Extensional Tectonic Controlling the Ordovician Evolution of Precordillera Terrane, Western Argentine.

PERALTA, SILVIO H.; ROSALES, C.V.

Nanjing, China

Simposio; Proceedings 10th International Symposium on the Ordovician System, Nanjing, China. 27-30 June, 2007; 2007

International Subcommission on Ordovician System-International Subcommission on Silurian Stratigraphy

Regional analysis of the Ordovician stratigraphic framework of the Precordillera terrane suggest it has been controlled by extensional tectonic from Lower through Middle-Upper Ordovician. Diachronic top of the San Juan Formation indicates a northward basin tilting, as result of which a south-directed transgressive event took place during Middle Ordovician, driving to basin deepening and drowning of carbonate platform, as it is evidenced by black shale deposition during Darriwilian. Coeval shallower pelagic carbonate facies evolved as result of structural block rising. Extensional structures of horst (?seamount?) and graben basin are formed as result of regional collapse of the carbonate platform in the Early Sandbian. Such event is continuous through Sandbian as it is indicated by the N. gracilis zone in the Villicum graben, and the C. bicornis zone in the Guandacol graben. However, coeval sedimentation occurs on horst in shallower environment, which is documented by fine grained carbonate, siliciclastic and mixed deposits, interpreted as Pelagic Carbonate Platform Facies or PCPs facies. Extensional regime continues active into the Katian, as it is evidenced by coarse-grained deposits of the Villicum graben and mixed sequence of the Sassito Block. This extensional process is incongruent with a foreland model as it is proposed by others authors for the Ordovician basin.