A palaeomagnetic survey of South American rock formations - Palaeomagnetic studies on rock formations from Northwest Argentina

Cambrian, Cambro-Ordovician and Ordovician formations of red sandstones and siltstones from the sub-Andean regionso f the Provinces of Salta and Jujuy have been studied. The grouping of n.r.m . directions suggests partial remagnetization after folding in the Tertiary or Quaternary geomagnetic field. Thermal cleaning at 300 °C and higher temperatures destroys this secondary magnetization leaving a magnetization which is accepted as representing the lower Palaeozoic geomagnetic field. Palaeomagnetic south pole positions have been computed and lie in the Atlantic Ocean to the NNE of Brazil. The period between the Carboniferous and Triassic is covered by the Paganzo formation which is exposed in La Rioja Province. The older part (Paganzo II) is reversely magnetized with a south p.m . pole in the South Atlantic, while the upper part (Paganzo III) is normally magnetized with a south palaeomagnetic pole in the vicinity of poles obtained for Triassic rock formations from elsewhere in S. America. Formations of red beds from Salta province regarded as Upper Palaeozoic or Mesozoic yield a south palaeomagnetic pole in the South Atlantic corresponding to a Triassic age. Ore microscope and thermal decay curves suggest that the remanent magnetism is due to haematite.