Sedimentología de las formaciones Lemaire y Yahgán (Jurásico-Cretácico) en Tierra del Fuego.

E. B. OLIVERO; D. R. MARTINIONI

Buenos Aires

Congreso; XIII Congreso Geológico Argentino y III Congreso de Exploración de Hidrocarburos; 1996

Asociación Geológica Argentina

ABSTRACT:   In southern Tierra del Fuego, the thick Jurassic-Cretaceous Lemaire and Yahgán formations are interpreted as the filling of a small marginal basin. Deformation and metamorphism during subsequent closure of the basin darken depositional features and the sedimentology of these units is poorly known. Detailed field and petrographic studies allow for a better understanding of their lithology, sedimentary facies, provenance, and paleoenvironmental settings. The Lemaire Formation is a complex association of acidic volcanic and volcaniclastic, marine rocks. The sedimentary rocks include breccias, resedimented conglomerates with well rounded schist clasts, classical turbidites, massive sandstone, and black, probably radiolarian mudstone. Dominant composition is quartz, alkaline feldspar, and rhyolitic fragments, suggesting derivation from acidic volcanic rocks. Previously included in the Yahgán Formation, they are interpreted here as resedimented volcaniclastic rocks related to the submarine, rhyolitic volcanism of the Lemaire Formation. Three main facies are recognized within the Yahgán Formation: a) black mudstone, fine-grained turbidites and tuff; b) classical turbidites; and c) massive to graded sandstone. The basal Yahgán Formation is characterized by thick packages of mudstone, fine-grained turbidites and tuff. These fine-grained rocks are also recurrently associated with classical turbidites and massive sandstone in the mid and upper part of the unit. Petrographic composition of sandstone is uniform and dominated by lithic andesitic fragments and plagioclase, suggesting derivation from the volcanic arc, located to the South in the Fuegian Archipelago. Paleocurrent indicators also suggest currents flowing with a North-directed component. Most of the lower fine-grained rocks have a similar composition; the basal mudstone and tuff, however, are rich in quartz and feldspar reflecting the last stages of the Lemaire volcanism. Thicker and coarser facies of the Yahgán Formation are restricted to the South, nearby the volcanic arc. This clastic-wedge geometry suggests a strong tectonic control and a sedimentary system dominated by aggradation, not by progradation of submarine fans as it was previously interpreted.