CALCIFICATION PROCESSES OF SILICEOUS SPONGES IN VISEAN LIMESTONES (COUNTIES SLIGO AND LEITRIM, NORTHWESTERN IRELAND)

In the Lower Carboniferous limestones and shales of the Benbulben Rang e, Counties Sligo and Leitrim, north-western Ireland, a suite of carbo nate nodules, about 1 to 4 cm in diameter, has been sampled and invest igated by thin sectioning. The nodules consist of micritic,peloidal an d fenestral fabrics. Many of them contain relies of desma bearing demo sponges and hexactinellid sponge skeletons. The nodules are interprete d as calcified siliceous sponges. Micrite and peloids have been formed via microbial activity during the decay of the soft sponge tissue. Th e actual processes are deduced from Recent examples investigated at Li zard Island, Autralia, by REITNER (1993). The skeletal opal was dissol ved very early. In places where the skeleton was already embedded in m icrite the spicules are preserved as molds cemented by granular ferroa n calcite. The nodules were extensively inhabited by agglutinating pol ychaetes and bored by sponges. Micrite clasts have been exported to th e surrounding seafloor before the sponges were completely covered by s ediments. Fenestral fabrics represent primary sponge cavities, that ma y be enlarged due to volume reduction of the soft tissue during calcif ication. Some originated from non-calcification of decaying tissue. Th e granular calcite cement, filling the fenestral fabrics, contains rel ies of spicules and faintly visible peloids floating unsupportedly in the cement. These peloids were probably produced in situ by calcificat ion of organic mucilages that filled the cavities almost entirely. It is evident that most diagenetic processes occurring within the sponges happened on the seafloor, most likely within the still living individ uals. Possibly the nodules represent a precursor stage of mud mound de velopment.