bluuebunny/crossref_metadata_2025 huggingface dataset:
The Saharan debris flow: an insight into the mechanics of long runout submarine debris flows
Authors: GEE  , MASSON  , WATTS  , ALLEN
Published: March 2003

New 3·5 kHz profiles and a series of piston cores from the north‐west African margin provide evidence that the Saharan debris flow travelled for more than 400 km on a highly fluid, low‐friction layer of poorly sorted sediment. Data suggest that the Saharan debris flow is a two‐phase event, consisting of a basal, volcaniclastic debris flow phase overlain by a pelagic debris flow phase. Both phases were emplaced on the lower continental rise by a single large debris flow at around 60 ka. The volcaniclastic flow left a thin deposit less than 5 m thick. This contrasts with the much thicker (over 25 m) deposit left by the pelagic debris flow phase. We suggest that pelagic sediment, sourced and mobilized as debris flow from the African continental margin, loaded and destabilized volcaniclastic material in the vicinity of the western Canaries. When subjected to this loading, the volcaniclastic material appears to have formed a highly fluid sandy debris flow, capable of transporting with it the huge volumes of pelagic debris, and contributing to a runout distance extending over 400 km downslope of the Canary Islands on slopes that decrease to as little as 0·05°. It is likely that the pelagic debris formed a thick impermeable slab above the volcanic debris, thus maintaining high pore pressures generated by loading and giving rise to low apparent friction conditions. The distribution of the two debris phases indicates that the volcaniclastic debris flow stopped within a few tens of kilometres after escaping from beneath the pelagic debris flow, probably because of dissipation of excess pore pressure when the seal of pelagic material was removed.
Depositional architecture of a rimmed carbonate platform (Albian, Gorbea, western Pyrenees)
Authors: GÓMEZ‐PÉREZ IRENE , FERNÁNDEZ‐MENDIOLA PEDRO A. , GARCÍA‐MONDÉJAR JOAQUÍN
Published: March 2003

A Lower Cretaceous carbonate platform depositional system with a rimmed margin and an adjacent oversteepened slope was analysed in order to determine its depositional architecture and major depositional controls. The platform is made up of coral, rudist, orbitolinid and algal limestones and, in a 12‐km dip transect, there is a gradation from lagoon to platform margin, slope and basin environments, each characterized by distinctive sedimentological features and facies associations. The rimmed platform is an aggradational system developed during approximately 4·2 million years of fluctuating relative sea‐level rise, and it is bounded by unconformities at its base and top. Internal cyclicity in the construction of the system is evident, mainly in platform interior and slope settings. The seven recognized sequences average 0·6 million years in duration and are related to minor relative sea‐level changes. Carbonate deposition occurred in shallow‐ and deep‐water settings during periods of high relative sea level. Reduced rates of sea‐level rise led to the development of shallowing upward sequences and, eventually, to the exposure of the shallowest parts of the platform during relative sea‐level falls. During low relative sea level, erosion surfaces developed on the slope, and gravitational resedimentation occurred at the toe of slope. Basinwards, resedimented units pinch out over distances of a few hundred metres. Active faults controlled sedimentation at the platform margin, promoting the development of steep slopes (up to 35°) and preventing progradation of the shallow‐water platform, despite high sediment production rates. The development of sequences is interpreted to be related to tectonic activity.
Aeolian system sediment state: theory and Mojave Desert Kelso dune field example
Authors: Kocurek Gary , Lancaster Nicholas
Published: March 2003

The sediment state of aeolian dune fields and sand seas at a basinal scale is defined by the separate components of sediment supply, sediment availability and the transport capacity of the wind. The sediment supply for aeolian systems is the sediment that contemporaneously or at some later point serves as the source material for the aeolian system. Numerous factors impact the susceptibility of grains on a surface to transport, but these are cumulatively manifested by the actual transport rate, which serves as a proxy for sediment availability. Transport capacity is the potential sediment transport rate of the wind. Because the three aspects of sediment state can be given as a volumetric rate, they are directly comparable. Plotted simultaneously against time, the generated curves define nine possible classes of sediment state. Sediment supply that is stored occurs because it is transport or availability limited, or generated at a rate greater than the potential or actual transport rates respectively. Contemporaneous or lagged influx to an aeolian system may be limited by sediment availability, but cannot exceed the transport capacity of the wind. For the Kelso dune field in the Mojave Desert of California, a variety of stratigraphic and geomorphic evidence is used to approximate the sediment state of the system. The sediment supply was generated during the latest Pleistocene and earliest Holocene during humid periods of enhanced discharge by the Mojave River to form the Lake Mojave fan delta or terminal fan, and has been calculated over time from the sedimentation rate and the frequency of floods. Estimation of transport capacity over time was based upon modern wind data, with an allowance for greater winds during the Pleistocene based upon climatic models. Sediment availability was approximated by calculation of a modern dune mobility index, with variation over time based upon climatic inferences. While quantifying the Kelso or any natural system is subject to numerous uncertainties, the sediment state approach reflects the temporal and spatial disjointed nature of accumulations at Kelso, as well as illuminating questions for future research.
Soft‐sediment deformation during thawing of ice‐rich frozen soils: results of scaled centrifuge modelling experiments
Authors: Harris Charles , Murton Julian , Davies Michael C. R.
Published: March 2003

The origin of periglacial involutions remains uncertain, largely because of the difficulties of field monitoring in modern permafrost regions. This paper describes an alternative approach, in which process studies are based on scaled centrifuge modelling of thawing ice‐rich soils. Centrifuge scaling laws allow similitude in self‐weight stresses between the model scale and the prototype (field) scale to be achieved. In these experiments, 120‐ to 130‐mm‐thick frozen models comprising a sand unit overlying ice‐rich kaolinite clay (three models) or ice‐rich silt (one model) were thawed under an acceleration of 20 gravities. The models were therefore equivalent to 2·4–2·6 m of frozen sediments (permafrost) at the prototype scale. Temperature profiles and porewater pressures during the thawing of each model are described. Porewater pressures significantly in excess of hydrostatic were not observed in the sand/silt model. In the sand/clay models, however, excess pressures developed rapidly after thawing, and observed fluctuations in pressure were interpreted as water‐escape events. After thawing, careful sectioning of the models revealed small‐scale deformation structures at the clay–sand interface, resulting from loading of the upper sand layer into very soft fluid‐like clay and injection of clay upwards into the base of the sand. It is concluded that these experiments provide analogues for some Pleistocene involutions. Such involutions therefore mark phases of permafrost degradation when high porewater pressures caused loading and injection along sedimentary boundaries.
Catastrophic flooding of an aeolian dune field: Jurassic Entrada and Todilto Formations, Ghost Ranch, New Mexico, USA
Authors: Ahmed Benan Cheikh A. , Kocurek Gary
Published: March 2003

Surveyed outcrops of the Middle Jurassic Entrada Sandstone at Ghost Ranch, New Mexico, show the unusual occurrence of preserved aeolian dune palaeotopography buried beneath subaqueous strata. The preserved dune remnants have relief up to 35 m, trend NNW, and show internal scalloped cross‐strata dipping to the WSW, with small sets occurring as both topsets and bottomsets. Outcrop data are best satisfied in computer models by 50 m high, sinuous bedforms that migrated to the WSW, while the sinuosity migrated alongcrest to the NNW. Superimposed small dunes occurred upon the stoss slope, and at the basal lee of the main bedform where they migrated alongslope to the NNW. Remnant dune palaeotopography is buried by onlapping, subaqueous, largely structureless sandstones believed to be derived by mass wasting of the upper portions of the dunes and deposited as sediment‐gravity flows that infilled between the dunes. Preservation of dune palaeotopography beneath mass‐flow deposits, with no evidence for gradually rising water, argues that flooding of the Entrada dune field was geologically instantaneous. The thickness and lithology of the overlying Todilto Formation conform to slight remnant palaeotopography on the Entrada surface. The Todilto is a laminated limestone and thinnest over remnant dune crestal areas, but thickens and increases in gypsum content downslope until it abruptly yields to a gypsum mound positioned over a remnant interdune hollow. The Todilto laminations are interpreted as seasonal varves deposited below wave base in a density‐stratified water body. The flooding event that gave rise to the controversial Todilto water body occurred during Entrada time, with Todilto deposition occurring within an already substantial water body.
Sedimentology and sequence stratigraphy of a transfer zone coarse‐grained delta, Miocene Suez Rift, Egypt
Authors: Young Mike J. , Gawthorpe Rob L. , Sharp Ian R.
Published: March 2003

This study focuses on Miocene sedimentation and stratigraphic evolution in a major transfer zone at the northern tip of the Thal Fault segment, Gulf of Suez. The succession generally shoals upwards from offshore mudstone containing pro‐delta turbidites, into conglomeratic delta foresets and topsets, with sandstone‐dominated shoreface facies coexisting laterally. Despite this upward shoaling, key stratal surfaces marking abrupt changes in relative sea‐level allow the succession to be divided into four stratal units. The stacking pattern of the stratal units suggests an initial relative sea‐level rise that generated a major marine flooding surface. A relative sea‐level fall followed, resulting in widespread exposure and incision. During the ensuing relative sea‐level rise a lowstand coarse‐grained delta and coeval shoreface succession prograded several kilometres basinward. The stratigraphic development of the transfer zone delta is in marked contrast to that of aggradationally stacked deltas that occur near the centre of the Baba‐Sidri fault segment, further south. At the transfer zone, low rates of subsidence and accommodation development coupled with a high sediment supply derived from a large fault tip drainage catchment have produced a strongly progradational delta subject to marked changes in relative sea‐level. In the fault centre location, however, higher rates of accommodation development coupled with lower rates of sediment supply from footwall catchments have produced aggradationally stacked deltas. The results from this study have implications for sequence stratigraphic models and hydrocarbon exploration within extensional basins.
Flow behaviour of the submarine glacigenic debris flows on the Bear Island Trough Mouth Fan, western Barents Sea
Authors: Laberg Jan Sverre , Vorren Tore O.
Published: March 2003

Using 3·5 kHz high‐resolution seismic data, gravity cores and side‐scan sonar imagery, the flow behaviour of submarine, glacigenic debris flows on the Bear Island Trough Mouth Fan, western Barents Sea was studied. During their downslope movement, the sediments within the uppermost part of the debris flows (<3 m) are inferred to have been deformed as a result of the shear stress at the debris–water interface. Thus, the uppermost part of the flow did not move downslope as a rigid plug. If present, a rigid part of the flow was located at least some metres below the surface. At c. 1000 to at least 1600 m water depth, the debris flows eroded and probably incorporated substrate debris. Further downslope, the debris flows moved passively over substrate sediments. The hypothesis of hydroplaning of the debris flow front may explain why the debris flows moved across the lower fan without affecting the underlying sediments. Detailed morphological information from the surface of one of the debris flow deposits reveals arcuate ridges. These features were probably formed by flow surge. Hydroplaning of the debris flow front may also explain the formation of flow surge. The long runout distance of some of the large debris flows could be due to accretion of material to the base of the debris flow, thereby increasing in volume during flow, and/or to hydroplaning suppressing deceleration of the flow.
Pleistocene decalcification of Late Pliocene Red Crag shelly sands from Walton‐on‐the‐Naze, England
Authors: Kendall Alan C. , Clegg Nigel M.
Published: March 2003

Uppermost sands of the Red Crag at Walton‐on‐the‐Naze (Essex) and elsewhere in East Anglia have been decalcified to iron‐stained quartz sands. In contrast, lower sands are only minimally altered and contain aragonitic and calcitic shells. Aragonitic shells are slightly dissolved (chalkified), but calcitic shells are unaffected. Cementation is limited to an addition of iron oxides, now mainly haematite, which also coat carbonate grains. Abundant iron‐oxide fines in the upper decalcified sands were liberated from the coatings of shells; shells that have since dissolved. The diagenetic nature of the contact between decalcified upper and unaffected lower sands is evident where it transects cross‐bedding. The contact is knife‐sharp, even smoothly truncating large shells, and is usually planar and subhorizontal. Shelly sands immediately beneath the boundary contain similar amounts of aragonitic material, as do sands further below. Locally the decalcification boundary has been contorted by cryoturbation, implying that carbonate dissolution was a Pleistocene event. Decalcification probably occurred when the area was affected by permafrost. Lower sands were cemented by ice and protected from dissolution. Upper sands were above the ice table and subject to chemically aggressive waters during summer thaws. Decalcification is believed to have taken place during an episode of climate amelioration when downward retreat of the ice table accompanied replacement of tundra by boreal forest. Highly acidic and podzolic soils developed, beneath which shell‐carbonate dissolved. Sharp based decalcified zones in Lowestoft Till and Devensian deposits in other parts of England can also be attributed to dissolution associated with permafrost.
Sharp‐based, tide‐dominated deltas of the Sego Sandstone, Book Cliffs, Utah, USA
Authors: Willis Brian J. , Gabel Sharon
Published: March 2003

The lower part of the Cretaceous Sego Sandstone Member of the Mancos Shale in east‐central Utah contains three 10‐ to 20‐m thick layers of tide‐deposited sandstone arranged in a forward‐ and then backward‐stepping stacking pattern. Each layer of tidal sandstone formed during an episode of shoreline regression and transgression, and offshore wave‐influenced marine deposits separating these layers formed after subsequent shoreline transgression and marine ravinement. Detailed facies architecture studies of these deposits suggest sandstone layers formed on broad tide‐influenced river deltas during a time of fluctuating relative sea‐level. Shale‐dominated offshore marine deposits gradually shoal and become more sandstone‐rich upward to the base of a tidal sandstone layer. The tidal sandstones have sharp erosional bases that formed as falling relative sea‐level allowed tides to scour offshore marine deposits. The tidal sandstones were deposited as ebb migrating tidal bars aggraded on delta fronts. Most delta top deposits were stripped during transgression. Where the distal edge of a deltaic sandstone is exposed, a sharp‐based stack of tidal bar deposits successively fines upward recording a landward shift in deposition after maximum lowstand. Where more proximal parts of a deltaic‐sandstone are exposed, a sharp‐based upward‐coarsening succession of late highstand tidal bar deposits is locally cut by fluvial valleys, or tide‐eroded estuaries, formed during relative sea‐level lowstand or early stages of a subsequent transgression. Estuary fills are highly variable, reflecting local depositional processes and variable rates of sediment supply along the coastline. Lateral juxtaposition of regressive deltaic deposits and incised transgressive estuarine fills produced marked facies changes in sandstone layers along strike. Estuarine fills cut into the forward‐stepped deltaic sandstone tend to be more deeply incised and richer in sandstone than those cut into the backward‐stepped deltaic sandstone. Tidal currents strongly influenced deposition during both forced regression and subsequent transgression of shorelines. This contrasts with sandstones in similar basinal settings elsewhere, which have been interpreted as tidally influenced only in transgressive parts of depositional successions.
Palaeogeographical influence on Late Eocene biosiliceous sponge‐rich sedimentation, southern Western Australia
Authors: Gammon P. R. , James N. P.
Published: March 2003

Late Eocene nearshore shallow‐marine environments within the Bremer and western Eucla Basins of southern Western Australia were characterized by the thick deposition of spongolite and spiculite deposits. Epibenthic sponge communities dominated estuaries and topographically complex basin margin embayments‐archipelagos, while cool‐water carbonates with up to 10% sponges accumulated in open‐shelf environments. The transition from a biosiliceous to calcareous epibenthos was related to the degree of palaeogeographical ‘protection’. Within basement‐protected embayments there was an offshore gradation from shoreface spongolite and pure spiculite to a muddy spiculite facies towards central embayment areas. Calcareous fossils are rare throughout embayment facies, but rapidly increase in more open outer archipelago areas. This depositional relationship occurred along 2000 km of the Late Eocene southern Australian coastline. Palaeogeographical protection from strong currents acted in concert with: (1) a planar, low‐gradient inland topography with sluggish run‐off, supplying fine‐grained sediment, nutrients, and abundant dissolved silica; and (2) a microtidal setting, weak to moderate swells and opposing wind and Coriolis surface current forcing, which inhibited water exchange between embayments‐estuaries and the open shelf. This situation led to an embayment water chemistry that encouraged prolific sponge growth. Calcareous spiculites record the mixing front between these embayment waters and normal open‐shelf waters supporting cool‐water carbonates.
Internal structure of a trough blowout, determined from migrated ground‐penetrating radar profiles
Authors: Neal Adrian , Roberts Clive L.
Published: March 2003

Ground‐penetrating radar (GPR) was used to investigate the relationship between the geomorphological development of a large aeolian trough blowout and the stratigraphy and internal sedimentary structure of its associated deposits. Although analogous, many of the data‐processing techniques routinely applied in seismic reflection are very rarely applied in GPR studies. In this study, a simple migration program was used that significantly enhanced the quality of GPR images from a large trough blowout at Raven Meols on the Sefton coast, northwest England. These improvements aided subsequent data interpretation, which was achieved through application of the principles of radar stratigraphy. GPR shows the pre‐blowout dunes to have a complex internal structure that suggests they were formed in the presence of at least a partial vegetation cover. Subsequent to stabilization of these dunes a thin soil developed. This dune soil forms an important radar sequence boundary and delineates a complex topography beneath the depositional lobe of the blowout. The internal structure of the depositional lobe of the blowout does not conform to a model of simple radial foreset deposition, as derived from contemporary process studies reported in the literature. Instead, the pattern of deposition has been extensively modified by the antecedent dune topography and by varying spatial and temporal exposure to important sand‐transporting winds that is partly controlled by interactions between the regional wind pattern and local dune morphology. Trough blowout deposits in coastal aeolian sedimentary sequences are likely to be recognized by the presence of laterally continuous packets of relatively high‐angle cross‐strata, which often display a spatially‐variable radial dip pattern that is only very poorly or partially developed. In addition, a soil, or other surface representing a significant hiatus in dune deposition, is likely to underlie the blowout deposits, the topography of which will show a clear relationship to the dip and orientation of the overlying cross‐strata.
Triassic carbonate buildup flanks in the Dolomites, northern Italy: breccias, boulder fabric and the importance of early diagenesis
Authors: Blendinger Wolfgang
Published: March 2003

The flanks of Middle Triassic carbonate buildups in the Dolomites show well‐developed clinostratification, with typical angles of 30–40°. This paper focuses on the metre‐scale fabric of these clinoforms and sets these within the context of their large‐scale and microscopic features. Clinoform stratification is caused by fibrous cement crusts, by stylolites paralleling a vague stratification and by pelagic limestone interbedded with the lower portions of the clinoforms. These parts of the clinoforms locally exhibit a boulder fabric. A fracture system, subvertical to clinostratification, is filled by fibrous cements and marine internal sediment. The analysis of clinoform fabric indicates that parts consist ofin situautomicrite. Other parts of the clinoforms are made up of breccia. The breccias occur as isolated pockets and lenses with random orientation and dimensions ranging from a few square decimetres to hundreds of square metres. The breccias have gradual contacts with the unbrecciated host rock. Breccia components are mostly angular, show a microfacies similar to that of the unbrecciated host rock and are composed of millimetre‐ to decimetre‐sized particles that generally float in radiaxial‐fibrous cement. Matching grain boundaries are common. Calcite cement typically makes up 20 and 40 vol.% of the brecciated areas. Clinostratification, the fracture system, brecciation and boulder fabric point to gravity induced deformation ofin situflank deposits rather than gravity induced depositional processes. Brecciation appears to result from translational sliding (millimetres to metres) on the steep buildup flanks, which caused fracturing of the vaguely stratified automicrite, followed by displacive growth of fibrous cement. Cementation occurred in a (shallow) burial, marine phreatic environment, because cement clasts are virtually absent from the flank‐derived gravity flows in the adjoining basinal sediments. Displacive cement growth indicates a volume increase of the clinoforms during diagenesis of up to 20–40 vol.% and can account for the local drag of buildup interior limestones. Similarly, the boulder fabric appears to be a diagenetic feature, which resulted from differential settling of incompletely lithified boundstone and grainstone, and the interbedded pelagic limestone.
Stable isotopic signature of a palaeoaquifer, Mississippian Alamogordo Member limestones, Sacramento Mountains, New Mexico, USA
Authors: Wu Yongqiang , Chafetz Henry S.
Published: March 2003

Nine stratigraphic sections, each ≈5 m thick, were sampled from the Alamogordo Member limestones of the Lake Valley Formation, Sacramento Mountains, New Mexico, USA. Four stratigraphic sections consist entirely of lime mudstone and wackestone, whereas the other five sections have a prominent layer of crinoidal packstone about 1 m thick at their base. Stable isotopic analyses reveal that the lime muds in the sections with basal packstone layers show a downward decrease in δ18O and constant δ13C values, whereas those in the sections solely composed of lime mudstone and wackestone have, in general, relatively uniform δ18O and δ13C values. The diagenesis of the Alamogordo Member limestones was previously believed to have been governed by the downward percolation of meteoric water from a regional pre‐Pennsylvanian exposure surface ≈100 m above this unit. However, the uniform δ13C and downward decrease in δ18O values in the lime muds in the sections with basal packstones indicate that the meteoric water ascended within the Alamogordo Member, rather than descended from the overlying exposure surface. This indicates that the basal packstones were probably a conduit for meteoric water. This is further supported indirectly by the relatively uniform δ18O and δ13C values of the lime mud in the sections without basal packstones. The implications are that the oxygen isotopic gradients may be used to identify palaeoaquifers, flow directions within these aquifers and that meteoric diagenesis below an exposure surface could be governed by flow through a palaeoaquifer.
Palaeoenvironmental significance of rounded pyrite in siliciclastic sequences of the Late Archaean Witwatersrand Basin: oxygen‐deficient atmosphere or hydrothermal alteration?
Authors: England Gavin L. , Rasmussen Birger , Krapez Bryan , Groves David I.
Published: March 2003

Petrographic and sulphur isotope studies support the long‐held contention that rounded grains of pyrite in siliciclastic sequences of the Late Archaean Witwatersrand Supergroup originated as placer grains. The grains are concentrated at sites where detrital heavy minerals are abundant within quartz‐pebble conglomerates and quartzose sandstones. Depositional sites with abundant pyrite are: (1) within the matrix of bar‐type, clast‐supported conglomerates; (2) on scoured or winnowed surfaces; and (3) on stratification planes. The grains are internally compact or porous, with truncation of internal structure at outer margins indicating fragmentation and rounding of pyritic source‐rocks during erosion and sediment transport. A large range in textures reflects source‐rock lithologies, with known varieties linked to sedimentary‐hosted diagenetic pyrite, volcanic‐hosted massive sulphide deposits and hydrothermal pyrite. Laser ablation sulphur isotope analysis of pyrite reveals a broader range in δ34S values (− 5·3 to + 6·7‰) than that of previously reported conventional bulk‐grain analyses (− 1 to + 4‰). Rounded pyrite from the Steyn Reef has significant variation in δ34S values (− 4·7 to + 6·7‰) that establishes heterogeneous sulphur compositions, with even adjacent grains having diverse isotopic signatures. The heterogeneity supports a placer origin for rounded pyrite. Euhedral pyrite and pyrite overgrowths which are undoubtedly authigenic have restricted δ34S values (− 0·5 to + 2·5‰), are chemically distinct from rounded pyrite and are probably the products of metamorphism or hydrothermal alteration. The placer origin of rounded pyrite indicates that pyrite was a stable heavy mineral during erosion and transport in the early atmosphere. Its distribution in three sequences (Witwatersrand Supergroup, Ventersdorp Contact Reef and Black Reef), and in other sequences not linked to Witwatersrand‐type Au‐U ore deposits, implies deposition of redox‐sensitive detrital heavy minerals during the Late Archaean. Consequently, rounded grains of detrital pyrite are strong indicators of an oxygen‐poor atmosphere. While not confirming a placer origin for gold in Witwatersrand Au‐U ore deposits, the palaeoenvironmental significance of rounded pyrite negates its link to hydrothermal mineralization.
Factors governing subaqueous siliceous sinter precipitation in hot springs: examples from Yellowstone National Park, USA
Authors: Guidry Sean A. , Chafetz Henry S.
Published: March 2003

Abstract Siliceous sinter precipitation within hot spring systems has been attributed to a variety of mechanisms: evaporative concentration, cooling, changes in pH and cation effects. Repetitive in situ (T, pH, alkalinity, etc.) and laboratory (major, minor and trace elemental, stable isotopic) analyses of the waters plus observations of silica precipitation on natural (e.g. twigs, pine cones) as well as artificial substrates (glass slides and copper plates) in the waters substantiate that subaqueous precipitation is occurring throughout the vent to distal end of flow in both Cistern Spring (Norris Geyser Basin) and Deerbone Spring (Lower Geyser Basin), Yellowstone National Park, Wyoming, USA. Quartz and sodium–potassium geothermometers indicate that Cistern Spring is fed by a subsurface reservoir that is between 232 and 272 °C. Calculated reservoir temperatures are significantly lower at Deerbone Spring (182–197 °C). Based on a suite of measured and theoretical saturation indices, downflow changes in the system resulting from evaporative concentration (e.g. Cl increases 10%), changes in pH (e.g. 5·6–7·1) and cation effects (Al and Fe) are of negligible importance in the subaqueous precipitation of hot spring opal‐A. Similarly, at the macroenvironmental scale, potential biotic effects on opal‐A precipitation appear to be minimal. Modelling of the two active siliceous sinter precipitating systems indicates that cooling (e.g. 80–17 °C) is the predominant process governing subaqueous mineral precipitation.
Wave‐influenced deltas: geomorphological implications for facies reconstruction
Authors: Bhattacharya Janok P. , Giosan Liviu
Published: March 2003

ABSTRACT A process‐based facies model for asymmetric wave‐influenced deltas predicts significant river‐borne muds with potentially lower quality reservoir facies in prodelta and downdrift areas, and better quality sand in updrift areas. Many ancient barrier‐lagoon systems and ‘offshore bars’ may be better reinterpreted as components of large‐scale asymmetric wave‐influenced deltaic systems. The proposed model is based on a re‐evaluation of several modern examples. An asymmetry index A is defined as the ratio between the net longshore transport rate at the mouth (in m3 year−1) and river discharge (in 106 m3 month−1). Symmetry is favoured in deltas with an index below ≈ 200 (e.g. Tiber, lobes of the Godavari delta, Rosetta lobe of the Nile, Ebro), whereas deltas with a higher index are asymmetric (e.g. Danube – Sf. Gheorghe lobe, Brazos, Damietta lobe of the Nile). Periodic deflection of the river mouth for significant distances in the downdrift direction occurs in extreme cases of littoral drift dominance (e.g. Mahanadi), resulting in a series of randomly distributed, quasi‐parallel series of sand spits and channel fills. Asymmetric deltas show variable proportions of river‐, wave‐ and tide‐dominated facies both among and within their lobes. Bayhead deltas, lagoons and barrier islands form naturally in prograding asymmetric deltas and are not necessarily associated with transgressive systems. This complexity underlines the necessity of interpreting ancient depositional systems in a larger palaeogeographic context.
Phylogeny and classification of the Elachistidaes.s. (Lepidoptera: Gelechioidea)
Authors: Kaila LaurI.
Published: March 2003

Abstract.The classification of the gelechioid family Elachistidae (Lepidoptera) is revised on the basis of a phylogenetic analysis. Pee‐Wee analysis of 131 characters of adult and pupal morphology and larval mode of life, coded for seventy elachistid species, results in a classification with three recognized genera:Perittia,StephensiaandElachista.Elachistais further divided into four subgenera. The phylogenetic relationships of the genera and subgenera are (Perittia(Stephensia((E. sg.Dibrachia(E. sg.Hemiprosopa,E. sg.Aphelosetia)) (E. sg.Elachista)))). Twenty‐four new generic synonyms and thirty‐eight new generic combinations of species are proposed. A checklist is given for the species of Elachistidae in their revised generic combinations, including nine new synonymies. Due to secondary homonymy,Elachista dasycaranom. n. is proposed as a new name forEurynome albellaChambers.
World revision of Ortheziola Šulc (Homoptera: Coccoidea: Ortheziidae) with descriptions of eleven new species
Authors: Kozár Ferenc , Miller Douglass R.
Published: March 2003

Soil samples and museum collections were analysed from all zoogeographic regions of the world. From this material eleven new species of OrtheziolaŠulc are described from Africa and the U.K., five additional species are redescribed and a key is presented for the identification of all sixteen species. A slightly modified concept of the genus is presented and several new characters are given as diagnostic of the genus. A phylogenetic hypothesis is given based on analysis of the morphological features of adult females. Results show that Ortheziola is most diverse in eastern Africa where relatively more advanced species occur. Basal species occur primarily in western Africa and Asia.
Phylogenetic relationships among genera of Aphidiinae (Hymenoptera: Braconidae) based on DNA sequence of the mitochondrial 16S rRNA gene
Authors: Kambhampati Srinivas , Völkl Wolfgang , Mackauer Manfred
Published: March 2003

Phylogenetic relationships among forty‐nine taxa representing twenty‐four genera of Aphidiinae (Hymenoptera: Braconidae) were investigated using DNA sequence of a portion of the mitochondrial 16S rRNA gene and parsimony analysis. Seven species in six other subfamilies of Braconidae were used as outgroup. The results suggested that members of Aphidiinae are monophyletic. The basal lineage of Aphidiinae was Aclitus in weighted and unweighted parsimony analyses and Praini was basal relative to Ephedrini. With the exception of Pauesia and Aphidius, all genera were monophyletic. The results support generic status for Euaphidius, but not for Lysaphidus. Diaeretus leucopterus was internal to a clade composed of three Pauesia species, suggesting that the latter genus may be paraphyletic. A combined analysis that included DNA sequence of 16S rRNA, NADH1 dehydrogenase and 28S rRNA resulted in more robust cladograms with topologies similar to those inferred from the 16S rRNA gene sequence alone. The results are compared to previously proposed phylogenies of Aphidiinae based on morphological and molecular characters.
A 3.1 billion year old marble and the87Sr/86Sr of late‐Archean seawater
Authors: Zachariah
Published: March 2003

A marble band in the ≈ 2.75 Gyr old Ramagiri schist belt in the Dharwar craton of south India gave a Pb–Pb age of 3.075 ± 0.095 Gyr. The geochemical data, including high Sr and low Ba and Mn indicate seawater origin for the parent rock, and that there was insignificant geochemical exchange between the marble and the surrounding rocks. The calculated initial Nd isotopic composition and μ1indicate an older continental crustal source for the Nd and Pb. The initial87Sr/86Sr of the marble is 0.70128, which is higher than the calculated mantle value at ≈ 3 Ga. Although pre‐3 Gyr old marine carbonate rocks are thought to be buffered by mantle Sr, the Ramagiri marble contains evolved, crustal Sr. Despite this, the marble has the lowest measured87Sr/86Sr among carbonates and represents one of the least radiogenic periods in seawater Sr isotope composition.
Early Oligocene stratigraphic turnover on the west Africa continental margin: a signature of the Tertiary greenhouse‐to‐icehouse transition?
Authors: Séranne
Published: March 2003

The post‐rift stratigraphy on the west African margin is characterized by aggradation of a carbonate ramp during Late Cretaceous to Eocene epochs and progradation of a terrigenous wedge from Oligocene to the Present. Such first‐order structure has been attributed in the past to geodynamic forcing. However, comparison of the stratigraphic record of the margin with eustasy, δ18O and 87Sr/86Sr curves, shows a close temporal relationship with the Tertiary climate cooling, an increase of continental weathering, and a long‐term lowering of sea level. We suggest that the transition from low‐amplitude, high‐frequency sea‐level changes during the greenhouse period to high‐amplitude, high‐frequency sea‐level changes during the icehouse period may account for: (i) the switch from an aggrading carbonate ramp to a prograding clastic wedge, and (ii) the enhanced continental weathering and increased terrigenous influx to the margin.
Ductile rebound of an orogenic root: case study and numerical model of gravity tectonics in the Western Gneiss Complex, Caledonides, southern Norway
Authors: Milnes  , Koyi
Published: March 2003

The main Caledonian deformation in the Western Gneiss Complex (southern Norway) has been interpreted as resulting from the gravity‐driven ductile rebound of the orogenic root. Detailed field data from the Sognefjord profile, a 60‐km long, continuously exposed shore section through the complex, are compared with the results of a numerical scale model of root rebound dimensioned according to reconstructions of the overall tectonic evolution. The strain pattern in the model can be closely tied to the previously recognized structural regimes, from east to west: R1 – little deformed Precambrian basement (stiff orogenic lid, basal part of upper crust); R2 – heterogeneously sheared basement (‘simple shear’ domain at upper crust/lower crust boundary); R3 – completely overprinted basement with eclogitic remnants (‘pure shear’ regime in the lower crust of the rebounded root). Subsequent to root rebound the profile remained practically unaffected by later tectonic events, except for tilting/uplift/erosion, providing a unique example of deep‐seated gravity tectonics.
Marine biotic signals across a late Eocene impact layer at Massignano, Italy: evidence for long‐term environmental perturbations?
Authors: Coccioni R. , Basso D. , Brinkhuis H. , Galeotti S. , Gardin S. , Monechi S. , Spezzaferri S.
Published: March 2003

The Eocene–Oligocene transition marks the passage from ‘greenhouse’ conditions to an ‘icehouse’ state, with progressive global cooling starting in the early middle Eocene. The late Eocene presents substantial evidence for extraterrestrial impacts whose effects on living organisms and climatic changes are still not completely clear. A high‐resolution, microfloral and faunal investigation has been carried out in a 4‐m‐thick segment of the Massignano Global Stratotype Section and Point for the Eocene–Oligocene boundary. The studied interval includes a late Eocene (35.7 ± 0.4 Myr old) impactoclastic layer containing several cosmic signatures. The impact event recorded at Massignano had no abrupt, dramatic effects on marine biota in terms of extinction. However, significant quantitative changes in the calcareous plankton and dinoflagellate cyst assemblages occurred 60 kyr after the impact event. The observed pattern is intepreted as reflecting a long‐term re‐organization of water structure.
Dating cleavage formation in slates and phyllites with the 40Ar/39Ar laser microprobe: an example from the western New England Appalachians, USA
Authors: Chan Yu‐Chang , Crespi Jean M. , Hodges Kip V.
Published: March 2003

Determinations of the absolute age of cleavage formation can provide fundamental information about the evolution of orogenic belts. However, when applied to cleavages in slates and phyllites, conventional dating methods are complicated by problems related to mineral separation and the presence of multiple cleavage generations. In situ high‐spatial‐resolution 40Ar/39Ar laser microprobe geochronology and microstructural observations indicate that the age of cleavage formation in slates and phyllites can be constrained by analysing zones of tightly packed cleavage domains. Three regionally developed cleavages (S2, S3, and S4) are present in the northern Taconic Allochthon of Vermont and New York. Representative samples were studied from a variety of localities where these cleavages, which are defined by white micas, are well developed. In the suite of samples, only S3 and S4 are expressed as domains that are sufficiently wide and spatially isolated in thin section to permit quantitative 40Ar/39Ar geochronology. Mean 40Ar/39Ar laser microprobe ages for these domains are 370.7 ± 1.0 Myr for S3 and 345.5 ± 1.7 Myr for S4. Because estimates of the Ar closure temperature for white micas are substantially higher than the inferred growth temperatures of the micas defining S3 and S4, these values are interpreted as periods since cleavage formation. This interpretation is consistent with independent geochronological constraints on the age of the Acadian orogeny in the region.
Third‐order depositional sequences reflecting Milankovitch cyclicity
Authors: Strasser André , Hillgärtner Heiko , Hug Wolfgang , Pittet Bernard
Published: March 2003

The origin of third‐order depositional sequences remains debatable, and in many cases it is not clear whether they were controlled by tectonic activity and/or by eustatic sea‐level changes. In Oxfordian and Berriasian–Valanginian carbonate‐dominated sections of Switzerland, France, Germany and Spain, high‐resolution sequence‐stratigraphic and cyclostratigraphic analyses show that the sedimentary record reflects Milankovitch cyclicity. Orbitally induced insolation changes translated into sea‐level fluctuations, which in turn controlled accommodation changes. Beds and bedsets formed in rhythm with the precession and 100‐kyr eccentricity cycles, whereas the 400‐kyr eccentricity cycle contributed to the creation of major depositional sequences. Biostratigraphical data allow the correlation of many of the 400‐kyr sequence boundaries with third‐order sequence boundaries recognized in European basins. This implies that climatically controlled sea‐level changes contributed to the formation of third‐order sequences. Furthermore, this cyclostratigraphical approach improves the relative dating of stratigraphic intervals.
Shoreface tongue geometry constrains history of relative sea‐level fall: examples from Late Cretaceous strata in the Book Cliffs area, Utah
Authors: Hampson Gary J. , Burgess Peter M. , Howell John A.
Published: March 2003

Progradational shoreface tongues preserve a near‐complete depositional record of relative sea‐level highstands, falls and lowstands. Two distinct styles of progradational shoreface tongue are examined in an extensive outcrop and subsurface dataset from Late Cretaceous strata of the Book Cliffs area, Utah, representing (i) highstand through attached lowstand progradation and (ii) highstand through detached lowstand progradation. Using this dataset, key geometrical attributes of the shoreface tongues and their internal facies architecture are identified and quantified that enable the reconstruction of relative sea‐level fall history. For example, attached, wave‐dominated lowstand shoreface deposits record a slow (0.2– 0.3 mm yr–1), low‐magnitude (> 14 m) relative sea‐level fall punctuated by minor rises. Detached, weakly wave‐influenced lowstand shoreface deposits record a more rapid (0.4–0.5 mm yr–1), high‐magnitude (> 45 m) relative sea‐level fall synchronous with a marked change in sediment delivery and depositional process regime at the shoreline.
Granites built by sheeting in a fault stepover (the Sanabria Massifs, Variscan Orogen, NW Spain)
Authors: Vegas N. , Aranguren A. , Tubía J. M.
Published: March 2003

The Sanabria region (Central Iberian Zone, Variscan belt of Spain) shows an asymmetric thermal dome marked by migmatites accompanied by the Ribadelago and the Sotillo plutons. These small plutons display pronounced mineralogical variations. Biotite granodiorites and tonalites prevail, and granites and gabbros are common. Both plutons are confined in the releasing stepover of a transpressional shear zone that strikes 120°E and dips 70°SW. Most of the igneous rocks form sheets parallel to the shear zone. Magnetic foliations and lineations in the igneous rocks are parallel, respectively, to the shear bands and stretching lineations observed in the shear zone. The formation of these igneous sheets at high angle to the main axis of the regional field stress is explained by a combination of the fault‐valve behaviour of the shear zone with the power of melt overpressure to open and ascend through previously formed planar structures, like S‐ or C‐planes.
Middle Miocene high‐pressure metamorphism and fast exhumation of the Nevado‐Filábride Complex, SE Spain
Authors: Sánchez‐Vizcaíno V. López , Rubatto D. , Gómez‐Pugnaire M. T. , Trommsdorff V. , Müntener O.
Published: March 2003

This study provides new constraints on fast cooling and exhumation rates of high‐pressure metamorphic rocks in young active mountain belts. Ion microprobe (SHRIMP) U–Pb analysis of zircon in a pyroxenite layer of the Cerro del Almirez ultramafic rocks (Nevado‐Filábride Complex, southern Spain) gave an age of 15.0 ± 0.6 Myr (95% c.l.). Mineral inclusions demonstrate that zircon formed close to the high‐pressure peak. Combined with previous fission track data, the 15 Myr age suggests high cooling (˜ 80 °C Myr−1) and exhumation (˜1.2 cm yr−1) rates for the unit. The new results indicate that both the Nevado‐Filábride Complex and the overlying Alpujárride Complex, with somewhat higher ages and exhumation rates, underwent similar metamorphic evolutions at different times. This implies that the Alpujárride rocks were exhumed when the Nevado‐Filábride was subducting and that the same tectonic scenario propagated from one portion of the Betic Cordilleras to another.
Control of bioclast abundance on natural joint density in carbonate rocks: data from Oman, Provence and Languedoc (France)
Authors: Eyssautier‐Chuine Stéphanie , Odonne Francis , Massonnat Gérard
Published: March 2003

Joint density in sedimentary rocks is highly dependent on many factors such as tectonic stress, fluid pressure, interbedded layers and bed thickness. Data from Cretaceous carbonate rocks in Oman, Provence and Languedoc (France) show that bed thickness has a general control on joint density in all areas but other parameters also influence joint density. In order to understand the influence of textural parameters on volume joint density, all joint sets were measured in several beds. Data show that joint density decreases when the percentage of figured grains increases in Oman and Provence but that this relationship is more complex in Languedoc. The major textural parameter controlling joint density is the percentage of bioclasts: joint density decreases with an increase of the percentage of bioclasts in the three sectors.
Melt and fluid inclusions from volcanic quartz phenocrysts in the Shila gold / base metal ore district, Peru: precursors to ore‐forming epithermal solutions?
Authors: Grancea Luminita , André‐Mayer Anne‐Sylvie , Cuney Michel , Leroy Jacques L.
Published: March 2003

ABSTRACT Despite the close association with volcanic activity, the source of metals and ligands in the epithermal ore deposits is still controversial. In order to explore the magmatic–hydrothermal connection further, silicate melt, saline‐ and water‐rich fluids, and CO2 vapours are documented that are trapped as inclusions in quartz phenocrysts from dacitic dykes associated with epithermal gold/base metal mineralization in the Shila district (Peru). Melt inclusion characteristics, and microthermometric and laser Raman fluid inclusion data are presented. The investigation of melt and fluid inclusions reveals that the volatile phase of magmas might represent the precursors to the early chlorine‐rich ore‐forming epithermal solutions. Microthermometric investigations in magmatic quartz crystals and data on quartz mineralized veins suggest that the fluid evolution and ore deposition may be the result of several processes including: release of an evolving magmatic fluid, and/or boiling, and/or mixing.