Stratigraphy of unconformable Upper Cretaceous - Paleogene units in central Tierra del Fuego, Argentina.

D. R. MARTINIONI; E. B. OLIVERO; S. PALAMARCZUK

Buenos Aires

Simposio; Simposio “Paleógeno de América del Sur”; 1996

IUGS-IGCP - Project 301 South American Regional Comittee on Paleogene Stratigraphy

ABSTRACT:   Fairly continuous outcrops of Upper Cretaceous to Paleogene marine rocks are first described in the northern hillside of the highest peaks of Sierra de Apen. The exposed rocks form part of a thrust-sheet, uplifted along a north vergent fracture. Bedding strike has an overall E-W trend and dip is around 50° to the South. The section comprises a lower mudstone dominated unit, a middle sandier unit and an upper conglomeratic unit, respectively bounded by erosive unconformities, The lower unit, presumably over 700 m thick, is characterized by massive to faintly laminated silty to sandy mudstone. Some burrowed intervals display a variety of trace fossils, commonly preserved in calcareous concretions. Intercalated beds, up to 50 cm thick, of tabular, massive or parallel laminated and/or cross-stratified sandstone are less frequent. Cross-stratification foresets indicate NNW directed paleocurrents. Poorly preserved ammonite imprints are rare. Dinocysts samples in this unit indicate a Maastrichtian age. The top of the unit is cut by an erosion surface, above which lays the sandier middle unit, about 100 m thick. It starts with a thin conglomerate layer, followed by a succession of alternating sandstone, siltstone and mudstone beds. The cong1omerate consists of well rounded clasts of black mudstone and calcareous concretion, eroded from the underlaying unit, and quartz, indurated mudstone and acidic volcanics, probably derived from Lemaire and Yahgan formations. The dinocysts assemblage in this middle unit has a probable Early Paleocene age. The overlaying upper unit has a very coarse and thick basalt polymictic orthoconglomerate resting on an apparently major erosive unconformity that laterally eliminates all the middle and part of the lower units within a distance of some hundreds of meters. The upper unit, over 200 m thick, is made up of lenticular conglomerates, tens of meters wide and several meters thick, that form a fining and thinning upward succession. These lenses are encased in very thin bedded mudstone and clean very tine sandstone ritmites with small-scale synsedimentary folds. The conglomerate clasts are composed of bluish gray sandstone and marine fossil invertebrates coquina -presumably derived from underlaying basal Tertiary strata-, similar components to that of the middle unit and sparse granite. Clast imbrication reflects NE directed paleoflows. Dynocysts in this unit suggest an Upper Paleocene age. The Maastrichtian lower unit is best explained as deposited below wave base level in an outer shelf. Both Paleogene units are believed to form part of a fan-deltaic succession. Though apparently somewhat older, the conglomerate upper unit corresponds to part of the Chilean Ballena Formation and roughly age equivalent comparable rocks recently described in Argentina by the authors just NE of Hito XIX and nearby Lago Yehuin. The unconformities recorded in outcrop appear to be in agreement with coeval horizons in the subsurface of both Austral-Magallanes and western Malvinas basins. The NNW directed sediment dispersal pattern of the Upper Cretaceous unit probably mirrors early stages of the foreland basin development, whereas the coarser Paleogene units represent deposition in the already well defined foreland basin in front of the rising cordillera.