Falling stage and Lowstand Systems Tract development in a ramp setting. Barremian-Aptian sequence stratigraphy in the central Neuquén basin, Argentina.

VEIGA, G.D.; HOWELL, J.A.

Encuentro; 2001 Annual Meeting. British Sedimentological Research Group; 2001

The Neuquén basin is located in west central Argentina and is characterised by a 2,500 m-thick Jurassic-Cretaceous sedimentary succession developed in a back-arc setting in the south-western margin of Gondwana. During the Early Cretaceous the basin had a ramp geometry. Clear evidence exists for several episodes of relative sea-level fall. One of these episodes resulted in a sequence boundary, spectacularly marked by the superimposition of fluvial and aeolian deposits (Lower Troncoso Member of the Huitrín Formation) on top of offshore shales (Upper Member of the Agrio Formation) during Barremian-Aptian times. However, the detailed description of this transition in basinward localities shows a more complex record. Intercalation of shallow marine deposits with these two units indicating active sedimentation during falling sea-level. The development of this Falling Stage Systems Tract is characterised by the presence of a series of detached shorefaces that record the successive stages of sea level fall. A thick succession of limestones is also locally present. The limestones have a very low bioclastic concentration and limited bioturbation indicating that they accumulated in a restricted shallow marine environment, during the final stages of the sea level fall but prior to the development of the main sequence boundary. The more widespread fluvial and aeolian deposits of the Lower Troncoso Member represent the final stage of relative sea-level fall. They are associated basinwards with evaporitic deposits and suggest the complete desiccation of the basin. Driving mechanisms for this desiccation may be either eustatic or related to the completion of the Andean magmatic arc isolating the basin from the Pacific Ocean during lowstand periods.