bluuebunny/crossref_metadata_2025 huggingface dataset:
‘Sediment’–cement relationships in a Pleistocene speleothem from Italy: a possible analogue for ‘replacement’ cements and Archaeolithoporella in ancient reefs
Authors: Kendall Alan C. , Iannace Alessandro
Published: March 2003

Ancient carbonate buildups may contain extraordinarily large amounts of early diagenetic precipitates. In some, host rock lamination may be traced into inclusion bands within the ‘cement’ crystals, suggesting that the crystals are replacive. By analogy with a Pleistocene speleothem from the Sorrento Peninsula, however, these relationships can be explained differently. In the speleothem, large, repeatedly split and dendritic calcite crystals occur within a laminated carbonate. Lamination consists of sub‐mm alternations of micrite and microspar. Micritic laminae pass laterally into inclusion‐rich growth bands in the dendritic calcite crystals, and have replaced an aragonitic cement, whereas the microspar laminae were primary calcite cements. Three types of inclusion‐rich bands occur in the dendrite crystals: (1) with aragonite relicts, (2) ‘ribbon calcite’ and (3) with oriented micropores. When aragonite precipitated, the calcite dendrite branches were unable to keep growing as single crystals and split into crystallites (separated by micropores, some forming ribbon calcite), whereas during episodes of calcite lamina precipitation, the larger crystals were regenerated by crystallite coalescence. Calcite crystals are primary: they did not replace a micritic precursor. By analogy with the Italian speleothem, some ancient reefal sparry carbonates may not be replacements of earlier laminated sediments, but may have grown concurrently with them. It is also probable that some ancient laminated sediments were instead sea‐floor precipitates, and that stromatolites containing cross‐cutting crystal fabrics, and the alternating micrite‐microspar laminae typical of Archaeolithoporella, could be largely abiotic crystal growths.
Palaeokarst in the Ordovician of the southern Great Basin, USA: implications for sea‐level history
Authors: Cooper John D. , Keller Martin
Published: March 2003

Abundant and regionally unique dolostone lithoclast breccias occur throughout the shallow‐marine, Lower to Middle Ordovician Pogonip Group in the Nopah Range and adjacent ranges in eastern California and southern Nevada. Breccia bodies display sharply cross‐cutting relationships with host dolostone bedrock stratigraphy. They also show stratigraphic variability in size, shape and dolostone clast composition, but similarity in breccia matrix composition and framework texture and fabric. These characteristics are consistent with a palaeokarst origin. Upsection changes in breccia clast lithology as well as multiple occurrences of associated quartz sand‐filled grikes (solution‐widened fissures) indicate multiple episodes of carbonate platform exposure and karstification. Repeated karstification is also indicated by stratiform bodies of quartz sand and thin terra rossa palaeosols that locally truncate breccias and grike systems, thus bracketing karstified exposure surfaces. Facies successions and stacking patterns between recognized exposure surfaces are developed as transgressive–regressive cycles and thus show depositional sequence architecture. Hence, these breccias and other associated palaeokarst features are related to a succession of disconformities that provide a sequence‐stratigraphic framework for assessing Ordovician relative sea‐level history of the south‐western Cordilleran margin of Laurentia.
Modelling of the construction of the Rhône alluvial plain since 15 000 years BP
Authors: Teles V. , Bravard J. P. , De Marsily G. , Perrier E.
Published: March 2003

A numerical model presented here develops a three‐dimensional image of alluvial media on an elementary scale significant for groundwater flow modelling. The model was tested on the alluvial plain of the Rhône River (France), on a scale of several kilometres and, from geomorphological observations and dating, reproduced the construction of this alluvial plain from ≈15 000 years BP to the present. The history of the alluvial plain during the Late Glacial and Holocene periods is summarized. Through most of this time, the River Rhône has maintained a braided pattern, with the exception of two incising phases with a meander pattern. The model does not use any physically based equations or water representation. The main processes governing the construction of the plain are modelled by simple rules chosen according to geometrical or empirical laws taken from the literature or as modelling assumptions. Using multi‐agent concepts of distribution and interaction of elementary entities, these sedimentary rules are applied to ‘sediment’ entities or to conceptual ‘erosion’ entities that simulate local deposition and erosion of sediments. The sedimentation model reproduces the various climatic periods during which the sediments were deposited by simulating genetic periods and associated modelled processes. For each period, the model was constrained by quantitative field data such as altitude of ancient channels and deposits or thickness of sediments. The general geometry of the alluvial deposits was satisfactorily reproduced. During the simulation, characteristic large‐scale features emerge despite the use of local rules. The model results are discussed with reference to other approaches, such as geostatistical or Boolean models, and the applicability of the model to other less documented alluvial plains is outlined.
Sedimentological characteristics, heavy metal distribution and magnetic properties in subtidal sediments, Ria de Pontevedra, NW Spain
Authors: Rubio B. , Pye K. , Rae J. E. , Rey D.
Published: March 2003

The rias of NW Spain are coastal ecosystems of high biological productivity and great economic importance. They are intensively exploited by man for fish and shellfish. There are a number of important centres of population and industrial activity along their margins, which serve as sources of contamination. In this context, it is desirable to achieve the best possible understanding of the physico‐chemical processes that control spatial and temporal variations in the geochemical, mineralogical and sedimentological characteristics of near‐surface sediments in the Ria de Pontevedra and, in particular, the distribution and mobility of heavy metal contaminants. Thus, adequate environmental planning can be achieved for this site and understanding gained for comparable contexts. Core samples were examined from the inner, middle and outer parts of the ria. Grain‐size distributions reflect the presence of two main populations, one dominated by silt and clay, derived mainly from terrestrial sources, and the other by fine sand to coarse silt, which is derived mainly from continental shelf and ria mouth sources. Mineralogical analysis shows an abundance of terrestrial intensive‐weathering products near the ria head, a dominance of shelf‐derived sediment towards the mouth and the presence of several diagenetic minerals whose nature varies with location within the ria. In the inner ria, the near‐surface sediments are slightly enriched in Pb, Cu and Zn from anthropogenic sources. These sediments are fine grained and have a high organic content; hence, they have a higher potential to sorb contaminants than the coarser grained, less organic‐rich sediments of the mid and outer ria. The estimated sedimentation rates for the fine‐grained organic‐rich sediments from the inner part of the ria are about 1 mm year–1. The dominant authigenic minerals in the inner ria are iron sulphides, whereas in the mid and outer ria, iron silicates and oxyhydroxides are more important. These differences in authigenic iron mineralogy are clearly reflected by the magnetic properties of the sediments.
Combined tide and wave influence on sedimentation patterns in the Upper Jurassic Swift Formation, south‐eastern Alberta
Authors: Molgat M. , Arnott R. W. C.
Published: March 2003

In south‐eastern Alberta, the Oxfordian Swift Formation comprises two unconformity‐bounded sequences. Sequence 1 consists of the shale member. It unconformably overlies Bathonian calcareous shale of the Rierdon Formation and consists dominantly of shale with uncommon hum‐ mocky cross‐stratified siltstone. The shale member comprises up to four parasequences that show a subtle upward‐fining stacking pattern, suggesting overall deepening in a storm‐dominated fully marine basin. Deposition of the shale member was terminated by a fall in relative sea‐level, which in turn initiated sequence 2. During lowstand, a network of north‐east‐ to south‐west‐trending meandering channels incised the top of sequence 1. Despite the preserved morphology of these channels, observations in core suggest that lowstand deposits are absent. These strata were most likely thoroughly reworked by ravinement processes associated with the ensuing transgression and are recorded only by a thin chert pebble lag. Also associated with transgression were significant changes in the physical and ecological conditions in the local Swift basin. These changes were related to modifications in basin configuration and the development of a low‐energy strait, which probably formed as a result of uplift of the Sweetgrass Arch. Within this newly configured basin, brackish‐water conditions prevailed, and sedimentation was dominated by suspension deposition of mud. Bed‐load sediment, composed mostly of very fine and fine sand, made up only a small part of the total sediment flux into the area and was transported principally by low‐energy combined flows with variable‐speed microtidal currents and a low‐energy oscillatory component related to storm waves. Sand was generally deposited in the form of small, discontinuous sand ridges that developed locally throughout the study area – it is these features that form the principal hydrocarbon reservoir bodies in the Swift Formation in the study area. After initiation, ridges built upwards and migrated laterally in response to transport and preferential local deposition of bed‐load sediment. Although common near the unconformity along the top of sequence 1, sand ridges also occur at several higher stratigraphic levels in sequence 2. These latter ridges, however, are not associated with regionally correlatable discontinuities but, instead, were initiated by local bed irregularities, possibly related to breaking internal waves, and were supplied with new sediment transported into the study area. The origin of these sand ridges is therefore not related to changes in relative sea‐level but, instead, to intrabasinal processes in a low‐energy strait.
Neogene evaporites in desert volcanic environments: Atacama Desert, northern Chile
Authors: Pueyo Juan José , Chong Guillermo , Jensen Arturo
Published: March 2003

The Upper Miocene and Pliocene evaporite deposits of the Atacama Desert of northern Chile (Hilaricos and Soledad Formations) are among the few non‐marine evaporites in which aridity not only formed the deposits, but has also preserved them almost unaltered under near‐surface conditions. These deposits are largely composed of displacive Ca sulphate and halite together with minor amounts of glauberite, thenardite and polyhalite. However, at the base and top of these deposits, there are also beds of gypsum crystal pseudomorphs that originally formed as free‐growth forms within shallow brine bodies, rather than as displacive sediments. The halite is present as interstitial cement, displacive cubes and shallow‐water, bottom‐growth chevron crusts. Most of the calcium sulphate is presently anhydrite, pseudomorphous after gypsum, that was the primary depositional sulphate mineral. The secondary anhydrite formed under early diagenetic conditions after slight burial (some metres) resulting from the effect of strongly evolved pore brines. The anhydrite has been preserved without rehydration during late diagenetic and exhumation stages on account of the arid environment of the Atacama Desert. Both the Hilaricos and the Soledad Formations contain geochemical markers indicating that these Neogene evaporites had a largely non‐marine origin. Bromine content in the halite is very low (few p.p.m.), indicating neither a sedimentological relation with sea water nor the likelihood of direct recycling of prior marine halites. Moreover, the δ34S of sulphates (+4·5‰ to +9‰) also reflects a non‐marine origin, with a strong volcanic influence, although some recycling of Mesozoic marine sulphates cannot be ruled out. δ34S of dissolved sulphate from hot springs and streams in the area commonly displays positive values (+2‰ to +10‰). Leaching of oxidized sulphur and chlorine compounds from volcanoes and epithermal ore bodies, very common in the associated drainage areas, have been the main contribution to the accumulation of evaporites. The sedimentary and diagenetic evolution of the Hilaricos and Soledad evaporites (based on lithofacies analysis) provides information about the palaeohydrological conditions in the Central Depression of northern Chile during the Neogene. In addition, the diagenesis and exhumation history of these evaporites confirms the persistence of strongly arid conditions from Late Miocene until the present. A final phase of tectonism took place permitting the internal drainage to change and open to the sea, resulting in dissolution and removal of a significant portion of these deposits. Despite the extensive dissolution, the remaining evaporites have undergone little late exhumational hydration.
Sedimentary facies and landform genesis at a temperate outlet glacier: Soler Glacier, North Patagonian Icefield
Authors: Glasser Neil F. , Hambrey Michael J.
Published: March 2003

This paper focuses on the structural glaciology, dynamics, debris transport paths and sedimentology of the forefield of Soler Glacier, a temperate outlet glacier of the North Patagonian Icefield in southern Chile. The glacier is fed by an icefall from the icefield and by snow and ice avalanches from surrounding mountain slopes. The dominant structures in the glacier are ogives, crevasses and crevasse traces. Thrusts and recumbent folds are developed where the glacier encounters a reverse slope, elevating basal and englacial material to the ice surface. Other debris sources for the glacier include avalanche and rockfall material, some of which is ingested in marginal crevasses. Debris incorporated in the ice and on its surface controls both the distribution of sedimentary facies on the forefield and moraine ridge morphology. Lithofacies in moraine ridges on the glacier forefield include large isolated boulders, diamictons, gravel, sand and fine‐grained facies. In relative abundance terms, the dominant lithofacies and their interpretation are sandy boulder gravel (ice‐marginal), sandy gravel (glaciofluvial), angular gravel (supraglacial) and diamicton (basal glacial). Proglacial water bodies are currently developing between the receding glacier and its frontal and lateral moraines. The presence of folded sand and laminites in moraine ridges in front of the glacier suggests that, during a previous advance, Soler Glacier over‐rode a former proglacial lake, reworking lacustrine deposits. Post‐depositional modification of the landform/sediment assemblage includes melting of the ice‐core beneath the sediment cover, redistribution of finer material across the proglacial area by aeolian processes and fluvial reworking. Overall, the preservation potential of this landform/sediment assemblage is high on the centennial to millennial timescale.
Settling velocities and entrainment thresholds of biogenic sands (shell fragments) under unidirectional flow
Authors: Paphitis D. , Collins M. B. , Nash L. A. , Wallbridge S.
Published: March 2003

Settling velocities and entrainment thresholds of biogenic sedimentary particles, under unidirectional flow conditions, are derived on the basis of settling tower and laboratory flume experiments. Material consisting predominantly of equant blocks (shell fragments ofCerastoderma edule, density,ρs=2800 kg m−3) or of mica‐like flakes and elongate rods (Mytilus edulisfragments,ρs=2720 kg m−3) are used in separate series of experiments. Differences in the measured settling and threshold properties are related primarily to particle shape. The selection of a characteristic length scale for non‐spherical grains is investigated by comparing two approaches used to define the grain size (D) of the sediment samples: grain settling and sieve analysis that are used to derive data for the threshold criteria, in terms of the Shields and Movability diagrams. The empirical curves effectively predict the threshold conditions for any grain shape, provided that grain size is defined in terms of grain settling velocity. However, a functional distinction is made between the characteristic `hydraulic' grain size, defined by grain settling for grain transport applications, and the actual (physical) grain size defined by sieve analysis.
Recent colluvial sedimentation in Jordan: fans evolving into sand ramps
Authors: Turner Brian R. , Makhlouf Issa
Published: March 2003

Abstract High‐angle accumulations of sand and escarpment‐derived gravel along the outcrop walls of Plio‐Pleistocene sandstones, eastern Jordan, form small, coalesced colluvial fans, built by rockfalls, rockfall‐derived debris flows, dry sandfalls and sandy grainflows. These deposits are sourced through wind erosion of fault‐controlled outcrops of weakly cemented sandstone and a hard, gypsum‐cemented sandstone and fine conglomerate caprock exposed in sandpits. Eroded sediment is supplied to the fans directly as rockfalls and sandfalls, and indirectly as gully‐confined sandy grainflows. The preserved colluvium fans comprise sandy, matrix‐rich rockfall, rockfall‐derived, dry debris‐flow lenticular gravel deposits and minor lenticular sandy grainflow deposits. The fans develop initially against the footwall escarpment and, as erosion continues, the outcrop and the fans become covered by stable sand sheet ramps in a self‐regulatory geomorphic system. Preserved fan–sand ramp systems in eastern Jordan are characterized by a threefold hierarchy of genetically related bounding surfaces, which develop over short time scales. Rapid fault‐controlled uplift and/or rapid stream incision may produce non‐equilibrium scarp faces, identical to those in the sandpits, associated with the colluvial fan–sand ramp systems. Thus, such systems have the potential to identify fault‐related unconformities, rapid uplift events and episodes of rapid downcutting in the rock record. Colluvium deposits have good preservation potential, but are often associated with complex, coarse, basin‐margin facies, and are thus difficult to identify in the stratigraphic record; a problem exacerbated by the lack of adequate colluvium facies models and diagnostic sedimentary criteria.
Phylogeny and classification of the Culicidae (Diptera)
Authors: HARBACH RALPH E. , KITCHING IAN J.
Published: March 2003

The generic relationships and higher classification of the family Culicidae are examined on the basis of a phylogenetic analysis. New and traditional morphological characters studied and compared throughout the Culicidae resulted in the acquisition of character data relative to the majority of species within each genus. Polymorphisms and morphological observations are discussed and additional information and illustrations are provided for the majority of characters and their character states. The analysis of seventy‐three adult, pupal and fourth‐instar larval characters coded for the thirty‐eight currently recognized genera of mosquitoes resulted in relationships and groupings which differ significantly from traditional hypotheses. The analysis supports the monophyly of the subfamily Anophelinae and the tribes Culicini and Sabethini. The Anophelinae form the most basal clade of the family. The results indicate that Aedini is a paraphyletic assemblage with respect to the Mansoniini, each of which is monophyletic in itself. The Aedini + Mansoniini form a sister group to the Culicini + Sabethini, with the Aedini and the Culicini placed in ancestral relationships to the Mansoniini and the Sabethini, respectively. Based on the topography of generic relationships among more ‘generalized’ mosquitoes, the boundaries and relationships of the tribes Aedeomyiini, Uranotaeniini, Ficalbiini, Hodgesiini, Orthopodomyiini and Culisetini appear to be problematic. Relationships between genera of the tribe Aedini are generally poorly resolved due to a significant amount of polymorphism, especially within the genus Aedes as currently defined. There is no support for separate subfamily recognition for the genus Toxorhynchites, which is downgraded to tribal status as a result of the analysis. The results are discussed in relation to previous hypotheses based on subjective inference and cladistic analyses.
Phylogenetics and evolution of the aphid genus Uroleucon based on mitochondrial and nuclear DNA sequences
Authors: Moran Nancy A. , Kaplan Matthew E. , Gelsey Michael J. , Murphy Troy G. , Scholes Edwin A.
Published: March 2003

The genus Uroleucon, and the related genus Macrosiphoniella, represent a large Tertiary radiation of aphids, with a total of about 300 species distributed throughout the world, primarily on host plant species in the family Asteraceae. A molecular phylogenetic study was conducted to identify major clades within Uroleucon and to address the cladistic validity of current subgeneric categories, the evolution of host plant associations, the age of origin, and intercontinental movements in this genus. The seventeen study species included members of the three major subgenera of Uroleucon, species from Europe and North America, one member of Macrosiphoniella, and two outgroups. Data consisted of DNA sequences for three mitochondrial regions and the nuclear gene EF1alpha, for a total of 4287 sites. Nodes supported strongly in both parsimony and maximum likelihood analyses suggest that: (1) Nearctic Uromelan are a monophyletic group branching near the base of the genus and not related to European Uromelan, (2) the New World subgenus Lambersius is possibly monophyletic but is not a tightly related group and is not closely related to other North American species, and (3) Nearctic members of subgenus Uroleucon are a closely related monophyletic group not allied with Nearctic Uromelan or Lambersius. Instead they represent a separate colonization by an Old World ancestor, as they are nested within a strongly supported clade containing European members of both subgenera Uroleucon and Uromelan. Neither of these subgenera is monophyletic. Molecular clock calculations, based on calibrations of mitochondrial divergences from other insects, suggest that Uroleucon + Macrosiphoniella is a relatively recent radiation, probably no more than 5–10 million years old. Although largely confined to Asteraceae, this clade did not radiate in parallel with its host plants. Rather, lateral movement between lineages of Asteraceae must have occurred repeatedly.
Phylogeny of carabid beetles as inferred from 18S ribosomal DNA (Coleoptera: Carabidae)
Authors: Maddison DaviD. R.. , And MichaeL. D. BakeR. , Ober KareN. A..
Published: March 2003

The phylogeny of carabid tribes is examined with sequences of 18S ribosomal DNA from eighty‐four carabids representing forty‐seven tribes, and fifteen outgroup taxa. Parsimony, distance and maximum likelihood methods are used to infer the phylogeny. Although many clades established with morphological evidence are present in all analyses, many of the basal relationships in carabids vary from analysis to analysis. These deeper relationships are also sensitive to variation in the sequence alignment under different alignment conditions. There is moderate evidence against the monophyly of Migadopini + Amarotypini, Scaritini + Clivinini, Bembidiini and Brachinini. Psydrini are not monophyletic, and consist of three distinct lineages (Psydrus,Laccocenusand a group of austral psydrines, from the Southern Hemisphere consisting of all the subtribes excluding Psydrina). The austral psydrines are related to Harpalinae plus Brachinini. The placements of many lineages, includingGehringia,Apotomus,Omophron,PsydrusandCymbionotum, are unclear from these data. One unexpected placement, suggested with moderate support, isLoriceraas the sister group toAmarotypus. Trechitae plus Patrobini form a monophyletic group. Brachinini probably form the sister group to Harpalinae, with the latter containingPseudomorpha,MorionandCnemalobus. The most surprising, well supported result is the placement of four lineages (Cicindelinae, Rhysodinae, Paussinae and Scaritini) as near relatives of Harpalinae + Brachinini. Because these four lineages all have divergent 18S rDNA, and thus have long basal branches, parametric bootstrapping was conducted to determine if their association and placement could be the result of long branch attraction. Simulations on model trees indicate that, although their observed association might be due to long branch attraction, there was no evidence that their placement near Harpalinae could be so explained. These simulations also suggest that 18S rDNA might not be sufficient to infer basal carabid relationships.
Phylogenetic analysis of the New World Ptininae (Coleoptera: Bostrichoidea)
Authors: Philips T. Keith
Published: March 2003

A phylogenetic analysis of the New World Ptininae (Anobiidae) was conducted with representatives of nine of ten New World genera, several Old World genera and seven more of the ten subfamilies of Anobiidae. One hundred and two characters (forty‐three multistate) from thirty‐four taxa were used. The single cladogram shows Ptininae as monophyletic and the sister group of the remaining Anobiidae, supporting their placement as subfamilies of a monophyletic Bostrichidae. Genus Niptus Boieldieu is polyphyletic supporting recognition of Pseudeurostus Heyden and the creation of a new genus to encompass the remaining New World species of Niptus. Flightlessness has evolved a minimum of three times within Ptininae and myrmecophily has probably evolved three times within just the New World taxa. The classifications of Ptininae and the remaining Anobiidae are examined and the evolution of feeding habits, myrmecophily and wing loss are discussed.
Chromosomes and identification of the sibling species Pterostichus nigrita (Paykull) and P. rhaeticus Heer (Coleoptera: Carabidae)
Authors: Angus R. B. , Brown R. E. , Bryant L. J.
Published: March 2003

The karyotypes of Pterostichus nigrita (Paykull) and P. rhaeticus Heer are described. Both species have eighteen pairs of autosomes, sex chromosomes which are XO (♂) and XX (♀), plus a variable number of totally heterochromatic B chromosomes. Males may be identified by the form of the inflated endophallus, but the shape of the right paramere is not a reliable character. Females may be identified by the form of the eighth abdominal sternite.
Phylogenetic studies of Meloidae (Coleoptera), with emphasis on the evolution of phoresy
Authors: Bologna Marco A. , Pinto John D.
Published: March 2003

One of the most recent classifications of Meloidae is based on the assumption that phoretic first‐instar larvae evolved twice in the family, once in Meloinae and again in Nemognathinae. Within Meloinae, this scheme places all presumed phoretic taxa in Meloini regardless of other characteristics. This paper challenges this classification with a cladistic analysis of all meloid genera whose first‐instar larvae were available for study. It concludes that phoresy evolved several times in Meloinae alone and that Meloini, when defined to include all phoretic genera, is polyphyletic. Cladistic support also is presented for four subfamilies of Meloidae and for several of the traditional tribes recognized in recent classifications.
Trans‐Tasman Platycoelostoma Morrison (Hemiptera: Coccoidea: Margarodidae) on endemic Cupressaceae, and the phylogenetic history of margarodids
Authors: Gullan P. J. , Sjaarda A. W.
Published: March 2003

A new species of Platycoelostoma Morrison (Hemiptera: Coccoidea: Margarodidae) is described from the Tasmanian endemic conifer, Diselma archeri (Cupressaceae), growing in alpine heathland. Its sister species, P. compressa (Maskell), occurs in alpine and subalpine New Zealand on Libocedrus bidwilli (Cupressaceae). Vicariance dating provides a minimum age for Platycoelostoma of about 80 Ma. Cladistic analysis of morphological data from adult females and first‐instar nymphs suggests that Platycoelostoma is the sister genus of Callipappus Guérin‐Méneville and also is more closely related to members of subfamilies Margarodinae and Xylococcinae than to the New Zealand and South American genera of Coelostomidiinae, to which Platycoelostoma traditionally has been linked. The analysis also indicates that most traditional tribal groupings within Margarodidae are monophyletic, but that three (Coelostomidiinae, Margarodinae and Xylococcinae) of the five subfamilies in the widely used higher classification of Morrison may be either paraphyletic or polyphyletic as currently defined. Only Monophlebinae and Steingeliinae have adequate support from morphological characters. Relationships among subfamilies and many tribes are unresolved. These results lend some support to the classification previously proposed by Koteja, in which all margarodid subfamilies and a few tribes are elevated to the rank of family. Furthermore, the analysis suggests that Marchalini and Monophlebinae (in which all immature female instars have well developed appendages) are derived with respect to other margarodids and thus development via legless intermediate female instars (‘cyst’ form) may be plesiomorphic within margarodids. This hypothesis is weakly supported and requires corroboration from independent data. Platycoelostoma is transferred to tribe Callipappini, which is redefined, but the subfamily placement of this tribe must remain uncertain until the higher classification and rank of the various margarodid taxa can be re‐evaluated. A revised generic description of Platycoelostoma, based on the adult and intermediate female instars and the first instar of both species, and a key to separate the female instars of both species are provided. The first‐instar nymph, all female instars and the third‐instar male (the prepupa) of Platycoelostoma tasmanicum sp.n. are described and illustrated. Scanning electron micrographs depict the unusual cuticular features of the third‐instar female.
Variations in basaltic geochemistry along a propagating rift of the late Ordovician marginal basin of the West Norwegian Caledonides
Authors: Furnes  , Dilek  , Skjerlie  , Ryttvad  , Fonneland
Published: March 2003

The late Ordovician Solund‐Stavfjord ophiolite complex in the western Norwegian Caledonides records a multi stage seafloor spreading history of an Iapetus marginal basin and contains three structural domains with distinctive tectonomagmatic evolutionary paths. The NNE‐trending Domain 1 consisting of high‐level gabbro, sheeted dykes and extrusive rocks is interpreted to represent fossil oceanic crust developed along a spreading centre that propagated northwards into pre‐existing oceanic crust in the marginal basin. Dyke swarms at the head of this inferred propagating rift range from primitive, high‐MgO basalts to highly fractionated quartz‐diorites. Southwards along‐strike of Domain 1, the abundance of primitive basalts decreases and the proportion of FeTi‐basalts increases to become predominant furthest behind the tip of the propagating rift. This geochemical evolution is comparable to that of the basalts of modern propagating rifts at the East Pacific Rise and the Galapagos spreading ridge. We suggest that the chemical variations of the metabasalts reflect changes in magma supply rates and in the increasing size of magma chamber(s) along‐strike of the spreading centres.
Evolution of oceanic crust on the Kolbeinsey Ridge, north of Iceland, over the past 22 Myr
Authors: Kodaira  , Mjelde  , Gunnarsson  , Shiobara  , Shimamura
Published: March 2003

The evolution of oceanic crust on the Kolbeinsey Ridge, north of Iceland, is discussed on the basis of a crustal transect obtained by seismic experiment from the Kolbeinsey Ridge to the Jan Mayen Basin. The crustal model indicates a relatively uniform structure; no significant lateral velocity variations are observed, especially in the lower crust. The uniform velocity structure suggests that the postulated extinct axis does not exist over the oceanic crust formed at the Kolbeinsey Ridge, but supports a model of continuous spreading along the ridge after oceanic spreading started west of the Jan Mayen Basin. The oceanic crust formed at Kolbeinsey Ridge is 1–2.5 km thicker than normal oceanic crust due to hotter‐than‐normal mantle from the Iceland Mantle Plume. The observed generally uniform thickness throughout the transect might also indicate that the temperatures of the astheno‐spheric mantle ascending along the Kolbeinsey Ridge have not changed significantly since the age of magnetic anomaly 6B.
Vaalbara, Earth’s oldest assembled continent? A combined structural, geochronological, and palaeomagnetic test
Authors: Zegers  , De Wit  , Dann  , White
Published: March 2003

The only remaining areas of pristine 3.6–2.7 Ga crust on Earth are parts of the Kaapvaal and Pilbara cratons. General similarities of their rock records, especially of the overlying late Archean sequences, suggest that they were once part of a larger Vaalbara supercontinent. Here we show that the present geochronological, structural and palaeomagnetic data support such a Vaalbara model at least as far back as 3.1 Ga, and possibly further back to 3.6 Ga. Vaalbara fragmented prior to 2.1 Ga, and possibly as early as 2.7 Ga, suggesting supercontinent stability of at least 400 Myr, consistent with Neoproterozoic and Phanerozoic analogues.
Oceanic circulation as an element in palaeogeographical reconstructions: the Arenig (early Ordovician) as an example
Authors: Christiansen J.L. , Stouge S.
Published: March 2003

A new conceptual palaeo‐oceanographical model is outlined in this paper. The model differs from previous models by using shifts in Hadley circulation caused by orbital variations, and the rotation rate of the Earth, to locate the position of the planetary oceanic low‐and high‐pressure systems, around which the planetary ocean surface currents flow. Adapting the model to the Arenig (early Ordovician) the temperate low pressure zones were found to be located at 50° latitude and the subtropical high pressure zones at 25° latitude.

Traditionally, most Palaeozoic palaeogeographical recon‐structions are reconstructed using palaeomagnetic data supplemented with data from climate‐sensitive lithofacies and palaeo‐biogeographical distributions. However, as a new approach in palaeogeographical reconstructions, the con‐ceptual palaeo‐oceanographical model is combined with palaeobiogeographical data for the Arenig series, comple‐menting the palaeomagnetic data, and resulting in a new, refined palaeogeographical reconstruction.
Subsidence and extension at a convergent plate margin: evidence for subduction erosion off Costa Rica
Authors: Meschede M. , Zweigel P. , Kiefer E.
Published: March 2003

Subduction erosion rather than subduction accretion is proposed as the dominant process currently occurring at the Nicoya segment of the convergent plate margin off Costa Rica. Based on new results from ODP drilling cores and our interpretations of published seismic data we present a tectonic model of subsidence and extension due to tectonic erosion of the forearc wedge and landward migration of the Nicoya coastline. High seismic velocities in the outermost part of the forearc wedge off the Nicoya Peninsula below the BOSS (bottom‐of‐slope‐sediment) reflector indicate the seaward continuation of the Nicoya ophiolite complex into the basement of the forearc wedge. ODP Site 1042 revealed neritic sediment that points to strong subsidence of the forearc basement, progradation of the sedimentary succession, and landward migration of the coastline. Tilted block structures are explained by substantial fore‐arc extension. In our interpretation, the removal of material from the base of the forearc wedge by the process of basal subduction erosion leads to progressive subsidence and thus to landward migration of the coastline. Landward shift of the active volcano chain in the same order of magnitude as the coastline migration is consistent with this interpretation.
Stable chlorine isotope evidence for non‐marine chloride in Badenian evaporites, Carpathian mountain region
Authors: Eastoe C.J. , Peryt T.
Published: March 2003

Routine trace‐element geochemistry suggests that components in putative marine halite evaporites may be partly of nonmarine origin, but such interpretations are commonly ambiguous. Stable chlorine isotopes may provide a less‐ambiguous marker of chloride origin where δ37Cl departs from the range predicted for evaporite formation from seawater. Bedded halite with primary sedimentary textures preserves original δ37Cl values. Measurable change in δ37Cl can be generated by incongruent dissolution of halite, but only if less than half the original halite remains. Badenian (middle Miocene) halite from the Forecarpathian and from the East Slovakian and Transcarpathian basins has a δ37Cl range of – 0.2 to 0.8‰. Two phenomena cannot be explained by simple evaporation of 0.0‰ seawater. At Wieliczka, the Shaft Salt has distinctive δ37Cl values (– 0.2 to 0.0‰) relative to neighbouring salt beds (0.2 to  0.6‰), requiring a large, abrupt input of brine with negative δ37Cl. Halite with high (0.6 – 0.8‰) δ37Cl near the base of the East Slovakian and Transcarpathian evaporites requires a large input of chloride with positive δ37Cl into the basins. Expulsion of basin brine with non‐0‰δ37Cl into the evaporite basins may account for the nonmarine chloride sources.
Birth of a major strike‐slip fault in SW Japan
Authors: Gutscher  , Lallemand
Published: March 2003

A 500 km‐long strike‐slip fault, the North Chugoku Shear Zone (NCSZ) is identified in SW Japan, responsible for four M > 7 earthquakes in the past 130 years. A new geodynamic mechanism of increased interplate coupling above an obliquely subducting flat slab is presented to explain the transfer of trench parallel motion 400 km inland from the Nankai Trough, to the level of the Quaternary adakitic volcanic arc. While Quaternary dextral strike‐slip motion is widely believed to have occurred along the Median Tectonic Line (MTL), near the Miocene volcanic arc, the central segment in Shikoku has shown little seismicity for the last 1000 years. Pure dextral strike‐slip focal mechanisms for 15 modern and historical events indicate that part of the transcurrent motion is being transferred from the MTL to the NCSZ.
Convective support of long‐wavelength topography in the Apennines (Italy)
Authors: D’Agostino  , McKenzie
Published: March 2003

Two different free‐air gravity datasets from the Apennines have been analysed in the frequency domain to investigate the mechanisms of support of topography. At short wavelength (λ<150 km) the best fit between the observed and the calculated admittance gives a value of the elastic thickness of 3.7 km. The observed behaviour of the admittance at larger wavelengths (λ>150 km) shows that such topography is not supported by elastic stresses in the lithosphere but is dynamically supported by mantle convection. The distribution of gravity anomalies and topography suggests that the mantle is rising beneath the main topographic bulge of the Apennines and that this flow is maintained by buoyancy forces, rather than being driven by movement of the subducted slab.
Control on the emplacement of the andesite lava dome of the Soufriere Hills volcano, Montserrat by degassing‐induced crystallization
Authors: Sparks  , Murphy  , Lejeune  , Watts  , Barclay  , Young
Published: March 2003

Lava solidification is controlled by two mechanisms: external cooling and gas exsolution, the latter inducing crystallization due to increasing liquidus temperature. The andesite lava dome of the Soufriere Hills Volcano, Montserrat, is an extrusion dominated by crystallization caused by gas exsolution where cooling is unimportant in controlling emplacement. In the magma chamber the magma has an estimated viscosity of 7 × 106 Pa s. During ascent, gas exsolution caused the magma to extrude in a highly crystalline state, with only 5–15% residual melt, viscosities in the range 1013–1014 Pa s and mechanical strength > 1 MPa. Deformation can be heterogeneous with extrusion along shear zones. Rheological stiffening in the upper conduit also causes large overpressures, shallow seismicity, and cyclic patterns of dome extrusion. Gas‐rich porphyritic andesites tend to be the least mobile kind of lava, because transition from magma into hot crystalline material was reached during ascent.
Continuous vs. discontinuous melt segregation in migmatites: insights from a cellular automaton model
Authors: Vigneresse J.L. , Burg J.P.
Published: March 2003

A cellular automaton model is presented that simulates melt extraction from migmatites. The varying parameters are the relative amount of melt, thresholds for melt connection leading to movement and melt escape, and type of deformation (pure and/or simple shear). It is shown that when the melt percentage is about half that of the melt escape threshold, the competition between production and deformation‐assisted extraction results in a discontinuous rate of melt extraction. The process can be compared to stick‐slip motion during frictional experiments. At higher melt fraction, it is continuously extracted. Deformation assists significantly in segregating melt, particularly when a pure shear (compaction) component is added to simple shear.
Europrobe seismic reflection profiling across the eastern Middle Urals and West Siberian Basin
Authors: Friberg M. , Juhlin C. , Green A.G. , Horstmeyer H. , Roth J. , Rybalka A. , Bliznetsov M.
Published: March 2003

New deep seismic reflection data provide images of the crust and uppermost mantle underlying the eastern Middle Urals and adjacent West Siberian Basin. Distinct truncations of reflections delineate the late‐orogenic strike‐slip Sisert Fault extending vertically to ∼28 km depth, and two gently E‐dipping reflection zones, traceable to 15–18 km depth, probably represent normal faults associated with the opening of the West Siberian Basin. A possible remnant Palaeozoic subduction zone in the lower crust under the West Siberian Basin is visible as a gently SW‐dipping zone of pronounced reflectivity truncated by the Moho. Continuity of shallow to intermediate‐depth reflections suggest that Palaeozoic accreted island‐arc terranes and overlying molasse sequences exposed in the hinterland of the Urals form the basement for Triassic and younger deposits in the West Siberian Basin. A highly reflective lower crust overlies a transparent mantle at about 43 km depth along the entire 100 km long seismic reflection section, suggesting that the lower crust and Moho below the eastern Middle Urals and West Siberian Basin have the same origin.
Tying platform drowning to perturbations of the global carbon cycle with a δ13COrg‐curve from the Valanginian of DSDP Site 416
Authors: Wortmann Ulrich G. , Weissert Helmut
Published: March 2003

The carbonate carbon isotope record of the Phanerozoic is marked by repeated high‐amplitude excursions towards more positive values. Although the interpretation of C‐isotope events remains controversial, they are regarded as a proxy of the global C‐cycle. Using δ13COrg‐measurements of samples from DSDP Site 416, it is shown that a classic example of platform drowning coincided with the onset of the mid‐Valanginian carbon‐cycle excursion. Widespread carbonate platform drowning would have contributed to the observed positive shift in the C‐isotope record. As choking of carbonate production was compensated by increased organic production, both processes favoured a shift in marine carbon partitioning from the oxidized to the reduced carbon reservoir. This would have resulted in an increased flux of CO2 from the atmosphere into the marine and sedimentary carbon reservoir.
Modelling karst denudation on a synthetic landscape
Authors: Kaufmann Georg , Braun Jean
Published: March 2003

This contribution presents the results of a numerical study of karst denudation on limestone plateaux. The landscape evolution model used incorporates not only long‐range fluvial and short‐range hill‐slope processes, but also large‐scale chemical dissolution of limestone surfaces. The relative efficiencies of fluvial and chemical processes are of equal importance to the landscape evolution of a plateau dropping to sea level along an escarpment. While fluvial processes have an impact confined mostly to river channels, the karst denudation process is more uniform, removing material also from the plateau surface. The combined effect of both processes results in a landscape evolution almost twice as effective as the purely erosional evolution of an insoluble landscape.
Lead incorporation during crystal growth and the misinterpretation of geochronological data from low‐238U/204Pb metamorphic minerals
Authors: Romer R. L.
Published: March 2003

The isotopic composition of lead available for incorporation during metamorphic reactions is heterogeneous, depends on the reaction history of the metamorphic rock, and is commonly not accessible for measurement as the precursor minerals have been consumed during the growth of the metamorphic phases. The initial lead composition has a significant effect on the age of low‐238U/204Pb metamorphic phases (e.g. garnet, rutile, titanite, staurolite, vesuvianite and ilmenite). Using a distinct value (e.g. leached K‐feldspar Pb, model Pb) rather than a geologically reasonably constrained range may result in apparently precise, yet inaccurate ages. Since age data from metamorphic minerals are widely used to unravel the P–T–t–d evolution of orogens, inaccurate ages result in: (1) incorrect timing (duration) of P–T loops and associated with it the heat budget and mass transfer in orogens; (2) arbitrary rates (based on the age difference between core and rim) for mineral growth, P–T evolution and deformation; and (3) apparent sequences of isotopic closure for the U–Pb system of contrasting minerals.
Middle Miocene graben development in Crete and its possible relation to large‐scale detachment faults in the southern Aegean
Authors: Ring U. , Brachert T. , Fassoulas C.
Published: March 2003

The linkage between the development of south‐facing Cretan graben and large‐scale detachment faulting in the southern Aegean is unknown. Widespread Serravallian deposits in the Ierapetra graben of Crete supply constraints to Middle Miocene graben development in the southern Aegean. The Ierapetra graben, and by inference the Cretan graben in general, were hitherto believed to have formed as a result of sinistral transpression during N–S shortening. We argue that the formation of the Cretan graben is due to N–S extension. The south‐dipping, N–S‐extending Kritsa normal fault served as the master fault controlling graben development in the Ierapetra graben. The Kritsa normal fault is either an antithetic fault related to the top‐N Cretan detachment or a synthetic fault associated with the top‐S Ios detachment. The overall geometry and timing relationships lead us to favour a connection with the Ios detachment, which in turn implies a large‐scale displacement on the Ios detachment.