Tectonic and sedimentary significance of Cretaceous Tekenika Beds of Tierra del Fuego

Abstract

STRATA called Tekenika Beds have been considered the oldest post-orogenic rocks within the southernmost Andean Cordillera. Their significance in terms of regional tectonic history outweighs their small outcrop area on the east side of Hardy Peninsula on Hoste Island, Chile, about 160 km north-northwest of Cape Horn (Fig. 1). The Tekenika Beds, which are 2–3 km thick, are best known on Burleigh Peninsula on the south side of Tekenika Bay (Tekenika derives from two Yahgan Indian words Teke uneka¹), where they are largely sandstone and conglomerate (Figs 1 and 2). The basal contact has not been seen; however, the strata, although folded (ref. 2 and unpublished reports of Empresa National del Petroles), are less deformed than the tightly folded Lower Cretaceous Yahgan Formation just north of Tekenika Bay less than 5 km away. Therefore an unconformity is implied. Coarseness of the strata, fossil wood (some bored by worms or molluscs), thin lignitic coal beds, shallow marine molluscs and presence of some plutonic clasts led to the view that the Tekenika is a nearshore, post-orogenic sequence². Suarez and Pettigrew recognise it as pre-orogenic, but still interpret it as a shallow marine deposit³. We report here structural evidence indicating that the Tekenika Beds are pre-orogenic and deep water deposits.