bluuebunny/crossref_metadata_2025 huggingface dataset:
Controls on synorogenic alluvial‐fan architecture, Beartooth Conglomerate (Palaeocene), Wyoming and Montana
Authors: DeCELLES P. G. , GRAY M. B. , RIDGWAY K. D. , COLE R. B. , PIVNIK D. A. , PEQUERA N. , SRIVASTAVA P.
Published: June 2006

Late Palaeocene uplift of the Beartooth Range in northwestern Wyoming and southwestern Montana generated the Beartooth Conglomerate along the eastern and northeastern flanks of the range. Systematic unroofing sequences and intraformational unconformities, folds, and faults in the conglomerate attest to deposition during uplift. Along the eastern flank, at least three ancient alluvial‐fan systems and a braidplain system can be distinguished on the bases of petrofacies and lithofacies. The two southern fans consist of 700+ m of sedimentary‐clast conglomerate and subordinate sandstone, dominated by hyperconcentrated‐flow and stream‐flow facies. The next fan to the north is dominated by plutonic and metamorphic clasts and contains abundant mud‐matrix‐supported debris‐flow facies, as well as stream‐flow facies. The northernmost depositional system consists of arkosic, channellized fluvial conglomerate and sandstone, overbank mudstone, and crevasse‐splay sandstone units. Palaeocurrent data indicate eastward dispersal, away from the Beartooth Range.

Outstanding exposure of the Beartooth Conglomerate allows facies to be mapped on lateral photographic mosaics. A seven‐fold hierarchy of bounding surfaces and enclosed lithosomes exists in the Beartooth Conglomerate. First‐ through fourth‐order surfaces are analogous to first‐ through fourth‐order surfaces that recently have been documented in sandy fluvial facies, with one exception: sediment gravity flows are bounded by first‐order surfaces. Fifth‐order surfaces are either erosional (e.g. lateral migration of fanhead trench) or accretionary (e.g. aggradation of fan surface during backfilling of trench, and construction of lobes on lower fan during entrenchment on upper fan). Some fifth‐order surfaces coincide with intraformational angular unconformities and are thus the result of long‐term fanhead entrenchment following uplift of the upper part of the fan. Sixth‐order surfaces bound individual fan packages that are several hundred metres thick and ∼ 10 km2 in area. The enclosed sixth‐order lithosomes are distinguishable in terms of petrofacies and lithofacies. A single seventh‐order surface bounds the entire Beartooth Conglomerate. Lower‐order lithosomes are produced by intrinsic processes of fan construction. Fifth‐order lithosomes can be attributed to both extrinsic and intrinsic controls. Sixth‐ and seventh‐order lithosomes are generated by extrinsic controls.
Environmental gradients in carbonate sediments and rocks detected by correspondence analysis: examples from the Recent of Norway and the Dinantian of southwest England
Authors: HENNEBERT M. , LEES A.
Published: June 2006

Continuous sedimentary gradients are only crudely expressed by standard facies and microfacies methods which are more appropriate to situations where changes occur in relatively discrete steps. In carbonate sediments and rocks, continuous gradients are often represented by the arrangement of component grain types in a relay, that is, a systematic shifting of the relative importance of the components. Subdivision of such relays into (micro)facies can only be arbitrary.

Correspondence analysis is shown to be useful for detecting, isolating and describing relays. Particular use is made of the arch effect in which samples and components from data sets with a strong unidimensional structure (a relay) plot in the form of an arch in the plane of the first two factor axes. A relay index, indicating the position of samples in the relay, can be extracted from the analysis and plotted on maps and stratigraphic logs to reveal details of the sedimentary gradient in areal and/or stratigraphic context.

Examples are given from: (i) Recent shallow‐marine carbonate sediments from northern Norway, illustrating a relatively simple depositional setting where surface sediments are viewed in plan; and (ii) Lower Carboniferous carbonates of southwest England, representing a more complex regional study of a particular stratigraphic interval viewed in cross‐section. In both examples the relays can be related to identifiable environmental gradients.
Syntectonic carbonate veins from the Barbados accretionary prism (ODP Leg 110): record of palaeohydrology
Authors: VROLIJK PETER , SHEPPARD SIMON M. F.
Published: June 2006

Syntectonic carbonate veins were found along the décollement zone between the Atlantic and Caribbean plates and in two sites 12 and 17 km landward of the deformation front of the Barbados accretionary prism (ODP Leg 110). Both calcite and lesser amounts of rhodochrosite (MnCO3) occur in microcrystalline, fibrous, and blocky mosaic forms in veins deposited along scaly shear zones. Descriptions of the occurrences, textures, mineralogy, and stable isotope geochemistry of these veins presented herein help trace the hydrological evolution of the Barbados accretionary prism, which has seen the impact of more deeply derived fluids on elevated sediment temperatures and altered geochemistry of interstitial fluids. At one site, for example, vein‐forming fluids have been as much as 10°C warmer and 1‰ more enriched in 18O than expected.

Carbon isotope ratios of veins indicate a mixture of carbon derived from seawater and from oxidized organic matter during sulphate reduction, although dissolution of calcareous skeletons may be an additional component. The presence of methane is recorded in one vein sample in the accretionary prism, indicating that either the plumbing of the prism has changed with time, allowing more fluid to leak into the prism in the past, or that materials currently 1 km above the décollement zone were once deformed in the décollement.

Using oxygen isotope analyses of rhodochrosite samples from two sites in conjunction with measurements of fluid temperature and δ18O values of interstitial fluids, it is proposed that the fractionation of oxygen isotopes between rhodochrosite and water (1000 1nα)∼38‰ at 25°C, 10‰ greater than the fractionation between calcite and water. However, it appears that substitution of 10‐20 mol.% Ca into rhodochrosite reduces the fractionation factor by 2‐9‰ so impure diagenetic rhodochrosite samples from the deep ocean will rarely be this enriched in 18O.
Sheet slides and rotational failures on a convergent margin: the Kidnappers Slide, New Zealand
Authors: BARNES PHILIP M. , LEWIS KEITH B.
Published: June 2006

The Kidnappers Slide, on the upper continental slope of the convergent margin off eastern North Island, New Zealand, has been re‐examined using a grid of high‐resolution seismic reflection profiles. The slide is not a single feature but a complex of sheet slides and rotational failures ranging from 20 to 140 m thick, and covering a total area of 720 km2. Failures occurred in several phases, on slopes of 1–5°, in late Quaternary, muddy, shelf‐edge clinoforms that have prograded into an accretionary, trench‐slope basin. Piston cores and seismic stratigraphy show that the main failure probably occurred in early Holocene times but that movements ranged from mid last glacial to late Holocene times.

The sheet slides exhibit tensional collapse via numerous listric normal faults that sole out on glide planes; there is no clear evidence of compressional structures anywhere within the complex. The glide planes occur at progressively deeper stratigraphic levels towards the northeastern end of the complex, and near the steep slope that defines the seaward edge of the trench‐slope basin. There is retrogressive failure at the top of the slope. The surficial slides are being deformed by growth of active tectonic faults and folds associated with the convergent plate margin.

This type of slope failure may be partially related to metastable sandy layers within the last glacial age progradational sequence, and possibly to formation of bubble phase gas at shallow depths. Failure was probably triggered by earthquake loading of sediments in this highly seismic region.
Microenvironmental controls on mineralogy and habit of CaCO3 precipitates: an example from an active travertine system
Authors: CHAFETZ HENRYS. , RUSH PATRICK F. , UTECH NANCY M.
Published: June 2006

Analysis of water and associated carbonate precipitates from a small, warm‐spring travertine system in SW Colorado, USA, provide an example of the: (i) great variability of the geochemical parameters within these dynamic systems, and (ii) significance of the microenvironment in controlling mineralogy and morphology of carbonate precipitates. Waters emerged from the springs highly charged in CO2, with an initial pCO2 of 1.2 × 105 Pa.

Degassing of the CO2 from the waters decreased the pH from 6.1 to 8.0, resulting in an increase of 8%‰ in δ13C values downflow in the total CO2 in solution and an increase in the ISAT from 2.1 to as high as 63 times supersaturation with respect to calcite. Due to changes in the stable isotopic composition of the waters downflow as well as changes in the degree of supersaturation, stable isotopic analyses range greatly from locale to locale within this small system.

Near the spring vents, at relatively low ISAT levels, well‐developed rhombohedra of calcite formed as biotically induced precipitates around diatom stalks and other algae as well as abiotic crusts. In contrast, near the distal end of the system, very high ISAT levels were reached and resulted in the precipitation of skeletal‐dendritic crystals of calcite on copper substrates, floating rafts of laterally linked hemispheres of aragonite crystals, and bimineralic carbonate‐encrusted bubbles. Microenvironmental parameters control the mineralogy and habit of these precipitates.
The concretions of the Bearreraig Sandstone Formation: geometry and geochemistry
Authors: WILKINSON MARK
Published: June 2006

The sandbodies of the Bearreraig Sandstone Formation (Inner Hebrides, UK) are cemented by two generations of calcite. The first generation, an inhomogeneous ferroan calcite (0.05−3.28 mol% FeCo3) formed during sulphate reduction (δ13C =−24 to −32%o PDB) in marine porewaters (δ18O of cement from −1 to −4%o PDB) at very shallow burial depths (a few centimetres). These cements are rare but form millimetre‐scale clusters of crystals which acted as nuclei to the later, concretionary cements.

The second generation of cements are more homogeneous ferroan calcites (mean 1−58% mol% FeCo3) which evolve to progressively higher Fe/Mg ratios. They are sourced by shell dissolution (δ13C of cement from +1 to −3%o PDB) into meteoric (δ18O of cement from −6 to −10%o PDB) or mixed marine meteoric waters (δ18O of cement from −4 to −6%o SMOW). These were introduced into the formation either during Bathonian times as a freshwater lens, or, subsequent to partial inversion, by confined aquifer flow. Corroded feldspars within the concretions suggest that an interval of at least 8 Ma separated the deposition of the sediments from the onset of concretion growth.

Abundant concretions are preferentially developed at certain horizons within the sandbodies, where the early generation of ferroan calcite cements provided nuclei. The latter formed close to the sediment‐water interface, the concentration of cement within the sediment being related to sedimentation rate. The relatively high concentrations of the first generation of cement, upon which the concretionary horizons are nucleated, formed during periods of minimal sedimentation.
Significance of molluscan shell beds in sequence stratigraphy: an example from the Lower Cretaceous Mannville Group of Canada1
Authors: BANERJEE INDRANIL , KIDWELL SUSAN M.
Published: June 2006

Detailed study of marine shales (the Ostracod zone) within a Cretaceous, third‐order transgressive‐regressive sequence in the Alberta Foreland Basin reveals a systematic association between shell beds and parasequence‐scale flooding surfaces, including surfaces of maximum flooding. The Ostracod zone (a subsurface lithostratigraphic unit known as the Calcareous Member in outcrop) consists of 10‐20 m of black shale and bioturbated sandstones with many thin, fossiliferous limestones. Parasequences (shallowing‐up cycles 2–3 m thick) were delineated within this transgressive unit based on lithology, sedimentary structures, degree of bioturbation, dinoflagellate diversity, total organic carbon and carbon/sulphur ratios; many flooding surfaces are firmgrounds or hardgrounds.

Shell‐rich limestones occur in three different positions relative to these flooding surfaces, and each has a distinctive bioclastic fabric and origin. (i) Base‐of‐parasequence shell beds (BOPs) lie on or just above flooding surfaces in the deepest water part of a parasequence; they are thin (up to a few centimetres), graded or amalgamated skeletal packstones/wackestones composed of well‐sorted granular shell, and are interpreted as hydraulic event concentrations of exotic shell debris. (ii) Top‐of‐parasequence shell beds (TOPs) are capped by flooding surfaces at the top, shallowest water part of a parasequence; they typically are several decimetres thick, are physically amalgamated packstones/grainstones or bioturbated wackestones, and contain abundant whole as well as comminuted shells; these are composite, multiple‐event concentrations of local shells. (iii) Mid‐sequence shell beds rest on as well as are capped by firmgrounds or hardgrounds, and are intercalated between parasequences in the deepest water part of the larger sequence; they are laterally extensive lime mudstones a few decimetres thick, with sparse shells in various states of dissolution, recrystallization and replacement; these beds are terrigenous‐starved hiatal concentrations and record maximum flooding within the Ostracod zone. Offshore sections of the Ostracod zone typically contain several starved mid‐sequence shell beds, underscoring the difficulty of identifying a single‘maximum flooding surface’ within a third‐order sequence.
Shallow‐water gravity‐flow deposits, Chapel Island Formation, southeast Newfoundland, Canada
Authors: MYROW PAUL M. , HISCOTT RICHARD N.
Published: June 2006

A remarkable suite of shallow‐water, gravity‐flow deposits are found within very thinly‐bedded siltstones and storm‐generated sandstones of member 2 of the Chapel Island Formation in southeast Newfoundland. Medium to thick siltstone beds, termed unifites, range from non‐graded and structureless (Type 1) to slightly graded with poorly developed lamination (Type 2) to well graded with lamination similar to that described for fine‐grained turbidites (Type 3). Unifite beds record deposition from a continuum of flow types from liquefied flows (Type 1) to turbidity currents (Type 3). Calculations of time for pore‐fluid pressure dissipation support the feasibility of such transitions. Raft‐bearing beds consist of siltstone with large blocks or‘rafts’ of thinly bedded strata derived from the underlying and adjacent substrate. Characteristics suggest deposition from debris flows of variable strength. Estimates of debris strength and depositional slope are calculated for a pebbly mudstone bed using measurable and assumed parameters. An assumed density of 2.0 g cm‐1 and a compaction estimate of 50% gives a strength estimate of 79.7 dyn cm‐2 and a depositional slope estimate of 0.77°.

The lithologies and sedimentary structures in member 2 indicate an overall grain‐size distribution susceptible to liquefaction. Inferred high sediment accumulation rates created underconsolidated sediments (metastable packing). Types of sediment failure included in situ liquefaction (‘disturbed bedding’), sliding and slumping. Raft‐bearing debrites resulted from sliding and incorporation of water. Locally, hummocky cross‐stratified sandstone directly overlies slide deposits and raft‐bearing beds, linking sediment failure to the cyclical wave loading associated with large storms.

The gravity flows of the Chapel Island Formation closely resemble those described from the surfaces of modern, mud‐rich, marine deltas. Details of deltaic gravity‐flow deposition from this and other outcrop studies further our understanding of modern deposits by adding a third dimension to studies primarily carried out with side‐scan sonar.
Soil‐landscape and climatic relationships in the middle Miocene of the Madrid Basin
Authors: ZARZA A. M. ALONSO , WRIGHT V. P. , CALVO J. P. , DEL CURA M. A. GARCÍA
Published: June 2006

The Miocene alluvial‐lacustrine sequences of the Madrid Basin, Spain, formed in highly varied landscapes. The presence of various types of palaeosols allows assessment of the effects of local and external factors on sedimentation, pedogenesis and geomorphological development. In the northern, more arid, tectonically active area, soils were weakly developed in aggrading alluvial fans, dominated by mass flows, reflecting high sedimentation rates. In more distal parts of the fans and in playa lakes calcretes and dolocretes developed; the former were associated with Mg‐poor fan sediments while the latter formed on Mg‐rich lake clays exposed during minor lake lowstands.

The north‐east part of the basin had a less arid climate. Alluvial fans in this area were dominated by stream flood deposits, sourced by carbonate terrains. Floodplain and freshwater lake deposits formed in distal areas. The high local supply of calcium carbonate may have contributed to the preferential development of calcretes on the fans. Both the fan and floodplain palaeosols exhibit pedofacies relationships and more mature soils developed in settings more distant from the sediment sources. Palaeosols also developed on pond and lake margin carbonates, and led to the formation of palustrine limestones.

The spatial distributions and stratigraphies of palaeosols in the Madrid Basin alluvial fans suggest that soil formation was controlled by local factors. These palaeosols differ from those seen in Quaternary fans, which are characterized by climatically induced periods of stability and instability.
Sedimentology, geochemistry and origin of phosphatic chalks: the Upper Cretaceous deposits of NW Europe
Authors: JARVIS IAN
Published: June 2006

Santonian‐Lower Campanian and Lower Maastrichtian phosphatic chalks in northern France, southern England and Belgium are Europe's largest sedimentary phosphatc deposits. The stratigraphy, sediment‐ology, petrography, mineralogy and geochemistry of the lithofacies are reviewed and new data presented. Depositional and diagenetic models for phosphatic chalk deposits are developed using published experimental work and from observations of modern high‐ and low‐productivity phosphogenic systems. It is concluded that phosphatic chalks were deposited in well‐oxygenated, current‐swept environments. Phosphatization required a delicate balance to be maintained between moderate organic carbon and carbonate sedimentation rates, reduced bulk sediment accumulation rate and an enhanced rate of bioturbation. Precipitation of carbonate‐fluorapatite (francolite) accompanied the bacterially mediated decomposition of organic matter, occurring within centimetres of the sediment‐seawater interface, and taking place preferentially within microbial bodies and coatings. In addition to the organically derived component, pore water phosphate levels were enhanced by phosphate absorbed on ferric oxyhydroxides which was liberated during iron reduction. Mineralization was probably a dominantly post‐oxic process, but occurred in a thick sediment mixed layer in which marine organic matter was undergoing intense mixed aerobic and anaerobic microbial degradation. Phosphogenesis occurred predominantly on the NE margins of the Anglo‐Paris Basin where shallower sea floors and suitable palaeoceanographic conditions prevailed. Phosphogenic episodes were limited by sea level fluctuations'which controlled the effectiveness of the erosional currents that formed and maintained the phosphatic basins and may have stimulated local productivity.
A numerical model for porosity modification at a sandstone‐mudstone boundary by quartz pressure dissolution and diffusive mass transfer
Authors: MULLIS ANDREW M.
Published: June 2006

In many sandstones quartz cements significantly reduce porosity. The origin of these cements is often unclear. This paper investigates a possible mechanism for the generation of silica for quartz cement by pressure dissolution in interbedded mudstones.

Theoretical models of quartz pressure dissolution, including the effects of silica precipitation kinetics, show that the concentration of dissolved silica in the pore fluids of a compacting sediment increases with decreasing grain size and silica precipitation constant. Quartz precipitation is strongly inhibited by the presence of small amounts of clay within a sediment, suggesting that siltstones and quartz‐rich mudstones which are undergoing pressure dissolution may act as a source of dissolved silica for export to nearby, coarser sediments.

A computational model for the diagenetic modification of a sandstone‐mudstone interface due to pressure dissolution is described. Both sandstone and mudstone layers are assumed to be actively compacting by pressure dissolution, and mass transport by molecular diffusion is considered. As quartz precipitation in the mudstone layer is relatively slow compared to that in the sandstone, significant amounts of dissolved silica become available in the mudstone, and may be exported into the adjacent sandstone. In the absence of pore‐fluid advection, this may result in the formation of extensive secondary quartz within the sandstone, close to the interface.

The volume of silica exported from the mudstone is limited by the length scale over which diffusion through the mudstone is effective. This is typically 3–5 m. The volume of silica available therefore suggests that extensive porosity modification within the adjacent sandstone can only occur close to the mudstone. Thus it is possible that thin sandstones could become cemented by slow diffusive transfer of silica, but that in thicker sandstones the silica may become dispersed by pore‐fluid advection.
Plio‐Pleistocene sedimentation in Ferrar Fiord, Antarctica
Authors: BARRETT P. J. , HAMBREY M. J.
Published: June 2006

A 166 m thick Plio‐Pleistocene sequence of glacial sediments has been cored in Ferrar Fiord in the southwestern corner of the Ross Sea, Antarctica. The core has the following lithofacies: massive diamictite (33% of the core; interpreted as lodgement or waterlain till), weakly stratified diamictite (25%; waterlain till or proximal glaciomarine sediment), well‐stratified diamictite (8%; proximal glaciomarine or glaciolacustrine sediment), sandstone (25%; sand of aeolian or supraglacial origin), mudstone(7%; derived from subglacial debris and transported offshore in suspension), and minor amounts of rhythmite and tuff. The range of facies in this polar setting differs from those normally found in subpolar and temperate glacier fiord settings in the high proportion of aeolian‐derived sand and the low proportion of mudstone facies.

The core can be divided into two sequences based on composition and texture. The sequence from 162 to 100 mbsf (metres below the sea floor) comprises alternations of diamictite dominated by basement lithologies and thin marine mudstone beds. It is Pliocene in age (4.9–2.0 Ma) and records several advances and retreats of ice through the Transantarctic Mountains and across the drill site from the west. The sequence from 100 mbsf to the sea floor, of Pleistocene age, consists of alternations of diamictite, interpreted as lodgement and waterlain till, and sandstone of aeolian origin deposited in a glaciolacustrine setting, similar to ice‐covered lakes in the Dry Valleys today. These sediments have a high volcanic component, and hence are thought to have been derived by the grounding and advance of the Ross Ice Shelf from the east past volcanic Ross Island. This change in source is attributed to the rising Transantarctic Mountains increasingly containing East Antarctic ice.

The Pleistocene sequence above 100 mbsf clearly represents polar glacial sedimentation, with alternations of till and glaciolacustrine sand. Mudstones from the Pliocene sequence beneath include palynomorphs, indicating times when the landscape was at least partially vegetated, but contain no evidence of meltwater influence.
A new location of Holocene dolomite formation, Lake Hayward, Western Australia
Authors: ROSEN MICHAEL R. , COSHELL LEE
Published: June 2006

Holocene dolomite forms in the sediment of Lake Hayward, a small permanent hypersaline lake in the Clifton‐Preston Lakeland System, Western Australia. The geomorphological setting of dolomite formation in Lake Hayward is similar to the Coorong region in South Australia. Unlike in the Coorong region, dolomite in Lake Hayward does not form as a direct precipitate from the lake water, but is of diagenetic origin. This can be deduced from the following features: (1) the dolomite occurs only below 60–70 cm from the sediment‐water interface, (2) dolomite occurs as luminescing cement, and (3) dolomite has pristine well‐formed rhomb‐shaped crystals. The source of magnesium for dolomitization is probably from the concentration on inflowing groundwater by evaporation and the selective removal of calcium by chemical and biological aragonite/calcite precipitation.
Earthquake‐induced resedimentation in the Badenian (middle Miocene) gypsum of southern Poland
Authors: PERYT TADEUSZ MAREK , KASPRZYK ALICJA
Published: June 2006

In the Badenian (middle Miocene) basin of the Carpathian foreland of southern Poland, gypsum breccias occur associated with laminated gypsum deposits. These breccias consist of large clasts of gypsum, carbonates, marls and clay chips of variable size embedded in a gypsarenitic matrix. Constituent gypsum grains and clasts commonly appear to be mechanically abraded and chemically corroded crystals or fragments of selenitic, laminated and alabastrine gypsum. Gypsorudites are commonly accompanied by laminated gypsarenites and gypsolutites which show graded bedding; a vertical sequence of graded gypsum beds showing Bouma sequences may be recognized in borehole sections. Microfolding is common within the folded laminated gypsum, and is closely associated with expressions of extensional strain. Both are accompanied by pervasive microfaulting, suggesting a semi‐coherent downslope mass movement. The stratiform geometry of the breccias, together with the intensity of slumping relatively independent of the palaeoslope, suggest earthquake shocks as the initial, main cause.

Gypsum deposits form a constant, laterally extensive sequence of different lithofacies. The occurrence of the same lithologies and shallowing‐up cycles over a wide area reflects thrusting of the Carpathians over the Carpathian foredeep. Local tectonism has also played a significant role. The tectonic framework favoured activation of dip‐slip faults promoting shallow‐focus earthquakes. These in turn resulted in the resedimentation of gypsum by slumps, debris flows and turbidites. A similar basinward resedimentation of clastic material by gravity flows initiated by fault‐induced earthquakes could be of great importance in the foreland geological setting, and may explain some phenomena observed in other evaporite formations from different geological settings, especially of rift type.
Half‐graben lacustrine sedimentary rocks of the lower Carboniferous Strathlorne Formation, Horton Group, Cape Breton Island, Nova Scotia, Canada
Authors: HAMBLIN ANTHONY P.
Published: June 2006

The Strathlorne Formation is the middle formation of a three‐part Horton Group stratigraphy present throughout the post‐Acadian Orogeny Maritimes Basin in Atlantic Canada. It is up to 600 m in thickness and is of Tournaisian age. The formation was deposited in a complex lacustrine system during the period of maximum fault‐bounded extensional subsidence within two asymmetric half‐graben sub‐basin segments of a large rift. This rift was located at a palaeolatitude of 10–15°S. Four facies assemblages are identified and interpreted: (1) dark grey mudstone (open lacustrine), (2) grey, very fine to fine‐grained sandstone (nearshore/shoreline), (3) grey, medium‐grained sandstone to conglomerate (fan delta) and (4) red siltstone to fine‐grained sandstone (interdeltaic mudflat). Interpreted structural asymmetry of the fault‐bounded sub‐basins is evidenced by asymmetry of sediment input, facies distribution and palaeoflow in the lacustrine sedimentary fill. These indicators suggest that the sub‐basins, which were linked end‐to‐end, had opposed polarity of structural asymmetry during deposition of the Strathlorne Formation. Open lacustrine sediments are typified by stacked shallowing‐upward sequences, each representing deepening due to sub‐basin‐wide subsidence events followed by gradual infilling to shallow water depths. Sub‐basin asymmetry is also reflected in the contrast of thick sequences and grouped thinner sequences at marginal and axial positions, respectively. The lakes which occupied the sub‐basins were large (up to 100 × 50 km), tens to hundreds of metres deep and periodically stratified (presence of an anoxic hypolimnion, at least near sub‐basin axes).
Sediment transport over salt‐wedge intrusions: Fraser River estuary, Canada
Authors: KOSTASCHUK R. A. , CHURCH M. A. , LUTERNAUER J. L.
Published: June 2006

A field investigation was conducted in the Fraser River estuary, Canada, to examine flow and suspended sediment concentration above salt‐wedge intrusions. The thickness of the upper layer above the intrusion decreased with distance seaward of the salt‐wedge tip, but salinity and velocity increased. A seaward decline in the suspended sediment concentration of the upper layer is believed to result from interference in the exchange of sediment between the flow and the bed, reduced turbulence, flocculation of fine sediment, and dilution of the sediment‐water mixture.

Multiple regression models based on simple physical reasoning and easily obtained data provide relations for the suspended sediment concentration of the upper layer. Scaling the models did not produce notable improvements over the dimensioned results and introduced numerical instability. The models showed that sediment concentration was controlled by riverine processes and that the rate of seaward decline in concentration was exponential. These observations support a descriptive salt‐wedge sedimentary model.
Aeolian granule ripple deposits, Namibia
Authors: FRYBERGER STEVEN G. , HESP PATRICK , HASTINGS KATHLEEN
Published: June 2006

Granule ripples are a common feature of most dunefields, yet they have seldom been recognized in ancient deposits. Although granule ripples are common in erosional settings, such as windward slopes of dunes, or scour surfaces in interdunes, they nevertheless migrate laterally and leave distinctive deposits that can be recognized in ancient rocks. These deposits have characteristics of ‘type B’sand sheet deposits, including: ‘poured‐in’texture; curving ripple trough; tangential, coarse‐grained foresets; irregular silty layers; well‐sorted coarse and fine layers (either horizontal or within foresets); and fine layers in ripple troughs. Wind tunnel experiments suggest that under low‐velocity wind conditions, granule ripples grow to a significant degree as parasites dependent on saltation of fine sand grains whose impact moves the larger grains of the granule ripple. Although the depositional surface of granule ripples is commonly coated with a layer of coarse grains, this is in most places only a few grains thick. Underlying deposits commonly have a poorly sorted, or ‘poured‐in’texture. This texture results from an admixture of fine grains that fall among the spaces between the larger grains during deposition.
Halite saltern in the Canning Basin, Western Australia: a sedimentological analysis of drill core from the Ordovician‐Silurian Mallowa Salt
Authors: CATHRO DONNA L. , WARREN JOHN K. , WILLIAMS GEORGE E.
Published: June 2006

The Late Ordovician‐Early Silurian Mallowa Salt of the Carribuddy Group, Canning Basin, north‐west Australia, is the largest halite deposit known in Australia, attaining thicknesses of 800 m or more within an area of approximately 200 000 km2. Study of 675 m of drill core from BHP‐Utah Minerals’ Brooke No. 1 well in the Willara Sub‐basin indicates that the Mallowa Salt accumulated within a saltern (dominantly subaqueous evaporite water body) that was subject to recurrent freshening, desiccation and exposure. Textures and bromine signatures imply a shallow water to ephemeral hypersaline environment typified by increasing salinity and shallowing into evaporitic mudflat conditions toward the top of halite‐mudstone cycles (Type 2) and the less common dolomite/anhydrite‐halite‐mudstone cycles (Type 1). The borate mineral priceite occurs in the capping mudstones of some cycles, reinforcing the idea of an increasing continental influence toward the top of mudstone‐capped halite cycles.

The rock salt in both Type 1 and Type 2 cycles typically comprises a mosaic of large, randomly orientated, interlocking halite crystals that formed during early diagenesis. It only partially preserves a primary sedimentary fabric of vertically elongate crystals, some with remnant aligned chevrons. Intraformational hiati, halite karst tubes and solution pits attest to episodic dissolution. Stacked Type 2 cycles dominate; occasional major recharges of less saline, perhaps marine, waters in the same area produced Type I cycles.

The envisaged saltern conditions were comparable in many ways to those prevailing during the deposition of halite cycles of the Permian Salado Formation in New Mexico and the Permian San Andres Formation of the Palo Duro Basin area in Texas. However, in the Canning Basin the cycles are characterized by a much lower proportion of anhydrite, implying perhaps a greater degree of continental restriction to the basin. The moderately high level of bromine in the Mallowa Salt (156·5 ± 43·5 ppm Br for primary halite, 146·1 ± 54·7 ppm Br for secondary halite) accords with evolved continental brines, although highly evaporative minerals such as polyhalite and magnesite are absent. The bromine levels suggest little or no dissolution/reprecipitation of primary halite and yet, paradoxically, there is little preservation of the primary depositional fabric. The preservation of early halite cements and replacement textures supports the idea of an early shutdown of brine flow paths, probably at burial depths of no more than a few metres, and the resultant preservation of primary bromine values in the secondary halite.
Theoretical and measured aeolian sand transport on a barrier island, Louisiana, USA
Authors: DINGLER JOHN R. , HSU S. A. , REISS THOMAS E.
Published: June 2006

Over the past 100 years, the Isles Dernieres, a low lying barrier island chain along the coast of central Louisiana, Usa, has undergone more than 1 km of northward beach face retreat with the loss of 70% of its surface area. The erosion results from a long term relative sea level rise coupled with day to day wind and wave action that ultimately favours erosion over deposition. At a site in the central Isles Dernieres, 8 days of wind and beach profile measurements during the passage of one winter cold front documented aeolian erosion and deposition patterns under both onshore and offshore winds. For offshore winds, the theoretical erosion rate, based on wind shear velocity, closely matched the measured erosion rate; for onshore winds, the theoretical rate matched the measured rate only after being corrected by a factor that accounted for beach face morphology.

In late February 1989, a strong cold front moved into coastal Louisiana. That cold front stalled over the Gulf of Mexico, resulting in 4 days of strong northerly winds at a study site on the Isles Dernieres. During those 4 days, the wind moved sand from the backshore to the upper beach face. When the cold front finally moved out of the area, the wind shifted to the south and decreased in strength. The onshore wind then restored some of the upper beach face sand to the backshore while increased wave activity moved the rest into the nearshore.

The theoretical estimate of 1·28 m3 m−1 for the rate of sand transport by the northerly wind compares well with the measured backshore erosion rate of 1·26 m3 m−1, which was determined by comparing beach profiles from the start and end of the period of northerly winds. The theoretical estimate of 0·04 m3 m−1 for the rate of sand transport by the southerly wind, however, is notably less than the measured rate of 0·45 m3 m−1. The large discrepancy between the two rates can be explained by a difference in the shear velocity of the wind between the beach face, where the erosion occurred, and the backshore, where the wind stress was measured. Using an empirical relationship for the wind shear drag coefficient as a function of coastal environment, the theoretical estimate for the rate of sand transport by the southerly wind becomes 0·44 m3 m−1
Computer simulation of reef growth
Authors: BOSSCHER HEMMO , SCHLAGER WOLFGANG
Published: June 2006

Light is one of the major controls on reef growth and carbonate production. The growth of present reef builders depends largely upon the amount of light available for photosynthesis. As light decreases with water depth, so does reef growth.

The computer model presented extends this principle by combining two functions, one for photosynthesis and the other for the extinction of light in water. The model is used to simulate the growth of Alacran Reef, Mexico, two reefs of the Great Barrier Reef and the reefs of the windward platform of St Croix, US Virgin Islands. The model also gives an accurate simulation of the growth of fore‐reef walls in Belize, in agreement with the accretion hypothesis developed for this feature.
The Ilchulbong tuff cone, Cheju Island, South Korea
Authors: SOHN Y. K. , CHOUGH S. K.
Published: June 2006

The Ilchulbong mount of Cheju Island, South Korea, is an emergent tuff cone of middle Pleistocene age formed by eruption of a vesiculating basaltic magma into shallow seawater. A sedimentological study reveals that the cone sequence can be represented by nine sedimentary facies that are grouped into four facies associations.

Facies association I represents steep strata near the crater rim composed mostly of crudely and evenly bedded lapilli tuff and minor inversely graded lapilli tuff. These facies suggest fall‐out from tephra finger jets and occasional grain flows, respectively. Facies association II represents flank or base‐of‐slope deposits composed of lenticular and hummocky beds of massive or backset‐stacked deposits intercalated between crudely to thinly stratified lapilli tuffs. They suggest occasional resedimentation of tephra by debris flows and slides during the eruption. Facies association III comprises thin, gently dipping marginal strata, composed of thinly stratified lapilli tuff and tuff. This association results from pyroclastic surges and cosurge falls associated with occasional large‐scale jets. Facies association IV comprises a reworked sequence of massive, inversely graded and cross‐bedded (gravelly) sandstones. These facies represent post‐eruptive reworking of tephra by debris and stream flows.

The facies associations suggest that the Ilchulbong tuff cone grew by an alternation of vertical and lateral accumulation. The vertical buildup was accomplished by plastering of wet tephra finger jets. This resulted in oversteepening and periodic failure of the deposits, in which resedimentation contributed to the lateral growth. After the eruption ceased, the cone underwent subaerial erosion and faulting of intracrater deposits. A volcaniclastic apron accumulated with erosion of the original tuff cone; the faulting was caused by subsidence of the subvolcanic basement within the crater.
Distal alluvial deposits in a foreland basin setting—the Lower Freshwater Miocene), Switzerland: sedimentology, architecture and palaeosols
Authors: PLATT NIGEL H. , KELLER BEAT
Published: June 2006

The Lower Freshwater Molasse (Untere Susswasser Molasse) crops out over a wide area of the Swiss Molasse Basin. Coarse grained alluvial fan conglomerates dominate in proximal basin areas along the Alpine front. These conglomerates pass northwards into sandstones and mudstones of an extensive northeastward draining meandering river system which ran parallel to the basin axis. Sedimentological study of outcrops, quarry exposures and boreholes in the basal Miocene (‘Aquitanian‘) has permitted detailed facies analysis of this distal alluvial sequence. The distal Aquitanian is made up of distinct ‘architectural elements’characterized by their geometries and sedimentary structures. Each may be assigned to a particular depositional setting: meander belt, levees, crevasse channels and splays, overbank fines and palaeosols, and lacustrine.

Meander belt sandstones were deposited in mixed load channels with a dominant bedload component. Sandstones commonly comprise amalgamated and locally stacked ribbon bodies 2–15 m thick and 150–1500 m wide. Interbedded rippled, laminated and mottled fine grained levee sandstones and siltstones form lenticular packages up to 3 m thick and 30–100 m across. Small scale crevasse channel sandstones 2–4 m thick and 5–10 m across pass laterally into metre scale, medium to fine grained crevasse sandstone sheets. Rare laminated lacustrine siltstones occur only in the north‐east part of the basin. Floodplain mudstones and marls make up the remainder of the succession. These display a variety of pedogenic features recording cyclical palaeosol development. Palaeosols show strong variations in morphology and maturity both laterally across the floodplain and downstream along the basin axis, reflecting local variation in aggradation rate associated with proximity to alluvial channel courses as well as regional variation in subsidence and floodplain drainage.

Analysis of the organization and distribution of the various sediment bodies permits reconstruction of the fluvial system and allows development of a model for the sedimentary architecture of the Lower Freshwater Molasse in the study area. Integration of palaeosol studies into a well defined architectural framework assists recognition of areal facies belts and may aid location of sand‐prone sequences in the subsurface.
A sedimentological and stable isotopic study of travertines and associated sediments within Upper Triassic lacustrine limestones, South Wales, UK
Authors: LESLIE ALICK B. , TUCKER MAURICE E. , SPIRO BARUCH
Published: June 2006

The Upper Triassic in South Wales is composed of up to 100 m of red, dolomitic mudstones of the Mercia Mudstone Group which overlie and are laterally equivalent to basin margin coarse clastic deposits. In the Sully Island and Dinas Powys areas, a series of carbonate deposits was laid down within small basins adjacent to the main Bristol Channel Basin. The rocks consist of dolomites containing replaced evaporites, overlain by perilittoral freshwater limestones. The limestones are fenestral intrasparites and contain abundant pedogenic and stromatolitic horizons, as well as locally developed travertines. Evidence for vadose diagenesis within the limestones is common. The travertines consist of sheets of fibrous calcite (flowstone) associated with pisoids and flöe calcite. Most of the travertines consist of single sheets several millimetres in thickness although ‘mounds’up to 1 m in height and 5 m in diameter are also present. The topmost metre of the limestones, which has been dolomitized, is deformed into tepee and megapolygonal structures.

The carbon and oxygen stable isotopic composition of the limestones suggests that they were precipitated in low salinity waters. Successive samples from individual bands of flowstone show a covariance of δ18C and δ13C which is consistent with the mixing of resurgent groundwaters with pools of more evolved waters at the surface. The sedimentological and geochemical evidence suggests that the limestones were deposited in a small, enclosed basin fed by upwelling meteoric groundwaters in an environment distinct from that in which the laterally equivalent gypsiferous red mudstones were formed.
The morphology and dynamics of low amplitude bedwaves upon upper stage plane beds and the preservation of planar laminae
Authors: BEST JIM , BRIDGE JOHN
Published: June 2006

Experimental studies of subcritical, unidirectional flow over upper stage plane beds of medium grained sand reveal the ubiquitous presence of low amplitude bedwaves. Flow depth was 0·11 m, mean flow velocities were 0·86–1·0 m s−1, shear velocities were 0·058–0·71 m s−1 and dimensionless shear stresses were 0·56–0·87. Bedwaves are asymmetrical in profile and range from 0·75 to 11 mm in height (mainly 2–6 mm), from 0·7 to 1·3 m in wavelength and have mean celerities of 10 mm s−1. Flow records suggest that the bedwaves are associated with accelerating flow over the bedwave crests and flow which decelerates and diverges laterally over the troughs. High resolution bed profiling during aggradation of the bed combined with subsequent box coring illustrates that these bedwaves are responsible for the planar laminae characteristic of upper stage plane beds. Lamina preservation is dependent upon the mean aggradation rate and the sequence of bedwaves of different height crossing any point; individual laminae are more readily preserved at higher aggradation rates where the possibility of reworking by later bedwaves is reduced. Laminae are recognized by small changes in grain size and commonly a fining upwards at the top of laminae which is generated by fine grained material infiltrating a lower lamina in the leeside of a bedwave.
On the entrainment of sediment and initiation of bed defects: insights from recent developments within turbulent boundary layer research
Authors: BEST JIM
Published: June 2006

Sediment entrainment and the initiation of bed defects has commonly been ascribed to the impact of high velocity sweeps upon mobile sand beds. Results of visualization experiments suggest that these sweep impacts are grouped and may define ‘patches’of entrainment upon a mobile bed that may be wider than an individual sweep impact. Additionally, the presence of longitudinal ribs of sediment generated by sweep impacts may stabilize the position of sweeps and low speed streaks. These observations are interpreted in the light of recent boundary layer research which suggests the formation of multiple hairpin shaped vortices within turbulent flows: these are postulated to generate multiple sweeps that manifest themselves as entrainment patches on a mobile bed. These features of the turbulent boundary layer and its modification by sweep generated sediment ridges can be used to propose a model for bed defect formation and the subsequent development of current ripples.