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11.
DANIELA A. VALLINI BIRGER RASMUSSEN BRYAN KRAPE&#; IAN R. FLETCHER NEAL J. MCNAUGHTON 《Sedimentology》2005,52(1):101-122
An ≈ 26 m thick unit of phosphatic sandstone and black shale (the Phosphatic Unit) in the Palaeoproterozoic Mount Barren Group of south-western Australia contains abundant authigenic xenotime crystals showing well-preserved diagenetic textures. Despite extensive regional deformation and thermal metamorphism, the peak of which occurred at ≈ 1205 Ma, the Phosphatic Unit was preserved as a low-strain envelope because of its pre-compaction carbonate and phosphate cementation. In situ U–Pb geochronology of xenotime reveals four discrete age populations at 1693 ± 4, 1645 ± 3, 1578 ± 10 and 1481 ± 21 Ma. When integrated with petrography, the age data place a timeframe on: (i) sediment deposition; (ii) phosphogenesis; (iii) diagenetic cement infilling; (iv) diagenetic pyrite formation; (v) secondary porosity generation; (vi) hydrocarbon migration; (vii) burial compaction; and (viii) hydrothermal alteration, up until peak thermal metamorphism. Xenotime growth at ≈ 1693 Ma occurred prior to compaction, whereas xenotime growth at ≈ 1645 Ma occurred during burial. Xenotime growth at ≈ 1580 Ma and at ≈ 1480 Ma appears to be the far-field record of thermotectonic events associated with intracontinental extension and magmatism recorded elsewhere in Australia. Geochemical analysis, integrated with geochronology, shows a systematic increase in MREE/HREE in xenotime crystals with decreasing age and with increasing stratigraphic depth. Coupled with a decrease in xenotime abundance and age with depth, it suggests that: (i) the main focus of porosity infilling was at the top of the Phosphatic Unit and progressed downwards over the > 200 Myr period of porosity infilling, and (ii) the changes in xenotime REE chemistry may be due to an influx of MREE from increasing amounts of dissolved apatite or changes, with respect to REE solubility, in the physiochemical nature of the fluids with burial depth. 相似文献
12.
The Upper Oligocene–Lower Miocene succession in eastern Jylland can be subdivided into three sequences (A–C from older to younger) deposited on and around the Ringkøbing-Fyn High. The development of the sequences reflects a complex interaction between eustatic sea-level changes, physiography and variable sediment supply. Superimposed on this, frequent storms promoted longshore sediment transport and the development of spit systems adjacent to structural highs. As a consequence, sequence boundaries and flooding surfaces are not always expressed as portrayed in conventional sequence models; sequence boundaries or flooding surfaces may only be marked by subtle changes in depositional environment that can only be revealed by careful integration of sedimentological observations with palynological data. The influence of the topography resulted in the development of brackish water basins that were sufficiently large to permit the deposition of hummocky cross-stratified sands with muds. These deposits are overlain by clean hummocky and swaley cross-stratified sands that were deposited in a fully marine, high-energy environment. This evolution from mud-rich, storm-influenced sediments to sand-dominated shoreface sediments resulted from a rise in sea level and was not the result of shoreface progradation and downstepping during a sea level fall. In addition to the topographic control on sequence development, sediment supply to the study area changed significantly during the deposition of the three sequences. Initially the basin was sediment-starved, favouring the formation of glaucony-rich sediments. The sediment input gradually increased and the influence of structural highs and lows became less significant with time. Consequently, both sequence boundaries and flooding surfaces are characterized by more conventional features in the younger part of the succession, where a basinward displacement of the shoreline resulted in thick lowstand delta deposits. 相似文献
13.
JESPER OLSEN PETER RASMUSSEN JAN HEINEMEIER 《Boreas: An International Journal of Quaternary Research》2009,38(3):458-470
Holocene temporal and spatial reservoir age variations have been investigated in three Danish fjords (Horsens Fjord, Tempelkrog in Isefjord and Skælskør Nor) by 14 C dating paired samples of terrestrial plant material and marine molluscs in sediment cores. The local reservoir age offsets at all three sites show significant temporal and local variations, with ΔR(t) values ranging from −425 to 620 yr. The ΔR(t) values for Horsens Fjord range from −425 to 0 yr, for Tempelkrog from −230 to 360 yr and for Skælskør Nor from 30 to 620 yr. The differences are best explained by differences in local hydrographic configuration, catchment characteristics and connection with the inner Danish seas. The observed temporal ΔR(t) variations are not synchronous, which is probably due to the overriding role played by numerous local factors affecting the ΔR(t) values. Based on the reservoir data from Horsens Fjord and Tempelkrog, it is suggested as a working hypothesis that the ΔR value for Kattegat prior to 4500 cal. yr BP was negative, i.e. between −130 and −200 yr. The temporal and local variations in the reservoir age underline the problems and uncertainties connected with radiocarbon-dating fossil human skeletons with marine diets. Overall, our data show that reservoir ages in Danish fjords and estuaries are both site-specific and time-specific, implying that modern reservoir ages cannot simply be applied back in time. This conclusion may also be valid in the case of fjords and estuaries in large parts of the Baltic too. 相似文献
14.
Kaare L. RASMUSSEN Henning HAACK Finn ULFF‐M
LLER 《Meteoritics & planetary science》2001,36(7):883-896
Abstract— Metallographic cooling rates have been calculated for all five members of the iron meteorites group IIF using two different techniques. We have determined cooling rates of ～5 °C/Ma based on Ni profiles through the taenite rim enclosing kamacite spindles. Ni profiles through the kamacite phase are less precise cooling rate indicators, but suggest a cooling rate of ～1 °C/Ma within an order of magnitude at lower temperatures (360–400 °C). Based on the kamacite bandwidth and the Ni profiles through the taenite, we estimate that the kamacite nucleated 130–200 °C below the temperature predicted from the phase diagram. The size of and the distance between the large kamacite spindles is found to be consistent with the thermal history that we have determined on the basis of Ni profiles in kamacite and taenite. We find that previously published kamacite bandwidth cooling rates for the five group IIF members are most likely in error because of the presence of large schreibersite spindles in some kamacite spindles and because undercooling of kamacite was ignored. Contrary to previous workers we find that the metallographic cooling rates are consistent with cooling in a common core. 相似文献
15.
KELD RØMER RASMUSSEN WERNER BAI 《Boreas: An International Journal of Quaternary Research》1987,16(4):393-404
During a search for unpolluted resources, new wells have been drilled to a deep confined aquifer in central Jutland. Since little is known about the extent and vulnerability of the deep reservoir, geophysical logging was used to supplement hydraulic well-testing. Gamma-ray, SP and resistivity logs were recorded in the boreholes. Gamma-logs were also recorded in two cased wells. Analysis of the logs, combined with information from samples, shows an unconfined aquifer of quartz and mica sand to a depth of about 55 m at Grindsted. Below this level there is a 40 m thick aquitard with an apparent southerly dip. The aquitard is composed of interbedded sand, silt and clay. The deep confined aquifer extends from 95 to 110 m below the surface at Grindsted. Both the aquitard and the deep aquifer are provisionally dated as Early/Middle Miocene. The electrical logs discriminate well between sand, silt and clay beds. If a suitable mud is used in the well it is possible to calculate the formation water resistivities in good agreement with water samples obtained after completion. From the resistivity logs formation factors were calculated in the range 3–5 using no-invasion departure curves. The high content of heavy mineralsin the Tertiary deposits makes it impossible to obtain precise lithological information from the gamma-ray log. On the other hand this log provides clear markers for identifying the hydrogeological units in the well-bores. 相似文献
16.
The distribution patterns of benthonic and planktonic foraminifera in cores from the Ionian Basin, central Mediterranean, were investigated in relation to the deposition of sapropel S-1. The sapropel is a dark organic-rich sediment deposited under anoxic conditions during the last marine stagnation in the Early Holocene. The major divisions between both benthonic and planktonic faunas correspond to changes in the core lithology and coincide with the transitions between pre-sapropel, sapropel/'oxidized layer' and post-sapropel sediments. The faunal evidence shows that the oxidized layer belongs to the sapropel sequence. The planktonic faunas have the same species composition as in the sapropel sediment and the high density of planktonic species continues into the oxidized layer. The oxidized layer is devoid of a benthonic fauna or contains a 'sapropel-associated' fauna composed of infaunal species with an affinity to high supply of organic material material and low oxygen. High depletion in the heavy oxygen isotope in the oxidized layer substantiates the faunal evidence. A peak in abundance of the planktonic species Globorotalia inflata at the top of the oxidized layer marks the time when turnover of the water masses ended the stagnation phase and sapropel sedimentation in the Ionian Basin at about 6000 BP. The distribution of the benthonic and planktonic foraminifera shows that the sapropel in the central part of the Ionian Basin was originally almost twice as thick as it is today. When oxygen returned to the deep sediments the top of the sapropel was oxidized to 4–7 cm below its original surface. Only the lower part of the sapropel is preserved. The remainder is now a red laminated layer, the 'oxidized layer'. 相似文献
17.
MARGOT JESPERSEN M.Sc. ERIK RASMUSSEN M.Sc. 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2013,113(1):13-23
Jespersen, Margot und Rasmussen, Erik: Jordsand—Erosion und Akkumulation einer Hallig. Geografisk Tidsskrift 75: 13–23. København, juni 1, 1976. Since 1972 the authors have carried out investigations of erosion and accumulation on the island of Jordsand in the Danish Wadden Sea. The investigations supported by the Danish State Research Council for Natural Sciences are not yet finished, and this paper is a midway report including preliminary results and working hypotheses. 相似文献
18.
J. CAMPSIE M. H. RASMUSSEN N. HANSEN C. J. LIEBE J. LAURSEN W. BROCHWICZ-LEVINSKI L. JOHNSON 《Polar research》1988,6(2):173-177
Age determinations for rock samples collected by Fridtjof Nansen during his trans-Arctic expedition from 1893-1896 have yielded additional information on the tectonic chronology of the Arctic Basin. The data suggest pulses of volcanic activity in the Frans Josef Land Archipelago with approximate averages of 120 ma and 135 ma. These ages are consistent with postulated opening dates for the western Arctic and thus suggest that initial volcanism affected the entire Arctic margin. 相似文献
19.
OLE BENNIKE PETER RASMUSSEN KIM AARIS-SØRENSEN 《Boreas: An International Journal of Quaternary Research》2008,37(2):263-272
The harp seal ( Phoca groenlandica ) is a low-arctic species that is currently a rare visitor to Danish waters. However, bone remains from archaeological and geological deposits in Denmark and the Baltic Sea testify to a regular presence of harp seals in this region during the mid-Holocene. The paradox of the presence of a low-arctic seal species in southern Scandinavia during the mid-Holocene thermal maximum has been widely discussed. In order to improve the Holocene chronology for the presence of harp seal in Denmark, 24 bone remains of the species were radiocarbon dated. The oldest date is around 4100 cal. yr BC, indicating that the harp seal arrived several millennia after fully marine conditions were established in Danish waters. The majority of the dated specimens fall within two age groups, one centred around 3900 cal. yr BC (11 dates), the other around 2700 cal. yr BC (7 dates). It is argued that these two groups may reflect periods with suitable living conditions for the harp seal in Danish waters and that this is connected with an enhanced inflow of high-salinity North Sea water and higher biological productivity. Six dates show a scattered distribution between c . 1400 cal. yr BC and c . AD 1000, suggesting sporadic visits of the harp seal to Danish waters during the late Holocene. 相似文献
20.
KJELD RASMUSSEN M.Sc. ANETTE REENBERG M.Sc. 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2013,113(1):81-88
Rasmussen, Kjeld and Anette Reenberg: Ecological Human Geography—some considerations of concepts and methods. Geografisk Tidsskrift 80: 81–88. Copenhagen, June 1980. The field of interest for ecological human geograhy is defined as the study of utilization systems. The application of different ecological concepts in human geography is shown, and the fertility of using different types of systems approaches in the analysis is demonstrated on a simplied, closed agricultural system. 相似文献