UPPER CRETACEOUS PLATFORM-DERIVED CONGLOMERATES IN CENTRAL TUNISIA: SIGNIFICANCE AND GENESIS MODE

摘要

In Central Tunisia, conglomerates are volumetrically well represented within and/or around Campanian platform environments. In Jebel el Kebar, conglomeratic carbonates rich in rudist blocks which are associated to massive rudist limestones, are intercalated within well bedded pelagic limestones. Basinward, the Campanian pelagic carbonates constituting the Abiod Formation, also include frequent conglomeratic intercalations. The conglomerate boulders are varied in size and composition. Most of them are bioclastic and derive from platform carbonates such as in other localities. Others are micritic and appear to derive from carbonates deposited on palaeoslopes or in more depressed areas. Diagenetic analyses show that rudist-rich blocks and bioclastic blocks preferentially exhibit an intergranular sparitic cement. This means that platform bioclastic limestones were, at least partly, cemented before their early disintegration into olistoliths, blocks and pebbles. Syn sedimentary fracturing occurs enhancing their fragmentation and erosion. In contrast, within micritic blocks, components (calcispheres, essentially) are not cemented. In addition, limestones containing micritic blocks generally fossilise slump marks. It means that micritic blocks containing calcispheres were not subjected to early lithification. Limestones generating micritic blocks which are non lithified to semi-consolidated, slipped down a slope, folded and fragmented into blocks.