Alteración diagenética en conchas de rudistas Monopleúridos del Complejo Urgoniano de Ereño (Aptiense – Albiense Inferior, Bizkaia) (original) (raw)
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The Urgonian limestones of El Castillo Cape (lower Albian) were deposited in a shallow platform in which developed rudist and coral constructions. A further karstification created caves at metrical scale filled by sandy deposits. The rudist assemblages consist largely on bouquets and thickets of caprotinids in whose interspaces grew bouquets of monopleurids. A complex diagenetic history occurred upon both caprotinid and monopleurid shells, from early stages (vadose meteoric dissolution) to more advanced ones (neomorphism and selective dolomitization) to later ones (stylolitization and tectonic fracturing). The monopleurid hipostracum seems to be the more alterable part of the shell, because it appears frequently dissolved and filled by micrite. On the other hand, the hipostracum of caprotinids shows in some cases its original aragonitic prismatic polygonal microstructure partially preserved, in a major degree when close to micritic zones in which the water movement was restrained.
Alteración diagenética en conchas de rudistas caprínidos del Cretácico Medio de Urdax (Navarra)
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Cathodoluminescence and microprobe analysis in monopleurid shells from Aptian -Lower Albian limestones in Ereño (Bizkaia) has allowed to us to realize that different rudist shells show different diagenetic responses in front of a similar degree of diagenetic alteration. Monopleurid shells show a relatively high diagenetic degree when compared to that of requienids. Early neomorphism processes were dissimilar for different shells. The relatively thin monopleurid prisms could allow a diagenetic alteration in a higher degree than that affected requienid shells.
Alteración diagenética en conchas de rudistas radiolítidos del Cretácico Medio de Urdax (Navarra)
Geogaceta, 2004
Cathodoluminescence and microprobe analysis in monopleurid shells from Aptian -Lower Albian limestones in Ereño (Bizkaia) has allowed to us to realize that different rudist shells show different diagenetic responses in front of a similar degree of diagenetic alteration. Monopleurid shells show a relatively high diagenetic degree when compared to that of requienids. Early neomorphism processes were dissimilar for different shells. The relatively thin monopleurid prisms could allow a diagenetic alteration in a higher degree than that affected requienid shells.
Chondrodonta sp. shells from Aptian-Lower Albian red limestones of Ereño (Bizkaia, northern Spain) have proved to be excellent indicators of a high resistance to the entrance of diagenetic fluids and further diagenetic alteration. Chondrodonta sp. shells, when compared to their coeval rudist shells, show higher Sr +2 and Na + contents, together with very dull or even null luminescence. For this reason they could be very appropriate for further C, O an Sr isotopic studies as a tool for reconstruction of palaeotemperatures and palaeoenvironmental conditions.
The petrographic analysis of the inoceramid shells allows to identify some microstructures, which suggest their early modification, previously and during the first burial stages. In this sense, it is important to emphasize the borings produced by epibionts and the formation of an early ferrous-manganese coating in the outer surface of the shell. The desegregation of the prismatic framework and its dispersion in the sediment are other processes that took place during the early stages of burial. The study of the post-mortem modification of the shells brings forward some evidences about the marine bottom oxygenation in the Basque-Cantabrian Basin, just before the inoceramid extinction near the lower-upper Maastrichtian boundary (~69,3 m.a.).
2009
The realization of geochemical and diagenetical studies on fossil oyster's shells from the «Calizas Nodulosas con Ostreidos y Limolitas de Sopuerta unit» has allowed demonstrating the variations in the luminescence of different microstructures, mainly in the Regular Foliated one (RF), as a result of different burial depths. The structural properties of the distinguished microstructures made possible, to a greater or lesser extent, the circulation of diagenetic fluids which provided the shell with diagenetic elements like Fe, Mn or Mg. As a result of this cationic exchange, the shell enriched the fluids with Na or Sr, considered primitive elements in oyster's shells. The importance of the rocky matrix in the geochemical alterations has been demonstrated, comparing the results of this study with the results obtained by Higuera-Ruiz and Elorza (2006).
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