bluuebunny/crossref_metadata_2025 huggingface dataset:
Fibrous calcite from the Ordovician of Tennessee: preservation of marine oxygen isotopic composition and its implications
Authors: TOBIN KENNETH J. , WALKER KENNETH R. , STEINHAUFF D. MARK , MORA CLAUDIA I.
Published: November 2003

Three categories of fibrous calcite from early to middle Caradoc platform‐marginal buildups in east Tennessee can be delineated using cathodoluminescent microscopy, minor element chemistry and stable C‐O isotopic composition. Bright luminescent fibrous cement has elevated Mn (>1000 p.p.m.), negative δ13C and intermediate δ18O values relative to other types of fibrous calcite. This cement reflects fibrous calcite that interacted with reducing Mn‐rich fluids. Dully luminescent fibrous cement has elevated Fe (>400 p.p.m.), positive δ13C and negative δ18O values relative to other fibrous cements. This cement was stabilized by burial fluids. Nonluminescent fibrous cement has low Mn and Fe (generally below 400 p.p.m.) and positive δ13C and δ18O values relative to other types of fibrous calcite. The latter cement is interpreted to be the best material for determining the isotopic composition of calcite precipitated in equilibrium with early to middle Caradoc seawater, which is δ13C=1% PDB and δ18O=−4 to −5‰ PDB. Results from this study and Ashgillian brachiopods indicate that the average δ18O composition of the Ordovician ocean, during nonglacial periods, was probably never more negative than −3‰ SMOW. Assuming an Ordovician seawater δ18O value of −1‰ SMOW, Holston Formation fibrous cements would have precipitated at temperatures between 27 and 36 °C, which is near the upper temperature limit for metazoans. A seawater δ18O value of −2‰ SMOW yields temperatures ranging from 23 to 31 °C, while a −3‰ SMOW value yields temperatures of 18–26 °C.
Experimental turbidity currents entering density‐stratified water: analogues for turbidites in Mediterranean hypersaline basins
Authors: RIMOLDI BIANCA , ALEXANDER JAN , MORRIS STEPHEN
Published: November 2003

Experimental turbidity currents entering two‐layer density‐stratified water behave differently from similar currents flowing over the same topography into non‐stratified water. Experiments were designed as analogues for flows entering Mediterranean hypersaline pools. In both the hypersaline pools and the experiments, the water density changes abruptly across a pycnocline. Turbidity currents generated on a platform at the level of the pycnocline behaved in one of three ways as they flowed from the platform into deeper stratified water. (1) When the bulk density of the current was less than the dense water layer, the current spread at the pycnocline. The head of the current advanced rapidly when it lost contact with the bed. Grains settling out of the current fell through the dense water layer forming an extensive deposit. In nature this behaviour will lead to ‘turbidites’ with sharp but non‐erosive bases, strongly developed grading and no traction features. (2) When the bulk density of the current was greater than the dense water layer, the current continued as an underflow, plunging into the deeper water. Sedimentation lowered the bulk density of the current and the low‐density interstitial fluid caused the head to loft. Low‐density interstitial fluid convected from the body of the current, lofting particles into the water column. These particles were hydraulically sorted during upward transport and subsequent settling to the floor. The resulting turbidites had a more limited extent than the deposits of either non‐lofting underflows or interflows. By inference from the experiments, natural deposits of this type may have local (proximal) erosion and traction features at the base and strongly graded tops. (3) In some of the currents with high bulk density, the rising turbid water reached the pycnocline and spread at that level as a secondary interflow. The tail of the turbidity current, which was less dense than the head and body of the current, flowed above the pycnocline adding momentum to the secondary interflow. The thin non‐erosive graded deposit from the secondary interflow may extend beyond the deposits of the primary underflow. In all three cases (but more pronounced in cases 2 and 3) the interaction of the current with the pycnocline displaced that surface and generated a wave that was reflected back and forth from each end of the pool. The waves remobilized sediment on the ramp.
Sedimentology of Badenian (middle Miocene) gypsum in eastern Galicia, Podolia and Bukovina (West Ukraine)
Authors: PERYT TADEUSZ MAREK
Published: November 2003

Primary gypsum is the main evaporite mineral in the middle Miocene (Badenian) of the West Ukraine. The lower part of the gypsum sequence is built of autochthonous gypsum while the upper part is composed of allochthonous gypsum that formed following a major, tectonically induced, change in basin morphology. This change resulted in the destruction of the gypsum deposited on the margins of the basin and formation of redeposition features.

Autochthonous gypsum facies were deposited in two main environments: (1) giant gypsum intergrowths precipitated from highly concentrated brines; (2) very shallow subaqueous gypsum deposited in a vast brine pan. The brine pan was characterized by a facies mosaic that reflects an interplay of concentrated brines from the central part of the evaporite basin and diluted brines due to the influx of continental meteoric waters. The facies continuum, microbial gypsum ‐ bedded selenite ‐ massive selenite ‐ sabre gypsum, indicates increasing salinity of the brine with time. This type of facies pattern has been established in recent salinas that are analogous to Badenian gypsum in their lateral facies changes. However, the pattern of facies distribution with respect to the open sea in the Badenian basin is opposite to that found in recent salinas.

The pattern of the Badenian gypsum facies in the Ukraine indicates that facies repetition may have been related to climatically controlled salinity changes and not to depth changes, as is commonly used to explain the repetition of sulphate facies in a vertical succession.
Grain‐size characteristics of fine‐grained unflocculated sediments I:‘one‐round’ distributions
Authors: KRANCK KATE , SMITH P. C. , MILLIGAN T. G.
Published: November 2003

A simple physical model of gravitational settling from an unsorted, unflocculated source suspension is presented and an equation derived to describe the grain‐size spectra of the resulting bottom sediment. Results of grain‐size analyses of sediments from a variety of environments and geographical locations are shown to conform with the postulated model. The characteristic size spectrum, termed ‘one‐round’ sediment, identifies a deposit which has settled from suspension with no subsequent reworking resulting in modification of the grain‐size distribution. The distribution of settling rates of grains in the suspension may be inferred from an analytical form fit to the bottom sediment grain‐size spectrum, along with knowledge of certain physical characteristics of the fluid (e.g. mean velocity profile).
Significance of fan deltas without toe‐sets within rift and piggy‐back basins: examples from the Corinth graben and the Mesohellenic trough, Central Greece
Authors: ZELILIDIS ABRAHAM , KONTOPOULOS NIKOLAOS
Published: November 2003

Trapezoidal‐type fan deltas, lacking bottomset deposits, were studied in two different tectonic settings: extensional (rifted), and compressional (piggy‐back) basins. In both cases studied fan deltas were characterized by: (i) an absence of bottomsets; (ii) development in protected or narrow basins and sub‐basins confined by intrabasinal basement highs or by topographic highs, respectively; (iii) coarse‐grained sediment fluxes, dominated by mass‐flows forming fan deltas that prograded from steep nearshore slopes basinwards; and (iv) a high‐energy environment, with powerful underflows that probably bypassed the basins and transported fine‐grained sediments outside the basins. The location of channels cut by such underflows is influenced by local tectonic style. When the supplied sediments in the rifted basins overstepped the intrabasinal basement highs, trapezoidal fan deltas were replaced upwards by Gilbert‐type deltas, with bottomsets.
Facies and cyclicity of the Late Permian Bainmedart Coal Measures in the Northern Prince Charles Mountains, MacRobertson Land, Antarctica
Authors: FIELDING CHRISTOPHER R. , WEBB JOHN A.
Published: November 2003

The Late Permian Bainmedart Coal Measures form part of the Permo‐Triassic Amery Group, which crops out in the Beaver Lake area of the Northern Prince Charles Mountains, MacRobertson Land, Antarctica. The exposed strata are believed to have formed in graben or half‐graben sub‐basins on the western edge of the Lambert Graben, a major failed rift system. Sedimentological analysis has revealed that these rocks formed in alluvial environments in which swiftly flowing rivers of low sinuosity (represented by Facies A1 and A2) flowed northward down the axis of the basin, and were associated with waterlogged floodbasin and peat‐forming wetlands (Facies B1‐B4). A third Facies Association (comprising Facies C1‐C3), interpreted as the deposits of lake floor and delta environments, is exclusively developed within a distinctive, fine‐grained interval here named the Dragon's Teeth Member. The proportion of Association B facies within the succession increases markedly above the level of the Dragon's Teeth Member (at about 300 m above the base of the formation).

Flat, low‐angle and undulatory bedding structures preserved within channel deposits are suggestive of sediment deposition in flow conditions which were often critical or supercritical. Presence of massive and chaotic intervals of sandstone further implies some deposition from high‐concentration aqueous flows. Alluvial channel bodies show evidence of incision into underlying substrates, both during initiation and at later stages in channel belt construction. The lack of interfingering between channel deposits and coals suggests that thick peats formed only in areas and at times of minimal clastic sediment supply.

Analysis of well‐developed cyclicity within the coal measures suggests that the dominant control on sequence architecture was climatic, related to precessional Milankovitch fluctuations of c. 19‐kyr periodicity. Cycles began abruptly with the deposition of coarse‐grained material in high‐energy alluvial channels, which contracted with time in response to changes in water supply (rainfall). Upper parts of cycles are dominated by finer‐grained sediments and then coal, indicative of progressively reduced coarse sediment input. Tectonic processes overprinted this pattern at least once during the period of sediment accumulation, to form the Dragon's Teeth Member.
Morphology and growth of aragonite crystals in hot‐spring travertines at Lake Bogoria, Kenya Rift Valley
Authors: JONES BRIAN , RENAUT ROBIN W.
Published: November 2003

Pseudohexagonal aragonite crystals are common components in some hot‐spring travertines at Chemurkeu on the western shore of Lake Bogoria, Kenya. Beds, lenses and pods of aragonite crystals are intercalated with beds of white non‐crystallographic calcite dendrites. The pseudohexagonal aragonite crystals, which are up to 4 cm long and 4 mm wide, are formed of nested skeletal crystals. Each skeletal crystal is formed of cyclical twinned crystals that are constructed of stacked subcrystals. The latter are inclined at a consistent angle of 40° to the long axis of the pseudohexagonal aragonite crystal. Intense competition for space during growth modified the crystal morphology with the result that many of the pseudohexagonal crystals are distorted. Intercrystalline and intracrystalline pores are filled or partly filled by epitaxial aragonite overgrowths and/or reticulate microbial coatings that have a high concentration of Si and Mg. In places, this extracellular mucus induced etching of the underlying aragonite crystal.

Today the hot (T>95 °C) Na‐HCO3‐Cl spring waters at Chemurkeu have a salinity of 5–6 g L−1 TDS, a pH of 8·1–9·1, Ca2+ concentrations of <2 mg L−1 and Mg2+ concentrations of <0·7 mg L−1, The springs of the Lake Bogoria Geothermal Field are fed by a shallow aquifer (T∼100 °C) and a deeper aquifer (T∼170 °C). Springs at Chemurkeu derive from meteoric groundwater, lake water and condensed steam, and are fed mainly from the shallow thermal aquifer. Much of the aragonite may have formed when the spring waters contained more dissolved Ca2+ than today, possibly under more humid conditions during the Holocene.
Holocene Saharan dust deposition on the Cape Verde Islands: sedimentological and Nd‐Sr isotopic evidence
Authors: ROGNON P. , COUDÉ‐GAUSSEN G. , REVEL M. , GROUSSET F. E. , PEDEMAY P.
Published: November 2003

Holocene aeolian silts deposited on the Cape Verde Islands provide information about the origin of African palaeodusts that have fallen on the north‐eastern Atlantic ocean over the last 10 000 years. Sedimentological composition indicates that most of these aeolian silts are unquestionably of continental origin. Their Sr and Nd isotopic composition identifies a Saharan origin‐suggesting transport by Harmattan winds. We estimate that Saharan dust comprises 75–95% of material in these Holocene silts, the rest coming from the weathering of local basaltic bedrock.
Sedimentological responses to basin initiation in the Devonian of East Greenland
Authors: MARSHALL J. E. A. , STEPHENSON B. J.
Published: November 2003

The Devonian of East Greenland comprises a thick sequence of continental clastic sediments infilling an extensional basin. West of the main basin bounding fault (Western Fault Zone) are scattered outliers of Devonian conglomerate which accumulated in small basins such as found on the island, Ella Ø. The Ella Ø Basin formed by extensional movement along the Narhval Sund Fault accompanied by the formation of a prominent bedding parallel detachment surface which was subsequently modified by sub‐aerial exposure to become the unconformity surface. Mapping of this unconformity surface shows major vertical relief. A thick sequence of conglomerate occurs on Ella Ø, which, close to its exposed basal unconformity, has three lacustrine beds intercalated within it. Detailed analysis of one lake unit shows it to have significant lateral variation. At proximal localities it largely comprises lacustrine turbidites, whereas more distal locations were within a stratified lake. The interpreted sequence of events on Ella Ø is an interval of fluvial sedimentation followed by rapid drowning of the topography with lacustrine sediments onlapping onto basement. After an interval of deeper lacustrine sedimentation including laminites, the lake shallowed, the lithology changed to limestone and the lake dried out. Conglomerate deposition then recommenced. Maximum lake water depth of 100 m is estimated following correction (7°) for post‐Devonian rotation, both determined using the distribution of lacustrine sediments on the unconformity surface. The preferred mechanism for flooding of the Ella Ø Basin is episodic flooding of the entire basin system. Lacustrine sediment preservation results purely from its deposition amongst topography at the edge of the active fluvial system. In such fluvial systems lacustrine sediments may be part of the normal sedimentary cycle but were almost always removed by reworking.
The geometry and petrogenesis of hydrothermal dolomites at Navan, Ireland
Authors: BRAITHWAITE C. J. R. , RIZZI GIANCARLO
Published: November 2003

The dolomites at Navan, Ireland, formed in Courceyan peritidal and shallow‐shelf limestones. The dolomite body has a plume‐like geometry, cross‐cutting both lithological boundaries and diagenetic barriers generated by sea‐floor cementation and emergence. The dolomitizing fluids rose parallel to major faults to diffuse laterally through the succession, controlled by variations in permeability that reflect both facies variation on deposition and pre‐dolomitization diagenesis.

Cathodoluminescent zones reveal three principal stages of dolomite emplacement, separated by dissolution surfaces, with each stage reflecting several changes in the character of dolomitizing solutions. The predominance of dull zones indicates burial rather than surface conditions. The dolomites formed some time after burial in response to an areally limited hydrothermally‐driven flow. Isotope values (σ18O of — 6σ6 to — 10.4%δ and σ13C of — 0σ2 to +2σ5%δ PDB), and fluid inclusion data, suggest that these fluids had compositions similar to those of Carboniferous seawater. However, these became hotter with time, with temperature increasing from 60 to 160δC.

The Navan dolomites are closely associated with Europe's largest zinc‐lead deposit. The distribution of the ores follows the same trend as that of the dolomites and paragenetic relationships indicate that dolomitization and mineralization were temporally and genetically related.
Late Miocene Halimeda alga‐microbial segment reefs in the marginal Mediterranean Sorbas Basin, Spain
Authors: MARTÍN JOSÉ M. , BRAGA JUAN C. , RIDING ROBERT
Published: November 2003

A ∼6 Ma Messinian (late Miocene) Bioherm Unit on the southern slope of the Sorbas Basin, SE Spain, contains numerous biotically diverse lensoid patch reefs that formed on a shelf to basin slope during a cycle of relative sea‐level change.

Halimeda reefs are the largest and most complex of the patch reefs and are divisible into core, cap, and flank facies. On the upper and midslope they are up to 40 m thick and 400 m long. They become smaller downslope. The core consists of jumbled Halimeda segments, released by spontaneous disaggregation of the alga. The segments were stabilized close to their sites of growth and rapidly lithified by micritic and peloidal microbial crusts. Residual cavities were further veneered by isopachous marine cements. Flank facies, consisting of bedded packstones to rudstones, form wedge‐shaped units lateral to the mounds. Cap facies consist of bioclastic calcarenites/calcirudites and microbial carbonates. Synsedimentary lithification assisted rapid accretion and inhibited off‐mound export of sediment. Allochthonous reef‐derived blocks on the mid‐slope reflect penecontemporaneous rigidity of the Halimeda bioherms.

Proximal Porites coral frame patch reefs associated with calcarenites were located near the shelf margin during the initial lowstand stage. Halimeda segment reefs associated with calcarenites and silty marls developed on the midslope and bivalve‐bryozoan‐serpulid reefs formed on the lower slope in silty marls with occasional turbidites. During the transgressive stage, coral patch reefs near the shelfbreak were overgrown by Halimeda. During highstand progradation, cap facies spread basinward as a sheet connecting many of the midslope patch reefs.

These ancient analogues differ from most modern Halimeda reefs in being discrete laterally restricted patch reefs, surrounded by marly sediment, and located on a slope. They are, however, broadly comparable in biota, thickness, and depositional depth. Intense early lithification by microbial crusts and marine cements is an important feature of these Messinian segment reefs. It has not been reported from modern examples.
Hurricane control on shelf‐edge‐reef architecture around Grand Cayman
Authors: BLANCHON PAUL , JONES BRIAN
Published: November 2003

Rimming the outer shelf of Grand Cayman is a submerged, 87 km long shelf‐edge reef that rises to within 12 m of mean sea level. It consists of an array of coral‐armoured buttresses aligned perpendicular to shore and separated by steep‐sided sediment‐floored canyons. Individual buttresses have a diverse coral‐dominated biota and consist of three architectural elements: a shield‐like front wall colonized by platy corals, a dome‐shaped crown colonized by head corals, and a shoreward‐projecting spur covered by varying amounts of branching coral. Buttresses are commonly fronted by coral pinnacles that, in some areas, have amalgamated with buttress walls to produce pinnacle‐and‐arch structures.

As margin orientation changes, shelf‐edge‐reef architecture shows systematic variations that are consistent with changes in fetch and height of hurricane waves. Along margins exposed to fully developed storm waves, shelf‐edge‐reef buttresses are deep, have large amplitudes, and are dominated by robust head corals. These characteristics are consistent with hurricane‐induced pruning of branching corals and the flushing of significant quantities of sand from buttress canyons by return flows. Along margins impacted by fetch‐limited storm waves, reef buttresses are shallower, have intermediate‐amplitudes, and have a significantly higher proportion of branching corals. These characteristics are consistent with less coral pruning and sand flushing by weaker hurricane waves. Along margins fully protected from storm waves, the buttresses‐canyon architecture of the shelf‐edge reef breaks down producing a series of shallow, undulating, branching‐coral‐dominated ridges that merge laterally into an unbroken belt of coral. These characteristics correspond with negligible amounts of pruning and flushing during hurricanes.

In addition to differences between margins, local intra‐marginal changes in shelf‐edge reef architecture are consistent with changes in the angle of hurricane‐wave approach. Open sections of the shelf‐edge reef, which face directly into storm waves, are pruned of branching corals and the fragments swept back onto the shelf producing extensive spurs. By contrast, on more sheltered, obliquely orientated sections, storm‐waves sweep debris along and off shelf producing little or no spur development. Instead, the debris shed seawards accumulates in front of the buttress walls and initiates the development of coral pinnacles.

Over time, repeated buttress pruning and canyon flushing during hurricanes not only controls reef architecture but may also influence accretion patterns. Vertical accretion is limited by the effective depth of storm‐wave fragmentation. Once this hurricane‐accretion threshold is reached the reef moves into a shedding phase and accretes laterally via pinnacle growth, amalgamation, and infilling. Consequently, the reef steps out over its own debris in a kind of balancing act between lateral growth and slope failure — a pattern widely recognized in ancient reefs.
Early diagenesis in Pleistocene coral reefs, southern Sinai, Egypt: response to tectonics, sea‐level and climate
Authors: STRASSER ANDRÉ , STROHMENGER CHRISTIAN
Published: November 2003

The uplifted Pleistocene terraces along the coast of southern Sinai exhibit a well developed reef system formed during isotope stage 9, and a younger one formed during isotope stage 5. An intermediate reef corresponding to isotope stage 7 occurs only as an erosional relic in the study area. The sediments comprise reefal framestones, peri‐reefal facies, coral rubble, and siliciclastic‐dominated beach and aeolian facies. The compositional and textural complexity of the sediments leads to a highly variable spatial distribution of diagenetic features. However, the geometric relationships and elemental analyses allow a reconstruction of the general diagenetic evolution: during the major eustatic sea‐level highstand of isotope stage 9, the Older Reef was constructed and cemented with aragonite and high‐Mg calcite. Climate was probably semiarid with some rainy periods which permitted the installation of ephemeral freshwater lenses, especially during the minor sea‐level lowstand within isotope stage 9. In these lenses, and during the subsequent major sea‐level lowstand, some freshwater dissolution occurred. The highstand during isotope stage 7 led to the construction of the Intermediate Reef. In the Older Reef, some high‐Mg calcite precipitated at that time. Dolomite cement formed either in marine interstitial waters modified by some freshwater input, or in a hypersaline context. Phreatic‐meteoric low‐Mg calcite cement covers, and partly replaces, previous marine cements and dolomite, but is still attributed to the major highstand of isotope stage 7 when freshwater lenses could develop during minor sea‐level lowstands. The subsequent major sea‐level lowstand was dominated by an arid climate, and only a little freshwater corrosion occurred. The Younger Reef formed during the major highstand of isotope stage 5. Aragonite and high‐Mg calcite cements, as well as some dolomite, are common within the reef, whereas freshwater cements are limited to beach and aeolian facies. Due to tectonic uplift, only the lower part of the Older Reef was reflooded during isotope stage 5, and only some aragonite crystals precipitated on top of dolomite or low‐Mg calcite. The interrelationships between tectonics, sea‐level variations of different orders, and climatic changes thus had a profound impact on the diagenetic history of these reef systems.
Facies distribution and retreat of Middle Triassic platform margin, Guizhou province, south China
Authors: ENOS PAUL , JIAYONG WEI , YANGJI YAN
Published: November 2003

The southern margin of the vast Yangtze platform in central Guizhou Province, China, retreated during the Anisian (early Middle Triassic) by shedding skeletal debris and boundstone blocks into the margin of the adjacent basin. Anisian platform deposits are shoaling‐up cycles that commonly terminated in subaerial exposure. Platform‐margin facies are obscured by massive dolomitization and mechanical erosion. Distal basin deposits are terrigenous mudstone and siltstone. At the basin margin a wedge of mixed carbonate and terrigenous rocks consists of (A) thin‐bedded dolostone and limestone, (B) lime breccia with thin‐bedded mudstone, and (C) lime mudstone with breccia. Blocks within the breccias indicate that the shelf margin contained extensive boundstone formed by ‘Tubiphytes,’encrusting organisms, and early marine cement. Interspersed thin beds of skeletal packstone represent unlithified skeletal debris at the platform margin.

The profile of the shelf margin from detailed mapping indicates 1.7–2.7 km of platform‐margin retreat during deposition of a basin‐margin wedge 250 m thick. Intertonguing of various basin‐margin facies reflects alternating minor episodes of advance and retreat of the margin. Near‐parallelism of the tongues suggests low relief at the platform margin. An upward stratigraphic progression to more distal, carbonate‐free, terrigenous basin facies indicates a cessation of carbonate production on the platform owing to emergence during early Anisian time. Retreat may have occurred entirely by collapse of blocks less than 100 m wide and 30 m thick, the largest observed dimensions. A re‐entrant in the margin 7 km wide and 10 km deep could also reflect collapse. Retreat occurred along 175 km of the platform margin. The lack of platform‐margin facies along this front suggests 3–7 km of retreat and a total area of 875 km2.

The Anisian platform‐margin retreat in Guizhou is one of relatively few examples of platform‐margin retreat in the geological record.
Deglaciation sequences in the Permo‐Carboniferous Karoo and Kalahari basins of southern Africa: a tool in the analysis of cyclic glaciomarine basin fills
Authors: VISSER JOHAN N. J.
Published: November 2003

The Late Westphalian to Artinskian glaciomarine deposits of the Karoo and Kalahari basins of southern Africa consist of massive and stratified diamictite, mudrock with ice‐rafted material, sandstone, silty rhythmite, shale and subordinate conglomerate forming a cyclic succession recognizable across both basins. A complete cycle comprises a resistant basal unit of apparently massive diamictite overlain by softer, bedded stratified diamictite, sandstone and mudrock with a total thickness of as much as 350 m. Four major cycles are observed each separated by bounding surfaces. Lateral facies changes are present in some cycles.

The massive diamictites formed as aprons and fans in front of the ice‐grounding line, whereas the stratified diamictites represent more distal debris‐flow fans. The sandstones originated in different environments as turbidite sands, small subaqueous outwash channel sands and delta front sands. The rhythmites and mudrock represent blanket deposits derived from turbid meltwater plumes.

Cycles represent deglaciation sequences which formed during ice retreat phases caused by eustatic changes in the Karoo and Kalahari basins. Evidence for shorter‐term fluctuation of the ice margin is present within the major advance‐retreat cycles. Hardly any sediment was deposited during lowstand ice sheet expansion, whereas a deglaciation sequence was laid down during a sea‐level rise and ice margin retreat with the volume of meltwater and sediment input depending on temporary stillstands of the ice margin during the retreat phase. The duration of the cycles is between 9 and 11 Ma suggesting major global tectono‐eustatic events. Smaller cycles probably linked to orbital forcing were superimposed on the longer‐term events. A sequence stratigraphic approach using the stacking of deglaciation sequences with the ice margin advance phases forming bounding surfaces, can be a tool in the framework analysis of ancient glaciomarine basin fills.
Possible seismic origin of molar tooth structures in Neoproterozoic carbonate ramp deposits, north China
Authors: FAIRCHILD IAN J. , EINSEL GERHARD , SONG TIANRUI
Published: November 2003

The Xinmincun Formation forms the uppermost unit of a thick Neoproterozoic section which accumulated near the east margin of the North China Block and is overlain by two thin nearshore to continental formations below fossiliferous Lower Cambrian sediments. Although tectonically deformed, sedimentary structures are preserved undeformed on cleavage‐parallel surfaces, and an 80 m section has been reconstructed by correlation across minor folds and faults in the Golden Stone beach area, 50 km NE of the city of Dalian, southern Liaoning province.

The measured section shows 65 m of storm‐dominated deposits, consisting of alternations of micrites and sharp‐based graded intraclastic grainstone beds (tempestites), some with rudaceous, commonly erosional or guttered, bases. The top 15 m of section shows three alternations of similar subtidal lithofacies with partly to completely dolomitized peritidal deposits (laminated, sometimes fenestral and desiccated, micrite beds, and intraclastic rudite and grainstone beds). Tempestite beds become thinner and less abundant upwards towards a muddy upper shoreface zone. This environment was characterized in part by the occurrence of micrite with thin or streaky lamination (probable storm‐resuspended sediment), interbedded tempestites, numerous erosion surfaces, and evidence of liquidization and sediment slumping into hollows.

Molar tooth structures are widespread in micrite beds of the Xinmincun Formation and are present in lesser abundance in tempestites and liquefied channel‐fills. Originally the structures were sub‐vertical cracks, 1–20 cm long, tapering upwards and downwards. Subsequently they became filled with microspar cement and buckled rigidly during compaction of surrounding sediment during burial. Evidence of repeated episodes of cracking, presence of brecciated cracks and localization of cracking within beds, together with variable degree of development and variation of preferential alignment in plan indicates a mechanical origin. Crack generation may have been by seismic surface waves generated by movement along faults defining either the basin's margins or its internal structure, or possibly by wave action during storms. A seismic origin for molar tooth structure is consistent with other Neoproterozoic occurrences. Their preferential occurrence in Precambrian deposits arises from the relative rigidity of micritic sediment at this time related to lack of bioturbation, incipient cementation and possibly microbial binding.
Early Cambrian braid‐delta deposits, MacKenzie Mountains, north‐western Canada
Authors: MACNAUGHTON ROBERT B. , DALRYMPLE ROBERT W. , NARBONNE GUY M.
Published: November 2003

Latest Neoproterozoic to earliest Cambrian strata in north‐western Canada provide an example of a pre‐vegetation braid‐delta depositional system. Depositional environments represented in the succession include braided fluvial and braid‐delta distributary channels, aeolian dune fields and interdistributary lagoons/bays, as well as mouth bar, beach to shoreface, and prodelta to distal shelf settings. Three formations have been investigated: the Ingta Formation formed in wave‐dominated nearshore to offshore shelf environments with little or no apparent deltaic influence, whereas the overlying Backbone Ranges and Vampire formations contain an extensive record of braid‐delta deposits ranging from braidplain to distal prodelta facies. On the braid‐plain, river channels reached widths of up to several kilometres. Such channels terminated seaward in braid deltas that showed some shoreline protuberance and were characterized by fluvial‐dominated mouth‐bar deposition with lesser wave influence; wave‐dominated deltaic successions are rare in the succession. Interdeltaic areas were characterized by wave‐dominated prograding shorelines. Interdistributary lagoons probably formed primarily in abandoned distributary channels. Delta‐front/prodelta deposits are silt‐rich and contain abundant soft‐sediment deformation, including slumps. The deposits in these formations illustrate the significantly different nature of sedimentation prior to the advent of land plants. This is illustrated in the dominance of braided fluvial deposition and of silt‐rich offshore facies that may have resulted from enhanced aeolian transport of loess. The non‐actualistic effects of limited bioturbation and extensive microbial binding apparently exerted relatively little control on the distribution of facies. However, the absence of extensive bioturbation is manifest in pristine preservation of primary sedimentary structures, while the hypothesized latest Proterozoic‐earliest Cambrian decline in microbial binding may be reflected in the upward increase in the abundance of sole marks in the succession.
Effect of bacteria on the elemental composition of early diagenetic siderite: implications for palaeoenvironmental interpretations
Authors: MORTIMER R. J. G. , COLEMAN M. L. , RAE J. E.
Published: November 2003

Many ancient early diagenetic siderite concretions are thought to have been precipitated by microbially‐mediated reactions, and their trace element chemistry has been used to infer palaeoenvironments of formation (e.g. marine versus freshwater). In this study, pure cultures of the microorganism Geobacter metallireducens were used to precipitate siderite in the laboratory at a range of temperatures (18–40°C). Magnesium and calcium, in three different ratios (3:1, as in seawater, 1:1, and 1:3 as in freshwater), or manganese were added to some cultures at each incubation temperature to study trace element incorporation. The siderite produced exhibited the rhombohedral crystal form typical of concretionary siderite. However, this microbial siderite did not simply retain the trace element chemistry of the water from which it precipitated, as is assumed in palaeoenvironmental interpretations. Instead, manganese and calcium incorporation were found to be inversely proportional to the rate of microbial activity, which itself is dependent upon the nutritional status of the microorganisms. Magnesium incorporation was found to be rate‐independent, and at high Mg/Ca ratios, it caused inhibition of calcium incorporation. In short, the influence of microorganisms on the trace element composition of early diagenetic siderite must be taken into account in order to produce valid palaeoenvironmental interpretations.
Spillover models for axial rivers in regions of continental extension: the Rio Mimbres and Rio Grande in the southern Rio Grande rift, USA
Authors: MACK GREG H. , LOVE DAVID W. , SEAGER WILLIAM R.
Published: November 2003

The Pliocene‐early Pleistocene history of the ancestral Rio Grande and Quaternary history of the Rio Mimbres in the southern Rio Grande rift, New Mexico, illustrate how axial rivers may alternately spill into and subsequently abandon extensional basins. Three types of spillover basins are recognized, based on the angle at which the axial river enters the basin and whether it descends the hanging wall dip slope or footwall scarp to reach the basin floor. In the Mimbres basin type, the axial river enters and flows through the spillover basin nearly parallel to the footwall scarp, resulting in a narrow belt of basin‐axis‐parallel channel sand bodies located near the footwall scarp. In contrast, an axial river may enter a spillover basin at a high angle to its axis, either descending the hanging wall dip slope (Columbus basin type) or footwall scarp (Tularosa basin type), and construct a fluvial fan, consisting of radiating distributary channels orientated nearly perpendicular to the basin axis. Faulting exerts significant control on river spillover by creating the topographic gaps through which the axial river moves and by terminating spillover by subsequently uplifting or tilting the gap. Spillover may also be autocyclic in origin as a result of aggradation to the level of a pre‐existing gap, headward erosion creating and/or intersecting a gap, or simple river avulsion upstream of a gap. Predicting facies architecture in the three types of spillover basins is critical to successful subsurface exploration for hydrocarbon reservoirs, groundwater aquifers or placer mineral deposits.
Contrasting flooding histories of Mississippian carbonate platforms revealed by marine alteration effects in palaeosols
Authors: WRIGHT V. PAUL , VANSTONE SIMON D. , MARSHALL JAMES D.
Published: November 2003

Palaeosols associated with exposure surfaces in Mississippian platform carbonate sequences in Britain invariably show evidence for later alteration by sea water. These alteration effects can be attributed to flooding of the emergent platforms during transgressions that terminated exposure surface development. A study of 230 palaeosol profiles representing 60 stratigraphic levels has revealed a two fold division of these marine hydromorphic effects. Palaeosols in ramp sequences (Chadian‐Arundian stages) are capped by ferroan dolomite horizons with carbonized rootlets, pyrite and thin coals. The ferroan dolomites exhibit δ13C and δ18O values indicative of formation in brackish waters. These are interpreted as coastal marshes that developed landward of a transgressive shoreline. Younger Asbian‐Brigantian palaeosols lack these dolomites but have been extensively pyritised. The pyrite also developed through marine hydromorphic alteration but flooding was relatively instantaneous over the flat topped platforms. These differences in flooding history reflect both different platform configuration and more rapid transgressions during the Asbian‐Brigantian, likely a result of glacio‐eustatic effects. Flooding characteristics of the Asbian‐Brigantian platforms differ from those associated with late Cainozoic examples, apparently because complete platform rims were not developed. Similar mineralogical alteration effects are likely to be common in other platform sequences in the geological record, but have not been documented.
Messinian stromatolite‐thrombolite associations, Santa Pola, SE Spain: an analogue for the Palaeozoic?
Authors: FELDMANN MARK , McKENZIE JUDITH A.
Published: November 2003

Stromatolite‐thrombolite associations are the dominant facies forming large portions of the Santa Pola carbonate platform (SE Spain) during deposition of the Terminal Carbonate Complex (TCC). The TCC, the last period of marine sedimentation in the Western Mediterranean associated with the Messinian Salinity Crisis, comprises a NE‐SW trending thrombolite reef with occasionally interlayered stromatolite horizons and a predominantply stromatolite and oolite facies in the back‐reef area. The stromatolites are mainly dome shaped, but fine‐columnar or wavy‐undulose forms can occur. The stromatolites form huge bioherms, extending tens to hundreds of metres. They are finely laminated with alternating layers of dolomicrite and dolomicrospar. The dolomicrite layers appear to be a primary dolomite precipitate, whereas the dolomite crystals in the dolomicrospar layers apparently formed around a meta‐stable nuclei which was subsequently dissolved or degraded. The low content of sand‐sized particles in the stromatolitic layers indicates formation under low‐energy conditions, possibly on a tidal flat. As reported from other areas in the Western Mediterranean, deposition of the TCC at Santa Pola was apparently cyclic, whereby stromatolites generally terminate each depositional cycle. Subtidal Conophyton stromatolites, possibly the only known occurrence younger than Palaeozoic, are, however, found on the reef slope at the base of the first TCC depositional cycle. The dolomitic nature of the unadulterated stromatolitic laminations and the association of stromatolites and thrombolites as platform builders were a common feature in the Early Palaeozoic but are unusual in post‐Ordovician carbonate facies. We propose that the conditions during TCC deposition were very restricted, possibly reflecting an environment similar to that of the Early Palaeozoic.
Intermittent saltation
Authors: STOUT J. E. , ZOBECK T. M.
Published: November 2003

During a typical wind erosion event, large variations in wind strength produce temporal variations in saltation activity. The focus of this paper is on a special type of unsteady behaviour ‐ intermittent saltation ‐ a process characterized by bursts of blowing soil interspersed with periods of inactivity. We report here measurements from a field study designed to measure intermittent saltation during three separate 1‐h periods. Our measurements show that natural wind erosion events consist of intermittent bursts of blowing soil often occupying a small fraction of the total time. We have managed to describe the level of intermittency by a simple and universal mathematical expression. We find that the level of intermittency is governed by whether typical wind fluctuations span the gap between the mean wind speed and threshold wind speed. We propose a nondimensional number which expresses the ratio of these velocity scales, called the relative wind strength, and find that the level of intermittency can be described by a simple distribution function of the relative wind strength.
High‐frequency cycles within Pleistocene forced‐regressive conglomerate wedges (Bradanic area, southern Italy) filling collapse scars
Authors: MASSARI FRANCESCO
Published: November 2003

Middle Pleistocene sedimentation in the Bradanic area (foreland basin of the southern Apennines) took place in a setting in which tectonic tilting created anomalous shelf gradients and where slope failures repeatedly occurred at the front of quickly prograding sedimentary wedges. A coarse‐grained wave‐dominated coastal/deltaic complex consisting of two downstepping, forced‐regressive wedges prograded during the falling stage of a middle Pleistocene glacio‐eustatic fluctuation (P. lacunosa Zone). Conglomerate clinoforms show a dramatic increase in relief and steepness as a result of the infill of collapse scars.

Compaction of prodelta mudstones, due to the load of the scar‐filling conglomerates, resulted in a slight deformation of the fills into broad synformal structures, with gentle anticlinal closures at their distal ends. Accommodation space was created, resulting in splitting of the prograding bodies into high‐frequency cycles which are thought to reflect the interaction of localized compactional subsidence with autocyclic shifting of river mouths and/or climatically induced variations in sediment supply. Available chronologic constraints suggest average durations of small‐scale cycles on time‐scales shorter than those of Milankovitch orbital frequencies.

This case history highlights the importance of some factors which are often neglected in sequence stratigraphic models, e.g. physiography of the sea floor and localized sediment loading. On one hand, the bulk of falling‐stage wedges can be preserved as they prograde into intraplatform depressions represented by collapse scars; on the other, significant accommodation space is created by compactional subsidence, leading to the splitting of a simple progradational unit into multiple, small‐scale cycles.
Cross‐stratified calcarenites from New Zealand: subaqueous dunes in a cool‐water, Oligo‐Miocene seaway
Authors: ANASTAS ALEXIS S. , DALRYMPLE ROBERT W. , JAMES NOEL P. , NELSON CAMPBELL S.
Published: November 2003

Cross‐bedded, cool‐water, bioclastic limestones of the Te Kuiti Group on the North Island of New Zealand are composed primarily of bryozoans, echinoderms, and benthic foraminifers. Their prominent, large‐scale, unidirectional cross‐stratification is interpreted as produced by migrating subaqueous dunes on the floor of a 50–100 km wide, north‐east‐trending seaway in water depths of 40–60 m. These dunes are thought to have developed in response to strong, seaway‐parallel, tidal currents combined with a north‐east‐directed, set‐up or oceanic current.

Cross‐stratification is organized into four hierarchical levels: (1) cross‐lamination; (2) first‐order sets; (3) second‐order sets; and (4) cross‐stratified successions. The levels are based on increasing degrees of internal complexity. Distinct attributes such as internal organization, cross‐set thickness, foreset shape, and lower bounding‐surface shape are used to describe and interpret the cross‐stratification. All these attributes are here integrated in a new and expanded classification of unidirectional cross‐stratification that emphasizes flow and bedform dynamics rather than overall set shape.

Individual cross‐stratified successions are interpreted to have formed by dunes with varying sinuosity, superposition, and flow history, under conditions of different current strength but constant sediment production. Horizontally bedded successions are the result of robust, active dune fields that grew during times of vigorous sediment transport. Formset successions were produced from large compound dunes and are the expression of languid and decaying dune fields that developed during times of decreasing sediment transport. These decaying dunes were gradually smothered by continuously and locally produced bioclastic sediment. Formset cross‐stratified successions are most likely to develop in carbonates, where the sediment is produced in place, than in terrigenous clastics where the sediment is imported.