摘要:

ABSTRACTA recent (100 yr old) turbidite is described from Hueneme Fan, California Continental Borderland. Dense sampling over the fan surface has allowed excellent delineation of the characteristics of this deposit. It exhibits Bouna DE sequences and has a distinctly bimodal, sandy silt grain size distribution. Through the use of generalized fluid dynamics equations, it is possible to reconstruct original flow properties of the current which deposited this material. The calculated velocities ranged from 10–90 cm s‐1and excess density (above ambient seawater) from 0·001–0·005 g cm‐3in the lower midfan and upper fan channel regions, respectively. Height of the current ranged from 5–15 m, on slopes from 1·5 to 0·15°. A total of 107m3of sediment was deposited during 10 days. The turbidity current is conjectured to have originated from direct river input during the floods of 1884.An older event is also described, which has distinctly different properties and origins. The grain sizes of this older deposit are much coarser, and sedimentary structures suggest higher flow regimes. This turbidite is conjectured to have been deposited from a higher density, faster current thought to have been generated by slumping. The need for a better understanding of the controls on the characteristics of turbidity currents and their effect on fan morphology

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00565.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTAlthough contemporary dust storms are frequent in arid and semi‐arid areas, desert loess deposits are poorly developed. Much of the World's loess occurs in mid‐latitude areas which experienced glaciation during the Pleistocene. Ocean core evidence indicates that dust transport from sub‐tropical deserts increased during cold stages of the Pleistocene, but loess formed only on certain desert margins, for reasons which have not been fully explained. This paper re‐examines the mechanisms of dust transport and deposition, and the circumstances leading to the accumulation of thick loess. Typical loess is composed mainly of medium silt grains which are transported in short‐term suspension a few metres above the ground. Significant thicknesses of loess form only when dust is trapped within a limited area, often relatively close to the source. Dust particles finer than 20 μm are transported mainly in long‐term suspension over a greater height range and may be widely dispersed.The availability of silt and the frequency, magnitude and direction of dust‐transporting winds are important factors governing the potential for loess formation, but the existence of a suitable dust trap is particularly important. Traps may be formed by topographic obstacles, areas of moist ground, or vegetated surfaces. Vegetation adjacent to glacial outwash plains and braided meltwater streams trapped dust in mid‐latitudes during the Pleistocene. Dust blown during glacial periods from certain deserts, notably in Sinai, Soviet Central Asia and China, accumulated as loess in neighbouring semi‐arid regions. On the margins of other deserts loess formation was inhibited partly by the absence of vegetation traps. During most of the Holocene net dust deposition rates in all desert‐marginal areas have been too low for significant loess accumulation. This is mainly due to a reduction in silt availability and a tendency towards landscape stability. Reported dust storm frequencies during the past 50 years over‐estimate the longer‐term Holocene dust flux due to the effects of human activities. Much modern dust owes its origin to erosion of cultivated soils in semi‐arid areas and i

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00566.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTIn this paper interference phenomena between particle flattening and particle rounding during the free vertical sedimentation of natural particles are investigated. Particle flattening is measured by the Corey Shape Factor (CSF), particle rounding by the Powers Index (P). Using Dietrich's equation, the effect of the flattening and the effect of the rounding on the terminal fall velocity of the particle can be studied separately. This is done by examining the influence of particle flattening and particle rounding on the Particle Sedimentation Coefficient (PSC), which is the ratio of the nominal diameterDnof the particle to the equivalent sedimentation diameterDb. It appears that particle flattening and particle rounding are interdependent so that interference phenomena occur. In the case of positive interference, the combined effect of flattening and rounding on the PSC is larger than the sum of the effects of both parameters separately. If, on the other hand, the combined effect is smaller than the sum of the single effects, negative interference occurs.For fine‐grained particles, the more the shape of a particle differs from a sphere in one variable (flattening or rounding), the higher will be the influence of the other variable on the free vertical fall. For coarse‐grained particles, the inverse occurs. For very fine‐grained particles (Dn<200 μm (air) or<370 μm (water)) the influence of particle rounding on free vertical settling is negligibly small. For coarser grained particles, flattening is the most important parameter if the grains are strongly flattened. If not, rounding will be the governing pa

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00567.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

摘要:

ABSTRACTA new automatic method for quantitative mineralogical analysis of carbonate sediments by X‐Ray diffraction is described. Percentage analysis has been carried out by means of the reference intensities method from integrated areas of the diffraction peaks. Diffraction peaks were fitted by means of Lorentzian profiles. The addition of an internal standard allows the determination of data on the stoichiometric disequilibrium of the carbonate phases and the existence of non‐carbonate phases and/or non‐crystalline phases simultaneously. Relative errors of the quantitative estimates of carbonate phases are always less than 15%, even though real percentage differences between actual and calculated values are less than 3·8% in all

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00568.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00569.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00570.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00571.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00572.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00573.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY

ISSN:0037-0746    

DOI:10.1111/j.1365-3091.1987.tb00574.x

出版商:Blackwell Publishing Ltd    

年代:1987

数据来源: WILEY