Author Thierry P.A. Sempere, Independent Consulting Geologist
A giant salt basin extended over what is now western Peru and southwestern Bolivia in the Late Triassic. The basin formed from ~240 to ~210 Ma through backarc rifting along the axis of what is now the Eastern Cordillera (which results from the transpressional to compressional inversion of this rift system). Salt and other evaporites were originally deposited during the Early ‘short Norian’ as seawater entered grabens; the eastern limit of thick autochthonous evaporites ran approximately along the current Subandean-Lowlands transition; these evaporites subsequently extended westwards, in part allochthonously. Salt is however absent east of the Shira-Otishi uplift and from the Camisea area; only two salt features, including a dome, are known along the southern Subandean of Peru. In Andean central Peru, a salt dome proves however that salt was originally deposited ~100 km west of the present-day Subandean belt.
Salt is notoriously prone to dissolution, especially in outcrops and shallow subsurface. The original extension of the salt basin is outlined using a number of proxies, such as the associated evaporites and the structural effects of salt tectonics; evaporites formed a weak level that was used as a tectonic detachment by later deformations, and as preferential level of emplacement of magmatic sills. In Bolivia most of these evaporites were eroded during the inversion that formed the Eastern Cordillera, but are preserved in a number of Mesozoic synclines in the east, and in large portions of the Altiplano.
A significant part of the deformation currently interpreted as classical shortening originated in fact by salt tectonics. A variety of salt-tectonic structures is indeed observed, opening many new avenues for oil exploration, in particular in Peru.
Thierry P.A. Sempere, Independent Consulting Geologist
Thierry is a Doctor in Geology and Engineer from the Paris School of Mines. He has 38 years of experience, and worked as researcher at ORSTOM/IRD from 1982 to 2017. He currently is independent consulting geologist at Lima, Peru. Thierry is a member of AAPG.