SILICIFICATION OF CARBONATE CLASTS IN A MARINE-ENVIRONMENT (UPPER MIOCENE, VERA BASIN, SE SPAIN)

Upper Miocene marine sediments in the Vera Basin (SE Spain) contain ab undant silicified marble pebbles and bivalves. The silicification of t hese marble pebbles and bivalves represents one of the stages of their sedimentary history, associated with small fluctuations in relative s ea level. During the late Tortonian alluvial fan deposits containing t he marble pebbles were invaded by the sea, which caused reworking of t he pebbles and the establishment of a marine biota in the environment. After shallow burial numerous marble pebbles and bivalve remains were silicified. Both types of clasts were partially replaced by opal-CT o r quartz from opaline silica derived from dissolution of skeletal debr is. The silicified zones (rinds) now consist of quartzine or quartz. S ilicification of the clasts must have taken place under shallow burial conditions during early diagenesis, while pyrite formed in favourable anoxic sites. Iron oxides also precipitated during early diagenesis, but after silicification. Finally, part of the deposits containing the silicified clasts were transported by gravitational mass flows and re deposited in deeper areas of the basin. If the hypothesis proposed for the diagenetic process is correct, it indicates that the uncommon sil icification of carbonate clasts does not only occur in continental env ironments, in which it has previously been described.