LITHOFACIES AND DEPOSITIONAL ENVIRONMENT OF MIDDLE CAMBRIAN SAN-ISIDRO FORMATION, MENDOZA, ARGENTINA

Outcrops of the San Isidro Formation (32 m) in west Mendoza province, consist of monotonous, nearly homogeneous sequences of dark-gray to da rk brown, thin-bedded limestones with shales, siltstones and interbedd ed medium to fine grained sandstones. This area is known for its fossi liferous localities of Middle Cambrian trilobites. Fossils include tri lobites, particularly planktonic agnostids, sponge spicules, foraminif era and calcispheres. Both fossils and sedimentologic characteristics indicate relatively deepwater marine sedimentation. The overall pictur e of depositional history represents a transgressive-regressive-transg ressive sequence, as the result of moderate sea-level change. During t he sea-level low-stands siliciclastic sediments were transported on th e shelf and into the adjacent basin. It would seem possible that the p resence of mudstone-wackestone over the low-stand sandstones represent s a rapid sea-level rise. Some aspects about diagenetic effects are di scussed with special reference to cementation, neomorphism, dolomitiza tion, pressure-solution, and silicification.