PERSONAL DE APOYO
MARTINIONI daniel Roberto
congresos y reuniones científicas
Título:
Cretaceous-Paleogene surface stratigraphy of the Austral Basin in the southernmost Andes: new evidences from central Tierra del Fuego, Argentina.
Autor/es:
D. R. MARTINIONI
Lugar:
Heidelberg
Reunión:
Congreso; 18th International Association of Sedimentologists Regional European Meeting of Sedimentology; 1997
Institución organizadora:
International Association of Sedimentologists
Resumen:
ABSTRACT:   Early attempts to depict the surface geology of the northern front of the Argentinean Andes of Tierra del Fuego were only made by extrapolating data from nearby localities across the island, but in place supporting evidence was lacking. Although the geological picture is still far from being complete, recent research by the author introduces significant new evidence for the stratigraphy and tectonic evolution of the previously unexplored mountainous area north of Lago Fagnano. The lithology is essentially dominated by mudstone facies; sandier sections and interbedded sandstone are less frequent, and conglomerate packages are restricted to a few stratigraphic intervals. The general trend suggests that the outcrops are progressively younger to the north, nevertheless, there are north verging reverse faults that locally repeat and invert the stratigraphic column. The oldest rocks in the area conform the Upper Jurassic Lemaire (or Tobífera) Formation, cropping out in the Río Azopardo - western Lago Fagnano surroundings. Black slates prevail along the northern shore of Lago Fagnano - southern Sierra de Beauvoir. These rocks only provide scarce inoceramid fragments and radiolarians. A ?Late Jurassic-Early Cretaceous age (s. l.) is ascribed to them considering the field relationships with the Upper Jurassic Lemaire Formation, and their strong slaty cleavage, typical of the former and almost trivial in the younger units. Similar black mudstones, without a metamorphic imprint, form the thick pile of sedimentary rocks in the centre of Sierra de Beauvoir. A poorly preserved ?Barremian-Albian fossil assemblage suggests a late Early Cretaceous (s. l.) age. Basaltic dykes, to be isotopically dated, cut through this unit after the main deformation phase. At Monte Taarsh - Hito XIX a thrust sheet involving roughly age equivalent rocks in the northern edge of the focused zone, the Hito XIX (or Vicuña) Formation with diagnostic upper Early Cretaceous fossil invertebrates, can be partly correlated with this interval, though there is some lithologic dissimilarity. A slightly different succession is recorded in part of northern Sierra de Beauvoir and along the east-west trending Río Mio and Río Claro valleys. This unit can roughly be divided into a lower mudstone dominated part and an upper succession of mudstone with intercalated sandstone. The lower portion is composed by massive to faintly laminated mudstone and siltstone resembling the rocks of Sierra de Beauvoir, but containing distinct possibly younger fossil invertebrates tentatively attributable to the Late Cretaceous (s. l.). The upper section shows a definite change in the homogeneous Cretaceous lithostratigraphy of Sierra de Beauvoir. It has alternating packages of moderately burrowed mudstone and massive mudstone with typical calcareous concretions and randomly intercalated parallel laminated, cross-stratified, or partially ripple cross-laminated fine to very coarse sandstone beds. Their lower contacts are sharp and mainly erosive, bed geometry ranges from lenticular to tabular, and ca. 50 cm is the average maximum thickness. Paleocurrents point to the north-northwest. Uppermost strata of this younger unit show up in Sierra de Apen, just north of Sierra de Beauvoir, along a section ca. 700 m thick, that provided Maastrichtian dinocysts, as well as ammonite fragments. The top of this unit is cut by an erosive surface that might have a regional extent. A sandier unit rests above the erosive unconformity in Sierra de Apen. Thickness involves about 100 m of a thin basal conglomerate layer succeeded by alternating sandstone with lesser siltstone and mudstone beds. The conglomerate consists of well rounded clasts of black mudstone and calcareous concretions, eroded from the underlaying uppermost Cretaceous unit, and quartz, indurated mudstone and acidic volcanics, probably derived from Lemaire and Yahgan Formations. The dinocysts assemblage in this unit has an Early Palaeocene age. Coarser clastics with a thick basal polymictic orthoconglomerate rest in Sierra de Apen on an apparently major erosive unconformity that laterally eliminates all the Lower Palaeocene and part of the Upper Cretaceous. There, this unit, over 200 m thick, is made up of lenticular conglomerates that form a fining and thinning upward succession. These lenses are encased in thin bedded mudstone and very fine sandstone rhythmites. The conglomerate clasts are composed of bluish grey sandstone and coquina possibly derived from the lowermost Tertiary, plus similar components to those in the Lower Palaeocene unit and sparse granite. Paleoflows were directed towards the northeast. Dynocysts in this unit suggest an Early to Late Palaeocene age. Lithologically and age equivalent successions appear nearby Lago Yehuin in the southeastern portion of the study area. A detached comparable conglomerate unit shows up just north of Hito XIX. These rocks were assigned to the Eocene Ballena Formation (surface and subsurface of Chilean Tierra del Fuego) on the basis of lithological affinities and the presence of a dinocysts assemblage also recorded in the Eocene Leticia Formation (eastern Tierra del Fuego). Austral (or Magallanes) and western Malvinas basins subsurface stratigraphy is roughly consistent with the observed surface geology. The stratigraphy of Sierra de Beauvoir reflects a long period of basin fill during the Early Cretaceous and the onset of the transition to a foreland basin foredeep starting in the Late Cretaceous with regionally significant major breaks during the uppermost Cretaceous-Paleogene.