PALAEOMAGNETIC STUDIES OF A CRETACEOUS MOLASSE SEQUENCE IN THE CENTRAL ANDES (COLOSO FORMATION, NORTHERN CHILE).

The Coloso Formation is an early Cretaceous conglomerate-dominated molasse 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 arid-zone alluvium. Diagenetic studies of the orebody and associated sediments reveal a number of features characteristic of the shallow burial diagenesis of arid-zone continental sediments. These include dissolution and clay replacement of detrital silicate grains and a suite of authigenic minerals including quartz, feldspar, calcite and hematite.