Eocene stratigraphy of southeastern Tierra del Fuego Island, Argentina

Recognizing folded and thrusted Eocene foreland strata of the oil-producing Austral Basin provides new insights on the Paleogene evolution of Tierra d el Fuego Island and the adjacent Atlantic shelf. Field mapping and petrogra phic and micropaleontologic studies at the Punta Torcida anticline depict a n Eocene composite section about 1600 m thick assigned to the La Despedida Group. This group is divided into three formations: Punta Torcida (215 m), Leticia (520 m), and Cerro Colorado (855 m). These formations are subdivide d into informal members. In the Punta Torcida Formation the lower member is composed of dark-gray mudstones with thin sandstone beds. The middle membe r of the Punta Torcida contains mudstones with small sandstone lenses. Both the lower and middle members have a high radiolarian content. The upper me mber contains mudstones. Lithologic succession, presence of pyrite, and a w ell-preserved benthic assemblage having little diversification suggest a re gressive sequence formed in shelf settings with a dysaerobic to anoxic bott om. Planktonic foraminifera indicate a Ypresian age (equivalent to upper Zo ne P6 to basal Zone P9 tropical planktonic zones). In the Leticia Formation the lower and upper members are glauconite rich, highly bioturbated sandst ones; in the middle member, rich in pyrite and carbonized plant material, t he glauconite is rare or absent. Large channels, filled with cross-stratifi ed sandstone lenses and having a general absence of planktonic foraminifera , suggest estuarine, restricted-marine settings. Planktonic foraminifera in dicate a late-middle Eocene age (basal Zone P12 to middle of Zone P14) indi cating a hiatus with the underlying Punta Torcida Formation. The Cerro Colo rado Formation, a coarsening- and thickening-upward succession of mudstones and sandstones, is a major, shelfal transgressive-regressive sequence punc tuated by minor deepening and shallowing. Its four members record different benthic assemblages that reflect cooling waters. The uppermost member bear s abundant radiolarians and planktonic foraminifera typical of an oxygen-mi nimum habitat and indicating a late Eocene age (upper Zone P15 to upper Zon e P16). The microfauna and stratigraphic position of the lower three member s are consistent with a latest middle Eocene age (middle of Zone P14 to low er Zone P15). In preliminary observations on the outcrop area of Late Cretaceous-Paleogen e strata between Cabo San Pablo and Bahia Thetis, two important angular unc onformities were recorded: one at the contact between the Estratos de Polic arpo and the Rio Bueno-La Despedida Group, and the other at the contact bet ween the La Despedida Group and the Cabo Pena-Rio Leona formations.