62,874 research outputs found
Stratigrafia ad alta risoluzione nei depositi di piattaforma distale: esempi dell'Ordoviciano armoricano e sardo
No full textZircon typology and geochemistry in the palaeogeographic reconstruction of the Late Ordovician of Sardinia (Italy)
No full textThe present study concerns the Upper Ordovician sedimentary succession which crops out in the Sulcis-Iglesiente area
autochthon and in the: Sarrabus-Arburese allochthonous units, with the aim of contributing to a better understanding of the
pre-Hercynian palaeogeography. This study is based on the typology of zircon and trace-element geochemistry of zircon
and titaniferous minerals.
The Upper Ordovician rocks cropping out in the Sarrabus-Arburese area (allochthon belt) mainly result from the
erosion of pre-Caradocian volcanics (the so-called 'Porfidi grigi'), which were emplaced in a magmatic arc setting. The
lateral and vertical homogeneity of the terrigenous supply during the Caradoc and early Ashgill, and the argillaceouscarbonate
sediments ,with shallow shelf faunas during Ashgillian times, suggest that stable geodynamic conditions persisted
throughout this time ,;pan.
In the Sulcis-Iglesiente area (autochthon belt), the nature of the terrigenous influx and the source-rocks remained the
same from the Late Cambrian to the early Ashgill. The constant nature of the detrital zircon grains rules out any possibility
that the 'Porfidi grigi' might have contributed to the sedimentary supply; the source-rocks were probably located within
an internal cratonic area, distinct from the magmatic arc of the Sarrabus area. In the early Hirnantian, new source-rocks
contributed to the terrigenous supply. They were volcanics with a strong mantle component. This new material was later
progressively diluted by terrigenous supply of cratonic origin.
The significant differences in the composition and origin of the terrigenous material deposited in the autochthonous
and the allochthonous units may reflect distinct source-areas with different geodynamic histories. It is therefore possible
that the two depositional areas were geographically separated. Location of the Sulcis-Iglesiente area within the North
Gondwanan shelf is ~ plausible scenario. But the occurrence of Baltic taxa in the Sarrabus area suggests that this area was
originally located in an intermediate position between North Gondwana and Baltica
Stratigraphic significance of siliceous-argillaceous nodules in Ordovician formations of the Armorican Massif (France) and Sardinia (Italy)
No full textControls of sea-level fluctuations on the formation of Ordovician siliceous nodules in terrigenous offshore environments
No full textAbstract
Siliceous nodules of the Ordovician Period, of the Armorican Massif and of southwest Sardinia are made of a mixture of
terrigenous origin material (quartz and phyllosilicates) and of biogenic origin components (microcrystalline quartz, calcium
phosphate and calcite). They are associated with fine-grained sequences, which were deposited between the distal part of the
inner shelf and the outer shelf. Within these sequences, two facies can be distinguished, clayey–silty facies without nodules
related to episodes of high sedimentation rate and nodule-bearing clayey facies linked to episodes of reduced sedimentation.
The genesis of the siliceous nodules can be integrated into a model taking account of the fluctuations in terrigenous flux due to
eustatic variations: during periods of rising sea level, the terrigenous flux decreases and the sedimentation is dominated by
bioclastic elements (calcareous, siliceous and phosphatic). Stability of the sediment–water interface facilitates biogenic silica
dissolution. During early diagenesis, silica precipitation leads to the formation of proto-nodules. The distribution of the two
facies can be explained by the superposition of two eustatic signals, one of very high frequency (VHF) and the other of high
frequency (HF): the silty–clayey facies could be accumulated during the period of falling sea level within the HF cycle and the
nodule-bearing clayey facies could correspond to amalgamation of several VHF cycles related to periods of low sedimentation
rate in the HF cycle
Concrétions des dépôts silico-clastiques: origine et apport en stratigraphie génétique
No full textBenthic fauna and relative sea-level variations in the Ordovician of the Armorican Massif: a disruption at the Darriwilian-Sandbian transition
No full textSignification stratigraphique des accumulations à phosphates dans l'Ordovicien du Massif armoricain
No full text
Preservation of the Miocene Atacama Gravels in Vallenar area, Northen Chilean Andes: Climate, Stratigraphic or Tectonic control?
No full textU-Pb ages and Hf isotopes on detrital zircons from Neoproterozoic sediments from the Armorican Massif (NW France): western Gondwana as possible source
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