OBSIDIAN SOURCES AND DISTRIBUTION IN SOUTHERNMOST PATAGONIA, SOUTH AMERICA

Several different types of rhyolite obsidian, with different geologic ages and source areas, have been documented in archeological sites in southernmost Patagonia south of 44ºS. The most widely distributed obsidian in southernmost Patagonia are two chemically distinct types of black obsidian, both found as cobbles concentrated in fluvial drainage systems on and around Pampa del Asador (48ºS; 71.4ºW), Santa Cruz, Argentina. These two chemically distinct types of black obsidian can be distinguished based solely on the trace-elements Rb, Sr, Zr and Y determined by XRF analysis. They have been referred to as obsidian types PDA-I and PDA-II, and have geologic ages of 6.2 and 6.4 Ma. In more detail, two chemically different types of PDA-II (PDA-IIa and PDA-IIb) can be distinguished based on rare-earth-element compositions determined by more complete ICP-MS analysis. Both among the cobbles of black obsidian from Pampa del Asador, and among artifacts fashioned from this obsidian, >80% are type PDA-I and <20% are type PDA-II. Artifacts fashioned from these black obsidians occur, within occupational levels dated in some cases as old as >9,700 BP, in archeological sites up to >600 km to the south in Tierra del Fuego and >800 km to the northeast on Peninsula Valdes along the Atlantic coast. A banded grey-green obsidian, with geologic age of 2.3 Ma, derived from somewhere within Cordillera Baguales (50.5ºS; 72ºW), Santa Cruz, Argentina, occurs as artifacts up to >250 km east along the Atlantic coast. Green obsidian, with geologic age of 17.1 Ma, derived from somewhere around Seno Otway (53ºS; 71.5ºW), Magallanes, Chile, occurs as artifacts in both marine hunter-gatherer sites up to >350 km southeast in Tierra del Fuego and >250 km north along the Pacific coast, as well as in terrestrial hunter-gatherer sites up to >250 km east near the Atlantic coast. Obsidian in archaeological sites is scarce in areas remote from these three known sources of obsidian. For example, in the Pali-Aike and Fell's Cave sites near the Atlantic coast in Magallanes, Chile, obsidian makes up only 1.3% of all the artifacts excavated by Junius Bird (59 of 4,900 artifacts). The three different obsidian types found in these sites (black, green and banded grey-green) have come from the three different sources described above, which occur between >250 and >500 km away.