Péter Ozsvárt

Radiolarites are traditionally described as hard and very fine-grained cherts containing more or less recrystallised radiolarian skeletons. Radiolarites consists of beds of red or green cherts, measured in centimetres, alternating with shale beds, measured in millimetres. Radiolarites are of huge importance for paleoceanographic reconstruction, because their bathymetric significance and their frequent association with ophiolites are used to date old oceanic crusts and to propose geodynamic models. There are many discussions in the literature about the conditions of deposition of radiolarites. For a long time it was thought that these sediments accumulated in deep (>3000 m) oceanic basins enriched in silica by volcanism along spreading ridges. We know, today, that the deposition of old siliceous sediments was not necessarily characteristic of deep environments and nerby volcanism. The most convincing hypothesis infers large oceanic radiolarian blooms related to nutrient input as strong as that found in upwelling processes. The radiolarites are relatively common in the sedimentary record until the Cretaceaus time all over the world. They first occur in the Paleozoic time in the norteastern part of Balaton Highland and in the Uppony Mountains, Northeastern Hungary but they are extreamly rare. The second oldest occur of radiolarites are in Middle Triassic sediments and continue through Jurassic sections. The Jurassic radiolarites are the most famous and the best known of all radiolarites in the Alpine-Carpathian-Dinaridic orogenic system and also in the Carpathian Basin. The so-called “Lókút Radiolarite Formation” is one of the most widespread radiolarite formation in the Transdanubian Range. It is well-known from the Bakony Mountains (radiolarite), the Gerecse Mountains (radiolaritic cherts) and the Pilis Mountains (clayey radiolarite). The thickness of the Lókút Radiolarite changes within rather wide borders. It exceeds onehundred meters in the vicinity in the South-Western part of the Transdanubian Central Range. It is thinner in the central and eastern part of the Bakony Mountains (few tens of meters) and only few meters in the Gerecse Mountains. Age of Jurassic radiolarites is from the Bajocian (167.4 Ma) to Oxfordian – Kimmeridgian (~ 155 Ma).