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Diagenetic signatures of stratal surfaces in the Upper Jurassic Fulmar Formation, Central North Sea, UKCS 总被引：2，自引：0，他引：2
FIONA E. BURNS STUART D. BURLEY ROB L. GAWTHORPE JOHN E. POLLARD 《Sedimentology》2005,52(6):1155-1185
Sedimentation rate and changes in relative sea level affect early diagenetic cementation along key stratal surfaces within the deeply buried Upper Jurassic Fulmar Formation, South Central Graben, UKCS. As a result of the bioturbated nature of the shallow marine strata, variations in ichnofabrics and cross-cutting relationships between trace fossils are important in identifying and correlating stratal surfaces. Two transgressive and two regressive surfaces/sequence boundaries were studied, using petrographical, mineralogical and geochemical techniques. Cements, including early diagenetic pyrite framboids, grain-rimming apatite, microcrystalline dolomite and ankerite, occur at, above and immediately below both transgressive and forced regressive surfaces/sequence boundaries. Breaks or subdued rates of sedimentation associated with both types of surfaces meant that the sediment package resided within early diagenetic zones for prolonged periods of time, enhancing diagenetic reactions at and below the surfaces. The distribution of ankerite, despite being a deep-burial cement, was primarily controlled by concentrations of bioclasts that are particularly abundant at transgressive surfaces. The diagenetic character of the forced regressive surfaces/sequence boundaries is more complex than that of the transgressive surfaces, both in terms of mineralogy and paragenesis. This is attributed to the superimposition of diagenetic assemblages as a result of erosion, re-sedimentation and sediment by-passing. Although the diagenetic signature of the surfaces has been modified by dissolution/replacement reactions during deep burial, early diagenetic signatures can still be distinguished. 相似文献
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PETER M. ABBOTT WILLIAM E. N. AUSTIN SIWAN M. DAVIES NICHOLAS J. G. PEARCE FIONA D. HIBBERT 《第四纪科学杂志》2013,28(5):501-514
The Eemian interglacial and the onset of the subsequent glacial period serve as the most recent analogue for the natural operation of the climate system during the current interglacial. Pronounced climatic oscillations occurred during this period, but their nature and pattern are poorly understood due to dating limitations and unknown phase relationships between different regions and archives. Tephrochronology offers considerable potential for precise correlation of disparate palaeoclimatic archives preserving evidence of these rapid climatic transitions through the tracing of common isochronous tephra horizons. We outline the identification of three previously unknown cryptotephra horizons within a marine core from the Rockall Trough, North East Atlantic. This sequence preserves a high‐resolution record of this interval and shard size, geochemical heterogeneity and the co‐variance of shard concentrations with ice‐rafted debris data are utilized to demonstrate that primary airfall was the most likely transport and depositional pathway. The main geochemical populations of these horizons have similar transitional alkali major and trace element compositions, suggesting that they were derived from a common Icelandic source, potentially the Öræfajökull volcanic system. These tephra horizons represent additions to the North Atlantic event stratigraphy for this period and tentative correlations to Icelandic terrestrial deposits are proposed for two horizons. 相似文献
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LEE J. ARNOLD RICHARD G. ROBERTS ROSS D. E. MACPHEE JAMES S. HAILE FIONA BROCK PER MÖLLER DUANE G. FROESE ALEXEI N. TIKHONOV ALLAN R. CHIVAS M. THOMAS P. GILBERT ESKE WILLERSLEV 《Boreas: An International Journal of Quaternary Research》2011,40(3):417-445
Arnold, L. J., Roberts, R. G., MacPhee, R. D. E., Haile, J. S., Brock, F., Möller, P., Froese, D. G., Tikhonov, A. N., Chivas, A. R., Gilbert, M. T. P. & Willerslev, E. 2010: Paper II – Dirt, dates and DNA: OSL and radiocarbon chronologies of perennially frozen sediments in Siberia, and their implications for sedimentary ancient DNA studies. Boreas, Vol. 40, pp. 417–445. 10.1111/j.1502‐3885.2010.00181.x. ISSN 0300‐9483 The sedimentary ancient DNA (sedaDNA) technique offers a potentially invaluable means of investigating species evolution and extinction dynamics in high‐latitude environments. An implicit assumption of the sedaDNA approach is that the extracted DNA is autochthonous with the host deposit and that it has not been physically transported from older source deposits or reworked within the sedimentary profile by postdepositional mixing. In this paper we investigate whether these fundamental conditions are upheld at seven perennially frozen wetland sites across the Taimyr Peninsula and coastal lowlands of north‐central Siberia. Optically stimulated luminescence (OSL) and radiocarbon (14C) dating are used to constrain the ages of both the inorganic and organic fractions of perennially frozen deposits from which sedaDNA of extinct and extant species have been recovered. OSL and 14C age/depth profiles, as well as single‐grain equivalent dose (De) distribution characteristics, are used to assess the stratigraphic integrity of these sedaDNA sequences by (i) identifying the presence of primary or reworked organic and inorganic material, and (ii) examining the types of depositional and postdepositional processes that have affected specific sedimentary facies. The results of this study demonstrate that even though DNA preservation and stratigraphic integrity are commonly superior in perennially frozen settings, this does not, in itself, guarantee the suitability of the sedaDNA approach. The combined OSL and 14C chronologies reveal that certain perennially frozen sites may be poorly suited for sedaDNA analysis, and that careful site selection is paramount to ensuring the accuracy of any sedaDNA study – particularly for ‘latest appearance date’ estimates of extinct taxa. 相似文献
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M. BRÖCKER R. KLEMD M. COSCA W. BROCK A. N. LARIONOV N. RODIONOV 《Journal of Metamorphic Geology》2009,27(5):385-403
The Orlica–?nie?nik complex (OSC) is a key geological element of the eastern Variscides and mainly consists of amphibolite facies orthogneisses and metasedimentary rocks. Sporadic occurrences of eclogites and granulites record high‐pressure (HP) to ultrahigh‐pressure (UHP) metamorphic conditions. A multimethod geochronological approach (40Ar–39Ar, Rb–Sr, Sm–Nd, U–Pb) has been used to gain further insights into the polymetamorphic evolution of eclogites and associated country rocks. Special attention was given to the unresolved significance of a 370‐ to 360 Ma age group that was repeatedly described in previous studies. Efforts to verify the accuracy of c. 370 Ma K–Ar phengite and biotite dates reported for an eclogite and associated country‐rock gneiss from the location Nowa Wie? suggest that these dates are meaningless, due to contamination with extraneous Ar. Extraneous Ar is also considered to be responsible for a significantly older 40Ar–39Ar phengite date of c. 455 Ma for an eclogite from the location Wojtowka. Attempts to further substantiate the importance of 370–360 Ma zircon dates as an indicator for a melt‐forming high‐temperature (HT) episode did not provide evidence in support of anatectic processes at this time. Instead, SHRIMP U–Pb zircon dating of leucosomes and leucocratic veins within both orthogneisses and (U)HP granulites revealed two age populations (490–450 and 345–330 Ma respectively) that correspond to protolith ages of the magmatic precursors and late Variscan anatexis. The results of this study further underline the importance of Late Carboniferous metamorphic processes for the evolution of the OSC that comprise the waning stages of HP metamorphism and lower pressure HT overprinting with partial melting. Eclogites and their country rocks provided no chronometric evidence for an UHP and ultrahigh‐temperature episode at 387–360 Ma, as recently suggested for granulites from the OSC, based on Lu–Hf garnet ages ( Anczkiewicz et al., 2007 ). 相似文献
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JANE K. HART FIONA CANE ROBERT J. WATTS 《Boreas: An International Journal of Quaternary Research》1996,25(2):101-114
At the Dänischer Wohld Peninsula coastal sections (North West Germany), subglacial deformation was found at three scales. At the smallest scale, features typical of deforming bed tills were found, i.e. small boudins, tectonic laminations and low fabric strength till. At an intermediate scale, large lenses of glaciolacustrine sediments were found within subglacially deformed till. At the largest scale, there were large (over 5 m high) subglacial folds. We suggest that these styles of sedimentation/deformation were associated with a series of readvances during overall glacial retreat: subglacial deformation occurred during each advance and glaciolacus trine sedimentation occurred during each retreat. This led to glaciolacustrine sediments and deforming bed tills being folded together during subsequent readvances. Where the rheology was relatively weak, the lacustrine sediments were totally incorporated into the diamicton and lost their previous identity. However, where the glaciolacustrine sediments were relatively strong, they survived. We suggest that this style of deformation is typical of the conditions just upglacier from the ice margin and is associated with a relatively thick deforming layer and a high input of subglacial sediment. We conclude that the evidence found at this site provides further indications that the southern margins of the Fenno-Scandinavian ice sheet were coupled with the glacier bed and underwent deforming bed conditions. 相似文献
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