bluuebunny/crossref_metadata_2025_split huggingface dataset:
Sedimentation in the lakes and marshes (Ahwar) of the Tigris‐Euphrates Delta, southern Mesopotamia
Authors: AQRAWI A. A. M. , EVANS G. 
Published: June 2006

ABSTRACTThe fluvial plains of lower Mesopotamia crossed by the Tigris and Euphrates rivers are occupied by fresh‐brackish water lakes and extensive reed‐marshes dominated by Phragmites sp. and Typha sp. (together locally called Ahwar). Allochthonous sediment is mostly supplied by the Tigris and Euphrates seasonal floodwaters, and by winds which bring aeolian dust from the north‐west and sand during storms from the western deserts in summer. Other contributions to the sediment budget are made by in situ biological activity and by biochemical‐chemical processes within the sediments.The sediments are dominantly silts, with silt comprising more than 60% of the sediment. The remainder of the sediment consists of varying quantities of clay and sand. Clay is more abundant in the lower parts of most of the analysed cores (30–80 cm in length), indicating a change in sediment supply and/or sedimentary environment. Well developed stratification is found only on the river levees and in some clay‐rich lake and marsh sediments. The remainder of the sediments are generally poorly stratified with irregular stratification and considerable bioturbation.Mineralogically, the sediments consist mainly of calcite, quartz, dolomite and feldspar (particularly albite). Aragonite is present as a result of in situ production by macrofauna. The clay fraction (<2 μn) consists of smectite, illite‐smectite, illite, palygorskite, kaolinite and chlorite.The total organic carbon (TOC) content is usually <5% except for some peaty surface layers of the freshwater lakes and marshes where TOC may rise to 10–20%. The sediments have a relatively high calcium carbonate content. 20–80%, the highest values only being found in mollusc‐rich horizons. The sediments contain high amounts of Ca, Mg and Sr and are generally low in Al, Ti. K. and V. Values for Fe, Co, Pb, Zn and Mn are close to the average for mudstones, whereas Cu and P are relatively low. The high Ni and Cr contents of the sediments are related to high contents in the source material of the major rivers and aeolian deposits. Multivariate statistical analysis shows that elemental components of surface and source materials correlate well with other sedimentary properties.Aragonitic molluscs of various species dominate the macrofauna, but a microfauna of foraminifers, ostracods and diatoms is also present in the sediments.
Rare earth element geochemistry of dolomites in the Middle Devonian Presqu'ile barrier, Western Canada Sedimentary Basin: implications for fluid‐rock ratios during dolomitization
Authors: QING HAIRUO , MOUNTJOY ERIC W. 
Published: June 2006

ABSTRACTRare earth elements (REE) were determined in fine, medium and coarse crystalline replacement dolomites, and for saddle dolomite cements from the Middle Devonian Presqu'ile barrier from Pine Point and the subsurface of the Northwest Territories and north‐eastern British Columbia. REE patterns of the fine crystalline dolomite are similar to those of Middle Devonian limestones from the Presqu'ile barrier. Fine crystalline dolomite occurs in the back‐barrier facies and may represent penecontemporaneous dolomitization at, or just below, the sea floor. Medium crystalline dolomite is widespread in the lower southern and lower central barrier. Medium crystalline dolomite is slightly depleted in heavy REE compared with Devonian marine limestones and fine crystalline dolomite, and has negative Ce and Eu anomalies. Medium crystalline dolomites replaced pre‐existing limestones or were recrystallized from earlier fine crystalline dolomites. During these processes, the REE patterns of their precursors were modified. Late stage, coarse crystalline replacement dolomite and saddle dolomite cements occur together in the upper barrier and have similar geochemical signatures. Coarse crystalline dolomites have negative Eu anomalies, and those from the Pine Point area also have positive La anomalies. Saddle dolomites are enriched in light REE and have positive La anomalies. The REE patterns of coarse crystalline dolomite and saddle dolomite differ from those of marine limestones and fine and medium crystalline dolomites, suggesting that different diagenetic fluids were responsible for these later dolomites.Although massive dolomitization requires relatively large volumes of fluids in order to provide the necessary amounts of Mg2‐. dolomitization and subsequent recrystallization may not necessarily modify the REE signatures of the precursor limestones because of the low concentrations of REE in most natural fluids. Thus, relative fluid‐rock ratios during diagenesis may be estimated from REE patterns in the diagenetic and precursor minerals. Fine crystalline dolomites retain the REE patterns of their limestone precursors. In the medium and coarse crystalline dolomites the precursor REE patterns were apparently altered by large volumes of fluids involved during dolomitization. This study suggests that REE compositions of dolomites and their limestone precursors may provide important information about the relative amounts of fluids involved during diagenetic processes, such as dolomitization.
Subaerial exposure and deposition of shallowing upward sequences: evidence from stable isotopes of Purbeckian peritidal carbonates (basal Cretaceous), Swiss and French Jura Mountains
Authors: JOACHIMSKI MICHAEL M. 
Published: June 2006

ABSTRACTPurbeckian (lowermost Cretaceous) peritidal carbonates are characterized by open marine, lagoonal, intertidal and lacustrine facies arranged in Milankovitch‐type shallowing upward sequences. Shallowing upward sequences typically consist of 2–6 individual beds. The sequences may be (i) complete, (ii) incomplete or (ii) pedogenetically overprinted, reflecting the duration of subaerial exposure and/or the extent of erosion and pedogenetic modification at the cycle tops.The stable isotopic composition of the peritidal micrites reveals homogenous δ18O values attributed to diagenetic stabilization in a meteoric, water‐buffered system. Carbon isotopes show three distinctly different carbon isotope patterns dependent on the completeness of the shallowing upward sequences. Complete shallowing upward sequences consist of 4–6 individual carbonate beds. The carbon isotope values show a facies‐dependent pattern: open marine carbonate muds record enriched δ13C values of +0·28‰ while lagoonal (−0·82‰), intertidal (−2·46‰) and lacustrine micrites (−2·96‰) are increasingly depleted. This distinct pattern is explained by carbonate mud deposition in environments of differing salinity and marine influence. Incomplete sequences (2–5 carbonate beds) are characterized by depleted δ13C values below subaerial exposure surfaces that become progressively enriched in 13C with increasing depth. Pedogenetically overprinted sequences (1–3 carbonate beds) show strong 13C depletion throughout the sequence with little variation in the carbon isotopic composition. The depleted values (−4·5‰) of the pedogenetically altered micrites suggest that modification during subaerial exposure was associated with equilibration with meteoric solutions enriched in isotopically light soil gas CO2.The duration of subaerial exposure is the most crucial factor determining the extent of pedogenetic alterations, the completeness of the shallowing upward sequences and the carbon isotope pattern. The recorded patterns clearly illustrate that micrites have a good potential for the preservation of their primary carbon isotopic composition if the duration of subaerial exposure is rather brief. Otherwise, the recorded carbon isotope patterns may support sequence stratigraphic analysis by providing a refinement of the time‐stratigraphic interpretation.
Factors controlling aeolian sequence stratigraphy: clues from super bounding surface features in the Middle Jurassic Page Sandstone
Authors: HAVHOLM K. G. , KOCUREK G. 
Published: June 2006

ABSTRACTSuper bounding surfaces (super surfaces) facilitate delineation of internal genetic architecture, or sequence stratigraphy, of aeolian sandstones and help in distinguishing controls on aeolian accumulation, hiatuses in accumulation and preservation of such sandstones. The Middle Jurassic Page Sandstone is an amalgamation of such sandstone units accumulated in a dry erg system, with water table below the surface, separated by super surfaces with features indicating an arid climate but a near surface water table. The Page Sandstone accumulated episodically with periods of aeolian accumulation followed by deflation to the water table; the water table fluctuated up and down through the sediment package. This rising water table did not directly control accumulation, but placed a limit on extent of deflation by stabilizing the substrate, thereby directly controlling preservation of the aeolian unit. A combination of relative water table behaviour, sediment supply and aerodynamic conditions upwind and within the erg controlled the thickness of genetic packages within the erg. A change in the pattern of deflation associated with super surface formation late in deposition of the Page Sandstone reflects a change in tectonic regime in the basin. The number of super surfaces and features on the super surfaces with the Page Sandstone suggest that the super surfaces represent more time than do the accumulations they enclose.
Threshold studies of gravel size particles under the co‐linear combined action of waves and currents
Authors: PANAGIOTOPOULOS I. , SYLAIOS G. , COLLINS M. B. 
Published: June 2006

ABSTRACTGravel size sediment beds are tested under the combined influence of simulated wave action and co‐linear currents in a laboratory flume. Critical current speed, at threshold, increases with increasing size. Superimposed wave energy causes a small reduction in the unidirectional current energy. For low values of wave‐induced near bed current velocities, the resistance to erosion increases when the wave period decreases from 10 to 6 s. Finally, combined critical shear stresses are found to be lower than those predicted using the Shields curve, as modified for oscillatory flow. Grain protrusion is suggested as a mechanism to explain this divergence.
Determination of turbidity current erosional characteristics from reworked coccolith assemblages, Canary Basin, north‐east Atlantic
Authors: WEAVER P. P. E. 
Published: June 2006

ABSTRACTTurbidites contain mixtures of sediments of various ages. These sediments can include both material that was eroded to form the initial turbidity current plus additions derived from erosion of the sea floor during transport. It has been possible to interpret the age range of sediments incorporated into individual turbidites from the Madeira Abyssal Plain by examining the proportions of microfossil (coccolith) species that they contain. The pelagic record of coccoliths is well known for the Quaternary period and shows a succession of dominant species or acmes each lasting a few tens of thousand years. Hence, erosion of sediment representing more than a few tens of thousand years will produce coccolith mixtures not seen in the pelagic record, but dependent upon the age range of the sediments that were eroded. This age range can be estimated by comparison with synthetic ratios of coccolith species created by combining ratios of species from successively older layers in the pelagic record. These can then be compared with the ratios found in individual turbidites until a match is found. The results show age ranges of 54–500 kyr for the sediment mixture in seven turbidites from the Madeira Abyssal Plain. Since the volumes of these turbidites are also known, and accumulation rates in their source areas can be estimated, it is possible to determine both the thickness and the area of the eroded sediment mass that generated the turbidity current. Minimum depths of erosion on the north‐west African continental margin vary from 8 to 50 m and minimum areas eroded from 1600 to 5800 km2. None of the turbidites examined contains a significant excess of surface sediment, suggesting that, once formed, the turbidity currents that transported them were virtually non‐erosional, and that they travelled several hundred kilometres in this state.
Sediment particle size in the Hudson River Estuary
Authors: GIBBS R. J. , JHA P. K. , CHAKRAPANI G. J. 
Published: June 2006

ABSTRACTThe particle sizes in bottom and suspended sediments from the ocean upstream for 80 km in the Hudson River Estuary were analysed. Several modes at 4, 8 and 30 μm were prevalent throughout the estuary. The average size of particles in bottom samples varied from 150 μm near the ocean to 15–20 μm in Haverstraw Bay. The bottom sediment size appears to be controlled by two components; (1) sand from the ocean and (2) the particles in the flocs.
High‐resolution sequence stratigraphy of a complex, incised valley succession, Cobequid Bay — Salmon River estuary, Bay of Fundy, Canada
Authors: DALRYMPLE ROBERT W. , ZAITLIN BRIAN A. 
Published: June 2006

ABSTRACTThe post‐glacial succession in the Cobequid Bay — Salmon River incised valley contains two sequences, the upper one incomplete. The lower sequence contains only highstand system tracts (HST) deposits which accumulated under microtidal, glacio‐marine deltaic conditions. The upper sequence contains two, retrogradationally stacked parasequences. The lower one accumulated in a wave‐dominated estuarine environment under micro‐mesotidal conditions. It belongs to the lowstand system tract (LST) or early transgressive system tract (TST) depending on the timing and location of the lowstand shoreline, and contains a gravel barrier that has been overstepped and preserved with little modification. The upper parasequence accumulated in the modern, macrotidal estuary, and is assignable to the late TST. Recent, net progradation of the fringing marshes indicates that a new HST has begun.The sequence boundary separating the two sequences was formed by fluvial incision, and perhaps also by subtidal erosion during the relative sea level fall. Additional local erosion by waves and tidal currents occurred during the transgression. The base of the macrotidal sands is a prominent tidal ravinement surface which forms the flooding surface between the backstepping estuarine parasequences. Because fluvial deposition continued throughout the transgression, the fluvial‐estuarine contact is diachronous and cannot be used as the transgressive surface. The maximum flooding surface will be difficult to locate in the macrotidal sands, but is more easily identified in the fringing muddy sediments.These observations indicate that: (1) large incised valleys may contain a compound fill that consists of more than one sequence; (2) relative sea level changes determine the stratal stacking patterns, but local environmental factors control the nature of the facies and surfaces; (3) these surfaces may have complex origins, and commonly become amalgamated; (4) designation of the transgressive surface (and thus the LST) is particularly difficult as many of the prominent surfaces in the valley fill are diachronous facies boundaries; and (5) the transgression of complex topography may cause geologically instantaneous changes in tidal range, due to resonance under particular geographical configurations.
On the prediction of bed‐load sand transport rate in air
Authors: McEWAN I. K. , WILLETTS B. B. 
Published: June 2006

ABSTRACTThis paper discusses the parameterization of aeolian mass transport rate beginning with a critical discussion of four different forms of the prediction equation. Following this, recent progress in understanding the physics of the aeolian system is used to suggest an alternative approach which provides some new insights into the prediction of mass flux. The primary object of the paper is not to add another transport rate equation to those already present in the literature but rather to discuss the transport system in relation to the physical processes showing, to some extent, why reliable transport rate prediction has so far been elusive.
Diagenetically altered sabkha‐type Pleistocene dolomite from the Arabian Gulf
Authors: CHAFETZ HENRY S. , RUSH PATRICK F. 
Published: June 2006

ABSTRACTDiagenetically altered Pleistocene dolomite occurs in the shallow subsurface of the Arabian Gulf, offshore of Al Jubayl, Saudi Arabia. This dolomite accumulated in relatively shallow marine to sabkha depositional environments. In contrast with the thin extent of most other Quaternary sabkha and sabkha‐related dolomite deposits, these deposits comprise a thick (>56 m) accumulation. Additionally, this Pleistocene dolomite displays a high degree of ordering and has a more nearly ideal stoichiometric composition than the dolomite from the depositionally and diagenetically analogous Abu Dhabi sabkha complex. The Pleistocene dolomite also has lower δ13 and δ18O values than the modern Abu Dhabi sabkha dolomite, and higher values than those commonly reported for analogous dolomite from the ancient rock record. The low δ18O values, in conjunction with the geological setting, indicate that the diagenetic waters were meteoric or mixed meteoric and marine in composition. Thus, the degree of ordering, stoichiometric and stable isotopic values indicate that this dolomite has undergone diagenetic alteration relative to its presumed Holocene precursor.
Microbiological effects on slope stability: an experimental analysis
Authors: MEADOWS AZRA , MEADOWS PETER S. , WOOD DAVID MUIR , MURRAY JOHN M. H. 
Published: June 2006

ABSTRACTA natural, pure quartz sand has been seeded with the bacterium Pseudomonas atlantica and the fungus Penicillium chrysogenum, and angles of avalanche and repose have been measured under water using a laboratory clinometer. The lowest angles of avalanche occur in freshly packed clean sediment (control), with the seeded sediments having higher values. Among the latter, the lowest angles of repose occur in the bacterial seeded sediments, and the highest in the fungal seeded sediments. The largest differences between the angle of avalanche and angle of repose occur in the bacterial seeded and media control sediments. The smallest differences occur in the fungal seeded sediment. In most cases the second angle of avalanche is lower than the first angle of avalanche, whilst the second angle of repose is higher than the first angle of repose.The bacteria bind particles together with their extracellular polymeric material, while the fungus binds particles by holding them together with a network of hyphal filaments. In the bacterial seeded sediment growth is uniform over the sediment surface. In the fungal seeded sediment growth occurs as discrete colonies separated by bare sediment, and the fungal hyphae penetrate the sediment to a significant depth. On avalanching, the fungal colonies move down the slope with the hyphal filaments trailing behind them in the sediment. Overall, both the bacterium and the fungus increase slope stability. However, the fungal colonies maintain slope stability after avalanching more effectively than does the uniform bacterial growth. The results are discussed in relation to the wide range of biological effects that stabilize flat sediments and to laboratory and field studies on the stability of sediment slopes.
Lithofacies relations and palaeoecology of a Late Jurassic to Early Cretaceous fan delta to shelf depositional system in the Sierra Madre Oriental of north‐east Mexico
Authors: MICHALZIK D. , SCHUMANN D. 
Published: June 2006

ABSTRACTThe La Casita Formation in north‐eastern Mexico represents a period of increased terrigenous clastic supply that is characteristic for the entire northern and western margins of the Gulf of Mexico Basin during latest Jurassic and earliest Cretaceous time. A prograding delta complex developed at the south‐eastern margin of the Coahuila Peninsula. Delta plain, delta front, prodelta and shelf areas show distinct lithofacies associations. Some of the most characteristic elements are fining upward and coarsening upward grain size trends, syndepositional deformational features, storm layers, phosphorites, fossiliferous carbonate concretions and an anaerobic biofacies.A possible break in sedimentation is inferred for parts of the Valanginian. This may be interpreted as a sequence boundary between second order cycles. The Kimmeridgian ‐ Berriasian cycle appears to correlate with the Cotton Valley cycle of the Texas Gulf Coast.
Stratigraphical architecture at the muddy margin of the Cretaceous Western Interior Seaway, southern Utah
Authors: LEITHOLD ELANA L. 
Published: June 2006

ABSTRACTThe Tropic Shale and correlative Tununk Shale Member of the Mancos Shale accumulated during Cenomanian‐Turonian time, within prodeltaic environments near the western margin of the Western Interior Seaway of North America. Stratigraphical and sedimentological analysis has revealed a detailed history of relative sea level change in the thick, fine grained succession. The Tropic and Tununk shales were deposited during the Greenhorn second‐order sea level cycle, over a time span of about 2–5 million years. In southern Utah, six depositional sequences are superposed upon the record of this long term sea level change. The sequences developed during third‐order relative sea level cycles of hundreds of thousands of years duration and are composed of at least 37 parasequences, arranged in retrogradational, aggradational and progradational parasequence sets.The Tropic Shale and Tununk Shale Member accumulated just basinward of the axis of maximum subsidence of a foreland basin. Stratal geometries and facies distribution patterns in the succession indicate that in southern Utah the Greenhorn cycle was tectonically controlled. During the Greenhorn transgression and highstand, rapid rates of tectonic subsidence trapped terrigenous sediment to the west of the study area, in the more proximal foreland. At this time, hemipelagic facies accumulated at relatively slow rates in southern Utah and type 2 sequences developed during third‐order sea level cycles. In contrast, during the Greenhorn regression rates of thrust‐induced subsidence in the proximal foreland basin evidently slowed, and deltaic clinoforms prograded across the study area. At least one forced regression occurred in southern Utah at this time, and type 1 sequences developed.The formation of type 1 sequence boundaries in the upper part of the Tropic Shale and Tununk Shale Member points to episodes of base level fall and indicates that the six third‐order sea level cycles recorded in the succession were not the result of changes in sediment supply alone. The third‐order cycles may have been a consequence of episodic tectonism. The timing of these cycles, however, suggests that development of sequences and parasequences in the Tropic Shale and Tununk Shale Member may have been related to orbital forcing in the Milankovitch band. Glacioeustasy or climatically related fluctuations in the amount of groundwater stored on continents may explain these high frequency sea level changes.