Sedimentary response to a tectonically-induced sea-level fall: the Mulichinco Formation, Neuquén Basin, Argentina

SCHWARZ, E.; SPALLETTI, L.A.; HOWELL, J.A.

Coimbra

Congreso; 23rd Meeting of Sedimentology; 2004

International Association of Sedimentologists

This study examines the sedimentary response of a major tectonically- driven relative sea-level drop occurred in the Neuquén Basin (west-central Argentina) during the late Early Valanginian. The basin occupied a back-arc setting with an emergent magmatic arc towards the west at that time. The drastic relative sea-level fall allowed deposition only towards the central part of the basin where the Mulichinco Formation, a mainly clastic, continental to shallow marine wedge, sharply developed upon basinal black shales. The unit comprises a complete third-order sequence lasting about 2 m.y. where lowstand, transgressive and highstand deposits were recogni-sed. However, major along-strike facies changes within the sequence are evident, as it is a depocentre shifting during the sequence evolution. These peculiarities are attributed to differences in sediment flux, basin gradient, and fault tilting, ultimately related to tectonic activity that ?asymmetrically? concentrated towards the west (magmatic arc). The low-relief sequence boundary of the Mulichinco Sequence without incised valleys also departs from classic sequence stratigraphic models. Shallow alluvial incision during relative sea-level fall is probably associated with the ramp-type margin of the basin and the fact that rapid uplift likely developed high regression rates that limited adjustment to base-level shifts. These attributes encountered in the Mulichinco Sequence that depart from standard sequence models, could be more common that recorded until present in tectonically active semi-enclosed basins (e.g. shallow back-arc basins).