Tuesday, June 18, 2013

The Salto group

This spectacular formation is part of the Salto group of rocks in northern Argentina, dating from the Cretaceous and early Tertiary. It shows the angular unconformity (a period of non deposition of sediment) between the Pirgua and Balbuena de Tres Cruces members. The Hill of Seven Colours we covered recently athttp://tinyurl.com/mq8xpmp is also within this group. The sediments were deposited in a graben, which is a block of rock that has collapsed down between two normal faults during crustal extension, in this case, during the rifting apart of the supercontinent Gondwana during the Mezozoic.

The rocks filling the pull-apart basin date from the Cretaceous to the Quaternary and are up to 5Km thick. Several different sedimentary environments are found within the group, each prevailing in a different climatic and tectonic regime, ranging from lake deposits and alluvial fans to marine rocks from a transgression of the sea onto the land. There were several of these during this period, related to sea level rise caused by the heat from rifting making the spreading ridges under the sea rise, literally pushing the sea onto the land. These events are common whenever rifting rates increase.

There were different environments in varied parts of the basin, reflecting differential levels of subsidence, but the climate as the terrestrial sediments were deposited stayed generally arid throughout. The package was then uplifted and tilted after lithification and jumbled about somewhat during the Andean orogeny, until the layers show the sub-vertical dip you can see on the image. There are several unconformities within the package, representing erosive gaps in sedimentation, where parts of the record disappeared. The Pirgua formation represents the rifting stage, and the Tres Cruces member the post-rift period. In the photo, the angular unconformity between sandy mudstones and carbonates and redbeds (oxidised terrestrial desert sandstones) is obvious. Some of the redbeds represent sheet floods, others dune fields.

Image credit: http://www.unsa.edu.ar/
In Spanish: http://www.scielo.org.ar/scielo.php?script=sci_arttext&pid=S0004-48222010000600007
For those with firewall access: http://link.springer.com/article/10.1007/s00531-004-0443-2?LI=true

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