Retreat dynamics of the eastern sector of the British–Irish Ice Sheet during the last glaciation     

David J. A. Evans  David H. Roberts  Mark D. Bateman  Chris D. Clark  Alicia Medialdea  Louise Callard  Elena Grimoldi  Richard C. Chiverrell  Jeremy Ely  Dayton Dove  Colm Ó Cofaigh  Margot Saher  Tom Bradwell  Steven G. Moreton  Derek Fabel  Sarah L. Bradley 《第四纪科学杂志》2021,36(5):723-751

The findings of BRITICE-CHRONO Transect 2 through the North Sea Basin and eastern England are reported. We define ice-sheet marginal oscillation between ~31 and 16 ka, with seven distinctive former ice-sheet limits (L1–7) constrained by Bayesian statistical analysis. The southernmost limit of the North Sea Lobe is recorded by the Bolders Bank Formation (L1; 25.8–24.6 ka). L2 represents ice-sheet oscillation and early retreat to the northern edge of the Dogger Bank (23.5–22.2 ka), with the Garret Hill Moraine in north Norfolk recording a significant regional readvance to L3 at 21.5–20.8 ka. Ice-marginal oscillations at ~26–21 ka resulted in L1, L2 and L3 being partially to totally overprinted. Ice-dammed lakes related to L1–3, including Lake Humber, are dated at 24.1–22.3 ka. Ice-sheet oscillation and retreat from L4 to L5 occurred between 19.7 and 17.3 ka, with grounding zone wedges marking an important transition from terrestrial to marine tidewater conditions, triggered by the opening of the Dogger Lake spillway between 19.9 and 17.5 ka. L6 relates to ice retreat under glacimarine conditions and final ice retreat into the Firth of Forth by 15.8 ka. L7 (~15 ka) represents an ice retreat from Bosies Bank into the Moray Firth.  相似文献   

Metamorphism and kinematics of the early deformation in the Variscan suture of SW Iberia     

C. PONCE  J. F. SIMANCAS  A. AZOR  D. J. MARTÍNEZ POYATOS  G. BOOTH‐REA  I. EXPÓSITO 《Journal of Metamorphic Geology》2012,30(7):625-638

The early (Devonian) collisional stage in SW Iberia has been investigated through the analysis of deformation in the Cubito‐Moura schists, the main lithology of an Allochthonous Complex putatively rooted in the suture between the Ossa‐Morena and South Portuguese zones. The first deformation in these schists (D₁) is recorded as a S₁‐L₁ mylonitic fabric well preserved in early quartz veins. Subsequent D₂ deformation caused the main folds and the main (S₂) foliation. After restoration, the stretching lineation (L₁) trends at a small angle with the Ossa‐Morena/South Portuguese suture. This trend, together with the top‐to‐the‐east kinematics determined from quartz microfabric is indicative of an oblique left‐lateral collisional scenario in SW Iberia. Chlorite–white K‐mica–quartz ± chloritoid multi‐equilibrium calculations yield P–T conditions in the range 0.9–1.2 GPa and 300–400 °C, during the first collisional stage. P–T conditions during D₂ were 0.3–0.8 GPa and 400–450 °C, thus indicating an important stage of exhumation of the Allochthonous Complex during these two collisional events, after subduction of the Ossa‐Morena Zone margin under the South Portuguese Zone continental crust. In the general context of the Variscan orogen, dominated by dextral collision, the left‐lateral convergence in SW Iberia can be explained in terms of the Avalonian salient represented by the South Portuguese Zone, which would impinge between Iberia and Morocco.  相似文献   

Biochronological data inferred from the Early Pleistocene small mammals of the Barranc de la Boella site (Tarragona,north‐eastern Spain)          下载免费PDF全文

IVÁN LOZANO‐FERNÁNDEZ  SANDRA BAÑULS‐CARDONA  HUGUES‐ALEXANDRE BLAIN  JUAN MANUEL LÓPEZ‐GARCÍA  JOSEP VALLVERDÚ  JORDI AGUSTÍ  GLORIA CUENCA‐BESCÓS 《第四纪科学杂志》2014,29(7):722-728

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Formation of a stratified subglacial ‘till’ assemblage by ice‐marginal thrusting and glacier overriding     

COLM Ó COFAIGH  DAVID J. A. EVANS  JOHN F. HIEMSTRA 《Boreas: An International Journal of Quaternary Research》2011,40(1):1-14

Ó Cofaigh, C., Evans, D. J. A. & Hiemstra, J. F. 2010: Formation of a stratified subglacial ‘till’ assemblage by ice‐marginal thrusting and glacier overriding. Boreas, 10.1111/j.1502‐3885.2010.00177.x. ISSN 0300‐9483. A thick sequence of glaciotectonically stacked till and outwash is preserved in a coastal embayment at Feohanagh, southwest Ireland. The sequence contains a variety of diamicton lithofacies, including laminated, stratified and massive components, but stratified diamictons dominate. Stratification/lamination is imparted by the presence of numerous closely spaced subhorizontal and anastomosing partings, which give a fissile appearance to the diamictons. Many partings are the result of sandy or thin gravelly layers within the diamictons. Some diamictons contain interbeds and lenses of sand, mud and gravel, which still preserve the original stratification. The sequence at Feohanagh is the product of a two‐stage depositional process in which initial glaciolacustrine sedimentation in an ice‐dammed lake was followed by glaciotectonic thrusting and overriding, during which the lake sediments were reworked and variably deformed. Similar late Quaternary sequences of glaciotectonically stacked stratified sediments and till have been described from around the coastal margins of Ireland and Britain, where they constitute glaciotectonite–subglacial traction till continuums rather than true lodgement tills as traditionally implied. Thick stratified diamicton assemblages are likely to occur in areas where steep topography provides pinning points for the glacier margin to stabilize and deliver large volumes of sediment into a glaciolacustrine or glaciomarine setting before proglacial and subglacial reworking of the sediment pile. The resulting geological–climatic unit, often defined as ‘till’, will contain a large amount of stratified and variably deformed material (laminated and stratified diamictons will be common), including intact sediment rafts, reflecting low strain rates and short sediment transport distances.  相似文献   

The first description and confirmation of the Vista Alegre impact structure in the Paraná flood basalts of southern Brazil     

Alvaro P. CRÓSTA  Christian KOEBERL  Rafael A. FURUIE  Cesar KAZZUO‐VIEIRA 《Meteoritics & planetary science》2010,45(2):181-194

Abstract– The Vista Alegre structure, centered at 25°57′S and 52°41′W, has been recently proposed as a meteorite impact structure. The 9.5 km‐diameter structure is located in the Paraná state of southern Brazil, within the Paraná Basin, which contains one of the largest and most extensive flood basalt provinces on Earth. The Paraná flood basalts belong to the Serra Geral Formation and are temporally related to the opening of the South Atlantic Ocean, having been dated at about 133–132 Ma. Tholeiitic basalts dominate the western portion of Paraná state, with some minor rhyodacites. Morphologically, Vista Alegre has a prominent circular outline, in the form of an incomplete ring of escarpments, and an inner depression. The presence of a central uplift is not obvious, but it is inferred by the occurrence of deformed sandstone blocks near the center of the structure. These sandstones are possibly related to the Triassic Pirambóia Formation and/or to the Cretaceous Botucatu Formation. These units are normally at stratigraphic depths of about 700–800 m below the present surface in this portion of the Paraná Basin. The structure appears to be in an advanced erosion stage and its interior is occupied by a soil cover several meters thick, extensively used for agriculture. As a result there are limited outcrops in the interior of the structure, all of polymict breccias, some of them melt‐bearing. We report the extensive occurrence of shatter cones, in the form of fine‐grained rock clasts within the polymict breccias. The shatter cone‐bearing breccias occur at different locations within the structure, separated by several kilometers. The nested shatter cones range in size from about 0.5 to 20 cm for individual cones, and up to half a meter for complete assemblages. The shatter cones formed in fine‐grained Parana flood basalt and might be the first examples of shatter cones in such a rock type. In addition, planar deformation features (PDFs) were found in quartz grains within sedimentary rock clasts of the polymict breccia. These findings confirm the impact origin of the Vista Alegre structure.  相似文献   

The Colônia structure,São Paulo,Brazil     

Claudio RICCOMINI  Alvaro P. CRÓSTA  Renato L. PRADO  Marie‐Pierre LEDRU  Bruno J. TURCQ  Lucy G. SANT’ANNA  José A. FERRARI  W. Uwe REIMOLD 《Meteoritics & planetary science》2011,46(11):1630-1639

Abstract– The near‐circular Colônia structure, located in the southern suburbs of the mega‐city of São Paulo, Brazil, has attracted the attention of geoscientists for several decades due to its anomalous character and the complete absence of any plausible endogenous geologic explanation for its formation. Origin by impact cratering has been suggested repeatedly since the 1960s, but no direct evidence for this has been presented to date. New seismic data have been recently acquired at Colônia, providing new insights into the characteristics and possible layering of infill of the structure, as well as into the depth to the underlying basement. We review the current knowledge about the Colônia structure, present the new seismic data, and discuss the existing—as yet still indirect—evidence for a possible origin by an impact. The new data suggest the existence of a sedimentary fill of approximately 275 m thickness and also the presence of two intermediate zones between sediment and basement: an upper zone that is approximately 65 m thick and can be interpreted as a possible crater‐fill breccia, whereas the other zone possibly represents fractured/brecciated basement, with a thickness of approximately 50 m. Although this depth to basement seems to be inconsistent with the expected geometry of a simple, bowl‐shape impact structure of such diameter, there are a number of still unconstrained parameters that could explain this, such as projectile nature, size and velocity, impact angle, and particularly the current erosion depth.  相似文献   

The complex impact structure Serra da Cangalha,Tocantins State,Brazil     

Thomas KENKMANN  Marcos A. R. VASCONCELOS  Alvaro P. CRÓSTA  Wolf U. REIMOLD 《Meteoritics & planetary science》2011,46(6):875-889

Abstract– Serra da Cangalha is a complex impact structure with a crater diameter of 13,700 m and a central uplift diameter of 5800 m. New findings of shatter cones, planar fractures, feather features, and possible planar deformation features are presented. Several ring‐like features that are visible on remote sensing imagery are caused by selective erosion of tilted strata. The target at Serra da Cangalha is composed of Devonian to Permian sedimentary rocks, mainly sandstones that are interlayered with siltstone and claystones. NNE–SSW and WNW–ESE‐striking joint sets were present prior to the impact and also overprinted the structure after its formation. As preferred zones of weakness, these joint sets partly controlled the shape of the outer perimeter of the structure and, in particular, affected the deformation within the central uplift. Joints in radial orientation to the impact center did not undergo a change in orientation during tilting of strata when the central uplift was formed. These planes were used as major displacement zones. The asymmetry of the central uplift, with preferred overturning of strata in the northern to western sector, may suggest a moderately oblique impact from a southerly direction. Buckle folding of tilted strata, as well as strata overturning, indicates that the central uplift became gravitationally unstable at the end of crater formation.  相似文献   

'On glaciers in general and particular…': The life and works of an Icelandic pioneer in glacial research     

ÓLAFUR INGÓLFSSON 《Boreas: An International Journal of Quaternary Research》1991,20(1):79-84

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Site Response as a Function of Near-Surface Geology in the South Iceland Seismic Zone     

K. Atakan  B. BrandsdÓttir  P. HalldÓrsson  G.O. Fridleifsson 《Natural Hazards》1997,15(2-3):139-164

Site response measurements provide information on the amplification of ground motions generated by local conditions. Recent studies of large destructive earthquakes have shown that damage during the earthquakes are often caused by the amplification of seismic waves in near-surface sedimentary layers. The estimation of site response is therefore critical, in order to evaluate the true seismic hazard potential of a given area. We investigated local site amplifications in the South Iceland Seismic Zone (SISZ). Nine digital seismographs were deployed, temporarily, in an area of approximately 400 km2, in the westernmost part of the SISZ. Among the 90 events recorded, 15 were used in this study, including a magnitude 3.1 (ML) event and selected aftershocks, which occurred in the northern outskirts of the village Hveragerdi. Single Station Spectral Ratios (SSSR) of the recorded earthquakes revealed some of the effects of local site conditions. Spectral amplification factors of 2–5 on average, can be expected in the SIL area, depending on the sediment type and thickness. Higher site amplifications occur in the southern part of the study region, where the thickest sedimentary cover is found. Spectral amplification, related to topographical effects, is observed at the bedrock reference station, Bjarnastadir. Standard Spectral Ratios (SSR), with respect to the bedrock reference station, Bjarnastadir, were also calculated for some stations, in order to compare the two spectral ratio results. The two methods show a good correlation at some stations, whereas at others they vary considerably. The comparison between the earthquake and ambient noise data, on the other hand, gave better correlation when the SSSR method is used.  相似文献   

Experimental studies on the evolution process of microcrack patterns in concrete samples con-taining hard inclusions   总被引：1，自引：0，他引：1  

许昭永  王彬  赵晋明  胡毅力  梅世蓉 《地震学报(英文版)》1997,10(3):397-403

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