CABO BLANCO: AN UNKNOWN SILURIAN-DEVONIAN ?? FRAGMENT OF WEST GONDWANA IN SOUTHERN PATAGONIA, ARGENTINA

MARCELO MÁRQUEZ; CÉSAR NAVARRETE

Buzios

Congreso; Gondwana 14° ?East meets West"; 2011

Cabo Blanco is a small outcrop (1km long for a 0,3km wide) located in the northeastern part of Deseado Massif (47°12´20"SL - 65°44´22"WL), on the Atlantic coast. In this geological context there is no record of Early-Palaeozoic sedimentary rocks implying a gap in the geological evolution of the Deseado Massif in comparison with adjacent geological provinces such Malvinas Islands, NorthPatagonian Massif, Ventania and South Africa.As described by Darwin (1846) the Cabo Blanco outcrop is composed of quartzites. It is a folded sedimentary succession mainly composed of finely stratified fine-grained quartz-rich sandstones intercalated with shales displaying an apparent tickness close to 120 m. Shales predominate in the lower part of the succession, with some beds bearing abundant indetermined gastropods, bivalves and trace fossils, whereas the upper part of the succession includes a 5 meter thick clast-supported conglomerate overlie by an oligostrome with a minimum thickness of 10 m. The sedimentary structures recognized include parallel layering and convolute stratification. The succession is strongly folded, with a general north-south strike and a ca 45º dip to the west with tight folds with metric wavelength and almost parallel limbs. Whilst the southernmost parts of the outcrop display an assimetric anticline with N60°W strike. Another important characteristic is the intense contact metamorphism (hornblende hornfels) that affects all the rocks, converting the original textures to a granophyric one with neocrystallization of rutile and almost destroying the carbonate fossils. In summary the main features of Cabo Blanco outcrop indicate that is part of a marine sedimentary sequence deposited in shallow water conditions that was folded and affected by contact metamorphism. There are no similar units combining such geological conditions in the Deseado Massif, where the following stratigraphic record is present: 1) Ductile deformed metamorphic and igneous rocks from Cambrian (540 My) to Ordovician (470-406 My) age, 2) Early Permian to Triassic continental sediments and tuffs bearing remains of Glossopteris, Gangamopteris and Dicroidium, 3) I-type Triassic granitoids (208 My) that intrude and metamorphose the sediments, 4) Widespread extensional volcanism that generated mesosilicic-basic to acidic dominated related rocks, exposed as ignimbrites flows, tuffs, dome complexes, laharic diamictons flows, etc. 4) Early cretaceous continental tuffs and paleosoils. 6) Tertiary marine/continental sediments and basalt flows and finally diverse Quaternary deposits.The nearest unit with comparable facies is the Silurian-Devonian Bahía Fox formation of Malvinas Islands.If this correlation, previously made by Darwin, is correct, the marine succession of Cabo Blanco may represent and important keystone to reconstruct the fragmented western border of Gondwana.