Abstract

Recognizing folded and thrusted Eocene foreland strata of the oil-producing Austral Basin provides new insights on the Paleogene evolution of Tierra del Fuego Island and the adjacent Atlantic shelf. Field mapping and petrographic and micropaleontologic studies at the Punta Torcida anticline depict an 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 subdivided into informal members. In the Punta Torcida Formation the lower member is composed of dark-gray mudstones with thin sandstone beds. The middle member of the Punta Torcida contains mudstones with small sandstone lenses. Both the lower and middle members have a high radiolarian content. The upper member contains mudstones. Lithologic succession, presence of pyrite, and a well-preserved benthic assemblage having little diversification suggest a regressive sequence formed in shelf settings with a dysaerobic to anoxic bottom. Planktonic foraminifera indicate a Ypresian age (equivalent to upper Zone P6 to basal Zone P9 tropical planktonic zones). In the Leticia Formation the lower and upper members are glauconite rich, highly bioturbated sandstones; in the middle member, rich in pyrite and carbonized plant material, the glauconite is rare or absent. Large channels, filled with cross-stratified sandstone lenses and having a general absence of planktonic foraminifera, suggest estuarine, restricted-marine settings. Planktonic foraminifera indicate a late-middle Eocene age (basal Zone P12 to middle of Zone P14) indicating a hiatus with the underlying Punta Torcida Formation. The Cerro Colorado Formation, a coarsening- and thickening-upward succession of mudstones and sandstones, is a major shelfal transgressive-regressive sequence punctuated by minor deepening and shallowing. Its four members record different benthic assemblages that reflect cooling waters. The uppermost member bears abundant radiolarians and planktonic foraminifera typical of an oxygen-minimum habitat and indicating a late Eocene age (upper Zone P15 to upper Zone P16). The microfauna and stratigraphic position of the lower three members are consistent with a latest middle Eocene age (middle of Zone P14 to lower Zone P15). In preliminary observations on the outcrop area of Late Cretaceous-Paleogene strata between Cabo San Pablo and Bahia Thetis, two important angular unconformities were recorded: one at the contact between the Estratos de Policarpo and the Rio Bueno-La Despedida Group, and the other at the contact between the La Despedida Group and the Cabo Pena-Rio Leona formations.

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