The marine Lower Cretaceous to Upper Jurassic(?) Yahgan Formation of southern Chile was orogenically folded between late Early Cretaceous and middle Late Cretaceous time. Biotite and hornblende from diorite bodies intrusive into the folded Yahgan Formation have been dated at 79 ± 5 m.y. by the Rb-Sr and K-Ar methods, thus setting the younger limit to the age of folding of the formation. Hornblende from greenstone at Bertrand Island, which is considered contemporaneous with deposition of the Yahgan Formation, but recrystallized during the folding of the sedimentary sequence (Katz and Walters, 1966), has a K-Ar date of 92 ± 4 m.y.
At Burleigh Peninsula, the Tekenika conglomeratic beds, generally accepted as Jurassic in age, are considered as Late Cretaceous to early Tertiary. Paleontologic evidence and a K-Ar total rock date of 116 ± 5 m.y. for a boulder of dioritic gabbro from the Tekenika beds support this stratigraphic revision.