La Formazione dei Calcari a Briozoi e Litotamni nell'area di Tagliacozzo (Appennino Centrale) e considerazioni paleoambientali sulle facies rodalgali [The Calcari a Briozoi e Litotamni Formation in the Tagliacozzo area (Central Apennines); with paleoenvironmental consideration to rodalgal facies]

This paper describe a rhodolitic level in the Lower-Middle Miocene succession («Calcari a Briozoi e Litotamni» Formation) outcropping near the village of Tagliacozzo (Carseolani Mts., Central Apennines). The «Calcari a Briozoi e Litotamni» Formation represents the Cenozoic portion of the Latium-Abruzzi carbonate platform, that paraconformably overlies the Cretaceous platform limestones. The Miocene limestones consist predominantly of biogenic elements belonging to rhodalgal/molechfor associations (CARANNANTE et alii, 1988) deposited on a low-angle carbonate ramp (BRANDANO, 2001). They are overlain by hemipelagic marls rich in planktonic foraminifers (Tortonian Orbulina Marls). In the Tagliacozzo area (Carseolani Mountains) the «Calcari a Briozoi e Litotamni» formation can be subdivided into three main lithostratigraphic units. The lowest unit (UL1) consists of 1 to 2 m thick beds composed of fine bioclastic wackestone and packstone unconformably overlying the Upper Cretaceous carbonate platform deposits. Its thickness is about 10 m. The second unit (UL2), up to 90 m thick, is made up of poorly stratified, white coarse-grained echinoid-foraminifer packstone and pectinidbryozoan floatstone; the third unit (UL3) is composed of 8 meters of finely stratified, with planar bedding planes, fine to medium-grained echinoid-planktonic foraminiferal packstone. A «mineralized» hardground marks the boundary between UL3 and «Orbulina marls». The analysed rhodoliths occur in the middle part of UL2, forming a thick 0.6 m level. The rhodoliths have mainly spheroidal shape, a diameter of 4-5 cm and a laminar structure. The rare elliptical rhodoliths show major axis measuring 8-9 cm and minor axis measuring 5-6 cm. These last rhodoliths show an internal laminar structure with a tendency to be columnar towards the exterior. The nucleus is generally made up of bryozoan colonies. Borings (Entobia) on bivalve shells and red-algal thalli are also visible. Bryozoans, echinoid fragments, bivalves, benthic and plancktonic foraminifera form the fossil skeletal fraction. This study provides some new paleoecological data about the «Calcari a Briozoi e Litotamni» Formation. The algal association is mainly dominated by Lithothamnion (50%) and secondarily by Mesophyllum (30%) and Sporolithon (20%). The coralline algae association clearly indicates a circalittoral environment. The laminar structure of the rhodoliths is related to the high energy conditions at the bottom (BOSENCE, 1983). The presence of bioerosion indicates low rates of sedimentation. The occurrence of Sporolithon in the red algae associations and the palaeoclimate reconstruction in Early Miocene times (VANNUCCI et alii, 1996; ESTEBAN, 1996; POMAR et alii, 2001) suggest that deposition took place in the Miocene under subtropical-tropical climatic conditions.