Abstract
The process of siliceous sponge skeleton dissolution, followed by precipitation of silica polymorphs, was part of the pre-Eocene marine Si cycle and has been linked to chert formation (so-called ‘chertification’). This study was conducted to test the proposed key role of siliceous sponges as a main dissolved silica (dSi) supplier for chert formation in carbonate environments, as suggested by Maliva & Siever (1989a). The study is based on advanced mineralogical (XRD, SEM-EDS) and microtextural (SEM) analyses of fossil sponge remnants and the surrounding rocks from a Late Cretaceous marine section deposited in a similar palaeoenvironment of carbonate deposition in outer shelf of epicontinental basins. Taphonomic studies of fossil sponges, combined with mineralogical and microtextural observations of the surrounding rocks, were carried out to develop diagenetic models of opal-A sponge skeleton dissolution, silica migration, and subsequent precipitation of all mineralogical phases observed in the studied samples. The adopted approach is based on the assumption of an open system of porewaters circulating within seabed mud, in which sponge remnants were buried, and on the geochemical setting within the sediment column, which is controlled by microbial decomposition of organic matter.