Silicification of carbonate clasts in a marine environment (Upper Miocene, Vera Basin, SE Spain)

Abstract

Upper Miocene marine sediments in the Vera Basin (SE Spain) contain abundant silicified marble pebbles and bivalves. The silicification of these marble pebbles and bivalves represents one of the stages of their sedimentary history, associated with small fluctuations in relative sea level. During the late Tortonian alluvial fan deposits containing the marble pebbles were invaded by the sea, which caused reworking of the 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 or quartz from opaline silica derived from dissolution of skeletal debris. The silicified zones (rinds) now consist of quartzine or quartz. Silicification 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 redeposited in deeper areas of the basin. If the hypothesis proposed for the diagenetic process is correct, it indicates that the uncommon silicification of carbonate clasts does not only occur in continental environments, in which it has previously been described.