首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 22 毫秒
1.
This paper records the findings at a temporary exposure at Thorpe St Andrew near Norwich, Norfolk, UK in Early and early Middle Pleistocene Crag deposits. The British Geological Survey (BGS) describes the particular formation exposed as Norwich Crag consisting of Early Pleistocene shallow marine sediments. The section shows a succession of sorted sands and gravels overlain by a sandy diamicton. Based on field evidence and clast analysis, the sands and gravels are interpreted as the product of point bar and overbank sedimentation and represent the product of a river cutting into and aggrading within the more widespread shallow marine deposits. Composition of the sediments indicates derivation, primarily from Wroxham Crag Formation, with a contribution from Norwich Crag. The sandy diamicton is interpreted as late Middle Pleistocene Corton Till that is recorded in the area. A distinct pattern of colour changes at the top of the sands and gravels is interpreted as a soil that developed on the fluvial sediments before being overridden by the glacier that deposited the Corton Till. The existence of the fluvial sediments within the regional shallow marine deposits suggests that a fall of sea-level, possibly due to climate cooling, while the elevation of the sediments and the adjacent Crag implies that the site has been uplifted since sedimentation. This is the first observation of terrestrial sediments within the shallow marine Crag. The paper also makes a contribution to understanding the diagenetic processes that give deposits within this region some distinctive colour and sediment patterns.  相似文献   

2.
Erratic clasts with a mass of up to 15 kg are described from preglacial shallow marine and coastal deposits (Wroxham Crag Formation) in northeast Norfolk. Detailed examination of their petrology has enabled them to be provenanced to northern Britain and southern Norway. Their clustered occurrence in coastal sediments in Norfolk is believed to be the product of ice-rafting from glacier incursions into the North Sea from eastern Scotland and southern Norway, and their subsequent grounding and melting within coastal areas of what is now north Norfolk. The precise timing of these restricted glaciations is difficult to determine. However, the relationship of the erratics to the biostratigraphic record and the first major expansion of ice into the North Sea suggest these events occurred during at least one glaciation between the late Early Pleistocene and early Middle Pleistocene (c. 1.1–0.6 Ma). In contrast to the late Middle (Anglian) and Late Pleistocene (Last Glacial Maximum) glaciations, where the North Sea was largely devoid of extensive marine conditions, the presence of far-travelled ice-rafted materials implies that earlier cold stage sea-levels were considerably higher.  相似文献   

3.
The Norwich Crag of north-eastern Suffolk is mainly composed of near-shore sands representing several sequences (cycles of transgression and regression). It is difficult to separate out the different sequences but the final sequence here is well known for the localised development of beds of flint gravel that have been interpreted as the in-situ remnants of prograding beaches. While a review of the evidence supports the involvement of this sedimentary environment in the overall processes, the evidence shows that virtually only gravels associated with rip-channels represent in-situ beach gravels and that thicker gravel beds are the infill of much larger channels. From consideration of the characteristics of the large channels it is concluded that these large channels were tidal-inlets between prograding barrier islands and that the gravels were derived from the adjacent up-drift beach faces of the barrier-islands.  相似文献   

4.
The Oligocene depositional history of the Thrace Basin documents a unique paleogeographic position at a junction between the Western Tethys and the Eastern Paratethys. As part of the Tethys, shallow marine carbonate platforms prevailed during the Eocene. Subsequently, a three-staged process of isolation started with the Oligocene. During the Early Rupelian, the Thrace Basin was still part of the Western Tethys, indicated by typical Western Tethyan marine assemblages. The isolation from the Tethys during the Early Oligocene is reflected by oolite formation and endemic Eastern Paratethyan faunas of the Solenovian stage. The third phase reflects an increasing continentalisation of the Thrace Basin with widespread coastal swamps during the Late Solenovian. The mollusc assemblages are predominated by mangrove dwelling taxa and the mangrove plant Avicennia is recorded in the pollen spectra. The final continentalisation is indicated by the replacement of the coastal swamps by pure freshwater swamps and fluvial plains during the Late Oligocene (mammal zone MP 26). This paleogeographic affiliation of the Thrace Basin with the Eastern Paratethys after ~32 Ma contrasts all currently used reconstructions which treat the basin as embayment of the Eastern Mediterranean basin.  相似文献   

5.
Uppermost sands of the Red Crag at Walton-on-the-Naze (Essex) and elsewhere in East Anglia have been decalcified to iron-stained quartz sands. In contrast, lower sands are only minimally altered and contain aragonitic and calcitic shells. Aragonitic shells are slightly dissolved (chalkified), but calcitic shells are unaffected. Cementation is limited to an addition of iron oxides, now mainly haematite, which also coat carbonate grains. Abundant iron-oxide fines in the upper decalcified sands were liberated from the coatings of shells; shells that have since dissolved. The diagenetic nature of the contact between decalcified upper and unaffected lower sands is evident where it transects cross-bedding. The contact is knife-sharp, even smoothly truncating large shells, and is usually planar and subhorizontal. Shelly sands immediately beneath the boundary contain similar amounts of aragonitic material, as do sands further below. Locally the decalcification boundary has been contorted by cryoturbation, implying that carbonate dissolution was a Pleistocene event. Decalcification probably occurred when the area was affected by permafrost. Lower sands were cemented by ice and protected from dissolution. Upper sands were above the ice table and subject to chemically aggressive waters during summer thaws. Decalcification is believed to have taken place during an episode of climate amelioration when downward retreat of the ice table accompanied replacement of tundra by boreal forest. Highly acidic and podzolic soils developed, beneath which shell-carbonate dissolved. Sharp based decalcified zones in Lowestoft Till and Devensian deposits in other parts of England can also be attributed to dissolution associated with permafrost.  相似文献   

6.
Two types of fracture occur in the Pliocene (Red Crag and Coralline Crag) shelly sands of south-east Suffolk. Post-lithification, planar fractures of tectonic origin are confined to the more lithified Coralline Crag. More irregular fractures containing calcretized sediment-fills occur in both the Coralline and the more friable Red Crags, and are of less certain origin. The calcrete-filled fractures formed when the Red Crag sands were overpressured (with the fracture-filling sediment injected upward). Calcretes are associated with roots (rhizoliths) and are confined to fracture fills, despite the high porosity and permeability of the host sands. This confinement remains unexplained. Root penetration and calcrete formation may have occurred when the host sediments were frozen — but this would imply the fissure fills remained open because of upward water flow. This possibility intensifies a problem of finding a source for the overpressured water. If, alternatively, roots and accompanying calcretes were introduced after permafrost conditions had disappeared, their precise confinement to the fracture fills (with calcrete carbonates disappearing within only a few pore diameters into the host sediment) remains inexplicable. If both fractures and their calcrete fills formed during non-permafrost conditions, both the origin of the fractures and their precise filling by calcrete are problematic.  相似文献   

7.
The Pliocene Norwest Bend Formation is a well‐preserved succession of terrestrial and shallow‐marine deposits in the Murray Basin, South Australia. Sediments in this unit consist of two discrete terrigenous clastic‐rich, decametre‐scale sequences, or informal members, which record episodes of marine incursion during the Early and Late Pliocene respectively. The base of each sequence is a transgressive lag and/or strandline deposit that is transitional upwards into a highstand, subtidal, terrigenous clastic and cool‐water carbonate sediment accumulation. The top of each sequence is incised by fluvial channels that are filled by river deposits which formed as relative sea‐level fell and terrestrial environments prograded basinward. Sedimentological data suggest that gross stratigraphic architecture was primarily determined by glacioeustasy. Differences in sedimentary style between these two sequences, however, reflect a major climatic change that took place in southern Australia during the mid‐Pliocene. The lower quartzose sand member is formed of siliciclastic sediment derived from prolonged, deep, subaerial weathering and contains a bivalve‐dominated, cool‐temperate, open‐marine mollusc assemblage. These sediments accumulated under an equitable, relatively warm, humid climate. The Murray Basin during this time, because of high fluvial discharge, was a salt‐wedge estuary with typical estuarine circulation. In contrast, the upper, oyster‐rich member is typified by large monospecific oyster buildups that grew in restricted coastal environments. Strandline deposits contain a warm‐temperate skeletal assemblage. Contemporaneous aeolian sediments accumulated under warm, semi‐arid climatic conditions. Well‐developed ferricrete, silcrete and calcrete horizons reflect cyclic conditions of rainwater infiltration and evaporation in the seasonally dry climate that typifies southern Australia today. Highly seasonal rainfall produced an estuary that fluctuated annually from being well to partially mixed. These Pliocene sediments support the notion that mollusc‐rich facies are the signature of cool‐water carbonate accumulations in inboard neritic environments. Unlike bryozoans that dominate the outer parts of Cenozoic cool‐water carbonate shelves, molluscs evolved to exploit an array of coastal ecosystems with wide salinity variations and variable sedimentation rates.  相似文献   

8.
The biostratigraphic revision of the benthic foraminifera present in the coastal Cenozoic quartzose and shelly sands (crags) at Fécamp and Valmont (Seine-Maritime) reveals Early Pliocene (Fécamp) and Early Pleistocene (Valmont) ages. The Tortonian-Messinian thanatocœnosis contained in the Fécamp Crag shows the presence of a former bryozoan-rich platform on the floor of the Channel that was reworked during the Lower Pliocene transgression. Tortonian-Messinian and Lower Pliocene deposits have been found in Belgium, England, Brittany, and at Fécamp, but are absent in Cotentin (North-West Normandy), which was uplifted at this period. The Lower Pleistocene tidal sands and crags described in Cotentin, Upper Normandy and the southern North Sea Basin indicate a marine passage between the Channel and the North Sea.  相似文献   

9.
Vertical or inclined planar fissures that may for the most part be classified as joints are commonly seen in exposures of both the Coralline and Red Crag in East Anglia. Measurements of fissure orientation reveal orthogonal patterns of alignment in both formations. The field relations of the fissures to the host Crag sediment suggest a tectonic rather than periglacial origin. It is proposed that the fissure system is the product of early Pleistocene tectonic flexuring in the area, on the western margin of the subsiding southern North Sea basin.  相似文献   

10.
Pleistocene sediments and soils exposed at Stebbing in central Essex, England are described, analysed and interpreted. The sand and gravel units above Eocene London Clay and Upper Pliocene Red Crag are shown to be a high level member of the Kesgrave Formation, with a surface immediately beneath that of the Westland Green Gravels, which are tentatively assigned to the Pre-Pastonian ‘a’ Stage of the British Quaternary succession. The rubified, argillic soil developed in the surface of these fluvial deposits is a composite of the Valley Farm and Barham Soils and displays micromorphological evidence of several phases of clay illuviation, gleying and clay coating disruption. Originally truncated and buried beneath Anglian gelifluction deposits, cover sand and till, the soil has been exhumed in most places by subsequent erosion. The full succession, however, is preserved within large gulls that formed by periglacial cambering prior to this erosion. More recent loess incorporation and pedogenesis have modified the exhumed soil and the materials within the gulls.  相似文献   

11.
Although substantial work has been done on the pre-glacial terraces of East Anglia, very little systematic work has been done to understand the origin of river terraces in East Anglia that have formed since ice last covered the region. This paper records the results of studies of exposures and borehole records in ‘classical’ Quaternary terrace landforms that are considered to have formed since the Anglian (MIS 12) Glaciation, in the middle Waveney Valley. These features have been examined in terms of their morphological and sedimentological properties, in order to provide a detailed record of their form and composition, understand their processes of formation, and identify their stratigraphical status. The results show that the main body of the highest terrace (Homersfield Terrace, Terrace 3) is not composed of river sediments, but of shallow marine sediments, and is a remnant of early Middle Pleistocene Wroxham Crag. River sediments, in the form of Anglian age (MIS 12) glaciofluvial Aldeby Sands and Gravels also exist in the area as a channel fill, cut through the Wroxham Crag, and reflect outwash erosion and sedimentation from a relatively proximal ice margin to the west. The results mean that the interpretations previously presented for the terrace landforms of the middle Waveney valley are not applicable. The issue of why the terrace stratigraphy, hitherto identified in East Anglia cannot be related to that for the River Thames to the south and the rivers of Midland England to the west, still requires further research.  相似文献   

12.
This paper outlines evidence from Pakefield (northern Suffolk), eastern England, for sea‐level changes, river activity, soil development and glaciation during the late Early and early Middle Pleistocene (MIS 20–12) within the western margins of the southern North Sea Basin. During this time period, the area consisted of a low‐lying coastal plain and a shallow offshore shelf. The area was drained by major river systems including the Thames and Bytham. Changes in sea‐level caused several major transgressive–regressive cycles across this low‐relief region, and these changes are identified by the stratigraphic relationship between shallow marine (Wroxham Crag Formation), fluvial (Cromer Forest‐bed and Bytham formations) and glacial (Happisburgh and Lowestoft formations) sediments. Two separate glaciations are recognised—the Happisburgh (MIS 16) and Anglian (MIS 12) glaciations, and these are separated by a high sea level represented by a new member of the Wroxham Crag Formation, and several phases of river aggradation and incision. The principal driving mechanism behind sea‐level changes and river terrace development within the region during this time period is solar insolation operating over 100‐kyr eccentricity cycles. This effect is achieved by the impact of cold climate processes upon coastal, river and glacial systems and these climatically forced processes obscure the neotectonic drivers that operated over this period of time. © British Geological Survey/Natural Environment Research Council copyright 2005. Reproduced with the permission of BGS/NERC. Published by John Wiley & Sons, Ltd.  相似文献   

13.
Middle Pleistocene preglacial and glacial sediments are described from Sidestrand in north Norfolk, UK. The sequence consists of estuarine and fluvial deposits of the Wroxham Crag and Cromer Forest-bed formations that were deposited by, and adjacent to, a major river system that drained northern and central England during the ‘Cromerian Complex’. These preglacial sediments were subsequently overridden and partially tectonised during a glaciation that deposited till of the Happisburgh Formation associated with the first lowland glaciation of eastern England. Detailed examination of the stratigraphy and structural evolution of the sequence reveals that glaciotectonic rafts of Sidestrand Unio Bed material, a regionally important biostratigraphic marker horizon, have been remobilised and partially mixed with other lithologies whilst being transported and emplaced further up-sequence by glaciotetconic processes. Caution should therefore be exercised when examining this deposit for biostratigraphic and palaeoenvironmental purposes to ensure that sampling is from in situ material.  相似文献   

14.
The Plio-Pleistocene Crag deposits of East Anglia include a wealth of shelly remains, including barnacles, preserved variously as complete shells, their disarticulated plates and trace fossils. Herein, we present a field guide to these distinctive fossils, with diagnoses of all known taxa recorded from the Crags of East Anglia, supported by both line drawings and photographs. The known stratigraphic and geographic distribution within the study area are tabulated. Recognised species include the sessile barnacles Armatobalanus bisulcatus (Darwin), A. dolossus (Darwin), Balanus balanus (Linné), B. crenatus Bruguière, B. inclusus Darwin, Concavus concavus (Bronn), Chirona hameri (Ascanius), Megabalanus tintinnabulum (Linné), Conopea calceola (Ellis), Co. spongicola (Brown), Acasta undulata Darwin, Coronula barbara Darwin, Megatrema anglicum (G.B. Sowerby) and Verruca stroemia (Müller) (=14 species); two pedunculate forms, Scalpellum magnum Darwin and Lepas delicatula Withers; and the boring Rogerella isp. The greatest diversity of species is found in the Coralline Crag and Red Crag formations, both yielding 11 species, although only four are common to both. Barnacles are poorly represented in other Crag deposits.  相似文献   

15.
The marine Pliocene-Pleistocene sequence of Iceland contains four main assemblages of mollusks: (1) Venerupis rhomboides et al., (2) Serripes groenlandicus et al., (3) Portlandia arctica et al., and (4) recent species. Analysis of the stratigraphic distribution of 100 species shows that the percentage of extinct species in the assemblages decreases from 40% in the oldest to 4–8% in the youngest. There is also a change from south boreal affinities in the oldest assemblage to north boreal in the youngest. The molluskan assemblages of Iceland can be correlated with Pliocene-Pleistocene assemblages of England and North Europe: (1) Coralline Crag, (2) Red Crag—lower part of the Icenian strata, (3) upper part of the Icenian-Cromerian strata. The upper boundaries of the Pliocene and Eopleistocene are defined by paleomagnetic data and radiological dating. The upper boundary of the Pliocene presumably should be established in the basalts lying between the Tjörnes and the Breidavik deposits by changes between the second and third mollusk assemblages and by the Gilsa paleomagnetic event.  相似文献   

16.
An account is given of a Geologists’ Association meeting in the Isle of Purbeck held on 28th–30th September 2012 and the stratigraphy and structures of the rocks examined during the weekend are described. Uppermost Jurassic Stage nomenclature and recent changes to stratigraphical nomenclature in the uppermost part of the Kimmeridge Clay Formation are discussed and the conclusion reached that the long-established divisions (Members) of this Formation are both readily recognisable and have nomenclatorial priority. The recent change to the position of Pallasioides-Rotunda zonal boundary ignores the ammonite fauna and is inappropriate. For the Lulworth district the stratigraphy of the uppermost Jurassic (Portlandian) through Lower and Upper Cretaceous formations are described and their associated structures discussed. The coastal evolution of the Lulworth coast is briefly discussed.  相似文献   

17.
Deer species provide a valuable biostratigraphical tool through Cromer Forest-bed times, due particularly to species turnover between the Early and early Middle Pleistocene. This study is based on the reidentification of 348 fossil antlers. The provenance of most large mammal fossils from the CF-bF, collected over 150 years, was recorded only by the nearest coastal village. None the less, analysis of the cervid taxa by these ‘localities’ reveals interesting patterns. The fauna of the West Runton Freshwater Bed is of early Middle Pleistocene complexion, and that from the foreshore at East Runton is of Early Pleistocene. Pure or nearly pure early Middle Pleistocene assemblages also occur at Kessingland-Pakefield and at Trimingham. At Overstrand, Sidestrand, Mundesley, Bacton and Happisburgh, there is a mixture of Early and early Middle Pleistocene elements. Analysis of Savin's data shows that fossils of earlier species were generally found further down the beach than those of later ones. Late nineteenth century geological surveys, made when bone collecting was at its peak, give additional information about fossil horizons, which in several cases can be related to modern stratigraphical units. Most Early Pleistocene large-mammal bones came from Pastonian conglomerates, in contrast to small-mammal and molluscan assemblages mostly extracted from Pre-Pastonian Crag. However, the diversity of Early Pleistocene CF-bF cervid species in comparison with continental faunas, and their pattern of distribution between sites, suggests they may span more than one chronostratigraphic stage. Early Middle Pleistocene assemblages came from strata now referred to the Cromerian, and the differing proportions of taxa between sites provide limited evidence of time-transgression.  相似文献   

18.
This paper revisits the utility of sodium (Na) content in aragonite and calcite mollusc shells as an indicator of palaeosalinity. The data come mainly from a related suite of Middle Pleistocene marine and freshwater fossils that have been subject to broadly similar diagenetic histories. Environmental salinity is re-affirmed as the primary factor in determining the sodium content of modern and ancient mollusc shells: values <2000 ppm Na are generally indicative of non-marine environments while values >2000 Na ppm are typically from marine shells. There is a positive relationship between Na (salinity) and Sr which is a helpful discriminator of palaeosalinity in the fossil data set. The Na and Sr data give confidence that the fossil shells have not suffered pervasive diagenetic alteration and that the marine fossils lived in fully marine conditions. Oxygen isotope values in the best-preserved, fully marine fossil shells, suggest Middle Pleistocene ‘eastern England’ seawater temperatures were broadly similar to those of the modern North Sea.  相似文献   

19.
本文综述了欧洲白垩纪非海相软体动物群,列出了主要产自英格兰南部、法国和西班牙早白垩世的16个不同沉积层的59个分类单元。淡水动物群以珠蚌类双壳类和田螺类腹足类为主,但在有些地点也存有肺螺类腹足类。这些化石类群与现代类型很相似,说明白垩纪淡水中的水草、氧气与营养环境良好。在欧特里沃期和巴列姆期,淡水与边缘海环境中的动物群组成都发生了显著的变化。欧洲的淡水生物群落早在巴列姆期就已存在,此时的有些类群,如著名的Mar garitifera(s.l.)valdensis在欧洲西部有着广泛的分布。英格兰南部的早白垩世韦尔登群被认为是欧洲最连续的非海相白垩纪地层,其上部的生物群可与西班牙的LasHoyas动物群和法国的Wassy动物群相对比。这3个动物群,以及法国侏罗(汝拉)和英格兰南部波倍克的侏罗纪-白垩纪的过渡生物群——Purbeck动物群,是了解欧洲白垩纪淡水动物群的关键动物群。  相似文献   

20.
Marine clay from two cores (50 and 36m deep) from Gothenburg, southwestern Sweden, have been analysed using different stratigraphic methods. Foraminiferal stratigraphy complemented withe lithostratigraphy, pollen and mollusc analyses show an environmental succession from arctic conditions with water depths up to 100m during Late Welchselian time, to a boreal shallow water environment in early Holocene time. A comparison of the foraminiferal faunas with those from corresponding investigations from southern Bohuslän, NW of Gothenburg, shows a similar development in the two areas. The sudden environmental change around the Pleistocene/Holocene boundary (10,000 years B.P.) along the Swadish west coast is attributed to changes of the hydrographic patterns; a general shift of the circulation pattern of the Skagerrak/ Kattegat at that time and or a large supply of fresh water flowing into the area from the Lake Vanern basin.  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号