Journal of Conference Abstracts

Vestiges of New Type Cretaceous Atolls from the Mecsek Mountains, Hungary

Geza Csaszar (csaszar@mafi.hu)

Stefania ut 14, Budapest, H-1143, Hungary

The Late Jurassic thinning of the continental crust was followed by rifting and a heavy, long lasting alkaline magmatic activity, mainly basaltic volcanism in the Mecsek Zone, South Hungary in the Early Cretaceous.

In the studied sections representing erosional remnants of a volcano-sedimentary succession of several hundred metres, fossils of shallow marine, mainly platform carbonate origin (corals, chaetetids, stromatoporoids, coralline algae, rudists, ostreids, other bivalves and gastropods) are often found together with pelagic fossils (ammonites, calpionellids, and brachiopods) in monomictic coarse-grained volcanoclastics. In one section Ammonite-rich marly sequences are intercalated by breccia horizons with fossils of shallow water origin. As a result of a detailed study a new model of sedimentation was accomplished (Császár & Turnsek, 1996). In the Early Cretaceous giant volcanoes grew above the sea level while among volcanoes basinal sedimentation continued. Due to continuous subsidence at Darwin atolls (Darwin 1842) after settling down of the colonial organisms new generations followed each other without significant erosion of the volcanic build ups. In the Mecsek zone the settlement of colonial organisms was preceded by an incision into the body of volcanoes at the sea level. The volcanoes were surrounded by narrow zones of the following sedimentary environments: gravel beaches, sandy and silty "lagoons", and the edge of submarine slopes were crowned by colonial organisms forming atoll type build-ups. Rock fragments and fossils were transported gravitationally (mass flow and slumping) downslope around volcanoes and mixed with pelagic fossils. The reason of different behaviour of Darwin's atolls and the Mecsek type atolls is a lack of notable subsidence of the of the basement at the latter one.

In the following the link between the atoll and basin, a swell and the basin from the Mecsek area will be outlined. At the end the history of the Early Cretaceous atolls in the South Pacific (Grötsch. & Flügel. 1992, McNutt et al. 1990) will be compared with that one in the Mecsek Mountains.

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Grötsch J & Flügel E, Facies, Inst. Pal. Univ. Erlangen-Nürnberg, 27, 153-174, (1992).

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