bluuebunny/crossref_metadata_2025 huggingface dataset:
Fringing reef growth on a terrigenous mud foundation, Fantome Island, central Great Barrier reef, Australia
Authors: JOHNSON D. P. , RISK M. J.
Published: June 2006

The leeward fringing reef at Fantome Island (central Great Barrier Reef province) is a carbonate body which has developed under the influence of terrigenous sedimentation. The reef flat is up to 1000 m wide and is surfaced by mobile sand and gravel, with almost all live corals restricted to the seaward rim. The reef slope has coral columns and heads on the upper part, but below 5 m water depth it is a muddy substrate with scattered mounds of branching corals.

Three high recovery cores show the reef is up to 10 m thick and developed over a gently sloping terrace of weathered Pleistocene alluvium. Three post‐glacial stratigraphic units are recognised: (1) carbonate reef top unit of coral rudstone and framestone including Sinularia spiculite; (2) lower slope unit of coral floatstone in a terrigenous muddy matrix; and (3) transgressive basal unit of skeletal arkosic sand. The acid insoluble content of matrix and of individual corals increases downwards. Coral growth rates decrease downwards, reflecting slower growth in muddier environments.

Radiocarbon dating shows that the reef prograded seaward at almost stable sea level. An average vertical accumulation rate of 6.7 mm yr‐1 is indicated. Two age reversals are interpreted as material transported by storms or by erosion in response to a late Holocene sea‐level fall.

The carbonate reef top unit has developed adjacent to, and is environmentally compatible with a muddy terrigenous, lower slope unit. Terrigenous influx has not changed during the Holocene, and terrigenous content of sediments is controlled by deposition on the reef slope of fine sediment winnowed from the reef flat and concentration of coarse sediment in the transgressive basal sheet.
Steady state saltation in air
Authors: UNGAR J. E. , HAFF P. K.
Published: June 2006

Coupled equations of motion for steady state saltation over an infinite plane are derived and solved for a simplified model of the grain‐surface impact process. Experimentally observed features of the wind velocity profile in saltation are qualitatively reproduced, including a diminution of the sub‐saltation layer mean wind speed, as the friction speed increases. In this model the surface impact velocity of the saltating grains remains relatively constant over a wide range of free‐stream shear stresses, and the grain mass flux increases with friction speed uf* less rapidly than uf3.
Bedform migration and bed‐load transport in some rivers and tidal environments
Authors: VAN DEN BERG JAN H.
Published: June 2006

Reliable field data obtained by directly measuring bed‐load transport of fine‐ to coarse‐grained bed material are extremely scarce, mainly because of the difficulty of sampling accurately. Therefore, the verification of bed‐load transport formulae is largely based on flume experiments, which refer to unrealistic shallow‐water conditions. In this study, some bed‐load transport formulae were tested against data from natural environments. As an alternative to ascertaining the bed‐load transport rate by sampling the bed‐load, the transport rate was deduced from data on bedform height and bedform celerity. For this purpose, 43 sets of data from rivers, representing a wide range of bed material, bedform dimensions and hydraulic conditions were collected as were some sets of data from tidal settings. Two formulae were used for the prediction of the bed‐load transport: the formula of Van Rijn (1981) and the Kalinske (1947) formula as approximated by Elzerman & Frijlink (1951) (and, in the present study, slightly modified for application to tidal waters). Both the bed‐load function of Van Rijn and the modified formula of Kalinske‐Frijlink require data which are easily obtained and that can be measured accurately.

At those stages of the flow when bed‐load transport was high the Van Rijn function tended to overestimate that transport. For flow stages when bed‐load transport was low the opposite was true. The modified Kalinske‐Frijlink function gave consistently good results: 86% of the transport rates predicted using the river data were within 0·5–2·0 times the values actually measured.
Authigenic natroalunite in middle Miocene evaporites from the Gulf of Suez (Gemsa, Egypt)
Authors: ROUCHY JEAN‐MARIE , PIERRE CATHERINE
Published: June 2006

This paper deals with a new occurrence of sedimentary natroalunite (Na, K) Al3, (SO4)2 (OH)6, in the caprock of a diapir of middle Miocene evaporites from the Gemsa peninsula, located on the southwestern coast of the Gulf of Suez. Field observations, petrographic examinations and stable isotope (18O, 34S) measurements on the associated authigenic phases of anhydrite and native sulphur argue for the genesis of natroalunite at a rather high temperature (i.e. higher than 75°C) by the reaction of clay minerals with sulphuric acid. The sulphuric acid was produced by the concatenation of the following reactions which are thought to increase the diagenetic temperature: bacterial reduction of sulphate evaporites, and oxidation of hydrogen sulphide to native sulphur and sulphate where aerated conditions prevailed in the pore fluids. These changes through time from reducing to oxidizing diagenetic conditions were controlled by the progressive uplift of the diapiric system toward the land surface so that the draining ground waters became progressively oxygenated.
Deep marine arc apron deposits and syndepositional magmatism in the Alisitos group at Punta Cono, Baja California, Mexico
Authors: WHITE JAMES D. L. , BUSBY‐SPERA CATHY J.
Published: June 2006

Rocks exposed at Punta Cono include very fine‐grained to coarse‐grained tuffs, lapilli tuffs, and tuff breccias deposited in a deep marine environment. Syndepositional basaltic intrusive activity was common. In one locality a hyaloclastite‐peperite complex formed. Slumped sections with fluidal basalt ‘clasts’, derived from intrusions that entered the sediment pile from below, are present elsewhere. Abundant soft‐sediment folds in fine‐grained laminated subaqueous fall‐out tuff suggest steep gradients; these are cut by shallow channels filled with coarse‐grained tuff, lapilli tuff, and rare tuff breccia. The combination of marine fossils, extreme textural immaturity, abundant slump features, and syndepositional magmatism indicates deposition upon the submarine flanks of an active volcano.

Recognition of magma‐wet sediment interaction is hampered in volcaniclastic rocks because of the similarity between host and intrusive fragments. Products of magma‐water‐sediment interactions at Punta Maria include: (1) jigsaw‐puzzle hyaloclastite, formed by non‐explosive hydroclastic fragmentation of magma upon contact with water and water‐bearing sediment; (2) peperites, produced by mixing of magma with sediment; and (3) an unusual tuff breccia unit, the result of non‐explosive mixing of ‘wisps’of lava with sediment during remobilization of an unconsolidated section. Low‐explosivity magma‐water‐sediment interactions are favoured by relatively high hydrostatic pressures in sub‐wave base settings.
A computer‐interfaced sedigraph for modal size analysis of fine‐grained sediment
Authors: JONES K. P. N. , McCAVE I. N. , PATEL D.
Published: June 2006

ABSTRACT

A system has been developed allowing the direct storage on disc of size data from the X‐ray scanning settling tube SediGraph 5000ET. Processing of the cumulative percentage data output to produce smooth frequency curves is described. This allows rapid (15 min) and precise (±M 0.1ø) discrimination of size modes, peak heights, median and silt and clay percentages of fine‐grained sediments. An example of a mud turbidite is given showing systematic changes in bulk and modal properties, not visible by eye. Lack of size segregation in the > 1 m thickness of this turbidite shows that it may have been deposited from a highly concentrated flow, rather than from a dilute, waning turbidity current. Such flows may explain ungraded/poorly graded, muddy sediments also known as homogenites, unifites or megaturbidites by other authors.
The Hunghae Formation, SE Korea: Miocene debris aprons in a back‐arc intraslope basin
Authors: CHOE MOON YOUNG , CHOUGH SUNG KWUN
Published: June 2006

ABSTRACT

During early to middle Miocene times a sudden opening of the Ulleung (Tsushima) back‐arc basin in the East Sea (Sea of Japan) led to the development of intraslope basins along the rifted southwestern margin (southeast Korea). Abrupt subsidence resulted in the deposition of the 200 m thick Hunghae Formation (middle Miocene), a sand/mudstone sequence that can be divided into five facies. Facies I (sand and mudstone couplet) and II (coarse sand) are turbiditic in origin, as evidenced by massive, graded, crudely‐layered and parallel‐laminated sand beds. Facies III (homogeneous mudstone) is characterized by various lignite and plant fragments, clastic and biogenic grains that are randomly oriented, suggestive of hemipelagic deposition. Facies IV (chaotic deposit) is characterized by the disruption of beds, the presence of isolated siltstone blocks (or balls) and large clasts in the muddy matrix, indicative of retrogressive rockfall and slide/slump. Facies V (conglomerate) is of debris flow origin, as evidenced by clast‐ and matrix‐supported features, floating large clasts and absence of traction structures.

Individual facies are organized into two types of facies association: (1) homogeneous mudstone (facies III) associated randomly with the rest (facies I, II, IV and V), indicative of hemipelagic and episodic sediment‐gravity flow processes, respectively; (2) conglomerate (facies V), coarse sand (facies II) and sand/mudstone couplet (facies I), representing the flow transformation from debris flow to high‐ and low‐concentration turbidity currents. These facies associations are similar in many respects to modern and ancient debris (or slope) aprons found elsewhere. Numerous isolated slide/slump blocks, wedged conglomerates with armoured mudstone balls, discontinuous lignite‐containing sand/mudstone beds, chaotic structure and growth faults suggest that the deposition occurred on a steep slope (intraslope basin) off coalescing fan‐deltas, mainly by unchannellized sediment‐gravity flows. Ancient deposits with irregular facies sequences can be viewed as debris‐apron systems, which provide alternatives to submarine‐fan models in many clastic basins with a line rather than point source.
Soft‐sediment deformation (fluid escape) features in a coarse‐grained pyroclastic‐surge deposit, north‐central New Mexico
Authors: Nocita Bruce W.
Published: June 2006

ABSTRACT

The Puye Formation in north‐central New Mexico is a very coarse‐grained fanglomerate which was deposited on the eastern flank of the Jemez caldera. Pyroclastic deposits occur within the Puye in the form of airfall pumice beds and the remnant of at least one pyroclastic‐surge deposit. This pyroclastic‐surge deposit shows the effects of fluidization and soft‐sediment deformation in the form of: (1) intrusive sedimentary plumes; (2) upwardly injected gravelly pipes; (3) ‘pocket structures’ similar to those of Postma (1983), and; (4) oversteepened and deformed cross‐stratification.

Fluidization and soft‐sediment deformation resulted from a combination of the mechanical instability and high, possibly pressurized, fluid content of the deposit. This metastable condition was a consequence of the nature of the flow which deposited the sediment: a rapidly depositing, high‐velocity sediment gravity flow. The fluids in pyroclastic surges may be either gas or liquid. However, because of the coarse grain‐size of the fluidized sediment, it is suggested that liquids were responsible for the features described in this paper. Evidence also suggests that locally fluidization, liquefaction, and soft‐sediment deformation took place penecontemporaneously with deposition.
Late Pleistocene mixing zone dolomitization, southeastern Barbados, West Indies
Authors: HUMPHREY JOHN D.
Published: June 2006

ABSTRACT

Field, geochemical, and petrographic data for late Pleistocene dolomites from southeastern Barbados suggest that the dolomite precipitated in the zone of mixing between a coastal meteoric phreatic lens and normal marine waters. The dolomite is localized in packstones and wackestones from the algalAmphistegina fore‐reef calcarenite facies.

Stable isotopic evidence suggests that meteoric water dominated the diagenetic fluids responsible for dolomitization. Carbon isotopes in pure dolomite phases average about ‐15%0 PDB. This light carbon is attributed to the influence of soil gas CO2, and precludes substantial mixing with seawater. A narrow range of oxygen isotopic compositions coupled with a wide range of carbon compositions attest to the meteoric diagenetic overprint. Dolomitization likely occurred with as little as a five per cent admixture of seawater.

Strontium compositions of the dolomites indicate probable replacement dolomitization of original unstable mineralogy. The dolomite is characterized by low sodium values. Low concentrations of divalent manganese and iron suggest oxidizing conditions at the time of dolomitization.

A sequence of petrographic features suggests a progression of diagenetic fluids from more marine to more meteoric. Early marine diagenesis was followed by replacement dolomitization of skeletal grains and matrix. Limpid, euhedral dolomite cements precipitated in primary intra‐ and interparticle porosity subsequent to replacement dolomitization. As waters became progressively less saline, dolomite cements alternated with thin bands of syntaxial calcite cement. The final diagenetic phase precipitated was a blocky calcite spar cement, representing diagenesis in a fresh‐water lens. This sequence of diagenetic features arose as the result of a single fall in eustatic sea‐level following deposition.

A stratigraphic‐eustatic‐diagenetic model constrains both the timing and rate of dolomitization in southeastern Barbados. Dolomitization initiated as sea‐level began to fall immediately following the oxygen isotope stage 7–3 high stand, some 216 000 yr bp. Due to the rapidity of late Pleistocene glacio‐eustasy, dolomitization (locally complete) is constrained to have occurred within about 5000 yr.
The diagenetic history of some septarian concretions from the Kimmeridge Clay, England
Authors: ASTIN T. R. , SCOTCHMAN I. C.
Published: June 2006

ABSTRACT

Large septarian concretions from the Kimmeridge Clay, up to 1.2 m in diameter, have centres comprising anhedral calcite microspar passing into margins of radiating fibrous calcite microspar, with a pyrite‐rich zone at the transition. Septarian veins formed and were lined with brown calcite synchronously with fibrous matrix growth, with white calcite precipitated in septarian cavities after concretion growth ceased. Septarian veins, filled only with white calcite, formed later, at the same time as the outermost calcite microspar crystals were enlarged.

The concretions were buried in the Late Jurassic to about 130 m, and in the Late Cretaceous to about 550 m, with uplift between. Oxygen isotopes show that the concretion grew throughout the first burial, with septarian veins forming from about 30 m depth onwards. Later septarian veins formed between about 200 and 500 m during the second burial.

Carbon isotopes show that the compact inner matrix grew in the sulphate reduction zone, the end of which is marked by the pyrite‐enriched zone. Dissolving shells, and possibly minor methanogenic carbonate, slowly diluted sulphate reduction‐zone carbonate during deeper burial. During early concretion growth, Mg and Sr were depleted in the pore water. During later stages of the first burial, Mg, Sr, Mn and Fe all increased, especially after concretion growth ceased. During the second burial, Fe, Mn and Mg decreased as calcite precipitated, implying relatively closed systems for these elements.

Synchronous formation of septarian fractures and fibrous calcite matrix shows that the Kimmeridge Clay became overpressured during the later stages of both burials.
Textural and geochemical alternations in Late Cenozoic Bahamian dolomites
Authors: DAWANS JOHN M. , SWART PETER K.
Published: June 2006

Dolomitized intervals of a core from San Salvador Island, Bahamas, exhibit variations of two texturally and geochemically distinct end‐members. In the Pliocene section of the dolomitized interval, the two end‐members alternate in a pattern that may reflect originally and/or diagenetically modified depositional facies. Formerly mud‐free intervals, locally capped by exposure surfaces are massive crystalline, mimetic dolomites (CM). Muddier sediments are replaced by friable microsucrosic dolomites (MS). CM and MS dolomites also differ in porosity (< 10% vs > 30%), permeability (< 10 md vs > 100 md), mol% MgCO3 (44–9 vs 47–7) mol%), oxygen isotopic composition (1–7 vs 2–7‰) and strontium content (241 vs 106 ppm).

These data indicate that depositional and diagenetic fabric are the principal controls governing the distribution of dolomite types. Differences in texture and geochemistry are suggested as arising through differential rates of crystallization produced as a result of variations in permeability and reactivity of the precursor sediments and rocks.
The genesis of nodular limestones in the Ordovician and Silurian of the Oslo Region (Norway)
Authors: MÖLLER NICOLA KERSTIN , KVINGAN KNUT
Published: June 2006

Nodular limestones have been studied from the Lower Chasmops Shale (Middle Ordovician) and the Rytteráker Formation (Lower Silurian). Observations on nodule‐host‐rock relations and variations of ferroan/non‐ferroan calcite cements help explain the role of precipitation, dissolution and redistribution of carbonate.

Distribution and frequency of nodules depends on environmental parameters such as carbonate/clay ratio, grain size distribution and bioturbation, though final shapes are the result of pressure‐dissolution and cleavage.
Evidence for surface reaction‐controlled growth of carbonate concretions in shales
Authors: RAISWELL R.
Published: June 2006

Estimates for the rate of concretionary growth in shales are based on models which assume that growth is diffusion‐controlled. However, laboratory and field studies of CaCO3 precipitation in organic carbon‐rich sediments indicate that surface reactions control growth, due to inhibition by various dissolved species. The spatial distribution of carbonate concretions in the Jet Rock (Lower Jurassic, England) is also inconsistent with diffusion‐controlled precipitation of CaCO3 into concretions, and growth must have been at least partly surface reaction‐controlled.
Sur submarine slide, Monterey Fan, central California
Authors: NORMARK WILLIAM R. , GUTMACHER CHRISTINA E.
Published: June 2006

The Sur debris slide and associated debris flow cover more than 1000 km2 of the eastern part of Monterey submarine fan and extend from the base of the continental slope near the apex of the fan to the Monterey fan valley. The flow is generally confined between the continental slope and remnants of an older channel (Monterey East fan valley). The hummocky surface of the debris slide and diffuse echo returns from the surface of the nearly acoustically transparent debris flow, as seen in 3.5 kHz profiles, are common to large submarine slides described from passive‐margin continental‐slope and rise deposits. Piston cores from the Sur slide recovered contorted and sheared fan turbidite units as well as clast‐ and matrix‐supported mudlump debris flows. The cores show that the slide debris is overlain by 0.23–0.87 m of mud and turbidite sand that limit the age of the slide to latest Holocene, although more than one stage of emplacement is possible.
Magnesian calcite cements and their diagenesis: dissolution and dolomitization, Mururoa Atoll
Authors: AISSAOUI DJAFAR M.
Published: June 2006

The most ubiquitous syn‐sedimentary cements affecting Mururoa atoll are composed of magnesian calcite. Three main types are distinguished: fibrous, bladed and sparitic on the basis of petrography, morphology and MgCO3 concentration of the constituting crystals, while peloid infills, a particular form of HMC chemical precipitation, also exist. Petrographic evidence and isotopic signatures are compatible with marine precipitation.

Mururoa atoll was exposed several times to meteoric diagenesis resulting in varied diagenetic alterations including selective dissolution and partial dolomitization of Mg‐calcite cements. These alterations are responsible for substantial modifications of the initial cement fabrics and may introduce unconformities in the diagenetic chronology. The first stage of the partial dissolution of Mg‐calcite induces the development of chalky, white friable zones within the initially crystalline, hard cement layers. At ultrascale, this is due to the creation of micro‐voids along the elongate cement fibres. Advanced dissolution includes total disappearance of cement portions as attested to by large voids within the cement crust and/or between superposed cement layers. Mg‐calcite dissolution is related to meteoric diagenesis during periods of Quaternary exposure. The creation of voids within Mg‐calcite layers is due to the mechanical removal of previously altered calcium carbonate, a process suggesting marine or non‐marine water flow, probably in the vadose environment.

Selective dolomitization of Mururoa cements involves alternations of calcite and dolomite which form successive cement‐like rinds within primary cavities. At Mururoa, these alternations are the result of selective dolomitization of the pre‐existing Mg‐calcite cements rather than successive precipitation of calcite and dolomite. Selective dolomitization of Mg‐calcite cements at Mururoa indicates that a given cement succession is not necessarily a simple chronological sequence. Oxygen isotope values of dolomites are enriched in δ186 by about 3‰ PDB within calcite‐dolomite pseudo‐alternations. The dolomitizing fluid at Mururoa seems similar to present marine water although some mixture with meteoric water is probable to favour dissolution associated with dolomitization.
Cathodoluminescent bimineralic ooids from the Pleistocene of the Florida continental shelf
Authors: MAJOR R. P. , HALLEY R. B. , LUKAS K. J.
Published: June 2006

A bored and encrusted late Pleistocene ooid grainstone was recovered from the seafloor at a depth of approximately 40 m on the outer continental shelf of eastern Florida. Ooid cortices are dominantly bimineralic, generally consisting of inner layers of radial magnesian calcite and outer layers of tangential aragonite. Ooid nuclei are dominantly rounded cryptocrystalline grains, although quartz grains and a variety of skeletal grains also occur as nuclei. Ooids are partially cemented by blocky calcite, and interparticle porosity is partially filled by micrite.

Radial cortex layers are composed of brightly cathodoluminescent magnesian calcite having a composition of approximately 12 mol% MgCO3 and 1000 ppm strontium. The iron and manganese concentrations in radial cortex layers are generally in the range of 500–1000 ppm and 100–250 ppm, respectively. Tangential cortex layers are composed of noncathodoluminescent aragonite containing approximately 11 500 ppm strontium and less than 0.5 mol% MgCO3. Iron concentrations in tangential cortex layers are generally in the range of 150–400 ppm, and manganese concentrations are generally below the detection limit of 100 ppm. Echinoderm skeletal fragments, which are present as accessory grains, are composed of brightly cathodoluminescent magnesian calcite. Some ooid nuclei and the thin outer edges of some blocky calcite cement are cathodoluminescent; micrite matrix and the bulk of blocky calcite cement are noncathodoluminescent. Ooids do not exhibit textural evidence of recrystallization.

The ooid grainstone underwent an episode of meteoric diagenesis. but ooid cortices were not affected by the event. We propose a previously unrecognized process by which the magnesian calcite cortex layers underwent diagenetic alteration in oxygen‐depleted seawater. During this diagenesis, magnesium was lost and manganese was incorporated without apparent textural alteration and without mineralogical stabilization. Thus, we Suggest that cathodoluminescence may result from diagenetic alteration on the sea‐floor.
Sedimentology of proximal to distal volcaniclastics dispersed across an active foldbelt: Ellensburg Formation (late Miocene), central Washington
Authors: SMITH GARY A.
Published: June 2006

Late Miocene volcaniclastics of the Ellensburg Formation (Washington, northwestern USA) are exposed in basins within an integrated palaeodrainage over the depositional reach from 15 to 120 km eastward from a Cascade Range source area. Two facies associations are recognized. The first is composed of laterally restricted, well‐sorted, polymictic conglomerates representing a gravel‐bedload regime during inter‐eruption periods. The second comprises laterally extensive sheets, bounded by deeply‐developed paleosols, composed of monomictic pebbly, pumiceous dacitic sandstones with intercalated debris‐flow and hyperconcentrated flood‐flow deposits. These sheets aggraded in response to eruptive events that are sometimes recorded by air‐fall tephras at the bases of sequences. Debris‐flow and hyperconcentrated flood‐flow deposits occur as far as 120 km from source, but are uncommon beyond 50 km. Hyperconcentrated flood‐flow deposits initially increase in abundance away from source as debris‐flow deposits diminish, suggesting formation of hyperconcentrated flood flows by dilution of debris flows. Sandy facies form broad sheets dominated by scour‐fill bedding, in proximal and medial settings, and grade to narrower, trough cross‐bedded sheets in distal settings, suggesting moderation of flood discharges with distance. Base‐level changes associated with episodic sediment influx caused incision‐aggradation cycles in mainstream settings, and episodic impoundment of tributaries to form lakes or rapidly aggrading, poorly drained floodplains. Although volcanism was the primary control on depositional style, concurrent development of the Yakima fold belt produced a structure‐consequent drainage pattern that determined sediment dispersal, and basin subsidence permitted preservation of both syn‐eruption and inter‐eruption facies. Detritus from rising anticlines was generally diluted by the volumetrically superior extrabasinal volcaniclastics, but dominates deposits of small tributary streams flowing toward, rather than from, the Cascades.
Depositional processes on an ancient and modern muddy shelf, northern California
Authors: LEITHOLD ELANA L.
Published: June 2006

The modern Eel River shelf and analogous Pleistocene Rio Dell Formation in northern California provide an ideal opportunity to combine the advantages of studying a modern environment with those of studying an ancient sequence, and thereby enables further understanding of muddy‐shelf processes. The modern shelf is the site of accumulation of a thick deposit of Holocene mud. Both large‐scale sediment distribution patterns and small‐scale stratigraphy on the shelf indicate that river floods play an important role in sediment accumulation, even on a high‐energy, ‘storm‐dominated’ coast. The major factors in preservation of this flood ‘signature’ are the cohesive behaviour of fine‐grained sediments and episodically rapid rates of sediment input.

The Rio Dell Formation includes approximately 400 m of mostly fine‐grained shelf deposits that accumulated offshore from a palaeo‐Eel River mouth. The shelf sediments comprise four depositional sequences. Sequence 1 records progradation from outer to inner shelf depths. Facies trends closely resemble across‐shelf trends on the modern shelf, suggesting that processes were similar. Detailed examination of these deposits provides insight into the nature and role of various processes on both the ancient and modern shelf.

Muddy facies of the Rio Dell sequence are characterized by bioturbated, clayey silts, interbedded with event layers of several types. Clay‐rich silt layers are interpreted as flood deposits and physically stratified, coarse‐silt layers are interpreted to record transport and deposition of coarse silt on the midshelf during storms. Sediment‐transport calculations and consideration of grain‐size distributions of bioturbated sediments, which form the bulk of the Rio Dell sequence, suggest that these sediments are the result of biological homogenization of the fine‐grained flood deposits and of the coarser‐grained storm deposits. The results of this study in general indicate that fine‐grained shelf deposits do preserve a distinguishable, if subtle, record of depositional processes.
Bedload transport of mud as pedogenic aggregates in modern and ancient rivers
Authors: RUST BRIAN R. , NANSON GERALD C.
Published: June 2006

Abundant sand‐sized mud aggregates in the Cooper and Diamantina Rivers, Lake Eyre Basin, Australia are attributed to bedload transport of aggregates formed in deeply‐cracked floodplain soils. The conditions required for formation of pedogenic mud aggregates are: (i) abundant clay containing at least minor swelling clay, and (ii) a climate with at least seasonally hot dry periods. The worldwide distribution of these soils (Vertisols) suggests that a significant amount of mud is transported as pedogenic aggregates by modern rivers.

Ancient analogues in which mud aggregates and Vertisol profiles have been recognized are the Jurassic East Berlin Formation (Connecticut, USA) and the Carboniferous Maringouin Formation (New Brunswick and Nova Scotia, Canada). The dominant red mudstones of these formations are interpreted as mainly bedload sediments deposited by sheet floods in semi‐arid palaeoclimates. The Triassic Hawkesbury Sandstone (NSW, Australia) also contains sand‐sized mudstone aggregates, thought to be pedogenic, but its paleosol and other facies point to formation in a wetter palaeoclimate. The indications are that bedload transport of mud as pedogenic aggregates was as significant a process in ancient rivers as it is at present.
Early Cambrian reefs, reef complexes, and associated lithofacies of the Shackleton Limestone, Transantarctic Mountains
Authors: REES MARGARET N. , PRATT BRIAN R. , ROWELL A. J.
Published: June 2006

The Shackleton Limestone formed a carbonate platform that bordered part of the Greater Antarctic craton in middle and late Early Cambrian time. In the Holyoake Range of the central Transantarctic Mountains, this unit records deposition on a stable shelf on which flourished ecological reefs composed of microorganisms and archaeocyathans. Burrow‐mottled lime mudstone, wackestone and packstone with patch reefs represent accumulation in shelf areas of relatively low to moderate energy. Thick ooidal grainstone units reflect deposition in higher energy shoals and as sand sheets that were associated with extensive reef complexes.

The framework of these reefs was principally the product of micro‐organisms, by inference mostly cyanobacteria. Archaeocyathans constitute as much as 30% of some reefs, but commonly they form less than 10% and are absent from some. On the basis of microbial composition, three reef types are recognized. The first type is a Renalcis boundstone that lacks archaeocyathans. Within these, abundant upward‐directed thalii of Renalcis formed a framework that trapped fine‐grained sediment. The second type, which forms the core of some larger reefs, is composed of stromatactis‐bearing, microbial boundstone. The third, yet most common, reef type is variable in composition. It is characterized by the presence of abundant Epiphyton, but may include archaeocyathans, and the microbial microfossils Girvanella and Renalcis as well as cryptomicrobial clotted micrite. In this type of reef, frame‐building organisms typically constructed highly porous structures that had small interparticle and fenestral pores and large growth‐framework cavities, as well as rare metre‐sized caverns. Within these spaces, Epiphyton and, less commonly Renalcis, encrusted framework elements, fine‐grained sediments accumulated, and pervasive sea‐floor cements were precipitated.

Boundstone fabrics in the Shackleton Limestone are highly complex, with fabrics analogous to younger, more metazoan‐rich reefs, as well as deep‐water stromatactis‐bearing mud‐mounds. The Epiphyton‐Girvanella‐archaeocyathan frameworks and stromatactis‐bearing boundstones, both of which seemingly first appeared in the middle Early Cambrian, are regarded as the precursors, in structure, composition, and preferred hydrologic setting, of the more extensive reefs and complex framework styles of later Phanerozoic time.
Environmental control on diverse stromatolite morphologies in the 3000 Myr Pongola Supergroup, South Africa
Authors: BEUKES NICOLAS J. , LOWE DONALD R.
Published: June 2006

A unique outcrop of partly silicified dolomite in the White Umfolozi section of the Pongola Supergroup, South Africa indicates that stromatolites were diverse and adapted to a range of shallow, tidal depositional settings 3000 Myr ago. Composite columnar stromatolitic bioherms 0.7‐1.6m high and 0.4‐1.0m in diameter formed along the margins of a tidal channel. They were flanked, away from the channel, by flat stratiform and small domical stromatolites growing in low energy tidal flat environments. Conical stromatolites, 0.05‐0.30m high and 0.03‐0.10m in diameter, accreted in high‐energy coarse‐grained carbonate sand along the bottom of the tidal channel. The stromatolites probably formed through the activities of filamentous, oxygen‐producing, photoautotrophic cyanobacteria.
Anatomy of an avulsion
Authors: SMITH NORMAN D. , CROSS TIMOTHY A. , DUFFICY JOSEPH P. , CLOUGH STEPHEN R.
Published: June 2006

The Cumberland Marshes in east‐central Saskatchewan, Canada, occupy over 5000 km2and contain a variety of active and abandoned fluvial features, including straight to sinuous isolated channels, anastomosed channel systems, levees, and crevasse splays in addition to marshes, lakes and bogs. In 1873, an avulsion of the Saskatchewan River diverted most of its flow into a portion of the Cumberland Marshes (locally termed the breakout area), and altered the alluvial terrain as the invaded wetlands adjusted to the influx of sediment and water. These adjustments continue today, and so far over 500 km2of wetlands have been affected by the avulsion.

Avulsion‐controlled modification of the wetlands involves the initiation and evolution of crevasse splays and splay complexes. Three intergradational forms are recognized, each associated with characteristic sand‐body geometries. Stage I splays are small, lobate in plan, crossed by unstable distributary channels, and form wedge‐shaped sheets which depositionally overlie fine‐grained, organic‐rich wetland sediments. Stage II splays and splay complexes evolve both spatially and temporally from Stage I splays. They are larger, contain dense networks of anastomosed channels, and form disconnected tabular sand bodies or continuous sand sheets, some of which incise underlying wetland sediments. Stage III splays develop from either Stage I or II splays and contain few but stable anastomosed channels that deposit isolated stringer sands encased in fine‐grained floodplain sediments. Although sand bodies deposited by splays comprise important components of the evolving floodplain, various fine‐grained facies occurring in levees, shallow lakes, abandoned splay channels, and interchannel floodplains dominate the avulsion deposits.

The post‐1873 record of deposition and terrain modification in the breakout area suggests four stages of floodplain evolution following avulsion. In theinitial avulsion stage,new channels and splay complexes increase in numbers rapidly as diverted discharge of water and sediment overwhelm the adjacent floodbasin. Theanastomosed stageis characterized by an approximate balance between rates of new channel and splay development, and abandonment of old ones. This stage continues for as long as new floodplain areas are invaded. The rate of new splay development eventually decreases as accessible floodplain becomes aggraded, forcing a higher rate of channel abandonment and concentration of remaining flow into fewer but larger channels (reversion stage). The result of reversion is eventual return to asingle channel stage,completing the avulsive sequence and initiating a new alluvial ridge. For the Saskatchewan River, this final‐stage single channel will likely produce a meander belt which occupies only a portion of the more extensive avulsion belt which preceded it.
Marine pisolites from Upper Proterozoic carbonates of East Greenland and Spitsbergen
Authors: SWETT KEENE , KNOLL ANDREW H.
Published: June 2006

Upper Proterozoic carbonate successions from central East Greenland (the Limestone‐Dolomite ‘Series’ of the Eleonore Bay Group) and Svalbard (the Backlundtoppen Formation of the Akademikerbreen Group, Spitsbergen, and the Upper Russö Formation of the Raoldtoppen Group, Nordaustlandet) contain thick sequences dominated by pisolites. These rocks were generated in shallow marine enviroments, and the pisoids are essentially oversized ooids. A marine environment is supported by the thickness and lateral extent of the carbonates; by a sedimentary association of pisolites with stromatolites, flake‐conglomerates, calcarenites, calcilutites, microphytolites, and ooids similar to that found in numerous other Proterozoic carbonate successions; by sedimentary structures, including cross‐beds and megaripples that characterize the pisolitic beds; and by microfossils of endolithic cyanobacteria that are specifically comparable to microorganisms that inhabit modern marine ooids of the Bahama Banks. Petrographic features and strontium abundances suggest that the pisoids were originally aragonitic, but neomorphism, silicification, calcitization, and dolomitization have extensively modified original mineralogies and fabrics. The East Greenland and Svalbard pisolitic carbonates reflect similar depositional environments and diagenetic histories, reinforcing previous bio‐, litho‐, and chemostratigraphic interpretations that the two sequences accumulated contiguously in a coastal zone of pisoid genesis which extended for at least 600, and probably 1000 or more, kilometres.
Coagulation and transport of sediments in the Gironde Estuary
Authors: GIBBS R. J. , TSHUDY D. M. , KONWAR L. , MARTIN JEAN MARIE
Published: June 2006

The distribution of suspended particle size and concentration were measured along the Gironde Estuary, France, from the river seaward to the ocean. The suspended particle size and volume concentration were measured using in situ holography and onboard optical techniques utilizing special procedures in order to avoid floc breakage.

Sediments discharged by the rivers coagulate upon encountering the very low salinities (0.2‰ of the upper estuary (confirmed with laboratory experiments), and are then transported and deposited by currents in the remainder of the estuary. This coagulation, coupled with estuarine circulation, produces a turbidity maximum which is offset between the surface and bottom waters. The floc size maximum is oceanward of the turbidity maximum and is, likewise, offset along the estuary by about 30 km. The estuary can be subdivided into the following zones: (1) coagulation; (2) hydrodynamic, landward of the null point; and (3) hydrodynamic, seaward of the null point. Initial coagulation appears to be completed in coagulation zone (1), and particles are transported and settled (with very little floc breakage and recoagulation) in zones (2) and (3) only. The floc settling velocities, coupled with estuarine circulation, control the concentration and size distributions of flocs in the water column, and eventually control the deposition of sediments.
Fluvial‐aeolian interactions: Part II, ancient systems
Authors: LANGFORD R. P. , CHAN M. A.
Published: June 2006

An understanding of fluvial‐aeolian deposition derived from modern case‐examples in a previous study is applied to the Permian Cutler Formation and Cedar Mesa Sandstone on the Colorado Plateau. These formations supply an excellent three‐dimensional exposure of intertonguing fluvial and aeolian strata. Four distinct facies associations form the bulk of the Cutler Formation and Cedar Mesa Sandstone: (1) aeolian dune deposits; (2) wet interdune deposits; (3) fluvial channel deposits; and (4) overbank‐interdune deposits. In addition, two distinctive types of erosion surfaces are found within the Cutler Formation and Cedar Mesa Sandstone: pebble‐ to granule‐rich erosion surfaces (aeolian deflation surfaces) and flood surfaces.

Fluvial and aeolian intertonguing result in extensive tabular sheets of aeolian sandstone separated by flood surfaces and overbank‐interdune deposits. Fluvial channels are associated with the deposits overlying flood surfaces and are incised into the underlying aeolian sandstones. Overbank‐interdune deposits and wet interdune deposits cover flood surfaces and intertongue with overlying aeolian sandstones.

The primary characteristics of ancient fluvial‐aeolian deposition are overbank‐interdune deposits and pronounced extensive erosion surfaces (flood surfaces), which are parallel to underlying fluvial sandstones and thus trend parallel to the palaeoslope and palaeohydrological gradient.
Submarine sedimentation on a developing Holocene fan delta
Authors: PRIOR DAVID B. , BORNHOLD BRIAN D.
Published: June 2006

The submarine morphology, sediments, and three‐dimensional geometry of a developing fan delta are described using data from acoustic surveys, bottom sampling, and observations from a manned submersible. The fan system is being built in a British Columbian fjord (water depth 410 m) supplied with coarse‐grained sediments from a fjord‐side river.

Construction of the subaqueous fan began about 10–12,000 yr BP and is ongoing. The system is analogous to part of one fault‐uplift sedimentation cycle in ancient fan deltas. Initially, when offshore relief was at a maximum, acoustically chaotic sediment wedges were emplaced over fjord‐bottom glaciomarine deposits. Subsequent aggradation/progradation resulted in moderately dipping sequences interrupted by local chaotic units. The present fan surface (average slope 13°) is divided into six zones arranged concentrically from the fan apex, on the basis of form, sediment and process interpretations. Continued subaqueous fan growth results from settling of river‐derived sediments from suspension and downslope sediment dispersal by episodic gravity flows, apparently fed by underflows from the river.
Depositional environments and diagenesis in Lake Parakeelya: a Cambrian alkaline playa from the Officer Basin, South Australia
Authors: SOUTHGATE P. N. , LAMBERT I. B. , DONNELLY T. H. , HENRY R. , ETMINAN H. , WESTE G.
Published: June 2006

Laminated and desiccated siliceous dolostones, dolomitic mudstones and dolomitic sandstones in the Cambrian Parakeelya Alkali Member of the Observatory Hill Formation accumulated in an alkaline playa. Four facies are recognized (from lake centre to lake edge): lake, saline mudflat, dry mudflat and sandflat facies. The facies occur in cycles. Cycles of tens of metres thickness record the gradual expansion and contraction of the playa. Superimposed smaller cycles of tens of centimetres thickness record minor oscillations in the position of the strandline in a shallow lake. The dominance of saline mudflat, dry mudflat and sandflat facies indicates that the lake was rimmed by broad flat areas with negligible relief. The high δ18O values of primary and penecontemporaneous diagenetic carbonates of +24 to +28‰ (SMOW) indicate strong evaporation of ground and surface waters within the lake system. Calcite pseudomorphs of the sodium carbonate minerals trona and shortite have δ18O values between + 19 and + 22.5‰, and contain fluid inclusions with variable salinites and homogenization temperatures up to around 110°C. This suggests that the euhedral alkaline evaporites were dissolved by heated waters; calcite pseudomorphs then precipitated from a mixed solution formed by the interaction of these incoming fluids with the relatively saline interstitial brines. The sodium bicarbonate solutions formed by dissolution of the evaporites would have been dispersed by the basinal brines so that despite the closed drainage, further groundwater concentration did not take place.
Depositional model of a macrotidal estuary and floodplain, South Alligator River, Northern Australia
Authors: WOODROFFE C. D. , CHAPPELL J. , THOM B. G. , WALLENSKY E.
Published: June 2006

The South Alligator River, Northern Territory of Australia, has a macrotidal estuary. Tidal influence (spring tidal range 5–6 m at the mouth) extends 105 km up the channel. It is dominated by freshwater in the wet season (December‐April) with a salt wedge near the mouth, but is well mixed and becomes saline throughout the dry season. The tidal channel can be divided into four different channel types: an estuarine funnel, a sinuous meandering segment, a cuspate meandering segment (in which the inside of bends are pointed) and an upstream tidal channel. The distribution of morphologically defined land classes and morphological units within each land class on the floodplain flanking the estuary differs from one channel type to another. Several stratigraphic and morphostratigraphic units have been recognized from drill holes on the coastal and deltaic‐estuarine plains, and a model of development is proposed on the basis of extensive radiocarbon chronology and palynology.

The coastal plain has prograded with most rapid sedimentation between 5000 and 3000 yr BP. A similar pattern of progradation is identified in the estuarine funnel. In the sinuous segment of the estuary the channel has migrated laterally across the floodplain. Previous channel positions are indicated by palaeochannels and the meander tract is occupied by laminated channel sediments. Within the cuspate segment there are numerous sinuous palaeochannels on the plains. In the upstream segment, the channel and palaeochannels have long straight reaches with irregular bends and discontinuous levées, and channel avulsion is indicated.

Mangrove mud is a widespread stratigraphic unit throughout the plains. The initial phase of development is a transgressive phase. 8000–6800 yr BP, when mangrove forests extended landwards into a pre‐existing valley as sea‐level rose. As sea‐level stabilized, the transgressive phase was followed by a widespread mangrove phase, termed the ‘big swamp’ 6800–5300 yr BP. The mangrove forests disappeared from most of the plains as vertical accretion continued, and were replaced by grass and sedge‐covered floodplains. During the sinuous phase, about 5300–2500 yr BP, the channel migrated laterally and eroded the deltaicestuarine plain and deposited lateral accretion deposits (laminated channel sediments). Part of the channel of the South Alligator River has then progressed from sinuous to cuspate in form, and erosion of river banks has occurred. Transgressive and big swamp phases occurred under rising and stabilizing sea‐level, respectively. Later morphodynamic channel adjustments occurred under conditions of stable sea‐level. The depositional model has direct application to other estuaries in northern Australia, and may be applied to other areas where sea‐level change has been similar.