Geological investigation of the Yahgan Formation (Upper Mesozoic) and associated igneous rocks of Navarino Island, Southern Chile

Abstract

Navarino Island is composed mainly of a sequence of geosynclinal sediments—the Yahgan Formation—at least 3,000 metres thick and of probable Early Cretaceous age. The main sediments are greywackes, argillites, and cherts. Calcareous concretions are characteristic of some horizons but are nowhere abundant. Many coarser-grained sediments contain abundant andesitic to basaltic debris, indicating widespread contemporaneous volcanic activity. Fossils are rare and comprise poorly preserved Radiolaria, fragmentary plant remains, and imprints of Inoceramus. Recrystallisation is evident in many rocks; in particular, secondary prehnite is characteristic, especially in the coarser sediments. The formation includes several thick quartz dolerite sills. The largest of these forms a conspicuous east-west chain of serrated peaks, the so-called Dientes de Navarino. The south-west part of the island and the nearby Bertrand Island are composed of altered basic igneous rocks, believed to be contemporaneous with the Yahgan Formation sediments. Dioritic rock, belonging to the Andean intrusive series (Patagonian Batholith) and considered to be Upper Cretaceous, cuts the folded Yahgan Formation, more especially in the west and south parts of the island. The sediments are moderately to strongly folded and dip mainly towards the south. In many places, however, minor thrusts and asymmetric folds are present, and their attitudes suggest that orogenic stress was towards the north. The relation of the Yahgan Formation to possible correlatives in Tierra Del Fuego and Patagonia is discussed. The striking similarity of the formation, as examined on Navarino Island, to the Cumberland Bay greywacke series of South Georgia is stressed.