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  • 1
    ISSN: 1365-3121
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Geosciences
    Notes: During a recent expedition at the Gorringe Bank (eastern Atlantic, 150 miles SW off Portugal), one of the rare sites in the ocean where mantle rocks crop out at very shallow depths (− 30 m), the Gettysburg and Ormonde seamounts, the two summits on the Gorringe Bank, were surveyed in detail. At Gettysburg seamount, within the modern bioclastic material, which is continually produced on the summit and exported to deep water, several examples of Mesozoic cephalopods were found. These fossils, reworked ‘in situ’, gave an age spanning from Kimmeridgian–Tithonian to Hauterivian (145–155 Ma) and recall some condensed Jurassic sequences of the Thetyan region compatible with shallow water (〈 200 m). The serendipitous discovery of such ancient faunas within modern sediments suggests that Gorringe Bank was a seamount at the early opening of the Atlantic Ocean and requires us to reassess the age of rifting along the Iberian margin and the importance of vertical tectonics for non-volcanic, mantle-rooted seamounts.
    Type of Medium: Electronic Resource
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  • 2
    Publication Date: 2020-02-01
    Description: Mass-transport deposits (MTDs) intercalated in slope/basinal successions, produced by submarine collapses and mass flows, are generally constituted of allochthonous elements sourced from platform margins. Here we present a noticeable exception where Toarcian calciclastic deposits made of pelagic carbonate elements are embedded in other pelagic carbonates. Selected outcrops pertaining to the Rosso Ammonitico Fm of three different pelagic carbonate platform (PCP)-basin systems were studied. The pelagic successions of the study areas onlap Early Jurassic structural highs. These clastic bodies partially replace the typical Toarcian reddish marls and shales of the Umbria-Marche-Sabina palaeogeographic Domain at different stratigraphic levels. The clasts range from blocks to megablocks; extraclasts of Corniola facies (Pliensbachian) and, sporadically, of Calcare Massiccio peritidal carbonates (Hettangian) are associated with Rosso Ammonitico intraclasts. The internal architecture of the MTDs and their emplacement processes, as well as their source and accumulation areas, were identified. Three distinct lithofacies characterise the clastic bodies, each one corresponding to a different emplacement process or to a different portion of the flow. The occurrence of lithified megablocks (〉20 m across) of Corniola Fm suggests the exhumation of the buried portion of the unit. Synsedimentary extensional tectonics is the most likely triggering mechanism. Sedimentological analysis, coupled with geological mapping of the study areas, reveals the key role played by PCPs in the genesis of these MTDs. The early Toarcian reactivation of Hettangian palaeofaults bounding the structural highs is inferred. These faults, characterised by moderate offsets, crosscut the onlap wedges of the hangingwall successions, exhuming the older and lithified portion of Corniola Fm. Once became inactive, the fault planes were then eroded producing palaeoescarpments. Their backstepping, coupled with seismic shocks, produced the accumulation of the study clastic bodies.
    Print ISSN: 2038-1719
    Electronic ISSN: 2038-1727
    Topics: Geosciences
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