The measurement and significance of displacements on Laxfordian Shear Zones,North-West Scotland     

Alastair Beach 《Proceedings of the Geologists' Association. Geologists' Association》1974,85(1):13-IN1

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Lateglacial and Holocene talus slope development and rockwall retreat on Mynydd Du, UK   总被引：1，自引：0，他引：1  

Alastair M. Curry  Chris J. Morris 《Geomorphology》2004,58(1-4):85-106

Modification of Lateglacial and Holocene talus sheets by debris flows and gully incision on Mynydd Du, Wales, has resulted in a convergence of upper slope form characterised by an upper rectilinear slope gradient of 36°±3° and a range of concavities of c. 0.1–0.2. In most cases, gully incision and accumulation of debris cones have led to an increase in slope concavity. Evidence for talus erosion, reworking and redeposition on the upper slope emphasises secondary reworking processes, as well as primary talus accumulation on the upper slope, and permits construction of a model of talus development at Mynydd Du. On the basis of talus volume, calculation of the first rockwall retreat data set for southern Britain suggests that c. 7.1 m (84%) of overall rockwall retreat (8.5 m) took place during the Lateglacial, and only c. 1.4 m (16%) occurred during the Holocene. These figures imply that Lateglacial retreat rates ranged from 1.01 to 2.44 m ka⁻¹, with an overall mean rate of 1.23 m ka⁻¹. In contrast, Holocene rockwall retreat rates range from 0.10 to 0.17 m ka⁻¹, with a mean rate of 0.12 m ka⁻¹. Approximately 27% of cliff retreat is attributed to microgelivation. While similar to Holocene and present-day alpine environments, these Lateglacial retreat rates are one order of magnitude higher than most equivalent values for arctic sites. This reflects both ‘alpine-style’ diurnal freeze–thaw activity on Mynydd Du during the Younger Dryas and paraglacial rock-mass instability following deglaciation. Assuming an exponential decline in rockwall sediment release, it is estimated that approximately half the talus had accumulated within c. 1 ka of deglaciation. At one site, paraglacial talus accumulation appears to have contributed significantly to the glacial sediment transport system of a subsequent ice advance. Present-day rates of rockwall retreat and talus accumulation by rockfall are estimated to be 0.014 and 0.022 mm yr⁻¹ (m ka⁻¹), respectively, similar to values for other British sites and markedly lower than Holocene rates of cliff recession due to microgelivation. By implication, the geomorphic significance of microgelivation may have been greatly underestimated in studies of inland rock-slope evolution in temperate, mid-latitude environments.  相似文献   

Seismic stratigraphic analysis of non-marine Lower Cretaceous strata in the Wessex and North Celtic Sea Basins     

Alastair H. Ruffell 《Cretaceous Research》1995,16(6)

Non-marine strata of Early Cretaceous age ('Wealden facies') are found at outcrop in the type localities of the Wessex Basins, southern England (and adjacent subsurface) and in extensive and thick successions filling the North Celtic Sea Basin. Sedimentology, paleontology, petrology and geochemistry have traditionally been used as evidence in determining the climatic, tectonic and sea level controls on Wealden facies, sedimentary processes and stratigraphy. Analysis of seismic data, through seismic facies and sequence stratigraphic analysis, allows direct comparison of the Wealden in these basins and new interpretations to be made of the tectonic and depositional influences. In the north-eastern end of the North Celtic Sea Basin, tectonic controls on seismic facies can be demonstrated and are related to coarse-grained fan-delta horizons documented in core. In the south-western North Celtic Sea Basin, adjacent to the Cretaceous proto-Atlantic, tectonic controls are less apparent, and changes in relative sea level were probably more important in controlling the preservation of stratigraphic sequences. Where the non-marine Lower Cretaceous succession is imaged clearly, the stratigraphic similarity between parts of the North Celtic Sea and the Weald and Channel Basins suggests a wider control by relative sea-level fluctuations. However, important variations in seismic facies within the basins indicate localized, dominant tectonic control. The recognition of a very distinctive 'lower' Wealden seismic facies, observed as undulatory (?channelized) and downlapping reflections, is contrasted with either the parallel or synsedimentary fault-dominated 'upper' Wealden facies. These seismic characteristics reflect the previously recognized climatic/tectonic change from Hastings to Weald Clay Group environments. The debate on tectonics versus eustacy is further complicated: palaeoclimate appears to be a third process responsible for stratigraphic variation.  相似文献   

The influence of intrabasinal faults on the development of a linked thrust system     

Alastair Welbon 《International Journal of Earth Sciences》1988,77(1):11-24

The Mesozoic and Cenozoic rocks exposed in the Arve valley region of the External French Alps are used to assess the role of early intrabasinal faults on later thrust fault evolution. The early intrabasinal faults produced at some time from latest Upper Cretaceous to Tertiary strike parallel or subparallel to later Neo-Alpine thrusts. Where early faults dip away from the thrusts they are generally cut through and occasionally are overturned during this process. In one example extreme overturning has allowed partial reactivation. Early faults dipping in the same direction as thrusts may: a) be reactivated b) initiate ramping of the thrust ahead of the preexisting fault c) be cut through by the thrust d) cause pinning of the thrust at the footwall of the fault and folding against the fault as displacement continues (buttressing). From this work it is evident that intrabasinal faults exerted a major influence on the distribution of mechanical heterogeneities. These heterogeneities include variations of stratigraphic thickness and type across faults, fault-related unconformities and the presence of the fault itself. During the period of contraction such features strongly controlled the development of the stress field produced ahead of an advancing thrust and hence influenced the position of thrust fault propagation within the stratigraphy.  相似文献   

Preface     

Alastair Dingwall 《Marine Policy》1984,8(2):82

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Holocene reworking of drift‐mantled hillslopes in the Scottish Highlands     

Alastair M. Curry 《第四纪科学杂志》2000,15(5):529-541

Episodic erosion and resedimentation of valley‐side glacigenic drift is evident at three sites in the Scottish Highlands in the form of buried in situ palaeosols intercalated with stacked sediment units. Radiocarbon dating of 10 palaeosols exposed in sections in debris cones suggests that gully erosion and redeposition of drift represents asynchronous debris flow and slopewash activity within the past 6⋅5 cal.kyr. Palynological and charcoal analyses confirm that reworking was unrelated to burning of vegetation, and post‐dated the main phase of anthropogenic woodland clearance at two sites, although the possibility that recent debris flow activity in Glen Docherty may have been associated with removal of woodland cover cannot be dismissed. However, the collective evidence points to discrete, extreme storm‐generated events of random occurrence as the most likely cause of episodes of enhanced slope reworking at the sites investigated. Evidence for periodic climatic deterioration is more equivocal. By implication, the accumulation of substantial debris cones in upland Britain may reflect primarily the reworking of sediment during extremely rare, intense rainstorms, essentially independent of progressive climate changes. Consequently, caution is necessary in using debris flow stratigraphies alone as a means of reconstructing former long‐term climatic changes. Copyright © 2000 John Wiley & Sons, Ltd.  相似文献   

The effect of willow control using a glyphosate formulation on aquatic invertebrates within a New Zealand wetland     

Janine Wech  Alastair Suren  Matthew Brady  Cathy Kilroy 《新西兰海洋与淡水研究杂志》2018,52(1):16-41

Exotic trees, such as willow (Salix spp.), have become invasive around New Zealand waterways. Lowland wetlands in particular are vulnerable, with invasions changing hydrology and causing biodiversity loss. Applying herbicide to the willow canopy is a cost-effective control method, however uncertainty remains around effects on wetland aquatic invertebrates. Within a North Island, New Zealand, wetland, we conducted a paired before-after, control-impact study to determine if aerially applied glyphosate (with adjuvants) affected invertebrate assemblages. No effect on communities was detected during the first year after glyphosate treatment, however significant differences between control and spray-exposed communities were found during the second year, coinciding with a significant drought event in March 2013. By the third year, no statistically significant differences were found between communities from control and spray-exposed sites. This study suggests that glyphosate use, in comparable environments, is unlikely to affect aquatic invertebrates. Furthermore, the study illustrates invertebrate communities’ resilience to drought events.  相似文献   

Linking tsunami deposits, submarine slides and offshore earthquakes     

Alastair G. Dawson   《Quaternary International》1999,60(1):141

The recognition that many tsunamis are associated with coastal sedimentation has been of great value in the study of tsunamis prehistorically. Geological investigation of such sediments has resulted in the identification of a series of palaeotsunamis that appear to have taken place in different areas of the world. In most cases, however, it has proved difficult to link former tsunamis to specific source mechanisms. Studies of modern tsunamis have also faced difficulties in the recognition of the specific source mechanisms. For example, offshore earthquakes may trigger submarine slides that combine to produce complex patterns of tsunami flooding at the coast.  相似文献   

HgO at high pressures: the transition to the NaCl structure (HgO-III) and the equation of state of tetragonal HgO-II     

Jinyuan Yan  Bin Chen  Selva Vennila Raju  Budhiram K. Godwal  Alastair A. MacDowell  Jason Knight  Hongwei Ma  Quentin Williams 《Physics and Chemistry of Minerals》2012,39(4):269-275

The high-pressure behavior of HgO-montroydite was investigated up to 36.5 GPa using angle-dispersive X-ray diffraction. The tetragonal phase of this material (HgO-II), a distortion of the NaCl structure, transforms into the cubic NaCl structure (HgO-III) above ~31.5 GPa. The transformation of mercury oxide from the orthorhombic Pnma (HgO-I) structure to a tetragonal I4/mmm structure (HgO-II) is confirmed to occur at 13.5 ± 1.5 GPa. Neither of the high-pressure phases, HgO-II nor HgO-III, is quenchable in pressure. The derived isothermal bulk modulus of HgO-II and its pressure derivative strongly depend on the assumed zero-pressure volume of this phase, but our elasticity results on HgO-II nevertheless lie significantly closer to theoretical calculations than prior experimental results, and the measured pressure of the phase transformation to the NaCl structure is also in agreement with recent theoretical results. The general accord with theory supports the existence of significant relativistic effects on the high-pressure phase transitions of HgO.  相似文献   

Spectral gamma‐ray logs and palaeoclimate change? Permian–Triassic,Persian Gulf     

Ebrahim Ghasemi‐Nejad  Alastair Ruffell  Hossein Rahimpour‐Bonab  Mohammed Sharifi  Behzad Soltani  Ebrahim Sfidari 《Geological Journal》2015,50(2):210-219

Spectral gamma ray (SGR) logs are used as stratigraphic tools in correlation, sequence stratigraphy and most recently, in clastic successions as a proxy for changes in hinterland palaeoweathering. In this study we analyse the spectral gamma ray signal recorded in two boreholes that penetrated the carbonate and evaporate‐dominated Permian–Triassic boundary (PTB) in the South Pars Gasfield (offshore Iran, Persian Gulf) in an attempt to analyse palaeoenvironmental changes from the upper Permian (Upper Dalan Formation) and lower Triassic (Lower Kangan Formation). The results are compared to lithological changes, total organic carbon (TOC) contents and published stable isotope ( δ ¹⁸O, δ ¹³C) results. This work is the first to consider palaeoclimatic effects on SGR logs from a carbonate/evaporate succession. While Th/U ratios compare well to isotope data (and thus a change to less arid hinterland climates from the Late Permian to the Early Triassic), Th/K ratios do not, suggesting a control not related to hinterland weathering. Furthermore, elevated Th/U ratios in the Early Triassic could reflect a global drawdown in U, rather than a more humid episode in the sediment hinterlands, with coincident changes in TOC. Previous work that used spectral gamma ray data in siliciclastic successions as a palaeoclimate proxy may not apply in carbonate/evaporate sedimentary rocks. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献