CONGLOMERADOS DEL PALEÓGENO EN TIERRA DEL FUEGO: EVIDENCIAS DE DISCORDANCIA ENTRE EL CRETÁCICO SUPERIOR- (PALEOCENO) Y EL EOCENO DE CUENCA AUSTRAL

Abstract

A 40 m thick Paleogene conglomerate marine succession located just northeast of Hito XIX is described for the first time. A faulted block of Aptian-Albian strata rests on a north vergent reverse fault above this succession which, in turn, is thrust against younger rocks on its northern front. Coarse polymictic orthoconglomerate is dominant, with minor interbedded pebbly sandstone, fine sandstone, and mudstone. Conglomerate clast imbrication indicates northwest-directed palaeocurrents. Prevalent clasts are black shale and acidic volcanics; less common are bluish-gray sandstone, calcareous concretion, quartz, and granite. Autochtonous Eocene and reworked, Late Cretaceous (Santonian to Campanian) dinocyst were recovered from the interbedded fine-grained facies. To the south-southeast, in Sierra de Apen, similar rocks lay disconformably on basal Cenozoic and on uppermost Cretaceous rocks, suggesting an unconformity with a regional extension. To the east, near Lago Yehuin, a conglomerate succession previously assigned to the Beauvoir Formation (Lower Cretaceous), also bears Paleogene microfossils. Lithofacies and fossil evidence suggest that these conglomerates correlate with the Chilean Ballena Formation (Eocene). Based on subsurface data, the latter unit was interpreted as fandelta deposits fed from an uplifted area located to the south. Stratigraphic relationships, clast composition, paleocurrents, and inferred correlation of these deposits with the Ballena Formation are all consistent, and indicate that they represent a major break in the sedimentation of the Austral Basin following a marked tectonic uplift of upper Mesozoic rocks.

KEY WORDS. Paleogene. Stratigraphy. Conglomerates. Dinoflagellates. Austral Basin. Ballena Formation. Tierra del Fuego. Argentina.