Stratigraphic Architecture of Pre-salt Microbial Series Deposited during the Sag Phase of the Salta Rift System: The Yacoraite Formation (NW Argentina)
The present study is focused on the Yacoraite Fm that has been deposited during the sag phase of the Salta rift system, NW Argentina. The Salta rift (Early Cretaceous to Early Tertiary) is a huge rift system with several sub-basins and branches (100's x 100's km) where the sedimentary fill can reach a thickness of 5,000m including syn- to post-rift sequences. The Yacoraite Fm (ca. 76-62 Ma) was deposited during a transitional phase characterized by shallow marine to lacustrine mixed deposits.
The depositional model of the Yacoraite Fm includes proximal mudflats carbonate-dominated facies to lagoonal and back-barrier facies that laterally pass to oolitic shoal facies associated with large stromatolites. Toward the edge of the rift, especially in hangingwall setting, siliciclastic-dominated facies (fluvial, aeolian, tidal, shoreface) commonly occurred.
The Yacoraite Fm records an overall transgressive trend from fluvio-eolian sandstones passing upward to shallow marine/lacustrine carbonates in turn overlaid by lacustrine dark shales hosting halite, anhydrite and gypsum. It can be subdivided into four third-order sequences. During the deposition of Sequence 1, the sedimentation was characterized by mixed depositional environments dominated by restricted facies. Thickness variations and facies distribution suggest an influence of inherited structures, previously active during the syn-rift phase. During the deposition of Sequence 2, the sedimentation was mainly characterized by high energy carbonate-dominated depositional environments. Sequence 2 records a flooding of the basin, probably coming from the west. Facies distribution is still influenced by syn-rift structures. During the deposition of Sequence 3, the sedimentation was mainly characterized by mixed to siliciclastic-dominated depositional environments. Following a major maximum flooding surface, that probably induced a shut down in the carbonate production, Sequence 3 records a progressive abandonment of the basin ended by a major sequence boundary. During the deposition of Sequence 4, the sedimentation was mainly characterized by carbonate-dominated depositional environments. Following a first transgressive trend draping the previous paleotopography, Sequence 4 is characterized by widely distributed restricted facies with very large domal stromatolites. The lacustrine vs marine influences during each sequence is matter of debate and will be finally discussed.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California