bluuebunny/crossref_metadata_2025_split huggingface dataset:
The fluvial architecture of the upper Buntsandstein in the Iberian Basin, central Spain
Authors: GARCÍA‐GIL SOLEDAD 
Published: June 2006

ABSTRACTThe Cercadillo Sandstone and Siltstone (CSS) Formation forms the upper part of the Buntsandstein fluvial facies in the western margin of the Iberian Basin. Sedimentation was controlled by two major pre‐Triassic tectonic systems: the Iberian Ranges System (NW‐SE) and the Central System (NE‐SW). Fluvial sedimentation during this period took place in coexisting braided and meandering channels, together with associated ephemeral episodes. This unusual association was due to a combination of structural control by the main normal faults coupled with large discharge fluctuations.The CSS Formation consists of two major fluvial sequences with very similar vertical patterns comprising low sinuosity stream deposits that pass upwards into high sinuosity deposits associated with extensive floodplain sediments. A widespread calcrete horizon is developed at the top of the lower sequence and a surface with pedogenic features is present at the top of the upper sequence. These horizons are interpreted to be the result of subaerial exposure during a period of no subsidence and equilibrium in the erosion‐sedimentation processes in the area, probably due to quiescent periods on the master faults that delineate the basin.Minor changes within the sequences are probably due to discharge fluctuations of a climatic origin whereas the general vertical trend of the sequences is suggested to be tectonically controlled. Palaeocurrent data show a general flux of sediments to the south‐east, parallel to the axis of the Iberian Basin, but the final part of the upper sequence palaeocurrents to the north‐east are interpreted to indicate a local reactivation of secondary, transverse faults in the south‐east part of the study area.
Milankovitch cyclicity in Purbeck peritidal limestones of the Prebetic (Berriasian, southern Spain)
Authors: DE CISNEROS C. JIMENEZ , VERA J. A. 
Published: June 2006

ABSTRACTPeritidal carbonate rocks (Purbeck facies) of the uppermost Portlandian to Berriasian in the type section of the Sierra del Pozo Formation in the Prebetic Zone, southern Spain, are divisible into 141 shallowing upward cycles averaging 2 m in thickness. The subtidal facies in these cycles consist of micritic or marly limestones with dasycladacean algae and lituolids; the intertidal facies are micritic limestones containing birdseyes and miliolids; the supratidal facies comprise laminated algal limestones, the tops of which display desiccation cracks and rhizocretions, or more locally palaeosols, calcretes, or palaeokarst surfaces.A statistical study, using power spectra of the Fast Fourier Transform, demonstrates that the periodicity of these cycles is in the Milankovitch frequency band. Most sedimentary cycles correspond to the obliquity cycles; eccentricity and precession cycles have also been recognized. Using a Fischer plot, third‐order tectono‐eustatic cycles are recognized, which can be correlated with the eustatic curve of the Exxon chart.The shallowing upward sequences are characterized by a distinctive pattern of geochemical parameters. Carbon and oxygen isotopic (δ13C and δ18O) variations, calcium and magnesium carbonate contents and the abundance of organic matter and trace elements (Mn and Sr) all have predictable patterns of distribution within the sequences. The Sr content of the subtidal facies is relatively high whereas the δ13C and δ18O ratios are quite low; in the intertidal facies the Sr and Mn levels fall concomitantly with a rise in δ13C and δ18O. The highest δ13C and δ18O values occur in the lower part of the supratidal facies, whereas in the upper part of δ values and Sr contents drop sharply.Cyclic variations in evaporation and in meteoric water influence, determined from oxygen isotopic composition, reveal that the cyclicity of the beds containing the most limestones (supratidal) and those with the most marls (subtidal) is related to climatic changes. The coldest periods are those represented by supratidal deposits, when the sea level was at its lowest. During the warmest periods, when the overall sea level was higher, subtidal deposits accumulated in the region.A genetic model is proposed, according to which the asymmetrical sedimentary cycles occur in response to glacio‐eustatic changes with a periodicity similar to that of Plio‐Pleistocene sea‐level variations, but with a much lower range due to the smaller extent of polar ice caps during the Early Cretaceous. The glacio‐eustatic changes involved a rapid sea‐level rise and a slow sea‐level fall.
The surface texture of the Saharan Debris Flow deposit and some speculations on submarine debris flow processes
Authors: MASSON D. G. , HUGGETT Q. J. , BRUNSDEN D. 
Published: June 2006

ABSTRACTDeep towed 30 kHz sidescan sonar data from the Saharan Debris Flow deposit, west of the Canary Islands, show spectacular backscatter patterns which are interpreted in terms of flow banding, longitudinal shears, lateral ridges (levees) and transported blocks. Identification of these features is based on high resolution seismic profiles and on a comparison with similar structures seen in better known environments including other marine debris flows and slides, subaerial sediment failures (particularly rock fall avalanches), glaciers and lava flows. Flow banding in the debris flow, picked out by bands of differing backscatter intensity, is on a scale of tens to hundreds of metres. It is considered to result from flow streaming of clasts, with variation in clast size between bands. This primary fabric is cut by a series of distinct flow‐parallel longitudinal shears. Broad, high backscatter longitudinal bands along the edge of and within the debris flow are interpreted as lateral ridges associated with multiple flow pulses; the high backscatter possibly reflects either a concentration of coarse grained material or chaotic sediments deposited from a turbulent flow. Coherent, low backscatter patches are interpreted as rafted blocks, although streamlined haloes of high backscatter material around some blocks indicates differential movement between block and flow, possibly during the waning stages of the flow.A non‐turbulent debris flow model is preferred, in which a raft of more or less coherent material is carried along by a base undergoing laminar flow. Speculatively, the lack of turbulent mixing preserves original sedimentological heterogeneity from the debris flow source area, possibly in the form of clast size distributions. These heterogeneous sediments are drawn out into a flow‐parallel banding which is imaged as the flow‐parallel backscatter intensity banding. The upper raft of material responds to cross‐flow velocity differences, and perhaps to variations in the timing of flow movement, primarily by longitudinal shearing. More complex deformation of the flow banding occurs at the flow margins and around obstacles in the flow, where lateral velocity shear would be expected to be highest.
Facies analysis and high‐resolution sequence stratigraphy of the Lower Eocene shallow marine Ametlla Formation, Spanish Pyrenees
Authors: DREYER TOM , FÄLT LARS‐M. 
Published: June 2006

ABSTRACTThe Lower Eocene Ametlla Formation of the Ager Basin, Spanish Pyrenees, is a rapidly deposited shallow marine unit formed in a setting characterized by syn‐sedimentary tectonic activity. Mapping of the formation over a distance of 25 km was conducted according to sequence stratigraphical principles with emphasis on facies analysis. Twelve facies, grouped in five facies associations, have been recognized in the Ametlla Formation. The studied succession records a vertical transition from deltaic systems prograding onto a sediment‐starved shelf, via estuarine deposits associated with incised valleys, to sandbar complexes in a tidal seaway.In terms of sequence stratigraphy, three scales of genetic sedimentary units were recognized.(1) At the regional scale, elements of two 3rd‐order composite sequences (sensu Exxon) have been recognized. These include a 3rd‐order highstand sequence set encompassing the lowermost part of the Ametlla Formation and the underlying Passarella Formation, and a 3rd‐order transgressive sequence set that constitutes the middle parts of the Ametlla Formation. The sequence sets are separated by an unconformity with up to 35 m of incision that is interpreted as a major sequence boundary. It is argued that the incised valleys associated with this unconformity were infilled during landward‐stepping of the shelfal depositional system. Basinwards, the unconformable surface becomes subhorizontal and is overlain by a 2 m thick oyster bed formed in a sediment‐starved setting subsequent to flooding of the incised valleys (which still acted as sediment conduits). Sandstones dominate the transgressive sequence set, whereas the highstand sequence set is dominated by siltstones, particularly in the lower part. In the transgressive sequence set, an upward increase in sand content and calibre is observed, relatable to punctuations of the transgressive trend by high‐frequency sea‐level fluctuations, and to downslope redistribution of sand.(2) At the subregional scale, detailed mapping indicates the presence of five 4th‐order sequences. The 4th‐order sequence boundaries are associated with sediment bypassing and minimal erosional relief, and were created by forced regressions during periods of relative sea‐level fall. Sharp‐based sandstones overlying these unconformities are believed to have accumulated during subsequent rise of relative sea‐level. Where 4th‐order maximum flooding surfaces can be recognized, the sequences may be subdivided into a sandstone‐dominated transgressive systems tract and a siltstone‐dominated highstand systems tract.(3) At the local scale, 2–9 5th‐order parasequences are present within the 4th‐order sequences. Superimposed parasequences are separated by flooding surfaces characterized by bioclastic accumulations, pervasive burrowing and extensive calcite cementation. The parasequences are commonly stacked in a landward‐stepping manner.
Sedimentation and extensional basin evolution in a Miocene metamorphic core complex setting, Alvord Mountain, central Mojave Desert, California, USA
Authors: FILLMORE ROBERT P. 
Published: June 2006

ABSTRACTThe Lower Miocene Clews Formation at Alvord Mountain, Mojave Desert, California, comprises an upward coarsening sequence of synextensional continental deposits of variable thickness; this sequence pinches out to the west against the Alvord Mountain pre‐Tertiary basement complex and thickens eastward to c.300 m over a distance of 7 km. Two stages of sedimentation are recognized in the formation. During the initial stage of sedimentation, the depocentre was delineated by a lacustrine system that was bounded to the west by small, sheetflood‐dominated alluvial fans and to the north by a southerly flowing fluvial braidplain. Lacustrine mudstone, siltstone and carbonate suggest an initially closed basin. Conglomerate to the west represents small, sheetflood‐dominated fans. Igneous clasts and a south‐east palaeoflow direction suggest a proximal, low‐relief source in the western Alvord Mountain area. Pebbly sandstone of the southward‐prograding braidplain has a metasedimentary provenance in the Paradise Range to the north. The second stage of deposition was dominated by coarse conglomerate and breccia. West to southwest palaeotransport indicators and a distinctive metaigneous petrofacies indicate a provenance in the Cronese Hills, 8 km to the east. These strata reflect rapid westward progradation of sheetflood‐ and debris flow‐dominated alluvial fans that advanced across the axial braidplain and lacustrine system.Basin development is interpreted to have been controlled by regional NE‐directed extension on a detachment fault associated with the central Mojave metamorphic core complex. 30 km to the west. The Alvord Mountain ‐ Cronese Hills region was initially transported as a single hangingwall block on the E‐dipping detachment. Initial lacustrine sedimentation reflects the development of a flexural or sag basin in the hangingwall. Subsequent westward progradation of alluvial fans out of the Cronese Hills is believed to record the propagation of a NW‐striking, SW‐dipping normal fault antithetic to the low‐angle E‐dipping detachment fault.
Grain size, composition, porosity and permeability contrasts within cross‐bedded sandstones in Tertiary fluvial deposits, central Spain
Authors: HARTKAMP CHRISTEL A. , ARRIBAS JOSÉ , TORTOSA AMPARO 
Published: June 2006

ABSTRACTPermeability measured with a portable probe permeameter on outcrops of cross‐bedded sandstones ranges between 0·9 and 19 D. The highest permeability (2–19 D with an average of 8·5 D) occurs in the coarsest grained foreset laminae (CFL), intermediate values (2–12 D with an average of 5·3 D) occur in finer grained foreset laminae (FFL) and the lowest values (0·9–10 D with an average of 4·8 D) occur in bottomset layers (BL). In the cross‐beds the average grain size ranges from medium grained sand in the CFL to fine grained sand in the FFL and BL. In all three subfacies, the average size of the primary pores is approximately 1φ unit smaller than the average grain size. The abundance of unstable carbonate clasts correlates with increasing average grain size, micritic clasts being most abundant in the CFL. Conversely, quartz content increases with decreasing grain size and is highest in the FFL and BL. Diagenetic destruction of primary porosity by compaction and cementation, as well as generation of secondary porosity through dissolution, were controlled by the original mineralogical composition of the sand. Contrasts in grain size determine the primary pore size contrasts and differences in composition between CFL, FFL and BL. Permeability contrasts reflect variations in average primary pore size rather than differences in total porosity. Probe permeability contrasts between CFL, FFL and BL depend on contrasts in average pore size and contrasts in mineralogical composition between the subfacies.
Description and interpretation of fluvial deposits: a critical perspective
Authors: BRIDGE JOHN S. 
Published: June 2006

ABSTRACTA critique is given of recent methods proposed for the standardized description, classification and interpretation of fluvial deposits in terms of: (1) hierarchies of strata and their bounding surfaces; (2) lithofacies; (3) lithofacies associations (architectural elements); (4) geometry of sedimentary bodies. Much of the descriptive information can be conveyed using clear, explicit diagrams without recourse to complicated terminology. Any classification that is used should be based on easily measurable parameters which are used to define mutually exclusive classes. Terms used to refer to these classes should be explicit. Little agreement exists on the best terms to describe strata, mainly because existing terminology was not designed to account for all the different superimposed stratal scales. A simple terminology is suggested here whereby the relative scales of hierarchies of strata and cross‐strata are specified. Methods of describing hierarchies of strata by numerically ordering stratal bounding surfaces (rather than the strata themselves) are difficult to use in practice. The use of standardized lithofacies codes tends to discourage close observation and recognition of varieties and superpositions of lithofacies, has led to proliferation of acronyms, and to the suggestion that a particular lithofacies has a unique interpretation. In fact, many interpretations associated with these lithofacies codes are misleading or wrong. In a commonly used classification system of lithofacies associations (architectural elements) the different classes are not mutually exclusive, they are referred to using a mixture of descriptive and interpretive terms, and each element is represented graphically by only a single two‐dimensional section. Terms used to describe the three‐dimensional geometry of a sediment body should only be used to refer to two‐dimensional sections if three‐dimensional form can be reconstructed unambiguously. Fluvial ‘facies models’ are constructed to relate lithofacies associations and geometry to parameters such as channel and floodplain geometry, modes of channel migration and deposition rates. Most are of limited use or misleading because insufficient three‐dimensional information is shown. In addition, they commonly incorrectly associate channel planform (e.g. meandering, braided) with a characteristic lithofacies association and geometry, whereas other factors may play a dominant role.
The ecology of Lower Cambrian buildups from Zuune Arts, Mongolia: implications for early metazoan reef evolution
Authors: WOOD RACHEL , ZHURAVLEV ANDREY Yu. , ANAAZ CHIMED TSEREN 
Published: June 2006

ABSTRACTReefal buildups in western Mongolia of mid‐Early Cambrian (late Atdabanian) age flourished during a period when shelf seas were globally widespread. The succession at Zuune Arts records the transition from shallow marine siliciclastic sediments (Bayan Gol Formation) to shallow marine, but still clastic‐influenced, carbonates (Salaany Gol Formation). The Salaany Gol Formation is interpreted as having been deposited as a series of shallowing upwards cycles on a shallow, gently inclined shelf in a rapidly subsiding epicontinental sea. Cycles commenced with the growth of subtidal metazoan‐calcimicrobe aggregative communities on an open shelf. The resultant buildups were commonly engulfed by extensive, massive microbial stromatolites when they grew in agitated intertidal conditions. Latterly, they were smothered by ooid shoals in response to rapid sea level rise.Four types of reefal buildup are distinguished: (1) green‐coloured calcimicrobe (Tarthinia, Epiphyton, Gordonophyton and Renalcis) boundstones; (2) red‐or green‐coloured Cambrocyathellus‐Tarthinia‐Epiphyton bafflestones; (3) red‐coloured Okulitchicyathus bindstones; and (4) red‐coloured radiocyath‐archaeocyath‐cribricyath bioherms. Each is interpreted as having grown at increasing depths and possibly sedimentation rates. The buildups are commonly enclosed within graded and planar bedded bioclastic grainstones and packstones, and are best developed towards the top of the formation, when sea level was high. Thickets of large, solitary archaeocyaths are also inferred in the deeper interbiohermal areas.These buildups had synoptic relief and constructed porous structures with growth‐framework cavities housing diverse coelobiontic communities. Extensive synsedimentary cements are present, including pseudomorphed aragonitic fans and possible pseudomorphed aragontic botryoids. These early reefs thus have geological fabrics similar to later Phanerozoic representatives. It is proposed however, that this ecosystem was largely composed of generalist and opportunistic filter and suspension feeders which were dependent upon a far higher input of nutrients than modern day reefal developments. Bacteria were probably the main primary producers, from both planktic and benthic cyanobacterial communities. The diversity of each buildup assemblage appears to be controlled by primary cavity size, the richest fauna belonging to the highly tiered radiocyath‐dominated community inferred to have lived in the deepest waters. The communities at Zuune Arts can be compared with other buildups from the early Cambrian, and with Lower Ordovician receptaculid‐calcimicrobe‐solitary sponge bioherms known from the USA and Siberia.
Dust emission and transport in Mali, West Africa
Authors: NICKLING W. G. , GILLIES J. A. 
Published: June 2006

ABSTRACTVertical dust fluxes were measured in the Inland Delta region of the Niger River, Mali, West Africa, during April‐June, 1989 and 1990. Measurements of dust flux represent, for the most part, non‐dust storm conditions or ‘dust haze’ periods. The observed concentration versus height relationships are similar to data presented by other investigators. The relationship between wind shear velocity (u*, m s−1) and vertical dust flux (F, μg m−2 s−1) can be described by a relationship in which F is proportional to u*4. However, there is considerable scatter within the data set which is attributable to textural controls and surface conditions. The vertical dust fluxes measured in Mali are compared to dust fluxes measured in Texas, USA, and Yukon Territories, Canada. The significantly different values for the constant of proportionality (a) in the F α a u*4 relationship for these geographically diverse areas is a function of surficial controls on the release of sediments to the air stream. Dust concentrations measured in Mali were found to be uniformly high and in general exceed WHO health standards for acceptable total suspended particulate loadings. As a result background dust may be considered a long term stress on health for the people of this region of Mali.
The effect of sampling scale on actualistic sandstone petrofacies
Authors: INGERSOLL RAYMOND V. , KRETCHMER ANDREA G. , VALLES PETER K. 
Published: June 2006

ABSTRACTEmpirical correlations between plate tectonic setting and sand/sandstone composition have been the basis for large scale petrological models. These models do not explicitly treat sampling scale. Four areas from the western USA with diverse tectonic settings and rock types provide a natural laboratory for sampling sand at three different scales: talus piles to small drainages (first order), streams and rivers draining mountain ranges (second order), and large rivers and marine environments (third order). Existing plate tectonic petrofacies models should only be applied to third order settings because the data were derived from studies of such settings. This is especially true in tectonic settings with diverse source rocks (e.g. continental rifts and transform settings). On the other hand, some settings, such as active magmatic arcs and foreland fold‐thrust belts, provide uniform results at any sampling scale because of homogeneity of source rocks.The Rio Grande drainage area is especially complex, with diverse igneous, metamorphic and sedimentary source areas. Some components (e.g. basalt) are destroyed with minimal transport, whereas others (e.g. quartz) are relatively enriched with greater transport. In this complex continental rift setting, first and second order sand is diverse and heterogeneous due to input from tributaries. The Santa Clara River of southern California also has heterogeneous sand due to diverse source rocks in this transform setting. It is only after considerable homogenization and stabilization due to weathering and mixing with more stable components, and/or considerable transport, that homogeneous compositions are produced in these two settings.In contrast, the Cascade magmatic arc and the Canadian Rocky Mountain fold‐thrust belt have uniform source rocks (dominantly volcanic in the former and dominantly sedimentary in the latter). Uniform sand composition that is unique to each of these tectonic settings results at any sampling scale in these two cases.Uniformity of data collection and analysis is essential for reproducible results. Use of the Gazzi‐Dickinson point counting method allows direct comparison among source rocks (zero order samples), modern sand of any order and ancient sandstone of unknown provenance.Lack of recognition of the effect of sampling scale in the development of actualistic petrofacies models has led to incorrect rejection of many existing models. Third order sands are excellent predictors of plate tectonic setting, but first and second order sands can provide ambiguous plate tectonic interpretations in many settings. More complex actualistic petrofacies models based on diverse sampling scales are needed.