Magallanes Basin, Geological structure in Patagonia, Argentina and Chile.

The Magallanes Basin is a sedimentary basin in southern Patagonia covering about 170,000 to 200,000 square kilometres, with a NNW-SSE oriented shape. It is bounded to the west by the Andes and separated from the Malvinas Basin to the east by the Río Chico-Dungeness High. Its Jurassic rocks include the Cerro Toro Formation, and the Aysén Basin forms its northwesternmost sub-basin.

The basin formed roughly 150 million years ago during the Jurassic period and went through several phases of geological reshaping. This transformation from an extensional basin to one under compression shaped the landscape of southernmost South America.

The basin's rock formations provided shelters and caves for early humans; the Cueva del Milodón is the best known, with archaeological finds confirming ancient occupation. Near Cerro Guido and the Las Chinas river valley lies an important fossil and palaeontological site.

The basin stretches about 250 km north to south and reaches from the Andes foothills in Chile into Argentine Patagonia, so access points differ by country. Cueva del Milodón, near Puerto Natales, is one of the few easily reached sites, with rock shelters and caves once used by early humans. The fossil beds around Cerro Guido and the Las Chinas river valley lie on working estancia land and are best visited with local guides.

The basin holds most of Chile's coal reserves, far exceeding those of the Arauco Basin or the deposits around Valdivia such as Catamutún and Mulpún; the coals range from lignitic to sub-bituminous. Its sedimentary sequence also yielded three SALMA mammal ages: the Santacrucian and Friasian of the Early Miocene and the Pleistocene Ensenadan.