bluuebunny/crossref_metadata_2025 huggingface dataset:
Evolving turbidite systems on a deforming basin floor, Tabernas, SE Spain
Authors: Haughton Peter D. W.
Published: March 2003

The Tabernas‐Sorbas basin was a narrow, east‐west trending, marine trough of Late Miocene age. Sediment gravity flow deposits dominate the basin fill and provide a record of changing bathymetry in response to tectonically induced sea bed deformation. A reanalysis of the western end of the basin in the vicinity of Tabernas establishes an upward evolution involving: (1) sand‐starved marls that were incised by axial channels recording a period of bypass, during which sand deposition took place in a depocentre further to the east; (2) punctuated infilling of the incisions, locally by high‐sinuosity embedded channels. Channel filling is related to a gradient reduction, which presaged collapse of the axial slope as the depocentre began to migrate westwards into the Tabernas area; (3) draping of the earlier incision fills by laterally extensive sheet turbidites, which were initially contained in structurally controlled depressions. These ‘deeps’ opened up as active faults propagated through the former axial slope. Flow containment is inferred on account of the unusual structure of the sheet sandstone beds, complex palaeoflow relationships and thick mudstone caps; (4) fault‐controlled topography was subsequently healed, and further sheet turbidites showing evidence of longer range containment and progressive slope onlap were emplaced. These record mixed supply from both seismically trigged ‘axial’ failures and a reactivated, fault‐controlled slope building out from the northern margin of the basin. Flows traversing the trough floor were strongly reflected off slopes marking the southern limit of the basin. The studied succession is capped by (5) the Gordo megabed event, a large, probably seismically triggered, failure which blanketed the basin floor, demonstrating an enlarged but still contained basin now devoid of significant intrabasinal fault topography. Tectonics played a key role in driving the evolution of the turbidite systems in this basin. Deformation of the basin floor had an important impact on gradients, slope stability, bathymetry and the ability of flows to bypass along the trough axis. Westward migration of the depocentre into the Tabernas area led to a change from incision and bypass to conduit backfilling to flow containment, as fault‐induced subsidence generated a ‘sump’, which trapped flows moving along the basin axis.
Sedimentary record of explosive silicic volcanism in a Cretaceous deep‐marine conglomerate succession, northern Antarctic Peninsula
Authors: Hathway  , Kelley
Published: March 2003

Lower Cretaceous conglomeratic strata exposed on southern Sobral Peninsula were deposited on a deep‐marine apron in the back‐arc Larsen Basin close to its faulted boundary with the Antarctic Peninsula magmatic arc. The succession is dominated by amalgamated beds of clast‐supported conglomerate, which, together with minor intercalated sandstones, consist of varied, but largely basaltic to andesitic, volcanic material and clasts derived from the Palaeozoic–Triassic (meta)sedimentary basement of the arc. Most of the volcanic clasts are thought to have been derived from lithified volcanic successions or older synvolcanic deposits, rather than from sites of coeval eruption. These mixed‐provenance strata enclose a number of intervals, consisting mainly of inverse–normally graded conglomerate and graded–stratified pebbly sandstone, in which the sand fraction is dominated by crystals and vitric grains considered to have been redeposited in the immediate aftermath of explosive silicic arc volcanism. Like syneruption deposits on non‐marine volcaniclastic aprons, these intervals are more sand‐prone than the enclosing strata and appear to show evidence of unusually rapid aggradation. Plagioclase from one such interval has yielded 40Ar/39Ar ages concordant at ≈121 Ma, similar to those obtained from the non‐marine Cerro Negro Formation, deposited within the magmatic arc. It is suggested that the two successions can be viewed as counterparts, both recording a history of mainly basaltic to andesitic volcanism, punctuated by relatively infrequent, explosive silicic eruptions. Whereas the Cerro Negro Formation consists mainly of syneruption deposits, most of the volcaniclastic material delivered to the eruption‐distal, deep‐marine apron appears to have been derived by normal degradation processes. Only rare silicic eruptions were capable of supplying pyroclastic material rapidly enough and in sufficient quantities to produce compositionally distinct syneruption intervals.
Neogene fjordal sedimentation on the western margin of the Lambert Graben, East Antarctica
Authors: Hambrey Michael J. , McKelvey Barrie
Published: March 2003

The Lambert Graben is occupied by the world’s largest fjord system, through which flows the Lambert Glacier, the Amery Ice Shelf and their tributaries. Along the western margin of the graben, in the northern Prince Charles Mountains, remnants of uplifted Miocene and Pliocene strata of the glacigenic fjordal Pagodroma Group total more than 800 m in thickness. These sediments provide evidence for a dynamic East Antarctic ice sheet during the Neogene Period. Each of the four Pagodroma Group formations defined from this region rests unconformably on either Proterozoic or Permo‐Triassic rocks. The unconformity surfaces represent parts of the walls and floors of Neogene fjords. For these surfaces to have been eroded, the ice must have been grounded out as far as the continental shelf in Prydz Bay. The Pagodroma Group was deposited by wet‐based glaciers discharging into a fjordal setting and includes lithofacies that are quite different from those produced by modern Antarctic ice masses. The principal lithofacies are massive diamicts and soulder gravels, deposited both close to a calving, grounded glacier terminus and from icebergs. The few stratified diamicts are the product of more distal iceberg sedimentation. An ice‐transported gravel lithofacies includes rockfall debris derived from palaeofjord walls and mixed with subglacially derived diamicts. Some lithofacies contain evidence of subaquatic slumping and gravity flowage. Volumetrically minor lithofacies include laminites, with some exposures exhibiting large ice‐rafted clasts. The laminites represent less proximal, mainly ice‐free fjordal sediments, resulting either from tidal‐current sorting of suspended sediment originating from subaquatic glaciofluvial discharge, or from turbidity currents derived from unstable subaquatically deposited glacigenic sediment. The Pagodroma Group provides a record of multiple glaciation by dynamic, sliding glaciers carrying large amounts of both basal and supraglacial debris. The closest modern analogues, in terms of the thermal and dynamic characteristics of the Neogene Lambert Glacier, appear to be the fast‐flowing tidewater glaciers of East Greenland. These glaciers originate from the interior ice sheet and discharge large volumes of icebergs; the resulting lithofacies are predominantly diamicts.
A comparative study of Pleistocene phosphorites from the continental slope off western India
Authors: Purnachandra Rao V. , Naqvi S. W. A. , Dileep Kumar M. , Cardinal D. , Michard A. , Borole D. V. , Jacobs E. , Natarajan R.
Published: March 2003

Two types of phosphorite recovered from the continental slope off western India are described. The first type, phosphorite 1, comprises a hard, grey nodule composed of carbonate fluorapatite (CFA) and calcite as major minerals. The phosphorite consists of light‐brown microcrystalline apatite containing a few skeletal fragments and planktonic foraminifera. Scanning electron microscope (SEM) studies show evidence of dissolution of skeletal calcite and filling of the resulting cavities by phosphate composed of ovoid to rod‐shaped apatite microparticles. Apatite also occurs as coatings on these particles. The P2O5 content of the phosphorite is 29%, and the CO2 content of the CFA is about 4·5%. The rare‐earth element (REE) abundance (ΣREE=2·02 μg g–1) is lower than in other modern phosphorites. The 87Sr/86Sr ratio and ɛNd value of this sample are 0·70921 and –9·9 respectively. The 14C age found through accelerator mass spectrometry (AMS) dating (18 720 ± 120 years BP) is much younger than that determined by the U‐series method (100 ka). The second type, phosphorite 2, comprises a friable, light‐brown nodule consisting of CFA as the only major mineral, with a CO2 content of the CFA of 4·5%. In thin section, the phosphate is light brown and homogeneous, and a few bone fragments are present. The P2O5 content is 33%, and REE contents (ΣREE = 0·18 μg g–1) are lower than in phosphorite 1. The age of phosphorite 2 is >300 ka. Phosphorite 1 appears to have formed during the late Pleistocene through replacement of carbonate by phosphate; phosphorite 2 is also of Pleistocene age but is much older than phosphorite 1. The initial substrate for phosphorite 2 was a fish coprolite, which was subsequently phosphatized during slow sedimentation under low‐energy conditions. Microbial mediation is evident in both phosphorites. The colour, density and P2O5 content of the phosphorites are found to be dependent on the nature of the initial substrates and physico‐chemical conditions during phosphatization. The CO2 content of the CFA is not related to the precursor carbonate phase. The nature of sediments, rates of sedimentation and the time spent undergoing phosphogenesis at the sediment–water interface may control REE concentrations in phosphorites.
Open‐coast intertidal deposits and the preservation potential of individual laminae: a case study from east‐central China
Authors: Li Congxian , Wang Ping , Daidu Fan , Bing Dang , Tiesong Li
Published: March 2003

Monitoring of sedimentation and erosion was conducted on an open coastal tidal flat on the southern flank of the Yangtze delta. Various elevation references were established in the intertidal zone and monitored intensively for 4 months in order to examine fortnightly and seasonal (calm weather and storm season) sedimentation and erosion. Longer term (100 years) sedimentation and preservation were investigated through examination of cores and trenches. Two different vertical grouping patterns of tidal bedding were distinguished with thinner and thicker sandy laminae. The number of sand‐dominated layers and individual muddy and sandy lamina in the cores were compared with theoretically derived sedimentation rates in order to assess long‐term preservation potential. Waves, especially high storm waves, have a significant influence on sedimentation and the preservation of intertidal deposits along the open‐coast tidal flat. Monitoring during one season indicated that the sand‐dominated layer was directly related to storm deposits, while the mud‐dominated layer was deposited during calm weather conditions. The variation in sandy lamina thickness was not related to spring–neap tidal cycles during the monitoring period. The assumption of 100% preservation of sandy laminae deposited during every tidal cycle, which has been assumed in previous time‐series analyses for the identification of palaeotidal periodicity, was found to be unrealistic along this open‐coast tidal flat. Preservation potential decreases as temporal scale increases. During one neap–spring tidal cycle, the preservation potential of individual sandy and muddy laminae was of the order of 10%. Over a period of 100 years, the estimated preservation potential of individual laminae, including both calm weather and storm deposits, decreased to 0·2%. The 100‐year preservation potential of storm‐induced, sand‐dominated layers was estimated to be of the order of 10%.
Evaporites and siliciclastics of the Permian Nippewalla Group of Kansas, USA: a case for non‐marine deposition in saline lakes and saline pans
Authors: Benison Kathleen Counter , Goldstein Robert H.
Published: March 2003

The mid‐Permian Nippewalla Group of Kansas consists of bedded evaporites, red‐bed siliciclastics and grey siliciclastics deposited in a non‐marine environment. Lithologies and sedimentary features indicate lacustrine and aeolian deposition, subaerial exposure and palaeosol formation. Grey siliciclastic mudstones characterized by planar and convolute laminations, ostracods, peloids and plant material represent a freshwater‐brackish perennial lake facies. Bedded anhydrites containing gypsum‐crystal pseudomorphs, clastic anhydrite grains and grey mud drapes and partings suggest deposition in saline lakes. Bedded halites consist of chevron and cumulate crystals, dissolution surfaces and pipes and mudcracked microcrystalline salt crusts, which were deposited in saline pans dominated by flooding, evaporative concentration and desiccation. Chaotic halite, composed of red‐bed mudstone and siltstone with displacive halite crystals, formed in saline mudflats. Red‐bed mudstone and siltstone with little or no displacive halite, but with abundant cracking, root and plant features, suggest deposition in a dry mudflat. Red‐bed sandstone, composed of well‐sorted, well‐rounded quartz grains cemented with halite, indicate aeolian and rare shallow‐water deposition. Most deposition took place in halite‐dominated ephemeral saline lakes surrounded by saline and dry mudflats, sandflats and sand dunes. Evaporation, desiccation, flooding and wind played significant roles in this environment. The Nippewalla Group siliciclastics and evaporites represent an evolution from a perennial lacustrine system to a non‐marine, acidic saline pan system in the mid‐continent of North America. The problem of distinguishing between ancient marine and non‐marine evaporites, as well as recognizing those evaporites deposited in acid settings, with detailed field, core and petrographical study of both evaporite deposits and associated sedimentary rocks has successfully been addressed. In addition, interpretations of mid‐Permian palaeoclimate data in the form of short‐term air temperature proxies within longer‐term wet–dry trends have been made. These data provide a new palaeogeographic and palaeoclimatic model for the mid‐Permian of western Pangaea.
Gypsum facies transitions in basin‐marginal evaporites: middle Miocene (Badenian) of west Ukraine
Authors: Marek Peryt Tadeusz
Published: March 2003

In the middle Miocene Badenian gypsum basin of the Carpathian Foredeep, west Ukraine, three main zones of gypsum development occur in the peripheral parts of the basin. Zone I consists entirely of stromatolitic gypsum formed in a nearshore zone. Zone II is located more basinward and is characterized by stromatolitic gypsum in the lower part of the section, overlain by a sabre gypsum unit. Zone III occurs in still more basinward areas and is characterized by giant gypsum intergrowths (or secondary nodular gypsum pseudomorphs of these) in the lowermost part, overlain by stromatolitic gypsum, sabre gypsum and then by clastic gypsum units. Correlation between these facies and zones has been achieved using lithological marker beds and surfaces. Of particular importance for correlation is a characteristic marker bed (usually 20–40 cm thick) of cryptocrystalline massive gypsum occurring in zones II and III. The marker was not distinguished in zone I, possibly because this bed is older than the entire gypsum section of that zone. These new results strongly suggest that the deposition of giant gypsum intergrowth facies and stromatolitic gypsum facies was coeval. In some sections of zones I and II, limestone intercalations have been recorded within the upper part of the gypsum sections. Considerable scatter of the δ18O and δ13C values of these limestones indicates variable diagenetic overprints of marine carbonates, but a marine provenance of the limestones is confirmed by microfacies analysis. Some of the limestones are coeval with an intercalation of gypsarenitic, mostly laminated gypsum occurring in the sabre gypsum unit of zones II and III. Badenian gypsum formed in extremely shallow‐water to subaerial environments on broad, very low relief areas of negligible brine depth, which could be affected by rapid transgressions. Stable isotope (δ34S, δ18O) studies of the gypsum demonstrate that the sulphate was of sea‐water origin or was derived from dissolution of Miocene marine evaporites. Investigations of individual inclusions in the gypsum indicate decreased water salinity when compared with modern marine‐derived, calcium sulphate‐saturated water. Groundwater influences are indicated by high calcium sulphate contents of the brines in the evaporite basin. The chemical composition of Badenian waters was thus a mixture of relic sea water (depleted in NaCl), groundwater (enriched in calcium sulphate) and surface run‐off.
Flow structure of turbidity currents
Authors: Felix M.
Published: March 2003

A two‐dimensional numerical model is used to describe the flow structure of turbidity currents in a vertical plane. To test the accuracy of the model, it is applied to historical flows in Bute Inlet and the Grand Banks flow. The two‐dimensional spatial and temporal distributions of velocity and sediment concentration and non‐dimensionalized vertical profiles of velocity, turbulent kinetic energy and sediment concentration are discussed for several simple computational currents. The flows show a clear interaction between velocity, turbulence and sediment distribution. The results of the numerical tests show that flows with fine‐grained sediment have low vertical and high horizontal gradients of velocity and sediment concentration, show little increase in flow thickness and decelerate slowly. Steadiness and uniformity in these flows are comparable for velocity and concentration. In contrast, flows with coarse‐grained sediment have high vertical and low horizontal velocity gradients and high horizontal concentration gradients. These flows grow considerably in thickness and decelerate rapidly. Steadiness and uniformity in flows with coarse‐grained sediment are different for velocity and concentration. The results show the influence of spatial and temporal flow structure on flow duration and sediment transport.
Hoanib River flood deposits of Namib Desert interdunes asanalogues for thin permeability barrier mudstone layers inaeolianite reservoirs
Authors: Stanistreet Ian G. , Stollhofen Harald
Published: March 2003

ABSTRACT The ephemeral braided Hoanib River of NW Namibia flows for a few days a year, and only high discharges enable the river to pass through interdunal depressions within the northern Namib Desert dune field to the Atlantic. The dune field comprises mainly large transverse dunes resulting from predominant SSW winds. River flood deposits between aeolian dunes are analogous to mudstone layers conformably interbedded with ancient aeolianite dune foresets. Deep floods pond laterally to considerable depths (metres to >10 m) in adjacent interdunes, depositing mud layers (1–50 cm) to considerable heights on avalanche and stoss faces of bounding dunes. Fairly passive flooding only disturbs aeolian stratification minimally. Floodwater clay infiltrates and settles as an impermeable seal, with a flood pond on top, perched, above regional groundwater. Flood ponds evaporate slowly for long periods (>3 years). Early emergence desiccates higher parts of a mud layer. Subsequent floods can refill a predecessor pond, benefiting from the existing impervious seal. Potential preservation of such mud layers is lower on the stoss face, but high on the avalanche face after burial by subsequent dune reactivation and migration. The leeward (right) Hoanib bank, a dune stoss face, is river and wind eroded to exhume fossil interdune pond mud layers of an earlier Hoanib channel. The highly inclined layers are interbedded with dune avalanche foresets and represent the edges of two successive fossil ponds exposed in plan. Ancient flood pond mudstones occur in the Permian–Triassic hydrocarbon reservoir, the Sherwood Sandstone Group of the Cheshire Basin (Kinnerton Formation) and Irish Sea Basin and were previously used erroneously to argue against the aeolian origin of cross‐bed sets. Hoanib studies show that primary river interaction with a dune field might preserve only localized erosional omission surfaces in ancient aeolianites, with little sandy barform preservation, prone to aeolian reworking. Around the main fluvial channel locus, however, flood pond mudstone layers should form a predictable halo, within which fluid permeability will decrease.
Lithofacies modelling and sequence stratigraphy in microtidal cool‐water carbonates: a case study from the Pleistocene of Sicily, Italy
Authors: Pedley Martyn , Grasso Mario
Published: March 2003

ABSTRACT Quaternary carbonates in SE Sicily were deposited in seamount and short ramp settings during glacio‐eustatically driven highstand conditions. They provide an excellent opportunity to investigate the depositional and erosional aspects of cool‐water carbonate sedimentation in a microtidal marine water body. The derived ramp facies model differs significantly from modern‐day, open‐ocean ramp scenarios in projected facies depth ranges and in the preservation of inshore facies. A sequence stratigraphic study of the carbonates has confirmed many established aspects of carbonate sedimentation (e.g. production usually only occurred during highstands). It has also revealed several new features peculiar to water bodies with little tidal influence, including ‘catch‐up’ surfaces taking the place of transgressive facies, second‐order sequence boundary events being most important as triggers for initiating resedimentation and a virtual absence of sediment shedding to the basin during the terminal lowstand. Production in the carbonate factory lasted for about 0·5 Myr. Despite this, carbonate production was considerable and included both bioconstructional and bioclastic‐dominated facies and the production of abundant lime muds. A model for eustatically controlled cool‐water carbonate production and resedimentation in microtidal marine water bodies is presented. This is considered to be more applicable to Neogene and Quaternary strata in the Mediterranean region than are current open‐ocean models.
Avulsions, channel evolution and floodplain sedimentation rates of the anastomosing upper Columbia River, British Columbia, Canada
Authors: Makaske Bart , Smith Derald G. , Berendsen Henk J. A.
Published: March 2003

Ages of channels of the anastomosing upper Columbia River, south‐eastern British Columbia, Canada, were investigated in a cross‐valley transect by 14C dating of subsurface floodplain organic material from beneath levees. The avulsion history within the transect was deduced from these data, and morphological stages in channel development were recognized. Additionally, floodplain sedimentation rates were established. The new data demonstrate that the upper Columbia River is a long‐lived, dynamic anastomosing system. Results show that anastomosis at the study location has persisted since at least 2700 cal. years BP, with avulsions occurring frequently. At least nine channels have formed in the studied cross‐valley transect within the past 3000 years. Channel lifetimes from formation to abandonment appear to be highly variable, ranging from approximately 800 to 3000 years. Log jams provoking avulsions and/or silting up of old channels are proposed as reasons for this variability. Long‐term average floodplain sedimentation rates appear to be significantly lower than previously proposed by Smith (1983, Int. Assoc. Sedimentol. Spec. Publ., 6, 155–168). A long‐term (4550 years) average of 1·75 mm year−1 (after compaction) was based on 14C dates, while a short‐term sedimentation rate of 0·8 mm was determined for a single, relatively small, seasonal flood in 1994 using sediment traps. However, short‐term sedimentation rates vary considerably over the floodplain, with levees aggrading up to four times faster than floodbasins. Channels of the upper Columbia River anastomosed reach follow a consistent pattern in their development, with each stage being characterized by different morphology and processes. Channel evolution comprises the following succession: (1) avulsion stage, in which a crevasse splay channel deepens by scour and levee sedimentation; (2) widening and deepening stage, in which bank slumping and bed scouring dominates; (3) infilling stage, in which either channel narrowing (bank accretion) or channel shallowing (bed accretion) takes place; and (4) abandonment stage, in which the residual (remnant) channel is filled exclusively by silt, clay and organic material. Vertical stacking (super‐ imposition) of active channels on recent channel‐fill sand bodies is a notable feature of the upper Columbia River, which suggests that reoccupation of residual channels is a common process.
Sedimentological and palaeohydrological responses to tectonics and climate in a small, closed, lacustrine system: Oligocene As Pontes Basin (Spain)
Authors: Sáez A. , Cabrera L.
Published: March 2003

ABSTRACT A small, closed, lacustrine system developed during the restraining overstep stages of the Oligocene As Pontes strike‐slip basin (Spain). The increase in basin accommodation and the headward spread of the drainage, which increased the water input, triggered a change from shallow, holomictic to deeper, meromictic conditions. The lower, shallow, lacustrine assemblage consists of mudstone–carbonate cycles recording lacustrine–palustrine ramp deposition in a saline lake. High Sr content in some early diagenetic calcites suggests that aragonite and calcite made up the primary carbonate muds. Early dolomitization took place together with widespread pedogenic activity. The upper, deep, freshwater, lacustrine assemblage includes bundles of carbonate–clay rhythmites and fine‐grained turbidite beds. Primary calcite and diagenetic siderite make up the carbonate laminae. The Mg content of the primary carbonates records variations in Mg/Ca ratios in lacustrine waters. δ18O and δ13C covariance trends in calcite reinforce closed drainage conditions. δ18O data indicate that the lake system changed rapidly from short‐lived isotopically light periods (i.e. from seasonal to pluriannual) to longer steady‐state periods of heavier δ18O (i.e. from pluriannual to millennial). The small δ13C changes in the covariant trends were caused by dilute inflow, changing the contributions of dissolved organic carbon in the system and/or internal variations in lacustrine organic productivity and recycling. In both shallow and deep carbonate facies, sulphate reduction and methanogenesis may account, respectively, for the larger negative and positive δ13C shifts recorded in the early diagenetic carbonates (calcite, dolomite and siderite). The lacustrine system was very susceptible to high‐frequency, climatically forced water balance variations. These climatic oscillations interfered with the low‐frequency tectonic and morphological changes in the basin catchment. This resulted in the superposition of high‐order depositional, mineralogical and geochemical cycles and rhythms on the lower order lacustrine infill sequence.
A consideration of the dune:antidune transition in fine gravel
Authors: Carling Paul A. , Shvidchenko Andrey B.
Published: March 2003

Hydraulic data defining the dune:antidune transition in fine gravel are compared with potential flow theory, and information is drawn from published experiments and field‐based studies. Attention is given to both transitional bedforms and the development of downstream‐migrating antidunes. In the latter case, most data pertain to sand beds and not to gravel. Empirical data provide some weak support for the theoretical notion that the transition occurs at progressively lower Froude numbers at greater relative depths. Although a critical Froude number of 0·84 may reasonably be applied for the beginning of the dune to antidune transformation, lag effects (and a possible depth limitation) ensure that transitional bedforms may persist across a broad range of Froude numbers from 0·5 to 1·8. This latter observation has great relevance for palaeohydraulic estimates derived from outcrop data. Whereas the application of theoretical bedform existence fields, based upon potential flow theory, to fine gravel was previously purely speculative, the addition of experimental and field data to these plots provides a degree of confidence in applying stability theory to practical geological problems. For the first time, laboratory data pertaining to downstream‐migrating gravel antidunes are compared with theory. These bedforms have been reported from certain experimental near‐critical flows above sand or gravel beds, but have been observed infrequently in natural streams. However, there are no detailed studies from natural rivers and only a few contentious identifications from outcrops. Nevertheless, the limited hydraulic data conform to theoretical expectations.
Longitudinal profile evolution of the Rhine–Meuse system during the last deglaciation: interplay of climate change and glacio‐eustasy?
Authors: Törnqvist
Published: March 2003

Fluvial longitudinal profiles reconstructed from abandoned floodplains contain significant evidence about the role of relative sea level vs. climatic and tectonic controls on depositional systems. Two Weichselian floodplain surfaces that occur as terraces updip of the hinge zone of the Rhine–Meuse system have recently been mapped beneath the Holocene Rhine–Meuse Delta (The Netherlands). Their vertical offset is several metres in the upstream area and decreases to only 0.4 m in the central part of the delta. The older and higher of the two floodplain surfaces is generally assumed to have been formed around the Last Glacial Maximum, whereas the younger dates to the Younger Dryas, following a phase of climatically induced fluvial incision during the Bølling‐Allerød. The downstream convergence of these two floodplain surfaces may be related to the relative rise of sea level, forcing the Rhine–Meuse system to become graded to a higher base level during the Younger Dryas. The upper Weichselian Rhine–Meuse system then provides an example of a basin‐marginal fluvial system that responds, in terms of its longitudinal profile, to the combined effects of upstream control (primarily climate change affecting water and sediment flux from the hinterland) and downstream control (glacio‐eustatically driven relative sea‐level change). This new evidence may therefore revitalize the presently unfashionable concept of relative sea‐level control penetrating many hundreds of kilometres inland.
Palaeomagnetic evidence for late Miocene rotation of the Swiss Alps: results from the north Alpine foreland basin
Authors: Kempf  , Schlunegger  , Strunck  , Matter
Published: March 2003

Magnetostratigraphic studies in the Oligocene to Miocene north Alpine foreland basin of Switzerland suggest a post‐middle Miocene (< 13 Ma) clockwise rotation of the Swiss Alps. The angle of rotation is 16–17° with respect to the present‐day earth's magnetic field. This rotation can be observed in 12 sections analysed for palaeomagnetic directions which cover a lateral distance of ≈ 250 km (SW–NE extension). The rotation angle shows neither a significant change throughout the examined period of deposition, nor is it dependent on the tectonic position of the individual regions in the basin (autochthonous or allochthonous Molasse).
Validated surface mineralogy from high‐spectral resolution remote sensing: a review and a novel approach applied to gold exploration using AVIRIS data
Authors: Van Der Meer  , Bakker
Published: March 2003

Remote sensing’s main limitation to date was that geological surface information lacked detail due to the broad bandwidth of the operational scanners. Imaging spectrometers now allow acquisition of image data in many contiguous spectral bands, with the aim of producing laboratory‐like reflectance spectra for each pixel in the scene which can be directly compared with spectra of known materials measured in the field or laboratory to allow mineral identification on the Earth surface. The advent of satellite‐based imaging spectrometer systems justifies the surge for techniques that allow the automatic preparation of validated surface mineralogy maps. We show an extension of a procedure known as cross‐correlogram spectral matching to retrieve accuracy assessed surface mineralogy from Airborne Visible/InfraRed Imaging Spectrometer data (AVIRIS). The procedure is based on the spectral cross‐correlation of known mineral spectra with unknown pixel spectra from AVIRIS. The validation is performed through a root‐mean square calculation of expected correlograms and pixel correlograms. Results obtained in the study outlined allow an accurate delineation of zones of hydrothermal alteration in an area of active gold exploration.
Soft‐sediment deformation structures induced by cyclic stress of storm waves in tempestites (Miocene, Guadalquivir Basin, Spain)
Authors: Molina  , Alfaro  , Moretti  , Soria
Published: March 2003

The identification of triggering agents for soft‐sedimentation structures is an enigmatic geological problem. Mainly seismic‐induced soft‐sediment structures have been recognized in ancient sediments, rather than those resulting from storm waves. We analyse soft‐sediment deformation structures in Upper Miocene calcarenitic tempestites of the Guadalquivir Basin (Southern Spain). The most common structures are load‐casts which vary in height and width from 10 centimetres to several metres. The structures are always restricted to a small part of the stratigraphic sections, in exclusive association with tempestites. The analysed soft‐deformation structures are interpreted to be the result of liquidization processes. Chiefly from their inferred depositional environment, and subordinately from the deformation style, we have devised basic criteria to identify the trigger mechanism. In these soft‐sediment deformation structures the liquefaction was triggered by pore pressure changes induced by cyclic and residual stress of storm waves.
An early Miocene age for a high‐temperature event in gneisses from Zabargad Island (Red Sea, Egypt): mantle diapirism?
Authors: Bosch  , Bruguier
Published: March 2003

U–Pb zircon data from a felsic gneiss located at the contact zone with the central peridotite body of Zabargad Island (Red Sea, Egypt) provide an age of 23.2 ± 5.9 Myr consistent with the 238U–206Pb age of the youngest concordant grain (22.4 ± 1.3 Myr). Concordant grains indicate new zircon growth and/or resetting whereas slightly discordant analyses suggest participation of an older zircon component whose age cannot be defined precisely. SEM back‐scattered imaging further reveals the occurrence of zoned domains almost completely erased by complex internal structures attributed to extensive recrystallization under metamorphic conditions. The 22.4 Myr Miocene age is thus interpreted as dating a high‐temperature metamorphic event. The proximity between the studied sample and the peridotite supports previous conclusions which regard parts of the peridotites from Zabargad Island as an asthenopheric mantle diapir which intruded the thinned Pan‐African continental crust during the early stages of the Red Sea opening.
Detrital pyrite in Witwatersrand gold reefs: X‐ray diffraction evidence and implications for atmospheric evolution
Authors: Fleet
Published: March 2003

Pyrite is easily oxidized, and therefore unambiguous evidence of detrital pyrite grains in metasediments is a significant constraint on when an oxygenated atmosphere developed. Compact rounded pyrite in the Witwatersrand gold reefs of South Africa has a detrital habit and is texturally equivalent to and spatially associated with detrital zircon and chromite. X‐ray precession photography reveals that petrographically featureless As‐poor grains are untwinned single crystals of high diffraction quality. This new evidence from crystallography is consistent with mechanically abraded pyrite from primary lode gold deposits, and excludes an origin by replacement of a pre‐existing detrital phase. Further evidence of a detrital origin for the compact rounded pyrite is afforded by isolated grains of arsenian pyrite displaying truncated As‐rich growth bands. The geographically extensive Witwatersrand fluvial conglomerates evidently had a matrix of quartz and pyrite sand and pyritic mud in their unconsolidated state and, thus, the late Archean atmosphere of Earth was likely essentially anoxic.
Concept and styles of reciprocal stratigraphies: Western Canada foreland system
Authors: Catuneanu  , Sweet  , Miall
Published: March 2003

The late Cretaceous–Palaeocene Western Canada foreland system provides a record of high‐frequency cycles of reciprocal flexural tectonics superimposed on longer term changing dynamic subsidence. Initial dominance of dynamic subsidence during the deposition of the Bearpaw marine succession resulted in continual subsidence with differential rates across the flexural hinge line. Subsequent dominance of flexural tectonics resulted in opposite base‐level changes between the proximal and distal reaches of the foreland system during the deposition of the post‐Bearpaw nonmarine succession. In both cases, the contrasting base‐level changes generated out‐of‐phase stratigraphic sequences, which defines the concept of reciprocal stratigraphies. Two styles of reciprocal stratigraphies have been identified in relation to the pattern of base‐level changes across the foreland system. The Bearpaw style consists of a conformable succession of correlative transgressive and regressive systems tracts, suggesting continuous basin‐wide sedimentation with the rates within the range of variation of the rates of base‐level rise. The post‐Bearpaw style shows sequences correlative to age‐equivalent sequence boundaries related to coeval rising and falling base‐level, respectively. The succession of Bearpaw and post‐Bearpaw sequences corresponds to a cycle of marine to nonmarine foreland sedimentation controlled by changing ratios between dynamic subsidence and flexural tectonics. The amount of sediment supply derived from the orogen to the foreland system may also reflect the rates of dynamic subsidence, as a decrease in dynamic loading may lead to accelerated denudation of the sediment source areas.
From extension to contraction in syn‐orogenic foredeep basins: the Contessa section, Umbria‐Marche Apennines, Italy
Authors: Tavarnelli E. , Peacock D.C.P.
Published: March 2003

Syn‐orogenic deposits that occupy foredeep basins commonly experience contraction related to the migration of fold‐and‐thrust systems toward the foreland. This contraction may overprint the earlier extensional deformation that is related to the initiation of the basin. Although predicted by models for foredeep development, evidence for extension predating contraction at different scales is not extensively recorded in syn‐orogenic deposits. Mesoscopic structures from the Contessa section, in the Umbria‐Marche Apennines, Italy, reveal a complex history, characterized by extension soon before the contractional deformation. Normal faults predate the folds and thrusts that are related to the Miocene‐age orogenic event responsible for the development of the Apennines. Extensional deformation may have resulted from flexuring of a lithospheric plate induced by the load of a stacking thrust pile. The transition from extension to contraction could play an important role in the evolution of belt–foredeep–foreland systems, as it could reflect the migration of advancing thrust fronts toward the foreland.
Tectonics of the Dabieshan (eastern China) and possible exhumation mechanism of ultra high‐pressure rocks
Authors: Faure M. , Lin W. , Shu L. , Sun Y. , Schärer U.
Published: March 2003

The Dabieshan is divided into three tectonic domains. The Southern Dabieshan is a stack of allochtons, from top to bottom: (i) unmetamorphosed Cambrian–Early Triassic series unconformably covered by Jurassic sandstone; (ii) weakly metamorphosed Proterozoic slate; (iii) HP rocks mostly retrogressed into greenschist facies; (iv) UHP allochton; (v) UHP‐free gneisses. These units exhibit a NW–SE lineation and top‐to‐the‐NW shearing reworked by S‐verging folds. The Central Dabieshan is a migmatitic dome superimposed on UHP metamorphism and bounded by a detachement fault responsible for the exhumation of the UHP unit during its retrogression into amphibolite facies. In the Northern Dabieshan, early foliation and N–S trending compressional lineation are deformed by N‐verging folds coeval to the syn‐exhumation ductile structures of the Central Dabieshan. A geodynamic model involving coeval thrusting and normal faulting is discussed.
Early Ordovician terrane accretion along the Gondwanian margin of the East Antarctic Craton: new Pb/Pb titanite ages from the Tonalite Belt, North Victoria Land, Antarctica
Authors: Musumeci  , Kramers  , Pertusati
Published: March 2003

Early Palaeozoic subduction of the palaeo‐Pacific plate and terrane accretion along the palaeomargin of the East Antarctic Craton is well‐documented in North Victoria Land, where the Tonalite Belt is a complex of synkinematic intrusions emplaced within the Lanterman–Murchison Shear Zone at the boundary between the Wilson Terrane and the allochthonous Bowers Terrane. Stepwise leaching Pb/Pb and U–Pb studies of titanite separates carried out on two well‐foliated samples of tonalites yielded ages of deformation bracketed between 490 and 480 Ma with an isochron age of 480 ±13 Myr. Ar/Ar and K–Ar ages of 477 Myr in the metamorphic rocks of accreted terranes point to fast cooling and uplift after accretion. The new titanite ages, compared with a regional distribution of magmatic and metamorphic ages, indicate an early Ordovician age for terrane collision and amalgamation. As a consequence of collision, subduction shifted to an outward position along the palaeomargin of the East Antarctic Craton.
Space accommodation by roof lifting during pluton emplacement at Amiata (Italy)
Authors: Acocella V.
Published: March 2003

The Amiata area is characterized by an uplift attributed to pluton emplacement during the Pliocene epoch. The shape of the buried pluton is constrained by gravity data. Fieldwork was carried out to identify possible deformations within the overburden, in order to evaluate the mechanism of emplacement. The attitudes of Lower Pliocene marine sediments, deposited immediately before the uplift, were measured also. The overall pattern of the deposits defines a dome, approximately 2.5 km high and 35 × 50 km wide. Consistency between these field data and the existing geophysical data suggests that the dome results from the emplacement of a laccolith‐like intrusion which was responsible for lifting and flexuring the overburden. The Amiata case shows that roof lifting is a viable process by which space in the upper crust, to a depth of 7 km, can be accommodated during pluton emplacement.
The Hercynian orogeny in the South Armorican Massif (Saint‐Georges‐sur‐Loire Unit, Ligerian Domain, France): rifting and welding of continental stripes
Authors: Cartier C. , Faure M. , Lardeux H.
Published: March 2003

A new approach to explain the Western Hercynian Belt is given by a lithological and structural study of the St‐Georges‐sur‐Loire Unit (southern part of the French Armorican Massif). This unit is interpreted as a Devonian olistostrome sheared to the North. It is formed in an accretionary prism linked with a south‐dipping subduction zone, called the Layon subduction zone. It results from a chaotic sedimentation including exotic blocks of various lithologies and ages in a Middle Devonian pebbly mudstone matrix. An inovative geodynamic model involving microblock rifting and collision is discussed to explain the presence of the St‐Georges‐sur‐Loire Unit in the Armorican Massif. This model involves two Palaeozoic subductions and emphasizes the role of the Nort‐sur‐Erdre fault, considered as a major polyphase suture zone separating South and Central Armorica.
Benthic foraminifera from the upper Collio Formation (Lower Permian, Lombardy Southern Alps): implications for the palaeogeography of the peri‐Tethyan area
Authors: Sciunnach Dario
Published: March 2003

New palaeontological evidence points to a temporary marine transgression in the Early Permian into the Collio Basin, a major palaeogeographic feature of the present‐day Southern Alps. The thick volcaniclastic succession filling the basin (Collio Formation) is widely held as deposited in alluvial to lacustrine settings. Rare calcareous foraminifers were recently found in a single sandstone interval, containing phosphate nodules, from the uppermost Collio Formation. A temporary seaway, necessary for the foraminifera to spread into a continental basin, implies that (i) the Collio Basin lake was not only an intramontane (as commonly viewed), but also a coastal lake, and (ii) its altitude did not exceed the amplitude of a first‐order sea‐level rise, that is, about 100 m. These constraints, along with striking similarities as to tectonic context, accumulation rates and geochemical signature, suggest that the Collio Basin was a California‐type basin, resembling in particular the present‐day Salton Sea (CA, USA).
Hercynian compressional deformations of the Ahnet–Mouydir Basin, Algerian Saharan Platform: far‐field stress effects of the Late Palaeozoic orogeny
Authors: Haddoum H. , Guiraud R. , Moussine‐Pouchkine A.
Published: March 2003

N–S to NW–SE‐trending faults and reverse faults occur within the Palaeozoic Ahnet–Mouydir Basin of the Algerian Saharan Platform, located to the east of the West African Pan‐African suture zone. Deformation and stratigraphic analysis show that this basin underwent a NNE–SSW to ENE–WSW shortening at about the Carboniferous/Permian transition or, more probably, during the Early Permian. A brief review of the more or less synchronous deformations registered in the neighbouring regions, and more specifically around the West African Craton, illustrates the mechanical coupling between intraplate tectonics and the Hercynian plate margins orogenies.
Holocene extension direction along the Main Ethiopian Rift, East Africa
Authors: Acocella V. , Korme T.
Published: March 2003

Assessment of the extension direction and its spatial and temporal variations is critical for evaluating a rifting process. The extension direction in the East African Rift System is a matter of debate and the NE–SW‐trending Ethiopian portion is one of the most controversial areas, for which several extension directions have been proposed. Field analysis was performed along the axis of the Main Ethiopian Rift (MER), aiming at recognizing the opening direction of the NNE–SSW‐trending Holocene extension fractures. The matching of pairs of asperities along the sides of these fractures allowed evaluation of the horizontal displacement and, thus, the extension direction. The collected data reveal a consistent Holocene extension direction, with a mean value of N52°W ± 20°. This NW–SE direction is constant along the MER, despite the ∼15° variations observed in its trend and the NNE–SSW trend of the extension fractures, oblique to the NE–SW trend of the MER.
Radiolarian biochronology of Mesozoic deep‐water successions in NW Syria and Cyprus: implications for south‐Tethyan evolution
Authors: Al‐Riyami Khalil K. , Danelian Taniel , Robertson Alastair H. F.
Published: March 2003

ABSTRACT New radiolarian biostratigraphical data have shed light on the Mesozoic tectonic evolution of South‐Tethys in the Baer–Bassit region of NW Syria. Radiolarian assemblages of Late Triassic, Middle Jurassic and Early Cretaceous age were extracted from radiolarites in five measured sections. The results are compared with published radiolarian ages from the Mamonia Complex, western Cyprus. These two areas are interpreted as preserved fragments of the conjugate margins of a small South Tethyan oceanic basin formed by Triassic rifting. In the southerly (i.e. Arabian) margin, proximal successions were dominated by shallow‐water‐derived carbonate, whereas distal successions reveal seamount‐type alkaline/peralkaline volcanism, dated as both Late Triassic and Middle Jurassic–Early Cretaceous. Along the inferred northern margin (i.e. western Cyprus) proximal successions are dominantly terrigenous, whereas distal settings include Late Triassic oceanic crust and seamount‐type lavas.