Abstract

A subhorizontal angular erosional unconformity that separates subhorizontal Upper Palaeocene-Lower Eocene fluvial deposits of the Ligorio Marquez Formation from folded and faulted Lower Cretaceous tuffs and sedimentary rocks in the area south of Chile Chico, in the northern Patagonian fold-and-thrust belt of southern Chile (46 degrees 45'S), indicates tectonism prior to the previously recognized post-Early Miocene contractional event. Similar observations between latitudes 45 degrees 30' and 47 degrees 30'S in Argentina support the existence of a pre-Palaeocene-Eocene contractional tectonic phase and, locally, of a late Early Cretaceous event. This indicates that the northern Patagonian Fold-and-thrust belt includes structures earlier than previously thought, probably masked by the later Neogene ones. The timing of this earlier tectonism is not precisely known, beyond being pre-Upper Palaeocene-Lower Eocene and post-Barremian. One possibility is that it may be the earlier stages of a progressive deformation that in its later stages is represented by the post-Lower Miocene tectonism previously documented in the region. In this case it may represent the latest Cretaceous?-Palaeocene major contractional tectonism and uplift well documented along the southernmost Patagonian Cordillera, south of latitude 51 degrees S. The Upper Palacocene-Lower Eocene Ligorio Marquez Formation may represent the first accumulation in a foreland (wedge top) basin developed cast of the orogenic Front. Subsequently, and probably during the Middle Miocene, these beds were folded and faulted during the tectonism that generated the foreland basin where the fluvial deposits of the Galera Formation accumulated. Different speculative scenarios for the origin of this compression can be invoked depending on its age.