| Abstrakt: |
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 arc distinguished: (1) Dolomitized 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; (2) Dolomitized 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 1we 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. (3) Ferruginous 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 learns reworked on the continent during interglacial stages). These data point to the following sequence of events: (1) At the end of the Pliocene the sea-level must have been between -100 and -150 m with respect to present-day sea-level. (2) A 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. (3) Ferruginization is more recent. It may be tentatively connected with erosion of the last continental red loams, where they overlie the Tyrrhenian. [ABSTRACT FROM AUTHOR] |
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