THE CALCIMICROBES-ARCHAEOCYATHAN MOUNDS OF THE MATOPPA FM. AND EUSTATIC CONTROL

The mounds of the Matoppa Fm. from SW Sardinia are mainly built up by Epiphyton, Renalcis, rare Girvanella and a variable amount of Archaeocyathans. The reoccurrence of these discontinuous calcareous bodies within a cyclic siliciclastic dominated succession has been studied in order to clarify the mechanisms of the onset of carbonate deposition (Pillola et al., 2000). The deposits show different orders of cyclically arranged depositional sequences; their hierarchy indicates eustatic control of depositional dynamics. More developed mounds occur in correspondence of sensible acceleration of sea level rise of the medium frequency sequences. Finer terrigenous facies, eteropic with the mounds (middle and distal portions of the upper offshore), testify to rapid deepening conditions as compared to the underlying deposits (coarse to medium grained sandstones with HCS, SCS, etc., indicating a shoreface environment). In this context, the mounds can be considered as an expression of sedimentary condensation linked to strong eustatic rises. Surfaces at the top of each buildup system represent maximum flooding surfaces, which are of high stratigraphic value (isochronous and correlatable surfaces). The onset of widespread development of the mounds is triggered by rapid eustatic rise which induce the reduction of terrigenous input and the stabilization of the sea bottom. Calcimicrobes and Archaeocyathans settle on this firm bottom and may coexist with silty-clay material which can fill empty spaces not previously occupied by primary constructors and/or cements. Later on the massive lenticular limestones, dominated by Epiphyton, Renalcis, with minor Archaeocyathans develop. These massive bodies may also have reoccurrences within interconnected Archaeocyathan floatstones in more complex bioconstructions.

PILLOLA, G.L., FLORIS G.G. & LOI, A. 2000. Risposta delle comunità bentoniche del Cambriano inferiore alle variazioni eustatiche ad alta frequenza. Accad. Naz. Sci. Lett. Arti di Modena, Collana Studi, 21: 197-203.