Palaeomagnetic studies of a Cretaceous molasse sequence in the central Andes (Coloso Formation, Northern Chile)

sequence in northern Chile. The sequence was deposited rapidly and records the unroofing of granodioritic and andesitic source rocks. Thin sand units throughout the sequence have been studied palaeomagnetically; the results are complex but can be related to the diagenesis of and-zone alluvium. Natural remanent magnetization (NRM) may be normal or reversed but is usually a combination of the two. Isolation of magnetic components by vector analysis of thermal and AF demagnetization data yields both normal and reversed components which are approximately antiparallel and yield an average pole position at 703, 192"E. This is consistent with late Cretaceous data from northern Chile but not with the established apparent polar wander (APW) path for South America. It may indicate late Cretaceous remagnetization but is considered to be more likely an indication that the present APW path assumes no tectonic rotation and that important relative movements have taken place between northern Chile and Argentina. The Coloso Formation in Antofagasta Province (Fig. 1) is a conglomerate-dominated alluvial fan succes-sion, 1500 m thick, which passes conformably upward