Geological history of the Precordillera terrane

The Cambrian-Ordovian sedimentology in the Precordillera terrane has its source neither from old Andes nor nearby country rock, but shares similar characteristics with the Grenville orogeny of eastern North America.

The Precordillera is a moving micro-continent which started from the southeast part of the ancient continent Laurentia (current location: North American Plate).

[7] The Early-Cambrian to Ordovician deposits of Precordillera are composed of 3000 m thickness of marine carbonate sections with multiple phases of grain size variation.

The upper formation comprises Early Cambrian olenellid trilobites which indicates the region was a normal marine environment.

It shows two transgressive system tracks indicating drowning events, or in other words the eustatic rise of sea level.

The significant reef accumulations can be found in two intervals – the packstone at the boundary above La Silla Formation, and during the sudden change from marine environment to shallow-water grainstone.

[9] As sea level rises, fine and dark mudstone and shale deposit as an indication of approaching the subduction zone beneath Gondwana.

On top of the black shale the eastern Precordillera crust received a continental rise, inducing the rapid intensive events shown by the rifting structure on the deposits above the unconformity.

The western basin consists of black graptolitic shale with non-constant thickness which contains silicate clasts, believed to be a kind of rifting-related deposit.

[9] The unconformity symbolizes the increasing tectonic activity, probably the start of rifting events and the collision between Precordillera and Gondwana.The olenellid trilobites found in early Cambrian sequences are identical to those in the fragments in Laurentia.

[7] Regarding stratigraphical records and fossil evidence, multiple geological models has been published to explain the evolution of the Precordillera contributing to the Pre-Andean history of Gondwana.

After the collision, the continent rifted away during late Ordovician, so that Precordillera was no longer attached to Laurentia and stuck to the western Gondwana.

He proposed another model saying the Precordillera, or Cuyania was at the Southern margin of Gondwana and started drifting along the transform fault in mid-late Ordovician.

[2] During Silurian to Devonian periods, increasing metamorphism or magmatic activities with structural deformation showed the approach of the Chilenia terrace from the west and Precordillera finally locked into the position with Gondwana.