bluuebunny/crossref_metadata_2025 huggingface dataset:
Late Mississippian bryozoan/microbial build‐ups on a drowned karst terrain: Port au Port Peninsula, western Newfoundland
Authors: DIX GEORGE R. , JAMES NOEL P.
Published: June 2006

Bryozoan and microbial carbonate build‐ups occur within the Late Mississippian (mid‐Viséan) Codroy Group on the Port au Port Peninsula, western Newfoundland. Build‐ups formed only adjacent to a rocky, cliffed, shoreline filling narrow submerged palaeovalleys of a well‐developed Late Devonian‐Early Mississippian karst terrain. The nearshore setting was a stressed environment as indicated by (1) the absence of normal marine biota, such as corals, echinoderms, and calcareous algae, (2) high numbers and low species diversity of dominant taxa (bryozoans, brachiopods, and microbial communities), and (3) abundant plant‐bearing siliciclastics deposited by the episodic influx of fresh‐water from adjacent uplands. Build‐up development was terminated by Meramec time, due to falling sea‐level and seaward progradation of terrestrial sediments.

Preserved structural and constructional fabrics within the build‐ups include (1) thickets of bryozoans and microbial microthrombolites, structurally enhanced by probable marine Mg‐calcite and aragonite cements, (2) multigeneration internal sediment, and (3) small colonies of serpulid worms and terebratulid brachiopods. Submarine cementation within the build‐ups appears to have been abundant during arrested sedimentation, whereas intermound sediments (carbonates and siliciclastics) were lithified only during burial diagenesis. Shallow‐burial fracturing, stylolitization, sulphide and sulphate mineralization, and precipitation of phreatic, iron‐poor, calcite cement occurred during burial diagenesis.

Codroy build‐ups are distinct from the more common Mississippian mud mounds. Their internal characteristics and the geologic setting are more like other Late Palaeozoic and Holocene bryozoan‐rich build‐ups.
The strontium isotopic composition and origin of burial cements in the Lincolnshire Limestone (Bajocian) of central Lincolnshire, England
Authors: EMERY D. , DICKSON J. A. D. , SMALLEY P. C.
Published: June 2006

Strontium isotopic composition (87Sr/86Sr) of two petrographically, chemically and isotopically (δ18O and δ13C) distinct phases of burial calcites from the Lincolnshire Limestone are indistinguishable (0.70820± 26). The mean 87Sr/86Sr ratio of these phases is considerably more radiogenic than 87Sr/86Sr ratios of Bajocian marine waters (∼0.70725). Neither Bajocian marine waters nor meteoric waters buffered by host marine carbonate in the Limestone could have precipitated the burial spars. Radiogenic strontium may have been contributed from K‐feldspar dissolution and/or clay recrystallization, either within clastic portions of the Limestone itself, or from major clastic units adjacent to the Limestone. Alternatively, Palaeozoic marine waters or remobilized Palaeozoic marine carbonate and/or sulphate could have supplied the necessary radiogenic strontium.
Biogenic chert in the Cow Head Group (Cambro‐Ordovician), western Newfoundland
Authors: CONIGLIO MARIO
Published: June 2006

Chert in the Cow Head Group is mainly a replacement of limestone and shale and, to a lesser extent, an interparticle cement. Its field occurrences are distinct as: (1) silicified margins on coarse conglomerates and thinly bedded limestones; (2) nodules within limestone and shale; (3) pervasively silicified beds of limestone and shale; and (4) clasts or partial replacement of clasts within conglomerate.

Radiolarians and sponge spicules are composed of microquartz or calcite and are particularly common in the Ordovician part of the succession where most chert occurs. In limestone spatially associated with chert, the use of cathodoluminescence demonstrates that calcite‐replaced radiolarians and spicules are volumetrically more important than realized through transmitted‐light petrography. Petrographic relations between siliceous and rare pyritized radiolarians further indicate that these particles may be dissolved prior to compaction. No trace of their former existence remains, other than indirectly through the presence of silicified limestone and shale. Crushed grains cemented by chalcedony indicate that chert was precipitated during or after compaction. The history of silicification and the replacement or dissolution of siliceous bioclasts is protracted, ranging from near the sediment‐water interface, where it is concomitant with early limestone lithification, to deeper burial, postdating mechanical compaction.
Siliciclastic grain breakage and displacement due to carbonate crystal growth: an example from the Lueders Formation (Permian) of north‐central Texas, U.S.A.
Authors: BUCZYNSKI CHRIS , CHAFETZ HENRY S.
Published: June 2006

The Lueders Formation (mid‐Permian) in Baylor County, Texas, is an intercalated suite of fluvial siliciclastic, shallow marine siliciclastic, and shallow marine carbonate strata. There are at least two generations of carbonate cements (probably originally composed of calcite) in the fluvial sandstones where fractured grains are observed. These cements represent the initial stages of caliche formation.

Cementation is envisioned as a two step process. In the first step, calcite cements form from supersaturated fluids in a freshwater, vadose environment as a meniscus cement at grain contacts. Areas of cement formation are restricted to these sites because fluid distribution is restricted to these sites. Stresses generated by the growth of cements at grain contacts are transmitted through and concentrated at quartz/quartz grain point‐contacts until the stress is sufficient to fracture quartz grains, even though the ultimate strength of calcite is less than that of quartz, per unit area. This process occurs too rapidly to be accommodated by pressure solution.

In the second phase of cementation, cement nucleation is no longer restricted by vadose conditions. In this phase, calcite growth can no longer result in quartz grain breakage; rather, the quartz grains are dispersed in poikilotopic calcite cement.
A debris‐flow deposit with multiple plug‐flow channels and associated side accretion deposits
Authors: CAS R. A. F. , LANDIS C. A.
Published: June 2006

A grain‐dominant debris‐flow deposit of juvenile, basaltic volcaniclastic detritus from a late Eocene‐early Oligocene, marine surtseyan volcanic pile in north Otago, New Zealand, contains multiple, semi‐circular to U‐shaped channel‐like structures, or relics thereof, associated with wedges of asymptotic layers of fine and coarse debris. The U‐shaped structures are interpreted as channel‐like plug flow zones and the associated wedges are interpreted as abandoned parts of the marginal laminar shear flow zone (fine layers) and adjacent plug zone (coarse layers). These appear to have been abandoned by frictional freezing as the plug flow ‘channels’ migrated laterally in an apparently step‐like fashion. The debris flow and its deposit appear to have been tabular in geometry, and the development of multiple transport pathways in debris flows is considered in terms of the geometrical ratio of the width of the flow to the dynamic thickness.
Ice margin influence on glaciomarine sedimentation in the Permo—Carboniferous Dwyka Formation from the southwestern Karoo, South Africa
Authors: VISSER J. N. J. , LOOCK J. C.
Published: June 2006

A persistent sedimentary unit, interbedded in massive diamictite over a distance of almost 400 km near the top of the Permo‐Carboniferous Dwyka Formation in the southwestern Karoo, consists of stratified diamictite, rhythmite, lonestone argillite and black shale. The stratified diamictite facies association is interpreted as ice‐marginal debris‐flow, the diamictite‐lonestone argillite facies association as proximal to intermediate debris‐flow, debris rain and suspension settling, and the shale‐diamictite facies association as distal debris‐flow and suspension settling deposits. An analysis of the mudrock and diamictite facies relationships suggests deposition from a rapidly calving oscillating ice margin in the east and at a consistently retreating grounded ice margin with few icebergs in the west. Sediment sources, volume of ice rafting, resedimentation processes, ice marginal recession and advance, and configuration of the ice margin influenced the distribution of debris‐flow deposits and bergstone mud in the east and resulted in a near‐random facies arrangement. Due to a lack of icebergs in the west, debris‐flow deposits and bergstone mud were poorly developed which reduced the number of facies transitions and variation, resulting in a more systematic upward‐fining sequence.
A theoretical model for aeolian impact ripples
Authors: ANDERSON ROBERT S.
Published: June 2006

New insights into the grain‐bed impact process arising from both numerical and physical experiments involving single grain impacts lead to a more complete conceptual model of the aeolian saltation process that in turn allows a simple model of aeolian impact ripples to be developed. The saltating population may be idealized as consisting of (1) long trajectory, high impact‐energy, constant impact‐angle ‘successive saltations’, and (2) short trajectory, low impact‐energy ‘reptations’. It is argued that the spatial variations in mass flux due to the reptating population lead to the growth and translation of impact ripples.

Using the sediment continuity equation, an expression for the spatial variation in the ejection rate of reptating grains from a sinusoidally perturbed bed, and a probability distribution for the reptation lengths, a simple stability analysis demonstrates that the flat bed is unstable to small amplitude perturbations. A fastest‐growing wavelength emerges that is roughly six times the mean reptation length, and is only weakly dependent upon the detailed shape of the probability distribution of reptation lengths. The results match well with the observed initial wavelengths in wind tunnel experiments.
Internal structure of subtidal sandwaves revealed by high‐resolution seismic reflection
Authors: BERNÉ SERGE , AUFFRET JEAN‐PAUL , WALKER P.
Published: June 2006

ABSTRACT

Bedforms of the Surtainville area, off the Cherbourg peninsula, include subtidal sandwaves, which are a good example of mega‐structures associated with the strong tidal currents prevailing in the English Channel. A fine‐scale study using a high‐accuracy echo‐sounder and side‐scan sonar shows that some of these sandwaves have a crescentic shape and a strong asymmetry indicating a sand movement toward the north. The sandwaves range in height from about 3.5 m to 7.5 m, in width from 100 m to 500 m and in length from 70 m to 200 m; their internal structure, revealed by the simultaneous use of a high‐resolution seismic source, is characterized by large ‘foreset’ beds dipping in the same direction as the lee sides of the sandwaves. Groups of foresets are limited by reactivation surfaces which we interpret as erosional surfaces created by subordinate tides. The presence of horizontal erosional reflectors inside the sandwaves and the truncation of the present‐day profiles may reflect the effects of storms. The asymmetry of the tide in the area studied, shown by long‐term current measurements, indicates that these sandwaves belong to classes III or IV of Allen's (1980a) classification; the observed structures correspond very well to the prediction of Allen's conceptual model, but we suggest that long term phenomena like equinox cyclicity, associated with storms, may be responsible for their origin rather than the neap‐spring‐neap tidal cycles responsible for the internal structure of intertidal bedforms.
Stokes surfaces and the effects of near‐surface groundwater‐table on Aeolian deposition
Authors: FRYBERGER STEVEN G. , SCHENK CHRISTOPHER J. , KRYSTINIK LEE F.
Published: June 2006

ABSTRACT

Stokes surfaces in aeolian deposits are caused by wind scour of unconsolidated material to a roughly planar horizon controlled by near‐surface water‐tables (Stokes, 1968). A water‐table forms a downward limit of scour through the cohesion of damp or wet sand near water‐table, and through early cementation by evaporites precipitated in the sediments as water evaporates near the sand‐air interface. Study of modern analogues reveals that Stokes surfaces exist in a variety of depositional settings, including a coastal offshore prograding sand sea (Jafurah, Saudi Arabia); a coastal onshore prograding sand sea (Guerrero Negro, Mexico) and a continental sand sea (White Sands, New Mexico, USA). These modern analogues indicate that our concept of Stokes surfaces must be broadened to include the following: (i) modern analogues for Stokes surfaces described here cover areas on the order of 25 km2. These may be as representative of similar surfaces in ancient rocks as hypothesized plains of deflation requiring removal of entire sand seas; (ii) Stokes surfaces occupy a continuum in scale from local to extensive, and erosional surfaces of different magnitude may be stacked closely in the sediments; (iii) Stokes surfaces, although erosional in nature, are commonly associated with deposits both above and below the Stokes bounding surface which plainly reveal the influence of a near‐surface groundwater control on wind sedimentation. Moreover, the erosional relief of the bounding surface itself (as well as other features) reveals the influence of a groundwater‐table; (iv) Stokes surfaces may be diachronous, representing the lateral shift of a zone of scour within a sand sea rather than simultaneous removal of all dunes from the area encompassed by the erosional surface; (v) Stokes surfaces and associated deposits are often laterally transitional to surfaces and deposits of adjacent depositional environments, including interdunes, tidal flats, lagoons, beaches, lakes and non‐aeolian sabkhas. Finally, modern examples from different depositional settings suggest that while most Stokes surfaces have many features in common (such as erosional ridges due to early cementation), there are some features which may, with further study, be revealed to be distinctive of an individual depositional setting.
Density surges: two‐dimensional experiments
Authors: LAVAL A. , CREMER M. , BEGHIN P. , RAVENNE C.
Published: June 2006

ABSTRACT

Some new aspects of the development of density surges in channels are developed from recent experimental results. A density surge is characterized by the flow of a finite amount of dense fluid. This dense fluid may be a saline‐water solution, a saline‐water solution with tracers (sand in suspension) or a pure suspension (sand in suspension in fresh water).

The experimental results show in particular that: (1) the velocity of the surge is effectively proportional to the square root of the initial volume, (2) the velocity of the surge increases with the increase of the initial density of the heavy fluid, proportionally to the square root of the ratio of density difference to the ambient fluid density, (3) the speed of the surge increases again with an increase of the slope.

For pure suspensions the experiments show that velocities slow down much more quickly than for saline solutions with the same initial density, due to loss of negative buoyancy.

The effects of the initial density and of the size of the grains on the distribution of sediments are described.
Comparative evolution of the Senegal and eastern central Atlantic Basins, from mineralogical and geochemical investigations
Authors: CHAMLEY HERVÉ , DEBRABANT PIERRE , FLICOTEAUX RENÉ
Published: June 2006

ABSTRACT

Cretaceous to Palaeogene sedimentary series in two long, continously recovered boreholes from the Senegal coastal basin and adjacent Cape Verde deep‐sea basin have been compared by mineralogical, geochemical, electron‐microscope and microprobe investigations. The transition between the diagenetic influences related to the depth of burial and the palaeoenvironmental influences, identified in the thick coastal‐basin successions, is complicated by the tectonic instability of the margin during periods of high subsidence rates and crustal thinning. As the deep‐sea basin sediments are only slightly affected by diagenetic changes, the comparison between the deep sea and coastal series allows the relative effects of lithostatic pressure, tectonics and other palaeoenvironmental factors to be evaluated. The disappearance of synsedimentary signatures (tectonics, climate, depositional conditions, etc.) expressed by clay mineralogy occurs below 4.5 km; there are no significant diagenetic changes in sediments at less than 2 km depth. By comparing identical time slices in coastal and deep‐sea basins, it is possible to recognize from the clay stratigraphic record the main African detrital sources, the hot and often semi‐arid character of the continental climate during the Cretaceous, the existence of minor but extensive stages of tectonic activity (Barremian‐late Aptian, late Albian‐early Cenomanian), the temporary existence of semi‐closed basins on the shelf (Late Cretaceous, early Palaeogene) and the complexity of transport and sedimentation relationships between proximal and distal environments.
Rudist‐coral frameworks associated with submarine volcanism in the Maastrichtian of the Pachino area (Sicily)
Authors: CAMOIN GILBERT , BERNET‐ROLLANDE MARIE‐CLAIRE , PHILIP JEAN
Published: June 2006

ABSTRACT

Maastrichtian strata from the Pachino area (SE Sicily) provide a model of association between rudist‐coral frameworks and submarine volcanic activity.

Two successive carbonate units are distinguished: (a) Coral‐rudist bioherms and biostromes developed in a high‐energy environment, (b) Hippuritid build‐ups and banks overlain by rudist‐coral clusters which grew under harsher ecological conditions, in a weaker current regime.

At the top of the Maastrichtian sequence, the growth of rudist‐coral frameworks ceased as ecological conditions shifted toward restricted environments. The inferred succession reflects a gradual decrease in current strengths and water depths, related to the Maastrichtian regressive phase.

The frameworks display consistent evolutionary sequences reflecting progressive changes in their biota, structure, size and shape.

Occurrences of rudist‐coral frameworks are clearly linked with submarine volcanoes which provided opportunities for their development; they sometimes grew in the vicinity of active‐vent centres, a feature reported for the first time in the Upper Cretaceous.
Washover fan and brackish bay sedimentation in the Berriasian‐Valanginian of Bornholm, Denmark
Authors: NOE‐NYGAARD NANNA , SURLYK FINN
Published: June 2006

ABSTRACT

The Robbedale and Jydegård Formations (Berriasian‐Valanginian) of the Danish island of Bornholm represent a 100 m thick vertical sequence from shoreface, foreshore and beach sands of a high‐energy coast through backbarrier flat, bay margin pond and distal washover fan sand and clay, brackish bay clay, to fluvial sands. The longevity of the backbarrier‐bay system (c. 10 Myr), thickness (100 m) of the bay deposits and apparently stacked nature of the facies belts suggest a relatively stationary position of the individual subenvironments, with only minor progradation. This reflects strong tectonic control of the depositional system during an important phase of synsedimentary block faulting and wrenching along the Tornquist Zone. The importance of washover fan sands in the backbarrier deposits, and the lack of tidal indications in the whole sequence, suggest a microtidal regime. A system of migrating mud banks formed in shallow water on the landward side of the barrier. The bay waters varied from almost fresh to brackish, and anoxic conditions commonly occurred at the bottom. Adverse living conditions for most organisms in the bay caused seasonal, possibly toxic, dinoflagellate blooms resulting in mass mortality of infaunal bivalves. Bay‐margin ponds underwent periodic desiccation, leading to mass mortality of freshwater gastropods. As a general scenario it is envisaged that longshore currents redistributed bedload from a major delta and formed an extensive NW‐SE barrier‐spit which partly enclosed a major bay to the NE. The barrier was breached during heavy storms and the sand transported along the resulting washover channels was deposited on the backbarrier flat made up of the subaerial parts of coalescent washover fans. Enormous amounts of suspension load from the delta travelled further along the barrier to be deposited in the lee‐side bay.
Shallowing‐upward sequences in Purbeckian peritidal carbonates (lowermost Cretaceous, Swiss and French Jura Mountains)
Authors: STRASSER ANDRÉ
Published: June 2006

Purbeckian carbonates in the Swiss and French Jura (Goldberg Formation, lower Berriasian) comprise shallow‐subtidal, intertidal, supratidal, low‐energy, high‐energy, marine, brackish, freshwater, and hypersaline facies. These facies are arranged in small (0–2–1.5 m thick) sequences which display a dominant shallowing‐upward component, and which form the fundamental units of the highly structured Purbeckian sedimentary record.

Six types of small‐scale sequences can be recognized. A: intertidal to supratidal overprinting of shallow lagoonal facies; B: algal‐marsh sequence with frequent dolomitization; C: sabkha sequence, often associated with collapse breccia; D: tidal‐flat sequence with desiccation features; E: lacustrine sequence; F: terrestrial overprinting of subtidal or intertidal facies. Episodic event deposits such as tempestites are superimposed] Thin transgressive beds which rework elements of the underlying facies are frequently found at the base of the sequences. Green marls and black pebbles are common at the top and indicate long subaerial exposure. The sequences are often incomplete, as subtidal facies may be absent, or their upper part can be eroded. Lateral facies changes are common, which is due to the very shallow and partly emergent Purbeckian platform where various depositional environments were juxtaposed. However, many sequence boundaries are well developed and can be correlated over large parts of the study area.

The Purbeckian shallowing‐upward sequences were generated by climatically controlled sea‐level changes. Autocyclic processes occurred locally, but were overprinted by drops of sea‐level affecting the entire platform. The small‐scale sequences are most probably related to the 20 000‐year cycle of the precession of the equinoxes. Larger sequences with usually well‐developed emersion surfaces are attributed to the 100 000 and 400 000‐year eccentricity cycles of the Earth's orbit. Identification and correlation of sequence boundaries makes it possible to set up a framework of isochronous surfaces (which often cut across facies boundaries), and thus to interpret in detail the palaeogeographic, sedimentological and diagenetic evolution of the Purbeckian peritidal carbonate environments.
Sediment transport and bed morphology at river channel confluences
Authors: BEST JAMES L.
Published: June 2006

River channel confluences form important morphological elements of every river system, being points at which rapid changes in flow, sediment discharge and hydraulic geometry must be accommodated. This article presents results of a quantitative investigation of sediment transport at channel confluences accomplished through both scaled laboratory flume simulation and complementary monitoring of a natural channel confluence.

Bed morphology at channel confluences is characterized by three distinct elements: avalanche faces at the mouth of each confluent channel, a deep central scour and a bar within the separation zone formed at the downstream junction corner. These elements are controlled predominantly by the confluence angle and the ratio of discharges between the tributary and mainstream channels. As confluence angle and discharge ratio increase, the sediment contributions from the confluent channels are progressively segregated in their paths through the junction, with sediment being transported around rather than through the centre of the confluence. This segregation of sediment loads is accompanied by the retreat of the main channel avalanche face from the confluence, an increase in the scour depth, a change in the orientation of the scour and an increase in the size of the separation zone bar. Field measurements closely replicate the flume simulation.

A model of sediment transport and bed morphology links these features to the fluid dynamics of these sites. An understanding of confluence dynamics is important not only in considerations of channel morphology and design criteria but must form the basis for the interpretation of confluence sediments in the ancient record.
Contrasting diagenesis of two Carboniferous Oolites from South Wales: a tale of climatic influence
Authors: HIRD KEVIN , TUCKER MAURICE E.
Published: June 2006

Two oolites in the Dinantian (Mississippian/Lower Carboniferous) of Glamorgan, SW Britain, were deposited in similar depositional environments but have contrasting diagenetic histories. The Brofiscin and Gully Oolites occur in the upper parts of shallowing‐upward sequences, formed through strandplain progradation and sand shoal and barrier growth upon a southward‐dipping carbonate ramp. The Brofiscin Oolite is characterized by a first‐generation cement of equant calcite spar, preferentially located at grain‐contacts and forming non‐isopachous fringes around grains, interpreted as meteoric vadose and phreatic in origin. Isopachous fibrous calcite fringes of marine origin are rather rare and occur only at a few horizons. Burial compaction was not important and porosity was occluded by poikilotopic calcite spar. Fitted grain‐grain contacts locally occur and could be the result of near‐surface vadose dissolution‐compaction. Syntaxial overgrowths on echinoderm debris are common. Pre‐compaction overgrowths are cloudy (inclusion‐rich) and probably of meteoric origin, and post‐compaction overgrowths are inclusion‐free. By contrast, the Gully Oolite has little first‐generation cement. However, marine fibrous calcite is common in oolitic intraclasts, as isopachous fringes of acicular calcite crystals closely associated with peloidal internal sediment; and early equant, drusy calcite spar occurs in the uppermost part of the Gully, beneath a prominent palaeokarst where pedogenic cements also occur. The major feature of Gully diagenesis is burial compaction, resulting in extensive grain‐grain dissolution and microstylolitic grain contacts, and post‐compaction poikilotopic spar occluded remaining porosity.

The Brofiscin Oolite is pervasively dolomitized up‐dip but the Gully Oolite for the most part only contains scattered pre‐compaction dolomite rhombs and late veins of baroque dolomite, with less pervasive dolomitization.

The difference in diagenetic style of the two Dinantian oolites is attributed to prevailing climate. The paucity of early meteoric cements in the Gully is a result of an arid climate, and this is supported by the nature of the capping palaeokarst. The abundant meteoric cements in the Brofiscin reflect a more humid climate, and effective meteoric recharge also resulted in up‐dip pervasive mixing‐zone dolomitization. The style of early diagenesis in these two oolites exerted a major control on the later burial diagenesis: in the Brofiscin, the early cements inhibited grain‐grain dissolution and pressure solution, while these processes operated extensively in the Gully Oolite. Thus, prevailing climate can influence a limestone's diagenetic history from near‐surface through into deep burial.
Architecture of the Castissent fluvial sheet sandstones, Eocene, South Pyrenees, Spain
Authors: MARZO MARIANO , NIJMAN WOUTER , PUIGDEFABREGAS CAI
Published: June 2006

The Castissent Formation represents a phase of strong fluvial progradation within the fluvio‐deltaic Montanyana Group, probably enhanced by the late Ypresian sea‐level fall. The structural setting is characterized by the emplacement of the South Pyrenean nappes. The major architectural feature of the Formation is the superposition of three multilateral and multistorey sheet sandstone complexes (A, B and C), interbedded in finer floodplain deposits and dark‐grey mudstones of brackish transitional origin. The structure of the sheet sandstone complexes is exemplified by the detailed analysis of the lower sheet sandstone complex (A), 25 m thick, 6 km wide and exposed over 25 km of its length. Three types of channel‐fill sandstones serve as components: (1) lenticular‐bedded bodies, interpreted as braided stream deposits, (2) lateral accretion‐bedded bodies, intepreted as meandering stream deposits, and (3) ribbons, originating from laterally stable and short‐lived offshoot or crevasse channels. By coalescence and/or stacking of these channel‐fill types, higher‐order architectural units (A1, A2, A3) have been formed that together build the sheet sandstone complex. On the basis of downstream variation in proportion and geometrical relation between the channel‐fill components, three depositional zones have been differentiated within the fluvial system. Each zone is characterized by specific architectural processes: (1) alternation of minor aggradational‐degradational cycles in the proximal valley, (2) generation of new alluvial belts in the intermediate nodal avulsion zone, and (3) successive random avulsion processes with development of semipermanent crevasse channels or minor distributaries in the distal distributive zone. The sedimentation of the Castissent Formation was structurally controlled by an interplay of vertical basement movement due to thrust stacking in the hinterland and surficial thrust displacement to the foreland resulting in alternating southward and northward shift of the fluvial system.
Suspended‐load fallout rate as an independent variable in the analysis of current structures
Authors: LOWE DONALD R.
Published: June 2006

The dynamic interpretation of most current‐structure sequences derives directly from experiments on the succession of bedforms produced by flows in flumes. The results of these and related studies have been used to construct stability field diagrams in which the fields of individual bedforms are usually expressed as a function of flow intensity (power, velocity, bed shear stress, etc.) and grain size.

The data underlying existing stability‐field diagrams were collected largely from the study of flows carrying coarse‐grained sediment entrained through particle‐by‐particle bed erosion. Many flows, however, do not entrain sediment through simple bed erosion. Most turbidity currents originate by the development of turbulence in slumps, slides, and other slope failures. Such flows generally form with highly concentrated suspended loads and their bed‐load layers derive sediment from the collapsing suspended‐sediment clouds. Because the collapse properties of such clouds may be related as much to suspended particle concentration, size distribution, particle interactions, and other factors as to flow intensity, the stability fields of bedforms developed beneath such flows may differ in flow intensity‐grain‐size relationships from those beneath flows deriving sediment from bed erosion alone.

Useful stability‐field diagrams for turbidity currents must include suspended‐load fallout rate as a third variable, independent of flow intensity and mean grain size. A preliminary stability‐field diagram of this type indicates that Bouma Tabc sequences may theoretically form with essentially no velocity variation of the attendant flow. This type of analysis may have considerable relevance to the interpretation not only of turbidites but also of other deposits formed where bed‐load layers are fed from above rather than below. These include shallow‐shelf storm units deposited from highly concentrated flows and volcaniclastic layers formed where pyroclastic debris falls directly into moving water.
Paleosols of Late Pennsylvanian cyclic strata, New Mexico*
Authors: GOLDSTEIN ROBERTH.
Published: June 2006

Cyclically deposited siliciclastic and carbonate rocks of the Late Pennsylvanian (Virgilian, Holder Formation) of south‐central New Mexico were deposited on a narrow shelf between the Pedernal uplift and the Orogrande basin. In the Dry Canyon area, Sacramento Mountains, shelf‐to‐basin palaeotopography is indicated by onlapping beds and primary dips. Calcareous alteration zones are developed on the tops of major carbonate units. These detailed studies confirm earlier interpretations that alteration zones represent subaerial exposure events and provide new evidence for and further documentation of a paleosol environment. The paleosols contain: (1) rhizoliths; (2) tangential needle fibres of calcite; (3) five types of alveolar texture; (4) upward oriented ‘ribbon spar’; (5) brownish and clear isopachous spar; (6) blackened grains; (7) random needle fibres of calcite; (8) coated grains; (9) micritized grains; (10) glaebules; (11) desiccation cracks; (12) possible Microcodium; and (13) well‐developed laminated crusts. Isopachous cements, upward‐oriented cements (which mark the former tops of capillary fringes), dark colour and shallow depth of penetration of rhizoliths (less than 1 m) indicate significant paleosol formation in waterlogged (hydromorphic) and drier conditions. Pennsylvanian palaeohydrogeology of the system is reflected in the type of paleosol alteration.

Paleosols developed on subtidal strata and paleosols which drape tens of metres of palaeotopography indicate cyclic sedimentation was not controlled by delta lobe switching. Local tectonic up and down movement would be an unlikely mechanism to explain the repeated tens of metres of relative sea‐level fluctuation. Eustatic sea‐level fluctuations, induced by Late Pennsylvanian Gondwana glaciation, probably were of the correct magnitude and frequency to have caused the Holder cycles.
The dynamics of star dunes: an example from the Gran Desierto, Mexico
Authors: LANCASTER N.
Published: June 2006

Observations of patterns of erosion and deposition and surface wind velocity and direction on a 40 m high star dune in the Gran Desierto sand sea indicate that interactions between dune form and airflow as winds change direction seasonally play a major role in the formation of this dune type. Such interactions lead to deposition of sand in the central parts of the dune, giving rise to its pyramidal shape, as well as to some extension of the linear arms.

The major arms of the dune studied are oriented NE‐SW, or transverse to summer SSE and winter NNW winds. An avalanche face up to 10 m high develops during the course of each season. Flow separation at the main crestline gives rise to a wide zone of lee side secondary flow which moves sand along the base of the avalanche face towards the central part of the dune, where it is deposited as wind ripples migrate into zones of locally reduced flow velocity. Reattachment of the separated flow occurs on the lower part of the N or S arms, parallel to the flow. Spring westerly winds move sand obliquely up the S and N arms of the dune and outwards on the E arm. Large scale flow separation and diversion are replaced by the development of strong helical eddies in the immediate lee of the main crestline which move sand along avalanche faces and into zones of lower flow velocity at the end of dune arms.

Formation of star dunes in the Gran Desierto follows a sequence in which crescentic dunes migrating into areas of opposed winds first develop a reversing crestal ridge. Convergent leeside secondary flows are developed, which result in the formation of linear elements parallel to each major wind direction and the concentration of sand in the central part of the dune. Examples of star dunes at different stages of their development can be documented.
Glacial advance and retreat sequences in a Permo‐Carboniferous section, central Transantarctic Mountains
Authors: MILLER JULIA M. G.
Published: June 2006

Episodes of glacial advance and retreat can be recognized through analysis of vertical facies sequences in the Permo‐Carboniferous Pagoda Formation of the Beardmore Glacier area, Antarctica. The formation includes a remarkably complete record of continental sedimentation near the terminus of a temperate glacier. Facies sequence is pre‐eminent for inferring glacial advance and retreat. Other important criteria are abundance and geometry of sandstone interbedded with diamictite, diamictite character and nature of bed contacts. Using these characteristics advance and retreat sequences 5–60 m thick are recognized. A sharp contact, with a striated surface and erosional relief, overlain by structureless diamictite (lodgement till) is typical of grounded ice advance. Grounded ice retreat is characterized by structureless diamictite (lodgement till), overlain by crudely stratified diamictite (melt‐out till) and then by diamictite interbedded with sandstone and conglomerate (flow till and glacio‐fluvial or glacio‐lacustrine deposits). Gradational contacts between shale overlain by diamictite and diamictite overlain by shale characterize advance and retreat, respectively, in subaqueous settings.

Pauses in sediment accumulation, minor(?) fluctuations of the ice margin, and/or changes in subglacial dynamics are indicated by specific features within diamictite units such as probable frost‐wedge casts, single layer boulder beds, sharp sedimentary contacts and changes in diamictite character. These minor(?) events are superimposed upon the main advance‐retreat cycles. Study of both the overall facies sequence and of individual diagnostic structures, albeit in an incomplete stratigraphic record, permits a distinction between major and minor advance‐retreat events. As many as six major advance‐retreat cycles exist in some Pagoda sections, but the number of cycles present varies in different sections.
Glaciomarine facies within subglacial tunnel valleys: the sedimentary record of glacioisostatic downwarping in the Irish Sea Basin
Authors: EYLES N. , MCCABE A. M.
Published: June 2006

Coastal exposures of Late Pleistocene sediments deposited after 19 000 yr BP near Dublin, Ireland, provide a window into the infill of a subglacially‐cut tunnel valley. Exposures close to the steeply dipping bedrock wall of the valley show boulder gravels within multi‐storey U‐shaped channels cut and filled by subglacial meltwaters driven by a high hydrostatic head. Gravels are truncated by poorly sorted ice‐proximal glaciomarine sediments that record the pumping of large volumes of subglacial debris along the tunnel valley to a tidewater ice sheet margin. The sedimentary succession is dominated by sediment gravity flow facies comprising interbedded diamict and massive, poorly sorted gravel facies interpreted as subaqueous debris flow deposits. Gravel beds show local inverse and normal coarse‐tail graded facies recording the restricted development of turbulent flow. Sediment gravity flow deposits fill broad (<2 km) shallow (10 m) and overlapping channels. Penetrative deformation structures (e.g. dykes) are common at the base of channels.

The same subglacially‐eroded topography and glaciomarine infill stratigraphy can be identified on high resolution seismic profiles across nearly 600 km2 of the western Irish Sea. Tunnel valleys are argued to have been exposed to glaciomarine processes by the rapid retreat of a calving tidewater ice sheet margin in response to marine flooding caused by glacio‐isostatic downwarping below the last British Ice Sheet. The facies associations described in this paper comprise an event stratigraphy that may be found on other glaciated continental shelves.
Erg deposits in the Lower Jurassic Wingate Sandstone, northeastern Arizona: oblique dune sedimentation
Authors: CLEMMENSEN LARS B. , BLAKEY RONALD C.
Published: June 2006

The Lower Jurassic erg (aeolian sand sea) deposits of the Wingate Sandstone on the Colorado Plateau are beautifully exposed near Many Farms, Arizona. These 3‐D outcrops allow a detailed study of structures and sequenses in the erg body.

The erg sequence comprises chiefly oblique dune deposits. The dune facies are in most cases characterized by a well‐developed tripartite upbuilding. Each dune coset contains unusually thick and intricate bottomsets, medial low‐angle dipping toesets, and upper steeply dipping foresets.

The foresets reveal significant across‐crest transport of sand and dip within a narrow range of directions towards the ESE. The bottomset beds are composed of compound cross‐bedding that documents strong along‐crest transport towards the SSW, whereas the toeset beds reveal upslope, downslope, and along‐crest transport of sand.

The ancient dunes apparently formed in a directionally varying wind flow with prevailing winds (early summer) from the NW and periodic strong winds (late summer) from the SW. The dunes were oblique not only to seasonal transport directions, but also to the resultant annual transport direction and dune migration direction. This was caused by the interaction of the dune system with the primary winds which resulted in secondary airflow and significant along‐crest transport of sand.

The erg deposits at Many Farms are separated by a number of super bounding surfaces suggesting several episodes of erg formation and destruction. The initial erg system was dominated by transverse dunes, but overlying ergs only contained oblique dunes. All erg systems were bounded to the SW by wide regions of erg margin environments in which aeolian sand sheet, fluvial, and lacustrine facies were deposited. Even though fluvial deposits are absent from the main part of the sequence at the study area, the effects of this system are reflected within the erg deposits at Many Farms.
Franciscan Complex olistostrome at Crescent City, northern California
Authors: AALTO K. R.
Published: June 2006

An olistostrome and bounding turbidites are exposed within the late Mesozoic Franciscan Complex along the Crescent City (California) coastline. Facies grade in character from Mutti & Ricci Lucchi (1978) mixed facies B, C and D, to F (the olistostrome), to mixed A, B and E, progressing upwards within the Franciscan stratigraphic section. The facies F unit outcrop is up to 600 m thick and extends 12 km along strike. It consists of oblate to tabular blocks, up to 200 m in maximum dimension, of greenstone, tonalite, radiolarian chert, limestone, phyllite and greywacke dispersed in a scaly argillite matrix. The olistostromal origin of the unit is indicated by depositional contacts with bounding turbidites, by the presence of abundant recycled sedimentary clasts within the unit, and by the presence of sedimentary breccias and associated dismembered, slump‐folded turbidites both within the olistostrome and among subjacent turbidites. Sandstones are chiefly feldspathic litharenites that were very likely derived from the partially dissected, late Mesozoic Sierran‐Klamath magmatic are.

Franciscan rocks record an early pervasive, layer‐parallel flattening strain in such features as extensional faults, necking and pinch‐and‐swell structures. Several scales of extensional faulting account for the juxtaposition of turbidites of different facies and/or with varying degrees of stratal disruption, the formation of sandstone lozenges, and the formation of scaly foliation in the olistostrome matrix. The latter resulted from the juxtaposition of lenticles with varying concentrations of silt and clay. These were ultimately derived from the finer divisions of turbidite beds that were incorporated into the olistostrome. The presence of gradational contacts between some sandstone olistoliths and the olistostrome matrix, and of sandstone dykes that intrude fractures and associated drag‐folded turbidite beds indicate that Franciscan sediments were not lithified during their early deformation. These sediments were deposited in either a trench or trench slope basin, and were first deformed either by gravity collapse of the trench slope cover or, less likely, by vertical loading beneath the toe of the accretionary wedge. They later were folded during internal shortening of the growing Franciscan accretionary prism.
In situ acoustic measurements of marine gravel threshold and transport
Authors: THORNE PETER D. , WILLIAMS JON J. , HEATHERSHAW ANTHONY D.
Published: June 2006

Measurements of the nearbed turbulent current flow and the bedload transport of marine gravel have been made over three tidal cycles. The turbulence in the bottom boundary layer was measured using two electromagnetic current meters, and the gravel transport was measured using a passive acoustic system which monitored the interparticle collision noise of locally mobile material. Visual estimates of bedload were also obtained with an underwater TV camera. The acoustic technique, unlike a conventional bedload sampler, has allowed estimates of transport to be obtained with a temporal resolution comparable with the turbulence data collected. This has enabled a detailed comparison to be made between the turbulent flow and the sediment response to the instantaneous flow conditions. The results of the study show that of the turbulent bursting events which contribute towards the Reynolds stress, only the sweeps and outward interactions play a significant role in the transport of coarse sedimentary material. The measurements show that it is the instantaneous increases in the horizontal turbulent velocity fluctuations that generate excess shear stresses which drive the transport process.
Travertines in the northern Hula Valley, Israel
Authors: HEIMANN ARIEL , SASS EYTAN
Published: June 2006

This study describes the field and petrographic relationships of a widespread deposit of Pleistocene travertines in the northern Hula Valley, northern Israel. The travertines interfinger with conglomerate deposits and basaltic lava flows. Field relationships and radiometric dating indicate that the travertines accumulated intermittently over the past million years, and their formation virtually stopped 25 000 years ago.

The travertines are characteristically highly porous. Some of the pores, as well as some spar‐filled voids, preserve the shapes of stalks and leaves. The abundance of plant material suggests that photosynthesis, rather than bacterial or abiogenic processes, was the main mechanism which induced carbonate precipitation.

The reasons for travertine accumulation in the past and for the cessation of its formation today are ascribed to differences in the palaeogeographic setting. In the past, water flow is viewed as having been mostly sluggish, and a widespread and shallow sheet of water was formed. The lush vegetation, combined with the relatively long residence time of the water in the area, led to increased efficiency of the calcium carbonate precipitation. Today, by contrast, water flows rapidly in gorges, precipitating only a small fraction of its load of dissolved calcium carbonate. The conjectured change in hydrological conditions is ascribed to rejuvenation of faulting activity in the Late Quaternary.
Comparisons between acoustic measurements and predictions of the bedload transport of marine gravels
Authors: WILLIAMS JON J. , THORNE PETER D. , HEATHERSHAW ANTHONY D.
Published: June 2006

Continuous, detailed records of marine gravel transport have been obtained acoustically and compared with bedload transport rates (qb) predicted by five bedload transport equations using measurements of the near‐bed turbulent current flow. When mean flow data are used in these equations, total qb estimates are similar to those measured. However, when instantaneous flow data are used, total qb is over‐estimated by approximately one order‐of‐magnitude. Based on the acoustic measurements, an empirical equation has been obtained that gives accurate estimates of total qb over a tidal cycle and simulates well the intermittent characteristics of marine bedload transport.