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    Wyświetlanie 1-4 z 4
    Tytuł:
    The stratigraphy of the Upper Greensand Formation (Albian, Cretaceous) of the Isle of Purbeck, Dorset, UK
    Autorzy:
    Gallois, R.
    Owen, H.
    Powiązania:
    https://bibliotekanauki.pl/articles/138644.pdf
    Data publikacji:
    2017
    Wydawca:
    Polska Akademia Nauk. Czytelnia Czasopism PAN
    Tematy:
    Upper Greensand Formation
    Dorset
    Isle of Purbeck
    Cretaceous
    Albian Stage
    stratigraphy
    palaeontology
    ammonites
    Wyspa Purbeck
    kreda
    stratygrafia
    paleontologia
    amonity
    Opis:
    At its maximum development in the type area on the Devon coast, the Upper Greensand Formation comprises up to 55 m of sandstones and calcarenites with laterally and stratigraphically variable amounts of carbonate cement, glauconite and chert that were deposited in fully marine, shallow-water environments. The formation is divided into three members, in ascending order the Foxmould, Whitecliff Chert and Bindon Sandstone, each of which is bounded by a prominent erosion surface that can be recognised throughout the western part of the Wessex Basin. The full thickness of the formation, up to 60 m, was formerly well exposed in cliffs in the Isle of Purbeck in the steeply dipping limb of the Purbeck Monocline. The upper part of the succession is highly condensed in comparison with the Devon succession and exhibits lateral variations over distances of hundreds of metres that are probably related to penecontemporaneous fault movements. Much of the fauna is not age-diagnostic with the result that the ages of parts of the succession are still poorly known. However, the Isle of Purbeck sections contain diverse ammonite faunas at a few stratigraphically well-defined levels that enable the succession to be correlated with that of east Devon and west Dorset.
    Źródło:
    Acta Geologica Polonica; 2017, 67, 3; 405-424
    0001-5709
    Pojawia się w:
    Acta Geologica Polonica
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Geochemical and stable isotope patterns of calcite cementation in the Upper Cretaceous Chalk, UK: Direct evidence from calcite-filled vugs in brachiopods
    Autorzy:
    Hu, X.
    Jeans, C.
    Dickson, T.
    Powiązania:
    https://bibliotekanauki.pl/articles/139259.pdf
    Data publikacji:
    2012
    Wydawca:
    Polska Akademia Nauk. Czytelnia Czasopism PAN
    Tematy:
    cement kalcytowy
    diageneza
    historia
    izotopy stabilne
    kreda
    pierwiastki śladowe
    wpływ drobnoustrojów
    anoxia
    calcite cement
    chalk
    diagenesis
    history
    Microbial influence
    oxia
    stable isotopes
    suboxia
    trace elements
    Opis:
    The history of research into the cementation of the Upper Cretaceous Chalk of the UK is reviewed. Calcite-filled vugs within the shell cavities of terebratulid brachiopods from the Cenomanian Chalk of eastern england have been investigated by cathodoluminesence imaging, staining, electron microprobe and stable isotope analysis. This has provided the first detailed analysis of the geochemistry of the Chalk.s cement. two cement series, suboxic and anoxic, are recognized. Both start with a Mg-rich calcite with positive [delta^13]C values considered to have been precipitated under oxic conditions influenced by aerobic ammonification. The suboxic series is characterized by positive [delta^13]C values that became increasingly so as cementation progressed, reaching values of 3.5[per mil]. Manganese is the dominant trace element in the earlier cement, iron in the later cement. Mn-and Fe-reducing microbes influenced cement precipitation and the trace element and [delta^13]C patterns. The anoxic series is characterized by [delta^13]C values that became increasingly negative as cementation progressed, reaching values of .6.5[per mil]. Trace elements are dominated by iron and manganese. Sulphate-reducing microbes influenced cement precipitation and the trace element and [delta^13]C patterns. Both cement series are related closely to lithofacies and early lithification pre-dating the regional hardening of the Chalk. The suboxic series occurs in chalk which was continuously deposited and contained hematite pigment and limited organic matter. The anoxic series was associated with slow to nil deposition and hardground development inc halks that originally contained hematite pigment but no longer do so, and an enhanced supply of organic matter.
    Źródło:
    Acta Geologica Polonica; 2012, 62, 2; 143-172
    0001-5709
    Pojawia się w:
    Acta Geologica Polonica
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    The stratigraphy of the mid Cretaceous (Albian) Upper Greensand Formation of the Wessex Basin and South West England, UK
    Autorzy:
    Gallois, R.
    Owen, H.
    Powiązania:
    https://bibliotekanauki.pl/articles/139028.pdf
    Data publikacji:
    2018
    Wydawca:
    Polska Akademia Nauk. Czytelnia Czasopism PAN
    Tematy:
    stratigraphy
    Upper Greensand Formation
    Cretaceous
    Devon
    Dorset
    Wessex
    Albian Stage
    Cenomanian Stage
    sequence stratigraphy
    stratygrafia
    formacja
    kreda
    dewon
    alb
    cenoman
    Opis:
    The Upper Greensand Formation, mostly capped by the Chalk, crops out on the edges of a broad, dissected plateau in Devon, west Dorset and south Somerset and has an almost continuous outcrop that runs from the Isle of Purbeck to the Vale of Wardour in south Wiltshire. The Formation is well exposed in cliffs in east Devon and the Isle of Purbeck, but is poorly exposed inland. It comprises sandstones and calcarenites with laterally and stratigraphically variable amounts of carbonate cement, glauconite and chert. The sedimentology and palaeontology indicate deposition in marginal marine-shelf environments that were at times subject to strong tidal and wave-generated currents. The formation of the Upper Greensand successions in the region was influenced by penecontemporaneous movements on major fault zones, some of which are sited over E-W trending Variscan thrusts in the basement rocks and, locally, on minor faults. Comparison of the principal sedimentary breaks in the succession with the sequence boundaries derived from world-wide sea-level curves suggests that local tectonic events mask the effects of any eustatic changes in sea level. The preserved fauna is unevenly distributed, both laterally and stratigraphically. Bivalves, gastropods and echinoids are common at some horizons but are not age-diagnostic. Ammonites are common at a few stratigraphically narrowly defined horizons, but are rare or absent throughout most of the succession. As a result, the age of parts of the succession is still poorly known.
    Źródło:
    Acta Geologica Polonica; 2018, 68, 2; 161-180
    0001-5709
    Pojawia się w:
    Acta Geologica Polonica
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
    Tytuł:
    Regional hardening of Upper Cretaceous Chalk in eastern England, UK: trace element and stable isotope patterns in the Upper Cenomanian and Turonian Chalk and their significance
    Autorzy:
    Jeans, C. V.
    Long, D.
    Hu, X.-F.
    Mortimore, R. N.
    Powiązania:
    https://bibliotekanauki.pl/articles/139436.pdf
    Data publikacji:
    2014
    Wydawca:
    Polska Akademia Nauk. Czytelnia Czasopism PAN
    Tematy:
    chalk hardening
    trace elements
    stable isotopes
    cement modelling
    reservoir diagenesis
    history
    kreda
    hartowanie
    pierwiastki śladowe
    izotopy trwałe
    cement
    historia
    Opis:
    The regional hardening of the Late Cenomanian to Early Turonian Chalk of the Northern Province of eastern England has been investigated by examining the pattern of trace elements and stable carbon and oxygen isotopes in the bulk calcite of two extensive and stratigraphically adjacent units each 4 to 5 m thick of hard chalk in Lincolnshire and Yorkshire. These units are separated by a sequence, 0.3–1.3 m thick, of variegated marls and clayey marls. Modelling of the geochemistry of the hard chalk by comparison with the Standard Louth Chalk, combined with associated petrographic and geological evidence, indicates that (1) the hardening is due to the precipitation of a calcite cement, and (2) the regional and stratigraphical patterns of geochemical variation in the cement are largely independent of each other and have been maintained by the impermeable nature of the thin sequence of the clay-rich marls that separate them. Two phases of calcite cementation are recognised. The first phase was microbially influenced and did not lithify the chalk. It took place predominantly in oxic and suboxic conditions under considerable overpressure in which the Chalk pore fluids circulated within the units, driven by variations in compaction, temperature, pore fluid pressure and local tectonics. There is evidence in central and southern Lincolnshire of the loss of Sr and Mg-enriched pore fluids to the south during an early part of this phase. The second phase of calcite precipitation was associated with the loss of overpressure in probably Late Cretaceous and in Cenozoic times as the result of fault movement in the basement penetrating the overlying Chalk and damaging the seal between the two chalk units. This greatly enhanced grain pressures, resulting in grain welding and pressure dissolution, causing lithification with the development of stylolites, marl seams, and brittle fractures. Associated with this loss of overpressure was the penetration of the chalk units by allochthonous fluids, rich in sulphate and hydrocarbons, derived probably from the North Sea Basin. Microbial sulphate-reduction under anoxic conditions within these allochthonous fluids has been responsible for dissolving the fine-grained iron and manganese oxides within the chalk, locally enriching the Fe and Mn content of the calcite cement. The possibility is discussed that the pattern of cementation preserved in these regionally hard chalks of Late Cenomanian and Early Turonian age may be different from that preserved in the younger (late Turonian to Campanian) more basinal chalks of eastern England.
    Źródło:
    Acta Geologica Polonica; 2014, 64, 4; 419-455
    0001-5709
    Pojawia się w:
    Acta Geologica Polonica
    Dostawca treści:
    Biblioteka Nauki
    Artykuł
      Wyświetlanie 1-4 z 4

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