摘要:

ABSTRACTRhythmic changes were observed in the quantity of radiolarians and calpionellids preserved in an Upper Jurassic to Lower Cretaceous deep water succession in the Bakony Mountains of Hungary. Statistical analysis revealed that for short periods (order of 100 kyr) the correlation of the two planktonic groups is negative. Over longer time intervals definite trends are recognized, which can be explained by the combined effect of facies transition, evolution of the calpionellids and rhythmic environmental (climatic) changes. Spectral analysis and time span calculations indicate that orbital forcing in the Milankovitch frequency band influenced the observed changes.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01415.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTDrill cores from Holocene reefs on Tahiti (French Polynesia) reveal a framework composed of massive branching acroporids encrusted by coralline algae associated with sessile vermetid gastropods and arborescent foraminifers. Laminated micritic crusts form coatings over coral branches or, more commonly, over related encrusting organisms throughout the cored reef sections; these crusts appear as a major structural and volumetric component of the reef framework. The microbial nature of these micritic crusts is inferred from their typical organic growth forms and geometry, the occurrence of microbial remains and stable isotope measurements. The reef communities accumulated at depths less than 5 m below mean sea level in a high energy environment throughout vertical growth from 7140 ± 170 yrbpto the present. The nature of the involved benthic communities, stable isotope data and high calcification rates of microbially encrusted corals strongly suggest that local environmental conditions have been optimal for reef development for the last 7000 years. The causes of the predominance of microbial communities over actual encrusters (red algae, foraminifers) remain problematic and could be related to short term fluctuations in ecological parameters. Microbial micritic crusts seemingly played a prominent role in protecting the coralgal colonies from bioeroders and grazers and, possibly, in strengthening the framework, due to rapid lithification. The record of similar microbial crusts in other Quaternary reef tracts suggests that microbial communities may have played a more prominent role in Quaternary reefs than presently recognized

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01416.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

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 a

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01417.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe early Stephanian Bonar Cyclothem of the Sydney Basin, Nova Scotia, contains an erosional surface cut through coastal plain strata with economic coals and distributary channel bodies. The erosion surface is interpreted as a palaeovalley 20 m deep and at least 7 km wide that marks a sequence boundary formed during relative fall in sea level. The palaeovalley is filled with stacked alluvial channel bodies which become more isolated as the valley fill passes upward into red, alluvial plain deposits, probably laid down in an anastomosed river system. In an adjacent, interfluve area, calcretes and red, vertic palaeosols cap coastal strata. Assemblage analysis of agglutinated foraminifera and thecamoebians indicates that the palaeovalley was filled with freshwater sediments before an initial marine transgression flooded the alluvial surface and adjacent interfluve.Valley incision probably reflects glacioeustatic sea level fall. However, the alluvial nature of the valley deposits suggests that valley filling reflects an abundant sediment supply during lowstand and/or transgressive stages and was not a direct consequence of sea level rise. During the subsequent transgression phase, aggradation was rapid as sediment supply apparently kept pace with rising sea level. Features of both channel and extra‐channel facies suggest that seasonality intensified during the transition from coastal plain to palaeovalley and alluvial plain depositio

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01418.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTA new wind tunnel has been constructed to study the mass transport properties of wind‐blown sand. This report on the first set of experiments in the new wind tunnel concerns the effect of slope on threshold friction speed. The results of this series of static threshold experiments (and one dynamic threshold experiment) for a range of particle diameters and bed slope angles show that, provided the effects of Reynolds number variation and interparticle cohesive force are accounted for, the static friction angle α is independent of slope and close in value to the measured static angle of respon

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01419.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTSandbodies from storm‐dominated marine and marginal marine environments commonly contain intervals of laminated fine sandstones. A characteristic of such lamination is the presence of low angle cross lamination. In order to model correctly the effects of such lamination on a waterflood of an oil‐bearing shoreface sequence it was necessary to quantify the geometry of the laminaset elements.This challenge has been greatly complicated by the lack of outcrop of the formation of interest. The Middle Jurassic Rannoch Formation of the North Sea only occurs in the subsurface where it is not possible in core to measure the aspect ratio of laminasets directly. In this study, the laminaset geometry data that can be obtained from core (e.g. apparent set thicknesses) were collected for the Rannoch Formation. These data were compared with similar data from potential outcrop analogues in (1) the Cretaceous Kennilworth Member of the Blackhawk Formation in Utah, USA and (2) the Upper Jurassic Bencliff Grit from the Dorset coast, UK.A quantitative analysis of laminaset geometries has been used to compare subsurface core with potential outcrop analogues. The Rannoch Formation core is characterized by numerous low angle truncations. We have measured these features in two wells (means of 7.2° and 12.1°). Mean apparent set thicknesses were 0.24 and 0.19 m.In the outcrop sections studied, truncation angles ranged from 9.6° to 13.4° and mean set thicknesses from 0.24 to 0.34 m. Mean bounding surface dips of 5.8° and 8.6°, and mean laminaset lengths of 2.3 and 4.1 m were also measured. directly in the field and by using photomosaics.On the basis of this comparison, the Kennilworth Member in Utah was found to be the most suitable and the geometries (i.e. aspect ratios) measured there were used to generate an appropriate geometry of Rannoch laminaset geometries for use in engineering studies: laminaset length, 2.0 m; laminaset thickne

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01420.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTError polygons and error bars are commonly used graphical methods for showing the precision associated with means on triangular diagrams and orthogonal plots, respectively. If the error or precision associated with a mean, based either upon counts of individual components or upon count ratios, is to be defined using basic statistics, the confidence interval method should be used. Standard deviation alone, which has been frequently in the literature, is not an appropriate statistic to use for this purpose. The graphical use of confidence intervals permits a rapid visualization of statistically significant differences between means, and thus more quantitative description of detrital clastic composition. Previous investigators using the standard deviation method have underestimated the precision of their data when sample sizes are large, and overestimated precision with small samples.

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01421.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

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 byPhragmitessp. andTyphasp. (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 byin situbiological 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 ofin situproduction 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 pres

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01422.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe only reported occurrence of quartz‐rich sands in Jamaica is on the plains of Black River, St Elizabeth, where they outcrop in isolated pockets. The sands overlie limestones of the White Limestone Supergroup and the Coastal Group, and partly underlie Holocene peat deposits.Monocrystalline quartz, containing fluid and mineral inclusions, is the dominant mineral of the sands, with the heavy minerals. magnetite, haematite, epidote, rutile, anatase, zircon and tourmaline, occurring in minor amounts. Grain surface texture studies of the quartz grains have identified a series of mechanical and chemical features. Microtextures depicting marine, aeolian and pedological environments have been retained on the grains and represent the last three stages in the history of the grains.The petrography of the sands indicates an igneous provenance, with the major source of the detrital minerals being the Cretaceous granitoids that outcrop in the eastern half of the island. These rocks were unroofed during the uplift of Jamaica in the Late Tertiary and were subjected to extensive chemical weathering, which accounted for the breakdown of the majority of their primary silicate minerals. Fluvial transportation and deposition of detritus onto the shelf of the south coast commenced during this period and continued into the Pleistocene, providing terrigenous sand that was modified along the coast to quartz arenite by moderate to high energy marine conditions. Tidal currents, east‐west longshore currents and weathering contributed to the maturity of the sand by the time it had been transported along the shelf to the vicinity of Black River. During low sea level stands in the Pleistocene, some of this sediment that became trapped on the shelf was blown inland by south‐east

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01423.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

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 dolom

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1994.tb01424.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY