Stratigraphy of Coastal Range in Tarapacá Province, Chile

The stratigraphy of the Jurassic sediments of the Coastal Range in Tarapacá Province is examined although the geology of this area is relatively unknown. Strata range in age from Bajocian to Oxfordian, but no ammonites of definite Bathonian age have been found. The continental Atajaña formation of Neocomian age overlaps at about the same topographic level several Jurassic formations which are quite different in age and lithologic character. The Atajana formation grades upward into the marine sandy Blanco formation which contains Berriasian ammonites. The Jurassic sediments are folded into a general northerly trend, whereas the Cretaceous sediments have a general easterly strike. Between the Upper Oxfordian and the basal Cretaceous there is evidence of a major orogeny, which by definition is Nevadian. The Nevadian folding in northern Chile, as in Canada and Alaska, has been gentler than in western United States. The Andean diorite was locally intruded after the Nevadian orogeny and was subsequently eroded, supplying the pebbles of the Cretaceous basal conglomerates. A working hypothesis of a gradually younger age of the Andean diorite toward the south of the Chilean Andes is proposed.