Taking varves to bits: Scanning electron microscopy in the study of laminated sediments and varves   总被引：2，自引：0，他引：2  

Jean M. Dean  Alan E.S. Kemp  David Bull  Jennifer Pike  Gunilla Patterson  Bernd Zolitschka 《Journal of Paleolimnology》1999,22(2):121-136

Conventional high resolution studies of varved sediments are able to identify clastic and biogenic laminae, but are often unable to resolve the nature of fine-scale lamination contained therein. This intra-annual signal provides us with the highest potential resolution from the sedimentary record and can be resolved using scanning electron microscopy (SEM). Six case studies from lacustrine and marine settings are presented to illustrate the combination of clastic and biogenic fabric types typically found in laminated sediments. Clastic laminae fabrics include those which originate through grain settling and those which are event deposits. The correct identification of event deposits is essential if varves are to be used chronologically. SEM-based biogenic laminae fabric studies have identified seasonal faunal successions where individual laminae may be less than 100 thick and most recently, deep chlorophyll maxima (DCM) summer diatom floras, providing an insight into seasonal scale processes. High resolution lamina fabric studies can provide a basis for generating records of seasonal and inter-annual variability, thus contributing to our understanding of lacustrine and marine processes and palaeoenvironmental interpretation.  相似文献   

Cosmic ray tracks in the Shalka meteorite     

R.K. Bull  S.A. Durrani 《Earth and Planetary Science Letters》1976,32(1):35-39

Track densities of heavy cosmic rays have been studied in hypersthene crystals taken from seven known depths below a fusion crust in the Shalka meteorite. By comparing these with a theoretical model, a preatmospheric radius of 20 ± 2cm has been estimated for the meteorite, assuming it to have been originally spherical. The fossil track-length distributions show a prominent peak in the region of 12–15 μm; and this has been attributed, on the basis of accelerated-ion calibration experiments, to cosmic ray Fe ions. The value (≥0.38 ± 0.04) of relative abundance of Fe ions to the total Fe-group ions (20 ≤ Z. ≤ 28) found in the cosmic ray flux averaged over the last ～19 m.y. is shown to be broadly similar to those given by present-day measurements.  相似文献   

Fault and magmatic interaction within Iceland's western rift over the last 9 kyr   总被引：2，自引：0，他引：2  

J. M. Bull  T. A. Minshull  N. C. Mitchell  K. Thors  J. K. Dix  A. I. Best 《Geophysical Journal International》2003,154(1):F1-F8

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Field meeting at Croydon,Surrey: Saturday, 20th May, 1944     

F. Gossling  A.J. Bull 《Proceedings of the Geologists' Association. Geologists' Association》1945,56(2):140-141

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Buried scour marks as indicators of palaeo-current direction at the Mary Rose wreck site     

R. Quinn  J. M. Bull  J. K. Dix 《Marine Geology》1997,140(3-4):405-413

A 3-dimensional seismic reflection survey (using a Chirp source) of the excavated Mary Rose wreck site (King Henry VIII's flagship, wrecked in 1545) was conducted in the East Solent, off the south coast of England. The high resolution geophysical survey identified two ‘brightspot' anomalies, buried to a depth of 4–5 m, trending east–west adjacent to the western margin of the excavation hole. These anomalous reflectors are interpreted as infilled palaeo-scour features associated with the wrecking and subsequent degradation of the Mary Rose. The features were previously unrecognized on the site, and to the authors' knowledge this is the first time that such preserved longitudinal palaeo-scour marks have been recognized in the sedimentary record. Implications associated with the preservation and identification of the scour features are discussed.  相似文献   

The fission track records of the Estherville,Nakhla and Odessa meteorites     

P.F. Green  R.K. Bull  S.A. Durrani 《Geochimica et cosmochimica acta》1978,42(9):1359-1366

Investigation of fossil charged-particle track densities in various mineral phases of three meteorites—Estherville, Nakhla and Odessa—coupled with U content determination, has led to the evaluation of various contributions to the total fossil track density, including those due to the spontaneous fission of ²³⁸U and ²⁴⁴Pu. A fission-track age for Estherville of around 4 × 10⁹ yr is found, which is thought to reflect slow cooling of the parent body. A Pu track excess of (106 ± 9): 1 over the spontaneous fission of ²³⁸U is found in Odessa diopside, which is larger than may be allówed on a simple ‘continuous synthesis’ model for the production of heavy elements prior to solar system formation. Possible explanations for this value are discussed, including fractionation of Pu relative to U. No detectable U was found in Nakhla diopside, and a measurable contribution of track densities from the fission of superheavy elements is ruled out on the basis of track-length measurements and laboratory calibration with Fe ions.  相似文献   

The sedimentology of river confluences     

Gregory H. Sambrook Smith  Andrew P. Nicholas  James L. Best  Jonathan M. Bull  Simon J. Dixon  Steven Goodbred  Maminul H. Sarker  Mark E. Vardy 《Sedimentology》2019,66(2):391-407

Channel confluences are key nodes within large river networks, and yet surprisingly little is known about their spatial and temporal evolution. Moreover, because confluences are associated with vertical scour that typically extends to several times the mean channel depth, the deposits associated with such scours should have a high preservation potential within the rock record. Paradoxically, such scours are rarely observed, and their preservation and sedimentological interpretation are poorly understood. The present study details results from a physically‐based morphodynamic model that is applied to simulate the evolution and alluvial architecture of large river junctions. Boundary conditions within the model were defined to approximate the junction of the Ganges and Jamuna rivers, Bangladesh, with the model output being supplemented by geophysical datasets collected at this junction. The numerical simulations reveal several distinct styles of sedimentary fill that are related to the morphodynamic behaviour of bars, confluence scour downstream of braid bars, bend scour and major junction scour. Comparison with existing, largely qualitative, conceptual models reveals that none of these can be applied simply, although elements of each are evident in the deposits generated by the numerical simulation and observed in the geophysical data. The characteristics of the simulated scour deposits are found to vary according to the degree of reworking caused by channel migration, a factor not considered adequately in current conceptual models of confluence sedimentology. The alluvial architecture of major junction scours is thus characterized by the prevalence of erosion surfaces in conjunction with the thickest depositional sets. Confluence scour downstream of braid bar and bend scour sites may preserve some large individual sets, but these locations are typically characterized by lower average set thickness compared to major junction scour and by a lack of large‐scale erosional surfaces. Areas of deposition not related to any of the specific scour types highlighted above record the thinnest depositional sets. This variety in the alluvial architecture of scours may go some way towards explaining the paradox of ancient junction scours, that while abundant large scours are likely in the rock record, they have been reported rarely. The present results outline the likely range of confluence sedimentology and will serve as a new tool for recognizing and interpreting these deposits in the ancient fluvial record.  相似文献   

西藏高原洞穴碎屑沉积的电镜扫描研究     

P.A.Bull  A.W.Magee  M.M.Sweeting  章典  施振斌 《中国岩溶》1990,9(1):76-87

应用电镜扫描石英颗粒结构分析,对西藏高原洞穴碎屑沉积及其相关的地表沉积物进行了研究。其结论是:绝大部分样品的物质来源于附近的火成岩和变质岩基岩,这些沉积物的石英颗粒表面并未显示经受冰川的、河流的或风的搬运作用所造成的变形痕迹,也缺乏成土作用或热带湿热环境的化学变形证据。很明显,目前的环境难于在石英颗粒表面产生物理的或化学的结构形态。   相似文献   

An investigation of the effects of mass distribution on pounding structures     

Gregory Cole  Rajesh Dhakal  Athol Carr  Des Bull 《地震工程与结构动力学》2011,40(6):641-659

The effects of diaphragm mass distribution are investigated for building pounding. Elastic diaphragm‐to‐diaphragm collisions are explained by considering the total momentum over the length of each diaphragm at three critical instants during collision. Expressions for collision force and collision duration are produced, providing additional information about the collision process. Equations for the post collision velocity of each diaphragm are produced and are found to appreciably differ from conventional impact—momentum equations under certain conditions. The change in post collision velocity is found to be dependent on the ratio of the axial periods of free vibration of the two diaphragms and the ratio of their masses. An equivalent lumped mass model is proposed and assessed against simplified distributed mass models with numerical modelling of two two‐storey buildings. Finally, a new parameter is introduced to represent the plasticity of an inelastic collision between the two distributed masses. This paper highlights the significant influence that diaphragm mass distribution may have on the analysis of pounding structures. Copyright © 2010 John Wiley & Sons, Ltd.  相似文献   

Volcanic influences in a storm‐ and tide‐dominated shallow marine depositional system: The late permian broughton formation,southern Sydney Basin,Kiama, NSW     

Stuart W. Bull  R.A.F. Cas 《Australian Journal of Earth Sciences》2013,60(4):569-584

Shallow marine sediments of the Broughton Formation are dominated by immature volcanic debris of intermediate to basic composition, generated in an adjacent subaerial environment by volcanism responsible for the nine shoshonite units intercalated within sediments of the Kiama region. Sediment was supplied to the offshore environment via periodic storm‐generated, expanded high density turbidity currents. Initial deposition, represented by the Westley Park Sandstone Member, was below storm wave base, during which time the depositional surface was subjected to post‐depositional tractional reworking by northerly directed, tidally influenced bottom currents. The resulting positive‐relief sand bodies on the seafloor contain tractional sedimentary structures (the ‘tractional facies association'). Areas of the substrate between these sand bodies retained their turbidite bedding structure (the ‘rhythmically bedded facies association') but were extensively bioturbated by a diverse deposit‐feeding biomass.

Upon emplacement of the lowest of the nine shoshonite units as a tri‐composite, locally intrusive lava flow, the depositional surface was elevated, transgressing storm wave base. The body of the shoshonite flow also shielded the substrate from the northerly directed tractional currents, allowing the development and preservation of the hummocky cross‐stratified sandstone facies in the Kiama Sandstone Member. Following burial of the shoshonite flow by continued deposition, this local shielding effect was overcome and tractional currents again reworked the entire depositional surface.  相似文献