The Chachil depocentre is one of a number of early Mesozoic extensional basins that form the early depocentres of the southern Neuquén Basin in Argentina. The synrift volcanic fill is composed of andesites, rhyolites and volcaniclastic deposits. Coarse-grained, non-marine facies dominate the sedimentary fill, mainly in the form of sediment gravity flow deposits. Stream flow deposits and minor non-marine carbonates are also locally present. The evolution of the graben border system was mainly controlled by subsidence along the main boundary fault (the Chihuido Bayo fault system) and recurrent volcanic activity. Marked changes in the thickness of the synrift megasequence indicate that episodic normal faulting in the hanging wall was also important. The integration of structural, magmatic and sedimentary data from the study area has led to the definition of three stages in the evolution of the synrift succession. The early rift stage is defined by the interplay between bimodal volcanism and gravity-driven sedimentation. The mid-rift stage is marked by the transition to acidic magmatism (rhyolitic and pyroclastic flows), also associated with coarse-grained non-marine deposition. The late-rift stage is dominated by fine-grained turbidites and pyroclastic falls related to the first marine sedimentation in the Neuquén Basin.

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