, the higher frequency terrigenous mud signal of the basin is thought to have been climatically modulated; fluctuations of a shorter period than those predicted by the Milankovitch theory affected hinterland precipitation and runoff. Particularly rapid subsidence and high depositional rates may have allowed the preservation of this signal.
Authigenic aluminium phosphate‐sulphates in sandstones of the Mitterberg Formation, Northern Calcareous Alps, Austria
Authors: SPÖTL CHRISTOPH
Published: June 2006

Complex aluminium phosphate‐sulphate minerals of the hinsdalite group have been identified as early diagenetic precipitates in Late Permian sandstones of the Northern Calcareous Alps, western Austria. According to their chemical composition they can be regarded as solid solutions between woodhouseite, svanbergite, crandallite and goyazite. The model proposed for their origin involves the dissolution of detrital apatite in a low‐pH environment and subsequent precipitation of aluminium phosphate‐sulphate minerals, which clearly pre‐date syntaxial quartz cementation of the sediment. Their occurrence probably has been overlooked in other sandstones showing diagenetic apatite dissolution. The recognition of such minerals could provide a significant insight into early phosphate diagenesis.
Late Holocene estuarine phosphogenesis in Raglan Harbour, New Zealand
Authors: CULLEN DAVID J. , CHALLIS G. A. , DRUMMOND G. W.
Published: June 2006

A unique instance of Holocene estuarine phosphogenesis has been investigated in Raglan Harbour, on the west coast of North Island, New Zealand. Nodular calcareous concretions, which formed within surficial estuarine muds between 6500 and 3600 years BP, were superficially impregnated by carbonate fluorapatite between 3600 and 1600 years BP. The phosphatized nodules lie beneath a late Quaternary shore platform, which is reinforced by a mass of intertwined bioturbation structures and tubular rootlet casts impregnated by limonite and goethite. Large polygonal cracks, indicative of a desiccatory regime, are locally conspicuous on the platform surface and in the immediately subjacent muds.

The phosphogenesis in Raglan Harbour is here related to an episode of lowered sea‐level, during which a paludal environment succeeded the estuarine, to be followed, in turn, by a positively and interlude. The subsequent rise of sea‐level to its present position over the last 1600 years or so has reinstated the shore platform in its present intertidal situation.
Sedimentation by river‐induced turbidity currents: field measurements and interpretation
Authors: CHIKITA KAZUHISA
Published: June 2006

Turbidity currents, initiated from spring runoffs of an influent river, were observed in the upper region of a reservoir in Hokkaido, Japan, by measuring water temperature, velocity and suspended‐sediment concentration. Their profiles offer some physical parameters for the sedimentary conditions, assuming the turbidity currents to be quasi‐uniform. The bottom sediment deposited by the turbidity currents was then collected by a portable core sampler. The bottom sediment consists of more than 90% silt and clay, and thus offers a hydraulically smooth bed for shear flow; a plane bed as a bed configuration was formed on the reservoir bed, probably because of the low shear velocity and small grain size of sediment. Using a graphic method with log‐normal probability paper, the bottom sediment is divided into several overlapping log‐normal subpopulations. Grain‐size analysis indicates that the bottom sediment may be regarded as cohesionless; criteria for ‘complete deposition’ of transported grains can then be incorporated into the ‘extended Shields diagram’ giving the minimum shear stress to erode bottom sediment. Applying the new diagram to the grain size distribution of the bottom sediment, it is suggested that each of the log‐normal subpopulations was deposited in each of four different ‘modes of deposition’, i.e. ‘traction’, ‘saltation (or intermittent suspension)’, ‘suspension’ and ‘suspension under equilibrium’. The last mode may be observed under a sedimentary condition where upward flux of suspended sediment by eddy diffusion is almost equal to its depositional flux due to gravity. The mean and critical grain sizes for bottom sediment and each of the corresponding subpopulations decrease consistently with an increase of Ψ=Fd2 log10Re (Fd is the densimetric Froude number and Re is the flow Reynolds number). Ψ correlates inversely with shear velocity, which bears a linear relationship to mean velocity. These results lead to the conclusion that relatively fine suspended sediment is deposited as a result of decreasing bottom friction with a relative decrease of turbulent energy.
An empirical model of aeolian dune lee‐face airflow
Authors: SWEET M. L. , KOCUREK GARY
Published: June 2006

Airflow data, gathered over dunes ranging from 60‐m tall complex‐crescentic dunes to 2‐m tall simplecrescentic dunes, were used to develop an empirical model of dune lee‐face airflow for straight‐crested dunes. The nature of lee‐face flow varies and was found to be controlled by the interaction of at least three factors (dune shape, the incidence angle between the primary wind direction and the dune brinkline and atmospheric thermal stability). Three types of lee‐face flow (separated, attached and deflected along slope, or attached and undeflected) were found to occur. Separated flows, characterized by a zone of low‐speed (0–3O% of crestal speed) back‐eddy flow, typically occur leeward of steep‐sided dunes in transverse flow conditions. Unstable atmospheric thermal stability also favours flow separation. Attached flows, characterized by higher flow speeds (up to 84% of crestal speed) that are a cosine function of the incidence angle, typically occur leeward of dunes that have a lower average lee slope and are subject to oblique flow conditions. Depending on the slope of the lee face, attached flow may be either deflected along slope (lee slopes greater than about 20°), or have the same direction as the primary flow (lee slopes less than about 20°). Neutral atmospheric thermal stability also favours flow attachment.

As each of the three types of lee‐face flow is defined by a range of wind speeds and directions, the nature of lee‐face flow is intimately tied to the type of aeolian depositional process (i.e. wind ripple or superimposed dune migration, grainflow, or grainfall) that occurs on the lee slope and the resulting pattern of dune deposits. Therefore, the model presented in this paper can be used to enhance the interpretation of palaeowind regime and dune type from aeolian cross‐strata.
Clarification of some misconceptions about antidune geometry and flow character
Authors: PRAVE ANTHONY R.
Published: June 2006

Estimates of palaeoflow conditions based on antidune geometries generally are obtained using equations derived from potential flow analysis. Unfortunately, in several previous studies and review articles, misunderstanding and improper substitution of density terms has resulted in two misconceptions (a third is discussed in detail elsewhere). Both concern the commonly derived ratio of antidune wavelength to critical flow depth (L/D). Previously published results which suggest that, for a given set of flow conditions in a two‐layer system, L/D increases with increasing density contrast (Δρ) are wrong. Correct substitutions indicate the opposite—there is a slightly inverse relationship between the value of L/D and Δρ. The second misconception concerns the two values reported for critical flows: L/D≥ 6·3 (or ≥ 2γ) and L/D≥ 12·6 (or ≥ 4γ). Despite insinuations that the latter is wrong by a factor of two, both are correct. In this case, what has been overlooked is that the former value is derived for antidunes on free surface flows (a ‘two‐layer’ system in which ρ2 << ρ1) and the latter for antidunes at the interface of a two‐layer system in which Δp→0 (i.e. ρ2SIMEρ1).
Depositional mechanics and sequences of base surges, Songaksan tuff ring, Cheju Island, Korea
Authors: CHOUGH S. K. , SOHN Y. K.
Published: June 2006

The Songaksan mount in the southwestern part of Cheju Island, Korea, is a Taalian tuff ring produced by phreatomagmatic explosions at an aquifer. A detailed analysis of proximal‐to‐distal facies changes reveals that the tuff ring sequence can be represented by 21 sedimentary facies; one lateral facies sequence (LFS) and three vertical facies sequences (VFS).

The VFS 1 and 2 are representative of facies relationships in horizontal near‐vent deposits. The VFS 1 comprises scour‐fill bedded tuff, inversely graded tuff, massive tuff and laminated tuff from base to top. The VFS 2 is a variant of the VFS 1, replaced by an inversely graded lapilli tuff unit at the base. The sequences suggest traction carpet, suspension and minor traction sedimentation from a high‐concentration near‐vent base surge.

The LFS 1 and the VFS 3 are distilled from outward‐dipping flank deposits. Both sequences begin with disorganized lapilli tuff, followed successively by stratified (lapilli) tuff, dune‐bedded (lapilli) tuff, very thinbedded tuff and accretionary lapilli. They are suggestive of waning base surge which decreases in particle concentration, suspended‐load fall‐out rate and flow regimes with an increase in traction and sorting.

These facies sequences suggest that a base surge experiences flow transformation with its flow characters changing with time and space. A near‐vent base surge is turbulent, uniformly mixed and highly concentrated and produces scour‐fill bedded tuff. As capacity decreases, the surge transforms into a dense and laminar underflow and a dilute and turbulent upper part (gravity transformation), depositing inversely graded, massive and normally graded (lapilli) tuff. Ensuing loss of sediment load and mixing of ambient air result in flow dilution (surface transformation). Stratified and dune‐bedded units are produced by tractional processes of turbulent and low‐concentration surge. Further dilution causes deceleration and cooling and results in precipitation of moistened ash and accretionary lapilli from suspension.
A comparison of an autosuspension criterion to field observations of five turbidity currents
Authors: STACEY MICHAEL W. , BOWEN ANTHONY J.
Published: June 2006

An autosuspension criterion that has been developed directly from the fluid dynamical equations, by taking into account the vertical structure of turbidity currents, is compared to field observations of five turbidity currents. It is found that the criterion is consistent with the motion of all five currents, which suggests that the criterion may, at least under certain circumstances, be a reasonable guide in estimating the conditions necessary for a turbidity current to be self‐sustaining.
Tidally flooded back‐barrier dunefield, Guerrero Negro area, Baja California, Mexico
Authors: FRYBERGER STEVEN G. , KRYSTINIK LEE F. , SCHENK CHRISTOPHER J.
Published: June 2006

A 45 km long barrier island exists west of the town of Guerrero Negro, Mexico, along the western coast of the Baja California peninsula, about 720 km south of San Diego, California. This barrier has developed in a mesotidal, arid‐climate regime characterized by steady, strong, onshore winds from the NW. The barrier island W of Guerrero Negro has prograded seaward about 1·6 km in the last 1800 years while an aeolian dunefield fed by sand blown from beaches has advanced inland up to 13 km. Landward progradation of the dune system from the barrier has occurred during relative rise in sea‐level; thus, aeolian sediments exist at or below the water table over a wide area. The progradation of dunes across marshes, tidal flats, and tidal channels, as well as the repeated submergence of interdune areas by tidal waters, has created a complex suite of mixed aeolian and subaqueous sediments in the back barrier. The complexity of the suites of aeolian sedimentary structures, together with the inclusion of subaqueously formed structures such as current and oscillation ripples, would make recognition of the aeolian origin of much of the sediments difficult in ancient rocks.

In addition to the scientific importance of recognizing the aeolian deposits, the sedimentation model represented by the Guerrero Negro barrier has applications in petroleum exploration and development. Currently, most preservational models for barrier islands attach little volumetric importance to aeolian deposits. This modern example suggests that volumetrically significant aeolian deposits can be preserved behind a barrier, particularly in an arid‐climate regime. If preserved and charged with oil, the resulting productive sandstone could have an extremely irregular landward edge comprised in part of onshore‐prograded aeolian dune sandstone with excellent reservoir characteristics. As with current barrier models, the reservoir would be sealed landward and above by lagoonal mudstone and silt, evaporites, or evaporitic, sandy sabkha deposits. High organic productivity occurs in lagoons immediately adjacent to the dunefields of Guerrero Negro, suggesting that organic‐rich source rock may exist near aeolian sandstone in ancient settings similar to Guerrero Negro.
Ribbon sandstones in the Pennsylvanian Waddens Cove Formation, Sydney Basin, Atlantic Canada: the influence of siliceous duricrusts on channel‐body geometry
Authors: GIBLING MARTIN R. , RUST BRIAN R.
Published: June 2006

Channelized bodies up to 9 m thick in the Pennsylvanian Waddens Cove Formation show low width: thickness ratios (8:1 to 14:1), up to four vertically stacked storeys, and deep incision into associated floodplain strata. The presence of lateral accretion sets up to 6 m thick indicates that the migration of bank‐attached bars was a major channel‐filling process. The progressive rise in elevation of the basal part of the accretion surfaces where the sets adjoin the steeply inclined sides of the channel bodies shows that the channel floor aggraded progressively as the bars approached the channel sides. In planform, the river system probably showed a low‐sinuosity, incised valley within which the channel followed a more sinuous and partially confined course.

The alluvium surrounding the channel bodies includes sandstone sheets attributed to crevasse splay and levee accumulation. The upper strata of many sheet's and some channel bodies contain siliceous duricrusts (ganisters) which formed by pedogenic lithification of sandy material commencing shortly after the landforms became inactive. The channel bodies contain duricrust‐bearing slumpblocks and show stepped margins over duricrusts. The stepped nature of the banks suggests that duricrusts were an important factor in confining the reach, and we suggest that channel‐body geometry was influenced, although not necessarily controlled, by the presence of the lithified layers.

Alternation in the Waddens Cove Formation of grey, coal‐bearing zones with red, duricrust‐rich zones is attributed to water‐table fluctuation in response to base‐level changes. The low width: thickness ratios and aggradation mode'of the channel bodies may in part reflect channel incision and subsequent aggradation resulting from these allocyclic events.
Aeolian landscapes in central Australia: gypsiferous and quartz dune environments from Lake Amadeus
Authors: CHEN XIANG YANG , BOWLER JAMES M. , MAGEE JOHN W.
Published: June 2006

There are two different dune systems in central Australia; regional quartz dunefields and transverse gypsiferous dunes associated with playa lakes. These two systems, especially gypsiferous dunes at Lake Amadeus, the largest playa in central Australia, provide a sedimentary, geomorphological and environmental history of the region during the late Quaternary. The gypsifierous dunes consist of a surficial gypcrete overlying an aeolian sediment sequence below, a mixture of gypsum sand and quartz sand. No clay pellets have been found in the dune sequence, in significant contrast to the gypsiferous clay dunes in other parts of Australia. Three possible models of the environmental controls of gypsiferous dune formation are discussed. The most plausible one suggests simultaneous gypsum precipitation and deflation. Sandsized gypsum was precipitated in a groundwater‐seepage zone around the playa margin during seasonally high water‐tables and these crystals were deflated onto land during dry intervals, forming the marginal gypsiferous dunes. These processes require conditions of high regional water‐table, strong climatic seasonality and probably a windier and overall wetter climate. At least two separate gypsiferous‐duneforming episodes can be recognized. The age of formation of the younger one has been dated by thermoluminescence at 44–54 ka. The gypcrete crust capping the dunes is characterized by intergrown microcrystalline gypsum crystals, showing evidence of leaching, dissolution and recrystallization. It is interpreted as a pedogenic product formed during a stable period after accumulation of the gypsiferous dune. After the construction of the younger gypsiferous dune, there was a major episode of activation of regional quartz dunefields which formed thick quartz sand mantles overlying gypsiferous dunes on both playa margins and the dune islands within the playa. An equivalent aeolian sand layer was deposited within the playa. Soil structures in this unit indicate that the sand sheet over the playa was later colonized by vegetation. Activation of the regional dunefields suggests a major period of dry climate, which, although not dated, may correlate with the last glacial maximum identified as a period of maximum aridity from 25 to 18 ka at other sites in Australia.
Grain‐size distributions of pyroclastic deposits
Authors: LIRER LUCIO , VINCI ANNA
Published: June 2006

A multivariate statistical technique (principal component analysis) has been used to investigate the granulometric features of 414 samples of pyroclastic rocks: 216 samples were collected from areas of recent and active volcanism in Central and Southern Italy; data for the remaining 198 samples were obtained from the literature and relate to products from Somma‐Vesuvio, Mount St Helens and the Japanese volcanic arc. The analysis offers an improved means of discriminating pyroclastic deposits using their grain‐size characteristics.
Detrital carbonate grains as provenance indicators in the Upper Cretaceous Pietraforte Formation (northern Apennines)
Authors: FONTANA DANIELA
Published: June 2006

The Upper Cretaceous Pietraforte Formation, an allochthonous unit of the Ligurian domain in the northern Apennines, provides a case study of the importance of detrital carbonate grains for provenance determination in sandstones.

The Pietraforte Formation is composed of turbidite sandstones with subordinate conglomerate, deposited in an external sector of the Ligurian ocean, close to the Adriatic margin. The sandstones have a lithic composition, characterized by abundant sedimentary and metasedimentary rock fragments (35–56% of the terrigenous framework), little feldspar (<7%) that is almost exclusively plagioclase, and a high ratio of fine‐ to coarse‐grained polycrystalline quartzose grains to total quartzose grains (average Qp/Qt=0.37).

Carbonate rock fragments dominate the lithic association of both sandstones and conglomerates and provide the most detailed information for provenance determination. They are composed primarily of dolostones and a wide variety of limestones containing identifiable age‐diagnostic microfossils. Fossils and rock textures of carbonate clasts document the erosion of Upper Triassic to Lower Cretaceous shelf and pelagic carbonate units which can be matched with Mesozoic rock types present in the Tuscan domain of the northern Apennines. Compositional results constrain the source of the Pietraforte Formation sandstones to the western margin of the Adriatic plate, from uplifted sedimentary and metasedimentary rocks of the Tuscan domain and its low‐grade metamorphic basement.

Coeval intrabasinal sources provided additional supplies to the depositional basin of the Pietraforte Formation; this intrabasinal supply consists of shelf carbonate allochems, planktonic foraminifera and argillaceous rip‐up clasts. The presence of carbonate grains from shallow‐water environments may indicate the existence during deposition of marginal shelf areas favourable for carbonate allochem production.
Pleistocene, multiple‐growth, lacustrine oncoids from the Poacher's Point Formation, Etosha Pan, northern Namibia
Authors: SMITH A. M. , MASON T. R.
Published: June 2006

The Poacher's Point Formation is located within the Etosha Pan, northern Namibia. It contains Pleistocene stromatolites, ooids and silica nodules. The stromatolites were constructed by cyanobacteria in very lowenergy lacustrine conditions. The internal structure of the oncoids is complex and subdivided into shells by unconformities. Each unconformity‐bounded shell probably represents stromatolite growth during a distinct lacustrine event. Shallower conditions are represented by colloform growth and deeper water by smooth oncoid mat morphology. The unconformities represent hiatuses of unknown duration. At least six stromatolite growth shells are recognized. Silica concretions which may have developed during shallowing lake stages are also present. The lack of clastic material in the carbonate lithologies suggests that the lake waters may have accumulated from collecting rainwater or have been spring fed. The lowest bed of the succession is a saline clay which represents a seasonally flooded salt pan deposited in an arid to semi‐arid climate similar to that of today. The overlying carbonates represent sedimentation during a series of transient lacustrine events, representing intermittent wetter periods. Finally, due to progressive aridity, the lacustrine succession was terminated by calcrete development.