bluuebunny/crossref_metadata_2025_split huggingface dataset:
Calcite from the Quaternary spring waters at Tylicz, Krynica, Polish Carpathians
Authors: KOSTECKA ALEKSANDRA 
Published: June 2006

ABSTRACTAt Tylicz, near Krynica Spa (Polish Carpathians), spelean deposits fill fissures and caverns in Eocene flysch rocks. They occur as: (1) clastic cave sediments transformed into hard crusts due to cementation by finely crystalline low‐Mg calcite, (2) drusy calcite that covers crust surfaces and fills voids in the crust and (3) colloform calcite.Two varieties of drusy calcite are distinguished: acicular and columnar. The acicular calcite is built up of crystallites forming spherulitic fans or cones. In places it is syntaxially covered with colloform calcite. The drusy calcite is low‐Mg ferroan calcite with non‐ferroan subzones, whereas the colloform calcite is a low‐Mg non‐ferroan variety.The columnar calcite crystals form fan‐like bundles. Cross‐sections cut perpendicular to the c‐axes of columnar crystals are equilateral triangular in shape, although some have slightly curved edges. The columnar crystals have steep rhombic terminations and most have curved triangular faces, i.e. gothic‐arch calcite. Saddle crystals have also been observed.The columnar crystals are composed of radially orientated crystallites whose long dimension is parallel to the c‐axis. The curved crystal faces of such polycrystals are interpreted as a result of differential growth rates of the crystallites.The spelean calcites precipitated from CO2‐saturated water. The high rate of CaCO3 precipitation is thought to be responsible for the formation of radial structures.Finely crystalline calcite formed within pore spaces of clastic sediments close to the water‐air interface, drusy calcite crystallized beneath the water‐air interface, and colloform calcite precipitated from thin films of water.
Anomalous pebble size variation in an erosive, Late Archaean braided stream: the Middelvlei gold placer, Witwatersrand, South Africa
Authors: ELS B. G. 
Published: June 2006

ABSTRACTA characteristic of fluvial conglomerates is a gradual decrease in pebble size in the direction of the palaeocurrents. However, for the ancient braided stream investigated in this study, a deviation from this norm was found.The Middelvlei gold placer is a quartz pebble conglomerate that is mined in a South African goldfield, known as the West Wits Line, situated west of Johannesburg. Stratigraphically the placer occurs near the base of the Central Rand Group of the Witwatersrand Supergroup.In previous studies, the placer and the underlying sequence have been shown to be fluvial in origin, constituting a coarsening upward genetic increment of strata. The complete conformable succession, however, is only preserved in the distal parts of the depositional environment. In most parts of the goldfield the placer is separated from the underlying sequence by a progressive unconformity, which disappears deeper into the basin. This unconformity is thought to have formed during a progradation.Palaeocurrents of the Middelvlei placer are towards the south‐southeast in the western parts of the study area and towards the south‐east in the east. Maximum pebble size, however, decreases towards the west‐southwest, that is in a direction roughly perpendicular to the palaeocurrents.A reconstruction of the palaeotopography of the base of the placer in the western parts of the study area revealed considerable variations in palaeoheight with a gradient of about 0·002 along depositional strike.The anomalous lateral pebble size variation of the placer is explained by postulating an eastward migration of the depositing braided stream, simultaneous with its progradation. During the eastward migration, erosion of the underlying sequence became progressively more severe. This created an eastward‐thinning wedge of underlying strata.The results of the study do not invalidate lateral pebble size variation as a regional palaeoslope indicator. However, the findings suggest that caution should be exercised in the application of this indicator.
Carboniferous calcretes in the Canadian Arctic
Authors: THERIAULT P. , DESROCHERS A. 
Published: June 2006

ABSTRACTCalcrete palaeosols have been found in the Upper Carboniferous Canyon Fiord Formation of southwestern Ellesmere Island, Canadian Arctic. These calcretes were developed in fluvial and shallow marine sediments that accumulated within two adjacent subbasins, in which the tectono‐sedimentary environment led to the deposition of five different sedimentary facies: (i) floodbasin sandstones; (ii) alluvial fan sandstones; (iii) alluvial fan conglomerates; (iv) braided fluvial sandstones; and (v) shallow marine limestones.Nodular/massive palaeosol profiles, consisting of cryptic, nodular, massive and laminar horizons, occur within the floodbasin sandstone and alluvial fan sandstone facies. Plugged palaeosol profiles, consisting of cryptic, plugged and laminar horizons, are restricted to the alluvial fan conglomerate facies. Massive/brecciated palaeosol profiles, consisting of cryptic, massive/brecciated and laminar horizons, occur essentially within the shallow marine limestone facies.The relationships between calcrete profiles and sedimentary facies suggest that profile types were controlled mainly by the texture and composition of the parent material: nodular/massive profiles are restricted to silicate‐rich sandstone hosts, plugged profiles are restricted to carbonate‐rich conglomerate hosts and massive/brecciated profiles are restricted to limestone hosts. Important relationships also exist between the maturity levels of nodular/massive profiles and sedimentary facies: profiles are generally mature in the floodbasin sandstones, invariably immature in the alluvial fan sandstones and absent from the braided fluvial sandstones. These different maturity levels were probably controlled mainly by exposure time, vegetation and substrate composition.
Sedimentology and isotope geochemistry of lacustrine carbonates of the Oligocene Campins Basin, north‐east Spain
Authors: ANADÓN P. , UTRILLA R. 
Published: June 2006

ABSTRACTThe non‐marine Campins Basin developed in the Oligocene, during a period of early rifting of the Catalan Coastal Ranges. Lacustrine deposits, interbedded between two alluvial units, comprise shallow and deep lacustrine facies. The lower, shallow lacustrine facies are made up of microbialite buildups and thin limestone beds. In the studied area, these facies are overlain by deep lacustrine facies which consist of alternations of several, metre‐thick carbonate‐ and mudstone‐dominated intervals. In addition to calcite, which is characteristic of the shallow lacustrine facies, aragonite and abundant dolomite are present in the deep lacustrine facies. This mineralogical change in the sequence reflects an overall increase in the Mg/Ca ratio of the lake waters. The deep lacustrine sequences are interpreted as having formed in a hydrologically closed basin that was subject to changes in the Mg/Ca ratio of the water, probably related to variations in the evaporation/precipitation rate.The sedimentological, mineralogical and isotopic characteristics of the Campins Basin dolomites suggest that, in general, they are primary in origin. The stable isotope data show an approximate covariance between δ13C and δ18O in the lower shallow lacustrine carbonates (calcite) which suggests that they formed during the onset of closure of the lake. The δ13C and δ18O values of the deep lacustrine carbonates display three different clusters that are roughly related to the carbonate mineralogy. Normalisation with respect to calcite of the isotopic compositions of dolomite and aragonite from the deep lacustrine carbonates allows the integration of all these isotope values into one covariant trend.The sequential appearance of different carbonate minerals and the isotopic covariant trend may indicate an overall evaporative concentration of the lake waters. The change in slope of the covariant trend for the isotope values between the shallow and the deep lacustrine carbonates might reflect the change in the waterbody morphology recorded in the basin fill sequence.
The Udo tuff cone, Cheju Island, South Korea: transformation of pyroclastic fall into debris fall and grain flow on a steep volcanic cone slope
Authors: SOHN YOUNG KWAN , CHOUGH SUNG KWUN 
Published: June 2006

ABSTRACTThe Udo tuff cone of Cheju Island, South Korea, is a middle Pleistocene basalt tuff cone that has formed by early Surtseyan‐type eruptions and later drier hydroclastic eruptions. The tuff cone comprises steep (20–30°) and planar beds of lapillistone, lapilli tuff and tuff that can be grouped into seven sedimentary facies (A‐G). Facies A and B comprise continuous to lenticular layers of grain‐supported and openwork lapillistone that are inversely graded and coarsen downslope. They suggest emplacement by grain flows that are maintained by gravity‐induced stress and grain collisions. Facies C includes poorly sorted, crudely bedded and locally inversely graded lapilli tuff, also suggestive of rapid deposition from highly concentrated grain flows. Facies D includes thinly stratified and mantle‐bedded tuff that was probably deposited by fallout of wind‐borne ash. Other facies include massive lapilli tuff (Facies E), chaotic lapilli tuff (Facies F) and cross‐bedded tuffaceous sandstone (Facies G) that were deposited by resedimentation processes such as debris flow, slide/slump and stream flow, respectively.The grain flows that produced Facies A, B and C are interpreted to have originated from falling pyroclasts, which initially generated highly dispersed, saltating avalanches, in which momentum was transferred by the particles themselves. This transport mechanism is similar to that of debris fall. As the slope gradient was too low to maintain a highly dispersed flow, the debris fall decelerated and contracted due to a decrease in dispersive pressure. The mode of momentum transfer changed to one of collision because contraction of the debris fall resulted in an increase in particle concentration. This transport mechanism is similar to that of common grain flows. Grain segregation occurred in several ways. Initial segregation of ash from lapilli occurred due to their differing terminal fall velocities, and their contrasting degrees of sliding friction with the bed. Percolation of ash into interstices of lapilli during flow (kinematic sieving) augmented further segregation of ash from lapilli. The latter process, along with a dispersive pressure effect, gave rise to vertical inverse size grading. Downdip inverse grading was produced by particle overpassing.
Shallow seismic stratigraphy and pockmarks of a hydrothermally influenced lake, Lake Rotoiti, New Zealand
Authors: PICKRILL R. A. 
Published: June 2006

ABSTRACTLake Rotoiti formed 11 850–20 000 years ago when lava flows dammed the drainage system through the Okataina caldera, one of the major rhyolitic centres of the intracontinental, back‐arc, Taupo Volcanic Zone of North Island, New Zealand. The lake morphology reflects this complex origin, with remnants of the old caldera rim, the lava dam and relict river system being preserved. High resolution seismic reflection profiles (7 kHz) show the drowned river system channelled inflowing water into the lake basin, scouring and depositing sediments and maintaining an active sublacustrine channel in the former river valley. Airfall tephras are preserved in lake sediments. The tephrachronology, established from the surrounding catchments, can be correlated with the siesmic stratigraphy for eight major eruptions over the last 11 850 years. Hydrothermal fields underlying the lake generate gaseous sediments which mask seismic penetration. This is indistinguishable from seismic masking caused by biogenic gas accumulation within the sediments. Venting of hydrothermal gases at the surface creates pockmarks. From the seismic stratigraphy beneath pockmarks seven stages of pockmark genesis have been identified which show pockmarks grow, decay, migrate and persist over several thousand years.
Sedimentology of subaqueous volcaniclastic sediment gravity flows in the Neogene Santa Maria Basin, California
Authors: COLE RONALD B. , STANLEY RICHARD G. 
Published: June 2006

ABSTRACTSubaqueous tuff deposits within the lower Miocene Lospe Formation of the Santa Maria Basin, California, are up to 20 m thick and were deposited by high density turbidity flows after large volumes of ash were supplied to the basin and remobilized. Tuff units in the Lospe Formation include a lower lithofacies assemblage of planar bedded tuff that grades upward into massive tuff, which in turn is overlain by an upper lithofacies assemblage of alternating thin bedded, coarse grained tuff beds and tuffaceous mudstone. The planar bedded tuff ranges from 0.3 to 3 m thick and contains 1‐8 cm thick beds that exhibit inverse grading, and low angle and planar laminations. The overlying massive tuff ranges from 1 to 10 m thick and includes large intraclasts of pumiceous tuff and stringers of pumice grains aligned parallel to bedding. The upper lithofacies assemblage of thin bedded tuff ranges from 0.4 to 3 m thick; individual beds are 6‐30 cm thick and display planar laminae and dewatering structures. Pumice is generally concentrated in the upper halves of beds in the thin bedded tuff interval.The association of sedimentary structures combined with semi‐quantitative analysis for dispersive and hydraulic equivalence of bubble‐wall vitric shards and pumice grains reveals that particles in the planar bedded lithofacies are in dispersive, not settling, equivalence. This suggests deposition under dispersive pressures in a tractive flow. Grains in the overlying massive tuff are more closely in settling equivalence as opposed to dispersive equivalence, which suggests rapid deposition from a suspended sediment load. The set of lithofacies that comprises the lower lithofacies assemblage of each of the Lospe Formation tuff units is analogous to those of traction carpets and subsequent suspension sedimentation deposits often attributed to high density turbidity flows. Grain distributions in the upper thin bedded lithofacies do not reveal a clear relation for dispersive or settling equivalence. This information, together with the association of sedimentary features in the thin bedded lithofacies, including dewatering structures, suggests a combination of tractive and liquefied flows.Absence of evidence for elevated emplacement temperatures (e.g. eutaxitic texture or shattered crystàls) suggests emplacement of the Lospe Formation tuff deposits in a cold state closely following pyroclastic eruptions. The tuff deposits are not only a result of primary volcanic processes which supplied the detritus, but also of processes which involved remobilization of unconsolidated ash as subaqueous sediment gravity flows. These deposits provide an opportunity to study the sedimentation processes that may occur during subaqueous volcaniclastic flows and demonstrate similarities with existing models for sediment gravity flow processes.
Late Quaternary sequence stratigraphy of Lake Malawi (Nyasa), Africa
Authors: SCHOLZ CHRISTOPHER A. , FINNEY BRUCE P. 
Published: June 2006

ABSTRACTHigh resolution seismic data, multichannel seismic data and sediment cores were used to examine the Songwe Sequence, the uppermost of four depositional sequences identifiable on multichannel seismic data from Lake Malawi (Nyasa). The sequence has a maximum thickness of about 115 m in two areas of the lake, but is typically less than 70 m thick over most of the basin. The sequence is distributed along the entire length of the 560 km long lake, and is concentrated in three main depocentres. 14C age dates from sediment piston cores are extrapolated to provide an age estimate of about 78 000 yr bp for the oldest sediments within the Songwe Sequence. In the North and Central bathymetric basins of the lake, high resolution seismic data indicate a dynamic depositional environment, dominated by turbidity and mass flow deposits. Seismic data from the southern basin show acoustically transparent sediments with relatively low amplitude internal reflections, indicative of pelagic and hemipelagic sedimentation. In many areas the Songwe Sequence is underlain by a pronounced angular unconformity, suggestive of a significant, prolonged, low lake stage prior to deposition of the sequence. Seismic reflectors within the Songwe Sequence can be correlated to younger low lake stages identified from sediment core data. Major late Quaternary low lake level stages in Lake Malawi, interpreted from features identified in the seismic data and sediment core analyses, are tentatively interpreted at 6000 to 10 000 yr bp, 28 000 to >40 000 yr bp, and prior to 78 000 yr bp. Budget calculations indicate mean sediment concentrations from catchment runoff during the period of deposition of the Songwe Sequence to be about 190 mg 1−1, comparable to estimates of modern rainy season discharges from the major river systems. Erosion rates within the drainage basin are estimated to be higher than the African average by a factor of three or more, probably due to the high relief within the Lake Malawi catchment.
Current crescents from the Herreria Formation (Lower Cambrian) of northern Spain
Authors: OWEN GERAINT 
Published: June 2006

ABSTRACTCurrent crescents are well preserved on the upper surfaces of quartzite beds in the Lower Cambrian Herreria Formation of the Cantabrian Mountains in northern Spain. Within a sequence of nearshore deposits, the current crescents are interpreted as having formed by flow separation of wave backwash around obstacles on a beach. The obstacles are not preserved and are inferred to have been perishable organic material, such as stranded algae or jellyfish, which subsequently decayed.
Diagenetic alteration of early marine cements of Upper Silurian stromatactis
Authors: BOURQUE PIERRE‐ANDRÉ , RAYMOND LYNDA 
Published: June 2006

ABSTRACTStromatactis is a spar network whose elements in cross section have flat to undulose lower surfaces and digitate upper surfaces. The network is composed principally of isopachous crusts of centripetal cement and commonly occurs embedded in finely crystalline limestone. It is the cement filling of interconnected cavities. Stromatactis of Upper Silurian red stromatactis limestone from Gaspé Peninsula, Québec Appalachians, exhibits two types of cements: (1) an isopachous cement that lined the walls of the conduits and is interpreted as early marine; and (2) a later blocky cement that occupies the centres of cavities. The first cement is composed exclusively of non‐ferroan calcite, whereas the second cement is mixed non‐ferroan and ferroan calcite. The early isopachous cement is white on polished slabs and has a turbid aspect under transmitted light. In a few samples, the relative homogeneity of this early cement is broken by the presence of distinctive grey clear calcite. Under cathodoluminscence, the grey clear calcite is non‐luminescent and exhibits well defined bladed crystal shapes, whereas the white turbid cement has a dull orange luminescence and indistinct crystal shapes. The relationships between the two cements indicate that the dull luminescent cement is a secondary form of the non‐luminescent cement, and it is concluded that the dull cement is the product of alteration of the non‐luminescent cement by burial or meteoric fluids. The later blocky cement has the same dull luminescence as the white turbid cement and is thought to have been precipitated from the same fluids as those responsible for the alteration of the early marine cements. Oxygen isotopic values of the dull cement of the early isopachous crusts (mean δ18O= ‐6.8%o are intermediate between those of the non‐luminescent early marine cement (mean δ18O= ‐5.3%o) and the dull luminescent blocky cement (mean δ18O= ‐11.8%o), while carbon isotopic values do not differ significantly (δ13C=+2.9, +2.4 and +2.6%o, respectively). The alteration also has affected the distribution of some trace elements, particularly Mg. Both unaltered and altered cements contain less than 1% microdolomite inclusions, but the Mg content of the background calcite of unaltered cement is three times that of altered cement (14171 vs. 5502 ppm). Precursor early marine cement is thought to have been low‐Mg calcite. The mean δ18O value (− 5.3%o) of unaltered early marine cement is higher than values for the oxygen isotopic signature of Silurian oceans provided by brachiopod shells.
Diachronous evolution of systems tracts in a depositional sequence from the middle Pleistocene palaeo‐Tokyo Bay, Japan
Authors: ITO MAKOTO , O'HARA SAKAE 
Published: June 2006

ABSTRACTPalaeo‐Tokyo Bay is a relic of the Plio‐Pleistocene Kazusa forearc basin in the Boso Peninsula of Japan. The sedimentary infill of palaeo‐Tokyo Bay is characterized by shallow marine to paralic sediments of the middle to upper Pleistocene Shimosa Group. Sequence stratigraphical analysis has been used to describe spatial and temporal variations in the depositional systems of the lowest units of the Shimosa Group, deposited during the early stage of development of palaeo‐Tokyo Bay.Three different type of depositional systems were recognized: sand ridge to shelf (SRS), shelf to delta (SDL) and shelf to non‐deltaic nearshore (SNS) systems. They overlie early transgressive estuarine deposits infilling lowstand valleys incised in the south‐eastern margin of palaeo‐Tokyo Bay. These systems were developed during late transgressive through highstand stages of a relative sea level cycle, which may have been controlled by a glacio‐eustatic sea level change at about 0·4 Ma. Spatial variation in depositional systems is largely identical to that in modern Tokyo Bay; environmental conditions similar to those prevailing at the present day probably characterized the early history of palaeo‐Tokyo Bay.The timing of highstand systems tracts within a high frequency depositional sequence was analysed in terms of the effect of sedimentation rate, based on the mapping of a chronostratigraphical surface marked by the Hy4 volcanic ash layer. From spatial variations in sedimentation rate, it was possible to identify the diachronous evolution of highstand systems tracts from the SDL system, through the SNS system, to the SRS system. Time lag is indicated by major bounding surfaces, such as maximum flooding or downlap surfaces associated with a condensed section, which developed immediately above or below the Hy4 volcanic ash layer. The lag may be of the order of a few thousands to tens of thousands of years within a depositional sequence with a total of duration of about 100 000 years.
Discrimination of accreted and eroded coasts using heavy mineral compositions of the Nile Delta beach sands, Egypt
Authors: FRIHY OMRAN E. 
Published: June 2006

ABSTRACTCoastal erosion has occurred on the beaches of the three major promontories of the Nile Delta of Egypt. The eroded sand is generally carried alongshore to the east, or locally to the west, from these promontories and is deposited in areas of beach accretion between promontories. In this study the heavy minerals of 150 samples have been investigated with the aim of differentiating between eroded and accreted beaches. Discrimination was achieved by using bivariate plots of the raw weight percentage data and Q‐mode factor analysis. Factor analysis yielded two ‘mineral assemblages’: Factor 1 is dominated by augite, hornblende and epidote, while Factor 2 consists of opaque minerals, garnet, zircon, rutile and monazite. These two mineral factors are produced by the selective sorting of heavy minerals by natural processes of waves and currents. Minerals of Factor 2 are associated with high density opaques which tend to concentrate as a lag in areas of erosion. Conversely, the lower density minerals of Factor 1 are preferentially transported away from areas of erosion and carried to the accreted shorelines.