EXEMPLES DE DIAGENÈSE SOUS-MARINE DANS LES SÈDIMENTS PLIOCÈNES ET PLÉISTOCÈNES: DOLOMITISATION, FERRUGINISATION (MÉDITERRANÉE NORD-OCCIDENTALE, SUD DE MARSEILLE)

Authors


ABSTRACT

An investigation based on thin section and X-ray analysis of lithified Pliocene and Pleistocene samples collected on the top of the continental slope between -150 and -300 m is presented. Three groups are distinguished:

  • 1Dolomitized and non-ferruginous Pliocene. Samples with Pliocene microfauna occur under two facies: algal calcirudites and argillaceous skeletal-detrital sands, both with micritic matrix. Dolomitization concerns preferentially cell-filling of coralline algae;
  • 2Dolomitized ferruginous Pleistocene. A sample (Halimeda limestone) is described in detail. Aragonite of Halimeda and low-magnesium calcite or aragonite of molluscs are dissolved, sparitic early cement and coralline algae are dolomitized; echinoderms are not affected by dolomitization. Goethite fills part of the voids. A mosaic of magnesium calcite marks the last stage of cementation.These facts lead the author to the conclusion that for the first two groups dolomitization took place in a supralittoral environment, during a raising of the sea contemporaneous with an interglacial stage, following a glacial low-level at about −300 m.
  • 3Ferruginous and non-dolomitized Pleistocene. This group includes one aragonite-cemented sample, and several samples with calcite cement. For the former, crystallization is referred to a shallow-marine environment, which suggests a sea-level at about -270 m as for dolomitization. Iron oxide (goethite) occurs either as impregnations of micrite cement, or as deposits alternating with layers of sparitic calcite. Its submarine deposition required favourable physical-chemical conditions, but also previous concentration, on the continent. This ferruginization is then related to washing of iron-rich soils (red loams reworked on the continent during interglacial stages).

These data point to the following sequence of events:

  • 1At the end of the Pliocene the sea-level must have been between -100 and -150 m with respect to present-day sea-level.
  • 2A period of dolomitization in the Early Pleistocene, indicating a lowering of sea-level down to -270 m. Where subsequent downwarping of the continental margin occurred, this figure would decrease further.
  • 3Ferruginization is more recent. It may be tentatively connected with erosion of the last continental red loams, where they overlie the Tyrrhenian.

RÉSUMÉ

L'article présente une étude minéralogique et pétrographique de fragments rocheux pliocènes et pléistocènes recueillis sur le sommet de la pente continentale. Cette étude amène à distinguer trois groupes: Pliocène dolomitisé non ferrugineux, Pléistocène dolomitisé et ferrugineux, Pléistocène ferrugineux non dolomitisé. La diagenèse s'est effectuée en milieu sous-marin. Les processus de dolomitisation sont discutés. La ferruginisation est rapportée au lessivage de sols riches en fer (limons rouges) élaborés sur le continent lors des périodes interglaciaires. Ces données sont utilisées pour tenter de classer les échantillons dans une chronologie relative.

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